FREQUENCY OF INTERGENERATIONAL CONTRACTIONS OF THE CTG REPEATS IN MYOTONIC-DYSTROPHY

Myotonic dystrophy (MD), an autosomal dominant multisystemic disorder with a high phenotypic variability, is the most common muscular dystro phy in adult life. The mutation underlying DM has been characterized a s an expanded CTG trinucleotide repeat sequence in the 3' untranslated region of a protein kinase gene on chromosome 19q13.2-13.3. We have a nalyzed the presence of CTG intergenerational variations on transmissi on in parent-child pairs affected with DM. The series includes 90% of all living affected descendants (symptomatic or asymptomatic) from a g iven myotonic dystrophy (DM) patient. A contraction of the CTG repeat size was observed in ten parent-child pairs (14.1%) and remained uncha nged in five (7%) pairs. The number of CTG repeats decreased in 2/30 m aternal transmissions (6.7%) and in 8/41 paternal transmissions (19.5% ). We found 14 asymptomatic individuals carrying the CTG expansion amo ng the offspring. In six of them, a contraction of the CTG repeat was observed, and in all six cases, the DM allele was paternally transmitt ed. Since nearly all the asymptomatic family members of DM patients we re analyzed in this series, the observed percentage of contractions ca n be considered more realistic, even though the number of parent-child pairs is small. (C) 1996 Wiley-Liss, Inc.