BRD- and EHMT-dependent immunosuppressive transcriptome in triple-mutant Astrocytoma

Astrocytomas (WHO grade II and III) primarily occur in early-middle adult life and frequently progress to more aggressive secondary GBM despite best standard-of-care therapy. Developing more effective therapeutic strategies requires a more complete understanding of the mechanisms employed by astrocytoma cells to escape anti-tumor immunity. The majority (~80%) of grade II & III astrocytoma carry mutations in isocitrate dehydrogenase (e.g., IDH1 R132H ), which typically occurs in combination with p53 mutation and ATRX inactivation (i.e., IDH1 R132H /p53 mut /ATRX loss, triple-mut for simplicity), indicating that this triple mutation background has an important role in tumor growth and tumor microenvironment reprogramming potentially including immune evasion mechanisms. Our recent findings show that triple-mut induces multiple pathways with anti-tumor immune modulating potential including checkpoint ligands, IFN-gamma signaling insensitivity, down-regulation of MHC-I and -II antigen presentation pathways (MHC APP), and dysregulated cytokine/chemokine profiles that modulate the astrocytoma immune microenvironment. Furthermore, we found that the components of the immune-modulating transcriptome associated with triple-mut astrocytom are augmented by standard-of-care radiation (IR) and temozolomide (TMZ). Epigenetic mechanisms orchestrate the tumor cell and the immunosuppressive microenvironment. BET proteins as epigenetic readers regulate gene expression by binding to acetylated lysine residues and/or protein partners, while EHMT1/2 as histone-lysine-N-methyltransferases associated with epigenetic silencing of genes. We show that BRD- and EHMT-dependent histone modifications are elevated in triple-mut astrocytoma cells and together generate the immune-modulating transcriptome. Pan-BET bromodomain inhibitors (e.g., JQ1) significantly inhibited the immunosuppressive transcriptome associated with triple-mut astrocytoma and induced by IR/TMZ, and the pan -EHMT inhibitor UNC0642 significantly up-regulated MHC APP genes and IFN signaling-related genes. SIGNIFICANCE: our findings identify targetable epigenetic determinants that induce an immunosuppressive transcriptome associated with the immunosuppressive microenvironment in triple-mut astrocytoma.