DDR1 regulates RUNX1-CBFβ 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 CBFβ, 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.