INTERGENERATIONAL CONTRACTION OF THE CTG REPEATS IN 2 FAMILIES WITH MYOTONIC DYSTROPHY TYPE 1

Abstract

Myotonic dystrophy type 1 (DM1) is an autosomal dominant multisystemic disorder caused by the expansion of an unstable CTG trinucleotide repeat at chromosome region 19q13.3.1 The number of CTG repeats is unstable in the abnormal range and usually increases in size in successive generations, in association with anticipation.2,3 A decrease in the CTG repeat size during transmission from parents to child can also occur in about 6.4% of transmissions, most frequently during paternal transmissions (10%).4 In the French-Canadian DM1 population, intergenerational contractions occur in about 7.4% of transmissions, all in cases of paternal transmission. We report here 2 French-Canadian DM1 families with paternal transmission, both originating from the Saguenay-Lac St-Jean, in which all affected children display CTG contractions. Although it was already reported that intergenerational contractions could be observed in several sibs in a same family, it was not noted whether this occurred in all affected sibs.4 Overall, these observations support the existence of a paternal factor that prevents CTG expansion. ### Case reports. The pedigree of the 2 families and Southern blot analysis are shown in the figure. Figure Family tree and Southern blot Pedigree of family (A, B). (C, D) Southern blot analysis. The DNA (3–5 μg) was …