HG-116: ONCOGENIC ACTIVITY OF HISTONE MUTATIONS IN THE MAMMALIAN BRAIN

Pediatric diffuse high-grade gliomas (HGGs) confer a devastatingly poor outcome. Diffuse intrinsic pontine gliomas (DIPGs), brainstem HGGs found almost exclusively in children, represent approximately 50 percent of all pediatric HGGs. Recurrent somatic mutations in histone H3 distinguish the genetic mechanisms underlying pediatric and adult HGG. H3 K27M mutations are found in nearly 80 percent of DIPGs and over half of non-brainstem HGGs occurring in thalamus or other midline structures. In contrast, H3.3 G34R/V mutations occur in approximately 14 percent of cortical HGG tumors in older children through young adulthood suggesting a distinct developmental origin compared to K27M mutant tumors. DNA methylation and gene expression patterns distinguish G34R/V and K27M tumors, but the oncogenic mechanisms and reasons for brain region selectivity of histone mutations remain uncertain. To study these mutations in the developing mammalian brain, we generated conditionally activated, epitope-tagged knock-in mice to express K27M, G34R or non-mutated H3.3 proteins from the endogenous H3f3a locus. Interrogation of untransformed embryonic neural precursor or astrocyte cells expressing H3.3 K27M demonstrated H3K27me3 loss and a growth advantage in vitro with brain region-specific, stage-dependent expression and epigenetic signatures. DIPGs frequently harbor genetic alterations in Tp53 and PDGFRa in addition to H3.3 K27M, and mice with induced brain-specific Trp53 deletion, and mutated PDGFRa^V544ins expression developed HGG in vivo. Induction of H3.3 K27M significantly accelerated PDGFRa^V544ins;Trp53 knockout HGG formation involving the brainstem. We will relate the genetic, epigenetic and transcriptional landscapes of these models to primary pediatric HGG to identify oncogenic mechanisms downstream of histone H3 mutations.