G9a Inhibition Enhances Checkpoint Inhibitor Blockade Response in Melanoma

Lysine methylation modifications in tumor immunomodulation and immunotherapy: regulatory mechanisms and perspectives

Lysine methylation is a crucial post-translational modification (PTM) that significantly impacts gene expression regulation. This modification not only influences cancer development directly but also has significant implications for the immune system. Lysine methylation modulates immune cell functions and shapes the anti-tumor immune response, highlighting its dual role in both tumor progression and immune regulation. In this review, we provide a comprehensive overview of the intrinsic role of lysine methylation in the activation and function of immune cells, detailing how these modifications affect cellular processes and signaling pathways. We delve into the mechanisms by which lysine methylation contributes to tumor immune evasion, allowing cancer cells to escape immune surveillance and thrive. Furthermore, we discuss the therapeutic potential of targeting lysine methylation in cancer immunotherapy. Emerging strategies, such as immune checkpoint inhibitors (ICIs) and chimeric antigen receptor T-cell (CAR-T) therapy, are being explored for their efficacy in modulating lysine methylation to enhance anti-tumor immune responses. By targeting these modifications, we can potentially improve the effectiveness of existing treatments and develop novel therapeutic approaches to combat cancer more effectively.

G9a in Cancer: Mechanisms, Therapeutic Advancements, and Clinical Implications

G9a, also named EHMT2, is a histone 3 lysine 9 (H3K9) methyltransferase responsible for catalyzing H3K9 mono- and dimethylation (H3K9me1 and H3K9me2). G9a contributes to various aspects of embryonic development and tissue differentiation through epigenetic regulation. Furthermore, the aberrant expression of G9a is frequently observed in various tumors, particularly in prostate cancer, where it contributes to cancer pathogenesis and progression. This review highlights the critical role of G9a in multiple cancer-related processes, such as epigenetic dysregulation, tumor suppressor gene silencing, cancer lineage plasticity, hypoxia adaption, and cancer progression. Despite the increased research on G9a in prostate cancer, there are still significant gaps, particularly in understanding its interactions within the tumor microenvironment and its broader epigenetic effects. Furthermore, this review discusses the recent advancements in G9a inhibitors, including the development of dual-target inhibitors that target G9a along with other epigenetic factors such as EZH2 and HDAC. It aims to bring together the existing knowledge, identify gaps in the current research, and suggest future directions for research and treatment strategies.

The Role and Mechanism of the Histone Methyltransferase G9a in Tumors: Update

Methylation-mediated gene silencing is closely related to the occurrence and development of human tumors. The euchromatic histone lysine methyltransferase 2 (EHMT2, also known as G9a) is highly expressed in many tumors and is generally considered to be an oncogene, which is associated with the poor outcome of many tumors. Combined immunotherapy and immune checkpoint blockade therapy also have good efficacy and certain safety. However, there are still many difficulties in the drugs targeting G9a, and the combined effect and safety of G9a with many drugs is still under study. This article aims to summarize the role and mechanism of G9a and its inhibitors in tumors in the past two years, and to understand the application prospect of G9a from the perspective of diagnosis and treatment.

Galectin-7 Induction by EHMT2 Inhibition Enhances Immunity in Microsatellite Stability Colorectal Cancer

BACKGROUND & AIMS

Although immunotherapy shows substantial advancement in colorectal cancer (CRC) with microsatellite instability high, it has limited efficacy for CRC with microsatellite stability (MSS). Identifying combinations that reverse immune suppression and prime MSS tumors for current immunotherapy approaches remains an urgent need.

METHODS

An in vitro CRISPR screen was performed using coculture models of primary tumor cells and autologous immune cells from MSS CRC patients to identify epigenetic targets that could enhance immunotherapy efficacy in MSS tumors.

RESULTS

We revealed EHMT2, a histone methyltransferase, as a potential target for MSS CRC. EHMT2 inhibition transformed the immunosuppressive microenvironment of MSS tumors into an immunomodulatory one by altering cytokine expression, leading to T-cell-mediated cytotoxicity activation and improved responsiveness to anti-PD1 treatment. We observed galectin-7 up-regulation upon EHMT2 inhibition, which converted a "cold" MSS tumor environment into a T-cell-inflamed one. Mechanistically, CHD4 repressed galectin-7 expression by recruiting EHMT2 to form a cotranscriptional silencing complex. Galectin-7 administration enhanced anti-PD1 efficacy in MSS CRC, serving as a potent adjunct cytokine therapy.

CONCLUSIONS

Our findings suggest that targeting the EHMT2/galectin-7 axis could provide a novel combination strategy for immunotherapy in MSS CRC.

The dopamine transporter antagonist vanoxerine inhibits G9a and suppresses cancer stem cell functions in colon tumors

Cancer stem cells (CSCs), functionally characterized by self-renewal and tumor-initiating activity, contribute to decreased tumor immunogenicity, while fostering tumor growth and metastasis. Targeting G9a histone methyltransferase (HMTase) effectively blocks CSC functions in colorectal tumors by altering pluripotent-like molecular networks; however, existing molecules directly targeting G9a HMTase activity failed to reach clinical stages due to safety concerns. Using a stem cell-based phenotypic drug-screening pipeline, we identified the dopamine transporter (DAT) antagonist vanoxerine, a compound with previously demonstrated clinical safety, as a cancer-specific downregulator of G9a expression. Here we show that gene silencing and chemical antagonism of DAT impede colorectal CSC functions by repressing G9a expression. Antagonizing DAT also enhanced tumor lymphocytic infiltration by activating endogenous transposable elements and type-I interferon response. Our study unveils the direct implication of the DAT-G9a axis in the maintenance of CSC populations and an approach to improve antitumor immune response in colon tumors.

Epigenetic modification in liver fibrosis: Promising therapeutic direction with significant challenges ahead

Liver fibrosis, characterized by scar tissue formation, can ultimately result in liver failure. It's a major cause of morbidity and mortality globally, often associated with chronic liver diseases like hepatitis or alcoholic and non-alcoholic fatty liver diseases. However, current treatment options are limited, highlighting the urgent need for the development of new therapies. As a reversible regulatory mechanism, epigenetic modification is implicated in many biological processes, including liver fibrosis. Exploring the epigenetic mechanisms involved in liver fibrosis could provide valuable insights into developing new treatments for chronic liver diseases, although the current evidence is still controversial. This review provides a comprehensive summary of the regulatory mechanisms and critical targets of epigenetic modifications, including DNA methylation, histone modification, and RNA modification, in liver fibrotic diseases. The potential cooperation of different epigenetic modifications in promoting fibrogenesis was also highlighted. Finally, available agonists or inhibitors regulating these epigenetic mechanisms and their potential application in preventing liver fibrosis were discussed. In summary, elucidating specific druggable epigenetic targets and developing more selective and specific candidate medicines may represent a promising approach with bright prospects for the treatment of chronic liver diseases.

EZH2/G9a interact to mediate drug resistance in non-small-cell lung cancer by regulating the SMAD4/ERK/c-Myc signaling axis

Drug resistance is the leading problem in non-small-cell lung cancer (NSCLC) therapy. The contribution of histone methylation in mediating malignant phenotypes of NSCLC is well known. However, the role of histone methylation in NSCLC drug-resistance mechanisms remains unclear. Here, our data show that EZH2 and G9a, two histone methyltransferases, are involved in the drug resistance of NSCLC. Gene manipulation results indicate that the combination of EZH2 and G9a promotes tumor growth and mediates drug resistance in a complementary manner. Importantly, clinical study demonstrates that co-expression of both enzymes predicts a poor outcome in patients with NSCLC. Mechanistically, G9a and EZH2 interact and promote the silencing of the tumor-suppressor gene SMAD4, activating the ERK/c-Myc signaling pathway. Finally, SU08, a compound targeting both EZH2 and G9a, is demonstrated to sensitize resistant cells to therapeutic drugs by regulating the SMAD4/ERK/c-Myc signaling axis. These findings uncover the resistance mechanism and a strategy for reversing NSCLC drug resistance.

Emerging prognostic biomarkers in advanced cutaneous melanoma: a literature update

INTRODUCTION

Over the past two years, the scientific community has witnessed an exponential growth in research focused on identifying prognostic biomarkers for melanoma, both in pre-clinical and clinical settings. This surge in studies reflects the need of developing effective prognostic indicators in the field of melanoma.

AREAS COVERED

The aim of this work is to review the scientific literature on the most recent findings on the development or validation of prognostic biomarkers in melanoma, in the attempt of providing both clinicians and researchers with an updated broad synopsis of prognostic biomarkers in cutaneous melanoma.

EXPERT OPINION

While the field of prognostic biomarkers in melanoma appears promising, there are several complexities and limitations to address. The interdependence of clinical, histological, and molecular features requires accurate classification of different biomarker families. Correlation does not imply causation, and adjustments for confounding factors are often overlooked. In this scenario, large-scale studies based on high-quality clinical trial data can provide more reliable evidence. It is essential to avoid oversimplification by focusing on a single biomarker, as the interactions among multiple factors contribute to define the disease course and patient's outcome. Furthermore, implementing well-supported evidence in real-life settings can help advance prognostic biomarker research in melanoma.

A Review of Advanced Cutaneous Melanoma Therapies and Their Mechanisms, from Immunotherapies to Lysine Histone Methyl Transferase Inhibitors

Advanced cutaneous melanoma is considered to be the most aggressive type of skin cancer and has variable rates of treatment response. Currently, there are some classes of immunotherapy and target therapies for its treatment. Immunotherapy can inhibit tumor growth and its recurrence by triggering the host's immune system, whereas targeted therapy inhibits specific molecules or signaling pathways. However, melanoma responses to these treatments are highly heterogeneous, and patients can develop resistance. Epigenomics (DNA/histone modifications) contribute to cancer initiation and progression. Epigenetic alterations are divided into four levels of gene expression regulation: DNA methylation, histone modification, chromatin remodeling, and non-coding RNA regulation. Deregulation of lysine methyltransferase enzymes is associated with tumor initiation, invasion, development of metastases, changes in the immune microenvironment, and drug resistance. The study of lysine histone methyltransferase (KMT) and nicotinamide N-methyltransferase (NNMT) inhibitors is important for understanding cancer epigenetic mechanisms and biological processes. In addition to immunotherapy and target therapy, the research and development of KMT and NNMT inhibitors is ongoing. Many studies are exploring the therapeutic implications and possible side effects of these compounds, in addition to their adjuvant potential to the approved current therapies. Importantly, as with any drug development, safety, efficacy, and specificity are crucial considerations when developing methyltransferase inhibitors for clinical applications. Thus, this review article presents the recently available therapies and those in development for advanced cutaneous melanoma therapy.

Distinct functions of EHMT1 and EHMT2 in cancer chemotherapy and immunotherapy

EHTM1 (GLP) and EHMT2 (G9a) are closely related protein lysine methyltransferases often thought to function together as a heterodimer to methylate histone H3 and non-histone substrates in diverse cellular processes including transcriptional regulation, genome methylation, and DNA repair. Here we show that EHMT1/2 inhibitors cause ATM-mediated slowdown of replication fork progression, accumulation of single-stranded replication gaps, emergence of cytosolic DNA, and increased expression of STING. EHMT1/2 inhibition strongly potentiates the efficacy of alkylating chemotherapy and anti-PD-1 immunotherapy in mouse models of tripe negative breast cancer. The effects on DNA replication and alkylating agent sensitivity are largely caused by the loss of EHMT1-mediated methylation of LIG1, whereas the elevated STING expression and remarkable response to immunotherapy appear mainly elicited by the loss of EHMT2 activity. Depletion of UHRF1, a protein known to be associated with EHMT1/2 and LIG1, also induces STING expression, and depletion of either EHMT2 or UHRF1 leads to demethylation of specific CpG sites in the STING1 promoter, suggestive of a distinct EHMT2-UHRF1 axis that regulates DNA methylation and gene transcription. These results highlight distinct functions of the two EHMT paralogs and provide enlightening paradigms and corresponding molecular basis for combination therapies involving alkylating agents and immune checkpoint inhibitors.

Epigenetics behind tumor immunology: a mini review

Immunogenic- and immune-therapies have become hot spots in the treatment of cancer. Although promising, these strategies are frequently associated with innate or acquired resistance, calling for combined targeting of immune inhibitory signals. Epigenetic therapy is attracting considerable attention as a combination partner for immune-based therapies due to its role in molding the state and fate of cancer and immune cells in the tumor microenvironment. Here, we describe epigenetic dysregulations in cancer, with a particular focus on those related to innate immune signaling and Type I interferons, and emphasize opportunities and current efforts to translate this knowledge into treatment regimens with improved clinical benefit.

The cancer epigenome: Non-cell autonomous player in tumor immunity

Dysregulation of the tumor-intrinsic epigenetic circuit is a key driver event for the development of cancer. Accumulating evidence suggests that epigenetic and/or genetic drivers stimulate intrinsic oncogenic pathways as well as extrinsic factors that modulate the immune system. These modulations indeed shape the tumor microenvironment (TME), allowing pro-oncogenic factors to become oncogenic, thereby contributing to cancer development and progression. Here we review the epigenetic dysregulation arising in cancer cells that disseminates throughout the TME and beyond. Recent CRISPR screening has elucidated key epigenetic drivers that play important roles in the proliferation of cancer cells (intrinsic) and inhibition of antitumor immunity (extrinsic), which lead to the development and progression of cancer. These epigenetic players can serve as promising targets for cancer therapy as a dual (two-in-one)-targeted approach. Considering the interplay between cancer and the immune system as a key determinant of immunotherapy, we discuss a novel lineage-tracing technology that enables longitudinal monitoring of cancer and immune phenotypic heterogeneity and fate paths during cancer development, progression, and therapeutic interventions.

Novel insights into histone lysine methyltransferases in cancer therapy: From epigenetic regulation to selective drugs

The reversible and precise temporal and spatial regulation of histone lysine methyltransferases (KMTs) is essential for epigenome homeostasis. The dysregulation of KMTs is associated with tumor initiation, metastasis, chemoresistance, invasiveness, and the immune microenvironment. Therapeutically, their promising effects are being evaluated in diversified preclinical and clinical trials, demonstrating encouraging outcomes in multiple malignancies. In this review, we have updated recent understandings of KMTs' functions and the development of their targeted inhibitors. First, we provide an updated overview of the regulatory roles of several KMT activities in oncogenesis, tumor suppression, and immune regulation. In addition, we summarize the current targeting strategies in different cancer types and multiple ongoing clinical trials of combination therapies with KMT inhibitors. In summary, we endeavor to depict the regulation of KMT-mediated epigenetic landscape and provide potential epigenetic targets in the treatment of cancers.

The role of histone methylase and demethylase in antitumor immunity: A new direction for immunotherapy

Epigenetic modifications may alter the proliferation and differentiation of normal cells, leading to malignant transformation. They can also affect normal stimulation, activation, and abnormal function of immune cells in the tissue microenvironment. Histone methylation, coordinated by histone methylase and histone demethylase to stabilize transcription levels in the promoter area, is one of the most common types of epigenetic alteration, which gained increasing interest. It can modify gene transcription through chromatin structure and affect cell fate, at the transcriptome or protein level. According to recent research, histone methylation modification can regulate tumor and immune cells affecting anti-tumor immune response. Consequently, it is critical to have a thorough grasp of the role of methylation function in cancer treatment. In this review, we discussed recent data on the mechanisms of histone methylation on factors associated with immune resistance of tumor cells and regulation of immune cell function.

Cinobufagin induces FOXO1-regulated apoptosis, proliferation, migration, and invasion by inhibiting G9a in non-small-cell lung cancer A549 cells

ETHNOPHARMACOLOGICAL RELEVANCE

Bufonis (VB), an animal drug called Chansu in China, is the product of the secretion of Bufo gargarizans Cantor or B. melanostictus Schneider. As a traditional Chinese medicine (TCM) for a long time, it has been widely used in the treatment of heart failure, ulcer, pain, and various cancers. Cinobufaginn (CNB), the cardiotonic steroid or bufalene lactone extracted from VB, has the effects of detoxification, detumescence, and analgesia.

AIM OF THE STUDY

The present study aimed to define the effects of CNB on non-small-cell lung cancer (NSCLC) and identify the potential molecular mechanisms.

MATERIALS AND METHODS

A549 cells were treated with cinobufagin and cell viability, apoptosis, migration, and invasion were then evaluated using Cell Counting Kit-8 (CCK8) assays, flow cytometry, and Transwell assays, respectively. Moreover, the levels of proliferating cell nuclear antigen (PCNA), cytokeratin8 (CK8), poly ADP-ribose polymerase (PARP), Caspase3, Caspase8, B-cell lymphoma/lewkmia-2(Bcl-2), Bcl2-Associated X(Bax), forkhead box O1 (FOXO1), and euchromatic histone-lysine N-methyltransferase2 (G9a, EHMT2) in A549 cells were evaluated using qRT-PCR and/or Western blot analysis (WB), Co-IP, immunofluorescence, and immunohistochemistry. An in vivo imaging system, TUNEL, Immunofluorescence, and immunohistochemistry were also used to detect proliferating cell nuclear antigen(PCNA), Ki67, E-Cadherin(E-Cad), FOXO1, and G9a in mouse xenograft model experiments.

RESULTS

CNB suppressed cell proliferation, migration, and invasion but promoted apoptosis in A549 cells in a dose- and time-dependent manner, while cinobufagin had no cytotoxic effect on BEAS-2B cells. In vivo, cinobufagin inhibited the proliferation, migration, and invasion of A549 cells and promoted their apoptosis. The occurrence of the above phenomena was accompanied by an increase in FOXO1 expression and a decrease in G9a expression. In A549 cells, CNB did not reverse the changes in the proliferation, migration, invasion, and apoptosis of A549 cells after FOXO1 was successfully silenced.

CONCLUSION

Our study provides the first evidence that cinobufagin suppresses the malignant biological behaviours of NSCLC cells in vivo and in vitro and suggests that mechanistically, this effect may be achieved by inhibiting the expression of the histone methyltransferase G9a and activating the tumour suppressor gene FOXO1. Taken together, our findings provide important insights into the molecular mechanism underlying cinobufagin's anticancer activity, and suggest that cinobufagin could be a candidate for targeted cancer therapy.

Targeting the LSD1-G9a-ER Stress Pathway as a Novel Therapeutic Strategy for Esophageal Squamous Cell Carcinoma

Despite recent advances in the management and treatment of esophageal squamous cell carcinoma (ESCC), the prognosis remains extremely poor, and current nonsurgical treatment options are limited. To identify new therapeutic targets, we screened a curated library of epigenetic compounds using a panel of cancer cell lines and found that coinhibiting the histone demethylase LSD1 and the histone methyltransferase G9a potently suppresses cell growth; similar results were obtained by knocking down both LSD1 and G9a expression. Importantly, we also found that inhibiting LSD1 and G9a significantly decreased tumor growth in a xenograft mouse model with ESCC cell lines. To examine the clinical relevance of these findings, we performed immunohistochemical analyses of microarray profiling data obtained from human esophageal squamous cancer tissues and found that both LSD1 and G9a are upregulated in cancer tissues compared to healthy tissues, and this increased expression was significantly correlated with poor prognosis. Mechanistically, we discovered that inhibiting LSD1 and G9a induces cell death via S-phase arrest and apoptosis, and cotargeting ER stress pathways increased this effect both in vitro and in vivo. Taken together, these findings provide compelling evidence that targeting LSD1, G9a, and ER stress-related pathways may serve as a viable therapeutic strategy for ESCC.

Inhibiting Histone and DNA Methylation Improves Cancer Vaccination in an Experimental Model of Melanoma

Immunotherapy has improved the treatment of malignant skin cancer of the melanoma type, yet overall clinical response rates remain low. Combination therapies could be key to meet this cogent medical need. Because epigenetic hallmarks represent promising combination therapy targets, we studied the immunogenic potential of a dual inhibitor of histone methyltransferase G9a and DNA methyltransferases (DNMTs) in the preclinical B16-OVA melanoma model. Making use of tumor transcriptomic and functional analyses, methylation-targeted epigenetic reprogramming was shown to induce tumor cell cycle arrest and apoptosis in vitro coinciding with transient tumor growth delay and an IFN-I response in immune-competent mice. In consideration of a potential impact on immune cells, the drug was shown not to interfere with dendritic cell maturation or T-cell activation in vitro. Notably, the drug promoted dendritic cell and, to a lesser extent, T-cell infiltration in vivo, yet failed to sensitize tumor cells to programmed cell death-1 inhibition. Instead, it increased therapeutic efficacy of TCR-redirected T cell and dendritic cell vaccination, jointly increasing overall survival of B16-OVA tumor-bearing mice. The reported data confirm the prospect of methylation-targeted epigenetic reprogramming in melanoma and sustain dual G9a and DNMT inhibition as a strategy to tip the cancer-immune set-point towards responsiveness to active and adoptive vaccination against melanoma.

Targeting Cancer Stem Cells through Epigenetic Modulation of Interferon Response

Cancer stem cells (CSCs) are a small subset of cancer cells and are thought to play a critical role in the initiation and maintenance of tumor mass. CSCs exhibit similar hallmarks to normal stem cells, such as self-renewal, differentiation, and homeostasis. In addition, CSCs are equipped with several features so as to evade anticancer mechanisms. Therefore, it is hard to eliminate CSCs by conventional anticancer therapeutics that are effective at clearing bulk cancer cells. Interferons are innate cytokines and are the key players in immune surveillance to respond to invaded pathogens. Interferons are also crucial for adaptive immunity for the killing of specific aliens including cancer cells. However, CSCs usually evolve to escape from interferon-mediated immune surveillance and to shape the niche as a “cold” tumor microenvironment (TME). These CSC characteristics are related to their unique epigenetic regulations that are different from those of normal and bulk cancer cells. In this review, we introduce the roles of epigenetic modifiers, focusing on LSD1, BMI1, G9a, and SETDB1, in contributing to CSC characteristics and discussing the interplay between CSCs and interferon response. We also discuss the emerging strategy for eradicating CSCs by targeting these epigenetic modifiers, which can elevate cytosolic nuclei acids, trigger interferon response, and reshape a “hot” TME for improving cancer immunotherapy. The key epigenetic and immune genes involved in this crosstalk can be used as biomarkers for precision oncology.

Combined inhibition of G9a and EZH2 suppresses tumor growth via synergistic induction of IL24-mediated apoptosis

G9a and EZH2 are two histone methyltransferases commonly upregulated in several cancer types, yet the precise roles that these enzymes play cooperatively in cancer is unclear. We demonstrate here that frequent concurrent upregulation of both G9a and EZH2 occurs in several human tumors. These methyltransferases cooperatively repressed molecular pathways responsible for tumor cell death. In genetically distinct tumor subtypes, concomitant inhibition of G9a and EZH2 potently induced tumor cell death, highlighting the existence of tumor cell survival dependency at the epigenetic level. G9a and EZH2 synergistically repressed expression of genes involved in the induction of endoplasmic reticulum (ER) stress and the production of reactive oxygen species. IL24 was essential for the induction of tumor cell death and was identified as a common target of G9a and EZH2. Loss-of-function of G9a and EZH2 activated the IL24-ER stress axis and increased apoptosis in cancer cells while not affecting normal cells. These results indicate that G9a and EZH2 promotes the evasion of ER stress-mediated apoptosis by repressing IL24 transcription, therefore suggesting that their inhibition may represent a potential therapeutic strategy for solid cancers.

Epigenetics in the Diagnosis and Therapy of Malignant Melanoma

Epigenetic mechanisms are fundamentally important for cancer initiation and development. However, a survey of the literature reveals that, to date, they appear less comprehensively investigated in melanoma than in many other cancers, e.g., prostate, breast, and colon carcinoma. The aim of this review is to provide a short summary of epigenetic aspects of functional relevance for melanoma pathogenesis. In addition, some new perspectives from epigenetic research in other cancers with potential for melanoma diagnosis and therapy are introduced. For example, the PrimeEpiHit hypothesis in urothelial carcinoma, which, similarly to malignant melanoma, can also be triggered by a single exogenous noxa, states that one of the first steps for cancer initiation could be epigenetic changes in key genes of one-carbon metabolism. The application of such insights may contribute to further progress in the diagnosis and therapy of melanoma, a deadly type of 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.

G9a: An Emerging Epigenetic Target for Melanoma Therapy

Epigenetic regulation is a crucial component of DNA maintenance and cellular identity. As our understanding of the vast array of proteins that contribute to chromatin accessibility has advanced, the role of epigenetic remodelers in disease has become more apparent. G9a is a histone methyltransferase that contributes to immune cell differentiation and function, neuronal development, and has been implicated in diseases, including cancer. In melanoma, recurrent mutations and amplifications of G9a have led to its identification as a therapeutic target. The pathways that are regulated by G9a provide an insight into relevant biomarkers for patient stratification. Future work is aided by the breadth of literature on G9a function during normal differentiation and development, along with similarities to EZH2, another histone methyltransferase that forms a synthetic lethal relationship with members of the SWI/SNF complex in certain cancers. Here, we review the literature on G9a, its role in melanoma, and lessons from EZH2 inhibitor studies.