Environmental chemical mixtures reprogram mammary epithelial development to epigenetic states associated with breast cancer

Environmental exposures occur as complex mixtures, yet the mechanisms by which they alter human tissue development and generate cancer-relevant tissue states remain poorly defined. Here, we establish a physiologically relevant 3D human breast organoid high-content screening platform that relates exposure-induced developmental disruption to cancer-relevant cell states. Screening of structurally diverse environmental chemicals as single agents and mixtures reveals heterogeneous but reproducible developmental responses, indicating that distinct environmental chemicals perturb different aspects of mammary morphogenesis. Focusing on bisphenols, we show that a low-dose mixture (BPA, BPS, and BPF; 3BPX) disrupts organoid development and induces transcriptional programs characterized by extracellular matrix remodeling, epithelial plasticity, and partial epithelial-mesenchymal transition, accompanied by widespread DNA methylation remodeling. A 3BPX exposure signature maps to ER+ luminal breast cancers and is preferentially enriched in invasive lobular carcinoma (ILC), a subtype characterized by epithelial plasticity. The 3BPX-associated methylation signature is detectable in primary tumors and adjacent normal tissue and is enriched in ILC. Together, these findings show that environmentally relevant chemical mixtures induce heterogeneous developmental perturbations that converge on conserved transcriptional and epigenetic programs associated with epithelial plasticity and tissue remodeling. These exposure-associated signatures are represented in defined epithelial and stromal compartments in human breast cancers, supporting a model in which developmental exposures may establish field-like, cancer-relevant tissue states.