Understanding the diverse roles of polycomb-like proteins in the regulation of chromatin and transcriptional landscapes

Abstract

The Polycomb repressive complex 2 (PRC2) is a multiprotein chromatin modifying complex that is essential for vertebrate development and frequently deregulated in cancer. In higher eukaryotes, PRC2 is responsible for all mono-, di- and tri-methylation of Histone H3 at Lysine 27, and is composed of core subunits EED, SUZ12, and EZH1/2, as well as accessory components, Polycomb-like 1-3 (PCL1-3), EPOP, PALI1/2, AEBP2 and JARID2. Two distinct subtypes of PRC2 exist, termed PRC2.1 (containing PCL1-3, EPOP and PALI1/2) and PRC2.2 (containing AEBP2 and JARID2), however, little is known about their respective overlapping and divergent functions. As critical components of the PRC2.1 complex, PCL1-3 represent an excellent avenue to further explore the roles of PRC2 in cell fate decisions during differentiation and development, but also holds promise for future targeted cancer therapies. This PhD thesis addresses the various roles of PCL proteins in both differentiated adult cells and pluripotent embryonic stem cells (ESCs). Firstly, I explore the biochemical functions of PCL1 in the maintenance of cellular quiescence through a novel PCL1-p53 regulatory axis. Secondly, through the analysis of all three PCL proteins in cycling and quiescent human cells, I have defined a novel catalytically inactive form of PRC2, lacking SUZ12, that exists in quiescent cells. Thirdly, by analysing the effects of genetic knockout of all three Pcl genes in ESCs I characterise roles for distinct classes of PRC2 subcomplex assemblies, PRC2.1 and PRC2.2. Taken together, I believe these data will contribute to a better mechanistic understanding of the roles of Polycomb proteins during cell fate decisions and in complex biological processes such as carcinogenesis and embryogenesis.