REGULATION OF BRCA1 AND BRCA2 EXPRESSION IN HUMAN BREAST-CANCER CELLSBY DNA-DAMAGING AGENTS

Germline mutations in the breast cancer susceptibility genes BRCA1 and BRCA2 have been linked Ito the development of breast cancer, ovarian cancer, and other malignancies. Recent studies suggest that the BRCA1 and BRCA2 gene products may function in the sensing and/or repair of D NA damage. To investigate this possibility, we determined the effects of various DNA-damaging agents and other cytotoxic agents on the mRNA levels of BRCA1 and BRCA2 in the MCF-7 and other human breast cancer c ell lines. We found that several agents, including adriamycin (a DNA i ntercalator and inhibitor of topoisomerase II), camptothecin (a topois omerase I inhibitor), and ultraviolet radiation induced significant de creases in BRCA1 and BRCA2 mRNA levels. Decreased levels of BRCA1 and BRCA2 mRNAs were observed within 6-12 h after treatment with adriamyci n and persisted for at least 72 h. Adriamycin also induced decreases i n BRCA1 protein levels; but these decreases required several days. U.V . radiation induced dose-dependent down-regulation of BRCA1 and BRCA2 mRNAs, with significant decreases in both mRNAs at doses as low as 2.5 J/m(2), a dose that yielded very little cytotoxicity. Adriamycin-indu ced down-regulation of BRCA1 and BRCA2 mRNAs was first observed at dos es that yielded relatively little cytotoxicity and little or no apopto tic DNA fragmentation, Adriamycin and U.V. radiation induced distinct dose- and time-dependent alterations in the cell cycle distribution; b ut these alterations did not correlate well with corresponding changes in BRCA1 and BRCA2 mRNA levels. However, the adriamycin-induced reduc tion in BRCA1 and BRCA2 mRNA levels was correlated with p53 functional status. MCF-7 cells transfected with a dominant negative mutant p53 ( 143 val-->ala) required at least tenfold higher doses of adriamycin to down-regulate BRCA1 and BRCA2 mRNAs than did parental MCF-7 cells or control-transfected MCF-7 clones. These results suggest that BRCA1 and BRCA2 may play roles in the cellular response to DNA-damaging agents and that there may be a p53-sensitive component to the regulation of B RCA1 and BRCA2 mRNA expression.