RAD51 up-regulation bypasses BRCA1 function and is a common feature of BRCA1-deficient breast tumors.
The breast cancer susceptibility gene BRCA1 encodes a large protein thought to contribute to a variety of cellular processes, although the critical determinants of BRCA1-deficient tumorigenesis remain unclear. Given that BRCA1 is required for cell proliferation, suppressor mutations are believed to modify BRCA1 phenotypes and contribute to the etiology of BRCA1-deficient tumors. Here, we show that overexpression of the homologous recombinase RAD51 in a DT40 BRCA1Delta/Delta mutant rescues defects in proliferation, DNA damage survival, and homologous recombination (HR). In addition, epistasis analysis with BRCA1 and the DNA end-joining factor KU70 indicates that these factors operate independently of one another to repair double-strand breaks. Consistent with this genetic finding, cell synchronization studies show that the ability of BRCA1 to promote radioresistance is restricted to the late S and G2 phases of the cell cycle, as predicted for genes whose function is specific to homology-mediated repair rather than nonhomologous end-joining. Notably, retrospective analyses of microarray expression data reveal elevated expression of RAD51 and two of its late-acting cofactors, RAD54 and RAD51AP1, in BRCA1-deficient versus sporadic breast tumors. Taken together, our results indicate that up-regulation of HR provides a permissive genetic context for cells lacking BRCA1 function by circumventing its requirement in RAD51 subnuclear assembly. Furthermore, the data support a model in which enhanced HR activity contributes to the etiology of BRCA1-deficient tumors.
Martin
Richard W
RW
Department of Radiation, Ludwig Center for Metastasis Research, University of Chicago, Chicago, Illinois 60637, USA.
Orelli
Brian J
BJ
Yamazoe
Mitsuyoshi
M
Minn
Andy J
AJ
Takeda
Shunichi
S
Bishop
Douglas K
DK
D000818:Animals; D034961:Antigens, Nuclear / Q000096:biosynthesis / Q000235:genetics; D019313:BRCA1 Protein / Q000172:deficiency* / Q000235:genetics; D001943:Breast Neoplasms / Q000201:enzymology / Q000235:genetics / Q000378:metabolism* / Q000473:pathology; D002453:Cell Cycle / Q000235:genetics; D048708:Cell Growth Processes / Q000235:genetics; D045744:Cell Line, Tumor; D002645:Chickens; D004249:DNA Damage; D004268:DNA-Binding Proteins / Q000096:biosynthesis / Q000235:genetics; D016195:G2 Phase / Q000235:genetics; D015971:Gene Expression Regulation, Enzymologic; D015972:Gene Expression Regulation, Neoplastic; D019398:Genes, BRCA1; D006801:Humans; D000072200:Ku Autoantigen; D051135:Rad51 Recombinase / Q000096:biosynthesis* / Q000235:genetics; D011836:Radiation Tolerance / Q000235:genetics; D016196:S Phase / Q000235:genetics; D015854:Up-Regulation
D034961:Antigens, Nuclear; D019313:BRCA1 Protein; D004268:DNA-Binding Proteins; C495153:RAD51 protein, human; D051135:Rad51 Recombinase; C000606509:Xrcc6 protein, human; D000072200:Ku Autoantigen