Cas9-induced single cut enables highly efficient and template-free repair of a muscular dystrophy causing founder mutation.

With thousands of patients worldwide, c.550delA is the most frequent mutation causing severe, progressive, and untreatable limb girdle muscular dystrophy. We aimed to genetically correct this founder mutation in primary human muscle stem cells. We designed editing strategies providing CRISPR-Cas9 as plasmid and mRNA first in patient-derived induced pluripotent stem cells and applied this strategy then in primary human muscle stem cells from patients. Mutation-specific targeting yielded highly efficient and precise correction of c.550delA to wild type for both cell types. Most likely a single cut generated by SpCas9 resulted in a 5' staggered overhang of one base pair, which triggered an overhang-dependent base replication of an A:T at the mutation site. This recovered the open reading frame and the DNA sequence was repaired template-free to wild type, which led to mRNA and protein expression. Off-target analysis using amplicon sequencing of 43 predicted sites demonstrates the safety of this approach. Our study extends previous usage of single cut DNA modification since our gene product has been repaired into the wild-type sequence with the perspective of a real cure.