Polycomb repressive complex 2 component Suz12 is required for hematopoietic stem cell function and lymphopoiesis

Characterization of gene regulatory networks underlying key properties in human hematopoietic stem cell ontogeny

Human hematopoiesis starts at early yolk sac and undergoes site- and stage-specific changes over development. The intrinsic mechanism underlying property changes in hematopoiesis ontogeny remains poorly understood. Here, we analyzed single-cell transcriptome of human primary hematopoietic stem/progenitor cells (HSPCs) at different developmental stages, including yolk-sac (YS), AGM, fetal liver (FL), umbilical cord blood (UCB) and adult peripheral blood (PB) mobilized HSPCs. These stage-specific HSPCs display differential intrinsic properties, such as metabolism, self-renewal, differentiating potentialities etc. We then generated highly co-related gene regulatory network (GRNs) modules underlying the differential HSC key properties. Particularly, we identified GRNs and key regulators controlling lymphoid potentiality, self-renewal as well as aerobic respiration in human HSCs. Introducing selected regulators promotes key HSC functions in HSPCs derived from human pluripotent stem cells. Therefore, GRNs underlying key intrinsic properties of human HSCs provide a valuable guide to generate fully functional HSCs in vitro.

DNA methylation-dependent and -independent binding of CDX2 directs activation of distinct developmental and homeostatic genes

Precise spatiotemporal and cell type-specific gene expression is essential for proper tissue development and function. Transcription factors (TFs) guide this process by binding to developmental stage-specific targets and establishing an appropriate enhancer landscape. In turn, DNA and chromatin modifications direct the genomic binding of TFs. However, how TFs navigate various chromatin features and selectively bind a small portion of the millions of possible genomic target loci is still not well understood. Here we show that Cdx2 - a pioneer TF that binds distinct targets in developing versus adult intestinal epithelial cells - has a preferential affinity for a non-canonical CpG-containing motif in vivo. A higher frequency of this motif at embryonic and fetal Cdx2 target loci and the specifically methylated state of the CpG during development allows selective Cdx2 binding and activation of developmental enhancers and linked genes. Conversely, demethylation at these enhancers prohibits ectopic Cdx2 binding in adult cells, where Cdx2 binds its canonical motif without a CpG. This differential Cdx2 binding allows for corecruitment of Ctcf and Hnf4, facilitating the establishment of intestinal superenhancers during development and enhancers mediating adult homeostatic functions, respectively. Induced gain of DNA methylation in the adult mouse epithelium or cultured cells causes ectopic recruitment of Cdx2 to the developmental target loci and facilitates cobinding of the partner TFs. Together, our results demonstrate that the differential CpG motif requirements for Cdx2 binding to developmental versus adult target sites allow it to navigate different DNA methylation profiles and activate cell type-specific genes at appropriate times.

Research advances of polycomb group proteins in regulating mammalian development

Polycomb group (PcG) proteins are a subset of epigenetic factors that are highly conserved throughout evolution. In mammals, PcG proteins can be classified into two muti-proteins complexes: Polycomb repressive complex 1 (PRC1) and PRC2. Increasing evidence has demonstrated that PcG complexes play critical roles in the regulation of gene expression, genomic imprinting, chromosome X-inactivation, and chromatin structure. Accordingly, the dysfunction of PcG proteins is tightly orchestrated with abnormal developmental processes. Here, we summarized and discussed the current knowledge of the biochemical and molecular functions of PcG complexes, especially the PRC1 and PRC2 in mammalian development including embryonic development and tissue development, which will shed further light on the deep understanding of the basic knowledge of PcGs and their functions for reproductive health and developmental disorders.

Epigenetic Mechanisms in Hematologic Aging and Premalignant Conditions

Hematopoietic stem cells (HSCs) are essential for maintaining overall health by continuously generating blood cells throughout an individual's lifespan. However, as individuals age, the hematopoietic system undergoes significant functional decline, rendering them more susceptible to age-related diseases. Growing research evidence has highlighted the critical role of epigenetic regulation in this age-associated decline. This review aims to provide an overview of the diverse epigenetic mechanisms involved in the regulation of normal HSCs during the aging process and their implications in aging-related diseases. Understanding the intricate interplay of epigenetic mechanisms that contribute to aging-related changes in the hematopoietic system holds great potential for the development of innovative strategies to delay the aging process. In fact, interventions targeting epigenetic modifications have shown promising outcomes in alleviating aging-related phenotypes and extending lifespan in various animal models. Small molecule-based therapies and reprogramming strategies enabling epigenetic rejuvenation have emerged as effective approaches for ameliorating or even reversing aging-related conditions. By acquiring a deeper understanding of these epigenetic mechanisms, it is anticipated that interventions can be devised to prevent or mitigate the rates of hematologic aging and associated diseases later in life. Ultimately, these advancements have the potential to improve overall health and enhance the quality of life in aging individuals.

Functional and molecular profiling of hematopoietic stem cells during regeneration

Hematopoietic stem cells (HSCs) enable hematopoietic stem cell transplantation (HCT) through their ability to replenish the entire blood system. Proliferation of HSCs is linked to decreased reconstitution potential, and a precise regulation of actively dividing HSCs is thus essential to ensure long-term functionality. This regulation becomes important in the transplantation setting where HSCs undergo proliferation followed by a gradual transition to quiescence and homeostasis. Although mouse HSCs have been well studied under homeostatic conditions, the mechanisms regulating HSC activation under stress remain unclear. Here, we analyzed the different phases of regeneration after transplantation. We isolated bone marrow from mice at 8 time points after transplantation and examined the reconstitution dynamics and transcriptional profiles of stem and progenitor populations. We found that regenerating HSCs initially produced rapidly expanding progenitors and displayed distinct changes in fatty acid metabolism and glycolysis. Moreover, we observed molecular changes in cell cycle, MYC and mTOR signaling in both HSCs, and progenitor subsets. We used a decay rate model to fit the temporal transcription profiles of regenerating HSCs and identified genes with progressively decreased or increased expression after transplantation. These genes overlapped to a large extent with published gene sets associated with key aspects of HSC function, demonstrating the potential of this data set as a resource for identification of novel HSC regulators. Taken together, our study provides a detailed functional and molecular characterization of HSCs at different phases of regeneration and identifies a gene set associated with the transition from proliferation to quiescence.

Deregulated Gene Expression Profiles and Regulatory Networks in Adult and Pediatric RUNX1/RUNX1T1-Positive AML Patients

Acute myeloid leukemia (AML) is a heterogeneous and complex disease concerning molecular aberrations and prognosis. RUNX1/RUNX1T1 is a fusion oncogene that results from the chromosomal translocation t(8;21) and plays a crucial role in AML. However, its impact on the transcriptomic profile of different age groups of AML patients is not completely understood. Here, we investigated the deregulated gene expression (DEG) profiles in adult and pediatric RUNX1/RUNX1T1-positive AML patients, and compared their functions and regulatory networks. We retrospectively analyzed gene expression data from two independent Gene Expression Omnibus (GEO) datasets (GSE37642 and GSE75461) and computed their differentially expressed genes and upstream regulators, using limma, GEO2Enrichr, and X2K. For validation purposes, we used the TCGA-LAML (adult) and TARGET-AML (pediatric) patient cohorts. We also analyzed the protein–protein interaction (PPI) networks, as well as those composed of transcription factors (TF), intermediate proteins, and kinases foreseen to regulate the top deregulated genes in each group. Gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways enrichment analyses were further performed for the DEGs in each dataset. We found that the top upregulated genes in (both adult and pediatric) RUNX1/RUNX1T1-positive AML patients are enriched in extracellular matrix organization, the cell projection membrane, filopodium membrane, and supramolecular fiber. Our data corroborate that RUNX1/RUNX1T1 reprograms a large transcriptional network to establish and maintain leukemia via intricate PPI interactions and kinase-driven phosphorylation events.

Polycomb Alterations in Acute Myeloid Leukaemia: From Structure to Function

Epigenetic dysregulation is a hallmark of many haematological malignancies and is very frequent in acute myeloid leukaemia (AML). A cardinal example is the altered activity of the Polycomb Repressive Complex 2 (PRC2) due to somatic mutations and deletions in genes encoding PRC2 core factors that are necessary for correct complex assembly. These genetic alterations typically lead to reduced histone methyltransferase activity that, in turn, has been strongly linked to poor prognosis and chemoresistance. In this review, we provide an overview of genetic alterations of PRC components in AML, with particular reference to structural and functional features of PRC2 factors. We further review genetic interactions between these alterations and other AML-associated mutations in both adult and paediatric leukaemias. Finally, we discuss reported prognostic links between PRC2 mutations and deletions and disease outcomes and potential implications for therapy.

Multi-lineage Differentiation from Hematopoietic Stem Cells

The hematopoietic stem cells (HSCs) have the ability to differentiate and give rise to all mature blood cells. Commitment to differentiation progressively limits the self-renewal potential of the original HSCs by regulating the level of lineage-specific gene expression. In this review, we will summarize the current understanding of the molecular mechanisms underlying HSC differentiation toward erythroid, myeloid, and lymphocyte lineages. Moreover, we will decipher how the single-cell technologies advance the lineage-biased HSC subpopulations and their differentiation potential.

Cell-intrinsic factors governing quiescence vis-à-vis activation of adult hematopoietic stem cells

Hematopoiesis is a highly complex process, regulated by both intrinsic and extrinsic factors. Often, these two regulatory arms work in tandem to maintain the steady-state condition of hematopoiesis. However, at times, certain intrinsic attributes of hematopoietic stem cells (HSCs) override the external stimuli and dominate the outcome. These could be genetic events like mutations or environmentally induced epigenetic or transcriptomic changes. Since leukemic stem cells (LSCs) share molecular pathways that also regulate normal HSCs, identifying specific, dominantly acting intrinsic factors could help in the development of novel therapeutic approaches. Here we have reviewed such dominantly acting intrinsic factors governing quiescence vis-à-vis activation of the HSCs in the face of external forces acting on them. For brevity, we have restricted our review to the articles dealing with adult HSCs of human and mouse origin that have been published in the last 10 years. Hematopoietic stem cells (HSCs) are closely associated with various stromal cells in their microenvironment and, thus, constantly receive signaling cues from them. The illustration depicts some dominantly acting intrinsic or cell-autonomous factors operative in the HSCs. These fall into various categories, such as epigenetic regulators, transcription factors, cell cycle regulators, tumor suppressor genes, signaling pathways, and metabolic regulators, which counteract the outcome of extrinsic signaling exerted by the HSC niche.

Emerging role of EZH2 in rheumatic diseases: A comprehensive review

Enhancer of zeste homolog 2 (EZH2) is a histone methylated enzyme. It trimethylates histone 3 lysine 27 (H3K27) to regulate epigenetic processes. Recently, studies showed excessive expression of EZH2 in rheumatic diseases, such as systemic lupus erythematosus, rheumatoid arthritis, osteoarthritis, and systemic sclerosis. Moreover, epigenetic modification of EZH2 regulates differentiation and proliferation of different immune cells. Therefore, in this review, we comprehensively discuss the role of EZH2 in rheumatic diseases. Collection of the evidence may provide a basis for further understanding the role of EZH2 and give potential for targeting these diseases.

Bioinformatics analysis of potential therapeutic targets for COVID-19 infection in patients with carotid atherosclerosis

BACKGROUND

COVID-19 is a new coronavirus that constitutes a great challenge to human health. At this stage, there are still cases of COVID-19 infection in some countries and regions, in which ischemic stroke (IS) is a risk factor for new coronavirus pneumonia, and patients with COVID-19 infection have a dramatically elevated risk of stroke. At the same time, patients with long-term IS are vulnerable to COVID-19 infection and have more severe disease, and carotid atherosclerosis is an early lesion in IS.

METHODS

This study used human induced pluripotent stem cell (hiPSC)-derived monolayer brain cell dataset and human carotid atherosclerosis genome-wide dataset to analyze COVID-19 infection and carotid atherosclerosis patients to determine the synergistic effect of new coronavirus infection on carotid atherosclerosis patients, to clarify the common genes of both, and to identify common pathways and potential drugs for carotid atherosclerosis in patients with COVID-19 infection RESULTS: Using several advanced bioinformatics tools, we present the causes of COVID-19 infection leading to increased mortality in carotid atherosclerosis patients and the susceptibility of carotid atherosclerosis patients to COVID-19. Potential therapeutic agents for COVID-19 -infected patients with carotid atherosclerosis are also proposed.

CONCLUSIONS

With COVID-19 being a relatively new disease, associations have been proposed for its connections with several ailments and conditions, including IS and carotid atherosclerosis. More patient-based data-sets and studies are needed to fully explore and understand the relationship.

Link between the EZH2 noncanonical pathway and microtubule organization center polarization during early T lymphopoiesis

EZH2 plays an essential role at the β-selection checkpoint of T lymphopoiesis by regulating histone H3 lysine 27 trimethylation (H3K27me3) via its canonical mode of action. Increasing data suggest that EZH2 could also regulate other cellular functions, such as cytoskeletal reorganization, via its noncanonical pathway. Consequently, we investigated whether the EZH2 noncanonical pathway could be involved in early T-cell maturation, which requires cell polarization. We observed that EZH2 localization is tightly regulated during the early stages of T-cell development and that EZH2 relocalizes in the nucleus of double-negative thymocytes enduring TCRβ recombination and β-selection processes. Furthermore, we observed that EZH2 and EED, but not Suz12, colocalize with the microtubule organization center (MTOC), which might prevent its inappropriate polarization in double negative cells. In accordance with these results, we evidenced the existence of direct or indirect interaction between EED and α-tubulin. Taken together, these results suggest that the EZH2 noncanonical pathway, in association with EED, is involved in the early stages of T-cell maturation.

Mechanisms of Polycomb group protein function in cancer

Cancer arises from a multitude of disorders resulting in loss of differentiation and a stem cell-like phenotype characterized by uncontrolled growth. Polycomb Group (PcG) proteins are members of multiprotein complexes that are highly conserved throughout evolution. Historically, they have been described as essential for maintaining epigenetic cellular memory by locking homeotic genes in a transcriptionally repressed state. What was initially thought to be a function restricted to a few target genes, subsequently turned out to be of much broader relevance, since the main role of PcG complexes is to ensure a dynamically choregraphed spatio-temporal regulation of their numerous target genes during development. Their ability to modify chromatin landscapes and refine the expression of master genes controlling major switches in cellular decisions under physiological conditions is often misregulated in tumors. Surprisingly, their functional implication in the initiation and progression of cancer may be either dependent on Polycomb complexes, or specific for a subunit that acts independently of other PcG members. In this review, we describe how misregulated Polycomb proteins play a pleiotropic role in cancer by altering a broad spectrum of biological processes such as the proliferation-differentiation balance, metabolism and the immune response, all of which are crucial in tumor progression. We also illustrate how interfering with PcG functions can provide a powerful strategy to counter tumor progression.

Insights into high-risk multiple myeloma from an analysis of the role of PHF19 in cancer

Despite  improvements in outcome, 15-25% of newly diagnosed multiple myeloma (MM) patients have treatment resistant high-risk (HR) disease with a poor survival. The lack of a genetic basis for HR has focused attention on the role played by epigenetic changes. Aberrant expression and somatic mutations affecting genes involved in the regulation of tri-methylation of the lysine (K) 27 on histone 3 H3 (H3K27me3) are common in cancer. H3K27me3 is catalyzed by EZH2, the catalytic subunit of the Polycomb Repressive Complex 2 (PRC2). The deregulation of H3K27me3 has been shown to be involved in oncogenic transformation and tumor progression in a variety of hematological malignancies including MM. Recently we have shown that aberrant overexpression of the PRC2 subunit PHD Finger Protein 19 (PHF19) is the most significant overall contributor to HR status further focusing attention on the role played by epigenetic change in MM. By modulating both the PRC2/EZH2 catalytic activity and recruitment, PHF19 regulates the expression of key genes involved in cell growth and differentiation. Here we review the expression, regulation and function of PHF19 both in normal and the pathological contexts of solid cancers and MM. We present evidence that strongly implicates PHF19 in the regulation of genes important in cell cycle and the genetic stability of MM cells making it highly relevant to HR MM behavior. A detailed understanding of the normal and pathological functions of PHF19 will allow us to design therapeutic strategies able to target aggressive subsets of MM.

Designing Myeloid Gene Panels

CONTEXT.—

Next-generation sequencing studies are increasingly used in the evaluation of suspected chronic myeloid neoplasms (CMNs), but there is wide variability among laboratories in the genes analyzed for this purpose. Recently, the Association for Molecular Pathology CMN working group recommended a core 34-gene set as a minimum target list for evaluation of CMNs. This list was recommended based on literature review, and its diagnostic yield in clinical practice is unknown.

OBJECTIVE.—

To determine the diagnostic yield of the core 34 genes and assess the potential impact of including selected additional genes.

DESIGN.—

We retrospectively reviewed 185 patients with known or suspected CMNs tested using a 62-gene next-generation sequencing panel that included all 34 core genes.

RESULTS.—

The Association for Molecular Pathology's core 34 genes had a diagnostic yield of 158 of 185 (85.4%) to detect at least 1 variant with strong/potential clinical significance and 107 of 185 (57.8%) to detect at least 2 such variants. The 62-gene panel had a diagnostic yield of 160 of 185 (86.5%) and 112 of 185 (60.5%), respectively. Variants of unknown significance were identified in 49 of 185 (26.5%) using the core 34 genes versus 76 of 185 (41.1%) using the 62-gene panel.

CONCLUSIONS.—

This study demonstrates that the Association for Molecular Pathology-recommended core 34-gene set has a high diagnostic yield in CMNs. Inclusion of selected additional genes slightly increases the rate of abnormal results, while also increasing the detection of variants of unknown significance. We recommend inclusion of CUX1, DDX41, ETNK1, RIT1, and SUZ12 in addition to the Association for Molecular Pathology's 34-gene core set for routine evaluation of CMNs.

Easy or Not-The Advances of EZH2 in Regulating T Cell Development, Differentiation, and Activation in Antitumor Immunity

Enhancer of zeste homolog 2 (EZH2) is the catalytic subunit of polycomb repressive complex 2 (PRC2), which regulates downstream gene expression by trimethylation of lysine 27 in histone H3 (H3K27me3). EZH2 mutations or overexpressions are associated with many types of cancer. As inhibition of EZH2 activity could upregulate the expression of tumor suppressor genes, EZH2 has recently become an interesting therapeutic target for cancer therapy. Moreover, accumulating evidence has shown that EZH2 may contribute to the regulation of immune cells, especially T cells. EZH2 regulates T cell development, differentiation, and function, suggesting that EZH2 also regulates immune homeostasis in addition to tumor suppressor genes. Moreover, EZH2 can regulate T cell fate by targeting non-T cell factors such as metabolism, cytokines, and myeloid-derived suppressor cells. The role of EZH2 in this process has not been fully addressed. This review discusses up-to-date research on EZH2-mediated regulation of immunological function and the progress of immunological therapeutic strategies based on this regulation.

INTS11 regulates hematopoiesis by promoting PRC2 function

INTS11, the catalytic subunit of the Integrator (INT) complex, is crucial for the biogenesis of small nuclear RNAs and enhancer RNAs. However, the role of INTS11 in hematopoietic stem and progenitor cell (HSPC) biology is unknown. Here, we report that INTS11 is required for normal hematopoiesis and hematopoietic-specific genetic deletion of Ints11 leads to cell cycle arrest and impairment of fetal and adult HSPCs. We identified a novel INTS11-interacting protein complex, Polycomb repressive complex 2 (PRC2), that maintains HSPC functions. Loss of INTS11 destabilizes the PRC2 complex, decreases the level of histone H3 lysine 27 trimethylation (H3K27me3), and derepresses PRC2 target genes. Reexpression of INTS11 or PRC2 proteins in Ints11-deficient HSPCs restores the levels of PRC2 and H3K27me3 as well as HSPC functions. Collectively, our data demonstrate that INTS11 is an essential regulator of HSPC homeostasis through the INTS11-PRC2 axis.

Immature acute leukaemias: lessons from the haematopoietic roadmap

It is essential to relate the biology of acute leukaemia to normal blood cell development. In this review, we discuss how modern models of haematopoiesis might inform approaches to diagnosis and management of immature leukaemias, with a specific focus on T-lymphoid and myeloid cases. In particular, we consider whether next-generation analytical tools could provide new perspectives that could improve our understanding of immature blood cancer biology.

Heterochromatin and Polycomb as regulators of haematopoiesis

Haematopoiesis is the process by which multipotent haematopoietic stem cells are transformed into each and every type of terminally differentiated blood cell. Epigenetic silencing is critical for this process by regulating the transcription of cell-cycle genes critical for self-renewal and differentiation, as well as restricting alternative fate genes to allow lineage commitment and appropriate differentiation. There are two distinct forms of transcriptionally repressed chromatin: H3K9me3-marked heterochromatin and H3K27me3/H2AK119ub1-marked Polycomb (often referred to as facultative heterochromatin). This review will discuss the role of these distinct epigenetic silencing mechanisms in regulating normal haematopoiesis, how these contribute to age-related haematopoietic dysfunction, and the rationale for therapeutic targeting of these pathways in the treatment of haematological malignancies.

Polycomb Factor PHF19 Controls Cell Growth and Differentiation Toward Erythroid Pathway in Chronic Myeloid Leukemia Cells

Polycomb group (PcG) of proteins are a group of highly conserved epigenetic regulators involved in many biological functions, such as embryonic development, cell proliferation, and adult stem cell determination. PHD finger protein 19 (PHF19) is an associated factor of Polycomb repressor complex 2 (PRC2), often upregulated in human cancers. In particular, myeloid leukemia cell lines show increased levels of PHF19, yet little is known about its function. Here, we have characterized the role of PHF19 in myeloid leukemia cells. We demonstrated that PHF19 depletion decreases cell proliferation and promotes chronic myeloid leukemia (CML) differentiation. Mechanistically, we have shown how PHF19 regulates the proliferation of CML through a direct regulation of the cell cycle inhibitor p21. Furthermore, we observed that MTF2, a PHF19 homolog, partially compensates for PHF19 depletion in a subset of target genes, instructing specific erythroid differentiation. Taken together, our results show that PHF19 is a key transcriptional regulator for cell fate determination and could be a potential therapeutic target for myeloid leukemia treatment.

Pathogenic Impacts of Dysregulated Polycomb Repressive Complex Function in Hematological Malignancies

Polycomb repressive complexes (PRCs) are epigenetic regulators that mediate repressive histone modifications. PRCs play a pivotal role in the maintenance of hematopoietic stem cells through repression of target genes involved in cell proliferation and differentiation. Next-generation sequencing technologies have revealed that various hematologic malignancies harbor mutations in PRC2 genes, such as EZH2, EED, and SUZ12, and PRC1.1 genes, such as BCOR and BCORL1. Except for the activating EZH2 mutations detected in lymphoma, most of these mutations compromise PRC function and are frequently associated with resistance to chemotherapeutic agents and poor prognosis. Recent studies have shown that mutations in PRC genes are druggable targets. Several PRC2 inhibitors, including EZH2-specific inhibitors and EZH1 and EZH2 dual inhibitors have shown therapeutic efficacy for tumors with and without activating EZH2 mutations. Moreover, EZH2 loss-of-function mutations appear to be attractive therapeutic targets for implementing the concept of synthetic lethality. Further understanding of the epigenetic dysregulation associated with PRCs in hematological malignancies should improve treatment outcomes.

Epigenetic activities in erythroid cell gene regulation

Interest in the role of epigenetic mechanisms in human biology has exponentially increased over the past several decades. The multitude of opposing and context-dependent chromatin-modifying enzymes/coregulator complexes is just beginning to be understood at a molecular level. This science has benefitted tremendously from studies of erythropoiesis, in which a series of β-globin genes are in sequence turned "on" and "off," serving as a fascinating model of coordinated gene expression. We, therefore, describe here epigenetic complexes about which we know most, using erythropoiesis as the context. The biochemical insights lay the foundation for proposing and developing novel treatments for diseases of red cells and of erythropoiesis, identifying for example epigenetic enzymes that can be drugged to manipulate β-globin locus regulation, to favor activation of unmutated fetal hemoglobin over mutated adult β-globin genes to treat sickle cell disease and β-thalassemias. Other potential translational applications are in redirecting hematopoietic commitment decisions, as treatment for bone marrow failure syndromes.

Molecular and cellular mechanisms of aging in hematopoietic stem cells and their niches

Aging drives the genetic and epigenetic changes that result in a decline in hematopoietic stem cell (HSC) functioning. Such changes lead to aging-related hematopoietic/immune impairments and hematopoietic disorders. Understanding how such changes are initiated and how they progress will help in the development of medications that could improve the quality life for the elderly and to treat and possibly prevent aging-related hematopoietic diseases. Here, we review the most recent advances in research into HSC aging and discuss the role of HSC-intrinsic events, as well as those that relate to the aging bone marrow niche microenvironment in the overall processes of HSC aging. In addition, we discuss the potential mechanisms by which HSC aging is regulated.

The functions of EZH2 in immune cells: Principles for novel immunotherapies

Enhancer of zeste homolog 2 (EZH2) is aberrantly expressed or mutated in multiple types of cancer cells and plays an oncogenic role in tumorigenesis and development in most cancers. Results from pilot clinical studies have implied that EZH2 inhibitors have therapeutic potential against some cancers. However, the exact mechanisms by which EZH2 plays oncogenic roles and EZH2 inhibition exerts anticancer effects are incompletely understood. To date, the findings of studies focusing on EZH2 and cancer cells have failed to fully explain the observations in preclinical and clinical studies. Therefore, recent studies about the roles of EZH2 in cancers have shifted from cancer cells to immune cells. The human immune system is a complex network comprising multiple subpopulations of immune cells. Immune cells communicate and interact with cancer cells during cancer development and treatment, dictating the fate of cancer cells. Elucidating the roles of EZH2 in immune cells, especially in cancer patients, promises the identification of novel immunotherapeutic strategies or priming of existing immunotherapies against cancer. Hence, we reviewed the studies focusing on the involvement of EZH2 in various immune cells, aiming to provide ideas for immunotherapies targeting EZH2 in immune cells.

The eroding chromatin landscape of aging stem cells

Adult stem cells undergo both replicative and chronological aging in their niches, with catastrophic declines in regenerative potential with age. Due to repeated environmental insults during aging, the chromatin landscape of stem cells erodes, with changes in both DNA and histone modifications, accumulation of damage, and altered transcriptional response. A body of work has shown that altered chromatin is a driver of cell fate changes and a regulator of self-renewal in stem cells and therefore a prime target for juvenescence therapeutics. This review focuses on chromatin changes in stem cell aging and provides a composite view of both common and unique epigenetic themes apparent from the studies of multiple stem cell types.

The Polycomb-associated factor PHF19 controls hematopoietic stem cell state and differentiation

Adult hematopoietic stem cells (HSCs) are rare multipotent cells in bone marrow that are responsible for generating all blood cell types. HSCs are a heterogeneous group of cells with high plasticity, in part, conferred by epigenetic mechanisms. PHF19, a subunit of the Polycomb repressive complex 2 (PRC2), is preferentially expressed in mouse hematopoietic precursors. Here, we now show that, in stark contrast to results published for other PRC2 subunits, genetic depletion of Phf19 increases HSC identity and quiescence. While proliferation of HSCs is normally triggered by forced mobilization, defects in differentiation impede long-term correct blood production, eventually leading to aberrant hematopoiesis. At molecular level, PHF19 deletion triggers a redistribution of the histone repressive mark H3K27me3, which notably accumulates at blood lineage-specific genes. Our results provide novel insights into how epigenetic mechanisms determine HSC identity, control differentiation, and are key for proper hematopoiesis.

EZH2 function in immune cell development

The polycomb repressive complex 2 (PRC2) consists of three core components EZH2, SUZ12 and EED. EZH2 catalyzes the methylation of lysine 27 of histone H3, a modification associated with gene silencing. Through gene duplication higher vertebrate genomes also encode a second partially redundant methyltransferase, EZH1. Within the mammalian immune system most research has concentrated on EZH2 which is expressed predominantly in proliferating cells. EZH2 and other PRC2 components are required for hematopoietic stem cell function and lymphocyte development, at least in part by repressing cell cycle inhibitors. At later stages of immune cell differentiation, EZH2 plays essential roles in humoral and cell-mediated adaptive immunity, as well as the maintenance of immune homeostasis. EZH2 is often overactive in cancers, through both gain-of-function mutations and over-expression, an observation that has led to the development and clinical testing of specific EZH2 inhibitors. Such inhibitors may also be of use in inflammatory and autoimmune settings, as EZH2 inhibition dampens the immune response. Here, we will review the current state of understanding of the roles for EZH2, and PRC2 more generally, in the development and function of the immune system.

Structural heterogeneity of the mammalian polycomb repressor complex in immune regulation

Epigenetic regulation is mainly mediated by enzymes that can modify the structure of chromatin by altering the structure of DNA or histones. Proteins involved in epigenetic processes have been identified to study the detailed molecular mechanisms involved in the regulation of specific mRNA expression. Evolutionarily well-conserved polycomb group (PcG) proteins can function as transcriptional repressors by the trimethylation of histone H3 at the lysine 27 residue (H3K27me3) and the monoubiquitination of histone H2A at the lysine 119 residue (H2AK119ub). PcG proteins form two functionally distinct protein complexes: polycomb repressor complex 1 (PRC1) and PRC2. In mammals, the structural heterogeneity of each PRC complex is dramatically increased by several paralogs of its subunit proteins. Genetic studies with transgenic mice along with RNA-seq and chromatin immunoprecipitation (ChIP)-seq analyses might be helpful for defining the cell-specific functions of paralogs of PcG proteins. Here, we summarize current knowledge about the immune regulatory role of PcG proteins related to the compositional diversity of each PRC complex and introduce therapeutic drugs that target PcG proteins in hematopoietic malignancy.

Protein complexes that suppress gene activity by remodeling chromatin, the substance that contains most of a cell’s DNA, play a critical role in regulating the immune system and provide a therapeutic target for treating blood cancers. Seok-Jin Kang and Taehoon Chun from Korea University in Seoul, South Korea, review how polycomb group proteins, best known for their function in embryonic development, also contribute to the formation of immune cells from blood stem cell precursors. Studies with stem cells and cancer cells have begun to reveal many targets of these proteins, and drug companies are evaluating candidate agents directed against some polycomb group proteins in patients with lymphoma and other cancers. More comprehensive profiling of protein function across a broad range of immune cell types could reveal new targets for additional diseases associated with immune dysfunction.

Polycythemia vera: the current status of preclinical models and therapeutic targets

INTRODUCTION

Polycythemia vera (PV) is the most common myeloproliferative neoplasm (MPN). PV is characterized by erythrocytosis, leukocytosis, thrombocytosis, increased hematocrit, and hemoglobin in the peripheral blood. Splenomegaly and myelofibrosis often occur in PV patients. Almost all PV patients harbor a mutation in the JAK2 gene, mainly represented by the JAK2^V617F point mutation.

AREAS COVERED

This article examines the recent in vitro and in vivo available models of PV and moreover, it offers insights on emerging biomarkers and therapeutic targets. The evidence from mouse models, resembling a PV-like phenotype generated by different technical approaches, is discussed. The authors searched PubMed, books, and clinicaltrials.gov for original and review articles and drugs development status including the terms Myeloproliferative Neoplasms, Polycythemia Vera, erythrocytosis, hematocrit, splenomegaly, bone marrow fibrosis, JAK2^V617F, Hematopoietic Stem Cells, MPN cytoreductive therapy, JAK2 inhibitor, histone deacetylase inhibitor, PV-like phenotype, JAK2^V617F BMT, transgenic JAK2^V617F mouse, JAK2 physiologic promoter.

EXPERT OPINION

Preclinical models of PV are valuable tools for enabling an understanding of the pathophysiology and the molecular mechanisms of the disease. These models provide new biological insights on the contribution of concomitant mutations and the efficacy of novel drugs in a 'more faithful' setting. This may facilitate an enhanced understanding of pathogenetic mechanisms and targeted therapy.

Sleeve gastrectomy enhances glucose utilization and remodels adipose tissue independent of weight loss

Sleeve gastrectomy (SG) induces weight loss-independent improvements in glucose homeostasis by unknown mechanisms. We sought to identify the metabolic adaptations responsible for these improvements. Nonobese C57BL/6J mice on standard chow underwent SG or sham surgery. Functional testing and indirect calorimetry were used to capture metabolic phenotypes. Tissue-specific glucose uptake was assessed by 18-fluorodeoxyglucose (18-FDG) PET/computed tomography, and RNA sequencing was used for gene-expression analysis. In this model, SG induced durable improvements in glucose tolerance in the absence of changes in weight, body composition, or food intake. Indirect calorimetry revealed that SG increased the average respiratory exchange ratio toward 1.0, indicating a weight-independent, systemic shift to carbohydrate utilization. Following SG, orally administered 18-FDG preferentially localized to white adipose depots, showing tissue-specific increases in glucose utilization induced by surgery. Transcriptional analysis with RNA sequencing demonstrated that increased glucose uptake in the visceral adipose tissue was associated with upregulation in transcriptional pathways involved in energy metabolism, adipocyte maturation, and adaptive and innate immune cell chemotaxis and differentiation. SG induces a rapid, weight loss-independent shift toward glucose utilization and transcriptional remodeling of metabolic and immune pathways in visceral adipose tissue. Continued study of this early post-SG physiology may lead to a better understanding of the anti-diabetic mechanisms of bariatric surgery.

Memory of Divisional History Directs the Continuous Process of Primitive Hematopoietic Lineage Commitment

Hematopoietic stem cells (HSCs) exist in a dormant state and progressively lose regenerative potency as they undergo successive divisions. Why this functional decline occurs and how this information is encoded is unclear. To better understand how this information is stored, we performed RNA sequencing on HSC populations differing only in their divisional history. Comparative analysis revealed that genes upregulated with divisions are enriched for lineage genes and regulated by cell-cycle-associated transcription factors, suggesting that proliferation itself drives lineage priming. Downregulated genes are, however, associated with an HSC signature and targeted by the Polycomb Repressive Complex 2 (PRC2). The PRC2 catalytic subunits Ezh1 and Ezh2 promote and suppress the HSC state, respectively, and successive divisions cause a switch from Ezh1 to Ezh2 dominance. We propose that cell divisions drive lineage priming and Ezh2 accumulation, which represses HSC signature genes to consolidate information on divisional history into memory.

•

Divisional history is a major source of gene expression variation across HSCs

•

Cell divisions themselves appear to drive lineage priming in HSCs

•

Comparative analysis suggests that chromatin marks are dynamic with cell divisions

•

An Ezh1-to-Ezh2 switch consolidates HSC divisional history information into memory

Suz12 inactivation cooperates with JAK3 mutant signaling in the development of T-cell acute lymphoblastic leukemia

The polycomb repressive complex 2, with core components EZH2, SUZ12, and EED, is responsible for writing histone 3 lysine 27 trimethylation histone marks associated with gene repression. Analysis of sequence data from 419 T-cell acute lymphoblastic leukemia (T-ALL) cases demonstrated a significant association between SUZ12 and JAK3 mutations. Here we show that CRISPR/Cas9-mediated inactivation of Suz12 cooperates with mutant JAK3 to drive T-cell transformation and T-ALL development. Gene expression profiling integrated with ChIP-seq and ATAC-seq data established that inactivation of Suz12 led to increased PI3K/mammalian target of rapamycin (mTOR), vascular endothelial growth factor (VEGF), and WNT signaling. Moreover, a drug screen revealed that JAK3/Suz12 mutant leukemia cells were more sensitive to histone deacetylase (HDAC)6 inhibition than JAK3 mutant leukemia cells. Among the broad genome and gene expression changes observed on Suz12 inactivation, our integrated analysis identified the PI3K/mTOR, VEGF/VEGF receptor, and HDAC6/HSP90 pathways as specific vulnerabilities in T-ALL cells with combined JAK3 and SUZ12 mutations.

Understanding intrinsic hematopoietic stem cell aging

Hematopoietic stem cells (HSC) sustain blood production over the entire life-span of an organism. It is of extreme importance that these cells maintain self-renewal and differentiation potential over time in order to preserve homeostasis of the hematopoietic system. Many of the intrinsic aspects of HSC are affected by the aging process resulting in a deterioration in their potential, independently of their microenvironment. Here we review recent findings characterizing most of the intrinsic aspects of aged HSC, ranging from phenotypic to molecular alterations. Historically, DNA damage was thought to be the main cause of HSC aging. However, over recent years, many new findings have defined an increasing number of biological processes that intrinsically change with age in HSC. Epigenetics and chromatin architecture, together with autophagy, proteostasis and metabolic changes, and how they are interconnected, are acquiring growing importance for understanding the intrinsic aging of stem cells. Given the increase in populations of older subjects worldwide, and considering that aging is the primary risk factor for most diseases, understanding HSC aging becomes particularly relevant also in the context of hematologic disorders, such as myelodysplastic syndromes and acute myeloid leukemia. Research on intrinsic mechanisms responsible for HSC aging is providing, and will continue to provide, new potential molecular targets to possibly ameliorate or delay aging of the hematopoietic system and consequently improve the outcome of hematologic disorders in the elderly. The niche-dependent contributions to hematopoietic aging are discussed in another review in this same issue of the Journal.

Epigenetics of paediatric acute myeloid leukaemia

Comprehensive cataloguing of the acute myeloid leukaemia (AML) genome has revealed a high frequency of mutations and deletions in epigenetic factors that are frequently linked to treatment resistance and poor patient outcome. In this review, we discuss how the epigenetic mechanisms that underpin normal haematopoiesis are subverted in AML, and in particular how these processes are altered in childhood and adolescent leukaemias. We also provide a brief summary of the burgeoning field of epigenetic-based therapies, and how AML treatment might be improved through provision of better conceptual frameworks for understanding the pleiotropic molecular effects of epigenetic disruption.

PRC2 is high maintenance

As the process that silences gene expression ensues during development, the stage is set for the activity of Polycomb-repressive complex 2 (PRC2) to maintain these repressed gene profiles. PRC2 catalyzes a specific histone posttranslational modification (hPTM) that fosters chromatin compaction. PRC2 also facilitates the inheritance of this hPTM through its self-contained "write and read" activities, key to preserving cellular identity during cell division. As these changes in gene expression occur without changes in DNA sequence and are inherited, the process is epigenetic in scope. Mutants of mammalian PRC2 or of its histone substrate contribute to the cancer process and other diseases, and research into these aberrant pathways is yielding viable candidates for therapeutic targeting. The effectiveness of PRC2 hinges on its being recruited to the proper chromatin sites; however, resolving the determinants to this process in the mammalian case was not straightforward and thus piqued the interest of many in the field. Here, we chronicle the latest advances toward exposing mammalian PRC2 and its high maintenance.

Polycomb repressive complex 2 is a critical mediator of allergic inflammation

Strategies that intervene with the development of immune-mediated diseases are urgently needed, as current treatments mostly focus on alleviating symptoms rather than reversing the disease. Targeting enzymes involved in epigenetic modifications to chromatin represents an alternative strategy that has the potential to perturb the function of the lymphocytes that drive the immune response. Here, we report that 2 major epigenetic silencing pathways are increased after T cell activation. By specific inactivation of these molecules in the T cell compartment in vivo, we demonstrate that the polycomb repressive complex 2 (PRC2) is essential for the generation of allergic responses. Furthermore, we show that small-molecule inhibition of the PRC2 methyltransferase, enhancer of zeste homolog 2 (Ezh2), reduces allergic inflammation in mice. Therefore, by systematically surveying the pathways involved in epigenetic gene silencing we have identified Ezh2 as a target for the suppression of allergic disease.

Emerging Roles for Polycomb-Group Proteins in Stem Cells and Cancer

Polycomb-group (PcG) complexes are multiprotein, evolutionarily conserved epigenetic machineries that regulate stem cell fate decisions and development, and are also implicated in cancer and other maladies. The PcG machinery can be divided into two major complexes: Polycomb repressive complex 1 and 2 (PRC1 and PRC2). Traditionally, PcG complexes have been associated with maintenance of gene repression mainly via histone-modifying activities. However, during the last years, increasing evidence indicates that the PcG complexes can also positively regulate gene transcription and modify non-histone substrates in multiple biological processes, cellular stages, and cancers. In this review, we will illustrate recent findings in PcG-mediated gene regulation, with special focus on the recently described non-classical functions of PcG complexes in stem cells and cancer.

Control of Intra-Thymic αβ T Cell Selection and Maturation by H3K27 Methylation and Demethylation

In addition to transcription factor binding, the dynamics of DNA modifications (methylation) and chromatin structure are essential contributors to the control of transcription in eukaryotes. Research in the past few years has emphasized the importance of histone H3 methylation at lysine 27 for lineage specific gene repression, demonstrated that deposition of this mark at specific genes is subject to differentiation-induced changes during development, and identified enzymatic activities, methyl transferases and demethylases, that control these changes. The present review discusses the importance of these mechanisms during intrathymic αβ T cell selection and late differentiation.

Chimeric antigen receptor T-cell toxicity

PURPOSE OF REVIEW

Chimeric antigen receptor -(CAR) T-cell therapy has become a commonly used immunotherapy originally used in the treatment of B-cell leukemias but which are now applied broadly across tumor classes. Although high rates of remission are associated with CAR T-cell therapy, toxicities associated with these novel treatment regimens can be lethal if not recognized in a timely manner.

RECENT FINDINGS

Cytokine release syndrome and neurotoxicity are the two most common toxicities associated with CAR T-cell therapy. Cytokine release syndrome is characterized by a flu-like illness accompanied by significant hemodynamic instability; treatments include administration of tocilizumab and corticosteroids. Neurotoxicity is associated with nonpattern-specific neurological changes and can rapidly progress to a comatose state from cerebral edema and death. Other potential toxicities from CAR T-cell therapy include tumor lysis syndrome, B-cell aplasia, graft versus host disease, and dermatological eruptions.

SUMMARY

Clinical awareness of CAR T-cell toxicities is important because prompt treatment leads to improved survival and remission rates.

Cytokine Release Syndrome With the Novel Treatments of Acute Lymphoblastic Leukemia: Pathophysiology, Prevention, and Treatment

PURPOSE OF REVIEW

T cell-based therapies (blinatumomab and CAR T cell therapy) have produced unprecedented responses in relapsed and refractory (r/r) acute lymphoblastic leukemia (ALL) but is accompanied with significant toxicities, of which one of the most common and serious is cytokine release syndrome (CRS). Here we will review the pathophysiology, prevention, and treatment of CRS.

RECENT FINDINGS

Efforts have been initiated to define and grade cytokine release syndrome (CRS), to identify patients at risk, to describe biomarkers that predict onset and severity, to understand the pathophysiology, and to prevent and treat severe cases to reduce T cell immunotherapy-related morbidity and mortality. Optimizing the timing of T cell-based therapies in ALL, identifying new biomarkers, and investigating novel anti-cytokine agents that have anti-CRS activity are likely to be fruitful avenues of study.

JARID2 Functions as a Tumor Suppressor in Myeloid Neoplasms by Repressing Self-Renewal in Hematopoietic Progenitor Cells

How specific genetic lesions contribute to transformation of non-malignant myeloproliferative neoplasms (MPNs) and myelodysplastic syndromes (MDSs) to secondary acute myeloid leukemia (sAML) are poorly understood. JARID2 is lost by chromosomal deletions in a proportion of MPN/MDS cases that progress to sAML. In this study, genetic mouse models and patient-derived xenografts demonstrated that JARID2 acts as a tumor suppressor in chronic myeloid disorders. Genetic deletion of Jarid2 either reduced overall survival of animals with MPNs or drove transformation to sAML, depending on the timing and context of co-operating mutations. Mechanistically, JARID2 recruits PRC2 to epigenetically repress self-renewal pathways in hematopoietic progenitor cells. These studies establish JARID2 as a bona fide hematopoietic tumor suppressor and highlight potential therapeutic targets.

Polycomb complexes in normal and malignant hematopoiesis

Di Carlo et al. discuss how the regulation/dysregulation of Polycomb group proteins contributes to hematopoiesis and hematological disorders.

Epigenetic mechanisms are crucial for sustaining cell type–specific transcription programs. Among the distinct factors, Polycomb group (PcG) proteins are major negative regulators of gene expression in mammals. These proteins play key roles in regulating the proliferation, self-renewal, and differentiation of stem cells. During hematopoietic differentiation, many PcG proteins are fundamental for proper lineage commitment, as highlighted by the fact that a lack of distinct PcG proteins results in embryonic lethality accompanied by differentiation biases. Correspondingly, proteins of these complexes are frequently dysregulated in hematological diseases. In this review, we present an overview of the role of PcG proteins in normal and malignant hematopoiesis, focusing on the compositional complexity of PcG complexes, and we briefly discuss the ongoing clinical trials for drugs targeting these factors.

Canonical PRC2 function is essential for mammary gland development and affects chromatin compaction in mammary organoids

Distinct transcriptional states are maintained through organization of chromatin, resulting from the sum of numerous repressive and active histone modifications, into tightly packaged heterochromatin versus more accessible euchromatin. Polycomb repressive complex 2 (PRC2) is the main mammalian complex responsible for histone 3 lysine 27 trimethylation (H3K27me3) and is integral to chromatin organization. Using in vitro and in vivo studies, we show that deletion of Suz12, a core component of all PRC2 complexes, results in loss of H3K27me3 and H3K27 dimethylation (H3K27me2), completely blocks normal mammary gland development, and profoundly curtails progenitor activity in 3D organoid cultures. Through the application of mammary organoids to bypass the severe phenotype associated with Suz12 loss in vivo, we have explored gene expression and chromatin structure in wild-type and Suz12-deleted basal-derived organoids. Analysis of organoids led to the identification of lineage-specific changes in gene expression and chromatin structure, inferring cell type-specific PRC2-mediated gene silencing of the chromatin state. These expression changes were accompanied by cell cycle arrest but not lineage infidelity. Together, these data indicate that canonical PRC2 function is essential for development of the mammary gland through the repression of alternate transcription programs and maintenance of chromatin states.

Cell-extrinsic hematopoietic impact of Ezh2 inactivation in fetal liver endothelial cells

Despite the well-established cell-intrinsic role of epigenetic factors in normal and malignant hematopoiesis, their cell-extrinsic role remains largely unexplored. Herein we investigated the hematopoietic impact of inactivating Ezh2, a key component of polycomb repressive complex 2 (PRC2), in the fetal liver (FL) vascular niche. Hematopoietic specific (Vav-iCre) Ezh2 inactivation enhanced FL hematopoietic stem cell (HSC) expansion with normal FL erythropoiesis. In contrast, endothelium (Tie2-Cre) targeted Ezh2 inactivation resulted in embryonic lethality with severe anemia at embryonic day 13.5 despite normal emergence of functional HSCs. Ezh2-deficient FL endothelium overexpressed Mmp9, which cell-extrinsically depleted the membrane-bound form of Kit ligand (mKitL), an essential hematopoietic cytokine, in FL. Furthermore, Mmp9 inhibition in vitro restored mKitL expression along with the erythropoiesis supporting capacity of FL endothelial cells. These data establish that Ezh2 is intrinsically dispensable for FL HSCs and provides proof of principle that modulation of epigenetic regulators in niche components can exert a marked cell-extrinsic impact.

Are whole-exome and whole-genome sequencing approaches cost-effective? A systematic review of the literature

PURPOSE

We conducted a systematic literature review to summarize the current health economic evidence for whole-exome sequencing (WES) and whole-genome sequencing (WGS).

METHODS

Relevant studies were identified in the EMBASE, MEDLINE, Cochrane Library, EconLit and University of York Centre for Reviews and Dissemination databases from January 2005 to July 2016. Publications were included in the review if they were economic evaluations, cost studies, or outcome studies.

RESULTS

Thirty-six studies met our inclusion criteria. These publications investigated the use of WES and WGS in a variety of genetic conditions in clinical practice, the most common being neurological or neurodevelopmental disorders. Study sample size varied from a single child to 2,000 patients. Cost estimates for a single test ranged from $555 to $5,169 for WES and from $1,906 to $24,810 for WGS. Few cost analyses presented data transparently and many publications did not state which components were included in cost estimates.

CONCLUSION

The current health economic evidence base to support the more widespread use of WES and WGS in clinical practice is very limited. Studies that carefully evaluate the costs, effectiveness, and cost-effectiveness of these tests are urgently needed to support their translation into clinical practice.

Regulation of embryonic haematopoietic multipotency by EZH1

All haematopoietic lineages circulating in the blood of adult mammals derive from multipotent haematopoietic stem cells (HSCs)¹. Haematopoiesis in the mammalian embryo stands in stark contrast, with lineage-restricted progenitors arising first, independently of HSCs, and HSCs emerging only later in gestation^2,3. As best defined in the mouse, “primitive” progenitors first appear in the yolk sac (YS) at 7.5 days post-coitum (dpc)^2,3. Subsequently, erythroid-myeloid progenitors (EMPs) that express fetal hemoglobin⁴, as well as fetal lymphoid progenitors⁵ develop in the YS and the embryo proper, but these cells lack HSC potential. Ultimately, “definitive” HSCs with long-term, multilineage potential and the capacity to engraft irradiated adults emerge at 10.5 dpc from arterial endothelium in the aorta-gonad-mesonephros (AGM) and other haemogenic vasculature³. The molecular mechanisms for reverse progression of haematopoietic ontogeny remain unexplained. We hypothesized that the definitive haematopoietic program might be actively repressed in early embryogenesis via epigenetic silencing⁶, and that alleviating this repression would elicit multipotency in otherwise restricted haematopoietic progenitors. Here, we demonstrate that reduced expression of the Polycomb group protein EZH1 uncovers multi-lymphoid output from human pluripotent stem cells (PSCs) and precocious emergence of functional definitive HSCs at sites of primitive and/or EMP-biased haematopoiesis in vivo, identifying EZH1 as a repressor of haematopoietic multipotency in the early mammalian embryo.

Loss of H3K27 trimethylation is not suitable for distinguishing malignant peripheral nerve sheath tumor from melanoma: a study of 387 cases including mimicking lesions

The diagnosis of malignant peripheral nerve sheath tumor remains challenging, especially in the sporadic setting. Malignant peripheral nerve sheath tumor is a rare malignancy, and owing to the lack of specific histological criteria, immunohistochemical and molecular diagnostic markers, several differential diagnoses must be considered, in particular melanoma. Recently, inactivation of the polycomb repressive complex 2 (PRC2), induced by inactivating mutations in two of its critical constituents SUZ12 and EED, was reported in a large subset of malignant peripheral nerve sheath tumors. Homozygous PRC2 inactivation induces complete loss of trimethylation at lysine 27 of histone 3 (H3K27me3). Recent studies suggest that complete loss of H3K27me3 is highly specific for malignant peripheral nerve sheath tumor and may be a useful immunohistochemical diagnostic marker. Therefore, to determine the specificity of the complete loss of H3K27me3 expression in the context of the differential diagnosis of malignant peripheral nerve sheath tumor from melanoma (its major potential mimic), we performed H3K27me3 immunohistochemistry in a pathologically and genetically well-characterized cohort of primary (neurofibromatosis type 1 (NF1), radiation-associated and sporadic context) malignant peripheral nerve sheath tumors (n=122) and in a cohort or primary (desmoplastic) and metastatic melanomas (n=265). In total, 88 (72%) malignant peripheral nerve sheath tumors, including 46 (71%) NF1-associated, 4 (100%) radiation-associated, and 38 (72%) sporadic tumors, showed complete loss of H3K27me3. We observed increased loss of H3K27me3 with increasing histological grade. Interestingly, we found complete loss of H3K27me3 in 37% (n=98) of all melanomas, including 25% (n=9) of primary desmoplastic melanomas. Moreover, partial loss ('mosaic' pattern) was observed in 23 (19%) of all malignant peripheral nerve sheath tumors and in 136 (51%) of all melanomas. Complete loss of H3K27me3 detected by immunohistochemistry is not specific for malignant peripheral nerve sheath tumor and cannot be used safely when distinguishing malignant peripheral nerve sheath tumor from melanoma.