Myotonic Dystrophy Type 2 Caused by a CCTG Expansion in Intron 1 of ZNF9

Myotonic dystrophy (DM), the most common form of muscular dystrophy in adults, can be caused by a mutation on either chromosome 19q13 (DM1) or 3q21 (DM2/PROMM). DM1 is caused by a CTG expansion in the 3′ untranslated region of the dystrophia myotonica–protein kinase gene ( DMPK ). Several mechanisms have been invoked to explain how this mutation, which does not alter the protein-coding portion of a gene, causes the specific constellation of clinical features characteristic of DM. We now report that DM2 is caused by a CCTG expansion (mean ∼5000 repeats) located in intron 1 of the zinc finger protein 9 ( ZNF9 ) gene. Parallels between these mutations indicate that microsatellite expansions in RNA can be pathogenic and cause the multisystemic features of DM1 and DM2.