Solid-phase synthesis of pseudo-complementary peptide nucleic acids

Pseudo-Complementary G:C Base Pair for Mixed Sequence dsDNA Invasion and Its Applications in Diagnostics (SARS-CoV-2 Detection)

Pseudo-complementary oligonucleotides contain artificial nucleobases designed to reduce duplex formation in the pseudo-complementary pair without compromising duplex formation to targeted (complementary) oligomers. The development of a pseudo-complementary A:T base pair, U^s:D, was important in achieving dsDNA invasion. Herein, we report pseudo-complementary analogues of the G:C base pair leveraged on steric and electrostatic repulsion between the cationic phenoxazine analogue of cytosine (G-clamp, C⁺) and N-7 methyl guanine (G⁺), which is also cationic. We show that while complementary peptide nucleic acids (PNA) form a much more stable homoduplex than the PNA:DNA heteroduplex, oligomers based on pseudo-C:G complementary PNA favor PNA:DNA hybridization. We show that this enables dsDNA invasion at physiological salt concentration and that stable invasion complexes are obtained with low equivalents of PNAs (2-4 equiv). We harnessed the high yield of dsDNA invasion for the detection of RT-RPA amplicon using a lateral flow assay (LFA) and showed that two strains of SARS-CoV-2 can be discriminated owing to single nucleotide resolution.

Supernatural: Artificial Nucleobases and Backbones to Program Hybridization-Based Assemblies and Circuits

The specificity and predictability of hybridization make oligonucleotides a powerful platform to program assemblies and networks with logic-gated responses, an area of research which has grown into a field of its own. While the field has capitalized on the commercial availability of DNA oligomers with its four canonical nucleobases, there are opportunities to extend the capabilities of the hardware with unnatural nucleobases and other backbones. This Topical Review highlights nucleobases that favor hybridizations that are empowering for assemblies and networks as well as two chiral XNAs than enable orthogonal hybridization networks.

Thiazole Containing PNA Mimic Regulates c-MYC Gene Expression through DNA G-Quadruplex

Peptide nucleic acids (PNAs), besides hybridizing to complementary DNA and RNAs, bind and stabilize DNA secondary structures. Herein, we illustrate the design and synthesis of PNA-like scaffolds by incorporating five-membered thiazole rings as modified bases instead of nucleobases and their subsequent effects on gene regulation by biophysical and in vitro assays. A thiazole-modified PNA trimer selectively recognizes c-MYC G-quadruplex (G4) DNA over other G4s and duplex DNA. It displays a high stabilization potential for the c-MYC G4 DNA and shows remarkable fluorescence enhancement with the c-MYC G4. It is flexible enough to bind at 5' and 3' ends as well as in the groove region of c-MYC G4. Furthermore, the PNA trimer easily permeates the cellular membrane and suppresses c-MYC mRNA expression in HeLa cells by targeting the promoter G4. This study illuminates modified PNAs as flexible molecular tools for selective targeting of noncanonical nucleic acids and modulating gene function.

Sequence-Specific Recognition of Double-Stranded DNA by Peptide Nucleic Acid Forming Double-Duplex Invasion Complex

Peptide nucleic acid (PNA) is an analog of natural nucleic acids, where the sugar-phosphate backbone of DNA is replaced by an electrostatically neutral N-(2-aminoethyl)glycine backbone. This unique peptide-based backbone enables PNAs to form a very stable duplex with the complementary nucleic acids via Watson–Crick base pairing since there is no electrostatic repulsion between PNA and DNA·RNA. With this high nucleic acid affinity, PNAs have been used in a wide range of fields, from biological applications such as gene targeting, to engineering applications such as probe and sensor developments. In addition to single-stranded DNA, PNA can also recognize double-stranded DNA (dsDNA) through the formation of a double-duplex invasion complex. This double-duplex invasion is hard to achieve with other artificial nucleic acids and is expected to be a promising method to recognize dsDNA in cellula or in vivo since the invasion does not require the prior denaturation of dsDNA. In this paper, we provide basic knowledge of PNA and mainly focus on the research of PNA invasion.

Inhibiting mutant KRAS G12D gene expression using novel peptide nucleic acid‑based antisense: A potential new drug candidate for pancreatic cancer

KRAS mutations, which are the main cause of the pathogenesis of lethal pancreatic adenocarcinomas, impair the functioning of the GTPase subunit, thus rendering it constitutively active and signaling intracellular pathways that end with cell transformation. In the present study, the AsPC-1 cell line, which has a G12D-mutated KRAS gene sequence, was utilized as a cellular model to test peptide nucleic acid-based antisense technology. The use of peptide nucleic acids (PNAs) that are built to exhibit improved hybridization specificity and have an affinity for complementary RNA and DNA sequences, as well as a simple chemical structure and high biological stability that affords resistance to nucleases and proteases, enabled targeting of the KRAS-mutated gene to inhibit its expression at the translation level. Because PNA-based antisense molecules should be capable of binding to KRAS mRNA sequences, PNAs were utilized to target the mRNA of the mutated KRAS gene, a strategy that could lead to the development of a novel drug for pancreatic cancer. Moreover, it was demonstrated that introducing new PNA to cells inhibited the growth of cancer cells and induced apoptotic death and, notably, that it can inhibit G12D-mutated KRAS gene expression, as demonstrated by RT-PCR and western blotting. Altogether, these data strongly suggest that the use of PNA-based antisense agents is an attractive therapeutic approach to treating KRAS-driven cancers and may lead to the development of novel drugs that target the expression of other mutated genes.

Peptide nucleic acids and their role in gene regulation and editing

The unique properties of peptide nucleic acid (PNA) makes it a desirable candidate to be used in therapeutic and biotechnological interventions. It has been broadly utilized for numerous applications, with a major focus in regulation of gene expression, and more recently in gene editing. While the classic PNA design has mainly been employed to date, chemical modifications of the PNA backbone and nucleobases provide an avenue to advance the technology further. This review aims to discuss the recent developments in PNA based gene manipulation techniques and the use of novel chemical modifications to improve the current state of PNA mediated gene targeting.

Investigation of the Characteristics of NLS-PNA: Influence of NLS Location on Invasion Efficiency

Peptide nucleic acid can recognise sequences in double-stranded DNA (dsDNA) through the formation of a double-duplex invasion complex. This double-duplex invasion is a promising method for the recognition of dsDNA in cellula because peptide nucleic acid (PNA) invasion does not require the prior denaturation of dsDNA. To increase its applicability, we developed PNAs modified with a nuclear localisation signal (NLS) peptide. In this study, the characteristics of NLS-modified PNAs were investigated for the future design of novel peptide-modified PNAs.

Progress in Biomedical Applications of Tetrahedral Framework Nucleic Acid-Based Functional Systems

The past decades have witnessed the development of DNA nanotechnology and the emergence of various spatial DNA nanostructures, from two-dimensions to three-dimensions. The typical example is the tetrahedral framework nucleic acid (tFNA). In this review, we summarize the progress in fabrication, modification of tFNA-based functional systems and their potentials in biomedical applications. Through a one-step assembly process, tFNA is synthesized via four single stranded DNAs with three short sequences complementary to the other sequence of another single strand. Characterizations including polyacrylamide gel electrophoresis, atomic force microscopy, and dynamic light scattering measurement show tFNA as a pyramid-like nanostructure with the size of around 10 nm. Feathered with intrinsic biocompatibility and satisfactory cellular membrane permeability, the first generation of tFNA shows promising capacities in regulating cell biological behavior, promoting tissue regeneration, and immunomodulation. Along with excellent editability and relative biostability in complicated conditions, tFNA could be modified via hanging functional domains on the vertex or side arm and incorporating small-molecular-weight drugs to form the second generation, for reversing multidrug resistance in tumor cells or microorganisms, target therapy, anticancer and antibacterial treatments. The third generation of tFNA is currently tried via a multistep assembly process for stimuli-response and precise drug release. Although tFNAs show promising potentials in cargo delivery, massive efforts still need to be made to improve biostability, maximal load, and structural controllability.

Peptide nucleic acid (PNA) and its applications in chemical biology, diagnostics, and therapeutics

Peptide nucleic acid (PNA) stands as one of the most successful artificial oligonucleotide mimetics. Salient features include the stability of hybridization complexes (either as duplexes or triplexes), metabolic stability, and ease of chemical modifications. These features have enabled important applications such as antisense agents, gene editing, nucleic acid sensing and as a platform to program the assembly of PNA-tagged molecules. Here, we review recent advances in these areas.

HIF-1α Stabilization Increases miR-210 Eliciting First Trimester Extravillous Trophoblast Mitochondrial Dysfunction

Preeclampsia is associated with first trimester placental dysfunction. miR-210, a small non-coding RNA, is increased in the preeclamptic placenta. The effects of elevated miR-210 on placental function remain unclear. The objectives of this study were to identify targets of miR-210 in first trimester primary extravillous trophoblasts (EVTs) and to investigate functional pathways altered by elevated placental miR-210 during early pregnancy. EVTs isolated from first trimester placentas were exposed to cobalt chloride (CoCl₂), a HIF-1α stabilizer and hypoxia mimetic, and miR-210 expression by qPCR, HIF1α protein levels by western blot and cell invasion were assessed. A custom TruSeq RNA array, including all known/predicted miR-210 targets, was run using miR-210 and miR-negative control transfected EVTs. Mitochondrial function was assessed by high resolution respirometry in transfected EVTs. EVTs exposed to CoCl₂ showed a dose and time-dependent increase in miR-210 and HIF1α and reductions in cell invasion. The TruSeq array identified 49 altered genes in miR-210 transfected EVTs with 27 genes repressed and 22 enhanced. Three of the top six significantly repressed genes, NDUFA4, SDHD, and ISCU, are associated with mitochondrial function. miR-210 transfected EVTs had decreased maximal, complex II and complex I+II mitochondrial respiration. This study suggests that miR-210 alters first trimester trophoblast function. miR-210 overexpression alters EVT mitochondrial function in early pregnancy. Mitochondrial dysfunction may lead to increased reactive oxygen species, trophoblast cell damage and likely contributes to the pathogenesis of preeclampsia.

Affinity Isolation of Defined Genomic Fragments Cleaved by Nuclease S1-based Artificial Restriction DNA Cutter

The human genome is highly susceptible to various modifications, lesions, and damage. To analyze lesions and proteins bound to a defined region of the human genome, the genome should be fragmented at desired sites and the region of interest should be isolated. The few available methods for isolating a desired region of the human genome have serious drawbacks and can only be applied to specific sequences or require tedious experimental procedures. We have recently developed a novel method to isolate a desired fragment of the genome released by site-specific scission of DNA using a pair of pseudo-complementary peptide nucleic acids (pcPNAs) and S1 nuclease. When conjugated to biotin, one of the pcPNAs can be used to affinity purify the cleavage product. Here we report a detailed protocol to isolate defined kilobase-length DNA fragments that can be applied to plasmid or genomic DNA and is not limited by sequence. © 2019 by John Wiley & Sons, Inc.

Artificial Restriction DNA Cutter Using Nuclease S1 for Site-Selective Scission of Genomic DNA

By combining a pair of pseudo-complementary peptide nucleic acids (pcPNAs) with S1 nuclease, a novel tool to cut DNA at a predetermined site can be obtained. Complementary pcPNAs invade the DNA duplex and base pair to each strand of a target site, creating single-stranded regions that are cleaved by S1 nuclease. The scission site can be freely modulated by the design of pcPNAs. This method can be used to cleave a single site in the human genome. This protocol presents experimental details for site-selective scission using this versatile new tool. © 2019 by John Wiley & Sons, Inc.

Inhibiting Methicillin-Resistant Staphylococcus aureus by Tetrahedral DNA Nanostructure-Enabled Antisense Peptide Nucleic Acid Delivery

One of the biggest obstacles for the use of antisense oligonucleotides as antibacterial therapeutics is their limited uptake by bacterial cells without a suitable carrier, especially in multi-drug-resistant bacteria with a drug efflux mechanism. Existing vectors, such as cell-penetrating peptides, are inefficient and nontargeting, and accordingly are not ideal carriers. A noncytotoxic tetrahedral DNA nanostructure (TDN) with a controllable conformation has been developed as a delivery vehicle for antisense oligonucleotides. In this study, antisense peptide nucleic acids (asPNAs) targeting a specific gene ( ftsZ) were efficiently transported into methicillin-resistant Staphylococcus aureus cells by TDNs, and the expression of ftsZ was successfully inhibited in an asPNA-concentration-dependent manner. The delivery system specifically targeted the intended gene. This novel delivery system provides a better platform for future applications of antisense antibacterial therapeutics and provides a basis for the development of a new type of antibacterial drug for multi-drug-resistant bacterial infections.

Peptide Nucleic Acid Conjugated with Ruthenium-Complex Stabilizing Double-Duplex Invasion Complex Even under Physiological Conditions

Peptide nucleic acid (PNA) can form a stable duplex with DNA, and, accordingly, directly recognize double-stranded DNA through the formation of a double-duplex invasion complex, wherein a pair of complementary PNA strands form two PNA/DNA duplexes. Because invasion does not require prior denaturation of DNA, PNA holds great potential for in cellulo or in vivo applications. To broaden the applicability of PNA invasion, we developed a new conjugate of PNA with a ruthenium complex. This Ru-PNA conjugate exhibits higher DNA-binding affinity, which results in enhanced invasion efficiency, even under physiological conditions.

One-Pot Isolation of a Desired Human Genome Fragment by Using a Biotinylated pcPNA/S1 Nuclease Combination

Scission of the human genome at predetermined sites and isolation of a particular fragment are of great interest for the analysis of lesion/modification sites, in proteomics, and for gene therapy. However, methods for human genome scission and specific fragment isolation are limited. Here, we report a novel one-pot method for the site-specific scission of DNA by using a biotinylated pcPNA/S1 nuclease combination and isolation of a desired fragment by streptavidin-coated magnetic beads. The proof of concept was initially demonstrated for the clipping of plasmid DNA and isolation of the required fragment. Our method was then successfully applied for the isolation of a fragment from the cell-derived human genome.

PNA monomers fully compatible with standard Fmoc-based solid-phase synthesis of pseudocomplementary PNA

Here we report the synthesis of new PNA monomers for pseudocomplementary PNA (pcPNA) that are fully compatible with standard Fmoc chemistry. The thiocarbonyl group of the 2-thiouracil (sU) monomer was protected with the 4-methoxy-2-methybenzyl group (MMPM), while the exocyclic amino groups of diaminopurine (D) were protected with Boc groups. The newly synthesized monomers were incorporated into a 10-mer PNA oligomer using standard Fmoc chemistry for solid-phase synthesis. Oligomerization proceeded smoothly and the HPLC and MALDI-TOF MS analyses indicated that there was no remaining MMPM on the sU nucleobase. The new PNA monomers reported here would facilitate a wide range of applications, such as antigene PNAs and DNA nanotechnologies.

Water-soluble clickable nucleic acid (CNA) polymer synthesis by functionalizing the pendant hydroxyl

Water-soluble single stranded DNA analogs are generated via thiol–ene photo-oligomerization.

Double duplex invasion of DNA induced by ultrafast photo-cross-linking using 3-cyanovinylcarbazole for antigene methods

New photoresponsive antigene probes containing^CNVK and^CNU have a high double-duplex invasion capability upon photoirradiation because of the inhibition of photo-cross-linking between the probes.

Fabrication of DNA/RNA Hybrids Through Sequence-Specific Scission of Both Strands by pcPNA-S1 Nuclease Combination

By combining two strands of pseudo-complementary peptide nucleic acid (pcPNA) with S1 nuclease, a tool for site-selective and dual-strand scission of DNA/RNA hybrids has been developed. Both of the DNA and the RNA strands in the hybrids are hydrolyzed at desired sites to provide unique sticky ends. The scission fragments are directly ligated with other DNA/RNA hybrids by using T4 DNA ligase, resulting in the formation of desired recombinant DNA/RNA hybrids.

Development of a Backbone Cyclic Peptide Library as Potential Antiparasitic Therapeutics Using Microwave Irradiation

Protein-protein interactions (PPIs) are intimately involved in almost all biological processes and are linked to many human diseases. Therefore, there is a major effort to target PPIs in basic research and in the pharmaceutical industry. Protein-protein interfaces are usually large, flat, and often lack pockets, complicating the discovery of small molecules that target such sites. Alternative targeting approaches using antibodies have limitations due to poor oral bioavailability, low cell-permeability, and production inefficiency. Using peptides to target PPI interfaces has several advantages. Peptides have higher conformational flexibility, increased selectivity, and are generally inexpensive. However, peptides have their own limitations including poor stability and inefficiency crossing cell membranes. To overcome such limitations, peptide cyclization can be performed. Cyclization has been demonstrated to improve peptide selectivity, metabolic stability, and bioavailability. However, predicting the bioactive conformation of a cyclic peptide is not trivial. To overcome this challenge, one attractive approach it to screen a focused library to screen in which all backbone cyclic peptides have the same primary sequence, but differ in parameters that influence their conformation, such as ring size and position. We describe a detailed protocol for synthesizing a library of backbone cyclic peptides targeting specific parasite PPIs. Using a rational design approach, we developed peptides derived from the scaffold protein Leishmania receptor for activated C-kinase (LACK). We hypothesized that sequences in LACK that are conserved in parasites, but not in the mammalian host homolog, may represent interaction sites for proteins that are critical for the parasites' viability. The cyclic peptides were synthesized using microwave irradiation to reduce reaction times and increase efficiency. Developing a library of backbone cyclic peptides with different ring sizes facilitates a systematic screen for the most biological active conformation. This method provides a general, fast, and facile way to synthesize cyclic peptides.

Clipping of Telomere from Human Chromosomes Using a Chemistry-Based Artificial Restriction DNA Cutter

The detection of individual telomere lengths of human chromosomes can provide crucial information on genome stability, cancer, and telomere-related diseases. However, current methods to measure telomere length entail shortcomings that have limited their use. Recently, we have developed a method for detection of individual telomere lengths (DITL) that uses a chemistry-based DNA-cutting approach. The most beneficial feature of the DITL approach is to cleave the sequence adjacent to the telomere followed by resolution of the telomere length at the nucleotide level of a single chromosome. In this unit, a protocol for successful detection of individual telomere lengths from individual chromosomes is described in detail.

Sequence selective tagging of 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodGuo) using PNAs

8-Oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodGuo) is a commonly formed DNA lesion that is useful as a biomarker for oxidative stress. Methods for detecting 8-oxodGuo at specific positions within DNA could be useful for correlating DNA damage with mutational hotspots and repair enzyme accessibility. We describe a method for covalently linking ('tagging') peptide nucleic acids (PNAs) containing terminal nucleophiles under oxidative conditions to 8-oxodGuo at specific sites within DNA. Several nucleophiles were examined and the ε-amine of lysine was selected for further studies. As little as 10 fmol of 8-oxodGuo were detected by gel shift using (32)P-labeled target DNA and no tagging of dG at the same site or 8-oxodGuo at a distal site was detected when potassium ferricyanide was used as oxidant in substrates as long as 221 bp.

Promotion of double-duplex invasion of peptide nucleic acids through conjugation with nuclear localization signal peptide

Pseudo-complementary peptide nucleic acid (pcPNA), as one of the most widely used synthetic DNA analogues, invades double-stranded DNA according to Watson-Crick rules to form invasion complexes. This unique mode of DNA recognition induces structural changes at the invasion site and can be used for a range of applications. In this paper, pcPNA is conjugated with a nuclear localization signal (NLS) peptide, and its invading activity is notably promoted both thermodynamically and kinetically. Thus, the double-duplex invasion complex is formed promptly at low pcPNA concentrations under high salt conditions, where the invasion otherwise never occurs. Furthermore, NLS-modified pcPNA is successfully employed for site-selective DNA scission, and the targeted DNA is selectively cleaved under conditions that are not conducive for DNA cutters using unmodified pcPNAs. This strategy of pcPNA modification is expected to be advantageous and promising for a range of in vitro and in vivo applications.

Chemical modifications of artificial restriction DNA cutter (ARCUT) to promote its in vivo and in vitro applications

Recently, completely chemistry-based tools for site-selective scission of DNA (ARCUT) have been prepared by combining 2 strands of pseudo-complementary PNA (pcPNA: site-selective activator) and a Ce(IV)-EDTA complex (molecular scissors). Its site-specificity is sufficient to cut the whole human genome at one predetermined site. In this first-generation ARCUT, however, there still remain several problems to be solved for wider applications. This review presents recent approaches to solve these problems. They are divided into (i) covalent modification of pcPNA with other functional groups and (ii) new strategies using conventional PNA, in place of pcPNA, as site-selective activator. Among various chemical modifications, conjugation with positively-charged nuclear localization signal peptide is especially effective. Furthermore, unimolecular activators, a single strand of which successfully activates the target site in DNA for site-selective scission, have been also developed. As the result of these modifications, the site-selective scission by Ce(IV)-EDTA was achieved promptly even under high salt conditions which are otherwise unfavourable for double-duplex invasion. Furthermore, it has been shown that "molecular crowding effect," which characterizes the inside of living cells, enormously promotes the invasion, and thus the invasion seems to proceed effectively and spontaneously in the cells. Strong potential of pcPNA for further applications in vivo and in vitro has been confirmed.

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.

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.

Clipping of predetermined fragments from the human genome by S1 nuclease-PNA combinations

By combining S1 nuclease with two strands of pseudo-complementary peptide nucleic acid (pcPNA), the whole human genome was selectively cut at targeted sites, and desired fragments were clipped from the genome.

PNA-NLS conjugates as single-molecular activators of target sites in double-stranded DNA for site-selective scission

Artificial DNA cutters have been developed by us in our previous studies by combining two strands of pseudo-complementary peptide nucleic acid (pcPNA) with Ce(IV)-EDTA-promoted hydrolysis. The pcPNAs have two modified nucleobases (2,6-diaminopurine and 2-thiouracil) instead of conventional A and T, and can invade double-stranded DNA to activate the target site for the scission. This system has been applied to site-selective scissions of plasmid, λ-phage, E. coli genomic DNA, and human genomic DNA. Here, we have reported a still simpler and more convenient DNA cutter obtained by conjugating peptide nucleic acid (PNA) with a nuclear localization signal (NLS) peptide. This new DNA cutter requires only one PNA strand (instead of two) bearing conventional (non-pseudo-complementary) nucleobases. This PNA-NLS conjugate effectively activated the target site in double-stranded DNA and induced site-selective scission by Ce(IV)-EDTA. The complex formation between the conjugate and DNA was concretely evidenced by spectroscopic results based on time-resolved fluorescence. The target scission site of this new system was straightforwardly determined by the Watson-Crick base pairing rule, and mismatched sequences were clearly discriminated. Importantly, even highly GC-rich regions, which are difficult to be targeted by a previous strategy using pcPNA, were successfully targeted. All these features of the present DNA cutter make it promising for various future applications.

Site-selective scission of human genome using PNA-based artificial restriction DNA cutter

Site-selective scission of genomes is quite important for future biotechnology. However, naturally occurring restriction enzymes cut these huge DNAs at too many sites and cannot be used for this purpose. Recently, we have developed a completely chemistry-based artificial restriction DNA cutter (ARCUT) by combining a pair of pseudo-complementary PNA (pcPNA) strands (sequence recognition moiety) and Ce(IV)/EDTA complex (molecular scissors). The scission site of ARCUT and its scission specificity can be freely modulated in terms of the sequences and lengths of the pcPNA strands so that even huge genomes can be selectively cut at only one predetermined site. In this chapter, the method of site-selective scission of human genomic DNA using ARCUT is described in detail.

Thiazole orange-conjugated peptide nucleic acid for fluorescent detection of specific DNA sequences and site-selective photodamage

Conjugates of thiazole orange (TO) with a pseudo-complementary peptide nucleic acid (pcPNA) functioned as (i) fluorescent detector of specific DNA and (ii) site-selective photodamage inducer through generation of ¹O₂.

Conjugation of peptide nucleic acid with a pyrrole/imidazole polyamide to specifically recognize and cleave DNA

Cut loose: A pseudocomplementary peptide nucleic acid was tethered to a pyrrole/imidazole hairpin polyamide, and was used to selectively target a specific DNA sequence. Binding even occurs under high salt conditions. Furthermore, the conjugate facilitated sequence-specific scission of long dsDNA. This simple approach promises to resolve the technical difficulties in targeting DNA sequences with PNA.

Pseudo-complementary PNA actuators as reversible switches in dynamic DNA nanotechnology

The structural reorganization of nanoscale DNA architectures is a fundamental aspect in dynamic DNA nanotechnology. Commonly, DNA nanoarchitectures are reorganized by means of toehold-expanded DNA sequences in a strand exchange process. Here we describe an unprecedented, toehold-free switching process that relies on pseudo-complementary peptide nucleic acid (pcPNA) by using a mechanism that involves double-strand invasion. The usefulness of this approach is demonstrated by application of these peptide nucleic acids (PNAs) as switches in a DNA rotaxane architecture. The monomers required for generating the pcPNA were obtained by an improved synthesis strategy and were incorporated into a PNA actuator sequence as well as into a short DNA strand that subsequently was integrated into the rotaxane architecture. Alternate addition of a DNA and PNA actuator sequence allowed the multiple reversible switching between a mobile rotaxane macrocycle and a stationary pseudorotaxane state. The switching occurs in an isothermal process at room temperature and is nearly quantitative in each switching step. pcPNAs can potentially be combined with light- and toehold-based switches, thus broadening the toolbox of orthogonal switching approaches for DNA architectures that open up new avenues in dynamic DNA nanotechnology.

Cut-and-Paste of DNA Using an Artificial Restriction DNA Cutter

 DNA manipulations using a completely chemistry-based DNA cutter (ARCUT) have been reviewed. This cutter, recently developed by the authors, is composed of Ce(IV)/EDTA complex and two strands of pseudo-complementary peptide nucleic acid. The site-selective scission proceeds via hydrolysis of targeted phosphodiester linkages, so that the resultant scission fragments can be easily ligated with other fragments by using DNA ligase. Importantly, scission-site and site-specificity of the cutter are freely tuned in terms of the Watson-Crick rule. Thus, when one should like to manipulate DNA according to the need, he or she does not have to think about (1) whether appropriate "restriction enzyme sites" exist near the manipulation site and (2) whether the site-specificity of the restriction enzymes, if any, are sufficient to cut only the aimed position without chopping the DNA at non-targeted sites. Even the human genome can be manipulated, since ARCUT can cut the genome at only one predetermined site. Furthermore, the cutter is useful to promote homologous recombination in human cells, converting a site to desired sequence. The ARCUT-based DNA manipulation should be promising for versatile applications.

A chemistry-based method to detect individual telomere length at a single chromosome terminus

The understanding of telomeres is expected to provide major insights into genome stability, cancer, and telomere-related diseases. In recent years, there have been considerable improvements in the technologies available to determine the length of telomeres of human chromosomes; however, the present methods for measuring telomere length are fraught with shortcomings that have limited their use. Here we describe a method for detection of individual telomere lengths (DITL) that uses a chemistry-based approach that accurately measures the telomere lengths from individual chromosomes. The method was successfully used to determine telomere DNA by breaking in the target sequence and producing a "real telomere fragment." The DITL approach involves cleavage of the sequence adjacent to the telomere followed by resolution of the telomere length at the nucleotide level of a single chromosome. Comparison of the DITL method and the traditional terminal restriction fragment (TRF) analysis indicates that the DITL approach appears to be promising for the quantification of telomere repeats in each chromosome and the detection of accurate telomere lengths that can be missed using TRF analysis.

G-rich sequence-specific recognition and scission of human genome by PNA/DNA hybrid G-quadruplex formation

Hole in one: A single peptide nucleic acid (PNA) effectively targets the G-rich region in double-stranded DNA through formation of a PNA/DNA hybrid G-quadruplex. Only one target site in the whole human genome was selectively cleaved by the hybrid G-quadruplex. Such site-selective scission of DNA is central to gene manipulation for molecular biology, biotechnology, and therapy.

Efficient synthesis of exo-N-carbamoyl nucleosides: application to the synthesis of phosphoramidate prodrugs

An efficient protection protocol for the 6-exo-amino group of purine nucleosides with various chloroformates was developed utilizing N-methylimidazole (NMI). The reaction of an exo-N(6)-group of adenosine analogue 1 with alkyl/and aryl chloroformates under optimized conditions provided the N(6)-carbamoyl adenosines (2a-j) in good to excellent yields. The reaction of N(6)-Cbz-protected nucleosides (5a-c) with phenyl phosphoryl chloride (7) using t-BuMgCl followed by catalytic hydrogenation afforded the corresponding phosphoramidate pronucleotides (8a-c) in excellent yield.

Introduction of multiphosphonate ligand to peptide nucleic acid for metal ion conjugation

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.