CTG instability in myotonic dystrophy: molecular genetic analysis of families from south-eastern France with characteristics of intergenerational variation in CGT repeat numbers

We report clinical, genetical and genealogical findings in 149 French famil ies from the Rhone-Alpes area studied over a 5-year period. There was a sig nificant excess of DM females compared to DM males with (CTG) repeat sizes between 1-2 kb. The mean maternal (CTG) repeat size was higher than paterna l repeat size. Anticipation phenomenom was significantly higher after mater nal than after paternal transmission. A significant correlation between par ental (CTG) repeat size and intergenerational variation both in paternal an d maternal transmissions was observed. The anticipation phenomenom was more important for sons than daughters particularly after maternal transmission . The mean (CTG) repeat size in mothers of CDM cases was about twice that o f mothers of NCDM children. The risk of giving birth to a CDM child increas ed considerably when the number of maternal (CTG) repeats was over 300 (CTG ), A significant excess of DM females was observed. They had on average 24% fewer children than male patients. Paternal transmission (63.6%) of DM occ ured more frequently than maternal transmission (52.7%).