DMG-28. Targeting H3K27 Acetylation Reverts the Differentiation Blockade in Histone Mutant Diffuse Midline Glioma

Diffuse midline glioma (DMG) is characterized by heterozygous H3K27M mutations that reprogram the epigenome and block terminal differentiation. These histone mutations impair PRC2 activity resulting in a global decrease in H3K27me3 and a concomitant increase in H3K27ac. Interestingly, in vitro, serum supplementation forces DMG cells to transition from a proliferative stem-like cell state to a more mature and noncycling astrocytic-like cell state. To understand how we can promote differentiation in DMG, we assessed for changes in the epigenome upon serum-induced differentiation and observed that differentiated cells lose H3K27 acetylation. Hence, we questioned whether loss of H3K27ac would reciprocally provoke differentiation. Pharmacological inhibition of EP300/CBP, the histone acetyltransferases responsible for H3K27 acetylation, drastically decreased DMG cell proliferation leading to cell cycle arrest. Besides loss of H3K27ac, H3K27me3 is rescued upon EP300/CBP inhibition suggesting that targeting these acetyltransferases can revert the epigenetic reprograming caused by the H3K27M mutation. ATAC sequencing revealed that the epigenetic changes caused by EP300/CBP inhibition mimic the ones obtained upon serum-induced differentiation. Moreover, similarly to serum treatment, EP300/CBP targeting shifts DMG cells towards a more differentiated astrocyte-like cell state. In contrast, H3G34R/V gliomas and non-malignant cells lines responded less to EP300/CBP inhibition, indicating a selective sensitivity of H3K27M gliomas. In summary, we showed that EP300/CBP inhibition can partially restore the epigenetic landscape in DMG cells. Further, our findings suggest that targeting H3K27 acetylation is a promising strategy to revert the activity of H3K27M, permit DMG cell differentiation, and suppress tumor progression.