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

From Collagen Mimetics to Collagen Hybridization and Back

ConspectusFacilitated by the unique triple-helical protein structure, fibrous collagens, the principal proteins in animals, demonstrate a dual function of serving as building blocks for tissue scaffolds and as a bioactive material capable of swift renewal in response to environmental changes. While studies of triple-helical collagen mimetic peptides (CMPs) have been instrumental in understanding the molecular forces responsible for the folding and assembly of triple helices, as well as identifying bioactive regions of fibrous collagen molecules, single-strand CMPs that can specifically target and hybridize to denatured collagens (i.e., collagen hybridizing peptides, CHPs) have proven useful in identifying the remodeling activity of collagen-rich tissues related to development, homeostasis, and pathology. Efforts to improve the utility of CHPs have resulted in the development of new skeletal structures, such as dimeric and cyclic CHPs, as well as the incorporation of artificial amino acids, including fluorinated proline and N-substituted glycines (peptoid residues). In particular, dimeric CHPs were used to capture collagen fragments from biological fluid for biomarker study, and the introduction of peptoid-based collagen mimetics has sparked renewed interest in peptidomimetic research because peptoids enable a stable triple-helical structure and the presentation of an extensive array of side chain structures offering a versatile platform for the development of new collagen mimetics.This Account will cover the evolution of our research from CMPs as biomaterials to ongoing efforts in developing triple-helical peptides with practical theranostic potential in targeting denatured and damaged collagens. Our early efforts in functionalizing natural collagen scaffolds via noncovalent modifications led to the discovery of an entirely new use of CMPs. This discovery resulted in the development of CHPs that are now used by many different laboratories for the investigation of pathologies associated with changes in the structures of extracellular matrices including fibrosis, cancer, and mechanical damage to collagen-rich, load-bearing tissues. Here, we delve into the essential design features of CHPs contributing to their collagen binding properties and practical usage and explore the necessity for further mechanistic understanding of not only the binding processes (e.g., binding domain and stoichiometry of the hybridized complex) but also the biology of collagen degradation, from proteolytic digestion of fibrils to cellular processing of collagen fragments. We also discuss the strengths and weaknesses of peptoid-based triple-helical peptides as applied to collagen hybridization touching on thermodynamic and kinetic aspects of triple-helical folding. Finally, we highlight current limitations and future directions in the use of peptoid building blocks to develop bioactive collagen mimetics as new functional biomaterials.

Recent advancements in bionanomaterial applications of peptide nucleic acid assemblies

Peptide nucleic acid (PNA) is a unique combination of peptides and nucleic acids. PNA can exhibit hydrogen bonding interactions with complementary nucleobases like DNA/RNA. Also, its polyamide backbone allows easy incorporation of biomolecules like peptides and proteins to build hybrid molecular constructs. Because of chimeric structural properties, PNA has lots of potential to build diverse nanostructures. However, progress in the PNA material field is still immature compared with its massive applications in antisense oligonucleotide research. Examples of well-defined molecular assemblies have been reported with PNA amphiphiles, self-assembling guanine-PNA monomers/dimers, and PNA-decorated nucleic acids/ polymers/ peptides. All these works indicate the great potential of PNA to be used as bionanomaterials. The review summarizes the recent reports on PNA-based nanostructures and their versatile applications. Additionally, this review shares a perspective to promote a better understanding of controlling molecular assembly by the systematic structural modifications of PNA monomers.

Unexpectedly stable homopurine parallel triplex of SNA:RNA*SNA and L-aTNA:RNA*L-aTNA

Homopurine strands are known to form antiparallel triplexes stabilized by G*G and A*A Hoogsteen pairs, which have two hydrogen bonds. But there has been no report on the parallel triplex formation of homopurine involving both adenosine and guanosine to the duplex. In this paper, we first report parallel triplex formation between a homopurine serinol nucleic acid (SNA) strand and an RNA/SNA duplex. Melting profiles revealed that the parallel SNA:RNA*SNA triplex was remarkably stable, even though the A*A pair has a single hydrogen bond. An L-acyclic threoninol nucleic acid (L-aTNA) homopurine strand also formed a stable parallel triplex with an L-aTNA/RNA duplex.

Antitumor efficacy of a sequence-specific DNA-targeted γPNA-based c-Myc inhibitor

Targeting oncogenes at the genomic DNA level can open new avenues for precision medicine. Significant efforts are ongoing to target oncogenes using RNA-targeted and protein-targeted platforms, but no progress has been made to target genomic DNA for cancer therapy. Here, we introduce a gamma peptide nucleic acid (γPNA)-based genomic DNA-targeted platform to silence oncogenes in vivo. γPNAs efficiently invade the mixed sequences of genomic DNA with high affinity and specificity. As a proof of concept, we establish that γPNA can inhibit c-Myc transcription in multiple cell lines. We evaluate the in vivo efficacy and safety of genomic DNA targeting in three pre-clinical models. We also establish that anti-transcription γPNA in combination with histone deacetylase inhibitors and chemotherapeutic drugs results in robust antitumor activity in cell-line- and patient-derived xenografts. Overall, this strategy offers a unique therapeutic platform to target genomic DNA to inhibit oncogenes for cancer therapy.

UltraFast PhotoInduced double duplex DNA invasion into a 400-mer dsDNA target

BACKGROUND

Natural DNA restriction enzymes bind duplex DNA with high affinity at multiple sites; however, for some of the artificial chemical-based restriction moieties, invasion of the double-strand for efficient cleavage is an obstacle. We have previously reported photo-induced double-duplex invasion (pDDI) using 3-cyanovinylcarbazole (K)-containing probes for both the target strands that photo-crosslink with pyrimidine bases in a sequence-specific manner on both the strands, stabilizing the opened double-strand for cleavage. The drawback of the pDDI was low efficiency due to inter-probe cross-linking, solved by the inclusion of 5-cyano-uridine at -1 position on the complimentary strand with respect to K in both probes. Although this led to reduced inter-probe cross-linking, the pDDI efficiency was still low.

RESULTS

Here, we report that inter-probe cross-linking and intra-probe cross-linking of a single probe is also leading to reduced pDDI efficiency. We addressed this problem by designing DDI probes to inhibit both inter-probe and intra-probe cross-linking.

CONCLUSION

Based on the new design of pDDI probe with 5-cyano uridine led to a drastic increase in the efficiency of pDDI in (400-mer) double-stranded DNA with only 1 s of photo-irradiation.

Pursuing excitonic energy transfer with programmable DNA-based optical breadboards

DNA nanotechnology has now enabled the self-assembly of almost any prescribed 3-dimensional nanoscale structure in large numbers and with high fidelity. These structures are also amenable to site-specific modification with a variety of small molecules ranging from drugs to reporter dyes. Beyond obvious application in biotechnology, such DNA structures are being pursued as programmable nanoscale optical breadboards where multiple different/identical fluorophores can be positioned with sub-nanometer resolution in a manner designed to allow them to engage in multistep excitonic energy-transfer (ET) via Förster resonance energy transfer (FRET) or other related processes. Not only is the ability to create such complex optical structures unique, more importantly, the ability to rapidly redesign and prototype almost all structural and optical analogues in a massively parallel format allows for deep insight into the underlying photophysical processes. Dynamic DNA structures further provide the unparalleled capability to reconfigure a DNA scaffold on the fly in situ and thus switch between ET pathways within a given assembly, actively change its properties, and even repeatedly toggle between two states such as on/off. Here, we review progress in developing these composite materials for potential applications that include artificial light harvesting, smart sensors, nanoactuators, optical barcoding, bioprobes, cryptography, computing, charge conversion, and theranostics to even new forms of optical data storage. Along with an introduction into the DNA scaffolding itself, the diverse fluorophores utilized in these structures, their incorporation chemistry, and the photophysical processes they are designed to exploit, we highlight the evolution of DNA architectures implemented in the pursuit of increased transfer efficiency and the key lessons about ET learned from each iteration. We also focus on recent and growing efforts to exploit DNA as a scaffold for assembling molecular dye aggregates that host delocalized excitons as a test bed for creating excitonic circuits and accessing other quantum-like optical phenomena. We conclude with an outlook on what is still required to transition these materials from a research pursuit to application specific prototypes and beyond.

Design and Synthesis of Artificial Nucleobases for Sequence-Selective DNA Recognition within the Major Groove

We present the design and synthesis of artificial specific nucleobases, each one recognizing a single base pair within the major groove of duplex DNA. Computational calculations indicate that PNAs modified with these nucleobases enable the formation of highly stable triple helices with no sequence restrictions through multiple hydrogen bonding and π⋅⋅⋅π stacking interactions, without significantly widening the DNA double helix. New synthetic routes were developed to the structures of these fused heterocycles which have rarely been described in the literature. NMR titration experiments indicate specific hydrogen bonding at the Hoogsteen sites. The new building blocks allow the construction of four PNA monomers for each canonic base pair and their covalent connection to PNA oligomers. These can be designed complementary to any given DNA sequence. With high efficiency and relative simplicity of operation, the described methodologies and strategies hence form the basis for a new supramolecular ligand system targeting double-stranded DNA without strand invasion.

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.

Syntheses of Base-Labile Pseudo-Complementary SNA and l-aTNA Phosphoramidite Monomers

We previously synthesized phosphoramidite monomers bearing Boc-protected 2,6-diaminopurine (D) and 2-methyl-4-methoxybenzyl-protected 2-thiouracil (sU) as building blocks for the preparation of pseudo-complementary serinol nucleic acids (SNAs). Since SNA is stable under acidic conditions, an acid-deprotection step could be inserted into the work-up. However, as the 4,4'-dimethoxytrityl group was concurrently removed at this step, purification of SNA by reversed-phase HPLC was difficult. Here, we report the syntheses of SNA and acyclic l-threoninol nucleic acid (l-aTNA) phosphoramidite monomers with bis(phenoxyacetyl)-protected D and 4-acetoxybenzyl-protected sU, both of which can be deprotected under mild basic conditions. Using these monomers, we prepared pseudo-complementary SNA and l-aTNA in high yield using conventional oligonucleotide synthesis protocols. These monomers can be used for large-scale syntheses of SNAs and l-aTNAs.

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.

Targeting a KRAS i-motif forming sequence by unmodified and gamma-modified peptide nucleic acid oligomers

Growing interest in i-motif DNA as a transcriptional regulatory element motivates development of synthetic molecules capable of targeting these structures. In this study, we designed unmodified peptide nucleic acid (PNA) and gamma-modified PNA (γPNA) oligomers complementary to an i-motif forming sequence derived from the promoter of the KRAS oncogene. Biophysical techniques such as circular dichroism (CD) spectroscopy, CD melting, and fluorescence spectroscopy demonstrated the successful invasion of the i-motif by PNA and γPNA. Both PNA and γPNA showed very strong binding to the target sequence with high thermal stability of the resulting heteroduplexes. Interestingly fluorescence and CD experiments indicated formation of an intermolecular i-motif structure via the overhangs of target-probe heteroduplexes formed by PNA/γPNA invasion of the intramolecular i-motif. Targeting promoter i-motif forming sequences with high-affinity oligonucleotide mimics like γPNAs may represent a new approach for inhibiting KRAS transcription, thereby representing a potentially useful anti-cancer strategy.

Factors Impacting Invader-Mediated Recognition of Double-Stranded DNA

The development of chemically modified oligonucleotides enabling robust, sequence-unrestricted recognition of complementary chromosomal DNA regions has been an aspirational goal for scientists for many decades. While several groove-binding or strand-invading probes have been developed towards this end, most enable recognition of DNA only under limited conditions (e.g., homopurine or short mixed-sequence targets, low ionic strength, fully modified probe strands). Invader probes, i.e., DNA duplexes modified with +1 interstrand zippers of intercalator-functionalized nucleotides, are predisposed to recognize DNA targets due to their labile nature and high affinity towards complementary DNA. Here, we set out to gain further insight into the design parameters that impact the thermal denaturation properties and binding affinities of Invader probes. Towards this end, ten Invader probes were designed, and their biophysical properties and binding to model DNA hairpins and chromosomal DNA targets were studied. A Spearman's rank-order correlation analysis of various parameters was then performed. Densely modified Invader probes were found to result in efficient recognition of chromosomal DNA targets with excellent binding specificity in the context of denaturing or non-denaturing fluorescence in situ hybridization (FISH) experiments. The insight gained from the initial phase of this study informed subsequent probe optimization, which yielded constructs displaying improved recognition of chromosomal DNA targets. The findings from this study will facilitate the design of efficient Invader probes for applications in the life sciences.

Serine-γPNA, Invader probes, and chimeras thereof: three probe chemistries that enable sequence-unrestricted recognition of double-stranded DNA

Three probe chemistries are evaluated with respect to thermal denaturation temperatures, UV-Vis and fluorescence characteristics, recognition of complementary and mismatched DNA hairpin targets, and recognition of chromosomal DNA targets in the context of non-denaturing fluorescence in situ hybridization (nd-FISH) experiments: (i) serine-γPNAs (SγPNAs), i.e., single-stranded peptide nucleic acid (PNA) probes that are modified at the γ-position with (R)-hydroxymethyl moieties, (ii) Invader probes, i.e., DNA duplexes modified with +1 interstrand zippers of 2'-O-(pyren-1-yl)methyl-RNA monomers, a molecular arrangement that results in a violation of the neighbor exclusion principle, and (iii) double-stranded chimeric SγPNAs:Invader probes, i.e., duplexes between complementary SγPNA and Invader strands, which are destabilized due to the poor compatibility between intercalators and PNA:DNA duplexes. Invader probes resulted in efficient, highly specific, albeit comparatively slow recognition of the model DNA hairpin targets. Recognition was equally efficient and faster with the single-stranded SγPNA probes but far less specific, whilst the double-stranded chimeric SγPNAs:Invader probes displayed recognition characteristics that were intermediate of the parent probes. All three probe chemistries demonstrated the capacity to target chromosomal DNA in nd-FISH experiments, with Invader probes resulting in the most favorable and consistent characteristics (signals in >90% of interphase nuclei against a low background and no signal in negative control experiments). These probe chemistries constitute valuable additions to the molecular toolbox needed for DNA-targeting applications.

Perspectives on conformationally constrained peptide nucleic acid (PNA): insights into the structural design, properties and applications

Peptide nucleic acid or PNA is a synthetic DNA mimic that contains a sequence of nucleobases attached to a peptide-like backbone derived from N-2-aminoethylglycine. The semi-rigid PNA backbone acts as a scaffold that arranges the nucleobases in a proper orientation and spacing so that they can pair with their complementary bases on another DNA, RNA, or even PNA strand perfectly well through the standard Watson-Crick base-pairing. The electrostatically neutral backbone of PNA contributes to its many unique properties that make PNA an outstanding member of the xeno-nucleic acid family. Not only PNA can recognize its complementary nucleic acid strand with high affinity, but it does so with excellent specificity that surpasses the specificity of natural nucleic acids and their analogs. Nevertheless, there is still room for further improvements of the original PNA in terms of stability and specificity of base-pairing, direction of binding, and selectivity for different types of nucleic acids, among others. This review focuses on attempts towards the rational design of new generation PNAs with superior performance by introducing conformational constraints such as a ring or a chiral substituent in the PNA backbone. A large collection of conformationally rigid PNAs developed during the past three decades are analyzed and compared in terms of molecular design and properties in relation to structural data if available. Applications of selected modified PNA in various areas such as targeting of structured nucleic acid targets, supramolecular scaffold, biosensing and bioimaging, and gene regulation will be highlighted to demonstrate how the conformation constraint can improve the performance of the PNA. Challenges and future of the research in the area of constrained PNA will also be discussed.

Nanoscale structures and mechanics of peptide nucleic acids

Peptide nucleic acids (PNAs) are charge-neutral polyamide oligomers having extremely favorable thermal stability and high affinity to cell membranes when coupled with cationic cell-penetrating peptides (CPPs), as well as the encouraging antisense and antigene activity in cell-free systems. The study of the mechanical properties of short PNA molecules is rare both in experiments and theoretical calculations. Here, we studied the microscopic structures and elastic properties; namely, persistence length, stretch modulus, twist-stretch coupling, and structural crookedness of double-stranded PNA (dsPNA) and their hybrid derivatives using all-atom MD simulation and compared them with those of double-stranded DNA (dsDNA) and double-stranded RNA (dsRNA). The stretch modulus of the dsPNA is found to be ∼160 pN, an order of magnitude lower than that of dsDNA and smaller than dsRNA, respectively. Similarly, the persistence length of dsPNA is found to be ∼35 nm, significantly smaller than those of dsDNA and dsRNA. The PNA-DNA and PNA-RNA hybrid duplexes have elastic properties lying between that of dsPNA and dsDNA/dsRNA. We argue that the neutral backbones of the PNA make it less stiff than dsDNA and dsRNA molecules. Measurement of structural crookedness and principal component analysis additionally support the bending flexibility of dsPNA. Detailed analysis of the helical-rise coupled to helical-twist indicates that the PNA-DNA hybrid over-winds like dsDNA, while PNA-PNA and PNA-RNA unwind like dsRNA upon stretching. Because of the highly flexible nature of PNA, it can bind other biomolecules by adopting a wide range of conformations and is believed to be crucial for future nanobiotechnology research studies.

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.

Nicked Invader probes: multistranded and sequence-unrestricted recognition of double-stranded DNA

Major efforts have been devoted to the development of constructs that enable sequence-specific recognition of double-stranded (ds) DNA, fueled by the promise for enabling tools for applications in molecular biology, diagnostics, and medicine. Towards this end, we have previously introduced Invader probes, i.e., short DNA duplexes with +1 interstrand zipper arrangements of intercalator-functionalized nucleotides. The individual strands of these labile probes display high affinity towards complementary DNA (cDNA), which drives sequence-unrestricted dsDNA-recognition. However, recognition of long targets is challenging due to the high stability of the corresponding probes. To address this, we recently introduced toehold Invader probes, i.e., Invader probes with 5'-single-stranded overhangs. The toehold architecture allows for shorter double-stranded segments to be used, which facilitates probe dissociation and dsDNA-recognition. As an extension thereof, we here report the biophysical and dsDNA-targeting properties of nicked Invader probes. In this probe architecture, the single-stranded overhangs of toehold Invader probes are hybridized to short intercalator-modified auxiliary strands, leading to formation of additional labile segments. The extra binding potential from the auxiliary strands imparts nicked Invader probes with greater dsDNA-affinity than the corresponding toehold or blunt-ended probes. Recognition of chromosomal DNA targets, refractory to recognition by conventional Invader probes, is demonstrated for nicked Invader probes in the context of non-denaturing FISH experiments, which highlights their utility as dsDNA-targeting tools.

Plasmonic Nanoparticle Conjugation for Nucleic Acid Biosensing

This chapter details the use of gold nanorods conjugated with peptide nucleic acid probes for sequence-specific detection of circulating tumor DNA (ctDNA). ctDNA is gaining increased attention as a biomarker for liquid biopsy, the process of detecting molecules in the peripheral blood rather than a tissue sample. It has wide ranging applications as a diagnostic and prognostic biomarker with a similar mutational profile as the tumor. Plasmonic nanoparticles offer a relatively rapid, amplification-free method for detection of ctDNA through the use of sequence-specific peptide nucleic acid (PNA) probes. In this chapter, we discuss methods for probe design, conjugation to plasmonic particles, and ctDNA quantitation with the resulting sensor. This chapter is a resource for those looking to use plasmonic gold particles for sensing in a solution format for a range of applications.

Xeno nucleic acids (XNAs) having non-ribose scaffolds with unique supramolecular properties

DNA and RNA have significance as a genetic materials, therapeutic potential, and supramolecular properties. Advances in nucleic acid chemistry have enabled large-scale synthesis of DNA and RNA oligonucleotides and oligomers...

The Inhibition Effect of Photo-Cross-Linking between Probes in Photo-Induced Double Duplex Invasion DNA

Double duplex invasion (DDI) DNA is a useful antigene method that inhibits expression of genomic DNA. We succeeded in performing photoinduced-DDI (pDDI) using ultrafast photo-cross-linking. 5-Cyanouracil (^CN U) has been used in pDDI to inhibit photo-cross-linking between probes, but its importance has not been clarified. Therefore, in this study, we evaluated the effect of spacer (S) and d-spacer (dS) that exhibit photo-cross-linking ability similar to that of ^CN U. ^CN U exhibited the highest pDDI efficiency, and S, dS, and T were not very different. The photo-cross-linking inhibitory effect was better with S and dS than with thymidine (T). Conversely, the thermal stability was significantly lower with S and dS than with T. The results suggest that the pDDI efficiency is determined by the balance between the photo-cross-linking inhibitory effect and the thermal stability, which is the introduction efficiency for double-stranded DNA. Therefore, ^CN U, which has a photo-cross-linking inhibitory effect and a high T_m value, showed the highest inhibitory efficiency.

Recognition of double-stranded DNA using LNA-modified toehold Invader probes

Development of molecules capable of binding to specific sequences of double-stranded (ds) DNA continues to attract considerable interest, as this may yield useful tools for applications in life science, biotechnology, and medicine. We have previously demonstrated sequence-unrestricted of dsDNA using Invader probes, i.e., DNA duplexes that are energetically activated through incorporation of +1 interstrand zipper arrangements of O2'-intercalator-functionalized RNA monomers. Nonetheless, recognition of extended dsDNA target regions remains challenging due to the high stability of the corresponding probes. To address this, we introduce toehold Invader probes, i.e., Invader probes with 5'-single-stranded overhangs. This design provides access to probes with shortened double-stranded segments, which facilitates probe denaturation. The single-stranded overhangs can, furthermore, be modified with affinity-enhancing modifications like LNA (locked nucleic acid) monomers to additionally increase target affinity. Herein, we report the biophysical and dsDNA-targeting properties of different toehold Invader designs and compare them to conventional Invader probes. LNA-modified toehold Invader probes display promising recognition characteristics, including greatly improved affinity to dsDNA, excellent binding specificity, and fast recognition kinetics, which enabled recognition of chromosomal DNA targets that have proven refractory to recognition by conventional Invader probes. Thus, toehold Invader probes represent another step toward a robust, oligonucleotide-based approach for sequence-unrestricted dsDNA-recognition.

A simple fluorescent assay for the detection of peptide nucleic acid-directed double strand duplex invasion

Peptide nucleic acid (PNA) is a mimic of nucleic acids that is able to bind complementary oligonucleotides with high affinity and excellent selectivity. As such, the use of PNA has been proposed in numerous applications in biochemistry, medicine, and biotechnology. Sequences of pseudo-complementary PNAs containing diaminopurine (D)-2-thiouracil (^S U) base pairs bind to complementary regions within double-stranded DNA targets. This type of binding is termed "double duplex invasion" and involves unwinding of the duplex accompanied by simultaneous hybridization of both DNA strands by the two pseudo-complementary PNAs. In this study, a simple method of assaying DNA strand invasion by pseudo-complementary PNAs was developed. This method is based on the incorporation of a single fluorescent cytidine analog, phenylpyrrolocytidine (PhpC), into the double-stranded DNA target such that upon invasion by PNA, the PhpC is displaced to a single-stranded region resulting in the turn-on of fluorescence emission. This fluorescent assay is applicable to studies both at equilibrium and approach-to-equilibrium (time-dependent) conditions.

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.

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.

Versatile Target-Guided Screen for Discovering Bidirectional Transcription Inhibitors of a Trinucleotide Repeat Disease

Myotonic dystrophy type 1 originates from d(CTG·CAG) repeats that undergo aberrant expansion during normal processing because the d(CTG) repeat forms stable hairpin structures. Bidirectional transcription of d(CTG·CAG) yields two RNA transcripts that undergo repeat-associated non-ATG (RAN) translation to form homopolymeric proteins. Thus, both the r(CUG) transcript and the r(CAG) transcript are known to be toxic. We report a pairwise fragment-based, target-guided approach to screen for proximity-induced click dimers formed on the nucleic acid template. This screen uses an azide/alkyne clickable fragment library of nucleic acid-binding ligands incubated in parallel, pairwise reactions as an alternative to our previously reported one-pot screening method. MALDI-TOF mass spectroscopy was used to detect template assisted click products. Hit compounds inhibited the in vitro transcription of d(CTG·CAG)₉₀ bidirectionally with IC₅₀ values in the low micromolar range. This approach may be broadly applicable to other trinucleotide repeat diseases and in targeting other disease-associated nucleic acid sequences.

DNA with zwitterionic and negatively charged phosphate modifications: Formation of DNA triplexes, duplexes and cell uptake studies

Two phosphate modifications were introduced into the DNA backbone using the Staudinger reaction between the 3’,5’-dinucleoside β-cyanoethyl phosphite triester formed during DNA synthesis and sulfonyl azides, 4-(azidosulfonyl)-N,N,N-trimethylbutan-1-aminium iodide (N+ azide) or p-toluenesulfonyl (tosyl or Ts) azide, to provide either a zwitterionic phosphoramidate with N+ modification or a negatively charged phosphoramidate for Ts modification in the DNA sequence. The incorporation of these N+ and Ts modifications led to the formation of thermally stable parallel DNA triplexes, regardless of the number of modifications incorporated into the oligodeoxynucleotides (ONs). For both N+ and Ts-modified ONs, the antiparallel duplexes formed with complementary RNA were more stable than those formed with complementary DNA (except for ONs with modification in the middle of the sequence). Additionally, the incorporation of N+ modifications led to the formation of duplexes with a thermal stability that was less dependent on the ionic strength than native DNA duplexes. The thermodynamic analysis of the melting curves revealed that it is the reduction in unfavourable entropy, despite the decrease in favourable enthalpy, which is responsible for the stabilisation of duplexes with N+ modification. N+ONs also demonstrated greater resistance to nuclease digestion by snake venom phosphodiesterase I than the corresponding Ts-ONs. Cell uptake studies showed that Ts-ONs can enter the nucleus of mouse fibroblast NIH3T3 cells without any transfection reagent, whereas, N+ONs remain concentrated in vesicles within the cytoplasm. These results indicate that both N+ and Ts-modified ONs are promising for various in vivo applications.

Peptide nucleic acid Hoogsteen strand linker design for major groove recognition of DNA thymine bases

Sequence-specific targeting of double-stranded DNA and non-coding RNA via triple-helix-forming peptide nucleic acids (PNAs) has attracted considerable attention in therapeutic, diagnostic and nanotechnological fields. An E-base (3-oxo-2,3-dihydropyridazine), attached to the polyamide backbone of a PNA Hoogsteen strand by a side-chain linker molecule, is typically used in the hydrogen bond recognition of the 4-oxo group of thymine and uracil nucleic acid bases in the major groove. We report on the application of quantum chemical computational methods, in conjunction with spatial constraints derived from the experimental structure of a homopyrimidine PNA·DNA-PNA hetero-triplex, to investigate the influence of linker flexibility on binding interactions of the E-base with thymine and uracil bases in geometry-optimised model systems. Hydrogen bond formation between the N2 E-base atom and target pyrimidine base 4-oxo groups in model systems containing a β-alanine linker (J Am Chem Soc 119:11116, 1997) was found to incur significant internal strain energy and the potential disruption of intra-stand aromatic base stacking interactions in an oligomeric context. In geometry-optimised model systems containing a 3-trans olefin linker (Bioorg Med Chem Lett 14:1551, 2004) the E-base swung out away from the target pyrimidine bases into the solvent. These findings are in qualitative agreement with calorimetric measurements in hybridisation experiments at T-A and U-A inversion sites. In contrast, calculations on a novel 2-cis olefin linker design indicate that it could permit simultaneous E-base hydrogen bonding with the thymine 4-oxo group, circumvention and solvent screening of the thymine 5-methyl group, and maintenance of triplex intra-stand base stacking interactions.

Conformational constraints of cyclopentane peptide nucleic acids facilitate tunable binding to DNA

We report a series of synthetic, nucleic acid mimics with highly customizable thermodynamic binding to DNA. Incorporation of helix-promoting cyclopentanes into peptide nucleic acids (PNAs) increases the melting temperatures (Tm) of PNA+DNA duplexes by approximately +5°C per cyclopentane. Sequential addition of cyclopentanes allows the Tm of PNA + DNA duplexes to be systematically fine-tuned from +5 to +50°C compared with the unmodified PNA. Containing only nine nucleobases and an equal number of cyclopentanes, cpPNA-9 binds to complementary DNA with a Tm around 90°C. Additional experiments reveal that the cpPNA-9 sequence specifically binds to DNA duplexes containing its complementary sequence and functions as a PCR clamp. An X-ray crystal structure of the cpPNA-9-DNA duplex revealed that cyclopentanes likely induce a right-handed helix in the PNA with conformations that promote DNA binding.

Synthesis of Pyrrolidinyl PNA and Its Site-Specific Labeling at Internal Positions by Click Chemistry

Pyrrolidinyl PNA with an α-/β-dipeptide backbone consisting of alternating nucleobase-modified D-proline and (1S,2S)-2-aminocyclopentanecarboxylic acid (also known as acpcPNA) is a class of conformationally constrained PNA that shows exceptional DNA hybridization properties including very high specificity and the inability to form self-pairing hybrids. In this chapter, details of the syntheses of acpcPNA as well as its monomers and a protocol for site-specific labeling with a fluorescent dye via click chemistry are reported.

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.

Tertiary Base Triple Formation in the SRV-1 Frameshifting Pseudoknot Stabilizes Secondary Structure Components

Minor-groove base triples formed between stem 1 and loop 2 of the simian retrovirus type 1 (SRV-1) mRNA frameshifting pseudoknot are essential in stimulating -1 ribosomal frameshifting. How tertiary base triple formation affects the local stabilities of secondary structures (stem 1 and stem 2) and thus ribosomal frameshifting efficiency is not well understood. We made a short peptide nucleic acid (PNA) that is expected to invade stem 1 of the SRV-1 pseudoknot by PNA-RNA duplex formation to mimic the stem 1 unwinding process by a translating ribosome. In addition, we used a PNA for invading stem 2 in the SRV-1 pseudoknot. Our nondenaturing polyacrylamide gel electrophoresis data for the binding of PNA to the SRV-1 pseudoknot and mutants reveal that mutations in loop 2 disrupting base triple formation between loop 2 and stem 1 in the SRV-1 pseudoknot result in enhanced invasion by both PNAs. Our data suggest that tertiary stem 1-loop 2 base triple interactions in the SRV-1 pseudoknot can stabilize both of the secondary structural components, stem 1 and stem 2. Stem 2 stability is thus coupled to the structural stability of stem 1-loop 2 base triples, mediated through a long-range effect. The apparent dissociation constants of both PNAs are positively correlated with the pseudoknot mechanical stabilities and frameshifting efficiencies. The relatively simple PNA local invasion experiment may be used to characterize the energetic contribution of tertiary interactions and ligand binding in many other RNA and DNA structures.

Impact of non-nucleotidic bulges on recognition of mixed-sequence dsDNA by pyrene-functionalized Invader probes

Invader probes, i.e., DNA duplexes modified with +1 interstrand zippers of intercalator-functionalized nucleotides like 2'-O-(pyren-1-yl)methyl-RNA monomers, are energetically activated for sequence-unrestricted recognition of double-stranded DNA (dsDNA) as they are engineered to violate the neighbor exclusion principle, while displaying high affinity towards complementary DNA sequences. The impact on Invader-mediated dsDNA-recognition upon additional modification with different non-nucleotidic bulges is studied herein, based on the hypothesis that bulge-containing Invader probes will display additionally disrupted base-stacking, more extensive denaturation, and improved dsDNA-recognition efficiency. Indeed, Invader probes featuring a single central large bulge - e.g., a nonyl (C9) monomer - display improved recognition of model DNA hairpin targets vis-à-vis conventional Invader probes (C50 values ∼1.5 μM vs. ∼3.9 μM). In contrast, probes with two opposing central bulges display less favorable binding characteristics. Remarkably, C9-modified Invader probes display perfect discrimination between fully complementary dsDNA and dsDNA differing in only one of eighteen base-pairs, underscoring the high binding specificity of double-stranded probes. Cy3-labeled bulge-containing Invader probes are demonstrated to signal the presence of gender-specific DNA sequences in fluorescent in situ hybridization assays (FISH) performed under non-denaturing conditions, highlighting one potential application of dsDNA-targeting Invader probes.

Effect of linker length on photo-cross-linking position mediated by click chemistry via [2 + 2] photocycloaddition†

Ultrafast reversible DNA/RNA photo-cross-linking is a powerful tool for regulating the target strand in living cells. In particular, 3-cyanovinylcarbazole (CNVK) and 3-cyanovinylcarbazole modified by D-threoninol (CNVD) can photo-cross-link to pyrimidine bases within a few seconds of photoirradiation. However, these photo-cross-linkers can only cross-link to the counter base if it is adjacent to the 5'-side (-1 position). In this study, we synthesized novel photo-cross-linkers with varying linker lengths capable of photo-cross-linking with pyrimidine bases at locations other than the -1 position via click chemistry. The photo-cross-linking site was dependent on linker length.

Cationic guanine: positively charged nucleobase with improved DNA affinity inhibits self-duplex formation

Oligonucleotides represent powerful DNA-recognition tools, but the formation of undesirable "self-duplexes" becomes more probable with increasing DNA affinity. Herein, we have developed a modified nucleobase with "self-avoiding" properties. Facile methylation of guanine yields a cationic N7-methylguanine, which suppresses the formation of self-duplexes whilst improving DNA affinity through electrostatic interaction.

Head-to-head comparison of LNA, MPγPNA, INA and Invader probes targeting mixed-sequence double-stranded DNA

Four probe chemistries are characterized and compared with respect to thermal denaturation temperatures (Tms), thermodynamic parameters associated with duplex formation, and recognition of mixed-sequence double-stranded (ds) DNA targets: (i) oligodeoxyribonucleotides (ONs) modified with Locked Nucleic Acid (LNA) monomers, (ii) MPγPNAs, i.e., single-stranded peptide nucleic acid (PNA) probes that are functionalized at the γ-position with (R)-diethylene glycol (mini-PEG, MP) moieties, (iii) Invader probes, i.e., DNA duplexes modified with +1 interstrand zipper arrangements of 2'-O-(pyren-1-yl)methyl-RNA monomers, and (iv) intercalating nucleic acids (INAs), i.e., DNA duplexes with opposing insertions of 1-O-(1-pyrenylmethyl)glycerol bulges. Invader and INA probes, which are designed to violate the nearest-neighbor exclusion principle, denature readily, whereas the individual probe strands display exceptionally high affinity towards complementary DNA (cDNA) as indicated by increases in Tms of up to 8 °C per modification. Optimized Invader and INA probes enable efficient and highly specific recognition of mixed-sequence dsDNA targets with self-complementary regions (C50 = 30-50 nM), whereas recognition is less efficient with LNA-modified ONs and fully modified MPγPNAs due to lower cDNA affinity (LNA) and a proclivity for dimerization (LNA and MPγPNA). A Cy3-labeled Invader probe is shown to stain telomeric DNA of individual chromosomes in metaphasic spreads under non-denaturing conditions with excellent specificity.

Incorporation of Pseudo-complementary Bases 2,6-Diaminopurine and 2-Thiouracil into Serinol Nucleic Acid (SNA) to Promote SNA/RNA Hybridization

Serinol nucleic acid (SNA) is a promising candidate for nucleic acid-based molecular probes and drugs due to its high affinity for RNA. Our previous work revealed that incorporation of 2,6-diaminpurine (D), which can form three hydrogen bonds with uracil, into SNA increases the melting temperature of SNA-RNA duplexes. However, D incorporation into short self-complementary regions of SNA promoted self-dimerization and hindered hybridization with RNA. Here we synthesized a SNA monomer of 2-thiouracil (sU), which was expected to inhibit base pairing with D by steric hindrance between sulfur and the amino group. To prepare the SNA containing D and sU in high yield, we customized the protecting groups on D and sU monomers that can be readily deprotected under acidic conditions. Incorporation of D and sU into SNA facilitated stable duplex formation with target RNA by suppressing the self-hybridization of SNA and increasing the stability of the heteroduplex of SNA and its complementary RNA. Our results have important implications for the development of SNA-based probes and nucleic acid drugs.

Peptide Nucleic Acids and Gene Editing: Perspectives on Structure and Repair

Unusual nucleic acid structures are salient triggers of endogenous repair and can occur in sequence-specific contexts. Peptide nucleic acids (PNAs) rely on these principles to achieve non-enzymatic gene editing. By forming high-affinity heterotriplex structures within the genome, PNAs have been used to correct multiple human disease-relevant mutations with low off-target effects. Advances in molecular design, chemical modification, and delivery have enabled systemic in vivo application of PNAs resulting in detectable editing in preclinical mouse models. In a model of β-thalassemia, treated animals demonstrated clinically relevant protein restoration and disease phenotype amelioration, suggesting a potential for curative therapeutic application of PNAs to monogenic disorders. This review discusses the rationale and advances of PNA technologies and their application to gene editing with an emphasis on structural biochemistry and repair.

Peptide nucleic acids harness dual information codes in a single molecule

Nature encodes the information required for life in two fundamental biopolymers: nucleic acids and proteins. Peptide nucleic acid (PNA), a synthetic analog comprised of nucleobases arrayed along a pseudopeptide backbone, has the ability to combine the power of nucleic acids to encode information with the versatility of amino acids to encode structure and function. Historically, PNA has been perceived as a simple nucleic acid mimic having desirable properties such as high biostability and strong affinity for complementary nucleic acids. In this feature article, we aim to adjust this perception by highlighting the ability of PNA to act as a peptide mimic and showing the largely untapped potential to encode information in the amino acid sequence. First, we provide an introduction to PNA and discuss the use of conjugation to impart tunable properties to the biopolymer. Next, we describe the integration of functional groups directly into the PNA backbone to impart specific physical properties. Lastly, we highlight the use of these integrated amino acid side chains to encode peptide-like sequences in the PNA backbone, imparting novel activity and function and demonstrating the ability of PNA to simultaneously mimic both a peptide and a nucleic acid.

Antibacterial Peptide Nucleic Acids-Facts and Perspectives

Antibiotic resistance is an escalating, worldwide problem. Due to excessive use of antibiotics, multidrug-resistant bacteria have become a serious threat and a major global healthcare problem of the 21st century. This fact creates an urgent need for new and effective antimicrobials. The common strategies for antibiotic discovery are based on either modifying existing antibiotics or screening compound libraries, but these strategies have not been successful in recent decades. An alternative approach could be to use gene-specific oligonucleotides, such as peptide nucleic acid (PNA) oligomers, that can specifically target any single pathogen. This approach broadens the range of potential targets to any gene with a known sequence in any bacterium, and could significantly reduce the time required to discover new antimicrobials or their redesign, if resistance arises. We review the potential of PNA as an antibacterial molecule. First, we describe the physicochemical properties of PNA and modifications of the PNA backbone and nucleobases. Second, we review the carriers used to transport PNA to bacterial cells. Furthermore, we discuss the PNA targets in antibacterial studies focusing on antisense PNA targeting bacterial mRNA and rRNA.

Chimeric γPNA-Invader probes: using intercalator-functionalized oligonucleotides to enhance the DNA-targeting properties of γPNA

Gamma peptide nucleic acids (γPNAs), i.e., single-stranded PNA strands that are modified at the γ-position with (R)-diethylene glycol, and Invader probes, i.e., DNA duplexes with +1 interstrand zipper arrangements of 2'-O-(pyren-1-yl)methyl-RNA monomers, are two types of nucleic acid mimics that are showing promise for sequence-unrestricted recognition of double-stranded (ds) DNA targets. We recently demonstrated that recognition of dsDNA targets with self-complementary regions is challenging for single-stranded high-affinity probes like γPNAs due to their proclivity for secondary structure formation, but not so for Invader probes, which are engineered to form readily denaturing duplexes irrespective of the target sequence context. In the present study, we describe an approach that mitigates these limitations and improves the dsDNA-recognition properties of γPNAs in partially self-complementary target contexts. Chimeric probes between γPNAs and individual Invader strands are shown to form metastable duplexes that (i) are energetically activated for recognition of complementary mixed-sequence dsDNA target regions, (ii) reduce γPNA dimerization, and (iii) substantially improve the fidelity of the dsDNA-recognition process. Chimeric γPNA-Invader probes are characterized with respect to thermal denaturation properties, thermodynamic parameters associated with duplex formation, UV-Vis and fluorescence trends to establish pyrene binding modes, and dsDNA-recognition properties using DNA hairpin model targets.

Recognition of mixed-sequence DNA targets using spermine-modified Invader probes

Double-stranded oligodeoxyribonucleotides with +1 interstrand zipper arrangements of 2'-O-(pyren-1-yl)methyl-RNA monomers are additionally activated for highly specific recognition of mixed-sequence DNA targets upon incorporation of non-nucleotidic spermine bulges.

Poly(Lactic-co-Glycolic Acid) Nanoparticle Delivery of Peptide Nucleic Acids In Vivo

Many important biological applications of peptide nucleic acids (PNAs) target nucleic acid binding in eukaryotic cells, which requires PNA translocation across at least one membrane barrier. The delivery challenge is further exacerbated for applications in whole organisms, where clearance mechanisms rapidly deplete and/or deactivate exogenous agents. We have demonstrated that nanoparticles (NPs) composed of biodegradable polymers can encapsulate and release PNAs (alone or with co-reagents) in amounts sufficient to mediate desired effects in vitro and in vivo without deleterious reactions in the recipient cell or organism. For example, poly(lactic-co-glycolic acid) (PLGA) NPs can encapsulate and deliver PNAs and accompanying reagents to mediate gene editing outcomes in cells and animals, or PNAs alone to target oncogenic drivers in cells and correct cancer phenotypes in animal models. In this chapter, we provide a primer on PNA-induced gene editing and microRNA targeting-the two PNA-based biotechnological applications where NPs have enhanced and/or enabled in vivo demonstrations-as well as an introduction to the PLGA material and detailed protocols for formulation and robust characterization of PNA/DNA-laden PLGA NPs.

On the Necessity of Nucleobase Protection for 2-Thiouracil for Fmoc-Based Pseudo-Complementary Peptide Nucleic Acid Oligomer Synthesis

A selection of benzyl-based protecting groups for thiouracil (^SU) for the synthesis of pseudo-complementary peptide nucleic acid (PNA) has been evaluated. The 4-methoxybenzyl-protecting group that has found use for ^SU during Boc-based oligomerization is also suitable for Fmoc-based oligomerization. Furthermore, it is demonstrated that ^SU protection is unnecessary for the successful synthesis of thiouracil-containing PNA. The new 2-thiothymine (^ST) PNA monomer has also been prepared and incorporated into an oligomer and its binding to complementary PNA evaluated.

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.

RNA Secondary Structure-Based Design of Antisense Peptide Nucleic Acids for Modulating Disease-Associated Aberrant Tau Pre-mRNA Alternative Splicing

Alternative splicing of tau pre-mRNA is regulated by a 5' splice site (5'ss) hairpin present at the exon 10-intron 10 junction. Single mutations within the hairpin sequence alter hairpin structural stability and/or the binding of splicing factors, resulting in disease-causing aberrant splicing of exon 10. The hairpin structure contains about seven stably formed base pairs and thus may be suitable for targeting through antisense strands. Here, we used antisense peptide nucleic acids (asPNAs) to probe and target the tau pre-mRNA exon 10 5'ss hairpin structure through strand invasion. We characterized by electrophoretic mobility shift assay the binding of the designed asPNAs to model tau splice site hairpins. The relatively short (10-15 mer) asPNAs showed nanomolar binding to wild-type hairpins as well as a disease-causing mutant hairpin C+19G, albeit with reduced binding strength. Thus, the structural stabilizing effect of C+19G mutation could be revealed by asPNA binding. In addition, our cell culture minigene splicing assay data revealed that application of an asPNA targeting the 3' arm of the hairpin resulted in an increased exon 10 inclusion level for the disease-associated mutant C+19G, probably by exposing the 5'ss as well as inhibiting the binding of protein factors to the intronic spicing silencer. On the contrary, the application of asPNAs targeting the 5' arm of the hairpin caused an increased exon 10 exclusion for a disease-associated mutant C+14U, mainly by blocking the 5'ss. PNAs could enter cells through conjugation with amino sugar neamine or by cotransfection with minigene plasmids using a commercially available transfection reagent.

Incorporating 2-Thiouracil into Short Double-Stranded RNA-Binding Peptide Nucleic Acids for Enhanced Recognition of A-U Pairs and for Targeting a MicroRNA Hairpin Precursor

Chemically modified short peptide nucleic acids (PNAs) recognize RNA duplexes under near physiological conditions by major-groove PNA·RNA-RNA triplex formation and show great promise for the development of RNA-targeting probes and therapeutics. Thymine (T) and uracil (U) are often incorporated into PNAs to recognize A-U pairs through major-groove T·A-U and U·A-U base triple formation. Incorporation of a modified nucleobase, 2-thiouracil (s²U), into triplex-forming oligonucleotides stabilizes both DNA and RNA triplexes. Thiolation of uracil causes a decrease in the dehydration energy penalty for triplex formation as well as a decrease in the pK_a of the N3 atom, which may result in improved hydrogen bonding in addition to enhanced base stacking interactions, similar to the previously reported thiolation effect of pseudoisocytosine (J to L substitution). Here, we incorporated s²U into short PNAs, followed by binding studies of a series of s²U-modified PNAs. We demonstrated by nondenaturing polyacrylamide gel electrophoresis and thermal melting experiments that s²U and L incorporated into dsRNA-binding PNAs (dbPNAs) enhance the recognition of A-U and G-C pairs, respectively, in RNA duplexes in a position-independent manner, with no appreciable binding to the DNA duplex. Combining s²U and L modifications in dbPNAs facilitates enhanced recognition of dsRNAs and maintains selective binding to dsRNAs over ssRNAs. We further demonstrated through a cell-free assay the application of the s²U- and L-modified dbPNAs (8-mer, with a molecular mass of ∼2.3 kDa) in the inhibition of the pre-microRNA-198 maturation in a substrate-specific manner. Thus, s²U-modified dbPNAs may be generally useful for the enhanced and selective recognition of RNA duplexes and for the regulation of RNA functions.