Chemical probes for methyl lysine reader domains

Discovery of a Potent, Selective, and Cell-Active SPIN1 Inhibitor

The methyl-lysine reader protein SPIN1 plays important roles in various human diseases. However, targeting methyl-lysine reader proteins has been challenging. Very few cellularly active SPIN1 inhibitors have been developed. We previously reported that our G9a/GLP inhibitor UNC0638 weakly inhibited SPIN1. Here, we present our comprehensive structure-activity relationship study that led to the discovery of compound 11, a dual SPIN1 and G9a/GLP inhibitor, and compound 18 (MS8535), a SPIN1 selective inhibitor. We solved the cocrystal structure of SPIN1 in complex with 11, confirming that 11 occupied one of the three Tudor domains. Importantly, 18 displayed high selectivity for SPIN1 over 38 epigenetic targets, including G9a/GLP, and concentration dependently disrupted the interactions of SPIN1 and H3 in cells. Furthermore, 18 was bioavailable in mice. We also developed 19 (MS8535N), which was inactive against SPIN1, as a negative control of 18. Collectively, these compounds are useful chemical tools to study biological functions of SPIN1.

Trimethyllysine Reader Proteins Exhibit Widespread Charge-Agnostic Binding via Different Mechanisms to Cationic and Neutral Ligands

In the last 40 years, cation-π interactions have become part of the lexicon of noncovalent forces that drive protein binding. Indeed, tetraalkylammoniums are universally bound by aromatic cages in proteins, suggesting that cation-π interactions are a privileged mechanism for binding these ligands. A prominent example is the recognition of histone trimethyllysine (Kme3) by the conserved aromatic cage of reader proteins, dictating gene expression. However, two proteins have recently been suggested as possible exceptions to the conventional understanding of tetraalkylammonium recognition. To broadly interrogate the role of cation-π interactions in protein binding interactions, we report the first large-scale comparative evaluation of reader proteins for a neutral Kme3 isostere, experimental and computational mechanistic studies, and structural analysis. We find unexpected widespread binding of readers to a neutral isostere with the first examples of readers that bind the neutral isostere more tightly than Kme3. We find that no single factor dictates the charge selectivity, demonstrating the challenge of predicting such interactions. Further, readers that bind both cationic and neutral ligands differ in mechanism: binding Kme3 via cation-π interactions and the neutral isostere through the hydrophobic effect in the same aromatic cage. This discovery explains apparently contradictory results in previous studies, challenges traditional understanding of molecular recognition of tetraalkylammoniums by aromatic cages in myriad protein-ligand interactions, and establishes a new framework for selective inhibitor design by exploiting differences in charge dependence.

Discovery of a 53BP1 Small Molecule Antagonist Using a Focused DNA-Encoded Library Screen

Methyl-lysine reader p53 binding protein 1 (53BP1) is a central mediator of DNA break repair and is associated with various human diseases, including cancer. Thus, high-quality 53BP1 chemical probes can aid in further understanding the role of 53BP1 in genome repair pathways. Herein, we utilized focused DNA-encoded library screening to identify the novel hit compound UNC8531, which binds the 53BP1 tandem Tudor domain (TTD) with an IC50 of 0.47 ± 0.09 μM in a TR-FRET assay and Kd values of 0.85 ± 0.17 and 0.79 ± 0.52 μM in ITC and SPR, respectively. UNC8531 was cocrystallized with the 53BP1 TTD to guide further optimization efforts, leading to UNC9512. NanoBRET and 53BP1-dependent foci formation experiments confirmed cellular target engagement. These results show that UNC9512 is a best-in-class small molecule 53BP1 antagonist that can aid further studies investigating the role of 53BP1 in DNA repair, gene editing, and oncogenesis.

Discovery of hit compounds for methyl-lysine reader proteins from a target class DNA-encoded library

Methyl-lysine (Kme) reader domains are prevalent in chromatin regulatory proteins which bind post-translational modification sites to recruit repressive and activating factors; therefore, these proteins play crucial roles in cellular signaling and epigenetic regulation. Proteins that contain Kme domains are implicated in various diseases, including cancer, making them attractive therapeutic targets for drug and chemical probe discovery. Herein, we report on expanding the utility of a previously reported, Kme-focused DNA-encoded library (DEL), UNCDEL003, as a screening tool for hit discovery through the specific targeting of Kme reader proteins. As an efficient method for library generation, focused DELs are designed based on structural and functional features of a specific class of proteins with the intent of novel hit discovery. To broadly assess the applicability of our library, UNCDEL003 was screened against five diverse Kme reader protein domains (53BP1 TTD, KDM7B JmjC-PHD, CDYL2 CD, CBX2 CD, and LEDGF PWWP) with varying structures and functions. From these screening efforts, we identified hit compounds which contain unique chemical scaffolds distinct from previously reported ligands. The selected hit compounds were synthesized off-DNA and confirmed using primary and secondary assays and assessed for binding selectivity. Hit compounds from these efforts can serve as starting points for additional development and optimization into chemical probes to aid in further understanding the functionality of these therapeutically relevant proteins.

Recognition of stapled histone H3K4me3 peptides by epigenetic reader proteins

Epigenetic reader proteins can display stronger or weaker binding affinities for cyclic histone peptides relative to linear histones, indicating that selectivity of biomolecular recognition can be achieved.

A Peptidomimetic Ligand Targeting the Chromodomain of MPP8 Reveals HRP2's Association with the HUSH Complex

The interpretation of histone post-translational modifications (PTMs), specifically lysine methylation, by specific classes of "reader" proteins marks an important aspect of epigenetic control of gene expression. Methyl-lysine (Kme) readers often regulate gene expression patterns through the recognition of a specific Kme PTM while participating in or recruiting large protein complexes that contain enzymatic or chromatin remodeling activity. Understanding the composition of these Kme-reader-containing protein complexes can serve to further our understanding of the biological roles of Kme readers, while small molecule chemical tools can be valuable reagents in interrogating novel protein-protein interactions. Here, we describe our efforts to target the chromodomain of M-phase phosphoprotein 8 (MPP8), a member of the human silencing hub (HUSH) complex and a histone 3 lysine 9 trimethyl (H3K9me3) reader that is vital for heterochromatin formation and has specific roles in cancer metastasis. Utilizing a one-bead, one-compound (OBOC) combinatorial screening approach, we identified UNC5246, a peptidomimetic ligand capable of interacting with the MPP8 chromodomain in the context of the HUSH complex. Additionally, a biotinylated derivative of UNC5246 facilitated chemoproteomics studies which revealed hepatoma-derived growth factor-related protein 2 (HRP2) as a novel protein associated with MPP8. HRP2 was further shown to colocalize with MPP8 at the E-cadherin gene locus, suggesting a possible role in cancer cell plasticity.

Profiling and Validation of Live-Cell Protein Methylation with Engineered Enzymes and Methionine Analogues

Protein methyltransferases (PMTs) regulate many aspects of normal and disease processes through substrate methylation, with S-adenosyl-L-methionine (SAM) as a cofactor. It has been challenging to elucidate cellular protein lysine and arginine methylation because these modifications barely alter physical properties of target proteins and often are context dependent, transient, and substoichiometric. To reveal bona fide methylation events associated with specific PMT activities in native contexts, we developed the live-cell Bioorthogonal Profiling of Protein Methylation (lcBPPM) technology, in which the substrates of specific PMTs are labeled by engineered PMTs inside living cells, with in situ-synthesized SAM analogues as cofactors. The biorthogonality of this technology is achieved because these SAM analogue cofactors can only be processed by the engineered PMTs-and not native PMTs-to modify the substrates with distinct chemical groups. Here, we describe the latest lcBPPM protocol and its application to reveal proteome-wide methylation and validate specific methylation events. © 2021 Wiley Periodicals LLC. Basic Protocol 1: Live-cell labeling of substrates of protein methyltransferases GLP1 and PRMT1 with lcBPPM-feasible enzymes and SAM analogue precursors Support Protocol: Gram-scale synthesis of Hey-Met Basic Protocol 2: Click labeling of lcBPPM cell lysates with a biotin-azide probe Alternate Protocol: Click labeling of small-scale lcBPPM cell lysates with a TAMRA-azide dye for in-gel fluorescence visualization Basic Protocol 3: Enrichment of biotinylated lcBPPM proteome with streptavidin beads Basic Protocol 4: Proteome-wide identification of lcBPPM targets with mass spectrometry Basic Protocol 5: Validation of individual lcBPPM targets by western blot.

Mimicking the Nucleosomal Context in Peptide-Based Binders of a H3K36me Reader Increases Binding Affinity While Altering the Binding Mode

Targeting of proteins in the histone modification machinery has emerged as a promising new direction to fight disease. The search for compounds that inhibit proteins that readout histone modification has led to several new epigenetic drugs, mostly for proteins involved in recognition of acetylated lysines. However, this approach proved to be a challenging task for methyllysine readers, which typically feature shallow binding pockets. Moreover, reader proteins of trimethyllysine K36 on the histone H3 (H3K36me3) not only bind the methyllysine but also the nucleosomal DNA. Here, we sought to find peptide-based binders of H3K36me3 reader PSIP1, which relies on DNA interactions to tightly bind H3K36me3 modified nucleosomes. We designed several peptides that mimic the nucleosomal context of H3K36me3 recognition by including negatively charged Glu-rich regions. Using a detailed NMR analysis, we find that addition of negative charges boosts binding affinity up to 50-fold while decreasing binding to the trimethyllysine binding pocket. Since screening and selection of compounds for reader domains is typically based solely on affinity measurements due to their lack of enzymatic activity, our case highlights the need to carefully control for the binding mode, in particular for the challenging case of H3K36me3 readers.

Getting a handle on chemical probes of chomatin readers

The dynamic nature of histone post-translational modifications such as methylation or acetylation makes possible the alteration of disease associated epigenetic states through the manipulation of the associated epigenetic machinery. One approach is through small molecule perturbation. Chemical probes of epigenetic reader domains have been critical in improving our understanding of the biological consequences of modulating their targets, while also enabling the development of novel probe-based reagents. By appending a functional handle to a reader domain probe, a chemical toolbox of reagents can be created to facilitate chemiprecipitation of epigenetic complexes, evaluate probe selectivity, develop in vitro screening assays, visualize cellular target localization, enable target degradation and recruit epigenetic machinery to a site within the genome in a highly controlled fashion.

Discovery of a Potent and Selective Fragment-like Inhibitor of Methyllysine Reader Protein Spindlin 1 (SPIN1)

By screening an epigenetic compound library, we identified that UNC0638, a highly potent inhibitor of the histone methyltransferases G9a and GLP, was a weak inhibitor of SPIN1 (spindlin 1), a methyllysine reader protein. Our optimization of this weak hit resulted in the discovery of a potent, selective, and cell-active SPIN1 inhibitor, compound 3 (MS31). Compound 3 potently inhibited binding of trimethyllysine-containing peptides to SPIN1, displayed high binding affinity, was highly selective for SPIN1 over other epigenetic readers and writers, directly engaged SPIN1 in cells, and was not toxic to nontumorigenic cells. The crystal structure of the SPIN1-compound 3 complex indicated that it selectively binds tudor domain II of SPIN1. We also designed a structurally similar but inactive compound 4 (MS31N) as a negative control. Our results have demonstrated for the first time that potent, selective, and cell-active fragment-like inhibitors can be generated by targeting a single tudor domain.

Engineering Methyllysine Writers and Readers for Allele-Specific Regulation of Protein-Protein Interactions

Protein-protein interactions mediated by methyllysine are ubiquitous in biological systems. Specific perturbation of such interactions has remained a challenging endeavor. Herein, we describe an allele-specific strategy toward an engineered protein-protein interface orthogonal to the human proteome. We develop a methyltransferase (writer) variant that installs aryllysine moiety on histones that can only be recognized by an engineered chromodomain (reader). We establish biochemical integrity of the engineered interface, provide structural evidence for orthogonality and validate its applicability to identify transcriptional regulators. Our approach provides an unprecedented strategy for specific manipulation of the methyllysine interactome.

Targeting protein methylation: from chemical tools to precision medicines

The methylation of proteins is integral to the execution of many important biological functions, including cell signalling and transcriptional regulation. Protein methyltransferases (PMTs) are a large class of enzymes that carry out the addition of methyl marks to a broad range of substrates. PMTs are critical for normal cellular physiology and their dysregulation is frequently observed in human disease. As such, PMTs have emerged as promising therapeutic targets with several inhibitors now in clinical trials for oncology indications. The discovery of chemical inhibitors and antagonists of protein methylation signalling has also profoundly impacted our general understanding of PMT biology and pharmacology. In this review, we present general principles for drugging protein methyltransferases or their downstream effectors containing methyl-binding modules, as well as best-in-class examples of the compounds discovered and their impact both at the bench and in the clinic.

Investigation of Trimethyllysine Binding by the HP1 Chromodomain via Unnatural Amino Acid Mutagenesis

Trimethyllysine (Kme3) reader proteins are targets for inhibition due to their role in mediating gene expression. Although all such reader proteins bind Kme3 in an aromatic cage, the driving force for binding may differ; some readers exhibit evidence for cation-π interactions whereas others do not. We report a general unnatural amino acid mutagenesis approach to quantify the contribution of individual tyrosines to cation binding using the HP1 chromodomain as a model system. We demonstrate that two tyrosines (Y24 and Y48) bind to a Kme3-histone tail peptide via cation-π interactions, but linear free energy trends suggest they do not contribute equally to binding. X-ray structures and computational analysis suggest that the distance and degree of contact between Tyr residues and Kme3 plays an important role in tuning cation-π-mediated Kme3 recognition. Although cation-π interactions have been studied in a number of proteins, this work is the first to utilize direct binding assays, X-ray crystallography, and modeling, to pinpoint factors that influence the magnitude of the individual cation-π interactions.

Emerging Chemistry Strategies for Engineering Native Chromatin

Chromosomes present one of most challenging of all substrates for biochemical study. This is because genomic DNA is physically associated with an astonishing collection of nuclear factors, which serve to not only store the nucleic acid in a stable form, but also grant access to the information it encodes when needed. Understanding this complex molecular choreography is central to the field of epigenetics. One of the great challenges in this area is to move beyond correlative type information, which is now in abundant supply, to the point where we can truly connect the dots at the molecular level. Establishing such causal relationships requires precise manipulation of the covalent structure of chromatin. Tools for this purpose are currently in short supply, creating an opportunity that, as we will argue in this Perspective, is well suited to the sensibilities of the chemist.

Selective targeting of epigenetic reader domains

INTRODUCTION

Epigenetic regulators including writers, erasers, and readers of chromatin marks have been implicated in numerous diseases and are therefore subject of intense academic and pharmaceutical research. While several small-molecule inhibitors targeting writers or erasers are either approved drugs or are currently being evaluated in clinical trials, the targeting of epigenetic readers has lagged behind. Proof-of-principle that epigenetic readers are also relevant drug targets was provided by landmark discoveries of selective inhibitors targeting the BET family of acetyl-lysine readers. More recently, high affinity chemical probes for non-BET acetyl- and methyl-lysine reader domains have also been developed. Areas covered: This article covers recent advances with the identification and validation of inhibitors and chemical probes targeting epigenetic reader domains. Issues related to epigenetic reader druggability, quality requirements for chemical probes, interpretation of cellular action, unexpected cross-talk, and future challenges are also discussed. Expert opinion: Chemical probes provide a powerful means to unravel biological functions of epigenetic readers and evaluate their potential as drug targets. To yield meaningful results, potency, selectivity, and cellular target engagement of chemical probes need to be stringently validated. Future chemical probes will probably need to fulfil additional criteria such as strict target specificity or the targeting of readers within protein complexes.

Structure-Activity Relationships of Cbx7 Inhibitors, Including Selectivity Studies against Other Cbx Proteins

The five human polycomb (Pc) paralog proteins, chromobox homolog (Cbx) 2/4/6/7/8, are a family of chromodomain containing methyllysine reader proteins that are canonical readers of trimethyllysine 27 on histone 3 (H3K27me3). The aberrant expression of the Cbx7 gene is implicated in several cancers including prostate, gastric, thyroid, pancreas, and colon cancer. Previous reports on antagonizing the molecular recognition of Cbx7-H3K27me3 with chemical inhibitors showed an impact on prostate cancer cell lines. We report here on the design, synthesis, and structure-activity relationships of a series of potent peptidomimetic antagonists that were optimized on a trimethyllysine-containing scaffold to target Cbx7. The ligands were characterized using fluorescence polarization (FP) for their binding efficiency and selectivity against the Pc paralog Cbx proteins. The most selective ligand 9, as indicated by the FP data analysis, was further characterized using the isothermal titration calorimetry (ITC). Compound 9 exhibits a 220 nM potency for Cbx7 and exhibits 3.3, 1.8, 7.3 times selective for Cbx7 over Cbx2/4/8 and 28-fold selective over the HP1 family member Cbx1. Our research provides several potent and partially selective inhibitors for Cbx2/4/7 that do not contain trimethyllysine. Our models and binding data suggest that the aromatic cages of Cbx7/Cbx4 can accommodate larger alkyl groups such as diisobutyl substitution on the lysine nitrogen.