Proof of genetic heterogeneity in the proximal myotonic myopathy syndrome (PROMM) and its relationship to myotonic dystrophy type 2 (DM2).

Recently, myotonic dystrophy type 2 has been described as a separate disease entity that is distinctive from classical Steinert's disease since it lacks a CTG repeat expansion on chromosome 19q. A gene locus for myotonic dystrophy type 2 has been mapped to chromosome 3q. Independently, proximal myotonic myopathy has been recognized as yet another form of a multisystem myotonic disorder. Its relationship to myotonic dystrophy type 2 remains to be clarified. In our linkage study of 17 German proximal myotonic myopathy families nine of them mapped to the myotonic dystrophy type 2 locus (LOD score 18.9). However, two families with a typical proximal myotonic myopathy phenotype were excluded from this locus (LOD score -7.4). These results confirm genetic heterogeneity in the proximal myotonic myopathy syndrome. Furthermore, in the majority of the proximal myotonic myopathy families the disease phenotype may be caused by allelic mutations in the putative myotonic dystrophy type 2 gene.