Double duplex invasion by peptide nucleic acid: a general principle for sequence-specific targeting of double-stranded DNA

The impact of nucleic acid secondary structure on PNA hybridization

Hybridization of oligonucleotides and their analogues to complementary DNA or RNA sequences is complicated by the presence of secondary and tertiary structure in the target. In particular, folding of the target nucleic acid imposes substantial thermodynamic penalties to hybridization. Slower kinetics for hybridization can also be observed, relative to an unstructured target. The development of high affinity oligonucleotide analogues such as peptide nucleic acid (PNA) can compensate for the thermodynamic and kinetic barriers to hybridization. Examples of structured targets successfully hybridized by PNA oligomers include DNA duplexes, DNA hairpins, DNA quadruplexes and an RNA hairpin embedded within a mRNA.

Cellular delivery of peptide nucleic acid (PNA)

Peptide nucleic acid (PNA) is a DNA mimic having a pseudopeptide backbone that makes it extremely stable in biological fluids. PNA binds complementary RNA and DNA with high affinity and specificity. These qualities make PNA a leading agent among "third generation" antisense and antigene agents. Unfortunately, fast progress in the exploration of PNA as an experimental and therapeutical regulator of gene expression has been hampered by the poor cellular uptake of PNA. However, a number of transfection protocols for PNA have now been established. These include microinjection, electroporation, co-transfection with DNA, conjugation to lipophilic moieties, conjugation to peptides, etc. Here we give a short introduction to the basic findings on PNA as an antisense and antigene agent in cell-free in vitro systems. This is followed by a comprehensive evaluation of the most interesting literature concerning cellular delivery and the intracellular effect of PNA. Also the current progress as regards using PNA as co-factor in DNA delivery is reviewed.

High-affinity peptide nucleic acid oligomers containing tricyclic cytosine analogues

[structure: see text] Peptide nucleic acid (PNA) monomers containing the tricyclic cytosine analogues phenoxazine, 9-(2-aminoethoxy)phenoxazine (G-clamp), and 9-(3-aminopropoxy)phenoxazine (propyl-G-clamp) have been synthesized. The modified nucleobases were incorporated into PNA oligomers using Boc-chemistry for solid-phase synthesis. PNAs containing single G-clamp modifications exhibit significantly enhanced affinity toward RNA and DNA targets relative to unmodified PNA while maintaining mismatch discrimination. These PNA G-clamp modifications exhibit the highest increase in affinity toward nucleic acid targets reported so far for PNA modifications.

Protein-cleaving catalyst selective for protein substrate

A protein-cleaving catalyst specific for a disease-related protein can be used as a catalytic drug. As the first protein-cleaving catalyst selective for a protein substrate, a catalyst for myoglobin was designed by attaching Cu(II) or Co(III) complex of cyclen to a binding site searched by a combinatorial method using peptide nucleic acid monomers as building units. [reaction: see text]

Tailoring the activity of restriction endonuclease PleI by PNA-induced DNA looping

DNA looping is one of the key factors allowing proteins bound to different DNA sites to signal one another via direct contacts. We demonstrate that DNA looping can be generated in an arbitrary chosen site by sequence-directed targeting of double-stranded DNA with pseudocomplementary peptide-nucleic acids (pcPNAs). We designed pcPNAs to mask the DNA from cleavage by type IIs restriction enzyme PleI while not preventing the enzyme from binding to its primary DNA recognition site. Direct interaction between two protein molecules (one bound to the original recognition site and the other to a sequence-degenerated site) results in a totally new activity of PleI: it produces a nick near the degenerate site. The PNA-induced nicking efficiency varies with the distance between the two protein-binding sites in a phase with the DNA helical periodicity. Our findings imply a general approach for the fine-tuning of proteins bound to DNA sites well separated along the DNA chain.

Kinetics and mechanism of the DNA double helix invasion by pseudocomplementary peptide nucleic acids

If adenines and thymines in two mutually complementary mixed-base peptide nucleic acid (PNA) oligomers are substituted with diaminopurines and thiouracils, respectively, so-called pseudocomplementary PNAs (pcPNAs) are created. Pairs of pcPNAs have recently demonstrated an ability to highly selectively target essentially any designated site on double-stranded DNA (dsDNA) by forming very stable PNA-DNA strand-displacement complexes via double duplex invasion (helix invasion). These properties of pcPNAs make them unique and very promising ligands capable of denying the access of DNA-binding proteins to dsDNA. To elucidate the sequence-unrestricted mechanism of sequence-specific dsDNA recognition by pcPNAs, we have studied the kinetics of formation of corresponding PNA-DNA complexes at various temperatures by the gel-shift assay. In parallel, the conditions for possible self-hybridization of pcPNA oligomers have been assayed by mixing curve (Job plot) and thermal melting experiments. The data indicate that, at physiological temperatures ( approximately 37 degrees C), the equilibrium is shifted toward the pairing of corresponding pcPNAs with each other. This finding explains a linear concentration dependence, within the submicromolar range, of the pcPNA invasion rate into dsDNA at 37 degrees C. At elevated temperatures (>50 degrees C), the rather unstable pcPNA duplexes dissociate, yielding the expected quadratic dependence for the rate of pcPNA invasion on the PNA concentration. The polycationic character of pcPNA pairs, carrying the duplicated number of protonated terminal PNA residues commonly used to increase the PNA solubility and binding affinity, also explains the self-inhibition of pcPNA invasion observed at higher PNA concentrations. Melting of pcPNA duplexes occurs with the integral transition enthalpies ranged from -235 to -280 kJ.mol(-1), contributing to an anomalously high activation energy of approximately 150 kJ.mol(-1) found for the helix invasion of pcPNAs carrying four different nucleobases. A simplified kinetic model for pcPNAs helix invasion is proposed that interprets all unusual features of pcPNAs binding to dsDNA. Our findings have important implications for rational use of pcPNAs.

Substituted 1,8-naphthyridin-2(1H)-ones are superior to thymine in the recognition of adenine in duplex as well as triplex structures

The synthesis and evaluation of a series of novel nucleobases based on substituted 1,8-naphthyridin-2(1H)-ones are reported. The nucleobases were designed to meet the requirements for incorporation into peptide nucleic acids (PNAs) and were evaluated as part of PNA duplex and triplex nucleic acid recognition systems. Of the various nucleobases tested, only the 7-chloro-1,8-naphthyridin-2(1H)-one (7-Cl-bT) nucleobase led to consistently increased affinity in all recognition systems, duplex (Watson-Crick) as well as triplex (Hoogsteen). For multiply modified systems, the increase in thermal stability per modification was dependent on the sequence context, ranging from 2.0 degrees C (in separate positions) to 3.5 degrees C (in adjacent positions) in PNA-DNA duplexes and from 1.2 degrees C (in separate positions) to 3.2 degrees C (in adjacent positions) in PNA-RNA duplexes. Singly mismatched oligonucleotide targets were employed to demonstrate uncompromised sequence discrimination. When part of multiply modified triplex (Hoogsteen) recognition systems, the 7-Cl-bT unit gave rise to increases in the thermal stability ranging from 2.7 to 3.5 degrees C when incorporated into separated and adjacent positions, respectively. Our results furthermore indicate that the duplex stabilization is predominantly enthalpic and therefore most likely not a consequence of single-strand preorganization. Finally, and most surprisingly, we find no direct correlation between the end-stacking efficiency of this type of nucleobase and its helix stabilization when involved in Watson-Crick base pairing within a helix.

DNA and its associated processes as targets for cancer therapy

DNA is the molecular target for many of the drugs that are used in cancer therapeutics, and is viewed as a non-specific target of cytotoxic agents. Although this is true for traditional chemotherapeutics, other agents that were discovered more recently have shown enhanced efficacy. Furthermore, a new generation of agents that target DNA-associated processes are anticipated to be far more specific and effective. How have these agents evolved, and what are their molecular targets?

Hybridization of DNA and PNA molecular beacons to single-stranded and double-stranded DNA targets

Molecular beacons are sensitive fluorescent probes hybridizing selectively to designated DNA and RNA targets. They have recently become practical tools for quantitative real-time monitoring of single-stranded nucleic acids. Here, we comparatively study the performance of a variety of such probes, stemless and stem-containing DNA and PNA (peptide nucleic acid) beacons, in Tris-buffer solutions containing various concentrations of NaCl and MgCl(2). We demonstrate that different molecular beacons respond differently to the change of salt concentration, which could be attributed to the differences in their backbones and constructions. We have found that the stemless PNA beacon hybridizes rapidly to the complementary oligodeoxynucleotide and is less sensitive than the DNA beacons to the change of salt thus allowing effective detection of nucleic acid targets under various conditions. Though we found stemless DNA beacons improper for diagnostic purposes due to high background fluorescence, we believe that use of these DNA and similar RNA constructs in molecular-biophysical studies may be helpful for analysis of conformational flexibility of single-stranded nucleic acids. With the aid of PNA "openers", molecular beacons were employed for the detection of a chosen target sequence directly in double-stranded DNA (dsDNA). Conditions are found where the stemless PNA beacon strongly discriminates the complementary versus mismatched dsDNA targets. Together with the insensitivity of PNA beacons to the presence of salt and DNA-binding/processing proteins, the latter results demonstrate the potential of these probes as robust tools for recognition of specific sequences within dsDNA without denaturation and deproteinization of duplex DNA.

Hybridization of PNA to structured DNA targets: quadruplex invasion and the overhang effect

Peptide nucleic acid (PNA) probes have been synthesized and targeted to quadruplex DNA. UV-vis and CD spectroscopy reveal that the quadruplex structure of the thrombin binding aptamer (TBA) is disrupted at 37 degrees C by a short PNA probe. The corresponding DNA probe fails to bind to the stable secondary structure at this temperature. Thermal denaturation experiments indicate surprisingly high thermal and thermodynamic stabilities for the PNA-TBA hybrid. Our results point to the nonbonded nucleobase overhangs on the DNA as being responsible for this stability. This "overhang effect" is found for two different PNA-DNA sequences and a variety of different overhang lengths and sequences. The stabilization offered by the overhangs assists the PNA in overcoming the stable secondary structure of the DNA target, an effect which may be significant in the targeting of biological nucleic acids, which will always be much longer than the PNA probe. The ability of PNA to invade a structured DNA target expands its potential utility as an antigene agent or hybridization probe.

Synthesis and monitored selection of nucleotide surrogates for binding T:A base pairs in homopurine-homopyrimidine DNA triple helices

A total of 16 oligodeoxyribonucleotides of general sequence 5'-TCTTCTZTCTTTCT-3', where Z denotes an N-acyl-N-(2-hydroxyethyl)glycine residue, were prepared via solid phase synthesis. The ability of these oligonucleotides to form triplexes with the duplex 5'-AGAAGATAGAAAGA-HEG-TCTTTCTATCTTCT-3', where HEG is a hexaethylene glycol linker, was tested. In these triplexes, an 'interrupting' T:A base pair faces the Z residue in the third strand. Among the acyl moieties of Z tested, an anthraquinone carboxylic acid residue linked via a glycinyl group gave the most stable triplex, whose UV melting point was 8.4 degrees C higher than that of the triplex with 5'-TCTTCTGTCTTTCT-3' as the third strand. The results from exploratory nuclease selection experiments suggest that a combinatorial search for strands capable of recognizing mixed sequences by triple helix formation is feasible.

Hybridization between oxy-peptide nucleic acids and DNAs: dependence of hybrid stabilities on the chain-lengths, types of base pairs, and the chain directions

Oxy-peptide nucleic acids (OPNAs) of [-NH-CH(CH2-CH2-Base)-CH2-O-CH2-CO-]-type main chain with four different types of nucleobases (Base = A, G, C, and U) or with an abasic side group (X) were synthesized. Melting curves of the 1:1 hybrids of o(A(n))-d(Tn)) pairs with n = 6, 9, 12, and 15 showed very sharp transitions at high Tm values, particularly for long chains, indicating that nearly optimum matching is attained in the structure of the o(A(n))-d(Tn) hybrids. Effect of different types of base pairs on the hybrid stabilities was examined for the o(A4NA4)-d(T4N'T4) 1:1 mixtures where N is A, G, C, U, or X and N' is A, G, C, or T. In all series of the hybrids the complementary pairs showed the highest Tm values. The Tm values of the complementary pairs were about 35 degrees C when purine bases were inserted as the N group in the OPNA, but they were 20-23 degrees C when pyrimidine bases were inserted. The melting curves of the hybrids with a single mismatch were similar to those with a single X-N' pair, suggesting that the mismatch base pairs have been ignored in the hybrids. All complementary OPNA-DNA hybrids showed higher Tm values and sharper transitions than the corresponding DNA-DNA hybrids. The OPNA-DNA hybrids favor a parallel direction i.e., the N-terminal of OPNA is directed to the 5'-terminal of DNA.

An artificial primosome: design, function, and applications

Double-stranded (ds) DNA is capable of the sequence-specific accommodation of an additional oligodeoxyribonucleotide strand by the peptide nucleic acid(PNA)-assisted formation of a so-called PD-loop. We demonstrate here that the PD-loop may function as an artificial primosome within linear, nonsupercoiled DNA duplexes. DNA polymerase with its strand displacement activity uses this construct to initiate the primer extension reaction at a designated dsDNA site. The primer is extended by several hundred nucleotides. The efficiency of dsDNA priming by the artificial primosome assembly is comparable to the single-stranded DNA priming used in various assays. The ability of the PD-loop structure to perform like an artificial primosome on linear dsDNA may find applications in biochemistry, molecular biology, and molecular biotechnology, as well as for DNA diagnostics. In particular, multiple labels can be incorporated into a chosen dsDNA site resulting in ultrasensitive direct quantification of specific sequences. Furthermore, nondenaturing dsDNA sequencing proceeds from the PD-loop. This approach opens the way to direct isothermal reading of the DNA sequence against a background of unrelated DNA, thereby eliminating the need for purification of the target DNA.

Peptide nucleic acids as antibacterial agents via the antisense principle

Peptide nucleic acid (PNA) is a peptide-like DNA mimic that was introduced almost ten years ago. It was immediately predicted that PNA would have a bright future in gene therapeutic drug development, but progress in this direction has been rather modest thus far. This is predominantly due to inefficient uptake of PNA by most living cells. However, within the past couple of years a variety of methods have been devised to address this problem and the stage should now be set for more rapid progress. Several studies have demonstrated antisense effects ex vivo in cells in culture and two reports on direct injection of PNA into the brain of rats are also interesting. Only a few studies have addressed the possible exploitation of the antisense principle for development of antibacterial drugs. However, the first in vitro results using antiribosomal RNA PNAs and antisense PNAs targeted to the beta-lactamase gene on Escherichia coli cultures were quite promising. Most recently, these preliminary studies have been extended to demonstrate in vivo efficacy of antibacterial PNAs in an E. coli peritonitis/sepsis mouse model. Therefore, PNA drug development again is rapidly picking up pace.

Sequence-specific targeting of duplex DNA by peptide nucleic acids via triplex strand invasion

Because of a set of exceptional chemical, physical, and biological properties, polyamide or peptide nucleic acids (PNAs) hold a distinctive position among various synthetic ligands designed for DNA-targeting purposes. Cationic pyrimidine PNAs (cpyPNAs) represent a special group of PNAs, which effectively form strand invasion triplexes with double-stranded DNA (dsDNA) also known as P-loops. Extraordinary stability of the invasion triplexes and high sequence specificity of their formation combined with local opening of the DNA double helix within the P-loops make these complexes very attractive for sequence-specific manipulation with dsDNA. Important for applications is the fact that the discrimination between correct and mismatched binding sites in dsDNA by cpyPNAs is a nonequilibrium, kinetically controlled process. Therefore, a careful choice of experimental conditions that are optimal for the kinetic discrimination of correct versus mismatched cpyPNA binding is crucial for sequence-specific recognition of dsDNA by cpyPNAs. The experimental and theoretical data presented make it possible to select those solution parameters and cpyPNA constructions that are most favorable for sequence specificity without compromising the affinity of dsDNA targeting.

Peptide nucleic acid-assisted topological labeling of duplex dna

Peptide nucleic acids (PNAs) are a family of synthetic polyamide mimics of nucleic acids that offer a variety of applications. Pyrimidine bis-PNAs can be used for rational design of novel interlocked DNA nanostructures, earring labels, representing locked pseudorotaxanes or locked catenanes. These structures are created through DNA ligase-mediated catenation of duplex DNA with a circularized oligonucleotide tag at a designated DNA site. The assembly is performed via formation of the PD-loop consisting of a pair of bis-PNA openers and the probe oligonucleotide. The openers locally expose one of the two strands of duplex DNA for hybridizing the probe, whose termini are complementary to the displaced DNA strand. After hybridization, they are in juxtaposition and can subsequently be linked by DNA ligase. As a result, a true topological link forms at a precise position on the DNA double helix yielding locked, earring-like label. DNA topological labeling can be done both in solution and, for longer templates, within the agarose gel plug. Accordingly, highly localized DNA detection with rolling circle amplification of hybridization signal and effective micromanipulations with DNA duplexes become possible through precise spatial positioning of various ligands on the DNA scaffold.

Peptide nucleic acid: a versatile tool in genetic diagnostics and molecular biology

During the past ten years, the DNA mimic peptide nucleic acid has inspired the development of a variety of hybridisation-based methods for detection, quantification, purification and characterisation of nucleic acids. Most of these methods have taken advantage of the very favourable DNA and RNA hybridisation properties of peptide nucleic acids combined with the unique properties and opportunities offered by peptide chemistry. Within the past year, significant progress in in situ hybridisation technology has been achieved, which has resulted, in particular, in reliable and sensitive methods for detection of bacteria in clinical samples, as well as in environmental samples. Furthermore, applications of the polymerase chain reaction clamping method have been expanded, and novel ways of exploiting complexes of peptide nucleic acids with double-stranded DNA, such as double duplex invasion complexes and PD loops, have been developed.

Cyanuryl-PNA monomer: synthesis and crystal structure

[structure: see text] The chemical synthesis and crystal structure of the peptide nucleic acid (PNA) monomer 11 having cyanuric acid as the nucleobase is reported. The crystal structure of 11 shows molecular tapes arising from continuous intermolecular dimeric hydrogen bonding, with successive tapes held by single hydrogen bonds in the backbone.

Strand invasion by mixed base PNAs and a PNA-peptide chimera

Peptide nucleic acid oligomers (PNAs) have a remarkable ability to invade duplex DNA at polypurine-polypyrimidine target sequences. Applications for PNAs in medicine and biotechnology would increase if the rules governing their hybridization to mixed base sequences were also clear. Here we describe hybridization of PNAs to mixed base sequences and demonstrate that simple chemical modifications can enhance recognition. Easily synthesized and readily soluble eight and 10 base PNAs bind to plasmid DNA at an inverted repeat that is likely to form a cruciform structure, providing convenient tags for creating PNA-plasmid complexes. PNAs also bind to mixed base sequences that cannot form cruciforms, suggesting that recognition is a general phenomenon. Rates of strand invasion are temperature dependent and can be enhanced by attaching PNAs to positively charged peptides. Our results support use of PNAs to access the information within duplex DNA and demonstrate that simple chemical modifications can make PNAs even more powerful agents for strand invasion. Simple strategies for enhancing strand invasion should facilitate the use of PNAs: (i) as biophysical probes of double-stranded DNA; (ii) to target promoters to control gene expression; and (iii) to direct sequence-specific mutagenesis.

Synthesis and properties of 2'-O-methyl-2-thiouridine and oligoribonucleotides containing 2'-O-methyl-2-thiouridine

A new method for the synthesis of 2'-O-methyl-2-thiouridine (s2Um) found in thermophilic bacterial tRNA was developed. Structural properties of s2Um and s2Um(p)U were studied by using 1H NMR spectroscopy. A modified nonaribonucleotide (RNA*: 5'-CGUUs2UmUUGC-3') was synthesized to study the base-recognition ability of s2Um in formation of RNA-RNA and RNA DNA duplexes. The UV melting experiments revealed that RNA*-RNA and RNA*-DNA duplexes having an s2U-A base pair are more stable than those having a U-A base pair. On the contrary, the thermal stability of RNA*-RNA and RNA*-DNA duplexes having an s2U-G wobble base pair was much lower than that of the unmodified duplexes having a natural U-G base pair. It is concluded that s2Um has higher selectivity toward A over G than unmodified U.

Chemical approaches to control gene expression

A current goal in molecular medicine is the development of new strategies to interfere with gene expression in living cells in the hope that novel therapies for human disease will result from these efforts. This review focuses on small-molecule or chemical approaches to manipulate gene expression by modulating either transcription of messenger RNA-coding genes or protein translation. The molecules under study include natural products, designed ligands, and compounds identified through functional screens of combinatorial libraries. The cellular targets for these molecules include DNA, messenger RNA, and the protein components of the transcription, RNA processing, and translational machinery. Studies with model systems have shown promise in the inhibition of both cellular and viral gene transcription and mRNA utilization. Moreover, strategies for both repression and activation of gene transcription have been described. These studies offer promise for treatment of diseases of pathogenic (viral, bacterial, etc.) and cellular origin (cancer, genetic diseases, etc.).