Abstract 896: Correlation between modulation of RNA expression and the preventive and therapeutic efficacy of Tamoxifen in an ER+ model of breast cancer

Abstract

Abstract The SERMs, primarily Tamoxifen, have been employed in both the therapy and prevention of ER+ breast cancer for many decades. In the present study we examined the effects of the SERM tamoxifen as a preventive and therapeutic agent in an ER+ Methylnitrosourea (MNU) -induced model of mammary cancer in rats. Fifty day old female Sprague Dawley rats were administered MNU i.v. and tamoxifen was administered continually in diet beginning either 5 days post MNU (prevention) or when an animal developed its first palpable tumor (150mm2) [therapy]. The approximate ED50 (dose required to reduce tumor multiplicity 50%) and ED90 in the preventive assays were 0.66 and 3.3 ppm (mg of tamoxifen per g of diet) for Tamoxifen. Interestingly the 3.3 ppm dose was ineffective as a therapy at these doses although it slowed growth of palpable lesions. Rather one had to administer a dose of roughly 100 ppm achieve a strong therapeutic response. We also examined the short term effects (5 days of treatment) of 3.3 and 100 ppm on gene expression in palpable lesions. We compared each treatment group with the control to find the up or down regulated genes. The number of differentially expressed (DE) genes selected at FDR = 0.2 increased from 584 to 2267 when the dose increased from 3.3ppm to 100ppm. The 3.3ppm treatment yielded fewer DE genes compared to the 100ppm treatment, but the majority (60%) of the genes identified in the 3.3 ppm group were identified in the high dose group. The higher dose profoundly decreased a variety of specific genes in the proliferative pathways, particularly in APC (Anaphase Prometaphase Condensation Complex), which includes Ccnb1 and Top2A. In contrast, the low dose failed to significantly change expression of these specific genes. The effects of the higher dose of tamoxifen were confirmed by the use of RT-PCR. Using the human Tamoxifen treatment dataset GSE6532 in GEO, we found 633 DE genes by comparing the treated (n = 277) and untreated (n = 237) patients. We found 68 genes shared between human and rat derived from 100ppm treatment and 11 genes from 3.3ppm treatment. 9 of the 11 genes regulated by the low dose were also regulated by the high dose. These genes included RAB5B, a member of the RAS oncogene family, and GDF9, the growth differentiation factor 9. By gene enrichment analysis (GSEA), we found several pathways, including RAS, TGF beta signaling, DNA damage signaling, and cell cycle pathways, which were affected by the tamoxifen treatment. Our study suggests that the rat gene expression was regulated by tamoxifen even at low dose, and the DE genes at the low dose are a subset of these DE genes altered at the high dose. These data provide a rationale for testing low dose tamoxifen as a chemo preventive agent for women at high risk of developing breast cancer and offer potential genomic biomarkers associated with efficacy at these lower doses. Citation Format: Howard H. Yang, Huaitian Liu, Ronald Lubet, Clinton J. Grubbs, Holly Nicastro, Maxwell P. Lee. Correlation between modulation of RNA expression and the preventive and therapeutic efficacy of Tamoxifen in an ER+ model of breast cancer. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 896. doi:10.1158/1538-7445.AM2015-896