SHLD2/FAM35A co-operates with REV7 to coordinate DNA double-strand break repair pathway choice

DNA double-strand breaks (DSBs) can be repaired by two major pathways: non-homologous end-joining (NHEJ) and homologous recombination (HR). DNA repair pathway choice is governed by the opposing activities of 53BP1, in complex with its effectors RIF1 and REV7, and BRCA1. However, it remains unknown how the 53BP1/RIF1/REV7 complex stimulates NHEJ and restricts HR to the S/G2 phases of the cell cycle. Using a mass spectrometry (MS)-based approach, we identify 11 high-confidence REV7 interactors and elucidate the role of SHLD2 (previously annotated as FAM35A and RINN2) as an effector of REV7 in the NHEJ pathway. FAM35A depletion impairs NHEJ-mediated DNA repair and compromises antibody diversification by class switch recombination (CSR) in B cells. FAM35A accumulates at DSBs in a 53BP1-, RIF1-, and REV7-dependent manner and antagonizes HR by limiting DNA end resection. In fact, FAM35A is part of a larger complex composed of REV7 and SHLD1 (previously annotated as C20orf196 and RINN3), which promotes NHEJ and limits HR Together, these results establish SHLD2 as a novel effector of REV7 in controlling the decision-making process during DSB repair.

Cloning of canine Ku80 and its localization and accumulation at DNA damage sites

Molecularly targeted therapies have high specificity and significant cancer‐killing effect. However, their antitumor effect might be greatly diminished by variation in even a single amino acid in the target site, as it occurs, for example, as a consequence of SNPs. Increasing evidence suggests that the DNA repair protein Ku80 is an attractive target molecule for the development of next‐generation radiosensitizers for human cancers. However, the localization, post‐translational modifications (PTMs), and complex formation of Ku80 have not been elucidated in canines. In this study, for the first time, we cloned, sequenced, and characterized canine Ku80 cDNA. Our data show that canine Ku80 localizes in the nuclei of interphase cells and is quickly recruited at laser‐induced double‐strand break sites. Comparative analysis shows that canine Ku80 had only 82.3% amino acid identity with the homologous human protein, while the nuclear localization signal (NLS) in human and canine Ku80 is evolutionarily conserved. Notably, some predicted PTM sites, including one acetylation site and one sumoylation site within the NLS, are conserved in the two species. These findings suggest that the spatial and temporal regulation of Ku80 might be conserved in humans and canines. However, our data indicate that the expression of Ku80 is considerably lower in the canine cell lines examined than in human cell lines. These important findings might be useful to better understand the mechanism of the Ku80‐dependent DNA repair and for the development of potential next‐generation radiosensitizers targeting common targets in human and canine cancers.

Double-strand break repair based on short-homology regions is suppressed under terminal deoxynucleotidyltransferase expression, as revealed by a novel vector system for analysing DNA repair by nonhomologous end joining

We have constructed a novel, nonhomologous end‐joining (NHEJ) assay vector (NAV), containing mKate2, Venus and ccdB genes. Cotransfection of NAV with a construct expressing the restriction enzyme I‐SceI generated a double‐strand break (DSB) in NAV that excised mKate2 and ccdB. Repair of this DSB produced an intact vector that expressed Venus, a green fluorescent protein. Because cells bearing the repaired NAV lacked the ccdB gene which slows cell proliferation, the cultures were enriched in cells containing repaired DSBs. DNA sequence analysis of the DSB junctions indicated that the repair was carried out mainly by using the closest homology sequence. Use of the NAV yielded rapid results within 3 days after transfection. We then used the NAV to analyse NHEJ in cells overexpressing terminal deoxynucleotidyltransferase (TdT). The results indicated that TdT suppresses DNA repair that is based on short (one‐ or two‐base) homology regions, to efficiently add deoxynucleotides during VDJ recombination in lymphoid cells.