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Driver mutations in genes encoding histone H3 proteins resulting in p.Lys27Met substitutions (H3-K27M) are frequent in pediatric midline brain tumors. However, the precise mechanisms by which H3-K27M causes tumor initiation remain unclear. Here, we use human hindbrain neural stem cells to model the consequences of H3.3-K27M on the epigenomic landscape in a relevant developmental context. Genome-wide mapping of epitope-tagged histone H3.3 revealed that both the wild type and the K27M mutant incorporate abundantly at pre-existing active enhancers and promoters, and to a lesser extent at Polycomb repressive complex 2 (PRC2)-bound regions. At active enhancers, H3.3-K27M leads to focal H3K27ac loss, decreased chromatin accessibility and reduced transcriptional expression of nearby neurodevelopmental genes. In addition, H3.3-K27M deposition at a subset of PRC2 target genes leads to increased PRC2 and PRC1 binding and augmented transcriptional repression that can be partially reversed by PRC2 inhibitors. Our work suggests that, rather than imposing de novo transcriptional circuits, H3.3-K27M drives tumorigenesis by locking initiating cells in their pre-existing, immature epigenomic state, via disruption of PRC2 and enhancer functions.

Polycomb group proteins are repressive chromatin modifiers with essential roles in metazoan development, cellular differentiation and cell fate maintenance. How Polycomb proteins access active chromatin to confer transcriptional silencing during lineage transitions remains unclear. Here we show that the Polycomb repressive complex 2 (PRC2) component PHF19 binds trimethylated histone H3 Lys36 (H3K36me3), a mark of active chromatin, via its Tudor domain. PHF19 associates with the H3K36me3 demethylase NO66, and it is required to recruit the PRC2 complex and NO66 to stem cell genes during differentiation, leading to PRC2-mediated trimethylation of histone H3 Lys27 (H3K27), loss of H3K36me3 and transcriptional silencing. We propose a model whereby PHF19 functions during mouse embryonic stem cell differentiation to transiently bind the H3K36me3 mark via its Tudor domain, forming essential contact points that allow recruitment of PRC2 and H3K36me3 demethylase activity to active gene loci during their transition to a Polycomb-repressed state.

Cellular senescence is an irreversible arrest of proliferation. It is activated when a cell encounters stress such as DNA damage, telomere shortening or oncogene activation. Like apoptosis, it impedes tumour progression and acts as a barrier that pre-neoplastic cells must overcome during their evolution toward the full tumourigenic state. This review focuses on the role of transcriptional regulators in the control of cellular senescence, explores how their function is perturbed in cancer and discusses the potential to harness this knowledge for future cancer therapies.

Organization of chromatin by epigenetic mechanisms is essential for establishing and maintaining cellular identity in developing and adult organisms. A key question that remains unresolved about this process is how epigenetic marks are transmitted to the next cell generation during cell division. Here we provide a model to explain how trimethylated Lys 27 of histone 3 (H3K27me3), which is catalysed by the EZH2-containing Polycomb Repressive Complex 2 (PRC2), is maintained in proliferating cells. We show that the PRC2 complex binds to the H3K27me3 mark and colocalizes with this mark in G1 phase and with sites of ongoing DNA replication. Efficient binding requires an intact trimeric PRC2 complex containing EZH2, EED and SUZ12, but is independent of the catalytic SET domain of EZH2. Using a heterologous reporter system, we show that transient recruitment of the PRC2 complex to chromatin, upstream of the transcriptional start site, is sufficient to maintain repression through endogenous PRC2 during subsequent cell divisions. Thus, we suggest that once the H3K27me3 is established, it recruits the PRC2 complex to maintain the mark at sites of DNA replication, leading to methylation of H3K27 on the daughter strands during incorporation of newly synthesized histones. This mechanism ensures maintenance of the H3K27me3 epigenetic mark in proliferating cells, not only during DNA replication when histones synthesized de novo are incorporated, but also outside S phase, thereby preserving chromatin structure and transcriptional programs.

Nature Cell Biol. 10, 1291–1300 (2008); published online 19 October 2008; corrected after print 28 October 2008 In the version of this article initially published, the order of the labels siRNA cyclophilin B and siRNA SUZ12 in figure 5 and GAL4-EZH2WT and GAL4-EZH2ΔSET in Figure 7a were reversed. The corrected panels are shown below.