Crystal structure, stability and in vitro RNAi activity of oligoribonucleotides containing the ribo-difluorotoluyl nucleotide: insights into substrate requirements by the human RISC Ago2 enzyme

RNA oligomers at atomic resolution containing 1-methylpseudouridine, an essential building block of mRNA vaccines

All widely used mRNA vaccines against COVID-19 contain in their sequence 1-methylpseudouridine (m1Ψ) instead of uridine. In this publication, we report two high resolution crystal structures (at up to 1.01 and 1.32 Å, respectively) of one such double-stranded 12-mer RNA sequence crystallized in two crystal forms. The structures are compared with similar structures which do not contain this modification. Additionally, the X-ray structure of 1-methyl-pseudouridine itself was determined.

Conformational Morphing by a DNA Analogue Featuring 7-Deazapurines and 5-Halogenpyrimidines and the Origins of Adenine-Tract Geometry

Several efforts are currently directed at the creation and cellular implementation of alternative genetic systems composed of pairing components that are orthogonal to the natural dA/dT and dG/dC base pairs. In an alternative approach, Watson-Crick-type pairing is conserved, but one or all of the four letters of the A, C, G, and T alphabet are substituted by modified components. Thus, all four nucleobases were altered to create halogenated deazanucleic acid (DZA): dA was replaced by 7-deaza-2'-deoxyadenosine (dzA), dG by 7-deaza-2'-deoxyguanosine (dzG), dC by 5-fluoro-2'-deoxycytidine (FdC), and dT by 5-chloro-2'-deoxyuridine (CldU). This base-pairing system was previously shown to retain function in Escherichia coli. Here, we analyze the stability, hydration, structure, and dynamics of a DZA Dickerson-Drew Dodecamer (DDD) of sequence 5'-FdC-dzG-FdC-dzG-dzA-dzA-CldU-CldU-FdC-dzG-FdC-dzG-3'. Contrary to similar stabilities of DDD and DZA-DDD, osmotic stressing revealed a dramatic loss of hydration for the DZA-DDD relative to that for the DDD. The parent DDD 5'-d(CGCGAATTCGCG)-3' features an A-tract, a run of adenosines uninterrupted by a TpA step, and exhibits a hallmark narrow minor groove. Crystal structures─in the presence of RNase H─and MD simulations show increased conformational plasticity ("morphing") of DZA-DDD relative to that of the DDD. The narrow dzA-tract minor groove in one structure widens to resemble that in canonical B-DNA in a second structure. These changes reflect an indirect consequence of altered DZA major groove electrostatics (less negatively polarized compared to that in DNA) and hydration (reduced compared to that in DNA). Therefore, chemical modifications outside the minor groove that lead to collapse of major groove electrostatics and hydration can modulate A-tract geometry.

Anionic G•U pairs in bacterial ribosomal rRNAs

Wobble GU pairs (or G•U) occur frequently within double-stranded RNA helices interspersed between standard G=C and A-U Watson-Crick pairs. Another type of G•U pair interacting via their Watson-Crick edges has been observed in the A site of ribosome structures between a modified U34 in the tRNA anticodon triplet and G + 3 in the mRNA. In such pairs, the electronic structure of the U is changed with a negative charge on N3(U), resulting in two H-bonds between N1(G)…O4(U) and N2(G)…N3(U). Here, we report that such pairs occur in other highly conserved positions in ribosomal RNAs of bacteria in the absence of U modification. An anionic cis Watson-Crick G•G pair is also observed and well conserved in the small subunit. These pairs are observed in tightly folded regions.

Towards a comprehensive understanding of RNA deamination: synthesis and properties of xanthosine-modified RNA

Nucleobase deamination, such as A-to-I editing, represents an important posttranscriptional modification of RNA. When deamination affects guanosines, a xanthosine (X) containing RNA is generated. However, the biological significance and chemical consequences on RNA are poorly understood. We present a comprehensive study on the preparation and biophysical properties of X-modified RNA. Thermodynamic analyses revealed that base pairing strength is reduced to a level similar to that observed for a G•U replacement. Applying NMR spectroscopy and X-ray crystallography, we demonstrate that X can form distinct wobble geometries with uridine depending on the sequence context. In contrast, X pairing with cytidine occurs either through wobble geometry involving protonated C or in Watson–Crick-like arrangement. This indicates that the different pairing modes are of comparable stability separated by low energetic barriers for switching. Furthermore, we demonstrate that the flexible pairing properties directly affect the recognition of X-modified RNA by reverse transcription enzymes. Primer extension assays and PCR-based sequencing analysis reveal that X is preferentially read as G or A and that the ratio depends on the type of reverse transcriptase. Taken together, our results elucidate important properties of X-modified RNA paving the way for future studies on its biological significance.

Fluorobenzene Nucleobase Analogues for Triplex-Forming Peptide Nucleic Acids

2,4-Difluorotoluene is a nonpolar isostere of thymidine that has been used as a powerful mechanistic probe to study the role of hydrogen bonding in nucleic acid recognition and interactions with polymerases. In the present study, we evaluated five fluorinated benzenes as nucleobase analogues in peptide nucleic acids designed for triple helical recognition of double helical RNA. We found that analogues having para and ortho fluorine substitution patterns (as in 2,4-difluorotoluene) selectively stabilized Hoogsteen triplets with U-A base pairs. The results were consistent with attractive electrostatic interactions akin to non-canonical F to H-N and C-H to N hydrogen bonding. The fluorinated nucleobases were not able to stabilize Hoogsteen-like triplets with pyrimidines in either G-C or A-U base pairs. Our results illustrate the ability of fluorine to engage in non-canonical base pairing and provide insights into triple helical recognition of RNA.

Overcoming GNA/RNA base-pairing limitations using isonucleotides improves the pharmacodynamic activity of ESC+ GalNAc-siRNAs

We recently reported that RNAi-mediated off-target effects are important drivers of the hepatotoxicity observed for a subset of GalNAc–siRNA conjugates in rodents, and that these findings could be mitigated by seed-pairing destabilization using a single GNA nucleotide placed within the seed region of the guide strand. Here, we report further investigation of the unique and poorly understood GNA/RNA cross-pairing behavior to better inform GNA-containing siRNA design. A reexamination of published GNA homoduplex crystal structures, along with a novel structure containing a single (S)-GNA-A residue in duplex RNA, indicated that GNA nucleotides universally adopt a rotated nucleobase orientation within all duplex contexts. Such an orientation strongly affects GNA-C and GNA-G but not GNA-A or GNA-T pairing in GNA/RNA heteroduplexes. Transposition of the hydrogen-bond donor/acceptor pairs using the novel (S)-GNA-isocytidine and -isoguanosine nucleotides could rescue productive base-pairing with the complementary G or C ribonucleotides, respectively. GalNAc-siRNAs containing these GNA isonucleotides showed an improved in vitro activity, a similar improvement in off-target profile, and maintained in vivo activity and guide strand liver levels more consistent with the parent siRNAs than those modified with isomeric GNA-C or -G, thereby expanding our toolbox for the design of siRNAs with minimized off-target activity.

Beyond ribose and phosphate: Selected nucleic acid modifications for structure-function investigations and therapeutic applications

Over the past 25 years, the acceleration of achievements in the development of oligonucleotide-based therapeutics has resulted in numerous new drugs making it to the market for the treatment of various diseases. Oligonucleotides with alterations to their scaffold, prepared with modified nucleosides and solid-phase synthesis, have yielded molecules with interesting biophysical properties that bind to their targets and are tolerated by the cellular machinery to elicit a therapeutic outcome. Structural techniques, such as crystallography, have provided insights to rationalize numerous properties including binding affinity, nuclease stability, and trends observed in the gene silencing. In this review, we discuss the chemistry, biophysical, and structural properties of a number of chemically modified oligonucleotides that have been explored for gene silencing.

Thioguanosine Conversion Enables mRNA-Lifetime Evaluation by RNA Sequencing Using Double Metabolic Labeling (TUC-seq DUAL)

Temporal information about cellular RNA populations is essential to understand the functional roles of RNA. We have developed the hydrazine/NH₄ Cl/OsO₄ -based conversion of 6-thioguanosine (6sG) into A', where A' constitutes a 6-hydrazino purine derivative. A' retains the Watson-Crick base-pair mode and is efficiently decoded as adenosine in primer extension assays and in RNA sequencing. Because 6sG is applicable to metabolic labeling of freshly synthesized RNA and because the conversion chemistry is fully compatible with the conversion of the frequently used metabolic label 4-thiouridine (4sU) into C, the combination of both modified nucleosides in dual-labeling setups enables high accuracy measurements of RNA decay. This approach, termed TUC-seq DUAL, uses the two modified nucleosides in subsequent pulses and their simultaneous detection, enabling mRNA-lifetime evaluation with unprecedented precision.

Re-Engineering RNA Molecules into Therapeutic Agents

Efforts to chemically modify nucleic acids got underway merely a decade after the discovery of the DNA double helix and initially targeted nucleosides and nucleotides. The origins of three analogues that remain staples of modification strategies and figure prominently in FDA-approved nucleic acid therapeutics can be traced to the 1960s: 2'-deoxy-2'-fluoro-RNA (2'-F RNA), 2'- O-methyl-RNA (2'- OMe RNA), and the phosphorothioates (PS-DNA/RNA). Progress in nucleoside phosphoramidite-based solid phase oligonucleotide synthesis has gone hand in hand with the creation of second-generation (e.g., 2'- O-(2-methoxyethyl)-RNA, MOE-RNA) and third-generation (e.g., bicyclic nucleic acids, BNAs) analogues, giving rise to an expanding universe of modified nucleic acids. Thus, beyond site-specifically altered DNAs and RNAs with a modified base, sugar, and/or phosphate backbone moieties, nucleic acid chemists have created a host of conjugated oligonucleotides and artificial genetic polymers (XNAs). The search for oligonucleotides with therapeutic efficacy constitutes a significant driving force for these investigations. However, nanotechnology, diagnostics, synthetic biology and genetics, nucleic acid etiology, and basic research directed at the properties of native and artificial pairing systems have all stimulated the design of ever more diverse modifications. Modification of nucleic acids can affect pairing and chemical stability, conformation and interactions with a flurry of proteins and enzymes that play important roles in uptake, transport or processing of targets. Enhancement of metabolic stability is a central concern in the design of antisense, siRNA and aptamer oligonucleotides for therapeutic applications. In the antisense approach, uniformly modified oligonucleotides or so-called gapmers are used to target a specific RNA. The former may sterically block transcription or direct alternative splicing, whereas the latter feature a central PS window that elicits RNase H-mediated cleavage of the target. The key enzyme in RNA interference (RNAi) is Argonaute 2 (Ago2), a dynamic multidomain enzyme that binds multiple regions of the guide (antisense) and passenger (sense) siRNAs. The complexity of the individual interactions between Ago2 and the siRNA duplex provides significant challenges for chemical modification. Therefore, a uniform (the same modification throughout, e.g., antisense) or nearly uniform (e.g., aptamer) modification strategy is less useful in the pursuit of siRNA therapeutic leads. Instead, unique structural features and protein interactions of 5'-end (guide/Ago2MID domain), seed region, central region (cleavage site/Ago2 PIWI domain), and 3'-terminal nucleotides (guide/Ago2 PAZ domain) demand a more nuanced approach in the design of chemically modified siRNAs for therapeutic use. This Account summarizes current siRNA modification strategies with an emphasis on the regio-specific interactions between oligonucleotide and Ago2 and how these affect the choice of modification and optimization of siRNA efficacy. In addition to standard assays applied to measure the effects of modification on the stability of pairing and resistance against nuclease degradation, structural insights based on crystallographic data for modified RNAs alone and in complex with Ago2 from molecular modeling studies are a valuable guide in the design of siRNA therapeutics. Thus, this comprehensive approach is expected to result in accelerated generation of new siRNA-based therapies against various diseases, now that the first siRNA has obtained approval by the US FDA for treatment of hereditary hATTR amyloidosis.

Clinical potential of oligonucleotide-based therapeutics in the respiratory system

The discovery of an ever-expanding plethora of coding and non-coding RNAs with nodal and causal roles in the regulation of lung physiology and disease is reinvigorating interest in the clinical utility of the oligonucleotide therapeutic class. This is strongly supported through recent advances in nucleic acids chemistry, synthetic oligonucleotide delivery and viral gene therapy that have succeeded in bringing to market at least three nucleic acid-based drugs. As a consequence, multiple new candidates such as RNA interference modulators, antisense, and splice switching compounds are now progressing through clinical evaluation. Here, manipulation of RNA for the treatment of lung disease is explored, with emphasis on robust pharmacological evidence aligned to the five pillars of drug development: exposure to the appropriate tissue, binding to the desired molecular target, evidence of the expected mode of action, activity in the relevant patient population and commercially viable value proposition.

Computational Methods for RNA Structure Validation and Improvement

With increasing recognition of the roles RNA molecules and RNA/protein complexes play in an unexpected variety of biological processes, understanding of RNA structure-function relationships is of high current importance. To make clean biological interpretations from three-dimensional structures, it is imperative to have high-quality, accurate RNA crystal structures available, and the community has thoroughly embraced that goal. However, due to the many degrees of freedom inherent in RNA structure (especially for the backbone), it is a significant challenge to succeed in building accurate experimental models for RNA structures. This chapter describes the tools and techniques our research group and our collaborators have developed over the years to help RNA structural biologists both evaluate and achieve better accuracy. Expert analysis of large, high-resolution, quality-conscious RNA datasets provides the fundamental information that enables automated methods for robust and efficient error diagnosis in validating RNA structures at all resolutions. The even more crucial goal of correcting the diagnosed outliers has steadily developed toward highly effective, computationally based techniques. Automation enables solving complex issues in large RNA structures, but cannot circumvent the need for thoughtful examination of local details, and so we also provide some guidance for interpreting and acting on the results of current structure validation for RNA.

Delivery of RNAi therapeutics: work in progress

RNA interference (RNAi) therapeutics appear to offer substantial opportunities for future therapy. However, post-administration RNAi effectors are typically unable to reach disease target cells in vivo without the assistance of a delivery system or vector. The main focus of this review is on lipid-based nanoparticle (LNP) delivery systems in current research and development that have at least been shown to act as effective delivery systems for functional delivery of RNAi effectors to disease target cells in vivo. The potential utility of these LNP delivery systems is growing rapidly, and LNPs are emerging as the preferred synthetic delivery systems in preclinical studies and current nonviral RNAi effector clinical trials. Moreover, studies on LNP-mediated delivery in vivo are leading to the emergence of useful biophysical parameters and physical organic chemistry rules that provide a framework for understanding in vivo delivery behaviors and outcomes. These same parameters and rules should also suggest ways and means to develop next generations of LNPs with genuine utility and long-term clinical viability.

Crystal structure, stability and Ago2 affinity of phosphorodithioate-modified RNAs

The high Ago2 affinity of siRNAs with combined 2′-O-methyl and phosphorodithioate backbone modifications (MePS2) in the 3′-terminal region of the sense strand is likely the result of enhanced hydrophobic interactions with the protein's PAZ domain.

Computational approaches to predicting the impact of novel bases on RNA structure and stability

The use of computational modeling techniques to gain insight into nucleobase interactions has been a challenging endeavor to date. Accurate treatment requires the tackling of many challenges but also holds the promise of great rewards. The development of effective computational approaches to predict the binding affinities of nucleobases and analogues can, for example, streamline the process of developing novel nucleobase modifications, which should facilitate the development of new RNAi-based therapeutics. This brief review focuses on available computational approaches to predicting base pairing affinity in RNA-based contexts such as nucleobase-nucleobase interactions in duplexes and nucleobase-protein interactions. The challenges associated with such modeling along with potential future directions for the field are highlighted.

Short interfering RNA guide strand modifiers from computational screening

Short interfering RNAs (siRNAs) are promising drug candidates for a wide range of targets including those previously considered "undruggable". However, properties associated with the native RNA structure limit drug development, and chemical modifications are necessary. Here we describe the structure-guided discovery of functional modifications for the guide strand 5'-end using computational screening with the high-resolution structure of human Ago2, the key nuclease on the RNA interference pathway. Our results indicate the guide strand 5'-end nucleotide need not engage in Watson-Crick (W/C) H-bonding but must fit the general shape of the 5'-end binding site in MID/PIWI domains of hAgo2 for efficient knockdown. 1,2,3-Triazol-4-yl bases formed from the CuAAC reaction of azides and 1-ethynylribose, which is readily incorporated into RNA via the phosphoramidite, perform well at the guide strand 5'-end. In contrast, purine derivatives with modified Hoogsteen faces or N2 substituents are poor choices for 5'-end modifications. Finally, we identified a 1,2,3-triazol-4-yl base incapable of W/C H-bonding that performs well at guide strand position 12, where base pairing to target was expected to be important. This work expands the repertoire of functional nucleotide analogues for siRNAs.

Carbon-oxygen hydrogen bonding in biological structure and function

Carbon-oxygen (CH···O) hydrogen bonding represents an unusual category of molecular interactions first documented in biological structures over 4 decades ago. Although CH···O hydrogen bonding has remained generally underappreciated in the biochemical literature, studies over the last 15 years have begun to yield direct evidence of these interactions in biological systems. In this minireview, we provide a historical context of biological CH···O hydrogen bonding and summarize some major advancements from experimental studies over the past several years that have elucidated the importance, prevalence, and functions of these interactions. In particular, we examine the impact of CH···O bonds on protein and nucleic acid structure, molecular recognition, and enzyme catalysis and conclude by exploring overarching themes and unresolved questions regarding unconventional interactions in biomolecular structure.

Promiscuous 8-alkoxyadenosines in the guide strand of an siRNA: modulation of silencing efficacy and off-pathway protein binding

8-Alkoxyadenosines have the potential to exist in anti or syn conformations around the glycosidic bond when paired opposite to U or G in the complementary strands, thereby placing the sterically demanding 8-alkoxy groups in the major or minor groove, respectively, of duplex RNA. These modified bases were used as "base switches" in the guide strands of an siRNA to prevent off-pathway protein binding during delivery via placement of the alkoxy group in the minor groove, while maintaining significant RNAi efficacy by orienting the alkoxy group in the major groove. 8-Alkoxyadenosine phosphoramidites were synthesized and incorporated into the guide strand of caspase 2 siRNA at four different positions: two in the seed region, one at the cleavage junction, and another nearer to the 3'-end of the guide strand. Thermal stabilities of the corresponding siRNA duplexes showed that U is preferred over G as the base-pairing partner in the complementary strand. When compared to the unmodified positive control siRNAs, singly modified siRNAs knocked down the target mRNA efficiently and with little or no loss of efficacy. Doubly modified siRNAs were found to be less effective and lose their efficacy at low nanomolar concentrations. SiRNAs singly modified at positions 6 and 10 of the guide strand were found to be effective in blocking binding to the RNA-dependent protein kinase PKR, a cytoplasmic dsRNA-binding protein implicated in sequence-independent off-target effects.

The steric hypothesis for DNA replication and fluorine hydrogen bonding revisited in light of structural data

In DNA, bases pair in a molecular interaction that is both highly predictable and exquisitely specific. Therefore researchers have generally believed that the insertion of the matching nucleotide opposite a template base by DNA polymerases (pols) required Watson-Crick (W-C) hydrogen bond formation. However pioneering work by Kool and co-workers using hydrophobic base analogs such as the thymine (T) isostere 2,4-difluorotoluene (F) showed that shape rather than H-bonding served as the primary source of specificity in DNA replication by certain pols. This steric hypothesis for DNA replication has gained popularity, perhaps discouraging further experimental studies to address potential limitations of this new idea. The idea that shape trumps H-bonding in terms of pol selectivity largely hinges on the belief that fluorine is a poor H-bond acceptor. However, the shape complementarity model was embraced in the absence of any detailed structural data for match (F:A) and mismatch pairs (F:G, F:C, F:T) in DNA duplexes or at active sites of pols. Although the F and T nucleosides are roughly isosteric, it is unclear whether F:A and T:A pairs exhibit similar geometries. If the F:A pair is devoid of H-bonding, it will be notably wider than a T:A pair. Because shape and size and H-bonding are intimately related, it may not be possible to separate these two properties. Thus the geometries of an isolated F:A pair in water may differ considerably from an F:A pair embedded in a stretch of duplex DNA, at the tight active site of an A-family replicative pol, or within the spacious active site of a Y-family translesion pol. The shape complementarity model may have more significance for pol accuracy than efficiency: this model appears to be most relevant for replicative pols that use specific residues to probe the identity of the nascent base pair from the minor groove side. However, researchers have not fully considered the importance of such interactions that include H-bonds compared with W-C H-bonds in terms of pol fidelity and the shape complementarity model. This Account revisits the steric hypothesis for DNA replication in light of recent structural data and discusses the role of fluorine as an H-bond acceptor. Over the last 5 years, crystal structures have emerged for nucleic acid duplexes with F paired opposite to natural bases or located at the active sites of DNA pols. These data permit a more nuanced understanding of the role of shape in DNA replication and the capacity of fluorine to form H-bonds. These studies and additional research involving RNA or other fluorine-containing nucleoside analogs within duplexes indicate that fluorine engages in H-bonding in many cases. Although T and F are isosteric at the nucleoside level, replacement of a natural base by F in pairs often changes their shapes and sizes, and dF in DNA behaves differently from rF in RNA. Similarly, the pairing geometries observed for F and T opposite dATP, dGTP, dTTP, or dCTP and their H-bonding patterns at the active site of a replicative pol differ considerably.

Hydration changes upon DNA folding studied by osmotic stress experiments

The thermal stability of nucleic acid structures is perturbed under the conditions that mimic the intracellular environment, typically rich in inert components and under osmotic stress. We now describe the thermodynamic stability of DNA oligonucleotide structures in the presence of high background concentrations of neutral cosolutes. Small cosolutes destabilize the basepair structures, and the DNA structures consisting of the same nearest-neighbor composition show similar thermodynamic parameters in the presence of various types of cosolutes. The osmotic stress experiments reveal that water binding to flexible loops, unstable mismatches, and an abasic site upon DNA folding are almost negligible, whereas the binding to stable mismatch pairs is significant. The studies using the basepair-mimic nucleosides and the peptide nucleic acid suggest that the sugar-phosphate backbone and the integrity of the basepair conformation make important contributions to the binding of water molecules to the DNA bases and helical grooves. The study of the DNA hydration provides the basis for understanding and predicting nucleic acid structures in nonaqueous solvent systems.

2'-Azido RNA, a versatile tool for chemical biology: synthesis, X-ray structure, siRNA applications, click labeling

Chemical modification can significantly enrich the structural and functional repertoire of ribonucleic acids and endow them with new outstanding properties. Here, we report the syntheses of novel 2′-azido cytidine and 2′-azido guanosine building blocks and demonstrate their efficient site-specific incorporation into RNA by mastering the synthetic challenge of using phosphoramidite chemistry in the presence of azido groups. Our study includes the detailed characterization of 2′-azido nucleoside containing RNA using UV-melting profile analysis and CD and NMR spectroscopy. Importantly, the X-ray crystallographic analysis of 2′-azido uridine and 2′-azido adenosine modified RNAs reveals crucial structural details of this modification within an A-form double helical environment. The 2′-azido group supports the C3′-endo ribose conformation and shows distinct water-bridged hydrogen bonding patterns in the minor groove. Additionally, siRNA induced silencing of the brain acid soluble protein (BASP1) encoding gene in chicken fibroblasts demonstrated that 2′-azido modifications are well tolerated in the guide strand, even directly at the cleavage site. Furthermore, the 2′-azido modifications are compatible with 2′-fluoro and/or 2′-O-methyl modifications to achieve siRNAs of rich modification patterns and tunable properties, such as increased nuclease resistance or additional chemical reactivity. The latter was demonstrated by the utilization of the 2′-azido groups for bioorthogonal Click reactions that allows efficient fluorescent labeling of the RNA. In summary, the present comprehensive investigation on site-specifically modified 2′-azido RNA including all four nucleosides provides a basic rationale behind the physico- and biochemical properties of this flexible and thus far neglected type of RNA modification.

Novel modifications in RNA

The past several years have seen numerous reports of new chemical modifications for use in RNA. In addition, in that time period, we have seen the discovery of several previously unknown naturally occurring modifications that impart novel properties on the parent RNAs. In this review, we describe recent discoveries in these areas with a focus on RNA modifications that introduce spectroscopic tags, reactive handles, or new recognition properties.

Nucleic acid-metal ion interactions in the solid state

Metal ions play a key role in nucleic acid structure and activity. Elucidation of the rules that govern the binding of metal ions is therefore an essential step for better understanding of the nucleic acid functions. This review is as an update to a preceding one (Metal Ions Biol. Syst., 1996, 32, 91-134), in which we offered a general view of metal ion interactions with mono-, di-, tri-, and oligonucleotides in the solid state, based on their crystal structures reported before 1994. In this chapter, we survey all the crystal structures of metal ion complexes with nucleotides involving oligonucleotides reported after 1994 and we have tried to uncover new characteristic metal bonding patterns for mononucleotides and oligonucleotides with A-RNA and A/B/Z-DNA fragments that form duplexes. We do not cover quadruplexes, duplexes with metal-mediated base-pairs, tRNAs, rRNAs in ribosome, ribozymes, and nucleic acid-drug and -protein complexes. Factors that affect metal binding to mononucleotides and oligonucleotide duplexes are also dealt with.

Synthesis, physicochemical and biological properties of oligonucleotides incorporated with amino-isonucleosides

Antisense oligonucleotides (ASONs) and siRNAs have been applied extensively for the regulation of cellular and viral gene expression, and RNAi is currently one of the most promising new approaches for anti-tumor and anti-viral therapy. In order to improve bioactivity properties and physicochemical properties of siRNA, we synthesized a novel class of ASONs II–VII incorporated with amino-isonucleoside (isoA₁ and isoA₂) for investigation on basic physicochemical properties. Then we designed amino-isonucleoside (isoA₁, isoA₂ and isoT₁) incorporated siRNA 2–7. Some meaningful results have been obtained from the physicochemical property experiments in ASONs. In RNAi potency experiments, we investigated RNAi potency of each strand of the siRNA. These amino-isonucleosides incorporated siRNAs showed promising bioactivity properties and had position specificity. Reduced off target effect from sense strand loading in siRNA application was observed.

Chemical modification of siRNA bases to probe and enhance RNA interference

Considerable attention has focused on the use of alternatives to the native ribose and phosphate backbone of small interfering RNAs for therapeutic applications of the RNA interference pathway. In this synopsis, we highlight the less common chemical modifications, namely, those of the RNA nucleobases. Base modifications have the potential to lend insight into the mechanism of gene silencing and to lead to novel methods to overcome off-target effects that arise due to deleterious protein binding or mis-targeting of mRNA.

Molecular dynamics simulation and binding energy calculation for estimation of oligonucleotide duplex thermostability in RNA-based therapeutics

For oligonucleotide-based therapeutics, a thorough understanding of the thermodynamic properties of duplex formation is critical to developing stable and potent drugs. For unmodified small interfering RNA (siRNA), DNA antisense oligonucleotide (AON) and locked nucleic acid (LNA), DNA/LNA modified oligonucleotides, nearest neighbor (NN) methods can be effectively used to quickly and accurately predict duplex thermodynamic properties such as melting point. Unfortunately, for chemically modified olignonucleotides, there has been no accurate prediction method available. Here we describe the potential of estimating melting temperature (T(m)) for nonstandard oligonucleotides by using the correlation of the experimental T(m) with the calculated duplex binding energy (BE) for oligonucleotides of a given length. This method has been automated into a standardized molecular dynamics (MD) protocol through Pipeline Pilot (PP) using the CHARMm component in Discovery Studio (DS). Results will be presented showing the correlation of the predicted data with experiment for both standard and chemically modified siRNA and AON.

Unexpected origins of the enhanced pairing affinity of 2'-fluoro-modified RNA

Various chemical modifications are currently being evaluated for improving the efficacy of short interfering RNA (siRNA) duplexes as antisense agents for gene silencing in vivo. Among the 2'-ribose modifications assessed to date, 2'deoxy-2'-fluoro-RNA (2'-F-RNA) has unique properties for RNA interference (RNAi) applications. Thus, 2'-F-modified nucleotides are well tolerated in the guide (antisense) and passenger (sense) siRNA strands and the corresponding duplexes lack immunostimulatory effects, enhance nuclease resistance and display improved efficacy in vitro and in vivo compared with unmodified siRNAs. To identify potential origins of the distinct behaviors of RNA and 2'-F-RNA we carried out thermodynamic and X-ray crystallographic analyses of fully and partially 2'-F-modified RNAs. Surprisingly, we found that the increased pairing affinity of 2'-F-RNA relative to RNA is not, as commonly assumed, the result of a favorable entropic contribution ('conformational preorganization'), but instead primarily based on enthalpy. Crystal structures at high resolution and osmotic stress demonstrate that the 2'-F-RNA duplex is less hydrated than the RNA duplex. The enthalpy-driven, higher stability of the former hints at the possibility that the 2'-substituent, in addition to its important function in sculpting RNA conformation, plays an underappreciated role in modulating Watson-Crick base pairing strength and potentially π-π stacking interactions.

Short RNA duplexes guide sequence-dependent cleavage by human Dicer

Dicer is a member of the double-stranded (ds) RNA-specific ribonuclease III (RNase III) family that is required for RNA processing and degradation. Like most members of the RNase III family, Dicer possesses a dsRNA binding domain and cleaves long RNA duplexes in vitro. In this study, Dicer substrate selectivity was examined using bipartite substrates. These experiments revealed that an RNA helix possessing a 2-nucleotide (nt) 3'-overhang may bind and direct sequence-specific Dicer-mediated cleavage in trans at a fixed distance from the 3'-end overhang. Chemical modifications of the substrate indicate that the presence of the ribose 2'-hydroxyl group is not required for Dicer binding, but some located near the scissile bonds are needed for RNA cleavage. This suggests a flexible mechanism for substrate selectivity that recognizes the overall shape of an RNA helix. Examination of the structure of natural pre-microRNAs (pre-miRNAs) suggests that they may form bipartite substrates with complementary mRNA sequences, and thus induce seed-independent Dicer cleavage. Indeed, in vitro, natural pre-miRNA directed sequence-specific Dicer-mediated cleavage in trans by supporting the formation of a substrate mimic.

Determination of nucleic acid hydration using osmotic stress

Water plays an important role in structure and molecular recognition of biopolymers. Understanding hydration of biopolymers is a significant problem in structural chemistry and biology. However, hydration is a dynamic process that is difficult to study. While X-ray crystallography, NMR, and molecular modeling have provided structural detail on nucleic acid hydration and valuable insights into water dynamics, the thermodynamic contribution of water molecules to conformational equilibria and recognition of nucleic acids remains poorly understood. This unit describes a thermodynamic analysis of nucleic acid hydration using osmotic stress. Osmotic stress monitors the depression of melting temperature upon decreasing water activity, and calculates the number of thermodynamically unique water molecules associated with the double helix and released from single strands upon melting. Comparison of the number of water molecules released upon melting of nucleic acids with different sequences and chemical modifications provides insights that complement and enhance information obtained by other methods.

Modulation of thermal stability can enhance the potency of siRNA

During RNA-induced silencing complex (RISC) assembly the guide (or antisense) strand has to separate from its complementary passenger (or sense) strand to generate the active RISC complex. Although this process was found to be facilitated through sense strand cleavage, there is evidence for an alternate mechanism, in which the strands are dissociated without prior cleavage. Here we show that the potency of siRNA can be improved by modulating the internal thermodynamic stability profile with chemical modifications. Using a model siRNA targeting the firefly luciferase gene with subnanomolar IC50, we found that placement of thermally destabilizing modifications, such as non-canonical bases like 2,4-difluorotoluene or single base pair mismatches in the central region of the sense strand (9-12 nt), significantly improve the potency. For this particular siRNA, the strongest correlation between the decrease in thermal stability and the increase in potency was found at position 10. Controls with stabilized sugar-phosphate backbone indicate that enzymatic cleavage of the sense strand prior to strand dissociation is not required for silencing activity. Similar potency-enhancing effects were observed as this approach was applied to other functional siRNAs targeting a different site on the firefly luciferase transcript or endogenously expressed PTEN.

Exploring chemical modifications for siRNA therapeutics: a structural and functional outlook

RNA interference (RNAi) is a post-transcriptional gene silencing mechanism induced by small interfering RNAs (siRNAs) and micro-RNAs (miRNAs), and has proved to be one of the most important scientific discoveries made in the last century. The robustness of RNAi has opened up new avenues in the development of siRNAs as therapeutic agents against various diseases including cancer and HIV. However, there had remained a lack of a clear mechanistic understanding of messenger RNA (mRNA) cleavage mediated by Argonaute2 of the RNA-induced silencing complex (RISC), due to inadequate structural data. The X-ray crystal structures of the Argonaute (Ago)-DNA-RNA complexes reported recently have proven to be a breakthrough in this field, and the structural details can provide guidelines for the design of the next generation of siRNA therapeutics. To harness siRNAs as therapeutic agents, the prudent use of various chemical modifications is warranted to enhance nuclease resistance, prevent immune activation, decrease off-target effects, and to improve pharmacokinetic and pharmacodynamic properties. The focus of this review is to interpret the tolerance of various chemical modifications employed in siRNAs toward RNAi by taking into account the crystal structures and biochemical studies of Ago-RNA complexes. Moreover, the challenges and recent progress in imparting druglike properties to siRNAs along with their delivery strategies are discussed.

Crystallographic studies of chemically modified nucleic acids: a backward glance

Chemically modified nucleic acids (CNAs) are widely explored as antisense oligonucleotide or small interfering RNA (siRNA) candidates for therapeutic applications. CNAs are also of interest in diagnostics, high-throughput genomics and target validation, nanotechnology and as model systems in investigations directed at a better understanding of the etiology of nucleic acid structure, as well as the physicochemical and pairing properties of DNA and RNA, and for probing protein-nucleic acid interactions. In this article, we review research conducted in our laboratory over the past two decades with a focus on crystal-structure analyses of CNAs and artificial pairing systems. We highlight key insights into issues ranging from conformational distortions as a consequence of modification to the modulation of pairing strength, and RNA affinity by stereoelectronic effects and hydration. Although crystal structures have only been determined for a subset of the large number of modifications that were synthesized and analyzed in the oligonucleotide context to date, they have yielded guiding principles for the design of new analogs with tailor-made properties, including pairing specificity, nuclease resistance, and cellular uptake. And, perhaps less obviously, crystallographic studies of CNAs and synthetic pairing systems have shed light on fundamental aspects of DNA and RNA structure and function that would not have been disclosed by investigations solely focused on the natural nucleic acids.

Pairing geometry of the hydrophobic thymine analogue 2,4-difluorotoluene in duplex DNA as analyzed by X-ray crystallography

Certain DNA polymerases (pols) were found to efficiently insert A opposite the hydrophobic T isostere 2,4-difluorotoluene (F) and vice versa, resulting in the widely held belief that some pols rely on shape rather than H-bonding for accurate replication. Using X-ray crystallography we have analyzed the geometry of F:A pairs in duplex DNA and observed a distance between fluorine and the exocyclic amino group of A that is consistent with a H-bond, thus challenging the assumption that the F analogue is unable to engage in H-bonding as well as the steric hypothesis of DNA replication. Therefore, shape and H-bonding are inherently related, and steric constraints at a pol active site, or conferred by stacking or the DNA backbone conformation, may enable H-bonding by F.

Effects of N2,N2-dimethylguanosine on RNA structure and stability: crystal structure of an RNA duplex with tandem m2 2G:A pairs

Methylation of the exocyclic amino group of guanine is a relatively common modification in rRNA and tRNA. Single methylation (N(2)-methylguanosine, m(2)G) is the second most frequently encountered nucleoside analog in Escherichia coli rRNAs. The most prominent case of dual methylation (N(2),N(2)-dimethylguanosine, m(2) (2)G) is found in the majority of eukaryotic tRNAs at base pair m(2) (2)G26:A44. The latter modification eliminates the ability of the N(2) function to donate in hydrogen bonds and alters its pairing behavior, notably vis-à-vis C. Perhaps a less obvious consequence of the N(2),N(2)-dimethyl modification is its role in controlling the pairing modes between G and A. We have determined the crystal structure of a 13-mer RNA duplex with central tandem m(2) (2)G:A pairs. In the structure both pairs adopt an imino-hydrogen bonded, pseudo-Watson-Crick conformation. Thus, the sheared conformation frequently seen in tandem G:A pairs is avoided due to a potential steric clash between an N(2)-methyl group and the major groove edge of A. Additionally, for a series of G:A containing self-complementary RNAs we investigated how methylation affects competitive hairpin versus duplex formation based on UV melting profile analysis.

High-resolution NMR analysis of the conformations of native and base analog substituted retroviral and LTR-retrotransposon PPT primers

A purine-rich region of the plus-strand RNA genome of retroviruses and long terminal repeat (LTR)-containing retrotransposons, known as the polypurine tract (PPT), is resistant to hydrolysis by the RNase H domain of reverse transcriptase (RT) and ultimately serves as a primer for plus-strand DNA synthesis. The mechanisms underlying PPT resistance and selective processing remain largely unknown. Here, two RNA/DNA hybrids derived from the PPTs of HIV-1 and Ty3 were probed using high-resolution NMR for preexisting structural distortions in the absence of RT. The PPTs were selectively modified through base-pair changes or by incorporation of the thymine isostere, 2,4-difluoro-5-methylbenzene (dF), into the DNA strand. Although both wild-type (WT) and mutated hybrids adopted global A-form-like helical geometries, observed structural perturbations in the base-pair and dF-modified hybrids suggested that the PPT hybrids may function as structurally coupled domains.

Structure and activity of Y-class DNA polymerase DPO4 from Sulfolobus solfataricus with templates containing the hydrophobic thymine analog 2,4-difluorotoluene

The 2,4-difluorotoluene (DFT) analog of thymine has been used extensively to probe the relative importance of shape and hydrogen bonding for correct nucleotide insertion by DNA polymerases. As far as high fidelity (A-class) polymerases are concerned, shape is considered by some as key to incorporation of A(T) opposite T(A) and G(C) opposite C(G). We have carried out a detailed kinetic analysis of in vitro primer extension opposite DFT-containing templates by the trans-lesion (Y-class) DNA polymerase Dpo4 from Sulfolobus solfataricus. Although full-length product formation was observed, steady-state kinetic data show that dATP insertion opposite DFT is greatly inhibited relative to insertion opposite T (approximately 5,000-fold). No products were observed in the pre-steady-state. Furthermore, it is noteworthy that Dpo4 strongly prefers dATP opposite DFT over dGTP (approximately 200-fold) and that the polymerase is able to extend an A:DFT but not a G:DFT pair. We present crystal structures of Dpo4 in complex with DNA duplexes containing the DFT analog, the first for any DNA polymerase. In the structures, template-DFT is either positioned opposite primer-A or -G at the -1 site or is unopposed by a primer base and followed by a dGTP:A mismatch pair at the active site, representative of a -1 frameshift. The three structures provide insight into the discrimination by Dpo4 between dATP and dGTP opposite DFT and its inability to extend beyond a G:DFT pair. Although hydrogen bonding is clearly important for error-free replication by this Y-class DNA polymerase, our work demonstrates that Dpo4 also relies on shape and electrostatics to distinguish between correct and incorrect incoming nucleotide.