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.

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Over the past several years, the importance of regulated nuclear transport processes for tumor suppressors has become evident. Proteins with a molecular mass greater than 40 kDa can enter the nucleus only by active transport across the nuclear membrane. The most common pathway by which this occurs is via the importin alpha/beta pathway, whereby the cargo protein binds importin alpha. This heterodimer binds importin beta and the heterotrimer passes through nuclear pores at the expense of GTP. Breast cancer susceptibility gene 1 (BRCA1) is one such protein. As a mediator of transcription and DNA repair, two exclusively nuclear functions, BRCA1, at 220 kDa, can enter the nucleus only via active transport mechanisms. In addition to the classical importin alpha/beta pathway, BRCA1 can also enter the nucleus in a piggyback mechanism with BRCA1‐associated RING domain protein 1 (BARD1). The interaction between BRCA1 and BARD1 is also important in the retention of BRCA1 in the nucleus. This is important because BRCA1 also undergoes active nuclear export. BRCA1 is also involved in apoptotic processes. Whether this occurs within the nucleus or cytoplasm is still unclear; thus, the consequences of BRCA1 nuclear export have not been clearly elucidated. This review will discuss the literature regarding the subcellular localization of BRCA1, with particular emphasis on its nuclear import and export processes.

Mutations in the breast cancer susceptibility gene 1 (BRCA1) account for a substantial percentage of familial breast and ovarian cancers. Although BRCA1 is thought to function within the nucleus, it has also been located in the cytoplasm. In addition, BRCA1 accumulates in the nucleus of cells treated with leptomycin B, an inhibitor of chromosome region maintenance 1-mediated nuclear export, indicative of its active nuclear export via this pathway. The nuclear export signal in BRCA1 has been described as consisting of amino acid residues 81-99. However, a number of other tumor suppressors have multiple nuclear export sequences, and we sought to determine whether BRCA1 did also. Here, we report that BRCA1 contains a second nuclear export sequence that comprises amino acid residues 22-30. By use of the human immunodeficiency virus-1 Rev complementation assay, this sequence was shown to confer export capability to an export-defective Rev fusion protein. The level of export activity was comparable with that of residues 81-99 comprising the previously reported nuclear export sequence in BRCA1. Mutation of leucine 28 to an alanine reduced nuclear export by ϳ75%. In MCF-7 cells stably transfected with a BRCA1 cDNA containing mutations in this novel sequence or the previously reported export sequence, BRCA1 accumulated in the nucleus. These data imply that BRCA1 contains at least two leucine-dependent nuclear export sequences.Germ line mutations in the two known breast cancer susceptibility genes, BRCA1 and BRCA2, contribute to the development of a large percentage of hereditary breast cancers. Approximately 20% of families with an inherited susceptibility to breast cancer have a mutation in BRCA1 (1), whereas about 90% of the families with an increased prevalence of both breast and ovarian cancers have mutations within this gene (2). In contrast, mutations in BRCA1 do not appear to contribute significantly to the development of nonfamilial cancers. Rather, there is a reduced level of BRCA1 expression in sporadic breast cancer (3).The primary structure of BRCA1 contains several features that support a nuclear function for the protein. First, BRCA1 has two carboxyl-terminal domains, sequences present in several cell cycle checkpoint proteins, such as p53-binding protein 1 (4), DNA repair protein XRCC (5), and the retinoblastoma family of proteins (6). This structural feature, combined with the reported binding of BRCA1 to Rad 50 (7) and/or Rad 51 (8) as well as its ability to facilitate transcription-coupled DNA repair (9), implicates BRCA1 in the cellular response pathway for repair of DNA damage. Second, BRCA1 has an acidic carboxyl terminus that acts as a transactivation domain (10) and supports the role of BRCA1 as a transcriptional co-activator. Third, BRCA1 also has two nuclear localization sequences (11, 12), suggestive of a nuclear localization and function for the protein.Like several other tumor suppressors, including adenomatous polyposis coli (13, 14), p53 (15, 16) and Ini (integrase interactor 1)/hSNF5 (17), ...

Signaling pathways involved in regulating nuclear-cytoplasmic distribution of BRCA1 have not been previously reported. Here, we provide evidence that heregulin β1-induced activation of the Akt pathway increases the nuclear content of BRCA1. First, treatment of T47D breast cancer cells with heregulin β1 results in a two-fold increase in nuclear BRCA1 as assessed by FACS analysis, immunoblotting and immunofluorescence. This heregulin-induced increase in nuclear BRCA1 is blocked by siRNA-mediated down-regulation of Akt. Second, mutation of threonine 509 in BRCA1, the site of Akt phosphorylation, to an alanine, attenuates the ability of heregulin to induce BRCA1 nuclear accumulation. These data suggest that Akt-catalyzed phosphorylation of BRCA1 is required for the heregulin-regulated nuclear concentration of BRCA1. Because most functions ascribed to BRCA1 occur within the nucleus, we postulated that phosphorylation-dependent nuclear accumulation of BRCA1 would result in enhanced nuclear activity, specifically transcriptional activity, of BRCA1. This postulate is affirmed by our observation that the ability of BRCA1 to transactivate GADD45 promoter constructs was enhanced in T47D cells treated with heregulin β1. Furthermore, the heterologous expression of BRCA1 in HCC1937 human breast cancer cells, which have constitutively active Akt, also induces GADD45 promoter activity, whereas the expression of BRCA1 in which threonine 509 has been mutated to an alanine is able to only minimally induce promoter activity. These findings implicate Akt in upstream events leading to BRCA1 nuclear localization and function.