LNA (locked nucleic acid) and analogs as triplex-forming oligonucleotides

Lowering Entropic Barriers in Triplex DNA Switches Facilitating Biomedical Applications at Physiological pH

Triplex DNA switches are attractive allosteric tools for engineering smart nanodevices, but their poor triplex-forming capacity at physiological conditions limited the practical applications. To address this challenge, we proposed a low-entropy barrier design to facilitate triplex formation by introducing a hairpin duplex linker into the triplex motif, and the resulting triplex switch was termed as CTNSds. Compared to the conventional clamp-like triplex switch, CTNSds increased the triplex-forming ratio from 30 % to 91 % at pH 7.4 and stabilized the triple-helix structure in FBS and cell lysate. CTNSds was also less sensitive to free-energy disturbances, such as lengthening linkers or mismatches in the triple-helix stem. The CTNSds design was utilized to reversibly isolate CTCs from whole blood, achieving high capture efficiencies (>86 %) at pH 7.4 and release efficiencies (>80 %) at pH 8.0. Our approach broadens the potential applications of DNA switches-based switchable nanodevices, showing great promise in biosensing and biomedicine.

Three's a crowd - stabilisation, structure, and applications of DNA triplexes

DNA is a strikingly flexible molecule and can form a variety of secondary structures, including the triple helix, which is the subject of this review. The DNA triplex may be formed naturally, during homologous recombination, or can be formed by the introduction of a synthetic triplex forming oligonucleotide (TFO) to a DNA duplex. As the TFO will bind to the duplex with sequence specificity, there is significant interest in developing TFOs with potential therapeutic applications, including using TFOs as a delivery mechanism for compounds able to modify or damage DNA. However, to combine triplexes with functionalised compounds, a full understanding of triplex structure and chemical modification strategies, which may increase triplex stability or in vivo degradation, is essential - these areas will be discussed in this review. Ruthenium polypyridyl complexes, which are able to photooxidise DNA and act as luminescent DNA probes, may serve as a suitable photophysical payload for a TFO system and the developments in this area in the context of DNA triplexes will also be reviewed.

Synthesis and Properties of Oligonucleotides Containing 2'-O,4'-C-Methyleneoxy-Bridged Pyrimidine Derivatives

In this study, 2'-O,4'-C-methyleneoxy-bridged nucleic acid, a unique consecutive three-acetal-containing nucleic acid (TaNA), was designed. Pyrimidine derivatives of methylated TaNA (Me-TaNA) were also synthesized and introduced into oligonucleotides via solid-phase synthesis. The Me-TaNA-modified oligonucleotides exhibited higher stabilities when forming duplexes with single-stranded RNA or triplexes with double-stranded DNA, relative to the natural oligonucleotides and modified oligonucleotides containing another 2',4'-bridged 5-methyluridine, such as 2',4'-BNA/LNA and 2',4'-ENA. Furthermore, Me-TaNA within oligonucleotides significantly enhanced nuclease resistance.

Convertible and Constrained Nucleotides: The 2'-Deoxyribose 5'-C-Functionalization Approach, a French Touch

Many strategies have been developed to modulate the biological or biotechnical properties of oligonucleotides by introducing new chemical functionalities or by enhancing their affinity and specificity while restricting their conformational space. Among them, we review our approach consisting of modifications of the 5'-C-position of the nucleoside sugar. This allows the introduction of an additional chemical handle at any position on the nucleotide chain without disturbing the Watson-Crick base-pairing. We show that 5'-C bromo or propargyl convertible nucleotides (CvN) are accessible in pure diastereoisomeric form, either for nucleophilic displacement or for CuAAC conjugation. Alternatively, the 5'-carbon can be connected in a stereo-controlled manner to the phosphate moiety of the nucleotide chain to generate conformationally constrained nucleotides (CNA). These allow the precise control of the sugar/phosphate backbone torsional angles. The consequent modulation of the nucleic acid shape induces outstanding stabilization properties of duplex or hairpin structures in accordance with the preorganization concept. Some biological applications of these distorted oligonucleotides are also described. Effectively, the convertible and the constrained approaches have been merged to create constrained and convertible nucleotides (C2NA) providing unique tools to functionalize and stabilize nucleic acids.

Polyamine-Functionalized 2'-Amino-LNA in Oligonucleotides: Facile Synthesis of New Monomers and High-Affinity Binding towards ssDNA and dsDNA

Attachment of cationic moieties to oligonucleotides (ONs) promises not only to increase the binding affinity of antisense ONs by reducing charge repulsion between the two negatively charged strands of a duplex, but also to augment their in vivo stability against nucleases. In this study, polyamine functionality was introduced into ONs by means of 2'-amino-LNA scaffolds. The resulting ONs exhibited efficient binding towards ssDNA, ssRNA and dsDNA targets, and the 2'-amino-LNA analogue carrying a triaminated linker showed the most pronounced duplex- and triplex-stabilizing effect. Molecular modelling revealed that favourable conformational and electrostatic effects led to salt-bridge formation between positively charged polyamine moieties and the Watson-Hoogsteen groove of the dsDNA targets, resulting in the observed triplex stabilization. All the investigated monomers showed increased resistance against 3'-nucleolytic digestion relative to the non-functionalized controls.

Oligonucleotides Containing Phenoxazine Artificial Nucleobases: Triplex-Forming Abilities and Fluorescence Properties

1,3-Diaza-2-oxophenoxazine ("phenoxazine"), a tricyclic cytosine analogue, can strongly bind to guanine moieties and improve π-π stacking effects with adjacent bases in a duplex. Phenoxazine has been widely used for improving duplex-forming abilities. In this study, we have investigated whether phenoxazine and its analogue, 1,3,9-triaza-2-oxophenoxazine (9-TAP), could improve triplex-forming abilities. A triplex-forming oligonucleotide (TFO) incorporating a phenoxazine component was found to show considerably decreased binding affinity with homopurine/homopyrimidine double-stranded DNA, so the phenoxazine system was considered not to function as either a protonated cytosine or thymine analogue. Alternatively, a 9-TAP-containing artificial nucleobase developed by us earlier as a new phenoxazine analogue functioned as a thymine analogue with respect to AT base pairs in a parallel triplex DNA motif. The fluorescence of the 9-TAP moiety was maintained even in triplex (9-TAP:AT) formation, so 9-TAP might be useful as an imaging tool for various oligonucleotide nanotechnologies requiring triplex formation.

Synthesis of 2'-C,4'-C-Methyleneoxy-Bridged Thymidine Derivatives and Properties of Modified Oligonucleotides

2',4'-Bridged nucleic acid (2',4'-BNA) analogues are used for therapeutic oligonucleotides, owing to their excellent hybridizing ability with complementary RNA and high resistance toward enzymatic degradation. We developed 2',4'-BNA analogues with oxygen atoms at 6'-positions (e.g., EoNA and EoDNAs) and demonstrated that the presence of 6'-oxygen atoms in the bridge structure could show positive effect on the properties of the modified oligonucleotides. Herein, we designed and synthesized 7'-methyl derivatives of methyleneoxy-bridged 2'-deoxyribonucleic acid (MoDNA), possessing a five-membered bridge with 6'-oxygen atom via radical cyclization for the bridge construction. The synthesized monomers were incorporated into the oligonucleotides by solid-phase oligonucleotide synthesis. The MoDNA-modified oligonucleotides showed high affinity toward single-stranded RNA and double-stranded DNA, as well as excellent resistance toward nuclease compared with the corresponding natural oligonucleotide.

Quantification of siRNA-Antibody Conjugates in Biological Matrices by Triplex-Forming Oligonucleotide ELISA

The potential repertoire of short interfering RNA (siRNA) therapeutics is expanding as targeting strategies evolve. One approach to enable organ-specific delivery has been to directly conjugate siRNA to a monoclonal antibody (siRNA-mAb), analogous to antibody-drug conjugates. Detection of intact siRNA-mAb conjugates presents a bioanalytical challenge given that certain synthetic nucleotide chemical modifications and low-temperature requirements render common oligonucleotide detection assays, such as reverse transcription-polymerase chain reaction, incompatible with the immunoassay component. To circumvent these issues, we developed a triplex-forming oligonucleotide ELISA using locked nucleic acid (LNA) containing oligonucleotide probes. We demonstrate that the incorporation of these LNAs allow for an enrichment and immobilization of siRNA directly conjugated to an antibody at nondenaturing temperatures. Without further requirement for extraction or amplification, we can sensitively and specifically detect intact siRNA-mAb conjugates in complex matrices such as serum and tissue homogenate.

A double-headed nucleotide with two cytosines: DNA with condensed information and improved duplex stability

Double-headed nucleotide monomers are capable of condensing the genetic information of DNA. Herein, a double-headed nucleotide with two cytosine bases (C_C) is constructed. The additional cytosine is connected through a methylene linker to the 2'-position of arabinocytidine. The nucleotide is incorporated into oligonucleotides and its effect on duplex stability is studied. For single incorporations, a thermal stabilization of 4.0 °C is found as compared to the unmodified duplex and it is shown that both nucleobases of C_C participate in Watson-Crick base pairing. In combination with the previously published U_T monomer, it is also shown that multiple incorporations are tolerated. For instance, a 16-mer sequence is targeted by a 13-mer oligonucleotide by using one C_C and two U_T monomers without compromising the overall duplex stability. Finally, the potential of double-headed nucleotides in triplex-forming oligonucleotides is studied, however, with the conclusion that the present design is not well-suited for this function.

Role of Pseudoisocytidine Tautomerization in Triplex-Forming Oligonucleotides: In Silico and in Vitro Studies

Pseudoisocytidine (^ΨC) is a synthetic cytidine analogue that can target DNA duplex to form parallel triplex at neutral pH. Pseudoisocytidine has mainly two tautomers, of which only one is favorable for triplex formation. In this study, we investigated the effect of sequence on ^ΨC tautomerization using λ-dynamics simulation, which takes into account transitions between states. We also performed in vitro binding experiments with sequences containing ^ΨC and furthermore characterized the structure of the formed triplex using molecular dynamics simulation. We found that the neighboring methylated or protonated cytidine promotes the formation of the favorable tautomer, whereas the neighboring thymine or locked nucleic acid has a poor effect, and consecutive ^ΨC has a negative influence. The deleterious effect of consecutive ^ΨC in a triplex formation was confirmed using in vitro binding experiments. Our findings contribute to improving the design of ^ΨC-containing triplex-forming oligonucleotides directed to target G-rich DNA sequences.

Folding Topology of a Short Coiled-Coil Peptide Structure Templated by an Oligonucleotide Triplex

The rational design of a well-defined protein-like tertiary structure formed by small peptide building blocks is still a formidable challenge. By using peptide-oligonucleotide conjugates (POC) as building blocks, we present the self-assembly of miniature coiled-coil α-helical peptides guided by oligonucleotide duplex and triplex formation. POC synthesis was achieved by copper-free alkyne-azide cycloaddition between three oligonucleotides and a 23-mer peptide, which by itself exhibited multiple oligomeric states in solution. The oligonucleotide domain was designed to furnish a stable parallel triplex under physiological pH, and to be capable of templating the three peptide sequences to constitute a small coiled-coil motif displaying remarkable α-helicity. The formed trimeric complex was characterized by ultraviolet thermal denaturation, gel electrophoresis, circular dichroism (CD) spectroscopy, small-angle X-ray scattering (SAXS), and molecular modeling. Stabilizing cooperativity was observed between the trimeric peptide and the oligonucleotide triplex domains, and the overall molecular size (ca. 12 nm) in solution was revealed to be independent of concentration. The topological folding of the peptide moiety differed strongly from those of the individual POC strands and the unconjugated peptide, exclusively adopting the designed triple helical structure.

Oligonucleotides Containing Aminated 2'-Amino-LNA Nucleotides: Synthesis and Strong Binding to Complementary DNA and RNA

Mono- and diaminated 2'-amino-LNA monomers were synthesized and introduced into oligonucleotides. Each modification imparts significant stabilization of nucleic acid duplexes and triplexes, excellent sequence selectivity, and significant nuclease resistance. Molecular modeling suggested that structural stabilization occurs via intrastrand electrostatic attraction between the protonated amino groups of the aminated 2'-amino-LNA monomers and the host oligonucleotide backbone.

Peptide-oligonucleotide conjugates as nanoscale building blocks for assembly of an artificial three-helix protein mimic

Peptide-based structures can be designed to yield artificial proteins with specific folding patterns and functions. Template-based assembly of peptide units is one design option, but the use of two orthogonal self-assembly principles, oligonucleotide triple helix and a coiled coil protein domain formation have never been realized for de novo protein design. Here, we show the applicability of peptide–oligonucleotide conjugates for self-assembly of higher-ordered protein-like structures. The resulting nano-assemblies were characterized by ultraviolet-melting, gel electrophoresis, circular dichroism (CD) spectroscopy, small-angle X-ray scattering and transmission electron microscopy. These studies revealed the formation of the desired triple helix and coiled coil domains at low concentrations, while a dimer of trimers was dominating at high concentration. CD spectroscopy showed an extraordinarily high degree of α-helicity for the peptide moieties in the assemblies. The results validate the use of orthogonal self-assembly principles as a paradigm for de novo protein design.

Peptide and oligonucleotide systems are known to self-assemble both in nature and artificial systems. Here, the authors combine both forms of self-assembly through the synthesis of peptideoligonucleotide conjugates and show formation of a three-helix structure that dimerises at higher concentrations.

Next-generation bis-locked nucleic acids with stacking linker and 2'-glycylamino-LNA show enhanced DNA invasion into supercoiled duplexes

Targeting and invading double-stranded DNA with synthetic oligonucleotides under physiological conditions remain a challenge. Bis-locked nucleic acids (bisLNAs) are clamp-forming oligonucleotides able to invade into supercoiled DNA via combined Hoogsteen and Watson–Crick binding. To improve the bisLNA design, we investigated its mechanism of binding. Our results suggest that bisLNAs bind via Hoogsteen-arm first, followed by Watson–Crick arm invasion, initiated at the tail. Based on this proposed hybridization mechanism, we designed next-generation bisLNAs with a novel linker able to stack to adjacent nucleobases, a new strategy previously not applied for any type of clamp-constructs. Although the Hoogsteen-arm limits the invasion, upon incorporation of the stacking linker, bisLNA invasion is significantly more efficient than for non-clamp, or nucleotide-linker containing LNA-constructs. Further improvements were obtained by substituting LNA with 2′-glycylamino-LNA, contributing a positive charge. For regular bisLNAs a 14-nt tail significantly enhances invasion. However, when two stacking linkers were incorporated, tail-less bisLNAs were able to efficiently invade. Finally, successful targeting of plasmids inside bacteria clearly demonstrates that strand invasion can take place in a biologically relevant context.

2′-N-Guanidino,4′-C-ethylene bridged thymidine (GENA-T) modified oligonucleotide exhibits triplex formation with excellent enzymatic stability

We present the synthesis and characterization of 2′-N-guanidino,4′-C-ethylene bridged thymidine (GENA-T) modified oligo-DNA that forms a stable triplex (ΔT_m +9.5 °C) with exceptional nuclease stability.

Harnessing the Therapeutic Potential of MicroRNAs for Cardiovascular Disease

Cardiovascular diseases are one of the most common causes of death in humans and are responsible for billions of dollars in health care expenditures. As the molecular basis of cardiac diseases continues to be explored, there remains the hope for identification of more effective therapeutics. MicroRNAs (miRNAs) are recognized as important regulators of numerous biological pathways and stress responses, including those found in cardiovascular diseases. MicroRNA signatures of cardiovascular diseases can provide targets for miRNA adjustment and offer the possibility of changing gene and protein expression to treat certain pathologies. These adjustments can be conferred using advances in oligonucleotide delivery methods, which can target single miRNAs, families of miRNAs, and certain tissue types. In this review, we will discuss the use of miRNAs in vivo and recent advances in their use for cardiovascular disease in mammalian models.

Scaffolding along nucleic acid duplexes using 2'-amino-locked nucleic acids

CONSPECTUS: Incorporation of chemically modified nucleotide scaffolds into nucleic acids to form assemblies rich in function is an innovative area with great promise for nanotechnology and biomedical and material science applications. The intrinsic biorecognition potential of nucleic acids combined with advanced properties of the locked nucleic acids (LNAs) provide opportunities to develop new nanomaterials and devices like sensors, aptamers, and machines. In this Account, we describe recent research on preparation and investigation of the properties of LNA/DNA hybrids containing functionalized 2'-amino-LNA nucleotides. By application of different chemical reactions, modification of 2'-amino-LNA scaffolds can be efficiently performed in high yields and with various tags, postsynthetically or during the automated oligonucleotide synthesis. The choice of a synthetic method for scaffolding along 2'-amino-LNA mainly depends on the chemical nature of the modification, its price, its availability, and applications of the product. One of the most useful applications of the product LNA/DNA scaffolds containing 2'-amino-LNA is to detect complementary DNA and RNA targets. Examples of these applications include sensing of clinically important single-nucleotide polymorphisms (SNPs) and imaging of nucleic acids in vitro, in cell culture, and in vivo. According to our studies, 2'-amino-LNA scaffolds are efficient within diagnostic probes for DNA and RNA targets and as therapeutics, whereas both 2'-amino- and isomeric 2'-α-l-amino-LNA scaffolds have promising properties for stabilization and detection of DNA nanostructures. Attachment of fluorescent groups to the 2'-amino group results in very high fluorescent quantum yields of the duplexes and remarkable sensitivity of the fluorescence signal to target binding. Notably, fluorescent LNA/DNA probes bind nucleic acid targets with advantages of high affinity and specificity. Thus, molecular motion of nanodevices and programmable self-assembly of chemically modified LNA/DNA nanomaterials can be followed by bright fluorescence signaling from the functionalized LNA units. Another appealing aspect of the amino-LNA scaffolds is specific targeting of nucleic acids and proteins for therapeutic applications. 2'-Amino-LNA/DNA conjugates containing peptide and polyaromatic hydrocarbon (PAH) groups are promising in this context as well as for advanced imaging and diagnostic purposes in vivo. For imaging applications, photostability of fluorescence dyes is of crucial importance. Chemically stable and photostable fluorescent PAH molecules attached to 2'-amino functionality of the 2'-amino-LNA are potent for in vitro and in vivo imaging of DNA and RNA targets. We believe that rational evolution of the biopolymers of Nature may solve the major challenges of the future material science and biomedicine. However, this requires strong scientific progress and efficient interdisciplinary research. Examples of this Account demonstrate that among other synthetic biopolymers, synthetic nucleic acids containing functionalized 2'-amino-LNA scaffolds offer great opportunities for material science, diagnostics, and medicine of the future.

Biological activity and biotechnological aspects of locked nucleic acids

Locked nucleic acid (LNA) is one of the most promising new nucleic acid analogues that has been produced under the past two decades. In this chapter, we have tried to cover many of the different areas, where this molecule has been used to improve the function of synthetic oligonucleotides (ONs). The use of LNA in antisense ONs, including gapmers, splice-switching ONs, and siLNA, as well as antigene ONs, is reviewed. Pharmacokinetics as well as pharmacodynamics of LNA ONs and a description of selected compounds in, or close to, clinical testing are described. In addition, new LNA modifications and the adaptation of enzymes for LNA incorporation are reviewed. Such enzymes may become important for the development of stabilized LNA-containing aptamers.

Fluorescent intercalator displacement replacement (FIDR) assay: determination of relative thermodynamic and kinetic parameters in triplex formation--a case study using triplex-forming LNAs

Triplex forming oligonucleotides (TFOs) are the most commonly used approach for site-specific targeting of double stranded DNA (dsDNA). Important parameters describing triplex formation include equilibrium binding constants (K(eq)) and association/dissociation rate constants (k(on) and k(off)). The 'fluorescent intercalator displacement replacement' (FIDR) assay is introduced herein as an operationally simple approach toward determination of these parameters for triplexes involving TC-motif TFOs. Briefly described, relative rate constants are determined from fluorescence intensity changes upon: (i) TFO-mediated displacement of pre-intercalated and fluorescent ethidium from dsDNA targets (triplex association) and (ii) Watson-Crick complement-mediated displacement of the TFO and replacement with ethidium (triplex dissociation). The assay is used to characterize triplexes between purine-rich dsDNA targets and TC-motif TFOs modified with six different locked nucleic acid (LNA) monomers, i.e. conventional and C5-alkynyl-functionalized LNA and α-L-LNA pyrimidine monomers. All of the studied monomers increase triplex stability by decreasing the triplex dissociation rate. LNA-modified TFOs form more stable triplexes than α-L-LNA-modified counterparts owing to slower triplex dissociation. Triplexes modified with C5-(3-aminopropyn-1-yl)-LNA-U monomer Z are particularly stable. The study demonstrates that three affinity-enhancing features can be combined into one high-affinity TFO monomer: conformational restriction of the sugar ring, expansion of the pyrimidine π-stacking surface and introduction of an exocyclic amine.

Duplex and triplex formation of mixed pyrimidine oligonucleotides with stacking of phenyl-triazole moieties in the major groove

5-(1-Phenyl-1,2,3-triazol-4-yl)-2'-deoxycytidine was synthesized from a modified CuAAC protocol and incorporated into mixed pyrimidine oligonucleotide sequences together with the corresponding 5-(1-phenyl-1,2,3-triazol-4-yl)-2'-deoxyuridine. With consecutive incorporations of the two modified nucleosides, improved duplex formation with a complementary RNA and improved triplex formation with a complementary DNA duplex were observed. The improvement is due to π-π stacking of the phenyl-triazole moieties in the major groove. The strongest stacking and most pronounced positive influence on thermal stability was found in between the uridine analogues or with the cytidine analogue placed in the 3' direction to the uridine analogue. Modeling indicated a different orientation of the phenyl-triazole moieties in the major groove to account for the difference between the two nucleotides. The modified oligonucleotides were all found to be significantly stabilized toward nucleolytic degration.

Monitoring triplex DNA formation with fluorescence resonance energy transfer between a fluorophore-labeled probe and intercalating dyes

Triplex-forming oligonucleotides (TFOs) are sequence-dependent DNA binders that may be useful for DNA targeting and detection. A sensitive and convenient method to monitor triplex formation by a TFO and its target DNA duplex is required for the application of TFO probes. Here we describe a novel design by which triplex formation can be monitored homogeneously without prelabeling the target duplex. The design uses a TFO probe tagged with a fluorophore that undergoes fluorescence resonance energy transfer with fluorescent dyes that intercalate into the target duplex. Through color compensation analysis, the specific emission of the TFO probe reveals the status of the triple helices. We used this method to show that triple helix formation with TFOs is magnesium dependent. We also demonstrated that the TFO probe can be used for detection of sequence variation in melting analysis and for DNA quantitation in real-time polymerase chain reaction.

Cationic modified nucleic acids for use in DNA hairpins and parallel triplexes

Non-nucleosidic DNA monomers comprising partially protonated amines at low pH have been designed and synthesized. The modifications were incorporated into DNA oligonucleotides via standard DNA phosphoramidite synthesis. The ability of cationic modifications to stabilize palindromic DNA hairpins and parallel triplexes were evaluated using gel electrophoresis, circular dichroism and thermal denaturation measurements. The non-nucleosidic modifications were found to increase the thermal stability of palindromic hairpins at pH 8.0 as compared with a nucleosidic tetraloop (TCTC). Incorporation of modifications at the 5'-end of a triplex forming oligonucleotide resulted in a significant increase in thermal stability at low pH when the modifications were placed as the 5'-dangling end.

A 4-[(3R,4R)-dihydroxypyrrolidino]pyrimidin-2-one nucleobase for a CG base pair in triplex DNA

In order to expand target sequences in triplex DNA formation, the development of a nucleobase that recognizes a CG base pair in dsDNA was attempted. A 4-[(3R,4R)-dihydroxypyrrolidino]pyrimidin-2-one nucleobase was found to recognize a CG base pair with high sequence-selectivity.

Optimizing anti-gene oligonucleotide 'Zorro-LNA' for improved strand invasion into duplex DNA

Zorro-LNA (Zorro) is a newly developed, oligonucleotide (ON)-based, Z-shaped construct with the potential of specific binding to each strand of duplex DNA. The first-generation Zorros are formed by two hybridized LNA/DNA mixmers (2-ON Zorros) and was hypothesized to strand invade. We have now established a method, which conclusively demonstrates that an LNA ON can strand invade into duplex DNA. To make Zorros smaller in size and easier to design, we synthesized 3′–5′–5′–3′ single-stranded Zorro-LNA (ssZorro) by using both 3′- and 5′-phosphoramidites. With ssZorro, a significantly greater extent and rate of double-strand invasion (DSI) was obtained than with conventional 2-ON Zorros. Introducing hydrophilic PEG-linkers connecting the two strands did not significantly change the rate or extent of DSI as compared to ssZorro with a nucleotide-based linker, while the longest alkyl-chain linker tested (36 carbons) resulted in a very slow DSI. The shortest alkyl-chain linker (3 carbons) did not reduce the extent of DSI of ssZorro, but significantly decreased the DSI rate. Collectively, ssZorro is smaller in size, easier to design and more efficient than conventional 2-ON Zorro in inducing DSI. Analysis of the chemical composition of the linker suggests that it could be of importance for future therapeutic considerations.

Synthesis and biophysical studies of coronene functionalized 2'-amino-LNA: a novel class of fluorescent nucleic acids

Incorporation of 2'-N-(coronen-1-yl)methyl-2'-amino-LNA monomer X or 2'-N-4-(coronen-1-yl)-4-oxobutanoyl-2'-amino-LNA monomer Y into short DNA strands induces high binding affinity toward DNA or RNA and a marked red-shift in steady-state fluorescence emission upon hybridization to cDNA or RNA.

Synthesis of functionalized carbocyclic locked nucleic acid analogues by ring-closing diene and enyne metathesis and their influence on nucleic acid stability and structure

A series of bicylic 2'-deoxynucleosides that are locked in the N-type conformation due to three-carbon linkages between the 2'- and 4'-positions have been prepared by ring-closing diene or enyne metathesis. The alkene or 1,3-diene hereby introduced in the bicyclic system is further derivatized, the latter showing the expected potential for Diels-Alder reactions. Four derivatives that are saturated or unsaturated as well as functionalized at the 2'-4'-linkage are incorporated into oligodeoxynucleotides, and the affinity of these for complementary RNA and DNA is studied. Substantially increased affinity for complementary RNA is observed, especially with additional hydroxyl groups attached to the bicyclic system. On the other hand, decreased affinity for complementary single-stranded DNA is obtained, whereas only a very small influence on a triplex-forming oligonucleotide sequence is found. Hence, a strong RNA-selective nucleic acid recognition is seen, and it can be concluded that the 2'-oxygen atom is less important for the formation of DNA:RNA duplexes than for the formation of DNA:DNA duplexes. However, the lack of a 2'-oxygen in the duplex formation can be partly compensated by other hydrophilic moieties around the 2'-4'-linkages indicating structural water binding to be of significant importance.

Optimized DNA-targeting using triplex forming C5-alkynyl functionalized LNA

Triplex forming oligonucleotides (TFOs) modified with C5-alkynyl functionalized LNA (locked nucleic acid) monomers display extraordinary thermal affinity toward double stranded DNA targets, excellent discrimination of Hoogsteen-mismatched targets, and high stability against 3?-exonucleases.

The stability of triplex DNA is affected by the stability of the underlying duplex

We have studied the formation of DNA triple helices in different sequence contexts and show that, for the most stable triplexes, their apparent stability is affected by the stability of the underlying duplex. For a 14-mer parallel triplex-forming oligonucleotide (generating C(+).GC and T.AT triplets) at pH 5.0 the T(m) is more than 10 degrees C lower with an intermolecular 14-mer duplex target, than it is with an intramolecular duplex, or one which is flanked by 6 GC base pairs at either end. A similar effect is seen with triplex-forming oligonucleotides that contain stabilising analogues, for which the T(m) is higher for an intramolecular than an intermolecular duplex target. These results suggest that the use of simple intermolecular duplex targets may underestimate the triplex stabilisation that is produced by some nucleotide analogues.

Nanotechnology approaches for gene transfer

In both basic research as well as experimental gene therapy the need to transfer genetic material into a cell is of vital importance. The cellular compartment, which is the target for the genetic material, depends upon application. An siRNA that mediates silencing is preferably delivered to the cytosol while a transgene would need to end up in the nucleus for successful transcription to occur. Furthermore the ability to regulate gene expression has grown substantially since the discovery of RNA interference. In such diverse fields as medical research and agricultural pest control, the capability to alter the genetic output has been a useful tool for pushing the scientific frontiers. This review is focused on nanotechnological approaches to assemble optimised structures of nucleic acid derivatives to facilitate gene delivery as well as promoting down regulation of endogenous genes.

Synthesis of bicyclic 2'-deoxynucleosides with alpha-L-ribo- and beta-D-xylo-configurations and restricted S- and N-type conformations

Two bicyclic 2'-deoxynucleoside analogues are synthesized in 12 steps each from thymidine. With a six-membered ring fused to the C3'-C4' bond and an alpha-L-ribo- and a beta-D-xylo-configuration, these are conformationally restricted in an S- and an N-type conformation, respectively. The constitutions were proven by X-ray crystallography. The beta-D-xylo-configured analogue is successfully converted to a 3'-phosphoramidite and incorporated into oligodeoxynucleotides, which are found to hybridize to DNA and RNA complements with decreased affinity.

The triple helix: 50 years later, the outcome

Triplex-forming oligonucleotides constitute an interesting DNA sequence-specific tool that can be used to target cleaving or cross-linking agents, transcription factors or nucleases to a chosen site on the DNA. They are not only used as biotechnological tools but also to induce modifications on DNA with the aim to control gene expression, such as by site-directed mutagenesis or DNA recombination. Here, we report the state of art of the triplex-based anti-gene strategy 50 years after the discovery of such a structure, and we show the importance of the actual applications and the main challenges that we still have ahead of us.

DNA triple helices: biological consequences and therapeutic potential

DNA structure is a critical element in determining its function. The DNA molecule is capable of adopting a variety of non-canonical structures, including three-stranded (i.e. triplex) structures, which will be the focus of this review. The ability to selectively modulate the activity of genes is a long-standing goal in molecular medicine. DNA triplex structures, either intermolecular triplexes formed by binding of an exogenously applied oligonucleotide to a target duplex sequence, or naturally occurring intramolecular triplexes (H-DNA) formed at endogenous mirror repeat sequences, present exploitable features that permit site-specific alteration of the genome. These structures can induce transcriptional repression and site-specific mutagenesis or recombination. Triplex-forming oligonucleotides (TFOs) can bind to duplex DNA in a sequence-specific fashion with high affinity, and can be used to direct DNA-modifying agents to selected sequences. H-DNA plays important roles in vivo and is inherently mutagenic and recombinogenic, such that elements of the H-DNA structure may be pharmacologically exploitable. In this review we discuss the biological consequences and therapeutic potential of triple helical DNA structures. We anticipate that the information provided will stimulate further investigations aimed toward improving DNA triplex-related gene targeting strategies for biotechnological and potential clinical applications.

Recent highlights in modified oligonucleotide chemistry

The synthesis of modified nucleic acids has been the subject of much study ever since the structure of DNA was elucidated by Watson and Crick at Cambridge and Wilkins and Franklin at King's College over half a century ago. This review describes recent developments in the synthesis and application of these artificial nucleic acids, predominantly the phosphoramidites which allow for easy inclusion into oligonucleotides, and is divided into three separate sections. Firstly, modifications to the base portion will be discussed followed secondly by modifications to the sugar portion. Finally, changes in the type of nucleic acid linker will be discussed in the third section. Peptide Nucleic Acids (PNAs) are not discussed in this review as they represent a separate and large area of nucleic acid mimics.