Disruption of basal cell identity by Rank signaling drives mammary tumorigenesis

Rank signaling regulates mammary gland development and epithelial cell differentiation. Rank receptor is expressed by mammary basal and luminal populations, but, unlike luminal Rank, the contribution of basal Rank signaling to MG homeostasis remains poorly studied. We have combined timely regulated basal specific Rank expression with lineage tracing models and unveiled that basal Rank signaling controls basal cell identity in postnatal mammary glands. Ectopic basal Rank disrupts basal but also luminal cell identity, resulting in aberrant luminal-like differentiation of basal cells and impaired lactogenesis. Mechanistically, overactivation of basal Rank signaling leads to basal cell lineage infidelity, illustrated by the appearance of premalignant lesions composed by a basal-derived hybrid population with alveolar features which ultimately generates basal and luminal breast adenocarcinomas. Proteomic, transcriptomic and chromatin analyses support that the loss of tumor suppressive epigenetic regulators driven by basal Rank contributes to epithelial cell dedifferentiation and tumorigenesis. The basal Rank signature generated associates to poor prognosis particularly in human adenocarcinomas of the luminal subtype stressing the clinical relevance of our findings. Interestingly, our results reinforce the idea that luminal breast tumors might originate from basal cells that have suffered a luminal-like aberrant dedifferentiation triggered by Rank signaling