Anticipation in myotonic dystrophy type 1 parents with small CTG expansions.

Myotonic dystrophy type 1 is the most common form of adult muscular dystrophy and has the world's highest prevalence in the Saguenay-Lac-St-Jean region, due to a founder effect. This autosomal dominant disorder results from an unstable CTG repeat expansion in DMPK. This region of Canada has had a family screening and predictive testing program for this disorder since 1988. Heterozygotes for small expansions (50-100 CTG repeats) can be asymptomatic or minimally affected. The aim of this study was to assess anticipation for these individuals. At the time of this study, the molecular data of 40 individuals and their 76 affected children were available. We compared 76 parent-child pairs. Most offspring (92.1%) had a larger number of repeats than their parent and the median number of repeats in the offspring was 325 (range, 57-2000). The number of CTG repeats was significantly greater when the mutation was transmitted by a father (median, 425 repeats; range, 70-2000), than when it was transmitted by a mother (median, 200 repeats; range, 57-1400). The majority (65.8%) of children also had a more severe phenotype than their parent but the sex of the parent had no significant influence on the severity of the child's phenotype. No congenital phenotype was observed. These results confirm that anticipation is present even when the parent is heterozygous for a small CTG expansion. The parental sex has an impact on the size of the repeat in the next generation, larger increases being transmitted by males with a small expansion.