Association of BRCA1 with the hRad50-hMre11-p95 complex and the DNA damage response

Novel NBS1 heterozygous germ line mutation causing MRE11-binding domain loss predisposes to common types of cancer

DNA damage response (DDR) pathways maintain genomic stability. A 657del5 mutation of NBS1, a key DDR component, causing the rare cancer-predisposing Nijmegen breakage syndrome has been reported nearly exclusively in Slavic populations. In this study, we describe the first identification in a Japanese population of an unprecedented type of heterozygous NBS1 mutant, termed IVS11+2insT, lacking the MRE11- and ATM-binding site at the COOH terminus. Profoundly defective in crucial binding to MRE11, MDC1, BRCA1, and wild-type NBS1, the mutant caused impaired ATM phosphorylation in response to low-dose irradiation in a heterozygous state. Importantly, whereas IVS11+2insT was found in only 2 (0.09%) of 2,348 control subjects, it was identified in 2% (2 of 96) of heterozygotes with gastric cancer, 0.8% (3 of 376) of those with colorectal cancer, and 0.4% (2 of 532) of those with lung cancer, which were comparable to frequencies reported for other DDR-related genes known to confer cancer susceptibility. The presence of the heterozygous IVS11+2insT mutation seemed to be associated with an increased risk for gastrointestinal cancers, with an odds ratio of 12.6 and 95% confidence interval (95% CI) of 2.05 to 132.1 (P = 0.0001). The odds ratios separately calculated for gastric and colorectal cancers were 25.0 (95% CI, 1.78-346.0) and 9.43 (95% CI, 1.08-113.1), respectively. These findings suggest that IVS11+2insT is associated with an increased risk for the development of certain types of common cancers, warranting future investigation including detailed phenotypic characterization of age of onset and penetrance in heterozygotes, as well as screening in other ethnic groups.

Characterization of breast cancer by array comparative genomic hybridization

Cancer progression is due to the accumulation of recurrent genomic alterations that induce growth advantage and clonal expansion. Most of these genomic changes can be detected using the array comparative genomic hybridization (CGH) technique. The accurate classification of these genomic alterations is expected to have an important impact on translational and basic research. Here we review recent advances in CGH technology used in the characterization of different features of breast cancer. First, we present bioinformatics methods that have been developed for the analysis of CGH arrays; next, we discuss the use of array CGH technology to classify tumor stages and to identify and stratify subgroups of patients with different prognoses and clinical behaviors. We finish our review with a discussion of how CGH arrays are being used to identify oncogenes, tumor suppressor genes, and breast cancer susceptibility genes.

Molecular biosensing system based on intrinsically disordered proteins

Intrinsically disordered proteins (IDPs) that undergo structural transition upon binding their target molecules are becoming increasingly known. IDPs, because of their binding specificity and induced folding properties, can serve as biological recognition elements for sensing applications. In this paper, BRCA1, an IDP, was utilized as the biological recognition element to detect tumor suppressor protein p53 through the BRCA1/p53 binding interaction to serve as a proof-of-concept for the use of IDPs as recognition elements. The binding resulted in a disordered-to-ordered BRCA1 conformational change, as seen in our circular dichroism (CD) measurements. This conformational change in BRCA1 (residues 219-498) was utilized in the detection of p53 (residues 311-393) via both intrinsic and extrinsic fluorescent probes. Intrinsic tryptophan residues within the BRCA1 sequence detected p53 (311-393) with a detection limit of 0.559 nM (0.112 pmol). Two environmentally sensitive fluorophores, tetramethylrhodamine-5-maleimide (TMR) and 6-((5-dimethylaminonaphthalene-1-sulfonyl)amino)hexanoic acid, succinimidyl ester (dansyl-X, SE) were conjugated to BRCA1 (219-498). Dansyl-X, SE-conjugated BRCA1 (219-498) detected p53 (311-393) with a detection limit of 1.50 nM (0.300 pmol). The sensitivities for TMR and dansyl-X, SE-conjugated BRCA1 for the detection of p53 were nearly threefold and twofold higher, respectively, than the sensitivity reported using intrinsic BRCA1 tryptophan fluorescence. CD measurements did not reveal a disruption of p53/dye-conjugated BRCA1 binding, thus validating the applicability of environmentally sensitive fluorophores as transduction moieties to detect molecules which bind to IDPs and induce a structural change.

The ubiquitin E3 ligase activity of BRCA1 and its biological functions

The basal-like breast cancer, a new category of breast cancer associated with poor prognosis and possibly unique chemosensitivity, is a current topic in the breast cancer field. Evidence from multiple sources strongly indicate that impairment of BRCA1 pathways is responsible for this phenotype, implying the importance of BRCA1 not only in familial breast cancers but also in sporadic cancers. BRCA1 acts as a hub protein that coordinates a diverse range of cellular pathways to maintain genomic stability. BRCA1 participates in multiple cellular supercomplexes to execute its tasks and, in most of the complexes, BRCA1 exists as a RING heterodimer with BARD1 to provide ubiquitin E3 ligase activity that is required for its tumor suppressor function. It was revealed recently that the BRCA1 RING finger is capable of catalyzing multiple types of ubiquitination depending upon the interacting E2, the ubiquitin carrier protein. BRCA1 may catalyze distinct ubiquitination on different substrates as the situation demands. On the other hand, in response to DNA double-strand breaks where BRCA1 plays its major role for homologous recombination repair, recent evidence showed that ubiquitination is a critical step to recruit BRCA1 to the damaged site through UIM (ubiquitin interacting motif) containing protein RAP80. Thus, ubiquitin and BRCA1 likely affect each other in many ways to perform cellular functions. Elucidation of this mechanism in relation to cell survival is now much anticipated because it could be a key to predict chemosensitivity of basal-like breast cancer.

DNA strand break repair and human genetic disease

Each day tens of thousands of DNA single-strand breaks (SSBs) arise in every cell from the attack of deoxyribose and DNA bases by reactive oxygen species and other electrophilic molecules. DNA double-strand breaks (DSBs) also arise, albeit at a much lower frequency, from similar attacks and from the encounter of unrepaired SSBs and possibly other DNA structures by DNA replication forks. DSBs are also created during normal development of the immune system. Defects in the cellular response to DNA strand breaks underpin many human diseases, including disorders associated with cancer predisposition, immune dysfunction, radiosensitivity, and neurodegeneration. Here we provide an overview of the genetic diseases associated with defects in the repair/response to DNA strand breaks.

Brca1 in immunoglobulin gene conversion and somatic hypermutation

Defects in Brca1 confer susceptibility to breast cancer and genomic instability indicative of aberrant repair of DNA breaks. Brca1 was previously implicated in the homologous recombination pathway via effects on the assembly of recombinase Rad51. Activation-induced cytidine deaminase (AID) deaminates C to U in B lymphocyte immunoglobulin (Ig) DNA to initiate programmed DNA breaks. Subsequent uracil-glycosylase mediated U removal, and perhaps further processing, leads to four known classes of mutation: Ig class switch recombination that results in a region-specific genomic deletion, Ig somatic hypermutation that introduces point mutations in Ig V-regions, Ig gene conversion in vertebrates that possess Ig pseudo-V genes, and translocations common to B cell lymphomas. We tested the involvement of Brca1 in AID-dependent Ig diversification in chicken DT40 cells. The DT40 cell line diversifies IgVlambda mainly by gene conversion, and less so by point mutation. Brca1-deficiency caused a shift in Vlambda diversification, significantly reducing the proportion of gene conversions relative to point mutations. Thus, Brca1 regulates AID-dependent DNA lesion repair. Interestingly, while Brca1 is required to recruit ubiquitinated FancD2 to DNA damage, the phenotype of Brca1-deficient DT40 differs from the one of FancD2-deficient DT40, in which both gene conversion and non-templated mutations are impaired.

Negative selection on BRCA1 susceptibility alleles sheds light on the population genetics of late-onset diseases and aging theory

The magnitude of negative selection on alleles involved in age-specific mortality decreases with age. This is the foundation of the evolutionary theory of senescence. Because of this decrease in negative selection with age, and because of the absence of reproduction after menopause, alleles involved in women's late-onset diseases are generally considered evolutionarily neutral. Recently, genetic and epidemiological data on alleles involved in late onset-diseases have become available. It is therefore timely to estimate selection on these alleles. Here, we estimate selection on BRCA1 alleles leading to susceptibility to late-onset breast and ovarian cancer. For this, we integrate estimates of the risk of developing a cancer for BRCA1-carriers into population genetics frameworks, and calculate selection coefficients on BRCA1 alleles for different demographic scenarios varying across the extent of human demography. We then explore the magnitude of negative selection on alleles leading to a diverse range of risk patterns, to capture a variety of late-onset diseases. We show that BRCA1 alleles may have been under significant negative selection during human history. Although the mean age of onset of the disease is long after menopause, variance in age of onset means that there are always enough cases occurring before the end of reproductive life to compromise the selective value of women carrying a susceptibility allele in BRCA1. This seems to be the case for an extended range of risk of onset functions varying both in mean and variance. This finding may explain the distribution of BRCA1 alleles' frequency, and also why alleles for many late-onset diseases, like certain familial forms of cancer, coronary artery diseases and Alzheimer dementia, are typically recent and rare. Finally, we discuss why the two most popular evolutionary theories of aging, mutation accumulation and antagonistic pleiotropy, may underestimate the effect of selection on survival at old ages.

Clinically relevant biology of hereditary breast cancer

The identification of the two major breast cancer susceptibility genes BRCA1 and BRCA2 has led to an explosion of work covering the biology and epidemiology of these genes. Clinically, much is known about the risks associated with mutations in these genes, but less is certain with regards to their impact on treatment. This review will touch upon several important aspects of the biology of BRCA1 and BRCA2, but comments regarding these areas will be restricted to those most relevant to the clinician. The three main sections are molecular pathology, DNA repair, and finally other functions of the BRCA1 and BRCA2 proteins.

Activation of BRCA1/BRCA2-associated helicase BACH1 is required for timely progression through S phase

BACH1 (also known as FANCJ and BRIP1) is a DNA helicase that directly interacts with the C-terminal BRCT repeat of the breast cancer susceptibility protein BRCA1. Previous biochemical and functional analyses have suggested a role for the BACH1 homolog in Caenorhabditis elegans during DNA replication. Here, we report the association of BACH1 with a distinct BRCA1/BRCA2-containing complex during the S phase of the cell cycle. Depletion of BACH1 or BRCA1 using small interfering RNAs results in delayed entry into the S phase of the cell cycle. Such timely progression through S phase requires the helicase activity of BACH1. Importantly, cells expressing a dominant negative mutation in BACH1 that results in a defective helicase displayed increased activation of DNA damage checkpoints and genomic instability. BACH1 helicase is silenced during the G(1) phase of the cell cycle and is activated through a dephosphorylation event as cells enter S phase. These results point to a critical role for BACH1 helicase activity not only in the timely progression through the S phase but also in maintaining genomic stability.

The interaction of PP1 with BRCA1 and analysis of their expression in breast tumors

Background

The breast cancer susceptibility gene, BRCA1, is implicated in multiple cellular processes including DNA repair, the transactivation of genes, and the ubiquitination of proteins; however its precise functions remain to be fully understood. Identification and characterization of BRCA1 protein interactions may help to further elucidate the function and regulation of BRCA1. Additionally, detection of changes in the expression levels of BRCA1 and its interacting proteins in primary human breast tumors may further illuminate their role in the development of breast cancer.

Methods

We performed a yeast two-hybrid study to identify proteins that interact with exon11 of BRCA1 and identified Protein Phosphatase 1β (PP1β), an isoform of the serine threonine phosphatase, PP1. GST-pull down and co-immunoprecipitation assays were performed to further characterize this interaction. Additionally, Real-Time PCR was utilized to determine the expression of BRCA1, PP1α, β and γ in primary human breast tumors and normal breast tissue to identify alterations in the expression of these genes in breast cancer.

Results

PP1 and BRCA1 co-immunoprecipitate and the region within BRCA1 as well as the specific PP1 interacting domain mediating this interaction were identified. Following mRNA expression analysis, we identified low levels of BRCA1 and variable levels of PP1α and β in primary sporadic human breast tumors. Furthermore, BRCA1, PP1β and PP1γ were significantly higher in normal tissue specimens (BRCA1 p = 0.01, PP1β: p = 0.03, PP1γ, p = 1.9 × 10^-6) compared to sporadic breast tumor samples. Interestingly, we also identified that ER negative tumors are associated with low levels of PP1α expression.

Conclusion

The identification and characterization of the interaction of BRCA1 with PP1 and detection of changes in the expression of PP1 and genes encoding other BRCA1 associated proteins identifies important genetic pathways that may be significant to breast tumorigenesis. Alterations in the expression of genes, particularly phosphatases that operate in association with BRCA1, could negatively affect the function of BRCA1 or BRCA1 associated proteins, contributing to the development of breast cancer.

Identification of sequences that target BRCA1 to nuclear foci following alkylative DNA damage

BRCA1 is a tumor suppressor involved in the maintenance of genome integrity. BRCA1 co-localizes with DNA repair proteins at nuclear foci in response to DNA double-strand breaks caused by ionizing radiation (IR). The response of BRCA1 to agents that elicit DNA single-strand breaks (SSB) is poorly defined. In this study, we compared chemicals that induce SSB repair and observed the most striking nuclear redistribution of BRCA1 following treatment with the alkylating agent methyl methanethiosulfonate (MMTS). In MCF-7 breast cancer cells, MMTS induced movement of endogenous BRCA1 into distinctive nuclear foci that co-stained with the SSB repair protein XRCC1, but not the DSB repair protein gamma-H2AX. XRCC1 did not accumulate in foci after ionizing radiation. Moreover, we showed by deletion mapping that different sequences target BRCA1 to nuclear foci induced by MMTS or by ionizing radiation. We identified two core MMTS-responsive sequences in BRCA1: the N-terminal BARD1-binding domain (aa1-304) and the C-terminal sequence aa1078-1312. These sequences individually are ineffective, but together they facilitated BRCA1 localization at MMTS-induced foci. Site-directed mutagenesis of two SQ/TQ motif serines (S1143A and S1280A) in the BRCA1 fusion protein reduced, but did not abolish, targeting to MMTS-inducible foci. This is the first report to describe co-localization of BRCA1 with XRCC1 at SSB repair foci. Our results indicate that BRCA1 requires BARD1 for targeting to different types of DNA lesion, and that distinct C-terminal sequences mediate selective recruitment to sites of double- or single-strand DNA damage.

Brca1 heterozygous mice have shortened life span and are prone to ovarian tumorigenesis with haploinsufficiency upon ionizing irradiation

BRCA1 mutation carriers have an 85% lifetime risk of breast cancer and 60% for ovarian cancer. BRCA1 facilitates DNA double-strand break repair, and dysfunction of BRCA1 leads to hypersensitivity to DNA damaging agents and consequently genomic instability of cells. In this communication, we have examined the tumor incidence and survival of Brca1 heterozygous female mice. Brca1 heterozygotes appear to have a shortened life span with 70% tumor incidence. Lymphoma, but not ovarian and mammary gland tumors, occurs commonly in these mice. After a whole-body exposure to ionizing radiation, Brca1 heterozygous mice have a 3-5-fold higher incidence specific to ovarian tumors, but not lymphoma, when compared with the Brca1+/+ mice. All the tumors from heterozygous mice examined retain the wild-type allele and the cancer cells express Brca1 protein, precluding the chromosomal mechanism for loss of heterozygosity of Brca1 locus. Although the manifestation of BRCA1 haploinsufficiency may be different between human and mouse, this study suggests that women carrying Brca1 mutations may be more prone to ovarian tumor formation after IR exposure than nonmutation carriers.

BRCA1 contributes to cell cycle arrest and chemoresistance in response to the anticancer agent irofulven

Tumor suppressor gene BRCA1 is frequently mutated in familial breast and ovarian cancer. BRCA1 plays pivotal roles in maintaining genomic stability by interacting with numerous proteins in cell cycle control and DNA repair. Irofulven (6-hydroxymethylacylfulvene, HMAF, MGI 114, NSC 683863) is one of a new class of anticancer agents that are analogs of mushroom-derived illudin toxins. Preclinical studies and clinical trials have demonstrated that irofulven is effective against several tumor cell types. The exact nature of irofulven-induced DNA damage is not completely understood. We demonstrated previously that irofulven activates ATM and its targets, NBS1, SMC1, CHK2, and p53. In this study, we hypothesize that irofulven induces DNA double-strand breaks and that BRCA1 may affect chemosensitivity by controlling cell cycle checkpoints, DNA repair, and genomic stability in response to irofulven treatment. We observed that irofulven induces the formation of chromosome breaks and radials and the activation and foci formation of gamma-H2AX, BRCA1, and RAD51. We also provided evidence that irofulven induces the generation of DNA double-strand breaks. By using BRCA1-deficient or -proficient cells, we demonstrated that in response to irofulven, BRCA1 contributes to the control of S and G(2)/M cell cycle arrest and is critical for repairing DNA double-strand breaks and for RAD51-dependent homologous recombination. Furthermore, we found that BRCA1 deficiency results in increased chromosome damage and chemosensitivity after irofulven treatment.

BRCA1 ubiquitinates RPB8 in response to DNA damage

The breast and ovarian tumor suppressor BRCA1 catalyzes untraditional polyubiquitin chains that could be a signal for processes other than proteolysis. However, despite intense investigations, the mechanisms regulated by the enzyme activity remain only partially understood. Here, we report that BRCA1-BARD1 mediates polyubiquitination of RPB8, a common subunit of RNA polymerases, in response to DNA damage. A proteomics screen identified RPB8 as a protein modified after epirubicin treatment in BRCA1-dependent manner. RPB8 interacted with BRCA1-BARD1 and was polyubiquitinated by BRCA1-BARD1 in vivo and in vitro. BRCA1-BARD1 did not destabilize RPB8 in vivo but rather caused an increase in the amount of soluble RPB8. Importantly, RPB8 was polyubiquitinated immediately after UV irradiation in a manner sensitive to BRCA1 knockdown by RNA interference. Substitution of five lysine residues of RPB8 with arginine residues abolished its ability to be ubiquitinated while preserving its polymerase activity. HeLa cell lines stably expressing this ubiquitin-resistant form of RPB8 exhibited UV hypersensitivity accompanied by up-regulated caspase activity. Our findings suggest that ubiquitination of a common subunit of RNA polymerases is a mechanism underlying BRCA1-dependent cell survival after DNA damage.

BRCA1 , a Potential Predictive Biomarker in the Treatment of Breast Cancer

To date, estrogen receptor, progestogen receptor, and HER2/neu represent molecular biomarkers currently used in routine clinical practice to aid treatment decisions. Over the last few years, a large body of preclinical and retrospective clinical data has accumulated that suggests that BRCA1 mutation functions as a novel predictive marker of response to chemotherapy. This article reviews the role of BRCA1 as a predictive marker of chemotherapy response in breast cancer and examines the link between BRCA1 deficiency and the basal-like phenotype. Search strategy. Data for this article were identified through MEDLINE and PubMed searches for published reports using the terms BRCA1, breast cancer, basal-like, chemotherapy, prognosis, and predictive markers. In some cases, due to the restriction of space, readers are referred to review articles to allow further reading. Only articles published in English were included.

Genetic instability in the RAD51 and BRCA1 regions in breast cancer

Breast cancer is the most prevalent cancer type in women. Accumulating evidence indicates that the fidelity of double-strand break repair in response to DNA damage is an important step in mammary neoplasias. The RAD51 and BRCA1 proteins are involved in the repair of double-strand DNA breaks by homologous recombination. In this study, we evaluated loss of heterozygosity (LOH) in the RAD51 and BRCA1 regions, and their association with breast cancer. The polymorphic markers D15S118, D15S214 and D15S1006 were the focus for RAD51, and D17S855 and D17S1323 for BRCA1. Genomic deletion detected by allelic loss varied according to the regions tested, and ranged from 29 to 46% of informative cases for the RAD51 region and from 38 to 42% of informative cases for the BRCA1 region. 25% of breast cancer cases displayed LOH for at least one studied marker in the RAD51 region exclusively. On the other hand, 31% of breast cancer cases manifested LOH for at least one microsatellite marker concomitantly in the RAD51 and BRCA1 regions. LOH in the RAD51 region, similarly as in the BRCA1 region, appeared to correlate with steroid receptor status. The obtained results indicate that alteration in the RAD51 region may contribute to the disturbances of DNA repair involving RAD51 and BRCA1 and thus enhance the risk of breast cancer development.

E2F6 negatively regulates ultraviolet-induced apoptosis via modulation of BRCA1

E2F6 is believed to repress E2F-responsive genes and therefore plays an important role in cell-cycle regulation. However, the role of E2F6 in the control of apoptosis remains unknown. We show here that the expression of E2F6 was downregulated with a concurrent increase in BRCA1 mRNA and cleaved protein during ultraviolet (UV)-induced apoptosis in human embryonic kidney 293 cells. Moreover, E2F6 overexpression distinctly inhibited UV-induced apoptosis as well as UV-induced increases in BRCA1 expression and cleavage, accompanied with increases of the full-length BRCA1 and BRCA1 nuclear foci. In contrast, knockdown of E2F6 by small interfering RNA had opposite effects. Furthermore, these effects of E2F6 on BRCA1 depended upon the association of E2F6 with BRCA1 via its C-terminus in a UV-triggered manner and upon the transcriptional repression by E2F6 on the BRCA1 promoter. These findings provide the first demonstration of the important role for E2F6 in the control of apoptosis via targeting of BRCA1.

BRCA1 at the crossroad of multiple cellular pathways: approaches for therapeutic interventions

Approximately 10% of the cases of breast cancer and invasive ovarian cancer are hereditary, occurring predominantly in women with germ-line mutations in the BRCA1 or BRCA2 genes. Low expression of these genes in sporadic tumors extends their significance to sporadic breast and ovarian cancers as well. For over a decade since its identification, extensive research has been directed toward understanding the function of the breast and ovarian tumor suppressor gene BRCA1. The long-term goal has been to identify the biochemical pathways reliant on BRCA1 that can be exploited for developing targeted therapies and benefit mutation carriers. To date, no one specific role has been identified, but rather it is clear that BRCA1 has significant roles in multiple fundamental cellular processes, including control of gene expression, chromatin remodeling, DNA repair, cell cycle checkpoint control, and ubiquitination, and overall is important for maintenance of genomic stability. Major findings and potential BRCA1-dependent therapies will be discussed.

Contribution of germline BRCA1 and BRCA2 sequence alterations to breast cancer in Northern India

Background

A large number of distinct mutations in the BRCA1 and BRCA2 genes have been reported worldwide, but little is known regarding the role of these inherited susceptibility genes in breast cancer risk among Indian women. We investigated the distribution and the nature of BRCA1 and BRCA2 germline mutations and polymorphisms in a cohort of 204 Indian breast cancer patients and 140 age-matched controls.

Method

Cases were selected with regard to early onset disease (≤40 years) and family history of breast and ovarian cancer. Two hundred four breast cancer cases along with 140 age-matched controls were analyzed for mutations. All coding regions and exon-intron boundaries of the BRCA1 and BRCA2 genes were screened by heteroduplex analysis followed by direct sequencing of detected variants.

Results

In total, 18 genetic alterations were identified. Three deleterious frame-shift mutations (185delAG in exon 2; 4184del4 and 3596del4 in exon 11) were identified in BRCA1, along with one missense mutation (K1667R), one 5'UTR alteration (22C>G), three intronic variants (IVS10-12delG, IVS13+2T>C, IVS7+38T>C) and one silent substitution (5154C>T). Similarly three pathogenic protein-truncating mutations (6376insAA in exon 11, 8576insC in exon19, and 9999delA in exon 27) along with one missense mutation (A2951T), four intronic alterations (IVS2+90T>A, IVS7+75A>T, IVS8+56C>T, IVS25+58insG) and one silent substitution (1593A>G) were identified in BRCA2. Four previously reported polymorphisms (K1183R, S1613G, and M1652I in BRCA1, and 7470A>G in BRCA2) were detected in both controls and breast cancer patients. Rare BRCA1/2 sequence alterations were observed in 15 out of 105 (14.2%) early-onset cases without family history and 11.7% (4/34) breast cancer cases with family history. Of these, six were pathogenic protein truncating mutations. In addition, several variants of uncertain clinical significance were identified. Among these are two missense variants, one alteration of a consensus splice donor sequence, and a variant that potentially disrupts translational initiation.

Conclusion

BRCA1 and BRCA2 mutations appear to account for a lower proportion of breast cancer patients at increased risk of harboring such mutations in Northern India (6/204, 2.9%) than has been reported in other populations. However, given the limited extent of reported family history among these patients, the observed mutation frequency is not dissimilar from that reported in other cohorts of early onset breast cancer patients. Several of the identified mutations are unique and novel to Indian patients.

The roles of BRCA1 and BRCA2 and associated proteins in the maintenance of genomic stability

The BRCA1 and BRCA2 proteins are important in maintaining genomic stability by promoting efficient and precise repair of double-strand breaks. The main role of BRCA2 appears to involve regulating the function of RAD51 in the repair by homologous recombination. BRCA1 has a broader role upstream of BRCA2, participating in various cellular processes in response to DNA damage. The DNA repair defect associated with mutations in BRCA1 or BRCA2 could be exploited to develop new targeted therapeutic approaches for cancer occurring in mutation carriers.

BRCA1 suppresses osteopontin-mediated breast cancer

BRCA1 is a well described breast cancer susceptibility gene thought to be involved primarily in DNA repair. However, mutation within the BRCA1 transcriptional domain is also implicated in neoplastic transformation of mammary epithelium, but responsible mechanisms are unclear. Here we show in a rat mammary model system that wild type (WT) BRCA1 specifically represses the expression of osteopontin (OPN), a multifunctional estrogen-responsive gene implicated in oncogenic transformation, particularly that of the breast. WT.BRCA1 selectively binds OPN-activating transcription factors estrogen receptor alpha, AP-1, and PEA3, inhibits OPN promoter transactivation, and suppresses OPN mRNA and protein both from an endogenous gene and a relevant model inducible gene. WT.BRCA1 also inhibits OPN-mediated neoplastic transformation characterized by morphology change, anchorage-independent growth, adhesion to fibronectin, and invasion through Matrigel. A mutant BRCA1 allele (Mut.BRCA1) associated with familial breast cancer lacks OPN suppressor effects, binds to WT.BRCA1, and impedes WT.BRCA1 suppression of OPN. Stable transfection of rat breast tumor cell lines with Mut.BRCA1 dramatically up-regulates OPN protein and induces anchorage independent growth. In human primary breast cancer, BRCA1 mutation is significantly associated with OPN overexpression. Taken together, these data suggest that BRCA1 mutation may confer increased tissue-specific cancer risk, in part by disruption of BRCA1 suppression of OPN gene transcription.

Exploiting the DNA repair defect in BRCA mutant cells in the design of new therapeutic strategies for cancer

Individuals harboring germ-line mutations in the BRCA1 or BRCA2 genes are at highly elevated risk of a variety of cancers. Ten years of research has revealed roles for BRCA1 and BRCA2 in a wide variety of cellular processes. However, it seems likely that the function of these proteins in DNA repair is critically important in maintaining genome stability. Despite this increasing knowledge of the defects present in BRCA-deficient cells, BRCA mutation carriers developing cancer are still treated similarly to sporadic cases. Here we describe our efforts, based on understanding the DNA repair defects in BRCAdeficient cells, to define the optimal existing treatment for cancers arising in BRCA mutation carriers and, additionally, the development of novel therapeutic approaches. Finally, we discuss how therapies developed to treat BRCA mutant tumors might be applied to some sporadic cancers sharing similar specific defects in DNA repair.

DNA repair pathways in clinical practice: lessons from pediatric cancer susceptibility syndromes

Human cancers exhibit genomic instability and an increased mutation rate due to underlying defects in DNA repair. Cancer cells are often defective in one of six major DNA repair pathways, namely: mismatch repair, base excision repair, nucleotide excision repair, homologous recombination, nonhomologous endjoining and translesion synthesis. The specific DNA repair pathway affected is predictive of the kinds of mutations, the tumor drug sensitivity, and the treatment outcome. The study of rare inherited DNA repair disorders, such as Fanconi anemia, has yielded new insights to drug sensitivity and treatment of sporadic cancers, such as breast or ovarian epithelial tumors, in the general population. The Fanconi anemia pathway is an example of how DNA repair pathways can be deregulated in cancer cells and how biomarkers of the integrity of these pathways could be useful as a guide to cancer management and may be used in the development of novel therapeutic agents.

The role of BRCA1 in non-homologous end-joining

From the genotypic viewpoint, single nucleotide polymorphisms in the genes of the non-homologous end-joining (NHEJ) pathway, which is important in the repair of DNA double-strand breaks, have been shown to be associated with increased breast cancer risk. However, more phenotypic evidence is needed to strengthen the link between defective NHEJ genes and breast cancer development. Recently, BRCA1-deficient mouse embryonic fibroblasts were found to have significantly reduced NHEJ activity, suggesting an accessory role of BRCA1 in NHEJ. Since BRCA1 is a well-documented breast cancer susceptibility gene, this association between NHEJ and BRCA1 not only suggests a role of BRCA1 in NHEJ, but also provides support for the tumorigenic contribution of the NHEJ pathway to breast cancer development. Interestingly, the phenotypic data show that BRCA1 may promote only specific subtypes of NHEJ, e.g. in vivo precise and terminal end-joining capacities, and have either a suppressive or no effect on others. However, these findings have remained inconclusive, and the lack of consistency between these results may be at least partly explained by the use of different assays, which may measure different subtypes of NHEJ, and of different cell lines investigated. Although some insights have been obtained, the whole picture of NHEJ repair in mammalian cells is far from complete, and the questions of how many subpathways are involved or how we can investigate each subpathway have not yet been adequately addressed.

DNA mismatch repair and p53 function are major determinants of the rate of development of cisplatin resistance

As opposed to factors that control sensitivity to the acute cytotoxic effect of cisplatin, little is known about the factors that determine the rate at which resistance develops. This study examined how loss of p53 or DNA mismatch repair (MMR) function affected the rate of development of resistance to cisplatin in human colon carcinoma cells during sequential cycles of cisplatin exposure that mimic the way the drug is used in the clinic. We used a panel of sublines molecularly engineered to express either the MMR- and p53-proficient phenotype or singly or doubly deficient phenotypes. Loss of either MMR or p53 alone increased the rate of development of resistance to cisplatin by 1.8- and 2.4-fold, respectively; however, loss of both MMR and p53 increased the rate by 4.8-fold. Inhibition of DNA polymerase zeta by suppression of the expression of its REV3 subunit eliminated the increased rate of development of resistance observed in the MMR-deficient cells. Loss of p53 or MMR increased the steady-state level of REV3 and of REV1 mRNA; loss of both functions increased these levels much further by a factor of 20.2-fold for REV3 and 10.3-fold for REV1. The basal level of homologous recombination measured using a reporter vector was 1.3- to 1.7-fold higher in cells that had lost either p53 or MMR function, and 2.6-fold higher in cells that had lost both. In the p53- and MMR-proficient cells, cisplatin induced a 17-fold increase in homologous recombination even when the recombining sequences that did not contain cisplatin adducts; the magnitude of induction was even greater in cells that had lost either one or both functions. We conclude that separate from effects on sensitivity to the acute cytotoxic effect of cisplatin, loss of MMR, especially when combined with loss of p53, results in rapid evolution of cisplatin resistance during sequential rounds of drug exposure that is likely mediated by enhanced mutagenic translesion synthesis. The DNA damage response activated by cisplatin is accompanied by a p53- and MMR-dependent increase in homologous recombination even between adduct-free sequences.

BRIT1 regulates early DNA damage response, chromosomal integrity, and cancer

BRIT1, initially identified as an hTERT repressor, has additional functions at DNA damage checkpoints. Here, we demonstrate that BRIT1 formed nuclear foci minutes after irradiation. The foci of BRIT1 colocalized with 53BP1, MDC1, NBS1, ATM, RPA, and ATR. BRIT1 was required for activation of these elements, indicating that BRIT1 is a proximal factor in the DNA damage response pathway. Depletion of BRIT1 increased the accumulation of chromosomal aberrations. In addition, decreased levels of BRIT1 were detected in several types of human cancer, with BRIT1 expression being inversely correlated with genomic instability and metastasis. These results identify BRIT1 as a crucial DNA damage regulator in the ATM/ATR pathways and suggest that it functions as a tumor suppressor gene.

Removal of BRCA1/CtIP/ZBRK1 repressor complex on ANG1 promoter leads to accelerated mammary tumor growth contributed by prominent vasculature

BRCA1 exerts transcriptional repression through interaction with CtIP in the C-terminal BRCT domain and ZBRK1 in the central domain. A dozen genes, including angiopoietin-1 (ANG1), a secreted angiogenic factor, are corepressed by BRCA1 and CtIP based on microarray analysis of mammary epithelial cells in 3D culture. BRCA1, CtIP, and ZBRK1 form a complex that coordinately represses ANG1 expression via a ZBRK1 recognition site in the ANG1 promoter. Impairment of this complex upregulates ANG1, which stabilizes endothelial cells that form a capillary-like network structure. Consistently, Brca1-deficient mouse mammary tumors exhibit accelerated growth, pronounced vascularization, and overexpressed ANG1. These results suggest that, besides its role in maintaining genomic stability, BRCA1 directly regulates the expression of angiogenic factors to modulate the tumor microenvironment.

The tumor suppressor activity induced by adenovirus-mediated BRCA1 overexpression is not restricted to breast cancers

The BRCA1 (breast cancer 1) breast cancer susceptibility gene is recognized as responsible for most familial breast and ovarian cancers and is suggested to be a tissue-specific tumor suppressor gene. In this report, we investigated the tissue specificity of tumor inhibitory activities induced by a recombinant adenovirus coding for wild-type BRCA1 (wtAdBRCA1). We demonstrated a pronounced in vitro antiproliferative effect on H1299 lung and HT29 colon cells upon infection with AdBRCA1. We describe a prolonged G1 cell cycle arrest associated with a decrease in the hyperphosphorylated form of Rb, suggesting that the Rb/E2F pathway is implicated in BRCA1-induced cell growth arrest. We also observed a significant antitumor effect in these pre-established subcutaneous tumors after in situ delivery of AdBRCA1, although these two tumors do not express wt p53, and also estrogen alpha and beta, progesterone and androgen receptors. Moreover, BRCA1 can induce a strong prolonged cell cycle arrest and apoptotic cell death but no significant antiangiogenic effect in H1299 tumors. Finally, our data indicate that intratumor administration of wtAdBRCA1 significantly inhibits growth of lung and colon steroid hormone-independent tumors.

Analysis of DNA double-strand break repair pathways in mice

During the last years significant new insights have been gained into the mechanism and biological relevance of DNA double-strand break (DSB) repair in relation to genome stability. DSBs are a highly toxic DNA lesion, because they can lead to chromosome fragmentation, loss and translocations, eventually resulting in cancer. DSBs can be induced by cellular processes such as V(D)J recombination or DNA replication. They can also be introduced by exogenous agents DNA damaging agents such as ionizing radiation or mitomycin C. During evolution several pathways have evolved for the repair of these DSBs. The most important DSB repair mechanisms in mammalian cells are nonhomologous end-joining and homologous recombination. By using an undamaged repair template, homologous recombination ensures accurate DSB repair, whereas the untemplated nonhomologous end-joining pathway does not. Although both pathways are active in mammals, the relative contribution of the two repair pathways to genome stability differs in the different cell types. Given the potential differences in repair fidelity, it is of interest to determine the relative contribution of homologous recombination and nonhomologous end-joining to DSB repair. In this review, we focus on the biological relevance of DSB repair in mammalian cells and the potential overlap between nonhomologous end-joining and homologous recombination in different tissues.

Collaboration of Werner syndrome protein and BRCA1 in cellular responses to DNA interstrand cross-links

Cells deficient in the Werner syndrome protein (WRN) or BRCA1 are hypersensitive to DNA interstrand cross-links (ICLs), whose repair requires nucleotide excision repair (NER) and homologous recombination (HR). However, the roles of WRN and BRCA1 in the repair of DNA ICLs are not understood and the molecular mechanisms of ICL repair at the processing stage have not yet been established. This study demonstrates that WRN helicase activity, but not exonuclease activity, is required to process DNA ICLs in cells and that WRN cooperates with BRCA1 in the cellular response to DNA ICLs. BRCA1 interacts directly with WRN and stimulates WRN helicase and exonuclease activities in vitro. The interaction between WRN and BRCA1 increases in cells treated with DNA cross-linking agents. WRN binding to BRCA1 was mapped to BRCA1 452–1079 amino acids. The BRCA1/BARD1 complex also associates with WRN in vivo and stimulates WRN helicase activity on forked and Holliday junction substrates. These findings suggest that WRN and BRCA1 act in a coordinated manner to facilitate repair of DNA ICLs.

Mapping and conformational characterization of the DNA-binding region of the breast cancer susceptibility protein BRCA1

The breast cancer susceptibility gene, BRCA1, encodes a large nuclear phosphoprotein, the major isoform of which is 1863 amino acids in size. Structure-function studies have been largely restricted to the only two domains identified by homology searches: the RING (really interesting new gene) and BRCT (BRCA1 C-terminus) domains. However, we have recently reported the identification of a large central soluble region of BRCA1 (residues 230-534) that binds specifically to four-way junction DNA, a property that potentially facilitates its role in the repair of DNA lesions by homologous recombination. We have now used a combination of limited proteolysis and extension cloning to identify more accurately the DNA-binding region of BRCA1. Limited trypsinolysis of BRCA1-(230-534) resulted in the production of a soluble domain identified as residues 230-339. However, after cloning, expression and purification of this region, studies revealed that it was unable to bind to four-way junctions, suggesting that the DNA-binding activity, in part, resides within residues 340-534. A series of fragments extending from residue 340 were produced, and each was tested for its ability to bind to four-way junction DNA in gel retardation assays. In these experiments, residues 340-554 of BRCA1 were identified as the minimal DNA-binding region. We then went on to characterize the conformation of this region using CD spectroscopy and analytical centrifugation.

BRCA1 Modulates Xenobiotic Stress-inducible Gene Expression by Interacting with ARNT in Human Breast Cancer Cells

Previously, we have reported that BRCA1 regulates the expression of various classes of genes, including genes involved in xenobiotic stress responses (Bae, I., Fan, S., Meng, Q., Rih, J. K., Kim, H. J., Kang, H. J., Xu, J., Goldberg, I. D., Jaiswal, A. K., and Rosen, E. M. (2004) Cancer Res. 64, 7893-7909). In the present study, we have investigated the effects of BRCA1 on xenobiotic stress-inducible gene expression. In response to aryl hydrocarbon receptor (AhR) ligands, cytoplasmic AhR becomes activated and then translocates to the nucleus where it forms a complex with the aryl hydrocarbon receptor nuclear translocator (ARNT). Subsequently, the AhR.ARNT complex binds to the enhancer or promoter of genes containing a xenobiotic stress-responsive element and regulates the expression of multiple target genes including cytochrome P450 subfamily polypeptide 1 (CYP1A1). In this study, we have found that endogenous and overexpressed exogenous wild-type BRCA1 affect xenobiotic stress-induced CYP1A1 gene expression. Using a standard chromatin immunoprecipitation assay, we have demonstrated that BRCA1 is recruited to the promoter regions of CYP1A1 and CYP1B1 along with ARNT and/or AhR following xenobiotic exposure. Our findings suggest that BRCA1 may be physiologically important for mounting a normal response to xenobiotic insults and that it may function as a coactivator for ARNT activity. Using immunoprecipitation, Western blotting, and glutathione S-transferase capture assays, a xenobiotic-independent interaction between BRCA1 and ARNT has been identified, although it is not yet known whether this is a direct or indirect interaction. We have also found that the inducibility of CYP1A1 and CYP1B1 transcripts following xenobiotic stress was significantly attenuated in BRCA1 knockdown cells. This reduced inducibility is associated with an altered stability of ARNT and was almost completely reversed in cells transfected with an ARNT expression vector. Finally, we have found that xenobiotic (TCDD) treatments of breast cancer cells containing reduced levels of BRCA1 cause the transcription factor ARNT to become unstable.

BRCA1 DNA-binding activity is stimulated by BARD1

The breast- and ovarian-specific tumor suppressor BRCA1 has been implicated in numerous cellular processes, including transcription, ubiquitination, and DNA repair. Its tumor suppression activity is tightly linked to that of BARD1, a protein that heterodimerizes with BRCA1. It has been previously shown that BRCA1 binds to DNA, an interesting functional observation in light of the genetic data linking BRCA1 to DNA repair pathways. In this work, we reexamine the DNA-binding properties of BRCA1, comparing them with the DNA-binding properties of the BRCA1/BARD1 heterodimer. Because nuclear BRCA1 exists as a heterodimer with BARD1, it is likely that in vitro studies of the heterodimer will provide a more accurate model of physiologic conditions. Our results indicate that whereas BARD1 cannot directly bind DNA, it does enhance DNA binding by BRCA1. This is a surprising observation as both DNA-binding domains are distal to the BARD1-interacting RING domain of BRCA1. Further analysis of the dimerization reveals that the BRCA1/BARD1 interaction is not limited to the amino-terminal RING domains of each protein. The carboxyl terminus of BRCA1 contributes significantly to the stability of the heterodimer. We also show that the presence of BARD1 has a secondary effect, as autoubiquitination of BRCA1/BARD1 heterodimers additionally enhances the affinity of BRCA1 for DNA. Together, these data suggest that BRCA1 and BARD1 heterodimerization is stabilized via domains not previously thought to interact and that BARD1 acts in both ubiquitination-dependent and ubiquitination-independent ways to influence the role of BRCA1 in DNA repair.

Ataxia telangiectasia mutated and checkpoint kinase 2 regulate BRCA1 to promote the fidelity of DNA end-joining

Homologous recombination (HR) and nonhomologous end-joining (NHEJ) are the two mechanisms responsible for repairing DNA double-strand breaks (DSBs) and act in either a collaborative or competitive manner in mammalian cells. DSB repaired by NHEJ may be more complicated than the simple joining of the ends of DSB, because, if nucleotides were lost, it would result in error-prone repair. This has led to the proposal that a subpathway of precise NHEJ exists that can repair DSBs with higher fidelity; this is supported by recent findings that the expression of the HR gene, BRCA1, is causally linked to in vitro and in vivo precise NHEJ activity. To further delineate this mechanism, the present study explored the connection between NHEJ and the cell-cycle checkpoint proteins, ataxia telangiectasia mutated (ATM) and checkpoint kinase 2 (Chk2), known to be involved in activating BRCA1, and tested the hypothesis that ATM and Chk2 promote precise end-joining by BRCA1. Support for this hypothesis came from the observations that (a) knockdown of ATM and Chk2 expression affected end-joining activity; (b) in BRCA1-defective cells, precise end-joining activity was not restored by a BRCA1 mutant lacking the site phosphorylated by Chk2 but was restored by wild-type BRCA1 or a mutant mimicking phosphorylation by Chk2; (c) Chk2 mutants lacking kinase activity or with a mutation at a site phosphorylated by ATM had a dominant negative effect on precise end-joining in BRCA1-expressing cells. These results suggest that the other two HR regulatory proteins, ATM and Chk2, act jointly to regulate the activity of BRCA1 in controlling the fidelity of DNA end-joining by precise NHEJ.

Increased level of chromosomal damage after irradiation of lymphocytes from BRCA1 mutation carriers

Deleterious mutations in the BRCA1 gene predispose women to an increased risk of breast and ovarian cancer. Many functional studies have suggested that BRCA1 has a role in DNA damage repair and failure in the DNA damage response pathway often leads to the accumulation of chromosomal aberrations. Here, we have compared normal lymphocytes with those heterozygous for a BRCA1 mutation. Short-term cultures were irradiated (8Gy) using a high dose rate and subsequently metaphases were analysed by 24-colour chromosome painting (M-FISH). We scored the chromosomal rearrangements in the metaphases from five BRCA1 mutation carriers and from five noncarrier control samples 6 days after irradiation. A significantly higher level of chromosomal damage was detected in the lymphocytes heterozygous for BRCA1 mutations compared with normal controls; the average number of aberrations per mitosis was 3.48 compared with 1.62 in controls (P=0.0001). This provides new evidence that heterozygous mutation carriers have a different response to DNA damage compared with noncarriers and that BRCA1 has a role in DNA damage surveillance. Our finding has implications for treatment and screening of BRCA1 mutation carriers using modalities that involve irradiation.

BRCA1-associated complexes: new targets to overcome breast cancer radiation resistance

Since BRCA1 was cloned a decade ago, significant progress has been made in defining its biochemical and biological functions, as well as its role in breast and ovarian cancers. BRCA1 has been implicated in many cellular processes, including DNA repair, cell cycle checkpoint control, protein ubiquitination and chromatin remodeling. This review examines the role(s) of BRCA1 in mediating these cellular processes, and discusses its potential involvement in the resistance of breast cancer to radiation-based therapies. Finally, the possibility that BRCA1-associated proteins may serve as new targets for breast cancer radiation therapy is explored. The activation or inactivation of these BRCA1-associated proteins may modify both the risk of developing cancers in BRCA1 mutation carriers and the efficacy of breast cancer therapy, including radiation.

BRCA1 regulates RAD51 function in response to DNA damage and suppresses spontaneous sister chromatid replication slippage: implications for sister chromatid cohesion, genome stability, and carcinogenesis

The breast/ovarian cancer susceptibility proteins BRCA1 and BRCA2 maintain genome stability, at least in part, through a functional role in DNA damage repair. They both colocalize with RAD51 at sites of DNA damage/replication and activate RAD51-mediated homologous recombination repair of DNA double-strand breaks (DSB). Whereas BRCA2 interacts directly with and regulates RAD51, the role of BRCA1 in this process is unclear. However, BRCA1 may regulate RAD51 in response to DNA damage or through its ability to interact with and regulate MRE11/RAD50/NBS1 (MRN) during the processing of DSBs into single-strand DNA (ssDNA) ends, prerequisite substrates for RAD51, or both. To test these hypotheses, we measured the effect of BRCA1 on the competition between RAD51-mediated homologous recombination (gene conversion and crossover) versus RAD51-independent homologous recombination [single-strand annealing (SSA)] for ssDNA at a site-specific chromosomal DSB within a DNA repeat, a substrate for both homologous recombination pathways. Expression of wild-type BRCA1 in BRCA1-deficient human recombination reporter cell lines promoted both gene conversion and SSA but greatly enhanced gene conversion. In addition, BRCA1 also suppressed both spontaneous gene conversion and deletion events, which can arise from either crossover or sister chromatid replication slippage (SCRS), a RAD51-independent process. BRCA1 does not seem to block crossover. From these results, we conclude that (a) BRCA1 regulates RAD51 function in response to the type of DNA damage and (b) BRCA1 suppresses SCRS, suggesting a role for this protein in sister chromatid cohesion/alignment. Loss of such control in response to estrogen-induced DNA damage after BRCA1 inactivation may be a key initial event that triggers genome instability and carcinogenesis.

The role of the BRCA1 tumor suppressor in DNA double-strand break repair

The tumor suppressor gene BRCA1 was cloned in 1994 based on its linkage to early-onset breast and ovarian cancer. Although the BRCA1 protein has been implicated in multiple cellular functions, the precise mechanism that determines its tumor suppressor activity is not defined. Currently, the emerging picture is that BRCA1 plays an important role in maintaining genomic integrity by protecting cells from double-strand breaks (DSB) that arise during DNA replication or after DNA damage. The DSB repair pathways available in mammalian cells are homologous recombination and nonhomologous end-joining. BRCA1 function seems to be regulated by specific phosphorylations in response to DNA damage and we will focus this review on the roles played by BRCA1 in DNA repair and cell cycle checkpoints. Finally, we will explore the idea that tumor suppression by BRCA1 depends on its control of DNA DSB repair, resulting in the promotion of error-free and the inhibition of error-prone recombinational repair.

The Fanconi Anemia/BRCA pathway: new faces in the crowd

Over the past few years, study of the rare inherited chromosome instability disorder, Fanconi Anemia (FA), has uncovered a novel DNA damage response pathway. Through the cooperation of multiple proteins, this pathway regulates a complicated cellular response to DNA cross-linking agents and other genotoxic stresses. In this article we review recent data identifying new components of the FA pathway that implicate it in several aspects of the DNA damage response, including the direct processing of DNA, translesion synthesis, homologous recombination, and cell cycle regulation. We also discuss new findings that explain how the FA pathway is regulated through the processes of ubiquitination and deubiquitination. We then consider the clinical implications of our current understanding of the FA pathway, particularly in the development and treatment of malignancy in heterozygous carriers of FA mutations or in patients with sporadic cancers. We consider how recent studies of p53-mediated apoptosis and loss of p53 function in models of FA may help explain the clinical features of the disease and finally present a hypothesis to account for the specificity of the FA pathway in the response to DNA cross-links.

BACH1 is a DNA repair protein supporting BRCA1 damage response

The link between defects in BRCA1 and breast cancer development may be best understood by deciphering the role of associated proteins. BRCA1 associated C-terminal helicase (BACH1) interacts directly with the BRCA1 C-terminal BRCT repeats, which are important for BRCA1 DNA repair and are mutated in the majority of BRCA1 familial cancers. Thus, BACH1 is a likely candidate for mediating BRCA1 DNA repair and tumor suppression functions. Although previous evidence using overexpression of a dominant negative BACH1 has suggested that BACH1 is involved in BRCA1-DNA repair function, our results using BACH1 deficient cells provide direct evidence for involvement of BACH1 in DNA repair as well as for localizing BRCA1. Following DNA damage BACH1 is modified by phosphorylation, displays a BRCA1-like nuclear foci pattern and colocalizes with gamma-H2AX. Given that the BACH1/BRCA1 complex is unaltered by DNA damage and the intensity of BRCA1 foci is diminished in BACH1 deficient cells, BACH1 may serve to not only facilitate DNA repair, but also maintain BRCA1 in DNA damage foci.

Checkpoint Kinase 2–Mediated Phosphorylation of BRCA1 Regulates the Fidelity of Nonhomologous End-Joining

The tumor suppressor gene BRCA1 maintains genomic integrity by protecting cells from the deleterious effects of DNA double-strand breaks (DSBs). Through its interactions with the checkpoint kinase 2 (Chk2) kinase and Rad51, BRCA1 promotes homologous recombination, which is typically an error-free repair process. In addition, accumulating evidence implicates BRCA1 in the regulation of nonhomologous end-joining (NHEJ), which may involve precise religation of the DSB ends if they are compatible (i.e., error-free repair) or sequence alteration upon rejoining (i.e., error-prone or mutagenic repair). However, the precise role of BRCA1 in regulating these different subtypes of NHEJ is not clear. We provide here the genetic and biochemical evidence to show that BRCA1 promotes error-free rejoining of DSBs in human breast carcinoma cells while suppressing microhomology-mediated error-prone end-joining and restricting sequence deletion at the break junction during repair. The repair spectrum in BRCA1-deficient cells was characterized by an increase in the formation of >2 kb deletions and in the usage of long microhomologies distal to the break site, compared with wild-type (WT) cells. This error-prone repair phenotype could also be revealed by disruption of the Chk2 phosphorylation site of BRCA1, or by expression of a dominant-negative kinase-dead Chk2 mutant in cells with WT BRCA1. We suggest that the differential control of NHEJ subprocesses by BRCA1, in concert with Chk2, reduces the mutagenic potential of NHEJ, thereby contributing to the prevention of familial breast cancers.

A role for Brca1 in chromosome end maintenance

The role of BRCA1 in breast and ovarian tumor suppression has been primarily ascribed to the maintenance of genome integrity. BRCA1 interacts with components of the non-homologous end-joining pathway previously shown to play a role in telomere maintenance in yeast. Here, we provide evidence that links Brca1 with telomere integrity. Brca1(-/-) T-cells display telomere dysfunction in both loss of telomere repeats as well as defective telomere capping. Loss of Brca1 synergizes with p53 deficiency in the onset and frequency of tumorigenesis. Karyotyping of tBrca1(-/-)p53(-/-) thymic lymphomas revealed the presence of telomere dysfunction accompanied by clonal chromosomal translocations. The telomere dysfunction phenotype in Brca1-deficient cells suggests that loss of telomere integrity might contribute to chromosome end dysfunction and permit the formation of potentially oncogenic translocations.

BRCA1 phosphorylation regulates caspase-3 activation in UV-induced apoptosis

Apoptosis is implemented by death machinery that involves the caspase family of proteins, under the condition of a variety of stresses. Previous studies have shown that mouse embryonic stem cells deficient for caspase-3 are resistant to UV-induced apoptosis and that the active form of caspase-3 translocates to the nucleus. It has also been shown that the breast cancer tumor suppressor, BRCA1, is phosphorylated on Ser1423 and Ser1524 by the ataxia telangiectasia mutated-related kinase (ATR) after UV damage. Here, we show that activation of caspase-3 by UV is abrogated in BRCA1-mutated SNU251 and HCC1937 cells but was restored by reexpressing wild-type (wt) BRCA1, but not phosphorylation-deficient BRCA1Ser1423Ala/Ser1524Ala (BRCA1(S1423/1524A)). In SNU251 cells expressing BRCA1(S1423/1524A), the inhibitory interaction between X-linked inhibitor of apoptosis protein (XIAP) and caspase-9 remains intact after UV, compared with cells expressing wt BRCA1. We determined that XIAP and BRCA1 interact and upon phosphorylation this complex is disrupted. Nuclear translocation of active caspase-3 is detected only when wt BRCA1 is reexpressed. Consistent with this, Rad21, a known substrate of caspase-3, is cleaved only when wt BRCA1 is expressed in vivo. These results propose a model that inhibition of BRCA1 phosphorylation leads to the abrogation of a specific form of apoptosis that is mediated by caspase-3.

Multifactorial contributions to an acute DNA damage response by BRCA1/BARD1-containing complexes

The BRCA1 gene product and its stoichiometric binding partner, BARD1, play a vital role in the cellular response to DNA damage. However, how they acquire specific biochemical functions after DNA damage is poorly understood. Following exposure to genotoxic stress, DNA damage-specific interactions were observed between BRCA1/BARD1 and the DNA damage-response proteins, TopBP1 and Mre11/Rad50/NBS1. Two distinct DNA damage-dependent super complexes emerged; their activation was dependent, in part, on the actions of specific checkpoint kinases, and each super complex contributed to a distinctive aspect of the DNA damage response. The results support a new, multifactorial model that describes how genotoxic stress enables BRCA1 to execute a diverse set of DNA damage-response functions.

An hGCN5/TRRAP Histone Acetyltransferase Complex Co-activates BRCA1 Transactivation Function through Histone Modification

It is well established that genetic mutations that impair BRCA1 function predispose women to early onset of breast and ovarian cancer. However, the co-regulatory factors that support normal BRCA1 functions remain to be identified. Using a biochemical approach to search for such co-regulatory factors, we identified hGCN5, TRRAP, and hMSH2/6 as BRCA1-interacting proteins. Genetic mutations in the C-terminal transactivation domain of BRCA1, as found in breast cancer patients (Chapman, M. S., and Verma, I. M. (1996) Nature 382, 678-679), caused the loss of physical interaction between BRCA1 and TRRAP and significantly reduced the co-activation of BRCA1 transactivation function by hGCN5/TRRAP. The reported transcriptional squelching between BRCA1 and estrogen receptor alpha (Fan, S., Wang, J., Yuan, R., Ma, Y., Meng, Q., Erdos, M. R., Pestell, R. G., Yuan, F., Auborn, K. J., Goldberg, I. D., and Rosen, E. M. (1999) Science 284, 1354-1356) was rescued by the overexpression of TRRAP or hGCN5. Histone acetyltransferase hGCN5 activity appeared to be indispensable for coregulator complex function in both BRCA1-mediated gene regulation and DNA repair. Biochemical purification of the hGCN5/TRRAP-containing complex suggested that hGCN5/TRRAP formed a complex with hMSH2/hMSH6, presumably as a novel subclass of hGCN5/TRRAP-containing known TFTC (TBP-free TAF-containing)-type histone acetyltransferase complex (hTFTC, hPCAF, and hSTAGA) (Yanagisawa, J., Kitagawa, H., Yanagida, M., Wada, O., Ogawa, S., Nakagomi, M., Oishi, H., Yamamoto, Y., Nagasawa, H., McMahon, S. B., Cole, M. D., Tora, L., Takahashi, N., and Kato, S. (2002) Mol. Cell 9, 553-562). Unlike other subclasses, the isolated complex harbored a previously unknown combination of components including hMSH2 and hMSH6, major components of the BRCA1 genome surveillance repair complex (BASC). Thus, our results suggested that the multiple BRCA1 functions require a novel hGCN5/TRRAP histone acetyltransferase complex subclass.

BRCA1 and BRCA2: the genetic testing and the current management options for mutation carriers

Approximately 5-10% of breast carcinomas and 10% of ovarian carcinomas are ascribable to a genetic susceptibility. Of these, about 40% are related to genetic mutations in the genes BRCA1 and BRCA2. Despite the increasing demand for genetic testing arising from the patients and their relatives, the genetic testing can be offered yet only to individuals belonging to high-risk families in which the probability that there is a germline mutation in a BRCA gene is high and thus cancer occurrence is likely the expression of a highly penetrant genetic predisposition. In this article, we review how the current knowledge on the biological mechanisms underlying BRCA1 and BRCA2 dysfunction may contribute to the understanding of breast and ovarian cancer predisposition. The most currently employed methods for genetic testing are critically overviewed, together with some indications for the interpretation of the test outcome and the clinical management of mutation carriers.

BRCA1 associates with the inactive X chromosome in late S-phase, coupled with transient H2AX phosphorylation

The BRCA1 tumor suppressor gene encodes an E3-ubiquitin ligase that has been implicated in several distinct biochemical processes. As the cell cycle progresses, BRCA1 proteins interact transiently with nuclear foci containing DNA replication and DNA double-strand repair machinery. A hallmark of these foci is the presence of S139 phosphorylated histone H2AX. BRCA1 was recently shown to associate with facultative heterochromatin at the inactive X chromosome (Xi), where it may play a role in maintaining gene silencing. As the kinetics of this interaction has not been described, we sought to establish whether association of BRCA1 with the Xi also correlated with replication. Here we demonstrate that the interaction of BRCA1 and the Xi is transient, occurring during late S-phase. This interaction is concomitant with the presence of distinct foci of S139 phospho-H2AX and specifically corresponds with late replication of the Xi. BRCA1 and phospho-H2AX appear on the Xi immediately adjacent to CAF-1, a known marker of replication fork activity. Taken together, these data implicate BRCA1 and the H2AX kinase in replication of facultative heterochromatin on the Xi, most likely in a fashion similar to that performed at sites of DNA replication and double-strand break repair observed on somatic chromosomes.