Mutant BRCA1 genes antagonize phenotype of wild-type BRCA1

Systematic profiling of dominant ubiquitin variants reveals key functional nodes contributing to evolutionary selection

Dominant-negative mutations can help to investigate the biological mechanisms and to understand the selective pressures for multifunctional proteins. However, most studies have focused on recessive mutant effects that occur in the absence of a second functional gene copy, which overlooks the fact that most eukaryotic genomes contain more than one copy of many genes. We have identified dominant effects on yeast growth rate among all possible point mutations in ubiquitin expressed alongside a wild-type allele. Our results reveal more than 400 dominant-negative mutations, indicating that dominant-negative effects make a sizable contribution to selection acting on ubiquitin. Cellular and biochemical analyses of individual ubiquitin variants show that dominant-negative effects are explained by varied accumulation of polyubiquitinated cellular proteins and/or defects in conjugation of ubiquitin variants to ubiquitin ligases. Our approach to identify dominant-negative mutations is general and can be applied to other proteins of interest.

MTAP-ANRIL gene fusion promotes melanoma epithelial-mesenchymal transition-like process by activating the JNK and p38 signaling pathways

Gene fusions caused by cytogenetic aberrations play important roles in the initiation and progression of cancers. The recurrent MTAP-ANRIL fusion gene was reported to have a frequency of greater than 7% in melanoma in our previous study. However, its functions remain unclear. Truncated MTAP proteins resulting from point mutations in the last three exons of MTAP can physically interact with the wild-type MTAP protein, a tumor suppressor in several human cancers. Similarly, MTAP-ANRIL, which is translated into a truncated MTAP protein, would influence wild-type MTAP to act as an oncogene. Here, we found that MTAP-ANRIL gene fusion downregulated the expression of wild-type MTAP and promoted epithelial-mesenchymal transition-like process through the activation of JNK and p38 MAPKs in vitro and in vivo. Our results suggest that MTAP-ANRIL is a potential molecular prognostic biomarker and therapeutic target for melanoma.

FRG1 is a direct transcriptional regulator of nonsense-mediated mRNA decay genes

FRG1 is the primary candidate gene for Fascioscapulohumeral Muscular Dystrophy. So far, its role has been reported in muscle development, vasculogenesis, angiogenesis, and tumorigenesis. Mechanistically studies suggest FRG1's role in RNA biogenesis which may have implications in multiple physiological processes and diseases, including tumorigenesis. Its probable role as hnRNP and association with NMD-related genes prompted us to look into FRG1's effect on NMD gene expression and the mechanism. Using microarray profiling in cell lines, we found that FRG1 altered the mRNA surveillance pathway and associated pathways, such as RNA transport and spliceosome machinery molecules. Multiple sequence alignment of core factors, namely, UPF1, UPF3B, and SMG1, showed conserved stretches of nucleotide sequence 'CTGGG'. Structural modeling followed by EMSA, ChIP-qPCR, and luciferase reporter assays showed 'CTGGG' as a FRG1 binding site. Analysis of the publicly available datasets showed that the expression of FRG1 correlates with NMD genes in different tissue types. We validated the effect of FRG1 on NMD gene transcription by qRT-PCR. Overall, FRG1 might be a transcriptional regulator of NMD genes.

IL-6/STAT3 signaling in tumor cells restricts the expression of frameshift-derived neoantigens by SMG1 induction

BACKGROUND

The quality and quantity of tumor neoantigens derived from tumor mutations determines the fate of the immune response in cancer. Frameshift mutations elicit better tumor neoantigens, especially when they are not targeted by nonsense-mediated mRNA decay (NMD). For tumor progression, malignant cells need to counteract the immune response including the silencing of immunodominant neoantigens (antigen immunoediting) and promoting an immunosuppressive tumor microenvironment. Although NMD inhibition has been reported to induce tumor immunity and increase the expression of cryptic neoantigens, the possibility that NMD activity could be modulated by immune forces operating in the tumor microenvironment as a new immunoediting mechanism has not been addressed.

METHODS

We study the effect of SMG1 expression (main kinase that initiates NMD) in the survival and the nature of the tumor immune infiltration using TCGA RNAseq and scRNAseq datasets of breast, lung and pancreatic cancer. Different murine tumor models were used to corroborate the antitumor immune dependencies of NMD. We evaluate whether changes of SMG1 expression in malignant cells impact the immune response elicited by cancer immunotherapy. To determine how NMD fluctuates in malignant cells we generated a luciferase reporter system to track NMD activity in vivo under different immune conditions. Cytokine screening, in silico studies and functional assays were conducted to determine the regulation of SMG1 via IL-6/STAT3 signaling.

RESULTS

IL-6/STAT3 signaling induces SMG1, which limits the expression of potent frameshift neoantigens that are under NMD control compromising the outcome of the immune response.

CONCLUSION

We revealed a new neoantigen immunoediting mechanism regulated by immune forces (IL-6/STAT3 signaling) responsible for silencing otherwise potent frameshift mutation-derived neoantigens.

Deciphering the nonsense-mediated mRNA decay pathway to identify cancer cell vulnerabilities for effective cancer therapy

Nonsense-mediated mRNA decay (NMD) is a highly conserved cellular surveillance mechanism, commonly studied for its role in mRNA quality control because of its capacity of degrading mutated mRNAs that would produce truncated proteins. However, recent studies have proven that NMD hides more complex tasks involved in a plethora of cellular activities. Indeed, it can control the stability of mutated as well as non-mutated transcripts, tuning transcriptome regulation. NMD not only displays a pivotal role in cell physiology but also in a number of genetic diseases. In cancer, the activity of this pathway is extremely complex and it is endowed with both pro-tumor and tumor suppressor functions, likely depending on the genetic context and tumor microenvironment. NMD inhibition has been tested in pre-clinical studies showing favored production of neoantigens by cancer cells, which can stimulate the triggering of an anti-tumor immune response. At the same time, NMD inhibition could result in a pro-tumor effect, increasing cancer cell adaptation to stress. Since several NMD inhibitors are already available in the clinic to treat genetic diseases, these compounds could be redirected to treat cancer patients, pending the comprehension of these variegated NMD regulation mechanisms. Ideally, an effective strategy should exploit the anti-tumor advantages of NMD inhibition and simultaneously preserve its intrinsic tumor suppressor functions. The targeting of NMD could provide a new therapeutic opportunity, increasing the immunogenicity of tumors and potentially boosting the efficacy of the immunotherapy agents now available for cancer treatment.

Nonsense-mediated RNA decay and its bipolar function in cancer

Nonsense-mediated decay (NMD) was first described as a quality-control mechanism that targets and rapidly degrades aberrant mRNAs carrying premature termination codons (PTCs). However, it was found that NMD also degrades a significant number of normal transcripts, thus arising as a mechanism of gene expression regulation. Based on these important functions, NMD regulates several biological processes and is involved in the pathophysiology of a plethora of human genetic diseases, including cancer. The present review aims to discuss the paradoxical, pro- and anti-tumorigenic roles of NMD, and how cancer cells have exploited both functions to potentiate the disease. Considering recent genetic and bioinformatic studies, we also provide a comprehensive overview of the present knowledge of the advantages and disadvantages of different NMD modulation-based approaches in cancer therapy, reflecting on the challenges imposed by the complexity of this disease. Furthermore, we discuss significant advances in the recent years providing new perspectives on the implications of aberrant NMD-escaping frameshifted transcripts in personalized immunotherapy design and predictive biomarker optimization. A better understanding of how NMD differentially impacts tumor cells according to their own genetic identity will certainly allow for the application of novel and more effective personalized treatments in the near future.

Nonsense-Mediated mRNA Decay: Pathologies and the Potential for Novel Therapeutics

Nonsense-mediated messenger RNA (mRNA) decay (NMD) is a surveillance pathway used by cells to control the quality mRNAs and to fine-tune transcript abundance. NMD plays an important role in cell cycle regulation, cell viability, DNA damage response, while also serving as a barrier to virus infection. Disturbance of this control mechanism caused by genetic mutations or dys-regulation of the NMD pathway can lead to pathologies, including neurological disorders, immune diseases and cancers. The role of NMD in cancer development is complex, acting as both a promoter and a barrier to tumour progression. Cancer cells can exploit NMD for the downregulation of key tumour suppressor genes, or tumours adjust NMD activity to adapt to an aggressive immune microenvironment. The latter case might provide an avenue for therapeutic intervention as NMD inhibition has been shown to lead to the production of neoantigens that stimulate an immune system attack on tumours. For this reason, understanding the biology and co-option pathways of NMD is important for the development of novel therapeutic agents. Inhibitors, whose design can make use of the many structures available for NMD study, will play a crucial role in characterizing and providing diverse therapeutic options for this pathway in cancer and other diseases.

BRCA1 mutation spectrum, functions, and therapeutic strategies: The story so far

BRCA1 gene mutations account for about 25-28% of hereditary Breast Cancer as BRCA1 is included in the category of high penetrance genes. Except for few commonmutations, there is a heterogenous spectrum of BRCA1 mutations in various ethnic groups. 185AGdel and 5382ins Care the most common BRCA1 alterations (founder mutations) which have been identified in most of the population. This review has been compiled with an aim to consolidate the information on genetic variants reported in BRCA1 found in various ethnic groups, their functional implications if known; involvement of BRCA1 in various cellular pathways/processes and potential BRCA1 targeted therapies. The pathological variations of BRCA1 vary among different ethical groups. A systematic search in PubMed and Google scholar for the literature on BRCA1 gene was carried out to figure out structure and function of BRCA1 gene. BRCA1 is a large protein having 1863 amino acids with multiple functional domains and interacts with multiple proteins to carry out various crucial cellular processes. BRCA1 plays a major role in maintaining genome integrity, transcription regulation, chromatin remodeling, cell cycle checkpoint control, DNA damage repair, chromosomal segregation, and apoptosis. Studies investigating the phenotypic response of mutant BRCA1 protein and comparing it to wildtype BRCA1 protein are clinically important as they are involved in homologous recombination and other repair mechanisms. These studies may help in developing more targetted therapies, detecting novel interacting partners, identification of new signaling pathways that BRCA1 is a part of or downstream target genes that BRCA1 affects.

Nonsense-mediated mRNA Decay and Cancer

Nonsense-mediated mRNA decay (NMD) is a conserved mRNA surveillance pathway that cells use to ensure the quality of transcripts and to fine-tune transcript abundance. The role of NMD in cancer development is complex. In some cases, tumors have exploited NMD to downregulate gene expression by apparently selecting for mutations causing destruction of key tumor-suppressor mRNAs. In other cases, tumors adjust NMD activity to adapt to their microenvironment. Understanding how particular tumors exploit NMD for their benefit may augment the development of new therapeutic interventions.

The retinoblastoma homolog RBR1 mediates localization of the repair protein RAD51 to DNA lesions in Arabidopsis

The retinoblastoma protein (Rb), which typically functions as a transcriptional repressor of E2F‐regulated genes, represents a major control hub of the cell cycle. Here, we show that loss of the Arabidopsis Rb homolog RETINOBLASTOMA‐RELATED 1 (RBR1) leads to cell death, especially upon exposure to genotoxic drugs such as the environmental toxin aluminum. While cell death can be suppressed by reduced cell‐proliferation rates, rbr1 mutant cells exhibit elevated levels of DNA lesions, indicating a direct role of RBR1 in the DNA‐damage response (DDR). Consistent with its role as a transcriptional repressor, we find that RBR1 directly binds to and represses key DDR genes such as RADIATION SENSITIVE 51 (RAD51), leaving it unclear why rbr1 mutants are hypersensitive to DNA damage. However, we find that RBR1 is also required for RAD51 localization to DNA lesions. We further show that RBR1 is itself targeted to DNA break sites in a CDKB1 activity‐dependent manner and partially co‐localizes with RAD51 at damage sites. Taken together, these results implicate RBR1 in the assembly of DNA‐bound repair complexes, in addition to its canonical function as a transcriptional regulator.

BRCA1-hapoinsufficiency: Unraveling the molecular and cellular basis for tissue-specific cancer

Over the past 20 years tremendous progress has been made in understanding the function of BRCA1 gene products. Yet one question still remains: why is mutation of BRCA1 typically associated with preferential development of breast and ovarian cancers and not tumors in other tissues? Here we discuss recent evidence documenting the effect of BRCA1-haploinsufficiency in different cells and tissues and synthesize a model for how mutations in a single BRCA1 allele in human cells might preferentially confer increased cancer risk in breast epithelial cells.

DNA repair capacity is impaired in healthy BRCA1 heterozygous mutation carriers

BRCA1 germline mutations increase the lifetime risk of developing breast and ovarian cancers. However, taking into account the differences in disease manifestation among mutation carriers, it is probable that different BRCA1 mutations have distinct haploinsufficiency effects and lead to the formation of different phenotypes. Using lymphoblastoid cell lines derived from heterozygous BRCA1 mutation carriers and non-carriers, we investigated the haploinsufficiency effects of various mutation types using qPCR, immunofluorescence, and microarray technology. Lymphoblastoid cell lines carrying a truncating mutation showed significantly lower BRCA1 mRNA and protein levels and higher levels of gamma-H2AX than control cells or those harboring a missense mutation, indicating greater spontaneous DNA damage. Cells carrying either BRCA1 mutation type showed impaired RAD51 foci formation, suggesting defective repair in mutated cells. Moreover, compared to controls, cell lines carrying missense mutations displayed a more distinct expression profile than cells with truncating mutations, which is consistent with different mutations giving rise to distinct phenotypes. Alterations in the immune response pathway in cells harboring missense mutations point to possible mechanisms of breast cancer initiation in carriers of these mutations. Our findings offer insight into how various heterozygous mutations in BRCA1 could lead to impairment of BRCA1 function and provide strong evidence of haploinsufficiency in BRCA1 mutation carriers.

MicroRNA-mediated repression of nonsense mRNAs

Numerous studies have established important roles for microRNAs (miRNAs) in regulating gene expression. Here, we report that miRNAs also serve as a surveillance system to repress the expression of nonsense mRNAs that may produce harmful truncated proteins. Upon recognition of the premature termination codon by the translating ribosome, the downstream portion of the coding region of an mRNA is redefined as part of the 3' untranslated region; as a result, the miRNA-responsive elements embedded in this region can be detected by miRNAs, triggering accelerated mRNA deadenylation and translational inhibition. We demonstrate that naturally occurring cancer-causing APC (adenomatous polyposis coli) nonsense mutants which escape nonsense-mediated mRNA decay (NMD) are repressed by miRNA-mediated surveillance. In addition, we show that miRNA-mediated surveillance and exon-exon junction complex-mediated NMD are not mutually exclusive and act additively to enhance the repressive activity. Therefore, we have uncovered a new role for miRNAs in repressing nonsense mutant mRNAs.

Dissecting disease inheritance modes in a three-dimensional protein network challenges the "guilt-by-association" principle

To better understand different molecular mechanisms by which mutations lead to various human diseases, we classified 82,833 disease-associated mutations according to their inheritance modes (recessive versus dominant) and molecular types (in-frame [missense point mutations and in-frame indels] versus truncating [nonsense mutations and frameshift indels]) and systematically examined the effects of different classes of disease mutations in a three-dimensional protein interactome network with the atomic-resolution interface resolved for each interaction. We found that although recessive mutations affecting the interaction interface of two interacting proteins tend to cause the same disease, this widely accepted "guilt-by-association" principle does not apply to dominant mutations. Furthermore, recessive truncating mutations in regions encoding the same interface are much more likely to cause the same disease, even for interfaces close to the N terminus of the protein. Conversely, dominant truncating mutations tend to be enriched in regions encoding areas between interfaces. These results suggest that a significant fraction of truncating mutations can generate functional protein products. For example, TRIM27, a known cancer-associated protein, interacts with three proteins (MID2, TRIM42, and SIRPA) through two different interfaces. A dominant truncating mutation (c.1024delT [p.Tyr342Thrfs*30]) associated with ovarian carcinoma is located between the regions encoding the two interfaces; the altered protein retains its interaction with MID2 and TRIM42 through the first interface but loses its interaction with SIRPA through the second interface. Our findings will help clarify the molecular mechanisms of thousands of disease-associated genes and their tens of thousands of mutations, especially for those carrying truncating mutations, often erroneously considered "knockout" alleles.

Regulation of nonsense-mediated mRNA decay: implications for physiology and disease

Nonsense-mediated mRNA decay (NMD) is an mRNA quality control mechanism that destabilizes aberrant mRNAs harboring premature termination (nonsense) codons (PTCs). Recent studies have shown that NMD also targets mRNAs transcribed from a large subset of wild-type genes. This raises the possibility that NMD itself is under regulatory control. Indeed, several recent studies have shown that NMD activity is modulated in specific cell types and that key components of the NMD pathway are regulated by several pathways, including microRNA circuits and NMD itself. Cellular stress also modulates the magnitude of NMD by mechanisms that are beginning to be understood. Here, we review the evidence that NMD is regulated and discuss the physiological role for this regulation. We propose that the efficiency of NMD is altered in some cellular contexts to regulate normal biological events. In disease states-such as in cancer-NMD is disturbed by intrinsic and extrinsic factors, resulting in altered levels of crucial NMD-targeted mRNAs that lead to downstream pathological consequences. This article is part of a Special Issue entitled: RNA Decay mechanisms.

BRCA1 16 years later: risk-associated BRCA1 mutations and their functional implications

Mutations in the tumor suppressor breast cancer susceptibility gene 1 (BRCA1), an important player in the DNA damage response, apoptosis, cell cycle regulation and transcription, confer a significantly elevated lifetime risk for breast and ovarian cancer. Although the loss of wild-type BRCA1 function is an important mechanism by which mutations confer increased cancer risk, multiple studies suggest mutant BRCA1 proteins may confer functions independent of the loss of wild-type BRCA1 through dominant negative inhibition of remaining wild-type BRCA1, or through novel interactions and pathways. These functions impact various cellular processes and have the potential to significantly influence cancer initiation and progression. In this review, we discuss the functional classifications of risk-associated BRCA1 mutations and their molecular, cellular and clinical impact for mutation carriers.

Transcriptional regulation of the base excision repair pathway by BRCA1

Inactivation of the breast cancer susceptibility gene BRCA1 plays a significant role in the development of a subset of breast cancers, although the major tumor suppressor function of this gene remains unclear. Previously, we showed that BRCA1 induces antioxidant-response gene expression and protects cells against oxidative stress. We now report that BRCA1 stimulates the base excision repair pathway, a major mechanism for the repair of oxidized DNA, by stimulating the activity of key base excision repair (BER) enzymes, including 8-oxoguanine DNA glycosylase (OGG1), the DNA glycosylase NTH1, and the apurinic endonuclease redox factor 1/apurinic endonuclease 1 (REF1/APE1), in human breast carcinoma cells. The increase in BER enzyme activity appears to be due, primarily, to an increase in enzyme expression. The ability of BRCA1 to stimulate the expression of the three BER enzymes and to enhance NTH1 promoter activity was dependent upon the octamer-binding transcription factor OCT1. Finally, we found that OGG1, NTH1, and REF1/APE1 each contribute to the BRCA1 protection against oxidative stress due to hydrogen peroxide and that hydrogen peroxide stimulates the expression of BRCA1 and the three BER enzymes. These findings identify a novel mechanism through which BRCA1 may regulate the repair of oxidative DNA damage.

Germline mutations in MEN1 and BRCA1 genes in a woman with familial multiple endocrine neoplasia type 1 and inherited breast-ovarian cancer syndromes: a case report

The simultaneous occurrence of mutations in two different tumor suppressor genes in the same individual is a very rare event. Here we report the case of a woman in whom germline mutations in both MEN1 and BRCA1 were identified. The severity of MEN1-related biochemical and clinical findings did not significantly differ from that for other affected family members lacking the BRCA1 mutation, except for the development of an extremely large visceral lipoma; the proband has not developed any BRCA1-related malignancies. We explore genetic and molecular rationales for an association between these neoplastic processes.

BRCA1 185delAG mutant protein, BRAt, up-regulates maspin in ovarian epithelial cells

OBJECTIVE

Aggressive clinical course and difficult detection of ovarian cancer are major challenges to improving patient survival and necessitate avid investigation into more effective therapeutic approaches. Understanding early molecular and pathological changes in high risk patients, such as BRCA1 mutation carriers, can provide candidates for molecular profiling and novel targets for effective therapies.

METHODS

Using a culture model system for normal human ovarian surface epithelial cells with and without the BRCA1 185delAG frameshift mutation for the truncated protein product, BRAt, we investigated the role of BRAt in enhanced chemosensitivity. We used MTS, Western immunoblot, semi-quantitative RT-PCR, luciferase reporter and siRNA assays, to identify novel downstream targets of BRAt that promote apoptosis following chemotherapeutic treatment.

RESULTS

We identified maspin as a novel downstream target of BRAt. BRAt increases maspin expression with preferential nuclear localization of maspin. Further, Brat-mediated maspin expression is transcriptionally regulated through an AP1 site within the (-520) to (-297) region of the promoter. Lastly, BRAt, enhances chemosensitivity in normal ovarian surface epithelial cells through c-Jun by a mechanism that may involve maspin.

CONCLUSIONS

BRAt-mediated enhanced chemosensitivity correlates clinically with enhanced chemotherapeutic response in BRCA1 mutation carriers. BRAt-mediated maspin expression also correlates with improved prognostic outlook for ovarian tumors with high levels of nuclear maspin. Consequently, understanding early genotypic and phenotypic changes in the context of high risk disease may provide a better understanding of the mechanism of mutation-associated ovarian cancer and provide new targets for therapeutic intervention.

Inhibition of RNA polymerase III transcription by BRCA1

RNA polymerase III (RNA pol III) transcribes structural RNAs involved in RNA processing (U6 snRNA) and translation (tRNA), thereby regulating the growth rate of cells. Proper initiation by RNA pol III requires the transcription factor TFIIIB. Gene-external U6 snRNA transcription requires TFIIIB consisting of Bdp1, TBP, and Brf2. Transcription from the gene-internal tRNA promoter requires TFIIIB composed of Bdp1, TBP, and Brf1. TFIIIB is a target of tumor suppressors, including PTEN, ARF, p53, and RB, and RB-related pocket proteins. Breast cancer susceptibility gene 1 (BRCA1) tumor suppressor plays a role in DNA repair, cell cycle regulation, apoptosis, genome integrity, and ubiquitination. BRCA1 has a conserved amino-terminal RING domain, an activation domain 1 (AD1), and an acidic carboxyl-terminal domain (BRCA1 C-terminal region). In Saccharomyces cerevisiae, TFIIB interacts with the BRCA1 C-terminal region domain of Fcp1p, an RNA polymerase II phosphatase. The TFIIIB subunits Brf1 and Brf2 are structurally similar to TFIIB. Hence, we hypothesize that RNA pol III may be regulated by BRCA1 via the TFIIB family members Brf1 and Brf2. Here we report that: (1) BRCA1 inhibits both VAI (tRNA) and U6 snRNA RNA pol III transcription; (2) the AD1 of BRCA1 is responsible for inhibition of U6 snRNA transcription, whereas the RING domain and AD1 of BRCA1 are required for VAI transcription inhibition; and (3) overexpression of Brf1 and Brf2 alleviates inhibition of U6 snRNA and VAI transcription by BRCA1. Taken together, these data suggest that BRCA1 is a general repressor of RNA pol III transcription.

An integrative hypothesis about the origin and development of sporadic and familial breast cancer subtypes

Do breast cancer tumours have a common cell origin? Do different breast cancer molecular phenotypes arise from distinct cell types? The studies we have performed during the last few years in familial breast tumours (BRCA1, BRCA2 and non-BRCA1/2) widen questions about the development of sporadic breast cancer to hereditary breast cancer. Array-comparative genomic hybridisation (CGH) studies show universal genomic aberrations in both familial and sporadic breast cancer subtypes that may be selected in the breast tumour development. The inactivation of BRCA1 seems to play a critical role in oestrogen receptor (ER)-negative cancer stem cells (CSCs), driving the tumour development mostly towards a basal-like or, in some cases, to a luminal B phenotype, but other carcinogenetic events are proposed to explain the remaining tumour subtypes. The existence of common genomic alterations in basal-like, ERBB2 and luminal B breast tumours may suggest a common cell origin or clonal selection of these tumour subtypes, arising from an ER-negative CSC or from a progenitor cell (PC). Finally, specific genomic aberrations in ER-positive tumours could provide cellular proliferation advantages when the cells are exposed to oestrogen. We propose a combination of the CSC hypothesis (for the carcinogenesis processes) and the clonal selection model (in terms of tumour development). We uphold that the basal-like-, ERBB2- and luminal B-sporadic and familial tumour subtypes have an ER-negative breast stem/PC origin, whereas luminal A tumours arise from an ER-positive PC, supporting a hierarchical breast carcinogenesis model, whereas crucial genomic imbalances are clonally selected during the tumour development.

BRCA-FA pathway as a target for anti-tumor drugs

Promising research on DNA repair signaling pathways predicts a new age of anti-tumor drugs. This research was initiated through the discovery and characterization of proteins that functioned together in signaling pathways to sense, respond, and repair DNA damage. It was realized that tumor cells often lacked distinct DNA repair pathways, but simultaneously relied heavily on compensating pathways. More recently, researchers have begun to manipulate these compensating pathways to reign in and kill tumor cells. In a striking example it was shown that tumors derived from mutations in the DNA repair genes, of BRCA-FA pathway, were selectively sensitive to inhibition of the base excision repair pathway. These findings suggest that tumors derived from defects in DNA repair genes will be easier to treat clinically, providing a streamlined and targeted therapy that spares healthy cells. In the future, identifying patients with susceptible tumors and discovering additional DNA repair targets amenable to anti-tumor drugs will have a major impact on the course of cancer treatment.

Allelic imbalance in BRCA1 and BRCA2 gene expression is associated with an increased breast cancer risk

The contribution of BRCA1 and BRCA2 to familial and non-familial forms of breast cancer has been difficult to accurately estimate because of the myriad of potential genetic and epigenetic mechanisms that can ultimately influence their expression and involvement in cellular activities. As one of these potential mechanisms, we investigated whether allelic imbalance (AI) of BRCA1 or BRCA2 expression was associated with an increased risk of developing breast cancer. By developing a quantitative approach utilizing allele-specific real-time PCR, we first evaluated AI caused by nonsense-mediated mRNA decay in patients with frameshift mutations in BRCA1 and BRCA2. We next measured AI for BRCA1 and BRCA2 in lymphocytes from three groups: familial breast cancer patients, non-familial breast cancer patients and age-matched cancer-free females. The AI ratios of BRCA1, but not BRCA2, in the lymphocytes from familial breast cancer patients were found to be significantly increased as compared to cancer-free women (BRCA1: 0.424 versus 0.211, P = 0.00001; BRCA2: 0.206 versus 0.172, P = 0.38). Similarly, the AI ratios were greater for BRCA1 and BRCA2 in the lymphocytes of non-familial breast cancer cases versus controls (BRCA1: 0.353, P = 0.002; BRCA2: 0.267, P = 0.03). Furthermore, the distribution of under-expressed alleles between cancer-free controls and familial cases was significantly different for both BRCA1 and BRCA2 gene expression (P < 0.02 and P < 0.02, respectively). In conclusion, we have found that AI affecting BRCA1 and to a lesser extent BRCA2 may contribute to both familial and non-familial forms of breast cancer.

NMD: multitasking between mRNA surveillance and modulation of gene expression

Gene expression is a highly specific and regulated multilayer process with a plethora of interconnections as well as safeguard and feedback mechanisms. Messenger RNA, long neglected as a mere subcarrier of genetic information, is more recently recognized as a linchpin of regulation and control of gene expression. Moreover, the awareness of not only proteins but also mRNA as a modulator of genetic disorders has vastly increased in recent years. Nonsense-mediated mRNA decay (NMD) is a posttranscriptional surveillance mechanism that uses an intricate network of nuclear and cytoplasmic processes to eliminate mRNAs, containing premature termination codons. It thus helps limit the synthesis of potentially harmful truncated proteins. However, recent results suggest functions of NMD that go far beyond this role and affect the expression of wild-type genes and the modulation of whole pathways. In both respects--the elimination of faulty transcripts and the regulation of error-free mRNAs--NMD has many medical implications. Therefore, it has earned increasing interest from researchers of all fields of the life sciences. In the following text, we (1) present current knowledge about the NMD mechanism and its targets, (2) define its relevance in the regulation of important biochemical pathways, (3) explore its medical significance and the prospects of therapeutic interventions, and (4) discuss additional functions of NMD effectors, some of which may be networked to NMD. The main focus of this chapter lies on mammalian NMD and resorts to the features and factors of NMD in other organisms if these help to complete or illuminate the picture.

p53 alterations in human cancer: more questions than answers

The strongest and undisputed fact about p53 is the high frequency of p53 alterations in human cancer and that mutant p53 proteins constitute a complex family of several hundred proteins with heterogeneous properties. Beyond these observations, the p53 pathway and its regulation in a normal cell is like a desert trail, always moving with the wind of novel findings. The field is full of black boxes that are often ignored for sake of simplicity or because they do not fit with the current dominant view. Mutant p53 protein accumulation in tumours is the best example of a preconceived idea, as there is no experimental evidence to explain this observation. In this review, we will discuss several questions concerning the activity or selection of p53 mutations. The central domain of the p53 protein targeted by 80% of p53 mutations is associated with the DNA-binding activity of the p53 protein, but it is also the binding site for several proteins that play a key role in p53 regulation such as ASPP proteins or BclxL. The role of impaired DNA binding and/or protein interactions in tumour development has not been fully elucidated. Similarly, novel animal models carrying either missense p53 mutations or inducible p53 have provided abundant observations, some of which could challenge our view on p53 function as a tumour suppressor gene. Finally, the possible clinical applications of p53 will be discussed.

Gene signature of breast cancer cell lines treated with lycopene

Among the micronutrients studied in relation between nutrition and cancer, lycopene appears to be a breast cancer preventive phytochemical candidate found in raw tomatoes and tomato-derived products. In order to investigate the responsiveness of breast cancer genes to lycopene and to better understand the molecular mechanisms underlying the effects of lycopene, we used an oligonucleotide microarray approach. Human breast cancer cell lines (MCF-7 and MDA-MB-231) and a fibrocystic breast cell line (MCF-10a) were either exposed or not exposed to 10 microM lycopene for 48 h. Microarrays comprising 202 genes were used to identify genes responsive to lycopene supplementation. Hierarchical clustering revealed a cell line-specific lycopene modulation of breast cells. Based on the observed results, lycopene seems to exert regulation on apoptosis, cell cycle and DNA repair mechanisms according to estrogen and retinoic acid receptor cell status.

BRCA1 and FOXA1 proteins coregulate the expression of the cell cycle-dependent kinase inhibitor p27(Kip1)

We have previously shown that the breast cancer susceptibility gene, BRCA1, can transcriptionally activate the p27(Kip1) promoter. The BRCA1-responsive element was defined as a 35 bp region from position -545 to -511. We next determined that within this region is also a potential binding site for the transcription factor Forkhead box (FOX)A1. RNA and protein analysis as well as immunohistochemistry showed that expression of FOXA1 correlated with the expression of the estrogen receptor in a panel of breast cancer cell lines and tissues. In transient transfection reporter assays, FOXA1 could activate the p27(Kip1) promoter. Cotransfection of BRCA1 and FOXA1 resulted in a synergistic activation of the p27(Kip1) promoter. Mutation of the FOXA1 DNA-binding site in the p27(Kip1) promoter-luciferase construct significantly diminished the activity of FOXA1 alone or in combination with BRCA1. Cotransfection of FOXA1 and BRCA1 resulted in a greater amount of each protein compared to transfection of each expression vector alone. The half-life of FOXA1 was increased when coexpressed with BRCA1. Electrophoretic mobility shift assay analysis demonstrated that FOXA1 could bind to a wild-type oligonucleotide containing the FOXA1 binding site in the p27(Kip1) promoter, but this binding was lost upon mutation of this FOXA1 binding site. The protein-DNA binding complex could be supershifted with an antibody directed against FOXA1. The activity of the p27(Kip1) promoter as well as FOXA1 expression was reduced in cells treated with BRCA1 siRNA, thus silencing the expression of BRCA1 protein. In summary, we identified a FOXA1 binding site within the BRCA1-responsive element of the p27(Kip1) promoter and showed that FOXA1 activated the promoter alone and in conjunction with BRCA1. Furthermore, we identified high expression of FOXA1 in breast cancer cell lines and tissues, discovered a role for BRCA1 in the regulation of p27(Kip1) transcription and a possible interaction with BRCA1.

BRCA1 and BRCA2 as molecular targets for phytochemicals indole-3-carbinol and genistein in breast and prostate cancer cells

Indole-3-carbinol (I3C) and genistein are naturally occurring chemicals derived from cruciferous vegetables and soy, respectively, with potential cancer prevention activity for hormone-responsive tumours (e.g., breast and prostate cancers). Previously, we showed that I3C induces BRCA1 expression and that both I3C and BRCA1 inhibit oestrogen (E2)-stimulated oestrogen receptor (ER-α) activity in human breast cancer cells. We now report that both I3C and genistein induce the expression of both breast cancer susceptibility genes (BRCA1 and BRCA2) in breast (MCF-7 and T47D) and prostate (DU-145 and LNCaP) cancer cell types, in a time- and dose-dependent fashion. Induction of the BRCA genes occurred at low doses of I3C (20 μM) and genistein (0.5–1.0 μM), suggesting potential relevance to cancer prevention. A combination of I3C and genistein gave greater than expected induction of BRCA expression. Studies using small interfering RNAs (siRNAs) and BRCA expression vectors suggest that the phytochemical induction of BRCA2 is due, in part, to BRCA1. Functional studies suggest that I3C-mediated cytoxicity is, in part, dependent upon BRCA1 and BRCA2. Inhibition of E2-stimulated ER-α activity by I3C and genistein was dependent upon BRCA1; and inhibition of ligand-inducible androgen receptor (AR) activity by I3C and genistein was partially reversed by BRCA1-siRNA. Finally, we provide evidence suggesting that the phytochemical induction of BRCA1 expression is due, in part, to endoplasmic reticulum stress response signalling. These findings suggest that the BRCA genes are molecular targets for some of the activities of I3C and genistein.

New role for nuclear hormone receptors and coactivators in regulation of BRCA1-mediated DNA repair in breast cancer cell lines

INTRODUCTION

The breast cancer susceptibility gene BRCA1 is involved in the repair of double-strand breaks induced by ionizing radiation and chemotherapy drugs. BRCA1 interacts with coactivators such as p300 and CREB-binding protein (CBP) to activate target gene transcription. Estrogen and retinoic acid receptors (ER and RAR) also require coactivator proteins for their ligand-dependent functions. Few studies have suggested a role for nuclear hormone receptors in DNA repair.

METHODS

DNA damage and repair activity were quantified with the use of single-cell gel electrophoresis and plasmid end-joining assays. Cell cycle progression and apoptosis were determined by bromodeoxyuridine and TdT-mediated dUTP nick end labelling assays. Stable transfection was accomplished with the lipofection procedure. Protein interaction and expression were determined by immunoprecipitation and western blotting.

RESULTS

17beta-estradiol (E2) and all-trans retinoic acid (RA) had opposing effects on DNA damage and breast cancer cell survival after double-strand break damage. Treatment with E2, but not with RA, resulted in complex formation between ERalpha, CBP, and BRCA1 in ER-positive cell lines. Mutant BRCA1 reduced the expression and activity of DNA damage repair proteins but did not block nuclear hormone-dependent effects. Mutant BRCA1 failed to form complexes with ERalpha and CBP, which correlated with its ability to exert E2-independent effects on DNA repair. Mutant BRCA1 inhibited cell cycle progression and produced increased survival in cells with double-strand breaks. Ectopic ERalpha expression reproduced the E2-mediated effects on DNA damage, repair, and survival.

CONCLUSION

The present study proposes a new mechanism by which ER and RAR regulate BRCA1-mediated DNA repair by means of CBP.

BRCA1 Interaction with Human Papillomavirus Oncoproteins

Previously, we reported that BRCA1 strongly represses the transcriptional activity of estrogen receptor-alpha (ER-alpha) in human breast and prostate cancer cells but only weakly inhibits ER-alpha in cervical cancer cells. We now report that introduction of the human papillomavirus E7 or E6 oncogenes into human papillomavirus-negative cells rescues the BRCA1 repression of ER-alpha activity and that the E7 and E6 oncoproteins interact directly with BRCA1 in vitro and associate with BRCA1 in vivo in cultured cells. This interaction involves at least two contact points on BRCA1, one within an N-terminal site shown previously to interact with ER-alpha and the other in a C-terminal region of BRCA1 containing the first BRCA1 C-terminal domain. Point mutations within the zinc finger domains of E7 and E6 inactivated the binding to the N terminus of BRCA1 and reduced their ability to rescue BRCA1 inhibition of ER-alpha. E6 and E7 also antagonized the ability of BRCA1 to inhibit c-Myc E-box-mediated transactivation and human telomerase reverse transcriptase promoter activity, in a manner dependent upon the zinc finger domains. Finally, the ability of E6 and E7 to antagonize BRCA1 did not involve proteolytic degradation of BRCA1. These findings suggest functional interactions of BRCA1 with E7 and E6. The potential significance of these findings is discussed.

Structural determinants of the BRCA1 : estrogen receptor interaction

Previously, we showed that the BRCA1 protein interacts directly and functionally with estrogen receptor-alpha (ER-alpha), resulting in the inhibition of estradiol (E2)-stimulated ER-alpha transcriptional activity. The interaction sites were mapped to the N-terminus of BRCA1 (within amino acids (aa) 1-302) and the ligand-binding domain/activation function-2 (LBD/AF-2) region (within aa 282-420) of ER-alpha. In this study, we have further characterized the structure/function relationship for the BRCA1 : ER-alpha interaction. We found that the N-terminal RING domain (aa 20-64) is not required for the BRCA1 : ER-alpha interaction. We identified two separate contact points for ER-alpha, one within aa 1-100 and the other within aa 100-200 of BRCA1; and we showed that each of these BRCA1 peptides interacts with BRCA1 in vitro and in vivo. By using different fragments of the BRCA1 N-terminus, we found that aa 67-100 and 101-133 are required for the interaction with ER-alpha, but that aa 1-67 and 134-302 are dispensible. Previously, we showed that BRCA1 aa 1-302 does not inhibit E2-stimulated ER-alpha transcriptional activity but does bind to ER-alpha and acts as a dominant negative inhibitor of the full-length BRCA1 protein. Somewhat surprisingly, we found that BRCA1 aa 1-100 and BRCA1 aa 101-200 (but not aa 201-300) each inhibited ER-alpha activity, although not as efficiently as full-length BRCA1. Mutations within an HIV Rev-like nuclear export signal that resembles a nuclear receptor corepressor motif (aa 86-95) impaired the ability of both truncated (aa 1-100) and full-length (aa 1-1863) BRCA1 proteins to interact with and/or repress ER-alpha activity. Based on these findings, a partial BRCA1 : ER-alpha three-dimensional structure is proposed. The implications of these findings for understanding the BRCA1 : ER-alpha interaction are discussed.

Regulation of the estrogen-inducible gene expression profile by the breast cancer susceptibility gene BRCA1

The tumor suppressor gene BRCA1 functions in part as a caretaker in preserving the integrity of the genome, but also exhibits tissue-specific function by inhibiting estrogen receptor activity. Because estrogen (E2) induces a wide range of gene expression changes (by nongenomic and several transcriptional pathways), we sought to determine how comprehensive is the BRCA1-mediated inhibition of E2-induced gene expression alterations. Using cDNA-spotted microarrays, we identified a relatively large number of gene expression alterations (both increased and decreased expression) in MCF-7 cells caused by E2, some of which have been reported in previous studies. However, in the presence of exogenous wild-type BRCA1 (wtBRCA1), the response to E2 was severely blunted, with only about 10% the number of gene expression changes as that found in the absence of wtBRCA1. Examples of these findings were confirmed by semiquantitative and quantitative RT-PCR assays. In contrast to wtBRCA1, the induction by E2 of several E2-responsive genes was not inhibited by a full-length tumor-associated mutant BRCA1 protein [T300G (or (61)Cys-->Gly)]. For three E2-responsive genes whose induction by E2 was inhibited by wtBRCA1, wtBRCA1 had little or no effect on the mRNA half-life in the presence of E2. Consistent with these findings, wtBRCA1 inhibited E2-stimulated proliferation of MCF-7 cells, but wtBRCA1 failed to inhibit the proliferation of MCF-7 cells stimulated by IGF-I. Our findings suggest that BRCA1 globally inhibits the response to estrogen in a dose- and time-dependent fashion. The implications of these findings for understanding how BRCA1 may act to restrain E2 action in vivo are considered.

Brca1 inactivation induces p27(Kip1)-dependent cell cycle arrest and delayed development in the mouse mammary gland

One common characteristic of breast cancers arising in carriers of the predisposition gene BRCA1 is a loss of expression of the CDK inhibitor p27(Kip1) (p27), suggesting that p27 interacts epistatically with BRCA1. To investigate this relationship, we examined expression of p27 in mice expressing a dominant negative allele of Brca1 (MMTV-trBr) in the mammary gland. While these mice rarely develop tumors, they showed a 50% increase in p27 protein and a delay in mammary gland development associated with reduced proliferation. In contrast, on a p27 heterozygote background, MMTV-trBrca1 mice showed an increase in S phase cells, and normal mammary development. p27 was the only protein in the cyclin-cyclin-dependent kinase network to show altered expression, suggesting that it may be a central mediator of cell cycle arrest in response to loss of function of BRCA1. Furthermore, in human mammary epithelial MCF7 cells expressing BRCA1-specific RNAi and in the BRCA1-deficient human tumor cell line HCC1937, p27 is elevated at the mRNA level compared to cells expressing wild-type BRCA1. We hypothesize that disruption of BRCA1 induces an increase in p27 that inhibits proliferation. Accordingly, reduction in p27 expression leads to enhancement of cellular proliferation in the absence of BRCA1.

Disruption of BRCA1 function results in telomere lengthening and increased anaphase bridge formation in immortalized cell lines

BRCA1 is a tumor suppressor that functions in controlling cell growth and maintaining genomic stability. BRCA1 has also been implicated in telomere maintenance through its ability to regulate the transcription of hTERT, the catalytic subunit of telomerase, resulting in telomere shortening, and to colocalize with the telomere-binding protein TRF1. The high incidence of nonreciprocal translocations in tumors arising from BRCA1 mutation carriers and Brca1-null mice also raises the possibility that BRCA1 plays a role in telomere protection. To date, however, the consequences for telomere status of disrupting BRCA1 have not been reported. To examine the role of BRCA1 in telomere regulation, we have expressed a dominant-negative mutant of BRCA1 (trBRCA1), known to disrupt multiple functions of BRCA1, in telomerase-positive mammary epithelial cells (SVCT) and telomerase-negative ALT cells (GM847). In SVCT cells, expression of trBRCA1 resulted in an increased incidence of anaphase bridges and in an increase in telomere length, but no change in telomerase activity. In GM847 cells, trBRCA1 also increased anaphase bridge formation but did not induce any change in telomere length. BRCA1 colocalized with TRF2 in telomerase-positive cells and with a small subset of ALT-associated PML bodies (APBs) in ALT cells. Together, these results raise the possibility that BRCA1 could play a role in telomere protection and suggest a potential mechanism for one of the phenotypes of BRCA1-deficient cells.

BRCA1 induces antioxidant gene expression and resistance to oxidative stress

Mutations of the breast cancer susceptibility gene 1 (BRCA1), a tumor suppressor, confer an increased risk for breast, ovarian, and prostate cancers. To investigate the function of the BRCA1 gene, we performed DNA microarray and confirmatory reverse transcription-PCR analyses to identify BRCA1-regulated gene expression changes. We found that BRCA1 up-regulates the expression of multiple genes involved in the cytoprotective antioxidant response, including glutathione S-transferases, oxidoreductases, and other antioxidant genes. Consistent with these findings, BRCA1 overexpression conferred resistance while BRCA1 deficiency conferred sensitivity to several different oxidizing agents (hydrogen peroxide and paraquat). In addition, in the setting of oxidative stress (due to hydrogen peroxide), BRCA1 shifted the cellular redox balance to a higher ratio of reduced to oxidized glutathione. Finally, BRCA1 stimulated antioxidant response element-driven transcriptional activity and enhanced the activity of the antioxidant response transcription factor nuclear factor erythroid-derived 2 like 2 [also called NRF2 (NFE2L2)]. The ability of BRCA1 to stimulate antioxidant response element-dependent transcription and to protect cells against oxidative stress was attenuated by inhibition of nuclear factor erythroid-derived 2 like 2. These findings suggest a novel function for BRCA1, i.e., to protect cells against oxidative stress. This function would be consistent with the postulated role of BRCA1 as a caretaker gene in preserving genomic integrity.

Nonsense-mediated decay approaches the clinic

Nonsense-mediated decay (NMD) eliminates mRNAs containing premature termination codons and thus helps limit the synthesis of abnormal proteins. New results uncover a broader role of NMD as a pathway that also affects the expression of wild-type genes and alternative-splice products. Because the mechanisms by which NMD operates have received much attention, we discuss here the emerging awareness of the impact of NMD on the manifestation of human genetic diseases. We explore how an understanding of NMD accounts for phenotypic differences in diseases caused by premature termination codons. Specifically, we consider how the protective function of NMD sometimes benefits heterozygous carriers and, in contrast, sometimes contributes to a clinical picture of protein deficiency by inhibiting expression of partially functional proteins. Potential 'NMD therapeutics' will therefore need to strike a balance between the general physiological benefits of NMD and its detrimental effects in cases of specific genetic mutations.

Relevance of breast cancer cell lines as models for breast tumours: an update

The number of available breast cancer cell (BCC) lines is small, and only a very few of them have been extensively studied. Whether they are representative of the tumours from which they originated remains a matter of debate. Whether their diversity mirrors the well-known inter-tumoural heterogeneity is another essential question. While numerous similarities have long been found between cell lines and tumours, recent technical advances, including the use of micro-arrays and comparative genetic analysis, have brought new data to the discussion. This paper presents most of the BCC lines that have been described in some detail to date. It evaluates the accuracy of the few of them widely used (MCF-7, T-47D, BT-474, SK-BR-3, MDA-MB-231, Hs578T) as tumour models. It is concluded that BCC lines are likely to reflect, to a large extent, the features of cancer cells in vivo. The importance of oestrogen receptor-alpha (gene ESR1 ) and Her-2/ neu ( ERBB2 ) as classifiers for cell lines and tumours is underlined. The recourse to a larger set of cell lines is suggested since the exact origin of some of the widely used lines remains ambiguous. Investigations on additional specific lines are expected to improve our knowledge of BCC and of the dialogue that these maintain with their surrounding normal cells in vivo.

The Icelandic founder mutation BRCA2 999del5: analysis of expression

Introduction

A founder mutation in the BRCA2 gene (BRCA2 999del5) accounts for 7–8% of female breast cancers and for 40% of male breast cancers in Iceland. If expressed, the mutant gene would encode a protein consisting of the first 256 amino acids of the BRCA2 protein. The purpose of this study was to determine whether this mutant protein is produced in heterozygous individuals and, if so, what might be the functional consequences of mutant protein production.

Methods

The presence of BRCA2 999del5 transcripts in fibroblasts from heterozygous individuals was assayed by cDNA synthesis and sequencing. The potential protein-coding portion of BRCA2 999del5 was cloned into the pIND(SP1)/V5-His vector and expressed in COS7 cells. The presence of the mutant protein in cell lysates from heterozygous fibroblasts and from COS7 cells was tested by a number of methods including immunoprecipitation, affinity purification with nickel-coated agarose beads, Western blotting and ELISA, using antibodies to the N-terminal end of BRCA2, antiserum specific for the 16 nonrelevant amino acids at the carboxyl end and antibodies to fusion partners of recombinant proteins.

Results

The frequency of the BRCA2 999del5 transcript in heterozygous fibroblasts was about one-fifth of the wild-type transcript; however, no mutant protein could be detected. Overexpression of BRCA2 999del5 mRNA in COS7 cells failed to produce a mutant protein unless degradation by proteasomes was blocked.

Conclusion

Our results show that the protein product of BRCA2 999del5 is extremely unstable. Therefore, an increase in breast cancer risk in BRCA2 999del5 carriers is due to haploinsufficiency at the BRCA2 locus.

Cytoplasmic mislocalization of BRCA1 caused by cancer-associated mutations in the BRCT domain

BRCA1 is inactivated by gene mutations in >50% of familial breast and ovarian cancers. BRCA1 is primarily a nuclear protein, although others previously reported cytoplasmic staining in breast tumor cells. In this study, we demonstrate the cytoplasmic mislocalization of BRCA1 caused by a subgroup of clinically relevant cancer mutations. We show that mutations that disrupt or delete the C-terminal BRCT domains, but not other regions of BRCA1, caused significant relocalization of BRCA1 from nucleus to cytoplasm. Two of the BRCT mutations tested (M1775R and Y1853X) are known to adversely affect BRCA1 protein folding and nuclear function. The BRCT mutations reduced BRCA1 nuclear import by a mechanism consistent with altered protein folding, as indicated by the restoration of nuclear staining by more extensive C-terminal deletions. Furthermore, we observed increased cytoplasmic staining of both the ectopic and endogenous forms of the BRCA1-5382insC mutant (deleted BRCT domain) in HCC1937 breast cancer cells. Unlike wild-type BRCA1, the BRCA1-5382insC mutant failed to form DNA damage-inducible foci when targeted to the nucleus by BARD1. We propose that BRCT mutations alter nuclear targeting of BRCA1, and that this may contribute to the inhibition of nuclear DNA repair and transcription function.

Survival and Recurrence After Breast Cancer in BRCA1/2 Mutation Carriers

BACKGROUND

Genetic mutation is responsible for approximately 10% of breast cancers. The purpose of this study was to compare breast cancer survival and recurrence rates between BRCA1/2 mutation carriers and noncarriers.

METHODS

Using the Columbia Presbyterian breast cancer database, we collected the tissue blocks of all patients younger than 65 years of age and of Jewish descent. The patients were contacted and the data updated. DNA was extracted from the tissue blocks and tested for the common mutations. The results of the genetic mutation and updated database were anonymized and merged. The survival and recurrence rates were compared between mutation carriers and noncarriers.

RESULTS

A total of 739 breast cancer cases in 715 patients were identified. We were able to test 487 patients. We identified 30 BRCA1 and 21 BRCA2 mutation carriers, for an incidence of 10.36%. The median follow-up for the patients tested was 50 months. BRCA1 patients more frequently had estrogen- and progesterone-negative tumors and had a higher incidence of positive nodes. BRCA1 patients received chemotherapy more frequently. The incidence of in situ disease was similar for mutation and non-mutation carriers. BRCA1/2 mutation carriers had a higher incidence of bilateral disease. There was no difference in 5- or 10-year overall and breast cancer-specific survival between mutation and non-mutation carriers.

CONCLUSIONS

Breast cancer patients with BRCA1/2 mutations have a similar outcome as non-mutation carriers.

BRCA1 inhibition of telomerase activity in cultured cells

Telomerase, an enzyme that maintains telomere length, plays major roles in cellular immortalization and cancer progression. We found that an exogenous BRCA1 gene strongly inhibited telomerase enzymatic activity in human prostate and breast cancer cell lines and caused telomere shortening in cell lines expressing wild-type BRCA1 (wtBRCA1) but not a tumor-associated mutant BRCA1 (T300G). wtBRCA1 inhibited the expression of the catalytic subunit (telomerase reverse transcriptase [TERT]) but had no effect on the expression of a subset of other components of the telomerase holoenzyme or on the expression of c-Myc, a transcriptional activator of TERT. However, endogenous BRCA1 associated and partially colocalized with c-Myc; exogenous wtBRCA1 strongly suppressed TERT promoter activity in various cell lines. The TERT inhibition was due, in part, to suppression of c-Myc E-box-mediated transcriptional activity. Suppression of TERT promoter and c-Myc activity required the amino terminus of BRCA1 but not the carboxyl terminus. Finally, endogenous BRCA1 and c-Myc were detected on transfected mouse and human TERT promoter segments in vivo. We postulate that inhibition of telomerase may contribute to the BRCA1 tumor suppressor activity.

Of mice and (wo)men: genotype-phenotype correlations in BRCA1

To date, over 6300 mutations in BRCA1, involving 1100 distinct sites, have been described and reported in the BIC (breast cancer information core) database. Since the first BRCA1 mutations in early-onset breast and ovarian cancer families were reported, several attempts to establish genotype-phenotype correlations for this gene have been reported. Moreover, in vitro data have suggested dominant-negative effects of putative mutant BRCA1 proteins over wild-type proteins. Genotype-phenotype correlations are not only important for predicting the clinical course of the disease and to allow tailor-made surveillance of individuals at risk, but also have implications for the elucidation of the molecular genetic mechanisms underlying BRCA1-mediated tumorigenesis and the development of gene transfer-based therapies. Here, we discuss genotype-phenotype correlations at the BRCA1 locus in mouse and man, and the functional aspects that may account for these observations.

BRCA1 in hormone-responsive cancers

Mutations of the breast cancer susceptibility gene BRCA1 are linked to hereditary early onset breast and breast-ovarian cancer syndromes. These mutations confer an increased risk for other hormone-dependent cancers, including those of the uterus, cervix and prostate. BRCA1 expression is decreased or absent in a significant proportion of sporadic breast and ovarian cancers, suggesting a wider role in these tumor types. Multiple functions for BRCA1 have been identified, including roles in DNA repair, cell-cycle progression and apoptosis. These functions are consistent with a tumor suppressor activity, but they do not explain why BRCA1 mutation carriers develop hormone-responsive cancer types. Recent studies indicate that BRCA1 interacts with and regulates the activity of estrogen receptor alpha (ER alpha) and the androgen receptor. Its expression is regulated by carcinogens and anticarcinogens that modulate ER alpha signaling, suggesting a molecular linkage between BRCA1 and hormone-responsive cancers.

BRCA1 gene in breast cancer

The BRCA1 gene was identified and cloned in 1994 based its linkage to early onset breast cancer and breast-ovarian cancer syndromes in women. While inherited mutations of BRCA1 are responsible for about 40-45% of hereditary breast cancers, these mutations account for only 2-3% of all breast cancers, since the BRCA1 gene is rarely mutated in sporadic breast cancers. However, BRCA1 expression is frequently reduced or absent in sporadic cancers, suggesting a much wider role in mammary carcinogenesis. Since BRCA1 was cloned in 1994, its molecular function has been the subject of intense investigation. These studies have revealed multiple functions of the BRCA1 that may contribute to its tumor suppressor activity, including roles in: cell cycle progression, several highly specialized DNA repair processes, DNA damage-responsive cell cycle check-points, regulation of a set of specific transcriptional pathways, and apoptosis. Many of these functions are linked to protein:protein interactions involving different portions of the 1,863 amino acid (aa) BRCA1 protein. BRCA1 functions in cell cycle progression and the DNA damage response appear to be regulated by distinct and specific phosphorylation events, but the molecular pathways activated by these phosphorylations are only beginning to be unraveled. In addition, the reason that BRCA1 mutation carriers develop specific tumor types (breast and ovarian cancers in women and possibly prostate cancers in men) is not clearly understood. Elucidation of the precise molecular functions of the BRCA1 gene product will greatly enhance our understanding of the pathogenesis of hereditary as well as sporadic mammary carcinogenesis.

Average age-specific cumulative risk of breast cancer according to type and site of germline mutations in BRCA1 and BRCA2 estimated from multiple-case breast cancer families attending Australian family cancer clinics

If the risk of disease is not the same for all germline mutations in a given gene, or if there are other familial modifiers of risk in carriers, then family-history-based estimates of average risk for detected mutations in that gene will depend on how carriers are sampled. Risk may also depend on the site or type of mutation. We studied 51 families with strong histories of breast cancer who attended Australian family cancer clinics and in which a germline mutation in BRCA1 or BRCA2 had been identified (28 and 23 families, respectively). Breast cancer risk in carriers was estimated under maximum likelihood theory, using information from all family members including those not tested, with adjustment for ascertainment by conditioning on genotype of the proband and family phenotype. The average cumulative risk of breast cancer for mutations in either BRCA1 or BRCA2 was 27% (95% confidence interval 16-43%) to age 50 and 64% (44-83%) to age 70. When grouped, the incidence in carriers was on average 17 (10-30) times that in non-carriers, independent of gene or mutation type (hazard ratios: 11 (4-29) for BRCA1, 23 (12-43) for BRCA2 (P for difference = 0.23); 13 (6-29) for protein-truncating mutations, 30 (9-104) for missense mutations and 30 (10-90) for splice-site mutations). For missense mutations, this was equivalent to a cumulative risk to age 70 of 83% (40-100%) and was due in part, but not totally, to the missense mutations 300 T>G in BRCA1 and 4486 G>T in BRCA2, which were individually found to be associated with high risk (P<0.001). Mutations in the central region of BRCA1 may be associated with a lower risk. The issue of the pathogenicity of specific variants may be addressed analytically providing there are one or more suitably informative families with that mutation.