Effects of the myotonic dystrophy mutation in muscle differentiation and apoptosis.

Myotonic dystrophy (DM) is the most common inherited neuromuscular disorder of adult life. The genetic defect for DM was identified as an unstable CTG trinucleotide repeat found in the 3' untranslated region (UTR) of a serine threonine p&barbelow;rotein k&barbelow;inase, DMPK. Normal individuals possess 5--35 CTG repeats, typical adult DM patients have repeat sizes ranging from 80 to 1000 while cDM patients have from 1000 to several thousand CTG repeats. This discovery provided a molecular basis to account for large variability of penetrance and age of onset in DM. Work in our laboratory progressed from ascertaining mRNA levels in patient tissues to testing the hypothesis that overexpression of DMPK might cause features of DM. Transgenic mice expressed the human DMPK mRNA and protein in the appropriate tissues and had many features of DM including type I fibre atrophy, central nuclei and ringed fibres. Induction of expression resulted in about three fold higher levels of CTG 99 mRNA over CTG 11 mRNA at 48 hours post induction. Levels of cell death were assayed following induction and CTG 99 cell lines showed a marked level of cell death while CTG 11 cell lines did not. The presence of CTG repeats within mRNA therefore appears to be very problematic for the cell. Furthermore, we found that patient amniocytes and myoblasts are susceptible to staurosporine induced cell death. Taken together, this data suggests that myoblasts expressing the DMPK 3' UTR are prone to cell death in an expression level and repeat length dependent manner. In addition, patient cells were found to be susceptible in much the same way. These experiments also revealed that myogenin levels in vivo were reduced in transgenic embryos compared with wild type embryos suggesting that expression of the DMPK 3' UTR in vivo inhibits accumulation of myogenin and perhaps myogenesis. In adult mice there was consistent muscle atrophy in CTG 91 but not in CTG 11 mice despite much lower expression levels of the CTG 91 transgene. Together, these results indicate that expression of the DMPK 3' UTR by itself may inhibit myogenesis in vivo and contribute to pathological features of DM-like muscle atrophy. (Abstract shortened by UMI.