FAM35A associates with REV7 and modulates DNA damage responses of normal and BRCA1‐defective cells

To exploit vulnerabilities of tumors, it is urgent to identify associated defects in genome maintenance. One unsolved problem is the mechanism of regulation of DNA double‐strand break repair by REV7 in complex with 53BP1 and RIF1, and its influence on repair pathway choice between homologous recombination and non‐homologous end‐joining. We searched for REV7‐associated factors in human cells and found FAM35A, a previously unstudied protein with an unstructured N‐terminal region and a C‐terminal region harboring three OB‐fold domains similar to single‐stranded DNA‐binding protein RPA, as novel interactor of REV7/RIF1/53BP1. FAM35A re‐localized in damaged cell nuclei, and its knockdown caused sensitivity to DNA‐damaging agents. In a BRCA1‐mutant cell line, however, depletion of FAM35A increased resistance to camptothecin, suggesting that FAM35A participates in processing of DNA ends to allow more efficient DNA repair. We found FAM35A absent in one widely used BRCA1‐mutant cancer cell line (HCC1937) with anomalous resistance to PARP inhibitors. A survey of FAM35A alterations revealed that the gene is altered at the highest frequency in prostate cancers (up to 13%) and significantly less expressed in metastatic cases, revealing promise for FAM35A as a therapeutically relevant cancer marker. Synopsis Suppression of DNA double strand break resection favors non‐homologous end‐joining over homologous recombination repair, and is mediated by 53BP1‐RIF1‐REV7 factors. Identification of the OB‐fold protein FAM35A as additionally required REV7 interactor suggests a possible link to single‐stranded DNA recognition in this process. The previously uncharacterized protein FAM35A interacts with REV7, 53BP1 and RIF1. FAM35A contains OB‐fold domains that may mediate binding to single‐stranded DNA. FAM35A depletion sensitizes cells to DNA‐damaging agents. FAM35A loss suppresses non‐homologous end joining and enhances homologous recombination markers. FAM35A is frequently deleted in prostate cancers and absent in a BRCA1‐mutant cancer cell line with anomalous resistance to PARP inhibitors. Graphical Abstract Suppression of double strand break resection by 53BP1‐RIF1‐REV7 to allow non‐homologous end‐joining repair additionally requires FAM35A, an OB‐fold protein that may mediate interaction with single‐stranded DNA.