H3K9/HP1 and Polycomb: two key epigenetic silencing pathways for gene regulation and embryo development.

Proper development of an embryo requires tightly controlled expression of specific sets of genes. In order to generate all the lineages of the adult, populations of pluripotent embryonic stem cells differentiate and activate specific transcriptional programs whereas others are shutdown. The role of transcription factors is obvious in promoting expression of such developmental genes; however maintenance of specific states throughout cell division needs additional mechanisms. Indeed, the nucleoprotein complex of DNA and histones, the chromatin, can act as a facilitator or barrier to transcription depending on its configuration. Chromatin-modifying enzymes regulate accessibility of DNA by establishing specific sets of chromatin, which will be either permissive or repressive to transcription. In this review, we will describe the H3K9/HP1 and Polycomb pathways, which mediate transcriptional repression by modifying chromatin. We discuss how these two major epigenetic silencing modes are dynamically regulated and how they contribute to the early steps of embryo development.