PM-13 * BIOLOGICAL CONSEQUENCES OF HISTONE MUTATIONS IN THE MAMMALIAN BRAIN

Abstract

Pediatric diffuse high-grade gliomas (HGG) account for 20 percent of primary brain tumors in children and have a devastating outcome with a 2-year survival rate of 10-30 percent. Diffuse intrinsic pontine gliomas (DIPG), brainstem HGG found almost exclusively in children, represent approximately 50 percent of all pediatric HGG. Frequent recurrent somatic mutations in histone H3 distinguish the genetic drivers among pediatric and adult HGG. Mutually exclusive somatic heterozygous H3F3A or HIST1H3B mutations encoding a K27M variant of histone H3.3 or H3.1, respectively, are found in nearly 80 percent of DIPGs and 22 percent of non-brainstem HGG, where they occur predominantly in thalamic and other midline tumors. Strikingly, heterozygous H3F3A mutations encoding histone H3.3 G34R or G34V occur in approximately 14 percent of non-brainstem HGG, are associated predominantly with cortical tumors arising in older children through young adulthood, and suggest a distinct developmental origin compared to the majority of tumors harboring K27M mutations. DNA methylation signatures and gene expression patterns distinguish G34R/V and K27M tumors, but the oncogenic mechanisms and brain region selectivity of these 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. When activated in neural and glial precursor cells, extensive H3f3a expression was observed throughout the embryonic brain, and H3.3-K27M induces loss of H3K27me3, as has been shown in DIPGs. Notably, we also found that neural progenitor cells derived from embryonic hindbrain and forebrain expressing H3.3-K27M demonstrate a striking in vitro growth advantage compared to H3.3-G34R or H3.3-WT expressing neural progenitor cells. We will present an analysis of the epigenetic landscape and gene expression patterns from various brain regions expressing these different histone H3 mutations as well as a phenotypic analysis of the mutant mice.