Roles of BRCA1 in centrosome duplication

PARP inhibitors suppress tumours via centrosome error-induced senescence independent of DNA damage response

BACKGROUND

Poly(ADP-ribose) polymerase (PARP) inhibitors have emerged as promising chemotherapeutic drugs primarily against BRCA1/2-associated tumours, known as synthetic lethality. However, recent clinical trials reported patients' survival benefits from PARP inhibitor treatments, irrelevant to homologous recombination deficiency. Therefore, revealing the therapeutic mechanism of PARP inhibitors beyond DNA damage repair is urgently needed, which can facilitate precision medicine.

METHODS

A CRISPR-based knock-in technology was used to establish stable BRCA1 mutant cancer cells. The effects of PARP inhibitors on BRCA1 mutant cancer cells were evaluated by biochemical and cell biological experiments. Finally, we validated its in vivo effects in xenograft and patient-derived xenograft (PDX) tumour mice.

FINDINGS

In this study, we uncovered that the majority of clinical BRCA1 mutations in breast cancers were in and near the middle of the gene, rather than in essential regions for DNA damage repair. Representative mutations such as R1085I and E1222Q caused transient extra spindle poles during mitosis in cancer cells. PAR, which is synthesized by PARP2 but not PARP1 at mitotic centrosomes, clustered these transient extra poles, independent of DNA damage response. Common PARP inhibitors could effectively suppress PARP2-synthesized PAR and induce cell senescence by abrogating the correction of mitotic extra-pole error.

INTERPRETATION

Our findings uncover an alternative mechanism by which PARP inhibitors efficiently suppress tumours, thereby pointing to a potential new therapeutic strategy for centrosome error-related tumours.

FUNDING

Funded by National Natural Science Foundation of China (NSFC) (T2225006, 82272948, 82103106), Beijing Municipal Natural Science Foundation (Key program Z220011), and the National Clinical Key Specialty Construction Program, P. R. China (2023).

Brca1 Mouse Models: Functional Insights and Therapeutic Opportunities

Brca1 mouse models were first reported in the mid-1990's shortly after cloning the human gene. Since then, many mouse models with a range of mutations have been generated, some mimic patient mutations, others are designed to probe specific protein domains and functions. In this review, we discuss early and recent studies using engineered Brca1 mouse alleles, and their implications for understanding Brca1 protein function in the context of DNA repair, tumorigenesis, and anti-cancer therapeutics.

The role of ubiquitin signaling pathway on liver regeneration in rats

The ubiquitin signalling pathway is a large system associated with numerous intracellular mechanisms. However, its function in the liver regeneration process remains unknown. This particular study investigates the intracellular effect mechanisms of the ubiquitin signalling pathway. It also determines the differences in the expression of 88 genes belonging to the ubiquitin pathway using the RT-PCR array method. To conduct this research, three genes-that differed in the expression analysis were selected. Moreover, their proteins were analysed by western blot analysis while using Ki67 immunohistochemical analysis that determines proliferation rates by hour. It was determined that BRCA1 and BARD1, which are effective in DNA repair, play an active role at PH24. Similarly, Ube2t expression, which belongs to the Fanconi anaemia pathway associated with DNA repair, was also found to be high at PH12-72 h. In addition, it was revealed that the expressions of Anapc2, Anapc11, Cdc20 belonging to the APC/C^Cdc20 complex, which participate in cell cycle regulation, differed at different hours after PH. Expression of Mul1, which is involved in mitochondrial biogenesis and mitophagy mechanisms, peaked at PH12 under the observation. Considering the Mul1 gene expression difference, MUL1-mediated mitophagy and mitochondrial fission mechanism may be associated with liver regeneration. It was also determined that PARKIN-mediated mitophagy mechanisms are not active in 0-72 h of liver regeneration since PARKIN expression did not show a significant change in PH groups.

GATA2-Mediated Transcriptional Activation of Notch3 Promotes Pancreatic Cancer Liver Metastasis

The liver is the predominant metastatic site for pancreatic cancer. However, the factors that determine the liver metastasis and the specific molecular mechanisms are still unclear. In this study, we used human pancreatic cancer cell line Hs766T to establish Hs766T-L3, a subline of Hs766T with stable liver metastatic ability. We performed RNA sequencing of Hs766T-L3 and its parental cell line Hs766T, and revealed huge differences in gene expression patterns and pathway activation between these two cell lines. We correlated the difference in pathway activation with the expression of the four core transcriptional factors including STAT1, NR2F2, GATA2, and SMAD4. Using the TCGA database, we examined the relative expression of these transcription factors (TFs) in pan-cancer and their relationship with the prognosis of the pancreatic cancer. Among these TFs, we considered GATA2 is closely involved in tumor metastasis and may serve as a potential metastatic driver. Further in vitro and in vivo experiments confirmed that GATA2-mediated transcriptional activation of Notch3 promotes the liver metastasis of Hs766T-L3, and knockdown of either GATA2 or Notch3 reduces the metastatic ability of Hs766T-L3. Therefore, we claim that GATA2 may serve as a metastatic driver of pancreatic cancer and a potential therapeutic target to treat liver metastasis of pancreatic cancer.

Estrogens—Origin of Centrosome Defects in Human Cancer?

Estrogens are associated with a variety of diseases and play important roles in tumor development and progression. Centrosome defects are hallmarks of human cancers and contribute to ongoing chromosome missegragation and aneuploidy that manifest in genomic instability and tumor progression. Although several mechanisms underlie the etiology of centrosome aberrations in human cancer, upstream regulators are hardly known. Accumulating experimental and clinical evidence points to an important role of estrogens in deregulating centrosome homeostasis and promoting karyotype instability. Here, we will summarize existing literature of how natural and synthetic estrogens might contribute to structural and numerical centrosome defects, genomic instability and human carcinogenesis.

Long Non-Coding RNA Neighbor of BRCA1 Gene 2: A Crucial Regulator in Cancer Biology

Long non-coding RNAs (lncRNAs) are involved in fundamental biochemical and cellular processes. The neighbor of BRCA1 gene 2 (NBR2) is a long intergenic non-coding RNA (lincRNA) whose gene locus is adjacent to the tumor suppressor gene breast cancer susceptibility gene 1 (BRCA1). In human cancers, NBR2 expression is dysregulated and correlates with clinical outcomes. Moreover, NBR2 is crucial for glucose metabolism and affects the proliferation, survival, metastasis, and therapeutic resistance in different types of cancer. Here, we review the precise molecular mechanisms underlying NBR2-induced changes in cancer. In addition, the potential application of NBR2 in the diagnosis and treatment of cancer is also discussed, as well as the challenges of exploiting NBR2 for cancer intervention.

The ciliary impact of nonciliary gene mutations

Mutations in genes encoding centriolar or ciliary proteins cause diseases collectively known as 'ciliopathies'. Interestingly, the Human Phenotype Ontology database lists numerous disorders that display clinical features reminiscent of ciliopathies but do not involve defects in the centriole-cilium proteome. Instead, defects in different cellular compartments may impair cilia indirectly and cause additional, nonciliopathy phenotypes. This phenotypic heterogeneity, perhaps combined with the field's centriole-cilium-centric view, may have hindered the recognition of ciliary contributions. Identifying these diseases and dissecting how the underlying gene mutations impair cilia not only will add to our understanding of cilium assembly and function but also may open up new therapeutic avenues.

Downregulation of ATM and BRCA1 Predicts Poor Outcome in Head and Neck Cancer: Implications for ATM-Targeted Therapy

ATM and BRCA1 are DNA repair genes that play a central role in homologous recombination repair. Alterations of ATM and BRCA1 gene expression are found in cancers, some of which are correlated with treatment response and patient outcome. However, the role of ATM and BRCA1 gene expression in head and neck cancer (HNC) is not well characterized. Here, we examined the prognostic role of ATM and BRCA1 expression in two HNC cohorts with and without betel quid (BQ) exposure. The results showed that the expression of ATM and BRCA1 was downregulated in BQ-associated HNC, as the BQ ingredient arecoline could suppress the expression of both genes. Low expression of either ATM or BRCA1 was correlated with poor overall survival (OS) and was an independent prognostic factor in multivariate analysis (ATM HR: 1.895, p = 0.041; BRCA1 HR: 2.163, p = 0.040). The combination of ATM and BRCA1 expression states further improved on the prediction of OS (HR: 4.195, p = 0.001, both low vs. both high expression). Transcriptomic analysis showed that inhibition of ATM kinase by KU55933 induced apoptosis signaling and potentiated cisplatin-induced cytotoxicity. These data unveil poor prognosis in the HNC patient subgroup with low expression of ATM and BRCA1 and support the notion of ATM-targeted therapy.

BRCA1 and BRCA2 Tumor Suppressor Function in Meiosis

Meiosis is a specialized cell cycle that results in the production of haploid gametes for sexual reproduction. During meiosis, homologous chromosomes are connected by chiasmata, the physical manifestation of crossovers. Crossovers are formed by the repair of intentionally induced double strand breaks by homologous recombination and facilitate chromosome alignment on the meiotic spindle and proper chromosome segregation. While it is well established that the tumor suppressors BRCA1 and BRCA2 function in DNA repair and homologous recombination in somatic cells, the functions of BRCA1 and BRCA2 in meiosis have received less attention. Recent studies in both mice and the nematode Caenorhabditis elegans have provided insight into the roles of these tumor suppressors in a number of meiotic processes, revealing both conserved and organism-specific functions. BRCA1 forms an E3 ubiquitin ligase as a heterodimer with BARD1 and appears to have regulatory roles in a number of key meiotic processes. BRCA2 is a very large protein that plays an intimate role in homologous recombination. As women with no indication of cancer but carrying BRCA mutations show decreased ovarian reserve and accumulated oocyte DNA damage, studies in these systems may provide insight into why BRCA mutations impact reproductive success in addition to their established roles in cancer.

Genetic interactions among Brca1, Brca2, Palb2, and Trp53 in mammary tumor development

Inherited mutations in BRCA1, BRCA2, and PALB2 cause a high risk of breast cancer. Here, we conducted parallel conditional knockout (CKO) of Brca1, Palb2, and Brca2, individually and in combination, along with one copy of Trp53, in the mammary gland of nulliparous female mice. We observed a functional equivalence of the three genes in their basic tumor-suppressive activity, a linear epistasis of Palb2 and Brca2, but complementary roles of Brca1 and Palb2 in mammary tumor suppression, as combined ablation of either Palb2 or Brca2 with Brca1 led to delayed tumor formation. Whole-exome sequencing (WES) revealed both similarities and differences between Brca1 and Palb2 or Brca2 null tumors. Analyses of mouse mammary glands and cultured human cells showed that combined loss of BRCA1 and PALB2 led to high levels of reactive oxygen species (ROS) and increased apoptosis, implicating oxidative stress in the delayed tumor development in Brca1;Palb2 double CKO mice. The functional complementarity between BRCA1 and PALB2/BRCA2 and the role of ROS in tumorigenesis require further investigation.

BRCA1 degradation in response to mitochondrial damage in breast cancer cells

BRCA1 is a well-studied tumor suppressor involved in the homologous repair of DNA damage, whereas PINK1, a mitochondrial serine/threonine kinase, is known to be involved in mitochondrial quality control. Genetic mutations of PINK1 and Parkin cause autosomal recessive early-onset Parkinson's disease. We found that in breast cancer cells, the mitochondrial targeting reagents, which all induce mitochondrial depolarization along with PINK1 upregulation, induced proteasomal BRCA1 degradation. This BRCA1 degradation was dependent on PINK1, and BRCA1 downregulation upon mitochondrial damage caused DNA double-strand breaks. BRCA1 degradation was mediated through the direct interaction with the E3 ligase Parkin. Strikingly, BRCA1 and PINK1/Parkin expression were inversely correlated in cancerous mammary glands from breast cancer patients. BRCA1 knockdown repressed cancer cell growth, and high BRCA1 expression predicted poor relapse-free survival in breast cancer patients. These observations indicate a novel mechanism by which mitochondrial damage is transmitted to the nucleus, leading to BRCA1 degradation.

Functional genomics identifies new synergistic therapies for retinoblastoma

Local intravitreal or intra-arterial chemotherapy has improved therapeutic success for the pediatric cancer retinoblastoma (RB), but toxicity remains a major caveat. RB initiates primarily with RB1 loss or, rarely, MYCN amplification, but the critical downstream networks are incompletely understood. We set out to uncover perturbed molecular hubs, identify synergistic drug combinations to target these vulnerabilities, and expose and overcome drug resistance. We applied dynamic transcriptomic analysis to identify network hubs perturbed in RB versus normal fetal retina, and performed in vivo RNAi screens in RB1^null and RB1^wt;MYCN^amp orthotopic xenografts to pinpoint essential hubs. We employed in vitro and in vivo studies to validate hits, define mechanism, develop new therapeutic modalities, and understand drug resistance. We identified BRCA1 and RAD51 as essential for RB cell survival. Their oncogenic activity was independent of BRCA1 functions in centrosome, heterochromatin, or ROS regulation, and instead linked to DNA repair. RAD51 depletion or inhibition with the small molecule inhibitor, B02, killed RB cells in a Chk1/Chk2/p53-dependent manner. B02 further synergized with clinically relevant topotecan (TPT) to engage this pathway, activating p53-BAX mediated killing of RB but not human retinal progenitor cells. Paradoxically, a B02/TPT-resistant tumor exhibited more DNA damage than sensitive RB cells. Resistance reflected dominance of the p53-p21 axis, which mediated cell cycle arrest instead of death. Deleting p21 or applying the BCL2/BCL2L1 inhibitor Navitoclax re-engaged the p53-BAX axis, and synergized with B02, TPT or both to override resistance. These data expose new synergistic therapies to trigger p53-induced killing in diverse RB subtypes.

Secreted Wnt6 mediates diabetes-associated centrosome amplification via its receptor FZD4

We recently published that type 2 diabetes promotes cell centrosome amplification via upregulation of Rho-associated protein kinase 1 (ROCK1) and 14-3-3 protein-σ (14-3-3σ). This study further investigates the molecular mechanisms underlying diabetes-associated centrosome amplification. We found that treatment of cells with high glucose, insulin, and palmitic acid levels increased the intracellular and extracellular protein levels of Wingless-type MMTV integration site family member 6 (Wnt6) as well as the cellular level of β-catenin. The treatment also activated β-catenin and promoted its nuclear translocation. Treatment of cells with siRNA species for Wnt6, Frizzled-4 (FZD4), or β-catenin as well as introduction of antibodies against Wnt6 or FZD4 to the cell culture medium could all attenuate the treatment-triggered centrosome amplification. Moreover, we showed that secreted Wnt6-FZD4-β-catenin was the signaling pathway that was upstream of ROCK1 and 14-3-3σ. We found that advanced glycation end products (AGEs) were also able to increase the cellular and extracellular levels of Wnt6, the cellular protein level of β-catenin, and centrosome amplification. Treatment of the cells with siRNA species for Wnt6 or FZD4 as well as introduction of antibodies against Wnt6 or FZD4 to the cell culture could all inhibit the AGEs-elicited centrosome amplification. In colon tissues from a diabetic mouse model, the protein levels of Wnt6 and 14-3-3σ were increased. In conclusion, our results showed that the pathophysiological factors in type 2 diabetes, including AGEs, were able to induce centrosome amplification. It is suggested that secreted Wnt6 binds to FZD4 to activate the canonical Wnt6 signaling pathway, which is upstream of ROCK1 and 14-3-3σ, and that this is the cell signaling pathway underlying diabetes-associated centrosome amplification.

Polo-like Kinase 1 Inhibition as a Therapeutic Approach to Selectively Target BRCA1-Deficient Cancer Cells by Synthetic Lethality Induction

PURPOSE

BRCA1 and BRCA2 deficiencies are widespread drivers of human cancers that await the development of targeted therapies. We aimed to identify novel synthetic lethal relationships with therapeutic potential using BRCA-deficient isogenic backgrounds.

EXPERIMENTAL DESIGN

We developed a phenotypic screening technology to simultaneously search for synthetic lethal (SL) interactions in BRCA1- and BRCA2-deficient contexts. For validation, we developed chimeric spheroids and a dual-tumor xenograft model that allowed the confirmation of SL induction with the concomitant evaluation of undesired cytotoxicity on BRCA-proficient cells. To extend our results using clinical data, we performed retrospective analysis on The Cancer Genome Atlas (TCGA) breast cancer database.

RESULTS

The screening of a kinase inhibitors library revealed that Polo-like kinase 1 (PLK1) inhibition triggers strong SL induction in BRCA1-deficient cells. Mechanistically, we found no connection between the SL induced by PLK1 inhibition and PARP inhibitors. Instead, we uncovered that BRCA1 downregulation and PLK1 inhibition lead to aberrant mitotic phenotypes with altered centrosomal duplication and cytokinesis, which severely reduced the clonogenic potential of these cells. The penetrance of PLK1/BRCA1 SL interaction was validated using several isogenic and nonisogenic cellular models, chimeric spheroids, and mice xenografts. Moreover, bioinformatic analysis revealed high-PLK1 expression in BRCA1-deficient tumors, a phenotype that was consistently recapitulated by inducing BRCA1 deficiency in multiple cell lines as well as in BRCA1-mutant cells.

CONCLUSIONS

We uncovered an unforeseen addiction of BRCA1-deficient cancer cells to PLK1 expression, which provides a new means to exploit the therapeutic potential of PLK1 inhibitors in clinical trials, by generating stratification schemes that consider this molecular trait in patient cohorts.

Genetic variation and radiation quality impact cancer promoting cellular phenotypes in response to HZE exposure

There exists a wide degree of genetic variation within the normal human population which includes disease free individuals with heterozygote defects in major DNA repair genes. A lack of understanding of how this genetic variation impacts cellular phenotypes that inform cancer risk post heavy ion exposure poses a major limitation in developing personalized cancer risk assessment astronauts. We initiated a pilot study with Human Mammary Epithelial Cell strains (HMEC) derived from wild type, a p16 silenced derivative of wild type, and various genetic variants that were heterozygote for DNA repair genes; BRCA1, BRCA2 and ATM. Cells strains were exposed to different high and low LET radiation qualities to generate both simple and complex lesions and centrosome aberrations were examined as a surrogate marker of genomic instability and cancer susceptibility post different exposures. Our results indicate that centrosome aberration frequency is higher in the genetic variants under study. The aberration frequency increases with dose, complexity of the lesion generated by different radiation qualities and age of the individual. This increase in genomic instability correlates with elevated check-point activation post radiation exposure. These studies suggest that the influence of individual genetics on cell cycle regulation could modify the degree of early genomic instability in response to complex lesions and potentially define cancer predisposition in response to HZE exposure. These results will have significant implications in estimating cancer susceptibility in genetically variant individuals exposed to HZE particles.

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.

Effect of TPA and HTLV-1 Tax on BRCA1 and ERE controlled genes expression

Interference with the expression and/or functions of the multifunctional tumor suppressor BRCA1 leads to a high risk of breast and ovarian cancers. BRCA1 expression is usually activated by the estrogen (E2) liganded ERα receptor. Activated ERα is considered as a potent transcription factor which activates various genes expression by 2 pathways. A classical pathway, ERα binds directly to E2-responsive elements (EREs) in the promoters of the responsive genes and a non-classical pathway where ERα indirectly binds with the appropriate gene promoter. In our previous study, HTLV-1Tax was found to strongly inhibit ERα induced BRCA1 expression while stimulating ERα induced ERE dependent genes. TPA is a strong PKC activator which found to induce the expression of HTLV-1. Here we examined the effect of TPA on the expression of BRCA1 and genes controlled by ERE region in MCF-7 cells and on Tax activity on these genes. Our results showed strong stimulatory effect of TPA on both BRCA1 and ERE expression without treatment with E2. Tax did not show any significant effect on these TPA activities. It seems that TPA activation of BRCA1 and ERE expression is dependent on PKC activity but not through the NFκB pathway. However, 53BP1 may be involved in this TPA activity because its overexpression significantly reduced the TPA stimulatory effect on BRCA1 and ERE expression. Additionally, our Chip assay results probably exclude possible involvement of ERα pathway in this TPA activity because TPA did not interfere with the binding of ERα to both BRCA1 promoter and ERE region.

Differential effects of HTLV-1 Tax oncoprotein on the different estrogen-induced-ER α-mediated transcriptional activities

The activated estrogen (E2) receptor α (ERα) is a potent transcription factor that is involved in the activation of various genes by 2 different pathways; a classical and non-classical. In classical pathway, ERα binds directly to E2-responsive elements (EREs) located in the appropriate genes promoters and stimulates their transcription. However, in non-classical pathway, the ERα can indirectly bind with promoters and enhance their activity. For instance, ERα activates BRCA1 expression by interacting with jun/fos complex bound to the AP-1 site in BRCA1 promoter. Interference with the expression and/or functions of BRCA1, leads to high risk of breast or/and ovarian cancer. HTLV-1Tax was found to strongly inhibit BRCA1 expression by preventing the binding of E2-ERα complex to BRCA1 promoter. Here we examined Tax effect on ERα induced activation of genes by the classical pathway by testing its influence on E2-induced expression of ERE promoter-driven luciferase reporter (ERE-Luc). Our findings showed that E2 profoundly stimulated this reporter expression and that HTLV-1Tax significantly induced this stimulation. This result is highly interesting because in our previous study Tax was found to strongly block the E2-ERα-mediated activation of BRCA1 expression. ERα was found to produce a big complex by recruiting various cofactors in the nucleus before binding to the ERE region. We also found that only part of the reqruited cofactors are required for the transcriptional activity of ERα complex. Chip assay revealed that the binding of Tax to the ERα complex, did not interfere with its link to ERE region.

E3 Ubiquitin ligase RNF183 Is a Novel Regulator in Inflammatory Bowel Disease

BACKGROUND AND AIMS

Specific members of the RING finger [RNF] protein family serve as E3 ubiquitin ligases and play important roles in the regulation of inflammation. However, their roles in the pathogenesis of inflammatory bowel disease [IBD] have not been explored.

METHODS

Genomic microarray of inflamed colon samples from Crohn's disease [CD] patients was performed to identify potential up-regulated genes. Expression of the identified highly up-regulated RNF183 gene was subsequently examined by quantitative reverse transcription polymerase chain reaction [qRT-PCR], western blotting and immunohistochemistry of the intestinal tissues of IBD patients and the colons of trinitrobenzene sulphonic acid [TNBS]-induced colitic mice. RNF183-mediated interaction with the NF-κB pathway and ubiquitination of IκBα were examined by siRNA, plasmid transfection, and immunoprecipitation. The miRNA predicted to target RNF183 was explored and its role in the RNF183/ NF-κB pathway was investigated.

RESULTS

RNF183 was up-regulated in intestinal epithelial cells in IBD patients and in colitic mice. RNF183 promoted intestinal inflammation via the activation of the NF-κB pathway by increasing the ubiquitination and degradation of IκBα. Computational analysis identified putative binding of miR-7 to RNF183. Transfection of intestinal cells with a miR-7 mimic or inhibitor confirmed its negative regulatory effect on RNF183 expression and ubiquitination of IκBα. miR-7 was down-regulated in inflamed colon tissues of IBD patients and colitic mice.

CONCLUSIONS

RNF183, which is negatively regulated by miR-7, is a novel regulator promoting intestinal inflammation by increasing the ubiquitination and degradation of IκBα, thereby inducing NF-κB activation. The interaction between RNF183-mediated ubiquitination and miRNA may be an important novel epigenetic mechanism in the pathogenesis of IBD.

Is homologous recombination really an error-free process?

Homologous recombination (HR) is an evolutionarily conserved process that plays a pivotal role in the equilibrium between genetic stability and diversity. HR is commonly considered to be error-free, but several studies have shown that HR can be error-prone. Here, we discuss the actual accuracy of HR. First, we present the product of genetic exchanges (gene conversion, GC, and crossing over, CO) and the mechanisms of HR during double strand break repair and replication restart. We discuss the intrinsic capacities of HR to generate genome rearrangements by GC or CO, either during DSB repair or replication restart. During this process, abortive HR intermediates generate genetic instability and cell toxicity. In addition to genome rearrangements, HR also primes error-prone DNA synthesis and favors mutagenesis on single stranded DNA, a key DNA intermediate during the HR process. The fact that cells have developed several mechanisms protecting against HR excess emphasize its potential risks. Consistent with this duality, several pro-oncogenic situations have been consistently associated with either decreased or increased HR levels. Nevertheless, this versatility also has advantages that we outline here. We conclude that HR is a double-edged sword, which on one hand controls the equilibrium between genome stability and diversity but, on the other hand, can jeopardize the maintenance of genomic integrity. Therefore, whether non-homologous end joining (which, in contrast with HR, is not intrinsically mutagenic) or HR is the more mutagenic process is a question that should be re-evaluated. Both processes can be "Dr. Jekyll" in maintaining genome stability/variability and "Mr. Hyde" in jeopardizing genome integrity.

Mouse models of BRCA1 and their application to breast cancer research

Germline mutations of human breast cancer-associated gene 1 (BRCA1) predispose women to breast and ovarian cancers. In mice, over 20 distinct mutations, including null, hypomorphic, isoform, conditional, and point mutations, have been created to study functions of Brca1 in mammary development and tumorigenesis. Analyses using these mutant mice have yielded an enormous amount of information that greatly facilitates our understanding of the gender- and tissue-specific tumor suppressor functions of BRCA1, as well as enriches our insights into applying these preclinical models of disease to breast cancer research. Here, we review features of these mutant mice and their applications to cancer prevention and therapeutic treatment.

Spontaneous slow replication fork progression elicits mitosis alterations in homologous recombination-deficient mammalian cells

Homologous recombination deficient (HR(-)) mammalian cells spontaneously display reduced replication fork (RF) movement and mitotic extra centrosomes. We show here that these cells present a complex mitotic phenotype, including prolonged metaphase arrest, anaphase bridges, and multipolar segregations. We then asked whether the replication and the mitotic phenotypes are interdependent. First, we determined low doses of hydroxyurea that did not affect the cell cycle distribution or activate CHK1 phosphorylation but did slow the replication fork movement of wild-type cells to the same level than in HR(-) cells. Remarkably, these low hydroxyurea doses generated the same mitotic defects (and to the same extent) in wild-type cells as observed in unchallenged HR(-) cells. Reciprocally, supplying nucleotide precursors to HR(-) cells suppressed both their replication deceleration and mitotic extra centrosome phenotypes. Therefore, subtle replication stress that escapes to surveillance pathways and, thus, fails to prevent cells from entering mitosis alters metaphase progression and centrosome number, resulting in multipolar mitosis. Importantly, multipolar mitosis results in global unbalanced chromosome segregation involving the whole genome, even fully replicated chromosomes. These data highlight the cross-talk between chromosome replication and segregation, and the importance of HR at the interface of these two processes for protection against general genome instability.

Opportunities and hurdles in the treatment of BRCA1-related breast cancer

BRCA1 functions as a classical tumor suppressor in breast and ovarian cancer. While the role of BRCA1 in homology-directed repair of DNA double-strand breaks contributes to its tumor suppressive activity, it also renders BRCA1-deficient cells highly sensitive to DNA-damaging agents. Although BRCA1 deficiency is therefore considered to be an attractive therapeutic target, re-activation of BRCA1 by secondary mutations has been shown to cause therapy resistance. In this review, we will assess the role of BRCA1 in both hereditary and sporadic breast cancer and discuss how different functionalities of the BRCA1 protein can contribute to its tumor suppressor function. In addition, we will discuss how this knowledge on BRCA1 function can help to overcome the hurdles encountered in the clinic and improve current treatment strategies for patients with BRCA1-related breast cancer.

Transcriptional program of ciliated epithelial cells reveals new cilium and centrosome components and links to human disease

Defects in the centrosome and cilium are associated with a set of human diseases having diverse phenotypes. To further characterize the components that define the function of these organelles we determined the transcriptional profile of multiciliated tracheal epithelial cells. Cultures of mouse tracheal epithelial cells undergoing differentiation in vitro were derived from mice expressing GFP from the ciliated-cell specific FOXJ1 promoter (FOXJ1:GFP). The transcriptional profile of ciliating GFP+ cells from these cultures was defined at an early and a late time point during differentiation and was refined by subtraction of the profile of the non-ciliated GFP- cells. We identified 649 genes upregulated early, when most cells were forming basal bodies, and 73 genes genes upregulated late, when most cells were fully ciliated. Most, but not all, of known centrosome proteins are transcriptionally upregulated early, particularly Plk4, a master regulator of centriole formation. We found that three genes associated with human disease states, Mdm1, Mlf1, and Dyx1c1, are upregulated during ciliogenesis and localize to centrioles and cilia. This transcriptome for mammalian multiciliated epithelial cells identifies new candidate centrosome and cilia proteins, highlights similarities between components of motile and primary cilia, and identifies new links between cilia proteins and human disease.

Human BRCA1-associated breast cancer: no increase in numerical chromosomal instability compared to sporadic tumors

BRCA1 is a major gatekeeper of genomic stability. Acting in multiple central processes like double-strand break repair, centrosome replication, and checkpoint control, BRCA1 participates in maintaining genomic integrity and protects the cell against genomic instability. Chromosomal instability (CIN) as part of genomic instability is an inherent characteristic of most solid tumors and is also involved in breast cancer development. In this study, we determined the extent of CIN in 32 breast cancer tumors of women with a BRCA1 germline mutation compared to 62 unselected breast cancers. We applied fluorescence in situ hybridization (FISH) with centromere-specific probes for the chromosomes 1, 7, 8, 10, 17, and X and locus-specific probes for 3q27 (BCL6), 5p15.2 (D5S23), 5q31 (EGR1), 10q23.3 (PTEN), and 14q32 (IGH@) on formalin-fixed paraffin-embedded tissue microarray sections. Our hypothesis of an increased level of CIN in BRCA1-associated breast cancer could not be confirmed by this approach. Surprisingly, we detected no significant difference in the extent of CIN in BRCA1-mutated versus sporadic tumors. The only exception was the CIN value for chromosome 1. Here, the extent of CIN was slightly higher in the group of sporadic tumors.

The role of centrosomal Nlp in the control of mitotic progression and tumourigenesis

The human centrosomal ninein-like protein (Nlp) is a new member of the γ-tubulin complexes binding proteins (GTBPs) that is essential for proper execution of various mitotic events. The primary function of Nlp is to promote microtubule nucleation that contributes to centrosome maturation, spindle formation and chromosome segregation. Its subcellular localisation and protein stability are regulated by several crucial mitotic kinases, such as Plk1, Nek2, Cdc2 and Aurora B. Several lines of evidence have linked Nlp to human cancer. Deregulation of Nlp in cell models results in aberrant spindle, chromosomal missegregation and multinulei, and induces chromosomal instability and renders cells tumourigenic. Overexpression of Nlp induces anchorage-independent growth and immortalised primary cell transformation. In addition, we first demonstrate that the expression of Nlp is elevated primarily due to NLP gene amplification in human breast cancer and lung carcinoma. Consistently, transgenic mice overexpressing Nlp display spontaneous tumours in breast, ovary and testicle, and show rapid onset of radiation-induced lymphoma, indicating that Nlp is involved in tumourigenesis. This review summarises our current knowledge of physiological roles of Nlp, with an emphasis on its potentials in tumourigenesis.

Delocalization of gamma-tubulin due to increased solubility in human breast cancer cell lines

The centrosome is the major organelle responsible for the nucleation and organization of microtubules into arrays. Recent studies demonstrate that microtubules can nucleate outside the centrosome. The molecular mechanisms controlling acentrosomal microtubule nucleation are currently poorly defined, and the function of this type of microtubule regulation in tumor cell biology is particularly unclear. Since microtubule nucleation is initiated by the gamma-tubulin protein, we examined the regulation of gamma-tubulin in a panel of human breast tumor cell lines, ranging from non-tumorigenic to highly aggressive. We have identified a more dispersive subcellular localization of gamma-tubulin in aggressive breast cancer cell lines, while gamma-tubulin localization remains largely centrosomal in non-aggressive cell lines. Delocalization of gamma-tubulin occurs independently from changes in protein expression and is therefore regulated at the post-translational level. Subcellular fractionation revealed that tumor cell lines show an aberrantly increased release of gamma-tubulin into a soluble cytoplasmic fraction, with the most dramatic changes observed in tumor cell lines of greater aggressiveness. Extraction of soluble gamma-tubulin revealed acentrosomal incorporation of gamma-tubulin in cytoplasmic microtubules and along cell junctions. Moreover, acentrosomal delocalization of gamma-tubulin yielded resistance to colchicine-mediated microtubule collapse. These findings support a model where the solubility of gamma-tubulin can be altered through post-translational modification and provides a new mechanism for microtubule dysregulation in breast cancer. Gamma-tubulin that is delocalized from the centrosome can still clearly be incorporated into filaments, and defines a novel mechanism for tumor cells to develop resistance to microtubule-targeted chemotherapies.

Centrosomal Nlp is an oncogenic protein that is gene-amplified in human tumors and causes spontaneous tumorigenesis in transgenic mice

Disruption of mitotic events contributes greatly to genomic instability and results in mutator phenotypes. Indeed, abnormalities of mitotic components are closely associated with malignant transformation and tumorigenesis. Here we show that ninein-like protein (Nlp), a recently identified BRCA1-associated centrosomal protein involved in microtubule nucleation and spindle formation, is an oncogenic protein. Nlp was found to be overexpressed in approximately 80% of human breast and lung carcinomas analyzed. In human lung cancers, this deregulated expression was associated with NLP gene amplification. Further analysis revealed that Nlp exhibited strong oncogenic properties; for example, it conferred to NIH3T3 rodent fibroblasts the capacity for anchorage-independent growth in vitro and tumor formation in nude mice. Consistent with these data, transgenic mice overexpressing Nlp displayed spontaneous tumorigenesis in the breast, ovary, and testicle within 60 weeks. In addition, Nlp overexpression induced more rapid onset of radiation-induced lymphoma. Furthermore, mouse embryonic fibroblasts (MEFs) derived from Nlp transgenic mice showed centrosome amplification, suggesting that Nlp overexpression mimics BRCA1 loss. These findings demonstrate that Nlp abnormalities may contribute to genomic instability and tumorigenesis and suggest that Nlp might serve as a potential biomarker for clinical diagnosis and therapeutic target.

Centriole separation in DNA damage-induced centrosome amplification

Altered centrosome numbers are seen in tumor cells in response to DNA damaging treatments and are hypothesised to contribute to cancer development. The mechanism by which the centrosome and chromosome cycles become disconnected after DNA damage is not yet clear. Here, we show that centrosome amplification occurs after ionising radiation (IR) in chicken DT40 cells that lack DNA-PK, Ku70, H2AX, Xpa, and Scc1, demonstrating that these activities are not required for centrosome amplification. We show that inhibition of topoisomerase II induces Chk1-dependent centrosome amplification, a similar response to that seen after IR. In the immortalised, nontransformed hTERT-RPE1 line, we observed centriole splitting, followed by dose-dependent centrosome amplification, after IR. We found that IR results in the formation of single, not multiple, daughter centrioles during centrosome amplification in U2OS osteosarcoma cells. Analysis of BRCA1 and BRCA2 mutant tumor cells showed high levels of centriole splitting in the absence of any treatment. IR caused pronounced levels of centrosome amplification in BRCA1 mutant breast cancer cells. These data show that centrosome amplification occurs after different forms of DNA damage in chicken cells, in nontransformed human cells and in human tumor cell lines, indicating that this is a general response to DNA damaging treatments. Together, our data suggest that centriole splitting is a key step in potentiation of the centrosome amplification that is a general response to DNA damage.

BRCA1 interaction of centrosomal protein Nlp is required for successful mitotic progression

Breast cancer susceptibility gene BRCA1 is implicated in the control of mitotic progression, although the underlying mechanism(s) remains to be further defined. Deficiency of BRCA1 function leads to disrupted mitotic machinery and genomic instability. Here, we show that BRCA1 physically interacts and colocalizes with Nlp, an important molecule involved in centrosome maturation and spindle formation. Interestingly, Nlp centrosomal localization and its protein stability are regulated by normal cellular BRCA1 function because cells containing BRCA1 mutations or silenced for endogenous BRCA1 exhibit disrupted Nlp colocalization to centrosomes and enhanced Nlp degradation. Its is likely that the BRCA1 regulation of Nlp stability involves Plk1 suppression. Inhibition of endogenous Nlp via the small interfering RNA approach results in aberrant spindle formation, aborted chromosomal segregation, and aneuploidy, which mimic the phenotypes of disrupted BRCA1. Thus, BRCA1 interaction of Nlp might be required for the successful mitotic progression, and abnormalities of Nlp lead to genomic instability.

AKT1 represses gene conversion induced by different genotoxic stresses and induces supernumerary centrosomes and aneuploidy in hamster ovary cells

The oncogenic kinase AKT1 is frequently overexpressed or activated in sporadic breast and ovarian cancers. In human breast tumors, we have previously shown that AKT1 represses homologous recombination (HR) induced by one double-strand break (DSB). To further analyze the impact of AKT1 on HR, we ectopically expressed wild-type or mutant forms of AKT1 in a hamster ovary cell line containing an intrachromosomal substrate for monitoring HR. In this cell line, AKT1 repressed HR induced by different genotoxic stresses including ionizing radiation, UV-C and one single DSB introduced into the intrachromosomal substrate. Consistently, AKT1 disrupted RAD51 foci formation, showing that AKT1 specifically affects gene conversion. Concomitantly, AKT1 represses both BRCA1 foci formation and HR stimulation resulting from BRCA1 overexpression, showing that AKT1 affects BRCA1-mediated HR functions, also in another species (hamster) and in another type of cell tissue (ovary cells). Finally, consistent with the HR defects, active AKT1 expression induces supernumerary centrosomes and aneuploidy. In addition to its impact on cell proliferation and apoptosis, the present data propose a novel oncogenic function for AKT1, by producing genomic instability as a consequence of HR repression.

Interplay among BRCA1, SIRT1, and Survivin during BRCA1-associated tumorigenesis

Germline mutations of BRCA1 predispose women to breast and ovarian cancers. However, the downstream mediators of BRCA1 function in tumor suppression remain elusive. We found that human BRCA1-associated breast cancers have lower levels of SIRT1 than their normal controls. We further demonstrated that mammary tumors from Brca1 mutant mice have low levels of Sirt1 and high levels of Survivin, which is reversed by induced expression of Brca1. BRCA1 binds to the SIRT1 promoter and increases SIRT1 expression, which in turn inhibits Survivin by changing the epigenetic modification of histone H3. Absence of SIRT1 blocks the regulation of Survivin by BRCA1. Furthermore, we demonstrated that activation of Sirt1 and inhibition of Survivin expression by resveratrol elicit a more profound inhibitory effect on Brca1 mutant cancer cells than on Brca1-wild-type cancer cells both in vitro and in vivo. These findings suggest that resveratrol treatment serves as an excellent strategy for targeted therapy for BRCA1-associated breast cancer.

Association of loss of BRCA1 expression with centrosome aberration in human breast cancer

PURPOSE

Centrosome aberration in number and/or size is reportedly often observed in human breast cancer. The aim of this study was to investigate the relationship between centrosome aberration and chromosomal instability as well as the expression of centrosome regulators such as BRCA1, Aurora-A, and p53.

METHODS

Centrosome aberration in number and size was determined immunohistochemically using the anti-gamma-tubulin antibody, and chromosomal instability was evaluated by fluorescence in situ hybridization analysis of chromosomes 1, 11, and 17 in paraffin sections from 50 human breast cancers. Immunohistochemical examination of BRCA1, Aurora-A, and p53 was also performed to examine the relationship of their expression with centrosome aberration.

RESULTS

Percentage of tumor cells with centrosome aberration in size varied from 0.9 to 30.4% (median 9.5%) and in number it varied from 0.5 to 86.5% (median 34.5%) in each tumor. No significant association in number or size, however, was observed between chromosomal instability and centrosome aberration. Numerical centrosome aberration was significantly associated with negative BRCA1 expression (P = 0.001). Breast tumors (n = 3) from patients with a proven BRCA1 germline mutation also showed a significant relationship with numerical centrosome aberration (P = 0.011). On the other hand, expression of Aurora-A or p53 was not significantly associated with centrosome aberration in either number or size.

CONCLUSIONS

Centrosome aberration is not associated with chromosomal instability, indicating the importance of other mechanisms in the induction of chromosomal instability in human breast cancer. BRCA1, but not Aurora-A and p53, is significantly involved in the pathogenesis of centrosome aberration.

BRCA1 is required for meiotic spindle assembly and spindle assembly checkpoint activation in mouse oocytes

BRCA1 as a tumor suppressor has been widely investigated in mitosis, but its functions in meiosis are unclear. In the present study, we examined the expression, localization, and function of BRCA1 during mouse oocyte meiotic maturation. We found that expression level of BRCA1 was increased progressively from germinal vesicle to metaphase I stage, and then remained stable until metaphase II stage. Immunofluorescent analysis showed that BRCA1 was localized to the spindle poles at metaphase I and metaphase II stages, colocalizing with centrosomal protein gamma-tubulin. Taxol treatment resulted in the presence of BRCA1 onto the spindle microtubule fibers, whereas nocodazole treatment induced the localization of BRCA1 onto the chromosomes. Depletion of BRCA1 by both antibody injection and siRNA injection caused severely impaired spindles and misaligned chromosomes. Furthermore, BRCA1-depleted oocytes could not arrest at the metaphase I in the presence of low-dose nocodazole, suggesting that the spindle checkpoint is defective. Also, in BRCA1-depleted oocytes, gamma-tubulin dissociated from spindle poles and MAD2L1 failed to rebind to the kinetochores when exposed to nocodazole at metaphase I stage. Collectively, these data indicate that BRCA1 regulates not only meiotic spindle assembly, but also spindle assembly checkpoint, implying a link between BRCA1 deficiency and aneuploid embryos.

Taxanes, microtubules and chemoresistant breast cancer

The taxanes, paclitaxel and docetaxel are microtubule-stabilizing agents that function primarily by interfering with spindle microtubule dynamics causing cell cycle arrest and apoptosis. However, the mechanisms underlying their action have yet to be fully elucidated. These agents have become widely recognized as active chemotherapeutic agents in the treatment of metastatic breast cancer and early-stage breast cancer with benefits gained in terms of overall survival (OS) and disease-free survival (DFS). However, even with response to taxane treatment the time to progression (TTP) is relatively short, prolonging life for a matter of months, with studies showing that patients treated with taxanes eventually relapse. This review focuses on chemoresistance to taxane treatment particularly in relation to the spindle assembly checkpoint (SAC) and dysfunctional regulation of apoptotic signaling. Since spindle microtubules are the primary drug targets for taxanes, important SAC proteins such as MAD2, BUBR1, Synuclein-gamma and Aurora A have emerged as potentially important predictive markers of taxane resistance, as have specific checkpoint proteins such as BRCA1. Moreover, overexpression of the drug efflux pump MDR-1/P-gp, altered expression of microtubule-associated proteins (MAPs) including tau, stathmin and MAP4 may help to identify those patients who are most at risk of recurrence and those patients most likely to benefit from taxane treatment.

AURKA F31I polymorphism and breast cancer risk in BRCA1 and BRCA2 mutation carriers: a consortium of investigators of modifiers of BRCA1/2 study

The AURKA oncogene is associated with abnormal chromosome segregation and aneuploidy and predisposition to cancer. Amplification of AURKA has been detected at higher frequency in tumors from BRCA1 and BRCA2 mutation carriers than in sporadic breast tumors, suggesting that overexpression of AURKA and inactivation of BRCA1 and BRCA2 cooperate during tumor development and progression. The F31I polymorphism in AURKA has been associated with breast cancer risk in the homozygous state in prior studies. We evaluated whether the AURKA F31I polymorphism modifies breast cancer risk in BRCA1 and BRCA2 mutation carriers from the Consortium of Investigators of Modifiers of BRCA1/2. Consortium of Investigators of Modifiers of BRCA1/2 was established to provide sufficient statistical power through increased numbers of mutation carriers to identify polymorphisms that act as modifiers of cancer risk and can refine breast cancer risk estimates in BRCA1 and BRCA2 mutation carriers. A total of 4,935 BRCA1 and 2,241 BRCA2 mutation carriers and 11 individuals carrying both BRCA1 and BRCA2 mutations was genotyped for F31I. Overall, homozygosity for the 31I allele was not significantly associated with breast cancer risk in BRCA1 and BRCA2 carriers combined [hazard ratio (HR), 0.91; 95% confidence interval (95% CI), 0.77-1.06]. Similarly, no significant association was seen in BRCA1 (HR, 0.90; 95% CI, 0.75-1.08) or BRCA2 carriers (HR, 0.93; 95% CI, 0.67-1.29) or when assessing the modifying effects of either bilateral prophylactic oophorectomy or menopausal status of BRCA1 and BRCA2 carriers. In summary, the F31I polymorphism in AURKA is not associated with a modified risk of breast cancer in BRCA1 and BRCA2 carriers.

CCDC98 is a BRCA1-BRCT domain-binding protein involved in the DNA damage response

The product of the breast cancer-1 gene, BRCA1, plays a crucial part in the DNA damage response through its interactions with many proteins, including BACH1, CtIP and RAP80. Here we identify a coiled-coil domain-containing protein, CCDC98, as a BRCA1-interacting protein. CCDC98 colocalizes with BRCA1 and is required for the formation of BRCA1 foci in response to ionizing radiation. Moreover, like BRCA1, CCDC98 has a role in radiation sensitivity and damage-induced G2/M checkpoint control. Together, these results suggest that CCDC98 is a mediator of BRCA1 function involved in the mammalian DNA damage response.

Identification and functional characterization of a PP1-binding site in BRCA1

The phosphorylation state of the tumor suppressor protein BRCA1 is tightly associated with its functions including cell cycle control and DNA repair. Protein kinases involved in the DNA damage checkpoint control, such as ATM, ATR, and hCds1/Chk2, have been shown to phosphorylate and activate BRCA1 upon DNA damage. We reported previously that protein phosphatase 1alpha (PP1alpha) interacts with and dephosphorylates hCds1/Chk2-phosphorylated BRCA1. This study demonstrates the identification of a PP1-binding motif 898KVTF901 in BRCA1. Mutation or deletion of critical residues in this PP1-binding motif substantially reduces the interaction between BRCA1 and PP1alpha. PP1alpha can also dephosphorylate ATM and ATR phosphorylation sites in BRCA1 and may serve as a general regulator for BRCA1 phosphorylation. Unlike wild-type BRCA1, expression of the PP1 non-binding mutant BRCA1 protein in BRCA1-deficient cells failed to enhance survival after DNA damage. Taken together, these results suggest that interaction with PP1alpha is important for BRCA1 function.

ATM is activated by mitotic stress and suppresses centrosome amplification in primary but not in tumor cells

Centrosome amplification has been proposed to contribute to the development of aneuploidy and genome instability. Here, we show that Ataxia-Telangiectasia Mutated (ATM) is localized to the centrosome and co-purified with gamma-tubulin. The importance of ATM in centrosome duplication is demonstrated in Atm-deficient primary mouse embryonic fibroblasts that display centrosome amplification. Interestingly, centrosome amplification was not observed in tumor cell lines derived from Atm and p21 double deficient mouse. Our results also indicate that both p53 and p21 operate in the same pathway as ATM in regulating centrosome biogenesis. Finally, a potential role of ATM in spindle checkpoint regulation is demonstrated by which ATM protein is activated by mitotic stress. These results suggest a role of ATM in spindle checkpoint regulation and indicate that ATM suppresses genome instability and cellular transformation by regulating centrosome biogenesis.

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

Background

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

Methods

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

Results

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

Conclusion

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

Phosphatidylinositol 3-kinase/Akt signaling enhances nuclear localization and transcriptional activity of BRCA1

Signaling pathways involved in regulating nuclear-cytoplasmic distribution of BRCA1 have not been previously reported. Here, we provide evidence that heregulin beta1-induced activation of the Akt pathway increases the nuclear content of BRCA1. First, treatment of T47D breast cancer cells with heregulin beta1 results in a two-fold increase in nuclear BRCA1 as assessed by FACS analysis, immunoblotting and immunofluorescence. This heregulin-induced increase in nuclear BRCA1 is blocked by siRNA-mediated down-regulation of Akt. Second, mutation of threonine 509 in BRCA1, the site of Akt phosphorylation, to an alanine, attenuates the ability of heregulin to induce BRCA1 nuclear accumulation. These data suggest that Akt-catalyzed phosphorylation of BRCA1 is required for the heregulin-regulated nuclear concentration of BRCA1. Because most functions ascribed to BRCA1 occur within the nucleus, we postulated that phosphorylation-dependent nuclear accumulation of BRCA1 would result in enhanced nuclear activity, specifically transcriptional activity, of BRCA1. This postulate is affirmed by our observation that the ability of BRCA1 to transactivate GADD45 promoter constructs was enhanced in T47D cells treated with heregulin beta1. Furthermore, the heterologous expression of BRCA1 in HCC1937 human breast cancer cells, which have constitutively active Akt, also induces GADD45 promoter activity, whereas the expression of BRCA1 in which threonine 509 has been mutated to an alanine is able to only minimally induce promoter activity. These findings implicate Akt in upstream events leading to BRCA1 nuclear localization and function.

BRCA1 ubiquitinates RPB8 in response to DNA damage

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

Poly(ADP-ribose) polymerase-1 plays a role in suppressing mammary tumourigenesis in mice

The DNA strand break-binding molecule, poly(ADP-ribose) polymerase-1 (PARP-1), plays a role in DNA repair, chromosomal stability, transcription and cell death. Accumulating evidence suggests that dysfunction of PARP-1 contributes to tumorigenesis. Here, we report that PARP-1 deficiency causes mammary carcinoma formation in female mice, and that the introduction of Trp53 mutations accelerates the onset and shortens the latency of mammary tumorigenesis. We show that PARP-1 deficiency results in chromosomal aneuploidy and centrosome amplification, which are substantiated by the inactivation of Trp53 in primary mammary epithelial (PME) cells. In addition, PARP-1 deficiency compromises p53 activation and impairs BRCA1 recruitment to the sites of DNA damage in PME cells. PARP-1 complementation partly rescues the defective DNA damage response mediated by p53 and BRCA1. The present study thus identifies a role of PARP-1 in suppressing mammary tumorigenesis in vivo and suggests that dysfunction of PARP-1 may be a risk factor for breast cancer in humans.

Hyperplasia and spontaneous tumor development in the gynecologic system in mice lacking the BRCA1-Delta11 isoform

Alternative splicing in the BRCA1 locus generates multiple protein products including BRCA1-Delta11, which is identical to the BRCA1 full-length isoform (BRCA1-FL) except for the absence of exon 11. Mutation analysis using gene targeting to create null mutations or disrupt BRCA-FL has provided much of our understanding of BRCA1 functions; however, targeted mutation of specific short forms of BRCA1 has not been reported. To understand the physiologic functions of BRCA1-Delta11, we used a knock-in approach that blocks alternative splicing between exons 10 and 12 to prevent the formation of this form of BRCA1. We showed that homozygous mutant mice (Brca1(FL/FL)) were born at a Mendelian ratio without obvious developmental defects. However, the majority of Brca1(FL/FL) female mice showed mammary gland abnormalities and uterine hyperplasia after one year of age with spontaneous tumor formation. Cultured Brca1(FL/FL) cells exhibited abnormal centrosome amplification and reduction of G(1) population that was accompanied by accumulation of cyclin E and cyclin A. Accumulation of cyclin E was also found in epithelial layers of dilated ducts and hyperproliferative lobular regions in the mammary glands of Brca1(FL/FL) mice. These observations provide evidence that BRCA1 splicing variants are involved in BRCA1 functions in modulating G(1)/S transition, centrosome duplication, and repressing tumor formation.

HTLV-1 Tax: centrosome amplification and cancer

During interphase, each cell contains a single centrosome that acts as a microtubule organizing center for cellular functions in interphase and in mitosis. Centrosome amplification during the S phase of the cell cycle is a tightly regulated process to ensure that each daughter cell receives the proper complement of the genome. The controls that ensure that centrosomes are duplicated exactly once in the cell cycle are not well understood. In solid tumors and hematological malignancies, centrosome abnormalities resulting in aneuploidy is observed in the majority of cancers. These phenotypes are also observed in cancers induced by viruses, including adult T cell lymphoma which is caused by the human T cell lymphotrophic virus Type 1 (HTLV-1). Several reports have indicated that the HTLV-1 transactivator, Tax, is directly responsible for the centrosomal abnormalities observed in ATL cells. A recent paper in Nature Cell Biology by Ching et al. has shed some new light into how Tax may be inducing centrosome abnormalities. The authors demonstrated that 30% of ATL cells contained more than two centrosomes and expression of Tax alone induced supernumerary centrosomes. A cellular coiled-coil protein, Tax1BP2, was shown to interact with Tax and disruption of this interaction led to failure of Tax to induce centrosome amplification. Additionally, down-regulation of Tax1BP2 led to centrosome amplification. These results suggest that Tax1BP2 may be an important block to centrosome re-duplication that is observed in normal cells. Presently, a specific cellular protein that prevents centrosome re-duplication has not been identified. This paper has provided further insight into how Tax induces centrosome abnormalities that lead to ATL. Lastly, additional work on Tax1BP2 will also provide insight into how the cell suppresses centrosome re-duplication during the cell cycle and the role that Tax1BP2 plays in this important cellular pathway.

Altered cellular distribution and subcellular sorting of gamma-tubulin in diffuse astrocytic gliomas and human glioblastoma cell lines

Centrosome amplification is a pivotal mechanism underlying tumorigenesis but its role in gliomas is underinvestigated. The present study specifically examines the expression and distribution of the centrosome-associated cytoskeletal protein gamma-tubulin in 56 primary diffuse astrocytic gliomas (grades II-IV) and in 4 human glioblastoma cell lines (U87MG, U118MG, U138MG, and T98G). Monoclonal anti-peptide antibodies recognizing epitopes in C-terminal or N-terminal domains of the gamma-tubulin molecule were used in immunohistochemical, immunofluorescence, and immunoblotting studies. In tumors in adults (n = 46), varying degrees of localization were detected in all tumor grades, but immunoreactivity was significantly increased in high-grade anaplastic astrocytomas and glioblastomas multiforme as compared to low-grade diffuse astrocytomas (p = 0.0001). A similar trend was noted in diffuse gliomas in children but the sample of cases was too small as to be statistically meaningful. Two overlapping patterns of ectopic cellular localization were identified in both primary tumors and glioblastoma cell lines: A punctate pattern, in which gamma-tubulin was partially co-distributed with pericentrin in the pericentriolar region, and a diffuse pattern, independent of pericentrin staining, denoting a soluble pool of gamma-tubulin. Cellular gamma-tubulin was detected in both soluble and insoluble (nocodazole-resistant) fractions of glioblastoma cells. Divergent localizations of gamma-tubulin and pericentrin suggest a differential distribution of these 2 centrosome-associated proteins in glioblastoma cell lines. Our results indicate that overexpression and ectopic cellular distribution of gamma-tubulin in astrocytic gliomas may be significant in the context of centrosome protein amplification and may be linked to tumor progression and anaplastic potential.

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.