HGG-40. UNRAVELING THE IMMUNOLOGIC VULNERABILITIES OF DIFFUSE HEMISPHERIC GLIOMA, H3 G34-MUTANT

Abstract

Abstract BACKGROUND Insights into the pathogenesis of diffuse hemispheric glioma, H3 G34-mutant (DHG-G34), an incurable high-grade glioma, have opened new therapeutic possibilities. The DHG-G34 oncohistone mutation impairs the epigenetic regulator SETD2, which normally orchestrates DNA-mismatch-repair and negatively regulates Polycomb Repressive Complex 2 (PRC2). In DHG-G34, a mismatch-repair-deficiency phenotype is predicted, but an anti-tumor immune response is notably absent, which we hypothesize is linked to the aberrant transcriptional repression by PRC2. Here, we investigate DHG-G34 immunosuppression mechanisms in a real-world multi-omics cohort and cell line model. METHODS Clinical, molecular, and immunologic characteristics of a DHG-G34 cohort, sequenced at Caris Life Sciences, are contrasted against diffuse midline glioma (DMG) and pediatric low-grade glioma (LGG). KNS-42 DHG-G34 cell line was exposed to valemetostat (a EZH1/2 inhibitor) or DMSO in-vitro. Flow cytometry and RNA-sequencing were utilized to determine differential immunologic characteristics. RESULTS 29 DHG-G34, 51 DMG, and 52 LGG were identified. DHG-G34 median OS was 15.4 mo., like DMG (HR=1.1, p=0.686). Common DHG-G34 alterations were TP53 (93.1%), ATRX (69.0%), PDGFRA (31.0%). Tumor mutational burden (TMB) was high, >=10mt/MB, in 10% of DHG-G34, with 0 DMG or LGG events (p<0.01, q>0.05), but innate and adaptive immune fractions and antigen-presenting genes were decreased (fold-change: 0.19–0.83, q<0.05). In KNS-42, PRC2 inhibition upregulated MHC class 1 (p<0.001). Of 402 missense variants, 43 were upregulated following EHZ2 inhibition, and 4 were downregulated (q<0.05). Among immunostimulatory genes, P2RX7, which engages T-cells, was downregulated in patient samples (q<0.001) and upregulated in EZH2 inhibited KNS-42 (q<0.001). CONCLUSIONS DHG-G34 has a propensity for high TMB but remains immunologically cold. Our pilot experiment suggests inhibiting PRC2’s unique role in DHG-G34 may sensitize DHG-G34 to an immune response by inducing MHC class 1 expression and transcriptional read-through of mutated transcripts. Understanding whether treatment induces TMB in DHG-G34 may affect timing of an immunotherapeutic intervention.