DDDR-23. Targeting H3K27M protein with inhibitors of histone deacetylases in H3K27-altered diffuse midline gliomas

Abstract H3K27-altered diffuse midline gliomas (DMGs) are the deadliest paediatric high-grade gliomas (pHGGs) with a median survival of 9-11 months after diagnosis. Over 80% of DMGs expresses a somatic heterozygous mutation in the H3 histone variants (H3.3, and less commonly H3.1/H3.2), which results in the H3 lysine 27 to methionine substitution (H3K27M). The presence of H3K27M causes a global loss of H3K27 trimethylation (H3K27me3), an increase in H3K27 acetylation (H3K27ac) and an overall chromatin dysregulation, leading to the expression of specific pro-tumorigenic genes. Due to occurrence in the midline structures of the brain (thalamus, brainstem and spinal cord), the resection of DMGs is practically impossible, allowing only limited treatment with radiotherapy. Huge efforts are focused on developing new therapies, especially those interfering with epigenetic abnormalities in DMGs. However, the mechanism of action of some leading compounds, e.g. histone deacetylase inhibitor (HDACi), panobinostat, is only partially understood. Cells eventually develop resistance to panobinostat, despite encouraging pre-clinical data. Importantly, we have recently shown that some HDACi (including panobinostat, pracinostat/SB939, vorinostat and entinostat) lead to the reduction in the H3K27M protein levels. This is a novel and unexpected function of HDACi, which is being further investigated in this study. Proteomic analysis is being carried out in order to reveal the mechanism of HDACi-dependent targeting of H3K27M protein and to reveal the key players involved in this process. Our exciting discovery suggests that cells have mechanisms in place, which are able to target H3K27M during HDAC inhibition, and do not affect the wild-type H3. We are hoping that our studies may pave the way for new therapeutic strategies for patients with DMGs, aiming at eradication of detrimental H3K27M oncohistone protein. The study is supported by the Sonata Bis-14 grant (2024/54/E/NZ3/00480) awarded by the National Science Centre, Poland.