DMG-18. Leveraging Viral Mimicry for the Treatment of H3K27M Pediatric Diffuse Midline Glioma

Abstract H3K27M pediatric diffuse midline glioma (DMG) is a universally lethal brain tumor with a median overall survival of less than one year. 80% of DMGs are driven by the mutant oncohistone, H3K27M, that leads to global downregulation of the repressive H3K27 trimethylation. In the context of H3K27me3 loss, H3K27M DMGs become vulnerable to reactivation of retrotransposons (RT). RTs comprise ∼45% of the human genome and are normally transcriptionally silenced in somatic cells to prevent their deleterious effects on genome integrity and regulation. RT repression is largely mediated by histone modifications (H3K9me3 and H3K27me3) and DNA methylation. However, reactivation of RTs beyond a threshold level of tolerance in cancer cells can augment anti-tumor immunogenicity and induce tumor cell killing by a process termed “viral mimicry”. Our study investigates the potential epigenetic vulnerability of H3K27M DMGs to RT reactivation. We hypothesize that H3K27me3 loss renders H3K27M DMG exquisitely dependent on H3K9me3 as the alternative repressive histone modification to silence RTs, and that targeting H3K9me3 leverages viral mimicry as a therapeutic strategy. Indeed, in our CUT&RUN mapping of H3K9me3 in mouse DMG cell lines, we observed higher H3K9me3 on RTs in the H3K27M DMGs relative to H3WT, implicating a compensatory H3K9me3 response for RT silencing. Accordingly in preliminary studies, targeting H3K9me3 histone methyltransferase SUV39H1 by shRNA reactivated RTs, induced expression of cytosolic dsRNA/dsRNA sensors and interferon stimulated genes in H3K27M relative to H3WT DMG cell lines. This viral mimicry response was further amplified on concurrent inhibition of DNA methylation via Decitabine treatment. Therefore, our data indicates that targeting H3K9me3 could potentially be leveraged therapeutically to induce viral mimicry and immune-mediated tumor cell killing in H3K27M DMGs. Identifying synergistic viral mimicry-inducing combinations that inhibit complementary RT silencing mechanisms may become a formidable immune-based therapeutic avenue in the treatment of H3K27M DMG.