Hotspot ESR1 mutations are multimodal and contextual modulators of breast cancer metastasis

Constitutively active estrogen receptor-α (ER/ESR1) mutations have been identified in approximately one third of ER+ metastatic breast cancers. Although these mutations are known mediators of endocrine resistance, their potential role in promoting metastatic disease has not yet been mechanistically addressed. In this study, we show the presence of ESR1 mutations exclusively in distant but not local recurrences in five independent breast cancer cohorts. In concordance with transcriptomic profiling of ESR1 mutant tumors, genome-edited ESR1 Y537S and D538G mutant cell models exhibited a reprogrammed cell adhesive gene network via alterations in desmosome/gap junction genes and the TIMP3/MMP axis, which functionally conferred enhanced cell-cell contacts while decreasing cell-extracellular matrix (ECM) adhesion. In vivo studies showed ESR1 mutant cells were associated with larger multi-cellular circulating tumor cell (CTC) clusters with increased compactness compared to ESR1 WT CTCs. These preclinical findings translated to clinical observations, where CTC clusters were enriched in patients with ESR1-mutated metastatic breast cancer. Conversely, context-dependent migratory phenotypes revealed co-targeting of Wnt and ER as a vulnerability in a D538G cell model. Mechanistically, mutant ESR1 exhibited non-canonical regulation of several metastatic pathways, including secondary transcriptional regulation and de novo FOXA1-driven chromatin remodeling. Collectively, these data provide evidence for ESR1 mutation-modulated metastasis and suggest future therapeutic strategies for targeting ESR1 mutant breast cancer.