Myotonic dystrophies--how many are there?

Myotonic dystrophy: RNA-mediated muscle disease

PURPOSE OF REVIEW

The aim of this review is to highlight recent progress in elucidating the disease mechanism in myotonic dystrophy type 1 and type 2.

RECENT FINDINGS

Research on myotonic dystrophy has led to the recognition of a novel RNA-mediated disease process. In myotonic dystrophy it is the RNA rather than protein product of a disease gene that has deleterious effects on muscle cells. These unusual RNAs, which contain a long expanse of CUG or CCUG repeats, have far reaching effects on cell function by influencing the biogenesis of other cellular RNAs. One aspect of RNA metabolism that is particularly affected is the regulation of alternative splicing. By this mechanism, effects of myotonic dystrophy repeat expansions impact many different pathways, triggering a complex set of signs and symptoms.

SUMMARY

The genetic lesion in myotonic dystrophy does not eliminate an essential muscle protein. Instead, it induces a defect of RNA processing that is potentially reversible. The nature of this disease process raises the possibility that myotonic dystrophy, among genetic disorders, may be unusually susceptible to treatment using non-gene-therapy approaches.

Myotonic dystrophy type 2

Myotonic dystrophy type 2 (DM2) is a clinically but not genetically heterogeneous, multisystem disorder, that is clinically similar to, but distinct from myotonic dystrophy type 1 (DM1). Initially, different phenotypes of DM2 were described by Ricker (proximal myotonic myopathy, PROMM), Ranum (myotonic dystrophy 2, DM2) and Udd (proximal myotonic dystrophy, PDM). Clinical features these three phenotypes had in common were diffuse, proximal or distal weakness, wasting, myotonia, cataract, cerebral, endocrine and cardiac abnormalities. Initially, the clinical differences between DM1 and PROMM seemed unmistakable, but meanwhile it has become apparent that the clinical differences between these entities are blurring. In 1999, Day et al., Meola et al. and Ricker et al. mapped the mutated gene of all three phenotypes to chromosome 3q. In 2001, the three different phenotypes were found to rely on the same mutation in the ZNF9 gene on chromosome 3q21.3. Although DM2 may be clinically heterogeneous, it is by result of a mutation in a single gene. The mutation responsible for DM2 is a CCTG-repeat expansion of 75-11 000 repeats in intron 1 of the ZNF9 gene on chromosome 3q21.3. Because of the clinical heterogeneity, the diagnosis of DM2 should rely on DNA analysis alone.

Clinical and genetic heterogeneity in myotonic dystrophies

This review of myotonic dystrophies primarily concentrates on the clinical and genetic findings that can distinguish a novel form of myotonic dystrophy, myotonic dystrophy type 2 (DM2); proximal myotonic myopathy (PROMM); and proximal myotonic dystrophy (PDM) from myotonic dystrophy type 1 (DM1). The multisystemic nature of these disorders leads to a spectrum of symptoms and signs. Careful clinical evaluation of patients with DM2/PROMM shows that the similarities among the multisystemic myotonic disorders outweigh the differences. An important point in the comparison of the phenotypes of DM1 and DM2/PROMM is that no severe congenital type of DM2/PROMM has yet been described. Genetic linkage analyses show that myotonic dystrophies can be divided into three types: the conventional Steinert type linked to chromosome 19q13.3 (DM1); DM2/PROMM and PDM linked to chromosome 3q21.3; and families not linked to either chromosomal site. Although the diagnosis may be clinically suspected, it depends on DNA analysis.

Myotonic dystrophies

This review demonstrates genetic and phenotypic heterogeneity in all of the multisystemic myotonic disorders collectively called 'myotonic dystrophies' according to the new nomenclature: myotonic dystrophy type 1, myotonic dystrophy type 2, proximal myotonic myopathy and proximal myotonic dystrophy. Only two loci have so far been assigned (19q 13.3 in myotonic dystrophy type 1, and 3q 21.3 in myotonic dystrophy type 2). Although the diagnosis of these disorders may be suspected clinically, it needs to be confirmed by DNA analysis.