An E2 ubiquitin-conjugating enzyme links diubiquitinated H2B to H3K27M oncohistone function.

in Proceedings of the National Academy of Sciences of the United States of America by Alan L Jiao, Erdem Sendinc, Barry M Zee, Felice Wallner, Yang Shi

TLDR

  • Unbiased genome-wide suppressor screens in H3K27M-expressing cells identified 20 suppressors that restored H3K27 trimethylation, including mutations in histone H3.3 and an E2 ubiquitin-conjugating enzyme.

Abstract

The H3K27M oncogenic histone (oncohistone) mutation drives ~80% of incurable childhood brain tumors known as diffuse midline gliomas (DMGs). The major molecular feature of H3K27M mutant DMGs is a global loss of H3K27 trimethylation (H3K27me3), a phenotype conserved in(). Here, we perform unbiased genome-wide suppressor screens inexpressing H3K27M and isolate 20 suppressors, all of which at least partially restore H3K27me3. 19/20 suppressor mutations map to the same histone H3.3 gene in which the K27M mutation was originally introduced. Most of these create single amino acid substitutions between residues R26-Y54, which do not disrupt oncohistone expression. Rather, they are predicted to impair interactions with the Polycomb Repressive Complex 2 (PRC2) and are functionally conserved in human cells. Further, we mapped a single extragenic H3K27M suppressor to, an E2 ubiquitin-conjugating enzyme, whose loss rescued H3K27me3 to nearly 50% wild-type levels despite continued oncohistone expression and chromatin incorporation. We demonstrate thatis the major enzyme responsible for generating diubiquitinated histone H2B. Our study provides in vivo support for existing models of PRC2 inhibition via direct oncohistone contact and suggests that the effects of H3K27M may be modulated by H2B ubiquitination.

Overview

  • This study aimed to identify genetic suppressors of diffuse midline gliomas (DMGs) driven by the H3K27M oncogenic histone mutation.
  • The researchers performed unbiased genome-wide suppressor screens in H3K27M-expressing cells and isolated 20 suppressors that at least partially restored H3K27 trimethylation.
  • The study's primary objective was to understand the molecular mechanisms underlying the loss of H3K27 trimethylation in H3K27M mutant DMGs and to identify potential therapeutic targets.

Comparative Analysis & Findings

  • The study identified 20 suppressors of H3K27M, with 19 of them mapping to the same histone H3.3 gene and creating single amino acid substitutions between residues R26-Y54.
  • These mutations were predicted to impair interactions with the Polycomb Repressive Complex 2 (PRC2) and were functionally conserved in human cells.
  • In addition, the study mapped a single extragenic H3K27M suppressor to, an E2 ubiquitin-conjugating enzyme, which was shown to generate diubiquitinated histone H2B.

Implications and Future Directions

  • The study provides in vivo support for existing models of PRC2 inhibition via direct oncohistone contact and suggests that the effects of H3K27M may be modulated by H2B ubiquitination.
  • Future studies could focus on exploring the potential therapeutic applications of these suppressors, particularly in the context of H3K27M-driven DMGs.
  • Elucidating the molecular mechanisms underlying the loss of H3K27 trimethylation may lead to the development of new treatments for these incurable childhood brain tumors.
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