Epigenetic Regulation of Epithelial-Mesenchymal Transition in Epithelial Stem Cells and Triple Negative Breast Cancer

Epithelial-mesenchymal transition (EMT) is a cellular program responsible for the conversion of stationary epithelial cells characterized by apical-basolateral polarity to motile mesenchymal cells with front-back end polarity. EMT drives the fundamental developmental processes of implantation, gastrulation and neural crest formation. This essential developmental program becomes reactivated during cancer progression to promote metastasis. These studies examine the molecular and epigenetic mechanisms responsible for the induction of EMT in several developmental epithelial stem cell models and Basal-like and Claudin-low breast cancer subtypes. Using trophoblast stem (TS) cells paused in EMT, we have defined the molecular and epigenetic mechanisms responsible for modulating the induction of EMT. Targeted inactivation of MAP3K4, knockdown of CBP, or overexpression of SNAI1 in TS cells induced intermediate EMT phenotypes. These TS cells exhibited epigenetic changes in the histone acetylation landscape that caused loss of epithelial maintenance while preserving self-renewal and multipotency. MAP3K4 controls the activity of the histone acetyltransferase CBP, and acetylation of histone H2B by CBP is required to maintain the epithelial phenotype of TS cells. Combined loss of MAP3K4/CBP activity represses expression of epithelial genes and causes TS cells to undergo EMT while maintaining self-renewal and multipotency. A similar EMT phenotype and an EMT gene signature shared with TS cells in EMT were found in Claudin-low breast cancer cells with properties of EMT and stemness. Therefore, it was hypothesized that the upregulated genes within the intersecting EMT signature define a novel set of genes critical to the induction of EMT and maintenance of the mesenchymal phenotype in breast cancer cells. We designed an RNAi screen in the Claudin-low EpCAM(-) population of SUM149 and SUM229 Basal-like breast cancer cells to identify genes responsible for maintaining the mesenchymal phenotype. Using this RNAi screening strategy for EMT-regulatory genes, the chromatin-remodeling factor Smarcd3/Baf60c was identified as a novel epigenetic regulator controlling the induction of EMT. Expression of Smarcd3/Baf60c in human mammary epithelial cells promoted EMT in a Wnt5a-dependent manner. These results highlight a new mechanism of epigenetic regulation of EMT, whereby Smarcd3/Baf60c promotes loss of epithelial cell polarity and dissolution of cell-cell junctions through activation of non-canonical WNT signaling.Doctor of Philosoph