Unexpected Mutations by CRISPR-Cas9 CTG Repeat Excision in Myotonic Dystrophy and Use of CRISPR Interference as an Alternative Approach.

Myotonic dystrophy type 1 is the most common type of adult-onset muscular dystrophy. This is an autosomal dominant disorder and caused by the expansion of the CTG repeat in the 3' untranslated region of the dystrophia myotonica protein kinase () gene. Messenger RNAs containing these expanded repeats form aggregates as nuclear RNA foci. Then, RNA binding proteins, including muscleblind-like 1, are sequestered to the RNA foci, leading to systemic abnormal RNA splicing. In this study, we used CRISPR-Cas9 genome editing to excise this CTG repeat. Dual cleavage at the 5' and 3' regions of the repeat using a conventional Cas9 nuclease and a double nicking with Cas9 nickase successfully excised the CTG repeat. Subsequently, the formation of the RNA foci was markedly reduced in patient-derived fibroblasts. However, contrary to expectations, a considerable amount of off-target digestions and on-target genomic rearrangements were observed using high-throughput genome-wide translocation sequencing. Finally, the suppression of transcripts using CRISPR interference significantly decreased the intensity of RNA foci. Our results indicate that close attention should be paid to the unintended mutations when double-strand breaks are generated by CRISPR-Cas9 for therapeutic purposes. Alternative approaches independent of double-strand breaks, including CRISPR interference, may be considered.