Identification of ZNF850 as a novel CTG repeat expansion-related gene in myotonic dystrophy type 1 patient-derived iPSCs

Myotonic dystrophy type 1 (DM1) is a dominantly inherited multi-system disease caused by expanded CTG repeats in the 3' untranslated region of the dystrophia myotonica protein kinase (DMPK) gene. Similar to other repeat disorders, the expanded trinucleotide repeat is unstable and demonstrates a tendency to increase repeat size with age in affected tissues. DNA mismatch repair system is implicated in somatic instability. It has been demonstrated that DM1 patient-derived induced pluripotent stem cells (DM1-iPSCs) show repeat instability, in which involvement of mismatch repair proteins has been suggested. Here we identified ZNF850 as a novel CTG repeat expansion-related molecule in DM1-iPSCs. ZNF850 was downregulated in a DM1-iPSC clone whose CTG repeat is exceptionally stable. We found that RNAi-mediated ZNF850 downregulation in DM1-iPSCs significantly reduced the repeat expansion and resulting instability. In adult skeletal muscle tissue of DM1 patients, ZNF850 expression levels were positively correlated with the repeat size. Furthermore, we found that ZNF850 protein can bind to the expanded CTG repeat sequence, and is located in proximity to MutSβ components. These results suggest that ZNF850 might play a role in repeat instability in DM1 by recruiting MutSβ to the repeat sequence.