Deletion of the CTG Expansion in Myotonic Dystrophy Type 1 Reverses DMPK Aberrant Methylation in Human Embryonic Stem Cells but not Affected Myoblasts

Abstract

Myotonic dystrophy type 1 (DM1) results from a CTG repeat expansion in the 3-UTR of DMPK. When the repeat extensively expands, this results in DMPK aberrant methylation, reduction in SIX5 transcription and the development of the congenital form of the disease. To explore whether hypermethylation could be reversed in DM1 embryonic stem cells (hESCs) and patient myoblasts, we monitored methylation levels following removal of the expanded repeat by CRISPR/Cas9-mediated editing. Excision of the repeat in undifferentiated hESCs (CTG2000) resets the locus by abolishing abnormal methylation and H3K9me3 enrichment, and rescues SIX5 transcription. In contrast, in affected myoblasts methylation levels remain unchanged following deletion of a large expansion (CTG2600). Altogether, this provides evidence for a transition from a reversible to an irreversible heterochromatin state by the DM1 mutation upon cell differentiation. These findings should be taken into account when considering gene correction in congenital DM1 and potentially other epigenetically regulated disorders.