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. Results: A novel mutation resulting in substitution of aspartic acid at position 538 to glycine was identified in 4 of 10 samples. Moreover, in one case in which samples were available prior to and following development of resistance, the D538G mutation was noted only in the latter sample. Structural modeling indicated that the substitution leads to a conformational change in the ligand binding domain which prevents binding of either estrogen or tamoxifen and mimics the conformation of activated receptor. Experiments in cell lines indicated constitutive ligand-independent transcriptional activity of the mutated receptor. Overexpression of the receptor in MCF-7 cells enhanced proliferation and conferred resistance to treatment with tamoxifen and fulvestarnt. Conclusions: We report here a novel mutation D538G of the ER in human breast cancer and demonstrate its role in mediating resistance to endocrine treatment. Our study also indicates, for the first time, acquired mutation of the ER as a novel mechanism of endocrine resistance. Further studies on larger populations and on less selected patients are needed in order to assess the accurate frequency of this novel mutation.