Molecular characterization of ESR1 variants in breast cancer

Purpose Estrogen receptor 1 ( ESR1 ) mutations and fusions typically arise in patients with hormone receptor-positive breast cancer after aromatase inhibitor therapy, whereby ESR1 is constitutively activated in a ligand-independent manner. These variants can impact treatment response. Herein, we characterize ESR1 variants among molecularly profiled advanced breast cancers. Methods DNA next-generation sequencing (592-gene panel) data from 9860 breast cancer samples were retrospectively reviewed. Gene fusions were detected using the ArcherDx fusion assay or whole transcriptome sequencing ( n = 344 and n = 4305, respectively). Statistical analyses included Chi-square and Fisher’s exact tests. Results An ESR1 ligand-binding domain (LBD) mutation was detected in 8.6% of tumors evaluated and a pathogenic ESR1 fusion was detected in 1.6%. Most ESR1 LBD mutations/fusions were from estrogen receptor (ER)-positive samples (20.1% and 4.9%, respectively). The most common ESR1 LBD mutations included D538G (3.3%), Y537S (2.3%), and E380Q (1.1%) mutations. Among biopsy sites, ESR1 LBD mutations were most observed in liver metastases. Pathogenic ESR1 fusions were identified in 76 samples (1.6%) with 40 unique fusion partners. Evaluating co-alterations, ESR1 variant (mutation/fusion) samples more frequently expressed androgen receptor (78.0% vs 58.6, P < 0.0001) and less frequently immune checkpoint proteins than ESR1 wild-type (PD-1 20.0% vs 53.4, P < 0.05; immune cell PD-L1 10.0% vs 30.2, P < 0.0001). Conclusion We have described one of the largest series of ESR1 fusions reported. ESR1 LBD mutations were commonly identified in ER-positive disease. Limited data exists regarding the clinical impact of ESR1 fusions, which could be an area for future therapeutic exploration.