New nomenclature and DNA testing guidelines for myotonic dystrophy type 1(DM1)

Myotonic dystrophy (DM; OMIM 160900, also known as dystrophia myotonica, myotonia atrophica and Steinert disease) is an autosomal dominant myotonic myopathy associated with abnormalities of other organs, including eyes, heart, endocrine system, central and peripheral nervous systems, gastrointestinal organs, bone, and skin.1 The mutation underlying DM is an expansion of an unstable cytosine-thymine-guanine (CTG) trinucleotide repeat in the 3' untranslated region of the myotonic dystrophy protein kinase (DMPK) gene in chromosome 19q13.3.2-4 In 1994, Thornton et al.5 described an autosomal dominant disorder similar to DM without CTG repeat expansion at the DM locus. Ricker et al.6 named this disease "proximal myotonic myopathy" (PROMM; OMIM 600109) because of predominantly proximal muscle weakness without atrophy as opposed to the distal muscle involvement seen in DM. Subsequently, Meola et al.7 described a variant of PROMM with unusual myotonic and myopathic features, which they named "proximal myotonic myopathy syndrome," and Udd et al.8 described a PROMM-like family with dystrophic features, which they named "proximal myotonic dystrophy" (PDM). Researchers at the University of Minnesota9,10 found another multisystemic myotonic disorder that closely resembles DM with distal muscle weakness but no CTG repeat expansion. Because of the close phenotypic resemblance to DM, they called this disease "myotonic dystrophy type 2" (DM2; OMIM 602668). In 1998, Ranum et al.9 assigned the DM2 locus to chromosome 3q in a large kindred. Shortly after that, Ricker et al.11 found that the majority of German PROMM families show linkage to the DM2 locus. PDM was also mapped to this region (Krahe and Udd, personal communication, 1999). Whether PROMM, PDM, and DM2 represent different phenotypic expressions of a disease caused by the same mutation or if they are allelic disorders remains to be determined. It is also possible that these disorders are caused by mutations in different genes that are closely linked in the chromosome 3q region.12 Furthermore, the disease loci in some typical PROMM families11 and other families with multisystemic myotonic disorders have been excluded from both DM and DM2 loci. Because of the genetic and phenotypic heterogeneity in this group of disorders, it became necessary to establish a new nomenclature foreseeing the future discovery of new disease loci and phenotypic variability