Tumor Suppressor Gene BRCA1 Research Articles

Arrest of the cell cycle by the tumour-suppressor BRCA1 requires the CDK-inhibitor p21WAF1/CiP1.

Much of the predisposition to hereditary breast and ovarian cancer has been attributed to inherited defects in the BRCA1 tumour-suppressor gene. The nuclear protein BRCA1 has the properties of a transcription factor, and can interact with the recombination and repair protein RAD51. Young women with germline alterations in BRCA1 develop breast cancer at rates 100-fold higher than the general population, and BRCA1-null mice die before day 8 of development. However, the mechanisms of BRCA1-mediated growth regulation and tumour suppression remain unknown. Here we show that BRCA1 transactivates expression of the cyclin-dependent kinase inhibitor p21WAF1/CIP1 in a p53-independent manner, and that BRCA1 inhibits cell-cycle progression into the S-phase following its transfection into human cancer cells. BRCA1 does not inhibit S-phase progression in p21-/- cells, unlike p21+/+ cells, and tumour-associated, transactivation-deficient mutants of BRCA1 are defective in both transactivation of p21 and cell-cycle inhibition. These data suggest that one mechanism by which BRCA1 contributes to cell-cycle arrest and growth suppression is through the induction of p21.

Mutational Analysis of the BRCA1 Tumor Suppressor Gene in Endometrial Carcinoma

Objective.To investigate whether alteration of BRCA1 tumor suppressor gene occurs in sporadic endometrial carcinomas.Methods.Genomic DNAs were prepared from 33 consecutively collected endometrial carcinoma tissues for BRCA1 mutational analysis. To screen for BRCA1 mutation, polymerase chain reaction (PCR) amplification and single strand conformation polymorphism (SSCP) technique were used with 41 overlapping PCR primer pairs for the 23 coding exons of BRCA1. Tumors that demonstrated SSCP variants were further subjected to direct DNA sequencing in the appropriate exons to identify the DNA alteration.Results.In addition to detecting a previously described polymorphism in exon 11, single strand conformation polymorphism analysis of the 33 endometrial cancers identified 3 tumors with mobility shifts. Two tumors shifted in exon 3 and showed the same pattern of band shift. The other tumor shifted in exon 9. DNA sequencing revealed sequence alterations in the 3 tumors; all appeared heterozygous. In the 2 tumors shifted in exon 3, the sequence alteration caused no amino acid change and was consistent with an infrequent silent polymorphism. In the third tumor, a missense alteration at codon 191 was detected and was recognized as germline in origin.Conclusions.Because a normal allele of BRCA1 was retained in the tumor where a germline missense alteration was detected, the heterozygous DNA alteration should not be cancer predisposing in terms of the two-hit model for inactivation of the tumor suppressor gene. We conclude that mutation of BRCA1 may not be involved in the development of sporadic endometrial cancer.

BRCA1 tumor suppressor gene product shares immunoreactive epitopes with a protein present in seminal plasma

Objectives: To develop immunofluorometric procedures for measuring BRCA1 protein in various biological fluids and tissue extracts. Design and Methods: Five commercially available monoclonal and polyclonal antibodies against BRCA1 were evaluated for developing competitive and non-competitive immunofluorometric procedures for BRCA1. Biotinylated and nonbiotinylated peptides were used to assess the specificity of the antibodies for blocking experiments and for the competitive immunoassay. Extensive studies to exclude cross-reactivity and non-specific effects in the non-competitive immunoassay were undertaken. Seminal plasmas as well as breast tumor extracts, amniotic fluids and cerebrospinal fluids were analyzed. Results: We designed novel methods for measuring BRCA1 immunoreactivity. One configuration based on the “sandwich-type” immunoassay principle was used for further studies. We discovered that seminal plasma contains an immunoreactive protein which appears to possess the D-20 (aminoterminal) and C-20 (carboxyterminal) epitopes of BRCA1. Molecular weight identification using gel filtration chromatography has shown that the immunoreactive species has a molecular weight between 660 and 160 KDa. Conclusions: We identified for the first time a protein in seminal plasma that shares immunoreactive epitopes with the BRCA1 tumor suppressor protein. We are currently purifying this protein in order to examine if it is homologous or identical to BRCA1.

Ethnic differences in cancer incidence: a marker for inherited susceptibility?

Cancer incidence varies markedly by ethnicity and geographic location. Ethnic variation in cancer occurrence has traditionally been ascribed to differences in social, cultural, economic, and physical environments. However, this interpretation of the epidemiologic evidence may need to be revised as a result of new biological evidence and theories of carcinogenesis. Carcinogenesis is now recognized to be a multistep process during which mutations or heritable changes in expression occur in genes involved in cellular growth control and genome stability. Inherited cancer susceptibility may be a stronger determinant of ethnic differences in cancer incidence than is currently appreciated. To examine the potential role of inherited susceptibility, the theoretical contribution of inherited susceptibility to ethnic differences in rates in considered using a simple probability model. Germline mutations in tumor suppressor genes BRCA1 and p53 are used to illustrate the magnitude of the ethnic differences for breast cancer that might arise from differences in inherited susceptibility. Our simple model suggests that ethnic differences in cancer occurrence can result from differences in genetic susceptibility. However, the magnitude of ethnic relative risk is likely to more strongly reflect differences in the distribution of susceptibility genotypes between groups than the magnitude of the disease risk associated with the genotypes. For many scenarios, the ethnic relative risk arising from differences in susceptibility may be bounded by the ratio of the proportion of susceptible individuals in each group.

Brca2 is required for embryonic cellular proliferation in the mouse.

Mutations of the tumor suppressor gene BRCA2 are associated with predisposition to breast and other cancers. Homozygous mutant mice in which exons 10 and 11 of the Brca2 gene were deleted by gene targeting (Brca2(10-11)) die before day 9.5 of embryogenesis. Mutant phenotypes range from severely developmentally retarded embryos that do not gastrulate to embryos with reduced size that make mesoderm and survive until 8.5 days of development. Although apoptosis is normal, cellular proliferation is impaired in Brca2(10-11) mutants, both in vivo and in vitro. In addition, the expression of the cyclin-dependent kinase inhibitor p21 is increased. Thus, Brca2(10-11) mutants are similar in phenotype to Brca1(5-6) mutants but less severely affected. Expression of either of these two genes was unaffected in mutant embryos of the other. This study shows that Brca2, like Brca1, is required for cellular proliferation during embryogenesis. The similarity in phenotype between Brca1 and Brca2 mutants suggests that these genes may have cooperative roles or convergent functions during embryogenesis.

Hereditary ovarian cancer

Ovarian cancer has been described in association with three autosomal dominant syndromes: familial site-specific ovarian cancer, familial breast and ovarian cancer, and the hereditary nonpolyposis colon cancer syndrome. It appears that most breast-ovarian and site-specific ovarian cancer families are explained by mutations in the BRCA1 tumor suppressor gene. Other genes associated with inherited susceptibility to ovarian cancer include BRCA2, p53, and the DNA mismatch repair genes.

Direct detection of mutations in the breast and ovarian cancer susceptibility gene BRCA1 by PCR-mediated site-directed mutagenesis

The tumor suppressor genes BRCA1 and BRCA2, which confer increased susceptibility to breast and (or) ovarian cancer, were recently identified. Mutation analysis of BRCA1 has demonstrated significant allelic heterogeneity; however, some distinct mutations have been detected in unrelated individuals. The most notable is the 185delAG mutation, which occurs at an estimated frequency of approximately 1% in individuals of Ashkenazi Jewish descent [1]. Although consensus has not been reached regarding clinical testing for mutations in BRCA1, a tiered strategy may be appropriate, in which direct testing for the more common mutations is one component. Specific alleles can be detected by using PCR-mediated site-directed mutagenesis (PSM), which alters the PCR products derived from either the wild-type or mutant allele to create or destroy a restriction endonuclease recognition site. Recognition sites are introduced by a base substitution in one of the primers. The alleles are then resolved by electrophoresis of the digested PCR products. We have applied this technique to the detection of four BRCA1 mutations: 185delAG, 5382insC, E1250X, and R1443X. Another mutation, 1294de140, can be resolved from the wild-type allele by high-resolution gel electrophoresis alone. The PSM technique is sensitive, does not require radioactivity, and is specific for individual mutations.

Mutation analysis of the BRCA1 gene in 23 families with cases of cancer of the breast, ovary, and multiple other sites.

Germline mutations in the BRCA1 tumour suppressor gene on chromosome 17q21 are responsible for approximately half of the cases of hereditary breast cancer, including the majority of familial breast/ovarian cancers. To increase our knowledge of the spectrum of BRCA1 mutations, we have extended our analysis to include patients with varied family histories of cancer of the breast, ovary, and at multiple other sites. We have analysed 23 unrelated familial cases using direct sequencing or a combination of dideoxy fingerprinting and sequencing procedures. Twenty one of these families contained three or more cases of breast or ovarian cancer and two families had one case of breast cancer diagnosed before the age of 40 and one case of ovarian cancer. The common frameshift mutation 5382insC was detected in two patients, and the 185delAG mutation was found in a family of Ashkenazi Jewish descent. The novel frameshift mutation 3450del4 (CAAG) was detected in a patient who developed breast cancer at the age of 28 and ovarian cancer at the age of 34. Three other women in this family were diagnosed with breast cancer at the ages of 26, 29, and 40. The novel framshift mutation 2953del3+C was found in a French Canadian woman who had developed two primary cancers of the breast at the age of 37 and 38 and renal cancer at the age of 38.

The Tumor Suppressor Gene Brca1 Is Required for Embryonic Cellular Proliferation in the Mouse

Mutations of the BRCA1 gene in humans are associated with predisposition to breast and ovarian cancers. We show here that Brca1 +/− mice are normal and fertile and lack tumors by age eleven months. Homozygous Brca1 5-6 mutant mice die before day 7.5 of embryogenesis. Mutant embryos are poorly developed, with no evidence of mesoderm formation. The extraembryonic region is abnormal, but aggregation with wild-type tetraploid embryos does not rescue the lethality. In vivo, mutant embryos do not exhibit increased apoptosis but show reduced cell proliferation accompanied by decreased expression of cyclin E and mdm-2, a regulator of p53 activity. The expression of cyclin-dependent kinase inhibitor p21 is dramatically increased in the mutant embryos. Buttressing these in vivo observations is the fact that mutant blastocyst growth is grossly impaired in vitro. Thus, the death of Brca1 5-6 mutant embryos prior to gastrulation may be due to a failure of the proliferative burst required for the development of the different germ layers.

Von Hippel-Lindau syndrome: hereditary cancer arising from inherited mutations of the VHL tumor suppressor gene.

Von Hippel-Lindau syndrome (VHL) is an autosomal-dominant, multiple-tumor syndrome in which affected individuals are predisposed, over the course of their lifetime, to the formation of tumors or cysts in specific target organs such as the kidney, central nervous system (CNS), retina, adrenal gland, and pancreas. VHL is the most common syndrome of hereditary kidney cancer [1,2]. Metastatic clear cell kidney cancer and the neurologic sequelae of tumor growth in the CNS are the most common causes of morbidity and mortality of VHL patients. The VHL disease gene functions as a tumor suppressor gene (see below) [3] and places the VHL syndrome into a select group with diseases such as Li-Fraumeni syndrome, retinoblastomatosis, and familial breast-ovarian cancer, which are caused by mutations of the tumor suppressor genes p53, Rb, and BRCA1, respectively. An advance in the understanding of VHL and of sporadic clear cell kidney cancer occurred in 1993 when the gene responsible for the VHL syndrome was cloned [4].

The molecular basis of ovarian cancer

Molecular genetic studies of ovarian cancer have been limited by the inaccessible location of the ovary, the advanced stage of tumors available for analysis, and the lack of a well-defined precursor lesion. However, genetic alterations important in ovarian tumorigenesis have been identified recently. Molecular genetic evaluation of ovarian cancer primarily has utilized mutation analysis, immunohistochemical techniques, and loss of heterozygosity (LOH) studies. Overexpression of the HER-2/neu oncogene is present in approximately one third of ovarian cancers and is associated with poor prognosis. Mutations of the K-ras oncogene have been identified in a similar proportion of mucinous ovarian tumors, including borderline tumors. The study authors as well as others have frequently detected LOH on chromosome 17, including the p53 and BRCA1 loci, and at 17p3.3 and 1717q22-23. Genetic linkage analysis indicates that the majority of inherited ovarian cancers are caused by mutations in the BRCA1 gene. Mutations in mismatch repair genes have been identified in ovarian cancers that occur as part of the hereditary nonpolyposis colon cancer syndrome. Sporadic ovarian tumors are the end result of a complex pathway involving multiple oncogenes and tumor suppressor genes, including HER-2/neu, K-ras, p53, BRCA1, and additional tumor suppressor genes on chromosome 17. The majority of inherited ovarian cancers are due to mutations in the BRCA1 gene, which appears to be a tumor suppressor gene. It is hoped that an increased understanding of the molecular basis of ovarian cancer will lead to advances in prevention, diagnosis, and treatment.

The Medical Injunction of Stimulants

Germline mutations in the BRCA1 tumour suppressor gene on chromosome 17q21 are responsible for approximately half of the cases of hereditary breast cancer, including the majority of familial breast/ovarian cancers. To increase our knowledge of the spectrum of BRCA1 mutations, we have extended our analysis to include patients with varied family histories of cancer of the breast, ovary, and at multiple other sites. We have analysed 23 unrelated familial cases using direct sequencing or a combination of dideoxy fingerprinting and sequencing procedures. Twenty one of these families contained three or more cases of breast or ovarian cancer and two families had one case of breast cancer diagnosed before the age of 40 and one case of ovarian cancer. The common frameshift mutation 5382insC was detected in two patients, and the 185delAG mutation was found in a family of Ashkenazi Jewish descent. The novel frameshift mutation 3450del4 (CAAG) was detected in a patient who developed breast cancer at the age of 28 and ovarian cancer at the age of 34. Three other women in this family were diagnosed with breast cancer at the ages of 26, 29, and 40. The novel framshift mutation 2953del3+C was found in a French Canadian woman who had developed two primary cancers of the breast at the age of 37 and 38 and renal cancer at the age of 38.