BRCA1 transcriptionally regulates genes associated with the basal-like phenotype in breast cancer.

Expression profiling of BRCA1-deficient tumours has identified a pattern of gene expression similar to basal-like breast tumours. In this study, we examine whether a BRCA1-dependent transcriptional mechanism may underpin the link between BRCA1 and basal-like phenotype. In methods section, the mRNA and protein were harvested from a number of BRCA1 mutant and wild-type breast cancer cell lines and from matched isogenic controls. Microarray-based expression profiling was used to identify potential BRCA1-regulated transcripts. These gene targets were then validated (by in silico analysis of tumour samples) by real-time PCR and Western blot analysis. Chromatin immunoprecipitation (ChIP) assays were used to confirm recruitment of BRCA1 to specific promoters. In results, we demonstrate that functional BRCA1 represses the expression of cytokeratins 5(KRT5) and 17(KRT17) and p-Cadherin (CDH3) in HCC1937 and T47D breast cancer cell lines at both mRNA and protein level. ChIP assays demonstrate that BRCA1 is recruited to the promoters of KRT5, KRT17 and CDH3, and re-ChIP assays confirm that BRCA1 is recruited independently to form c-Myc and Sp1 complexes on the CDH3 promoter. We show that siRNA-mediated inhibition of endogenous c-Myc (and not Sp1) results in a marked increase in CDH3 expression analogous to that observed following the inhibition of endogenous BRCA1. The data provided suggest a model whereby BRCA1 and c-Myc form a repressor complex on the promoters of specific basal genes and represent a potential mechanism to explain the observed overexpression of key basal markers in BRCA1-deficient tumours.

BRCA1 mRNA expression levels predict for overall survival in ovarian cancer after chemotherapy.

PURPOSE: We investigated whether BRCA1 mRNA expression levels may represent a biomarker of survival in sporadic epithelial ovarian cancer following chemotherapy treatment. EXPERIMENTAL DESIGN: The effect of loss of BRCA1 expression on chemotherapy response in ovarian cancer was measured in vitro using dose inhibition assays and Annexin V flow cytometry. Univariate and multivariate analyses were done to evaluate the relationship between BRCA1 mRNA expression levels and survival after chemotherapy treatment in 70 fresh frozen ovarian tumors. RESULTS: We show that inhibition of endogenous BRCA1 expression in ovarian cancer cell lines results in increased sensitivity to platinum therapy and decreased sensitivity to antimicrotubule agents. In addition, we show that patients with low/intermediate levels of BRCA1 mRNA have a significantly improved overall survival following treatment with platinum-based chemotherapy in comparison with patients with high levels of BRCA1 mRNA (57.2 versus 18.2 months; P = 0.0017; hazard ratio, 2.9). Furthermore, overall median survival for higher-BRCA1-expressing patients was found to increase following taxane-containing chemotherapy (23.0 versus 18.2 months; P = 0.12; hazard ratio, 0.53). CONCLUSIONS: We provide evidence to support a role for BRCA1 mRNA expression as a predictive marker of survival in sporadic epithelial ovarian cancer.

BRCA1 and c-Myc associate to transcriptionally repress psoriasin, a DNA damage-inducible gene.

Evidence is accumulating to suggest that some of the diverse functions associated with BRCA1 may relate to its ability to transcriptionally regulate key downstream target genes. Here, we identify S100A7 (psoriasin), S100A8, and S100A9, members of the S100A family of calcium-binding proteins, as novel BRCA1-repressed targets. We show that functional BRCA1 is required for repression of these family members and that a BRCA1 disease-associated mutation abrogates BRCA1-mediated repression of psoriasin. Furthermore, we show that BRCA1 and c-Myc form a complex on the psoriasin promoter and that BRCA1-mediated repression of psoriasin is dependent on functional c-Myc. Finally, we show that psoriasin expression is induced by the topoisomerase IIalpha poison, etoposide, in the absence of functional BRCA1 and increased psoriasin expression enhances cellular sensitivity to this chemotherapeutic agent. Therefore, we identified a novel transcriptional mechanism that is likely to contribute to BRCA1-mediated resistance to etoposide.

Molecular basis for estrogen receptor alpha deficiency in BRCA1-linked breast cancer.

BACKGROUND: BRCA1-mutant breast tumors are typically estrogen receptor alpha (ER alpha) negative, whereas most sporadic tumors express wild-type BRCA1 and are ER alpha positive. We examined a possible mechanism for the observed ER alpha-negative phenotype of BRCA1-mutant tumors. METHODS: We used a breast cancer disease-specific microarray to identify transcripts that were differentially expressed between paraffin-embedded samples of 17 BRCA1-mutant and 14 sporadic breast tumors. We measured the mRNA levels of estrogen receptor 1 (ESR1) (the gene encoding ER alpha), which was differentially expressed in the tumor samples, by quantitative polymerase chain reaction. Regulation of ESR1 mRNA and ER alpha protein expression was assessed in human breast cancer HCC1937 cells that were stably reconstituted with wild-type BRCA1 expression construct and in human breast cancer T47D and MCF-7 cells transiently transfected with BRCA1-specific short-interfering RNA (siRNA). Chromatin immunoprecipitation assays were performed to determine if BRCA1 binds the ESR1 promoter and to identify other interacting proteins. Sensitivity to the antiestrogen drug fulvestrant was examined in T47D and MCF-7 cells transfected with BRCA1-specific siRNA. All statistical tests were two-sided. RESULTS: Mean ESR1 gene expression was 5.4-fold lower in BRCA1-mutant tumors than in sporadic tumors (95% confidence interval [CI] = 2.6-fold to 40.1-fold, P = .0019). The transcription factor Oct-1 recruited BRCA1 to the ESR1 promoter, and both BRCA1 and Oct-1 were required for ER alpha expression. BRCA1-depleted breast cancer cells expressing exogenous ER alpha were more sensitive to fulvestrant than BRCA1-depleted cells transfected with empty vector (T47D cells, the mean concentration of fulvestrant that inhibited the growth of 40% of the cells [IC40] for empty vector versus ER alpha: >10(-5) versus 8.0 x 10(-9) M [95% CI = 3.1 x 10(-10) to 3.2 x 10(-6) M]; MCF-7 cells, mean IC40 for empty vector versus ER alpha: >10(-5) versus 4.9 x 10(-8) M [95% CI = 2.0 x 10(-9) to 3.9 x 10(-6) M]). CONCLUSIONS: BRCA1 alters the response of breast cancer cells to antiestrogen therapy by directly modulating ER alpha expression.