The breast cancer predisposition genes, BRCA1 and BRCA2, are responsible for the vast majority of hereditary breast cancer. Although BRCA2 functions to help the cell repair double-stranded DNA breaks, the function of BRCA1 remains enigmatic. Here, we develop a human genetic system to study the role of BRCA1 in oxidative DNA damage. We show that human cancer cells containing mutated BRCA1 are hypersensitive to ionizing radiation. This hypersensitivity can be reversed by the expression of forms

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... BRCA1 that are not growth suppressing. Reversal of hypersensitivity requires the ring finger of BRCA1, its transactivation domain, and its BRCT domain. Lastly, we show that unlike BRCA2, BRCA1 does not function in the repair of doublestranded DNA breaks. Instead, it functions in transcription-coupled DNA repair (TCR). TCR ability correlated with radioresistance as cells containing BRCA1 showed both increased TCR and radioresistance, whereas cells without BRCA1 showed decreased TCR and radiosensitivity. These findings give physiologic significance to the interaction of BRCA1 with the basal transcription machinery. BRCA1 and BRCA2, the breast cancer 1 and 2 genes, are responsible for over 90% of hereditary breast cancers (1-4). Although BRCA2 has been shown to affect the repair of doublestranded DNA breaks (Refs. 5-8; reviewed in Ref. 9), a clear consensus has not been reached on the function of BRCA1. Unfortunately, BRCA1 Ϫ/Ϫ mice die at day 6.5 to day 8.5 of embryonic gestation because of lack of proliferation of the mouse blastocyst (10 -12). Despite this embryonic lethality, work in the mouse systems has resulted in two suggestive findings. First, when BRCA1 Ϫ/Ϫ mice are mated with p53 Ϫ/Ϫ mice to generate BRCA1 Ϫ/Ϫ p53 Ϫ/Ϫ mice, these double-knock out mice show reduced embryonic lethality (13, 14), suggesting that BRCA1 and p53 may lie on a common functional pathway. The second finding is that cells from BRCA1 Ϫ/Ϫ mice have a defect in transcription-coupled DNA repair (TCR) 1 (15), implying that BRCA1 may be involved in DNA repair and/or the stress response of the cell. Despite these suggestive findings in the mouse system, there are such large differences in mouse and human BRCA1 biology that it is unclear whether the DNA repair function of mouse BRCA1 is applicable to human BRCA1. Mouse BRCA1 is only 57% homologous to human BRCA1 (10 -12), and BRCA1 appears to function differently in the two systems. Although BRCA1 has been shown to be required for cellular proliferation during mouse development, BRCA1 has been shown to be a powerful growth suppressor in both yeast and human systems (16 -20). Although there are reports of living humans who are homozygous for BRCA1 mutations (21), mice carrying homozygous BRCA1 mutations die early in gestation. Lastly, DNA repair in a mouse cell is not necessarily indicative of repair in a human cell (22) (23) (24) . Mouse and human cells show differences in the amount of damage sustained per given DNA-damaging dose, in the kinetics of DNA repair, and in cellular survival at a given dose of a DNA-damaging agent (22) (23) (24) . Other attempts to determine the function of BRCA1 have centered on finding functional domains of BRCA1. BRCA1 contains a Zn 2ϩ finger in its N terminus (2), a transcriptional activation domain in its C terminus (25, 26), and a BRCT domain at its far C terminus (27, 28) . BRCA1 has been shown to interact with the basal transcriptional machinery (RNA polymerase II, TFIIH, TFIIE, and RNA helicase A) (29, 30), BRCA2 (31), and Rad51 (29) . BRCA1 also colocalizes with Rad51 to discrete nuclear foci when exposed to UV radiation (32). Although BRCA1 and BRCA2 show no homology (2, 3), mutations of these genes both cause breast cancer, and these two genes show identical expression throughout development (33). Cells that lack BRCA2 have been shown to be hypersensitive to ionizing radiation and have a decreased capacity to repair double-stranded DNA breaks (5-8). In addition, BRCA2 interacts directly with Rad51 (5, 34). When coupled with the finding that BRCA1 Ϫ/Ϫ mouse cells are deficient in TCR, all these findings suggest a role for BRCA1 in the cellular response to DNA damage. To determine the role of BRCA1 in the cellular response to DNA damage, we have developed a human genetic system. HCC1937 is a human breast cancer cell line originally established from a family carrying a known cancer-causing BRCA1 mutation, 5283insC. We show that: 1) HCC1937 cells are hy-