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1.  Peptide Nucleic Acid-Mediated PCR Clamping as a Useful Supplement in the Determination of Microbial Diversity 
Peptide nucleic acid (PNA)-mediated PCR clamping (H. Ørum, P. E. Nielsen, M. Egholm, R. H. Berg, O. Buchardt, and C. Stanley, Nucleic Acids Res. 21:5332–5336, 1993) was introduced as a novel procedure to selectively amplify ribosomal DNAs (rDNAs) which are not frequently found in clone libraries generated by standard PCR from complex microbial consortia. Three different PNA molecules were used; two of these molecules (PNA-ALF and PNA-EUB353) overlapped with one of the amplification primers, whereas PNA-1114F hybridized to the middle of the amplified region. Thus, PCR clamping was achieved either by competitive binding between the PNA molecules and the forward or reverse primers (competitive clamping) or by hindering polymerase readthrough (elongation arrest). Gene libraries generated from mixed rDNA templates by using PCR clamping are enriched for clones that do not contain sequences homologous to the appropriate PNA oligomer. This effect of PCR clamping was exploited in the following two ways: (i) analysis of gene libraries generated by PCR clamping with PNA-ALF together with standard libraries reduced the number of clones which had to be analyzed to detect all of the different sequences present in an artificial rDNA mixture; and (ii) PCR clamping with PNA-EUB353 and PNA-1114F was used to selectively recover rDNA sequences which represented recently described phylogenetic groups (NKB19, TM6, cluster related to green nonsulfur bacteria) from an anaerobic, dechlorinating consortium described previously. We concluded that PCR clamping might be a useful supplement to standard PCR amplification in rDNA-based studies of microbial diversity and could be used to selectively recover members of undescribed phylogenetic clusters from complex microbial communities.
PMCID: PMC91862  PMID: 10653717
2.  Xenobiology: A new form of life as the ultimate biosafety tool 
Bioessays  2010;32(4):322-331.
Synthetic biologists try to engineer useful biological systems that do not exist in nature. One of their goals is to design an orthogonal chromosome different from DNA and RNA, termed XNA for xeno nucleic acids. XNA exhibits a variety of structural chemical changes relative to its natural counterparts. These changes make this novel information-storing biopolymer “invisible” to natural biological systems. The lack of cognition to the natural world, however, is seen as an opportunity to implement a genetic firewall that impedes exchange of genetic information with the natural world, which means it could be the ultimate biosafety tool. Here I discuss, why it is necessary to go ahead designing xenobiological systems like XNA and its XNA binding proteins; what the biosafety specifications should look like for this genetic enclave; which steps should be carried out to boot up the first XNA life form; and what it means for the society at large.
PMCID: PMC2909387  PMID: 20217844
auxotrophy; biosafety; synthetic biology; xenobiology; xeno nucleic acids
3.  Towards XNA nanotechnology: new materials from synthetic genetic polymers 
Trends in Biotechnology  2014;32(6):321-328.
•We review recent advances in nucleic acid chemistry and polymerase engineering that have enabled the synthesis, replication, and evolution of a wide range of nucleic acid-like synthetic genetic polymers (XNAs) with improved chemical and biological stability.•We discuss the likely biotechnological impact of the further development of XNA technology for the generation of novel ligands, enzymes, and nanostructures with tailor-made chemistry.
Nucleic acids display remarkable properties beyond information storage and propagation. The well-understood base pairing rules have enabled nucleic acids to be assembled into nanostructures of ever increasing complexity. Although nanostructures can be constructed using other building blocks, including peptides and lipids, it is the capacity to evolve that sets nucleic acids apart from all other nanoscale building materials. Nonetheless, the poor chemical and biological stability of DNA and RNA constrain their applications. Recent advances in nucleic acid chemistry and polymerase engineering enable the synthesis, replication, and evolution of a range of synthetic genetic polymers (XNAs) with improved chemical and biological stability. We discuss the impact of this technology on the generation of XNA ligands, enzymes, and nanostructures with tailor-made chemistry.
PMCID: PMC4039137  PMID: 24745974
4.  Synthetic genetic polymers capable of heredity and evolution 
Science (New York, N.Y.)  2012;336(6079):341-344.
Genetic information storage and processing rely on just two polymers, DNA and RNA. Whether their role reflects evolutionary history or fundamental functional constraints is unknown. Using polymerase evolution and design, we show that genetic information can be stored in and recovered from six alternative genetic polymers (XNAs) based on simple nucleic acid architectures not found in nature. We also select XNA aptamers, which bind their targets with high affinity and specificity, demonstrating that beyond heredity, specific XNAs have the capacity for Darwinian evolution and for folding into defined structures. Thus, heredity and evolution, two hallmarks of life, are not limited to DNA and RNA but are likely to be emergent properties of polymers capable of information storage.
PMCID: PMC3362463  PMID: 22517858
5.  A Novel Conversion Strategy using an Expanded Genetic Alphabet to Assay Nucleic Acids 
Analytical chemistry  2013;85(9):10.1021/ac400422r.
Methods to detect DNA and RNA (collectively xNA) are easily plagued by noise, false positives, and false negatives, especially with increasing levels of multiplexing in increasing complex assay mixtures. We describe assay architectures that mitigate these problems, here by converting standard xNA analyte sequences into sequences that incorporate non-standard nucleotides (Z and P). Z and P are extra DNA building blocks that form tight non-standard base pairs without cross-binding to natural oligonucleotides containing G, A, C, and T (GACT). The resulting improvements are assessed in an assay that inverts the standard Luminex xTAG® architecture, placing a biotin on a primer (rather than on a triphosphate). This primer is extended on the target to create a standard GACT extension product that is captured by a CTGA oligonucleotide attached to a Luminex bead. Using conversion, a polymerase incorporates dZTP opposite template dG in the absence of dCTP. This creates a Z-containing extension product that is captured by a bead-bound oligonucleotide containing P, which binds selectively to Z. The assay with conversion produces higher signals than the assay without conversion, possibly because the Z:P pair is stronger than the C:G pair. These architectures improve the ability of the Luminex instruments to detect xNA analytes, producing higher signals without the possibility of competition from any natural oligonucleotides, even in complex biological samples.
PMCID: PMC3868369  PMID: 23541235
Detection of Nucleic Acids; Expanded Genetic Alphabet; Conversion Strategy; ZiP-TAG Technology; Luminex xTAG Technology
6.  Inhibiting Gene Expression with Peptide Nucleic Acid (PNA)–Peptide Conjugates that Target Chromosomal DNA 
Biochemistry  2007;46(25):7581-7589.
Peptide nucleic acids (PNAs) are a nonionic DNA/RNA mimic that can recognize complementary sequences by Watson–Crick base–pairing. The neutral PNA backbone facilitates recognition of duplex DNA by strand invasion, suggesting that antigene PNAs (agPNAs) can be important tools for exploring the structure and function of chromosomal DNA inside cells. However, before agPNAs can enter wide use it will be necessary to develop straightforward strategies for introducing them into cells. Here we demonstrate that agPNA–peptide conjugates can target promoter DNA and block progesterone receptor (PR) gene expression inside cells. Thirty–six agPNA–peptide conjugates were synthesized and tested. We observed inhibition of gene expression using cationic peptides containing either arginine or lysine residues, with eight or more cationic amino acids being preferred. Both thirteen and nineteen base agPNA-peptide conjugates were inhibitory. Inhibition was observed in human cancer cell lines expressing either high or low levels of progesterone receptor. Modification of agPNA–peptide conjugates with hydrophobic amino acids or small molecule hydrophobic moities yielded improved potency. Inhibition by agPNAs did not require cationic lipid or any other additive, but adding agents to cell growth media that promote endosomal release caused modest increases in agPNA potency. These data demonstrate that chromosomal DNA is accessible to agPNA–peptide conjugates and that chemical modifications can improve potency.
PMCID: PMC2564818  PMID: 17536840
7.  Synthetic constructs in/for the environment: Managing the interplay between natural and engineered Biology 
Febs Letters  2012;586(15):2199-2206.
The plausible release of deeply engineered or even entirely synthetic/artificial microorganisms raises the issue of their intentional (e.g. bioremediation) or accidental interaction with the Environment. Containment systems designed in the 1980s–1990s for limiting the spread of genetically engineered bacteria and their recombinant traits are still applicable to contemporary Synthetic Biology constructs. Yet, the ease of DNA synthesis and the uncertainty on how non-natural properties and strains could interplay with the existing biological word poses yet again the challenge of designing safe and efficacious firewalls to curtail possible interactions. Such barriers may include xeno-nucleic acids (XNAs) instead of DNA as information-bearing molecules, rewriting the genetic code to make it non-understandable by the existing gene expression machineries, and/or making growth dependent on xenobiotic chemicals.
PMCID: PMC3396840  PMID: 22710182
Genetic containment; Biological containment; Horizontal gene transfer; Orthogonal systems; XNA; Alternative gene code
8.  Subnanomolar antisense activity of phosphonate-peptide nucleic acid (PNA) conjugates delivered by cationic lipids to HeLa cells 
Nucleic Acids Research  2008;36(13):4424-4432.
In the search of facile and efficient methods for cellular delivery of peptide nucleic acids (PNA), we have synthesized PNAs conjugated to oligophosphonates via phosphonate glutamine and bis-phosphonate lysine amino acid derivatives thereby introducing up to twelve phosphonate moieties into a PNA oligomer. This modification of the PNA does not interfere with the nucleic acid target binding affinity based on thermal stability of the PNA/RNA duplexes. When delivered to cultured HeLa pLuc705 cells by Lipofectamine, the PNAs showed dose-dependent nuclear antisense activity in the nanomolar range as inferred from induced luciferase activity as a consequence of pre-mRNA splicing correction by the antisense-PNA. Antisense activity depended on the number of phosphonate moieties and the most potent hexa-bis-phosphonate-PNA showed at least 20-fold higher activity than that of an optimized PNA/DNA hetero-duplex. These results indicate that conjugation of phosphonate moieties to the PNA can dramatically improve cellular delivery mediated by cationic lipids without affecting on the binding affinity and sequence discrimination ability, exhibiting EC50 values down to one nanomolar. Thus the intracellular efficacy of PNA oligomers rival that of siRNA and the results therefore emphasize that provided sufficient in vivo bioavailability of PNA can be achieved these molecules may be developed into potent gene therapeutic drugs.
PMCID: PMC2490735  PMID: 18596083
9.  Peptide nucleic acid (PNA) is capable of enhancing hammerhead ribozyme activity with long but not with short RNA substrates. 
Nucleic Acids Research  1997;25(14):2690-2693.
Long RNA substrates are inefficiently cleaved by hammerhead ribozymes in trans. Oligonucleotide facilitators capable of affecting the ribozyme activity by interacting with the substrates at the termini of the ribozyme provide a possibility to improve ribozyme mediated cleavage of long RNA substrates. We have examined the effect of PNA as facilitator in vitro in order to test if even artificial compounds have facilitating potential. Effects of 12mer PNA- (peptide nucleic acid), RNA- and DNA-facilitators of identical sequence were measured with three substrates containing either 942, 452 or 39 nucleotides. The PNA facilitator enhances the ribozyme activity with both, the 942mer and the 452mer substrate to a slightly smaller extent than RNA and DNA facilitators. This effect was observed up to PNA facilitator:substrate ratios of 200:1. The enhancement becomes smaller as the PNA facilitator:substrate ratio exceeds 200:1. With the 39mer substrate, the PNA facilitator decreases the ribozyme activity by more than 100-fold, even at PNA facilitator:substrate ratios of 1:1. Although with long substrates the effect of the PNA facilitator is slightly smaller than the effect of identical RNA or DNA facilitators, PNA may be a more practical choice for potential applications in vivo because PNA is much more resistant to degradation by cellular enzymes.
PMCID: PMC146807  PMID: 9207013
10.  End invasion of peptide nucleic acids (PNAs) with mixed-base composition into linear DNA duplexes 
Nucleic Acids Research  2005;33(17):e146.
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.
PMCID: PMC1243805  PMID: 16204449
11.  Chiral introduction of positive charges to PNA for double-duplex invasion to versatile sequences 
Nucleic Acids Research  2008;36(5):1464-1471.
Invasion of two PNA strands to double-stranded DNA is one of the most promising methods to recognize a predetermined site in double-stranded DNA (PNA = peptide nucleic acid). In order to facilitate this ‘double-duplex invasion’, a new type of PNA was prepared by using chiral PNA monomers in which a nucleobase was bound to the α-nitrogen of N-(2-aminoethyl)-d-lysine. These positively charged monomer units, introduced to defined positions in Nielsen's PNAs (poly[N-(2-aminoethyl)glycine] derivatives), promoted the invasion without impairing mismatch-recognizing activity. When pseudo-complementary nucleobases 2,6-diaminopurine and 2-thiouracil were bound to N-(2-aminoethyl)-d-lysine, the invasion successfully occurred even at highly G–C-rich regions [e.g. (G/C)7(A/T)3 and (G/C)8(A/T)2] which were otherwise hardly targeted. Thus, the scope of sequences available as the target site has been greatly expanded. In contrast with the promotion by the chiral PNA monomers derived from N-(2-aminoethyl)-d-lysine, their l-isomers hardly invaded, showing crucial importance of the d-chirality. The promotion of double-duplex invasion by the chiral (d) PNA monomer units was ascribed to both destabilization of PNA/PNA duplex and stabilization of PNA/DNA duplexes.
PMCID: PMC2275137  PMID: 18203747
12.  PNA microarrays for hybridisation of unlabelled DNA samples 
Nucleic Acids Research  2003;31(19):e119.
Several strategies have been developed for the production of peptide nucleic acid (PNA) microarrays by parallel probe synthesis and selective coupling of full-length molecules. Such microarrays were used for direct detection of the hybridisation of unlabelled DNA by time-of-flight secondary ion mass spectrometry. PNAs were synthesised by an automated process on filter-bottom microtitre plates. The resulting molecules were released from the solid support and attached without any purification to microarray surfaces via the terminal amino group itself or via modifications, which had been chemically introduced during synthesis. Thus, only full-length PNA oligomers were attached whereas truncated molecules, produced during synthesis because of incomplete condensation reactions, did not bind. Different surface chemistries and fitting modifications of the PNA terminus were tested. For an examination of coupling selectivity, bound PNAs were cleaved off microarray surfaces and analysed by MALDI-TOF mass spectrometry. Additionally, hybridisation experiments were performed to compare the attachment chemistries, with fully acetylated PNAs spotted as controls. Upon hybridisation of unlabelled DNA to such microarrays, binding events could be detected by visualisation of phosphates, which are an integral part of nucleic acids but missing entirely in PNA probes. Overall best results in terms of selectivity and sensitivity were obtained with thiol-modified PNAs on maleimide surfaces.
PMCID: PMC206485  PMID: 14500847
13.  Cellular antisense activity of peptide nucleic acid (PNAs) targeted to HIV-1 polypurine tract (PPT) containing RNA 
Nucleic Acids Research  2007;35(12):3907-3917.
DNA and RNA oligomers that contain stretches of guanines can associate to form stable secondary structures including G-quadruplexes. Our study shows that the (UUAAAAGAAAAGGGGGGAU) RNA sequence, from the human immunodeficiency virus type 1 (HIV-1 polypurine tract or PPT sequence) forms in vitro a stable folded structure involving the G-run. We have investigated the ability of pyrimidine peptide nucleic acid (PNA) oligomers targeted to the PPT sequence to invade the folded RNA and exhibit biological activity at the translation level in vitro and in cells. We find that PNAs can form stable complexes even with the structured PPT RNA target at neutral pH. We show that T-rich PNAs, namely the tridecamer-I PNA (C4T4CT4) forms triplex structures whereas the C-rich tridecamer-II PNA (TC6T4CT) likely forms a duplex with the target RNA. Interestingly, we find that both C-rich and T-rich PNAs arrested in vitro translation elongation specifically at the PPT target site. Finally, we show that T-rich and C-rich tridecamer PNAs that have been identified as efficient and specific blockers of translation elongation in vitro, specifically inhibit translation in streptolysin-O permeabilized cells where the PPT target sequence has been introduced upstream the reporter luciferase gene.
PMCID: PMC1919497  PMID: 17537815
14.  Incorporation of thio-pseudoisocytosine into triplex-forming peptide nucleic acids for enhanced recognition of RNA duplexes 
Nucleic Acids Research  2014;42(6):4008-4018.
Peptide nucleic acids (PNAs) have been developed for applications in biotechnology and therapeutics. There is great potential in the development of chemically modified PNAs or other triplex-forming ligands that selectively bind to RNA duplexes, but not single-stranded regions, at near-physiological conditions. Here, we report on a convenient synthesis route to a modified PNA monomer, thio-pseudoisocytosine (L), and binding studies of PNAs incorporating the monomer L. Thermal melting and gel electrophoresis studies reveal that L-incorporated 8-mer PNAs have superior affinity and specificity in recognizing the duplex region of a model RNA hairpin to form a pyrimidine motif major-groove RNA2–PNA triplex, without appreciable binding to single-stranded regions to form an RNA–PNA duplex or, via strand invasion, forming an RNA–PNA2 triplex at near-physiological buffer condition. In addition, an L-incorporated 8-mer PNA shows essentially no binding to single-stranded or double-stranded DNA. Furthermore, an L-modified 6-mer PNA, but not pseudoisocytosine (J) modified or unmodified PNA, binds to the HIV-1 programmed −1 ribosomal frameshift stimulatory RNA hairpin at near-physiological buffer conditions. The stabilization of an RNA2–PNA triplex by L modification is facilitated by enhanced van der Waals contacts, base stacking, hydrogen bonding and reduced dehydration energy. The destabilization of RNA–PNA and DNA–PNA duplexes by L modification is due to the steric clash and loss of two hydrogen bonds in a Watson–Crick-like G–L pair. An RNA2–PNA triplex is significantly more stable than a DNA2–PNA triplex, probably because the RNA duplex major groove provides geometry compatibility and favorable backbone–backbone interactions with PNA. Thus, L-modified triplex-forming PNAs may be utilized for sequence-specifically targeting duplex regions in RNAs for biological and therapeutic applications.
PMCID: PMC3973316  PMID: 24423869
15.  Antiproliferative effect in chronic myeloid leukaemia cells by antisense peptide nucleic acids 
Nucleic Acids Research  2002;30(17):3712-3721.
Peptide nucleic acid (PNA) is a synthetic DNA analogue that is resistant to nucleases and proteases and binds with exceptional affinity to RNA. Because of these properties PNA has the potential to become a powerful therapeutic agent to be used in vivo. Until now, however, the use of PNA in vivo has not been much investigated. Here, we have attempted to reduce the expression of the bcr/abl oncogene in chronic myeloid leukaemia KYO-1 cells using a 13mer PNA sequence (asPNA) designed to hybridise to the b2a2 junction of bcr/abl mRNA. To enhance cellular uptake asPNA was covalently linked to the basic peptide VKRKKKP (NLS-asPNA). Moreover, to investigate the cellular uptake by confocal microscopy, both PNAs were linked by their N-terminus to fluorescein (FL). Studies of uptake, carried out at 4 and 37°C on living KYO-1 cells stained with hexidium iodide, showed that both NLS-asPNA-FL and asPNA-FL were taken up by the cells, through a receptor-independent mechanism. The intracellular amount of NLS-asPNA-FL was about two to three times higher than that of asPNA-FL. Using a semi-quantitative RT– PCR technique we found that 10 µM asPNA and NLS-asPNA reduced the level of b2a2 mRNA in KYO-1 cells to 20 ± 5% and 60 ± 10% of the control, respectively. Western blot analysis showed that asPNA promoted a significant inhibition of p210BCR/ABL protein: residual protein measured in cells exposed for 48 h to asPNA was ∼35% of the control. Additionally, asPNA impaired cell growth to 50 ± 5% of the control and inhibited completion of the cell cycle. In summary, these results demonstrate that a PNA 13mer is taken up by KYO-1 cells and is capable of producing a significant and specific down-regulation of the bcr/abl oncogene involved in leukaemogenesis.
PMCID: PMC137404  PMID: 12202756
16.  Synthesis and evaluation of some properties of chimeric oligomers containing PNA and phosphono-PNA residues. 
Nucleic Acids Research  1998;26(2):566-575.
In an attempt to improve physico-chemical and biological properties of peptide nucleic acids (PNAs), particularly water solubility and cellular uptake, the synthesis of chimeric oligomers consisted of PNA and phosphono-PNA analogues (pPNAs) bearing the four natural nucleobases has been accomplished. To produce these chimeras, pPNA monomers of two types containing N-(2-hydroxyethyl)phosphonoglycine, or N-(2-aminoethyl)phosphonoglycine backbone, were used in conjunction with PNA monomers representing derivatives of N-(2-aminoethyl)glycine, or N-(2-hydroxyethyl)glycine. The oligomers obtained were composed of either PNA and pPNA stretches or alternating PNA and pPNA monomers. The examination of hybridization properties of PNA-pPNA chimeras to DNA and RNA complementary strands in comparison with pure PNAs, and pPNAs as well as DNA-pPNA hybrids and DNA fragments confirmed that these chimeras form stable complexes with complementary DNA and RNA fragments. They were found to be resistant to degradation by nucleases. All these properties together with good solubility in water make PNA-pPNA hybrids promising for further evaluation as potential therapeutic agents.
PMCID: PMC147292  PMID: 9421517
17.  Affinity Purification of DNA and RNA from Environmental Samples with Peptide Nucleic Acid Clamps 
Bispeptide nucleic acids (bis-PNAs; PNA clamps), PNA oligomers, and DNA oligonucleotides were evaluated as affinity purification reagents for subfemtomolar 16S ribosomal DNA (rDNA) and rRNA targets in soil, sediment, and industrial air filter nucleic acid extracts. Under low-salt hybridization conditions (10 mM NaPO4, 5 mM disodium EDTA, and 0.025% sodium dodecyl sulfate [SDS]) a PNA clamp recovered significantly more target DNA than either PNA or DNA oligomers. The efficacy of PNA clamps and oligomers was generally enhanced in the presence of excess nontarget DNA and in a low-salt extraction-hybridization buffer. Under high-salt conditions (200 mM NaPO4, 100 mM disodium EDTA, and 0.5% SDS), however, capture efficiencies with the DNA oligomer were significantly greater than with the PNA clamp and PNA oligomer. Recovery and detection efficiencies for target DNA concentrations of ≥100 pg were generally >20% but depended upon the specific probe, solution background, and salt condition. The DNA probe had a lower absolute detection limit of 100 fg of target (830 zM [1 zM = 10−21 M]) in high-salt buffer. In the absence of exogenous DNA (e.g., soil background), neither the bis-PNA nor the PNA oligomer achieved the same absolute detection limit even under a more favorable low-salt hybridization condition. In the presence of a soil background, however, both PNA probes provided more sensitive absolute purification and detection (830 zM) than the DNA oligomer. In varied environmental samples, the rank order for capture probe performance in high-salt buffer was DNA > PNA > clamp. Recovery of 16S rRNA from environmental samples mirrored quantitative results for DNA target recovery, with the DNA oligomer generating more positive results than either the bis-PNA or PNA oligomer, but PNA probes provided a greater incidence of detection from environmental samples that also contained a higher concentration of nontarget DNA and RNA. Significant interactions between probe type and environmental sample indicate that the most efficacious capture system depends upon the particular sample type (and background nucleic acid concentration), target (DNA or RNA), and detection objective.
PMCID: PMC92168  PMID: 10919804
18.  Evaluating the Effect of Ionic Strength on Duplex Stability for PNA Having Negatively or Positively Charged Side Chains 
PLoS ONE  2013;8(3):e58670.
The enhanced thermodynamic stability of PNA:DNA and PNA:RNA duplexes compared with DNA:DNA and DNA:RNA duplexes has been attributed in part to the lack of electrostatic repulsion between the uncharged PNA backbone and negatively charged DNA or RNA backbone. However, there are no previously reported studies that systematically evaluate the effect of ionic strength on duplex stability for PNA having a charged backbone. Here we investigate the role of charge repulsion in PNA binding by synthesizing PNA strands having negatively or positively charged side chains, then measuring their duplex stability with DNA or RNA at varying salt concentrations. At low salt concentrations, positively charged PNA binds more strongly to DNA and RNA than does negatively charged PNA. However, at medium to high salt concentrations, this trend is reversed, and negatively charged PNA shows higher affinity for DNA and RNA than does positively charged PNA. These results show that charge screening by counterions in solution enables negatively charged side chains to be incorporated into the PNA backbone without reducing duplex stability with DNA and RNA. This research provides new insight into the role of electrostatics in PNA binding, and demonstrates that introduction of negatively charged side chains is not significantly detrimental to PNA binding affinity at physiological ionic strength. The ability to incorporate negative charge without sacrificing binding affinity is anticipated to enable the development of PNA therapeutics that take advantage of both the inherent benefits of PNA and the multitude of charge-based delivery technologies currently being developed for DNA and RNA.
PMCID: PMC3590165  PMID: 23484047
19.  Biochemical and functional characterization of xenoreactive natural antibodies in hu-PBL-SCID mice. 
Journal of Clinical Investigation  1996;97(5):1267-1275.
An in vivo model system to understand the mechanism of xenograft rejection was established using human peripheral blood leukocyte-reconstituted SCID (hu-PBL-SCID) mice. Human xenoreactive natural antibodies (XNA), of IgM and IgG subtypes, capable of binding to pig aortic endothelial cells (PAEC) were detected in the sera of hu-PBL-SCID by ELISA and flowcytometric methods. Western blot analysis of PAEC lysates showed that IgM and IgG XNA from hu-PBL-SCID recognized xenoantigens with similar molecular mass as those recognized by XNA from normal human serum (NHS). This result demonstrated that hu-PBL-SCID contained XNA representing the same repertoire as that of the NHS. XNA from NHS and hu-PBL-SCID were also able to induce intracellular Ca2+ signals in cultured PAEC several fold above the basal level. This result revealed their functional similarity and demonstrated for the first time that XNA in the absence of C can activate PAEC, which may lead to the pathology of xenograft rejection. In vivo, PAEC transplanted under the kidney capsule of hu-PBL-SCID mice showed deposition of human IgM and mouse C. In summary, the present study demonstrates that hu-PBL-SCID can serve as a useful model to characterize innate immunity against xenograft.
PMCID: PMC507180  PMID: 8636439
20.  Targeting peptide nucleic acid (PNA) oligomers to mitochondria within cells by conjugation to lipophilic cations: implications for mitochondrial DNA replication, expression and disease 
Nucleic Acids Research  2001;29(9):1852-1863.
The selective manipulation of mitochondrial DNA (mtDNA) replication and expression within mammalian cells has proven difficult. One promising approach is to use peptide nucleic acid (PNA) oligomers, nucleic acid analogues that bind selectively to complementary DNA or RNA sequences inhibiting replication and translation. However, the potential of PNAs is restricted by the difficulties of delivering them to mitochondria within cells. To overcome this problem we conjugated a PNA 11mer to a lipophilic phosphonium cation. Such cations are taken up by mitochondria through the lipid bilayer driven by the membrane potential across the inner membrane. As anticipated, phosphonium–PNA (ph–PNA) conjugates of 3.4–4 kDa were imported into both isolated mitochondria and mitochondria within human cells in culture. This was confirmed by using an ion-selective electrode to measure uptake of the ph–PNA conjugates; by cell fractionation in conjunction with immunoblotting; by confocal microscopy; by immunogold-electron microscopy; and by crosslinking ph–PNA conjugates to mitochondrial matrix proteins. In all cases dissipating the mitochondrial membrane potential with an uncoupler prevented ph–PNA uptake. The ph–PNA conjugate selectively inhibited the in vitro replication of DNA containing the A8344G point mutation that causes the human mtDNA disease ‘myoclonic epilepsy and ragged red fibres’ (MERRF) but not the wild-type sequence that differs at a single nucleotide position. Therefore these modified PNA oligomers retain their selective binding to DNA and the lipophilic cation delivers them to mitochondria within cells. When MERRF cells were incubated with the ph–PNA conjugate the ratio of MERRF to wild-type mtDNA was unaffected, even though the ph–PNA content of the mitochondria was sufficient to inhibit MERRF mtDNA replication in a cell-free system. This unexpected finding suggests that nucleic acid derivatives cannot bind their complementary sequences during mtDNA replication. In summary, we have developed a new strategy for targeting PNA oligomers to mitochondria and used it to determine the effects of PNA on mutated mtDNA replication in cells. This work presents new approaches for the manipulation of mtDNA replication and expression, and will assist in the development of therapies for mtDNA diseases.
PMCID: PMC37250  PMID: 11328868
21.  Non-Watson–Crick interactions between PNA and DNA inhibit the ATPase activity of bacteriophage T4 Dda helicase 
Nucleic Acids Research  2002;30(4):950-957.
Peptide nucleic acid (PNA) is a DNA mimic in which the nucleobases are linked by an N-(2-aminoethyl) glycine backbone. Here we report that PNA can interact with single-stranded DNA (ssDNA) in a non-sequence-specific fashion. We observed that a 15mer PNA inhibited the ssDNA-stimulated ATPase activity of a bacteriophage T4 helicase, Dda. Surprisingly, when a fluorescein-labeled 15mer PNA was used in binding studies no interaction was observed between PNA and Dda. However, fluorescence polarization did reveal non-sequence-specific interactions between PNA and ssDNA. Thus, the inhibition of ATPase activity of Dda appears to result from depletion of the available ssDNA due to non-Watson–Crick binding of PNA to ssDNA. Inhibition of the ssDNA-stimulated ATPase activity was observed for several PNAs of varying length and sequence. To study the basis for this phenomenon, we examined self-aggregation by PNAs. The 15mer PNA readily self-aggregates to the point of precipitation. Since PNAs are hydrophobic, they aggregate more than DNA or RNA, making the study of this phenomenon essential for understanding the properties of PNA. Non-sequence-specific interactions between PNA and ssDNA were observed at moderate concentrations of PNA, suggesting that such interactions should be considered for antisense and antigene applications.
PMCID: PMC100347  PMID: 11842106
22.  Introduction of multiphosphonate ligand to peptide nucleic acid for metal ion conjugation 
Artificial DNA, PNA & XNA  2012;3(2):73-79.
Peptide nucleic acid (PNA) is one of the most widely used synthetic DNA analogs. Conjugation of functional molecules to PNA is very effective to further widen its potential applications. For this purpose, here we report the synthesis of several ligand monomers and introduced them to PNA. These ligand-modified PNAs attract cerium ion and are useful for site-selective DNA hydrolysis. It should be noted that these ligands on PNA are also effective even under the conditions of invasion complex.
PMCID: PMC3429533  PMID: 22772037
cerium; DNA; hydrolysis; ligand; metal ion; peptide nucleic acid
23.  Site-directed gene mutation at mixed sequence targets by psoralen-conjugated pseudo-complementary peptide nucleic acids 
Nucleic Acids Research  2007;35(22):7604-7613.
Sequence-specific DNA-binding molecules such as triple helix-forming oligonucleotides (TFOs) provide a means for inducing site-specific mutagenesis and recombination at chromosomal sites in mammalian cells. However, the utility of TFOs is limited by the requirement for homopurine stretches in the target duplex DNA. Here, we report the use of pseudo-complementary peptide nucleic acids (pcPNAs) for intracellular gene targeting at mixed sequence sites. Due to steric hindrance, pcPNAs are unable to form pcPNA–pcPNA duplexes but can bind to complementary DNA sequences by Watson–Crick pairing via double duplex-invasion complex formation. We show that psoralen-conjugated pcPNAs can deliver site-specific photoadducts and mediate targeted gene modification within both episomal and chromosomal DNA in mammalian cells without detectable off-target effects. Most of the induced psoralen-pcPNA mutations were single-base substitutions and deletions at the predicted pcPNA-binding sites. The pcPNA-directed mutagenesis was found to be dependent on PNA concentration and UVA dose and required matched pairs of pcPNAs. Neither of the individual pcPNAs alone had any effect nor did complementary PNA pairs of the same sequence. These results identify pcPNAs as new tools for site-specific gene modification in mammalian cells without purine sequence restriction, thereby providing a general strategy for designing gene targeting molecules.
PMCID: PMC2190703  PMID: 17977869
24.  The Crystal Structure of Non-Modified and Bipyridine-Modified PNA Duplexes 
Peptide nucleic acid (PNA) is a synthetic analogue of DNA that commonly has an N-aminoethlyl-glycine backbone. The crystal structure of two PNA duplexes, one containing eight standard nucleobase pairs (GGCATCGG)2 (pdb: 3MBS), and the other containing the same nucleobase pairs and a central pair of bipyridine ligands (pdb: 3MBU), has been solved with a resolution of 1.2 Å and 1.05 Å, respectively. The non-modified PNA duplex adopts a P-type helical structure s i m i l a r t o that of previously characterized PNAs. The atomic-level resolution of the structures allowed us to observe for the first time specific modes of interaction between the terminal lysines of the PNA and the backbone and nucleobases situated in the vicinity of the lysines, which are considered an important factor in the induction of a preferred handedness in PNA duplexes. These results support the notion that while PNA typically adopts a P-type helical structure, its flexibility is relatively high. For example, the base pair rise in the bipyridine-containing PNA is the largest measured to date in a PNA homoduplex. The two bipyridines are bulged out of the duplex and are aligned parallel to the minor groove of the PNA. In the case of the bipyridine-containing PNA, two bipyridines from adjacent PNA duplexes form a π-stacked pair that relates the duplexes within the crystal. The bulging out of the bipyridines causes bending of the PNA duplex, which is in contrast to the structure previously reported for biphenyl-modified DNA duplexes in solution, where the biphenyls are π-stacking with adjacent nucleobase pairs and adopt an intrahelical geometry [Johar et al., Chem. Eur. J., 2008, 14, 2080]. This difference shows that relatively small perturbations can significantly impact the relative position of nucleobase analogues in nucleic acid duplexes.
PMCID: PMC3194003  PMID: 20859960
PNA structure; X-ray crystallography; nucleic acids; bipyridine; nucleic acid bending
25.  Peptide Nucleic Acid-Based Array for Detecting and Genotyping Human Papillomaviruses▿  
Journal of Clinical Microbiology  2009;47(6):1785-1790.
We describe a novel array for accurate and reliable genotyping of human papillomavirus (HPV) using peptide nucleic acid (PNA) probes. In order to exploit the superior hybridization properties of PNA with target HPV DNAs, we developed a novel PNA array (PANArray HPV). PANArray HPV enables the detection and genotyping of HPVs using 32 type-specific PNA capture probes for medically important HPVs. All tested HPV types showed highly unique hybridization patterns with type-specific PNA probes. PNA array results showed stable specificities and sensitivities after up to 13 months of storage at room temperature. Also, we demonstrated the superior specificity, sensitivity, and stability of PNA arrays for HPV genotyping. We compared the genotyping results of the PNA array to sequencing with MY09/11 PCR products derived from 72 clinical samples. The results showed excellent agreement between the PNA array and sequencing, except for samples reflecting multiple infections. The results from the PNA array were compared with those of type-specific PCR when discrepant results occurred owing to multiple infections. The results for the PNA array matched those of type-specific PCR in all cases. Newly developed PNA arrays show excellent specificity and sensitivity and long shelf life. Our results suggest that the PNA array represents a reliable alternative to conventional DNA arrays for HPV genotyping, as well as for diagnostics.
PMCID: PMC2691110  PMID: 19369432

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