ERCC6L2 ensures repair fidelity for staggered-end DNA double-strand breaks

DNA double-strand breaks (DSBs) both pose threats to genome integrity and are commonly used for genome editing applications. Structural features of DSB ends play key roles in determining DNA repair pathway usage and outcomes during genome editing, but the cellular factors involved in these processes are only partially known. Through genome-wide CRISPRi screening, we identify ERCC6L2 as critical for repairing Cas12a-induced staggered DSBs but irrelevant for Cas9-induced blunt DSBs. We show that ERCC6L2 acts as a protection factor for staggered DSBs with either 5' or 3' polarity, preventing large deletions and translocations stemming from DNA damage induced by Cas12a, TALENs, or dual Cas9 nicks. Furthermore, ERCC6L2 loss hyper-sensitizes cells to multiple staggered DSBs induced by promiscuous Cas12a activity or etoposide-induced TOP2 trapping. By combining genetics and biochemical reconstitution, we find that ERCC6L2 counteracts MRE11-RAD50-NBS1 (MRN)-mediated resection by binding and melting staggered DNA ends, thereby promoting accurate end joining. Our data reveal a protective role of ERCC6L2 in staggered-end DSB repair, which suggests the molecular underpinnings of pathology in patients with ERCC6L2 mutations and cautions against using overhang-inducing genome editing tools for their treatment.