Error-free repair of DNA double-strand breaks (DSBs) is achieved by homologous recombination (HR), and BRCA1 is an important factor for this repair pathway. In the absence of BRCA1-mediated HR, the administration of PARP inhibitors induces synthetic lethality of tumour cells of patients with breast or ovarian cancers. Despite the benefit of this tailored therapy, drug resistance can occur by HR restoration. Genetic reversion of BRCA1-inactivating mutations can be the underlying mechanism of drug resistance, but this does not explain resistance in all cases. In particular, little is known about BRCA1-independent restoration of HR. Here we show that loss of REV7 (also known as MAD2L2) in mouse and human cell lines re-establishes CTIP-dependent end resection of DSBs in BRCA1-deficient cells, leading to HR restoration and PARP inhibitor resistance, which is reversed by ATM kinase inhibition. REV7 is recruited to DSBs in a manner dependent on the H2AX-MDC1-RNF8-RNF168-53BP1 chromatin pathway, and seems to block HR and promote end joining in addition to its regulatory role in DNA damage tolerance. Finally, we establish that REV7 blocks DSB resection to promote non-homologous end-joining during immunoglobulin class switch recombination. Our results reveal an unexpected crucial function of REV7 downstream of 53BP1 in coordinating pathological DSB repair pathway choices in BRCA1-deficient cells.

Danish Cancer Society Research Center Copenhagen Denmark

Deep Sequencing Core Facility The Netherlands Cancer Institute Plesmanlaan 121 1066CX Amsterdam The Netherlands

Department of Experimental Radiation Oncology University of Texas MD Anderson Cancer Center Houston TX 77030 USA

Department of Genetics Erasmus University Medical Center Rotterdam The Netherlands

Division of Cell Biology The Netherlands Cancer Institute Plesmanlaan 121 1066CX Amsterdam The Netherlands

Division of Molecular Carcinogenesis The Netherlands Cancer Institute Plesmanlaan 121 1066CX Amsterdam The Netherlands

Division of Molecular Oncology The Netherlands Cancer Institute Plesmanlaan 121 1066CX Amsterdam The Netherlands

Division of Molecular Pathology The Netherlands Cancer Institute Plesmanlaan 121 1066CX Amsterdam The Netherlands

DNA Damage Response Laboratory London Research Institute Cancer Research UK Clare Hall South Mimms EN6 3LD UK

Institute of Animal Pathology Vetsuisse Faculty University of Bern Laengassstrasse 122 3012 Bern Switzerland

Institute of Molecular and Translational Medicine Faculty of Medicine and Dentistry Palacky University Olomouc Czech Republic

Protein Facility The Netherlands Cancer Institute Plesmanlaan 121 1066CX Amsterdam The Netherlands

The Wellcome Trust Centre for Human Genetics Roosevelt Drive Oxford OX3 7BN United Kingdom

EMBO J. 2015 Jun 12;34(12):1609-11. doi: 10.15252/embj.201591697. PubMed

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