Recent studies have identified a Lys 27-to-methionine (K27M) mutation at one 
allele of H3F3A, one of the two genes encoding histone H3 variant H3.3, in 60% 
of high-grade pediatric glioma cases. The median survival of this group of 
patients after diagnosis is ∼1 yr. Here we show that the levels of H3K27 di- and 
trimethylation (H3K27me2 and H3K27me3) are reduced globally in H3.3K27M patient 
samples due to the expression of the H3.3K27M mutant allele. Remarkably, we also 
observed that H3K27me3 and Ezh2 (the catalytic subunit of H3K27 
methyltransferase) at chromatin are dramatically increased locally at hundreds 
of gene loci in H3.3K27M patient cells. Moreover, the gain of H3K27me3 and Ezh2 
at gene promoters alters the expression of genes that are associated with 
various cancer pathways. These results indicate that H3.3K27M mutation 
reprograms epigenetic landscape and gene expression, which may drive 
tumorigenesis.