Solid phase click ligation for the synthesis of very long oligonucleotides

An Update on Protection of 5'-Hydroxyl Functions of Nucleosides and Oligonucleotides

The synthesis of natural and chemically modified nucleosides and oligonucleotides is in great demand due to its increasing number of applications in diverse areas of research. These include tools for diagnostics and proteomics, research reagents for molecular biology, probes for functional genomics, and the design, discovery, development, and manufacture of new therapeutics. The likelihood of success in synthesizing these molecules is often dependent on the correct choice of a protection strategy to block the 5'-hydroxyl group of a carbohydrate moiety, nucleoside, or oligonucleotide. This topic was reviewed extensively in the year 2000. The purpose of this article is to complement and update the original review with recently published methodologies recommended for the protection and deprotection of the 5'-hydroxyl group. © 2024 Wiley Periodicals LLC.

Triazole-linked Nucleic Acids: Synthesis, Therapeutics and Synthetic Biology Applications

This article covers the triazole-linked nucleic acids where the triazole linkage (TL) replaces the natural phosphate backbone. The replacement is done at either a few selected linkages or all the phosphate linkages. Two triazole linkages, the four-atom TL1 and the six-atom TL2, have been discussed in detail. These triazole-modified oligonucleotides have found a wide range of applications, from therapeutics to synthetic biology. For example, the triazole-linked oligonucleotides have been used in the antisense oligonucleotide (ASO), small interfering RNA (siRNA) and clustered regularly interspaced short palindromic repeats (CRISPR)-Cas9 technology as therapeutic agents. Due to the ease of the synthesis and a wide range of biocompatibility, the triazole linkage TL2 has been used to assemble a functional 300-mer DNA from alkyne- and azide-functionalized 100-mer oligonucleotides as well as an epigenetically modified variant of a 335 base-pair gene from ten short oligonucleotides. These outcomes highlight the potential of triazole-linked nucleic acids and open the doors for other TL designs and artificial backbones to fully exploit the vast potential of artificial nucleic acids in therapeutics, synthetic biology and biotechnology.

Guide RNA modification as a way to improve CRISPR/Cas9-based genome-editing systems

Genome-editing technologies, in particular, CRISPR systems, are widely used for targeted regulation of gene expression and obtaining modified human and animal cell lines, plants, fungi, and animals with preassigned features. Despite being well described and easy to perform, the most common methods for construction and delivery of CRISPR/Cas9-containing plasmid systems possess significant disadvantages, mostly associated with effects of the presence of exogenous DNA within the cell. Transfection with active ribonucleoprotein complexes of Cas9 with single-guide RNAs (sgRNAs) represents one of the most promising options because of faster production of sgRNAs, the ability of a researcher to control the amount of sgRNA delivered into the cell, and consequently, fewer off-target mutations. Artificial-RNA synthesis strategies allow for the introduction of various modified components, such as backbone alterations, native structural motifs, and labels for visualization. Modifications of RNA can increase its resistance to hydrolysis, alter the thermodynamic stability of RNA-protein and RNA-DNA complexes, and reduce the immunogenic and cytotoxic effects. This review describes various approaches to improving synthetic guide RNA function through nucleotide modification.

ClickSeq: Replacing Fragmentation and Enzymatic Ligation with Click-Chemistry to Prevent Sequence Chimeras

We recently reported a fragmentation-free method for the synthesis of Next-Generation Sequencing libraries called "ClickSeq" that uses biorthogonal click-chemistry in place of enzymes for the ligation of sequencing adaptors. We found that this approach dramatically reduces artifactual chimera formation, allowing the study of rare recombination events that include viral replication intermediates and defective-interfering viral RNAs. ClickSeq illustrates how robust, bio-orthogonal chemistry can be harnessed in vitro to capture and dissect complex biological processes. Here, we describe an updated protocol for the synthesis of "ClickSeq" libraries.

Simultaneous multiple single nucleotide polymorphism detection based on click chemistry combined with DNA-encoded probes

Single nucleotide polymorphisms (SNPs) are emerging as important biomarkers for disease diagnosis, prognostics and disease pathogenesis. As one type of disease is always connected to several SNP sites, there is great demand for a reliable multiple SNP detection method. Herein, we mimicked a ligation reaction based on DNA ligase and originally utilized an enzyme-free DNA template-directed click reaction for SNP detection. With 5'-alkyne and 3'-azide groups labelled on two oligonucleotide probes, the target DNA-directed Cu(i)-catalyzed alkyne-azide cycloaddition (CuAAC) click reaction produced a new DNA strand with a triazole backbone, as a mimic of a DNA phosphodiester linkage. Trace amounts of the target (as low as 25 fmol in 50 μL) could be sensitively detected using capillary gel electrophoresis with laser-induced fluorescence (CGE-LIF). Meanwhile, SNP caused an obvious difference in the efficiency of the click reaction, and 0.5% SNP could be easily detected. More importantly, multiplexed SNP detection in a one tube reaction was successfully achieved only by encoding different lengths of the DNA probes for the different SNP sites.

Automated Solid-Phase Click Synthesis of Oligonucleotide Conjugates: From Small Molecules to Diverse N-Acetylgalactosamine Clusters

We developed a novel technique for the efficient conjugation of oligonucleotides with various alkyl azides such as fluorescent dyes, biotin, cholesterol, N-acetylgalactosamine (GalNAc), etc. using copper-catalysed alkyne-azide cycloaddition on the solid phase and CuI·P(OEt)₃ as a catalyst. Conjugation is carried out in an oligonucleotide synthesizer in fully automated mode and is coupled to oligonucleotide synthesis and on-column deprotection. We also suggest a set of reagents for the construction of diverse conjugates. The sequential double-click procedure using a pentaerythritol-derived tetraazide followed by the addition of a GalNAc or Tris-GalNAc alkyne gives oligonucleotide-GalNAc dendrimer conjugates in good yields with minimal excess of sophisticated alkyne reagents. The approach is suitable for high-throughput synthesis of oligonucleotide conjugates ranging from fluorescent DNA probes to various multi-GalNAc derivatives of 2'-modified siRNA.

Synthesis of chemically modified DNA

Naturally occurring DNA is encoded by the four nucleobases adenine, cytosine, guanine and thymine. Yet minor chemical modifications to these bases, such as methylation, can significantly alter DNA function, and more drastic changes, such as replacement with unnatural base pairs, could expand its function. In order to realize the full potential of DNA in therapeutic and synthetic biology applications, our ability to 'write' long modified DNA in a controlled manner must be improved. This review highlights methods currently used for the synthesis of moderately long chemically modified nucleic acids (up to 1000 bp), their limitations and areas for future expansion.

Parallel ClickSeq and Nanopore sequencing elucidates the rapid evolution of defective-interfering RNAs in Flock House virus

Defective-Interfering RNAs (DI-RNAs) have long been known to play an important role in virus replication and transmission. DI-RNAs emerge during virus passaging in both cell-culture and their hosts as a result of non-homologous RNA recombination. However, the principles of DI-RNA emergence and their subsequent evolution have remained elusive. Using a combination of long- and short-read Next-Generation Sequencing, we have characterized the formation of DI-RNAs during serial passaging of Flock House virus (FHV) in cell-culture over a period of 30 days in order to elucidate the pathways and potential mechanisms of DI-RNA emergence and evolution. For short-read RNAseq, we employed 'ClickSeq' due to its ability to sensitively and confidently detect RNA recombination events with nucleotide resolution. In parallel, we used the Oxford Nanopore Technologies's (ONT) MinION to resolve full-length defective and wild-type viral genomes. Together, these accurately resolve both rare and common RNA recombination events, determine the correlation between recombination events, and quantifies the relative abundance of different DI-RNAs throughout passaging. We observe the formation of a diverse pool of defective RNAs at each stage of viral passaging. However, many of these 'intermediate' species, while present in early stages of passaging, do not accumulate. After approximately 9 days of passaging we observe the rapid accumulation of DI-RNAs with a correlated reduction in specific infectivity and with the Nanopore data find that DI-RNAs are characterized by multiple RNA recombination events. This suggests that intermediate DI-RNA species are not competitive and that multiple recombination events interact epistatically to confer 'mature' DI-RNAs with their selective advantage allowing for their rapid accumulation. Alternatively, it is possible that mature DI-RNA species are generated in a single event involving multiple RNA rearrangements. These insights have important consequences for our understanding of the mechanisms, determinants and limitations in the emergence and evolution of DI-RNAs.

Conjugation and Evaluation of Triazole-Linked Single Guide RNA for CRISPR-Cas9 Gene Editing

The CRISPR-Cas9 gene editing system requires Cas9 endonuclease and guide RNAs (either the natural dual RNA consisting of crRNA and tracrRNA or a chimeric single guide RNA) that direct site-specific double-stranded DNA cleavage. This communication describes a click ligation approach that uses alkyne-azide cycloaddition to generate a triazole-linked single guide RNA (sgRNA). The conjugated sgRNA shows efficient and comparable genome editing activity to natural dual RNA and unmodified sgRNA constructs.

A clickable UTP analog for the posttranscriptional chemical labeling and imaging of RNA

The development of robust tools and practical RNA labeling strategies that would facilitate the biophysical analysis of RNA in both cell-free and cellular systems will have profound implications in the discovery of new RNA diagnostic tools and therapeutic strategies. In this context, we describe the development of a new alkyne-modified UTP analog, 5-(1,7-octadinyl)uridine triphosphate (ODUTP), which serves as an efficient substrate for the introduction of a clickable alkyne label into RNA transcripts by bacteriophage T7 RNA polymerase and mammalian cellular RNA polymerases. The ODU-labeled RNA is effectively used by reverse transcriptase to produce cDNA, a property which could be utilized in expanding the chemical space of a RNA library in the aptamer selection scheme. Further, the alkyne label on RNA provides a convenient tool for the posttranscriptional chemical functionalization with a variety of biophysical tags (fluorescent, affinity, amino acid and sugar) by using alkyne-azide cycloaddition reaction. Importantly, the ability of endogenous RNA polymerases to specifically incorporate ODUTP into cellular RNA transcripts enabled the visualization of newly transcribing RNA in cells by microscopy using click reactions. In addition to a clickable alkyne group, ODU contains a Raman scattering label (internal disubstituted alkyne), which exhibits characteristic Raman shifts that fall in the Raman-silent region of cells. Our results indicate that an ODU label could potentially facilitate two-channel visualization of RNA in cells by using click chemistry and Raman spectroscopy. Taken together, ODU represents a multipurpose ribonucleoside tool, which is expected to provide new avenues to study RNA in cell-free and cellular systems.

Synthesis of triazole-linked morpholino oligonucleotides via Cu(I) catalysed cycloaddition

Triazole-linked morpholino (^TLMO) oligonucleic acids were synthesised using the Cu^I catalysed (3 + 2) azide–alkyne cycloaddition (CuAAC) reaction.

Triazole-linked morpholino (^TLMO) oligonucleic acids were synthesised using the Cu^I catalysed (3 + 2) azide–alkyne cycloaddition (CuAAC) reaction. The modified DNA analogues were incorporated into 13-mer sequences via solid phase synthesis. UV melting experiments showed that the ^TLMO modification gives higher T_m values than the corresponding ^TLDNA modification.

Synthetic Method for Oligonucleotide Block by Using Alkyl-Chain-Soluble Support

A straightforward method for the synthesis of oligonucleotide blocks using a Cbz-type alkyl-chain-soluble support (Z-ACSS) attached to the 3'-OH group of 3'-terminal nucleosides was developed. The Z-ACSS allowed for the preparation of fully protected deoxyribo- and ribo-oligonucleotides without chromatographic purification and released dimer- to tetramer-size oligonucleotide blocks via hydrogenation using a Pd/C catalyst without significant loss or migration of protective groups such as 5'-end 4,4'-dimethoxtrityl, 2-cyanoethyl on internucleotide bonds, or 2'-TBS.

CuAAC: An Efficient Click Chemistry Reaction on Solid Phase

Click chemistry is an approach that uses efficient and reliable reactions, such as Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC), to bind two molecular building blocks. CuAAC has broad applications in medicinal chemistry and other fields of chemistry. This review describes the general features and applications of CuAAC in solid-phase synthesis (CuAAC-SP), highlighting the suitability of this kind of reaction for peptides, nucleotides, small molecules, supramolecular structures, and polymers, among others. This versatile reaction is expected to become pivotal for meeting future challenges in solid-phase chemistry.

Cycloadditions for Studying Nucleic Acids

Cycloaddition reactions for site-specific or global modification of nucleic acids have enabled the preparation of a plethora of previously inaccessible DNA and RNA constructs for structural and functional studies on naturally occurring nucleic acids, the assembly of nucleic acid nanostructures, therapeutic applications, and recently, the development of novel aptamers. In this chapter, recent progress in nucleic acid functionalization via a range of different cycloaddition (click) chemistries is presented. At first, cycloaddition/click chemistries already used for modifying nucleic acids are summarized, ranging from the well-established copper(I)-catalyzed alkyne-azide cycloaddition reaction to copper free methods, such as the strain-promoted azide-alkyne cycloaddition, tetrazole-based photoclick chemistry and the inverse electron demand Diels-Alder cycloaddition reaction between strained alkenes and tetrazine derivatives. The subsequent sections contain selected applications of nucleic acid functionalization via click chemistry; in particular, site-specific enzymatic labeling in vitro, either via DNA and RNA recognizing enzymes or by introducing unnatural base pairs modified for click reactions. Further sections report recent progress in metabolic labeling and fluorescent detection of DNA and RNA synthesis in vivo, click nucleic acid ligation, click chemistry in nanostructure assembly and click-SELEX as a novel method for the selection of aptamers.

Copper-free click chemistry for attachment of biomolecules in magnetic tweezers

BACKGROUND

Single-molecule techniques have proven to be an excellent approach for quantitatively studying DNA-protein interactions at the single-molecule level. In magnetic tweezers, a force is applied to a biopolymer that is anchored between a glass surface and a magnetic bead. Whereas the relevant force regime for many biological processes is above 20pN, problems arise at these higher forces, since the molecule of interest can detach from the attachment points at the surface or the bead. Whereas many recipes for attachment of biopolymers have been developed, most methods do not suffice, as the molecules break at high force, or the attachment chemistry leads to nonspecific cross reactions with proteins.

RESULTS

Here, we demonstrate a novel attachment method using copper-free click chemistry, where a DBCO-tagged DNA molecule is bound to an azide-functionalized surface. We use this new technique to covalently attach DNA to a flow cell surface. We show that this technique results in covalently linked tethers that are torsionally constrained and withstand very high forces (>100pN) in magnetic tweezers.

CONCLUSIONS

This novel anchoring strategy using copper-free click chemistry allows to specifically and covalently link biomolecules, and conduct high-force single-molecule experiments. Excitingly, this advance opens up the possibility for single-molecule experiments on DNA-protein complexes and molecules that are taken directly from cell lysate.

ClickSeq: Fragmentation-Free Next-Generation Sequencing via Click Ligation of Adaptors to Stochastically Terminated 3'-Azido cDNAs

We present a simple method called "ClickSeq" for NGS (next-generation sequencing) library synthesis that uses click chemistry rather than enzymatic reactions for the ligation of Illumina sequencing adaptors. In ClickSeq, randomly primed reverse transcription reactions are supplemented with azido-2',3'-dideoxynucleotides that randomly terminate DNA synthesis and release 3'-azido-blocked cDNA fragments in a process akin to dideoxy-Sanger sequencing. Purified fragments are "click ligated" via copper-catalyzed alkyne-azide cycloaddition to DNA oligos modified with a 5'-alkyne group. This generates ssDNA molecules containing an unnatural triazole-linked DNA backbone that is sufficiently biocompatible for PCR amplification to generate a cDNA library for RNAseq. Here, we analyze viral RNAs and mRNA to demonstrate that ClickSeq produces unbiased NGS libraries with low error rates comparable to standard methods. Importantly, ClickSeq is robust against common artifacts of NGS such as chimera formation and artifactual recombination with fewer than 3 aberrant events detected per million reads.

Combining triazole ligation and enzymatic glycosylation on solid phase simplifies the synthesis of very long glycoprotein analogues

The solid-phase chemical assembly of a protein through iterative chemoselective ligation of unprotected peptide segments can be followed with chemical and/or enzymatic transformations of the resulting immobilized protein, the latter steps thus benefitting from the advantages provided by the solid support. We demonstrate here the usefulness of this strategy for the chemo-enzymatic synthesis of glycoprotein analogues. A linker was specifically designed for application to the synthesis of O-glycoproteins: this new linker is readily cleaved under mild aqueous conditions compatible with very sensitive glycosidic bonds, but is remarkably stable under a wide range of chemical and biochemical conditions. It was utilized for solid-supported N-to-C peptidomimetic triazole ligation followed by enzymatic glycosylation, ultimately leading to a very large MUC1-derived glycoprotein containing 160 amino acid residues, 24 α-GalNAc moieties linked to Ser and Thr, and 3 triazoles as peptide bond mimetics.

A modified dinucleotide for site-specific RNA-labelling by transcription priming and click chemistry

An improved strategy for RNA labelling using an alkyne-carrying dinucleotide is reported. This involves near-quantitative priming by phage RNA-polymerases followed by conjugation of different labels using click chemistry. Moreover, these transcripts bear a ligation compatible 5'-end, and thus through ligation the terminal label can be transformed to an internal one.

Stable RNA nanoparticles as potential new generation drugs for cancer therapy

Human genome sequencing revealed that only ~1.5% of the DNA sequence coded for proteins. More and more evidence has uncovered that a substantial part of the 98.5% so-called "junk" DNAs actually code for noncoding RNAs. Two milestones, chemical drugs and protein drugs, have already appeared in the history of drug development, and it is expected that the third milestone in drug development will be RNA drugs or drugs that target RNA. This review focuses on the development of RNA therapeutics for potential cancer treatment by applying RNA nanotechnology. A therapeutic RNA nanoparticle is unique in that its scaffold, ligand, and therapeutic component can all be composed of RNA. The special physicochemical properties lend to the delivery of siRNA, miRNA, ribozymes, or riboswitches; imaging using fluogenenic RNA; and targeting using RNA aptamers. With recent advances in solving the chemical, enzymatic, and thermodynamic stability issues, RNA nanoparticles have been found to be advantageous for in vivo applications due to their uniform nano-scale size, precise stoichiometry, polyvalent nature, low immunogenicity, low toxicity, and target specificity. In vivo animal studies have revealed that RNA nanoparticles can specifically target tumors with favorable pharmacokinetic and pharmacodynamic parameters without unwanted accumulation in normal organs. This review summarizes the key studies that have led to the detailed understanding of RNA nanoparticle formation as well as chemical and thermodynamic stability issue. The methods for RNA nanoparticle construction, and the current challenges in the clinical application of RNA nanotechnology, such as endosome trapping and production costs, are also discussed.

Solid phase oxime ligations for the iterative synthesis of polypeptide conjugates

All on-resin! An efficient C-to-N iterative strategy for solid phase chemical ligations (SPCL).