The methyltransferase G9a regulates HoxA9-dependent transcription in AML

HOXA9 transcription factor is a double-edged sword: from development to cancer progression

The HOXA9 transcription factor serves as a molecular orchestrator in cancer stemness, epithelial-mesenchymal transition (EMT), metastasis, and generation of the tumor microenvironment in hematological and solid malignancies. However, the multiple modes of regulation, multifaceted functions, and context-dependent interactions responsible for the dual role of HOXA9 as an oncogene or tumor suppressor in cancer remain obscure. Hence, unravelling its molecular complexities, binding partners, and interacting signaling molecules enables us to comprehend HOXA9-mediated transcriptional programs and molecular crosstalk. However, it is imperative to understand its central role in fundamental biological processes such as embryogenesis, foetus implantation, hematopoiesis, endothelial cell proliferation, and tissue homeostasis before designing targeted therapies. Indeed, it presents an enormous challenge for clinicians to selectively target its oncogenic functions or restore tumor-suppressive role without altering normal cellular functions. In addition to its implications in cancer, the present review also focuses on the clinical applications of HOXA9 in recurrence and drug resistance, which may provide a broader understanding beyond oncology, open new avenues for clinicians for accurate diagnoses, and develop personalized treatment strategies. Furthermore, we have also discussed the existing therapeutic options and accompanying challenges in HOXA9-targeted therapies in different cancer types.

SETDB1 as a cancer target: challenges and perspectives in drug design

Genome stability is governed by chromatin structural dynamics, which modify DNA accessibility under the influence of intra- and inter-nucleosomal contacts, histone post-translational modifications (PTMs) and variations, besides the activity of ATP-dependent chromatin remodelers. These are the main ways by which chromatin dynamics are regulated and connected to nuclear processes, which when dysregulated can frequently be associated with most malignancies. Recently, functional crosstalk between histone modifications and chromatin remodeling has emerged as a critical regulatory method of transcriptional regulation during cell destiny choice. Therefore, improving therapeutic outcomes for patients by focusing on epigenetic targets dysregulated in malignancies should help prevent cancer cells from developing resistance to anticancer treatments. For this reason, SET domain bifurcated histone lysine methyltransferase 1 (SETDB1) has gained a lot of attention recently as a cancer target. SETDB1 is a histone lysine methyltransferase that plays an important role in marking euchromatic and heterochromatic regions. Hence, it promotes the silencing of tumor suppressor genes and contributes to carcinogenesis. Some studies revealed that SETDB1 was overexpressed in various human cancer types, which enhanced tumor growth and metastasis. Thus, SETDB1 appears to be an attractive epigenetic target for new cancer treatments. In this review, we have discussed the effects of its overexpression on the progression of tumors and the development of inhibitor drugs that specifically target this enzyme.

Discovery of the First-in-Class G9a/GLP PROTAC Degrader

Aberrantly expressed lysine methyltransferases G9a and GLP, which catalyze mono- and dimethylation of histone H3 lysine 9 (H3K9), have been implicated in numerous cancers. Recent studies have uncovered both catalytic and noncatalytic oncogenic functions of G9a/GLP. As such, G9a/GLP catalytic inhibitors have displayed limited anticancer activity. Here, we report the discovery of the first-in-class G9a/GLP proteolysis targeting chimera (PROTAC) degrader 10 (MS8709), as a potential anticancer therapeutic. 10 induces G9a/GLP degradation in a concentration-, time-, and ubiquitin-proteasome system (UPS)-dependent manner. Futhermore, 10 does not alter the mRNA expression of G9a/GLP and is selective for G9a/GLP over other methyltransferases. Moreover, 10 displays superior cell growth inhibition to the parent G9a/GLP inhibitor UNC0642 in prostate, leukemia, and lung cancer cells and has suitable mouse pharmacokinetic properties for in vivo efficacy studies. Overall, 10 is a valuable chemical biology tool to further investigate the functions of G9a/GLP and a potential therapeutic for treating G9a/GLP-dependent cancers.

EHMT2 Suppresses ARRB1 Transcription and Activates the Hedgehog Signaling to Promote Malignant Phenotype and Stem Cell Property in Oral Squamous Cell Carcinoma

Oral squamous cell carcinoma (OSCC) represents the primary subtype of head and neck squamous cell carcinoma (HNSCC), characterized by a high morbidity and mortality rate. Although previous studies have established specific correlations between euchromatic histone lysine methyltransferase 2 (EHMT2), a histone lysine methyltransferase, and the malignant phenotype of OSCC cells, its biological functions in OSCC remain largely unknown. This study, grounded in bioinformatics predictions, aims to clarify the influence of EHMT2 on the malignant behavior of OSCC cells and delve into the underlying mechanisms. EHMT2 exhibited high expression in OSCC tissues and demonstrated an association with poor patient outcomes. Artificial EHMT2 silencing in OSCC cells, achieved through lentiviral vector infection, significantly inhibited colony formation, migration, invasion, and cell survival. Regarding the mechanism, EHMT2 was found to bind the promoter of arrestin beta 1 (ARRB1), thereby suppressing its transcription through H3K9me2 modification. ARRB1, in turn, was identified as a negative regulator of the Hedgehog pathway, leading to a reduction in the proteins GLI1 and PTCH1. Cancer stem cells (CSCs) were enriched through repeated sphere formation assays in two OSCC cell lines. EHMT2 was found to activate the Hedgehog pathway, thus promoting sphere formation, migration and invasion, survival, and tumorigenic activity of the OSCC-CSCs. Notably, these effects were counteracted by the additional overexpression of ARRB1. In conclusion, this study provides novel evidence suggesting that EHMT2 plays specific roles in enhancing stem cell properties in OSCC by modulating the ARRB1-Hedgehog signaling cascade.

How CBX proteins regulate normal and leukemic blood cells

Hematopoietic stem cell (HSC) fate decisions are dictated by epigenetic landscapes. The Polycomb Repressive Complex 1 (PRC1) represses genes that induce differentiation, thereby maintaining HSC self-renewal. Depending on which chromobox (CBX) protein (CBX2, CBX4, CBX6, CBX7, or CBX8) is part of the PRC1 complex, HSC fate decisions differ. Here, we review how this occurs. We describe how CBX proteins dictate age-related changes in HSCs and stimulate oncogenic HSC fate decisions, either as canonical PRC1 members or by alternative interactions, including non-epigenetic regulation. CBX2, CBX7, and CBX8 enhance leukemia progression. To target, reprogram, and kill leukemic cells, we suggest and describe multiple therapeutic strategies to interfere with the epigenetic functions of oncogenic CBX proteins. Future studies should clarify to what extent the non-epigenetic function of cytoplasmic CBX proteins is important for normal, aged, and leukemic blood cells.

Discovery of the First-in-class G9a/GLP PROTAC Degrader

Aberrantly expressed lysine methyltransferases G9a and GLP, which catalyze mono- and di-methylation of histone H3 lysine 9 (H3K9), have been implicated in numerous cancers. Recent studies have uncovered both catalytic and non-catalytic oncogenic functions of G9a/GLP. As such, G9a/GLP catalytic inhibitors have displayed limited anticancer activity. Here, we report the discovery of the first-in-class G9a/GLP proteolysis targeting chimera (PROTAC) degrader, 10 (MS8709), as a potential anticancer therapeutic. 10 induces G9a/GLP degradation in a concentration-, time, and ubiquitin-proteasome system (UPS)-dependent manner, does not alter the mRNA expression of G9a/GLP and is selective for G9a/GLP over other methyltransferases. Moreover, 10 displays superior cell growth inhibition to the parent G9a/GLP inhibitor UNC0642 in prostate, leukemia, and lung cancer cells and has suitable mouse pharmacokinetic properties for in vivo efficacy studies. Overall, 10 is a valuable chemical biology tool to further investigate the functions of G9a/GLP and a potential therapeutic for treating G9a/GLP-dependent cancers.

DNA methylation of HOX genes and its clinical implications in cancer

Homeobox (HOX) genes encode highly conserved transcription factors that play vital roles in embryonic development. DNA methylation is a pivotal regulatory epigenetic signaling mark responsible for regulating gene expression. Abnormal DNA methylation is largely associated with the aberrant expression of HOX genes, which is implicated in a broad range of human diseases, including cancer. Numerous studies have clarified the mechanisms of DNA methylation in both physiological and pathological processes. In this review, we focus on how DNA methylation regulates HOX genes and briefly discuss drug development approaches targeting these mechanisms.

BRD4770 inhibits vascular smooth muscle cell proliferation via SUV39H2, but not EHMT2 to protect against neointima formation

The behavior of vascular smooth muscle cells (VSMCs) contributes to the formation of neointima. We previously found that EHMT2 suppressed autophagy activation in VSMCs. BRD4770, an inhibitor of EHMT2/G9a, plays a critical role in several kinds of cancers. However, whether and how BRD4770 regulates the behavior of VSMCs remain unknown. In this study, we evaluate the cellular effect of BRD4770 on VSMCs by series of experiments in vivo and ex vivo. We demonstrated that BRD4770 inhibited VSMCs' growth by blockage in G2/M phase in VSMCs. Moreover, our results demonstrated that the inhibition of proliferation was independent on autophagy or EHMT2 suppression which we previous reported. Mechanistically, BRD4770 exhibited an off-target effect from EHMT2 and our further study reveal that the proliferation inhibitory effect by BRD4770 was associated with suppressing on SUV39H2/KTM1B. In vivo, BRD4770 was also verified to rescue VIH. Thus, BRD4770 function as a crucial negative regulator of VSMC proliferation via SUV39H2 and G2/M cell cycle arrest and BRD4770 could be a molecule for the therapy of vascular restenosis.

The YEATS domain epigenetic reader proteins ENL and AF9 and their therapeutic value in leukemia

Recent studies have uncovered similarities and differences between 2 highly homologous epigenetic reading proteins, namely, ENL (MLLT1) and AF9 (MLLT3) with therapeutic implications. The importance of these proteins has traditionally been exemplified by their involvement in chromosomal translocations with the mixed-lineage leukemia gene (MLL; aka KMT2a). MLL rearrangements occur in a subset of acute leukemias and generate potent oncogenic MLL-fusion proteins that impact epigenetic and transcriptional regulation. Leukemic patients with MLL rearrangements display intermediate-to-poor prognoses, necessitating further mechanistic research. Several protein complexes involved in regulating RNA polymerase II transcription and the epigenetic landscape are hijacked in MLL-r leukemia, which include ENL and AF9. Recent biochemical studies have defined a highly homologous YEATS domain in ENL and AF9 that binds acylated histones, which aids in the localization and retention of these proteins to transcriptional targets. In addition, detailed characterization of the homologous ANC-1 homology domain (AHD) on ENL and AF9 revealed differential association with transcriptional activating and repressing complexes. Importantly, CRISPR knockout screens have demonstrated a unique role for wild-type ENL in leukemic stem cell function, whereas AF9 appears important for normal hematopoietic stem cells. In this perspective, we examine the ENL and AF9 proteins with attention to recent work characterizing the epigenetic reading YEATS domains and AHD on both wild-type proteins and when fused to MLL. We summarized the drug development efforts and their therapeutic potential and assess ongoing research that has refined our understanding of how these proteins function, which continues to reveal new therapeutic avenues.

HOXA9 forms a repressive complex with nuclear matrix-associated protein SAFB to maintain acute myeloid leukemia

HOXA9 is commonly upregulated in acute myeloid leukemia (AML), in which it confers a poor prognosis. Characterizing the protein interactome of endogenous HOXA9 in human AML, we identified a chromatin complex of HOXA9 with the nuclear matrix attachment protein SAFB. SAFB perturbation phenocopied HOXA9 knockout to decrease AML proliferation, increase differentiation and apoptosis in vitro, and prolong survival in vivo. Integrated genomic, transcriptomic, and proteomic analyses further demonstrated that the HOXA9-SAFB (H9SB)-chromatin complex associates with nucleosome remodeling and histone deacetylase (NuRD) and HP1γ to repress the expression of factors associated with differentiation and apoptosis, including NOTCH1, CEBPδ, S100A8, and CDKN1A. Chemical or genetic perturbation of NuRD and HP1γ-associated catalytic activity also triggered differentiation, apoptosis, and the induction of these tumor-suppressive genes. Importantly, this mechanism is operative in other HOXA9-dependent AML genotypes. This mechanistic insight demonstrates the active HOXA9-dependent differentiation block as a potent mechanism of disease maintenance in AML that may be amenable to therapeutic intervention by targeting the H9SB interface and/or NuRD and HP1γ activity.

Dual inhibition of EZH2 and G9A/GLP histone methyltransferases by HKMTI-1-005 promotes differentiation of acute myeloid leukemia cells

All-trans-retinoic acid (ATRA)-based differentiation therapy of acute promyelocytic leukemia (APL) represents one of the most clinically effective examples of precision medicine and the first example of targeted oncoprotein degradation. The success of ATRA in APL, however, remains to be translated to non-APL acute myeloid leukemia (AML). We previously showed that aberrant histone modifications, including histone H3 lysine 4 (H3K4) and lysine 27 (H3K27) methylation, were associated with this lack of response and that epigenetic therapy with small molecule inhibitors of the H3K4 demethylase LSD1/KDM1A could reprogram AML cells to respond to ATRA. Serving as the enzymatic component of Polycomb Repressive Complex 2, EZH2/KMT6A methyltransferase plays a critical role in normal hematopoiesis by affecting the balance between self-renewal and differentiation. The canonical function of EZH2 is methylation of H3K27, although important non-canonical roles have recently been described. EZH2 mutation or deregulated expression has been conclusively demonstrated in the pathogenesis of AML and response to treatment, thus making it an attractive therapeutic target. In this study, we therefore investigated whether inhibition of EZH2 might also improve the response of non-APL AML cells to ATRA-based therapy. We focused on GSK-343, a pyridone-containing S-adenosyl-L-methionine cofactor-competitive EZH2 inhibitor that is representative of its class, and HKMTI-1-005, a substrate-competitive dual inhibitor targeting EZH2 and the closely related G9A/GLP H3K9 methyltransferases. We found that treatment with HKMTI-1-005 phenocopied EZH2 knockdown and was more effective in inducing differentiation than GSK-343, despite the efficacy of GSK-343 in terms of abolishing H3K27 trimethylation. Furthermore, transcriptomic analysis revealed that in contrast to treatment with GSK-343, HKMTI-1-005 upregulated the expression of differentiation pathway genes with and without ATRA, while downregulating genes associated with a hematopoietic stem cell phenotype. These results pointed to a non-canonical role for EZH2, which was supported by the finding that EZH2 associates with the master regulator of myeloid differentiation, RARα, in an ATRA-dependent manner that was enhanced by HKMTI-1-005, possibly playing a role in co-regulator complex exchange during transcriptional activation. In summary, our results strongly suggest that addition of HKMTI-1-005 to ATRA is a new therapeutic approach against AML that warrants further investigation.

Discovery of Novel Substrate-Competitive Lysine Methyltransferase G9a Inhibitors as Anticancer Agents

Identification of structurally novel inhibitors of lysine methyltransferase G9a has been a subject of intense research in cancer epigenetics. Starting with the high-throughput screening (HTS) hit rac-10a obtained from the chemical library of the University of Tokyo Drug Discovery Initiative, the structure-activity relationship of the unique substrate-competitive inhibitors was established with the help of X-ray crystallography and fragment molecular orbital (FMO) calculations for the ligand-protein interaction. Further optimization of the in vitro characteristics and drug metabolism and pharmacokinetics (DMPK) properties led to the identification of 26j (RK-701), which is a structurally distinct potent inhibitor of G9a/GLP (IC50 = 27/53 nM). Compound 26j exhibited remarkable selectivity against other related methyltransferases, dose-dependent attenuation of cellular H3K9me2 levels, and tumor growth inhibition in MOLT-4 cells in vitro. Moreover, compound 26j showed inhibition of tumor initiation and growth in a carcinogen-induced hepatocellular carcinoma (HCC) in vivo mouse model without overt acute toxicity.

Targeting the super elongation complex for oncogenic transcription driven tumor malignancies: Progress in structure, mechanisms and small molecular inhibitor discovery

Oncogenic transcription activation is associated with tumor development and resistance derived from chemotherapy or target therapy. The super elongation complex (SEC) is an important complex regulating gene transcription and expression in metazoans closely related to physiological activities. In normal transcriptional regulation, SEC can trigger promoter escape, limit proteolytic degradation of transcription elongation factors and increase the synthesis of RNA polymerase II (POL II), and regulate many normal human genes to stimulate RNA elongation. Dysregulation of SEC accompanied by multiple transcription factors in cancer promotes rapid transcription of oncogenes and induce cancer development. In this review, we summarized recent progress in understanding the mechanisms of SEC in regulating normal transcription, and importantly its roles in cancer development. We also highlighted the discovery of SEC complex target related inhibitors and their potential applications in cancer treatment.

Targeting epigenetic regulation for cancer therapy using small molecule inhibitors

Cancer cells display pervasive changes in DNA methylation, disrupted patterns of histone posttranslational modification, chromatin composition or organization and regulatory element activities that alter normal programs of gene expression. It is becoming increasingly clear that disturbances in the epigenome are hallmarks of cancer, which are targetable and represent attractive starting points for drug creation. Remarkable progress has been made in the past decades in discovering and developing epigenetic-based small molecule inhibitors. Recently, epigenetic-targeted agents in hematologic malignancies and solid tumors have been identified and these agents are either in current clinical trials or approved for treatment. However, epigenetic drug applications face many challenges, including low selectivity, poor bioavailability, instability and acquired drug resistance. New multidisciplinary approaches are being designed to overcome these limitations, e.g., applications of machine learning, drug repurposing, high throughput virtual screening technologies, to identify selective compounds with improved stability and better bioavailability. We provide an overview of the key proteins that mediate epigenetic regulation that encompass histone and DNA modifications and discuss effector proteins that affect the organization of chromatin structure and function as well as presently available inhibitors as potential drugs. Current anticancer small-molecule inhibitors targeting epigenetic modified enzymes that have been approved by therapeutic regulatory authorities across the world are highlighted. Many of these are in different stages of clinical evaluation. We also assess emerging strategies for combinatorial approaches of epigenetic drugs with immunotherapy, standard chemotherapy or other classes of agents and advances in the design of novel epigenetic therapies.

Role of HOXA9 in solid tumors: mechanistic insights and therapeutic potential

HOXA9 functioning as a transcription factor is one of the members of HOX gene family, which governs multiple cellular activities by facilitating cellular signal transduction. In addition to be a driver in AML which has been widely studied, the role of HOXA9 in solid tumor progression has also received increasing attention in recent years, where the aberrant expression of HOXA9 is closely associated with the prognosis of patient. This review details the signaling pathways, binding partners, post-transcriptional regulation of HOXA9, and possible inhibitors of HOXA9 in solid tumors, which provides a reference basis for further study on the role of HOXA9 in solid tumors.

Understanding ER homeostasis and the UPR to enhance treatment efficacy of acute myeloid leukemia

Protein biogenesis, maturation and degradation are tightly regulated processes that are governed by a complex network of signaling pathways. The endoplasmic reticulum (ER) is responsible for biosynthesis and maturation of secretory proteins. Circumstances that alter cellular protein homeostasis, determine accumulation of misfolded and unfolded proteins in the ER, a condition defined as ER stress. In case of stress, the ER activates an adaptive response called unfolded protein response (UPR), a series of pathways of major relevance for cancer biology. The UPR plays a preeminent role in adaptation of tumor cells to the harsh conditions that they experience, due to high rates of proliferation, metabolic abnormalities and hostile environment scarce in oxygen and nutrients. Furthermore, the UPR is among the main adaptive cell stress responses contributing to the development of resistance to drugs and chemotherapy. Clinical management of Acute Myeloid Leukemia (AML) has improved significantly in the last decade, thanks to development of molecular targeted therapies. However, the emergence of treatment-resistant clones renders the rate of AML cure dismal. Moreover, different cell populations that constitute the bone marrow niche recently emerged as a main determinant leading to drug resistance. Herein we summarize the most relevant literature regarding the role played by the UPR in expansion of AML and ability to develop drug resistance and we discuss different possible modalities to overturn this adaptive response against leukemia. To this aim, we also describe the interconnection of the UPR with other cellular stress responses regulating protein homeostasis. Finally, we review the newest findings about the crosstalk between AML cells and cells of the bone marrow niche, under physiological conditions and in response to therapies, discussing in particular the importance of the niche in supporting survival of AML cells by favoring protein homeostasis.

Identification of an Epi-metabolic dependency on EHMT2/G9a in T-cell acute lymphoblastic leukemia

Genomic studies have identified recurrent somatic alterations in genes involved in DNA methylation and post-translational histone modifications in acute lymphoblastic leukemia (ALL), suggesting new opportunities for therapeutic interventions. In this study, we identified G9a/EHMT2 as a potential target in T-ALL through the intersection of epigenome-centered shRNA and chemical screens. We subsequently validated G9a with low-throughput CRISPR-Cas9-based studies targeting the catalytic G9a SET-domain and the testing of G9a chemical inhibitors in vitro, 3D, and in vivo T-ALL models. Mechanistically we determined that G9a repression promotes lysosomal biogenesis and autophagic degradation associated with the suppression of sestrin2 (SESN2) and inhibition of glycogen synthase kinase-3 (GSK-3), suggesting that in T-ALL glycolytic dependent pathways are at least in part under epigenetic control. Thus, targeting G9a represents a strategy to exhaust the metabolic requirement of T-ALL cells.

Expression and prognostic significance of chromatin modulators EHMT2/G9a and KDM2b in acute myeloid leukemia

Epigenetics factors are critical for normal cell function and their regulation is sensitive to malignancy development. EHMT2/G9a and KDM2b are key epigenetics players in different cancer types. However, their expression profiles and related consequences in acute myeloid leukemia (AML) patients have not been known yet. In addition to routine lab work, expression levels of EHMT2/G9a and KDM2b were determined in 110 adult and pediatric patients with De Novo AML. Relations between their expression and patients' clinical data were tested by statistical methods. EHMT2/G9a and KDM2b were highly expressed in AML patients against control cases and associated with the presence of adverse genomic alterations. In response to induction chemotherapy, EHMT2/G9a and KDM2b showed to be significantly high in resistant and relapsed patients in comparison to the complete remission group. KDM2b overexpression was associated with CD11c (integrin alpha X) downregulation. Kaplan-Meier analysis indicated that EHMT2/G9a and KDM2b overexpression was correlated with poor survival status in AML patients. We conclude that EHMT2/G9a and KDM2b expression levels could be used as independent prognostic factors for AML disease.

H3K9 dimethylation safeguards cancer cells against activation of the interferon pathway

Activation of interferon genes constitutes an important anticancer pathway able to restrict proliferation of cancer cells. Here, we demonstrate that the H3K9me3 histone methyltransferase (HMT) suppressor of variegation 3-9 homolog 1 (SUV39H1) is required for the proliferation of acute myeloid leukemia (AML) and find that its loss leads to activation of the interferon pathway. Mechanistically, we show that this occurs via destabilization of a complex composed of SUV39H1 and the two H3K9me2 HMTs, G9A and GLP. Indeed, loss of H3K9me2 correlated with the activation of key interferon pathway genes, and interference with the activities of G9A/GLP largely phenocopied loss of SUV39H1. Last, we demonstrate that inhibition of G9A/GLP synergized with DNA demethylating agents and that SUV39H1 constitutes a potential biomarker for the response to hypomethylation treatment. Collectively, we uncovered a clinically relevant role for H3K9me2 in safeguarding cancer cells against activation of the interferon pathway.

BIX-01294 enhances the effect of chemotherapy on colorectal cancer by inhibiting the expression of stemness genes

During the development of colorectal cancer, tumor cells will generate some cancer stem cells with self-renewal ability because they adapt to the environment. Therefore, in the treatment of colorectal cancer, it has certain potential clinical application value to effectively inhibit cancer stem cells. A small molecule EHMT-2 inhibitor, BIX-01294, was evaluated for its activity in inhibiting cancer stem cells in human colorectal cancer by in vitro and in vivo experiments. Transcriptome analysis was performed on BIX-01294 treated cells for holistic analysis to elucidate how BIX-01294 inhibits the expression of genes related to cancer stem cells. The results show that BIX-01294 significantly inhibited the proliferative phenotype of human colorectal cancer in vivo and in vitro, reduced the proportion of cancer stem cells, and inhibited some stemness-related gene. Morever, it is synergistic with 5-fluorouracil in inhibiting the proliferation of colorectal cancer. In summary, EHMT-2 is a novel target of anti-tumor drugs. The combination of BIX-01294 and 5-fluorouracil has a synergistic therapeutic effect on human colorectal cancer.

EHMT1/EHMT2 in EMT, Cancer Stemness and Drug Resistance: Emerging Evidence and Mechanisms

Metastasis, therapy failure and tumor recurrence are major clinical challenges in cancer. The interplay between tumor initiating cells (TICs) and Epithelial-Mesenchymal transition (EMT) drives tumor progression and spread. Recent advances have highlighted the involvement of epigenetic deregulation in these processes. The Euchromatin Histone Lysine Methyltransferase 1 (EHMT1) and Euchromatin Histone Lysine Methyltransferase 2 (EHMT2) that primarily mediate histone 3 lysine 9 di-methylation (H3K9me2), as well as methylation of non-histone proteins, are now recognized to be aberrantly expressed in many cancers. Their deregulated expression is associated with EMT, cellular plasticity and therapy resistance. In this review, we summarize evidence of their myriad roles in cancer metastasis, stemness and drug resistance. We discuss cancer-type specific molecular targets, context-dependent mechanisms and future directions of research in targeting EHMT1/EHMT2 for the treatment of cancer.

Emerging role of G9a in cancer stemness and promises as a therapeutic target

The histone methyltransferase G9a is well-documented for its implication in neoplastic growth. However, recent investigations have demonstrated a key involvement of this chromatin writer in maintaining the self-renewal and tumor-initiating capacities of cancer stem cells (CSCs). Direct inhibition of G9a’s catalytic activity was reported as a promising therapeutic target in multiple preclinical studies. Yet, none of the available pharmacological inhibitors of G9a activity have shown success at the early stages of clinical testing. Here, we discuss central findings of oncogenic expression and activation of G9a in CSCs from different origins, as well as the impact of the suppression of G9a histone methyltransferase activity in such contexts. We will explore the challenges posed by direct and systemic inhibition of G9a activity in the perspective of clinical translation of documented small molecules. Finally, we will discuss recent advances in drug discovery as viable strategies to develop context-specific drugs, selectively targeting G9a in CSC populations.

Structure, Activity, and Function of the Protein Lysine Methyltransferase G9a

G9a is a lysine methyltransferase catalyzing the majority of histone H3 mono- and dimethylation at Lys-9 (H3K9), responsible for transcriptional repression events in euchromatin. G9a has been shown to methylate various lysine residues of non-histone proteins and acts as a coactivator for several transcription factors. This review will provide an overview of the structural features of G9a and its paralog called G9a-like protein (GLP), explore the biochemical features of G9a, and describe its post-translational modifications and the specific inhibitors available to target its catalytic activity. Aside from its role on histone substrates, the review will highlight some non-histone targets of G9a, in order gain insight into their role in specific cellular mechanisms. Indeed, G9a was largely described to be involved in embryonic development, hypoxia, and DNA repair. Finally, the involvement of G9a in cancer biology will be presented.

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.

EHMT2 promotes the pathogenesis of hepatocellular carcinoma by epigenetically silencing APC expression

BACKGROUND

Hepatocellular carcinoma (HCC), the second leading cause of cancer death worldwide, alone accounts for over half (466,100) of new cancer cases and 422,100 deaths based on the average year incidence rates of 2009 to 2011 in China. Due to unclear and complex underlying mechanisms for HCC development, effective therapy for HCC is still unavailable. The Wnt-β-catenin pathway is a critical contributor of HCC pathogenesis: 40-70% of HCCs from patients harbor the nuclear accumulation of β-catenin protein. However, the mechanisms for β-catenin activation are not fully understood.

METHODS

The deletion of EHMT2 in Hep3B and Huh1 cells was achieved by transiently transfecting cells with pX459 plasmids, which carry EHMT2 specific small guide RNA (sgRNA) sequences for Cas9 protein. All experiments were performed in triplicate and repeated more than three times.

RESULTS

In the present study, we observed that EHMT2 (but not EHMT1) mRNA and protein levels were significantly elevated in HCC compared with normal controls. Next, the results of Ki67 staining, as well as MTT, soft-agar and xenograft assays, in wild-type and EHMT2^-/- Hep3B and Huh1 cancer stem cells collectively revealed that the elevation of EHMT2 expression is required for the tumorigenesis of HCC. Meanwhile, we found that elevated EHMT2 expression contributes to the activation of Wnt-β-catenin signaling: deletion of EHMT2 in Hep3B or Huh1 cells promoted the cytoplasmic localization of β-catenin and restrained the expression of Wnt-β-catenin signaling targets such as Myc, CCND1, MMP-7, etc. We demonstrated that EMHT2 directly mediates the H3K9me2 methylation of the APC promoter to epigenetically silence its expression. More intriguingly, our findings also showed that UNC0642, a specific inhibitor of EHMT2, exhibits anti-tumorigenesis effects in HCC both in vitro and in vivo, which were largely abolished by deletion of EHMT2 or overexpression of APC in Hep3B and Huh1 cells.

CONCLUSION

Altogether, our observations emphasize that the EHMT2-APC axis is a critical contributor to Wnt-β-catenin pathway activation in HCC, and UNC0642 may be a potential candidate for target drug treatment of HCC.

Gene Transcription as a Therapeutic Target in Leukemia

Blood malignancies often arise from undifferentiated hematopoietic stem cells or partially differentiated stem-like cells. A tight balance of multipotency and differentiation, cell division, and quiescence underlying normal hematopoiesis requires a special program governed by the transcriptional machinery. Acquisition of drug resistance by tumor cells also involves reprogramming of their transcriptional landscape. Limiting tumor cell plasticity by disabling reprogramming of the gene transcription is a promising strategy for improvement of treatment outcomes. Herein, we review the molecular mechanisms of action of transcription-targeted drugs in hematological malignancies (largely in leukemia) with particular respect to the results of clinical trials.

Epithelial-Mesenchymal Transition Associated with Head and Neck Squamous Cell Carcinomas: A Review

Head and neck squamous cell carcinomas (HNSCCs) are aggressive, recurrent, and metastatic neoplasms with a high occurrence around the world and can lead to death when not treated appropriately. Several molecules and signaling pathways are involved in the malignant conversion process. Epithelial-mesenchymal transition (EMT) has been described in HNSCCs, a major type of aggressive carcinoma. EMT describes the development of epithelial cells into mesenchymal cells, which depends on several molecular interactions and signaling pathways that facilitate mesenchymal conversion. This is related to interactions with the microenvironment of the tumor, hypoxia, growth factors, matrix metalloproteinases, and the presence of viral infections. In this review, we focus on the main molecules related to EMT, their interactions with the tumor microenvironment, plasticity phenomena, epigenetic regulation, hypoxia, inflammation, their relationship with immune cells, and the inhibition of EMT in the context of HNSCCs.

Escape From Treatment; the Different Faces of Leukemic Stem Cells and Therapy Resistance in Acute Myeloid Leukemia

Standard induction chemotherapy, consisting of an anthracycline and cytarabine, has been the first-line therapy for many years to treat acute myeloid leukemia (AML). Although this treatment induces complete remissions in the majority of patients, many face a relapse (adaptive resistance) or have refractory disease (primary resistance). Moreover, older patients are often unfit for cytotoxic-based treatment. AML relapse is due to the survival of therapy-resistant leukemia cells (minimal residual disease, MRD). Leukemia cells with stem cell features, named leukemic stem cells (LSCs), residing within MRD are thought to be at the origin of relapse initiation. It is increasingly recognized that leukemia "persisters" are caused by intra-leukemic heterogeneity and non-genetic factors leading to plasticity in therapy response. The BCL2 inhibitor venetoclax, combined with hypomethylating agents or low dose cytarabine, represents an important new therapy especially for older AML patients. However, often there is also a small population of AML cells refractory to venetoclax treatment. As AML MRD reflects the sum of therapy resistance mechanisms, the different faces of treatment "persisters" and LSCs might be exploited to reach an optimal therapy response and prevent the initiation of relapse. Here, we describe the different epigenetic, transcriptional, and metabolic states of therapy sensitive and resistant AML (stem) cell populations and LSCs, how these cell states are influenced by the microenvironment and affect treatment outcome of AML. Moreover, we discuss potential strategies to target dynamic treatment resistance and LSCs.

Targeting protein lysine methyltransferase G9A impairs self-renewal of chronic myelogenous leukemia stem cells via upregulation of SOX6

The application of tyrosine kinase inhibitors (TKIs) in clinic has revolutionized chronic myelogenous leukemia (CML) treatment, but fails to eliminate leukemia stem cells (LSCs), which are considered as roots of drug resistance and disease relapse. Thus, eradication of LSCs may be a promising strategy for curing CML. In this study, we found that protein lysine methyltransferase G9A was overexpressed in CML LSCs. The upregulation of G9A by BCR-ABL was independent on its tyrosine kinase activity. Knockdown of G9A by shRNAs or pharmacological inhibition of G9A by UNC0642 significantly suppressed survival and impaired self-renewal capacity of CML LSCs. Inhibition of G9a eradicated LSCs in CML mice driven by BCR-ABL gene and dramatically prolonged survival of the mice. Ex vivo treatment with G9A inhibitor inhibited long-term engraftment of CML CD34⁺ cells in immunodeficient mice. Mechanically, tumor suppressor SOX6 was identified as a direct target of G9A in CML LSCs by RNA-seq analysis. Silencing Sox6 at least partially rescued G9a knockdown-mediated LSCs elimination in vivo. Our findings improve the understanding of LSC regulation network and validate G9A as a therapeutic target in CML LSCs. Targeting G9A may be considered as an additional strategy for the treatment of patients with CML.

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.

Dual EZH2 and G9a inhibition suppresses multiple myeloma cell proliferation by regulating the interferon signal and IRF4-MYC axis

Epigenetic mechanisms such as histone modification play key roles in the pathogenesis of multiple myeloma (MM). We previously showed that EZH2, a histone H3 lysine 27 (H3K27) methyltransferase, and G9, a H3K9 methyltransferase, are potential therapeutic targets in MM. Moreover, recent studies suggest EZH2 and G9a cooperate to regulate gene expression. We therefore evaluated the antitumor effect of dual EZH2 and G9a inhibition in MM. A combination of an EZH2 inhibitor and a G9a inhibitor strongly suppressed MM cell proliferation in vitro by inducing cell cycle arrest and apoptosis. Dual EZH2/G9a inhibition also suppressed xenograft formation by MM cells in vivo. In datasets from the Gene Expression Omnibus, higher EZH2 and EHMT2 (encoding G9a) expression was significantly associated with poorer prognoses in MM patients. Microarray analysis revealed that EZH2/G9a inhibition significantly upregulated interferon (IFN)-stimulated genes and suppressed IRF4-MYC axis genes in MM cells. Notably, dual EZH2/G9a inhibition reduced H3K27/H3K9 methylation levels in MM cells and increased expression of endogenous retrovirus (ERV) genes, which suggests that activation of ERV genes may induce the IFN response. These results suggest that dual targeting of EZH2 and G9a may be an effective therapeutic strategy for MM.

EHMT2 inhibitor BIX-01294 induces endoplasmic reticulum stress mediated apoptosis and autophagy in diffuse large B-cell lymphoma cells

Despite advancement in the treatment of diffuse large B-cell lymphoma (DLBCL), many patients tend to relapse or become refractory after initial therapy. Therefore, it is essential to identify novel therapeutic targets and drugs, understand the molecular pathogenesis mechanism of DLBCL, and find ways to prevent and treat relapsed or refractory DLBCL. BIX-01294 is a small molecule compound that specifically inhibits EHMT2 activity. In this study, we demonstrate that BIX-01294 triggered the inhibition of human DLBCL cell proliferation, lead to G₁ phase arrest via increasing P21 level and reducing cyclin E level. BIX-01294 also induced apoptosis via endogenous and exogenous apoptotic pathways. Moreover, BIX-01294 triggered autophagy and activated ER stress in human DLBCL cells. Furthermore, we showed that both key components of ER stress, ATF3, and ATF4, are required for BIX-01294-induced apoptosis and autophagy. Hence, this study provides new evidence that EHMT2 may be a new therapeutic target, and BIX-01294 may be a potential therapeutic drug for treating DLBCL.

Insight into the multi-faceted role of the SUV family of H3K9 methyltransferases in carcinogenesis and cancer progression

Growing evidence implicates histone H3 lysine 9 methylation in tumorigenesis. The SUV family of H3K9 methyltransferases, which include G9a, GLP, SETDB1, SETDB2, SUV39H1 and SUV39H2 deposit H3K9me1/2/3 marks at euchromatic and heterochromatic regions, catalyzed by their conserved SET domain. In cancer, this family of enzymes can be deregulated by genomic alterations and transcriptional mis-expression leading to alteration of transcriptional programs. In solid and hematological malignancies, studies have uncovered pro-oncogenic roles for several H3K9 methyltransferases and accordingly, small molecule inhibitors are being tested as potential therapies. However, emerging evidence demonstrate onco-suppressive roles for these enzymes in cancer development as well. Here, we review the role H3K9 methyltransferases play in tumorigenesis focusing on gene targets and biological pathways affected due to misregulation of these enzymes. We also discuss molecular mechanisms regulating H3K9 methyltransferases and their influence on cancer. Finally, we describe the impact of H3K9 methylation on therapy induced resistance in carcinoma. Converging evidence point to multi-faceted roles for H3K9 methyltransferases in development and cancer that encourages a deeper understanding of these enzymes to inform novel therapy.

Targeting EHMT2/ G9a for cancer therapy: Progress and perspective

Euchromatic histone lysine methyltransferase-2, also known as G9a, is a ubiquitously expressed SET domain-containing histone lysine methyltransferase linked with both facultative and constitutive heterochromatin formation and transcriptional repression. It is an essential developmental gene and reported to play role in embryonic development, establishment of proviral silencing in ES cells, tumor cell growth, metastasis, T-cell immune response, cocaine induced neural plasticity and cognition and adaptive behavior. It is mainly responsible for carrying out mono, di and tri methylation of histone H3K9 in euchromatin. G9a levels are elevated in many cancers and its selective inhibition is known to reduce the cell growth and induce autophagy, apoptosis and senescence. We carried out a thorough search of online literature databases including Pubmed, Scopus, Journal websites, Clinical trials etc to gather the maximum possible information related to the G9a. The main messages from the cited papers are presented in a systematic manner. Chemical structures were drawn by Chemdraw software. In this review, we shed light on current understanding of structure and biological activity of G9a, the molecular events directing its targeting to genomic regions and its post-translational modification. Finally, we discuss the current strategies to target G9a in different cancers and evaluate the available compounds and agents used to inhibit G9a functions. The review provides the present status and future directions of research in targeting G9a and provides the basis to persuade the development of novel strategies to target G9a -related effects in cancer cells.

G9a controls pluripotent-like identity and tumor-initiating function in human colorectal cancer

Colorectal tumors are hierarchically organized and governed by populations of self-renewing cancer stem cells, representing one of the deadliest types of cancers worldwide. Emergence of cancer stemness phenotype depends on epigenetic reprogramming, associated with profound transcriptional changes. As described for pluripotent reprogramming, epigenetic modifiers play a key role in cancer stem cells by establishing embryonic stem-like transcriptional programs, thus impacting the balance between self-renewal and differentiation. We identified overexpression of histone methyltransferase G9a as a risk factor for colorectal cancer, associated with shorter relapse-free survival. Moreover, using human transformed pluripotent cells as a surrogate model for cancer stem cells, we observed that G9a activity is essential for the maintenance of embryonic-like transcriptional signature promoting self-renewal, tumorigenicity, and undifferentiated state. Such a role was also applicable to colorectal cancer, where inhibitors of G9a histone methyltransferase function induced intestinal differentiation while restricting tumor-initiating activity in patient-derived colorectal tumor samples. Finally, by integrating transcriptome profiling with G9a/H3K9me2 loci co-occupancy, we identified the canonical Wnt pathway, epithelial-to-mesenchyme transition, and extracellular matrix organization as potential targets of such a chromatin regulation mechanism in colorectal cancer stem cells. Overall, our findings provide novel insights on the role of G9a as a driver of cancer stem cell phenotype, promoting self-renewal, tumorigenicity, and undifferentiated state.

Deficiency of G9a Inhibits Cell Proliferation and Activates Autophagy via Transcriptionally Regulating c-Myc Expression in Glioblastoma

Glioblastoma is an aggressive and difficult to treat cancer. Recent data have emerged implicating that histone modification level may play a crucial role in glioma genesis. The histone lysine methyltransferase G9a is mainly responsible for the mono- and di-methylation of histone H3 lysine 9 (H3K9), whose overexpression is associated with a more aggressive phenotype in cancer. However, the detailed correlations between G9a and glioblastoma genesis remain to be further elucidated. Here, we show that G9a is essential for glioblastoma carcinogenesis and reveal a probable mechanism of it in cell proliferation control. We found that G9a was highly expressed in glioblastoma cells, and knockdown or inhibition of G9a significantly repressed cell proliferation and tumorigenesis ability both in vitro and in vivo. Besides, knockdown or inhibition of G9a led to a cell cycle arrest in G2 phase, as well as decreased the expression of CDK1, CDK2, Cyclin A2, and Cyclin B1, while it induced the activation of autophagy. Further investigation showed that G9a deficiency induced cell proliferation suppression, and activation of autophagy was rescued by overexpression of the full-length c-Myc. Chromatin immunoprecipitation (ChIP) assay showed that G9a was enriched on the −2267 to −1949 region of the c-Myc promoter in LN-229 cells and the −1949 to −1630 region of the c-Myc promoter in U-87 MG cells. Dual-luciferase reporter assay showed that c-Myc promoter activity was significantly reduced after knockdown or inhibition of G9a. Our study shows that G9a controls glioblastoma cell proliferation by transcriptionally modulating oncogene c-Myc and provides insight into the capabilities of G9a working as a potential therapeutic target in glioblastoma.

Evolving insights on histone methylome regulation in human acute myeloid leukemia pathogenesis and targeted therapy

Acute myeloid leukemia (AML) is an aggressive, disseminated hematological malignancy associated with clonal selection of aberrant self-renewing hematopoietic stem cells and progenitors and poorly differentiated myeloid blasts. The most prevalent form of leukemia in adults, AML is predominantly an age-related disorder and accounts for more than 10,000 deaths per year in the United States alone. In comparison to solid tumors, AML has an overall low mutational burden, albeit more than 70% of AML patients harbor somatic mutations in genes encoding epigenetic modifiers and chromatin regulators. In the past decade, discoveries highlighting the role of DNA and histone modifications in determining cellular plasticity and lineage commitment have attested to the importance of epigenetic contributions to tumor cell de-differentiation and heterogeneity, tumor initiation, maintenance, and relapse. Orchestration in histone methylation levels regulates pluripotency and multicellular development. The increasing number of reversible methylation regulators being identified, including histone methylation writer, reader, and eraser enzymes, and their implications in AML pathogenesis have widened the scope of epigenetic reprogramming, with multiple drugs currently in various stages of preclinical and clinical trials. AML methylome also determines response to conventional chemotherapy, as well as AML cell interaction within a tumor-immune microenvironment ecosystem. Here we summarize the latest developments focusing on molecular derangements in histone methyltransferases (HMTs) and histone demethylases (HDMs) in AML pathogenesis. AML-associated HMTs and HDMs, through intricate crosstalk mechanisms, maintain an altered histone methylation code conducive to disease progression. We further discuss their importance in governing response to therapy, which can be used as a biomarker for treatment efficacy. Finally we deliberate on the therapeutic potential of targeting aberrant histone methylome in AML, examine available small molecule inhibitors in combination with immunomodulating therapeutic approaches and caveats, and discuss how future studies can enable posited epigenome-based targeted therapy to become a mainstay for AML treatment.

SETDB1 mediated histone H3 lysine 9 methylation suppresses MLL-fusion target expression and leukemic transformation

Epigenetic regulators play a critical role in normal and malignant hematopoiesis. Deregulation, including epigenetic deregulation, of the HOXA gene cluster drives transformation of about 50% of acute myeloid leukemia. We recently showed that the Histone 3 Lysine 9 methyltransferase SETDB1 negatively regulates the expression of the pro-leukemic genes Hoxa9 and its cofactor Meis1 through deposition of promoter H3K9 trimethylation in MLL-AF9 leukemia cells. Here, we investigated the biological impact of altered SETDB1 expression and changes in H3K9 methylation on acute myeloid leukemia. We demonstrate that SETDB1 expression is correlated to disease status and overall survival in acute myeloid leukemia patients. We recapitulated these findings in mice, where high expression of SETDB1 delayed MLL-AF9 mediated disease progression by promoting differentiation of leukemia cells. We also explored the biological impact of treating normal and malignant hematopoietic cells with an H3K9 methyltransferase inhibitor, UNC0638. While myeloid leukemia cells demonstrate cytotoxicity to UNC0638 treatment, normal bone marrow cells exhibit an expansion of cKit+ hematopoietic stem and progenitor cells. Consistent with these data, we show that bone marrow treated with UNC0638 is more amenable to transformation by MLL-AF9. Next generation sequencing of leukemia cells shows that high expression of SETDB1 induces repressive changes to the promoter epigenome and downregulation of genes linked with acute myeloid leukemia, including Dock1 and the MLL-AF9 target genes Hoxa9, Six1, and others. These data reveal novel targets of SETDB1 in leukemia that point to a role for SETDB1 in negatively regulating pro-leukemic target genes and suppressing acute myeloid leukemia.

Novel therapeutic strategies for MLL-rearranged leukemias

MLL rearrangement is one of the key drivers and generally regarded as an independent poor prognostic marker in acute leukemias. The standard of care for MLL-rearranged (MLL-r) leukemias has remained largely unchanged for the past 50 years despite unsatisfying clinical outcomes, so there is an urgent need for novel therapeutic strategies. An increasing body of evidence demonstrates that a vast number of epigenetic regulators are directly or indirectly involved in MLL-r leukemia, and they are responsible for supporting the aberrant gene expression program mediated by MLL-fusions. Unlike genetic mutations, epigenetic modifications can be reversed by pharmacologic targeting of the responsible epigenetic regulators. This leads to significant interest in developing epigenetic therapies for MLL-r leukemia. Intriguingly, many of the epigenetic enzymes also involve in DNA damage response (DDR), which can be potential targets for synthetic lethality-induced therapies. In this review, we will summarize some of the recent advances in the development of epigenetic and DDR therapeutics by targeting epigenetic regulators or protein complexes that mediate MLL-r leukemia gene expression program and key players in DDR that safeguard essential genome integrity. The rationale and molecular mechanisms underpinning the therapeutic effects will also be discussed with a focus on how these treatments can disrupt MLL-fusion mediated transcriptional programs and impair DDR, which may help overcome treatment resistance.

A sensitive homogeneous enzyme assay for euchromatic histone-lysine-N-methyltransferase 2 (G9a) based on terbium-to-quantum dot time-resolved FRET

Introduction: Histone modifying enzymes include several classes of enzymes that are responsible for various post-translational modifications of histones such as methylation and acetylation. They are important epigenetic factors, which may involve several diseases and so their assay, as well as screening of their inhibitors, are of great importance. Herein, a bioassay based on terbium-to-quantum dot (Tb-to-QD) time-resolved Förster resonance energy transfer (TR-FRET) was developed for monitoring the activity of G9a, the euchromatic histone-lysine N-methyltransferase 2. Overexpression of G9a has been reported in some cancers such as ovarian carcinoma, lung cancer, multiple myeloma and brain cancer. Thus, inhibition of this enzyme is important for therapeutic purposes.

Methods: In this assay, a biotinylated peptide was used as a G9a substrate in conjugation with streptavidin-coated ZnS/CdSe QD as FRET acceptor, and an anti-mark antibody labeled with Tb as a donor. Time-resolved fluorescence was used for measuring FRET ratios.

Results: We examined three QDs, with emission wavelengths of 605, 655 and 705 nm, as FRET acceptors and investigated FRET efficiency between the Tb complex and each of them. Since the maximum FRET efficiency was obtained for Tb to QD705 (more than 50%), this pair was exploited for designing the enzyme assay. We showed that the method has excellent sensitivity and selectivity for the determination of G9a at concentrations as low as 20 pM. Furthermore, the designed assay was applied for screening of an enzyme inhibitor, S-(5’-Adenosyl)-L-homocysteine (SAH).

Conclusion: It was shown that Tb-to-QD FRET is an outstanding platform for developing a homogenous assay for the G9a enzyme and its inhibitors. The obtained results confirmed that this assay was quite sensitive and could be used in the field of inhibitor screening.

Unfolded Protein Response in Leukemia: From Basic Understanding to Therapeutic Opportunities

Understanding genetic and epigenetic changes that underlie abnormal proliferation of hematopoietic stem and progenitor cells is critical for development of new approaches to monitor and treat leukemia. The unfolded protein response (UPR) is a conserved adaptive signaling pathway that governs protein folding, secretion, and energy production and serves to maintain protein homeostasis in various cellular compartments. Deregulated UPR signaling, which often occurs in hematopoietic stem cells and leukemia, defines the degree of cellular toxicity and perturbs protein homeostasis, and at the same time, offers a novel therapeutic target. Here, we review current knowledge related to altered UPR signaling in leukemia and highlight possible strategies for exploiting the UPR as treatment for this disease.

Small molecule therapeutics to treat the β-globinopathies

PURPOSE OF REVIEW

The current review focuses on recent insights into the development of small molecule therapeutics to treat the β-globinopathies.

RECENT FINDINGS

Recent studies of fetal γ-globin gene regulation reveal multiple insights into how γ-globin gene reactivation may lead to novel treatment for β-globinopathies.

SUMMARY

We summarize current information regarding the binding of transcription factors that appear to be impeded or augmented by different hereditary persistence of fetal hemoglobin (HPFH) mutations. As transcription factors have historically proven to be difficult to target for therapeutic purposes, we next address the contributions of protein complexes associated with these HPFH mutation-affected transcription factors with the aim of defining proteins that might provide additional targets for chemical molecules to inactivate the corepressors. Among the enzymes associated with the transcription factor complexes, a group of corepressors with currently available inhibitors were initially thought to be good candidates for potential therapeutic purposes. We discuss possibilities for pharmacological inhibition of these corepressor enzymes that might significantly reactivate fetal γ-globin gene expression. Finally, we summarize the current clinical trial data regarding the inhibition of select corepressor proteins for the treatment of sickle cell disease and β-thalassemia.

CBX7 Induces Self-Renewal of Human Normal and Malignant Hematopoietic Stem and Progenitor Cells by Canonical and Non-canonical Interactions

In this study, we demonstrate that, among all five CBX Polycomb proteins, only CBX7 possesses the ability to control self-renewal of human hematopoietic stem and progenitor cells (HSPCs). Xenotransplantation of CBX7-overexpressing HSPCs resulted in increased multi-lineage long-term engraftment and myelopoiesis. Gene expression and chromatin analyses revealed perturbations in genes involved in differentiation, DNA and chromatin maintenance, and cell cycle control. CBX7 is upregulated in acute myeloid leukemia (AML), and its genetic or pharmacological repression in AML cells inhibited proliferation and induced differentiation. Mass spectrometry analysis revealed several non-histone protein interactions between CBX7 and the H3K9 methyltransferases SETDB1, EHMT1, and EHMT2. These CBX7-binding proteins possess a trimethylated lysine peptide motif highly similar to the canonical CBX7 target H3K27me3. Depletion of SETDB1 in AML cells phenocopied repression of CBX7. We identify CBX7 as an important regulator of self-renewal and uncover non-canonical crosstalk between distinct pathways, revealing therapeutic opportunities for leukemia.

PERK/NRF2 and autophagy form a resistance mechanism against G9a inhibition in leukemia stem cells

BACKGROUND

The histone methyltransferase G9a has recently been identified as a potential target for epigenetic therapy of acute myeloid leukemia (AML). However, the effect of G9a inhibition on leukemia stem cells (LSCs), which are responsible for AML drug resistance and recurrence, is unclear. In this study, we investigated the underlying mechanisms of the LSC resistance to G9a inhibition.

METHODS

We evaluated the effects of G9a inhibition on the unfolded protein response and autophagy in AML and LSC-like cell lines and in primary CD34+CD38- leukemic blasts from patients with AML and investigated the underlying mechanisms. The effects of treatment on cells were evaluated by flow cytometry, western blotting, confocal microscopy, reactive oxygen species (ROS) production assay.

RESULTS

The G9a inhibitor BIX-01294 effectively induced apoptosis in AML cell lines; however, the effect was limited in KG1 LSC-like cells. BIX-01294 treatment or siRNA-mediated G9a knockdown led to the activation of the PERK/NRF2 pathway and HO-1 upregulation in KG1 cells. Phosphorylation of p38 and intracellular generation of reactive oxygen species (ROS) were suppressed. Pharmacological or siRNA-mediated inhibition of the PERK/NRF2 pathway synergistically enhanced BIX-01294-induced apoptosis, with suppressed HO-1 expression, increased p38 phosphorylation, and elevated ROS generation, indicating that activated PERK/NRF2 signaling suppressed ROS-induced apoptosis in KG1 cells. By contrast, cotreatment of normal hematopoietic stem cells with BIX-01294 and a PERK inhibitor had no significant proapoptotic effect. Additionally, G9a inhibition induced autophagy flux in KG1 cells, while autophagy inhibitors significantly increased the BIX-01294-induced apoptosis. This prosurvival autophagy was not abrogated by PERK/NRF2 inhibition.

CONCLUSIONS

PERK/NRF2 signaling plays a key role in protecting LSCs against ROS-induced apoptosis, thus conferring resistance to G9a inhibitors. Treatment with PERK/NRF2 or autophagy inhibitors could overcome resistance to G9a inhibition and eliminate LSCs, suggesting the potential clinical utility of these unique targeted therapies against AML.

Histone Modifying Enzymes and Chromatin Modifiers in Glioma Pathobiology and Therapy Responses

Signal transduction pathways directly communicate and transform chromatin to change the epigenetic landscape and regulate gene expression. Chromatin acts as a dynamic platform of signal integration and storage. Histone modifications and alteration of chromatin structure play the main role in chromatin-based gene expression regulation. Alterations in genes coding for histone modifying enzymes and chromatin modifiers result in malfunction of proteins that regulate chromatin modification and remodeling. Such dysregulations culminate in profound changes in chromatin structure and distorted patterns of gene expression. Gliomagenesis is a multistep process, involving both genetic and epigenetic alterations. Recent applications of next generation sequencing have revealed that many chromatin regulation-related genes, including ATRX, ARID1A, SMARCA4, SMARCA2, SMARCC2, BAF155 and hSNF5 are mutated in gliomas. In this review we summarize newly identified mechanisms affecting expression or functions of selected histone modifying enzymes and chromatin modifiers in gliomas. We focus on selected examples of pathogenic mechanisms involving ATRX, histone methyltransferase G9a, histone acetylases/deacetylases and chromatin remodeling complexes SMARCA2/4. We discuss the impact of selected epigenetics alterations on glioma pathobiology, signaling and therapeutic responses. We assess the attempts of targeting defective pathways with new inhibitors.

Epigenetic polypharmacology: A new frontier for epi-drug discovery

Recently, despite the great success achieved by the so-called "magic bullets" in the treatment of different diseases through a marked and specific interaction with the target of interest, the pharmacological research is moving toward the development of "molecular network active compounds," embracing the related polypharmacology approach. This strategy was born to overcome the main limitations of the single target therapy leading to a superior therapeutic effect, a decrease of adverse reactions, and a reduction of potential mechanism(s) of drug resistance caused by robustness and redundancy of biological pathways. It has become clear that multifactorial diseases such as cancer, neurological, and inflammatory disorders, may require more complex therapeutic approaches hitting a certain biological system as a whole. Concerning epigenetics, the goal of the multi-epi-target approach consists in the development of small molecules able to simultaneously and (often) reversibly bind different specific epi-targets. To date, two dual histone deacetylase/kinase inhibitors (CUDC-101 and CUDC-907) are in an advanced stage of clinical trials. In the last years, the growing interest in polypharmacology encouraged the publication of high-quality reviews on combination therapy and hybrid molecules. Hence, to update the state-of-the-art of these therapeutic approaches avoiding redundancy, herein we focused only on multiple medication therapies and multitargeting compounds exploiting epigenetic plus nonepigenetic drugs reported in the literature in 2018. In addition, all the multi-epi-target inhibitors known in literature so far, hitting two or more epigenetic targets, have been included.

Upregulated expression of G9a is correlated with poor prognosis of gastric cancer patients

As one of the most serious cancers, gastric cancer (GC) represents the third leading cause of malignancy-related deaths. G9a is a histone lysine methyltransferase and has been reported to be involved in the progression of some human cancers. In the present study, we aimed to explore the expression patterns and clinical value of G9a in GC patients.The expression of G9a in 142 paired GC tissues and adjacent non-cancerous tissues (no less than 5 cm from tumor edge) was examined with quantitative real-time polymerase chain reaction (qRT-PCR). To estimate the association of G9a expression with clinical characteristics of GC patients, Chi-square test and t test were conducted. Kaplan-Meier survival and multivariate Cox regression analyses were performed to explore the prognostic value of G9a in GC.Upregulated expression of G9a was found in GC tissues compared with noncancerous tissues (P < .001). Elevated G9a expression was significantly correlated with patients' lymph node metastasis (P = .007) and TNM stage (P < .001). Kaplan-Meier survival curves demonstrated that patients with high G9a expression had shorter survival time than those with low expression (log-rank test, P < .05), reaching a median OS of 24 months. According to the results of Cox regression, G9a could be considered as an independent prognostic biomarker in patients with GC (HR = 3.912, 95% CI = 2.213-6.915, P < .001). Additionally, the diagnosis cut-off value of G9a in GC patients was 1.515.Taken together, G9a expression was upregulated in GC tissues and could be an effective prognostic biomarker for GC.

Epigenetic Therapies for Acute Myeloid Leukemia and Their Immune-Related Effects

Over the past decades, our molecular understanding of acute myeloid leukemia (AML) pathogenesis dramatically increased, thanks also to the advent of next-generation sequencing (NGS) technologies. Many of these findings, however, have not yet translated into new prognostic markers or rationales for treatments. We now know that AML is a highly heterogeneous disease characterized by a very low mutational burden. Interestingly, the few mutations identified mainly reside in epigenetic regulators, which shape and define leukemic cell identity. In the light of these discoveries and given the increasing number of drugs targeting epigenetic regulators in clinical development and testing, great interest is emerging for the use of small molecules targeting leukemia epigenome. Together with their effects on leukemia cell-intrinsic properties, such as proliferation and survival, epigenetic drugs may affect the way leukemic cells communicate with the surrounding components of the tumor and immune microenvironment. Here, we review current knowledge on alterations in the AML epigenetic landscape and discuss the promises of epigenetic therapies for AML treatment. Finally, we summarize emerging molecular studies elucidating how epigenetic rewiring in cancer cells may as well exert immune-modulatory functions, boost the immune system, and potentially contribute to better patient outcomes.