Epigenetic Regulation of Pluripotency by Polycomb Group Proteins

Lineage-specific phenotypes are the result of characteristic cellular gene expression patterns. Several epigenetic mechanisms have evolved that control the generation of these different phenotypes from the same genotype. Stem cells, in order to prevent differentiation, need to repress lineage-specific transcription factors and maintain the activity of stemness genes that promote self-renewal and pluripotency. In this context differentiation is basically a process governed by changes in gene activity during development that alter the stemness-specific epigenome towards lineage-specific patterns, often in response to transient factors or environmental stimuli. Sophisticated networks of protein complexes maintain epigenomic states in stem cells and determined cells after lineage decision and ensure their transmission through cell division. In addition, they are also essential for the epigenetic changes happening during differentiation induction that are crucial for lineage specification. The Polycomb group of genes codes for a variety of proteins that maintain repressive chromatin states. They are part of a complex cellular memory system that creates a layer of epigenetic information on top of the DNA sequence that ensures the maintenance and transmission of cell-specific expression patterns.