Abstract ES4-1: Adaptive Resistance to Hormonal Therapy

Abstract

Abstract While recurrences are delayed by the use of endocrine therapy (ET), resistance can evolve with the development of metastatic breast cancer (MBC) in about one-fourth of ER-positive patients. MBC is incurable with a median 5-year survival rate of less than 25%. We know that ET resistance can result from an adaptive process in which escape survival mechanisms are used to evade treatment. One of the best-characterized ET resistance mechanisms is transcriptional activation of growth factor receptor (GFR) signaling, and subsequent estrogen-independent activation of ER via “cross-talk” with downstream signaling from GFRs. We will discuss distinct mechanisms of ligand-independent activation of ER discovered using transcriptional profiling of metastatic, ET-resistant tumors. It is now estimated that ~20% of therapy-resistant MBCs contain ESR1 mutations. Recent data has also shown that mutations in the MAPK/RAS pathway and transcription factor regulatory programs (for example, MYC) only account for an additional 22% of recurrent breast tumors. Thus, acquired mutations alone do not account for the majority of ET resistance mechanisms in MBC. ER-positive breast cancer cell lines display profound transcriptional variability at the single cell level associated with the acquisition of therapy resistance. Recently, bulk and single cell RNA sequencing was used on sequential samples from patients with ER-positive, ET-resistant patients and identified RNA expression patterns that represented different cellular phenotypes, including an increased epithelial mesenchymal transition state that could provide a selective advantage for selective phenotypic subclones to dominate. We will focus our discussion of transcriptional reprograming as a major mechanism of ET resistance. Citation Format: S Fuqua. Adaptive Resistance to Hormonal Therapy [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr ES4-1.