Pediatrichigh-grade gliomas (pHGGs) are one of the most aggressive forms of brain cancer, with a median survival (MS) of ~18 months 1,2. The current standard of care for pHGG, consisting of tumor resection followed by radiation and chemotherapy, elicits only a modest increase in MS 1-4. The main reasons for the limited therapeutic outcomes are tumor recurrence, caused by the infiltrative nature of pHGG, and the development of an immune-suppressive tumor microenvironment (TME). One of the main subtypes of hemispherical pHGG encodes for G34R/V substitutions in the histone H3F3A 4, along with ATRX and TP53 inactivating mutations. The investigation of the molecular pathways which play a role in the pathogenesis of pHGG requires accurate mouse models which recapitulate the salient features of pHGG and develop within the brain's microenvironment in an immune-competent host. Our lab created genetically engineered immune competent pHGG mouse models employing the Sleeping Beauty (SB) transposase system 5,6. Tumors harbor genetic lesions encountered in a subtype of pHGG, i.e., H3.3G34R co-expressed with ATRX and TP53 knock down. The host in this pHGG model exhibits an intact immune system, thus enabling detailed mechanistic studies on all aspects of pHGG biology in vivo, including interactions with the TME immune cells. Our preliminary RNA-Seq data on H3.3-G34R versus H3.3-Wt pHGG revealed downregulation of gene ontologies (GO) related to DNA Damage Response (DDR) in H3.3-G34R pHGG. We also observed upregulation of GOs related to the activation of the immune response, such as “Regulation of Immune Response” and “Type I Interferon Production”, in the H3.3-G34R pHGG model. Herein we propose to determine the cellular andmolecular mechanisms by which H3.3-G34R regulates the response to radiotherapy and DDR inhibition in mouse and human H3.3G34R pHGG cells in vitro and in vivo. We aim to elucidate the role played by H3.3-G34R in DNA repair processes, responseto DNA damaging agents, and in TME immune cells' reprogramming. We propose to test the hypothesis that H3.3G34R reshapes the epigenetic landscape, resulting in alterations in chromatin states and transcriptional changes. These alterations cause DDR impairment and induce genomic instability, which in turn leads to cGAS-STING- Pathway-mediated activation of the immune system within the H3.3G34R pHGG TME. We will assess chromatin states by ATAC-seq, and establish whether reduced chromatin accessibility impairs DNA repair in G34R pHGG. We will also evaluate whether genomic instability in G34R pHGG mediates the immune system activation via cGAS-STING. We will also define at the molecular level the phenotypically diverse tumor and infiltrating immune cell clusters within the H3.3-G34R pHGG microenvironment using scRNA-seq; this will allow us to uncover mechanisms of therapeutic resistance. This information, will enable uncovering genetic makeup- tailored therapeutic modalities for H3.3-G34R pHGG, such as cell cycle checkpoint or DDR inhibitors combined with radiotherapy.

Project Narrative Pediatric high-grade gliomas (pHGGs) are one of the most common and aggressive forms of brain cancer, with a median survival of 9-15 months. We aim to study a subtype of pHGG which harbors H3.3-G34R, ATRX and TP53 inactivating mutations. Our goal is to assess the molecular mechanisms by which H3.3-G34R impacts the biology of pHGGs, influence the tumor immune microenvironment and the response to therapeutics, including immune-mediated mechanisms.