BRCA1 interaction with RNA polymerase II reveals a role for hRPB2 and hRPB10 in activated transcription

A mechanism for transcriptional repression dependent on the BRCA1 E3 ubiquitin ligase

Loss of function of the tumor suppressor protein BRCA1 is responsible for a high percentage of familial and also sporadic breast cancers. Early work identified a stimulatory transcriptional coactivator function for the BRCA1 protein, and more recently, BRCA1 has been implicated in transcriptional repression, although few examples of repressed genes have been characterized. We recently used an in vitro transcription assay to identify a biochemical mechanism that explained the BRCA1 stimulatory activity. In this study, we identified an ubiquitin-dependent mechanism by which BRCA1 inhibits transcription. BRCA1 ubiquitinates the transcriptional preinitiation complex, preventing stable association of TFIIE and TFIIH, and thus blocks the initiation of mRNA synthesis. What is striking about this mechanism of regulation by BRCA1 is that the ubiquitination of the preinitiation complex is not targeting proteins for degradation by the proteasome, nor are ubiquitin receptors modifying the activity, but rather the ubiquitin moiety itself interferes with the assembly of basal transcription factors at the promoter. Using RNAi to knockdown expression of the endogenous BRCA1 protein, we assessed the level of repression dependent on BRCA1 in the cell, and we found that BRCA1 is at least as significant a transcriptional repressor as it is an activator. These results define a biochemical mechanism by which the BRCA1 enzymatic activity regulates a key cellular process.

BRCA1 promotes induction of ssDNA by ionizing radiation

The BRCA1 tumor suppressor contributes to the repair of DNA double-strand breaks (DSB) through homologous recombination, but the mechanism is unknown. The rapid accumulation of BRCA1 into nuclear foci in response to induction of DNA breaks suggests that BRCA1 may function in an early step in the repair pathway. We examined the role of BRCA1 in one such early step, the resection of DSBs to generate ssDNA. The appearance of ssDNA in response to ionizing radiation is similar to that of BRCA1 foci formation, suggesting that the two processes are related. Furthermore, BRCA1 colocalizes to ssDNA sites induced by ionizing radiation. Overexpression of BRCA1 causes an increase in cells exhibiting ssDNA induced by ionizing radiation. Mutant BRCA1 that lacks the COOH-terminal BRCT domain also promotes ssDNA but fails to form nuclear foci. Knockdown of BRCA1 expression reduces ssDNA and Rad51 foci formation in response to ionizing radiation. These results indicate that BRCA1 promotes induction of ssDNA in response to ionizing radiation and accumulates at sites of ssDNA.

Expression of an amino-terminal BRCA1 deletion mutant causes a dominant growth inhibition in MCF10A cells

Expression of deletion mutants of the breast and ovarian cancer-specific tumor suppressor protein, BRCA1, in the mammary epithelial cell line MCF10A revealed a powerful growth suppressive effect by a mutant that has the amino-terminal 302 amino acids deleted (DeltaN-BRCA1). The growth suppression is associated with an increase in apoptosis and amplification in centrosome number. The growth inhibitory effect of DeltaN-BRCA1 was not observed in cervical epithelial HeLa cells, suggesting that the phenotypes of BRCA1 mutant proteins differ depending on the cell line being tested. An internal domain, including BRCA1 residues 303-1292, caused the suppression of MCF10A cell growth, and the amino terminus of BRCA1 autoinhibited the growth suppression. Single point mutations that disrupted the amino-terminal RING domain of BRCA1 caused significant suppression of growth in MCF10A cells. These results suggest that the proper function of the RING domain, likely to be ubiquitin ligase function, is important in regulating the growth of the mammary epithelial cell line and in autoregulating the powerful internal growth-inhibiting domain of the BRCA1 tumor suppressor.

BRCA1 cooperates with NUFIP and P-TEFb to activate transcription by RNA polymerase II

The tumor suppressor gene product BRCA1 is a component of the RNA polymerase II (pol II) holoenzyme that is involved, through binding to various regulatory proteins, in either activation or repression of transcription. Using a yeast two-hybrid screen, we have identified a human zinc-finger-containing protein NUFIP that interacts with BRCA1. The ubiquitous, stably expressed, nuclear protein NUFIP specifically stimulates activator-independent pol II transcription in vitro and in vivo. Immunodepletion of the endogenous NUFIP causes a marked decrease of pol II transcription, which is then shown to be restored by stable complex of ectopically produced NUFIP and associated factors. NUFIP not only interacts with BRCA1 but also associates with the positive elongation factor P-TEFb through interaction with the regulatory Cyclin T1 subunit. Cyclin T1 is required for BRCA1- and NUFIP-dependent synergistic activation of pol II transcription in 293 cells. Mutation of the zinc-finger domain abolishes the NUFIP-mediated transcriptional activation. We show that NUFIP is associated with preinitiation complexes, open transcription complexes, and elongation complexes. In addition, NUFIP facilitates ATP-dependent dissociation of hyperphosphorylated pol II from open transcription complexes in vitro.

Roles of BRCA1 and BRCA2 in homologous recombination, DNA replication fidelity and the cellular response to ionizing radiation

Inheritance of one defective copy of either of the two breast cancer susceptibility genes, BRCA1 and BRCA2, predisposes individuals to breast and ovarian cancers. Current progress in determining the function of these genes suggests that they participate in a common pathway to facilitate orderly homologous recombination and thereby maintain genomic integrity. As a consequence of this defect in homologous recombination, tumors that arise in BRCA carriers are likely to be more sensitive to ionizing radiation. This review summarizes recent investigations about the nature of the defect in DNA repair, and highlights the unanswered questions about the tumor suppressor paradox of BRCA genes. The unsolved mystery is the other genetic changes that must occur to turn a BRCA-deficient cell from a nonviable cell into a tumor cell capable of endless growth.

Overexpression of a protein fragment of RNA helicase A causes inhibition of endogenous BRCA1 function and defects in ploidy and cytokinesis in mammary epithelial cells

The breast- and ovarian-specific tumor suppressor, BRCA1, has been implicated to function in many nuclear processes, including DNA damage repair, recombination, transcription, ubiquitination, cell cycle checkpoint enforcement, and centrosome regulation. Utilizing a previously described interaction between BRCA1 and RNA helicase A (RHA), we have developed a dominant-negative approach to block BRCA1 function in human breast epithelial cells. Overexpression of a truncated RHA peptide that can bind to the BRCA1 carboxy-terminus prevents normal BRCA1 function, such as BRCA1 association with nuclear foci following DNA damage. Overexpression of this dominant-negative protein induces pleomorphic nuclei, aberrant mitoses with extra centrosomes, and tetraploidy. This model system allows us to observe changes to mammary epithelial cells that occur acutely following loss of BRCA1 function. Furthermore, inhibition of BRCA1 via overexpressing the RHA fragment coincides with a reduction in PARP-1 protein expression, suggesting a possible mechanism for BRCA1 in the maintenance of genomic integrity.

Inhibition of BRCA1 leads to increased chemoresistance to microtubule-interfering agents, an effect that involves the JNK pathway

We have developed ribozymes (Rz) that inhibit BRCA1 expression in order to study the role of this gene in chemosensitivity. Two Rz, targeting positions 358 or 5282 of the BRCA1 mRNA, were cloned into the retroviral vector LXSN and lipofected into the breast cancer cell-line HBL100. We obtained 79-99% inhibition of BRCA1 expression, as determined by real-time quantitative PCR and by Western blotting. Decreased expression of BRCA1 led to sensitivity to the DNA damaging agents cisplatin and etoposide, resistance to the microtubule-interfering agents (MIA) taxol and vincristine. The molecular mechanism of resistance to MIA was investigated further by determining the status of the JNK pathway. We found that JNK1 expression was elevated, while JNK2 expression was decreased in Rz-expressing clones compared to controls. We have quantified the mRNA levels of BRCA1, JNK1, 2, MEK-4, -7 and c-jun after treatment with MIA. Vincristine treatment of control cells resulted in transcriptional repression of BRCA1, while the JNK1, 2, MEK-4, -7 and c-jun genes were induced. In Rz-treated cells, only JNK1 and MEK-4 were expressed and none was induced after MIA treatment. We then studied the phosphorylation of c-jun, a downstream effector of the JNK pathway. We observed a strong increase in phosphorylated c-jun after MIA treatment of the control cells but not in BRCA1-Rz treated cells, suggesting inhibition of the JNK pathway. These results show that the BRCA1-JNK pathway is involved in the cytotoxic response to MIA treatment, and inhibition of BRCA1 leads to transcriptional modifications of the JNK pathway.

Lessons learned from BRCA1 and BRCA2

BRCA1 and BRCA2 are breast cancer susceptibility genes. Mutations within BRCA1 and BRCA1 are responsible for most familial breast cancer cases. Targeted deletion of Brca1 or Brca2 in mice has revealed an essential function for their encoded products, BRCA1 and BRCA2, in cell proliferation during embryogenesis. Mouse models established from conditional expression of mutant Brca1 alleles develop mammary gland tumors, providing compelling evidence that BRCA1 functions as a breast cancer suppressor. Human cancer cells and mouse cells deficient in BRCA1 or BRCA2 exhibit radiation hypersensitivity and chromosomal abnormalities, thus revealing a potential role for both BRCA1 and BRCA2 in the maintenance of genetic stability through participation in the cellular response to DNA damage. Functional analyses of the BRCA1 and BRCA2 gene products have established their dual participation in transcription regulation and DNA damage repair. Potential insight into the molecular basis for these functions of BRCA1 and BRCA2 has been provided by studies that implicate these two tumor suppressors in both the maintenance of genetic stability and the regulation of cell growth and differentiation.

BRCA1 and cell signaling

The breast cancer and ovarian cancer susceptibility gene BRCA1 encodes a nucleoprotein whose mutations or aberrant expression is associated with both inherited and sporadic cancers. Studies over the last 6 years have suggested that BRCA1 may function as a scaffold in the assembly of a multi-protein complex, which plays a role in gene transcription, DNA damage repair, and transcription-coupled DNA damage repair. In this review, we discuss the implications drawn from the studies of BRCA1-interacting proteins and the cellular signaling pathways that may be involved in controlling the functions of BRCA1.