PD-loop: a complex of duplex DNA with an oligonucleotide

Peptide nucleic acids: Recent advancements and future opportunities in biomedical applications

Peptide nucleic acids (PNA), synthetic molecules comprising a peptide-like backbone and natural and unnatural nucleobases, have garnered significant attention for their potential applications in gene editing and other biomedical fields. The unique properties of PNA, particularly enhanced stability/specificity/affinity towards targeted DNA and RNA sequences, achieved significant attention recently for gene silencing, gene correction, antisense therapy, drug delivery, biosensing and other various diagnostic aspects. This review explores the structure, properties, and potential of PNA in transforming genetic engineering including potent biomedical challenges. In Addition, we explore future perspectives and potential limitations of PNA-based technologies, highlighting the need for further research and development to fully realize their therapeutic and biotechnological potential.

Artificial genetic polymers against human pathologies

Originally discovered by Nielsen in 1991, peptide nucleic acids and other artificial genetic polymers have gained a lot of interest from the scientific community. Due to their unique biophysical features these artificial hybrid polymers are now being employed in various areas of theranostics (therapy and diagnostics). The current review provides an overview of their structure, principles of rational design, and biophysical features as well as highlights the areas of their successful implementation in biology and biomedicine. Finally, the review discusses the areas of improvement that would allow their use as a new class of therapeutics in the future.

Chemical approaches to discover the full potential of peptide nucleic acids in biomedical applications

Peptide nucleic acid (PNA) is arguably one of the most successful DNA mimics, despite a most dramatic departure from the native structure of DNA. The present review summarizes 30 years of research on PNA's chemistry, optimization of structure and function, applications as probes and diagnostics, and attempts to develop new PNA therapeutics. The discussion starts with a brief review of PNA's binding modes and structural features, followed by the most impactful chemical modifications, PNA enabled assays and diagnostics, and discussion of the current state of development of PNA therapeutics. While many modifications have improved on PNA's binding affinity and specificity, solubility and other biophysical properties, the original PNA is still most frequently used in diagnostic and other in vitro applications. Development of therapeutics and other in vivo applications of PNA has notably lagged behind and is still limited by insufficient bioavailability and difficulties with tissue specific delivery. Relatively high doses are required to overcome poor cellular uptake and endosomal entrapment, which increases the risk of toxicity. These limitations remain unsolved problems waiting for innovative chemistry and biology to unlock the full potential of PNA in biomedical applications.

Modulation of DNA and RNA by PNA

Peptide nucleic acid (PNA), a synthetic DNA mimic that is devoid of the (deoxy)ribose-phosphate backbone yet still perfectly retains the ability to recognize natural nucleic acids in a sequence-specific fashion, can be employed as a tool to modulate gene expressions via several different mechanisms. The unique strength of PNA compared to other oligonucleotide analogs is its ability to bind to nucleic acid targets with secondary structures such as double-stranded and quadruplex DNA as well as RNA. This digest aims to introduce general readers to the advancement in the area of modulation of DNA/RNA functions by PNA, its current status and future research opportunities, with emphasis on recent progress in new targeting modes of structured DNA/RNA by PNA and PNA-mediated gene editing.

An earring for the double helix: assembly of topological links comprising duplex DNA and a circular oligodeoxynucleotide

Abstract Novel DNA nanostructures, locked pseudorotaxane and locked catenane were assembled through topological linkage of a double-stranded target to a circular oligodeoxyribonucleotide (cODN)(+). The formation of these supramolecular complexes occurs with remarkable sequence specificity and is accomplished via local opening of duplex DNA by a pair of homopyrimidine bis-PNAs. The obtained cODN label, resembling an earring, forms a true topological link with the linear or closed circular (cc) target DNA and occupies a fixed position along the double helix. The PNA directed assembly described here introduces PNA oligomers into the repertoire of DNA nanotechnological tools.

Ultrasensitive detection of DNA and protein markers in cancer cells

Cancer cells differ from normal cells in various parameters, and these differences are caused by genomic mutations and consequential altered gene expression. The genetic and functional heterogeneity of tumor cells is a major challenge in cancer research, detection, and effective treatment. As such, the use of diagnostic methods is important to reveal this heterogeneity at the single-cell level. Droplet microfluidic devices are effective tools that provide exceptional sensitivity for analyzing single cells and molecules. In this review, we highlight two novel methods that employ droplet microfluidics for ultra-sensitive detection of nucleic acids and protein markers in cancer cells. We also discuss the future practical applications of these methods.

Visual detection of bacterial pathogens via PNA-based padlock probe assembly and isothermal amplification of DNAzymes

We have developed a self-reporting isothermal system for visual bacterial pathogen detection with single base resolution. The new DNA diagnostic is based on combination of peptide nucleic acid (PNA) technology, rolling circle amplification (RCA) and DNAzymes. PNAs are used as exceedingly selective chemical tools that bind genomic DNA at a predetermined sequence under nondenaturing conditions. After assembly of the PNA-DNA construct a padlock probe is circularized on the free strand. The probe incorporates a G-quadruplex structure flanked by nicking enzyme recognition sites. The assembled circle serves as a template for a novel hybrid RCA strategy that allows for exponential amplification and production of short single-stranded DNA pieces. These DNA fragments fold into G-quadruplex structures and when complexed with hemin become functional DNAzymes. The catalytic activity of each DNAzyme unit leads to colorimetric detection and provides the second amplification step. The combination of PNA, RCA, and DNAzymes allows for sequence-specific and highly sensitive detection of bacteria with a colorimetric output observed with the naked eye. Herein, we apply this method for the discrimination of Escherichia coli, Salmonella typhimurium, and Clostridium difficile genomes.

PNA-based microbial pathogen identification and resistance marker detection: An accurate, isothermal rapid assay based on genome-specific features

With the rapidly growing availability of the entire genome sequences of microbial pathogens, there is unmet need for increasingly sensitive systems to monitor the gene-specific markers for diagnosis of bacteremia that enables an earlier detection of causative agent and determination of drug resistance. To address these challenges, a novel FISH-type genomic sequence-based molecular technique is proposed that can identify bacteria and simultaneously detect antibiotic resistance markers for rapid and accurate testing of pathogens. The approach is based on a synergistic combination of advanced Peptide Nucleic Acid (PNA)-based technology and signal-enhancing Rolling Circle Amplification (RCA) reaction to achieve a highly specific and sensitive assay. A specific PNA-DNA construct serves as an exceedingly selective and very effective biomarker, while RCA enhances detection sensitivity and provide with a highly multiplexed assay system. Distinct-color fluorescent decorator probes are used to identify about 20-nucleotide-long signature sequences in bacterial genomic DNA and/or key genetic markers of drug resistance in order to identify and characterize various pathogens. The technique's potential and its utility for clinical diagnostics are illustrated by identification of S. aureus with simultaneous discrimination of methicillin-sensitive (MSSA) versus methicillin-resistant (MRSA) strains. Overall these promising results hint to the adoption of PNA-based rapid sensitive detection for diagnosis of other clinically relevant organisms. Thereby, new assay enables significantly earlier administration of appropriate antimicrobial therapy and may, thus have a positive impact on the outcome of the patient.

Sequence specificity at targeting double-stranded DNA with a γ-PNA oligomer modified with guanidinium G-clamp nucleobases

γ-PNA, a new class of peptide nucleic acids, promises to overcome previous sequence limitations of double-stranded DNA (dsDNA) targeting with PNA. To check the potential of γ-PNA, we have synthesized a biotinylated, pentadecameric γ-PNA of mixed sequence carrying three guanidinium G-clamp nucleobases. We have found that strand invasion reactions of the γ-PNA oligomer to its fully complementary target within dsDNA occurs with significantly higher binding rates than to targets containing single mismatches. Association of the PNA oligomer to mismatched targets does not go to completion but instead reaches a stationary level at or below 60%, even at conditions of very low ionic strength. Initial binding rates to both matched and mismatched targets experience a steep decrease with increasing salt concentration. We demonstrate that a linear DNA target fragment with the correct target sequence can be purified from DNA mixtures containing mismatched target or unrelated genomic DNA by affinity capture with streptavidin-coated magnetic beads. Similarly, supercoiled plasmid DNA is obtained with high purity from an initial sample mixture that included a linear DNA fragment with the fully complementary sequence. Based on the results obtained in this study we believe that γ-PNA has a great potential for specific targeting of chosen duplex DNA sites in a sequence-unrestricted fashion.

PNA openers and their applications for bacterial DNA diagnostics

The unique ability of triplex-forming PNAs to invade the double helix has made it possible to develop a highly specific and sensitive approach for bacterial detection. The method uses short, about 20-bp-long, signature sequences presented as a single copy in the bacterial genome. Bacterial cells are fixed on slides and the PD-loop structure is assembled on the signature site with the help of PNA openers, which includes the circular probe. The sensitivity of the method is achieved via Rolling Circle Amplification (RCA) of the circular probe. The obtained amplicon is detected using short ssDNA decorator probes carrying fluorophores and via standard fluorescent microscopy.

PPG peptide nucleic acids that promote DNA guanine quadruplexes

Recent studies have shown that guanine-rich (G-rich) sequences with the potential to form quadruplexes might play a role in normal transcription as well as overexpression of oncogenes. Chemical tools that allow examination of the specific roles of G-quadruplex formation in vivo, and their association with gene regulation will be essential to understanding the functions of these quadruplexes and might lead to beneficial therapies. Properly designed peptide nucleic acids (PNAs) can invade G-rich DNA duplexes and induce the formation of a G-quadruplex in the free DNA strand. Replacing guanines in the PNA sequence with pyrazolo[3,4-d]pyrimidine guanine (PPG) nucleobases eliminates G-quadruplex formation with PNA and promotes invasion of the target DNA.

Application of PNA openers for fluorescence-based detection of bacterial DNA

Peptide nucleic acid (PNA) openers have the unique ability to invade double-stranded DNA with high efficiency and sequence specificity, making it possible to detect short (about 20 bp), single-copy bacterial DNA sequences. PNA openers bind to a target signature site on one strand of bacterial DNA, leaving the other strand open for hybridization with a circularizable oligonucleotide probe. The assembled complex serves as a template for rolling circle amplification. The obtained amplicon is decorated with short, single-stranded DNA probes carrying fluorophores and detected via fluorescence microscopy.

Direct DNA and PNA probe binding to telomeric regions without classical in situ hybridization

BACKGROUND

Fluorescence in situ Hybridization (FISH) utilizes peptide nucleic acid (PNA) probes to identify specific DNA sequences. Traditional techniques have required the heat denaturing of the DNA in formamide followed by multiple hours at moderated temperatures to allow the probe to hybridize to its specific target. Over the past 30 years, advancements in both protocols and probes have made FISH a more reliable technique for both biological research and medical diagnostics, additionally the protocol has been shortened to several minutes. These PNA probes were designed to target and hybridize to both DNA and RNA, and PNA-protein interactions still remain unclear.

RESULTS

In this study we have shown that a telomeric single stranded specific PNA probe is able to bind to its target without heat denaturing of the DNA and without formamide. We have also identified a centromere specific probe, which was found to bind its target with only incubation with formamide.

CONCLUSIONS

Certain PNA probes are able to hybridize with their targets with minimal to no denaturing of the DNA itself. This limited denaturing preserves the chromosome structure and may lead to more effective and specific staining.

Fluorescence imaging of single-copy DNA sequences within the human genome using PNA-directed padlock probe assembly

We present an approach for fluorescent in situ detection of short, single-copy sequences within genomic DNA in human cells. The single-copy sensitivity and single-base specificity of our method is achieved due to the combination of three components. First, a peptide nucleic acid (PNA) probe locally opens a chosen target site, which allows a padlock DNA probe to access the site and become ligated. Second, rolling circle amplification (RCA) generates thousands of single-stranded copies of the target sequence. Finally, fluorescent in situ hybridization (FISH) is used to visualize the amplified DNA. We validate this technique by successfully detecting six single-copy target sites on human mitochondrial and autosomal DNA. We also demonstrate the high selectivity of this method by detecting X- and Y-specific sequences on human sex chromosomes and by simultaneously detecting three sequence-specific target sites. Finally, we discriminate two target sites that differ by 2 nt. The PNA-RCA-FISH approach is a distinctive in situ hybridization method capable of multitarget visualization within human chromosomes and nuclei that does not require DNA denaturation and is extremely sequence specific.

Target DNA detection and quantitation on a single cell with single base resolution

In this report, we present a new method for sensitive detection of short DNA sites in single cells with single base resolution. The method combines peptide nucleic acid (PNA) openers as the tagging probes, together with isothermal rolling circle amplification (RCA) and fluorescence-based detection, all performed in a cells-in-flow format. Bis-PNAs provide single base resolution, while RCA ensures linear signal amplification. We applied this method to detect the oncoviral DNA inserts in cancer cell lines using a flow-cytometry system. We also demonstrated quantitative detection of the selected signature sites within single cells in microfluidic nano-liter droplets. Our results show single-nucleotide polymorphism (SNP) discrimination and detection of copy-number variations (CNV) under isothermal non-denaturing conditions. This new method is ideal for many applications in which ultra-sensitive DNA characterization with single base resolution is desired on the level of single cells.

Targeting DNA G-quadruplex structures with peptide nucleic acids

Regulation of genetic functions based on targeting DNA or RNA sequences with complementary oligonucleotides is especially attractive in the post-genome era. Oligonucleotides can be rationally designed to bind their targets based on simple nucleic acid base pairing rules. However, the use of natural DNA and RNA oligonucleotides as targeting probes can cause numerous off-target effects. In addition, natural nucleic acids are prone to degradation in vivo by various nucleases. To address these problems, nucleic acid mimics such as peptide nucleic acids (PNA) have been developed. They are more stable, show less off-target effects, and, in general, have better binding affinity to their targets. However, their high affinity to DNA can reduce their sequence-specificity. The formation of alternative DNA secondary structures, such as the G-quadruplex, provides an extra level of specificity as targets for PNA oligomers. PNA probes can target the loops of G-quadruplex, invade the core by forming PNA-DNA guanine-tetrads, or bind to the open bases on the complementary cytosine-rich strand. Not only could the development of such G-quadruplex-specific probes allow regulation of gene expression, but it will also provide a means to clarify the biological roles G-quadruplex structures may possess.

Precise engineering and visualization of signs and magnitudes of DNA writhe on the basis of PNA invasion

It is demonstrated that the right and left handedness of DNA supercoils can be engineered precisely and readily at the molecular level in vitro through utilization of the invading property of peptide nucleic acid.

Targeted gene modification of hematopoietic progenitor cells in mice following systemic administration of a PNA-peptide conjugate

Hematopoietic stem cell (HSC) gene therapy offers promise for the development of new treatments for a variety of hematologic disorders. However, efficient in vivo modification of HSCs has proved challenging, thus imposing constraints on the therapeutic potential of this approach. Herein, we provide a gene-targeting strategy that allows site-specific in vivo gene modification in the HSCs of mice. Through conjugation of a triplex-forming peptide nucleic acid (PNA) to the transport peptide, antennapedia (Antp), we achieved successful in vivo chromosomal genomic modification of hematopoietic progenitor cells, while still retaining intact differentiation capabilities. Following systemic administration of PNA-Antp conjugates, sequence-specific gene modification was observed in multiple somatic tissues as well as within multiple compartments of the hematopoietic system, including erythroid, myeloid, and lymphoid cell lineages. As a true functional measure of gene targeting in a long-term renewable HSC, we also demonstrate preserved genomic modification in the bone marrow and spleen of primary recipient mice following transplantation of bone marrow from PNA-Antp-treated donor mice. Our approach offers a minimally invasive alternative to ex vivo gene therapy, by eliminating the need for the complex steps of stem cell mobilization and harvesting, ex vivo manipulation, and transplantation of stem cells. Therefore, our approach may provide new options for individualized therapies in the treatment of monogenic hematologic diseases such as sickle cell anemia and thalassemia.

Quadruplex formation is necessary for stable PNA invasion into duplex DNA of BCL2 promoter region

Guanine-rich sequences are highly abundant in the human genome, especially in regulatory regions. Because guanine-rich sequences have the unique ability to form G-quadruplexes, these structures may play a role in the regulation of gene transcription. In previous studies, we demonstrated that formation of G-quadruplexes could be induced with peptide nucleic acids (PNAs). PNAs designed to bind the C-rich strand upstream of the human BCL2 gene promoted quadruplex formation in the complementary G-rich strand. However, the question whether G-quadruplex formation was essential for PNA invasion remained unanswered. In this study, we compared PNA invasion in the native and mutant, i.e. not forming G-quadruplex, BCL2 sequences and showed that G-quadruplex is required for effective PNA invasion into duplex DNA. This finding provides strong evidence for not only sequence-specific, but also quadruplex specific, gene targeting with PNA probes. In addition, we examined DNA-duplex invasion potential of PNAs of various charges. Using the gel shift assay, chemical probing and dimethyl sulfate (DMS) protection studies, we determined that uncharged zwitterionic PNA has the highest binding specificity while preserving efficient duplex invasion.

Advances in imaging RNA in plants

Increasing evidence shows that many RNAs are targeted to specific locations within cells, and that RNA-processing pathways occur in association with specific subcellular structures. Compartmentation of mRNA translation and RNA processing helps to assemble large RNA-protein complexes, while RNA targeting allows local protein synthesis and the asymmetric distribution of transcripts during cell polarisation. In plants, intercellular RNA trafficking also plays an additional role in plant development and pathogen defence. Methods that allow the visualisation of RNA sequences within a cellular context, and preferably at subcellular resolution, can help to answer important questions in plant cell and developmental biology. Here, we summarise the approaches currently available for localising RNA in vivo and address the specific limitations inherent with plant systems.

Stabilization of G-quadruplex in the BCL2 promoter region in double-stranded DNA by invading short PNAs

Numerous regulatory genes have G-rich regions that can potentially form quadruplex structures, possibly playing a role in transcription regulation. We studied a G-rich sequence in the BCL2 gene 176-bp upstream of the P1 promoter for G-quadruplex formation. Using circular dichroism (CD), thermal denaturation and dimethyl sulfate (DMS) footprinting, we found that a single-stranded oligonucleotide with the sequence of the BCL2 G-rich region forms a potassium-stabilized G-quadruplex. To study G-quadruplex formation in double-stranded DNA, the G-rich sequence of the BCL2 gene was inserted into plasmid DNA. We found that a G-quadruplex did not form in the insert at physiological conditions. To induce G-quadruplex formation, we used short peptide nucleic acids (PNAs) that bind to the complementary C-rich strand. We examined both short duplex-forming PNAs, complementary to the central part of the BCL2 gene, and triplex-forming bis-PNAs, complementary to sequences adjacent to the G-rich BCL2 region. Using a DMS protection assay, we demonstrated G-quadruplex formation within the G-rich sequence from the promoter region of the human BCL2 gene in plasmid DNA. Our results show that molecules binding the complementary C-strand facilitate G-quadruplex formation and introduce a new mode of PNA-mediated sequence-specific targeting.

Nanopore based sequence specific detection of duplex DNA for genomic profiling

We demonstrate a purely electrical method for the single-molecule detection of specific DNA sequences, achieved by hybridizing double-stranded DNA (dsDNA) with peptide nucleic acid (PNA) probes and electrophoretically threading the DNA through sub-5 nm silicon nitride pores. Bis-PNAs were used as the tagging probes in order to achieve high affinity and sequence specificity. Sequence detection is performed by reading the ion current traces of individual translocating DNA molecules, which display a characteristic secondary blockade level, absent in untagged molecules. The potential for barcoding DNA is demonstrated through nanopore analysis of once-tagged and twice-tagged DNA at different locations on the same genomic fragment. Our high-throughput, long-read length method can be used to identify key sequences embedded in individual DNA molecules, without the need for amplification or fluorescent/radio labeling. This opens up a wide range of possibilities in human genomics as well as in pathogen detection for fighting infectious diseases.

PNA-based microbial pathogen identification and resistance marker detection: an accurate, isothermal rapid assay based on genome-specific features

With the rapidly growing availability of the entire genome sequences of microbial pathogens, there is unmet need for increasingly sensitive systems to monitor the gene-specific markers for diagnosis of bacteremia that enables an earlier detection of causative agent and determination of drug resistance. To address these challenges, a novel FISH-type genomic sequence-based molecular technique is proposed that can identify bacteria and simultaneously detect antibiotic resistance markers for rapid and accurate testing of pathogens. The approach is based on a synergistic combination of advanced Peptide Nucleic Acid (PNA)-based technology and signal-enhancing Rolling Circle Amplification (RCA) reaction to achieve a highly specific and sensitive assay. A specific PNA-DNA construct serves as an exceedingly selective and very effective biomarker, while RCA enhances detection sensitivity and provide with a highly multiplexed assay system. Distinct-color fluorescent decorator probes are used to identify about 20-nucleotide-long signature sequences in bacterial genomic DNA and/or key genetic markers of drug resistance in order to identify and characterize various pathogens. The technique's potential and its utility for clinical diagnostics are illustrated by identification of S. aureus with simultaneous discrimination of methicillin-sensitive (MSSA) versus methicillin-resistant (MRSA) strains. Overall these promising results hint to the adoption of PNA-based rapid sensitive detection for diagnosis of other clinically relevant organisms. Thereby, new assay enables significantly earlier administration of appropriate antimicrobial therapy and may, thus have a positive impact on the outcome of the patient.

Peptide nucleic acid (PNA) binding and its effect on in vitro transcription in friedreich's ataxia triplet repeats

Peptide nucleic acids (PNAs) are DNA mimics in which peptide-like linkages are substituted for the phosphodiester backbone. Homopyrimidine PNAs can invade double-stranded DNA containing the homologous sequence by displacing the homopyrimidine strand from the DNA duplex and forming a PNA/DNA/PNA triplex with the complementary homopurine strand. Among biologically interesting targets for triplex-forming PNA are (GAA/CTT)(n) repeats. Expansion of these repeats results in partial inhibition of transcription in the frataxin gene, causing Friedreich's ataxia. We have studied PNA binding and its effect on T7 RNA polymerase transcription in vitro for short repeats (n = 3) and for long repeats (n = 39), placed in both possible orientations relative to the T7 promoter such that either the GAA-strand, or the CTT-strand serves as the template for transcription. In all cases PNA bound specifically and efficiently to its target sequence. For the short insert, PNA binding to the template strand caused partial transcription blockage with well-defined sites of RNA product truncation in the region of the PNA-binding sequence, whereas binding to the nontemplate strand did not block transcription. However, PNA binding to long repeats, whether in the template or the nontemplate strand, resulted in a dramatic reduction of the amount of full-length transcription product, although in the case of the nontemplate strand there were no predominant truncation sites. Biological implications of these results are discussed.

Silencing c-MYC expression by targeting quadruplex in P1 promoter using locked nucleic acid trap

The nuclease hypersensitive element of P1 promoter in c-MYC gene harbors a potential of unusual structure called quadruplex, which is involved in molecular recognition and function. This Hoogsteen bonded structure is in dynamic equilibrium with the usual Watson-Crick duplex structure, and these competing secondary structures undergo interconversion for execution of their respective biological roles. Herein, we investigate the sensitivity of the c-MYC quadruplex-duplex equilibrium by employing a locked nucleic acid (LNA) modified complementary strand as a pharmacological agent. Our biophysical experiments indicate that the c-MYC quadruplex under physiological conditions is stable and dominates the quadruplex-WC duplex equilibrium in both sodium and potassium buffers. This equilibrium is perturbed upon introducing the LNA modified complementary strand, which demonstrates efficient invasion of stable c-MYC quadruplex and duplex formation in contrast to the unmodified complementary strand. Our data indicate that LNA modifications confer increased thermodynamic stability to the duplex and thus favor the predominance of the duplex population over that of the quadruplex. Further, we demonstrate that this perturbation of equilibrium by a pharmacological agent results in altered gene expression. Our in vivo experiment performed using the LNA modified complementary strand suggests the influence of the quadruplex-duplex structural switch in the modulation of gene expression. We believe that this exploratory approach utilizing the selectivity and specificity of Watson-Crick base pairing of LNA bases would allow the modulation of quadruplex regulated gene expression.

Labeling of unique sequences in double-stranded DNA at sites of vicinal nicks generated by nicking endonucleases

We describe a new approach for labeling of unique sequences within dsDNA under nondenaturing conditions. The method is based on the site-specific formation of vicinal nicks, which are created by nicking endonucleases (NEases) at specified DNA sites on the same strand within dsDNA. The oligomeric segment flanked by both nicks is then substituted, in a strand displacement reaction, by an oligonucleotide probe that becomes covalently attached to the target site upon subsequent ligation. Monitoring probe hybridization and ligation reactions by electrophoretic mobility retardation assay, we show that selected target sites can be quantitatively labeled with excellent sequence specificity. In these experiments, predominantly probes carrying a target-independent 3′ terminal sequence were employed. At target labeling, thus a branched DNA structure known as 3′-flap DNA is obtained. The single-stranded terminus in 3′-flap DNA is then utilized to prime the replication of an externally supplied ssDNA circle in a rolling circle amplification (RCA) reaction. In model experiments with samples comprised of genomic λ-DNA and human herpes virus 6 type B (HHV-6B) DNA, we have used our labeling method in combination with surface RCA as reporter system to achieve both high sequence specificity of dsDNA targeting and high sensitivity of detection. The method can find applications in sensitive and specific detection of viral duplex DNA.

Self-assembling supramolecular complexes by single-stranded extension from plasmid DNA

Self-assembling supramolecular complexes are of great interest for bottom-up research like nanotechnology. DNA is an inexpensive building block with sequence-specific self-assembling capabilities through Watson-Crick and/or Hoogsteen base pairing and could be used for applications in surface chemistry, material science, nanomechanics, nanoelectronics, nanorobotics, and of course in biology. The starting point is usually single-stranded DNA, which is rather easily accessible for base pairing and duplex formation. When long stretches of double-stranded DNA are desirable, serving either as genetic codes or electrical wires, bacterial expansion of plasmids is an inexpensive approach with scale-up properties. Here, we present a method for using double-stranded DNA of any sequence for generating simple structures, such as junctions and DNA lattices. It is known that supercoiled plasmids are strand-invaded by certain DNA analogs. Here we add to the complexity by using "Self-assembling UNiversal (SUN) anchors" formed by DNA analog oligonucleotides, synthesized with an extension, a "sticky-end" that can be used for further base pairing with single-stranded DNA. We show here how the same set of SUN anchors can be utilized for gene therapy, plasmid purification, junction for lattices, and plasmid dimerization through Watson-Crick base pairing. Using atomic force microscopy, it has been possible to characterize and quantify individual components of such supra-molecular complexes.

Fluorescence-based detection of short DNA sequences under non-denaturing conditions

The ability of peptide nucleic acid (PNA) to open up duplex DNA in a highly sequence-specific manner makes it possible to detect short DNA sequences on the background of or within genomic DNA under non-denaturing conditions. To do so, chosen marker sites in double-stranded DNA are locally opened by a pair of PNA openers, thus transforming one strand within the target region (20-30 bp) into the single-stranded form. Onto this accessible DNA sequence a circular oligonucleotide probe is assembled, which serves as a template for rolling circle amplification (RCA). Both homogeneous and heterogeneous assay formats are investigated, as are different formats for fluorescence-based amplicon detection. Our recent data with immobilized analytes suggest that marker sequences in plasmid and bacterial chromosomal DNA can be successfully detected.

Detection of low-copy-number genomic DNA sequences in individual bacterial cells by using peptide nucleic acid-assisted rolling-circle amplification and fluorescence in situ hybridization

An approach is proposed for in situ detection of short signature DNA sequences present in single copies per bacterial genome. The site is locally opened by peptide nucleic acids, and a circular oligonucleotide is assembled. The amplicon generated by rolling circle amplification is detected by hybridization with fluorescently labeled decorator probes.

Structural diversity of target-specific homopyrimidine peptide nucleic acid-dsDNA complexes

Sequence-selective recognition of double-stranded (ds) DNA by homopyrimidine peptide nucleic acid (PNA) oligomers can occur by major groove triplex binding or by helix invasion via triplex P-loop formation. We have compared the binding of a decamer, a dodecamer and a pentadecamer thymine–cytosine homopyrimidine PNA oligomer to a sequence complementary homopurine target in duplex DNA using gel-shift and chemical probing analyses. We find that all three PNAs form stable triplex invasion complexes, and also conventional triplexes with the dsDNA target. Triplexes form with much faster kinetics than invasion complexes and prevail at lower PNA concentrations and at shorter incubation times. Furthermore, increasing the ionic strength strongly favour triplex formation over invasion as the latter is severely inhibited by cations. Whereas a single triplex invasion complex is formed with the decameric PNA, two structurally different target-specific invasion complexes were characterized for the dodecameric PNA and more than five for the pentadecameric PNA. Finally, it is shown that isolated triplex complexes can be converted to specific invasion complexes without dissociation of the Hoogsteen base-paired triplex PNA. These result demonstrate a clear example of a ‘triplex first’ mechanism for PNA helix invasion.

Site-specific labeling of supercoiled DNA

Visualization of site-specific labels in long linear or circular DNA allows unambiguous identification of various local DNA structures. Here we describe a novel and efficient approach to site-specific DNA labeling. The restriction enzyme SfiI binds to DNA but leaves it intact in the presence of calcium and therefore may serve as a protein label of 13 bp recognition sites. Since SfiI requires simultaneous interaction with two DNA recognition sites for stable binding, this requirement is satisfied by providing an isolated recognition site in the DNA target and an additional short DNA duplex also containing the recognition site. The SfiI/DNA complexes were visualized with AFM and the specificity of the labeling was confirmed by the length measurements. Using this approach, two sites in plasmid DNA were labeled in the presence of a large excess of the helper duplex to compete with the formation of looped structures of the intramolecular synaptic complex. We show that the labeling procedure does not interfere with the superhelical tension-driven formation of alternative DNA structures such as cruciforms. The complex is relatively stable at low and high pH (pH 5 and 9) making the developed approach attractive for use at conditions requiring the pH change.

Biological activity and biotechnological aspects of peptide nucleic acid

During the latest decades a number of different nucleic acid analogs containing natural nucleobases on a modified backbone have been synthesized. An example of this is peptide nucleic acid (PNA), a DNA mimic with a noncyclic peptide-like backbone, which was first synthesized in 1991. Owing to its flexible and neutral backbone PNA displays very good hybridization properties also at low-ion concentrations and has subsequently attracted large interest both in biotechnology and biomedicine. Numerous modifications have been made, which could be of value for particular settings. However, the original PNA does so far perform well in many diverse applications. The high biostability makes it interesting for in vivo use, although the very limited diffusion over lipid membranes requires further modifications in order to make it suitable for treatment in eukaryotic cells. The possibility to use this nucleic acid analog for gene regulation and gene editing is discussed. Peptide nucleic acid is now also used for specific genetic detection in a number of diagnostic techniques, as well as for site-specific labeling and hybridization of functional molecules to both DNA and RNA, areas that are also discussed in this chapter.

[The peptide nucleic acids (PNAs): "high-tech" probes for genetic and molecular cytogenetic investigations]

The peptide nucleic acids (PNAs) constitute a remarkable new class of synthetic nucleic acids analogs, in which the sugar phosphate backbone is replaced by repeating N-(2-aminoethyl) glycine units linked by amine bonds and to which the nucleobases are fixed. This structure gives to PNAs the capacity to hybridize with high affinity and specificity to complementary RNA and DNA sequences, and a great resistance to nucleases and proteinases. Originally conceived as ligands for the study of double stranded DNA, the unique physico-chemical properties of PNAs have led to the development of a large variety of research and diagnostic assays, including antigene and antisense therapy and genome mapping. Several sensitive and robust PNA-dependent methods have been designed for modulating polymerase chain reactions, detecting genomic polymorphisms and mutations or capturing nucleic acids. Over the last few years, the use of PNAs has proven its powerful usefulness in cytogenetics for the rapid in situ identification of human chromosomes and the detection of aneuploidies. Recent studies have reported the successful use of chromosome-specific PNA probes on human lymphocytes, amniocytes, spermatozoa as well as on isolated oocytes and blastomeres. Muticolor PNA protocols have been described for the identification of several human chromosomes, indicating that PNAs could become a powerful tool for in situ chromosomal investigation.

Stem-loop oligonucleotides as tools for labelling double-stranded DNA

We report on a sequence-specific double-stranded DNA labelling strategy in which a stem-loop triplex forming oligonucleotide (TFO) is able to encircle its DNA target. Ligation of this TFO to either a short hairpin oligonucleotide or a long double-stranded DNA fragment leads to the formation of a topological complex. This process requires the hybridization of both extremities of the TFO to each other on a few base pairs. The effects of different factors on the formation of these complexes have been investigated. Efficient complex formation was observed using both GT or TC TFOs. The stem-loop structure enhances the specificity of the complex. The topologically linked TFO remains associated with its target even under conditions that do not favour triple-helix formation. This approach is sufficiently sensitive for detection of a 20-bp target sequence at the subfemtomolar level. This study provides new insights into the mechanics and properties of stem-loop TFOs and their complexes with double-stranded DNA targets. It emphasizes the interest of such molecules in the development of new tools for the specific labelling of short DNA sequences.

End invasion of peptide nucleic acids (PNAs) with mixed-base composition into linear DNA duplexes

Peptide nucleic acid (PNA) is a synthetic DNA mimic with valuable properties and a rapidly growing scope of applications. With the exception of recently introduced pseudocomplementary PNAs, binding of common PNA oligomers to target sites located inside linear double-stranded DNAs (dsDNAs) is essentially restricted to homopurine-homopyrimidine sequence motifs, which significantly hampers some of the PNA applications. Here, we suggest an approach to bypass this limitation of common PNAs. We demonstrate that PNA with mixed composition of ordinary nucleobases is capable of sequence-specific targeting of complementary dsDNA sites if they are located at the very termini of DNA duplex. We then show that such targeting makes it possible to perform capturing of designated dsDNA fragments via the DNA-bound biotinylated PNA as well as to signal the presence of a specific dsDNA sequence, in the case a PNA beacon is employed. We also examine the PNA-DNA conjugate and prove that it can initiate the primer-extension reaction starting from the duplex DNA termini when a DNA polymerase with the strand-displacement ability is used. We thus conclude that recognition of duplex DNA by mixed-base PNAs via the end invasion has a promising potential for site-specific and sequence-unrestricted DNA manipulation and detection.

The peptide nucleic acids (PNAs): a new generation of probes for genetic and cytogenetic analyses

Peptide nucleic acids (PNAs) are synthetic homologs of nucleic acids in which the phosphate-sugar polynucleotide backbone is replaced by a flexible pseudo-peptide polymer to which the nucleobases are linked. This structure gives PNAs the capacity to hybridize with high affinity and specificity to complementary sequences of DNA and RNA, and also confers remarkable resistance to DNAses and proteinases. The unique physico-chemical characteristics of PNAs have led to the development of a wide range of biological assays. Several exciting new applications of PNA technology have been published recently in genetics and cytogenetics. Also, PNA-based hybridization technology is developing rapidly within the field of in situ fluorescence hybridization, pointing out the great potential of PNA probes for chromosomal investigations.

The use of peptide nucleic acids for in situ identification of human chromosomes

The peptide nucleic acids (PNAs) constitute a remarkable new class of synthetic nucleic acid analogues, based on their peptide-like backbone. This structure gives to PNAs the capacity to hybridize with high affinity and specificity to complementary RNA and DNA sequences and a great resistance to nucleases and proteinases. Originally conceived as ligands for the study of double-stranded DNA, the unique physicochemical properties of PNAs have led to the development of a large variety of research and diagnostic assays, including antigene and antisense therapy, genome mapping, and mutation detection. Over the past few years, PNAs have been shown to be powerful tools in cytogenetics for the rapid in situ identification of human chromosomes and the detection of aneuploidies. Recent studies have reported the successful use of chromosome-specific PNA probes on human lymphocytes, amniocytes, and spermatozoa, as well as on isolated oocytes and blastomeres. Multicolor PNA protocols have been described for the identification of several human chromosomes, indicating that PNAs could become a powerful complement to FISH for in situ chromosomal investigation.

The peptide nucleic acids (PNAs), powerful tools for molecular genetics and cytogenetics

Peptide nucleic acids (PNAs) are synthetic mimics of DNA in which the deoxyribose phosphate backbone is replaced by a pseudo-peptide polymer to which the nucleobases are linked. PNAs hybridize with complementary DNAs or RNAs with remarkably high affinity and specificity, essentially because of their uncharged and flexible polyamide backbone. The unique physico-chemical properties of PNAs have led to the development of a variety of research assays, and over the last few years, the use of PNAs has proven their powerful usefulness in molecular biology procedures and diagnostic assays. The more recent applications of PNA involve their use as molecular hybridization probes. Thus, several sensitive and robust PNA-dependent methods have been designed for developing antigene and anticancer drugs, modulating PCR reactions, detecting genomic mutation or labelling chromosomes in situ.

The peptide nucleic acids (PNAs): introduction to a new class of probes for chromosomal investigation

Peptide nucleic acids (PNAs) are synthetic DNA mimics in which the sugar phosphate backbone is replaced by repeating N-(2-aminoethyl) glycine units linked by an amine bond and to which the nucleobases are fixed. Peptide nucleic acids hybridize with complementary nucleic acids with remarkably high affinity and specificity, essentially because of their uncharged and flexible polyamide backbone. The unique physicochemical properties of PNAs have led to the development of a large variety of biological research assays, and, over the last few years, PNAs have proved their powerful usefulness in genetic and cytogenetic diagnostic procedures. Several sensitive and robust PNA-dependent methods have been designed for modulating polymerase chain reactions, detecting genomic mutation or capturing nucleic acids. The more recent applications of PNA involve their use as molecular hybridization probes. Thus, the in situ detection of several human chromosomes has been reported in various types of tissues.

Two sides of the coin: affinity and specificity of nucleic acid interactions

During the past decade, synthetic nucleobase oligomers have found wide use in biochemical sciences, biotechnology and molecular medicine, both as research and/or diagnostic tools and as therapeutics. Numerous applications of common and modified oligonucleotides and oligonucleotide mimics rely on their ability to sequence-specifically recognize nucleic acid targets (DNA or RNA) by forming duplexes or triplexes. In general, these applications would benefit significantly from enhanced binding affinities of nucleobase oligomers in the formation of various secondary structures. However, for high-affinity probes, the selectivity of sequence recognition must also be improved to avoid undesirable associations with mismatched DNA and RNA sites. Here, we review recent progress in understanding the molecular mechanisms of nucleic acid interactions and the development of new high-affinity plus high-specificity oligonucleotides and their mimics, with particular emphasis on peptide nucleic acids.

Inducing and modulating anisotropic DNA bends by pseudocomplementary peptide nucleic acids

DNA bending is significant for various DNA functions in the cell. Here, we demonstrate that pseudocomplementary peptide nucleic acids (pcPNAs) represent a class of versatile, sequence-specific DNA-bending agents. The occurrence of anisotropic DNA bends induced by pcPNAs is shown by gel electrophoretic phasing analysis. The magnitude of DNA bending is determined by circular permutation assay and by electron microscopy, with good agreement of calculated mean values between both methods. Binding of a pair of 10-meric pcPNAs to its target DNA sequence results in moderate DNA bending with a mean value of 40-45 degrees, while binding of one self-pc 8-mer PNA to target DNA yields a somewhat larger average value of the induced DNA bend. Both bends are found to be in phase when the pcPNA target sites are separated by distances of half-integer numbers of helical turns of regular duplex DNA, resulting in an enhanced DNA bend with an average value in the range of 80-90 degrees. The occurrence of such a sharp bend within the DNA double helix is confirmed and exploited through efficient formation of 170-bp-long DNA minicircles by means of dimerization of two bent DNA fragments. The pcPNAs offer two main advantages over previously designed classes of nonnatural DNA-bending agents: they have very mild sequence limitations while targeting duplex DNA and they can easily be designed for a chosen target sequence, because their binding obeys the principle of complementarity. We conclude that pcPNAs are promising tools for inducing bends in DNA at virtually any chosen site.

Use of locked nucleic acid oligonucleotides to add functionality to plasmid DNA

The available reagents for the attachment of functional moieties to plasmid DNA are limiting. Most reagents bind plasmid DNA in a non-sequence- specific manner, with undefined stoichiometry, and affect DNA charge and delivery properties or involve chemical modifications that abolish gene expression. The design and ability of oligonucleotides (ODNs) containing locked nucleic acids (LNAs) to bind supercoiled, double-stranded plasmid DNA in a sequence-specific manner are described for the first time. The main mechanism for LNA ODNs binding plasmid DNA is demonstrated to be by strand displacement. LNA ODNs are more stably bound to plasmid DNA than similar peptide nucleic acid (PNA) 'clamps' for procedures such as particle-mediated DNA delivery (gene gun). It is shown that LNA ODNs remain associated with plasmid DNA after cationic lipid-mediated transfection into mammalian cells. LNA ODNs can bind to DNA in a sequence-specific manner so that binding does not interfere with plasmid conformation or gene expression. Attachment of CpG-based immune adjuvants to plasmid by 'hybrid' phosphorothioate-LNA ODNs induces tumour necrosis factor-alpha production in the macrophage cell line RAW264.7. This observation exemplifies an important new, controllable methodology for adding functionality to plasmids for gene delivery and DNA vaccination.

P-loop catalytically assisting the enzymatic cleavage of single-stranded DNA

We demonstrated that a P-loop, a looped complex formed inside duplex DNA by adding peptide nucleic acids (PNA), acts catalytically as a template for enzymatic cleavage of single-stranded probe oligodeoxynucleotides (ODN). A PD-loop complex formed from P-loop and probe ODN was digested efficiently by a restriction enzyme, and the truncated probe ODN was released. The P-loop nicked by the enzyme can form PD-loop again with another probe ODN, and then assisted the enzymatic cleavage of an excess of probe ODN. In addition, by using dumbbell-formed ODN as a probe ODN, the efficiency of the P-loop-assisted ODN cleavage was enhanced considerably as compared with that of linear ODN. Thus, the method utilizing P-loop will make it possible to amplify the sequence information of duplex DNA via a catalytic cleavage of probe ODNs.

Sequence-universal recognition of duplex DNA by oligonucleotides via pseudocomplementarity and helix invasion

The well-known Watson-Crick complementarity rules, which were discovered 50 years ago, elegantly direct the specific pairing of two DNA single strands. On the contrary, once formed, the double-stranded (ds) DNA lacks such a simple and sequence-universal recognition principle, since most of the characteristic chemical groups of nucleobases are now buried deep inside the double helix, the major DNA form. We report a promising versatile approach for highly selective recognition of designated sites within dsDNA featuring considerable practical potential for a variety of molecular-biological, biotechnological, gene-therapeutic, and diagnostic applications. It may also have implications for prebiotic evolution of genetic machinery at the primordial stages of the origin of life. Our design synergistically employs the robust helix-invasion ability of recently developed DNA mimics and analogs, pseudocomplementary peptide nucleic acids and pseudocomplementary oligonucleotides, thus enabling the sequence-unrestricted recognition of chosen DNA duplexes by nucleobase oligomers. Using this basically general approach, we selectively tagged a unique mixed-base site on the target dsDNA fragment with streptavidin and/or multiply labeled this site with fluorophores via the primer-extension reaction.

Superior duplex DNA strand invasion by acridine conjugated peptide nucleic acids

DNA helix invasion by P-loop forming peptide nucleic acids (PNAs) is extremely sensitive to increased ionic strength as this stabilizes the DNA duplex. To address this, the DNA intercalator 9-aminoacridine was conjugated to helix invading PNAs, and the duplex DNA binding efficiency of such constructs was measured at different ionic strength conditions by electrophoretic mobility shift analysis. Remarkably, at physiogically relevant ionic strength (140 mM K+/10 mM Na+, 2 mM Mg2+), acridine conjugated PNAs showed 20-150-fold superior binding to a cognate sequence target as compared to the conventional PNAs. This enhancement occurred without compromising the sequence specificity of binding. Thus, simply conjugating the DNA intercalator 9-aminoacridine to PNA represents a major step toward the development of helix invading constructs for in vivo applications such as gene targeting.