ChIP-seq profiling of Foxp1 binding in basal and luminal mouse mammary cells

Long-lived quiescent mammary stem cells (MaSCs) are presumed to coordinate the dramatic expansion of ductal epithelium that occurs through the different phases of postnatal development, but little is known about the molecular regulators that underpin the activation of MaSCs. Here we show that ablation of the transcription factor Foxp1 in the mammary gland profoundly impairs ductal morphogenesis, resulting in a rudimentary tree throughout adult life. Foxp1-deficient glands were highly enriched for quiescent Tspan8hi MaSCs, which failed to become activated, even in competitive transplantation assays, and therefore harbor a cell-intrinsic defect. Luminal cells aberrantly expressed basal genes, suggesting that Foxp1 may also contribute to cell-fate decisions. Notably, Foxp1 was uncovered as a direct repressor of the Tspan8 gene in basal cells and deletion of Tspan8 could rescue the profound defects in ductal morphogenesis elicited by Foxp1 loss. Thus, a single transcriptional regulator, Foxp1, can control the exit of MaSCs from dormancy to orchestrate differentiation and development. Overall design: Basal and luminal epithelial cells were extracted from the mammary glands of 10-12 week old FVB/N wild-type female mice. Foxp1 ChIP-seq and DNA input was sequenced for each sample.