Oligonucleotide-directed Triple Helix Formation Research Articles

Effect of dC → d(m5C) substitutions on the folding of intramolecular triplexes with mixed TAT and C+GC base triplets

Oligonucleotide-directed triple helix formation has been recognized as a potential tool for targeting genes with high specificity. Cystosine methylation in the 5′ position is both ubiquitous and a stable regulatory modification, which could potentially stabilize triple helix formation. In this work, we have used a combination of calorimetric and spectroscopic techniques to study the intramolecular unfolding of four triplexes and two duplexes. We used the following triplex control sequence, named Control Tri, d(AGAGAC5TCTCTC5TCTCT), where C5 are loops of five cytosines. From this sequence, we studied three other sequences with dC → d(m5C) substitutions on the Hoogsteen strand (2MeH), Crick strand (2MeC) and both strands (4MeHC). Calorimetric studies determined that methylation does increase the thermal and enthalpic stability, leading to an overall favorable free energy, and that this increased stability is cumulative, i.e. methylation on both the Hoogsteen and Crick strands yields the largest favorable free energy. The differential uptake of protons, counterions and water was determined. It was found that methylation increases cytosine protonation by shifting the apparent pKa value to a higher pH; this increase in proton uptake coincides with a release of counterions during folding of the triplex, likely due to repulsion from the increased positive charge from the protonated cytosines. The immobilization of water was not affected for triplexes with methylated cytosines on their Hoogsteen or Crick strands, but was seen for the triplex where both strands are methylated. This may be due to the alignment in the major groove of the methyl groups on the cytosines with the methyl groups on the thymines which causes an increase in structural water along the spine of the triplex.

ChemInform Abstract: Design of a Nonnatural Deoxyribonucleoside for Recognition of GC Base Pairs by Oligonucleotide-Directed Triple Helix Formation.

The deoxyribonucleoside 1-(2-deoxy-beta-D-ribofuranosyl)-3-methyl-5-amino-1H-pyrazolo4,3-d]py rimidin-7-one (P1) was designed such that two specific hydrogen bonds would form with guanine (G) of a Watson-Crick guanine-cytosine (GC) base pair in the major groove of double-helical DNA. One edge of the P1 heterocycle mimics N3-protonated cytosine, which would circumvent the pH dependence observed for the formation of triple helices containing C + GC base triplets. P1 was synthesized in five steps and incorporated by automated methods in pyrimidine oligodeoxyribonucleotides. From affinity cleaving analyses, the stabilities of base triplets decrease in the order P1.GC >> P1.CG >> P1.AT approximately P1.TA (pH 7.4, 35-degrees-C). P1 binds GC base pairs within a pyrimidinte triple-helix motif as selectively and strongly as C but over an extended pH range. Oligodeoxyribonucleotides containing P1 residues were shown to bind within plasmid DNA a single 15 base pair site containing five GC base pairs at pH 7.8 and a single 16 base pair site containing six contiguous GC base pairs at pH 7.4.

A novel DNA computing model based on RecA-mediated triple-stranded DNA structure

Abstract The field of DNA computing emerged in 1994 after Adleman's paper was published. Henceforth, a few scholars solved some noted NP-complete problems in this way. And all these methods of DNA computing are based on conventional Watson-Click hydrogen bond of doublehelical DNA molecule. In this paper, we show that the triple-stranded DNA structure mediated by RecA protein can be used for solving computational problems. Sequence-specific recognition of double-stranded DNA by oligonucleotide-directed triple helix (triplex) formation is used to carry out the algorithm. We present procedure for the 3-vertex-colorability problems. In our proposed procedure, it is suggested that it is possible to solve more complicated problems with more variables by this model. *Supported by National Natural Science Foundation of China (Grant Nos. 60533010, 60373089, 60674106), Science Foundation of Shaanxi Bureau of Education (Grant No. 06JK202), Program for New Century Excellent Talents in University (No. NCET-05-0612) an...

Triple‐Helix Directed Cleavage of Double‐Stranded DNA by Benzoquinoquinoxaline‐1,10‐phenanthroline Conjugates

Oligonucleotide-directed triple-helix formation provides a rational means to interfere with genomic DNA targets and to direct modifications at specific sites. We have developed a new class of compounds that, at low concentrations, efficiently targets and damages double-stranded DNA specifically at the site where a triple-helical structure is formed. In these new compounds, a triple-helix-specific intercalator-benzoquinoquinoxaline (BQQ)-was coupled to one of two isomeric 1,10-phenanthrolinecarboxaldehyde derivatives. 1,10-Phenanthroline derivatives are known to cleave DNA in the presence of copper ions. The obtained BQQ-1,10-phenanthroline (BQQ-OP) conjugates were compared with regard to their ability to cleave triple-helix DNA. Both conjugates displayed a sequence preference inside the triple-helical site, as judged from the more pronounced cleavage obtained at stretches of TAxT base triplets.

A new method (GOREC) for directed mutagenesis and gene repair by homologous recombination.

Directed mutagenesis in mammalian cells has been the focus of intense research because of its promising application for gene correction and engineering. Both natural and modified oligonucleotides (ODN), RNA-DNA chimeric oligonucleotide (RDO) and small fragment DNA (SFHR), as well as vector DNA were used for promoting homologous replacement with varying success. It was recently shown that a triple helix-forming oligonucleotide (TFO) tethered to an oligonucleotide (donor DNA) can enhance mutagenesis by homologous recombination in cells. The basic idea is to accelerate homology search by oligonucleotide-directed triple helix formation in the vicinity of the target site for donor DNA. Here we describe a new method named GOREC (guided homologous recombination) which shares similar gene targeting, but has notable difference in the concept with the previous method. It is made of a homing device (TFO) and a donor DNA for effecting distinct functions. They are linked together by non-covalent or covalent interaction. This modular concept allows guidance of either an oligonucleotide (ODN, RDO) or a small DNA fragment to the target site for homologous replacement. Therefore, the triple helix site can be hundreds of base pairs away from the target site. An episomal assay for proof-of-principle study will be presented and discussed.

Stimulation of RecA-mediated D-loop formation by oligonucleotide-directed triple-helix formation: guided homologous recombination (GOREC).

Oligonucleotide-directed triple helix formation provides an elegant rational basis for gene-specific DNA targeting and has been widely used to interfere with gene expression ("antigene" strategies) and as a molecular tool for biological studies. Various strategies have been developed to introduce sequence modifications in genomes. However, the low efficiency of the overall process in eucaryotic cells impairs efficient recovery of recombinant genomes. Since one limiting step in homologous recombination is the targeting to the homologous sequence, we have tested the contribution of an oligonucleotide-directed triple helix formation on the RecA-dependent association of an oligonucleotide and its homologous target on duplex DNA (D-loop formation). For this study, the recombinant ssDNA fragment was noncovalently associated to a triple helix-forming oligonucleotide. The physicochemical and biochemical characteristics of the triple helix and D-loop structures formed by the complex molecules in the presence or in the absence of RecA protein were determined. We have demonstrated that the triple helix-forming oligonucleotide increases the efficiency of D-loop formation and the RecA protein speeds up also the triple helix formation. The so-called "GOREC" (for guided homologous recombination) approach can be developed as a novel tool to improve the efficiency of directed mutagenesis and gene alteration in living organisms.

Incorporation of a novel nucleobase allows stable oligonucleotide-directed triple helix formation at the target sequence containing a purine.pyrimidine interruption.

Thermal denaturation experiments have established that an oligonucleotide incorporating the artificial nucleobase S, does form a stable triplex with a double stranded DNA which exhibits a pyrimidine interruption within the oligopurine sequence.

LMPCR for detection of oligonucleotide-directed triple helix formation: a cautionary note.

We note that precautions are necessary when ligation-mediated PCR (LMPCR) is applied to the detection of oligonucleotide-directed triple helix formation in vitro and in vivo. Synthetic oligonucleotides applied to cell cultures can persist after chemical treatment and genomic DNA isolation and inhibit a key step in LMPCR, causing an artifact that simulates a triplex footprint. Residual oligonucleotides apparently form triplexes during LMPCR, blocking ligation of the unidirectional linker in a site-specific manner. We show that careful removal of residual oligonucleotide prior to LMPCR alleviates this problem.

Optimization of alternate-strand triple helix formation at the 5'CpG3' and 5'GpC3' junction steps.

Oligonucleotide-directed triple helix formation normally requires a long tract of oligopyrimidine.oligopurine sequence. This limitation can be partially overcome by alternate-strand triple helix (or switch triple helix) formation which enables recognition of alternating oligopurine/oligopyrimidine sequences. The present work is devoted to the optimization of switch triple helix formation at the 5'CpG3' and 5'GpC3' junction steps by combination of base triplets in Hoogsteen and in reverse Hoogsteen configurations. Rational design by molecular mechanics was first carried out to study the geometrical constraints at different junction steps and to propose a "switch code" which would optimize the interactions at junctions. These predictions were further checked and validated experimentally by gel retardation and DNase I footprinting assays. It was shown that the choice of an appropriate linker nucleotide in the switching third strand plays an important role in the interaction between oligonucleotides and alternating oligopurine/oligopyrimidine target sequences at different junctions: (i) the addition of a cytosine at the junction level in the oligonucleotide optimizes the crossover at the 5'CpG3' junction, whereas (ii) the best crossover at the 5'GpC3' junction step is achieved without any additional nucleotide. These results provide a useful guideline to extend double-stranded DNA sequence recognition by switch triple helix formation.

Selection and characterization of RNAs that relieve transcriptional interference in Escherichia coli.

Oligonucleotide-directed triple helix formation offers a method for duplex DNA recognition, and has been proposed as an approach to the rational design of gene-specific repressors. Indeed, certain RNA and DNA oligonucleotides have previously been shown to bind duplex DNA and repress in vitro transcription by occluding the binding of transcription factors or RNA polymerase at target genes. While similar oligonucleotides have reportedly caused repression of target genes in cultured cells, physical evidence of triple helix formation in vivo is generally lacking. In the present study we wished to determine whether RNA transcripts could repress the activity of an Escherichia coli promoter in vivo by binding to the duplex promoter DNA. An in vivo genetic selection previously developed to identify DNA binding proteins was modified for this purpose. Using expression libraries encoding RNAs predisposed to forming triple helices with a DNA target site, we have selected RNA transcripts that confer survival to E.coli by disrupting transcriptional interference. Surprisingly, genetic and biochemical evidence shows that these RNAs do not form triple helices at the target promoter in vivo , despite the fact that they contain sequences capable of forming triple helices at the duplex DNA target in vitro . Rather, the selected RNAs appear to disrupt transcriptional interference via an antisense mechanism.

Triple helices formed at oligopyrimidine*oligopurine sequences with base pair inversions: effect of a triplex-specific ligand on stability and selectivity.

Oligonucleotide-directed triple helix formation is mostly restricted to oligopyrimidine*oligopurine sequences of double helical DNA. An interruption of one or two pyrimidines in the oligopurine target strand leads to a strong triplex destabilisation. We have investigated the effect of nucleotide analogues introduced in the third strand at the site opposite the base pair inversion(s). We show that a 3-nitropyrrole derivative (M) discriminates G*C from C*G, A*T and T*A in the presence of a triplex-specific ligand (a benzo[e]pyridoindole derivative, BePI). N6-methoxy-2,6-diaminopurine (K) binds to an A*T base pair better than a T*A, G*C or C*G base pair. Some discrimination is still observed in the presence of BePI and triplex stability is markedly increased. These findings should help in designing BePI-oligonucleotide conjugates to extend the range of DNA sequences available for triplex formation.

DNA Triplex Formation Selectively Inhibits Granulocyte-Macrophage Colony-stimulating Factor Gene Expression in Human T Cells

Granulocyte-macrophage colony-stimulating factor (GM-CSF) is a hemopoietic growth factor that is expressed in activated T cells, fibroblasts, macrophages, and endothelial cells. Although GM-CSF does not appear to be essential for normal hemopoiesis, overexpression of GM-CSF has been implicated in the pathogenesis of some diseases such as myeloid leukemia and chronic inflammation. An NF-kappaB/Rel binding site within the GM-CSF promoter, termed the kappaB element appears to be important for controlling expression in reporter gene assays in response to a number of stimuli in T cells. We investigated oligonucleotide-directed triple helix formation across this regulatory sequence as a potential tool to inhibit GM-CSF gene transcription. A 15-base oligonucleotide, GM3, was targeted to a purine-rich region in the GM-CSF proximal promoter, which overlaps the kappaB element. Gel mobility shift assays and DNase I footprinting demonstrated that GM3 formed a sequence-specific collinear triplex with its double-stranded DNA target. Triplex formation by GM3 blocked recombinant and nuclear NF-kappaB proteins binding to the GM-CSF element. GM3 also caused selective inhibition of the human T-cell lymphotrophic virus-1 Tax transactivator-induced luciferase activity from a reporter construct driven by the GM-CSF promoter in Jurkat T cells. Finally, GM3 greatly reduced the concentration of endogenous GM-CSF mRNA induced by different stimuli in Jurkat T cells but did not affect interleukin 3 mRNA levels in the same cells. We conclude that the kappaB element in the GM-CSF promoter plays a central role in the transcriptional activation of the endogenous GM-CSF gene. Colinear triplex formation acts as a selective transcriptional repressor of the GM-CSF gene and may have potential therapeutic application in cases of undesirable overexpression of this protein.

Binding of DNA Oligonucleotides to Sequences in the Promoter of the Human bcl-2 Gene

Duplex DNA recognition by oligonucleotide-directed triple helix formation is being explored as a highly specific approach to artificial gene repression. We have identified two potential triplex target sequences in the promoter of the human bcl-2 gene, whose product inhibits apoptosis. Oligonucleotides designed to bind these target sequences were tested for their binding affinities and specificities under pseudo-physiological conditions. Electrophoretic mobility shift and dimethyl sulfate footprinting assays demonstrated that an oligonucleotide designed for simultaneous recognition of homopurine domains on alternate duplex DNA strands had the highest affinity of any oligonucleotide tested. Modifications to render this oligonucleotide nuclease-resistant did not reduce its binding affinity or specificity. In additional studies under various pH conditions, pyrimidine motif complexes at these target sequences were found to be stable at pH 8.0, despite the presumed requirement for protonation of oligonucleotide cytidines. In contrast, purine motif complexes, typically considered to be pH independent, were highly destabilized at decreasing pH values. These results indicate that a natural sequence in the human bcl-2 promoter can form a stable triplex with a synthetic oligonucleotide under pseudo-physiological conditions, and suggest that triple helix formation might provide an approach to the artificial repression of bcl-2 transcription.

Prospects for the therapeutic use of antigene oligonucleotides.

An outgrowth of classic nucleic acid interaction studies, oligonucleotide-directed triple helix formation is a unique method for creating highly specific chemical ligands that recognize and bind to particular sequences of duplex DNA. Under permissive conditions, these oligonucleotide-based compounds can approach or exceed the binding affinity and sequence specificity of natural DNA-binding proteins. Triple helix recognition has been found to be useful in certain cell-free applications including precise chromosome fragmentation. It has been proposed that such oligonucleotides could also form the basis for gene-targeted (antigene) drugs that might repress transcription from undesired genes in living cells. However, current strategies for oligonucleotide-directed triple helix formation suffer from important constraints involving requirements for stabilizing binding conditions, restrictions on permitted target sequences, and inefficient nuclear delivery of oligonucleotides. Implementation of oligonucleotide-directed triple helix formation as a viable approach to cancer therapy must therefore await clever solutions to a series of fascinating problems.

Inhibition of Replication Initiation by Triple Helix-Forming Oligonucleotides

Oligonucleotide-directed triple helix formation constitutes a new approach to block gene expression via transcription inhibition. In addition triple helices might inhibit replication. We have examined the capacity of triple helix-forming oligonucleotides to inhibit the initiation of replication on a single stranded DNA template using T7 DNA polymerase (Sequenase). We show that triple helix formation at the primer initiation site efficiently inhibits DNA polymerization, by preventing binding of the polymerase. The effect is dependent on the distance between the 3′-end of the primer and the triple helix boundary. Inhibition becomes ineffective when this distance is greater than 3 nucleotides. The presence of three base-pairs outside the triple-helical region on the 3′-side of the primer is therefore sufficient to allow for initiation of DNA replication.

Competitive Triplex/Quadruplex Equilibria Involving Guanine-Rich Oligonucleotides

Oligonucleotide-directed triple helix formation in the purine motif involves the binding of guanine-rich oligonucleotides to duplex DNA. Although this approach has been proposed for in vivo gene inhibition, triple helix formation by guanine-rich oligonucleotides is severely inhibited by physiological concentrations of certain monovalent cations (M+), especially K+. To clarify the mechanism of this inhibition, electrophoretic gel mobility shift titrations were performed to analyze the formation and stability of a purine motif triple helix in the presence of M+ and to monitor oligonucleotide aggregation under these conditions. M+ inhibition of triplex formation exhibited a concentration and ionic radius dependence that correlates with the ability of M+ to stabilize guanine quartet structures. In the presence of inhibitory [M+], guanine-rich oligonucleotides formed aggregates having characteristics consistent with the involvement of guanine quartets. The inhibitory effects of K+ on triplex formation could not be reversed by addition of the physiological polyamines spermidine3+ or spermine4+. M+ reduced the equilibrium concentration of the triplex primarily by decreasing the rate of triplex formation, but M+ also caused a detectable increase in the rate of triplex dissociation. Together, these results suggest that triplex inhibition under physiological ionic conditions is caused by competing equilibria wherein guanine-rich oligonucleotides form aggregates involving guanine quartets. Approaches to destabilizing aggregates of guanine-rich oligonucleotides under physiological conditions will be required before in vivo applications can be realistically considered.

Overcoming potassium-mediated triplex inhibition.

Sequence-specific duplex DNA recognition by oligonucleotide-directed triple helix formation is a possible approach to in vivo gene inhibition. However, triple helix formation involving guanine-rich oligonucleotides is inhibited by physiological ions, particularly K+, most likely due to oligonucleotide aggregation via guanine quartets. Three oligodeoxynucleotide (ODN) derivatives were tested for their ability to resist guanine quartet-mediated aggregation, yet form stable triplexes. Electrophoretic mobility shift and dimethyl sulfate footprinting assays were used to analyze the formation of triplexes involving these oligonucleotide derivatives. In the absence of K+, all ODNs had similar binding affinities for the duplex target. Triplexes involving a 14mer ODN derivative containing 7-deazaxanthine substituted for three thymine bases or an 18mer ODN containing two additional thymines on both the 5' and 3' termini were abolished by 50 mM K+. Remarkably, triplexes involving an ODN derivative containing four 6-thioguanine bases substituted for guanine resisted K+ inhibition up to 200 mM. We hypothesize that the increased radius and decreased electronegativity of sulfur in the 6-position of guanine destabilize potential guanine quartets. These results improve the prospects for creating ODNS that might serve as specific and efficient gene repressors in vivo.

Temperature Dependence of the Energetics of Oligonucleotide-Directed Triple-Helix Formation at a Single DNA Site

The influence of temperature on the energetics of oligonucleotide-directed triple-helix formation has been investigated in mixed valence salt solutions at pH 7.0. Equilibrium constants for formation of the local pyrimidine.purine.pyrimidine structure afforded by binding of the oligonucleotide 5'-d(T*TTTTCTCTCTCTCT)-3' to a single 15-bp site within a 339-bp plasmid fragment were measured using quantitative affinity cleavage titrations between 8 and 37 °C. In three different solutions buffered by 10 mM Bis-Tris at pH 7.0, BTNS 100 mM NaCl and 1 mM spermine tetrahydrochloride (SpmCl_4)], BTP1 (10 mM NaCl, 140 mM KCl, 1 mM MgCl_2, and 1 mM SpmCl_4), and BTP4 (10 mM NaCl, 140 mM KCl, 1 mM MgCl_2, and 4 mM SpmCl_4), the equilibrium association constants decreased at least 100-fold (from > 10^7 M^(-1) to ca. 10^5 M^(-1)) as the temperature was increased from 8 to 37 °C. Least squares analysis of van't Hoff plots (In K versus 1/T) of the data revealed that in each solution the tripler is enthalpically stabilized by ca. 2 kcal per mol of base triplets. This average value for single-site tripler formation on large DNA at pH 7.0 in mixed valence salt solutions, which reflects contributions from both T•AT and C+GC base triplets, is consistent with those reported previously for a number of oligonucleotide triplexes in solutions containing single cationic species and in excellent agreement with the calorimetrically determined enthalpy for binding of this 15mer to a 21-bp oligonucleotide DNA duplex.

Phosphorothioate oligonucleotide-directed triple helix formation.

Phosphorothioate oligodeoxyribonucleotides were tested for their ability to recognize double-helical DNA in two distinct triple helix motifs. Purine-rich oligonucleotides containing a diastereomeric mixture of phosphorothioate or stereoregular (all RP) phosphorothioate linkages are shown to form triple-helical complexes with affinities similar to those of the corresponding natural phosphodiester oligonucleotides. In contrast, pyrimidine-rich phosphorothioate oligonucleotides containing a mixture of diastereomeric or stereoregular (all RP) linkages do not bind to double-helical DNA with measurable affinity. These observations have implications for triple helix structure and for biological applications.

Inhibition of Klenow fragment DNA polymerase on double-helical templates by oligonucleotide-directed triple-helix formation.

We have examined the capacity of oligonucleotide-directed triple helices to block the progress of primer extension by DNA polymerase. Occupancy of the major groove of a double-helical DNA substrate obstructed Klenow fragment progress at sites that map near the proximal boundary between duplex and triplex. Among a family of related third-strand oligonucleotides that all stably occupied the target duplex in the absence of polymerase, those forming longer triplexes were more effective polymerase inhibitors than shorter complexes. Kinetic analysis revealed that the triple-helical complex provided an effective blockade for times of at least 20 min. These observations provide the basis for considering and further dissecting repair DNA polymerase function and mechanism by using various defined local three-stranded DNA structures as probes.