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Nature. 2015 Dec 17;528(7582):422-6. doi: 10.1038/nature16142. Epub 2015 Dec 9.

A mechanism for the suppression of homologous recombination in G1 cells.

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The Lunenfeld-Tanenbaum Research Institute, Mount Sinai Hospital, 600 University Avenue, Toronto, Ontario M5G 1X5, Canada.
ETH Zurich, Institute of Biochemistry, Department of Biology, Otto-Stern-Weg 3, CH-8093 Zurich, Switzerland.
Department of Molecular Genetics, University of Toronto, Ontario M5S 3E1, Canada.
Departments of Pathology and Biochemistry &Molecular Biology, Dalhousie University, Halifax, Nova Scotia B3H 4R2, Canada.
Department of Radiation Oncology, Rutgers Cancer Institute of New Jersey and Robert Wood Johnson Medical School, Rutgers, The State University of New Jersey, New Brunswick, New Jersey 08901, USA.


DNA repair by homologous recombination is highly suppressed in G1 cells to ensure that mitotic recombination occurs solely between sister chromatids. Although many homologous recombination factors are cell-cycle regulated, the identity of the events that are both necessary and sufficient to suppress recombination in G1 cells is unknown. Here we report that the cell cycle controls the interaction of BRCA1 with PALB2-BRCA2 to constrain BRCA2 function to the S/G2 phases in human cells. We found that the BRCA1-interaction site on PALB2 is targeted by an E3 ubiquitin ligase composed of KEAP1, a PALB2-interacting protein, in complex with cullin-3 (CUL3)-RBX1 (ref. 6). PALB2 ubiquitylation suppresses its interaction with BRCA1 and is counteracted by the deubiquitylase USP11, which is itself under cell cycle control. Restoration of the BRCA1-PALB2 interaction combined with the activation of DNA-end resection is sufficient to induce homologous recombination in G1, as measured by RAD51 recruitment, unscheduled DNA synthesis and a CRISPR-Cas9-based gene-targeting assay. We conclude that the mechanism prohibiting homologous recombination in G1 minimally consists of the suppression of DNA-end resection coupled with a multi-step block of the recruitment of BRCA2 to DNA damage sites that involves the inhibition of BRCA1-PALB2-BRCA2 complex assembly. We speculate that the ability to induce homologous recombination in G1 cells with defined factors could spur the development of gene-targeting applications in non-dividing cells.

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