We have characterized expression of the familial breast and ovarian cancer gene, BRCA1, in cases of non-hereditary (sporadic) breast cancer and analyzed the effect of antisense inhibition of BRCA1 on the proliferative rate of mammary epithelial cells. BRCA1 mRNA levels are markedly decreased during the transition from carcinoma in situ to invasive cancer. Experimental inhibition of BRCA1 expression with antisense oligonucleotides produced accelerated growth of normal and malignant mammary cells, but had no effect on non-mammary epithelial cells. These studies suggest that BRCA1 may normally serve as a negative regulator of mammary epithelial cell growth whose function is compromised in breast cancer either by direct mutation or alterations in gene expression.

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 of 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 reviewed in Ref. 9), a clear consensus has not been reached on the function of BRCA1. Unfortunately, BRCA1Ϫ/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 p53mice 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 ...

Inherited mutations in BRCA1 predispose to breast and ovarian cancer, but the role of BRCA1 in sporadic breast and ovarian cancer has previously been elusive. Here, we show that retroviral transfer of the wild-type BRCA1 gene inhibits growth in vitro of all breast and ovarian cancer cell lines tested, but not colon or lung cancer cells or fibroblasts. Mutant BRCA1 has no effect on growth of breast cancer cells; ovarian cancer cell growth is not affected by BRCA1 mutations in the 5' portion of the gene, but is inhibited by 3' BRCA1 mutations. Development of MCF-7 tumours in nude mice is inhibited when MCF-7 cells are transfected with wild-type, but not mutant, BRCA1. Most importantly, among mice with established MCF-7 tumours, peritoneal treatment with a retroviral vector expressing wild-type BRCA1 significantly inhibits tumour growth and increased survival.

No abstract