Overexpression of Cbx4 wild type and of a Cbx4 sumoylation mutant in human epidermal stem cells

Human epidermal stem cells transit from a slow cycling to an actively proliferating state to contribute to homeostasis. Both stem cell states differ in their cell cycle profiles but must remain guarded from differentiation and senescence. Here we show that Cbx4, a Polycomb Repressive Complex-1 (PRC1)-associated protein, maintains human epidermal stem cells slow-cycling and undifferentiated, while protecting them from senescence. Interestingly, abrogating the polycomb activity of Cbx4 impairs its anti-senescent function without affecting stem cell differentiation, indicating that differentiation and senescence are independent processes in human epidermis. Conversely, Cbx4 inhibits stem cell activation and differentiation through its SUMO ligase activity. Global transcriptome and chromatin occupancy analyses indicate that Cbx4 regulates modulators of epidermal homeostasis and represses factors, such as Ezh2, Dnmt1, and Bmi1, to prevent the activate stem cell state. Our results suggest that distinct Polycomb complexes balance epidermal stem cell dormancy and activation, while continually preventing senescence and differentiation. Primary human keratinocytes were infected with control vector pBABE, Cbx4 wildtype and a Sumoylation mutant version of Cbx4 (the two Sumo Interaction Motifs were deleted), both fused to a Estrogen Receptor to render their activaty inducible by addition of 4OHT to the media. Cells were cultured for 5 days (after infection and selection) under 4OHT treatment, after which total RNA was collected.