Histone H3 mutant and Polycomb Repressive Complex 2 role in Diffuse Intrinsic Pontine Gliomas

The correct establishment of cell-type specific transcription programs is of fundamental importance for proper embryonic development as well as to ensure correct cellular differentiation for tissue homeostasis in adult organisms. Regulation of cellular identity is at the basis of several human pathologies with a particular importance in the development of human cancers. The correct regulation of chromatin modifications plays a crucial role in these processes. Indeed, chromatin-modifying factors (also defined as epigenetic factors) often play essential roles in regulating normal development and differentiation and have been recently classified among the most mutated pathways in human cancers. Ensuring maintenance of gene repression is as important as establishing the activation of lineage specific genes. Polycomb group proteins (PcG) are critical for this and are among the most important gatekeepers that ensure the correct establishment and maintenance of cellular identity in metazoans. Indeed, the activity of PcG proteins is frequently altered in several human tumours via different genetic, transcriptional and epigenetic alteration whose mechanistic role remains poorly understood. PcGs exist in two distinct multi-protein repressive complexes (PRC1 and PRC2) that, by post-transnationally modifying respectively histone H2A and histone H3, control in large part redundant transcriptional pathways. While PRC2 methylates (me) histone H3 on lysine (K) 27, the PRC1 complex mono-ubiquitylates histone H2A on K119 (H2Aubq). Both activities are linked with transcriptional repression and chromatin compaction. The catalytic subunit of PRC2 is the methyltrasferase EZH2 (in some cases compensated by the activity of its paralogue EZH1) and requires stable association with the core members of the complex Eed and Suz12. The PRC2 members are generally regarded as negative prognostic factors in different tumours such as brain tumours (e.g. glioblastoma). However, recently, a high recurrent lysine-to-methionine somatic mutation on histone H3.3 (K27M) has been reported to globally inhibit PRC2 enzymatic activity in pediatric Diffused Intrinsic Pontine Gliomas (DIPGs). These findings indirectly support a tumour-suppressive function of the PRC2 in those tumours. Furthermore this generates a potentially serious drawback on the efficacy (or dangerousness) of PRC2 inhibitors for these specific type of tumours. This project aims to characterize the molecular and biological proprieties of the frequent histone H3.3 somatic mutation identified in DIPGs (K27M), its oncogenic potential and its relationship with the PRC2 role in those tumours.