Exploring the functions of PRC2 in development and congenital disorders

Abstract

Polycomb group (PcG) proteins are key chromatin-based repressors of gene expression that play important roles in lineage-specification and cellular memory. PcG proteins assemble into multi-subunit complexes, termed Polycomb Repressive Complexes (PRCs), which elicit transcriptional silencing through chromatin condensation. PRC2 consists of three core PcG proteins (SUZ12, EED and EZH1/2) that together catalyse the addition of up to three methyl groups on histone H3 at lysine 27 (H3K27). This catalytic core interacts with accessory proteins that function to modulate and refine PRC2 activity. Both core and accessory PRC2 components are essential for normal development and their aberrant function thus results in a range of developmental phenotypes. The aim of this PhD thesis was to generate new insight into the function of PRC2 in development and, specifically, in PRC2-related overgrowth. Firstly, I explore the function of the relatively poorly characterised PALI-family of PRC2 accessory proteins using mouse and stem cell models of development. Both PALI1 and PALI2 were found to promote PRC2 methyltransferase activity in vivo, though Pali1 knockout mice exhibited a perinatal lethal phenotype while Pali2 knockout mice were viable and fertile. PALI1 was additionally found to be dispensable for the de novo recruitment of PRC2 to target gene promoters. Secondly, using mouse embryonic stem cells (mESCs), I investigate the molecular effects of EZH2 mutations found in Weaver syndrome a developmental overgrowth and intellectual disability (OGID) disorder. I show that Weaver-syndrome-associated EZH2 mutations are predominantly partial loss-of-function but with dominant negative effects on PRC2 methyltransferase activity. Finally, through the comparative analysis of isogenic mESC models of distinct congenital growth disorders, I identify key chromatin and transcriptional changes that are shared across genetically distinct OGID syndromes. Taken together, these data contribute to an improved understanding of the biological functions of the PRC2-associated PALI proteins, and provide important molecular insight into the aetiology of EZH2-mutant Weaver syndrome and related congenital growth disorders.