Myotonic dystrophy (DM) is a complex multisystemic disorder linked to two different genetic loci. DM1 is caused by an expansion of a CTG repeat located in the 3' untranslated region (UTR) of DMPK. DM2 is caused by an unstable CCTG repeat in intron 1 of ZNF9. Therefore, both DMs are caused by a repeat expansion in a region transcribed into RNA but not translated into protein. The mutant RNA transcripts aberrantly affect the splicing of the same target RNAs, such as chloride channel 1 (ClC-1) and insulin receptor (INSR), resulting in their shared myotonia and insulin resistance. Affected individuals express highly heterogeneous, multisystemic symptoms including myotonia (muscle hyperexcitability), progressive muscle weakness and wasting, cataract development, testicular atrophy, and cardiac conduction defects. It has an autosomal dominant mode of inheritance and disease severity generally correlates with repeat length.
