Arrest of the cell cycle by the tumour-suppressor BRCA1 requires the CDK-inhibitor p21WAF1/CiP1

pRB binds to and modulates the transrepressing activity of the E1A-regulated transcription factor p120E4F

The retinoblastoma protein pRB is involved in the transcriptional control of genes essential for cell cycle progression and differentiation. pRB interacts with different transcription factors and thereby modulates their activity by sequestration, corepression, or activation. We report that pRB, but not p107 and p130, binds to and facilitates repression by p120(E4F), a ubiquitously expressed GLI-Kruppel-related protein identified as a cellular target of E1A. The interaction involves two distinct regions of p120(E4F) and the C-terminal part of pRB. In vivo pRB-p120(E4F) complexes can only be detected in growth-arrested cells, and accordingly contain the hypophosphorylated form of pRB. Repression of an E4F-responsive promoter is strongly increased by combined expression of p120(E4F) and pRB, which correlates with pRB-dependent enhancement of p120(E4F) binding activity. Elevated levels of p120(E4F) have been shown to block growth of mouse fibroblasts in G(1). We find this requires pRB, because RB(-/-) fibroblasts are significantly less sensitive to excess p120(E4F).

p53 and WAF1 polymorphisms in Jewish-Israeli women with epithelial ovarian cancer and its association with BRCA mutations

OBJECTIVE

To investigate whether polymorphic p53 and WAF1 alleles are associated with clinical, demographic and histopathological features and BRCA mutation in women with ovarian cancer.

DESIGN

A cross-sectional study.

POPULATION

Two hundred and twenty-one nonselected Israeli women with epithelial ovarian cancer.

METHODS

DNA was analysed for known polymorphisms in intron 3 (a 16 nucleotide single repeat) and intron 6 (a G to A change at nucleotide 13,494) of the p53 gene, the S31R polymorphism in the WAF1 gene, and for three predominant Jewish mutations in the BRCA genes (185delAG and 5382insC in BRCA1, and 6174delT in BRCA2).

MAIN OUTCOME MEASURE

The rate of polymorphic p53 and WAF1 alleles and their association with BRCA mutation, ethnic origin, age and stage at diagnosis, and family history of cancer.

RESULTS

Of the tested women, 72 (32.6%) were either BRCA1 (n = 57) or BRCA2 (n = 15) mutation carriers. Sixty-eight of 213 (31.9%) were heterozygous for intron 3 polymorphism, 67/193 (34.7%) for intron 6 polymorphism, and 22/154 (14.3%) for S31R of the WAF1 gene. The p53 and WAF1 polymorphism rate did not differ between BRCA mutation carriers and noncarriers. No significant association between specific p53 or WAF1 genotypes, and clinical, histopathological or demographic variables was observed.

CONCLUSION

In Jewish-Israeli women with sporadic and familial ovarian cancer, p53 or WAF1 polymorphisms do not seem to affect the phenotype.

Nuclear localization and cell cycle-specific expression of CtIP, a protein that associates with the BRCA1 tumor suppressor

The BRCA1 tumor suppressor has been implicated in a diverse spectrum of cellular processes, including transcriptional regulation, DNA repair, and cell cycle checkpoint control. CtIP was recently identified as a protein that associates with BRCA1 and two other nuclear factors, CtBP1 and Rb1. To understand the functions of CtIP, we have evaluated its biological properties with respect to those of BRCA1. Our results show that CtIP, like its associated factors, is predominantly a nuclear protein. A subset of the endogenous pool of CtIP polypeptides exists in a protein complex that includes both BRCA1 and the BRCA1-associated RING domain protein (BARD1). At the protein level, CtIP expression varies with cell cycle progression in a pattern identical to that of BRCA1. Thus, the steady-state levels of CtIP polypeptides, which remain low in resting cells and G(1) cycling cells, increase dramatically as dividing cells traverse the G(1)/S boundary. In contrast to BRCA1, however, the G(1)/S induction of CtIP expression is mediated primarily by post-transcriptional mechanisms. Finally, the interaction between CtIP and BRCA1 is shown to be stable in the face of genotoxic stress elicited by treatment with UV light, adriamycin, or hydrogen peroxide. Together, these results indicate that CtIP can potentially modulate the functions ascribed to BRCA1 in transcriptional regulation, DNA repair, and/or cell cycle checkpoint control.

The gut-enriched Kruppel-like factor (Kruppel-like factor 4) mediates the transactivating effect of p53 on the p21WAF1/Cip1 promoter

An important mechanism by which the tumor suppressor p53 maintains genomic stability is to induce cell cycle arrest through activation of the cyclin-dependent kinase inhibitor p21(WAF1/Cip1) gene. We show that the gene encoding the gut-enriched Krüppel-like factor (GKLF, KLF4) is concurrently induced with p21(WAF1/Cip1) during serum deprivation and DNA damage elicited by methyl methanesulfonate. The increases in expression of both Gklf and p21(WAF1/Cip1) due to DNA damage are dependent on p53. Moreover, during the first 30 min of methyl methanesulfonate treatment, the rise in Gklf mRNA level precedes that in p21(WAF1/Cip1), suggesting that GKLF may be involved in the induction of p21(WAF1/Cip1). Indeed, GKLF activates p21(WAF1/Cip1) through a specific Sp1-like cis-element in the p21(WAF1/Cip1) proximal promoter. The same element is also required by p53 to activate the p21(WAF1/Cip1) promoter, although p53 does not bind to it. Potential mechanisms by which p53 activates the p21(WAF1/Cip1) promoter include a physical interaction between p53 and GKLF and the transcriptional induction of Gklf by p53. Consequently, the two transactivators cause a synergistic induction of the p21(WAF1/Cip1) promoter activity. The physiological relevance of GKLF in mediating p53-dependent induction of p21(WAF1/Cip1) is demonstrated by the ability of antisense Gklf oligonucleotides to block the production of p21(WAF1/Cip1) in response to p53 activation. These findings suggest that GKLF is an essential mediator of p53 in the transcriptional induction of p21(WAF1/Cip1) and may be part of a novel pathway by which cellular responses to stress are modulated.

Isolation of a functional copy of the human BRCA1 gene by transformation-associated recombination in yeast

The BRCA1 gene, mutations of which contribute significantly to hereditary breast cancer, was not identified in the existing YAC and BAC libraries. The gene is now available only as a set of overlapping fragments that form a contig. In this work we describe direct isolation of a genomic copy of BRCA1 from human DNA by transformation-associated recombination (TAR) cloning. Despite the presence of multiple repeats, most of the primary BRCA1 YAC isolates did not contain detectable deletions and could be stably propagated in a host strain with conditional RAD52. Similar to other circular YACs, approximately 90kb BRCA1 YACs were efficiently and accurately retrofitted into bacterial artificial chromosomes (BACs) with the Neo(R) mammalian selectable marker and transferred as circular BAC/YACs in E. coli cells. The BRCA1 BAC/YAC DNAs were isolated from bacterial cells and were used to transfect mouse cells using the Neo(R) gene as selectable marker. Western blot analysis of transfectants showed that BRCA1 YACs isolated by a TAR cloning contained a functional gene. The advantage of this expression vector is that the expression of BRCA1 is generated from its own regulatory elements and does not require additional promoter elements that may result in overexpression of the protein. In contrast to the results with cDNA expression vectors, the level of BRCA1 expression from this TAR vector is stable, does not induce cell death, maintains serum regulation, and approximates the level of endogenously expressed BRCA1 in human cells. The entire isolation procedure of BRCA1 described in this paper can be accomplished in approximately 10 days and can be applied to isolation of gene from clinical material. We propose that the opportunity to directly isolate normal and mutant forms of BRCA1 will greatly facilitate analysis of the gene and its contribution to breast cancer.

Collaboration of signal transducer and activator of transcription 1 (STAT1) and BRCA1 in differential regulation of IFN-gamma target genes

Most of the activities of IFN-gamma are the result of STAT1-mediated transcriptional responses. In this study, we show that the BRCA1 tumor suppressor acts in concert with STAT1 to differentially activate transcription of a subset of IFN-gamma target genes and mediates growth inhibition by this cytokine. After IFN-gamma treatment, induction of the cyclin-dependent kinase inhibitor, p21WAF1, was synergistically activated by BRCA1, whereas the IRF-1 gene was unaffected. Importantly, the differential induction of p21WAF1 was impaired in breast cancer cells homozygous for the mutant BRCA1 5382C allele. Biochemical analysis illustrated that the mechanism of this transcriptional synergy involves interaction between BRCA1 aa 502-802 and the C-terminal transcriptional activation domain of STAT1 including Ser-727 whose phosphorylation is crucial for transcriptional activation. Significantly, STAT1 proteins mutated at Ser-727 bind poorly to BRCA1, reinforcing the importance of Ser-727 in the recruitment of transcriptional coactivators by STAT proteins. These findings reveal a novel mechanism for BRCA1 function in the IFN-gamma-dependent tumor surveillance system.

p21 (WAF1/CIP1) protein expression is associated with prolonged survival but not with p53 expression in epithelial ovarian carcinoma

OBJECTIVE

The objective of this study was to clarify the influence of p21 protein expression in ovarian cancer. p21 (WAF1 [wild-type p53 activated fragment 1]/CIP1) is a universal cyclin-dependent kinase inhibitor and can be induced as a downstream effector of the p53 tumor suppressor gene.

METHODS

The expression of p21 was evaluated by immunohistochemical analysis with the monoclonal antibody WAF1 (Oncogene Science) on 106 formalin-fixed, paraffin-embedded tissue samples of epithelial ovarian cancer.

RESULTS

p21 was expressed in 65 (61%) of all cases. p21 expression was associated with early stage in FIGO classification (FIGO I and II, P = 0.003) and no tumor residues after primary tumor resection (P = 0.018). Immunohistochemical staining results were judged as negative if no tumor nuclei were stained, as weak positive if 1-49% were stained, and as strong positive if over 50% of nuclei were stained. Clinical follow-up showed a better overall survival for cases with strong p21 expression (79 months) versus 40 months for weak expression and 30 months for no expression (P = 0.033). Previously determined p53 expression of this cohort was compared with p21 status. p53 overexpression was observed in 49 cases (48%) and showed no association with p21 expression.

CONCLUSION

No correlation was found between p21 and p53 expression. p21 expression is a significant prognostic marker for improved survival in ovarian cancer and is associated with early FIGO stage and zero tumor residues after primary tumor resection.

BRCA1 expression status in relation to DNA methylation of the BRCA1 promoter region in sporadic breast cancers

To understand the biological role of BRCA1 in sporadic breast cancers, the relationship between DNA methylation of the BRCA1 promoter region and BRCA1 expression was studied using molecular biological and immunohistochemical methods. Furthermore, BRCA1 expression was compared with the expression of various cell cycle regulatory proteins and the morphological nuclear grade of cancer cells. Of 32 sporadic breast cancers investigated in this study, 10 (31%) revealed DNA methylation of the BRCA1 promoter region. The expression of BRCA1 was observed in the nuclei of cancer cells and 18 (56%) of 32 cancers were positive for BRCA1 immunoreactivity. Breast cancers with BRCA1 methylation lacked BRCA1 expression, except for only three cancers, and there was a significant inverse relationship between BRCA1 methylation and its expression in sporadic breast cancers (P = 0.043). Compared with the expression of various cell cycle regulatory proteins, breast cancers with BRCA1 methylation showed decreased expression of estrogen receptor (P = 0. 016) and p27 (P = 0.018) and increased expression of p21 (P = 0.011). Furthermore, breast cancers without BRCA1 expression or with BRCA1 methylation had a tendency to contain nuclei with higher grade. These findings indicate that BRCA1 methylation might greatly influence its expression and BRCA1 expression might play an important role in cell cycle regulation and influence the grade of malignancy of sporadic breast cancers.

Establishment and characterization of a breast cell strain containing a BRCA1 185delAG mutation

OBJECTIVE

The aim of this study was to examine whether cells containing the heterozygous form of a BRCA1 185delAG mutation would exhibit abnormal growth or an altered response to DNA damage.

METHODS

A primary culture of human mammary epithelial cells (90P) was obtained from the nontumor breast tissue of a 35-year-old patient who had undergone a mastectomy for removal of a breast tumor. These cells were immortalized (90PE6E7) following retroviral infection with HPV-16 viral E6/E7. genes. Both the 90P cell strain and the cell line were characterized for their ability to grow in culture, form colonies in soft agar, and produce tumors in athymic nude mice compared to normal breast epithelial cells containing wild-type BRCA1. 90P cells were also analyzed for cellular response to gamma radiation and H(2)O(2).

RESULTS

These cells were confirmed to contain a frameshift mutation, 185delAG, of the BRCA1 gene. Despite being heterozygous for wild-type BRCA1, the 220-kDa full-size BRCA1 protein was abundantly expressed. 90P and 90PE6E7 cells grew at a similar rate and were anchorage dependent. 90PE6E7 also failed to form tumors in athymic nude mice. Finally, 90P cells exhibited a survival response similar to that of normal mammary epithelial cells to radiation damage and exposure to oxidative stress.

CONCLUSION

To our knowledge the 90P cells and the 90PE6E7 cells are the first characterized, non-tumor-derived breast epithelial cells that are heterozygous for the BRCA1 germline mutation 185delAG. Our conclusion is that these BRCA1 mutant cells appear to have growth and stress response characteristics similar to those of normal human breast cells, which is consistent with the hypothesis that loss of heterozygosity must occur to impair putative BRCA1 function.

hCds1-mediated phosphorylation of BRCA1 regulates the DNA damage response

Mutations in the BRCA1 (ref. 1) tumour suppressor gene are found in almost all of the families with inherited breast and ovarian cancers and about half of the families with only breast cancer. Although the biochemical function of BRCA1 is not well understood, it is important for DNA damage repair and cell-cycle checkpoint. BRCA1 exists in nuclear foci but is hyperphosphorylated and disperses after DNA damage. It is not known whether BRCA1 phosphorylation and dispersion and its function in DNA damage response are related. In yeast the DNA damage response and the replication-block checkpoint are mediated partly through the Cds1 kinase family. Here we report that the human Cds1 kinase (hCds1/Chk2) regulates BRCA1 function after DNA damage by phosphorylating serine 988 of BRCA1. We show that hCds1 and BRCA1 interact and co-localize within discrete nuclear foci but separate after gamma irradiation. Phosphorylation of BRCA1 at serine 988 is required for the release of BRCA1 from hCds1. This phosphorylation is also important for the ability of BRCA1 to restore survival after DNA damage in the BRCA1-mutated cell line HCC1937.

DNA methyltransferase inhibition induces the transcription of the tumor suppressor p21(WAF1/CIP1/sdi1)

Previous lines of evidence have shown that inhibition of DNA methyltransferase (MeTase) can arrest tumor cell growth; however, the mechanisms involved were not clear. In this manuscript we show that out of 16 known tumor suppressors and cell cycle regulators, the cyclin-dependent kinase inhibitor p21 is the only tumor suppressor induced in the human lung cancer cell line, A549, following inhibition of DNA MeTase by a novel DNA MeTase antagonist or antisense oligonucleotides. The rapid induction of p21 expression points to a mechanism that does not involve demethylation of p21 promoter. Consistent with this hypothesis, we show that part of the CpG island upstream of the endogenous p21 gene is unmethylated and that the expression of unmethylated p21 promoter luciferase reporter constructs is induced following inhibition of DNA MeTase. These results are consistent with the hypothesis that the level of DNA MeTase in a cell can control the expression of a nodal tumor suppressor by a mechanism that does not involve DNA methylation.

BRCA1, C-erbB-2, and H-ras gene expressions in young women with breast cancer. An immunohistochemical study

Young age does not seem to be directly related to the aggressiveness of the disease among patients with breast cancer. Identification and analysis of the alterations in a susceptibility gene expression in breast cancer occurring in young women may allow identification of those patients in whom tumors will show an aggressive clinical course. The purpose of the present study was to evaluate the association of BRCA1, H-ras, and c-erbB-2 gene expression with clinicopathologic parameters of prognosis in breast cancer. Formalin-fixed, paraffin-embedded tissue from 35 patients with breast cancer younger than 35 years were immunohistochemically stained for BRCA1, H-ras, and c-erbB-2 expression with monoclonal antibodies. For each antibody, immunoreactivity was assessed by a semiquantitative scoring system. Each case was also graded according to the modified Bloom-Richardson criteria and evaluated for Ki-67 labeling index, hormonal status, tumor size, distant metastasis, and axillary lymph node involvement. Strong expression of c-erbB-2 and H-ras were observed in 9 cases (25.7%) and 13 cases (37.2%), respectively. Loss of BRCA1 expression was found in five cases (14.3%). Statistical analysis showed that loss of BRCA1 expression was significantly associated with higher Ki-67 labeling index and greater tumor size. In addition, stronger H-ras expression was significantly associated with lymph node involvement and distant metastasis. However, c-erbB-2 immunoreactivity did not show statistical significance with any prognostic parameters. We conclude that, although care must be taken not to overstate the importance of our results in view of the lack of information on clinical outcome, alterations in BRCA1 and H-ras gene expression might be of prognostic importance because of the role of H-ras protein on metastatic behavior and the role of BRCA1 protein on tumor growth. However, c-erbB-2 expression seems to be of no importance in the prognosis of breast cancer occurring in young women.

Role of the tumor suppressor gene Brca1 in genetic stability and mammary gland tumor formation

Germline mutations in the tumor suppressor BRCA1 predispose women to breast and ovarian cancers. Current evidence demonstrates that mutations in BRCA1 do not directly result in tumor formation, but instead cause genetic instability, subjecting cells to high risks of malignant transformation. In an animal model in which Brca1 is mutated specifically in mammary epithelium, tumorigenesis occurs in mutant glands at low frequency after a long latency. Notably, introduction of a p53-null allele significantly enhanced mammary gland tumor formation in Brca1 conditional mutant mice. These results are consistent with a model that Brca1 is a caretaker gene, whose absence causes genetic instability and triggers further alterations, including inactivation of tumor suppressor genes and/or activation of oncogenes, leading to tumor formation.

CBP/p300 interact with and function as transcriptional coactivators of BRCA1

BRCA1 is a breast and ovarian cancer-specific tumor suppressor, with properties of a transcription factor involved in DNA repair. We previously have shown the transactivation of heterologous promoters by the carboxyl terminus of BRCA1. We now describe that BRCA1-mediated transactivation is enhanced by p300/CBP (CREB binding protein) and that this effect was suppressed by the adenovirus E1A oncoprotein. We show a physical association of BRCA1 with the transcriptional coactivators/acetyltransferases p300 and CBP. Endogenous as well as overexpressed BRCA1 and p300 were found to associate in a phosphorylation-independent manner. BRCA1 interacts with the cAMP response element binding protein (CREB) domain of p300/CBP via both its amino and carboxyl termini. Finally, full-length BRCA1 is shown to transcriptionally activate the Rous sarcoma virus-long terminal repeat promoter, which was further stimulated by p300. Immunocolocalization analyses suggest that BRCA1 and p300 associate in a cell cycle-dependent manner. Our results support a role for BRCA1 in transcription.

Insights into the functions of BRCA1 and BRCA2

Since BRCA1 and BRCA2 were cloned five years ago, unraveling their normal functions has posed fascinating problems for cancer biologists. Both genes are novel, and little of their normal function was revealed by their sequence. Both genes contribute to homologous recombination and DNA repair, to embryonic proliferation, to transcriptional regulation and, for BRCA1, to ubiquitination. But questions regarding BRCA1 and BRCA2 biology remain, and their resolution is critical for clinical development. Why do ubiquitously expressed genes that participate in universal pathways lead, when mutant, specifically to breast and ovarian cancer? Why are the same genes required for embryonic proliferation and for tumor suppression?

BRCA1 suppresses insulin-like growth factor-I receptor promoter activity: potential interaction between BRCA1 and Sp1

The insulin-like growth factor I receptor (IGF-I-R) has an important role in breast cancer etiology. The receptor is overexpressed by most breast cancers, where it functions as a potent antiapoptotic agent. BRCA1 is a tumor suppressor gene that is mutated in a large fraction of familial breast and ovarian cancers. Cotransfection of Saos-2, MCF7, and CHO cells with IGF-I-R promoter constructs driving luciferase reporter genes, and with a BRCA1 expression vector, suppressed promoter activity in a dose-dependent manner. Functional interactions between BRCA1 and Sp1 in the regulation of the IGF-I-R gene were studied in Schneider cells, a Drosophila cell line which lacks endogenous Sp1. In these cells BRCA1 suppressed 45% of the Sp1-induced trans-activation of the IGF-I-R promoter. These results suggest that BRCA1 is capable of suppressing the IGF-I-R promoter in a number of cell lines, thus resulting in low levels of receptor mRNA and protein. Mutant versions of BRCA1 lacking trans-activational activity can potentially derepress the IGF-I-R promoter. Activation of the overexpressed receptor by locally produced or circulating IGFs may be a crucial step in breast and ovarian cancer progression.

BRCA1 effects on the cell cycle and the DNA damage response are linked to altered gene expression

The breast and ovarian cancer susceptibility gene product BRCA1 has been reported to be expressed in a cell cycle-dependent manner; possess transcriptional activity; associate with several proteins, including the p53 tumor suppressor; and play an integral role in certain types of DNA repair. We show here that ectopic expression of BRCA1 using an adenovirus vector (Ad-BRCA1) leads to dephosphorylation of the retinoblastoma protein accompanied by a decrease in cyclin-dependent kinase activity. Flow cytometric analysis on Ad-BRCA1-infected cells revealed a G(1) or G(2) phase accumulation. High density cDNA array screening of colon, lung, and breast cancer cells identified several genes affected by BRCA1 expression in a p53-independent manner, including DNA damage response genes and genes involved in cell cycle control. Notable changes included induction of the GADD45 and GADD153 genes and a reduction in cyclin B1 expression. Therefore, BRCA1 has the potential to modulate the expression of genes and function of proteins involved in cell cycle control and DNA damage response pathways.

Role of Gadd45 in apoptosis

gadd45 is a p53-regulated growth arrest and DNA-damage-inducible gene that is also regulated in a p53-independent manner. Whether Gadd45 plays a direct role in apoptosis remains unclear. Microinjection of the exogenous gadd45 expression vector into human fibroblasts has been shown to cause G2 arrest but not apoptosis. Recent studies suggest that Gadd45 may mediate genotoxic stress or Brca1-induced apoptosis via activation of c-Jun N-terminal kinase (JNK) and/or p38 mitogen-activated protein kinase (MAPK). Analyses of gadd45-deficient mice and cells have revealed that Gadd45 appears to exhibit pleiotropic effects, including cell cycle arrest at G2/M, DNA damage repair, and control of genomic stability, but is not required for radiation-induced apoptosis. Furthermore, stress-induced activation of JNK and p38 MAPK is not altered in gadd45-deficient embryonic fibroblasts, suggesting that the lack of Gadd45 may not affect the JNK and p38 MAPK activity. Thus, although the evidence from gadd45-null cells suggests that Gadd45 probably does not play a direct role in genotoxic stress-induced apoptosis, more in-depth studies are needed to firmly establish this contention.

Role of BRCA gene dysfunction in breast and ovarian cancer predisposition

Tumor suppressor genes that perform apparently generic cellular functions nonetheless cause tissue-specific syndromes in the human population when they are mutated in the germline. The two major hereditary breast/ovarian cancer predisposition genes, BRCA1 and BRCA2, appear to participate in a common pathway that is involved in the control of homologous recombination and in the maintenance of genomic integrity. How might such functions translate into the specific suppression of cancers of the breast and ovarian epithelia? Recent advances in the study of BRCA1 and BRCA2, discussed herein, have provided new opportunities to address this question.

Increased cell survival by inhibition of BRCA1 using an antisense approach in an estrogen responsive ovarian carcinoma cell line

STATEMENT OF FINDINGS: We tested the hypothesis that BRCA1 may play a role in the regulation of ovarian tumor cell death as well as the inhibition of ovarian cell proliferation. Introduction of BRCA1 antisense retroviral constructs into BG-1 estrogen-dependent ovarian adenocarcinoma cells resulted in reduced BRCA1 expression. BRCA1 antisense pooled populations and derived subclones were able to proliferate in monolayer culture without estrogen, whereas control cells began to die after 10 days of estrogen deprivation. In addition, both populations and subclones of BRCA1 antisense infected cells demonstrated a growth advantage in monolayer culture in the presence of estrogen and were able to proliferate in monolayer culture without estrogen, while control cells did not. Furthermore, clonal studies demonstrated that reduced levels of BRCA1 protein correlated with growth in soft agar and greater tumor formation in nude mice in the absence of estrogen. These data suggest that reduction of BRCA1 protein in BG-1 ovarian adenocarcinoma cells may have an effect on cell survival during estrogen deprivation both in vitro and in vivo.

Substrate specificities and identification of putative substrates of ATM kinase family members

Ataxia telangiectasia mutated (ATM) phosphorylates p53 protein in response to ionizing radiation, but the complex phenotype of AT cells suggests that it must have other cellular substrates as well. To identify substrates for ATM and the related kinases ATR and DNA-PK, we optimized in vitro kinase assays and developed a rapid peptide screening method to determine general phosphorylation consensus sequences. ATM and ATR require Mn(2+), but not DNA ends or Ku proteins, for optimal in vitro activity while DNA-PKCs requires Mg(2+), DNA ends, and Ku proteins. From p53 peptide mutagenesis analysis, we found that the sequence S/TQ is a minimal essential requirement for all three kinases. In addition, hydrophobic amino acids and negatively charged amino acids immediately NH(2)-terminal to serine or threonine are positive determinants and positively charged amino acids in the region are negative determinants for substrate phosphorylation. We determined a general phosphorylation consensus sequence for ATM and identified putative in vitro targets by using glutathione S-transferase peptides as substrates. Putative ATM in vitro targets include p95/nibrin, Mre11, Brca1, Rad17, PTS, WRN, and ATM (S440) itself. Brca2, phosphatidylinositol 3-kinase, and DNA-5B peptides were phosphorylated specifically by ATR, and DNA Ligase IV is a specific in vitro substrate of DNA-PK.

Gene therapy for carcinoma of the breast: Therapeutic genetic correction strategies

Gene therapy is a therapeutic approach that is designed to correct specific molecular defects that contribute to the cause or progression of cancer. Genes that are mutated or deleted in cancers include the cancer susceptibility genes p53 and BRCA1. Because mutational inactivation of gene function is specific to tumor cells in these settings, cancer gene correction strategies may provide an opportunity for selective targeting without significant toxicity for normal nontumor cells. Both p53 and BRCA1 appear to inhibit cancer cells that lack mutations in these genes, suggesting that the so-called gene correction strategies may have broader potential than initially believed. Increasing knowledge of cancer genetics has identified these and other genes as potential targets for gene replacement therapy. Initial patient trials of p53 and BRCA1 gene therapy have provided some indications of potential efficacy, but have also identified areas of basic and clinical research that are needed before these approaches may be widely used in patient care.

Genetic analysis of BRCA1 function in a defined tumor cell line

Retrovirally expressed, wild-type BRCA1 decreased the gamma radiation (IR) sensitivity and increased the efficiency of double-strand DNA break repair (DSBR) of the BRCA1-/- human breast cancer line, HCC1937. It also reduced its susceptibility to DSB generation by IR. By contrast, multiple, clinically validated, missense mutant BRCA1 products were nonfunctional in these assays. These data constitute the basis for a BRCA1 functional assay and suggest that efficient repair of double-strand DNA breaks is linked to BRCA1 tumor suppression function.

Emerging roles of BRCA1 in transcriptional regulation and DNA repair

BRCA1 was the first breast cancer susceptibility gene to be identified and cloned. In individuals from high-risk families, mutations in BRCA1 increase the lifetime risk of developing breast cancer eight to tenfold, compared to the general population. How the BRCA1 protein product normally functions to suppress tumor formation and how defects in the gene can ultimately lead to breast cancer have been the focus of intense scrutiny by the scientific and medical communities. BRCA1 has intrinsic transactivation activity and is able to activate the p21 promoter. In addition, BRCA1 is linked to a number of genes involved in transcriptional regulation, including CtIP, c-Myc, the RNA holoenzyme complex, and the histone deacetylase complex. Moreover, BRCA1 is essential for cellular response to DNA damage repair. Inactivation of Brca1 in mouse embryonic stem and fibroblast cells results in increased cell sensitivity to DNA-damaging agents. In human cells, BRCA1 binds to both Rad50 and Rad51 and colocalizes with these proteins at repair foci. Part of BRCA1's response to DNA damage may in fact be corroborated through transcriptional regulation. The expression of GADD45, a DNA damage-responsive gene, is increased immediately after induction of BRCA1. Recently, BRCA1 was shown to repress estradiol (E2)-responsive ER-alpha-mediated transcriptional activity, potentially linking the multiple functions of BRCA1 to specific tissue targets. These recent developments in BRCA1 function are an encouraging step toward understanding the role of BRCA1 in breast cancer formation.

BRCA1 signals ARF-dependent stabilization and coactivation of p53

The hereditary breast and ovarian tumor suppressor BRCA1 can activate p53-dependent gene expression. We show here that BRCA1 increases p53 protein levels through a post-transcriptional mechanism. BRCA1-stabilized p53 has increased sequence-specific DNA-binding and transcriptional activity. BRCA1 does not stabilize p53 in p14ARF-deficient cells. A deletion mutant of BRCA1 which inhibits p53-dependent transcription confers resistance to topoisomerase II-targeted chemotherapy. Our results suggest that BRCA1 may trigger the p53 pathway through two potentially separate mechanisms: accumulation of p53 through a direct or indirect induction of p14ARF as well as direct transcriptional coactivation of p53. BRCA1 may also enhance chemosensitivity and repair of DNA damage through binding to and coactivation of p53.

Regulation of BRCA1 by protein degradation

BRCA1, a tumor suppressor protein implicated in hereditary forms of breast and ovarian cancer, is transcriptionally regulated in a proliferation-dependent manner. In this study, we demonstrate a substantial role for proteolysis in regulating the BRCA1 steady-state protein level in several cell lines. N-acetyl-leu-leu-norleucinal (ALLN), an inhibitor of the proteasome, calpain, and cathepsins, caused BRCA1 protein to accumulate in the nucleus of several human breast, prostate, and melanoma cell lines which express low or undetectable basal levels of BRCA1 protein, but not in cells with high basal expression of BRCA1. Protease inhibition did not increase BRCA1 synthesis, nor change its mRNA level, but it dramatically prolonged the protein's half-life. In contrast to ALLN, lactacystin and PS341, two specific proteasome inhibitors, as well as calpastatin peptide and PD150606, two selective calpain inhibitors, had no effect on BRCA1 stability, whereas ALLM, an effective calpain and cathepsin inhibitor but weak proteasome inhibitor, did stimulate accumulation of BRCA1. Moreover, three inhibitors of acidic cysteine proteases, chloroquine, ammonium chloride and bafilomycin, were as effective as ALLN. These results demonstrate that degradation by a cathepsin-like protease in fine balance with BRCA1 transcription is responsible for maintaining the low steady-state level of BRCA1 protein seen in many cancer cells.

Requirement of ATM-dependent phosphorylation of brca1 in the DNA damage response to double-strand breaks

The Brca1 (breast cancer gene 1) tumor suppressor protein is phosphorylated in response to DNA damage. Results from this study indicate that the checkpoint protein kinase ATM (mutated in ataxia telangiectasia) was required for phosphorylation of Brca1 in response to ionizing radiation. ATM resides in a complex with Brca1 and phosphorylated Brca1 in vivo and in vitro in a region that contains clusters of serine-glutamine residues. Phosphorylation of this domain appears to be functionally important because a mutated Brca1 protein lacking two phosphorylation sites failed to rescue the radiation hypersensitivity of a Brca1-deficient cell line. Thus, phosphorylation of Brca1 by the checkpoint kinase ATM may be critical for proper responses to DNA double-strand breaks and may provide a molecular explanation for the role of ATM in breast cancer.

Heregulin induces phosphorylation of BRCA1 through phosphatidylinositol 3-Kinase/AKT in breast cancer cells

The breast cancer susceptibility gene BRCA1 encodes a nuclear phosphoprotein that acts as a tumor suppressor. Phosphorylation of BRCA1 has been implicated in altering its function, however, the pathway(s) that leads to the phosphorylation of BRCA1 has not been described. Here, a signaling pathway by which heregulin induces cell cycle-independent phosphorylation of BRCA1 was delineated. We showed that heregulin stimulation induced the phosphorylation of BRCA1 and concomitant activation of the serine/threonine kinase AKT in T47D human breast cancer cells. Heregulin-induced phosphorylation of BRCA1 was abrogated by phosphatidylinositol 3-kinase (PI3K) inhibitors and by a dominant-negative AKT. In the absence of heregulin, the ectopic expression of the constitutively active p110 subunit of PI3K was sufficient to induce BRCA1 phosphorylation. Furthermore, the purified glutathione S-transferase/AKT kinase phosphorylated BRCA1 in vitro. We have also shown that the phosphorylation of BRCA1 by AKT occurs on the residue Thr-509, which is located in the nuclear localization signal. These results reveal a novel signaling pathway that links extracellular signals to the phosphorylation of BRCA1 in breast cancer cells.

Ras induces p21Cip1/Waf1 cyclin kinase inhibitor transcriptionally through Sp1-binding sites

p21Cip1/Waf1 cyclin-dependent kinase inhibitor (p21) is inducible by Raf and mitogen-activated protein kinase kinase (MAPKK), but the level of regulation is unknown. We show here by conditional and transient Ras-expression models that Ras induces p21. Induction of p21 in conditionally Ras-expressing cells is posttranscriptional utilizing mitogen-activated protein kinase (MAPK) pathway. Transient, high-level Ras-expression induces transcriptional activation of p21 mediated by a GC-rich region in p21 promoter -83-54 bp relative to the transcription initiation site containing binding sites for Sp1-family transcription factors. Mutation of either Sp1-binding site 2 or 4 in this region decreases the magnitude of induction of promoter activity by Ras, but only the simultaneous mutation of both sites abolishes fully the induction. Electrophoretic mobility shift assays using an oligonucleotide corresponding to Sp1-binding site 2 indicate that both Sp1 and Sp3 transcription factors bind to this region. The results demonstrate that the central cytosolic growth regulator Ras is a potent transcriptional and posttranscriptional inducer of the nuclear growth inhibitor p21.

BRCA1 partially reverses the transforming activity of the ras oncogene

The BRCA1 gene is associated with hereditary breast and ovarian cancers. BRCA1 fits the model of a classic tumor suppressor gene, a hypothesis supported by recent work demonstrating that expression of BRCA1 inhibits growth of breast and ovarian cancer cell lines. The present study was designed to test the potential of BRCA1 to reverse the transforming activity of the ras oncogene. The v-Ha ras oncogene was cloned downstream of the retrovirus LTR and stably expressed in Rat-1 cells (Rat-1/ras). Rat-1/ras (R/R) cells were fully transformed as indicated by change in morphology, colony formation in soft-agarose and tumor induction in nude mice. BRCA1 was stably expressed in R/R cells under the CMV promoter (R/R-BRCA1). The expression of ras and BRCA1 was confirmed by Western blot using monoclonal antibodies (mAbs) specific to ras and BRCA1, respectively. R/R-BRCA1 cells grew slower than the negative control, which was R/R cells transfected with vector alone (R/R-pCEP4). R/R-BRCA1 cells generated approximately 5 to 10 times less colonies in a soft-agarose assay compared to the negative control. When injected into nude mice, R/R-BRCA1 cells exhibited a delayed onset of tumorigenesis and generated smaller tumors compared to R/R or R/R-pCEP4 cells. These data strongly suggest that BRCA1 partially reverses the transforming activity of the v-Ha ras oncogene indicating that BRCA1 can bypass the effects of the v-Ha ras oncogene on cell growth. BRCA1, therefore, may be used in therapy of tumors arising due to activation of v-Ha ras oncogene.

Transcription of BRCA1 is dependent on the formation of a specific protein-DNA complex on the minimal BRCA1 Bi-directional promoter

BRCA1 is the first tumor suppressor gene linked to hereditary breast and ovarian cancers. Its involvement in sporadic breast cancer, however, remains unclear. Recent studies showed that a loss or lowered expression of BRCA1 is not uncommon in nonfamilial breast cancers. In addition, there have been cases of inherited BRCA1-linked breast cancer with as yet unidentified mutation. Misregulation of BRCA1 at the transcription level is a possible mechanism for loss of BRCA1 expression. To understand transcriptional regulation of the BRCA1 gene, we cloned and examined the BRCA1 promoter, by both functional reporter gene analyses and protein-DNA complex formation electrophorectic mobility shift assays. A bi-directional promoter could be located within a 229-base pair (bp) intergenic region between BRCA1 and its neighboring gene, NBR2. Deletion analyses further delineated a minimal 56-bp EcoRI-HaeIII fragment, which could drive transcription in the NBR2 gene direction 2-4-fold higher than in the BRCA1 direction in all cell lines tested. Furthermore, transcriptional activity in the BRCA1 direction was undetectable in the muscle cell line C2C12, whereas activity in the NBR2 direction was maintained. These results were consistent with the expression pattern of the respective genes. A specific protein-DNA complex was detected when nuclear extracts from HeLa cells and Caco2, a colon cell line, were incubated with the 56-bp minimal promoter. This protein binding activity was further localized to an 18-bp fragment and might involve a tissue-specific factor, because binding was not detected in the C2C12 cell line. The correlation of the detection of this protein-DNA complex only in those cell lines that expressed the chloramphenicol acetyltransferase reporter gene in the BRCA1 direction suggests a significant positive role of this complex in the transcription of the BRCA1 gene.

BRCA1-associated growth arrest is RB-dependent

BRCA1 is a susceptibility gene for breast and ovarian cancer with growth-inhibitory activity for which the mechanism of action remains unclear. When introduced into cells, BRCA1 inhibits growth of some but not all cell lines. In an attempt to uncover the mechanism of growth suppression by BRCA1, we examined a panel of cell lines for their ability to reduce colony outgrowth in response to BRCA1 overexpression. Of all variables tested, only those cells with wild-type pRb were sensitive to BRCA1-induced growth suppression. In cells with an intact rb gene, inactivation of pRb by HPV E7 abrogates the growth arrest imposed by BRCA1. In accordance with these observations, we found that BRCA1 could not suppress BrdUrd uptake in primary fibroblasts from rb-/- mice and exhibited an intermediate ability to inhibit DNA synthesis in rb+/- as compared with rb+/+ cells. We further found that the BRCA1 protein complexes with the hypophosphorylated form of pRb. This binding is localized to amino acids 304-394 of BRCA1 protein and requires the ABC domain of pRb. In-frame deletion of BRCA1 fragment involved in interaction with pRb completely abolished the growth-suppressive property of BRCA1. Although it has been reported that BRCA1 interacts with p53, we find the p53 status did not affect the ability of BRCA1 to suppress colony formation. Our data suggest that the growth suppressor function of BRCA1 depends, at least in part, on Rb.

The risk of angiosarcoma following primary breast cancer

Lymphangiosarcoma of the upper extremity is a rare and aggressive tumour reported to occur following post-mastectomy lymphoedema (Stewart-Treves syndrome). Haemangiosarcoma, a related rare tumour, has occasionally been reported to occur in the breast following irradiation. We conducted a case-control study using the University of Southern California-Cancer Surveillance Program, the population-based cancer registry for Los Angeles County, to evaluate the relationship between invasive female breast cancer and subsequent upper extremity or chest lymphangiosarcoma and haemangiosarcoma together referred to as angiosarcoma. Cases were females diagnosed between 1972 and 1995 with angiosarcoma of the upper extremity (n = 20) or chest (n = 48) who were 25 years of age or older and residing in Los Angeles County when diagnosed. Other sarcomas at the same anatomic sites were also studied. Controls were females diagnosed with cancers other than sarcoma during the same time period (n = 266,444). Cases and controls were then compared with respect to history of a prior invasive epithelial breast cancer. A history of breast cancer increased the risk of upper extremity angiosarcoma by more than 59-fold (odds ratio [OR] = 59.3, 95% confidence interval [95% CI] = 21.9-152.8). A strong increase in risk after breast cancer was also observed for angiosarcoma of the chest and breast (OR = 11.6, 95% CI = 4.3-26.1) and for other sarcomas of the chest and breast (OR = 3.3, 95% CI = 1.1-1.7).

Detection of loss of heterozygosity at RAD51, RAD52, RAD54 and BRCA1 and BRCA2 loci in breast cancer: pathological correlations

Loss of heterozygosity (LOH) in loci of the 15q15.1, 12p13, 1p32, 17q21 and 13q12-13 regions may collaborate in the inactivation of RAD51, RAD52, RAD54, BRCA1, BRCA2 and possibly other genes implicated in the repair of double-stranded DNA and in DNA recombination. We investigate allelic losses in microsatellites of the RAD51, RAD52, RAD54, BRCA1 and BRCA2 regions, and their correlations with nine pathologic parameters in 127 breast carcinomas. The LOH analysis was performed by amplifying DNA by PCR, using 15 markers of the 15q15.1, 12p13.3, 1p32, 17q21 and 13q12-13 regions. LOH was found in the RAD51 region in 32% of tumours, in the RAD52 region in 16%, in RAD54 in 20% and in the BRCA1 and BRCA2 regions in 49% and 44% respectively. Significant correlations between one or more regions with concomitant LOH and pathologic parameters were observed with respect to age (P = 0.008), oestrogen receptor content (P = 0.03), progesterone receptors (P = 0.003), higher grade (P = 0.001), more advanced stage (P = 0.004) and peritumoural vessel involvement (P < 0.0001). The number of cases in which LOH was observed simultaneously in two or more regions was always higher than expected on the basis of their statistical probability, and curiously, the three patients with LOH at five regions concomitantly were under the age of 30 years. These results suggest that LOH at these regions could be related to breast cancer, and probably to a poor tumour prognosis.

Growth retardation, DNA repair defects, and lack of spermatogenesis in BRCA1-deficient mice

BRCA1 is a nuclear phosphoprotein expressed in a broad spectrum of tissues during cell division. The inheritance of a mutant BRCA1 allele dramatically increases a woman's lifetime risk for developing both breast and ovarian cancers. A number of mouse lines carrying mutations in the Brca1 gene have been generated, and mice homozygous for these mutations generally die before day 10 of embryonic development. We report here the survival of a small number of mice homozygous for mutations in both the p53 and Brca1 genes. The survival of these mice is likely due to additional unknown mutations or epigenetic effects. Analysis of the Brca1(-/-) p53(-/-) animals indicates that BRCA1 is not required for the development of most organ systems. However, these mice are growth retarded, males are infertile due to meiotic failure, and the mammary gland of the female mouse is underdeveloped. Growth deficiency due to loss of BRCA1 was more thoroughly examined in an analysis of primary fibroblast lines obtained from these animals. Like p53(-/-) fibroblasts, Brca1(-/-) p53(-/-) cells proliferate more rapidly than wild-type cells; however, a high level of cellular death in these cultures results in reduced overall growth rates in comparison to p53(-/-) fibroblasts. Brca1(-/-) p53(-/-) fibroblasts are also defective in transcription-coupled repair and display increased sensitivity to DNA-damaging agents. We show, however, that after continued culture, and perhaps accelerated by the loss of BRCA1 repair functions, populations of Brca1(-/-) p53(-/-) fibroblasts with increased growth rates can be isolated. The increased survival of BRCA1-deficient fibroblasts in the absence of p53, and with the subsequent accumulation of additional growth-promoting changes, may mimic the events that occur during malignant transformation of BRCA1-deficient epithelia.

Overexpression of kin17 protein disrupts nuclear morphology and inhibits the growth of mammalian cells

UVC or ionizing radiation of mammalian cells elicits a complex genetic response that allows recovery and cell survival. Kin17 gene, which is highly conserved among mammals, is upregulated during this response. Kin17 gene encodes a 45 kDa protein which binds to DNA and presents a limited similarity with a functional domain of the bacterial RecA protein. Kin17 protein is accumulated in the nucleus of proliferating fibroblasts and forms intranuclear foci. Using expression vectors, we show that overexpression of kin17 protein inhibits cell-cycle progression into S phase. Our results indicate that growth inhibition correlates with disruption of the nuclear morphology which seems to modify the intranuclear network required during the early steps of DNA replication. We report that a mutant encoding a protein deleted from the central domain of kin17 protein enhanced these effects whereas the deletion of the C-terminal domain considerably reduced them. These mutants will be used to elucidate the molecular mechanism by which kin17 protein alters cell growth and DNA replication.

Is TP53 dysfunction required for BRCA1-associated carcinogenesis?

The identification of the breast/ovarian susceptibility genes, BRCA1 and BRCA2 was an important advancement in the field of breast and ovarian cancer research. About 40-50% of site specific hereditary breast cancers and up to 80% of hereditary breast-ovarian cancers result from mutations in the BRCA1 gene. Although BRCA1 mediates multiple functions in the cell, including a role in DNA damage repair and gene transcription, the role of BRCA1 has not completely been elucidated yet. It has been suggested that mutational inactivation of TP53 may be required for BRCA1-associated tumorigenesis. Several studies have shown that TP53 is more frequently inactivated in BRCA1-associated tumors than in sporadic breast or ovarian cancer. Up to 90% of BRCA1-associated tumors harbor either a TP53 mutation and/or TP53 protein accumulation. The remaining tumors may well have other alterations affecting the cell cycle checkpoint. Loss of this checkpoint may be obligatory for BRCA1-tumorigenesis. In this review, we discuss recent advances in BRCA1-research and stress the pivotal role TP53 may play in BRCA1-associated carcinogenesis.

Mutation analysis of BRCA1, TP53, and KRAS2 in ovarian and related pelvic tumors

Cancer may be viewed as a genetic disease resulting from critical mutations that disrupt normal cell growth. To characterize the involvement of the BRCA1 and TP53 tumor suppressor genes and of the KRAS2 protooncogene in gynecologic cancer, mutation analysis of these genes was conducted in pelvic tumors of 85 patients that included 49 epithelial ovarian carcinoma cases. The 85 pelvic tumors contained 5 tumors with BRCA1 mutations, 33 with TP53 mutations, and 1 with a KRAS2 mutation. Each of the BRCA1 and KRAS2 mutations, and 25 of the TP53 mutations, were in ovarian carcinomas. Four of the BRCA1 mutations were germline and 1 was somatic. The 4 patients with germline BRCA1 mutations had an early age of disease onset (33-48 years) relative to the mean age of onset (58 years) of all 49 ovarian carcinoma patients, and 3 of these 4 patients had a family history of ovarian or breast cancer. None of the 4 tumors with germline BRCA1 mutations had a KRAS2 mutation or a TP53 mutation, despite a 51% frequency of TP53 mutations in the 49 ovarian carcinomas. Three of the 4 tumors with germline BRCA1 mutations retained a wild-type BRCA1 allele. The tumor with the somatic BRCA1 mutation contained a TP53 mutation and had no evidence for wild-type BRCA1 and TP53 alleles. These data suggest that both BRCA1 and TP53 were inactivated in 1 of 49 ovarian carcinomas. Moreover, mutational inactivation of both BRCA1 and TP53 did not occur in 4 tumors with a germline BRCA1 mutation. It has been proposed that tumorigenesis in cells with a heterozygous BRCA1 mutation requires inactivation of the wild-type BRCA1 and TP53 alleles, which results in genomic instability and acquisition of mutations in protooncogenes. Clearly, mutational inactivation of TP53 and the wild-type BRCA1 allele in ovarian tumors with a heterozygous, germline BRCA1 mutation is not an absolute requirement for tumor formation. It is possible that these alleles may be inactivated by nonmutational mechanisms or that other tumor formation pathways exist.

Casein kinase 2 binds to and phosphorylates BRCA1

The BRCA1 gene encodes a complex protein that appears to be involved in some aspects of DNA repair, transcription, or cell cycle regulation. The phosphorylation of BRCA1 is enhanced following episodes of DNA damage or during cell cycle progression, indicating that phosphorylation may be an important regulatory mechanism. Through a yeast two hybrid assay, we found that the beta-subunit of casein kinase 2 (CK2) associated with a carboxy-terminal region of BRCA1. This association was much weaker with the same fragment bearing a missense mutation (M1775R) that has been identified in breast tumors. The interaction was also evident in Sf9 cells. Subsequent studies showed that BRCA1 was phosphorylated in vitro by CK2. An analysis by site directed mutagenesis of BRCA1 showed that in vitro phosphorylation by CK2 required a serine at aa1572. These data implicate CK2 as a potential mediator of BRCA1 activity.

BRCA1 is phosphorylated at serine 1497 in vivo at a cyclin-dependent kinase 2 phosphorylation site

BRCA1 is a cell cycle-regulated nuclear protein that is phosphorylated mainly on serine and to a lesser extent on threonine residues. Changes in phosphorylation occur in response to cell cycle progression and DNA damage. Specifically, BRCA1 undergoes hyperphosphorylation during late G1 and S phases of the cell cycle. Here we report that BRCA1 is phosphorylated in vivo at serine 1497 (S1497), which is part of a cyclin-dependent kinase (CDK) consensus site. S1497 can be phosphorylated in vitro by CDK2-cyclin A or E. BRCA1 coimmunoprecipitates with an endogenous serine-threonine protein kinase activity that phosphorylates S1497 in vitro. This cellular kinase activity is sensitive to transfection of a dominant negative form of CDK2 as well as the application of the CDK inhibitors p21 and butyrolactone I but not p16. Furthermore, BRCA1 coimmunoprecipitates with CDK2 and cyclin A. These results suggest that the endogenous kinase activity is composed of CDK2-cyclin complexes, at least in part, concordant with the G1/S-specific increase in BRCA1 phosphorylation.

BRCA1 Gene Mutations in Sporadic Ovarian Carcinomas: Detection by PCR and Reverse Allele-specific Oligonucleotide Hybridization

Background: Although germline mutations in BRCA1 play a central role in familial breast and ovarian cancers, to date, no somatic mutations in BRCA1 have been reported in sporadic breast cancer, and only five somatic mutations have been identified in the sporadic ovarian carcinomas. Because loss of heterozygosity appears frequently at the BRCA1 locus in nonfamilial breast and ovarian carcinomas, we searched for mutations in the BRCA1 gene in sporadic ovarian tumors.

Methods: We developed a detection system based on PCR and reverse allele-specific oligonucleotide hybridization on membrane strips for the simultaneous detection of 17 frequently occurring mutations in the BRCA1 gene.

Results: As little as 2% mutant DNA in a sample could be detected. Two of 122 DNA samples isolated from sporadic ovarian tumor biopsies contained the Cys61Gly mutation. Both mutations were germline mutations. One of these was an ovarian metastasis of a primary fallopian tube carcinoma. The tubal carcinoma was also confirmed to contain the Cys61Gly mutation.

Conclusions: This is the first report that a germline BRCA1 mutation is associated with primary tubal carcinoma. The 17 specific mutations in the BRCA1 gene do not play a major role in the tumorigenesis and progression of sporadic ovarian cancer.

BRCA1 expression restores radiation resistance in BRCA1-defective cancer cells through enhancement of transcription-coupled DNA repair

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 double-stranded 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.

Coordinate alterations in the expression of BRCA1, BRCA2, p300, and Rad51 in response to genotoxic and other stresses in human prostate cancer cells

BACKGROUND

BRCA1 and BRCA2 participate in cell cycle progression, apoptosis, and DNA repair pathways. The latter role may be mediated by interaction with DNA recombinase Rad51. The purpose of this study was to evaluate the effects of genotoxic and other cytotoxic agents on expression of DNA damage-response genes (BRCA1, BRCA2, p300, and Rad51) in human prostate cancer cells.

METHODS

Subconfluent proliferating cultures of Tsu-Prl or DU-145 cells were treated with various stressful agents and assayed 24 hr later for alterations in: 1) mRNA expression (by semiquantitative reverse transcription-PCR); 2) cell viability (by trypan blue dye exclusion); and 3) protein expression (by Western blotting).

RESULTS

Of 26 agents screened, BRCA1 and BRCA2 mRNA reductions were observed in both cell lines after exposure to adriamycin (ADR), camptothecin (CPT), sodium selenite (SLN), and ultraviolet radiation (UV), while nitrogen mustard (HN2) caused mRNA reduction in DU-145 but not in Tsu-Prl. Inhibition of BRCA1/2 expression by ADR and HN2 was blocked by cycloheximide, suggesting that this requires new protein synthesis, while inhibition by CPT, SLN, and UV did not require protein synthesis. Reduction of p300 and Rad51 mRNA levels occurred in parallel with that of BRCA1/2, suggesting coordinate regulation of these genes. The ability of an agent to inhibit mRNA expression was not directly correlated with cytotoxicity. ADR, CPT, UV, and SLN also caused reduction of protein levels; but the kinetics of decreases in protein vs. mRNA differed. After ADR treatment, high molecular weight (Mr hyperphosphorylated) BRCA1 decreased more rapidly than the low Mr species. BRCA2 showed a more rapid decrease in protein than mRNA, while Rad51 showed the opposite. By 48 and 72 hr post-ADR, all four mRNAs and proteins were reduced to well below control levels, except for Rad51 protein, which was only moderately decreased.

CONCLUSIONS

Selected DNA-damaging agents (ADR, CPT, and UV) and a reducing agent (SLN) inhibited BRCA1/2, p300, and Rad51 expression in prostate cancer cells, although decreases in mRNA vs. protein did not coincide. We postulate that temporal changes in relative protein levels affect different phases of the stress response, and that the ultimate downregulation of all four genes promotes prostate cancer survival.

Induction of GADD45 and JNK/SAPK-dependent apoptosis following inducible expression of BRCA1

The breast cancer susceptibility gene BRCA1 encodes a protein implicated in the cellular response to DNA damage, with postulated roles in homologous recombination as well as transcriptional regulation. To identify downstream target genes, we established cell lines with tightly regulated inducible expression of BRCA1. High-density oligonucleotide arrays were used to analyze gene expression profiles at various times following BRCA1 induction. A major BRCA1 target is the DNA damage-responsive gene GADD45. Induction of BRCA1 triggers apoptosis through activation of c-Jun N-terminal kinase/stress-activated protein kinase (JNK/SAPK), a signaling pathway potentially linked to GADD45 gene family members. The p53-independent induction of GADD45 by BRCA1 and its activation of JNK/SAPK suggest a pathway for BRCA1-induced apoptosis.

BRCA1 inhibition of estrogen receptor signaling in transfected cells

Mutations of the breast cancer susceptibility gene BRCA1 confer increased risk for breast, ovarian, and prostatic cancers, but it is not clear why the mutations are associated with these particular tumor types. In transient transfection assays, BRCA1 was found to inhibit signaling by the ligand-activated estrogen receptor (ER-alpha) through the estrogen-responsive enhancer element and to block the transcriptional activation function AF-2 of ER-alpha. These results raise the possibility that wild-type BRCA1 suppresses estrogen-dependent transcriptional pathways related to mammary epithelial cell proliferation and that loss of this ability contributes to tumorigenesis.