Abstract IA016: Replication-coupled and canonical DNA double strand breaks are processed by distinct mechanisms

Abstract

Abstract Single strand breaks (SSBs), including DNA nicks, are the most common form of endogenous DNA damage. If they are not resolved before the passage of a replication fork, current models suggest that SSBs are converted to single-ended double strand breaks (seDSBs) and repaired by homologous recombination (HR). Here we show that when a replication fork encounters a Cas9-induced nick on the leading strand, the replisome is rapidly dislodged, and a highly resected seDSB is generated. In contrast, a Cas9-induced nick on the lagging strand rarely gives rise to a resected seDSB, either because Cas9 is actively evicted and the nick sealed prior to the arrival of the fork or because the nick is concealed as the replisome traverses the lesion, resulting in a double ended DSB (deDSB). Unlike canonical DSBs, end-resection at collapsed forks is BRCA1-independent. Instead, BRCA1 antagonizes 53BP1 to promote RAD51 filament formation and recombination. In the absence of strand invasion, exemplified by BRCA1-, RAD51-, and BRCA2- deficiencies, replication-coupled seDSBs are converted deDSBs by a converging fork. Together, our results demonstrate how nicks can give rise to DSBs in a strand asymmetrical manner, with implications for cancer-associated rearrangements and therapies that target the replication fork. Citation Format: Raphael Pavani, Veenu Tripathi, Dali Zong, Elsa Callen, Ajith Pankajam, Andre Nussenzweig, Wei Wu, Kyle Vrtis, Raj Chari, Johannes C Walter. Replication-coupled and canonical DNA double strand breaks are processed by distinct mechanisms [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: DNA Damage Repair: From Basic Science to Future Clinical Application; 2024 Jan 9-11; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2024;84(1 Suppl):Abstract nr IA016.