Histone H3.3 G34 mutations promote aberrant PRC2 activity and drive tumor progression [RNA-Seq]

A remarkably high frequency of pediatric gliomas and bone tumors harbor monoallelic, missense mutations in genes encoding histone variant H3.3 at Glycine 34. We find that G34 mutations distinctly impede H3K36 methylation by SETD2. Consequently, the reduction of H3K36 methylation leads to high levels of PRC2-mediated H3K27me3 on G34 mutant nucleosomes. We find that H3.3G34W expression in murine mesenchymal stem cells (mMSCs) promotes aberrant gain of H3K27me2/3 and loss of H3K27ac at active enhancers containing SETD2. mMSCs expressing H3.3G34W display a unique transcriptional program that is dependent on PRC2 activity *in cis* and exhibit enhanced tumor development *in vivo*. Altogether, we demonstrate that H3.3G34 oncohistones selectively promote PRC2 activity by interfering with SETD2-mediated H3K36 methylation. We propose that PRC2-mediated silencing of enhancers involved in cell differentiation represents a potential mechanism that drives these malignancies. To understand the transcriptional profile changes mediated by H3.3 G34 mutations in mouse mesenchymal stem cells Expression of various histone transgenes in C3H10T1/2 cells, followed by mRNA-Seq