CONTRIBUTION OF MOLECULAR ANALYSES TO THE ESTIMATION OF THE RISK OF CONGENITAL MYOTONIC-DYSTROPHY

A molecular analysis of the maternal and child CTG repeat size and int ergenerational amplification was performed in order to estimate the ri sk of having a child with congenital myotonic dystrophy (CMD). In a st udy of 124 affected mother-child pairs (42 mother-CMD and 82 mother-no n-CMD) the mean maternal CTG allele in CMD cases was three times highe r (700 repeats) than in non-CMD cases (236 repeats). When the maternal allele was in the 50-300 repeats range, 90% of children were non-CMD. In contrast, when the maternal allele was greater than 300 repeats, 5 9% inherited the congenital form. Furthermore, the risk of having a CM D child is also related to the intergenerational amplification, which was significantly greater in the mother-CMD pairs than in the mother-n on-CMD pairs. Although the risk of giving birth to a CMD child always exists for affected mothers, our data show that such a risk is conside rably higher if the maternal allele is greater than 300 repeats.