Investigating the effects of H3K27M on PRC1 in Diffuse Midline Glioma

Abstract

Diffuse midline glioma (DMG) is a universally lethal paediatric brain tumour characterised by a mutation in histone H3 (H3K27M). This mutation leads to a global reduction in the repressive histone modification H3K27me3. However, a subset of genes retain H3K27me3, keeping them aberrantly repressed. This repression is crucial for DMG oncogenesis as it restrains differentiation and promotes proliferation. It is known that some forms of Polycomb Repressive Complex 1 (PRC1), known as canonical PRC1 (cPRC1), contribute to gene repression by binding to H3K27me3. Here, I demonstrate that DMG cells exhibit a specific dependence on a form of cPRC1 containing CBX4 and PCGF4, but not their paralogs. Using a combination of CRISPR-Cas9 screens, quantitative chromatin mapping techniques, and functional perturbations, I demonstrate that cPRC1 complexes containing CBX4 and are essential in DMG disease biology, acting downstream of H3K27me3. I demonstrate that the altered H3K27me3 landscape in DMG cells leads to a reorganization of cPRC1 complex distribution on chromatin, highlighting previously unknown differences between CBX paralogs. The work presented here not only advances our understanding of cPRC1 complexes, but also provides new insights into the molecular mechanisms underlying DMG disease and highlights the potential therapeutic value of targeting cPRC1 complexes in the treatment of this devastating paediatric cancer.