Post-cleavage target residence determines asymmetry in non-homologous end joining of Cas12a-induced DNA double strand breaks.

BACKGROUND: After Cas12a cleaves its DNA target, it generates a DNA double strand break (DSB) with two compatible 5'-staggered ends. The Cas12a-gRNA complex remains at the protospacer adjacent motif (PAM)-proximal end (PPE) while releasing the PAM-distal end (PDE). The effects of this asymmetric retention on DSB repair are currently unknown. RESULTS: Post-cleavage retention of LbCas12a at PPEs suppresses the recruitment of classical non-homologous end joining (c-NHEJ) core factors, leading to longer deletions at PPEs compared to PDEs. This asymmetry in c-NHEJ engagement results in approximately tenfold more accurate ligation between two compatible PDEs induced by paired LbCas12a than ligation involving a compatible PPE. Moreover, ligation to a given end of SpCas9-induced DSBs demonstrates more efficient ligation with a PDE from Cas12a-induced DSBs than with a PPE. In LbCas12a-induced NHEJ-mediated targeted integration, only two compatible PDEs from LbCas12a-induced DSBs-one from donor templates and the other from target sites-promote accurate and directional ligation. Based on these findings, we developed a strategy called Cas12a-induced PDE ligation (CIPDEL) for NHEJ-mediated efficient and precise gene correction and insertion. CONCLUSIONS: The asymmetric retention of CRISPR-LbCas12a at DSB ends suppresses c-NHEJ at PPEs, not at PDEs. This unique repair mechanism can be utilized in the CIPDEL strategy, offering a potentially better alternative for homology-directed targeted integration.