THE DM MUTATION - DIAGNOSTIC APPLICATIONS IN THE FINNISH POPULATION

A pair of marker loci, D19S63 and D19S51, which are tightly linked to the myotonic dystrophy (DM) locus, were used to evaluate diagnostic ap plicability in the Finnish population. Results were then compared to d irect detection of the mutation. The D19S63 locus revealed a linkage d isequilibrium, since in 16 DM families as many as 75% of DM chromosome s carried the same allele 3 for D19S63, and 25% carried allele 1. Howe ver, when the data for D19S51 and D19S63 were considered together as a haplotype, the statistical significance of this linkage disequilibriu m was considerably reduced. As expected, the best method for reliable evaluation of the carrier risk was direct analysis of the mutation. Th irteen particularly difficult cases were resolved and in 46% of them t he decision could be made only by direct visualization of the mutation . However, in a few cases where the size of the CTG-repeat expansion w as close to the normal size range, linked markers proved to be useful to determine the affected chromosomes. Present findings indicate that analysis of the D19S63 locus coupled to direct demonstration of the mu tation provides the basis for DNA diagnostics of DM in the Finnish pop ulation.