Intergenerational stability of the myotonic dystrophy protomutation.

The amplification of the CTG trinucleotide repeat in myotonic dystrophy (DM) correlates with increasingly severe phenotypes. We designate its minimal amplification the 'protomutation' since it is the mutation itself at an early stage of intergenerational evolution and is associated with very mild clinical signs. From the study of 536 DM mutation carriers (from 158 affected families), a total of 60 DM-parent/DM-offspring pairings were identified in which the parent had the protomutation. We found a strong correlation between the protomutation length and the amplification observed in the next generation. We also observed the stable transmission of the protomutation through successive generations. This stability may explain the maintenance in the population of this autosomal dominant disease despite the low reproductive fitness of severe DM phenotypes.