Kinetic characterization of human mRNA guanine-N7 methyltransferase.

The 5'-mRNA-cap formation is a conserved process in protection of mRNA in eukaryotic cells, resulting in mRNA stability and efficient translation. In humans, two methyltransferases, RNA cap guanine-N7 methyltransferase (hRNMT) and cap-specific nucleoside-2'-O-methyltransferase 1 (hCMTr1) methylate the mRNA resulting in cap0 (N7mGpppN-RNA) and cap1 (N7mGpppN2'-Om-RNA) formation, respectively. Coronaviruses mimic this process by capping their RNA to evade human immune systems. The coronaviral nonstructural proteins, nsp14 and nsp10-nsp16, catalyze the same reactions as hRNMT and hCMTr1, respectively. These two viral enzymes are important targets for development of inhibitor-based antiviral therapeutics. However, assessing the selectivity of such inhibitors against human corresponding proteins is crucial. Human RNMTs have been implicated in proliferation of cancer cells and are also potential targets for development of anticancer therapeutics. Here, we report the development and optimization of a radiometric assay for hRNMT, full kinetic characterization of its activity, and optimization of the assay for high-throughput screening with a Z-factor of 0.79. This enables selectivity determination for a large number of hits from various screening of coronaviral methyltransferases, and also screening hRNMT for discovery of inhibitors and chemical probes that potentially could be used to further investigate the roles RNMTs play in cancers.

Comparative Study of Adenosine Analogs as Inhibitors of Protein Arginine Methyltransferases and a Clostridioides difficile-Specific DNA Adenine Methyltransferase.

S-Adenosyl-l-methionine (SAM) analogs are adaptable tools for studying and therapeutically inhibiting SAM-dependent methyltransferases (MTases). Some MTases play significant roles in host-pathogen interactions, one of which is Clostridioides difficile-specific DNA adenine MTase (CamA). CamA is needed for efficient sporulation and alters persistence in the colon. To discover potent and selective CamA inhibitors, we explored modifications of the solvent-exposed edge of the SAM adenosine moiety. Starting from the two parental compounds (6e and 7), we designed an adenosine analog (11a) carrying a 3-phenylpropyl moiety at the adenine N6-amino group, and a 3-(cyclohexylmethyl guanidine)-ethyl moiety at the sulfur atom off the ribose ring. Compound 11a (IC50 = 0.15 µM) is 10× and 5× more potent against CamA than 6e and 7, respectively. The structure of the CamA-DNA-inhibitor complex revealed that 11a adopts a U-shaped conformation, with the two branches folded toward each other, and the aliphatic and aromatic rings at the two ends interacting with one another. 11a occupies the entire hydrophobic surface (apparently unique to CamA) next to the adenosine binding site. Our work presents a hybrid knowledge-based and fragment-based approach to generating CamA inhibitors that would be chemical agents to examine the mechanism(s) of action and therapeutic potentials of CamA in C. difficile infection.

Discovery of a Potent and Selective Targeted NSD2 Degrader for the Reduction of H3K36me2.

Nuclear receptor-binding SET domain-containing 2 (NSD2) plays important roles in gene regulation, largely through its ability to dimethylate lysine 36 of histone 3 (H3K36me2). Despite aberrant activity of NSD2 reported in numerous cancers, efforts to selectively inhibit the catalytic activity of this protein with small molecules have been unsuccessful to date. Here, we report the development of UNC8153, a novel NSD2-targeted degrader that potently and selectively reduces the cellular levels of both NSD2 protein and the H3K36me2 chromatin mark. UNC8153 contains a simple warhead that confers proteasome-dependent degradation of NSD2 through a novel mechanism. Importantly, UNC8153-mediated reduction of H3K36me2 through the degradation of NSD2 results in the downregulation of pathological phenotypes in multiple myeloma cells including mild antiproliferative effects in MM1.S cells containing an activating point mutation and antiadhesive effects in KMS11 cells harboring the t(4;14) translocation that upregulates NSD2 expression.

SS148 and WZ16 inhibit the activities of nsp10-nsp16 complexes from all seven human pathogenic coronaviruses.

Seven coronaviruses have infected humans (HCoVs) to-date. SARS-CoV-2 caused the current COVID-19 pandemic with the well-known high mortality and severe socioeconomic consequences. MERS-CoV and SARS-CoV caused epidemic of MERS and SARS, respectively, with severe respiratory symptoms and significant fatality. However, HCoV-229E, HCoV-NL63, HCoV-HKU1, and HCoV-OC43 cause respiratory illnesses with less severe symptoms in most cases. All coronaviruses use RNA capping to evade the immune systems of humans. Two viral methyltransferases, nsp14 and nsp16, play key roles in RNA capping and are considered valuable targets for development of anti-coronavirus therapeutics. But little is known about the kinetics of nsp10-nsp16 methyltransferase activities of most HCoVs, and reliable assays for screening are not available. Here, we report the expression, purification, and kinetic characterization of nsp10-nsp16 complexes from six HCoVs in parallel with previously characterized SARS-CoV-2. Probing the active sites of all seven by SS148 and WZ16, the two recently reported dual nsp14 / nsp10-nsp16 inhibitors, revealed pan-inhibition. Overall, our study show feasibility of developing broad-spectrum dual nsp14 / nsp10-nsp16-inhibitor therapeutics.

Systematic Design of Adenosine Analogs as Inhibitors of a Clostridioides difficile-Specific DNA Adenine Methyltransferase Required for Normal Sporulation and Persistence.

Antivirulence agents targeting endospore-transmitted Clostridioides difficile infections are urgently needed. C. difficile-specific DNA adenine methyltransferase (CamA) is required for efficient sporulation and affects persistence in the colon. The active site of CamA is conserved and closely resembles those of hundreds of related S-adenosyl-l-methionine (SAM)-dependent methyltransferases, which makes the design of selective inhibitors more challenging. We explored the solvent-exposed edge of the SAM adenosine moiety and systematically designed 42 analogs of adenosine carrying substituents at the C6-amino group (N6) of adenosine. We compare the inhibitory properties and binding affinity of these diverse compounds and present the crystal structures of CamA in complex with 14 of them in the presence of substrate DNA. The most potent of these inhibitors, compound 39 (IC50 ∼ 0.4 µM and KD ∼ 0.2 µM), is selective for CamA against closely related bacterial and mammalian DNA and RNA adenine methyltransferases, protein lysine and arginine methyltransferases, and human adenosine receptors.

Crystal structure of SARS-CoV-2 nsp10-nsp16 in complex with small molecule inhibitors, SS148 and WZ16.

SARS-CoV-2 nsp10-nsp16 complex is a 2'-O-methyltransferase (MTase) involved in viral RNA capping, enabling the virus to evade the immune system in humans. It has been considered a valuable target in the discovery of antiviral therapeutics, as the RNA cap formation is crucial for viral propagation. Through cross-screening of the inhibitors that we previously reported for SARS-CoV-2 nsp14 MTase activity against nsp10-nsp16 complex, we identified two compounds (SS148 and WZ16) that also inhibited nsp16 MTase activity. To further enable the chemical optimization of these two compounds towards more potent and selective dual nsp14/nsp16 MTase inhibitors, we determined the crystal structure of nsp10-nsp16 in complex with each of SS148 and WZ16. As expected, the structures revealed the binding of both compounds to S-adenosyl-L-methionine (SAM) binding pocket of nsp16. However, our structural data along with the biochemical mechanism of action determination revealed an RNA-dependent SAM-competitive pattern of inhibition for WZ16, clearly suggesting that binding of the RNA first may help the binding of some SAM competitive inhibitors. Both compounds also showed some degree of selectivity against human protein MTases, an indication of great potential for chemical optimization towards more potent and selective inhibitors of coronavirus MTases.

Kinetic Characterization of SARS-CoV-2 nsp13 ATPase Activity and Discovery of Small-Molecule Inhibitors.

SARS-CoV-2 non-structural protein 13 (nsp13) is a highly conserved helicase and RNA 5'-triphosphatase. It uses the energy derived from the hydrolysis of nucleoside triphosphates for directional movement along the nucleic acids and promotes the unwinding of double-stranded nucleic acids. Nsp13 is essential for replication and propagation of all human and non-human coronaviruses. Combined with its defined nucleotide binding site and druggability, nsp13 is one of the most promising candidates for the development of pan-coronavirus therapeutics. Here, we report the development and optimization of bioluminescence assays for kinetic characterization of nsp13 ATPase activity in the presence and absence of single-stranded DNA. Screening of a library of 5000 small molecules in the presence of single-stranded DNA resulted in the discovery of six nsp13 small-molecule inhibitors with IC50 values ranging from 6 ± 0.5 to 50 ± 6 µM. In addition to providing validated methods for high-throughput screening of nsp13 in drug discovery campaigns, the reproducible screening hits we present here could potentially be chemistry starting points toward the development of more potent and selective nsp13 inhibitors, enabling the discovery of antiviral therapeutics.

Probing the SAM Binding Site of SARS-CoV-2 Nsp14 In Vitro Using SAM Competitive Inhibitors Guides Developing Selective Bisubstrate Inhibitors.

The COVID-19 pandemic has clearly brought the healthcare systems worldwide to a breaking point, along with devastating socioeconomic consequences. The SARS-CoV-2 virus, which causes the disease, uses RNA capping to evade the human immune system. Nonstructural protein (nsp) 14 is one of the 16 nsps in SARS-CoV-2 and catalyzes the methylation of the viral RNA at N7-guanosine in the cap formation process. To discover small-molecule inhibitors of nsp14 methyltransferase (MTase) activity, we developed and employed a radiometric MTase assay to screen a library of 161 in-house synthesized S-adenosylmethionine (SAM) competitive MTase inhibitors and SAM analogs. Among six identified screening hits, SS148 inhibited nsp14 MTase activity with an IC50 value of 70 ± 6 nM and was selective against 20 human protein lysine MTases, indicating significant differences in SAM binding sites. Interestingly, DS0464 with an IC50 value of 1.1 ± 0.2 µM showed a bisubstrate competitive inhibitor mechanism of action. DS0464 was also selective against 28 out of 33 RNA, DNA, and protein MTases. The structure-activity relationship provided by these compounds should guide the optimization of selective bisubstrate nsp14 inhibitors and may provide a path toward a novel class of antivirals against COVID-19, and possibly other coronaviruses.

Human MettL3-MettL14 RNA adenine methyltransferase complex is active on double-stranded DNA containing lesions.

MettL3-MettL14 methyltransferase complex has been studied widely for its role in RNA adenine methylation. This complex is also recruited to UV- and X-ray exposed DNA damaged sites, and its methyltransfer activity is required for subsequent DNA repair, though in theory this could result from RNA methylation of short transcripts made at the site of damage. We report here that MettL3-MettL14 is active in vitro on double-stranded DNA containing a cyclopyrimidine dimer - a major lesion of UV radiation-induced products - or an abasic site or mismatches. Furthermore, N6-methyladenine (N6mA) decreases misincorporation of 8-oxo-guanine (8-oxoG) opposite to N6mA by repair DNA polymerases. When 8-oxoG is nevertheless incorporated opposite N6mA, the methylation inhibits N6mA excision from the template (correct) strand by the adenine DNA glycosylase (MYH), implying that the methylation decreases inappropriate misrepair. Finally, we observed that the N6mA reader domain of YTHDC1, which is also recruited to sites of DNA damage, binds N6mA that is located across from a single-base gap between two canonical DNA helices. This YTHDC1 complex with a gapped duplex is structurally similar to DNA complexes with FEN1 and GEN1 - two members of the nuclease family that act in nucleotide excision repair, mismatch repair and homologous recombination, and which incise distinct non-B DNA structures. Together, the parts of our study provide a plausible mechanism for N6mA writer and reader proteins acting directly on lesion-containing DNA, and suggest in vivo experiments to test the mechanisms involving methylation of adenine.

A High-Throughput Radioactivity-Based Assay for Screening SARS-CoV-2 nsp10-nsp16 Complex.

Frequent outbreaks of novel coronaviruses (CoVs), highlighted by the current severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic, necessitate the development of therapeutics that could be easily and effectively administered worldwide. The conserved mRNA-capping process enables CoVs to evade their host immune system and is a target for antiviral development. Nonstructural protein (nsp) 16 in complex with nsp10 catalyzes the final step of coronaviral mRNA capping through its 2'-O-methylation activity. Like other methyltransferases, the SARS-CoV-2 nsp10-nsp16 complex is druggable. However, the availability of an optimized assay for high-throughput screening (HTS) is an unmet need. Here, we report the development of a radioactivity-based assay for the methyltransferase activity of the nsp10-nsp16 complex in a 384-well format, kinetic characterization, and optimization of the assay for HTS (Z' factor = 0.83). Considering the high conservation of nsp16 across known CoV species, the potential inhibitors targeting the SARS-CoV-2 nsp10-nsp16 complex may also be effective against other emerging pathogenic CoVs.

Probing the SAM Binding Site of SARS-CoV-2 nsp14 in vitro Using SAM Competitive Inhibitors Guides Developing Selective bi-substrate Inhibitors.

The COVID-19 pandemic has clearly brought the healthcare systems world-wide to a breaking point along with devastating socioeconomic consequences. The SARS-CoV-2 virus which causes the disease uses RNA capping to evade the human immune system. Non-structural protein (nsp) 14 is one of the 16 nsps in SARS-CoV-2 and catalyzes the methylation of the viral RNA at N7-guanosine in the cap formation process. To discover small molecule inhibitors of nsp14 methyltransferase (MT) activity, we developed and employed a radiometric MT assay to screen a library of 161 in house synthesized S-adenosylmethionine (SAM) competitive methyltransferase inhibitors and SAM analogs. Among seven identified screening hits, SS148 inhibited nsp14 MT activity with an IC 50 value of 70 ± 6 nM and was selective against 20 human protein lysine methyltransferases indicating significant differences in SAM binding sites. Interestingly, DS0464 with IC 50 value of 1.1 ± 0.2 µM showed a bi-substrate competitive inhibitor mechanism of action. Modeling the binding of this compound to nsp14 suggests that the terminal phenyl group extends into the RNA binding site. DS0464 was also selective against 28 out of 33 RNA, DNA, and protein methyltransferases. The structure-activity relationship provided by these compounds should guide the optimization of selective bi-substrate nsp14 inhibitors and may provide a path towards a novel class of antivirals against COVID-19, and possibly other coronaviruses.

A High-Throughput RNA Displacement Assay for Screening SARS-CoV-2 nsp10-nsp16 Complex toward Developing Therapeutics for COVID-19.

SARS-CoV-2, the coronavirus that causes COVID-19, evades the human immune system by capping its RNA. This process protects the viral RNA and is essential for its replication. Multiple viral proteins are involved in this RNA capping process, including the nonstructural protein 16 (nsp16), which is an S-adenosyl-l-methionine (SAM)-dependent 2'-O-methyltransferase. Nsp16 is significantly active when in complex with another nonstructural protein, nsp10, which plays a key role in its stability and activity. Here we report the development of a fluorescence polarization (FP)-based RNA displacement assay for nsp10-nsp16 complex in a 384-well format with a Z' factor of 0.6, suitable for high-throughput screening. In this process, we purified the nsp10-nsp16 complex to higher than 95% purity and confirmed its binding to the methyl donor SAM, the product of the reaction, S-adenosyl-l-homocysteine (SAH), and a common methyltransferase inhibitor, sinefungin, using isothermal titration calorimetry (ITC). The assay was further validated by screening a library of 1124 drug-like compounds. This assay provides a cost-effective high-throughput method for screening the nsp10-nsp16 complex for RNA competitive inhibitors toward developing COVID-19 therapeutics.

A chemical probe of CARM1 alters epigenetic plasticity against breast cancer cell invasion.

CARM1 is a cancer-relevant protein arginine methyltransferase that regulates many aspects of transcription. Its pharmacological inhibition is a promising anti-cancer strategy. Here SKI-73 (6a in this work) is presented as a CARM1 chemical probe with pro-drug properties. SKI-73 (6a) can rapidly penetrate cell membranes and then be processed into active inhibitors, which are retained intracellularly with 10-fold enrichment for several days. These compounds were characterized for their potency, selectivity, modes of action, and on-target engagement. SKI-73 (6a) recapitulates the effect of CARM1 knockout against breast cancer cell invasion. Single-cell RNA-seq analysis revealed that the SKI-73(6a)-associated reduction of invasiveness acts by altering epigenetic plasticity and suppressing the invasion-prone subpopulation. Interestingly, SKI-73 (6a) and CARM1 knockout alter the epigenetic plasticity with remarkable difference, suggesting distinct modes of action for small-molecule and genetic perturbations. We therefore discovered a CARM1-addiction mechanism of cancer metastasis and developed a chemical probe to target this process.

Drugs that are small molecules have the potential to block the individual proteins that drive the spread of cancer, but their design is a challenge. This is because they need to get inside the cell and find their target without binding to other proteins on the way. However, small molecule drugs often have an electric charge, which makes it hard for them to cross the cell membrane. Additionally, most proteins are not completely unique, making it harder for the drugs to find the correct target. CARM1 is a protein that plays a role in the spread of breast cancer cells, and scientists are currently looking for a small molecule that will inhibit its action. The group of enzymes that CARM1 belongs to act by taking a small chemical group, called a methyl group, from a molecule called SAM, and transferring it to proteins that switch genes on and off. In the case of CARM1, this changes cell behavior by turning on genes involved in cell movement. Genetically modifying cells so they will not produce any CARM1 stops the spread of breast cancer cells, but developing a drug with the same effects has proved difficult. Existing drugs that can inhibit CARM1 in a test tube struggle to get inside cells and to distinguish between CARM1 and its related enzymes. Now, Cai et al. have modified and tested a CARM1 inhibitor to address these problems, and find out how these small molecules work. At its core, the inhibitor has a structure very similar to a SAM molecule, so it can fit into the SAM binding pocket of CARM1 and its related enzymes. To stop the inhibitor from binding to other proteins, Cai et al. made small changes to its structure until it only interacted with CARM1.Then, to get the inhibitor inside breast cancer cells, Cai et al. cloaked its charged area with a chemical shield, allowing it to cross the cell membrane. Inside the cell, the chemical shield broke away, allowing the inhibitor to attach to CARM1. Analysis of cells showed that this inhibition only affected the cancer cells most likely to spread. Blocking CARM1 switched off genes involved in cell movement and stopped cancer cells from travelling through 3D gels. This work is a step towards making a drug that can block CARM1 in cancer cells, but there is still further work to be done. The next stages will be to test whether the new inhibitor works in other types of cancer cells, in living animals, and in human patient samples.

Discovery of Small-Molecule Antagonists of the H3K9me3 Binding to UHRF1 Tandem Tudor Domain.

Ubiquitin-like with PHD and RING finger domains 1 (UHRF1) is a multidomain protein that plays a critical role in maintaining DNA methylation patterns through concurrent recognition of hemimethylated DNA and histone marks by various domains, and recruitment of DNA methyltransferase 1 (DNMT1). UHRF1 is overexpressed in various cancers, including breast cancer. The tandem tudor domain (TTD) of UHRF1 specifically and tightly binds to histone H3 di- or trimethylated at lysine 9 (H3K9me2 or H3K9me3, respectively), and this binding is essential for UHRF1 function. We developed an H3K9me3 peptide displacement assay, which was used to screen a library of 44,000 compounds for small molecules that disrupt the UHRF1-H3K9me3 interaction. This screen resulted in the identification of NV01, which bound to UHRF1-TTD with a Kd value of 5 µM. The structure of UHRF1-TTD in complex with NV01 confirmed binding to the H3K9me3-binding pocket. Limited structure-based optimization of NV01 led to the discovery of NV03 (Kd of 2.4 µM). These well-characterized small-molecule antagonists of the UHRF1-H3K9me2/3 interaction could be valuable starting chemical matter for developing more potent and cell-active probes toward further characterizing UHRF1 function, with possible applications as anticancer therapeutics.

Targeting human SET1/MLL family of proteins.

The SET1 family of proteins, and in particular MLL1, are essential regulators of transcription and key mediators of normal development and disease. Here, we summarize the detailed characterization of the methyltransferase activity of SET1 complexes and the role of the key subunits, WDR5, RbBP5, ASH2L, and DPY30. We present new data on full kinetic characterization of human MLL1, MLL3, SET1A, and SET1B trimeric, tetrameric, and pentameric complexes to elaborate on substrate specificities and compare our findings with what has been reported before. We also review exciting recent work identifying potent inhibitors of oncogenic MLL1 function through disruption of protein-protein interactions within the MLL1 complex.

Structure-Based Optimization of a Small Molecule Antagonist of the Interaction Between WD Repeat-Containing Protein 5 (WDR5) and Mixed-Lineage Leukemia 1 (MLL1).

WD repeat-containing protein 5 (WDR5) is an important component of the multiprotein complex essential for activating mixed-lineage leukemia 1 (MLL1). Rearrangement of the MLL1 gene is associated with onset and progression of acute myeloid and lymphoblastic leukemias, and targeting the WDR5-MLL1 interaction may result in new cancer therapeutics. Our previous work showed that binding of small molecule ligands to WDR5 can modulate its interaction with MLL1, suppressing MLL1 methyltransferase activity. Initial structure-activity relationship studies identified N-(2-(4-methylpiperazin-1-yl)-5-substituted-phenyl) benzamides as potent and selective antagonists of this protein-protein interaction. Guided by crystal structure data and supported by in silico library design, we optimized the scaffold by varying the C-1 benzamide and C-5 substituents. This allowed us to develop the first highly potent (Kdisp < 100 nM) small molecule antagonists of the WDR5-MLL1 interaction and demonstrate that N-(4-(4-methylpiperazin-1-yl)-3'-(morpholinomethyl)-[1,1'-biphenyl]-3-yl)-6-oxo-4-(trifluoromethyl)-1,6-dihydropyridine-3-carboxamide 16d (OICR-9429) is a potent and selective chemical probe suitable to help dissect the biological role of WDR5.

A Radioactivity-Based Assay for Screening Human m6A-RNA Methyltransferase, METTL3-METTL14 Complex, and Demethylase ALKBH5.

N(6)-methyladenosine (m(6)A) is the most common reversible internal modification in mammalian RNA. Changes in m(6)A levels have been implicated in a variety of cellular processes, including nuclear RNA export, control of protein translation, and protein splicing, and they have been linked to obesity, cancer, and other human diseases. METTL3 and METTL14 are N(6)-adenosine methyltransferases that work more efficiently in a stable METTL3-METTL14 heterodimer complex (METTL3-14). ALKBH5 is an m(6)A-RNA demethylase that belongs to the AlkB family of dioxygenases. We report the development of radioactivity-based assays for kinetic characterization of m(6)A-RNA modifications by METTL3-14 complex and ALKBH5 and provide optimal assay conditions suitable for screening for ligands in a 384-well format with Z' factors of 0.78 and 0.77, respectively.

Design of a fluorescent ligand targeting the S-adenosylmethionine binding site of the histone methyltransferase MLL1.

The histone methyltransferase MLL1 has been linked to translocation-associated gene fusion in childhood leukemias and is an attractive drug target. High-throughput biochemical analysis of MLL1 methyltransferase activity requires the production of at least a trimeric complex of MLL1, RbBP5 and WDR5 to elicit robust activity. Production of trimeric and higher order MLL1 complexes in the quantities and reproducibility required for high-throughput screening presents a significant impediment to MLL1 drug discovery efforts. We present here a small molecule fluorescent ligand (FL-NAH, 6) that is able to bind to the S-adenosylmethionine (SAM) binding site of MLL1 in a manner independent of the associated complex members. We have used FL-NAH to develop a fluorescence polarization-based SAM displacement assay in a 384-well format targeting the MLL1 SET domain in the absence of associated complex members. FL-NAH competes with SAM and is displaced from the MLL1 SET domain by other SAM-binding site ligands with Kdisp values similar to the higher-order complexes, but is unaffected by the H3 peptide substrate. This assay enables screening for SAM-competitive MLL1 inhibitors without requiring the use of trimeric or higher order MLL1 complexes, significantly reducing screening time and cost.