Autophagy-cell death balance is maintained by Polycomb-mediated regulation during stem cell differentiation

Abstract

Autophagy is a conserved cytoprotective process, aberrations in which, lead to numerous degenerative disorders. While the cytoplasmic components of autophagy have been extensively studied, the epigenetic regulation of autophagy genes, especially in stem cells, is less understood. Deciphering the epigenetic regulation of autophagy genes becomes increasingly relevant given the therapeutic benefits of small-molecule epigenetic inhibitors in novel treatment modalities. We observe that, during retinoic acid-mediated differentiation of mouse embryonic stem cells (mESCs), autophagy is induced, and identify the Polycomb enzyme EZH2 as a regulator of this process. In mESCs, EZH2 represses several autophagy genes including the autophagy regulator Dram1. EZH2 facilitates the formation of a bivalent chromatin domain at the Dram1 promoter, which allows the expression of the gene and induction of autophagy during differentiation, while still retaining the repressive H3K27me3 mark. EZH2 inhibition leads to loss of the bivalent domain, and a consequential "hyper- expression" of Dram1, with extensive cell death. This study shows that Polycomb group proteins help maintain an autophagy-cell death balance during stem cell differentiation, in part, by regulating the expression of the Dram1 gene.