In vivo systematic detection of the outcomes of CRISPR/Cas9 mediated DNA repair in skeletal muscle stem cells

CRISPR/Cas9 has revolutionized genome editing with broad therapeutic applications, yet its repair patterns in vivo remain poorly understood. Here, we systematically profiled CRISPR/Cas9 editing outcomes at 95 loci using our established CRISPR/Cas9/AAV9-sgRNA system in skeletal muscle stem cells (MuSCs). Through comprehensive characterization of the repair outcomes, our findings demonstrate that the general rules governing CRISPR/Cas9-mediated editing in vivo largely align with those observed in vitro but with reduced editing precision. Additional to the anticipated small editing indels such as MMEJ mediated deletions and NHEJ mediated templated insertions, we uncovered a prevalent occurrence of large on-target modifications, including large deletions (LDs) characterized by microhomology (MH) and large insertions (LIs). Notably, the LIs comprise not only exogenous AAV vector integrations but also endogenous genomic DNA fragments (Endo-LIs). Endo-LIs preferentially originate from active genomic regions, with their integration shaped by three-dimensional chromatin architecture. By disrupting key components of the NHEJ and MMEJ repair pathways in vivo , we identified their distinct roles in regulating the large on-target modifications. Together, our work for the first time systematically profiles the CRISPR/Cas9 repair outcomes in vivo and offers valuable guidance for improving the safety of CRISPR/Cas9-based gene therapies.