Gene expression profiling of BRCA1-mutant human breast cancer cells after exposure to tamoxifen plus estrogen

Abstract

9620 Background: Tamoxifen has been shown to reduce the risk of estrogen receptor (ER)-positive breast cancer in high-risk patients, including BRCA mutation carriers. However, as tumors in BRCA mutation carriers are largely ER-negative, tamoxifen efficacy in this high-risk population remains controversial. Toward that end, we hypothesized that acute tamoxifen exposure may induce similar changes in the gene expression profiles of ER-negative, BRCA1-mutant cells and ER-positive, BRCA1 wild-type cells, which may account for a chemoprotective effect of tamoxifen in both these populations. Methods: ER-positive (MCF7, T-47D, ZR-75–1), ER-negative (MDA-MB-436, SK-BR-3), and BRCA1-mutant, ER-negative (HCC1937) human breast cancer cells were cultured in steroid hormone-free media for 5 days followed by treatment with 10 nM 17-beta estradiol plus 100 nM 5-hydroxytamoxifen or vehicle alone for 8 hours. Total RNA from each cell line was isolated and subjected to gene expression profiling using the Affymetrix U133A Gene Chip oligonucleotide array. Results: The impact of tamoxifen plus estrogen exposure on gene expression was estimated, on a gene by gene basis, by taking the difference between the expression of treated cells and the average expression of the untreated cells. For each gene, confidence intervals (CI) were determined for the mean difference in expression between HCC1937 and ER-positive cells and HCC1937 and ER-negative cells. Genes were identified for which the CI included zero for ER-positive cells and for which the CIs did not overlap. 334 genes were identified which followed this pattern. Over-represented groups of genes included those involved in tumor suppression such as MAPK9 and FAF1 and cell cycle arrest genes including CDKN1B and TSG101. Conclusions: We have identified genes for which the influence of tamoxifen plus estrogen in BRCA1-mutant, ER-negative cells is more similar to its influence in wild-type BRCA1, ER-positive cells than wild-type BRCA1, ER-negative cells. Investigation of these genes may present insight into the molecular mechanism through which tamoxifen may prevent breast cancer in BRCA1 mutation carriers. Author Disclosure Employment or Leadership Consultant or Advisory Stock Ownership Honoraria Research Funding Expert Testimony Other Remuneration AstraZeneca