Mutations in BRCA1 are responsible for most of the hereditary ovarian and breast cancers.
The ability of BRCA1 to transactivate the p21 promoter can be inactivated by mutation of the C-terminal conserved BRCT domains. To explore the mechanisms of this BRCA1 function, the BRCT domains were found to bind a known protein, CtIP, a co-repressor with CtBP. CtIP interacts specifically with the BRCT domains of BRCA1, both in vitro and in vivo, and tumor-derived mutations abolished these interactions. The association of BRCA1 with CtIP was also abrogated in cells treated with UV, gamma-irradiation and adriamycin, a response correlated with BRCA1 phosphorylation. The transactivation of the p21 promoter by BRCA1 was diminished by expression of exogenous CtIP and CtBP. These results suggest that the binding of the BRCT domains of BRCA1 to CtIP/CtBP is critical in mediating transcriptional regulation of p21 in response to DNA damage. To explore the molecular mechanism of how BRCA1 is critical for the cellular response to DNA damage, we showed that BRCA1 interacts in vitro and in vivo with hRad50, which forms a complex with hMre11 and p95/nibrin. BRCA1 was detected in discrete foci in the nucleus that colocalize with hRad50 after irradiation. Formation of irradiation-induced foci positive for BRCA1, hRad50, hMre11 or p95 were dramatically reduced in HCC1937 breast cancer cells carrying a homozygous mutation in BRCA1, but was restored by transfection of wild-type BRCA1. Ectopic expression of wild-type, but not mutated BRCA1 in these cells rendered them less sensitive to the DNA damage agent, methyl methanesulfonate. These data suggest that BRCA1 is important for the cellular responses to DNA damage that are mediated by the hRad50-hMre11-p95 complex.