The lysine 27 to methionine mutation of histone H3.3 (H3.3K27M) is detected in over 75% of diffuse intrinsic pontine glioma (DIPG). The H3.3K27M mutant proteins inhibit H3K27 methyltransferase complex PRC2, resulting in a global reduction of tri-methylation of H3K27 (H3K27me3). Paradoxically, high levels of H3K27me3 were also detected at hundreds of genomic loci. However, it is not known how and why H3K27me3 is redistributed in DIPG cells. Here we show that lower levels of H3.3K27M mutant proteins at some genomic loci contribute to the retention of H3K27me3 peaks. But more importantly, Jarid2, a PRC2-associated protein, strongly correlates the presence of H3K27me3 and relieves the H3.3K27M-mediated inhibition in vivo and in vitro. Furthermore, we show that H3K27me3-mediated silencing of tumor suppressor gene Wilms Tumor 1 (WT1) supports the proliferation of DIPG cells and reaction of WT1 inhibits DIPG proliferation. Together, these studies reveal mechanisms whereby H3K27me3 is retained in the environment of global loss of this mark, and how persistence of this mark contributes to DIPG tumorigenesis.