Gene Regulation by Epigenetic Modifiers in Stem Cells, Cancer, and Immunology

DNA methylation is an essential epigenetic mechanism to control gene expression, with important implications for development, cellular homeostasis, and cancer progression. A lack of DNA methylation is not compatible with embryonic development. Likewise, dysregulation in DNA methylation is hallmark associated with cancer development. This dissertation focuses on the role of two epigenetic modifying proteins: protein inhibitor of activated STAT1 (PIAS1) and ten-eleven translocation 2 (TET2). Here I describe a newly discovered role for PIAS1 as an epigenetic gene silencer, with important implications in immunosuppression, stem cell maintenance and differentiation, and breast tumorigenesis. First, PIAS1 recruits DNMT3 and HP1 to the Foxp3 promoter for epigenetic silencing in regulatory T cells (Tregs). In the absence of PIAS1, the Foxp3 promoter is hypomethylated with a corresponding increase in expression. This allows for an increase in the number of Treg cells in Pias1-/- mice that provides resistance to the development of experimental autoimmune encephalomyelitis (EAE). Second, PIAS1 plays an important role in the maintenance and differentiation of hematopoietic stem cells (HSCs) by mediating proper DNA methylation at key differentiation genes such as Gata1. Without PIAS1 mediated gene silencing, dormant stem cells enter the cell cycle and exhaust their self-renewal properties. Additionally, stem cells aberrantly produce more myeloid cells than lymphoid cells in Pias1-/- mice. Third, elevated PIAS1 expression is observed in human breast cancer samples and is associated with increased tumorigenicity. In the breast cancer cell line MDA-MD231, PIAS1 mediates the silencing of important tumor suppressor genes such as WNT5A and CCND2 by epigenetic mechanisms. Knockdown of PIAS1 expression decreases the tumorigenicity of this cell line. Lastly, I explore the role of TET proteins in macrophage gene activation. Little is known about the role of TET proteins in the innate immune response. Surprisingly, TET2 is a repressor of several M1 macrophage genes whereas is required for the expression of a subset of M2 macrophage genes. Correspondingly, Tet2-/- macrophages exhibit an overexpression of several inflammatory genes important for microbial resistance, which may account for the increased protection to Listeria monocytogenes infection in seen in Tet2-/- mice