Sequencing of pediatric gliomas has identified missense mutations Lys27Met (K27M) and Gly34Arg/Val (G34R/V) in genes encoding histone H3.3 (H3F3A) and H3.1 (HIST3H1B). We report that human diffuse intrinsic pontine gliomas (DIPGs) containing the K27M mutation display significantly lower overall amounts of H3 with trimethylated lysine 27 (H3K27me3) and that histone H3K27M transgenes are sufficient to reduce the amounts of H3K27me3 in vitro and in vivo. We find that H3K27M inhibits the enzymatic activity of the Polycomb repressive complex 2 through interaction with the EZH2 subunit. In addition, transgenes containing lysine-to-methionine substitutions at other known methylated lysines (H3K9 and H3K36) are sufficient to cause specific reduction in methylation through inhibition of SET-domain enzymes. We propose that K-to-M substitutions may represent a mechanism to alter epigenetic states in a variety of pathologies.

Diffuse Intrinsic Pontine Glioma (DIPG) is a fatal brain cancer that arises in the brainstem of children with no effective treatment and near 100% fatality. The failure of most therapies can be attributed to the delicate location of these tumors and choosing therapies based on assumptions that DIPGs are molecularly similar to adult disease. Recent studies have unraveled the unique genetic make-up of this brain cancer with nearly 80% harboring a K27M-H3.3 or K27M-H3.1 mutation. However, DIPGs are still thought of as one disease with limited understanding of the genetic drivers of these tumors. To understand what drives DIPGs we integrated whole-genome-sequencing with methylation, expression and copy-number profiling, discovering that DIPGs are three molecularly distinct subgroups (H3-K27M, Silent, MYCN) and uncovering a novel recurrent activating mutation in the activin receptor ACVR1, in 20% of DIPGs. Mutations in ACVR1 were constitutively activating, leading to SMAD phosphorylation and increased expression of downstream activin signaling targets ID1 and ID2. Our results highlight distinct molecular subgroups and novel therapeutic targets for this incurable pediatric cancer.

Preface We have extended our understanding of the molecular biology underlying adult glioblastoma over many years. In contrast, high-grade gliomas in children and adolescents have remained a relatively under-investigated disease. The latest large-scale genomic and epigenomic profiling studies have yielded an unprecedented abundance of novel data and revealed deeper insights into gliomagenesis across all age groups, highlighting key distinctions, but also some commonalities. As we are on the verge of dissecting glioblastomas into meaningful biological subgroups, this Review summarizes the hallmark genetic alterations associated with distinct epigenetic features and patient characteristics in both paediatric and adult disease, and examines the complex interplay between the glioblastoma genome and epigenome.

Medulloblastomas are brain tumors that arise in the cerebellum of children and contain stem cells in a perivascular niche thought to give rise to recurrence following radiation. We used several mouse models of medulloblastomas in parallel to better understand how the critical cell types in these tumors respond to therapy. In our models, the proliferating cells in the tumor bulk undergo radiation-induced, p53-dependent apoptotic cell death. Activation of Akt signaling via PTEN loss transforms these cells to a nonproliferating extensive nodularity morphology. By contrast, the nestin-expressing perivascular stem cells survive radiation, activate PI3K/Akt pathway, undergo p53-dependent cell cycle arrest, and re-enter the cell cycle at 72 h. Furthermore, the ability of these cells to induce p53 is dependent on the presence of PTEN. These cellular characteristics are similar to human medulloblastomas. Finally, inhibition of Akt signaling sensitizes cells in the perivascular region to radiation-induced apoptosis.[Keywords: p53; PTEN; PI3K/Akt; medulloblastoma] Supplemental material is available at http://www.genesdev.org. Received October 17, 2007; revised version accepted December 17, 2007. Previous studies have shown the existence of a small subpopulation of cells in brain tumors that share key characteristics with neuronal stem/progenitor cells. These findings suggest that brain tumors contain "cancer stem cells" that may be critical for tumorigenesis (Ignatova et al. 2002;Hemmati et al. 2003;Singh et al. 2003). Brain tumor stem cells positive for nestin and CD133 occupy a perivascular niche (PVN) and a disruption of that microenvironment ablates the self-renewing cell population in brain tumors and arrests tumor growth (Calabrese et al. 2007). Nestin has been shown to be a strong prognostic factor for glioma malignancy (Strojnik et al. 2007). Furthermore, brain tumor cells expressing the stem cell marker CD133 have been shown to be relatively resistant to radiation by preferential activation of the DNA damage response (Bao et al. 2006). However, the molecular pathways governing such stem-like behavior remain largely elusive.Many critical questions surrounding the biology of therapeutic response in tumor stem-like cells remain to be answered. For example, are the stem-like cells in the niche the ones resistant to therapy, and does the niche provide additional protection for the stem-like cells that is independent of the DNA damage response? Does the relatively low cycling of stem-like cells contribute to their resistant character, or does radiation cause cell cycle arrest in the stem-like cells that are cycling at the time of treatment? Furthermore, what is the time interval between radiation treatment and the point when the stem-like cells re-enter the cell cycle? Do the mutations commonly found in some types of brain tumors, such as p53 or PTEN loss, affect the therapeutic response of either the resistant stem-like cells or the non-stem-like cell populations? Finally, are there signaling pathways that contribut...