Abstract 3006: Oncogenic activity of H3.3 K27M in a spontaneous DIPG mouse model

Abstract

Abstract Pediatric diffuse high-grade gliomas (HGG) cause a devastatingly poor outcome. Diffuse intrinsic pontine gliomas (DIPG) are brainstem HGGs found almost exclusively in children and represent approximately 50 percent of all pediatric HGGs. Recurrent, clonal somatic mutations in histone H3 are a molecular hallmark that distinguish the genetic mechanisms underlying pediatric and adult HGG, and indicate a strong link between epigenetic dysfunction and pediatric brain tumorigenesis. H3 K27M mutations found in nearly 80 percent of DIPGs and over half of non-brainstem HGGs occurring in thalamus or other midline structures, induce a dominant loss of genome-wide H3K27me3. In contrast, mutually exclusive 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 and investigate how they contribute to oncogenesis, we generated conditionally activated, epitope-tagged knock-in mice to express K27M, G34R or non-mutated H3.3 proteins from the endogenous H3f3a locus. 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 PDGFRα in addition to H3.3 K27M, and mice with induced brain-specific Trp53 deletion plus mutated PDGFRαV544ins expression developed spontaneous HGG in vivo. Induction of H3.3 K27M significantly accelerated PDGFRαV544ins;Trp53 knockout HGG formation involving the brainstem. Importantly, the non-mutated H3.3 epitope-tagged protein did not affect tumor burden or latency. The H3.3 K27M tumors recapitulate characteristics observed in human DIPG including histopathological features, strong nuclear K27M expression and robust loss of H3K27me3 throughout the tumors. We relate the genetic, epigenetic and transcriptional landscapes of these models to primary pediatric DIPG to help identify their developmental origins and oncogenic mechanisms downstream of histone H3.3 K27M. Citation Format: Jon D. Larson, Lawryn H. Kasper, Gang Wu, Yiping Fan, David Finkelstein, Hongjian Jin, Barbara S. Paugh, Andre B. Silveira, Beisi Xu, Xiaoyan Zhu, Junyuan Zhang, Helen R. Russell, Peter J. McKinnon, David W. Ellison, Jinghui Zhang, Suzanne J. Baker. Oncogenic activity of H3.3 K27M in a spontaneous DIPG mouse model [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 3006. doi:10.1158/1538-7445.AM2017-3006