On the role of DEAR1 as a novel ubiquitin ligase for ER alpha and predictor of tamoxifen response in ER positive breast cancer

Estrogen receptor positive (ER+) breast cancer accounts for over 70% of breast cancers. Targeted adjuvant therapies aimed at estrogen receptor have changed the natural history of the disease. However, over time many tumors develop resistance mechanisms which result in overall poor outcome. Thus, biomarkers that could predict response to antiestrogens are critically needed and would have major clinical impact. Thus, it is imperative that additional novel mechanisms be discovered not only to understand the basic underlying biology of ER alpha (ERα) signaling in order to identify genetic alterations that deregulate ERα and its role in breast cancer. DEAR1 is a tumor suppressor gene which is mutated, undergoes loss of heterozygosity in breast cancer. Previously we observed a subset of DEAR1 mutations in early onset breast cancers from premenopausal women, therefore we hypothesized that DEAR1 may function in the regulation of ERα signaling and conversely that mutations in DEAR1 (observed in human breast cancers) will abrogate that effect and could play a role in resistance to antiestrogen therapies. To test this hypothesis we studied the effect of DEAR1 expression on ERα. DEAR1 overexpression downregulated ERα expression and stability suggestive that DEAR1 may regulate ER signaling. DEAR1 interacted with ERα by immunoprecipitation and GST pulldown assays. Next we studied whether E3 ligase DEAR1 facilitates the degradation of ERα by promoting the ubiquitination of ERα. Our results indicate that DEAR1 promoted the polyubiquitination of ERα. To assess the effect of DEAR1 on estrogen mediated response, we examined the effect of DEAR1 on ERα-mediated transactivation using ERE-luciferase reporters. Results indicate that DEAR1 significantly inhibits estrogen-mediated transcriptional activity of ERα. We also studied the effect of DEAR1 in antiestrogen resistance using tamoxifen resistant cells. Preliminary results suggest that the presence of DEAR1 promoted the action of antiestrogen in tamoxifen resistant cell line. We have identified a new novel E3 ligase with regulatory effect of ERα signaling and potential implications in antiestrogen resistance. Citation Format: Seetharaman Balasenthil, Nanyue Chen, Ann Killary. On the role of DEAR1 as a novel ubiquitin ligase for ER alpha and predictor of tamoxifen response in ER positive breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 1009.