Myotonic dystrophy type 1 [DM1] leads to maldevelopment, myotonia, and wasting of skeletal muscle. DM1 is caused by an unstable CTG repeat expansion in the 3' untranslated region of DMPK. Our central hypothesis is that skeletal muscle findings in DM1 result from a toxic effect of repeat expansion transcripts. Support for this hypothesis comes from studies of HSALR transgenic mice that express CUG expansion RNA in muscle. (CUG)n transcripts accumulate in nuclear foci, leading to a myotonic myopathy that is similar to DM1. Our working model postulates the following sequence of events: expression of CUG expansion RNA -> accumulation of (CUG)n RNA in nuclear foci -> sequestration of muscleblind [Mbnl] proteins in nuclear foci -> abnormal regulation of alternative splicing -> expression of inappropriate splice isoforms -> symptoms of DM. We now have evidence that this model can explain certain aspects of DM1, such as, chloride channelopathy and myotonia. We plan to extend this model and define its limits. First, we will compare patterns of alternative splicing in HSALR, Mbnll knockout, and wild-type mice and test the hypothesis that CUG expansion RNA compromises a specific developmental program of alternative splicing that depends on Mbnl1, the predominant Mbnl protein expressed in muscle. A striking example of aberrant splicing involves Serca1, the calcium re-uptake pump in sarcoplasmic reticulum. The physiologic significance of mis-splicing Serca1 will be determined by calcium imaging. Second, there is little information about metabolism of (CUG)n transcripts. We have derived transgenic mice for inducible expression of CUG expansion transcripts. These mice will be used to compare the accumulation and degradation of transcripts with or without an expanded CUG repeat. We also will test the hypothesis that overexpression of nuclear mRNA-degradases can accelerate clearance of poly-CUG RNA. Third, we will assess myonuclear morphology and bromodeoxyuridine incorporation in HSA(LR) mice to test the hypothesis that accumulation of CUG expansion RNA leads to nuclear demise. In related experiments we will investigate the mechanism of cell death that occurs in HSA(LR) myoblasts when growth factors are withdrawn. Fourth, we have derived transgenic mice with cre-activation alleles to develop models for DM1-related maldevelopment and wasting and test the hypothesis that CUG expansion RNA interferes with muscle differentiation.

Agency
National Institute of Health (NIH)
Institute
National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS)
Type
Research Project (R01)
Project #
5R01AR046806-06
Application #
7061702
Study Section
Skeletal Muscle and Exercise Physiology Study Section (SMEP)
Program Officer
Nuckolls, Glen H
Project Start
2000-04-01
Project End
2010-04-30
Budget Start
2006-05-01
Budget End
2007-04-30
Support Year
6
Fiscal Year
2006
Total Cost
$335,135
Indirect Cost
Name
University of Rochester
Department
Neurology
Type
Schools of Dentistry
DUNS #
041294109
City
Rochester
State
NY
Country
United States
Zip Code
14627
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Osborne, Robert J; Thornton, Charles A (2008) Cell-free cloning of highly expanded CTG repeats by amplification of dimerized expanded repeats. Nucleic Acids Res 36:e24

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