Analysis of the CTG repeat in skeletal muscle of young and adult myotonic dystrophy patients: when does the expansion occur?

The purpose of this investigation was to analyze the CTG expansion in muscle as compared to lymphocytes DNA in a sample of selected myotonic dystrophy (DM) patients of different ages and degrees of clinical severity, ranging from severe congenital to minimally affected. Results from the present study showed that the size of the CTG repeat was markedly larger in skeletal muscle than in lymphocytes in all DM patients. In contrast to lymphocytes, no significant correlation was found between the size of the CTG expansion in muscle and age at onset. In addition, large expansions were observed in muscle from all adult symptomatic patients independently of the presence of muscle weakness, which raises the question of the value of analyzing CTG expansions in muscle for predicting the severity of the phenotype. Differences between the size of the CTG expansions in muscle as compared to lymphocytes were smaller in affected children suggesting an apparent tendency to increase with aging and reaching a plateau in adulthood.