Tumor formation in Brca1 conditional mutant mice

Activation of FGFR2 Signaling Suppresses BRCA1 and Drives Triple-Negative Mammary Tumorigenesis That is Sensitive to Immunotherapy

Fibroblast growth factor receptor 2 (FGFR2) is a membrane-spanning tyrosine kinase that mediates FGF signaling. Various FGFR2 alterations are detected in breast cancer, yet it remains unclear if activation of FGFR2 signaling initiates tumor formation. In an attempt to answer this question, a mouse model berrying an activation mutation of FGFR2 (FGFR2-S252W) in the mammary gland is generated. It is found that FGF/FGFR2 signaling drives the development of triple-negative breast cancer accompanied by epithelial-mesenchymal transition that is regulated by FGFR2-STAT3 signaling. It is demonstrated that FGFR2 suppresses BRCA1 via the ERK-YY1 axis and promotes tumor progression. BRCA1 knockout in the mammary gland of the FGFR2-S252W mice significantly accelerated tumorigenesis. It is also shown that FGFR2 positively regulates PD-L1 and that a combination of FGFR2 inhibition and immune checkpoint blockade kills cancer cells. These data suggest that the mouse models mimic human breast cancers and can be used to identify actionable therapeutic targets.

Advances in Rodent Models for Breast Cancer Formation, Progression, and Therapeutic Testing

Breast cancer is a highly complicated disease. Advancement in the treatment and prevention of breast cancer lies in elucidation of the mechanism of carcinogenesis and progression. Rodent models of breast cancer have developed into premier tools for investigating the mechanisms and genetic pathways in breast cancer progression and metastasis and for developing and evaluating clinical therapeutics. Every rodent model has advantages and disadvantages, and the selection of appropriate rodent models with which to investigate breast cancer is a key decision in research. Design of a suitable rodent model for a specific research purpose is based on the integration of the advantages and disadvantages of different models. Our purpose in writing this review is to elaborate on various rodent models for breast cancer formation, progression, and therapeutic testing.

Loss of BRCA1 Spontaneously Induces the Tumorigenesis in Lacrimal Gland

Environmental and genetic factors exert important influences on lifespan and neoplastic transformation. We have previously shown that spontaneous tumors form frequently in mice homozygous for a full-length Brca1 deletion. In general, mutations of BRCA1 are closely associated with induction of breast and ovarian cancers but are also known to contribute to the incidence of other cancers at a low frequency. Female Brca1-mutant mice (Brca1^co/coMMTV-cre) were generated by crossing Brca1 conditional knockout mice and MMTV-cre mice, and the occurrence of lacrimal gland abnormalities and tumors was followed until mice reached 18 months of age. Lacrimal gland tumors, which occur at a very low frequency in the human population (1 per 1,000,000 per year), were detected in 7 cases of Brca1^co/coMMTV-cre mice (2.75%) older than 9 months of age. None of seven mice exhibited any abnormality in the mammary gland including neoplasia, suggesting lacrimal gland tumor is spontaneously and independently formed. These tumors, which were detected in seven mutant mice that displayed exophthalmoses, were malignant, originated from epithelial cells, and were identified as acinic cell carcinoma by pathological analysis. Further analysis revealed that tumorigenesis was accompanied by the accumulation of cyclin D1 and decreased expression of the cellular oncogenes, c-Myc, c-Jun, and c-Raf. Tumors also exhibited rearrangement of cytoskeletal proteins, including β-catenin, keratin 5, and vimentin, depending on tumor progression. These results suggest that BRCA1 is involved in genetic stability of the lacrimal gland, providing new insight into genomic instability in organism maintenance and tumorigenesis of the lacrimal gland.

The association between BRCA1 gene polymorphism and cancer risk: a meta-analysis

Many studies have reported that BRCA1 polymorphisms are associated with cancer risk, but the results remain controversial. The purpose of this meta-analysis is to evaluate the relationship between BRCA1 polymorphisms (rs799917, rs1799950, rs1799966, or rs16941) and cancer risk. Relevant studies were identified via a systematic search of the PubMed, Embase, and Web of Science databases up to July 31, 2017. Odds ratios (ORs) with 95% confidence intervals (CIs) were calculated to examine the strength of the associations. Thirty-five studies published in 19 publications involving 28,094 cases and 50,657 controls were included in this meta-analysis. There was no obvious association between rs799917, rs1799966, or rs16941 polymorphisms and overall cancer risk in any genetic models. However, subgroup analyses revealed that the rs799917 polymorphism could decrease the risk of cervical cancer, esophageal squamous cell carcinoma (ESCC), gastric cancer, and non-Hodgkin lymphoma (NHL) among Asian populations in one or more genetic models and that rs16941 could increase overall cancer risk among Caucasian populations in the homozygote and recessive models. Our meta-analysis also indicated that rs1799950 could decrease the breast cancer (BC) risk among Caucasian populations in the homozygote and recessive models. In summary, our results suggest that BRCA1 polymorphisms may play an important role in the etiology of cancer. However, due to the limited number of studies, these findings should be confirmed by new studies with larger sample sizes that address various types of cancer.

Loss of BRCA1 Spontaneously Induces the Tumorigenesis in Lacrimal Gland

Environmental and genetic factors exert important influences on lifespan and neoplastic transformation. We have previously shown that spontaneous tumors form frequently in mice homozygous for a full-length Brca1 deletion. In general, mutations of BRCA1 are closely associated with induction of breast and ovarian cancers but are also known to contribute to the incidence of other cancers at a low frequency. Female Brca1-mutant mice (Brca1^co/coMMTV-cre) were generated by crossing Brca1 conditional knockout mice and MMTV-cre mice, and the occurrence of lacrimal gland abnormalities and tumors was followed until mice reached 18 months of age. Lacrimal gland tumors, which occur at a very low frequency in the human population (1 per 1,000,000 per year), were detected in 7 cases of Brca1^co/coMMTV-cre mice (2.75%) older than 9 months of age. None of seven mice exhibited any abnormality in the mammary gland including neoplasia, suggesting lacrimal gland tumor is spontaneously and independently formed. These tumors, which were detected in seven mutant mice that displayed exophthalmoses, were malignant, originated from epithelial cells, and were identified as acinic cell carcinoma by pathological analysis. Further analysis revealed that tumorigenesis was accompanied by the accumulation of cyclin D1 and decreased expression of the cellular oncogenes, c-Myc, c-Jun, and c-Raf. Tumors also exhibited rearrangement of cytoskeletal proteins, including β-catenin, keratin 5, and vimentin, depending on tumor progression. These results suggest that BRCA1 is involved in genetic stability of the lacrimal gland, providing new insight into genomic instability in organism maintenance and tumorigenesis of the lacrimal gland.

Fibroblast Growth Factor Receptor 2 Signaling in Breast Cancer

Fibroblast growth factor receptor 2 (FGFR2) is a membrane-spanning tyrosine kinase that mediates signaling for FGFs. Recent studies detected various point mutations of FGFR2 in multiple types of cancers, including breast cancer, lung cancer, gastric cancer, uterine cancer and ovarian cancer, yet the casual relationship between these mutations and tumorigenesis is unclear. Here we will discuss possible interactions between FGFR2 signaling and several major pathways through which the aberrantly activated FGFR2 signaling may result in breast cancer development. We will also discuss some recent developments in the discovery and application of therapies and strategies for breast cancers by inhibiting FGFR2 activities.

Identification of direct target genes of miR-7, miR-9, miR-96, and miR-182 in the human breast cancer cell lines MCF-7 and MDA-MB-231

Some microRNAs have carcinogenic or tumor suppressive effects in breast cancer, which is the most common cancer in women worldwide. MiR-7 and miR-9 are tumor suppressor microRNAs, which induce apoptosis and inhibit proliferation in breast cancer cells. Moreover, miR-96 and miR-182 are onco-microRNAs that increase proliferation, migration, and tumorigenesis in breast cancer cells. This study aimed to identify the direct target genes of these four microRNAs in the human breast cancer cell lines MCF-7 and MDA-MB-231. Initially, bioinformatics tools were used to identify the target genes that have binding sites for miR-7, MiR-9, MiR-96, and miR-182 and are also associated with breast cancer. Subsequently, the findings of the bioinformatics analysis relating to the effects of these four microRNAs on the 3'-UTR activity of the potential target genes were confirmed using the dual luciferase assay in MCF-7 and MDA-MB-231 cells co-transfected with the vectors containing 3'-UTR segments of the target genes downstream of a luciferase coding gene and each of the microRNAs. Finally, the effects of microRNAs on the endogenous expression of potential target genes were assessed by the overexpression of each of the four microRNAs in MCF-7 and MDA-MB-231 cells. Respectively, three, three, three, and seven genes were found to have binding sites for miR-7, miR-9, miR-96, and miR-182 and were associated with breast cancer. The results of empirical studies including dual luciferase assays and real-time PCR confirmed that miR-7 regulates the expression of BRCA1 and LASP1; MiR-9 regulates the expression of AR; miR-96 regulates the expression of ABCA1; and miR-182 regulates the expression of NBN, TOX3, and LASP1. Taken together, our results suggest that the tumor suppressive effects of miR-7 may be mediated partly by regulating the expression of BRCA1 as a tumor suppressor gene in breast cancer. In addition, this microRNA and miR-182 may have effects on the nodal-positivity and tumor size of breast carcinoma through the regulation of LASP1. The tumor suppressive functions of miR-9 may be mediated partly by suppressing the expression of AR-an oncogene in breast cancer. Moreover, miR-96 may play an oncogenic role in breast cancer by suppressing the apoptosis through the regulation of ABCA1.

Tamoxifen in the Mouse Brain: Implications for Fate-Mapping Studies Using the Tamoxifen-Inducible Cre-loxP System

The tamoxifen-inducible Cre-loxP system is widely used to overcome gene targeting pre-adult lethality, to modify a specific cell population at desired time-points, and to visualize and trace cells in fate-mapping studies. In this study we focused on tamoxifen degradation kinetics, because for all genetic fate-mapping studies, the period during which tamoxifen or its metabolites remain active in the CNS, is essential. Additionally, we aimed to define the tamoxifen administration scheme, enabling the maximal recombination rate together with minimal animal mortality. The time window between tamoxifen injection and the beginning of experiments should be large enough to allow complete degradation of tamoxifen and its metabolites. Otherwise, these substances could promote an undesired recombination, leading to data misinterpretation. We defined the optimal time window, allowing the complete degradation of tamoxifen and its metabolites, such as 4-hydroxytamoxifen, N-desmethyltamoxifen, endoxifen and norendoxifen, in the mouse brain after intraperitoneal tamoxifen injection. We determined the biological activity of these substances in vitro, as well as a minimal effective concentration of the most potent metabolite 4-hydroxytamoxifen causing recombination in vivo. For this purpose, we analyzed the recombination rate in double transgenic Cspg4-cre/Esr1/ROSA26Sortm14(CAG-tdTomato) mice, in which tamoxifen administration triggers the expression of red fluorescent protein in NG2-expressing cells, and employed a liquid chromatography, coupled with mass spectrometry, to determine the concentration of studied substances in the brain. We determined the degradation kinetics of these substances, and revealed that this process is influenced by mouse strains, age of animals, and dosage. Our results revealed that tamoxifen and its metabolites were completely degraded within 8 days in young adult C57BL/6J mice, while the age-matched FVB/NJ male mice displayed more effective degradation. Moreover, aged C57BL/6J mice were unable to metabolize all substances within 8 days. The lowering of initial tamoxifen dose leads to a significantly faster degradation of all studied substances. A disruption of the blood-brain barrier caused no concentration changes of any tamoxifen metabolites in the ipsilateral hemisphere. Taken together, we showed that tamoxifen metabolism in mouse brains is age-, strain- and dose-dependent, and these factors should be taken into account in the experimental design.

Conditional knockout mouse models of cancer

In 2007, three scientists, Drs. Mario R. Capecchi, Martin J. Evans, and Oliver Smithies, received the Nobel Prize in Physiology or Medicine for their contributions of introducing specific gene modifications into mice. This technology, commonly referred to as gene targeting or knockout, has proven to be a powerful means for precisely manipulating the mammalian genome and has generated great impacts on virtually all phases of mammalian biology and basic biomedical research. Of note, germline mutations of many genes, especially tumor suppressors, often result in lethality during embryonic development or at developmental stages before tumor formation. This obstacle has been effectively overcome by the use of conditional knockout technology in conjunction with Cre-LoxP- or Flp-Frt-mediated temporal and/or spatial systems to generate genetic switches for precise DNA recombination. Currently, numerous conditional knockout mouse models have been successfully generated and applied in studying tumor initiation, progression, and metastasis. This review summarizes some conditional mutant mouse models that are widely used in cancer research and our understanding of the possible mechanisms underlying tumorigenesis.

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.

A critical role of CD29 and CD49f in mediating metastasis for cancer-initiating cells isolated from a Brca1-associated mouse model of breast cancer

Cancer metastasis is a lethal problem that claims the lives of over 90% of cancer patients. In this study, we have investigated metastatic potential of cancer stem cells (CSCs) isolated from mammary tumors of a Brca1-mutant mouse model. Our data indicated that CSCs, which are enriched in CD24(+)CD29(+)/CD49f(+) cell population, displayed much higher migration ability than CD24(-)CD29(-)/CD49f(-) cells in tissue culture and enhanced metastatic potential in allograft-nude mice. CD24(+)CD29(+) cells maintained the ability to differentiate and reconstitute heterogeneity in the metastatic tumors whereas CD24(-)CD29(-) cells could not. Corresponding to their enhanced metastatic ability, CD24(+)CD29(+) cells exhibited features of the epithelial to mesenchymal transition. Finally, using short hairpin RNA to knock down CD29 and/or CD49f in metastatic cancer cells, we demonstrated that while acute knockdown of CD29 or CD49f alone slightly decreased cell migration ability, knockdown of both genes generated a profound effect to block their migration, revealing an overlapping, yet critical function of both genes in the migration of CSCs. Our findings indicate that in addition to serving as markers of CSCs, CD29 and CD49f may also serve as potential therapeutic targets for cancer metastasis.

Increased incidence of brain metastases in BRCA1-related ovarian cancers

AIM

Brain metastasis from ovarian cancer is a very rare phenomenon. BRCA1-related ovarian cancers show specific pathobiological profiles, advanced stage, and sensitivity to chemotherapeutic agents. However, no clear relationship to any known metastatic behavior has yet been found, so we examined the BRCA1 mutation and expression profiles in ovarian cancer cases with brain metastases.

MATERIAL AND METHODS

We examined our clinical records of 340 ovarian cancer cases from 1983 to 2007 to ascertain cases with brain metastases. In the molecular genetic analyses, we performed loss of heterozygosity (LOH), direct sequence and immunohistochemical staining analysis of BRCA1.

RESULTS

We ascertained seven cases with brain metastases in 340 ovarian cancer cases (7/340=2.1%). Among the seven cases, three cases had ovarian and/or breast cancer patients in third-degree relatives. We detected four LOH-positive cases and a germline mutation of BRCA1 in two of the four cases. Furthermore, the remaining two cases showed absent staining of the BRCA1 protein. Therefore, four of seven cases with brain metastases were considered BRCA1-related ovarian cancers (4/7=57.1%). All four of the cases were serous adenocarcinoma.

CONCLUSION

Our results suggest that the loss of BRCA1 function may be involved in the phenomenon of brain metastasis from ovarian cancer. Further molecular biologic analyses will be required for a better understanding of this rare phenomenon.

Akt1 inhibits homologous recombination in Brca1-deficient cells by blocking the Chk1-Rad51 pathway

Brca1 deficiency leads to the development of breast cancer. We previously found that Brca1 deficiency activates the Akt oncogenic pathway. Reduced expression of Brca1 was highly correlated with increased activated Akt in human breast cancer samples. Furthermore, activation of Akt1 was involved in Brca1-deficiency-mediated tumorigenesis in mice. Defective homologous recombination (HR) is thought to be a major contributor to tumorigenesis in Brca1 deficiency. Here, we show that Akt1 promotes chromosome instability in Brca1-deficent cells. DNA breaks in Brca1-deficent cells are aberrantly joined into complex chromosome rearrangements by a process dependent on Akt1. Depletion of Akt1 increases HR in Brca1-mutant cells, which is rescued by expression of wild-type, but not mutant Akt1 with deletion of Brca1-binding domain. Mechanistically, activated Akt1 in Brca1-deficient cells impairs Chk1 nuclear localization and subsequently disrupts interaction of Chk1 and Rad51 leading to HR defects. Our results indicate that Brca1 deficiency might activate Akt1 contributing to tumorigenesis through regulation of the Chk1-Rad51 signaling.

The unliganded glucocorticoid receptor positively regulates the tumor suppressor gene BRCA1 through GABP beta

Loss of BRCA1 tumor suppressor function is a critical event in breast tumorigenesis. We have previously identified the stress hormone hydrocortisone as a negative regulator of BRCA1 expression in nonmalignant mammary cells. Here, we have identified a direct role for the unliganded glucocorticoid receptor (GR) in BRCA1 upregulation in the absence of hydrocortisone. The positive regulatory effect of GR is lost upon the addition of hydrocortisone. We have shown that GR interacts with the BRCA1 promoter only in the absence of hydrocortisone, and that this interaction is mediated through the β-subunit of the ets transcription factor GA-binding protein (GABP) at the RIBS promoter element. GR and GABPβ interact in both coimmunoprecipitation and mammalian two-hybrid assays, and this interaction involves the N-terminal to central regions of both proteins. This work presents the first evidence of a ligand-independent role for GR as a positive regulator of gene expression, and loss of GR from the BRCA1 promoter in response to stress hormones leads to decreased BRCA1 expression. Because low levels of BRCA1 have been implicated in the development of sporadic breast cancer, this may represent a novel mechanism through which prolonged stress signaling increases breast cancer risk.

The role of BRCA1 in DNA damage response

BRCA1 is a well-established tumor suppressor gene, which is frequently mutated in familial breast and ovarian cancers. The gene product of BRCA1 functions in a number of cellular pathways that maintain genomic stability, including DNA damage-induced cell cycle checkpoint activation, DNA damage repair, protein ubiquitination, chromatin remodeling, as well as transcriptional regulation and apoptosis. In this review, we discuss recent advances regarding our understanding of the role of BRCA1 in tumor suppression and DNA damage response, including DNA damage-induced cell cycle checkpoint activation and DNA damage repair.

BRCA1 affects global DNA methylation through regulation of DNMT1

Global DNA hypomethylation at CpG islands coupled with local hypermethylation is a hallmark for breast cancer, yet the mechanism underlying this change remains elusive. In this study, we showed that DNMT1, which encodes a methylation maintenance enzyme, is a transcriptional target of BRCA1. BRCA1 binds to the promoter of the DNMT1 gene through a potential OCT1 site and the binding is required for maintaining a transcriptional active configuration of the promoter in both mouse and human cells. We further demonstrated that impaired function of BRCA1 leads to global DNA hypomethylation, loss of genomic imprinting, and an open chromatin configuration in several types of tissues examined in a BRCA1 mutant mouse model at premaligant stages. BRCA1 deficiency is also associated with significantly increased expression levels of several protooncogenes, including c-Fos, Ha-Ras, and c-Myc, with a higher expression in tumors, while premalignant mammary epithelial cells displayed an intermediate state between tumors and controls. In human clinical samples, reduced expression of BRCA1 correlates with decreased levels of DNMT1, and reduced methylation of CpG islands. Thus, BRCA1 prevents global DNA hypomethylation through positively regulating DNMT1 expression, and this provides one of mechanisms for BRCA1-associated breast cancer formation.

Hereditary breast and ovarian cancer due to mutations in BRCA1 and BRCA2

Hereditary breast and ovarian cancer due to mutations in the BRCA1 and BRCA2 genes is the most common cause of hereditary forms of both breast and ovarian cancer. The overall prevalence of BRCA1/2 mutations is estimated to be from 1 in 400 to 1 in 800 with a higher prevalence in the Ashkenazi Jewish population (1 in 40). Estimates of penetrance (cancer risk) vary considerably depending on the context in which they were derived and have been shown to vary within families with the same BRCA1/2 mutation. This suggests there is no exact risk estimate that can be applied to all individuals with a BRCA1/2 mutation. The likelihood of harboring a BRCA1 or BRCA2 mutation is dependent on one's personal and/or family history of cancer and can be estimated using various mutation probability models. For those individuals who have a BRCA1 or BRCA2 mutation, several screening and primary prevention options have been suggested, including prophylactic surgery and chemoprevention. Once a BRCA1 or BRCA2 mutation has been identified in a family, testing of at-risk relatives can identify those family members who also have the familial mutation and thus need increased surveillance and early intervention when a cancer is diagnosed.

Crosstalk between the DNA damage response, histone modifications and neovascularisation

Neovascularisation is critical in several malignant and inflammatory conditions, as well as in the course of eye disorders. During new vessel formation, endothelial cell functions, such as proliferation and sprouting are very important and are regulated by a variety of growth factors. The DNA damage response machinery as well as factors regulating histone modifications, such as histone deacetylases, regulate cell fate as well as gene expression. Recent evidence has pointed to potential interactions among BRCA1, H2AX and SIRT1 in these intracellular pathways and neovascularisation, which will be reviewed here.

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

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

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

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

RNAi-based conditional gene knockdown in mice using a U6 promoter driven vector

RNA interference (RNAi) is a powerful tool widely used for studying gene function in a number of species. We have previously developed an approach that allows conditional expression of a polymerase III promoter based small hairpin RNA (shRNA) in mice using the Cre-LoxP system. This approach uses a U6 promoter, which is inactive due to the presence of a ploxPneo cassette in the promoter; this promoter can be activated after excision of the neo gene in transgenic mice that express a Cre recombinase transgene. As a proof of principle, we have previously knocked down over 95% of Fgfr2 transcripts in mouse germlines, leading to embryonic lethality, while restricting the knockdown to the progress zone of the limb results in live animals with malformation of digits of both the forelimbs and hindlimbs. We now provide a detailed protocol, including a simplified single-step cloning procedure for vector construction. This method provides a fast yet efficient way to decipher gene functions in vivo in a tissue specific manner.

A synthetic uracil derivative with antitumor activity through decreasing cyclin D1 and Cdk1, and increasing p21 and p27 in MCF-7 cells

The anticarcinogenic potential of (RS)-1-(2,3-dihydro-5H-1,4-benzodioxepin-3-yl)uracil (DBDU), with the naturally occurring pyrimidine base uracil, is reported against the MCF-7 cancer cell line. The arrest in the G0/G1 and G2/M cell cycle phases was accounted for by decrease in the expression of the cyclin D1 and Cdk1 proteins, and increase in p21 and p27 proteins. Using a reverse transcription-polymerase chain reaction-based assay at a dose of 5 muM of DBDU cyclin D1 mRNA was decreased, suggesting that DBDU exerts its regulatory action on cyclin D1 at the level of transcription. DNA fragmentation was performed and demonstrated that apoptosis occurred in the tumor cell line treated with DBDU. The G0/G1 arrest is an irreversible process and the cells undergo apoptosis in a p53-independent manner. DBDU administered intravenously twice a week (50 mg/kg dose each time) induced neither toxicity nor death in mice for 5 weeks.

Cellular functions of the BRCA tumour-suppressor proteins

Inherited germline mutations in either BRCA1 or BRCA2 confer a significant lifetime risk of developing breast or ovarian cancer. Defining how these two genes function at the cellular level is essential for understanding their role in tumour suppression. Although BRCA1 and BRCA2 were independently cloned over 10 years ago, it is only in the last few years that significant progress has been made towards understanding their function in cells. It is now widely accepted that both genes play critical roles in the maintenance of genome stability. Evidence implicates BRCA2 as an integral component of the homologous recombination machinery, whereas BRCA1 is an E3 ubiquitin ligase that has an impact on DNA repair, transcriptional regulation, cell-cycle progression and meiotic sex chromosome inactivation. In this article, I will review the most recent advances and provide a perspective of potential future directions in this field.

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

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

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.

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.

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

Germline mutations of the breast cancer associated gene 1 (BRCA1) predispose women to breast and ovarian cancers. BRCA1 is a large protein with multiple functional domains and interacts with numerous proteins that are involved in many important biological processes/pathways. Mounting evidence indicates that BRCA1 is involved in all phases of the cell cycle and regulates orderly events during cell cycle progression. BRCA1 deficiency, consequently causes abnormalities in the S-phase checkpoint, the G(2)/M checkpoint, the spindle checkpoint and centrosome duplication. The genetic instability caused by BRCA1 deficiency, however, also triggers cellular responses to DNA damage that blocks cell proliferation and induces apoptosis. Thus BRCA1 mutant cells cannot develop further into full-grown tumors unless this cellular defense is broken. Functional analysis of BRCA1 in cell cycle checkpoints, genome integrity, DNA damage response (DDR) and tumor evolution should benefit our understanding of the mechanisms underlying BRCA1 associated tumorigenesis, as well as the development of therapeutic approaches for this lethal disease.

Increased genomic instability is not a prerequisite for shortened lifespan in DNA repair deficient mice

Genetic defects in nucleotide excision repair (NER) are associated with premature aging, including cancer, in both humans and mice. To investigate the possible role of increased somatic mutation accumulation in the accelerated appearance of symptoms of aging as a consequence of NER deficiency, we crossed four different mouse mutants, Xpa-/-, Ercc6(Csb)-/-, Ercc2(Xpd)m/m and Ercc1-/m, with mice harboring lacZ-reporter genes to assess mutant frequencies and spectra in different organs during aging. The results indicate an accelerated accumulation of mutations in both liver and kidney of Xpa defective mice, which correlated with a trend towards a decreased lifespan. Until 52 weeks, Xpa deficiency resulted mainly in 1-bp deletions. At old age (104 weeks), the spectrum had undergone a shift, in both organs, to G:C-->T:A transversions, a signature mutation of oxidative DNA damage. Ercc1-/m mice, with their short lifespan of 6 months and severe symptoms of premature aging, especially in liver and kidney, displayed an even faster lacZ-mutant accumulation in liver. In this case, the excess mutations were mostly genome rearrangements. Csb-/- mice, with mild premature aging features and no reduction in lifespan, and Xpdm/m mice, exhibiting prominent premature aging features and about 20% reduction in lifespan, did not have elevated lacZ-mutant frequencies. It is concluded that while increased genomic instability could play a causal role in the mildly accelerated aging phenotype in the Xpa-null mice or in the severe progeroid symptoms of the Ercc1-mutant mice, shortened lifespan in mice with defects in transcription-related repair do not depend upon increased mutation accumulation.

The breast cancer susceptibility gene BRCA1 regulates progesterone receptor signaling in mammary epithelial cells

The progesterone receptor (PR) plays roles in normal mammary development and breast cancer formation, where it may exert both stimulatory and inhibitory actions. Previously, the breast cancer susceptibility gene product BRCA1 was found to interact with and inhibit the transcriptional activity of estrogen receptor-alpha. In this study, we found that exogenous wild-type BRCA1 inhibited the activity of the PR in transient transfection assays utilizing a mouse mammary tumor virus-Luc reporter. Wild-type BRCA1 inhibited the activity of endogenous PR in human breast cancer cells (T47D and MCF-7) and inhibited the activity of exogenous PR-A, PR-B, and [PR-A plus PR-B] isoforms. On the other hand, knockdown of endogenous BRCA1 using small interfering RNA enhanced the progesterone-stimulated activity of the PR by about 4-fold. We documented an in vivo association of the endogenous BRCA1 with PR isoforms A and B and a direct in vitro interaction between BRCA1 and PR, which was partially mapped. Whereas down-regulation of the coactivator p300 contributes to the BRCA1-mediated repression of estrogen receptor-alpha, this mechanism does not contribute to inhibition of PR activity, because exogenous p300 did not rescue the BRCA1 repression of PR activity. The BRCA1-PR interaction has functional consequences. Thus, we showed that BRCA1 inhibits the expression of various endogenous progesterone-responsive genes and inhibits progesterone-stimulated proliferation of T47D cells. Finally, exogenous progesterone caused an exaggerated proliferative response in the mammary glands of mice harboring a mammary-targeted conditional deletion of the full-length isoform of Brca1. These findings suggest that BRCA1 regulates the activity of progesterone, a major hormone of pregnancy that may also participate in mammary carcinogenesis.

Conditional knockdown of Fgfr2 in mice using Cre-LoxP induced RNA interference

RNA interference (RNAi)-mediated gene knockdown is a potent approach for studying gene function. We have previously reported a plasmid-based, tamoxifen-inducible gene knockdown system in cultured cells using a combined RNAi and Cre-LoxP system. Here, we validate this system in mouse and show that it can be used to suppress the expression of an endogenous gene (Fgfr2) with high efficiency. We show that transgenic mice carrying the U6-ploxPneo-Fgfr2 RNAi construct are normal, displaying Fgfr2 transcripts equivalent to those of wild-type controls, indicating that the U6 promoter is inactive in vivo due to the presence of the neo in the promoter. After excision of the neo by crossing with transgenic mice that express Cre in the mouse germline, the U6 promoter is activated, leading to over 95% reduction of Fgfr2 transcripts, and consequently, embryonic lethality. On the other hand, activation of the U6 promoter using transgenic mice that express Cre in the progress zone of the limb results in live mice with malformation of digits of both the forelimbs and hindlimbs. This method provides a fast, yet efficient way to decipher gene functions in vivo in a tissue-specific manner.

N-(4-Hydroxyphenyl)retinamide is more potent than other phenylretinamides in inhibiting the growth of BRCA1-mutated breast cancer cells

Women with germline mutations in the breast cancer susceptibility gene BRCA1 are at an increased risk of developing breast cancer. The synthetic retinoid N-(4-hydroxyphenyl)retinamide (4-HPR) has been shown to have a clinical chemopreventive activity in patients with premenopausal breast cancer. Since BRCA1 mutations are associated with an early-onset breast cancer, usually before menopause, we hypothesized that 4-HPR may be an effective chemopreventive agent against breast tumors exhibiting BRCA1 mutations. The objective of this study was to determine the effectiveness and mechanisms of action of 4-HPR and its phenylretinamide analogues in BRCA1-mutated breast cancer cells. At clinically relevant doses, 4-HPR induced apoptosis in human (HCC1937) and murine (W0069, W525) BRCA1-mutated breast cancer cells. Among the various phenylretinamides tested, N-(2-carboxyphenyl)retinamide (2-CPR) and 3-CPR significantly inhibited the growth of HCC1937 cells; however, they were not as potent as 4-HPR in this respect. We also determined the mechanisms by which 4-HPR induces apoptosis in BRCA1-mutated breast cancer cells. The extent to which 4-HPR induced apoptosis in BRCA1-mutated cells correlated with the increases in nitric oxide (NO) production and nitric oxide synthase (NOS) II and NOSIII expression. Use of a NOS inhibitor to block NO production suppressed the inhibitory effects of 4-HPR in all cell lines. These in vitro results suggest that 4-HPR may be an effective chemopreventive agent against breast tumors that exhibit BRCA1 mutations because of its ability to induce NO-mediated apoptosis in such tumors.

A requirement for breast-cancer-associated gene 1 (BRCA1) in the spindle checkpoint

BRCA1-associated breast cancer exhibits significantly higher levels of chromosomal abnormalities than sporadic breast cancers. However, the molecular mechanisms regarding the roles of BRCA1 in maintaining genome integrity remain elusive. By using a mouse model deficient for Brca1 full-length isoform (Brca1(Delta11/Delta11)), we found that Brca1(Delta11/Delta11) cells displayed decreased expression of a number of genes that are involved in the spindle checkpoint, including Mad2, which is a key component of spindle checkpoint that inhibits anaphase-promoting complex. We showed that Brca1(Delta11/Delta11) cells failed to arrest at metaphase in the presence of nocodazole and underwent apoptosis because of activation of p53. Consistently, reconstitution of Mad2 in Brca1(Delta11/Delta11) cells partially restored the spindle checkpoint and attenuated apoptosis. By using UBR60 cells, which carry tetracycline-regulated expression of BRCA1, we demonstrated that BRCA1 binds to transcription factor OCT-1 and up-regulates the transcription of MAD2. Furthermore, we showed that the induction of BRCA1 to endogenous MAD2 or transfected MAD2 luciferase reporter in UBR60 cells was completely inhibited by acute suppression of BRCA1 by RNA interference. These data reveal a role of BRCA1 in maintaining genome integrity by interplaying with p53 and genes that are involved in the spindle checkpoint and apoptosis.

Uterus hyperplasia and increased carcinogen-induced tumorigenesis in mice carrying a targeted mutation of the Chk2 phosphorylation site in Brca1

The tumor suppressor BRCA1 contains multiple functional domains that interact with many proteins. After DNA damage, BRCA1 is phosphorylated by CHK2 at serine 988, followed by a change in its intracellular location. To study the functions of CHK2-dependent phosphorylation of BRCA1, we generated a mouse model carrying the mutation S971A (S971 in mouse Brca1 corresponds to S988 in human BRCA1) by gene targeting. Brca1(S971A/S971A) mice were born at the expected ratio without a developmental defect, unlike previously reported Brca1 mutant mice. However, Brca1(S971A/S971A) mice suffered a moderately increased risk of spontaneous tumor formation, with a majority of females developing uterus hyperplasia and ovarian abnormalities by 2 years of age. After treatment with DNA-damaging agents, Brca1(S971A/S971A) mice exhibited several abnormalities, including increased body weight, abnormal hair growth pattern, lymphoma, mammary tumors, and endometrial tumors. In addition, the onset of tumor formation became accelerated, and 80% of the mutant mice had developed tumors by 1 year of age. We demonstrated that the Brca1(S971A/S971A) cells displayed reduced ability to activate the G(2)/M cell cycle checkpoint upon gamma-irradiation and to stabilize p53 following N-methyl-N'-nitro-N-nitrosoguanidine treatment. These observations suggest that Chk2 phosphorylation of S971 is involved in Brca1 function in modulating the DNA damage response and repressing tumor formation.

Molecular mechanisms underlying WOX1 activation during apoptotic and stress responses

Human WWOX gene encodes a putative tumor suppressor WW domain-containing oxidoreductase WOX1 (also known as WWOX or FOR). A high frequency of loss of heterozygosity (LOH) of this gene has been shown in prostate, lung, breast and other cancers. In addition, numerous aberrant WWOX mRNA transcripts have been found in cancer cells. WOX1 is a proapoptotic protein. In response to stress or apoptotic stimuli, WOX1 became phosphorylated at Tyr33, which enabled its complex formation with activated p53 and JNK1. The p53/WOX1 complex translocated to the mitochondria and further to the nuclei to mediate apoptosis. WOX1 mutants, which were inactivated for nuclear translocation or Tyr33 phosphorylation, failed to induce apoptosis, indicating that activation of WOX1 via Tyr33 phosphorylation, followed by nuclear translocation, is essential for inducing cell death. WOX1 induced apoptosis synergistically with p53. In contrast, transiently activated JNK1 induced anti-apoptotic response, and this protective activity inhibited WOX1-induced apoptosis. Taken together, WOX1 is involved in stress and apoptotic responses, and is likely to regulate the activation of both p53 and JNK1.

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.

Senescence, aging, and malignant transformation mediated by p53 in mice lacking the Brca1 full-length isoform

Senescence may function as a two-edged sword that brings unexpected consequences to organisms. Here we provide evidence to support this theory by showing that the absence of the Brca1 full-length isoform causes senescence in mutant embryos and cultured cells as well as aging and tumorigenesis in adult mice. Haploid loss of p53 overcame embryonic senescence but failed to prevent the adult mutant mice from prematurely aging, which included decreased life span, reduced body fat deposition, osteoporosis, skin atrophy, and decreased wound healing. We further demonstrate that mutant cells that escaped senescence had undergone clonal selection for faster proliferation and extensive genetic/molecular alterations, including overexpression of cyclin D1 and cyclin A and loss of p53. These observations provide the first in vivo evidence that links cell senescence to aging due to impaired function of Brca1 at the expense of tumorigenesis.

Early events in ovarian epithelial carcinogenesis: progress and problems in experimental approaches

The etiology and early events in the progression of epithelial ovarian carcinomas are among the least understood of all major human malignancies. There are no adequate means for early detection of these neoplasms and, as a result, they are usually diagnosed in late stages. The purpose of this review is to point out some of the peculiar problems and limitations that have hampered progress in ovarian carcinogenesis research and to summarize new approaches and recent advances in our understanding of this process. The review first presents an overview of the properties of the ovarian surface epithelium (OSE) which is thought to be the source of epithelial ovarian carcinomas, followed by a discussion of recent research based on human OSE. This includes sections on methodology for the attainment and study of OSE, investigations of OSE from women with predisposing mutations, and attempts to convert normal OSE to malignancy. This overview is followed by a discussion of the contributions, potential, and limitations of animal models. The knowledge gained by these approaches will likely lead to improvements in our ability to prevent, diagnose, and treat ovarian cancer.

Roles of BRCA1 in centrosome duplication

Centrosome duplication is under precise control and occurs only once in a normal mammalian cell cycle. Disruption of this process causes centrosome amplification, unequal segregation of chromosomes and, ultimately, tumorigenesis. Recent studies indicate that breast cancer suppressor gene 1 (BRCA1) plays an important role in regulating centrosome duplication. BRCA1 is located in the centrosome and binds to gamma-tubulin. It interacts with a variety of proteins that regulate centrosome duplication, including BRCA2, CDK2-Cyclin A, CDK2-Cyclin E, Gadd45, p21, p53 and Rb. Furthermore, targeted disruption of murine BRCA1 results in centrosome amplification, suggesting that BRCA1 serves as a negative regulator for centrosome duplication. This review will examine these data and discuss possible relationships between BRCA1 and its interacting proteins in centrosome duplication.

Genomic instability, centrosome amplification, cell cycle checkpoints and Gadd45a

Genomic instability has been a recognized feature of many human tumors for decades. Until recently, however, there was little insight into potential mechanisms for this phenomenon. Recent work has shown first, that increased centrosome numbers (also referred to as centrosome amplification) often accompany genomic instability and second, that when centrosome numbers are increased, cells become genetically unstable. Deletion of Gadd45a leads to centrosome amplification and consequent abnormal mitosis and aneuploidy. Gadd45a is known to be involved in a G2 checkpoint and may be involved in the normal progression from G2 to M and its coordination with S phase events. Whether these functions contribute to prevention of centrosome amplification is being investigated. However, potential mechanisms can be proposed based on known protein associations with Gadd45a, as well as proteins that regulate Gadd45a transcription and are also required for efficient coordination of centrosome duplication and DNA synthesis.