Improvement of a streptavidin-binding aptamer by LNA- and α-l-LNA-substitutions

"Click handle"-modified 2'-deoxy-2'-fluoroarabino nucleic acid as a synthetic genetic polymer capable of post-polymerization functionalization

The functions of natural nucleic acids such as DNA and RNA have transcended genetic information carriers and now encompass affinity reagents, molecular catalysts, nanostructures, data storage, and many others. However, the vulnerability of natural nucleic acids to nuclease degradation and the lack of chemical functionality have imposed a significant constraint on their ever-expanding applications. Herein, we report the synthesis and polymerase recognition of a 5-(octa-1,7-diynyl)uracil 2′-deoxy-2′-fluoroarabinonucleic acid (FANA) triphosphate. The DNA-templated, polymerase-mediated primer extension using this “click handle”-modified FANA (cmFANA) triphosphate and other FANA nucleotide triphosphates consisting of canonical nucleobases efficiently generated full-length products. The resulting cmFANA polymers exhibited excellent nuclease resistance and the ability to undergo efficient click conjugation with azide-functionalized molecules, thereby becoming a promising platform for serving as a programmable and evolvable synthetic genetic polymer capable of post-polymerization functionalization.

Polymerase-mediated incorporation of a “click handle”-modified fluoroarabinonucleic acid (cmFANA) triphosphate produces a new class of nuclease-resistant, evolvable genetic polymers that can be functionalized with azide-containing molecules.

Advances in Therapeutic L-Nucleosides and L-Nucleic Acids with Unusual Handedness

Nucleic-acid-based small molecule and oligonucleotide therapies are attractive topics due to their potential for effective target of disease-related modules and specific control of disease gene expression. As the non-naturally occurring biomolecules, modified DNA/RNA nucleoside and oligonucleotide analogues composed of L-(deoxy)riboses, have been designed and applied as innovative therapeutics with superior plasma stability, weakened cytotoxicity, and inexistent immunogenicity. Although all the chiral centers in the backbone are mirror converted from the natural D-nucleic acids, L-nucleic acids are equipped with the same nucleobases (A, G, C and U or T), which are critical to maintain the programmability and form adaptable tertiary structures for target binding. The types of L-nucleic acid drugs are increasingly varied, from chemically modified nucleoside analogues that interact with pathogenic polymerases to nanoparticles containing hundreds of repeating L-nucleotides that circulate durably in vivo. This article mainly reviews three different aspects of L-nucleic acid therapies, including pharmacological L-nucleosides, Spiegelmers as specific target-binding aptamers, and L-nanostructures as effective drug-delivery devices.

Enhanced duplex- and triplex-forming ability and enzymatic resistance of oligodeoxynucleotides modified by a tricyclic thymine derivative

We designed and synthesized an artificial nucleic acid, [3-(1,2-dihydro-2-oxobenzo[b][1,8]naphthyridine)]-2'-deoxy-D-ribofuranose (OBN), with a tricyclic structure in a nucleobase as a thymidine analog. Oligodeoxynucleotides (ODNs) containing consecutive OBN displayed improved duplex-forming ability with complementary single-stranded (ss) RNA and triplex-forming ability with double-stranded DNA in comparison with ODNs composed of natural thymidine. OBN-modified ODNs also displayed enhanced enzymatic resistance compared with ODNs with natural thymidine and phosphorothioate modification, respectively, due to the structural steric hindrance of the nucleobase. The fluorescence spectra of OBN-modified ODNs showed sufficient fluorescence intensity with ssDNA and ssRNA, which is an advantageous feature for fluorescence imaging techniques of nucleic acids with longer emission wavelengths than bicyclic thymine (bT).

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.

DNA Polymerase Variants with High Processivity and Accuracy for Encoding and Decoding Locked Nucleic Acid Sequences

Xenobiotic nucleic acids (XNAs) are chemically modified nucleic acid analogues with potential applications in nucleic acid-based therapeutics including nucleic acid aptamers, ribozymes, small interfering RNAs, and antisense oligonucleotides. We have developed a promising XNA for therapeutic uses, 2',4'-bridged nucleic acid (2',4'-BNA), also known as locked nucleic acid (LNA). Unlike the rational design of small interfering and antisense oligonucleotides, the development of LNA aptamers and catalysts requires genetically engineered polymerases that enable the synthesis of LNA from DNA and the converse reverse transcription. However, no LNA decoders or encoders with sufficient performance have been developed. In this study, we developed variants of KOD DNA polymerase, a family B DNA polymerase derived from Thermococcus kodakarensis KOD1, which are effective LNA decoders and encoders, via structural analyses. KOD DGLNK (KOD: N210D/Y409G/A485L/D614N/E664K) enabled LNA synthesis from DNA (DNA → LNA), and KOD DLK (KOD: N210D/A485L/E664K) enabled LNA reverse transcription to DNA (LNA → DNA). Both variants exhibited greatly improved efficiency and accuracy. Notably, we synthesized LNAs longer than one kilobase using KOD DGLNK. We also showed that these variants can accept 2'-O-methyl (2'-OMe), a common modification for therapeutic uses. Here, we also show that LNA and 2'-OMe mix aptamer can be practically obtained via SELEX. The variants can be used as powerful tools for creating XNA aptamers and catalysts to completely eliminate the natural species, DNA and RNA.

Carbazole modified oligonucleotides: synthesis, hybridization studies and fluorescence properties

Synthesis of the novel thiophenyl carbazole phosphoramidite DNA building block 5 was accomplished in four steps using a Suzuki-Miyaura cross-coupling reaction from the core carbazole and it was seamlessly accommodated into a 9-mer DNA-based oligonucleotide by incorporation at the flanking 5'-end in combination with a central insertion of an LNA-T nucleotide. The carbazole-containing oligonucleotide was combined in different duplex hybrids, which were characterized by thermal denaturation, circular dichroism and fluorescence studies. The carbazole monomer modulates the duplex stability in various ways. Thus, monomer Z increased the thermal stability of the 9-mer towards the complementary 9-mer/15-mer DNA duplex by 4.2 °C. Furthermore, indications of its intercalation into the duplex were obtained by modeling studies and robust decreases in fluorescence emission intensities upon duplex formation. In contrast, no clear intercalating tendency was corroborated for monomer Z within the DNA/RNA hybrid duplex as indicated by moderate quenching of the fluorescence and similar duplex thermal stabilities relative to the corresponding control duplex. The recognition efficiencies of the carbazole modified oligonucleotide toward single nucleotide mismatches were studied with two 15-mer model targets (DNA and RNA). For both systems, mismatches positioned at the juxtaposition of the carbazole monomer showed pronounced deceases in thermal denaturation temperature. Steady-state fluorescence emission studies of all mismatched duplexes with incorporation of Z monomer typically displayed efficient fluorescence quenching.

Chemical Modification of Aptamers for Increased Binding Affinity in Diagnostic Applications: Current Status and Future Prospects

Aptamers are short single stranded DNA or RNA oligonucleotides that can recognize analytes with extraordinary target selectivity and affinity. Despite their promising properties and diagnostic potential, the number of commercial applications remains scarce. In order to endow them with novel recognition motifs and enhanced properties, chemical modification of aptamers has been pursued. This review focuses on chemical modifications, aimed at increasing the binding affinity for the aptamer's target either in a non-covalent or covalent fashion, hereby improving their application potential in a diagnostic context. An overview of current methodologies will be given, thereby distinguishing between pre- and post-SELEX (Systematic Evolution of Ligands by Exponential Enrichment) modifications.

Alpha-l-Locked Nucleic Acid-Modified Antisense Oligonucleotides Induce Efficient Splice Modulation In Vitro

Alpha-l-Locked nucleic acid (α-l-LNA) is a stereoisomeric analogue of locked nucleic acid (LNA), which possesses excellent biophysical properties and also exhibits high target binding affinity to complementary oligonucleotide sequences and resistance to nuclease degradations. Therefore, α-l-LNA nucleotides could be utilised to develop stable antisense oligonucleotides (AO), which can be truncated without compromising the integrity and efficacy of the AO. In this study, we explored the potential of α-l-LNA nucleotides-modified antisense oligonucleotides to modulate splicing by inducing Dmd exon-23 skipping in mdx mouse myoblasts in vitro. For this purpose, we have synthesised and systematically evaluated the efficacy of α-l-LNA-modified 2'-O-methyl phosphorothioate (2'-OMePS) AOs of three different sizes including 20mer, 18mer and 16mer AOs in parallel to fully-modified 2'-OMePS control AOs. Our results demonstrated that the 18mer and 16mer truncated AO variants showed slightly better exon-skipping efficacy when compared with the fully-23 modified 2'-OMePS control AOs, in addition to showing low cytotoxicity. As there was no previous report on using α-l-LNA-modified AOs in splice modulation, we firmly believe that this initial study could be beneficial to further explore and expand the scope of α-l-LNA-modified AO therapeutic molecules.

Computational analysis and optimization of carcinoembryonic antigen aptamers and experimental evaluation

Carcinoembryonic antigen (CEA), a highly glycosylated protein, overexpresses in many cancers. In this study, computational methods were used to optimize CEA aptamers. Experimental evaluvation of selected aptamers were conducted through electrochemical impedance spectroscopy. After two and three-dimensional structure modeling, the complexes of twelve reported aptamers against CEA were simulated using the ZDOCK server. Based on docking scores, two aptamer sequences (CSR59 and CSR57.1) were selected and used to create a new library. This ssDNA aptamer library consisting of 91 sequences was created using diverse in silico mutational methods. We obtained seventeen sequences having higher binding scores than reported sequences. Based on ZDOCK scores, the interaction domain of CEA, and steric hindrance due to glycosylation, two aptamer sequences (G3S1.5 and G2S2.2) were selected. An impedimetric aptasensor was designed, and selected aptamers were used as biorecognition elements. Resistance to charge transfer (Rct) quantities confirmed the bioinformatic approach and molecular docking scores. The result showed that the interaction ability of selected aptamers was about 13.5 fold higher than the control. It can be concluded that the selected aptamers have good potential for detection of carcinoembryonic antigen biomarker.

Aptamer chemistry

Aptamers are single-stranded DNA or RNA molecules capable of tightly binding to specific targets. These functional nucleic acids are obtained by an in vitro Darwinian evolution method coined SELEX (Systematic Evolution of Ligands by EXponential enrichment). Compared to their proteinaceous counterparts, aptamers offer a number of advantages including a low immunogenicity, a relative ease of large-scale synthesis at affordable costs with little or no batch-to-batch variation, physical stability, and facile chemical modification. These alluring properties have propelled aptamers into the forefront of numerous practical applications such as the development of therapeutic and diagnostic agents as well as the construction of biosensing platforms. However, commercial success of aptamers still proceeds at a weak pace. The main factors responsible for this delay are the susceptibility of aptamers to degradation by nucleases, their rapid renal filtration, suboptimal thermal stability, and the lack of functional group diversity. Here, we describe the different chemical methods available to mitigate these shortcomings. Particularly, we describe the chemical post-SELEX processing of aptamers to include functional groups as well as the inclusion of modified nucleoside triphosphates into the SELEX protocol. These methods will be illustrated with successful examples of chemically modified aptamers used as drug delivery systems, in therapeutic applications, and as biosensing devices.

Oligonucleotides containing a piperazino-modified 2'-amino-LNA monomer exhibit very high duplex stability and remarkable nuclease resistance

Incorporation of a piperazino-modified 2'-amino-LNA monomer (PipLNA-T) into oligonucleotides conferred very high affinity and base-pairing selectivity towards complementary DNA and RNA strands. Furthermore, one PipLNA-T modification provided a robust nuclease resistance that safeguarded three neighbouring natural nucleosides from 3'-exonucleolytic degradation. These favourable properties render PipLNA-T a promising oligonucleotide modification for various biological applications.

Synthesis of locked cyclohexene and cyclohexane nucleic acids (LCeNA and LCNA) with modified adenosine units

We designed novel conformationally locked cyclohexene nucleic acid and studied their properties.

Towards applications of synthetic genetic polymers in diagnosis and therapy

Aptamers are a class of single-stranded nucleic acid ligands that can bind their targets with high specificity and affinities rivalling those of antibodies. First described over 20 years ago by Tuerk & Gold [1] and Ellington & Szostak [2] (who coined the name), their promise as both diagnostic and therapeutic agents remains to be realised. Key problems include the generally low biostability of the standard DNA/RNA or mixed RNA/2'F-DNA backbones under physiological conditions, limited chemical diversity of functional groups on the natural nucleobases, and the difficulty in reliably discovering aptamer ligands to some therapeutic targets. This review will describe recent progress in developing aptamer selection technology as well as expanding aptamer chemistry and informational complexity to improve aptamer discovery and properties.