Small Molecule Approaches for Targeting the Polycomb Repressive Complex 2 (PRC2) in Cancer

Targeting EED as a key PRC2 complex mediator toward novel epigenetic therapeutics

EED within the PRC2 complex is crucial for chromatin regulation particularly in tumor development, making its inhibition a promising epigenetic therapeutic strategy. Significant advancement in PRC2 inhibitor development has been achieved with an approved EZH2 inhibitor in the market and with others in the clinical trials. However, current EZH2 inhibitors are limited to specific blood cancers and encounter therapeutic resistance. EED stabilizes PRC2 complex and enhances its activity through unique allosteric mechanisms, thereby acting as both a scaffold protein and a recognizer of H3K27me3 making it an attractive drug target. This review provides an overview of epigenetic therapeutic strategies targeting EED, including allosteric inhibitors, PPI inhibitors, and PROTACs, together with brief discussions on the relevant challenges, opportunities, and future directions.

A miniaturized mode-of-action profiling platform enables high throughput characterization of the molecular and cellular dynamics of EZH2 inhibition

The market approval of Tazemetostat (TAZVERIK) for the treatment of follicular lymphoma and epithelioid sarcoma has established "enhancer of zeste homolog 2" (EZH2) as therapeutic target in oncology. Despite their structural similarities and common mode of inhibition, Tazemetostat and other EZH2 inhibitors display differentiated pharmacological profiles based on their target residence time. Here we established high throughput screening methods based on time-resolved fluorescence energy transfer, scintillation proximity and high content analysis microscopy to quantify the biochemical and cellular binding of a chemically diverse collection of EZH2 inhibitors. These assays allowed to further characterize the interplay between EZH2 allosteric modulation by methylated histone tails (H3K27me3) and inhibitor binding, and to evaluate the impact of EZH2's clinically relevant mutant Y641N on drug target residence times. While all compounds in this study exhibited slower off-rates, those with clinical candidate status display significantly slower target residence times in wild type EZH2 and disease-related mutants. These inhibitors interact in a more entropy-driven fashion and show the most persistent effects in cellular washout and antiproliferative efficacy experiments. Our work provides mechanistic insights for the largest cohort of EZH2 inhibitors reported to date, demonstrating that-among several other binding parameters-target residence time is the best predictor of cellular efficacy.

Conformationally constrained potent inhibitors for enhancer of zeste homolog 2 (EZH2)

The enhancer of zeste homolog 2 (EZH2) plays the role of the main catalytic subunit of polycomb repressive complex 2 (PRC2) that catalyzes the methylation of histone H3 lysine 27 (H3K27). Overexpression or mutation of EZH2 has been observed in many types of hematologic malignancies and solid tumors, such as myeloma, lymphoma, prostate, breast, kidney, and lung cancers. EZH2 has been demonstrated as a promising therapeutic target for the treatment of tumors. Based on the structure of 1 (EPZ-6438), a series of novel conformationally constrained derivatives were designed and synthesized aiming to improve the EZH2 inhibition activity, especially for mutated EZH2. Structure and activity relationship (SAR) exploration and optimization at both enzymatic and cellular levels led to the discovery of 28. In vitro, 28 displayed potent EZH2 inhibition activity with an IC50 value of 0.95 nM, which is comparable to EPZ-6438 (1). 28 exhibited high anti-proliferation activity against different lymphoma cell lines including WSU-DLCL2, Pfeiffer and Karpas-422 (IC50 = 2.36, 1.73, and 1.82 nM, respectively). In vivo, 28 showed acceptable pharmacokinetic characteristics (oral bioavailability F = 36.9%) and better efficacy than 1 in both Pfeiffer and Karpas-422 xenograft mouse models, suggesting that it can be further developed as a potential therapeutic candidate for EZH2-associated cancers.

Targeting polycomb repressor complex 2-mediated bivalent promoter epigenetic silencing of secreted frizzled-related protein 1 inhibits cholangiocarcinoma progression

BACKGROUND

Cholangiocarcinoma (CCA) refers to a collection of malignancies that are associated with a dismal prognosis. Currently, surgical resection is the only way to cure patients with CCA. Available systemic therapy is limited to gemcitabine plus cisplatin; however, this treatment is palliative in nature. Therefore, there is still a need to explore new effective therapeutic targets to intervene against CCA.

METHODS

We analyzed the expression of EZH2 and the prognosis of patients in CCA. The proliferation, migration and invasion of CCA cells after gene knockdown and overexpression were examined and validated by a xenograft model and a primary CCA mouse model with corresponding gene intervention. Targeting DNA methylation, and RNA-sequencing-based transcriptomic analysis in EZH2 and SUZ12 knockout CCA cells was performed. Bisulfite sequencing polymerase chain reaction (PCR), chromatin immunoprecipitation-quantitative PCR (ChIP-qPCR) and reverse-ChIP assays were performed for research purposes.

RESULTS

Increased expression of EZH2 in CCA exhibited a significantly poorer prognosis. DNA hypomethylation of the promoter and increased mRNA levels of secreted frizzled-related protein 1 (SFRP1) were observed in CCA cells following the inhibition of polycomb repressor complex 2 (PRC2), which was achieved through a knockout of EZH2, SUZ12 and EED, respectively, or treatment with GSK126 and GSK343. Targeting the SFRP1 promoter DNA hypermethylation with dCas9-DNMT3a decreased the mRNA level of SFRP1. The expression of SFRP1 is regulated by both H3K27me3 and DNA methylation and H3K27me3 plays a crucial role in promoting SFRP1 promotor DNA methylation. GSK343 is a small molecule inhibitor that targets the catalytic activity of EZH2. It effectively inhibits the progression and development of subcutaneous xenografts and primary CCA mouse models.

CONCLUSION

Overall, our data strongly suggested that targeting PRC2 promotes the expression of SFRP1, thereby inhibiting the progression of CCA.

KEY POINTS/HEADLIGHTS

Cholangiocarcinoma (CCA) exhibits elevated expression of EZH2, SUZ12 and EED, resulting in increased levels of H3K27me3. Targeting polycomb repressor complex 2 (PRC2) leads to the removal of H3K27me3 from the secreted frizzled-related protein 1 (SFRP1) promoter and DNA hypomethylation, thereby activating the transcription of SFRP1. Inhibiting PRC2, including the use of EZH2 inhibitors, holds promise as a potential strategy for developing anti-cancer drugs for CCA.

Drug Discovery Targeting Nuclear Receptor Binding SET Domain Protein 2 (NSD2)

Nuclear receptor binding SET domain proteins (NSDs) catalyze the mono- or dimethylation of histone 3 lysine 36 (H3K36me1 and H3K36me2), using S-adenosyl-l-methionine (SAM) as a methyl donor. As a key member of the NSD family of proteins, NSD2 plays an important role in the pathogenesis and progression of various diseases such as cancers, inflammations, and infectious diseases, serving as a promising drug target. Developing potent and specific NSD2 inhibitors may provide potential novel therapeutics. Several NSD2 inhibitors and degraders have been discovered while remaining in the early stage of drug development. Excitingly, KTX-1001, a selective NSD2 inhibitor, has entered clinical trials. In this Perspective, the structures and functions of NSD2, its roles in various human diseases, and the recent advances in drug discovery strategies targeting NSD2 have been summarized. The challenges, opportunities, and future directions for developing NSD2 inhibitors and degraders are also discussed.

Inhibition of EZH2 alleviates SAHA-induced senescence-associated secretion phenotype in small cell lung cancer cells

Chemotherapy has been widely used in small cell lung cancer (SCLC) treatment in the past decades. However, SCLC is easy to recur after chemotherapy. The senescence of cancer cells during chemotherapy is one of the effective therapeutic strategies to inhibit the progression of cancer. Nevertheless, the senescence-associated secretion phenotype (SASP) promotes chronic inflammation of the cancer microenvironment and further accelerates the progression of tumors. Therefore, inducing the senescence of cancer cells and inhibiting the production of SASP factors during anticancer treatment have become effective therapeutic strategies to improve the anticancer effect of drugs. Here we reported that SCLC cells treated with an FDA-approved HDAC inhibitor SAHA underwent senescence and displayed remarkable SASP. In particular, SAHA promoted the formation of cytoplasmic chromatin fragments (CCFs) in SCLC cells. The increased CCFs in SAHA-treated SCLC cells were related to nuclear porin Tpr, which activated the cGAS-STING pathway, and promoted the secretion of SASP in cancer cells. Inhibition of EZH2 suppressed the increase of CCFs in SAHA-treated SCLC cells, weakened the production of SASP, and increased the antiproliferative effect of SAHA. Overall, our work affords new insight into the secretion of SASP in SCLC and establishes a foundation for constructing a new therapeutic strategy for SCLC patients.

Combining EZH2 and HDAC inhibitors to target castration-resistant prostate cancers

Development of resistance in castration-resistant prostate cancer (CRPC) involves epigenetic pathways. A new study in PLOS Biology demonstrates that combined therapy targeting enhancer of zeste homolog 2 (EZH2) and histone deacetylases (HDACs) may sensitize CRPC to both epigenetic and standard therapies.

Gene-repressing epigenetic reader EED unexpectedly enhances cyclinD1 gene activation

Epigenetically switched, proliferative vascular smooth muscle cells (SMCs) form neointima, engendering stenotic diseases. Histone-3 lysine-27 trimethylation (H3K27me3) and acetylation (H3K27ac) marks are associated with gene repression and activation, respectively. The polycomb protein embryonic ectoderm development (EED) reads H3K27me3 and also enhances its deposition, hence is a canonical gene repressor. However, herein we found an unexpected role for EED in activating the bona fide pro-proliferative gene Ccnd1 (cyclinD1). EED overexpression in SMCs increased Ccnd1 mRNA, seemingly contradicting its gene-repressing function. However, consistently, EED co-immunoprecipitated with gene-activating H3K27ac reader BRD4, and they co-occupied at both mitogen-activated Ccnd1 and mitogen-repressed P57 (bona fide anti-proliferative gene), as indicated by chromatin immunoprecipitation qPCR. These results were abolished by an inhibitor of either the EED/H3K27me3 or BRD4/H3K27ac reader function. In accordance, elevating BRD4 increased H3K27me3. In vivo, while EED was upregulated in rat and human neointimal lesions, selective EED inhibition abated angioplasty-induced neointima and reduced cyclinD1 in rat carotid arteries. Thus, results uncover a previously unknown role for EED in Ccnd1 activation, likely via its cooperativity with BRD4 that enhances each other's reader function; i.e., activating pro-proliferative Ccnd1 while repressing anti-proliferative P57. As such, this study confers mechanistic implications for the epigenetic intervention of neointimal pathology.

Triazole-fused pyrimidines in target-based anticancer drug discovery

In recent decades, the development of targeted drugs has featured prominently in the treatment of cancer, which is among the major causes of mortality globally. Triazole-fused pyrimidines, a widely-used class of heterocycles in medicinal chemistry, have attracted considerable interest as potential anticancer agents that target various cancer-associated targets in recent years, demonstrating them as valuable templates for discovering novel anticancer candidates. The current review concentrates on the latest advancements of triazole-pyrimidines as target-based anticancer agents, including works published between 2007 and the present (2007-2022). The structure-activity relationships (SARs) and multiple pathways are also reviewed to shed light on the development of more effective and biotargeted anticancer candidates.

Structure-Based Design of the Indole-Substituted Triazolopyrimidines as New EED-H3K27me3 Inhibitors for the Treatment of Lymphoma

Interrupting the embryonic ectoderm development (EED)-H3K27me3 interaction represents a promising strategy to allosterically inhibit polycomb repressive complex 2 (PRC2) for cancer therapy. In this work, we report the structure-based design of new triazolopyrimidine-based EED inhibitors, which structurally feature the electron-rich indole ring at the C8 position. Particularly, ZJH-16 directly binds to EED (HTRF IC₅₀ = 2.72 nM, BLI K_D = 4.4 nM) and potently inhibits the growth of KARPAS422 and Pfeiffer cells. In both cells, ZJH-16 is selectively engaged with EED and reduces H3K27 trimethylation levels. ZJH-16 inhibits the gene silencing function of PRC2 in KARPAS422 cells. ZJH-16 possesses favorable pharmacokinetic (PK) profiles with an excellent oral bioavailability (F = 94.7%). More importantly, ZJH-16 shows robust tumor regression in the KARPAS422 xenograft model after oral administration with the tumor growth inhibition reaching nearly 100%. The robust antitumor efficacy and favorable PK profiles of ZJH-16 warrant further advanced preclinical development for lymphoma treatment.

Proteomic characterization of post-translational modifications in drug discovery

Protein post-translational modifications (PTMs), which are usually enzymatically catalyzed, are major regulators of protein activity and involved in almost all celluar processes. Dysregulation of PTMs is associated with various types of diseases. Therefore, PTM regulatory enzymes represent as an attractive and important class of targets in drug research and development. Inhibitors against kinases, methyltransferases, deacetyltransferases, ubiquitin ligases have achieved remarkable success in clinical application. Mass spectrometry-based proteomics technologies serve as a powerful approach for system-wide characterization of PTMs, which facilitates the identification of drug targets, elucidation of the mechanisms of action of drugs, and discovery of biomakers in personalized therapy. In this review, we summarize recent advances of proteomics-based studies on PTM targeting drugs and discuss how proteomics strategies facilicate drug target identification, mechanism elucidation, and new therapy development in precision medicine.

Polycomb Directed Cell Fate Decisions in Development and Cancer

The polycomb group (PcG) proteins are a subset of transcription regulators highly conserved throughout evolution. Their principal role is to epigenetically modify chromatin landscapes and control the expression of master transcriptional programs to determine cellular identity. The two mayor PcG protein complexes that have been identified in mammals to date are Polycomb Repressive Complex 1 (PRC1) and 2 (PRC2). These protein complexes selectively repress gene expression via the induction of covalent post-translational histone modifications, promoting chromatin structure stabilization. PRC2 catalyzes the histone H3 methylation at lysine 27 (H3K27me1/2/3), inducing heterochromatin structures. This activity is controlled by the formation of a multi-subunit complex, which includes enhancer of zeste (EZH2), embryonic ectoderm development protein (EED), and suppressor of zeste 12 (SUZ12). This review will summarize the latest insights into how PRC2 in mammalian cells regulates transcription to orchestrate the temporal and tissue-specific expression of genes to determine cell identity and cell-fate decisions. We will specifically describe how PRC2 dysregulation in different cell types can promote phenotypic plasticity and/or non-mutational epigenetic reprogramming, inducing the development of highly aggressive epithelial neuroendocrine carcinomas, including prostate, small cell lung, and Merkel cell cancer. With this, EZH2 has emerged as an important actionable therapeutic target in such cancers.

Critical Roles of Polycomb Repressive Complexes in Transcription and Cancer

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

Identification of Two Non-Peptidergic Small Molecule Inhibitors of CBX2 Binding to K27 Trimethylated Oligonucleosomes

The dysregulation of the PRC1/2 complex plays a key role in lineage plasticity in prostate cancer and may be required to maintain neuroendocrine phenotype. [1] CBX2, a key component of the canonical PRC1 complex, is an epigenetic reader, recognizing trimethylated lysine on histone 3 (H3K27me3) [2] and is overexpressed in metastatic neuroendocrine prostate cancer. [3,4] We implemented a screening strategy using nucleosome substrates to identify inhibitors of CBX2 binding to chromatin. Construct design and phosphorylation state of CBX2 were critical for successful implementation and execution of an HTS library screen. A rigorous screening funnel including counter and selectivity assays allowed us to quickly focus on true positive hit matter. Two distinct non-peptide-like chemotypes were identified and confirmed in orthogonal biochemical and biophysical assays demonstrating disruption of CBX2 binding to nucleosomes and direct binding to purified CBX2, respectively.

The Status and Prospects of Epigenetics in the Treatment of Lymphoma

The regulation of gene transcription by epigenetic modifications is closely related to many important life processes and is a hot research topic in the post-genomic era. Since the emergence of international epigenetic research in the 1990s, scientists have identified a variety of chromatin-modifying enzymes and recognition factors, and have systematically investigated their three-dimensional structures, substrate specificity, and mechanisms of enzyme activity regulation. Studies of the human tumor genome have revealed the close association of epigenetic factors with various malignancies, and we have focused more on mutations in epigenetically related regulatory enzymes and regulatory recognition factors in lymphomas. A number of studies have shown that epigenetic alterations are indeed widespread in the development and progression of lymphoma and understanding these mechanisms can help guide clinical efforts. In contrast to chemotherapy which induces cytotoxicity, epigenetic therapy has the potential to affect multiple cellular processes simultaneously, by reprogramming cells to achieve a therapeutic effect in lymphoma. Epigenetic monotherapy has shown promising results in previous clinical trials, and several epigenetic agents have been approved for use in the treatment of lymphoma. In addition, epigenetic therapies in combination with chemotherapy and/or immunotherapy have been used in various clinical trials. In this review, we present several important epigenetic modalities of regulation associated with lymphoma, summarize the corresponding epigenetic drugs in lymphoma, and look at the future of epigenetic therapies in lymphoma.

Discovery of the Clinical Candidate MAK683: An EED-Directed, Allosteric, and Selective PRC2 Inhibitor for the Treatment of Advanced Malignancies

Polycomb Repressive Complex 2 (PRC2) plays an important role in transcriptional regulation during animal development and in cell differentiation, and alteration of PRC2 activity has been associated with cancer. On a molecular level, PRC2 catalyzes methylation of histone H3 lysine 27 (H3K27), resulting in mono-, di-, or trimethylated forms of H3K27, of which the trimethylated form H3K27me3 leads to transcriptional repression of polycomb target genes. Previously, we have shown that binding of the low-molecular-weight compound EED226 to the H3K27me3 binding pocket of the regulatory subunit EED can effectively inhibit PRC2 activity in cells and reduce tumor growth in mouse xenograft models. Here, we report the stepwise optimization of the tool compound EED226 toward the potent and selective EED inhibitor MAK683 (compound 22) and its subsequent preclinical characterization. Based on a balanced PK/PD profile, efficacy, and mitigated risk of forming reactive metabolites, MAK683 has been selected for clinical development.

Downregulation of MEG3 and upregulation of EZH2 cooperatively promote neuroblastoma progression

Neuroblastoma (NB), an embryonic tumour originating from sympathetic crest cells, is the most common extracranial solid tumour type in children with poor overall prognosis. Accumulating evidence has demonstrated the involvement of long non‐coding RNA (lncRNA) in numerous biological processes and their associations with embryonic development and multiple diseases. Ectopic lncRNA expression is linked to malignant tumours. Previous studies by our team indicate that MEG3 attenuates NB autophagy through inhibition of FOXO1 and epithelial‐mesenchymal transition via the mTOR pathway in vitro. Moreover, MEG3 and EZH2 negatively regulate each other. In present study, we first collected 60 NB tissues and 20 adjacent tissues for Quantitative real‐time polymerase chain reaction (Q‐PCR) experiments and performed clinical correlation analysis of the results. At the same time, nude mice were used for subcutaneous tumour formation to detect the effect of MEG3 in vivo. Two NB cell lines, SK‐N‐AS and SK‐N‐BE(2)C, were overexpressed MEG3 and rescued with EZH2 and then were subjected to proliferation, migration, invasion, apoptosis and autophagy experiments. RNA‐binding protein immunoprecipitation (RIP) and Co‐Immunoprecipitation (Co‐IP) experiments were performed to explore the molecular mechanism of MEG3 and EZH2 interaction. Q‐PCR revealed that MEG3 expression was negatively correlated with INSS stage and risk grade of NB. Moreover, MEG3 overexpression was associated with inhibition of NB growth in vivo. MEG3 exerted an anti‐cancer effect via stimulatory effects on EZH2 ubiquitination leading to its degradation. Conversely, EZH2 interacted with DNMT1 and HDAC1 to induce silencing of MEG3. The EZH2 inhibitor, DZNep, and HDAC inhibitor, SAHA, displayed synergistic activity against NB. Combined treatment with DZNep and SAHA inhibited proliferation, migration and invasion of NB through suppression of the PI3K/AKT/mTOR/FOXO1 pathway. In conclusion, downregulation of MEG3 and upregulation of EZH2 forms a feedback loop that concertedly promotes the development of NB. Combined blockage of EZH2 and HDAC1 with the appropriate inhibitors may therefore present an effective treatment strategy for NB cases with low MEG3 and high EZH2 expression.

Epigenetics of Cutaneous Sarcoma

Epigenetic changes influence various physiological and pathological conditions in the human body. Recent advances in epigenetic studies of the skin have led to an appreciation of the importance of epigenetic modifications in skin diseases. Cutaneous sarcomas are intractable skin cancers, and there are no curative therapeutic options for the advanced forms of cutaneous sarcomas. In this review, we discuss the detailed molecular effects of epigenetic modifications on skin sarcomas, such as dermatofibrosarcoma protuberans, angiosarcoma, Kaposi's sarcoma, leiomyosarcoma, and liposarcoma. We also discuss the application of epigenetic-targeted therapy for skin sarcomas.

Small molecule targeting of chromatin writers in cancer

More than a decade after the launch of DNA methyltransferase and histone deacetylase inhibitors for the treatment of cancer, 2020 heralded the approval of the first histone methyltransferase inhibitor, revitalizing the concept that targeted manipulation of the chromatin regulatory landscape can have profound therapeutic impact. Three chromatin regulatory pathways-DNA methylation, histone acetylation and methylation-are frequently implicated in human cancer but hundreds of potentially druggable mechanisms complicate identification of key targets for therapeutic intervention. In addition to human genetics and functional screening, chemical biology approaches have proven critical for the discovery of key nodes in these pathways and in an ever-increasing complexity of molecularly defined human cancer contexts. This review introduces small molecule targeting approaches, showcases chemical probes and drug candidates for epigenetic writer enzymes, illustrates molecular features that may represent epigenetic dependencies and suggests translational strategies to maximize their impact in cancer therapy.

MicroRNA-214-5p involves in the protection effect of Dexmedetomidine against neurological injury in Alzheimer's disease via targeting the suppressor of zest 12

OBJECTIVE

Alzheimer's disease (AD) is a neurological disease. Dexmedetomidine (Dex) has been evidenced to exert neuroprotective effects on multiple neurological diseases, while the function of microRNA(miR)- 214-5p on Dex-mediated AD progression via targeting the suppressor of zest 12 (SUZ12) remains unclear. This study obligates to investigate the regulatory functions of Dex, miR-214-5p and SUZ12 on AD.

METHODS

The expression of miR-214-5p and SUZ12 in APPswe/PS1dE9 mice (hereinafter referred to as AD mice) was examined. Thereafter, the AD mice were treated with Dex or increased miR-214-5p or reduced SUZ12 to determine the spatial memory ability, apoptosis of hippocampal neurons and the contents of serum inflammatory and oxidative stress factors of AD mice. Finally, the target relationship between miR-214-5p and SUZ12 was detected.

RESULTS

MiR-214-5p was reduced and SUZ12 was elevated in AD mice. Dex administration reduced the apoptosis of hippocampal neurons, the contents of serum inflammatory factor and oxidative stress, and attenuated the cognitive impairment of AD mice accompanied by up-regulated miR-214-5p and down-regulated SUZ12, and the overexpression of miR-214-5p or reduction of SUZ12 could effectively enhance the Dex-treated effects on AD mice. MiR-214-5p targeted SUZ12.

CONCLUSION

Dex may have a potential neuroprotective effect on AD via the miR-214-5p/SUZ12 axis. This study provides novel therapeutic targets for AD treatment.

Discovery of EEDi-5273 as an Exceptionally Potent and Orally Efficacious EED Inhibitor Capable of Achieving Complete and Persistent Tumor Regression

Embryonic ectoderm development (EED) is a promising therapeutic target for human cancers and other diseases. We report herein the discovery of exceptionally potent and efficacious EED inhibitors. By conformational restriction of a previously reported EED inhibitor, we obtained a potent lead compound. Further optimization of the lead yielded exceptionally potent EED inhibitors. The best compound EEDi-5273 binds to EED with an IC₅₀ value of 0.2 nM and inhibits the KARPAS422 cell growth with an IC₅₀ value of 1.2 nM. It demonstrates an excellent PK and ADME profile, and its oral administration leads to complete and persistent tumor regression in the KARPAS422 xenograft model with no signs of toxicity. Co-crystal structures of two potent EED inhibitors with EED provide a solid structural basis for their high-affinity binding. EEDi-5273 is a promising EED inhibitor for further advanced preclinical development for the treatment of human cancer and other human diseases.

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

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

Going beyond Polycomb: EZH2 functions in prostate cancer

The Polycomb group (PcG) protein Enhancer of Zeste Homolog 2 (EZH2) is one of the three core subunits of the Polycomb Repressive Complex 2 (PRC2). It harbors histone methyltransferase activity (MTase) that specifically catalyze histone 3 lysine 27 (H3K27) methylation on target gene promoters. As such, PRC2 are epigenetic silencers that play important roles in cellular identity and embryonic stem cell maintenance. In the past two decades, mounting evidence supports EZH2 mutations and/or over-expression in a wide array of hematological cancers and solid tumors, including prostate cancer. Further, EZH2 is among the most upregulated genes in neuroendocrine prostate cancers, which become abundant due to the clinical use of high-affinity androgen receptor pathway inhibitors. While numerous studies have reported epigenetic functions of EZH2 that inhibit tumor suppressor genes and promote tumorigenesis, discordance between EZH2 and H3K27 methylation has been reported. Further, enzymatic EZH2 inhibitors have shown limited efficacy in prostate cancer, warranting a more comprehensive understanding of EZH2 functions. Here we first review how canonical functions of EZH2 as a histone MTase are regulated and describe the various mechanisms of PRC2 recruitment to the chromatin. We further outline non-histone substrates of EZH2 and discuss post-translational modifications to EZH2 itself that may affect substrate preference. Lastly, we summarize non-canonical functions of EZH2, beyond its MTase activity and/or PRC2, as a transcriptional cofactor and discuss prospects of its therapeutic targeting in prostate cancer.

Polycomb Repressive Complex 2 Modulation through the Development of EZH2-EED Interaction Inhibitors and EED Binders

Epigenetics is nowadays a well-accepted area of research. In the last years, tremendous progress was made regarding molecules targeting EZH2, directly or indirectly. Recently tazemetostat hit the market after FDA-approval for the treatment of lymphoma. However, the impairment of EZH2 activity by orthosteric intervention has proven to be effective only in a limited subset of cancers. Considering the multiproteic nature of the PRC2 complex and the marked dependence of EZH2 functions on the other core subunits such as EED, in recent years, a new targeting approach ascended to prominence. The possibility to cripple the function of the PRC2 complex by interfering with its multimeric integrity fueled the interest in developing EZH2–EED protein–protein interaction and EED inhibitors as indirect modulators of PRC2-dependent methyltransferase activity. In this Perspective, we aim to summarize the latest findings regarding the development and the biological activity of these emerging classes of PRC2 modulators from a medicinal chemist’s viewpoint.

Structure-Guided Development of Small-Molecule PRC2 Inhibitors Targeting EZH2-EED Interaction

Disruption of EZH2-embryonic ectoderm development (EED) protein-protein interaction (PPI) is a new promising cancer therapeutic strategy. We have previously reported the discovery of astemizole, a small-molecule inhibitor targeting the EZH2-EED PPI. Herein, we report the cocrystal structure of EED in complex with astemizole at 2.15 Å. The structure elucidates the detailed binding mode of astemizole to EED and provides a structure-guided design for the discovery of a novel EZH2-EED interaction inhibitor, DC-PRC2in-01, with an affinity K_d of 4.56 μM. DC-PRC2in-01 destabilizes the PRC2 complex, thereby leading to the degradation of PRC2 core proteins and the decrease of global H3K27me3 levels in cancer cells. The proliferation of PRC2-driven lymphomas cells is effectively inhibited, and the cell cycle is arrested in the G0/G1 phase. Together, these data demonstrate that DC-PRC2in-01 could be an effective chemical probe for investigating the PRC2-related physiology and pathology and providing a promising chemical scaffold for further development.

Pharmacological inhibition of noncanonical EED-EZH2 signaling overcomes chemoresistance in prostate cancer

Rationale: Chemoresistance is a major obstacle in prostate cancer (PCa) treatment. We sought to understand the underlying mechanism of PCa chemoresistance and discover new treatments to overcome docetaxel resistance. Methods: We developed a novel phenotypic screening platform for the discovery of specific inhibitors of chemoresistant PCa cells. The mechanism of action of the lead compound was investigated using computational, molecular and cellular approaches. The in vivo toxicity and efficacy of the lead compound were evaluated in clinically-relevant animal models. Results: We identified LG1980 as a lead compound that demonstrates high selectivity and potency against chemoresistant PCa cells. Mechanistically, LG1980 binds embryonic ectoderm development (EED), disrupts the interaction between EED and enhancer of zeste homolog 2 (EZH2), thereby inducing the protein degradation of EZH2 and inhibiting the phosphorylation and activity of EZH2. Consequently, LG1980 targets a survival signaling cascade consisting of signal transducer and activator of transcription 3 (Stat3), S-phase kinase-associated protein 2 (SKP2), ATP binding cassette B 1 (ABCB1) and survivin. As a lead compound, LG1980 is well tolerated in mice and effectively suppresses the in vivo growth of chemoresistant PCa and synergistically enhances the efficacy of docetaxel in xenograft models. Conclusions: These results indicate that pharmacological inhibition of EED-EZH2 interaction is a novel strategy for the treatment of chemoresistant PCa. LG1980 and its analogues have the potential to be integrated into standard of care to improve clinical outcomes in PCa patients.

Targeting Epigenetics to Cure HIV-1: Lessons From (and for) Cancer Treatment

The Human Immunodeficiency Virus type 1 (HIV-1) integrates in the host genome as a provirus resulting in a long-lived reservoir of infected CD4 cells. As a provirus, HIV-1 has several aspects in common with an oncogene. Both the HIV-1 provirus and oncogenes only cause disease when expressed. A successful cure of both cancer and HIV-1 includes elimination of all cells with potential to regenerate the disease. For over two decades, epigenetic drugs developed against cancer have been used in the HIV-1 field to modulate the state of the proviral chromatin. Cells with an intact HIV-1 provirus exist in three states of infection: productive, inducible latent, and non-inducible latent. Here focus is on HIV-1, transcription control and chromatin structure; how the inducible proviruses are maintained in a chromatin structure that allows reactivation of transcription; and how transcription switches between different stages to allow for an abundance of different transcripts from a single promoter. Recently it was shown that a functional cure of HIV can be achieved by encapsulating all intact HIV-1 proviruses in heterochromatin, giving hope that epigenetic interventions may be used to end the HIV-1 epidemic.

Allosteric regulation of histone lysine methyltransferases: from context-specific regulation to selective drugs

Histone lysine methyltransferases (HKMTs) are key regulators of many cellular processes. By definition, HKMTs catalyse the methylation of lysine residues in histone proteins. The enzymatic activities of HKMTs are under precise control, with their allosteric regulation emerging as a prevalent paradigm. We review the molecular mechanisms of allosteric regulation of HKMTs using well-studied histone H3 (K4, K9, K27 and K36) methyltransferases as examples. We discuss the current advances and future potential in targeting allosteric sites of HKMTs for drug development.

Targeting Epigenetic Regulators with Covalent Small-Molecule Inhibitors

Epigenetic regulation of gene expression plays a critical role in various physiological processes, and epigenetic dysregulation is implicated in a number of diseases, prominently including cancer. Epigenetic regulators have been validated as potential therapeutic targets, and significant progress has been made in the discovery and development of epigenetic-based inhibitors. However, successful epigenetic drug discovery is still facing challenges, including moderate selectivity, limited efficacy, and acquired drug resistance. Inspired by the advantages of covalent small-molecule inhibitors, targeted covalent inhibition has attracted increasing interest in epigenetic drug discovery. In this review, we comprehensively summarize the structure-based design and characterization of covalent inhibitors targeting epigenetic writers, readers, and erasers and highlight their potential benefits in enhancing selectivity across the enzyme family and improving in vivo efficacy. We also discuss the challenges and opportunities of covalent small-molecule inhibitors and hope to shed light on future epigenetic drug discovery.