Structure-Activity Relationships and Kinetic Studies of Peptidic Antagonists of CBX Chromodomains

Discovery of CHD1 Antagonists for PTEN-Deficient Prostate Cancer

CHD1 is a chromodomain-helicase DNA-binding protein that preferentially recognizes di- and trimethylated lysine 4 on histone H3 (H3K4me2/3). Genetic studies have established CHD1 as a synthetic lethal target in phosphatase and tensin homologue (PTEN)-deficient cancers. Despite this attractive therapeutic link, no inhibitors or antagonists of CHD1 have been reported to date. Herein, we report the discovery of UNC10142, a first-in-class small molecule antagonist of the tandem chromodomains of CHD1 that binds with an IC50 of 1.7 ± 0.2 μM. A cocrystal structure revealed a unique binding mode and competition pull-down experiments in cell lysates confirmed endogenous target engagement. Treatment of PTEN-deficient prostate cancer cells with UNC10142 led to a dose-dependent reduction in viability while PTEN-intact prostate cancer cells were unaffected, phenocopying genetic loss of CHD1. Overall, this study demonstrates the ligandability of the CHD1 chromodomains and suggests more potent and selective antagonists could translate to compounds of therapeutic value in PTEN-deficient cancers.

Chemical inhibitors targeting histone methylation readers

Histone methylation reader domains are protein modules that recognize specific histone methylation marks, such as methylated or unmethylated lysine or arginine residues on histones. These reader proteins play crucial roles in the epigenetic regulation of gene expression, chromatin structure, and DNA damage repair. Dysregulation of these proteins has been linked to various diseases, including cancer, neurodegenerative diseases, and developmental disorders. Therefore, targeting these proteins with chemical inhibitors has emerged as an attractive approach for therapeutic intervention, and significant progress has been made in this area. In this review, we will summarize the development of inhibitors targeting histone methylation readers, including MBT domains, chromodomains, Tudor domains, PWWP domains, PHD fingers, and WD40 repeat domains. For each domain, we will briefly discuss its identification and biological/biochemical functions, and then focus on the discovery of inhibitors tailored to target this domain, summarizing the property and potential application of most inhibitors. We will also discuss the structural basis for the potency and selectivity of these inhibitors, which will aid in further lead generation and optimization. Finally, we will also address the challenges and strategies involved in the development of these inhibitors. It should facilitate the rational design and development of novel chemical scaffolds and new targeting strategies for histone methylation reader domains with the help of this body of data.

Chemical tools targeting readers of lysine methylation

Reader domains that recognize methylated lysine and arginine residues on histones play a role in the recruitment, stabilization, and regulation of chromatin regulatory proteins. Targeting reader proteins with small molecule and peptidomimetic inhibitors has enabled the elucidation of the structure and function of specific domains and uncovered their role in diseases. Recent progress towards chemical probes that target readers of lysine methylation, including the Royal family and plant homeodomains (PHD), is discussed here. We highlight recently developed covalent cyclic peptide inhibitors of a plant homeodomain. Additionally, inhibitors targeting previously untargeted Tudor domains and chromodomains are discussed.

Systematic Variation of Both the Aromatic Cage and Dialkyllysine via GCE-SAR Reveal Mechanistic Insights in CBX5 Reader Protein Binding

Development of inhibitors for histone methyllysine reader proteins is an active area of research due to the importance of reader protein-methyllysine interactions in transcriptional regulation and disease. Optimized peptide-based chemical probes targeting methyllysine readers favor larger alkyllysine residues in place of methyllysine. However, the mechanism by which these larger substituents drive tighter binding is not well understood. This study describes the development of a two-pronged approach combining genetic code expansion (GCE) and structure-activity relationships (SAR) through systematic variation of both the aromatic binding pocket in the protein and the alkyllysine residues in the peptide to probe inhibitor recognition in the CBX5 chromodomain. We demonstrate a novel change in driving force for larger alkyllysines, which weaken cation-π interactions but increases dispersion forces, resulting in tighter binding. This GCE-SAR approach establishes discrete energetic contributions to binding from both ligand and protein, providing a powerful tool to gain mechanistic understanding of SAR trends.

Discovery of Potent Peptidomimetic Antagonists for Heterochromatin Protein 1 Family Proteins

The heterochromatin protein 1 (HP1) sub-family of CBX chromodomains are responsible for the recognition of histone H3 lysine 9 tri-methyl (H3K9me3)-marked nucleosomal substrates through binding of the N-terminal chromodomain. These HP1 proteins, namely, CBX1 (HP1β), CBX3 (HP1γ), and CBX5 (HP1α), are commonly associated with regions of pericentric heterochromatin, but recent literature studies suggest that regulation by these proteins is likely more dynamic and includes other loci. Importantly, there are no chemical tools toward HP1 chromodomains to spatiotemporally explore the effects of HP1-mediated processes, underscoring the need for novel HP1 chemical probes. Here, we report the discovery of HP1 targeting peptidomimetic compounds, UNC7047 and UNC7560, and a biotinylated derivative tool compound, UNC7565. These compounds represent an important milestone, as they possess nanomolar affinity for the CBX5 chromodomain by isothermal titration calorimetry (ITC) and bind HP1-containing complexes in cell lysates. These chemical tools provide a starting point for further optimization and the study of CBX5-mediated processes.

Reprogramming CBX8-PRC1 function with a positive allosteric modulator

Canonical targeting of Polycomb repressive complex 1 (PRC1) to repress developmental genes is mediated by cell-type-specific, paralogous chromobox (CBX) proteins (CBX2, 4, 6, 7, and 8). Based on their central role in silencing and their dysregulation associated with human disease including cancer, CBX proteins are attractive targets for small-molecule chemical probe development. Here, we have used a quantitative and target-specific cellular assay to discover a potent positive allosteric modulator (PAM) of CBX8. The PAM activity of UNC7040 antagonizes H3K27me3 binding by CBX8 while increasing interactions with nucleic acids. We show that treatment with UNC7040 leads to efficient and selective eviction of CBX8-containing PRC1 from chromatin, loss of silencing, and reduced proliferation across different cancer cell lines. Our discovery and characterization of UNC7040 not only reveals the most cellularly potent CBX8-specific chemical probe to date, but also corroborates a mechanism of Polycomb regulation by non-specific CBX nucleotide binding activity.

Polycomb Paralog Chromodomain Inhibitors Active against Both CBX6 and CBX8*

Methyllysine reader proteins bind to methylated lysine residues and alter gene transcription by changing either the compaction state of chromatin or by the recruitment of other multiprotein complexes. The polycomb paralog family of methyllysine readers bind to trimethylated lysine on the tail of histone 3 (H3) via a highly conserved aromatic cage located in their chromodomains. Each of the polycomb paralogs are implicated in several disease states. CBX6 and CBX8 are members of the polycomb paralog family with two structurally similar chromodomains. By exploring the structure-activity relationships of a previously reported CBX6 inhibitor we have discovered more potent and cell permeable analogs. Our current report includes potent, dual-selective inhibitors of CBX6 and CBX8. We have shown that the -2 position in our scaffold is an important residue for selectivity amongst the polycomb paralogs. Preliminary cell-based studies show that the new inhibitors impact cell proliferation in a rhabdoid tumor cell line.

Discovery of an H3K36me3-Derived Peptidomimetic Ligand with Enhanced Affinity for Plant Homeodomain Finger Protein 1 (PHF1)

Plant homeodomain finger protein 1 (PHF1) is an accessory component of the gene silencing complex polycomb repressive complex 2 and recognizes the active chromatin mark, trimethylated lysine 36 of histone H3 (H3K36me3). In addition to its role in transcriptional regulation, PHF1 has been implicated as a driver of endometrial stromal sarcoma and fibromyxoid tumors. We report the discovery and characterization of UNC6641, a peptidomimetic antagonist of the PHF1 Tudor domain which was optimized through in silico modeling and incorporation of non-natural amino acids. UNC6641 binds the PHF1 Tudor domain with a K_d value of 0.96 ± 0.03 μM while also binding the related protein PHF19 with similar potency. A crystal structure of PHF1 in complex with UNC6641, along with NMR and site-directed mutagenesis data, provided insight into the binding mechanism and requirements for binding. Additionally, UNC6641 enabled the development of a high-throughput assay to identify small molecule binders of PHF1.

Improved methods for targeting epigenetic reader domains of acetylated and methylated lysine

Responsible for interpreting histone post-translational modifications, epigenetic reader proteins have emerged as novel therapeutic targets for a wide range of diseases. Chemical probes have been critical in enabling target validation studies and have led to translational advances in cancer and inflammation-related pathologies. Here, we present the most recently reported probes of reader proteins that recognize acylated and methylated lysine. We will discuss challenges associated with achieving potent antagonism of reader domains and review ongoing efforts to overcome these hurdles, focusing on targeting strategies including the use of peptidomimetic ligands, allosteric modulators, and protein degraders.

Protein Interaction Domains and Post-Translational Modifications: Structural Features and Drug Discovery Applications

Background:

Many pathways regarding healthy cells and/or linked to diseases onset and progression depend on large assemblies including multi-protein complexes. Protein-protein interactions may occur through a vast array of modules known as protein interaction domains (PIDs).

Objective:

This review concerns with PIDs recognizing post-translationally modified peptide sequences and intends to provide the scientific community with state of art knowledge on their 3D structures, binding topologies and potential applications in the drug discovery field.

Method:

Several databases, such as the Pfam (Protein family), the SMART (Simple Modular Architecture Research Tool) and the PDB (Protein Data Bank), were searched to look for different domain families and gain structural information on protein complexes in which particular PIDs are involved. Recent literature on PIDs and related drug discovery campaigns was retrieved through Pubmed and analyzed.

Results and Conclusion:

PIDs are rather versatile as concerning their binding preferences. Many of them recognize specifically only determined amino acid stretches with post-translational modifications, a few others are able to interact with several post-translationally modified sequences or with unmodified ones. Many PIDs can be linked to different diseases including cancer. The tremendous amount of available structural data led to the structure-based design of several molecules targeting protein-protein interactions mediated by PIDs, including peptides, peptidomimetics and small compounds. More studies are needed to fully role out, among different families, PIDs that can be considered reliable therapeutic targets, however, attacking PIDs rather than catalytic domains of a particular protein may represent a route to obtain selective inhibitors.

Advances and Opportunities in Epigenetic Chemical Biology

The study of epigenetics has greatly benefited from the development and application of various chemical biology approaches. In this review, we highlight the key targets for modulation and recent methods developed to enact such modulation. We discuss various chemical biology techniques to study DNA methylation and the post-translational modification of histones as well as their effect on gene expression. Additionally, we address the wealth of protein synthesis approaches to yield histones and nucleosomes bearing epigenetic modifications. Throughout, we highlight targets that present opportunities for the chemical biology community, as well as exciting new approaches that will provide additional insight into the roles of epigenetic marks.

Discovery and Characterization of a Cellular Potent Positive Allosteric Modulator of the Polycomb Repressive Complex 1 Chromodomain, CBX7

Polycomb-directed repression of gene expression is frequently misregulated in human diseases. A quantitative and target-specific cellular assay was utilized to discover the first potent positive allosteric modulator (PAM) peptidomimetic, UNC4976, of nucleic acid binding by CBX7, a chromodomain methyl-lysine reader of Polycomb repressive complex 1. The PAM activity of UNC4976 resulted in enhanced efficacy across three orthogonal cellular assays by simultaneously antagonizing H3K27me3-specific recruitment of CBX7 to target genes while increasing non-specific binding to DNA and RNA. PAM activity thereby reequilibrates PRC1 away from H3K27me3 target regions. Together, our discovery and characterization of UNC4976 not only revealed the most cellularly potent PRC1-specific chemical probe to date, but also uncovers a potential mechanism of Polycomb regulation with implications for non-histone lysine methylated interaction partners.

Discovery of selective activators of PRC2 mutant EED-I363M

Many common disease-causing mutations result in loss-of-function (LOF) of the proteins in which they occur. LOF mutations have proven recalcitrant to pharmacologic intervention, presenting a challenge for the development of targeted therapeutics. Polycomb repressive complex 2 (PRC2), which contains core subunits (EZH2, EED, and SUZ12), regulates gene activity by trimethylation of histone 3 lysine 27. The dysregulation of PRC2 catalytic activity by mutations has been implicated in cancer and other diseases. Among the mutations that cause PRC2 malfunction, an I363M LOF mutation of EED has been identified in myeloid disorders, where it prevents allosteric activation of EZH2 catalysis. We describe structure-based design and computational simulations of ligands created to ameliorate this LOF. Notably, these compounds selectively stimulate the catalytic activity of PRC2-EED-I363M over wildtype-PRC2. Overall, this work demonstrates the feasibility of developing targeted therapeutics for PRC2-EED-I363M that act as allosteric agonists, potentially correcting this LOF mutant phenotype.

Robustness of In Vitro Selection Assays of DNA-Encoded Peptidomimetic Ligands to CBX7 and CBX8

The identification of protein ligands from a DNA-encoded library is commonly conducted by an affinity selection assay. These assays are often not validated for robustness, raising questions about selections that fail to identify ligands and the utility of enrichment values for ranking ligand potencies. Here, we report a method for optimizing and utilizing affinity selection assays to identify potent and selective peptidic ligands to the highly related chromodomains of CBX proteins. To optimize affinity selection parameters, statistical analyses (Z' factors) were used to define the ability of selection assay conditions to identify and differentiate ligands of varying affinity. A DNA-encoded positional scanning library of peptidomimetics was constructed around a trimethyllysine-containing parent peptide, and parallel selections against the chromodomains from CBX8 and CBX7 were conducted over three protein concentrations. Relative potencies of off-DNA hit molecules were determined through a fluorescence polarization assay and were consistent with enrichments observed by DNA sequencing of the affinity selection assays. In addition, novel peptide-based ligands were discovered with increased potency and selectivity to the chromodomain of CBX8. The results indicate low DNA tag bias and show that affinity-based in vitro selection assays are sufficiently robust for both ligand discovery and determination of quantitative structure-activity relationships.

Epigenetic small molecule modulators of histone and DNA methylation

DNA and histone methylation belong to the key regulatory components in the epigenetic machinery, and dysregulations of these processes have been associated with various human diseases. Small molecule modulators of these epigenetic targets are highly valuable both as chemical probes to study the biological roles of the target proteins, and as potential therapeutics. Indeed, recent years have seen the discovery of chemical modulators of several epigenetic targets, some of which are already marketed drugs or undergoing clinical trials. In this review, we will focus on small molecule modulators of DNA and histone methylation.