Katherine A. Dick

BACKGROUND: Myotonic dystrophy types 1 (DM1) and 2 (DM2/proximal myotonic myopathy PROMM) are dominantly inherited disorders with unusual multisystemic clinical features. The authors have characterized the clinical and molecular features of DM2/PROMM, which is caused by a CCTG repeat expansion in intron 1 of the zinc finger protein 9 (ZNF9) gene. METHODS: Three-hundred and seventy-nine individuals from 133 DM2/PROMM families were evaluated genetically, and in 234 individuals clinical and molecular features were compared. RESULTS: Among affected individuals 90% had electrical myotonia, 82% weakness, 61% cataracts, 23% diabetes, and 19% cardiac involvement. Because of the repeat tract's unprecedented size (mean approximately 5,000 CCTGs) and somatic instability, expansions were detectable by Southern analysis in only 80% of known carriers. The authors developed a repeat assay that increased the molecular detection rate to 99%. Only 30% of the positive samples had single sizeable expansions by Southern analysis, and 70% showed multiple bands or smears. Among the 101 individuals with single expansions, repeat size did not correlate with age at disease onset. Affected offspring had markedly shorter expansions than their affected parents, with a mean size difference of -17 kb (-4,250 CCTGs). CONCLUSIONS: DM2 is present in a large number of families of northern European ancestry. Clinically, DM2 resembles adult-onset DM1, with myotonia, muscular dystrophy, cataracts, diabetes, testicular failure, hypogammaglobulinemia, and cardiac conduction defects. An important distinction is the lack of a congenital form of DM2. The clinical and molecular parallels between DM1 and DM2 indicate that the multisystemic features common to both diseases are caused by CUG or CCUG expansions expressed at the RNA level.

The general model that dominant diseases are caused by mutations that result in a gain or change in function of the corresponding protein was challenged by the discovery that the myotonic dystrophy type 1 mutation is a CTG expansion located in the 3' untranslated portion of a kinase gene. The subsequent discovery that a similar transcribed but untranslated CCTG expansion in an intron causes the same multisystemic features in myotonic dystrophy type 2 (DM2), along with other developments in the DM1 field, demonstrate a mechanism in which these expansion mutations cause disease through a gain of function mechanism triggered by the accumulation of transcripts containing CUG or CCUG repeat expansions. A similar RNA gain of function mechanism has also been implicated in fragile X tremor ataxia syndrome (FXTAS) and may play a role in pathogenesis of other non-coding repeat expansion diseases, including spinocerebellar ataxia type 8 (SCA8), SCA10, SCA12 and Huntington disease-like 2.