Abstract 6151: ESR1 mutations rewire TGF-B and hedgehog signaling to drive endocrine resistance and osteolytic bone metastasis in estrogen receptor-positive breast cancer

Abstract Endocrine therapy-resistant ER+ breast cancer frequently acquired activating ESR1 mutations (e.g. Y537S) conferring ligand-independent ER activation and aggressive bone metastasis, yet underlying mechanisms remain unclear. We hypothesized that ESR1 mutations promote bone colonization by simultaneously dysregulating TGF-β/SMAD and Hedgehog/Gli2 pathways. Wild-type and mutant (Y537S) MCF7 and T47D cells with ERE-luciferase reporters showed Y537S cells maintained elevated basal PTHrP transcription independent of estrogen while responding to TGF-β. Intratibial xenografts demonstrated Y537S tumors induced greater osteolytic lesion burden than wild-type, with TGF-β neutralization significantly reducing lesion formation. RNA-seq revealed Y537S cells exhibited dramatic Gli2 and PTHLH upregulation. Critically, ligand-stimulation activated Gli2/PTHLH in mutant but not in wild-type cells. CRISPR-mediated Gli2 knockout reduced PTHrP production while Gli2 overexpression maximized osteolytic potential. These results demonstrate that ESR1 mutations reprogram ER to activate dual bone-remodeling pathways (TGF-β/SMAD and Hedgehog/Gli2) explaining aggressive bone metastatic phenotypes and identify ER, TGF-β/SMAD, and Gli2 as rational combination therapeutic targets. Citation Format: Erykah J. Coe, Julie Rhoades. ESR1 mutations rewire TGF-B and hedgehog signaling to drive endocrine resistance and osteolytic bone metastasis in estrogen receptor-positive breast cancer abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 6151.