Per2 regulation of mammary gland development and branching morphogenesis via cell fate determination

Circadian rhythms play key roles in daily physiological functions, development, and cancer. Period 2 (PER2) is a repressive element which inhibits transcription activated by positive clock elements resulting in diurnal cycling of genes. Here we show, that outside of time keeping, PER2 has a non-circadian function that is critical to mammary gland development. Virgin Per2 deficient mice, Per2ldc, have underdeveloped glands containing fewer bifurcations and terminal ducts. Using a transplantation model, we show that these changes are intrinsic to the gland and furthermore identify changes in cell fate commitment. Per2ldc mouse mammary glands have a luminal/basal bi-potent phenotype in cells of the ductal epithelium. We identified co-localization of E-cadherin and Keratin 14 in luminal cells and decreased p63 staining and gene expression in myoepithelial cells. Moreover, Per2ldc mice overexpress SLUG, which is related to mammary stem cell maintenance and EMT. Similar results were demonstrated using MCF10A and shPER2 MCF10A cell lines. Collectively this study reveals a critical non-circadian function of PER2 in mammary gland development, validates the Per2ldc model, and describes a potential role for PER2 in breast cancer.