Convergent myotonic dystrophy (DM) haplotypes: potential inconsistencies in human disease gene localization

Myotonic dystrophy (DM) is an autosomal dominant neuromuscular disease which has been shown to be caused by an unstable trinucleotide repeat located on chromosome 19q. We have conducted extensive haplotype analysis on 105 DM chromosomes using twelve 19q13.2 loci identifying 18 RFLPs, spanning a physical distance of 1.3 Mb containing the DM gene. Three major haplotypes (H1, H2 and H3) comprising 46.7% of the DM chromosomes in our population, were observed. With the exception of H1 and H2 derivatives (H4, H5 and H6), the remainder of the DM chromosomes analyzed were found to have unique haplotypes. Haplotypes H2 and H3 observed exclusively on DM chromosomes of French-Canadian origin contain identical 500-kb core regions. The low frequency of this core haplotype in normal chromosomes (0.8%) is consistent with a mapping of the DM gene within this region. However, the DM mutation is found 160 kb distal to the point of divergence between the two haplotypes. In contrast, the 450-kb region shared by haplotypes H1 and H2 contains the DM mutation. Further analysis of the DM region using a polymorphic microsatellite (GJ-VSSM2; D19S207) located 15 kb distal to the DM mutation revealed strong allelic association of one of the (CA)n repeat alleles to DM; allele 5 was observed on 88.2% of DM chromosomes and 6% of normal chromosomes. The fact that the (CA)n allele 5 was found on all 56 DM chromosomes containing the three major haplotypes indicates that DM chromosomes in our population, including the two French-Canadian haplotypes which have a common region outside the DM gene, are probably derived from the same mutational event.