BRCA1 gene mutations are responsible for hereditary breast and ovarian cancers. In sporadic breast tumors, BRCA1 dysfunction or aberrant subcellular localization is thought to be common. BRCA1 is a nuclear–cytoplasm shuttling protein and the reason for cytoplasmic localization of BRCA1 in young breast cancer patients is not yet known. We have previously reported BRCA1 proteins unlike K109R and cancer-predisposing mutant C61G to bind Ubc9 and modulate ER-α turnover. In the present study, we have examined the consequences of altered Ubc9 binding and knockdown on the subcellular localization and growth inhibitory function of BRCA1 proteins. Our results using live imaging of YFP, GFP, RFP-tagged BRCA1, BRCA1a and BRCA1b proteins show enhanced cytoplasmic localization of K109 R and C61G mutant BRCA1 proteins in normal and cancer cells. Furthermore, down-regulation of Ubc9 in MCF-7 cells using Ubc9 siRNA resulted in enhanced cytoplasmic localization of BRCA1 protein and exclusive cytoplasmic retention of BRCA1a and BRCA1b proteins. These mutant BRCA1 proteins were transforming and impaired in their capacity to inhibit growth of MCF-7 and CAL51 breast cancer cells. Interestingly, cytoplasmic BRCA1a mutants showed more clonogenicity in soft agar and higher levels of expression of Ubc9 than parental MCF7 cells. This is the first report demonstrating the physiological link between cytoplasmic mislocalization of mutant BRCA1 proteins, loss of ER-α repression, loss of ubiquitin ligase activity and loss of growth suppression of BRCA1 proteins. Thus, binding of BRCA1 proteins to nuclear chaperone Ubc9 provides a novel mechanism for nuclear import and control of tumor growth.
EMSY links the BRCA2 pathway to sporadic breast/ovarian cancer. It encodes a nuclear protein that binds to the BRCA2 N-terminal domain implicated in chromatin/transcription regulation, but when sporadically amplified/overexpressed, increased EMSY level represses BRCA2 transactivation potential and induces chromosomal instability, mimicking the activity of BRCA2 mutations in the development of hereditary breast/ovarian cancer. In addition to chromatin/transcription regulation, EMSY may also play a role in the DNA-damage response, suggested by its ability to localize at chromatin sites of DNA damage/repair. This implies that EMSY overexpression may also repress BRCA2 in DNA-damage replication/checkpoint and recombination/repair, coordinated processes that also require its interacting proteins: PALB2, the partner and localizer of BRCA2; RPA, replication/checkpoint protein A; and RAD51, the inseparable recombination/repair enzyme. Here, using a well-characterized recombination/repair assay system, we demonstrate that a slight increase in EMSY level can indeed repress these two processes independently of transcriptional interference/repression. Since EMSY, RPA and PALB2 all bind to the same BRCA2 region, these findings further support a scenario wherein: (a) EMSY amplification may mimic BRCA2 deficiency, at least by overriding RPA and PALB2, crippling the BRCA2/RAD51 complex at DNA-damage and replication/transcription sites; and (b) BRCA2/RAD51 may coordinate these processes by employing at least EMSY, PALB2 and RPA. We extensively discuss the molecular details of how this can happen to ascertain its implications for a novel recombination mechanism apparently conceived as checkpoint rather than a DNA repair system for cell division, survival, death, and human diseases, including the tissue specificity of cancer predisposition, which may renew our thinking about targeted therapy and prevention.
Tumor-suppressor genes; Oncogenes; Development/abortion; Aging; Estrogens
BRCA1 is a tumor suppressor gene that is mutated in families with breast and ovarian cancer. Several BRCA1 splice variants are found in different tissues, but their subcellular localization and functions are poorly understood at the moment. We previously described BRCA1 splice variant BRCA1a to induce apoptosis and function as a tumor suppressor of triple negative breast, ovarian and prostate cancers. In this study we have analyzed the function of BRCA1 isoforms (BRCA1a and BRCA1b) and compared them to the wild type BRCA1 protein using several criteria like studying expression in normal and tumor cells by RNase protection assays, sub cellular localization/fractionation by immunofluorescence microscopy and western blot analysis, transcription regulation of biological relevant proteins and growth suppression in breast cancer cells. We are demonstrating for the first time that ectopically expressed GFP-tagged BRCA1, BRCA1a, and BRCA1b proteins are localized to the mitochondria, repress ELK-1 transcriptional activity and possess antiproliferative activity on breast cancer cells. These results suggest that the exon 9,10 and 11 sequences (aa 263 – 1365) which contain two nuclear localization signals, p53, Rb, c-Myc, γ- tubulin, Stat, Rad 51, Rad 50 binding domains, angiopoietin-1 repression domain are not absolutely required for mitochondrial localization and growth suppressor function of these proteins. Since mitochondrial dysfunction is a hallmark of cancer, we can speculate that the mitochondrial localization of BRCA1 proteins may be functionally significant in regulating both the mitochondrial DNA damage as well as apoptotic activity of BRCA1 proteins and mislocalization causes cancer.
BRCA1/1a/1b proteins; breast cancers; mitochondria; growth suppression; transcriptional regulation; ELK-1
BRCA1 and BRCA2 breast cancer susceptibility genes encode proteins, the normal cellular functions of which are complex and multiple, and germ-line mutations in individuals predispose both to breast and to ovarian cancer. There is nevertheless substantial evidence linking BRCA1 and BRCA2 to homologous recombination and DNA repair, to transcriptional control and to tissue proliferation. There is controversy regarding the localization of BRCA1 and BRCA2 proteins to either nucleus or cytoplasm and whether the expression is present in premeiotic germ cells or can still be expressed in mitotic spermatogonia. We report herein an immunohistochemical study of BRCA1 and BRCA2 distribution in a rather unsual tissue (an ovotestis), which addresses this issue.
BRCA1; BRCA2; immunohistochemistry; ovotestis
BACKGROUND: Mutations in the BRCA1 or BRCA2 genes are responsible for up to 95% of hereditary ovarian cancer cases. Both genes function as tumour suppressor genes, and development of a cancer is thought to require an accumulation of somatic genetic events in addition to the inherited germline predisposition. It is unknown whether these somatic events in BRCA associated ovarian cancer are similar to or distinct from those in sporadic cases. The most frequent somatic genetic event in ovarian cancer is a mutation of the p53 gene. AIM: To study the role of p53 in hereditary ovarian cancer, by analysing accumulation of the p53 protein in ovarian cancers which occurred in BRCA1 or BRCA2 germline mutation carriers and comparing the results with a panel of ovarian cancers from patients who tested negative for both BRCA1 and BRCA2. METHODS: The study group consisted of 39 ovarian cancer patients in whom a BRCA mutation had been confirmed previously. p53 Immunohistochemistry was performed on archival tissue using a standard microwave antigen retrieval technique. The rate of p53 accumulation was compared with 40 ovarian cancer cases who tested negative for BRCA1 and BRCA2 germline mutations. RESULTS: P53 Accumulation was similar in BRCA related ovarian cancers and BRCA negative controls. Overall 27 of 39 BRCA1 or BRCA2 positive cases (69%) had evidence of p53 accumulation, compared with 24 of 40 invasive ovarian cancer cases (60%) which tested negative for BRCA1 and BRCA2 germline mutations. BRCA1 related ovarian cancers showed p53 accumulation in 22 of 30 cases (73%); p53 accumulation was present in five of nine BRCA2 related ovarian cancers. CONCLUSIONS: In addition to germline BRCA1 and BRCA2 mutations, somatic p53 alterations leading to p53 accumulation are an important event in hereditary ovarian cancer and are as frequent as in non-BRCA-related ovarian cancer.
Loss or decrease of wild type BRCA1 function, by either mutation or reduced expression, has a role in hereditary and sporadic human breast and ovarian cancers. We report here that the PI3K/AKT pathway is constitutively active in BRCA1-defective human breast cancer cells. Levels of phospho-AKT are sustained even after serum starvation in breast cancer cells carrying deleterious BRCA1 mutations. Knockdown of BRCA1 in MCF7 cells increases the amount of phospho-AKT and sensitizes cells to small molecule protein kinase inhibitors (PKIs) targeting the PI3K/AKT pathway. Restoration of wild type BRCA1 inhibits the activated PI3K/AKT pathway and de-sensitizes cells to PKIs targeting this pathway in BRCA1 mutant breast cancer cells, regardless of PTEN mutations. In addition, clinical PI3K/mTOR inhibitors, PI-103 and BEZ235, showed anti-proliferative effects on BRCA1 mutant breast cancer cell lines and synergism in combination with chemotherapeutic drugs, cisplatin, doxorubicin, topotecan, and gemcitabine. BEZ235 synergizes with the anti-proliferative effects of gemcitabine by enhancing caspase-3/7 activity. Our results suggest that the PI3K/AKT pathway can be an important signaling pathway for the survival of BRCA1-defective breast cancer cells and pharmacological inhibition of this pathway is a plausible treatment for a subset of breast cancers.
constitutive activation; PI3K/AKT pathway; BRCA1-defective breast cancer; kinase inhibitor; chemotherapeutic agents; synergism
Mutation of the BRCA1 tumor suppressor gene predisposes women to hereditary breast and ovarian cancers. BRCA1 forms a heterodimer with BARD1. The BRCA1/BARD1 heterodimer has ubiquitin ligase activity, considered to play crucial roles in tumor suppression and DNA damage response. Nevertheless, relevant BRCA1 substrates are poorly defined. We have developed a new approach to systematically identify the substrates of ubiquitin ligases by identifying proteins that display enhanced incorporation of His-tagged ubiquitin upon ligase co-expression; using this method, we identified several candidate substrates for BRCA1. These include scaffold attachment factor B2 (SAFB2), Tel2, as well as BARD1. BRCA1 was found to enhance SAFB protein expression and induce Tel2 nuclear translocation. Identification of the ubiquitination substrates has been a major obstacle to understanding the functions of ubiquitin ligases. The quantitative proteomics approach we devised for the identification of BRCA1 substrates will facilitate the identification of ubiquitin ligase-substrate pairs.
BRCA1; BARD1; ubiquitination; substrate; quantitative proteomics
BRCA1, a multi-domain protein, is mutated in a large percentage of hereditary breast and ovarian cancers. BRCA1 is most often mutated in three domains or regions: the N-terminal RING domain, exons 11–13, and the BRCT domain. The BRCA1 RING domain is responsible for the E3 ubiquitin ligase activity of BRCA1 and mediates interactions between BRCA1 and other proteins. BRCA1 ubiquitinates several proteins with various functions. The BRCA1 BRCT domain binds to phosphoproteins with specific sequences recognized by both BRCA1 and ATM/ATR kinases. Structural studies of the RING and BRCT domains have revealed the molecular basis by which cancer causing mutations impact the functions of BRCA1. While no structural data is available for the amino acids encoded by exons 11–13, multiple binding sites and functional domains exist in this region. Many mutations in exons 11–13 have deleterious effects on the function of these domains. In this mini-review, we examine the structure-function relationships of the BRCA1 protein and the relevance to cancer progression.
Inherited mutations in BRCA1 confer susceptibility to breast and ovarian neoplasms. However, the function of BRCA1 and the role of BRCA1 in noninherited cancer remain unknown. Characterization of alternately spliced forms of BRCA1 may identify functional regions; thus, we constructed expression vectors of BRCA1 and a splice variant lacking exon 11, designated BRCA1 delta 672-4095. Immunofluorescence studies indicate nuclear localization of BRCA1 but cytoplasmic localization of BRCA1 delta 672-4095. Two putative nuclear localization signals (designated NLS1 and NLS2) were identified in exon 11; immunofluorescence studies indicate that only NLS1 is required for nuclear localization. RNA analysis indicates the expression of multiple, tissue-specific forms of BRCA1 RNAs; protein analysis with multiple antibodies suggests that at least three BRCA1 isoforms are expressed, including those lacking exon 11. The results suggest that BRCA1 is a nuclear protein and raise the possibility that splicing is one form of regulation of BRCA1 function by alteration of the subcellular localization of expressed proteins.
Mutations of the breast and ovarian cancer susceptibility gene 1 (BRCA1) account for about 40–45% of hereditary breast cancer cases. Moreover, a significant fraction of sporadic (non-hereditary) breast and ovarian cancers exhibit reduced or absent expression of the BRCA1 protein, suggesting an additional role for BRCA1 in sporadic cancers. BRCA1 follows the classic pattern of a highly penetrant Knudsen-type tumor suppressor gene in which one allele is inactivated through a germ-line mutation and the other is mutated or deleted within the tumor. BRCA1 is a multi-functional protein but it is not fully understood which function(s) is (are) most important for tumor suppression, nor is it clear why BRCA1-mutations confer a high risk for breast and ovarian cancers and not a broad spectrum of tumor types. Here, we will review BRCA1 functions in the DNA damage response (DDR), which are likely to contribute to tumor suppression. In the process, we will highlight some of the controversies and unresolved issues in the field. We will also describe a recently identified and under-investigated role for BRCA1 in the regulation of telomeres and the implications of this role in the DDR and cancer suppression.
breast cancer susceptibility gene 1; DNA damage response; telomeres; ataxia-telangiectasia mutated; homology-directed repair; base excision repair; DNA damage signaling
Ovarian epithelial tumors are an hallmark of hereditary cancer syndromes which are related to the germ-line inheritance of cancer predisposing mutations in BRCA1 and BRCA2 genes. Although these genes have been associated with multiple different physiologic functions, they share an important role in DNA repair mechanisms and therefore in the whole genomic integrity control. These findings have risen a variety of issues in terms of treatment and prevention of breast and ovarian tumors arising in this context. Enhanced sensitivity to platinum-based anticancer drugs has been related to BRCA1/2 functional loss. Retrospective studies disclosed differential chemosensitivity profiles of BRCA1/2-related as compared to "sporadic" ovarian cancer and led to the identification of a "BRCA-ness" phenotype of ovarian cancer, which includes inherited BRCA1/2 germ-line mutations, a serous high grade histology highly sensitive to platinum derivatives. Molecularly-based tailored treatments of human tumors are an emerging issue in the "era" of molecular targeted drugs and molecular profiling technologies. We will critically discuss if the genetic background of ovarian cancer can indeed represent a determinant issue for decision making in the treatment selection and how the provocative preclinical findings might be translated in the therapeutic scenario. The presently available preclinical and clinical evidence clearly indicates that genetic background has an emerging role in treatment individualization for ovarian cancer patients.
DNA repair genes critically regulate the cellular response to chemotherapy and epigenetic regulation of these genes may be influenced by chemotherapy exposure. Restoration of BRCA1 and BRCA2 mediates resistance to platinum chemotherapy in recurrent BRCA1 and BRCA2 mutated hereditary ovarian carcinomas. We evaluated BRCA1, BRCA2, and MLH1 protein expression in 115 sporadic primary ovarian carcinomas, of which 31 had paired recurrent neoplasms collected after chemotherapy. Additionally, we assessed whether promoter methylation of BRCA1, MLH1 or FANCF influenced response to chemotherapy or explained alterations in protein expression after chemotherapy exposure.
Of 115 primary sporadic ovarian carcinomas, 39 (34%) had low BRCA1 protein and 49 (42%) had low BRCA2 expression. BRCA1 and BRCA2 protein expression were highly concordant (p < 0.0001). MLH1 protein loss occurred in 28/115 (24%) primary neoplasms. BRCA1 protein loss in primary neoplasms was associated with better survival (p = 0.02 Log Rank test) and remained significant after accounting for either stage or age in a multivariate model (p = 0.04, Cox proportional hazards). In paired specimens, BRCA1 protein expression increased in 13/21 (62%) and BRCA2 protein expression increased in 15/21 (71%) of recurrent carcinomas with low or intermediate protein in the paired primary. In contrast MLH1 expression was rarely decreased in recurrent carcinomas (1/33, 3%). Similar frequencies of MLH1, BRCA1, and FANCF promoter methylation occurred in primary carcinomas without previous chemotherapy, after neoadjuvant chemotherapy, or in recurrent neoplasms.
Low BRCA1 expression in primary sporadic ovarian carcinoma is associated with prolonged survival. Recurrent ovarian carcinomas commonly have increased BRCA1 and/or BRCA2 protein expression post chemotherapy exposure which could mediate resistance to platinum based therapies. However, alterations in expression of these proteins after chemotherapy are not commonly mediated by promoter methylation, and other regulatory mechanisms are likely to contribute to these alterations.
The breast cancer susceptibility gene 1 (BRCA1) is mutated in approximately 50% of hereditary breast cancers, and its expression is decreased in 30–40% of sporadic breast cancers, suggesting a general role in breast cancer development. BRCA1 physically and functionally interacts with estrogen receptor-alpha (ERα) and several transcriptional regulators. We investigated the relationship between cellular BRCA1 levels and tamoxifen sensitivity. Decreasing BRCA1 expression in breast cancer cells by small interfering RNA alleviated tamoxifen-mediated growth inhibition and abolished tamoxifen suppression of several endogenous ER-targeted genes. ER-stimulated transcription and cytoplasmic signaling was increased without detectable changes in ER or ER coregulator expression. Co-immunoprecipitation studies showed that with BRCA1 knockdown, tamoxifen-bound ERα was inappropriately associated with coactivators, and not effectively with corepressors. Chromatin immunoprecipitation studies demonstrated that with tamoxifen, BRCA1 knockdown did not change ERα promoter occupancy, but resulted in increased coactivator and decreased corepressor recruitment onto the endogenous cyclin D1 promoter. Our results suggest that decreased BRCA1 levels modify ERα-mediated transcription and regulation of cell proliferation in part by altering ERα-coregulator association. In the presence of tamoxifen, decreased BRCA1 expression results in increased coactivator and decreased corepressor recruitment on ER-regulated gene promoters.
tamoxifen resistance; BRCA1, ERα; coactivators; corepressors
Mutations in BRCA1 account for a significant proportion of hereditary breast and ovarian cancers, but analysis of BRCA1 function is complicated by pleiotropic effects and binding partners (Pol II holoenzyme and transcription factors, chromatin remodelers, recombination complexes and E3 ligases). In vertebrate cells, efforts to elucidate BRCA1 transcriptional effects have focused on specific genes or restricted portions of the genome—limiting analyses of BRCA1 effects on adjoining DNA sequences and along chromosome lengths. Here, we use microarray analyses on the genetically tractable yeast cell system to elucidate BRCA1-dependent genome-wide positional effects on both gene induction and repression. Yeast responses may be of clinical relevance based on findings that BRCA1 severely diminishes yeast growth kinetics but that BRCA1 mutated at sites identified from breast tumors is no longer able to retard yeast cell growth kinetics. Our analysis suggests that BRCA1 acts through both transcription factors to upregulate specific loci and chromatin remodeling complexes to effect global changes in gene expression. BRCA1 also exhibits gene repression activities. Cluster-functional analysis reveals that these repressed factors are required for mitotic stability and provide a novel molecular explanation for the conditional lethality observed between BRCA1 and chromosome segregation genes.
BRCA1; gene expression; chromosome segregation; aneuploidy; microarray; chromatin remodeling
Germline mutations in BRCA1 and BRCA2 genes predispose to hereditary breast and ovarian cancer. Our aim was to find associations between the clinical characteristics and positive mutation status in 148 breast cancer families in order to predict the probability of finding a BRCA mutation in a family. Several factors were associated with mutations in univariate analysis, whereas in multivariate analysis (logistic regression with backward selection) only the age of the youngest breast cancer patient and the number of ovarian cancer cases in a family were independent predictors of BRCA mutations. A logistic model was devised to estimate the probability for a family of harbouring a mutation in either BRCA1 or BRCA2. Altogether, 63 out of 148 families (43%) and 28 out of 29 (97%) mutation carrier families obtained probabilities over 10%. The mean probability was 55% for mutation-positive families and 11% for mutation-negative families. The models by Couch et al (1997) and Shattuck-Eidens et al (1997) previously designed for BRCA1 were also tested for their applicability to distinguish carrier families with mutations in either gene. The probability model should be a useful tool in genetic counselling and focusing the mutation analyses, and thus increasing also the cost-effectiveness of the genetic screening. © 2001 Cancer Research Campaign http://www.bjcancer.com
breast cancer; ovarian cancer; BRCA1; BRCA 2; mutation; probability model
BRCA1, a product of a familial breast and ovarian cancer susceptibility gene, localizes to centrosomes and physically interacts with γ-tubulin, a key centrosomal protein for microtubule nucleation and anchoring at centrosomes. Here, we performed a rigorous analysis of centrosome localization of BRCA1, and found that BRCA1 is specifically associated with mother centrioles in unduplicated centrosomes, and daughter centrioles acquire BRCA1 prior to initiation of duplication, and thus duplicated centrosomes are both bound by BRCA1. We further found that BRCA1 suppresses centrosomal aster formation. In addition, we identified a new domain of BRCA1 critical for γ-tubulin binding, which confers not only its localization to centrosomes, but also its activity to suppress centrosomal aster formation.
BRCA1; centrosome; centriole; microtubules; aster formation
Germline mutations in BRCA2 gene predispose women to early-onset familial breast and ovarian cancer. BRCA2 is a protein of multiple functions. In addition to its role in DNA double-strand breaks repair, BRCA2 also plays a role in stabilization of stalled DNA replication forks, cytokinesis, transcription regulation, mammalian gametogenesis, centrosome duplication, and suppression of cell proliferation. However, how BRCA2 mutations predispose women specifically to breast and ovarian cancer remains undefined. Here we found that BRCA2 binds and stabilizes MAGE-D1, a member of the MAGE gene family of proteins. Expression of BRCA2 and MAGE-D1 synergistically suppresses cell proliferation independently of the p53 pathway. Using two MAGE-D1 RNAis and two cell lines expressing low or undetectable levels of MAGE-D1, we further demonstrated that the expression of MAGE-D1 is required for BRCA2-mediated suppression of cell proliferation, indicating that MAGE-D1 is a downstream target of BRCA2 and that BRCA2 suppresses cell proliferation via stabilizing MAGE-D1. Importantly, MAGE-D1 protein expression was reduced in 6 out of 16 breast carcinoma cell lines tested as compared with untransformed immortal mammary epithelial cell lines, suggesting that suppression of MAGE-D1 expression may be involved in the tumorigenesis of a subset of sporadic breast cancers.
Multiple copies of the mitochondrial genome in eukaryotic cells are organized into protein–DNA complexes called nucleoids. Mitochondrial genome repair mechanisms have been reported, but they are less well characterized than their nuclear counterparts. To expand our knowledge of mitochondrial genome maintenance, we have studied the localization of the BRCA1 protein, known to be involved in nuclear repair pathways. Our confocal and immunoelectron microscopy results show that BRCA1 is present in mitochondria of several human cancer cell lines and in primary breast and nasal epithelial cells. BRCA1 localization in mitochondria frequently overlapped that of nucleoids. Small interfering RNA-mediated knockdown of BRCA1 in human cancer cells (confirmed by Western blot) results in decreased nuclear, cytoplasmic, and mitochondrial staining after immunofluorescence microscopy, establishing the specificity of the BRCA1 immunolabeling. Furthermore, using cell fractionation, dephosphorylation, and enzyme protection experiments, we show that a 220-kDa phosphorylated isoform of BRCA1 is enriched in mitochondrial and nuclear fractions but reduced in cytoplasmic subcellular fractions. Submitochondrial fractionation confirmed the presence of BRCA1 protein in isolated mitoplasts. Because phosphorylation of BRCA1 and subsequent changes in subcellular localization are known to follow DNA damage, our data support a universal role for BRCA1 in the maintenance of genome integrity in both mitochondria and nucleus.
BRCA1 is a tumor suppressor which plays a crucial role in the repair of DNA double-strand breaks, and its abnormality is responsible for hereditary ovarian cancer syndrome. It has recently been reported that reduced expression of BRCA1 is also common in sporadic ovarian carcinoma via its promoter hypermethylation, and that ovarian carcinoma patients negative for BRCA1 expression showed favorable prognosis. To address if BRCA1 expression plays a role in the chemotherapeutic response, we analyzed the effect of BRCA1 suppression on the sensitivity to cisplatin and paclitaxel in ovarian cancer cells. Specific siRNA for BRCA1 gene was transfected into 3 ovarian cancer cell lines with various p53 status. Reduced expression of BRCA1 by transfection of BRCA1-siRNA resulted in a 5.3-fold increase in sensitivity to cisplatin in p53-wild A2780 cells, but not in p53-mutated A2780/CDDP and p53-deleted SKOV3 cells. Regarding the sensitivity to paclitaxel, BRCA1 suppression caused no significant changes in all the 3 cell lines. For ionizing radiation sensitivity, BRCA1 suppression also showed a significant higher sensitivity in A2780 cells. Growth curve and cell cycle analyses showed no significant differences between BRCA1-siRNA-transfected A2780 cells and control cells. However, cisplatin treatment under suppression of BRCA1 showed a significantly increased apoptosis along with up-regulation of p53 and p21 in A2780 cells. Accordingly, reduced expression of BRCA1 enhances the cisplatin sensitivity and apoptosis via up-regulation of p53 and p21, but does not affect the paclitaxel sensitivity. Expression of BRCA1 might be an important biomarker for cisplatin resistance in ovarian carcinoma.
BRCA1; ovarian cancer; biomarker; chemosensitivity; cisplatin
The BRCA2 and MRE11 proteins participate in the repair of double-strand DNA breaks by homologous recombination. Germline BRCA2 mutations predispose to ovarian, breast and pancreatic cancer, while a germline MRE11 mutation is associated with an ataxia telangiectasia-like disorder. Somatic mutations of BRCA2 are rare in typical sporadic cancers. In tumors having microsatellite instability (MSI), somatic truncating mutations in a poly [A] tract of BRCA2 are reported on occasion.
We analyzed gastrointestinal MSI cancers by whole gene BRCA2 sequencing, finding heterozygous truncating mutations in seven (47%) of 15 patients. There was no cellular functional defect in RAD51 focus-formation in three heterozygously mutated lines studied, although other potential functions of the BRCA2 protein could still be affected. A prior report of mutations in primary MSI tumors affecting the IVS5-(5–15) poly [T] tract of the MRE11 gene was confirmed and extended by analysis of the genomic sequence and protein expression in MSI cancer cell lines. Statistical analysis of the published MRE11 mutation rate in MSI tumors did not provide evidence for a selective pressure favoring biallelic mutations at this repeat.
Perhaps conflicting with common suspicions, the data are not compatible with selective pressures during tumorigenesis promoting the functional loss of BRCA2 and MRE11 in MSI tumors. Instead, these data fit closely with an absence of selective pressures acting on BRCA2 and MRE11 gene status during tumorigenesis.
Here, we show for the first time, that the familial breast/ovarian cancer susceptibility gene BRCA1 activates the Notch pathway in breast cells by transcriptional upregulation of Notch ligands and receptors in both normal and cancer cells. We demonstrate through chromatin immunoprecipitation assays that BRCA1 is localized to a conserved intronic enhancer region within the Notch ligand Jagged-1 (JAG1) gene, an event requiring ΔNp63. We propose that this BRCA1/ΔNp63-mediated induction of JAG1 may be important the regulation of breast stem/precursor cells, as knockdown of all three proteins resulted in increased tumoursphere growth and increased activity of stem cell markers such as Aldehyde Dehydrogenase 1 (ALDH1). Knockdown of Notch1 and JAG1 phenocopied BRCA1 knockdown resulting in the loss of Estrogen Receptor-α (ER-α) expression and other luminal markers. A Notch mimetic peptide could activate an ER-α promoter reporter in a BRCA1-dependent manner, whereas Notch inhibition using a γ-secretase inhibitor reversed this process. We demonstrate that inhibition of Notch signalling resulted in decreased sensitivity to the anti-estrogen drug Tamoxifen but increased expression of markers associated with basal-like breast cancer. Together, these findings suggest that BRCA1 transcriptional upregulation of Notch signalling is a key event in the normal differentiation process in breast tissue.
BRCA1, the breast and ovarian cancer specific tumor suppressor, is a transcriptional repressor as well as a transcriptional activator dependent on the promoter context. In order to identify the genes activated or repressed by BRCA1, we have analyzed microarray results from cells depleted of BRCA1 and revealed a number of genes regulated by BRCA1 on the level of transcription. Among the genes repressed by BRCA1 we have identified Amphiregulin (AREG) and Early Growth Response-1 (EGR1). Results indicate that BRCA1 regulates AREG transcription directly through the binding to the AREG promoter, however we could not detect BRCA1 on the EGR1 promoter, suggesting that EGR1 is indirectly regulated by BRCA1. In an attempt to indentify the mechanism of the AREG transcriptional repression by BRCA1, we have mapped two independent BRCA1-response elements on the AREG promoter located at the positions −202/−182 and +19/+122. BRCA1 depletion leads to induction of AREG protein. Taken together, our data builds the connection between BRCA1 loss of function and AREG up-regulation, a change in gene expression often observed in breast cancer.
BRCA1; Amphiregulin (AREG); transcription regulation; breast cancer
This article presents a comprehensive review of the literature on the role of BRCAness in ovarian cancer with respect to BRCA function, methods of BRCA epigenetic defect detection and molecular profiling, and the implications of BRCA dysfunction for ovarian cancer treatment.
Ovarian cancer is the leading cause of death among gynecological cancers. It exhibits great heterogeneity in tumor biology and treatment response. Germline mutations of DNA repair genes BRCA1/2 are the fundamental defects in hereditary ovarian cancer that expresses a distinct phenotype of high response rates to platinum agents, improved disease-free intervals and survival rates, and high-grade serous histology. The term “BRCAness” describes the phenotypic traits that some sporadic ovarian tumors share with tumors in BRCA1/2 germline mutation carriers and reflects similar causative molecular abnormalities. BRCA pathway studies and molecular profiling reveal BRCA-related defects in almost half of the cases of ovarian cancer. BRCA-like tumors are particularly sensitive to DNA-damaging agents (e.g., platinum agents) because of inadequate BRCA-mediated DNA repair mechanisms, such as nucleotide-excision repair and homologous recombination (HR). Additional inhibition of other DNA repair pathways leads to synthetic lethality in HR-deficient cells; this has been employed in the treatment of BRCA-like ovarian tumors with poly(ADP-ribose) polymerase inhibitors with promising results. This article presents a comprehensive review of the relevant literature on the role of BRCAness in ovarian cancer with respect to BRCA function, methods of BRCA epigenetic defect detection and molecular profiling, and the implications of BRCA dysfunction in the treatment of ovarian cancer.
BRCAness; BRCA1/2; Ovarian cancer; PARP inhibitors; Synthetic lethality
Germline mutations in the BRCA1 and BRCA2 genes have been shown to account for the majority of hereditary breast and ovarian cancers. The purpose of our study was to estimate the incidence and spectrum of pathogenic mutations in BRCA1/2 genes in high-risk Czech families.
A total of 96 Czech families with recurrent breast and/or ovarian cancer and 55 patients considered to be at high-risk but with no reported family history of cancer were screened for mutations in the BRCA1/2 genes. The entire coding sequence of each gene was analyzed using a combination of the protein truncation test and direct DNA sequencing.
A total of 35 mutations in the BRCA1/2 genes were identified in high-risk families (36.5%). Pathogenic mutations were found in 23.3% of breast cancer families and in 59.4% of families with the occurrence of both breast and ovarian cancer. In addition, four mutations were detected in 31 (12.9%) women with early onset breast cancer. One mutation was detected in seven (14.3%) patients affected with both a primary breast and ovarian cancer and another in three (33.3%) patients with a bilateral breast cancer. A total of 3 mutations in BRCA1 were identified among 14 (21.4%) women with a medullary breast carcinoma. Of 151 analyzed individuals, 35 (23.2%) carried a BRCA1 mutation and 9 (6.0%) a BRCA2 mutation. One novel truncating mutation was found in BRCA1 (c.1747A>T) and two in BRCA2 (c.3939delC and c.5763dupT). The 35 identified BRCA1 mutations comprised 13 different alterations. Three recurrent mutations accounted for 71.4% of unrelated individuals with detected gene alterations. The BRCA1 c.5266dupC (5382insC) was detected in 51.4% of mutation positive women. The mutations c.3700_3704del5 and c.181T>G (300T>G) contributed to 11.4% and 8.6% of pathogenic mutations, respectively. A total of eight different mutations were identified in BRCA2. The novel c.5763dupT mutation, which appeared in two unrelated families, was the only recurrent alteration of the BRCA2 gene identified in this study.
Mutational analysis of BRCA1/2 genes in 151 high-risk patients characterized the spectrum of gene alterations and demonstrated the dominant role of the BRCA1 c.5266dupC allele in hereditary breast and ovarian cancer.
Whereas HER2 amplification is a well-known phenomenon in breast tumours, its frequency and clinical importance in ovarian cancer have not been established. The aim of the study was to compare the frequency of HER2 amplification in hereditary (BRCA-positive) and sporadic (BRCA-negative) ovarian tumours and to estimate the association of this gene alteration on clinical outcome in ovarian cancer patients. We analysed HER2 amplification in 53 ovarian tumours: 20 from mutation carriers (18 in BRCA1 and 2 in BRCA2 gene) and 33 from non-carriers. Fluorescence in situ hybridization for HER2 was performed on 'touch' slides from frozen tumour samples or formalin-fixed, paraffin-embedded tissue. Our results indicate that high amplification (HER2: centromere ratio>5) is an infrequent phenomenon in ovarian tumours (6/53 cases). It occurs in both hereditary (4/20) and sporadic (2/33) tumours and no difference in the frequency of HER2 amplification exists between these groups. There is no significant difference in the clinical outcome of patients with HER2 amplified and non-amplified tumours (p = 0.3). Our results suggest a different biological role of HER2 amplification in ovarian and breast cancer.
HER2; ovarian cancer; BRCA; outcome