Megabase-scale loss of heterozygosity provoked by CRISPR-Cas9 DNA double-strand breaks

Harnessing DNA double-strand breaks (DSBs) is a powerful approach for gene editing, but it may provoke loss of heterozygosity (LOH), a common feature of tumor genomes. To interrogate this risk, we developed a flow cytometry-based system (Flo-LOH), detecting LOH in ∼5% of mouse embryonic and human epithelial cells following a DSB. Inhibition of both non-homologous end joining (NHEJ) and microhomology-mediated end joining (MMEJ) massively increases LOH, although the dependence on individual pathways differs in the two cell types. Multiple mechanisms lead to LOH, including chromosome truncations with de novo telomere addition and whole chromosome loss. LOH spans megabases distal from the DSB but also frequently tens of megabases centromere-proximal, which can arise from breakage-fusion-bridge events. Unlike DSBs, Cas9 nicks and adenine base editing did not noticeably impact LOH. The capacity for large-scale LOH must therefore be considered when using DSB-based gene editing, especially in conjunction with end-joining inhibition.