The tumor suppressor BRCA1 functions in DNA homologous recombination, and mutations in BRCA1 increase the risk of breast and ovarian cancers. RAP80 is a component of BRCA1-containing complexes that is required for recruitment of BRCA1 to sites of DNA damage. To evaluate the role of RAP80 in DNA damage repair, we genetically disrupted both RAP80 alleles in the recombinogenic avian DT40 cell line. The resulting RAP80(-/-) cells were proficient at homologous recombination and nonhomologous end-joining (NHEJ), but were specifically sensitized to the topoisomerase II inhibitor etoposide. Notably, doubly mutant RAP80(-/-)BRCA1(-/-) cells were more sensitive to etoposide than were BRCA1(-/-) cells, revealing that RAP80 performs a BRCA1-independent repair function. Moreover, jointly impairing the function of CtIP, a distinct BRCA1 effector protein, rendered RAP80(-/-) cells more sensitive to etoposide compared with singly mutant cells, again illustrating a BRCA1-independent role of RAP80. Based on our findings, we propose that RAP80 exerts a specific function in repair of the topoisomerase-cleavage complex, such as the removal of covalently bound polypeptides from double-strand break ends independently of BRCA1.
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