Abstract B51: Genetic etiology of ER low/HER2- breast tumors

Abstract

Abstract Background: In 2010, the American Society of Clinical Oncology (ASCO) and the College of American Pathologists (CAP) issued guidelines that tumors with >1% positively staining cells should be considered ER positive. Numerous studies have demonstrated that tumors with 1-9% ER-positive cells (low staining) represent ER-negative intrinsic subtypes with outcomes similar to ER-negative tumors. Given the similarities between low-staining tumors and TNBC, we investigated whether low-staining tumors share a germline etiology with TNBC. Methods: The Clinical Breast Care Project was queried to identify all tumors with percent ER-positive cells and HER2 data available. Genomic DNA was isolated from available blood samples from patients with HER2-negative tumors with either negative (TNBC) or low-staining (1-9%) ER status. Targeted sequencing was performed using the Illumina TruSight Cancer panel. Pathogenic mutations were identified using VariantStudio and classified as pathogenic, likely pathogenic, uncertain significance (VUS), likely benign, or benign using ClinVar. Results: Low ER/HER2- tumors and TNBC represented 50 (2.7%) and 223 (12.2%) of the 1819 tumors in the database. Patients with low-staining tumors and TNBC did not differ significantly by age at diagnosis, ethnicity, family history, tumor stage, size, grade, or survival, and all the low ER/HER2- tumors with gene expression data available (n=17) were basal-like. Four (9%) of the women with low ER/HER2- tumors had pathogenic mutations in BRCA1 or BRCA2; an additional 4 (9%) had pathogenic mutations in the cancer predisposition genes BLM (n=2), FH (n=1), and MUTYH (n=1). Seven others had VUS in BRCA2, BRIP1, CHEK2, MSH2, MSH6, and SMAD4. Frequency of pathogenic mutations did not differ significantly (P=0.139) between low-staining tumors (18%) and TNBC (13%). Discussion: Low ER/HER2- tumors share similar clinicopathologic characteristics and outcomes with TNBC and have a significant genetic component, with 9% of patients having BRCA1 or BRCA2 germline mutations and an additional 9% having mutations in other cancer-predisposing genes. These data suggest that while ASCO/CAP guidelines define low ER tumors as ER positive, women with low ER/HER2- tumors may benefit from the clinical care and genetic testing provided to women with TNBC. The views expressed in this article are those of the author and do not reflect the official policy of the Department of Army/Navy/Air Force, the Department of Defense, or the U.S. Government. Citation Format: Leann A. Lovejoy, Craig D. Shriver, Rachel E. Ellsworth. Genetic etiology of ER low/HER2- breast tumors [abstract]. In: Proceedings of the AACR Special Conference: Advances in Breast Cancer Research; 2017 Oct 7-10; Hollywood, CA. Philadelphia (PA): AACR; Mol Cancer Res 2018;16(8_Suppl):Abstract nr B51.