DIPG-40. HYPOXIC STRESS EXPOSES EPIGENETIC TARGETS TO ENHANCE RADIOTHERAPY RESPONSE IN H3K27M-POSITIVE PEDIATRIC DIFFUSE MIDLINE GLIOMAS

Diffuse midline gliomas (DMGs) are the most challenging to treat pediatric high-grade gliomas, with a dismal prognosis of 9 to 15 months of median survival. The ones localized in the pons are not accessible to the surgery and rely on radiotherapy treatment. The majority of DMGs confer a mutation in histone 3 variants (H3.3 or H3.1), leading to lysine 27 to methionine substitution (H3K27M) and a consequent global decrease in H3K27 trimethylation (H3K27me3), increased H3K27 acetylation (H3K27ac) and oncogenic changes in gene expression. An intrinsic feature of solid tumors, including DMGs, is the occurrence of hypoxic (low oxygenated) regions, which interferes with the efficiency of radiotherapy. Identifying the vulnerabilities of hypoxia-exposed tumor cells might bring new approaches in radiosensitizing DMG tumors. Hypoxia increases histone methylation in multiple solid tumors but the consequences of hypoxia-induced histone hypermethylation in H3K27M bearing tumors are unknown. Using syngeneic patient-derived DMG cell lines, we show that the presence of H3K27M prevents the hypoxia-induced hypermethylation of H3K27me3, but also impairs the hypoacetylation of histones H3 and H4 normally caused by hypoxic stress. We show that enhancing histone acetylation in hypoxic conditions with histone deacetylase (HDAC) inhibitors has a stronger sensitization effect in cells expressing H3K27M than in H3 wild-type cells. While HDAC inhibitors have been shown problematic in pediatric DMGs due to high systemic toxicity, our data points out that HDAC inhibition in the context of hypoxia may improve the therapeutic efficacy. Recent studies showed the possibility of targeting hypoxic cells with hypoxia-activated pro-drugs, including hypoxia-activated panobinostat. Our data support that such approach might be an option to target DMGs and eliminate the radiotherapy-resistant hypoxic tumor fraction, while minimizing the systemic toxicity caused by all-time active HDAC inhibitors. Studies were supported by the grant UMO-2019/33/B/NZ1/01556 (KL) from National Science Center (Poland).