Variation of CTG-repeat number of the DMPK gene in muscle tissue

Myotonic dystrophy (DM) is associated with an unstable expansion of CTG repeats located in the 3' untranslated region of a protein kinase-encoding gene (DMPK) on chromosome 19 (19q13.3). The CTG repeat number varies between 5 and 37 in lymphocytes of normal individuals, whereas DM patients may have expansions from 50 to several thousand copies. Although the CTG expansions related to myotonic dystrophy (DM) are usually larger in muscle compared to peripheral blood, the variation in repeat number in non-dystrophic muscle is not known. In order to investigate if there is a variation, the CTG-repeat number was determined in percutaneous muscle biopsies obtained from 86 individuals without any muscle disorder or with a neuromuscular disorder without any clinical or histopathological signs of DM. The number of CTG repeats varied between 5 and 28, this being within the normal range reported for peripheral blood. A major sharp peak at n = 5 (27%) and a broader peak at n = 8-17 (56%) with peak values at n = 12 and 14 (11 and 14%, respectively) were observed. Alleles with 19 or more repeats amounted to 17% with a small peak at n = 20 and 21 (6 and 4%, respectively). It is concluded that the normal variation of CTG-repeat number in skeletal muscle is within the range found in peripheral blood, although there is a slight shift in the overall frequency distribution towards alleles with CTG repeat numbers in the higher range.