Reduced expression of the normal DMPK allele in a congenital DM patient

Both adult-onset and congenital myotonic dystrophy (DM) are autosomal dominant disorders caused by triplet repeat expansions within the 3{prime} untranslated region of the DM protein kinase (DMPK) gene. The size of the repeat region shows a positive correlation with disease severity; in general, the triplet repeat expansions in congenital DM patients are larger than those found in adult DM individuals. In an adult DM patient, the expanded allele of 133 repeats reduced both the synthesis and processing of DMPK mRNA, whereas expression from the unexpanded allele remained unaffected. However, in both muscle and skin tissues from a congenital DM individual, DMPK mRNA expression from the unexpanded allele was also reduced. This reduced expression was maintained in fibroblasts cultured from a skin biospy of the patient; however, normal expression of the unexpanded allele occurred in cultured myoblasts established from this patient`s muscle biopsy. To determine if the expanded repeat exerts a trans effect on DMPK gene expression, we have separated the normal and mutant DMPK alleles from the cogenital DM skin fibroblasts by somatic cell hybridization. Hybrid clones containing only the normal DMPK gene still produced reduced levels of DMPK mRNA, indicating that the reduced expression from the normal allele ismore » due to a cis effect. Cultured skin fibroblasts from the congenital DM patient were exposed to 5-azacytidine to determine if demethylation of the DMPK gene could restore proper expression of the normal allele. We are currently analyzing DMPK mRNA levels in these cells and determining if a difference in the methylation patterns of the normal DMPK alleles from adult and congenital DM patients accounts for this effect.« less