D538g Acquired Mutation Of Estrogen Receptor (Er) Alpha: A Novel Mechanism Of Endocrine Resistance In Metastatic Breast Cancer.

e11578 Background: Resistance to endocrine therapy occurs in virtually all patients with ER-positive metastatic breast cancer (MBC) and is attributed to various mechanisms including loss of ER expression, altered activity of coregulators and cross-talk between the ER and growth factor signaling pathways. To our knowledge, acquired mutations of the ER have not been described as mediating endocrine resistance to endocrine treatment. Methods: Deep sequencing of the ER was conducted, as part of a commercially available next-generation sequencing of 182 cancer-related genes, on samples obtained from 10 heavily pre-treated, endocrine resistant patients with ER positive MBC. In one patient, a sample obtained at diagnosis was also available. Mutated ER was cloned and overexpressed in breast cancer cells. Transcriptional activity was tested using estrogen response element (ERE)-Luciferase construct, effects on viability were tested using MTT assays and computerized modeling was used to assess structural effects. R...