DDR1 regulates RUNX1-CBFß to control breast stem cell differentiation
Understanding epithelial stem cell differentiation and morphogenesis during breast tissue development is essential, as disruption in these processes underlie breast cancer formation. Recent work has shown that epithelial stem and progenitor cells use the collagen receptor Discoidin Domain Receptor 1 (DDR1) for differentiation into both basal and luminal cell lineages, which together are necessary for complex ductal-lobular morphogenesis. Here, we used  a next-generation single cell derived organoid model that generates miniaturized breast tissue, to study how single stem cells can give rise to multiple cell types and compound tissue structures. We show that DDR1inhibition traps cells in a bipotent state, blocking alveolar morphogenesis and luminal cell expansion necessary for complex epithelium formation. Disrupting RUNX1 function produced nearly identical phenotypes, underscoring its critical role downstream of DDR1. Mechanistically, DDR1 affects the interaction and expression of RUNX1 and its cofactor CBFß, thereby regulating its activity. Mutational analyses in breast cancer patients reveal frequent alterations in the DDR1-RUNX1 signaling axis, particularly co-occurring mutations. Together, these findings uncover DDR1-RUNX1 as a central signaling pathway driving breast epithelial differentiation, whose dysregulation may contribute fundamentally to breast cancer pathogenesis. Overall design: Three primary patient samples were cultured in 3D hydrogels for 7 days bafore receiving chronic treatment (Vehicle, DDR1i, AI-10-104 (RUNXi) for an additional 14-21 days until control was fully developed. Bulk RNA-seq was performed on these samples, followed by gene expression analysis comparison of differential gene sets from RUNXi and DDR1i treated samples compared to the control.