Abstract B09: Epigenetic changes mediated by H3.3 G34R mutation in a CRISPR-edited pediatric glioblastoma cell line

Abstract Background: Pediatric glioblastomas (pGBM) are aggressive and lethal brain tumors with few effective treatment options. Since the discovery of recurrent histone H3 mutations in pGBMs, multiple groups have sought to uncover the mechanism by which these mutations drive tumorigenesis. One striking aspect of histone H3 mutations is their tumor location and age-specificity: the K27M mutation occurs in midline brain structures in infants and young children, whereas the G34R/V mutations occur in the cerebral hemisphere of adolescents. Efforts to elucidate the mechanism underlying K27M tumorigenesis have centered on K27M-mediated inhibition of the PRC2 complex and the resulting drastic loss of the repressive H3K27me3 mark in these tumors. Studies have shown that G34 mutations affect methylation of the adjacent K36 residue, resulting in a local decrease of the active H3K36me3 mark on the mutant histone. It remains uncertain whether the G34R/V mutations promote global changes in the epigenome that drive tumorigenesis. Methods: To investigate the epigenetic consequences of the G34R mutation, we generated an isogenic in vitro model using CRISPR/Cas9 to correct the G34R mutation in a pGBM-derived cell line. We then comprehensively characterized and compared the epigenome and transcriptome of CRISPR-unedited clones (carrying the G34R mutation) and CRISPR-edited (wild-type) clones. Using ChIP-seq, we profiled H3K36me3 and H3K27me3 marks to study G34R-mediated regulation of these antagonistic marks. Results: ChIP-seq profiling revealed that CRISPR editing of the G34R mutation had minimal effects on total levels or genomic distribution and profile of H3K36me3 and H3K27me3 marks. In contrast, we previously showed that CRISPR editing of the K27M mutation in pGBM cell lines rescued total H3K27me3 levels and genomic distribution to that of histone wild-type pGBMs. In agreement with their similar epigenetic profiles, there were very few transcriptomic differences between G34R and wildtype cells. Conclusions: Our results suggest that the epigenome in G34R-mutant pGBM cells may be less dynamic or amenable to epigenetic perturbation from removal of the initial driver oncomutation than K27M-mutant tumors. These findings underscore that different strategies may be required to target and treat G34R and K27M-mutant pGBM tumors. Citation Format: Shriya Deshmukh, Carol C.L. Chen, Amira Ouanouki, Nada Jabado. Epigenetic changes mediated by H3.3 G34R mutation in a CRISPR-edited pediatric glioblastoma cell line [abstract]. In: Proceedings of the AACR Special Conference on the Advances in Pediatric Cancer Research; 2019 Sep 17-20; Montreal, QC, Canada. Philadelphia (PA): AACR; Cancer Res 2020;80(14 Suppl):Abstract nr B09.

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