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1 ROLE OF ALTERNATIVE SPLICING FACTOR MBNL 1 IN PATHOGENESIS OF MYOTONIC DYSTROPHY By JIHAE SHIN A DISSERTATION PRESE NTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL F ULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORID A 2009
2 2009 Jihae Shin
3 To my parents
4 ACKNOWLEDGEMENTS First and foremost, I would like to thank my mentor Dr. Maurice Swanson. I have been amazingly f ortunate to have an advisor who is extremely patient, supportive and thinks the best of his students. He was always available to discuss research projects as well as other issues important to my career as a scientist. Once he told me that most students imp rint on their mentors. I just hope I have effectively imprinted during my thesis work since he is an exceptional scientist with persistence, passion, curiosity and creativity. I would like to thank my committee members, Drs. Brian Harfe, Hideko Kasahara a nd Paul Oh for their direction, dedication and invaluable suggestions during my graduate study. I am grateful to Dr. Laura Ranum at University of Minnesota for her encouragement and support for my career. I thank my colleagues in the Swanson lab who all be came great friends, especially Yuan Yuan, Michael Poulos, Jason ORourke, Konstantinos Charizanis and Ranjan Batra. Their friendship made my years in graduate school very enjoyable and their passion and enthusiasm for research was truly inspirational. I al so acknowledge Joyce Connors for her outstanding support in handling administration. This dissertation would not have been possible without the invaluable help and contributions of many people. I would like to thank former lab members Drs. Rahul Kanadia an d Dan Tuttle as well as my collaborators Drs. Stuart Beattie, Thurman Wheeler, Charles Thornton, Jennifer Embury, Changquing Xia and Glenn Walter. The technical support of Fan Ye was also essential for this study. I am also indebted to my wonderful advisor s during my undergraduate studies. Dr. Kunsoo Rhee at Seoul National University and Dr. Ryoichi Matsuda at the University of Tokyo gave me priceless advice and guidance when I was most uncertain about my future and career
5 Last but not least, I would like to thank m y family. I am grateful to my parents, Boky e ong H eo and Bukyun Shin for their unconditional love and support. They have always ha d faith and confidence in me, which has been my source of strength for these years. I also thank my loving brother and sister, Hyunseung and Jiyeon Shin who have been m y best friends I would like to thank my little dog Hiro Shin for making me smile and still feel at home when I am thousands of miles away.
6 TABLE OF CONTENTS page ACKNOWLE DGEMENTS ................................................................................................................. 4 LIST OF TABLES ................................................................................................................................ 8 LIST OF FIGURES .............................................................................................................................. 9 LIST OF ABBER E VIATIONS ......................................................................................................... 10 ABSTRACT ........................................................................................................................................ 12 CHAPTERS 1 GENETIC COMPLEMENTA TION OF MBNL1 REVERSES MYOTONIA AND MISSPLICING IN A POLY(CUG) MOUSE MODEL FOR MYOTONIC DYSTROPHY ............................................................................................................................. 14 Introduction ................................................................................................................................. 14 Alternative S plicing during Post -natal Development ........................................................ 14 My o tonic D ystrophy i s an RNA -mediated Disease .......................................................... 15 Molecular Defects in DM: Perturbation of Alternative Splicing ...................................... 17 Therapeutic Approaches for DM ........................................................................................ 21 Results .......................................................................................................................................... 22 Selection of the Mbnl Isoform for Overexpression in Skeletal Muscle ........................... 22 Reversal of Myotonia in HSALR mice after Overexpression of Mbnl1 ............................ 25 Reversal of Myotonia Correlates with Rescue of Miss plicing of Specific Developmentally Regulated Exons ................................................................................. 26 Systemic Delivery of Mbnl1 in Neonate Poly(CUG) Mice .............................................. 29 Discussion .................................................................................................................................... 31 2 GENETIC FACTORS AFFECT DISEASE -ASSOCIATED PHENOTYPES IN MBNL1 KNOCKOUT MOUSE MODEL FOR MYOTONIC DYSTROPHY ..................... 44 Introduction ................................................................................................................................. 44 Myotonic D ystrophy i s a Variable Disease ........................................................................ 44 Mouse Models to Understand the P athogenesis of DM .................................................... 45 Influence of Genetic Background on Disease -Associated Phenotypes in Genetically Engineered Mice .............................................................................................................. 48 Congenic M bnl1 E3/ Mice and the Role of Mbnl1 in DM Pathogenesis ..................... 49 Results .......................................................................................................................................... 50 Effect of Genetic Background on Survival of Mbnl1 Knockout Mice ............................. 50 Mbnl1 E3/ 129 Mice Have Defects in Thymus and Skin Epithelium ........................... 50 Mbnl1 E3/ Bl6 Mice Develop a Severe Movement Def ic it and Muscle Abnormalities ................................................................................................................... 52
7 Age D ependent P rogression of M yopathic C hanges and A berrant E ndplate T opology in Mbnl1 E3/ B l 6 M uscles .......................................................................... 54 Abnormal Contractile P roperties in Mbnl1 E3/ Bl6 M uscle s ........................................ 55 Aberrant S plicing of Ryr1 Correlates with M yopathic C hanges ...................................... 56 Mbnl1 Binds to Ryr1 Exon 70 RNA i n Vitro but Does Not Regulate Splicing of an Ryr1 Minigene in C2C12 Cells ....................................................................................... 57 Discussion .................................................................................................................................... 58 3 CONCLUDING REMARKS AND FUTURE DIRECTIONS ................................................ 78 4 MATERIALS AND METHODS ............................................................................................... 80 Mice .............................................................................................................................................. 80 AAV2 /1 -mycMbnl1/41 Virus Preparation and Injection Protocol .......................................... 80 Analysis of Mbnl1 Isoforms in Adult Muscle ........................................................................... 81 RNA Sp licing .............................................................................................................................. 82 Electromyography ....................................................................................................................... 84 Histology and Immunohistochemistry ....................................................................................... 84 Rotarod Test ................................................................................................................................ 84 Analysis of T Cell Population by Flow Cytometry ................................................................... 85 In Vitro Force Measurements ..................................................................................................... 85 Neuromuscular Junction Staining .............................................................................................. 86 Calpain Activity Assay ............................................................................................................... 86 Ryr1 Minigene and Photocrosslinking A ssay ........................................................................... 87 LIST OF REFERENCES ................................................................................................................... 90 BIOGRAPHICAL SKETCH ........................................................................................................... 102
8 LIST OF TABLES Table page 2 1 Genotypic evaluation of B l 6 and 129 congenic mice .......................................................... 68 4 1 Sequence of primers ............................................................................................................... 89
9 LIST OF FIGURES Figu re page 1 1 RNA -mediated pathogenesis in myotonic dystrophy .......................................................... 35 1 2 Loss -of -function of alternative splicing factor MBNL1 in myotonic dystrophy ............... 36 1 3 Expression of Mbnl1 isoforms in skeletal muscle ............................................................... 37 1 4 Mbnl1 isoforms with the 4XC3H motif has a hi gher splicing activity .............................. 38 1 5 Overexpression of Mbnl1 leads to free Mbnl1 protein in nucleoplasm ............................. 39 1 6 Reversal of myotonia follo wing Mbnl1 overexpression...................................................... 40 1 7 Mbnl1 overexpression promotes adult splicing patterns ..................................................... 41 1 8 Time course for reversal of RNA miss plicing ...................................................................... 42 1 9 Ectopic mycMbnl1/41 expression after AAV2/1 -mycMbnl1/41 injection ........................ 43 2 1 Mbnl1 E3/ mouse is a functional null ................................................................................ 67 2 2 Shortened life span in congenic Mbnl1 E3/ Bl6 and 129 mice ........................................ 69 2 3 Mbnl1 E3/ 129 congenics have defects in thymus and skin ............................................ 70 2 4 Severe movement deficit in Mbnl1 E3/ Bl6 m ouse is separable from myotonia ........... 71 2 5 Abnormal muscle structure and fiber type switch in Mbnl1 E3/ Bl6 muscle ................. 72 2 6 Age dependent progressive muscle histopathology and abnormal end plate topology ..... 73 2 7 Reduced force and abnormal contraction relaxation kinetics of Mbnl1 E3/ Bl6 soleus muscles. ....................................................................................................................... 74 2 8 Reduced force generation of Mbnl1 E3/ Bl6 extensor digitorum longus (EDL) muscles .................................................................................................................................... 75 2 9 Aberrant splicing of Ryr1 and Tnnt3 are correlated with myopathic changes of Mbnl1 E3/ Bl6 congenic mice. ........................................................................................... 76 2 10 M odel for association of MBNL1 and genetic modifier X with the spliceosomal subunit U1 snRNP .................................................................................................................. 77
10 LIST OF ABBER E VIATIONS DM myotonic dystrophy UTR untranslated region ds RNA double stranded RNA HSA human skeletal actin MBNL musclebli ndlike CELF CUGBP1/ETR 3 like factors Tnnt troponin T Clcn chloride channel NMD nonsense mediated decay AON antisense oligonucleotide AAV adeno associated virus CF cystic fibrosis P postnatal day TA tibialis anterior SR sarcoplasmic reticulum Ryr ryanodin e receptor Serca SR /endoplasmic reticulum Ca2+ ATPase Ca calcium f fetal H&E hematoxylin and eosin Bl6 C57BL/6 129 129/Sv T1/2R time to half relaxation NMJ neuromuscular junction
11 AChR acetylcholine receptor nt nucleotide KO knockout
12 Abstract of Dissert ation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy ROLE OF ALTERNATIVE SPLICING FACTOR MBNL 1 IN PATHOGENESIS OF MYOTONIC DYSTROPHY By Jihae S hin December 2009 Chair: Maurice Swanson Major: Medical Sciences Genetics Myotonic dystrophy (DM) is an RNA -mediated disease caused by a non -coding CTG repeat expansion in the DMPK gene (DM type 1, DM1) or a CCTG expansion in ZNF9 (DM type 2, DM2). Pat hogenesis of DM involves dysregulation of alternative splicing factors such as the MBNL1 protein. MBNL1 is sequestered by mutant transcripts containing C(C)UG repeat RNAs which form discrete nuclear RNA foci that, in turn, leads to the loss of MBNL1 functi on. Because MBNL1 is an RNA binding protein that regulates alternative splicing of a specific subset of pre -mRNAs during postnatal development, loss of this splicing factor due to its sequestration results in missplicing of MBNL1 target pre mRNAs and pertu rbation of developmental signals. DM is a systemic disease that affects multiple organs and key features of this disease include myotonia (hyperexcitability of muscle), myopathy (muscle weakness), cardiac conduction defects and subcapsular cataracts. Howev er DM1 is also remarkably variable in severity and penetrance mainly due to somatic mosaicism of CTG repeat size but also due to the variability in genetic background between individuals. We hypothesized that MBNL1 loss of function due to
13 sequestration is a primary pathogenic event in DM and is responsible for disease associated phenotypes owing to the failure of developmental transitions as a result of mistakes in alternative splicing. In this study, we show that over expression of Mbnl1 in vivo mediated b y transduction of skeletal muscle with a recombinant adeno associated viral vector is sufficient to rescue myotonia and missplicing in the HSALR poly(CUG) mouse model for DM, suggesting that loss of MBNL1 activity is primarily responsible for disease patho genesis. We also report that Mbnl1 deficiency leads to defects not only in skeletal muscle but also other organs like thymus and skin in Mbnl1 knockout mice depending on the genetic background. This observation indicates that MBNL1 has a broad role in deve lopmental pathways. Furthermore, using congenic Mbnl1 knockout mice, we provide evidence that functional and structural muscle abnormalities in DM may be separable from myotonia and may be attributed to the altered splicing of genes important for calcium h omeostasis, including the ryanodine receptor. Our results suggest a fundamental role of MBNL1 in development and disease and provide a theoretical and experimental basis for the development of novel therapies for this neuromuscular disease
14 CHAPTER 1 G ENETIC COMPLEMENTATI ON OF MBNL1 REVERSES MYOTONIA AND MISSPLICING IN A POL Y(CUG) MOUSE MODEL F OR MYOTONIC DYSTROPHY Introduction Alternative Splicing during Post -natal Development During mammalian postnatal development, many organs undergo dramatic morphol ogical changes, which require precise regulation of gene expression. For example, during embryonic myogenesis, myoblasts of somatic origin proliferate and fuse to form multinucleated myofibers, which require high expression of myogenic proteins. On the other hand, postnatal growth of skeletal muscle is achieved by hypertrophy rather than an increase in fiber numbers and this process demands high levels of contractile proteins. Regulation of gene expression during this developmental transition is controlled in a complex, yet very coordinated, manner with the involvement of multiple layers of regulatory machinery including transcription, mRNA processing as well as translation. Alternative splicing is a critical step for the expression of most genes in eukaryot ic cells and it is a very efficient way to generate a variety of proteins that are physiologically and functionally distinct from the limited gene pool. For many tissues including skeletal muscle, expression of subsets of developmentally regulated genes is controlled by temporal and spatial regulation of alternative splicing. Hence, disruption of normal alternative splicing events during development results in the expression of misspliced forms of proteins that cannot support the functional requirements of a specific developmental stage. In many cases, this is due to frameshifting that leads to the loss of protein products. A few of the inherited genetic disorders are caused by defects in the regulation of alternative
15 splicing. Myotonic dystrophy (DM) is a good example of a disease caused by perturbation of developmentally regulated splicing events. My o tonic D ystrophy is an RNA mediated Disease DM is a late onset neuromuscular disease that affects 1 in 8000 adults making it the most common form of adult on set muscular dystrophy (1). Clinical manifestations inc l ude delay ed muscle relaxation (myotonia), muscle weakness (myopathy), early -onset cataracts and cardiac c onduction defects. As the name suggests, myotonia is a very unique clinical feature of this disease, which is often described as stiffness by patients and it makes DM distinctive from other type of muscular dystrophy. At the molecular level, DM is caused by a CTG microsatellite expansion in the 3 untranslated region ( UTR ) of the DM protein kinase ( DMPK ) gene (DM1) and CCTG repeat expansions in the first intron of the ZNF9 gene (DM2) (Fig. 1 1 A ) (2, 3) In mitotic cells, the expansions of repeat s involve mistakes during lagging-strand DNA replication. The formation of unusual DNA structures and DNA slippage lead to the repeat expansions in the nascent strand of DNA. For post -mitotic cells, DNA repair involving DNA synthesis is responsible for expansion mutations (Fig. 1 1 B ) (4). Normal DMPK microsatellite lengths range from 5 37, whereas the DM1affected population carries expansions from 50 to thousands of CT G repeats In contrast, DM 2 expansions can be massive with a disease range of 75 to >11,000 CCTG repeats. DM2 patients are generally mildly affected compared to DM1 with slight differences in the presentation of phenotypes however, overall they share very similar symptoms (5).
16 DM is an autosomal dominant disease, which means one mutant allele is enough to cause this genetically inherited disorder (1). In general, the number of repeats has a positive correlation with the severity of disease and a negative correlation with the age of onset. CTG repeats can expand greatly in the germline leading to genetic ant icipation, which means that disease symptoms tend to be more severe and occur earlier in successive generations. Somatic mosaicism is also common, where expansion mutations can be significantly different in various tissues in a single individual. Even in t he same tissue, repeats tend to get larger over time, which makes the disease highly variable and complex. Several hypotheses, including DMPK haploinsufficiency or chromatin structur al change s at the DM 1 locus (DMWD DMPK and SIX5 ) hav e been proposed to explain how microsatellite expan sions in the non-coding region of a gene can cause a dominant ly inherited genetic disease (6 8) These three genes are tightly clustered within an ~20 kb region with very short inter genic regions and the CTG expansion induces local chromatin structure changes and represses the expression of the genes at this locus. Dmpk knockout mice have been generated to directly test this idea and they develop cardiac conduction defects similar to DM and a late onset mild myopathy (9, 10) In addition, heterozygous loss of Six5 in mice leads to ocular central cataracts even though they are different than the type of subcapsular cataract that DM patients com monly develop (11) However, these hypotheses based on the DM1 locus fail to explain why a related expansion in a different gene located on another chromosome (DM2) results in sim ilar phenotypes. On the other hand, a series of recent studies support a toxic RNA gain -of -function hypothesis as the strongest disease model In this model, mutant transcripts with C U G or
17 CCUG repeats are toxic because they fold i nto stable double strand ed (ds) RNA hairpins that are not able to be exported out of nucleus and accumulate in nuclear RNA foci These mutant RNAs disrupt RNA processing events in trans by altering the normal developmental regulation of alternative splicing (12 14) (Fig. 1 2). This idea is supported by the RNA fluorescence in situ hybridization (FISH) studies on DM1 skeletal muscle, which indicated that mutant DMPK transcripts are accumulated within nuclear foci whereas normal allele tran scripts are exported out of nucleus and translated. In addition, a c rystal structure of an 18-bp RNA containing six CUG repeats confirmed that CUG repeats RNA forms a pseudohelical structure similar to a standard A -form helix (15, 16) To directly test the hypothesis that CUG expansions in the pathogenic range are toxic independent of the gen e context, transgenic mouse lines expressing 250 CTG repeats in the 3 UTR of the human skeletal actin ( HSA ) gene (HSALR) were generated (17) HSALR transcripts formed multiple foci in skeletal muscle nuclei that are similar to those observed in DM patients. HSALR mice also show severe hindlimb myotonia and dystrophic muscle features de monstrating that CTG repeats alone are sufficient to recapitulate DM -like muscle pathology. Importantly, the expression level of the HSALR transgene was proportional to the severity of the phenotype. Molecular Defects in DM: Perturbation of Alternative Spl icing How do the mutant RNA transcript s retained in the nucleus exert a toxic effect? A key hypothesis in the field is that these mutant transcripts prohibit two RNA binding protein families, muscleblind like (MBNL) and CUGBP1/ETR 3 like factors (CELF), fr om performing their cellular functions (7, 8, 1820) Steady state levels of CUGBP1, which is
18 the most studied member of the CELF family of proteins, are increased in DM1 tissues due to hyperphosphorylation and st abilization through Protein Kinase C (PKC) activation (21 24) In contrast, MBNL activity is decreased because these proteins directly bind to the CUG repeats which leads to the loss of function due to sequestration (12, 2527) A recent study reported that MBNL1 proteins form a ring -like structure and preferentially bind to the GC rich ds RNA hairpin containing a pyrimidine mismatch, which is similar to the structure of tox ic CUG repeats (see Fig 1 2 ) (28) What are the cellular functions of these proteins? CELF and MBNL function antagonistically as a lternative splicing factors such that CELF promotes inclusion of fetal exons, while MBNL promotes exclusion of fetal exons and/or inclusion of adult specific exons during development (18 20, 29) As a result, embryonic splicing patterns are retained i n the DM patient cells because of increased CEL F and decreased MBNL activity (19, 23) Consequently, the embryonic isoforms of many proteins lead to a functional failure in adult tissues In a previous study done in our lab, we tested the hypothesis that loss of MBNL 1 function due to sequestration results in pathogenic effects by generating a mouse model in which the Mbnl1 isoforms that are capable of binding to CUG repeats were ablated (Mbnl1 E3/ ) (19) These mice recapitulate key features of DM including myotonia, subcapsular cataracts cardiac conduction defects and mis splicing of certain pre -mRNAs such as cardiac troponin T (Tnnt2), skeletal musc le troponin T (Tnnt3), insulin receptor (IR) and muscle specific chloride channel (Clcn1). This study suggests that loss of MBNL1 function is directly related to the pathogenesis of DM. In addition to MBNL1 humans and mice have two additional MBNL gene fa mily members, MBNL2 and MBNL3 MBNL
19 proteins share ~90% similarity in the RNA binding motif and all three MBNL proteins have similar splicing activities when ectopically expressed in cultured cells and they also colocalize with CUG repeat RNA in nuclear fo ci (18, 30) Mbnl3 is mostly expressed embryonically and is not expressed in adult tissues however Mbnl2 is highly expressed in multiple adult tissues (31) Mbnl2 knockout mice were generated using available gene trapped alleles inserted into ES cells to test the functional role of the protein in vivo, but the phenotypes of these mice are mild to non-existent (32, 33) This result argues against a primary role of MBNL2 in DM pathogenesis. Cugbp1 overexpression transgenic mice were also generated to determine whether increased CUGBP1 function is sufficient to recapitulate DM. These mice display neonatal lethality when Cugbp1 expression is 4 6 fold above endogenous levels in heart and skeletal muscle (34) and they show abnormal muscle s tructure and missplicing of Tnnt2 and Clcn1 pre -mRNAs. These results support the possibility that elevated CUGBP1 activity, in addition to loss of MBNL activity, contributes to DM pathogenesis. MBNL1 Regulates Alternative Splicing during Post -natal Develop ment In many cases of aberrant splicing events in DM, there is a failure to make the transition from the fetal to adult splicing pattern that results in a disruption of a developme ntally regulated isoform switch For example, aberrant splicing of the muscl e specific chloride c hannel ( CLCN1 ) cause s myotonia in DM, as it fails to switch to the adult splicing pattern during postnatal development (35, 36) In DM affected individuals, inclusion of exon 7a put s the mRNA o ut of frame and introduce s a premature stop codon. These mRNAs are either degraded by the nonsense mediated decay (NMD) pathway or
20 produce trun cated forms of the chloride channel. Since chloride channel proteins oligomer ize into functional units truncated protein isoform s generated by exon 7a inclusion exert a dominant -negative effect on the density of functional channels in the muscle membrane, consequently decreasing the ionic conductance (37, 38) Furthermore, m orpholino antisense oligonucleotide (AON) targeting of the 3 splice site of Clcn1 exon 7a reverses the defect of Clcn1 alternative splicing, increases expression of the protein in the surface membrane and ultimately rescues myotonia in mouse models for DM ( HSALR and Mbnl1 E3/ ) which indicates that aberrant fetal exon inclusion of Clcn1 is the molecular defect underlying the myotonic phenotype of DM (39) While a specific subset of developmentally regulated pre -mRNAs is misregulated in DM, this perturbation of splicing regulation seems to affect many genes. In a recent study that examined splicing in skeletal muscle of Mbnl1 E3/ mice using splicing sensitive microarrays, more than 200 novel splicing events wer e found altered. (Du et al. in press). Also, there was a striking concordance of missplicing events between HSALR and Mbnl1 E3/ muscles, which suggests that loss of MBNL1 is the primary event for splicing defects in DM muscle pathology. Although loss of MBNL1 protein seem s to play a major role in DM pathogenesis, t here are other RNA binding proteins that are implicated in DM pathogenesis such as hnRNP H and the transcription factor Sp1 (40, 41) even though thei r involvement in DM pathogenesis seems to be limited. HnRNP H proteins are colocalized with mutant transcripts in the nuclear foci of DM patient cells by RNA fluorescence in situ hybridization (RNA -FISH) staining which may cause decreased activity of the se proteins.
21 HnRNP H has also shown to interact with MBNL 1 and CUGBP1 to regulate i nsulin receptor (IR) splicing, which is mis regulated in DM patients (21) Therapeutic Approaches for DM W e hypothesized that dysregulation of development al splicing events by decreased MBNL1 activity plays a major role in DM pathogenesis, rather than increased CUGBP1 activity or decreased hnRNP H acti vity. Thus increasing nuclear MBNL 1 availability should be sufficient to reverse the splicing events and the DM muscle phenotype including myotonia and muscle histopathology This study provides a proof -of principle that MBNL1 loss -of -function is a pri mary pathogenic event in DM. This study will also examine the possibility of using a gene therapy approach to upregulate MBNL1 proteins as a therapeutic treatment. Many drugs have been used to treat DM patients including selenium, vitamin E, baclofen, ni fedipine, creatine monohydrate and testosterone but all have failed to show significant clinical benefits (42) Other compounds, including DHEA -S and bioflavonoids are able to ameliorate cytotoxicity induced by CTG repeat tracts in a cell culture system but the mechanism that triggers this beneficial effect is unclear. Treatment for DM so far has been focused on relieving muscle degeneration rather than resolving disease pathogenesis at the molecular level (43) New therapeutic approaches have emerged recently and they have proven to be very effective, at least in mouse models. Treatment of AON containing (CAG) 25 in the HSALR mouse blocks th e interaction of Mbnl1 with toxic CUG RNA and leads to the correction of defective Clcn1 alternative splicing and myotonia (44) In similar manner, administration of 2 O -methyl phophorothioate -modified (CAG)7 AON in a DM mouse model carrying a
22 human DMPK transgene with a CTG repeat expansion also significantly reduces aberrant alternative splicing (45) Efforts to screen small molecules that can disrupt MBNL1 binding to CUG repeats is also ongoing and one of these potential drugs, pentamidine, is effective in a cell culture model and partially rescues missplicing in the HSALR mouse (46) In this study, we used recombinant adeno associated virus ( r AAV) mediated gene transfer to upregulate Mbnl1 protein since r AAV is non -pathogenic which makes it an attractive vector for gene therapy (47) Early p hase clinical trials using r AAV have been performed for cystic fibrosis (CF) and hemophilia B (48 51) R esults were positive, generally indicating lack of vector -mediated toxicity, efficient rate s of DNA transfer, and transient decrease s in pathogenic effects In this stu dy, we address the role of M BNL1 sequestration in vivo by using adeno associated virus (AAV) -mediated transduction to overexpress the protein in HSALR skeletal muscle. Our results demonstrate that elevated expression of Mbnl1 alone is sufficient to rescue the myotonia and aberrant splicing of specific gen e transcripts that are characteristic manifestations of DM skeletal muscle. Results Selection of the Mbnl Isoform for Ov erexpression in Skeletal Muscle There are three MBNL genes in humans and mice (12, 30) They all have two pairs of zinc knuckle like (CCCH, C3H) motif s (4XC 3 H proteins) that are required for RNA binding and share 90% sequence similarity. C o -transfection analysis of HEK293 and HeLa cells with minigene splicing reporters and human MBNL expression pl asmids demonstrates that all three MBNL proteins (MBNL1, MBNL2, and MBNL3) promote fetal exon
23 exclusion to a similar extent (18) Nevertheless, we chose Mbnl1 for AAV-mediated over expression in HSALR skeletal muscle primarily because transgenic HSALR and Mbnl1 knockout models for DM both develop myotonia, distinctive morphological changes to muscle structure, and remarkably similar adult mis splicing patterns (17, 19, 33) Also recent data showed that two Mbnl2 genetrap mice fail to show dramatic dy strophic changes in muscles (32, 33) Further, expression of Mbnl3 is primarily expressed during embryonic skeletal muscle development so it is unlikely that this isoform regulates splicing during postnatal develop ment (31) Since loss o f Mbnl1 f unction and expression of C UG repeat expansions causes similar muscle phenotype s we expected to see a rescue of pathogenic phenotypes upon Mbnl1 overexpression either by upregulation of free Mbnl1 protein in the nucleus or by increased Mbnl1 bind ing to dsCUG RNA thereby releasing the sequestered endogenous proteins. The later possibility is supported by a fluo rescence recovery after photobleaching (FRAP) study on DM1 fibroblasts that reported relatively rapid exchange rates between MBNL proteins within ribonuclear foci and the surrounding nucleoplasmic pool (52) Another question focused on which Mbnl1 isoform should be overexpressed in HSALR skeletal muscle. The m ouse Mbnl1gene encodes at least 14 isoforms that use two different initiation codons (19) Isofor ms that initiate in exon 3 contain four copies of C3 H (4 XC 3 H) whereas t he isoforms that use the exon 4 initiation codon contain only two C 3 H motifs (2XC3 H) (Fig 2 1) (12, 28, 53) To determine the most effective is oform for AAV mediated muscle expression, we identified the predominant Mbnl1 isoforms expressed in adult mouse skeletal muscle. Becau se deletion of exon 3 in the mouse wa s sufficient to
24 cause the multisystemic DM -like phenotype in Mbnl1 mice (19) we used primers positioned in Mbnl1exons 3 and 13. The cDNAs from mouse postnatal day (P) 28 quadriceps were amplified to determine the coding sequences for th e isoforms initiating in exon 3.This time point was cho sen because the postnatal developmental transition of Mbnl1 isoforms occurs between P2 and P20 (33) P 2 hindlimb cDNAs were also analyzed to compare neonatal isoforms to their adult counterparts. For each time point, complete DNA sequences of 72 81 Mbnl1 cDNA clones were determined. At P2, the major Mbnl1 4XC 3 H isoforms are 41, 40, and 35 kDa whereas by P28 the 41 kDa isoform i s no longer expressed (Fig. 1 3 A ). Seven additional isoforms (43, 42, 38, 37, 36, 36*, a nd 32) were also identified at significantly lower levels. Immunoblot analysis using the A2764 polyclonal antibody directed against the Mbnl1 -specific carboxyl terminal peptide confirmed that the major adult protein in three different skeletal muscles (tib ialis anterior, gastrocne mius, and quadriceps) was of 40 kDa, whereas the major isoforms in cerebellum heart and lung are slightly larger (Fig. 1 3 B ). We pursued the possibility of using the Mbnl1 41kDa isoform (Mbnl1/41) for AAV mediated transduction because a previous study confirmed that MBNL1/41 has a high affinity for a CUG repeat expansion RNA [(CUG)54] with a Kd of 5.3 nM (28). To determine the splicing activity of Mbnl1/41 we used a HEK293T cotransfecti on assay to compare splicing activities of various Mbnl1 isoforms and unrelated RNA -binding proteins. F ast skeletal muscle troponin T (Tnnt3) was selec ted as the minigene reporter because splicing of its fetal exon is very sensitive to Mbnl1 levels in vivo (19, 28) For protein expression, GFP fusion expression plasmids were used so that the transfection efficiency
25 could be monitored readily. Although overexpression of other RNA -binding proteins, including splicing factors hnRNP A1 and CUGBP1 failed to alter the F exon -splicing pattern, all Mbnl1 isoforms tested significantly enhanced F exon skipping (Fig. 1 4) Mbnl1 4 1 and 4 0 kDa isoforms showed the highest Tnnt3 F exon skipping activity. Importantly, there was n o difference in the splicing activities of these 4XC 3 H isoforms in this assay with -Mbnl1/40 or GFP Mbnl1/41. Reversal of Myotonia in HSALR mice after Overexpression of Mbnl1 For Mbnl1 overexpression, we selected the tibialis anterior (TA) muscle because it is relatively small, readily accessible without surgery, and efficiently transduced by AAV (54) F our -week old HSALR mice were injected with 1 1011 vector genomes (vg) in the right TA with AAV2/1 (AAV2 ITR in an AAV1 capsid) modified to express the myc tagged Mbnl1 41 kDa isoform (AAV2/1 -mycMbnl1/41). Relative levels of endogenous 40kDa versus exoge nous mycMbnl1/41 expression were assessed by immunoblotting. At 23 weeks after injection, the mycMbnl1/41 protein was present in injected, but not in uninjected TA muscles (Fig. 1 5 A ). Compared with the uninjected TA, the level of the endogenous 40kDa pr otein in the injected muscle was r educed ~ 20% when normalized to Gapdh. Despite this reduction, there was an overall 2 -fold increase in Mbnl1 protein due to AAV2/1 -mycMbnl1/41 expression. Because of the high expression level of the HSALR transgene, numerou s discrete ribonuclear foci containing Mbnl1, which colocalizes with (CUG)250 RNA (27) were detectable in transgenic myonuclei (Fig. 1 5 B ). In contrast, Mbnl1 was more diffusely distributed in the nucleus after A AV-mediated overexpression,
26 suggesting that a subpopulation of this splicing factor was no longer sequestered in these foci. Electrical myotonia is a prominent pathological feature of both human DM and mouse HSALR skeletal muscle. Electromyography reveale d a striking reduction of myotonia specifically in the HSALR injected TA by 4 weeks after injection and a complete absence of myotonic discharges at 23 weeks (Fig. 1 6 A ). By 43 weeks after injection, muscle hyperexcitability was again detectable in some mice, whereas myotonia present in both uninjected HSALR TA and gastrocnemius muscles was unaffected at all time points assayed which indicates that overexpression of Mbnl1 is injection site specific. M yotonia in DM results from missplicing of the major skeletal mus cle chloride channel CLCN1 in adults, which results in loss of functional membrane associa ted chloride channels. Clcn1 protein levels were reduced markedly in HSALR muscle compared with control FVB muscle levels. After Mbnl1 overexpression, Clc n1 was restored to near wildtype levels in injected TA muscles at 23 weeks after injection (Fig. 1 6 B and C) Mouse HSALR muscles show DM relevant morphological abnormalities, including centralized nuclei, split fibers, and fiber size heterogeneity in th e absence of significant muscle necrosis (17) In contrast to the myotonia, these abnormalities were still present in injected TA muscles at all time points examined (Fig. 1 6 E and F ). Reversal of Myotonia Correla tes with Rescue of Missplicing of Specific Developmentally Regulated Exons Since myotonia was eliminated by 23 weeks after injection of AAV2/1 mycMbnl1/41, we characterized alt ernative splicing of fetal exon of Clcn1 in FV B, HSALR, and AAV2/1 -mycMbnl1/41 -injected mice. Whereas in wild type FVB mice exons 6, 7,
27 and 8 are spliced directly together, exon 7a is included in HSALR muscle (Fig. 1 7). As expected, o verexpression of Mbnl1 reversed this splicing defect by promoting Clcn1 exon 7a exclusion to the normal adult pattern. Another pre -mRNA that is misspliced in DM is Ldb3/Cypher/Zasp, which encodes a striated muscle PDZ actinin 2. Although Cypher expression is not essential for sarcome rogenesis or Z line function, Cypher proteins are important for normal muscle function and Cypher knockout mice die perinatally because of severe congenital myopathy Cypher is misspliced in DM1 and DM2 muscle (33, 55) Interestingly, human CYPHER/ ZASP mutations have been linked to a novel autosomal dominant muscular dystrophy (56) During developm ent, the embryonic Cypher isoform (Cypher1S) is replaced postnatally by Cypher 3S by skipping of Cypher exon 11. As expected, normal FVB adults showed exon 11 skipping, whereas HSALR mice recapitulated the wild type neonatal pattern with equivalent levels of exon 11 inclusion and exclusion. At 23 weeks after injection, the adult Cypher splicing pattern was restored in AAV2/1 -mycMbnl1/41 injected HSALR TA muscles, with the majority of mRNAs excluding exon 11. Intracellular skeletal muscle calcium homeostasi s is regulated by the sarcoplasmic reticulum (SR) proteins, ryanodine receptor 1 (Ry r1) and SR/endoplasmic reticulum Ca2+ ATPase (Serca) 1 Whereas Ry r1 releases Ca2+ from the SR, Serca is the skeletal muscle SR Ca2+ reuptake pump. A previous study demon strated that Serca1 exon 22 is included in adult muscle but excluded in neonatal, as well as adult DM1 and mouse HSALR muscle (57) In agreement, exon 22 was predominantly included in FVB adult Serca1 mRNA
28 whereas the neonatal pattern of 50% inclusion was observed in HSALR adult TA. Over expression of Mbnl1 after AAV2/1 -mycMbnl1/41 injection led to a significant increase in exon 22 inclusion and near -normal adult Serca1 splicing. Fast skeletal muscle troponin T (TNNT3) is the subunit of the troponin complex that binds to tropomyosin. Although TNNT3 contains several alternatively spliced exons (4, 6, 7, 8, F, 16, and 17), only F exon splicing is altered in DM (19) The requirement for F exon inclus ion in fetal muscle is unclear. In HSALR adult muscle, the F exon is included together with different combinations of alternatively spliced exons 4 8, which yields multiple cDNAs containing the F exon, whereas the F exon is excluded from normal FVB TA adul t muscle. In agreement with the hypothesis that Mbnl1 is the primary regulator of this exon, nearly all F exon inclusion was eliminated in injected TA muscle. In contrast, alternative splicing of Capzb exon 8 and Itgb1 exon 17, two pre -mRNAs whose splicin g is not affected by the DM expansion mutation (33) were unaffected in HSALR and AAV-transduced mice compared with FVB controls at all time points Control TA injections with AAV expressing only GFP driven by the same CBA promoter (AAV2/1 GFP) also showed no effec t in this model and failed to reverse the splicing pattern of Serca1 and Tnnt3, suggesting rescue of DM associated phenotypes after AAV 2/1 -myc Mbnl1 /41 injection is Mbnl1 protein specific event. Because myo tonia showed gradual rescue from the 4 week to 23 week time points followed by recurrence at 43 weeks, we speculated that we would see the corresponding reversal of alternative splicing of the target pre-mRNAs. We examined Serca1 exon 22 splicing patterns at each of the time points (4, 12, 23 and 43 weeks after injection) and as
29 we expected, a higher level of exon 22 inclusion was observed at 23 weeks after injection compared to 4 or 12 weeks (Fig. 18). The result from the 43 week time point was comparable to 23 weeks. A similar trend was observed for the reversal of Tnnt3 missplicing. The degree of rescue in missplicing also correlated with the expression level of ectopic mycMbnl1/41 expression in myonuclei. We quantified myonuclei expressing myc -tagged Mb nl1 after AAV2/1 -mycMbnl1/41 injection by counting myc positive and myc -negative nuclei using anti -myc monoclonal antibody 9E10 (Fig. 1 9). As expected from the splicing assay, t he percentage of myonuclei expressing mycMbnl1/41 was higher for 23-week posti njection time point compared to 4 -week, which provides explanation for more complete rescue of myotonia and missplicing at 23 weeks Systemic Delivery of Mbnl1 in Neonate Poly(CUG) Mice Whereas direct muscle injection of AAV2/1 -mycMbnl1/41 effectively demo nstrated a primary role of MBNL1 in myotonia and missplicing in DM, systemic intravenous transduction is a more useful approach for balanced and uniform expression of a transgene since DM skeletal muscles are systemically compromised This approach will al so enable us to analyze the change in muscle histopathology more accurately in the absence of any structural changes resulting from injury induced by direct injection. Therefore, we tested if Mbnl1 systemic overexpression would rescue the myotonia and RNA missplicing in the multiple skeletal muscles of the same mouse model using systemic gene transfer. We c hose AAV2/8 for systemic delivery because previous studies have indicated that AAV2/8 is superior to AAV2/1 for the transduction of skeletal muscles foll owing intravenous injection due to its ability to cross the blood vessel barrier (58) Before we proceeded to systemic
30 injection, we first confirmed that AAV2/8 -mycMbnl1/41 was able to switch splicing after direct muscle injection. AAV2/1 -mycMbnl1/41 and AAV2/8-mycMbnl1/41 (1 1010 vg each) were d elivered to TA muscle of HSALR mice to compare the efficacy of splicing reversal of Serca1 exon 22. We chose this amount of virus, which is 10 fold less than what we used earlier, because we expected 1 1011 vg of AAV-Mbnl1 to saturate binding sites on C UG repeats and prevent us from comparing the efficiency of different types of virus. Four weeks after injection, splicing patterns for alternative exon 22 were tested. As we expected, both AAV2/1 and AAV2/8 -mycMbnl1/41 were able to rescue Serca1 missplicin g. Indeed, AAV2/8 -mycMbnl1/41 showed activity compared to AAV2/1 -mycMbnl1/41. Because purified AAV2/8 -mycMbnl1/41 was active in splicing, we proceeded with systemic injections of HSALR poly(CUG) mice. O ne day old anesthetized HSALR mice were administered w ith 2.5 5 1011 vg of AAV2/8 -mycMbnl1/41 intravenously via the superficial temporal vein. Unfortunately, all mice died within 10 days of injection, suggesting that AAV2/8 -mycMbn1/41 expression in neonatal mice is toxic. We analyzed autopsy muscle RNA samp les and found the partial adult splicing pattern of Serca1 exon 22 in the injected muscle 6 days of post injection. This result indicates that inappropriate inclusion of the adult, and/or exclusion of the fetal, exon may underlie lethality after Mbnl1 ove rexpression in neonates. We also examined expression levels of Mbnl1 transcripts by RT PCR in multiple tissues and saw very high expression in the heart and liver, in agreement preferred transduction tropism for AAV8 (58) We speculate that inappropriate high expression of Mbnl1 in these tissues res ults in
31 aberrant alternative splicing that leads to early lethality in AAV2/8 -mycMbnl1/41 injected HSALR mice. Discussion We have shown that AAV -mediated gene transfer of Mbnl1/41 is able to rescue DM associated myotonia and missplicing events in a poly(C UG) mouse model for DM. Our initial concern was that CUG repeat expression driven by the HSA promoter is too high and it would be impossible to saturate Mbnl1 binding sites on CUG transcripts in myonuclei. However two fold overexpression of Mbnl1 was suffi cient to rescue myotonia and missplicing of Clcn1 as well as other aberrant alternative splicing events in poly(CUG) muscle. Interestingly we observed that endogenous Mbnl1/40 level was downregulated by ~20% upon ectopic overexpression of mycMbnl1/41, whic h suggests a possible regulatory mechanism to maintain a consistent level of Mbnl1. Mbnl1 has a long 3 UTR which contains cis regulatory elements for mRNA translation and turnover and it is tempting to speculate that Mbnl1 binds to its own 3 UTR to repre ss translation or regulate stability which results in downregulation of the protein. Since Mbnl2 and Mbnl3 have similar RNA binding domains, it is also possible that Mbnl1 overexpression affects the stoichiometry of other Mbnl gene family members in the nu cleus and regulates the endogenous pool of Mbnl1 indirectly. After two -fold overexpression in the TA muscle in the poly(CUG) mouse, Mbnl1 proteins were more diffusely distributed throughout the nucleoplasm, suggesting that a subpopulation of Mbnl1 proteins are no longer sequestered in nuclear foci and free Mbnl1 proteins promote the reversal of myotonia and missplicing. It is also possible that ectopic
32 expression of Mbnl1 releases other proteins such as Mbnl2 and hnRNP H for phenotypic reversal. However, w e prefer the idea that increased Mbnl1 in the nucleoplasm plays a major role in reversing the disease associated phenotypes for the following reasons. First, the Mbnl1 knockout mouse shows more disease associated phenotypes compared to Mbnl2 genetrap mice. Second, exon array anlaysis of HSALR and Mbnl1 muscle shared >80% similarity in specific missplicing events, suggesting that Mbnl1 loss of function is able to explain the majority of RNA splicing changes induced by toxic CUG repeats. Third, the rol e of hnRNP H in DM pathogenesis seems limited because there are only a few splicing targets affected in DM that have been shown to be regulated by hnRNP H in vitro On the other hand, it is tempting to speculate that Mbnl2 interacts with Mbnl1 to cause dis ease phenotypes considering that Mbnl1 proteins form oligomeric structures on CUG repeats via their C -terminal residues. This idea was tested by crossing Mbnl1 and Mbnl2 knockout mice and Mbnl1/Mbnl2 double knockout mice were embryonic lethal (Unpublished data, Thornton et al) suggesting possible synthetic effects from loss of function of both these proteins. It would be interesting to test whether these proteins form a heterotypic complex which regulates alternative splicing during development. Even though we observed significant reduction of myotonia at all time points tested (4, 12, 23 and 43 weeks after injection), reversal of myotonia was significant at 23 weeks post injection and correction of missplicing was more complete at this time point as well. Complete rescue at 23 weeks seems to be related to higher ectopic expression of mycMbnl1 at this time point (Fig. 1 9). Generally, AAV2/1 reaches maximum expression ~8 weeks after administration, and additional time is required to replace existing protein i soforms.
33 Therefore, maximum rescue of disease phenotypes at 23 weeks after injection seems to fit to our current knowledge of the AAV expression profile. Recurrence of myotonia could be due to transgene loss caused by myofiber turnover which is a character istic feature of HSALR mouse muscle. In addition, muscle cell turnover is initiated by the activation of satellite cells, myoblasts proliferation and the fusion of the latter cells to existing myofibers. Therefore insufficient transduction of muscle satell ite cells by AAV2/1 -mycMbnl1/41 could result in transgene loss of transgene in newly synthesized muscle cells. Histopathology of muscle cells, such as centralized nuclei, is another DM associated phenotype in HSALR mice. Unlike myotonia, we were not able to rescue the centralized myonuclei phenotype. Following AAV2/1 -mycMbnl1/41 administration, myofibers seem to have even more centralized nuclei. It is possible that the Mbnl1 level was not sufficient to rescue histopathology even though all of the tested alternative splicing patterns reverted to near normal adult patterns. We prefer the alternative that muscle pathology in the poly(CUG) mouse resulted not only from the missplicing events due to Mbnl1 sequestration but also from the perturbation of other re gulatory pathways due to loss of additional factors, such as Mbnl2. This idea is supported by a recent study that showed poly(CUG) mice display a transcriptional dysregulation that cannot be explained by loss of Mbnl1 and may be attributable to Mbnl2 loss of -function Transcription of a group of genes involved in the regulation of extracellular matrix structure and function was misregulated in HSALR, but not in Mbnl1, mouse muscle. Since Mbnl2 has been proposed to have a function in mRNA transport and localization (59) it would be interesting to perform genetic complementation of Mbnl2 in poly(CUG) mice to see if Mbnl2 overexpression will
34 ameliorate muscle pathology in f these mice. In addition, combinational overexpression of Mbnl1 and Mbnl2 can also be performed to see if this combination completely rescues muscle histopathology. An alternative explanation for the fai lure to rescue muscle histopathology is that AAV2/1 -mycMbnl1/41 administration triggers an immune response and accelerates the regeneration of myofibers due to expression of an antigen. This idea is supported by the result that control AAV2/1 GFP administr ation in TA muscle of HSALR showed the same histopathological changes, such as increased myofibers with centralized nuclei. Injection of tolerized untagged Mbnl1 may be able to solve this issue. However systemic delivery will be critical to test the idea b ecause direct muscle injection leads to a physical injury at the injection site. We made an attempt to deliver AAV2/8 -mycMbnl1/41 systemically in neonate poly(CUG) mice to rescue myotonia and missplicing in multiple muscle tissues. All of the injected mice showed early lethality, which suggests possible toxic effects of Mbnl1 overexpression during the neonatal period. While we believe this result reflects the perturbation of development signals by improper temporal and spatial expression of Mbnl1, we cannot exclude the possibility that toxic effects of AAV capsid proteins in the heart and liver as possible causes of death even though previous studies using AAV in neonate mice argue against this possibility (58) In the future, we will change our strategy and deliver AAV2/8 -Mbnl1 to adult mice
35 Fig ure 1 1. RNA -mediated pathogenesis in myotonic dystrophy ( A ) DM1 is cause by a CTG repeat expansion in the 3 UTR of the DMPK gene. Mutant DMPK mRNA (black line) with coding region (black box labeled DMPK) and CUG expansions in the 3 UTR are indicated. D M2 is cause d by a CCTG repeat expansion in the first intron of ZNF9 Non -coding (open boxes) and coding exons (black boxes) of mutant ZNF9 gene (black line) are indicated. (B) Microsatellite expansions in DM are associated with mistakes during DNA replication and repair. C(C)TG repeat s form unusual hairpin structure s (red line) and lead to the expansion mutation in nascent DNA strand s
36 Figure 1 2. Loss of -function of alternative splicing factor MBNL1 in myotonic dystrophy. (A) MBNL1 promotes fetal exo n skipping by binding to the 3 splice site of the exon. In normal neonatal tissues, MBNL1 is located cytoplasm and not available in the nucleus, which results in the inclusion of the fetal exon. In adult tissues, MBNL1 (red oval) binds to the target pre -mRNA promoting exclusion of the fetal exon. (B) In DM1, the fetal splicing pattern persists in the adult due to sequestration of MBNL1 by CUG expansion RNA. The RNA -protein complex accumulates in nuclear foci.
37 Figure 1 3 Expression of Mbnl1 isoforms in skeletal muscle. ( A ) Distribution of Mbnl1 isoforms in neonatal (P2) and P28 muscle. Ten hindlimb muscle isoforms were identified by RT -PCR followed by cDNA sequencing. Isoforms (color coded) are as follows (in kDa): 43, lime green; 42, black; 41, turquoise; 40, orange; 38, purple; 37, light blue; 36, red; a 36 spliced variant (36*, dark green); 35, yellow; and 32, blue. ( B ) The major Mbnl1 protein in adult skeletal muscle is 40 kDa. Immunoblot analysis was performed on three skeletal muscles (tibialis a nterior, gastrocnemius, and quadriceps), cerebellum, heart, and lung by using either anti Mbnl1 or anti Gapdh (loading control). Reprinted from Reversal of RNA missplicing and myotonia after muscleblind overexpression in a mouse poly(CUG) model for myotoni c dystrophy ; Kanadia RN, Shin J, Yuan Y, Beattie SG, Wheeler TM, Thornton CA, Swanson MS; Copyright 2006 Proc Natl Acad Sci U S A.
38 Fig ure 1 4 Mbnl1 isoforms with the 4XC3H motif has a higher splicing activity. (A Upper ) RNA splicing in HEK293T cell s transfected with a Tnnt3 minigene reporter and protein expression plasmids containing GFP, GFP hnRNP A1 and GFP CUGBP1, and GFP -Mbnl1 4XC3H (Mbnl1/40 and Mbnl1/41) and 2XC3H (Mbnl1/30 and Mbnl1/26) isoforms. ( A Lower ) An immunoblot of GFP fusion protein expression showing equivalent expression levels for GFP and GFP fusion proteins after transfection. Protein loading control is Gapdh. PCR primers (arrows) are located in Tnnt3 exons 8 and 9 (open boxes) bordering the alternatively spliced fetal (F) exon (f illed box). ( B ) Phosphorimager quantification of percent fetal exon (FE) exclusion from the PCR data shown in A Reprinted from Reversal of RNA missplicing and myotonia after muscleblind overexpression in a mouse poly(CUG) model for myotonic dystrophy ; Kan adia RN, Shin J, Yuan Y, Beattie SG, Wheeler TM, Thornton CA, Swanson MS; Copyright 2006 Proc Natl Acad Sci U S A.
39 Figure 1 5. Overexpression of Mbnl1 leads to free Mbnl1 protein in nucleoplasm ( A ) Mbnl1 is overexpressed after AAV2/1 -mycMbnl1/41 transduction of TA muscle. TA muscles (23 weeks after injection) were dissected from either uninjected (control) or injected HSALR mice (both injected and uninjected muscles are shown) and total protein immunoblotted with anti -MBNL1 antibody. Gapdh is the prot ein loading control. ( B ) Distribution of Mbnl1 protein in transverse sections of skeletal muscle. Shown are max -value projections of deconvolved images obtained under identical exposure settings. HSALR sections from uninjected ( Left ) and injected ( Right) T A muscles were stained by using the anti -Mbnl1 antibody. (Scale Reprinted from Reversal of RNA missplicing and myotonia after muscleblind overexpression in a mouse poly(CUG) model for myotonic dystrophy ; Kanadia RN, Shin J, Yuan Y, Beattie SG, Wheeler TM, Thornton CA, Swanson MS; Copyright 2006 Proc Natl Ac ad Sci U S A.
40 Figure 1 6. Reversal of myotonia following Mbnl1 overexpression ( A ) Myotonia was assessed by electromyography on HSALR mice injected in the right TA. The uninjected left TA and gastrocnemius muscles also were tested. The electromyography scale is as follows: 0, no myotonia; 1, occasional myotonic discharge in <50% of needle insertions; 2, myotonic discharge with >50% of insertions; 3, myotonic discharge with nearly all insertions. Uninjected control and injected HSALR mice were tested at 4 (purple), 12 (yellow), 23 (turquoise), and 43 (orange) weeks after injection. ( D and E ) Restoration of Clcn1 protein levels in myofiber membranes after Mbnl1 overexpression. Clcn1 protein levels were detected in uninjected ( D ) and injected ( E ) transverse muscle sections at 23 weeks after injection by using an anti Clcn1 polyclonal antibody (red). DNA F and G ) Muscle histology (H&E staining) of muscle sections from uninjected ( F ) and injec ted ( G ) TA at 43 weeks after injection. Reprinted from Reversal of RNA missplicing and myotonia after muscleblind overexpression in a mouse poly(CUG) model for myotonic dystrophy ; Kanadia RN, Shin J, Yuan Y, Beattie SG, Wheeler TM, Thornton CA, Swanson MS; Copyright 2006 Proc Natl Acad Sci U S A.
41 Figure 1 7. Mbnl1 overexpression promotes adult splicing patterns. RT -PCR splicing assays of uninjected FVB/n and HSALR (three mice each) or AAV2/1 mycMbnl1/41 (gene therapy group C, GTC) injected TA at 23 wee ks after injection (six mice). Developmentally regulated exons (filled boxes) are either dysregulated in DM (Clcn1, Ldb3/Cypher, Serca1, and Tnnt3) or not affected by the DM expansion mutations (Capzb and Itgb1). Primer positions (arrows) are illustrated b elow each autoradiograph within constitutively spliced exons (open boxes). The overexpressed mycMbnl1/41 is detectable only in the GTC mice (Bottom ). Reprinted from Reversal of RNA missplicing and myotonia after muscleblind overexpression in a mouse poly(C UG) model for myotonic dystrophy ; Kanadia RN, Shin J, Yuan Y, Beattie SG, Wheeler TM, Thornton CA, Swanson MS; Copyright 2006 Proc Natl Acad Sci U S A.
42 Figure 1 8. Time course for reversal of RNA missplicing. Complete recovery of myotonia at 23 weeks post injection shows a robust reversal of missplicing. RT -PCR splicing assays of uninjected FVB/n and HSALR (three mice each) or AAV2/ 1 mycMbnl1/41 (GT A, GTB, GTC and GTD) injected TA at 4, 12, 23 and 43 weeks after injection respectively (2 mice for GTC and 3 mice for other groups. For more results GTC group, see Figure 1 5 ). Developmentally regulated exon 22 of Serca1 (filled box) is dysregulated in DM
43 Figure 1 9 Ectopic mycMbnl1/41 expression after AAV2/1 -mycMbnl1/41 injection ( A ) Myonuclei exp ressing myc tagged Mbnl1 were detected by using the anti -myc monoclonal antibody 9E10 (green), whereas myofibers were visualized by using TRITClabeled wheat germ agglutinin (red), and nuclei were stained with DAPI (blue). ( B ) Quantification of myc -positiv e myonuclei and myofibers. The percentage of myonuclei expressing mycMbnl1/41 was determined by counting myc -positive and myc -negative nuclei in a representative region of each muscle (n = 2) for the 4 -week (GTA6 and GTA8) and 23-week (GTC1 and GTC2) posti njection time points. Myonuclei were distinguished from interstitial nuclei by their location inside fibers rather than between fibers. Both subsarcolemmal and centrally located myonuclei were quantitated (blue). The percentage of myofibers that contained at least one myc -positive nucleus also was determined (purple). Reprinted from Reversal of RNA missplicing and myotonia after muscleblind overexpression in a mouse poly(CUG) model for myotonic dystrophy ; Kanadia RN, Shin J, Yuan Y, Beattie SG, Wheeler TM, Thornton CA, Swanson MS; Copyright 2006 Proc Natl Acad Sci U S A.
44 CHAPTER 2 GENETIC FACTORS AFFE CT DISEASE -ASSOCIATED PHENOTYPE S IN MBNL1 KNOCKOUT MOUSE MODEL FOR MYOTONIC DYSTROP HY Introduction Myotonic D ystrophy is a Variable Disease One of the remark able characteristics of DM is the variability of disease phenotypes among patients Symptoms vary greatly not only in severity (expressivity) but also in the percentage of patients who show the mutant phenotype (penetrance) (1). Not everyone displays all the disease associated phenotypes and different patients are affected differently Age of -onset is also variable among patients even within the same family and some people are only affected mildly in later life whereas others develop problems much earlier. Generally, the number of CTG repeats has a positive correlation with the severity of disease. However, genetic and environmental variations also play important roles in disease penetrance (1, 2, 60) Myotonia and muscle weakness/wasting are the two major muscle symptoms that occur in DM and it is muscle wasting that is the more troublesome for patients (61) Eve n though muscle is prominently affected, DM is not just a muscle disease. DM also affects other parts of the body and clinical manifestations include cardiac conduction defects and arrhythmia, bowel disturbances, balding in males and hypersomnolence (exces sive daytime sleepiness). Less recognized diseaseassociated phenotypes include hormone problems, calcifying epithelioma and a higher incidence of cancer even though the association of DM with cancer is still a subject of debate (62 65)
45 Even with the significant progress in recent years, our knowledge is very limited in the understanding of the molecular defects underlying each one of these symptoms except for the example of myotonia and missplicing of CLCN1 M ost of the studies so far have focused on elucidating common pathogenesis mechanisms for the disease. However, it is just as intriguing to ask why the symptoms of the disease are so variable and what is the molecular defect un derlying such variability? In this study we will demonstrate that genetic factors play an important role in phenotypic variability of DM using a mouse model. We will also make an attempt to uncover molecular defects underlying DM associated phenotypes by correlating the severity of cer tain phenotypes to a specific missplicing event. Mouse Models to Understand the Pathogenesis of DM Mouse models have been valuable tools to understand the molecular mechanisms of inherited diseases. A few mouse models have been generated to study DM inclu ding the HSALR mouse discussed in Chapter 1 (17) HSALR is a great model to prove that CTG expansions are toxic independent of gene context but it has limitations. First, the expression of CTG repeats is controlled by the human skeletal actin ( HSA ) promoter therefore HSALR mice cannot model other non-skeletal muscle symptoms. Second, the number of repeats is only 250 and it does not recapitulate the whole spectrum of phenotypes observed for larger repeats. To overco me these limitations, additional transgenic mouse models have been developed. One is a human DMPK transgenic mouse which carries a larger CTG repeat (>300) in the DMPK 3 UTR (66 68) This mouse models the intergenerational repeat expansion (repeats tend to get larger in successive generations) that leads to the more severe form of disease. Histological abnormalities are seen in DMXL (CTG repeats longer
46 than 700) homozygous mice and severe body size retardation in D MXL/DMXXL mice (homozygous mice carrying 700 CTG s on one allele and > 900 CTG s on the other allele) (66, 67) Aberrant splicing of target pre -mRNAs of DM such as Clcn1 is mild probably due to the low expression of t ransgene. A more recently developed model is a tamoxifen inducible (interrupted) transgenic 960 CTG repeat mouse. The transgene contains a ubiquitously expressed CMV promoter, a floxed concatamer of the SV40 polyadenylation site, and human DMPK exon 15 containing 960 CTG repeats When CTG repeats were induced in cardiac tissues by crossing with the MerCreMer (MCM) transgenic line which expresses a heart -specific tamoxifen inducible Cre, bi transgenic mice developed severe cardiac conduction defects as we ll as cardiomyopathy and died within 2 weeks (69) When 960 CTG expression was induced in skeletal muscle by crossing with HSA -Cre -ERT2 line, in 4 weeks after tamoxifen administration these mice not only developed myotonia and muscle histopathology but also muscle wasting, which is a muscle phenotype that was absent in other DM mouse models (70) This result indicates that repeat number together with expression level are two important parameters for the muscle wasting phenotype. Whereas the transgenic mice mentioned above demonstrate a fundamental role of CTG repeats in DM toxicity, other mouse models are employed to ask more direct questions pertaining to the molecular mecha nism of DM. Based on the toxic RNA gain -of function model, the roles of two splicing factors that are affected in DM have been evaluated using mouse models. Steady -state levels of Cugbp1 are upregulated in multiple DM tissues due to the phosphorylation of this protein via PKC kinase, and overexpression
47 of CUGBP1 in mouse results in the missplicing of subset of pre -mRNAs that are affected in DM (24, 34) These mice die around P8 and the shortened life span prevents i nvestigation of the presence of other disease associated phenotypes. In addition to increased CUGBP1 activity, MBNL1 function is compromised in DM due to sequestration. Since MBNL1 isoforms initiating from exon 3 have two pairs of C3H motif s that are requi red for RNA binding, the Mbnl1 knockout ( Mbnl1 E3/ ) mouse, in which exon 3 is deleted, is functionally null (Fig. 2 1). This mouse develops key clinical manifestations including myotonia, histopathology, cataracts and cardiac conduction defects (19) Inter estingly, phenotypes of Mbnl1 knockout mice are variable in severity and penetrance. First of all, half of Mbnl1 E3/ mice die between 6 12 months of age whereas ~50% live a normal life span (~2 years). Attempts to determine the direct cause of death ha ve been unsuccessful Second, there is a penetrance issue for some disease associated phenotypes. Myotonia and muscle pathology are present in every mouse that was tested, however, cardiac conduction defects (1st degree AV block) are present in only ~50% o f mice tested at 8 weeks of age using telemetric electrocardiography (ECG). Aberrant splicing of a subset of pre -mRNAs affected in DM, such as Clcn1 and Tnnt3, is robust in every mouse tested. Physiological analysis for compromised muscle function has not been tested in these mice. Muscle wasting has not been observed, however, sudden death in ~50% of Mbnl1 E3/ mice prevented further investigation of the wasting phenotype. The Mbnl1 knockout mouse is a good model for DM pathogenesis because it recapitula tes most of the key features of disease but also models the incomplete penetrance of the disease.
48 Influence of Genetic B ackground on Disease Associated Phenotypes in Genetically Engineered Mice A number of studies have investigated the influence of genetic background on penetrance and expressivity in genetically engineered mice. (71) Differences between C57BL/6 ( Bl 6 ) and 129/Sv ( 129) genetic backgrounds are particularly interesting because null mutations in mice are usually generated with embryonic stem cells der ived from 129 inbred strains and the resulting chimeric males are mated with B l 6 females which results in a mixed genetic background (72) Each strain has unique characteristics. For example, 129 mice have a highanxiety phenotype with learning deficiencies and a high prevalence of developing gonadal teratomas (73, 74) while Bl 6 mice often show low bone density, ocular defects and ulcerative dermatitis (75 77) Generation of congenic strains for a mutant allele may lead to more consistent, or reveal additional, phenotypes. For example, caspase 3 deficient 129 mice are uniformly and severely affected by perinatal death and marked exencephaly whereas caspase 3 deficient Bl6 mice reach adulthood and show minimal brain pathology (78) Phenotypes of Hdh Q111 knock in mice, whic h are a model for Huntingtons disease (HD), show different intergenerational instability of the HD CAG repeat (CAG encodes glutamine) and intranuclear polyQ inclusion formation in Bl6 and 129 backgrounds (79) Congenic lines also are valuable genetic tools to identify genetic modifiers that are responsible for the variability between strains. Genetic modifiers, which are genes that modify the effect produced by another gene (e.g. null allele in knockout mouse), a re genetic variants that affect disease penetrance. For example, a single nucleotide polymorphism (SNP) of the MBL gene was identified as an important modifier in cystic fibrosis (CF). CF
49 patients carrying certain MBL alleles have a significantly less seve re pulmonary phenotype which suggests that the modifier locus could be a target for therapeutic intervention (80, 81) No significant genetic modifier has been identified for DM whereas animal studies suggest that transcription factor Nkx2.5 may play role in modulating the severity of the cardiac phenotype in DM (82) Congenic Mbnl1 E3/ Mice and the Role of Mbnl1 in DM Pathogenesis As stated earlier, DM is a remarkably variable disease and both somatic mosaicism and genetic factors contribute to variability among DM patients. Since Mbnl1 E3/ mice faithfully recapitulate DM ass ociated phenotypes with incomplete penetrance for certain phenotypes such as cardiac conduction defects, we hypothesized that genetic background plays an important role in the clinical presentation of DM. In this study, we generated congenic Mbnl1 E3/ m ice to ask several questions. Do different genetic backgrounds affect DM disease severity and penetrance? What is the consequence of loss of Mbnl1 protein on highly penetrant phenotypes such as myotonia and myopathy? What is the role of Mbnl1 in the development of non-muscle cells? We addressed these questions by characterizing Bl6 and 129 congenic mouse lines since this approach should enable us to understand the fundamental roles of Mbnl1 in DM pathogenesis as well as in the normal developmental process. We report that genetic background has a profound effect on the presentation of key disease features in Mbnl1 E3/ mice such as muscle weakness. Furthermore, we correlated muscle weakness with altered alternative splicing of specific genes that are import ant for calcium homeostasis, such as ryanodine receptor 1 ( Ry r1) Our results support the possibility that muscle weakness in DM is not secondary to myotonia
50 and caused by a separate molecular defect. Finally, congenic mice developed in this study will b e invaluable tools to identify genetic modifiers of DM disease. Results Effect of Genetic Background on Survival of Mbnl1 Knockout Mice T o uncover disease associated phenotypes that were masked in the original Mbnl1 E3/ mixed background, we generated congenic lines using a conventional backcross breeding strategy with either C57BL/6 (B l 6) or 129/Sv (129) inbred mice. After ten generations of backcrosses, we analyzed the genotype ratio of pups at weaning age (~3 w eeks) from heterozygous intercrosses for both genetic backgrounds by PCR analysis (Table 2 1). The genotype ratio for Bl6 mice was significantly skewed with a chi -square value of 32.67 (p<0.005). Only ~50% of Mbnl1 E3/ homozygous Bl6 mutants survived to weaning suggesting that Mbnl1 plays important roles during embryogenesis and/or the perinatal period Mbnl1 is expressed at a moderate to high level in multiple embryonic tissues (31) In contrast, 129 mice showed a normal Mendelian ratio of 1:2:1 (WT:het:hom). The absence of Mbnl1 protein in both of Bl6 and 129 homozygous muscle tissues was confirmed by western blotting using the polyclonal anti -Mbnl1 antibody A2764 which confirmed PCR genotyping (Fig. 2 2 A ). Mbnl1 E3/ 129 Mice Have Defects in Thymus and Skin Epithelium We next examined morphological and histological properties of adult Mbnl1 E3/ congenic mice. The Mbnl1 E3/ m ice backcrossed onto the 129 background developed normally during first 3 months of age After this period they displayed a high incidence of
51 sudden death and the survival rate of the se mice decreased dramatically between 3 7 months of age No Mbnl1 E3/ 129 mutant mice survived more than one year (Fig. 2 2 B ). A possible cause of t he high mortality rate was a massive enla rgement of thymus which occupi ed the entir e thoracic cavity and compresse d both the heart and lungs of Mbnl1 E3/ 129 m ice The mutant thymus was about 10 times larger (measured by wet weight ) compared to that fro m wild type littermate s at 24 weeks of age. Thymic enlargement was observed as early as 12 weeks of age and coincided with the onset of normal thymic involution in mice. Histological analysis revealed thymic hyperplasia demonstrating expansion of T lymphocytes (Fig 2 3 A ). Thymic T cell development involves di stinct differentiation stages to provide a diverse T cell repertoire to fight against external invasion and induce tolerance to self antigens which involves the major histocompatibility complex (MHC ) (83, 84) Double negative (DN) (CD4 CD8 ) progenitors entering the thymic cortex from the bone marrow become D P (CD4+CD8+) cells following a pre T cell receptor ( TCR) mediated selection signal Mature SP thymocyt es (CD4+CD8 or CD4 CD8+) that are released from the thymus then migrate to peripheral tissues. About 98% of thymocytes die during these developmental processes either by failing the positive or negative selection process. Preliminary studies on thymocytes from Mbnl1 E3/ 129 thymus indicated a dysregulation of the T cell maturation process (Fig 2 3 B ). Flow cytometry of thymocytes from Mbnl1 E3/ 129 mice showed that most T cells were stalled at the CD4+CD8+ DP stage whereas thymocytes from WT 129 mice showed a normal thymocyte distribution. Accumulation of DP T lymphocytes in Mbnl1 E3/ 129 mice is possibly due to the failure
52 of one of the selection steps, most likely negative selection, which deletes the majority of positively -selected T cells by re action with self antigen and apoptosis. In support of this hypothesis, in a preliminary study using anti nuclear antibodies (ANA) test on human epidermoid cancer (HEp2)cell, Mbnl1 E3/ serum was positive for auto antibodies even though we failed to iden tify the responsible antigen (data not shown). Mbnl1 E3/ 129 mice also developed visible skin lesions around the eye lid, muzzle, dorsal neck and back. These lesions were generally dry and non -pru ri tic however they became progressively worse to the poi nt that the affected mouse had to be sacrificed. Histological analysis revealed acanthosis or hyperplas ia of the epi d ermis and sever e inflammation of the dermal layer (Fig. 2 3 C). Mbnl1 E3/ Bl6 Mice Develop a Severe Movement Def icit and Muscle Abno rmalities Backcrossed Mbnl1 E3/ Bl6 mice were spared both thymic hype rplasia and dermatitis. Instead Mbnl1 E3/ mice developed severe movement deficit s and impaired motor coordination as early as 2 months of age These mutants were easily distinguis hable from their WT littermates by stiff extension hindlimb postures when the mouse was dropped from an ~ 20 cm height and movement slow with gait abnormalities characterized by a wobbling walk (Fig. 2 4 A ). Muscle weakness and abnormal motor coordination were quantified by the r otarod assay (Fig. 2 4 B ). During the four day training period, Mbnl1 E3/ 129 mice performed as well as WT littermates and were able to remain on the rotating / accelerating r od up to 2 min whereas Mbnl1 E3/ Bl6 mice with latency to fall of ~30 sec.
53 We analyzed the degree of electrical myotonia in multiple skeletal mu scles (quadriceps, gastrocnemius tibialis anterior) from Bl 6 and 129 mutant mice because the movement deficit of Mbnl1 E3/ Bl6 mice was similar to that of the myotonic Adr mouse which does not express any Clcn1 chloride channel (85) Electrical myography (EMG) was performed by inserting a needle electrode and reco rding the membrane potential of the stimulated muscle (Fig. 2 4 C). Surprisingly, 6 month old Mbnl1 E3/ Bl 6 and 129 mice exhibited similar degree o f myotonia in hindlimb (quadriceps, tibialis anterior gastrocnemius) muscles while WT control s showed no electrical myotonia (n = 4 6 for each genotype). Myotonia in myot onic dystrophy is caused by aberrant splicing of the major skeletal muscle chloride channel ( Clcn1 ) that retains fetal exon 7a in adult muscles (39) Clcn1 splicing patterns in B l 6 and 129 mutant mice were also comparable with an equal ratio of exon 7a inclusion /exclusion as expected from the EMG results suggesting that myotonia is not the cause of the Mbnl1 E3/ Bl6 movement deficit (Fig. 2 -4 D ). Histological examination of muscle from B l 6 mutant mice at 6 months of age showed distinct myopathic changes including centralized nuclei, split fibers and f iber size heterogeneity (Fig. 2 5 A upper ) as reported in a previous study using Mbnl1 E3/ mixed background muscle (19) In contrast, 129 muscles showed only very mild changes and the percentage of Mbnl1 E3/ 129 myofibers with centralized nuclei was significantl y lower than that of Bl6 mutants (5% and 16%, respectively) (Fig. 2 5 C). Unlike human, mouse skeletal muscles, such as quadriceps and TA, are mostly composed of type II fast twitch fibers while other muscles such as soleus are ~70% type I slow twitch fib ers (86) Type II muscles have two distinct classes. Type IIa fibers are more
54 oxidative with high mitochondrial content and are relatively slower in contractile properties compared to type IIb fibers which are more glycolytic (87) Immunofluorescence analysis of myosin heavy chain (MHC) isoforms revealed a change toward a more oxidative MHC with a large increase in type IIa fibers in Bl 6 mutant quadriceps muscles (Fig. 2 5 A lower ). A simi lar shift toward a more oxidative muscle phenotype was also observed for TA M itochondri al protein content is enriched in slow er twitch oxidative fibers and this was detected by western blotting (Fig. 2 5 B ) We observed an increase in cytochrome oxidase V a (Cox Va) level, which is a nuclear encoded protein that resides in the inner membrane of mitochondria, (88) by 3 -fold in the B l 6 mutant compared with normal quadriceps muscle at 6 months of age. In contrast, Cox Va content in 129 mutant muscle lysate did not differ from WT controls T he increase in mitochondrial protein level is consistent with our observation of a shift toward type IIa fibers for Bl 6 muta nt muscles. Age D ependent P rogression of M yop athic C hanges and A berrant E ndplate Topology in Mbnl1 E3/ Bl 6 M uscles The morphological abnormalities observed at 6 months of age in the B l 6 mutant progressed with aging, and 15 month old mice exhibited more prominent changes and the presence of necroti c fibers as well as nuclear clumps a sign of atrophy (Fig. 2 6 left). The percentage of myofibers with centralized nuclei increased up to ~50%. H owever, we did not observe any difference in muscle wet weight for the Bl6 mutant compared to WT suggesting n o obvious sign of wasting by 15 months In a mouse model for muscular dystrophy, chronic muscle damage often leads to the degeneration of neuromuscular junction (NMJ) (89) We analyzed the structure of the neurom us cular junction of 15 month old Mbnl1 E3/ Bl6 and WT muscles by labeling
55 acetylcholine receptor s -bungarotoxin (Fig. 2 6 right ). TA mus c les from WT Bl 6 mice had a pattern of AChR staining that was smooth, continuous with extensive arboriz ati on In contrast ~80% of NMJs from Bl6 mutant muscle displayed a fragmented pattern of AChR staining characteri zed by discontinuous nerve ends, although small populations of intact NMJs were present with a normal continuous pattern of the AChR. We also analyzed the architecture of pre -synaptic axons and termini with antibodies against neurofilament and synaptic vesicles I n WT mice, axonal branches innervated well -defined post -synaptic structure s showing complete overlap of pre and post synapses. In mutan t mice, however, we observed incomplete overlap as well as axonal swelling indicati ng progressive denervation Abnormal C ontractile P roperties in Mbnl1 E3/ Bl6 M uscle s T o investigate the physiological relevance of the dystrophic structural changes in Mbnl1 E3/ Bl6 muscles, we next evaluated muscle contractile properties of Mbnl1 E3/ Bl6 and 129 mice as well as WT controls. Our speculation was that the dystrophic histopathology correlates with muscle function and deficits in contractile properties would be seen in Mbnl1 E3/ Bl6 mice but not in 129 mutants. We isolated intact soleus and extensor digitorum longus (EDL) muscles from 3 month old mice and measured force generation of those muscle groups upon electric stimulation because they well rep resent slow type I muscle fibers and fast type II fibers, respectively. For soleus muscle (see Fig. 2 7), twitch force produced by direct stimulation with 1 -ms current pulses did not show a significant difference between genotypes. However, in response to tetanic stimulation at 100Hz, Mbnl1 E3/ Bl6 muscles generated 21% less force compared to WT controls
56 whereas Mbnl1 E3/ 129 muscles contracted 20% more forcefully than their control counterparts. Due to an increase of muscle mass in Mbnl1 E3/ mice (19% for B6, p=0.055 and 28% for 129, p<0.01) when calibrated to the unit cross -sectional area (specific force), force from Mbnl1 E3/ Bl6 soleus muscle was 34% less than WT control while specific force of Mbnl1 E3/ 129 and WT were equivalent. Furthermore, both rapidity of contraction (time to peak) and the time to half relaxation (T1/2R) were significantly decreased in Mbnl1 E3/ Bl6 soleus muscles (no change for Mbnl1 E3/ 129 muscles). EDL muscles showed similar force measurement results (Fig. 2 8). Mbnl1 E3/ Bl6 muscles showe d reduced force production both for twitch (21%) and tetanus (23%) (p=0.01 and 0.008 respectively) whereas 129 mice showed no difference. Specific force was reduced 27% in Mbnl1 E3/ Bl6 compared to WT even though the p-value was not statistically signif icant (p=0.07). These results indicated that Mbnl1 loss -of -function leads to abnormal muscle pathology as well as altered contractile properties in slow and fast muscles in the Bl6 genetic background whereas 129 background mice are spared from these def icits. In addition, these data suggest that the degree of electrical myotonia caused by aberrant splicing of Clcn1 is separable from histological and physiological abnormalities of skeletal muscles and it is likely that there are additional molecular defec ts that underlie muscle weakness in DM. Aberrant S plicing of Ryr1 C orrelates with M yopathic C hanges Mbnl1 is a splicing factor and the splicing of sp ecific pre -mRNAs is affected in Mbnl1 E3/ mice. In an attempt to elucidate molecular defects underlying myopathic
57 changes in B6 mutant muscles, we compared the alternative splicing of Mbnl1 E3/ in both B l 6 and 129 backgrounds for genes that are important for muscle structure and function (Fig. 2 9) We tested the alt ernative splicing of ~30 genes and found that most of the targets that were affected in Mbnl1 E3/ showed no difference bet ween genetic backgrounds, including SR /endoplasmic reticulum Ca2+ ATPase 1 ( Serca1 ) and myotubularinrelated protein 3 ( Mtmr 3) Only 3 alternative exons were found to be differentially spl iced between Mbnl1 E3/ Bl 6 and 129 i ncluding the fetal exon of fast twitch troponin T ( Tnnt3 ) and two exons of the ryanodine receptor 1 ( Ryr1 ). The most significant change was in the alternative splicing of Ryr1 exon 70. Alternative e xon 70 was pre dominantly excluded in B6 Mbnl1 E3/ mR NA (<1% inclusion of exon 70) while 8 % inclusion of the exon was observed in Mbnl1 E3/ 129 muscle. Interestingly, both Tnnt3 and Ryr1 proteins are important cellular factors which determine the kin etics of contraction and calcium homeostasis in muscle. Tnnt3 is the tropomyosin-binding subunit of troponin that confers calcium sensitivity to skeletal muscle ATPase activity and Ryr1 encodes the SR calcium channel which releases calcium to trigger contr action. Mbnl1 Binds to Ryr1 Exon 70 RNA i n Vitro but Does Not Regulate Splicing of an Ryr1 Minigene in C2C12 Cells We investigated the molecular mechanism of Ryr1 exon 70 splicing more carefully because p revious studies have shown a direct role of for the peptide sequence encoded by this exon in excitation -contraction coupling (90) E xon 70 encodes the regulatory domain of Ryr1 and its exclusion results in enhanced calcium ef flux from the SR in cultured myo tubes W hile Tnnt3 pre -mRNA is a well -characterized binding target of the Mbnl1 protein, with the binding motif YGCY in intron 8 near the 3 splice site, it is not clear if
58 Ryr1 pre -mRNA is a direct binding target even though alternative splicing of Ryr1 is altere d in DM patient muscle as well as HSALR mice (28, 57) We first asked whether Mbnl1 binds to Ryr1 RNA by performing a photocrosslinking assay in which 293T cells were transfected with protein expression plasmid enc oding myc tagged MBNL1. Whole cell lysates were photocrosslinked with a 32P radiolabeled 600 nucleotide (nt) Ryr1 RNA flanking exon 70, RNA -protein complexes were pulled down using the 9E10 anti -myc monoclonal antibody and then these complexes were resolve d by SDS -PAGE. Autoradiography showed that RNA for the region encompassing exon 70 crosslinked to the MBNL1 protein. Concurrently, the Ryr1 minigene was transfected into C2C12 cells to assay whether Mbnl1 overexpression changes Ryr1 splicing. The splicing pattern of Ryr1 minigene was not altered suggesting the possibility that Mbnl1 may be regulate Ryr1 splicing indirectly and thus other splicing factors are responsible for the aberrant splicing of Ryr1 in B6 Mbnl1 E3/ mice. Discussion Mbnl1 is highly expressed in skeletal muscle, heart, brain and lymphoid organs, such as thymus and spleen in the mouse (31) In a previous study, our lab showed that the loss of Mbnl1 results in myotonia and histopathological anomalies in skeletal musc le (19) However, other disease associated phenotypes, such as muscle wasting and cardiac conduction defects, were absent or less penetrant. Also the initial study lacked a thorough characterization of the mouse mo del and it was not clear as to what extent the loss of MBNL1 function has a role in DM pathogenesis. In this study, we generated congenic (either B6 or 129) M bnl1 knockout mice and our results provide evidence that two major
59 muscle phenotypes in DM, myoton ia and muscle weakness, are separable events and that Mbnl1 may be responsible for the muscle weakness in DM possibly through altered alternative splicing of Ryr1 exon 70. Furthermore, we uncovered additional phenotypes such as the abnormal morphology of N MJs in Mbnl1 knockout mice. The most striking observation that we have made during the course of this study is that genetic background has a profound effect on the presentation of DM associated phenotypes in these mice which suggests that other genetic fa ctors contribute to the variability of DM. The first noticeable difference between congenic mice in the two different backg rounds was the survival rate. For Mbnl1 E3/ Bl 6 mice, a high level of lethality occurred (~50%) by 3 weeks of age. We have not investigated the cause of early lethality. However, it would be interesting to see if this population shows a retardation in embryonic muscle development which can be seen in the congenital form of DM. Bl6 mice still have variability in the onset of lethality even after a 10 generation backcross possibly because they still carry a129 region flanking and including the Mbnl1 null allele. It is possible that a genetic modifier that affects Mbnl1 function is linked to Mbnl1 locus on chromosome 3, and the modifie r allele plays a role in determining the life span of the mutant mice. In contrast, 129 mice showed perfect 1:2:1 segregation ratio from heterozygous intercrosses and they w ere completely normal by 3 month of age until they developed phenotypes such as thymic hyperplasia and skin defects. Thymic enlargement occurs due to a large number of proliferating lymphocytes which are mostly CD4+ CD8+ DP cells. Other mouse models with DP T lymphocyte expansion include the Ft ( Fused toes ) mouse with
6 0 deletion of IrxB gene cluster caused by thymic stromal cell defects as well as acetylcholinesterase (AchE) R variant overexpression transgenic mice that are implicated in myasthenia gravis (MG). The thymocytes from this latter mouse were more resistant to apoptosis suggesting a failure in selection (91 93) It is not clear whether the accumulation of immature T lymphocytes is due to defective signaling from thymic epithelial cells or is a T cell autonomous problem. One possibilit y that explains immature T cells retained in the Mbnl1 E3/ thymus is disrupted regulation of cell migration and signaling pathways. For example, fibronectin (FN) is a multifunctional extracellular matrix glycoprotein, which plays a role not only in prov iding a scaffold for cells but also in cell migration and signaling to adhering cells (94). Mbnl1 E3/ mice have an aberrant inclusion of the extradomain EDA and EDB exons which are found in onco -fetal tissues. I t is tempting to speculate that such a defect in splicing leads to altered cell migration or apoptosis that results in thymic enlargement in Mbnl1 E3/ mice. Also, it is not clear why only Mbnl1 E3/ 129 male mice develop this phenotype (>90% penetrant ) while females develop skin lesions. One possibility is that dysregulation of a sex specific gene underlies the thymic phenotype. Indeed, c astration of normal male rodents results in a significant enlargement of the thymus, and androgen replacement can re scue the defect. It has been suggest ed that the androgen receptor (AR) expressed by thymic epithelial cells is an important player in thymocyte development even though we failed to detect any obvious missplicing of the AR gene (95) To identify molecular defects underlying thymic phenotypes of Mbnl1 E3/ mice, w e have tested alternative splicing patterns of the TCR receptor as well as other key
61 factors that are known to regulate DP to SP differentiation, including Zap70, Id2, Cd3d, Cd45, Ptcra and transcription factor Aire. Hwoever, we failed to identify any significant changes in splicing Regardless, involvement of Mbnl1 in T cell development is an interesting observation and worth further investigation since a fundamental role for other splicing factors, such as SC35 and HuR, in thymic T cell development has been observed (96, 97) Because Mbnl1 translocates to the nucleus during a specific developmental window (between P2 and P20 for skeletal muscle), it would be interesting to see if this translocation occurs during the same period in thymic T cells as well (33) There is no evidence for defective T cell development in DM. Howe ver, several case reports exist which show the development of benign thymomas in DM patients. Thymomas originate from epithelial cell populations in the thymus (98 100) In addition, a subpopulation of DM patients is affected with pilomatrixomas, which is a benign tumor of the face, neck or proximal upper extremity, possibly due to altered epithelial cell function (63, 64) This observation supports the idea that epithelial cell changes may underlie the thymic and skin phenotypes in Mbnl1 E3/ 129 knockout mice Generally, pilomatrixomas are associated with mutations in genes of the Wnt signaling pathway. However we failed to detect altered splicing of these genes although further studies will be required to confirm that the dermatitis in Mbnl1 E3/ 129 mice and pilomatrixomas in DM patients are similar (101) Also, there is a recent case report of a DM patient with basal and squamous cell carcinomas that are similar to the skin lesions noted i n Mbnl1 E3/ 129 mice (65) Muscle weakness is evaluated usin g a force measurement protocol. It is a commonly used method to evaluate muscle function in muscular dystrophy. In our study, we clearly
62 showed that Mbnl1 E3/ Bl6 mice have defective contractile properties, especially in specific force deficit. It was also striking that Mbnl1 E3/ 129 mice are spared muscle weakness at least until 3 months of age. It is possible that the 129 mice will develop weakness at later stage however the shortened life span prevented us from investigating this possibility. Mbnl1 E3/ Bl6 muscles were also enriched in oxidative fibers, which is indicated by an increase of type IIa MHC positive fibers and a high mitochondrial pro tein expression. It is well known that chronic stimulation can result in a fiber type switch toward more oxidative fibers (102) In this study we have shown that Mbnl1 E3/ Bl6 and 129 mice have equivalent degree s of myotonia not only by EMG but also by the Clcn1 splicing pattern, which suggests that myotonia is not the main cause of the abnormal structure nor muscle weakness in DM. This idea is supported by the observation that the myotonic adr mouse muscle has r elatively minor histological changes except the absence of type IIb fibers (85, 103, 104) Our results provide the first direct evidence that Mbnl1 loss -of function underlies muscle weakness by altering both struct ural and functional properties of skeletal muscles. Another interesting observation is the progressive muscle pathology. By 15 months of age, myofibers with dystrophic changes increased up to 50% in quadriceps and TA. These changes were accompanied by abnormal NMJ structure. The morphological NMJ abnormalities appear to be degenerative rather than developmental since we did not observe a similar discontinuous pattern of AChR staining in the young (2 month ol d) Mbnl1 E3/ Bl6 TA muscles. It is not clear whether these NMJ changes result from motor neuron defects or are am product of skeletal muscle defects. H owever, intact NMJs are essential to
63 propagate action potentials from motor neurons. Therefore, aberrant endpl ate topology in Bl 6 mutants suggest s that the movement deficits seen in these mice might result from abnormal NMJ formation and physiological dysfunction of the NMJ Discontinuous junctions in skeletal muscles have been observed in other mouse models for muscular dystrophy such as the mdx mouse and mdx/ utrophin double knockout mice and it was suggested that the absence of dystrophin leads to abnormal AChR cytoskeleton interactions (89) Recently, there was a repor t that human DMPK transgenic mice with 300 or more CTG repeats exhibit abnormal NMJ in the diaphragm muscle. However, this study is the first to report an abnormal end plate morphology in skeletal muscle of a mouse model for DM (105) Significant fragmentation of the AChR in Mbnl1 E3/ muscle could lead to a reduction in the transmission of action potentials from motor neurons to muscles, which may explain muscle wasting in DM. As shown for other muscular dystrophies, muscle atrophy is related to motor neuron denervation In sup port of this hypothesis, it has been demonstrated that the DMPK gene is expressed at high level in subsynaptic myonuclei in human skeletal muscles and toxic (CUG)n RNA and MBNL1 proteins are retained in postsynaptic nuclei in the NMJ region in DM muscle (106) Absence of a muscle wasting phenotype in Mbnl1 E3/ mice is possibly due to the high regeneration level observed in rodents. Similarly, in the mdx mouse, which is a model mouse for Duchenne muscular dystrophy, necrosis is compensated by active regeneration and these mice fail to develop muscle atrop hy (107, 108) Alternately, the muscle wasting phenotype may result from the loss of function of another factor unrelated to MBNL1. The
64 observation that (CUG)960 mice develop muscle wasting supports the possibility that large CUG repeats sequester other trans activating factors resulting in a wasting phenotype. Muscle weakness and structural defects in Mbnl1 E3/ Bl6 mice correlate with altered splicing of Tnnt3 fetal exon, Ryr1 exons 70 and 83. Mutations of Ryr1 have already been linked to other skeletal myopathies such as central core disease, which causes hypotonia and proximal muscle weakness (109) Among two alternative exons of Ryr1, exon 70 exclusion in Mbnl1 E3/ B6 muscle is of particular interest because the aberrant splicing of this exon has been demonstrated in DM and HSALR muscles (57) A previous study, Ryr1 peptides with or without exon 70 were expressed in dyspedic ( Ryr1 null) myotubes in vitro to test the functional readout of missplicing Calcium (Ca2+) release was enhanced by >50% in myotubes without exon 70 compared to myotubes with exon 70 (90) Increased Ca2+ rel ease can result in myopathy if it leads to sustained rise in intracellular Ca2+ in muscle. A profound alteration in Ca2+ homeostasis can lead to a disruption in Ca2+mediated cell signaling. For example calpain, Ca2+-dependent protease, activation can occu r. Supporting this hypothesis, Mbnl1 E3/ Bl6 quadriceps muscle showed considerably higher calpain activity compared to WT (p=0.065) whereas 129 mice showed no changes (n=3, each genotype). This assay was not sensitive enough to distinguish between diffe rent types of calpains and it should be repeated with more samples and a more sensitive assay for specific calpain activities. Moreover direct measurement of Ca2+ efflux from SR as well as Ca2+ sensitivity of Mbnl1 E3/ muscle should be performed to demo nstrate dysregulaiton of Ca2+ homeostasis.
65 What is the molecular mechanism underlying the different regulation of Ryr1 alternative splicing in different genetic backgrounds? One possible model which involves a genetic modifier that functions as a splicing factor is presented in Fig. 2 10. Both MBNL1 and this splicing factor ( X ) interact with a spliceosome factor such as a component of U1 snRNP and stabilize the interaction of the pre -mRNA/spliceosome complex at the 5 splice site of Ryr1 exon 70. In th e Bl6 background, splicing factor X is either absent or functionally compromised leading to decreased Ryr1 exon 70 inclusion (~80%) compared to 129 background (~90%). In Mbnl1 E3/ tissues, exon 70 is predominantly excluded due to loss of Mbnl1 function. The presence of the genetic modifier in the 129 background enhances the inclusion ratio to 9%, which might be result in the normal muscle structure and function of the Mbnl1 E3/ 129 mouse. However in Mbnl1 E3/ Bl6 tissues, inclusion of exon 70 is le ss than 1% due to the absence, or decreased activity, of the genetic modifier X. What is a possible candidate for the genetic modifier X? The Scnm1 locus is a trans activating modifier that determines the severity of a sodium channelopathy in mice (110) Scnm1 is an auxiliary spliceosomal protein that contributes to the recognition of non consensus 5 splice sites. The phenotype of the hypomorphic allele of Scn8a (medJ) is dependent upon the Scnm1 locus (111113) Bl6 carries a variant allele Scnm1R187X that results in a more severe phenotype compared to other strains because the C -terminal domain of Scnm1 is truncated in Bl6, and not in the other strains, and this leads to i ncreased exclusion of exon 2 and 3 of Scn8a because this domain mediates the interaction with LUC7L2, which is a mammalian homolog of a yeast protein involved in recognition of
66 non -consensus splice sites. Interestingly, only a 5% difference in exon exclusi on (10% correct transcripts in other strain s and 5% in B6) is enough to yield a significant difference in neurological phenotype, which suggests that slight changes in splicing due to genetic modifiers have profound effects on disease severity. It is an i nteresting idea to test whether Scnm1 is the genetic modifier X. Alternatively the genetic modifier X can be a different splicing factor or a novel locus. Since the expression of the modifier in 129 tissues was sufficient to repress the disease associated phenotypes such as centralized nuclei in myofibers as well as compromised force generation, it is important to identify the genetic modifier. This putative splicing factor is an interesting potential therapeutic target for muscle weakness in DM.
67 Fig ure 2 1. The Mbnl1 E3/ mouse is a functional null. Schematic representation of the Mbnl1 ge ne (black line). Exons that encode UTR s (open boxes) and ORF s (black boxes) are indicated. Isoforms that initiate in exon 3 contain four copies of C3 H (4 XC 3 H) whereas t he isoforms that use the exon 4 initiation codon contain only two C 3 H motifs (2XC 3 H) Mbnl1 E3/ mouse is a functional null since 4XC3H containing isofo r ms are missing
68 Table 2 1 Genotypic evaluation of B6 and 129 congenic mice. Pups from heterozygous intercro sses were genotyped at weaning age (~3 weeks) Comparison of observed and expected genotypic ratio is presented along with chi -square value and corresponding p -value. A significant percentage of Mbnl1B6 knockout mice showed embryonic to perinatal lethality
69 Figure 2 2. Shortened life span in congenic Mbnl1 E3/ Bl6 and 129 mice. ( A ) Mbnl1 protein is completely absent in quadriceps muscles of Mbnl1 E3/ mice in both backgrounds when total protein is blotted with anti -Mbnl1 polyclonal antibody. Gapdh is the protein loading control. ( B ) Kaplan -Meier surviv al curve is shown for Mbnl1+/+ 129 (yellow), Mbnl1 E3/ 129(red), Mbnl1+/+ Bl6 (light green) and Mbnl1 E3/ Bl6 green). Enhanced lethality of Mbnl1 E3/ Bl6 mice was seen between 12 weeks and 26 weeks of age whereas lethality of Mbnl1 E3/ Bl6 mice was gradual. 100% of WT mice survived for the same time period.
70 Figure 2 3. Mbnl1 E3/ 129 congenics have defects in thymus and skin. ( A ) Enlargement of the thymus in Mbnl1 E3/ 129 congenics at 24 weeks of age. The size of the thymus in mutant mice was comparable to WT at 12 weeks of age while there is massive enlargement of thymus at 24 weeks of age. Rectangle in the background is 1cm2. ( B ) Enlargement of the thymus in 129 knockout mice at 24 weeks of age is correlated with expansion of immatur e T lymphocytes. Flow cytometry of thymocytes from Mbnl1 E3/ 129 mice show defects in differentiation of T cells from double positive (CD4+CD8+) to single positive (CD4+CD8 or CD4 -CD8+) stage. Cells were gated for CD4+ positive and CD8+ positive population. ( C) Histological analysis of 129 skin by H&E stain ing. E=epidermis, D=dermis, A=adipose. Mbnl1 E3/ 129 skin shows severe hyperplasia of epidermis as well as inflammation of the dermal layer. The thick keratinous layer above the epidermis is also noticeable.
71 Figure 2 4. Severe movement deficit in B 6 Mbnl1 E3/ mice is separable from myotonia (A ) Mbnl1 Bl6 knockout mice developed severe movement deficit s shown by stiff hindlimb postures. Mbnl1 129 knockout mice are much less affected. ( B ) Abnormal motor coordination of 3 4 month old Mbnl1 E3/ B6 mice measured by lat e ncy to fall using the accelerating rotarod. The rotarod starts at 4 rpm and accelerates to 40 rpm for first 2 min. The performance of Mbnl1 E3/ 129 mice was comparable to WT controls. Genotypes (color coded) are: Mbnl1 E3/ Bl6 (green), Mbnl1+/+ Bl6 (light green), Mbnl1 E3/ 129 (red) and Mbnl1+/+ 129 (yellow). (C) Myotonia was assessed by EMG on B6 (green) or Mbnl1 E3/ 129 mice (red) in the multiple skeletal muscle tissues. Difference in the degree of myotonia in the two gr oups is not significant. The EMG scale is: 0, no myotonia; 1, occasional myotonic discharge in <50% of needle insertions; 2, myotonic discharge with >50% of insertions; 3, myotonic discharge with nearly all insertions. WT mice for both genetic backgrounds were also tested and they were all 0. ( D ) RT -PCR splicing assay of congenic muscles for Clcn1 agreed with the EMG results The ratio of alternative exon 7a inclusion was equivalent in Mbnl1 E3/ mice in both backgrounds. Alternative exon 7a (black box) is dysregulated in DM as well as DM mouse models.
72 Figure 2 5. Abnormal muscle structure and fiber type switch in Mbnl1 E3/ Bl6 muscle. (A ) Muscle histology (H&E staining) of quadrice ps muscle sections from 6 month old Bl6 and 129 WT and mutants Mbnl1 E3/ Bl6 mice show abnormal muscle structures, such as centralized nuclei and split fibers, whereas Mbnl1 E3/ 129 muscles are much less affected. Immunohistochemistry of muscle secti ons with an antibody specific for type IIa MHC (green) shows muscle fiber type changes in Mbnl1 E3/ Bl6 toward more oxidative fiber type IIa. Muscle sections were costained with laminin anitibody (red) and DAPI (blue) to localize individual myofibers an d nuclei. ( B ) Mbnl1 E3/ Bl6 muscles are enriched in mitochondrial protein. Total protein lysates of quadriceps muscles from Bl6 and 129 mice were probed with mitochondrial protein Cox Va specific antibody. Gapdh is a glycolytic protein and is slightly d ownregulated in Mbnl1 E3/ Bl6 muscles. ( C) Percentage of myofibers with centralized nuclei is quantified from the muscles sections from ( A ). Average of >1400 fibers were counted for each genotype.
73 Figure 2 6. Age -dependent progressive muscle histopathology and abnormal end plate topology. ( Left ) H&E staining of TA muscles of Mbnl1 E3/ Bl 6 mice at 2 months of age shows almost normal structure. Histopathological abnormalities such as centralized nuclei and split fibers progress with ageing. There i s also very high heterogeneity in myofiber size. ( Right) Structure of neuromuscular junctions (NMJs) in TA muscles was analyzed by labeling acetylcholine receptor s (AchR) with -bungarotoxin. At 2 months of age, the NMJ of Mbnl1 E3/ mouse had a smooth a nd continuous pattern of AChR staining whereas ~80% of NMJs of the 15 month old Bl6 mutant mouse showed a fragmented pattern of AChR staining characteri zed by discontinuous nerve ends, although a small population of intact NMJs was present Wild type litte rmates showed normal NMJ structures at all time points.
74 Figure 2 7. Reduced force and abnormal contractionrelaxation kinetics of Mbnl1 E3/ Bl6 soleus muscles. Three month old Mbnl1+/+ (dark grey) and Mbnl1 E3/ (light grey) soleus muscles in Bl6 and 129 backgrounds were subjected to force measurements. Single twitch contraction produced by a 1ms current pulse did not show a signific ant difference whereas titanic stimulation at 100Hz produced 21% less force in Mbnl1 E3/ Bl6 soleus compared to WT controls. Mbnl1 E3/ 129 muscles showed 20% more titanic force than their control muscles. Due to the increase of muscle mass in Mbnl1 E3/ mice (19% for B6, p=0.055 and 28% for 129, p<0.01) when calibrated to the unit cross -sectional area (specific force), force from Mbnl1 E3/ Bl6 soleus muscle was 34% less than WT control while specific force of Mbnl1 E3/ 129 and WT was equivale nt. Furthermore, time to peak and T1/2R decreased suggesting changes in kinetics in contraction and relaxation of Mbnl1 E3/ Bl6 soleus muscles.
75 Figure 2 8. Reduced force generation of Mbnl1 E3/ Bl6 extensor digitorum longus (EDL) muscles. Thre e month old Mbnl1+/+ (dark grey) and Mbnl1/ (light grey) EDL muscles in B6 and 129 backgrounds were subjected to measure force production. Single twitch contraction produced by a 1ms current pulse generated 21% less force in Mbnl1 E3/ Bl6 EDL compared to WT controls (p=0.008). Titanic stimulation at 100Hz produced 23% less force in Mbnl1 E3/ Bl6 EDL compared to WT controls (p=0.01). Mbnl1 E3/ 129 muscles were identical to their WT controls. When calibrated to the unit cross -sectional area (specif ic force), force from Mbnl1 E3/ Bl6 EDL muscle was considerably (not significantly) lower than WT control (p=0.07).
76 Figure 2 9. Aberrant splicing of Ryr1 and Tnnt3 are correlated with myopathic changes of Mbnl1 E3/ Bl6 congenic mice. ( A ) Splic ing of Serca1, Mbnl1 and Mtmr3 among other Mbnl1 target pre -mRNAs is equivalent between Mbnl1 E3/ Bl6 and 129 muscles by RT PCR, suggesting they are not molecular defects underlying the difference between backgrounds. Alternative exons are shown with bl ack boxes. ( B ) Splicing of Tnnt3 and Ryr1 are different in Mbnl1 E3/ B6 and 129 muscles and correlate with myopathic changes. Inclusion ratio of Ryr1 exon 70 is >10 times lower in Mbnl1 E3/ Bl6, compared to 129, mice.
77 Figure 2 10. M odel for as sociation of MBNL1 and genetic modifier X with U1 snRNP. MBNL1 (red oval with M) and the genetic modifier X (blue oval with X) stabilize U1 snRNP (open oval with U1) binding at the 5 splice site of Ryr1 exon 70 (lines, introns; open boxes, exons). In the Bl6 background, splicing factor X is either absent or functionally compromised. Ratio of correct and incorrect transcripts of Ryr1 for each genotype is indicated with percentage.
78 CHAPTER 3 CONCLUDING REMARKS A ND FUTURE DIRECTIONS DM is a complex disease with multi -systemic clinical manifestations as well as remarkable variability among patients and the molecular etiology of this disease has been unclear for a long time. We hypothesized that loss of MBNL1 function is a primary pathogenic event in DM and we tested this idea by performing genetic complementation of Mbnl1 in skeletal muscle of a poly(CUG) mouse model for DM. AAV -mediated gene transfer of Mbnl1 was sufficient to rescue DM associated phenotypes such as myotonia and missplicing in the TA muscle of the mouse even though it failed to reverse the muscle histopathology. Our results suggest that MBNL1 loss -of -function due to sequestration plays a major role in the development of myotonia and missplicing in DM. A revised experimental design, such as systemic delivery of untagged Mbnl1, will be required to test if muscle pathology is reversible. Alternatively, it will be interesting to see if genetic complementation of other MBNL family members, such as MBNL2 and MBNL 3 can rescue the structural changes of muscle induced by expression of toxic poly(CUG). We also investigated the consequence of Mbnl1 loss -of -function by examining congenic (either Bl6 or 129) Mbnl1 E3/ m ice Our data demonstrated that Mbnl1 deficiency in mice leads to defects in multiple tissues and the severity depends on genetic background, which suggests that other factors are important to explain the variability and incomplete penetrance observed in DM patients. Our results also suggest that myotonia and myopathic changes in DM are t wo separable events and probably result from independent molecular mechanisms. We propose that disruption of calcium homeostasis due to missplicing of Ryr1 leads to the myopathy in DM. Future studies should investigate the
79 functional relevance of Ryr1 miss plicing in vivo Furthermore, analysis of congenic Mbnl1 E3/ m ice identified defects in thymus and skin, which seem to involve hyperplasia of epithelial cells. Since there have been multiple case reports of tumors in individuals affected with DM, it will be interesting to investigate a potential role for MBNL1 dysregulation in tumorigenesis. Our results clearly demonstrate that the MBNL1 loss -of -function model for DM is valid and explains many of the major pathogenesis events in DM. On the other hand, genetic factors seem to contribute to disease variability, which suggests that combinatorial therapeutic approaches will be required. Identification of the genetic modifier(s) that determines the disease severity will be an important contribution. We are hopeful that our data highlights the necessity for further studies on the molecular defects associated with less recognized disease associated phenotypes as well as the mechanisms underlying the variability of this disease.
80 CHAPTER 4 MATERIALS AND METHOD S Mice The HSALR 20b line was used for the study in Chapter 1 Intergenerational stability of the (CTG)250 expansion was monitored by PCR with AmpliTaq Gold. The age of mice that were used for the experiments was typically 5 weeks old. Congenic Mbnl1 knockout mice were generated by conventional backcrosses to inbred C57BL/6J or 129/Sv mice. Mbnl1 genotyping was performed using genomic DNA from tail snips at 3 weeks of age. Tails (0.5cm) were lysed in 200 ul of 20 mM NaOH at 99C for 1 hr and neutralized wit h same volume of 40 mM Tris Cl (pH.8.0). Lysate (2 ul) was used to perform PCR. Primers used were MSS1382, MSS1383 and MSS884 and PCR cycles are as follows: 9 5 C for 3 0 sec, 60C for 45 sec, and 68C for 120 sec. 700bp band for PCR product is for WT allele and 800bp is for mutant allele. AAV2 /1 mycMbnl1/41 Virus Preparation and Injection Protocol A myc tag was added to the human MBNL1/41 ORF by using sequential PCR rounds with the same reverse primer (MSS1580) and the following four forward primers: 1, MSS 1576; 2, MSS1577; 3, MSS1578; 4, MSS1579. Amplification for each round was performed for 25 cycles (98C for 20 sec, 60C for 30 sec, and 72C for 90 sec). The PCR product was subcloned into pCR4 TOPO (Invitrogen, Carlsbad, CA) to create mycMBNL1/41. Human and mouse protein sequences vary by two a mino a cids in exon 5. Therefore, human cDNA was converted into a cDNA that encodes the mouse protein sequence by digestion of the mouse Mbnl1/41 ORF with HincII and HindIII followed by ligation into HincII/HindIII digested mycMBNL1/41.
81 The mycMbnl1/41 cDNA was subcloned into SpeI/ClaI -cut pTR GFP deleted) to create pTR -mycMbnl1/41. The plasmids pTR -mycMbnl1/41 and pXYZ1 were used for virus preparation in HEK293 cells by using a two plasmid cotransfection protocol (114) The AAV 2/1 -mycMbnl1/41 vector was purified further by using a HiTrap Q column (GE Healthcare, Piscataway, NJ) (115) and the concentration (1.24 1013 vector genomes/ml) was determined by using a standardized real time qPCR titration technique (116) The right TA muscles of 4to 5 -week -old HSALR 20b mice were inject ed with one vector dose (1 1011 gauge needle. AAV2/8 -mycMbnl1/41 vector was prepared using similar protocol by University of Florida vector core laboratory except that pDG -AAV8 was used which contains the AAV8 capsid protein sequence. The concentration of virus was 1.3 1013 vector genomes/ml One day old HSALR mice were anesthetized by hypothermia and administered with 2.5 5 1011 particles (30 l total volume) intravenously via the superficial temporal ve in. Analysis of Mbnl1 Isoforms in Adult Muscle To determine the major Mbnl1 isoforms expressed in C57BL6/J skeletal muscle, total RNA was isolated from P2 hindlimb muscle and P28 quadriceps by using TRI Reagent (Sigma, St. Louis, MO). cDNA was generated primed reverse transcription by using SuperScript II (Invitrogen) followed by digestion amplified through 35 cycles (95C for 30 sec, 50C for 30 sec, and 72C for 90 sec) by using forward (MSS2161) and reverse (MSS2162) primers, and the product was cloned into
82 pCR4 TOPO. Multiple clones from P2 and P28 were randomly picked, and full -length cDNA coding sequences were obtained for both P2 (n = 72) and P28 ( n = 81). For immunological detection of Mbnl1, 9 -month -old HSALR tissues (tibialis anterior, gastrocnemius, quadriceps, cerebellum, heart, and lung) were homogenized in 50 mM TrisCl, pH = 6.8/1 mM EDTA/2% SDS/0.5 mM phenylmethylsulfonyl p aminobenzamidine/1 g for 10 min, and supernatant proteins were fractionated on 12.5% SDS -polyacrylam protein per lane), electroblotted to nitrocellulose, immunoblotted, and visualized by ECL. Antibodies for immunoblotting included the rabbit anti -Mbnl1 polyclonal A2764 (1:1,000) and the anti -GAPDH monoclonal 6C5 (1:10,000; Novus Biological s, Littleton, CO). The same procedure was performed for immunological detection of mycMbnl1/41 in AAV transduced TA muscles. RNA Splicing For splicing analysis in cell culture, HEK293T cells were plated in six-well plates in DMEM (Invitrogen), supplemented with 10% FBS (Invitrogen) and 1% penicillinstreptomycin (Invitrogen). The next day, the cells were cotrans Tnnt3 minigene and 10 ng of protein expression plasmids by using Lipofectamine 2000 (Invitrogen) according to the manufacturers protocol. Protein and RNA were harvested 48 h after transfection. For analysis of GFP -fusion protein ex pression after transfection, GFP (1:1,000; Roche, Indianapolis, IN) and anti GAPDH mAb 6C5 (1:10,000). For RNA -
83 splicing analysis, first-strand cDNA was generated from tota l RNA by reverse transcription RT (Invitrogen). Subsequent PCR (28 cycles at 95C for 30 sec, 55C for 30 sec, and 72C for 30 sec) was performed by using 20% of the reverse transcription reaction as a temp 32P] dCTP (PerkinElmer, Wellesley, MA). Tnnt3 minigene mRNA was analyzed by using exon 8 forward (MSS1956) and exon 9 reverse (MSS1938) primers. PCR products were resolved on 10% nondenaturing polya crylamide gels followed by autoradiography with Biomax MS film (Eastman Kodak, Rochester, NY). Splicing shifts were measured by using a Pharox FX plus Molecular Imager (Bio-Rad, Hercules, CA). Splicing patterns were monitored by radioactive RT -PCR as previ ously described (19) Primers used for Chapter 1 were as follows: Serca1, exon 21 forward (MSS2761) and exon 23 reverse (MSS2762); Cypher, exon 8 forward (MSS2763) and exon 13 reverse (MSS2764); Tnnt3, exon 2 and 3 overlapping forward (MSS1677) and exon 11 reverse (MSS1678); Clcn1, exon 6 forward (MSS2788) and exon 8 reverse (MSS1653); Capzb, exon 7 forward (MSS2765) and exon 9 reverse (MSS2766); Itgb1, exon 16 forward (MSS2767) and exon 18 reverse (MSS2768); myc -Mb nl1. forward primers in myc (MSS1579) and reverse primers in Mbnl1 exon 6 (MSS1656). Primers used for chapter 2 were as follows: Clcn1 exon 6 forward (MSS3513) and exon 7 reverse (MSS3514); Mbnl1 exon 6 forward (MA1) and exon 8 reverse (MA2); Mtmr3 exon 16 forward (MSS3825) and exon 18 reverse (MSS3826); Ryr1 exon 69 forward (MSS3890) and exon 71 reverse (MSS3891); Ryr1 exon 82 forward (MSS3892) and exon 84 reverse (MSS3893); Serca1 and Tnnt3 are same as above.
84 Electromyography Before electromyography, mice were anesthetized by i.p. using 100 mg/kg ketamine, 10 mg/kg xylazine, and 3 mg/kg acepromazine or 250 mg/kg 2,2,2 tribromoethanol. Electromyography on tibialis anterior and gastrocnemius muscles was performed as described (19) by using 30 gauge concentric needle electrodes and a minimum of 10 needle insertions for each muscle. For each time point (4, 12, 23, and 43 weeks after injection), at least 6 mice were evaluated, and both uninjected and injected TA, as well as gastrocnemius, muscles were tested. For congenic mice used in Chapter 2, numbers were between 4 and 6 and typical age at the time of analysis was 6 month old. Histology and Immunohistochemistry skeletal muscle were either prepared for routine H&E staining or immunostained by using antibodies against the ClC 1 C terminus (Alpha Diagnostic, San Antonio, TX) or the Mbnl1 C terminal peptide (rabbit polyclonal antibody A2764) as described (36) with the following modifications. For Mbnl1, sections were fixed in 4% paraformaldehyde for 15 min at room temperature and permeabilized with 2% acetone in PBS for 5 min. For ClC 1, sections wer e unfixed. Primary antibody concentrations used were 1:10,000 for Mbnl1 and 1:50 for ClC 1. Secondary antibodies were goat anti rabbit Alexa Fluor 488 and 546 (Invitrogen) at 1:400. Stacks of Z plane images were deconvolved by using a blind point spread fu nction (117) Rotarod Test Performance on an accelerating rotarod was tested for congenic mice to assess motor co ordination. For four consecutive days, each subject was given four trials per day with a
85 minimum 10 min rest between trials. Revolutions per minute (rpm) were initially 4 rpm with a progressive increase to a maximum of 40 rpm for the first 2 min. Maximum t rial length was 4 min. Latency to fall from the rotating rod was automatically detected and recorded by photo laser beams in the apparatus. Time to fall ( Mean SEM) for each trial day was plotted. Analysis of T Cell Population by Flow Cytometry Flow cyto metry was performed by Dr. Changquing Xia in the Department of Pathology, University of Florida. Single thymocyte suspension was prepared using 10 m cell strainer. After washing with PBS, centrifugated cell pellets were resuspended in red blood cell (RBC) lysis buffer to remove RBCs. After centrifugation, cell pellets were resuspended in washing buffer (PBS with 1% FBS). Cells were stained at 4C for 1hr in washing buffer containing appropriate monoclonal antibodies. After a series of washing steps, thymoc ytes were subjected to flow cytometric anlysis on a Becton Dickinson FACScan apparatus. On the basis of forward angle light scatter gating, live cells were gated and analyzed according to their FSC and SSC profiles. In Vitro Force Measurements In vitro fo rce measurements were performed by Fan Ye in the Department of Physical therapy, University of Florida. Mice were anesthetized with isoflurane by inhalation and sacrificed by cervical dislocation. The soleus and EDL muscles were dissected with intact tendo n and mounted horizontally in the organ bath of Ringers solution gas -equilibrat ed with 95% O2 and 5% CO2 maintained at 25C. Muscle tendons were tied with surgical silk thread to attach muscle to the experimental apparatus. The
86 muscles were adjusted to th eir optimal length to produce twitch force from supramaximal stimulation. M aximal titanic contraction force was determined by using a 120 Hz ,500ms pulse delivered by two parallel platinum plate electrodes. After force measurements, muscles were removed fr om bath and blotted on kimwipes twice and weighed for wet weight. Some of the muscles were mounted for histological analyses and were frozen in liquid nitrogen cold isopentane solution. Other muscles were snap frozen for molecular analyses and stored at 8 0C. Neuromuscular Junction Staining TA muscles were dissected to stain neuromuscular junctions. After overnight incubation in 2% PFA at 4 C muscle samples were washed in PBS. Then they were teased into ~0.3mm diameter bundles using a No.5 forceps. Myofi ber bundles were permeabilized and blocked in 2% BSA 4% normal goat serum, 1% Triton X 100 in PBS for 1 hrs at room temperature and then incubated with Alexa Fluor 594 bungarotoxin (1:1,000) (Invitrogen B13423) in 1% Triton X 100 in PBS (wash buffer) for 1 hr at RT. Samples were washed with wash buffer 3 times before mounted on the microscope slide with a drop of DAPI solution (vector lab) and a No. 1 cover slip. Either confocal laser scanning microscope or regular Zeiss inverted microscope was used to ta ke images of NMJs. For pre-synaptic staining, -neurofillament (1:5,000 EnCor Biotechnology MCA 1H1) and -SV2 (1:200 Iowa hybridoma core) antibodies were used. Calpain Activity Assay Biovision calpain activity assay kit (K240100) was used to measure cal pain activity in congenic quadriceps muscle extracts. Muscle protein extracts (50 g) were incubated
87 with 10 l of 10X reaction buffer and 5 l of calpain substrates in total reaction volume of 100 l at 37C for 1 hr in the dark. Active calpain (2 l) was added for positive control and 1 l of calpain inhibitor was added for negative control. Samples were transferred to 96 well plate and read in a fluorometer equipped with a 400nm excitation filter and 505nm emission filter. The activity is expressed as rel ative fluorescent unit (RFU) per mg protein of each sample. Ryr1 Minigene and Photocrosslinking Assay pSG5 Ryr1 E69.71 minigene was constructed by amplifying the mouse Ryr1 genomic region between exons 69 and 71 using MSS3923 and MSS3924 and inserting the PCR product into pSG5 (Stratagene, La Jolla, CA, USA) at the EcoRI site. For alternative construct containing only exon 70 and flanking 600nt, 600bp region crossing exon 70 was amplified using primers MSS4160 and MSS4161. And then it was inserted into pRS V -cTnt minigene to swap the cTnt genomic region using SalI and XbaI restriction enzyme sites. Whole cell lysates for cross -linking was prepared as previously described (28) Briefly, HEK293T cells were transfected with 10 g of pcDNA3 MBNL1mycHis and harvested in 48 hours. Cells are washed with PBS and resuspended in 250 l of 20 mM HEPES KOH (pH 8.0), 100 mM KCl, 0.1% IGEPAL and protease inhibitors. After sonication, lysates were centrifuged and supernatants were collected and glycerol was added to 20% of total volume. 600nt genomic region encompassing exon 70 of Ryr1 was labeled with ( -32P) -UTP (800 Ci/mmol) in the presence of 0.5 mM ATP and CTP, 0.02 mM GTP and UTP. Cross -linking was performed by incubating 0.1 pmol RNA with 15 l of HEK293T whole cell lysate in 25 l reactions containing 16 mM HEPES KOH (pH 8.0),
88 65 mM potassium glutamate, 2 mM Mg(OAC)2, 0.4 mM DTT, 0.16 mM EDTA, 20 mM creatine phosphate, 2 mM ATP and 16% glycerol (final concentration). Reactions were incubated at 30C for 15 min, transferred to pre chilled PCR caps on ice and photocrosslinked three times in Stratalinker (Stratagene, La Jolla, CA, USA) for 2.5 min with a 2 min interval between each irradiation. Samples were digested with 5 g of RNase A for 20 min at 37C and immunopurified using the anti -myc monoclonal antibody 9E10 pre coated protein A Sepharose (Amersham). Purified proteins were fractionated on 12.5% SDS PAGE gels followed by autoradiography. For splicing analysis of Ryr1 minige ne, pRSV -Ryr1 -E70 and protein expression plasmids were transfected into HEK293T and analyzed by RT -PCR as described above using primers MSS3497 and MSS3499.
89 Table 4 1 Sequence of primers
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102 BIOGRAPHICAL SKETCH Jihae Shin was born in Inchon Korea in 19 79. Sh e is the e ldest born to Bokyeong Heo and Bukyun Shin. Jihae graduated from Incheon Science High School in 1998, and at tended Seoul National University from 1998 2003, where s he studied biological sciences and earned a Bachelor of Science in Biology. She was also a student at the University of Tokyo, Japan from 2000 2001, where she had her first research experience under D r. Ryoichi Matsuda. After graduation, J ihae moved to United States to join the interdisciplinary program in biomedical sciences at the University of Florida College of Medicine in Gainesville, Florida. She did h er graduate work in Dr. Maur ice Swan sons la boratory of the Molecular Genetics and Microbiology Department and completed h er Ph.D. dissertation in December 2009. Jihae has accepted a postdoctoral position with Dr. Joel Richter at the University of Massachusetts Medical Center where she will work on the regulation of RNA processing in the nervous system.