Formamide significantly enhances the efficiency of chemical adenylation of RNA sequencing ligation adaptors.

Preadenylated single-stranded DNA ligation adaptors are essential reagents in many next generation RNA sequencing library preparation protocols. These oligonucleotides can be adenylated enzymatically or chemically. Enzymatic adenylation reactions have high yield but are not amendable to scale up. In chemical adenylation, adenosine 5'-phosphorimidazolide (ImpA) reacts with 5' phosphorylated DNA. It is easily scalable but gives poor yields, requiring labor-intensive cleanup steps. Here, we describe an improved chemical adenylation method using 95% formamide as the solvent, which results in the adenylation of oligonucleotides with >90% yield. In standard conditions, with water as the solvent, hydrolysis of the starting material to adenosine monophosphate limits the yields. To our surprise, we find that rather than increasing adenylation yields by decreasing the rate of ImpA hydrolysis, formamide does so by increasing the reaction rate between ImpA and 5'-phosphorylated DNA by ∼10-fold. The method described here enables straightforward preparation of chemically adenylated adapters with higher than 90% yield, simplifying reagent preparation for NGS.

Double Click: Unexpected 1:2 Stoichiometry in a Norbornene-Tetrazine Reaction.

We report a new reactivity for the inverse electron demand Diels-Alder (iEDDA) reaction between norbornene and tetrazine. Instead of simple 1:1 condensation between norbornene- and tetrazine-conjugated biomolecules, we observed that dimeric products were preferentially formed. As such, an olefinic intermediate formed after the addition of the first tetrazine unit to norbornene rapidly undergoes a consecutive cycloaddition reaction with a second tetrazine unit to result in a conjugate with a 1:2 stoichiometric ratio. This unexpected dimer formation was consistently observed in the reactions of both small-molecule norbornenes and tetrazines, as well as oligonucleotide conjugates. When norbornene was replaced with bicyclononyne to bypass the formation of this olefinic reaction intermediate, the reactions resulted exclusively in rapid formation of the expected 1:1 stoichiometric conjugates.

Structure of the catalytically active APOBEC3G bound to a DNA oligonucleotide inhibitor reveals tetrahedral geometry of the transition state.

APOBEC3 proteins (A3s) are enzymes that catalyze the deamination of cytidine to uridine in single-stranded DNA (ssDNA) substrates, thus playing a key role in innate antiviral immunity. However, the APOBEC3 family has also been linked to many mutational signatures in cancer cells, which has led to an intense interest to develop inhibitors of A3's catalytic activity as therapeutics as well as tools to study A3's biochemistry, structure, and cellular function. Recent studies have shown that ssDNA containing 2'-deoxy-zebularine (dZ-ssDNA) is an inhibitor of A3s such as A3A, A3B, and A3G, although the atomic determinants of this activity have remained unknown. To fill this knowledge gap, we determined a 1.5 Å resolution structure of a dZ-ssDNA inhibitor bound to active A3G. The crystal structure revealed that the activated dZ-H2O mimics the transition state by coordinating the active site Zn2+ and engaging in additional stabilizing interactions, such as the one with the catalytic residue E259. Therefore, this structure allowed us to capture a snapshot of the A3's transition state and suggests that developing transition-state mimicking inhibitors may provide a new opportunity to design more targeted molecules for A3s in the future.

Deciphering the Molecular Mechanism of HCV Protease Inhibitor Fluorination as a General Approach to Avoid Drug Resistance.

Third generation Hepatitis C virus (HCV) NS3/4A protease inhibitors (PIs), glecaprevir and voxilaprevir, are highly effective across genotypes and against many resistant variants. Unlike earlier PIs, these compounds have fluorine substitutions on the P2-P4 macrocycle and P1 moieties. Fluorination has long been used in medicinal chemistry as a strategy to improve physicochemical properties and potency. However, the molecular basis by which fluorination improves potency and resistance profile of HCV NS3/4A PIs is not well understood. To systematically analyze the contribution of fluorine substitutions to inhibitor potency and resistance profile, we used a multi-disciplinary approach involving inhibitor design and synthesis, enzyme inhibition assays, co-crystallography, and structural analysis. A panel of inhibitors in matched pairs were designed with and without P4 cap fluorination, tested against WT protease and the D168A resistant variant, and a total of 22 high-resolution co-crystal structures were determined. While fluorination did not significantly improve potency against the WT protease, PIs with fluorinated P4 caps retained much better potency against the D168A protease variant. Detailed analysis of the co-crystal structures revealed that PIs with fluorinated P4 caps can sample alternate binding conformations that enable adapting to structural changes induced by the D168A substitution. Our results elucidate molecular mechanisms of fluorine-specific inhibitor interactions that can be leveraged in avoiding drug resistance.

Discovery of Quinoxaline-Based P1-P3 Macrocyclic NS3/4A Protease Inhibitors with Potent Activity against Drug-Resistant Hepatitis C Virus Variants.

The three pan-genotypic HCV NS3/4A protease inhibitors (PIs) currently in clinical use-grazoprevir, glecaprevir, and voxilaprevir-are quinoxaline-based P2-P4 macrocycles and thus exhibit similar resistance profiles. Using our quinoxaline-based P1-P3 macrocyclic lead compounds as an alternative chemical scaffold, we explored structure-activity relationships (SARs) at the P2 and P4 positions to develop pan-genotypic PIs that avoid drug resistance. A structure-guided strategy was used to design and synthesize two series of compounds with different P2 quinoxalines in combination with diverse P4 groups of varying sizes and shapes, with and without fluorine substitutions. Our SAR data and cocrystal structures revealed the interplay between the P2 and P4 groups, which influenced inhibitor binding and the overall resistance profile. Optimizing inhibitor interactions in the S4 pocket led to PIs with excellent antiviral activity against clinically relevant PI-resistant HCV variants and genotype 3, providing potential pan-genotypic inhibitors with improved resistance profiles.

Detecting chromatin interactions between and along sister chromatids with SisterC.

Chromosome segregation requires both compaction and disentanglement of sister chromatids. We describe SisterC, a chromosome conformation capture assay that distinguishes interactions between and along identical sister chromatids. SisterC employs 5-bromo-2'-deoxyuridine (BrdU) incorporation during S-phase to label newly replicated strands, followed by Hi-C and then the destruction of 5-bromodeoxyuridine-containing strands via Hoechst/ultraviolet treatment. After sequencing of the remaining intact strands, this allows assignment of Hi-C products as inter- and intra-sister interactions based on the strands that reads are mapped to. We performed SisterC on mitotic Saccharomyces cerevisiae cells. We find precise alignment of sister chromatids at centromeres. Along arms, sister chromatids are less precisely aligned, with inter-sister connections every ~35 kilobase (kb). Inter-sister interactions occur between cohesin binding sites that are often offset by 5 to 25 kb. Along sister chromatids, cohesin results in the formation of loops of up to 50 kb. SisterC allows study of the complex interplay between sister chromatid compaction and their segregation during mitosis.

Avoiding Drug Resistance by Substrate Envelope-Guided Design: Toward Potent and Robust HCV NS3/4A Protease Inhibitors.

Hepatitis C virus (HCV) infects millions of people worldwide, causing chronic liver disease that can lead to cirrhosis, hepatocellular carcinoma, and liver transplant. In the last several years, the advent of direct-acting antivirals, including NS3/4A protease inhibitors (PIs), has remarkably improved treatment outcomes of HCV-infected patients. However, selection of resistance-associated substitutions and polymorphisms among genotypes can lead to drug resistance and in some cases treatment failure. A proactive strategy to combat resistance is to constrain PIs within evolutionarily conserved regions in the protease active site. Designing PIs using the substrate envelope is a rational strategy to decrease the susceptibility to resistance by using the constraints of substrate recognition. We successfully designed two series of HCV NS3/4A PIs to leverage unexploited areas in the substrate envelope to improve potency, specifically against resistance-associated substitutions at D168. Our design strategy achieved better resistance profiles over both the FDA-approved NS3/4A PI grazoprevir and the parent compound against the clinically relevant D168A substitution. Crystallographic structural analysis and inhibition assays confirmed that optimally filling the substrate envelope is critical to improve inhibitor potency while avoiding resistance. Specifically, inhibitors that enhanced hydrophobic packing in the S4 pocket and avoided an energetically frustrated pocket performed the best. Thus, the HCV substrate envelope proved to be a powerful tool to design robust PIs, offering a strategy that can be translated to other targets for rational design of inhibitors with improved potency and resistance profiles.IMPORTANCE Despite significant progress, hepatitis C virus (HCV) continues to be a major health problem with millions of people infected worldwide and thousands dying annually due to resulting complications. Recent antiviral combinations can achieve >95% cure, but late diagnosis, low access to treatment, and treatment failure due to drug resistance continue to be roadblocks against eradication of the virus. We report the rational design of two series of HCV NS3/4A protease inhibitors with improved resistance profiles by exploiting evolutionarily constrained regions of the active site using the substrate envelope model. Optimally filling the S4 pocket is critical to avoid resistance and improve potency. Our results provide drug design strategies to avoid resistance that are applicable to other quickly evolving viral drug targets.