Abstract
<jats:p> Breast cancer is a heterogeneous disease in which a single oncogenic driver can give rise to divergent tumor phenotypes. How oncogenic mutations generate epithelial state plasticity and coordinately remodel the surrounding tissue remains incompletely understood. Here, we applied longitudinal single cell RNA-sequencing to trace the mammary landscape during <jats:italic>Pik3ca</jats:italic> <jats:sup>H1047R</jats:sup> -driven tumor progression in the mouse. We identify an expansion of the epithelial transcriptional state space, in which luminal cells lose lineage fidelity and activate ciliated, basal, and squamous-like gene expression programs. While oncogene-expressing cells lose features of luminal identity, they retain expression of hormone-sensing genes such as <jats:italic>Esr1</jats:italic> , <jats:italic>Pgr</jats:italic> , and <jats:italic>Foxa1</jats:italic> . These transcriptional states are established early, and the transition to overt tumors is marked by the emergence of cancer-associated fibroblasts rather than new epithelial states. We identify a <jats:italic> Postn <jats:sup>+</jats:sup> </jats:italic> fibroblast population enriched at the epithelial interface as a candidate progenitor of cancer-associated fibroblasts. <jats:italic> Postn <jats:sup>+</jats:sup> </jats:italic> fibroblasts express an ECM-remodeling program and display altered epithelial crosstalk in oncogenic glands. Altogether, <jats:italic>Pik3ca</jats:italic> <jats:sup>H1047R</jats:sup> activation initiates a tissue-level process beginning with epithelial lineage infidelity, followed by an altered stromal microenvironment, which together mark tumor initiation. </jats:p>