A FOXA1/SPDEF Co-regulatory Axis Drives ERBB2 and Suppresses TEAD/YAP-Driven EMT to Maintain Luminal Identity in HER2+ Breast Cancer

The function of the pioneer factor forkhead box A1 (FOXA1) in estrogen receptor (ER)-negative, human epidermal growth factor receptor 2 (HER2, encoded by the ERBB2 gene)-positive breast cancer represents a critical gap in our understanding of lineage identity in tumorigenesis. Here, we solve this enigma by addressing this question through the identification of an indispensable, co-dependent regulatory circuit formed between FOXA1 and the SAM pointed domain containing ETS-family transcription factor (SPDEF). Using integrative multi-omics analyses of patient tumors and experimental models, we demonstrate that this circuit functions as the master guardian of the HER2-positive luminal state. It executes this role via two distinct, essential mechanisms: it directly binds regulatory elements to drive high-level expression of the ERBB2 oncogene, and it simultaneously preserves epithelial identity by suppressing a latent program of epithelial-to-mesenchymal transition (EMT). Mechanistically, the FOXA1/SPDEF circuit promotes the expression of the transcriptional repressor TRPS1, which in turn antagonizes the pro-mesenchymal activity of the TEAD/YAP complex. Our findings define a central regulatory node that couples lineage fidelity to oncogenic output in HER2-positive cancer, revealing the FOXA1/SPDEF-TRPS1 axis as a critical determinant of tumor plasticity and a potential therapeutic target for overcoming resistance in HER2-positive breast cancer.