A mutation in the E(Z) methyltransferase that increases trimethylation of histone H3 lysine 27 and causes inappropriate silencing of active Polycomb target genes

A de novo missense variant in EZH1 associated with developmental delay exhibits functional deficits in Drosophila melanogaster

EZH1, a polycomb repressive complex-2 component, is involved in a myriad of cellular processes. EZH1 represses transcription of downstream target genes through histone 3 lysine27 (H3K27) trimethylation (H3K27me3). Genetic variants in histone modifiers have been associated with developmental disorders, while EZH1 has not yet been linked to any human disease. However, the paralog EZH2 is associated with Weaver syndrome. Here we report a previously undiagnosed individual with a novel neurodevelopmental phenotype identified to have a de novo missense variant in EZH1 through exome sequencing. The individual presented in infancy with neurodevelopmental delay and hypotonia and was later noted to have proximal muscle weakness. The variant, p.A678G, is in the SET domain, known for its methyltransferase activity, and an analogous somatic or germline mutation in EZH2 has been reported in patients with B-cell lymphoma or Weaver syndrome, respectively. Human EZH1/2 are homologous to fly Enhancer of zeste (E(z)), an essential gene in Drosophila, and the affected residue (p.A678 in humans, p.A691 in flies) is conserved. To further study this variant, we obtained null alleles and generated transgenic flies expressing wildtype [E(z)WT] and the variant [E(z)A691G]. When expressed ubiquitously the variant rescues null-lethality similar to the wildtype. Overexpression of E(z)WT induces homeotic patterning defects but notably the E(z)A691G variant leads to dramatically stronger morphological phenotypes. We also note a dramatic loss of H3K27me2 and a corresponding increase in H3K27me3 in flies expressing E(z)A691G, suggesting this acts as a gain-of-function allele. In conclusion, here we present a novel EZH1 de novo variant associated with a neurodevelopmental disorder. Furthermore, we found that this variant has a functional impact in Drosophila.

A de novo missense variant in EZH1 associated with developmental delay exhibits functional deficits in Drosophila melanogaster

EZH1 ( Enhancer of Zeste, homolog 1) , a Polycomb Repressive Complex-2 (PRC2) component, is involved in a myriad of cellular processes through modifying histone 3 lysine27 (H3K27) residues. EZH1 represses transcription of downstream target genes through H3K27 trimethylation (H3K27me3). Genetic mutations in histone modifiers have been associated with developmental disorders, while EZH1 has not yet been linked to any human disease. However, the paralog EZH2 is associated with Weaver syndrome. Here we report a previously undiagnosed individual with a novel neurodevelopmental phenotype identified to have a de novo variant in EZH1 , p.Ala678Gly, through exome sequencing. The individual presented in infancy with neurodevelopmental delay and hypotonia and was later noted to have proximal muscle weakness. The variant, p.A678G, is in the SET domain, known for its methyltransferase activity, and was the best candidate variant found in the exome. Human EZH1 / 2 are homologous to fly Enhancer of zeste E(z) , an essential gene in flies, and the residue (A678 in humans, A691 in Drosophila ) is conserved. To further study this variant, we obtained Drosophila null alleles and generated transgenic flies expressing wild-type (E(z) WT ) and the variant (E(z) A691G ) . The E(z) A691G variant led to hyper H3K27me3 while the E(z) WT did not, suggesting this is as a gain-of-function allele. When expressed under the tubulin promotor in vivo the variant rescued null-lethality similar to wild-type but the E(z) A691G flies exhibit bang sensitivity and shortened lifespan. In conclusion, here we present a novel EZH1 de novo variant associated with a neurodevelopmental disorder. Furthermore, we found that this variant has a functional impact in Drosophila . Biochemically this allele leads to increased H3K27me3 suggesting gain-of-function, but when expressed in adult flies the E(z) A691G has some characteristics of partial loss-of-function which may suggest it is a more complex allele in vivo .

The molecular principles of gene regulation by Polycomb repressive complexes

Precise control of gene expression is fundamental to cell function and development. Although ultimately gene expression relies on DNA-binding transcription factors to guide the activity of the transcription machinery to genes, it has also become clear that chromatin and histone post-translational modification have fundamental roles in gene regulation. Polycomb repressive complexes represent a paradigm of chromatin-based gene regulation in animals. The Polycomb repressive system comprises two central protein complexes, Polycomb repressive complex 1 (PRC1) and PRC2, which are essential for normal gene regulation and development. Our early understanding of Polycomb function relied on studies in simple model organisms, but more recently it has become apparent that this system has expanded and diverged in mammals. Detailed studies are now uncovering the molecular mechanisms that enable mammalian PRC1 and PRC2 to identify their target sites in the genome, communicate through feedback mechanisms to create Polycomb chromatin domains and control transcription to regulate gene expression. In this Review, we discuss and contextualize the emerging principles that define how this fascinating chromatin-based system regulates gene expression in mammals.

Reevaluating the roles of histone-modifying enzymes and their associated chromatin modifications in transcriptional regulation

Histone-modifying enzymes are implicated in the control of diverse DNA-templated processes including gene expression. Here, we outline historical and current thinking regarding the functions of histone modifications and their associated enzymes. One current viewpoint, based largely on correlative evidence, posits that histone modifications are instructive for transcriptional regulation and represent an epigenetic 'code'. Recent studies have challenged this model and suggest that histone marks previously associated with active genes do not directly cause transcriptional activation. Additionally, many histone-modifying proteins possess non-catalytic functions that overshadow their enzymatic activities. Given that much remains unknown regarding the functions of these proteins, the field should be cautious in interpreting loss-of-function phenotypes and must consider both cellular and developmental context. In this Perspective, we focus on recent progress relating to the catalytic and non-catalytic functions of the Trithorax-COMPASS complexes, Polycomb repressive complexes and Clr4/Suv39 histone-modifying machineries.

The SERTAD protein Taranis plays a role in Polycomb-mediated gene repression

The Polycomb group (PcG) proteins have been implicated in epigenetic transcriptional repression in development, stem cell maintenance and in cancer. The chromodomain protein Polycomb (Pc) is a key member of the PcG. Pc binds to the histone mark, trimethylated histone 3 lysine 27 (H3K27me3), to initiate transcriptional repression. How PcG proteins are recruited to target loci is not fully understood. Here we show that the Drosophila SERTA domain protein Taranis (Tara) is involved in transcriptional regulation of Pc target genes. Embryos lacking Tara exhibit a partial homeotic transformation of cuticular the segments, a phenotype associated with the loss of Pc function. Moreover, Drosophila embryos homozygous for a tara hypomorphic allele also misexpress engrailed, a Pc-regulated gene, and this phenotype is associated with the loss of Pc binding to the cis response element in the engrailed enhancer. In relation to that, Pc recruitment is reduced on the salivary gland polytene chromosomes and specifically at the engrailed locus. These results suggest that Tara might be required for positioning Pc to a subset of its target genes.

Depletion of polycomb repressive complex 2 core component EED impairs fetal hematopoiesis

Polycomb repressive complex 2 (PRC2), a H3K27me3 methyltransferase complex, promotes the development of many organs by silencing ectopic transcription program. However, currently little is known about the role of PRC2 in blood and vascular development. In this study, we interrogated the function of embryonic ectoderm development (EED), a core PRC2 component, in both endothelial and hematopoietic tissues by inactivating a floxed murine EED allele with Tie2Cre, which catalyzes recombination in endothelial and hematopoietic lineages. Murine EED^fl/fl;Tie2Cre (EED^CKO) embryos died at embryonic day (E) 13.5. We did not observe structural abnormalities of blood vessels or cardiac valves, suggesting that EED is dispensable in endothelial cells for initial steps of vascular development. EED^CKO embryos were pale and had abnormal livers. Flow cytometry of fetal liver cells showed that EED depletion significantly impeded erythroid maturation. There was a corresponding increase in myeloid progenitors and granulocytes and macrophages, suggesting an attenuated differentiation path in myeloid lineages. Moreover, EED depletion impaired the generation of hematopoietic stem cells. Collectively, our study demonstrates that within Tie2Cre-recombined embryonic cells, EED is required for proper erythropoiesis and for formation of hematopoietic progenitor and stem cells, but is dispensable for endothelial lineage commitment and early vascular patterning.

Role of the Polycomb Repressive Complex 2 (PRC2) in Transcriptional Regulation and Cancer

The chromatin environment is modulated by a machinery of chromatin modifiers, required for the specification and maintenance of cell fate. Many mutations in the machinery have been linked to the development and progression of cancer. In this review, we give a brief introduction to Polycomb group (PcG) proteins, their assembly into Polycomb repressive complexes (PRCs) and the normal physiological roles of these complexes with a focus on the PRC2. We review the many findings of mutations in the PRC2 coding genes, both loss-of-function and gain-of-function, associated with human cancers and discuss potential molecular mechanisms involved in the contribution of PRC2 mutations to cancer development and progression. Finally, we discuss some of the recent advances in developing and testing drugs targeting the PRC2 as well as emerging results from clinical trials using these drugs in the treatment of human cancers.

Arabidopsis Flower and Embryo Developmental Genes are Repressed in Seedlings by Different Combinations of Polycomb Group Proteins in Association with Distinct Sets of Cis-regulatory Elements

Polycomb repressive complexes (PRCs) play crucial roles in transcriptional repression and developmental regulation in both plants and animals. In plants, depletion of different members of PRCs causes both overlapping and unique phenotypic defects. However, the underlying molecular mechanism determining the target specificity and functional diversity is not sufficiently characterized. Here, we quantitatively compared changes of tri-methylation at H3K27 in Arabidopsis mutants deprived of various key PRC components. We show that CURLY LEAF (CLF), a major catalytic subunit of PRC2, coordinates with different members of PRC1 in suppression of distinct plant developmental programs. We found that expression of flower development genes is repressed in seedlings preferentially via non-redundant role of CLF, which specifically associated with LIKE HETEROCHROMATIN PROTEIN1 (LHP1). In contrast, expression of embryo development genes is repressed by PRC1-catalytic core subunits AtBMI1 and AtRING1 in common with PRC2-catalytic enzymes CLF or SWINGER (SWN). This context-dependent role of CLF corresponds well with the change in H3K27me3 profiles, and is remarkably associated with differential co-occupancy of binding motifs of transcription factors (TFs), including MADS box and ABA-related factors. We propose that different combinations of PRC members distinctively regulate different developmental programs, and their target specificity is modulated by specific TFs.

Polycomb group proteins (PcGs) are essential for development in both animals and plants. Studies in plants are advantageous for elucidation of specific effects of PcGs during development, since most PcG mutants are viable in plants but not in animals. Previous efforts in genetic study of plant PcGs revealed that different PcGs have both common and unique effects on plant development, but the mechanisms underlying the specific regulation of different developmental programs by PcGs are still far from clear. In this study, we quantitatively compared the change in H3K27me3 and gene expression profiles between mutants of key PcG members on a genome-wide scale in Arabidopsis seedlings, and successfully unraveled different developmental programs that are specifically regulated by different combinations of PcGs. This context specific effect of PcGs is closely associated with different sets of transcription factor binding motifs. Together, we revealed on a genome-wide scale that different combinations of PcGs, as well as their association with the binding sites of different TFs, serve to explain the specific regulation of different developmental programs by PcGs.

Chromatin signatures of cancer

Studies by cancer genome consortiums have identified frequent mutations in chromatin regulatory factors and histone proteins in human cancer, implicating them as major mediators in the pathogenesis of both hematological malignancies and solid tumors. Here, Morgan and Shilatifard review recent advances in our understanding of the role of chromatin in cancer, focusing on transcriptional regulatory complexes, enhancer-associated factors, histone point mutations, and alterations in heterochromatin-interacting factors.

Changes in the pattern of gene expression play an important role in allowing cancer cells to acquire their hallmark characteristics, while genomic instability enables cells to acquire genetic alterations that promote oncogenesis. Chromatin plays central roles in both transcriptional regulation and the maintenance of genomic stability. Studies by cancer genome consortiums have identified frequent mutations in genes encoding chromatin regulatory factors and histone proteins in human cancer, implicating them as major mediators in the pathogenesis of both hematological malignancies and solid tumors. Here, we review recent advances in our understanding of the role of chromatin in cancer, focusing on transcriptional regulatory complexes, enhancer-associated factors, histone point mutations, and alterations in heterochromatin-interacting factors.

Occupying chromatin: Polycomb mechanisms for getting to genomic targets, stopping transcriptional traffic, and staying put

Chromatin modification by Polycomb proteins provides an essential strategy for gene silencing in higher eukaryotes. Polycomb repressive complexes (PRCs) silence key developmental regulators and are centrally integrated in the transcriptional circuitry of stem cells. PRC2 trimethylates histone H3 on lysine 27 (H3K27me3), and PRC1-type complexes ubiquitylate histone H2A and compact polynucleosomes. How PRCs are deployed to select and silence genomic targets is the subject of intense investigation. We review advances on targeting, modulation, and functions of PRC1 and PRC2 and progress on defining the transcriptional steps they impact. Recent findings emphasize PRC1 targeting independent of H3K27me3, nonenzymatic PRC1-mediated compaction, and connections between PRCs and noncoding RNAs. Systematic analyses of Polycomb complexes and associated histone modifications during DNA replication and mitosis have also emerged. The stage is now set to reveal fundamental epigenetic mechanisms that determine how Polycomb target genes are silenced and how Polycomb silence is preserved through cell-cycle progression.

A complex Polycomb issue: the two faces of EZH2 in cancer

In the April 1, 2012, issue of Genes & Development, Simon and colleagues (pp. 651-656) demonstrated that the disruption of Ezh2 in mice is sufficient to cause T-acute lymphoblastic leukemia (T-ALL). Moreover, in concert with concurrent studies, the authors revealed that similar mechanisms are involved in human T-ALL. These data contrast with previous findings showing that increased EZH2 activity promotes cancer.

Inner workings and regulatory inputs that control Polycomb repressive complex 2

Polycomb repressive complex 2 (PRC2) is a conserved multisubunit enzyme that methylates histone H3 on lysine-27. This chromatin modification is a hallmark of target genes transcriptionally silenced by the Polycomb system. At its core, PRC2 activity depends upon the SET domain active site of its catalytic subunit, EZH2, as well as critical stimulatory inputs from noncatalytic subunits, especially EED and SU(Z)12. We review recent progress on this core PRC2 machinery, including key features of the active site, control mechanisms that operate via EZH2 phosphorylation, and subunit elements and architectures that influence PRC2 function. Among these, we highlight work identifying an EED regulatory site that enables PRC2 to bind pre-existing methylated H3-K27 and stimulate enzyme output. These advances illuminate basic inner workings of PRC2 and also provide insights that could aid design of PRC2 inhibitors. The chromatin landscape that PRC2 encounters in vivo is decorated with many histone modifications that accompany active transcription, such as H3-K4 methylation. It has long been assumed that these "active" modifications oppose PRC2 at some level but, until recently, mechanisms of this antagonistic cross-talk have been elusive. We discuss new findings that illuminate how H3-K4 and H3-K36 methylation, H3-K27 acetylation, and H3-S28 phosphorylation each exert a negative impact on PRC2 function. The emerging picture presents PRC2 as a cooperative multipart machine, intricately outfitted to sense and respond to the local chromatin environment and other cues. This PRC2 design ensures flexibility and fine tuning of its fundamental gene silencing roles in diverse biological contexts.