BRCA1: cell cycle checkpoint, genetic instability, DNA damage response and cancer evolution

Epigenetic modulation of estrogen receptor-alpha by pRb family proteins: a novel mechanism in breast cancer

Estrogen receptor-alpha (ER-alpha) plays a crucial role in normal breast development and has also been linked to mammary carcinogenesis and clinical outcome in breast cancer patients. However, ER-alpha gene expression can change during the course of disease and, consequently, therapy resistance can occur. The molecular mechanism governing ER-alpha transcriptional activity and/or silencing is still unclear. Here, we showed that the presence of a specific pRb2/p130 multimolecular complex on the ER-alpha promoter strongly correlates with the methylation status of this gene. Furthermore, we suggested that pRb2/p130 could cooperate with ICBP90 (inverted CCAAT box binding protein of 90 kDa) and DNA methyltransferases in maintaining a specific methylation pattern of ER-alpha gene. The sequence of epigenetic events for establishing and maintaining the silenced state of ER-alpha gene can be locus- or pathway- specific, and the local remodeling of ER-alpha chromatin structure by pRb2/p130 multimolecular complexes may influence its susceptibility to specific DNA methylation. Our novel hypothesis could provide a basis for understanding how the complex pattern of ER-alpha methylation and transcriptional silencing is generated and for understanding the relationship between this pattern and its function during the neoplastic process.

Structural requirements for the BARD1 tumor suppressor in chromosomal stability and homology-directed DNA repair

The BRCA1 tumor suppressor exists as a heterodimeric complex with BARD1, and this complex is thought to mediate many of the functions ascribed to BRCA1, including its role in tumor suppression. The two proteins share a common structural organization that features an N-terminal RING domain and two C-terminal BRCT motifs, whereas BARD1 alone also contains three tandem ankyrin repeats. In normal cells, the BRCA1/BARD1 heterodimer is believed to enhance chromosome stability by promoting homology-directed repair (HDR) of double strand DNA breaks. Here we have investigated the structural requirements for BARD1 in this process by complementation of Bard1-null mouse mammary carcinoma cells. Our results demonstrate that the ankyrin and BRCT motifs of BARD1 are each essential for both chromosome stability and HDR. Tandem BRCT motifs, including those found at the C terminus of BARD1, are known to form a phosphoprotein recognition module. Nonetheless, the HDR function of BARD1 was not perturbed by synthetic mutations predicted to ablate the phospho-recognition activity of its BRCT sequences, suggesting that some functions of the BRCT domains are not dependent on their ability to bind phosphorylated ligands. Also, cancer-associated missense mutations in the BRCT domains of BARD1 (e.g. C557S, Q564H, V695L, and S761N) have been observed in patients with breast, ovarian, and endometrial tumors. However, none of these was found to affect the HDR activity of BARD1, suggesting that any increased cancer risk conferred by these mutations is not because of defects in this repair mechanism.

Identification of an Integrated SV40 T/t-Antigen Cancer Signature in Aggressive Human Breast, Prostate, and Lung Carcinomas with Poor Prognosis

Understanding the genetic architecture of cancer pathways that distinguishes subsets of human cancer is critical to developing new therapies that better target tumors based on their molecular expression profiles. In this study, we identify an integrated gene signature from multiple transgenic models of epithelial cancers intrinsic to the functions of the Simian virus 40 T/t-antigens that is associated with the biological behavior and prognosis for several human epithelial tumors. This genetic signature, composed primarily of genes regulating cell replication, proliferation, DNA repair, and apoptosis, is not a general cancer signature. Rather, it is uniquely activated primarily in tumors with aberrant p53, Rb, or BRCA1 expression but not in tumors initiated through the overexpression of myc, ras, her2/neu, or polyoma middle T oncogenes. Importantly, human breast, lung, and prostate tumors expressing this set of genes represent subsets of tumors with the most aggressive phenotype and with poor prognosis. The T/t-antigen signature is highly predictive of human breast cancer prognosis. Because this class of epithelial tumors is generally intractable to currently existing standard therapies, this genetic signature identifies potential targets for novel therapies directed against these lethal forms of cancer. Because these genetic targets have been discovered using mammary, prostate, and lung T/t-antigen mouse cancer models, these models are rationale candidates for use in preclinical testing of therapies focused on these biologically important targets.

The molecular genetics of breast cancer and targeted therapy

Breast cancer is a complex, molecular disease, in which a number of cellular pathways involving cell growth and proliferation, such as the MAPK, RB/E2F, P13K/AKT/mTOR, and TP53 pathways, are altered. These pathways represent molecular mechanisms that are composed and regulated by various genes. The genes that are altered in terms of cell growth and proliferation include the oncogenes HER2, c-MYC, and RAS, the ER genes, and the genes for cell cyclin D1 and E, and the tumor suppressor genes RB, TP53, and PTEN, and the breast cancer susceptibility genes BRCA1 and BRCA2. Although the nature of breast cancer is complex and has frustrated previous attempts at treatment or prevention, the elucidation of its molecular nature over the last several decades is now providing targets for effective therapies to treat the disease and hopefully one day to prevent it.

Zebrafish as a model system to screen radiation modifiers

Zebrafish (Danio rerio) is a bona fide vertebrate model system for understanding human diseases. It allows the transparent visualization of the effects of ionizing radiation and the convenient testing of potential radioprotectors with morpholino-modified oligonucleotides (MO) knockdown. Furthermore, various reverse and forward genetic methods are feasible to decipher novel genetic modifiers of radioprotection. Examined in the review are the radioprotective effects of the proposed radiomodifiers Nanoparticle DF-1 (C-Sixty, Inc., Houston, TX) and Amifostine (WR-2721, Ethyol), the DNA repair proteins Ku80 and ATM, as well as the transplanted hematopoietic stem cells in irradiated zebrafish. The presence of any of these sufficiently rescued the radiation-induced damages in zebrafish, while its absence resulted in mutagenic phenotypes as well as an elevation of time- and dose-dependent radiation-induced apoptosis. Radiosensitizers Flavopiridol and AG1478, both of which block progression into the radioresistant S phase of the cell cycle, have also been examined in zebrafish. Zebrafish has indeed become a favorite model system to test for radiation modifiers that can potentially be used for radiotherapeutic purposes in humans.

The BRCA1/BRCA2/Rad51 complex is a prognostic and predictive factor in early breast cancer

BACKGROUND AND PURPOSE

The breast cancer susceptibility genes BRCA1 and BRCA2 interact with Rad51, one of the central components in the homologous recombination repair pathway. This study evaluates the prognostic and predictive role of BRCA1, BRCA2 and Rad51, individually and as a complex, in breast cancer.

MATERIALS AND METHODS

Expression of BRCA1, BRCA2 and Rad51 was investigated using immunohistochemistry in tumours from 224 women with early breast cancer, who were randomised to receive postoperative radiotherapy or adjuvant chemotherapy (CMF).

RESULTS

Fifty-three percent (112/212) of the tumours had reduced expression of the BRCA1/BRCA2/Rad51 complex. Low expression correlated to high histologic grade (p=0.05). Patients with low expression of the complex developed significantly more local recurrences as compared to patients with high expression (RR=3.20, 95% CI 1.48-6.88, p=0.003). Expression of the BRCA1/BRCA2/Rad51 complex was an independent prognostic factor in multivariate analysis (p=0.03). Patients with low expression of the complex responded well to radiotherapy (RR=0.31, 95% CI 0.14-0.70, p=0.005), whereas patients with high expression had few local recurrences and no additional benefit from radiotherapy (RR=1.08, 95% CI 0.40-2.90, p=0.88).

CONCLUSIONS

Low expression of the BRCA1/BRCA2/Rad51 complex is a marker of poor prognosis, but predicts good response to radiotherapy in patients with early breast cancer.

The BRCA1/2 pathway prevents hematologic cancers in addition to breast and ovarian cancers

BACKGROUND

The present study was designed to test the hypothesis that inactivation of virtually any component within the pathway containing the BRCA1 and BRCA2 proteins would increase the risks for lymphomas and leukemias. In people who do not have BRCA1 or BRCA2 gene mutations, the encoded proteins prevent breast/ovarian cancer. However BRCA1 and BRCA2 proteins have multiple functions including participating in a pathway that mediates repair of DNA double strand breaks by error-free methods. Inactivation of BRCA1, BRCA2 or any other critical protein within this "BRCA pathway" due to a gene mutation should inactivate this error-free repair process. DNA fragments produced by double strand breaks are then left to non-specific processes that rejoin them without regard for preserving normal gene regulation or function, so rearrangements of DNA segments are more likely. These kinds of rearrangements are typically associated with some lymphomas and leukemias.

METHODS

Literature searches produced about 2500 epidemiology and basic science articles related to the BRCA pathway. These articles were reviewed and copied to a database to facilitate access. Meta-analyses of statistical information compared risks for hematologic cancers vs. mutations for the components in a model pathway containing BRCA1/2 gene products.

RESULTS

Deleterious mutations of genes encoding proteins virtually anywhere within the BRCA pathway increased risks up to nearly 2000 fold for certain leukemias and lymphomas. Cancers with large increases in risk included mantle cell lymphoma, acute myeloid leukemia, acute lymphocytic leukemia, chronic lymphocytic leukemia, and prolymphocytic leukemia. Mantle cell lymphoma is defined by a characteristic rearrangement of DNA fragments interchanged between chromosomes 11 and 14. DNA translocations or rearrangements also occur in significant percentages of the other cancers.

CONCLUSION

An important function of the BRCA pathway is to prevent a subgroup of human leukemias and lymphomas that may involve non-random, characteristic gene rearrangements. Here, the genetic defect in BRCA pathway deficiencies is a chromosomal misrepair syndrome that may facilitate this subgroup of somatic cancers. Inactivation of a single gene within the pathway can increase risks for multiple cancers and inactivation of a different gene in the same pathway may have similar effects. The results presented here may have clinical implications for surveillance and therapy.

Centrosomal localization of DNA damage checkpoint proteins

During mitosis, the phosphatidylinositol-3 (PI-3) family-related DNA damage checkpoint kinases ATM and ATR were found on the centrosomes of human cells. ATRIP, an interaction partner of ATR, as well as Chk1 and Chk2, the downstream targets of ATR or ATM, were also localized to the centrosomes. Surprisingly, the DNA-PK inhibitor vanillin enhanced the level of ATM on centrosomes. Accordingly, DNA-PKcs, the catalytic subunit of DNA-PK, was also found on the centrosomes. Vanillin altered the phosphorylation of Chk2 in the centrosomes and in whole cell extracts. Nucleoplasmic ATM co-immunoprecipitated with Ku70/86, the DNA binding subunits of DNA-PK, while vanillin diminished this association. Vanillin did not affect microtubule polymerization at the centrosomes but, surprisingly, caused a transient enhancement of alpha-tubulin foci in the nucleus. Interestingly, gamma-tubulin was also present in the nucleus and co-immunoprecipitated with ATR or BRCA1. DNA damage led to a reduction of the mentioned checkpoint proteins on the centrosomes but increased the level of gamma-tubulin at this organelle. Taken together, these results indicate that DNA damage checkpoint proteins may control the formation of gamma-tubulin and/or the kinetics of microtubule formation at the centrosomes, and thereby couple them to the DNA damage response.

The impact of BRCA1 on spina bifida meningomyelocele lesions

We examined the BRCA1 gene in 268 patients, and their parents, with a specific diagnosis of spina bifida meningomyelocele (SBMM). We genotyped two intragenic microsatellite markers (BRCA1 D17S1323, BRCA1 D17S1322) and 2 single nucleotide polymorphisms (A1186G, A4956G) in our patients. Transmission disequilibrium testing (TDT) showed significant association with A4956G, but not with A1186G. Extended TDT demonstrated over-transmission of the 17GT allele in BRCA1 D17S1323 and the 14GTT allele in BRCA1 D17S1322, and under-transmission of the 20GT allele in BRCA1 D17S1323 and the 16GTT allele in BRCA1 D17S1322. Our data included location of the rostral edge of the lesion. Individuals homozygous for the 17GT allele for BRCA1 D17S1323 were more likely to have SB lesions located caudally, while heterozygotes with the 17GT allele for BRCA1 D17S1323 had a more rostral lesion. Individuals heterozygous for the 16GTT allele of BRCA1 D17S1322 were more likely to have rostral lesions. We measured gene expression in CEPH members and demonstrated differential expression levels of BRCA1 associated with these polymorphisms. Integrating our data with HapMap findings showed that the polymorphic markers are associated with distinct haplotypes. We conclude that the BRCA1 gene is associated with SBMM and participates in the phenotypic variability seen in SBMM.

The ubiquitin-interacting motif containing protein RAP80 interacts with BRCA1 and functions in DNA damage repair response

In this study, we examine the potential role of receptor-associated protein 80 (RAP80), a nuclear protein containing two ubiquitin-interacting motifs (UIM), in DNA damage response and double-strand break (DSB) repair. We show that following ionizing radiation and treatment with DNA-damaging agents, RAP80 translocates to discrete nuclear foci that colocalize with those of gamma-H2AX. The UIMs and the region of amino acids 204 to 304 are critical for the relocalization of RAP80 to ionizing radiation-induced foci (IRIF). These observations suggest that RAP80 becomes part of a DNA repair complex at the sites of IRIF. We also show that RAP80 forms a complex with the tumor repressor BRCA1 and that this interaction is mediated through the BRCA1 COOH-terminal repeats of BRCA1. The UIMs are not required for the interaction of RAP80 with BRCA1. Knockdown of RAP80 in HEK293 cells significantly reduced DSB-induced homology-directed recombination (HDR). Moreover, inhibition of RAP80 expression by small interfering RNA increased radiosensitivity, whereas increased radioresistance was observed in human breast cancer MCF-7 cells with overexpression of RAP80. Taken together, our data suggest that RAP80 plays an important role in DNA damage response signaling and HDR-mediated DSB repair. We further show that RAP80 can function as a substrate of the ataxia-telangiectasia mutated protein kinase in vitro, which phosphorylates RAP80 at Ser(205) and Ser(402). We show that this phosphorylation is not required for the migration of RAP80 to IRIF.

Synergistic activation of the tumor suppressor, HLJ1, by the transcription factors YY1 and activator protein 1

HLJ1 is a novel tumor and invasion suppressor that inhibits tumorigenesis and cancer metastasis. However, the mechanism of HLJ1 activation is currently unclear. Here, we identify an enhancer segment in the HLJ1 gene at -2,125 to -1,039 bp upstream of the transcription start site. A 50-bp element between -1,492 and -1,443 bp is the minimal enhancer segment, which includes the activator protein 1 (AP-1) site (-1,457 to -1,451 bp), an essential regulatory domain that binds the transcriptional factors FosB, JunB, and JunD. Chromatin immunoprecipitation assays confirm that these AP-1 family members bind to a specific site in the HLJ1 enhancer segment in vivo. Overexpression of either YY1 at promoter or AP-1 at enhancer results in a 3-fold increase in the transcriptional activity of HLJ1. We propose a novel mechanism whereby expression of the tumor suppressor, HLJ1, is up-regulated via enhancer AP-1 binding to promoter YY1 and the coactivator, p300, through DNA bending and multiprotein complex formation. The combined expression of AP-1 and YY1 enhances HLJ1 expression by more than five times and inhibits in vitro cancer cell invasion. Elucidation of the regulatory mechanism of HLJ1 expression may facilitate the development of personalized therapy by inhibiting cancer cell proliferation, angiogenesis, and metastasis.

Polymorphism of ZuBeCa3 microsatellite and its association with mammary tumor in dogs

A study was conducted to explore the genetic polymorphism at ZuBeCa3 microsatellite and to estimate the association between microsatellite polymorphism and mammary cancer in dogs. Three genotypes namely AA, AB and BB were observed both in dogs affected by mammary cancer and in non-affected dogs. In the affected group, the frequency of genotype and allele was 0.63 for AA, 0.25 for AB and 0.12 for BB genotype, and 0.76 for A and 0.24 for B allele. In the nonaffected group, the frequency of AA, AB and BB genotype was 0.39, 0.38 and 0.23 and the frequency of A and B allele was 0.58 and 0.42, respectively. Histopathological observation classified the cancer-affected animals into three groups namely, malignant solid mammary carcinoma, malignant papillary adenocarcinoma and benign papillary adenoma in which the frequency of A allele was relatively more predominant in benign tumor group, which is more than 80%. Statistical tests showed significant differences (P < 0.01) of allelic distribution between tumor-affected and non-affected group, which reveals the effect of polymorphism on occurrences of mammary cancer in dogs. Besides, chi(2) test also reflected the significant effect of genotypes on occurrences of three groups of mammary cancer in dogs.

High prevalence of BRCA1 deletions in BRCAPRO-positive patients with high carrier probability

Mutation screening of the BRCA1 and BRCA2 genes in probands with familial breast/ovarian cancer has been greatly improved by the multiplex ligation-dependent probe amplification (MLPA) assay able to evidence gene rearrangements not detectable by standard screening methods. However, no criteria for selection of cases to be submitted to the MLPA test have been reported yet. We used the BRCAPro software for the selection of familial breast/ovarian cancer probands investigated with the MLPA approach after negative BRCA1/2 conventional mutation screening. One hundred and seventy-seven probands were investigated for germline BRCA1/2 mutations after assessment of genetic risk using BRCAPro. Probands were classified as BRCAPro positive (n = 67) when the carrier probability (CP) was >10% and as BRCAPro negative (n = 110), when the CP was <10%. Conventional mutational analyses of the BRCA1/2 genes and, in one case, of p53 identified 22 pathogenetic germline mutations, 12 in BRCA1, 9 in BRCA2 and 1 in p53, in 22/177 (12.4%) probands. All the mutations except one were detected in BRCAPro-positive patients. In the 46 BRCAPro-positive cases that resulted negative by BRCA1/2 mutation, screening analysis of rearrangements within BRCA1/2 by MLPA was carried out. Three patients with a very high CP showed BRCA1 deletions, consisting of deletions of exons 1-2 in two probands and of exon 24 in the third proband. In one case, the exons 1-2 deletion was shown to cosegregate with disease in the family. No BRCA2 rearrangements were detected, but one patient showed the 1100delC of the CHEK2 gene, whose probe is present in the BRCA2 kit. In our series, the highest carrier detection rate of mutation screening plus MLPA analysis (52.3%) was in patients with a BRCAPro CP >50%.

Identification of candidate genes involved in neuroblastoma progression by combining genomic and expression microarrays with survival data

Identifying genes, whose expression is consistently altered by chromosomal gains or losses, is an important step in defining genes of biological relevance in a wide variety of tumour types. However, additional criteria are needed to discriminate further among the large number of candidate genes identified. This is particularly true for neuroblastoma, where multiple genomic copy number changes of proven prognostic value exist. We have used Affymetrix microarrays and a combination of fluorescent in situ hybridization and single nucleotide polymorphism (SNP) microarrays to establish expression profiles and delineate copy number alterations in 30 primary neuroblastomas. Correlation of microarray data with patient survival and analysis of expression within rodent neuroblastoma cell lines were then used to define further genes likely to be involved in the disease process. Using this approach, we identify >1000 genes within eight recurrent genomic alterations (loss of 1p, 3p, 4p, 10q and 11q, 2p gain, 17q gain, and the MYCN amplicon) whose expression is consistently altered by copy number change. Of these, 84 correlate with patient survival, with the minimal regions of 17q gain and 4p loss being enriched significantly for such genes. These include genes involved in RNA and DNA metabolism, and apoptosis. Orthologues of all but one of these genes on 17q are overexpressed in rodent neuroblastoma cell lines. A significant excess of SNPs whose copy number correlates with survival is also observed on proximal 4p in stage 4 tumours, and we find that deletion of 4p is associated with improved outcome in an extended cohort of tumours. These results define the major impact of genomic copy number alterations upon transcription within neuroblastoma, and highlight genes on distal 17q and proximal 4p for downstream analyses. They also suggest that integration of discriminators, such as survival and comparative gene expression, with microarray data may be useful in the identification of critical genes within regions of loss or gain in many human cancers.

Identification and characterization of a novel p42.3 gene as tumor-specific and mitosis phase-dependent expression in gastric cancer

Multiple genetic alterations are attributed to gastric cancer (GC); however, only a few critical genes have been identified so far. In this study, we isolated and characterized a novel gene p42.3, represented as tumor-specific and mitosis phase-dependent expression protein in GC cell line BGC823. Our data showed that the expression of p42.3 was cell cycle-dependent in GC cell lines. Moreover, p42.3 was specifically expressed in primary GC tissues but not in the matched normal mucosa of stomach, and this gene was expressed in diverse embryonic tissues. Furthermore, significant suppression of cell proliferation and tumorigenicity were detected and G(2)/M phase arrest was observed in cell line BGC823 depleted of p42.3 expression by RNAi technique, and we confirmed the expression changes of cyclin B1 and Chk2 following the silence of p42.3. Taken together, we cloned and characterized p42.3 gene that was specifically expressed in GC tumors but not in normal gastric mucosa, and the gene was associated with M-phase regulation. Moreover, p42.3 might be involved in cell proliferation and tumorigenesis; therefore, this gene might have potential applications in the diagnosis or treatment of GC.

A role of estrogen/ERalpha signaling in BRCA1-associated tissue-specific tumor formation

Estrogen and its receptor alpha (ERalpha) have been implicated in the tissue-specific tumorigenesis associated with BRCA1 mutations. However, the majority of breast cancers developed in human BRCA1 mutation carriers are ERalpha-negative, challenging the link between BRCA1 and estrogen/ERalpha in breast cancer formation. Using a mouse model lacking the full-length form of BRCA1, here we show that ERalpha is highly expressed in the premalignant mammary gland and initiation stages of tumorigenesis, although its expression is gradually diminished during mammary tumor progression. We demonstrate that the absence of full-length BRCA1 increases sensitivity of cells to estrogen-induced extracellular signal-regulated kinase 1/2 phosphorylation and cyclin D1 expression. The absence of BRCA1 turns the proliferation of ERalpha-positive cells from a paracrine fashion to an autocrine or endocrine fashion. Consequently, BRCA1-mutant cells are sensitized to estrogen-induced cell proliferation in vitro and mammary tumorigenesis in vivo. These findings illustrate a molecular mechanism for estrogen/ERalpha signals in BRCA1-associated tissue-specific tumor formation, and identify several key elements in the estrogen/ERalpha-signaling cascade that can serve as potential therapeutic targets for BRCA1-associated tumorigenesis.

RINT-1 serves as a tumor suppressor and maintains Golgi dynamics and centrosome integrity for cell survival

Faithful mitotic partitioning of the Golgi apparatus and the centrosome is critical for proper cell division. Although these two cytoplasmic organelles are probably coordinated during cell division, supporting evidence of this coordination is still largely lacking. Here, we show that the RAD50-interacting protein, RINT-1, is localized at the Golgi apparatus and the centrosome in addition to the endoplasmic reticulum. To examine the biological roles of RINT-1, we found that the homozygous deletion of Rint-1 caused early embryonic lethality at embryonic day 5 (E5) to E6 and the failure of blastocyst outgrowth ex vivo. About 81% of the Rint-1 heterozygotes succumbed to multiple tumor formation with haploinsufficiency during their average life span of 24 months. To pinpoint the cellular function of RINT-1, we found that RINT-1 depletion by RNA interference led to the loss of the pericentriolar positioning and dispersal of the Golgi apparatus and concurrent centrosome amplification during the interphase. Upon mitotic entry, RINT-1-deficient cells exhibited multiple abnormalities, including aberrant Golgi dynamics during early mitosis and defective reassembly at telophase, increased formation of multiple spindle poles, and frequent chromosome missegregation. Mitotic cells often underwent cell death in part due to the overwhelming cellular defects. Taken together, these findings suggest that RINT-1 serves as a novel tumor suppressor essential for maintaining the dynamic integrity of the Golgi apparatus and the centrosome, a prerequisite to their proper coordination during cell division.

The XIST noncoding RNA functions independently of BRCA1 in X inactivation

Females with germline mutations in BRCA1 are predisposed to develop breast and ovarian cancers. A previous report indicated that BRCA1 colocalizes with and is necessary for the correct localization of XIST, a noncoding RNA that coats the inactive X chromosome (Xi) to mediate formation of facultative heterochromatin. A model emerged from this study suggesting that loss of BRCA1 in female cells could reactivate genes on the Xi through loss of the XIST RNA. However, our independent studies of BRCA1 and XIST RNA revealed little evidence to support this model. We report that BRCA1 is not enriched on XIST RNA-coated chromatin of the Xi. Neither mutation nor depletion of BRCA1 causes significant changes in XIST RNA localization or X-linked gene expression. Together, these results do not support a role for BRCA1 in promoting XIST RNA localization to the Xi or regulating XIST-dependent functions in maintaining the stability of facultative heterochromatin.

Absence of full-length Brca1 sensitizes mice to oxidative stress and carcinogen-induced tumorigenesis in the esophagus and forestomach

Environmental and genetic factors are important both in affecting life span and neoplastic transformation. We have shown previously that mice, which are homozygous for full-length breast cancer-associated gene-1 (Brca1) deletion and heterozygous for a p53-null mutation (Brca1(Delta11/Delta11)p53(+/-)), display premature aging and high frequency of spontaneous lymphoma and mammary tumor formation. To investigate the role of Brca1 in regulation of organ homeostasis and susceptibility of Brca1 deficiency to environmental carcinogens, we examined biological function of Brca1 in maintaining organ homeostasis and carcinogen-induced tumorigenesis. Brca1(Delta11/Delta11)p53(+/-) mice showed altered gastrointestinal tract homeostasis, including hyperkeratosis in the esophagus and forestomach. At 6 months of age, most mutant mice displayed hyperplasia in their forestomach and esophagus, leading to dysplasia and carcinoma formation in older animals. Brca1 mutant mice exhibited increased expression of Redd1, elevated reactive oxygen species and are more sensitive to oxidative stress induced lethality. Upon methyl-N-amylnitrosamine (MNAN) treatment, 70% Brca1 mutant mice developed tumors within 4 months whereas only 14% control animals developed tumor at the same period of the time. Our further analysis revealed that the tumorigenesis is accompanied by the loss of p53 and increased expression of a number of oncogenes, including Cyclin D1, phosphorylated form of Akt, beta-catenin, Runx-2 and c-Myc. These results suggest that Brca1 is involved in renewable organ homeostasis, linking the environmental and genetic factors in carcinogenesis and aging, and providing new insights into genomic instability in organism maintenance and tumorigenesis.

Functional characterization of a novel BRCA1-null ovarian cancer cell line in response to ionizing radiation

The breast and ovarian cancer susceptibility gene BRCA1 plays a major role in the DNA damage response pathway. The lack of well-characterized human BRCA1-null cell lines has limited the investigation of BRCA1 function, particularly with regard to its role in ovarian cancer. We propagated a novel BRCA1-null human ovarian cancer cell line UWB1.289 from a tumor of papillary serous histology, the most common form of ovarian carcinoma. UWB1.289 carries a germline BRCA1 mutation within exon 11 and has a deletion of the wild-type allele. UWB1.289 is estrogen and progesterone receptor negative and has an acquired somatic mutation in p53, similar to the commonly used BRCA1-null breast cancer cell line HCC1937. We used ionizing radiation to induce DNA damage in both UWB1.289 and in a stable UWB1.289 line in which wild-type BRCA1 was restored. We examined several responses to DNA damage in these cell lines, including sensitivity to radiation, cell cycle checkpoint function, and changes in gene expression using microarray analysis. We observed that UWB1.289 is sensitive to ionizing radiation and lacks cell cycle checkpoint functions that are a normal part of the DNA damage response. Restoration of wild-type BRCA1 function in these cells partially restores DNA damage responses. Expression array analysis not only supports this partial functional correction but also reveals interesting new information regarding BRCA1-positive regulation of the expression of claudin 6 and other metastasis-associated genes and negative regulation of multiple IFN-inducible genes.

Haploinsufficiency of Parp1 accelerates Brca1-associated centrosome amplification, telomere shortening, genetic instability, apoptosis, and embryonic lethality

The breast tumor associated gene-1 (BRCA1) and poly(ADP-ribose) polymerase-1 (PARP1) are both involved in DNA-damage response and DNA-damage repair. Recent investigations have suggested that inhibition of PARP1 represents a promising chemopreventive/therapeutic approach for specifically treating BRCA1- and BRCA2-associated breast cancer. However, studies in mouse models reveal that Parp1-null mutation results in genetic instability and mammary tumor formation, casting significant doubt on the safety of PARP1 inhibition as a therapy for the breast cancer. To study the genetic interactions between Brca1 and Parp1, we interbred mice carrying a heterozygous deletion of full-length Brca1 (Brca1(+/Delta11)) with Parp1-null mice. We show that Brca1(Delta11/Delta11);Parp1(-/-) embryos die before embryonic (E) day 6.5, whereas Brca1(Delta11/Delta11) embryos die after E12.5, indicating that absence of Parp1 dramatically accelerates lethality caused by Brca1 deficiency. Surprisingly, haploinsufficiency of Parp1 in Brca1(Delta11/Delta11) embryos induces a severe chromosome aberrations, centrosome amplification, and telomere dysfunction, leading to apoptosis and accelerated embryonic lethality. Notably, telomere shortening in Brca1(Delta11/Delta11);Parp1(+/-) MEFs was correlated with decreased expression of Ku70, which plays an important role in telomere maintenance. Thus, haploid loss of Parp1 is sufficient to induce lethality of Brca1-deficient cells, suggesting that partial inhibition of PARP1 may represent a practical chemopreventive/therapeutic approach for BRCA1-associated breast cancer.

The BRCT domain of mammalian Pes1 is crucial for nucleolar localization and rRNA processing

The nucleolar protein Pes1 interacts with Bop1 and WDR12 in a stable complex (PeBoW-complex) and its expression is tightly associated with cell proliferation. The yeast homologue Nop7p (Yph1p) functions in both, rRNA processing and cell cycle progression. The presence of a BRCT-domain (BRCA1 C-terminal) within Pes1 is quite unique for an rRNA processing factor, as this domain is normally found in factors involved in DNA-damage or repair pathways. Thus, the function of the BRCT-domain in Pes1 remains elusive. We established a conditional siRNA-based knock-down-knock-in system and analysed a panel of Pes1 truncation mutants for their functionality in ribosome synthesis in the absence of endogenous Pes1. Deletion of the BRCT-domain or single point mutations of highly conserved residues caused diffuse nucleoplasmic distribution and failure to replace endogenous Pes1 in rRNA processing. Further, the BRCT-mutants of Pes1 were less stable and not incorporated into the PeBoW-complex. Hence, the integrity of the BRCT-domain of Pes1 is crucial for nucleolar localization and its function in rRNA processing.

Cancer as an evolutionary and ecological process

Neoplasms are microcosms of evolution. Within a neoplasm, a mosaic of mutant cells compete for space and resources, evade predation by the immune system and can even cooperate to disperse and colonize new organs. The evolution of neoplastic cells explains both why we get cancer and why it has been so difficult to cure. The tools of evolutionary biology and ecology are providing new insights into neoplastic progression and the clinical control of cancer.

Role of single nucleotide polymorphisms and haplotypes in BRCA1 in breast cancer: Czech case-control study

We aimed at determining whether any association exists between six single nucleotide polymorphisms in breast cancer associated gene (BRCA1) and the risk of breast cancer. We constructed haplotypes and analyzed their importance as well. Clinico-pathological characteristics of breast cancer patients were included in the study to evaluate the prognostic impact of BRCA1 polymorphisms and haplotypes. Polymerase chain reaction-restriction fragment length polymorphism-based genotyping assays were used to determine the frequency of polymorphisms in codons 356, 871, 1038, 1183, 1436, and 1613 of BRCA1 in a group of 306 incident breast cancer patients and 313 unaffected controls of Czech origin. Statistical analyses revealed that the BRCA1 Arg356 allele may play a protective role in breast cancer (age-adjusted OR = 0.61, CI = 0.39-0.94, p = 0.026). We also observed a significant correlation between polymorphism Gln356Arg and stage (p = 0.026) in premenopausal cases suggesting that carriers of the wild Gln356Gln allele are at significantly higher risk of advanced disease. The most common haplotypes of BRCA1 did not play a significant role in breast cancer either as risk factors or as prognostic factors. The study on rare BRCA1 haplotypes however should be repeated using larger groups. In conclusion, the BRCA1-Gln356 allele presents risk factor in the onset and progression of breast cancer in Czech population and its use as a possible screening tool should be considered.

Does aneuploidy cause cancer?

Aneuploidy has been recognized as a common characteristic of cancer cells for >100 years. Aneuploidy frequently results from errors of the mitotic checkpoint, the major cell cycle control mechanism that acts to prevent chromosome missegregation. The mitotic checkpoint is often compromised in human tumors, although not as a result of germline mutations in genes encoding checkpoint proteins. Less obviously, aneuploidy of whole chromosomes rapidly results from mutations in genes encoding several tumor suppressors and DNA mismatch repair proteins, suggesting cooperation between mechanisms of tumorigenesis that were previously thought to act independently. Cumulatively, the current evidence suggests that aneuploidy promotes tumorigenesis, at least at low frequency, but a definitive test has not yet been reported.

Mouse models of BRCA1 and BRCA2 deficiency: past lessons, current understanding and future prospects

Germline mutations in BRCA1 and BRCA2 are responsible for a large proportion of hereditary breast and ovarian cancers. Soon after the identification of both genes in the mid-1990s, investigators set out to develop mouse models for the associated disease. Whereas conventional Brca1 and Brca2 mouse mutants did not reveal a strong phenotype in a heterozygous setting, most homozygous mutations caused embryonic lethality. Consequently, development of mouse models for BRCA-associated tumorigenesis required the generation of tissue-specific conditional knockout animals. In this review, we give an overview of the conventional and the conditional mouse models of BRCA1 and BRCA2 deficiency generated over the last decade, as well as the contribution of these models to our understanding of the biological and molecular functions of BRCA1 and BRCA2. The most advanced mouse models for BRCA1- and BRCA2-associated tumorigenesis mimic human disease to the extent that they can be used in studies addressing clinically relevant questions. These models will help to resolve yet unanswered questions and to translate our increasing knowledge of BRCA1 and BRCA2 biology into clinical practice.

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.

Targeting the Double-Strand DNA Break Repair Pathway as a Therapeutic Strategy: Fig. 1

DNA repair pathways are crucial for the maintenance of genome integrity. The pathway that repairs DNA double-strand breaks (DSB) has components involved in both signaling and repairing DNA damage. Impairing DSB repair using specific inhibitors of signaling or repair might, in principle, sensitize tumor cells to particular DNA-damaging agents. Moreover, the existence of specific defects in DNA repair pathways in tumors provides the rationale for the use of "synthetic lethal" approaches targeting this cellular "Achilles' heel." Here, we discuss the mechanisms involved in DSB repair and detail potential therapeutic approaches based on targeting this pathway.

PARP-1 inhibitors: are they the long-sought genetically specific drugs for BRCA1/2-associated breast cancers?

Recent studies demonstrated that PARP-1 [poly(ADP-ribose) polymerase-1] inhibitors kill breast cancer associated gene-1 and -2 (BRCA1/2) deficient cells with extremely high efficiency while BRCA+/- and BRCA+/+ cells are relatively non-responsive to the treatment. It was therefore proposed that PARP-1 inhibitors might be the long-sought genetically specific drugs that are both safe and effective for treating BRCA1/2-associated breast cancers. However, a report published in a recent issue of the International Journal of Biological Sciences revealed that PARP-1 inhibitors, although able to kill naïve BRCA1 mutant cells with high specificity both in vitro and in vivo, exhibit minimal specificity in inhibiting the growth of mouse mammary tumor cells irrespective of their BRCA1 status in allograft nude mice. Non-specific inhibition in human BRCA1+/+, BRCA1+/-, and BRCA1-/- breast cancer cells by PARP-1 inhibitors was also observed. Additional mutations occurring during cancer progression may be a culprit, although the exact cause for the resistance of BRCA1-/- breast cancer cells to PARP-1 inhibitors remains elusive. These findings suggest that PARP inhibition may serve as an approach for the prevention of BRCA related breast cancer and may be useful in combination with other chemotherapeutic agents in the treatment of breast cancer.

Murine Wee1 plays a critical role in cell cycle regulation and pre-implantation stages of embryonic development

Wee1 kinase regulates the G2/M cell cycle checkpoint by phosphorylating and inactivating the mitotic cyclin-dependent kinase 1 (Cdk1). Loss of Wee1 in many systems, including yeast and drosophila, leads to premature mitotic entry. However, the developmental role of Wee1 in mammals remains unclear. In this study, we established Wee1 knockout mice by gene targeting. We found that Wee^-/- embryos were defective in the G2/M cell cycle checkpoint induced by γ-irradiation and died of apoptosis before embryonic (E) day 3.5. To study the function of Wee1 further, we have developed MEF cells in which Wee1 is disrupted by a tamoxifen inducible Cre-LoxP approach. We found that acute deletion of Wee1 resulted in profound growth defects and cell death. Wee1 deficient cells displayed chromosome aneuploidy and DNA damage as revealed by γ-H2AX foci formation and Chk2 activation. Further studies revealed a conserved mechanism of Wee1 in regulating mitotic entry and the G2/M checkpoint compared with other lower organisms. These data provide in vivo evidence that mammalian Wee1 plays a critical role in maintaining genome integrity and is essential for embryonic survival at the pre-implantation stage of mouse development.

ATM-Chk2-p53 activation prevents tumorigenesis at an expense of organ homeostasis upon Brca1 deficiency

BRCA1 is a checkpoint and DNA damage repair gene that secures genome integrity. We have previously shown that mice lacking full-length Brca1 (Brca1(delta11/delta11)) die during embryonic development. Haploid loss of p53 completely rescues embryonic lethality, and adult Brca1(delta11/delta11)p53+/- mice display cancer susceptibility and premature aging. Here, we show that reduced expression and/or the absence of Chk2 allow Brca1(delta11/delta11) mice to escape from embryonic lethality. Compared to Brca1(delta11/delta11)p53+/- mice, lifespan of Brca1(delta11/delta11)Chk2-/- mice was remarkably extended. Analysis of Brca1(delta11/delta11)Chk2-/- mice revealed that p53-dependent apoptosis and growth defect caused by Brca1 deficiency are significantly attenuated in rapidly proliferating organs. However, in later life, Brca1(delta11/delta11)Chk2-/- female mice developed multiple tumors. Furthermore, haploid loss of ATM also rescued Brca1 deficiency-associated embryonic lethality and premature aging. Thus, in response to Brca1 deficiency, the activation of the ATM-Chk2-p53 signaling pathway contributes to the suppression of neoplastic transformation, while leading to compromised organismal homeostasis. Our data highlight how accurate maintenance of genomic integrity is critical for the suppression of both aging and malignancy, and provide a further link between aging and cancer.