Ezh2 loss in hematopoietic stem cells predisposes mice to develop heterogeneous malignancies in an Ezh1-dependent manner

Targeting chromatin modifying complexes in acute myeloid leukemia

Acute myeloid leukemia (AML) is a devastating hematologic malignancy with high rates of relapse, which can, in part, be attributed to the dysregulation of chromatin modifications. These epigenetic modifications can affect the capacity of hematopoietic cells to self-renew or differentiate, which can lead to transformation. Aberrant histone modifications contribute to the derepression of self-renewal genes such as HOXA/B and MEIS1 in committed hematopoietic progenitors, which is considered a key mechanism of leukemogenesis in MLL-rearranged (MLL-r) and NPM1-mutated AML. As regulators of some of the key histone modifications in this disease, the menin-KMT2A and polycomb repressive (PRC1/2) complexes have been identified as promising targets for the treatment of AML. This review explores recent discoveries of how leukemic cells hijack these complexes and their interactions with other chromatin regulators to promote disease progression. We also discuss inhibitors targeting these complexes that have demonstrated therapeutic efficacy in preclinical and clinical studies and propose novel therapeutic combinations targeting the KMT2A and PRC1/2 broader interacting networks to overcome issues of resistance to existing monotherapies.

Developmental Stage and Cellular Context Determine Oncogenic and Molecular Outcomes of Ezh2 Y641F Mutation in Hematopoiesis

Mutations in the histone methyltransferase EZH2, particularly the Y641 hotspot mutation, have been implicated in hematologic malignancies, yet the effect of timing and cellular context on their oncogenic potential has remained unknown. In this study, we utilized a conditional allele with tissue-specific Cre drivers to investigate the effects of Ezh2 Y641F mutations at various stages of development, with a focus on the hematopoietic system. We found that ubiquitous heterozygous Ezh2 Y641F expression at birth, or conditional expression in hematopoietic or mesenchymal stem cells, led to decreased survival due to hematopoietic defects and bone marrow failure, with no evidence of malignancy. In contrast, Ezh2 Y641F expression in committed B cells drives lymphoma formation, highlighting the lineage-specific oncogenic activity of the mutation. Transcriptomic analysis of B cell progenitors revealed key pathway alterations between Cre models such as altered IL2-Stat5 signaling pathway, differential expression of E2F targets, and altered GTPase pathway expression driven by upregulation of Guanylate Binding Proteins (GBPs) in Mx1-Cre Ezh2 Y641F pro-B cells. We further found that the GBP locus is regulated by Ezh2-mediated H3K27me3, it is associated with poorer survival in Acute Myeloid Leukemia patients and has variable effects on apoptosis in human lymphoma and leukemia cell lines. These findings suggest that the Ezh2 Y641F mutation may alter immune regulatory pathways, cell differentiation and apoptosis, with potential implications for disease progression. Our results highlight the critical role of mutation timing and cellular context in EZH2-driven hematopoietic disease, resulting in distinct downstream changes that shape the oncogenic impact of EZH2.

Biallelic loss-of-function variants of EZH1 cause a novel developmental disorder with central precocious puberty

Pathogenic variants of polycomb repressive complex-2 (PRC2) subunits are associated with overgrowth syndromes and neurological diseases. EZH2 is a major component of PRC2 and mediates the methylation of H3K27 trimethylation (H3K27me3). Germline variants of EZH2 have been identified as a cause of Weaver syndrome (WS), an overgrowth/intellectual disability (OGID) syndrome characterized by overgrowth, macrocephaly, accelerated bone age, intellectual disability (ID), and characteristic facial features. Germline variants of SUZ12 and EED, other components of PRC2, have also been reported in the WS or Weaver-like syndrome. EZH1 is a homolog of EZH2 that interchangeably associates with SUZ12 and EED. Recently, pathogenic variants of EZH1 have been reported in individuals with dominant and recessive neurodevelopmental disorders. We herein present sisters with biallelic loss-of-function variants of EZH1. They showed developmental delay, ID, and central precocious puberty, but not the features of WS or other OGID syndromes.

Stat3-mediated Atg7 expression regulates anti-tumor immunity in mouse melanoma

Epigenetic modifications to DNA and chromatin control oncogenic and tumor-suppressive mechanisms in melanoma. Ezh2, the catalytic component of the Polycomb Repressive Complex 2 (PRC2), which mediates methylation of lysine 27 on histone 3 (H3K27me3), can regulate both melanoma initiation and progression. We previously found that mutant Ezh2Y641F interacts with the immune regulator Stat3 and together they affect anti-tumor immunity. However, given the numerous downstream targets and pathways affected by Ezh2, many mechanisms that determine its oncogenic activity remain largely unexplored. Using genetically engineered mouse models, we further investigated the role of pathways downstream of Ezh2 in melanoma carcinogenesis and identified significant enrichment in several autophagy signatures, along with increased expression of autophagy regulators, such as Atg7. In this study, we investigated the effect of Atg7 on melanoma growth and tumor immunity within the context of a wild-type or Ezh2Y641F epigenetic state. We found that the Atg7 locus is controlled by multiple Ezh2 and Stat3 binding sites, Atg7 expression is dependent on Stat3 expression, and that deletion of Atg7 slows down melanoma cell growth in vivo, but not in vitro. Atg7 deletion also results in increased CD8 + T cells in Ezh2Y641F melanomas and reduced myelosuppressive cell infiltration in the tumor microenvironment, particularly in Ezh2WT melanomas, suggesting a strong immune system contribution in the role of Atg7 in melanoma progression. These findings highlight the complex interplay between genetic mutations, epigenetic regulators, and autophagy in shaping tumor immunity in melanoma.

Role of EZH2-mediated epigenetic modification on vascular smooth muscle in cardiovascular diseases: A mini-review

Vascular smooth muscle cells (VSMCs) are integral to the pathophysiology of cardiovascular diseases (CVDs). Enhancer of zeste homolog 2 (EZH2), a histone methyltransferase, plays a crucial role in epigenetic regulation of VSMCs gene expression. Emerging researches suggest that EZH2 has a dual role in VSMCs, contingent on the pathological context of specific CVDs. This mini-review synthesizes the current knowledge on the mechanisms by which EZH2 regulates VSMC proliferation, migration and survival in the context of CVDs. The goal is to underscore the potential of EZH2 as a therapeutic target for CVDs treatment. Modulating EZH2 and its associated epigenetic pathways in VSMCs could potentially ameliorate vascular remodeling, a key factor in the progression of many CVDs. Despite the promising outlook, further investigation is warranted to elucidate the epigenetic mechanisms mediated by EZH2 in VSMCs, which may pave the way for novel epigenetic therapies for conditions such as atherosclerosis and hypertension.

Stat3-mediated Atg7 expression enhances anti-tumor immunity in melanoma

Epigenetic modifications to DNA and chromatin control oncogenic and tumor suppressive mechanisms in melanoma. EZH2, the catalytic component of the Polycomb repressive complex 2 (PRC2), which mediates methylation of lysine 27 on histone 3 (H3K27me3), can regulate both melanoma initiation and progression. We previously found that mutant Ezh2 Y641F interacts with the immune regulator Stat3 and together they affect anti-tumor immunity. However, given the numerous downstream targets and pathways affected by EZH2, many mechanisms that determine its oncogenic activity remain largely unexplored. Using genetically engineered mouse models we further investigated the role of pathways downstream of EZH2 in melanoma carcinogenesis and identified significant enrichment in several autophagy signatures, along with increased expression of autophagy regulators, such as Atg7. In this study, we investigated the effect of Atg7 on melanoma growth and tumor immunity within the context of an Ezh2 Y641F epigenetic state. We found that expression of Atg7 is largely dependent on Stat3 expression and that deletion of Atg7 slows down melanoma cell growth in vivo, but not in vitro. Atg7 deletion also results in increased CD8+ T cells and reduced myelosuppressive cell infiltration in the tumor microenvironment, suggesting a strong immune system contribution in the role of Atg7 in melanoma progression. These findings highlight the complex interplay between genetic mutations, epigenetic regulators, and autophagy in shaping tumor immunity in melanoma.

Epigenetic vulnerabilities of leukemia harboring inactivating EZH2 mutations

Epigenetic regulators, such as the polycomb repressive complex 2 (PRC2), play a critical role in both normal development and carcinogenesis. Mutations and functional dysregulation of PRC2 complex components, such as EZH2, are implicated in various forms of cancer and associated with poor prognosis. This study investigated the epigenetic vulnerabilities of acute myeloid leukemia (AML) and myelodysplastic/myeloproliferative disorders (MDS/MPN) by performing a chemical probe screen in patient cells. Paradoxically, we observed increased sensitivity to EZH2 and embryonic ectoderm development (EED) inhibitors in AML and MDS/MPN patient cells harboring EZH2 mutations. Expression analysis indicated that EZH2 inhibition elicited upregulation of pathways responsible for cell death and growth arrest, specifically in patient cells with mutant EZH2. The identified EZH2 mutations had drastically reduced catalytic activity, resulting in lower cellular H3K27me3 levels, and were associated with decreased EZH2 and PRC2 component EED protein levels. Overall, this study provides an important understanding of the role of EZH2 dysregulation in blood cancers and may indicate disease etiology for these poor prognosis AML and MDS/MPN cases.

Spontaneous remission and loss of monosomy 7: a window of opportunity for young children with SAMD9L syndrome

Monosomy 7 is the most common cytogenetic abnormality in pediatric myelodysplastic syndrome (MDS) and associated with a high risk of disease progression. However, in young children, spontaneous loss of monosomy 7 with concomitant hematologic recovery has been described, especially in the presence of germline mutations in SAMD9 and SAMD9L genes. Here, we report on our experience of close surveillance instead of upfront hematopoietic stem cell transplantation (HSCT) in seven patients diagnosed with SAMD9L syndrome and monosomy 7 at a median age of 0.6 years (range, 0.4-2.9). Within 14 months from diagnosis, three children experienced spontaneous hematological remission accompanied by a decrease in monosomy 7 clone size. Subclones with somatic SAMD9L mutations in cis were identified in five patients, three of whom attained hematological remission. Two patients acquired RUNX1 and EZH2 mutations during the observation period, of whom one progressed to myelodysplastic syndrome with excess of blasts (MDS-EB). Four patients underwent allogeneic HSCT at a median time of 26 months (range, 14-40) from diagnosis for MDSEB, necrotizing granulomatous lymphadenitis, persistent monosomy 7, and severe neutropenia. At last follow-up, six patients were alive, while one passed away due to transplant-related causes. These data confirm previous observations that monosomy 7 can be transient in young children with SAMD9L syndrome. However, they also indicate that delaying HSCT poses a substantial risk of severe infection and disease progression. Finally, surveillance of patients with SAMD9L syndrome and monosomy 7 is critical to define the evolving genetic landscape and to determine the appropriate timing of HSCT (clinicaltrials gov. Identifier: NCT00662090).

PHF6-altered T-ALL Harbor Epigenetic Repressive Switch at Bivalent Promoters and Respond to 5-Azacitidine and Venetoclax

PURPOSE

To assess the impact of PHF6 alterations on clinical outcome and therapeutical actionability in T-cell acute lymphoblastic leukemia (T-ALL).

EXPERIMENTAL DESIGN

We described PHF6 alterations in an adult cohort of T-ALL from the French trial Group for Research on Adult Acute Lymphoblastic Leukemia (GRAALL)-2003/2005 and retrospectively analyzed clinical outcomes between PHF6-altered (PHF6ALT) and wild-type patients. We also used EPIC and chromatin immunoprecipitation sequencing data of patient samples to analyze the epigenetic landscape of PHF6ALT T-ALLs. We consecutively evaluated 5-azacitidine efficacy, alone or combined with venetoclax, in PHF6ALT T-ALL.

RESULTS

We show that PHF6 alterations account for 47% of cases in our cohort and demonstrate that PHF6ALT T-ALL presented significantly better clinical outcomes. Integrative analysis of DNA methylation and histone marks shows that PHF6ALT are characterized by DNA hypermethylation and H3K27me3 loss at promoters physiologically bivalent in thymocytes. Using patient-derived xenografts, we show that PHF6ALT T-ALL respond to the 5-azacytidine alone. Finally, synergism with the BCL2-inhibitor venetoclax was demonstrated in refractory/relapsing (R/R) PHF6ALT T-ALL using fresh samples. Importantly, we report three cases of R/R PHF6ALT patients who were successfully treated with this combination.

CONCLUSIONS

Overall, our study supports the use of PHF6 alterations as a biomarker of sensitivity to 5-azacytidine and venetoclax combination in R/R T-ALL.

Noncoding rules of survival: epigenetic regulation of normal and malignant hematopoiesis

Hematopoiesis is an essential process for organismal development and homeostasis. Epigenetic regulation of gene expression is critical for stem cell self-renewal and differentiation in normal hematopoiesis. Increasing evidence shows that disrupting the balance between self-renewal and cell fate decisions can give rise to hematological diseases such as bone marrow failure and leukemia. Consequently, next-generation sequencing studies have identified various aberrations in histone modifications, DNA methylation, RNA splicing, and RNA modifications in hematologic diseases. Favorable outcomes after targeting epigenetic regulators during disease states have further emphasized their importance in hematological malignancy. However, these targeted therapies are only effective in some patients, suggesting that further research is needed to decipher the complexity of epigenetic regulation during hematopoiesis. In this review, an update on the impact of the epigenome on normal hematopoiesis, disease initiation and progression, and current therapeutic advancements will be discussed.

Predicting leukemic transformation in myelodysplastic syndrome using a transcriptomic signature

Background: For prediction on leukemic transformation of MDS patients, emerging model based on transcriptomic datasets, exhibited superior predictive power to traditional prognostic systems. While these models were lack of external validation by independent cohorts, and the cell origin (CD34+ sorted cells) limited their feasibility in clinical practice. Methods: Transformation associated co-expressed gene cluster was derived based on GSE58831 ('WGCNA' package, R software). Accordingly, the least absolute shrinkage and selection operator algorithm was implemented to establish a scoring system (i.e., MDS15 score), using training set (GSE58831 originated from CD34+ cells) and testing set (GSE15061 originated from unsorted cells). Results: A total of 68 gene co-expression modules were derived, and the 'brown' module was recognized to be transformation-specific (R2 = 0.23, p = 0.005, enriched in transcription regulating pathways). After 50,000-times LASSO iteration, MDS15 score was established, including the 15-gene expression signature. The predictive power (AUC and Harrison's C index) of MDS15 model was superior to that of IPSS/WPSS in both training set (AUC/C index 0.749/0.777) and testing set (AUC/C index 0.933/0.86). Conclusion: By gene co-expression analysis, the crucial gene module was discovered, and a novel prognostic system (MDS15) was established, which was validated not only by another independent cohort, but by a different cell origin.

Non-canonical functions of EZH2 in cancer

Mutations in chromatin modifying genes frequently occur in many kinds of cancer. Most mechanistic studies focus on their canonical functions, while therapeutic approaches target their enzymatic activity. Recent studies, however, demonstrate that non-canonical functions of chromatin modifiers may be equally important and therapeutically actionable in different types of cancer. One epigenetic regulator that demonstrates such a dual role in cancer is the histone methyltransferase EZH2. EZH2 is a core component of the polycomb repressive complex 2 (PRC2), which plays a crucial role in cell identity, differentiation, proliferation, stemness and plasticity. While much of the regulatory functions and oncogenic activity of EZH2 have been attributed to its canonical, enzymatic activity of methylating lysine 27 on histone 3 (H3K27me3), a repressive chromatin mark, recent studies suggest that non-canonical functions that are independent of H3K27me3 also contribute towards the oncogenic activity of EZH2. Contrary to PRC2's canonical repressive activity, mediated by H3K27me3, outside of the complex EZH2 can directly interact with transcription factors and oncogenes to activate gene expression. A more focused investigation into these non-canonical interactions of EZH2 and other epigenetic/chromatin regulators may uncover new and more effective therapeutic strategies. Here, we summarize major findings on the non-canonical functions of EZH2 and how they are related to different aspects of carcinogenesis.

CNL and aCML should be considered as a single entity based on molecular profiles and outcomes

Chronic neutrophilic leukemia (CNL) and atypical chronic myeloid leukemia (aCML) are rare myeloid disorders that are challenging with regard to diagnosis and clinical management. To study the similarities and differences between these disorders, we undertook a multicenter international study of one of the largest case series (CNL, n = 24; aCML, n = 37 cases, respectively), focusing on the clinical and mutational profiles (n = 53 with molecular data) of these diseases. We found no differences in clinical presentations or outcomes of both entities. As previously described, both CNL and aCML share a complex mutational profile with mutations in genes involved in epigenetic regulation, splicing, and signaling pathways. Apart from CSF3R, only EZH2 and TET2 were differentially mutated between them. The molecular profiles support the notion of CNL and aCML being a continuum of the same disease that may fit best within the myelodysplastic/myeloproliferative neoplasms. We identified 4 high-risk mutated genes, specifically CEBPA (β = 2.26, hazard ratio [HR] = 9.54, P = .003), EZH2 (β = 1.12, HR = 3.062, P = .009), NRAS (β = 1.29, HR = 3.63, P = .048), and U2AF1 (β = 1.75, HR = 5.74, P = .013) using multivariate analysis. Our findings underscore the relevance of molecular-risk classification in CNL/aCML as well as the importance of CSF3R mutations in these diseases.

Recent advances in EZH2-based dual inhibitors in the treatment of cancers

The enhancer of zeste homolog 2 (EZH2) protein is the catalytic subunit of one of the histone methyltransferases. EZH2 catalyzes the trimethylation of lysine 27 of histone H3 (H3K27me3) and further alters downstream target levels. EZH2 is upregulated in cancer tissues, wherein its levels correlate strongly with cancer genesis, progression, metastasis, and invasion. Consequently, it has emerged as a novel anticancer therapeutic target. Nonetheless, developing EZH2 inhibitors (EZH2i) has encountered numerous difficulties, such as pre-clinical drug resistance and poor therapeutic effect. The EZH2i synergistically suppresses cancers when used in combination with additional antitumor drugs, such as PARP inhibitors, HDAC inhibitors, BRD4 inhibitors, EZH1 inhibitors, and EHMT2 inhibitors. Typically, the use of dual inhibitors of two different targets mediated by one individual molecule has been recognized as the preferred approach for overcoming the limitations of EZH2 monotherapy. The present review discusses the theoretical basis for designing EZH2-based dual-target inhibitors, and also describes some in vitro and in vivo analysis results.

A patent review of EZH2 inhibitors from 2017 and beyond

INTRODUCTION

EZH2 is an important epigenetic regulator that forms the PRC2 complex with SUZ12, EED and RbAp46/48. As the key catalytic subunit of PRC2, EZH2 regulates the trimethylation of histone H3K27, which in turn promotes chromatin condensation and represses the transcription of relevant target genes. EZH2 overexpression and mutations are strictly related to tumor proliferation, invasion and metastasis. Currently, a large number of highly specific EZH2 inhibitors have been developed and some have already been in clinical trials.

AREAS COVERED

The aim of the present review is to provide an overview of the molecular mechanisms of EZH2 inhibitors and to highlight the research advances in the patent literature published from 2017 to date. A search of the literature and patents for EZH2 inhibitors and degraders was performed using the Web of Science, SCIFinder, WIPO, USPTO, EPO and CNIPA databases.

EXPERT OPINION

In recent years, a great number of structurally diverse EZH2 inhibitors have been identified, including EZH2 reversible inhibitors, EZH2 irreversible inhibitors, EZH2-based dual inhibitors and EZH2 degraders. Despite the multiple challenges, EZH2 inhibitors offer promising potential for the treatment of various diseases, such as cancers.

Epigenetic regulation by ASXL1 in myeloid malignancies

Myeloid malignancies are clonal hematopoietic disorders that are comprised of a spectrum of genetically heterogeneous disorders, including myelodysplastic syndromes (MDS), myeloproliferative neoplasms (MPN), chronic myelomonocytic leukemia (CMML), and acute myeloid leukemia (AML). Myeloid malignancies are characterized by excessive proliferation, abnormal self-renewal, and/or differentiation defects of hematopoietic stem cells (HSCs) and myeloid progenitor cells hematopoietic stem/progenitor cells (HSPCs). Myeloid malignancies can be caused by genetic and epigenetic alterations that provoke key cellular functions, such as self-renewal, proliferation, biased lineage commitment, and differentiation. Advances in next-generation sequencing led to the identification of multiple mutations in myeloid neoplasms, and many new gene mutations were identified as key factors in driving the pathogenesis of myeloid malignancies. The polycomb protein ASXL1 was identified to be frequently mutated in all forms of myeloid malignancies, with mutational frequencies of 20%, 43%, 10%, and 20% in MDS, CMML, MPN, and AML, respectively. Significantly, ASXL1 mutations are associated with a poor prognosis in all forms of myeloid malignancies. The fact that ASXL1 mutations are associated with poor prognosis in patients with CMML, MDS, and AML, points to the possibility that ASXL1 mutation is a key factor in the development of myeloid malignancies. This review summarizes the recent advances in understanding myeloid malignancies with a specific focus on ASXL1 mutations.

Polycomb Repressive Complex 2 in Oncology

Dynamic regulation of the chromatin state by Polycomb Repressive Complex 2 (PRC2) provides an important mean for epigenetic gene control that can profoundly influence normal development and cell lineage specification. PRC2 and PRC2-induced methylation of histone H3 lysine 27 (H3K27) are critically involved in a wide range of DNA-templated processes, which at least include transcriptional repression and gene imprinting, organization of three-dimensional chromatin structure, DNA replication and DNA damage response and repair. PRC2-based genome regulation often goes wrong in diseases, notably cancer. This chapter discusses about different modes-of-action through which PRC2 and EZH2, a catalytic subunit of PRC2, mediate (epi)genomic and transcriptomic regulation. We will also discuss about how alteration or mutation of the PRC2 core or axillary component promotes oncogenesis, how post-translational modification regulates functionality of EZH2 and PRC2, and how PRC2 and other epigenetic pathways crosstalk. Lastly, we will briefly touch on advances in targeting EZH2 and PRC2 dependence as cancer therapeutics.

Dual inhibition of EZH1/2 induces cell cycle arrest of B cell acute lymphoblastic leukemia cells through upregulation of CDKN1C and TP53INP1

Disease-risk stratification and development of intensified chemotherapy protocols have substantially improved the outcome of acute lymphoblastic leukemia (ALL). However, outcomes of relapsed or refractory cases remain poor. Previous studies have discussed the oncogenic role of enhancer of zeste homolog 1 and 2 (EZH1/2), and the efficacy of dual inhibition of EZH1/2 as a treatment for hematological malignancy. Here, we investigated whether an EZH1/2 dual inhibitor, DS-3201 (valemetostat), has antitumor effects on B cell ALL (B-ALL). DS-3201 inhibited growth of B-ALL cell lines more significantly and strongly than the EZH2-specific inhibitor EPZ-6438, and induced cell cycle arrest and apoptosis in vitro. RNA-seq analysis to determine the effect of DS-3201 on cell cycle arrest-related genes expressed by B-ALL cell lines showed that DS-3201 upregulated CDKN1C and TP53INP1. CRIPSR/Cas9 knockout confirmed that CDKN1C and TP53INP1 are direct targets of EZH1/2 and are responsible for the antitumor effects of DS-3201 against B-ALL. Furthermore, a patient-derived xenograft (PDX) mouse model showed that DS-3201 inhibited the growth of B-ALL harboring MLL-AF4 significantly. Thus, DS-3201 provides another option for treatment of B-ALL.

The genetics of myelodysplastic syndromes and the opportunities for tailored treatments

Genomic instability, microenvironmental aberrations, and somatic mutations contribute to the phenotype of myelodysplastic syndrome and the risk for transformation to AML. Genes involved in RNA splicing, DNA methylation, histone modification, the cohesin complex, transcription, DNA damage response pathway, signal transduction and other pathways constitute recurrent mutational targets in MDS. RNA-splicing and DNA methylation mutations seem to occur early and are reported as driver mutations in over 50% of MDS patients. The improved understanding of the molecular landscape of MDS has led to better disease and risk classification, leading to novel therapeutic opportunities. Based on these findings, novel agents are currently under preclinical and clinical development and expected to improve the clinical outcome of patients with MDS in the upcoming years. This review provides a comprehensive update of the normal gene function as well as the impact of mutations in the pathogenesis, deregulation, diagnosis, and prognosis of MDS, focuses on the most recent advances of the genetic basis of myelodysplastic syndromes and their clinical relevance, and the latest targeted therapeutic approaches including investigational and approved agents for MDS.

Critical Roles of Polycomb Repressive Complexes in Transcription and Cancer

Polycomp group (PcG) proteins are members of highly conserved multiprotein complexes, recognized as gene transcriptional repressors during development and shown to play a role in various physiological and pathological processes. PcG proteins consist of two Polycomb repressive complexes (PRCs) with different enzymatic activities: Polycomb repressive complexes 1 (PRC1), a ubiquitin ligase, and Polycomb repressive complexes 2 (PRC2), a histone methyltransferase. Traditionally, PRCs have been described to be associated with transcriptional repression of homeotic genes, as well as gene transcription activating effects. Particularly in cancer, PRCs have been found to misregulate gene expression, not only depending on the function of the whole PRCs, but also through their separate subunits. In this review, we focused especially on the recent findings in the transcriptional regulation of PRCs, the oncogenic and tumor-suppressive roles of PcG proteins, and the research progress of inhibitors targeting PRCs.

Chemical biology and pharmacology of histone lysine methylation inhibitors

Histone lysine methylation is a post-translational modification that plays a key role in the epigenetic regulation of a broad spectrum of biological processes. Moreover, the dysregulation of histone lysine methyltransferases (KMTs) has been implicated in the pathogenesis of several diseases particularly cancer. Due to their pathobiological importance, KMTs have garnered immense attention over the last decade as attractive therapeutic targets. These endeavors have culminated in tens of chemical probes that have been used to interrogate many aspects of histone lysine methylation. Besides, over a dozen inhibitors have been advanced to clinical trials, including the EZH2 inhibitor tazemetostat approved for the treatment of follicular lymphoma and advanced epithelioid sarcoma. In this Review, we highlight the chemical biology and pharmacology of KMT inhibitors and targeted protein degraders focusing on the clinical development of EZH1/2, DOT1L, Menin-MLL, and WDR5-MLL inhibitors. We also briefly discuss the pharmacologic targeting of other KMTs.

Maternal Ezh1/2 deficiency in oocyte delays H3K27me2/3 restoration and impairs epiblast development responsible for embryonic sub-lethality in mouse

How maternal Ezh1 and Ezh2 function in H3K27 methylation in vivo in pre-implantation embryos and during embryonic development is not clear. Here, we have deleted Ezh1 and Ezh2 alone or simultaneously from mouse oocytes. H3K27me3 was absent in oocytes without Ezh2 alone, while both H3K27me2 and H3K27me3 were absent in Ezh1/Ezh2 (Ezh1/2) double knockout (KO) oocytes. The effects of Ezh1/2 maternal KO were inherited in zygotes and early embryos, in which restoration of H3K27me3 and H3K27me2 was delayed by the loss of Ezh2 alone or of both Ezh1 and Ezh2. However, the ablation of both Ezh1 and Ezh2, but not Ezh1 or Ezh2 alone, led to significantly decreased litter size due to growth retardation post-implantation. Maternal Ezh1/2 deficiency caused compromised H3K27me3 and pluripotent epiblast cells in late blastocysts, followed by defective embryonic development. By using RNA-seq, we examined crucial developmental genes in maternal Ezh1/2 KO embryos and identified 80 putatively imprinted genes. Maternal Ezh1/2-H3K27 methylation is inherited in offspring embryos and has a critical effect on fetal and placental development. Thus, this work sheds light on maternal epigenetic modifications during embryonic development.

Epigenetic traits inscribed in chromatin accessibility in aged hematopoietic stem cells

Hematopoietic stem cells (HSCs) exhibit considerable cell-intrinsic changes with age. Here, we present an integrated analysis of transcriptome and chromatin accessibility of aged HSCs and downstream progenitors. Alterations in chromatin accessibility preferentially take place in HSCs with aging, which gradually resolve with differentiation. Differentially open accessible regions (open DARs) in aged HSCs are enriched for enhancers and show enrichment of binding motifs of the STAT, ATF, and CNC family transcription factors that are activated in response to external stresses. Genes linked to open DARs show significantly higher levels of basal expression and their expression reaches significantly higher peaks after cytokine stimulation in aged HSCs than in young HSCs, suggesting that open DARs contribute to augmented transcriptional responses under stress conditions. However, a short-term stress challenge that mimics infection is not sufficient to induce persistent chromatin accessibility changes in young HSCs. These results indicate that the ongoing and/or history of exposure to external stresses may be epigenetically inscribed in HSCs to augment their responses to external stimuli.

Haematopoietic stem cells (HSCs) exhibit considerable cell-intrinsic changes with age. Here the authors demonstrate that differentially accessible regions in aged HSC chromatin are enriched for stress-responsive enhancers and act as an epigenetic hub to augment transcriptional responses of aged HSCs to external stimuli.

The Coming of Age of Preclinical Models of MDS

Myelodysplastic syndromes (MDS) are a heterogeneous group of clonal bone-marrow diseases with ineffective hematopoiesis resulting in cytopenias and morphologic dysplasia of hematopoietic cells. MDS carry a wide spectrum of genetic abnormalities, ranging from chromosomal abnormalities such as deletions/additions, to recurrent mutations affecting the spliceosome, epigenetic modifiers, or transcription factors. As opposed to AML, research in MDS has been hindered by the lack of preclinical models that faithfully replicate the complexity of the disease and capture the heterogeneity. The complex molecular landscape of the disease poses a unique challenge when creating transgenic mouse-models. In addition, primary MDS cells are difficult to manipulate ex vivo limiting in vitro studies and resulting in a paucity of cell lines and patient derived xenograft models. In recent years, progress has been made in the development of both transgenic and xenograft murine models advancing our understanding of individual contributors to MDS pathology as well as the complex primary interplay of genetic and microenvironment aberrations. We here present a comprehensive review of these transgenic and xenograft models for MDS and future directions.

The significance of CUX1 and chromosome 7 in myeloid malignancies

PURPOSE OF REVIEW

Loss of chromosome 7 has long been associated with adverse-risk myeloid malignancy. In the last decade, CUX1 has been identified as a critical tumor suppressor gene (TSG) located within a commonly deleted segment of chromosome arm 7q. Additional genes encoded on 7q have also been identified as bona fide myeloid tumor suppressors, further implicating chromosome 7 deletions in disease pathogenesis. This review will discuss the clinical implications of del(7q) and CUX1 mutations, both in disease and clonal hematopoiesis, and synthesize recent literature on CUX1 and other chromosome 7 TSGs.

RECENT FINDINGS

Two major studies, including a new mouse model, have been published that support a role for CUX1 inactivation in the development of myeloid neoplasms. Additional recent studies describe the cellular and hematopoietic effects from loss of the 7q genes LUC7L2 and KMT2C/MLL3, and the implications of chromosome 7 deletions in clonal hematopoiesis.

SUMMARY

Mounting evidence supports CUX1 as being a key chromosome 7 TSG. As 7q encodes additional myeloid regulators and tumor suppressors, improved models of chromosome loss are needed to interrogate combinatorial loss of these critical 7q genes.

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.

Aging and Clonal Behavior of Hematopoietic Stem Cells

Hematopoietic stem cells (HSCs) are the only cell population that possesses both a self-renewing capacity and multipotency, and can give rise to all lineages of blood cells throughout an organism's life. However, the self-renewal capacity of HSCs is not infinite, and cumulative evidence suggests that HSCs alter their function and become less active during organismal aging, leading ultimately to the disruption of hematopoietic homeostasis, such as anemia, perturbed immunity and increased propensity to hematological malignancies. Thus, understanding how HSCs alter their function during aging is a matter of critical importance to prevent or overcome these age-related changes in the blood system. Recent advances in clonal analysis have revealed the functional heterogeneity of murine HSC pools that is established upon development and skewed toward the clonal expansion of functionally poised HSCs during aging. In humans, next-generation sequencing has revealed age-related clonal hematopoiesis that originates from HSC subsets with acquired somatic mutations, and has highlighted it as a significant risk factor for hematological malignancies and cardiovascular diseases. In this review, we summarize the current fate-mapping strategies that are used to track and visualize HSC clonal behavior during development or after stress. We then review the age-related changes in HSCs that can be inherited by daughter cells and act as a cellular memory to form functionally distinct clones. Altogether, we link aging of the hematopoietic system to HSC clonal evolution and discuss how HSC clones with myeloid skewing and low regenerative potential can be expanded during aging.

MicroRNA-20a Suppresses Tumor Proliferation and Metastasis in Hepatocellular Carcinoma by Directly Targeting EZH1

Purpose

Hepatocellular carcinoma (HCC), a worldwide leading cause of morbidity and mortality, is the most frequent primary liver tumor. Most HCC patients are diagnosed with advanced liver cancer, resulting in a very low 5-year survival rate. Thus, there is an urgent need for the development of targeted therapies. In this study, we aimed to investigate the effect and mechanism of the miR-20a/EZH1 axis on the proliferation and metastasis of HCC and the inhibitory effect of the EZH1/EZH2 inhibitor UNC1999 on HCC.

Materials and Methods

The expression of miR-20a in human HCC tissues and cell lines was detected using quantitative real-time PCR (qRT-PCR). The expressions of proteins were analyzed with immunohistochemistry and Western blotting. Luciferase assay was used to verify whether miR-20a targets EZH1 or EZH2. The effect of miR-20a on HCC progression was studied in vivo and in vitro. The tumor inhibitory effect of UNC1999 was confirmed in vivo. CCK8 assay, wound healing assay, cell migration and invasion assay were used to evaluate the synergistic effect of UNC1999 with sorafenib. RNA sequencing (RNA-seq) was performed to screen the differentially expressed genes in the Huh7 and SMMC7721 cell lines after UNC1999, sorafenib, and combination treatments.

Results

In this study, miR-20a showed a lower expression in both HCC tissues and cell lines. MiR-20a inhibited the proliferation and migration of SMMC7721 and Huh7 cells. The results of the luciferase assay and Western blot analysis revealed that miR-20a directly targeted EZH1, a histone methyltransferase. We demonstrated that miR-20a negatively regulated the expression of EZH1 and inhibited the proliferation and metastasis of HCC by reducing H3K27 methylation. We found UNC1999 inhibited tumor cells proliferation and enhanced the inhibitory effect of sorafenib.

Conclusion

We demonstrated that miR-20a suppresses the tumor proliferation and metastasis in HCC by directly targeting EZH1. UNC1999 can inhibit tumor proliferation in vivo and increase the sensitivity of hepatoma cell lines to sorafenib.

Mouse Models of Frequently Mutated Genes in Acute Myeloid Leukemia

Acute myeloid leukemia is a clinically and biologically heterogeneous blood cancer with variable prognosis and response to conventional therapies. Comprehensive sequencing enabled the discovery of recurrent mutations and chromosomal aberrations in AML. Mouse models are essential to study the biological function of these genes and to identify relevant drug targets. This comprehensive review describes the evidence currently available from mouse models for the leukemogenic function of mutations in seven functional gene groups: cell signaling genes, epigenetic modifier genes, nucleophosmin 1 (NPM1), transcription factors, tumor suppressors, spliceosome genes, and cohesin complex genes. Additionally, we provide a synergy map of frequently cooperating mutations in AML development and correlate prognosis of these mutations with leukemogenicity in mouse models to better understand the co-dependence of mutations in AML.

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.

Convergence of oncogenic cooperation at single-cell and single-gene levels drives leukemic transformation

Cancers develop from the accumulation of somatic mutations, yet it remains unclear how oncogenic lesions cooperate to drive cancer progression. Using a mouse model harboring NRasG12D and EZH2 mutations that recapitulates leukemic progression, we employ single-cell transcriptomic profiling to map cellular composition and gene expression alterations in healthy or diseased bone marrows during leukemogenesis. At cellular level, NRasG12D induces myeloid lineage-biased differentiation and EZH2-deficiency impairs myeloid cell maturation, whereas they cooperate to promote myeloid neoplasms with dysregulated transcriptional programs. At gene level, NRasG12D and EZH2-deficiency independently and synergistically deregulate gene expression. We integrate results from histopathology, leukemia repopulation, and leukemia-initiating cell assays to validate transcriptome-based cellular profiles. We use this resource to relate developmental hierarchies to leukemia phenotypes, evaluate oncogenic cooperation at single-cell and single-gene levels, and identify GEM as a regulator of leukemia-initiating cells. Our studies establish an integrative approach to deconvolute cancer evolution at single-cell resolution in vivo.

EZH1/2 inhibition augments the anti-tumor effects of sorafenib in hepatocellular carcinoma

Both EZH2 and its homolog EZH1 function as histone H3 Lysine 27 (H3K27) methyltransferases and repress the transcription of target genes. Dysregulation of H3K27 trimethylation (H3K27me3) plays an important role in the development and progression of cancers such as hepatocellular carcinoma (HCC). This study investigated the relationship between the expression of EZH1/2 and the level of H3K27me3 in HCC. Additionally, the role of EZH1/2 in cell growth, tumorigenicity, and resistance to sorafenib were also analyzed. Both the lentiviral knockdown and the pharmacological inhibition of EZH1/2 (UNC1999) diminished the level of H3K27me3 and suppressed cell growth in liver cancer cells, compared with EZH1 or EZH2 single knockdown. Although a significant association was observed between EZH2 expression and H3K27me3 levels in HCC samples, overexpression of EZH1 appeared to contribute to enhanced H3K27me3 levels in some EZH2lowH3K27me3high cases. Akt suppression following sorafenib treatment resulted in an increase of the H3K27me3 levels through a decrease in EZH2 phosphorylation at serine 21. The combined use of sorafenib and UNC1999 exhibited synergistic antitumor effects in vitro and in vivo. Combination treatment canceled the sorafenib-induced enhancement in H3K27me3 levels, indicating that activation of EZH2 function is one of the mechanisms of sorafenib-resistance in HCC. In conclusion, sorafenib plus EZH1/2 inhibitors may comprise a novel therapeutic approach in HCC.

Polycomb group proteins in cancer: multifaceted functions and strategies for modulation

Polycomb repressive complexes (PRCs) are a heterogenous collection of dozens, if not hundreds, of protein complexes composed of various combinations of subunits. PRCs are transcriptional repressors important for cell-type specificity during development, and as such, are commonly mis-regulated in cancer. PRCs are broadly characterized as PRC1 with histone ubiquitin ligase activity, or PRC2 with histone methyltransferase activity; however, the mechanism by which individual PRCs, particularly the highly diverse set of PRC1s, alter gene expression has not always been clear. Here we review the current understanding of how PRCs act, both individually and together, to establish and maintain gene repression, the biochemical contribution of individual PRC subunits, the mis-regulation of PRC function in different cancers, and the current strategies for modulating PRC activity. Increased mechanistic understanding of PRC function, as well as cancer-specific roles for individual PRC subunits, will uncover better targets and strategies for cancer therapies.

EZH2 Knockout in Oral Cavity Basal Epithelia Causes More Invasive Squamous Cell Carcinomas

Oral squamous cell carcinoma (oral SCC) is an aggressive disease and despite intensive treatments, 5-year survival rates for patients have remained low in the last 20 years. Enhancer of zeste homolog 2 (EZH2), part of polycomb repressive complex 2 (PRC2), is highly expressed in human oral SCC samples and cell lines and has been associated with greater epithelia-to-mesenchymal transition (EMT), invasion, and metastasis. Here we developed a tamoxifen-regulated, transgenic mouse line (KcEZH2) in which EZH2 is selectively knocked out (KO) in some tongue epithelial basal stem cells (SCs) in adult mice. EZH2 KO SCs do not show the H3K27me3 mark, as assessed by double-label immunofluorescence. We used this mouse line to assess EZH2 actions during oral tumorigenesis with our immunocompetent 4-nitroquinoline 1-oxide (4-NQO) model of oral SCC. We report that higher percentages of mice with invasive SCCs and high-grade neoplastic lesions are observed in mice containing EZH2 KO SCs (KcEZH2-2TΔ and KcEZH2-5TΔ mice). Moreover, EZH2 expression does not correlate with the expression of markers of invasive SCCs. Finally, EZH2 KO cells that are E-cadherin+ are present at invasion fronts infiltrating underlying muscle tissue. Our findings indicate that the knockout of EZH2 in basal SCs of tongue epithelia results in more aggressive carcinomas, and this should be considered when targeting EZH2 as a therapeutic strategy.

Molecular Targeted Therapy and Immunotherapy for Myelodysplastic Syndrome

Myelodysplastic syndrome (MDS) is a heterogeneous, clonal hematological disorder characterized by ineffective hematopoiesis, cytopenia, morphologic dysplasia, and predisposition to acute myeloid leukemia (AML). Stem cell genomic instability, microenvironmental aberrations, and somatic mutations contribute to leukemic transformation. The hypomethylating agents (HMAs), azacitidine and decitabine are the standard of care for patients with higher-risk MDS. Although these agents induce responses in up to 40-60% of patients, primary or secondary drug resistance is relatively common. To improve the treatment outcome, combinational therapies comprising HMA with targeted therapy or immunotherapy are being evaluated and are under continuous development. This review provides a comprehensive update of the molecular pathogenesis and immune-dysregulations involved in MDS, mechanisms of resistance to HMA, and strategies to overcome HMA resistance.

Differential requirement for the Polycomb repressor complex 2 in dendritic cell and tissue-resident myeloid cell homeostasis

[Figure: see text].

EZH2/EHMT2 Histone Methyltransferases Inhibit the Transcription of DLX5 and Promote the Transformation of Myelodysplastic Syndrome to Acute Myeloid Leukemia

Myelodysplastic syndrome (MDS) is characterized by clonal hematopoiesis and impaired differentiation, and may develop to acute myeloid leukemia (AML). We explored the mechanism of histone methyltransferase EZH2/EHMT2 during the transformation of MDS into AML. Expression of EZH2/EHMT2 in patients and NHD13 mice was detected. EZH2 and EHMT2 were silenced or overexpressed in SKM-1 cells. The cell proliferation and cycle were evaluated. Levels of DLX5, H3K27me3, and H3K9me2 in SKM-1 cells were detected. Binding of DLX5 promoter region to H3K27me3 and H3K9me2 was examined. Levels of H3K27me3/H3K9me2 were decreased by EZH2/EHMT2 inhibitor (EPZ-6438/BIX-01294), and changes of DLX5 expression and cell proliferation were observed. EZH2 was poorly expressed in MDS patients but highly expressed in MDS-AML patients. EHMT2 was promoted in both MDS and MDS-AML patients. EZH2 expression was reduced and EHMT2 expression was promoted in NHD13 mice. NHD13 mice with overexpressing EZH2 or EHMT2 transformed into AML more quickly. Intervention of EZH2 or EHMT2 inhibited SKM-1 cell proliferation and promoted DLX5 expression. When silencing EZH1 and EZH2 in SKM-1 cells, the H3K27me3 level was decreased. EZH2 silencing repressed the proliferation of SKM-1 cells. Transcription level of DLX5 in SKM-1 cells was inhibited by H3K27me3 and H3K9me2. Enhanced DLX5 repressed SKM-1 cell proliferation. In conclusion, EZH2/EHMT2 catalyzed H3K27me3/H3K9me2 to inhibit the transcription of DLX5, thus promoting the transformation from MDS to AML.

The histone H3.3 chaperone HIRA restrains erythroid-biased differentiation of adult hematopoietic stem cells

Histone variants contribute to the complexity of the chromatin landscape and play an integral role in defining DNA domains and regulating gene expression. The histone H3 variant H3.3 is incorporated into genic elements independent of DNA replication by its chaperone HIRA. Here we demonstrate that Hira is required for the self-renewal of adult hematopoietic stem cells (HSCs) and to restrain erythroid differentiation. Deletion of Hira led to rapid depletion of HSCs while differentiated hematopoietic cells remained largely unaffected. Depletion of HSCs after Hira deletion was accompanied by increased expression of bivalent and erythroid genes, which was exacerbated upon cell division and paralleled increased erythroid differentiation. Assessing H3.3 occupancy identified a subset of polycomb-repressed chromatin in HSCs that depends on HIRA to maintain the inaccessible, H3.3-occupied state for gene repression. HIRA-dependent H3.3 incorporation thus defines distinct repressive chromatin that represses erythroid differentiation of HSCs.

UTX maintains the functional integrity of the murine hematopoietic system by globally regulating aging-associated genes

Epigenetic regulation is essential for the maintenance of the hematopoietic system, and its deregulation is implicated in hematopoietic disorders. In this study, UTX, a demethylase for lysine 27 on histone H3 (H3K27) and a component of COMPASS-like and SWI/SNF complexes, played an essential role in the hematopoietic system by globally regulating aging-associated genes. Utx-deficient (UtxΔ/Δ) mice exhibited myeloid skewing with dysplasia, extramedullary hematopoiesis, impaired hematopoietic reconstituting ability, and increased susceptibility to leukemia, which are the hallmarks of hematopoietic aging. RNA-sequencing (RNA-seq) analysis revealed that Utx deficiency converted the gene expression profiles of young hematopoietic stem-progenitor cells (HSPCs) to those of aged HSPCs. Utx expression in hematopoietic stem cells declined with age, and UtxΔ/Δ HSPCs exhibited increased expression of an aging-associated marker, accumulation of reactive oxygen species, and impaired repair of DNA double-strand breaks. Pathway and chromatin immunoprecipitation analyses coupled with RNA-seq data indicated that UTX contributed to hematopoietic homeostasis mainly by maintaining the expression of genes downregulated with aging via demethylase-dependent and -independent epigenetic programming. Of note, comparison of pathway changes in UtxΔ/Δ HSPCs, aged muscle stem cells, aged fibroblasts, and aged induced neurons showed substantial overlap, strongly suggesting common aging mechanisms among different tissue stem cells.

Epigenetic "Drivers" of Cancer

Genetics is at the basis of cancer initiation and evolution, but emerging evidence indicates that mutations are not sufficient to produce cancer, indicating a role for epigenetic contributions to the different stages of tumorigenesis. While the genetic tracks of cancer have been widely investigated, the epigenetic "drivers" remain a vague definition. Gene-environment interactions can produce gene-regulatory programs that dictate pathogenesis; this implies a reciprocal relationship where environmental factors contribute to genetic mechanisms of tumorigenesis (i.e. mutagenesis) and genetic factors influence the cellular response to extrinsic stress. In this review article, we attempt to summarise the most remarkable findings demonstrating a contribution of epigenetic factors as proper "drivers" of tumorigenesis. We also try to pose attention on the relevance of epigenetic mechanisms as downstream consequences of genes versus environment interaction.

Dual role of EZH2 on megakaryocyte differentiation

EZH2, the enzymatic component of PRC2, has been identified as a key factor in hematopoiesis. EZH2 loss of function mutations have been found in myeloproliferative neoplasms, more particularly in myelofibrosis, but the precise function of EZH2 in megakaryopoiesis is not fully delineated. Here, we show that EZH2 inhibition by small molecules and shRNA induces MK commitment by accelerating lineage marker acquisition without change in proliferation. Later in differentiation, EZH2 inhibition blocks proliferation, polyploidization and decreases proplatelet formation. EZH2 inhibitors similarly reduce MK polyploidization and proplatelet formation in vitro and platelet level in vivo in a JAK2V617F background. In transcriptome profiling, the defect in proplatelet formation was associated with an aberrant actin cytoskeleton regulation pathway, whereas polyploidization was associated with an inhibition of expression of genes involved in DNA replication and repair, and an upregulation of CDK inhibitors, more particularly CDKN1A and CDKN2D. The knockdown of CDKN1A and at a lesser extend of CDKN2D could partially rescue the percentage of polyploid MKs. Moreover, H3K27me3 and EZH2 ChIP assays revealed that only CDKN1A is a direct EZH2 target while CDKN2D expression is not directly regulated by EZH2 suggesting that EZH2 controls MK polyploidization directly through CDKN1A and indirectly through CDKN2D.

CDKN1C-mediated growth inhibition by an EZH1/2 dual inhibitor overcomes resistance of mantle cell lymphoma to ibrutinib

Mantle cell lymphoma (MCL) is a rare subtype of non‐Hodgkin's lymphoma, which is characterized by overexpression of cyclin D1. Although novel drugs, such as ibrutinib, show promising clinical outcomes, relapsed MCL often acquires drug resistance. Therefore, alternative approaches for refractory and relapsed MCL are needed. Here, we examined whether a novel inhibitor of enhancer of zeste homologs 1 and 2 (EZH1/2), OR‐S1 (a close analog of the clinical‐stage compound valemetostat), had an antitumor effect on MCL cells. In an ibrutinib‐resistant MCL patient–derived xenograft (PDX) mouse model, OR‐S1 treatment by oral administration significantly inhibited MCL tumor growth, whereas ibrutinib did not. In vitro growth assays showed that compared with an established EZH2‐specific inhibitor GSK126, OR‐S1 had a marked antitumor effect on MCL cell lines. Furthermore, comprehensive gene expression analysis was performed using OR‐S1–sensitive or insensitive MCL cell lines and showed that OR‐S1 treatment modulated B‐cell activation, differentiation, and cell cycle. In addition, we identified Cyclin Dependent Kinase Inhibitor 1C (CDKN1C, also known as p57, KIP2), which contributes to cell cycle arrest, as a direct target of EZH1/2 and showed that its expression influenced MCL cell proliferation. These results suggest that EZH1/2 may be a potential novel target for the treatment of aggressive ibrutinib‐resistant MCL via CDKN1C‐mediated cell cycle arrest.

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.

Peritoneal Modulators of EZH2-miR-155 Cross-Talk in Endometriosis

Activation of trimethylation of histone 3 lysine 27 (H3K27me3) by EZH2, a component of the Polycomb repressive complex 2 (PRC2), is suggested to play a role in endometriosis. However, the mechanism by which this complex is dysregulated in endometriosis is not completely understood. Here, using eutopic and ectopic tissues, as well as peritoneal fluid (PF) from IRB-approved and consented patients with and without endometriosis, the expression of PRC2 complex components, JARID2, miR-155 (known regulators of EZH2), and a key inflammatory modulator, FOXP3, was measured. A higher expression of EZH2, H3K27me3, JARID2, and FOXP3 as well as miR-155 was noted in both the patient tissues and in endometrial PF treated cells. Gain-or-loss of function of miR-155 showed an effect on the PRC2 complex but had little effect on JARID2 expression, suggesting alternate pathways. Chromatin immunoprecipitation followed by qPCR showed differential expression of PRC2 complex proteins and its associated binding partners in JARID2 vs. EZH2 pull down assays. In particular, endometriotic PF treatment increased the expression of PHF19 (p = 0.0474), a gene silencer and co-factor that promotes PRC2 interaction with its targets. Thus, these studies have identified the potential novel crosstalk between miR-155-PRC2 complex-JARID2 and PHF19 in endometriosis, providing an opportunity to test other epigenetic targets in endometriosis.

The role and mechanisms of polycomb repressive complex 2 on the regulation of osteogenic and neurogenic differentiation of stem cells

The stem cells differentiate into osteoblasts or neurocytes is the key process for treatment of bone‐ or neural tissue‐related diseases which is caused by ageing, fracture, injury, inflammation, etc Polycomb group complexes (PcGs), especially the polycomb repressive complex 2 (PRC2), act as pivotal epigenetic regulators by modifying key developmental regulatory genes during stem cells differentiation. In this review, we summarize the core subunits, the variants and the potential functions of PRC2. We also highlight the underlying mechanisms of PRC2 associated with the osteogenic and neurogenic differentiation of stem cells, including its interaction with non‐coding RNAs, histone acetyltransferases, histone demethylase, DNA methyltransferase and polycomb repressive complex 1. This review provided a substantial information of epigenetic regulation mediated by PRC2 which leads to the osteogenic and neurogenic differentiation of stem cells.

The roles of Polycomb repressive complexes in mammalian development and cancer

More than 80 years ago, the first Polycomb-related phenotype was identified in Drosophila melanogaster. Later, a group of diverse genes collectively called Polycomb group (PcG) genes were identified based on common mutant phenotypes. PcG proteins, which are well-conserved in animals, were originally characterized as negative regulators of gene transcription during development and subsequently shown to function in various biological processes; their deregulation is associated with diverse phenotypes in development and in disease, especially cancer. PcG proteins function on chromatin and can form two distinct complexes with different enzymatic activities: Polycomb repressive complex 1 (PRC1) is a histone ubiquitin ligase and PRC2 is a histone methyltransferase. Recent studies have revealed the existence of various mutually exclusive PRC1 and PRC2 variants. In this Review, we discuss new concepts concerning the biochemical and molecular functions of these new PcG complex variants, and how their epigenetic activities are involved in mammalian development and cancer.

Loss-of-function mutations in the histone methyltransferase EZH2 promote chemotherapy resistance in AML

Chemotherapy resistance is the main impediment in the treatment of acute myeloid leukaemia (AML). Despite rapid advances, the various mechanisms inducing resistance development remain to be defined in detail. Here we report that loss-of-function mutations (LOF) in the histone methyltransferase EZH2 have the potential to confer resistance against the chemotherapeutic agent cytarabine. We identify seven distinct EZH2 mutations leading to loss of H3K27 trimethylation via multiple mechanisms. Analysis of matched diagnosis and relapse samples reveal a heterogenous regulation of EZH2 and a loss of EZH2 in 50% of patients. We confirm that loss of EZH2 induces resistance against cytarabine in the cell lines HEK293T and K562 as well as in a patient-derived xenograft model. Proteomics and transcriptomics analysis reveal that resistance is conferred by upregulation of multiple direct and indirect EZH2 target genes that are involved in apoptosis evasion, augmentation of proliferation and alteration of transmembrane transporter function. Our data indicate that loss of EZH2 results in upregulation of its target genes, providing the cell with a selective growth advantage, which mediates chemotherapy resistance.

Chromatin-Spliceosome Mutations in Acute Myeloid Leukemia

Simple Summary

Recent genomic studies have identified chromatin-spliceosome (CS)-acute myeloid leukemia (AML) as a new subgroup of AML. CS-AML is defined by several mutations that perturb epigenetic regulation, such as those affecting splicing factors, cohesin components, transcription factors, and chromatin modifiers, which are also frequently mutated in other myeloid malignancies, such as myelodysplastic syndrome and secondary AML. Thus, these mutations identify myeloid neoplasms that lie on the boundaries of conventional differential diagnosis. CS-AML shares several clinical characteristics with secondary AML. Therefore, the presence of CS-mutations may help to better classify and manage patients with AML and related disorders. The aim of this review is to discuss the genetic and clinical characteristics of CS-AML and roles of driver mutations defining this unique genomic subgroup of AML.

Abstract

Recent genetic studies on large patient cohorts with acute myeloid leukemia (AML) have cataloged a comprehensive list of driver mutations, resulting in the classification of AML into distinct genomic subgroups. Among these subgroups, chromatin-spliceosome (CS)-AML is characterized by mutations in the spliceosome, cohesin complex, transcription factors, and chromatin modifiers. Class-defining mutations of CS-AML are also frequently identified in myelodysplastic syndrome (MDS) and secondary AML, indicating the molecular similarity among these diseases. CS-AML is associated with myelodysplasia-related changes in hematopoietic cells and poor prognosis, and, thus, can be treated using novel therapeutic strategies and allogeneic stem cell transplantation. Functional studies of CS-mutations in mice have revealed that CS-mutations typically cause MDS-like phenotypes by altering the epigenetic regulation of target genes. Moreover, multiple CS-mutations often synergistically induce more severe phenotypes, such as the development of lethal MDS/AML, suggesting that the accumulation of many CS-mutations plays a crucial role in the progression of MDS/AML. Indeed, the presence of multiple CS-mutations is a stronger indicator of CS-AML than a single mutation. This review summarizes the current understanding of the genetic and clinical features of CS-AML and the functional roles of driver mutations characterizing this unique category of AML.

Murine Modeling of Myeloproliferative Neoplasms

Myeloproliferative neoplasms, such as polycythemia vera, essential thrombocythemia, and primary myelofibrosis, are bone marrow disorders that result in the overproduction of mature clonal myeloid elements. Identification of recurrent genetic mutations has been described and aid in diagnosis and prognostic determination. Mouse models of these mutations have confirmed the biologic significance of these mutations in myeloproliferative neoplasm disease biology and provided greater insights on the pathways that are dysregulated with each mutation. The models are useful tools that have led to preclinical testing and provided data as validation for future myeloproliferative neoplasm clinical trials.