Towards applications of synthetic genetic polymers in diagnosis and therapy

Emerging Strategies in Proteolysis-Targeting Chimeras (PROTACs): Highlights from 2022

Targeted protein degradation (TPD) is a promising therapeutic modality that has garnered attention in academic, industrial, and pharmaceutical research for treating diseases such as cancer, neurodegenerative disorders, inflammation, and viral infections. In this context, proteolysis-targeting chimeras (PROTACs) present a reliable technology for degrading disease-causing proteins. PROTACs complement small-molecule inhibitors, which primarily rely on direct protein regulation. From concept-to-clinic, PROTACs have evolved from cell impermeable peptide molecules to orally bioavailable drugs. Despite their potential in medicinal chemistry, certain aspects regarding PROTACs remain unclear. The clinical significance of PROTACs is primarily limited owing to their lack of selectivity and drug-like properties. This review focused on recently reported PROTAC strategies, particularly in 2022. It aimed to address and overcome the challenges posed by classical PROTACs by correlating them with emerging approaches with improved selectivity and controllability, cell permeability, linker flexibility, druggability, and PROTAC-based approaches, developed in 2022. Furthermore, recently reported PROTAC-based approaches are discussed, highlighting each of their advantages and limitations. We predict that several improved PROTAC molecules will be accessible for treating patients exhibiting various conditions, including cancer, neurodegenerative disorders, inflammation, and viral infections.

Inducible Degradation of Oncogenic Nucleolin Using an Aptamer-Based PROTAC

While proteolysis-targeting chimeras (PROTACs) are showing promise for targeting previously undruggable molecules, their application has been limited by difficulties in identifying suitable ligands and undesired on-target toxicity. Aptamers can virtually recognize any protein through their unique and switchable conformations. Here, by exploiting aptamers as targeting warheads, we developed a novel strategy for inducible degradation of undruggable proteins. As a proof of concept, we chose oncogenic nucleolin (NCL) as the target and generated a series of NCL degraders, and demonstrated that dNCL#T1 induced NCL degradation in a ubiquitin-proteasome-dependent manner, thereby inhibiting NCL-mediated breast cancer cell proliferation. To reduce on-target toxicity, we further developed a light-controllable PROTAC, opto-dNCL#T1, by introducing a photolabile complementary oligonucleotide to hybridize with dNCL#T1. UVA irradiation liberated dNCL#T1 from caged opto-dNCL#T1, leading to dNCL#T1 activation and NCL degradation. These results indicate that aptamer-based PROTACs are a viable alternative approach to degrade proteins of interest in a highly tunable manner.

Nanopore Detection Assisted DNA Information Processing

The deoxyribonucleotide (DNA) molecule is a stable carrier for large amounts of genetic information and provides an ideal storage medium for next-generation information processing technologies. Technologies that process DNA information, representing a cross-disciplinary integration of biology and computer techniques, have become attractive substitutes for technologies that process electronic information alone. The detailed applications of DNA technologies can be divided into three components: storage, computing, and self-assembly. The quality of DNA information processing relies on the accuracy of DNA reading. Nanopore detection allows researchers to accurately sequence nucleotides and is thus widely used to read DNA. In this paper, we introduce the principles and development history of nanopore detection and conduct a systematic review of recent developments and specific applications in DNA information processing involving nanopore detection and nanopore-based storage. We also discuss the potential of artificial intelligence in nanopore detection and DNA information processing. This work not only provides new avenues for future nanopore detection development, but also offers a foundation for the construction of more advanced DNA information processing technologies.

Introducing a New Bond-Forming Activity in an Archaeal DNA Polymerase by Structure-Guided Enzyme Redesign

DNA polymerases have evolved to feature a highly conserved activity across the tree of life: formation of, without exception, internucleotidyl O-P linkages. Can this linkage selectivity be overcome by design to produce xenonucleic acids? Here, we report that the structure-guided redesign of an archaeal DNA polymerase, 9°N, exhibits a new activity undetectable in the wild-type enzyme: catalyzing the formation of internucleotidyl N-P linkages using 3'-NH₂-ddNTPs. Replacing a metal-binding aspartate in the 9°N active site with asparagine was key to the emergence of this unnatural enzyme activity. MD simulations provided insights into how a single substitution enhances the productive positioning of a 3'-amino nucleophile in the active site. Further remodeling of the protein-nucleic acid interface in the finger subdomain yielded a quadruple-mutant variant (9°N-NRQS) displaying DNA-dependent NP-DNA polymerase activity. In addition, the engineered promiscuity of 9°N-NRQS was leveraged for one-pot synthesis of DNA─NP-DNA copolymers. This work sheds light on the molecular basis of substrate fidelity and latent promiscuity in enzymes.

Uptake mechanisms of cell-internalizing nucleic acid aptamers for applications as pharmacological agents

Nucleic acid aptamers, also regarded as chemical antibodies, show potential as targeted therapeutic and delivery agents since they possess unique advantages over antibodies. Generated by an iterative selection and amplification process from oligonucleotide libraries using cultured cells, the aptamers bind to their target molecules expressed on the cell surface. Excitingly, most aptamers also demonstrate a cell-internalizing property in native living cells, allowing them to directly enter the cells via endocytosis depending on the target. In this review, we discuss selection methods in generating cell-internalizing aptamers via a cell-based selection process, along with their challenges and optimization strategies. We highlight the cellular uptake routes adopted by the aptamers and also their intracellular fate after the uptake, to give an overview of their mechanism of action for applications as promising pharmacological agents.

The proto-Nucleic Acid Builder: a software tool for constructing nucleic acid analogs

The helical structures of DNA and RNA were originally revealed by experimental data. Likewise, the development of programs for modeling these natural polymers was guided by known structures. These nucleic acid polymers represent only two members of a potentially vast class of polymers with similar structural features, but that differ from DNA and RNA in the backbone or nucleobases. Xeno nucleic acids (XNAs) incorporate alternative backbones that affect the conformational, chemical, and thermodynamic properties of XNAs. Given the vast chemical space of possible XNAs, computational modeling of alternative nucleic acids can accelerate the search for plausible nucleic acid analogs and guide their rational design. Additionally, a tool for the modeling of nucleic acids could help reveal what nucleic acid polymers may have existed before RNA in the early evolution of life. To aid the development of novel XNA polymers and the search for possible pre-RNA candidates, this article presents the proto-Nucleic Acid Builder (https://github.com/GT-NucleicAcids/pnab), an open-source program for modeling nucleic acid analogs with alternative backbones and nucleobases. The torsion-driven conformation search procedure implemented here predicts structures with good accuracy compared to experimental structures, and correctly demonstrates the correlation between the helical structure and the backbone conformation in DNA and RNA.

Accurate and Efficient One-Pot Reverse Transcription and Amplification of 2'-Fluoro-Modified Nucleic Acids by Commercial DNA Polymerases

DNA is a foundational tool in biotechnology and synthetic biology but is limited by sensitivity to DNA-modifying enzymes. Recently, researchers have identified DNA polymerases that can enzymatically synthesize long oligonucleotides of modified DNA (M-DNA) that are resistant to DNA-modifying enzymes. Most applications require M-DNA to be reverse transcribed, typically using a RNA reverse transcriptase, back into natural DNA for sequence analysis or further manipulation. Here, we tested commercially available DNA-dependent DNA polymerases for their ability to reverse transcribe and amplify M-DNA in a one-pot reaction. Three of the six polymerases chosen (Phusion, Q5, and Deep Vent) could reverse transcribe and amplify synthetic 2'F M-DNA in a single reaction with <5 × 10^-3 error per base pair. We further used Q5 DNA polymerase to reverse transcribe and amplify M-DNA synthesized by two candidate M-DNA polymerases (SFP1 and SFM4-6), allowing for quantification of the frequency, types, and locations of errors made during M-DNA synthesis. From these studies, we identify SFP1 as one of the most accurate M-DNA polymerases identified to date. Collectively, these studies establish a simple, robust method for the conversion of 2'F M-DNA to DNA in <1 h using commercially available materials, significantly improving the ease of use of M-DNA.

Energy Landscapes of Deoxyxylo- and Xylo-Nucleic Acid Octamers

Artificial analogues of the natural nucleic acids have attracted interest as a diverse class of information storage molecules capable of self-replication. In this study, we use the computational potential energy landscape framework to investigate the structural and dynamical properties of xylo- and deoxyxylo-nucleic acids (XyNA and dXyNA), which are derived from their respective RNA and DNA analogues by inversion of a single chiral center in the sugar moiety of the nucleotides. For an octameric XyNA sequence and the analogue dXyNA, we observe facile conformational transitions between a left-handed helix, which is the free energy global minimum, and a ladder-type structure with approximately zero helicity. The competing ensembles are better separated in the dXyNA, making it a more suitable candidate for a molecular switch, whereas the XyNA exhibits additional flexibility. Both energy landscapes exhibit greater frustration than we observe in RNA or DNA, in agreement with the higher degree of optimization expected from the principle of minimal frustration in evolved biomolecules.

Highly stable hexitol based XNA aptamers targeting the vascular endothelial growth factor

Biomedical applications of nucleic acid aptamers are limited by their rapid degradation in biological fluids and generally demand tedious post-selection modifications that might compromise binding. One possible solution to warrant biostability is to directly evolve chemically modified aptamers from xenobiotic nucleic acids (XNAs). We have isolated fully modified 2'-O-methyl-ribose-1,5-anhydrohexitol nucleic acid (MeORNA-HNA) aptamers targeting the rat vascular endothelial growth factor 164 (rVEGF164). Three sequences have been identified that interact with the target protein with affinities in the low-nanomolar range and HNA modifications appeared to be mandatory for their tight binding. The evolution of these XNA aptamers was accomplished using an in vitro selection procedure starting from a fully sugar-modified library containing a 20mer 2'-OMe-ribonucleotide region followed by a 47mer HNA sequence. The high binding affinity and selectivity of the selected aptamers were confirmed by several methods including gel-shift, fluorescence polarisation, and enzyme-linked oligonucleotide assays. The isolated HNA ligands exhibited higher specificity to the rVEGF164 and human VEGF165 isoforms compared to rat VEGF120, while very low binding efficiencies were observed to streptavidin and thrombin. Furthermore, it was clearly demonstrated that the resulting aptamers possessed a superior stability to degradation in human serum and DNase I solutions.

Reprint of: Non Canonical Genetic Material

To increase the scope of natural biosystem, nucleic acids have been intensively modified. One direction includes the development of a synthetic alternative to the native DNA and RNA, denoted Xenobiotic nucleic acids (XNAs) that are able to store and transfer genetic information either by base-modification or backbone-modification. Another line of research aims to develop alternative third base pair additional to natural A:T and G:C. These unnatural base pairs (UBPs) can store increased information content encoded in three base pairs. This review outlines the recent progress made towards XNA and UBP applications as new components of the genomic DNA as well as biostable aptamers. New achievements in the replacement of a bacterial genome by unnatural non-canonical nucleotides are also described.

Non-covalent Methods of Engineering Optical Sensors Based on Single-Walled Carbon Nanotubes

Optical sensors based on single-walled carbon nanotubes (SWCNTs) demonstrate tradeoffs that limit their use in in vivo and in vitro environments. Sensor characteristics are primarily governed by the non-covalent wrapping used to suspend the hydrophobic SWCNTs in aqueous solutions, and we herein review the advantages and disadvantages of several of these different wrappings. Sensors based on surfactant wrappings can show enhanced quantum efficiency, high stability, scalability, and diminished selectivity. Conversely, sensors based on synthetic and bio-polymer wrappings tend to show lower quantum efficiency, stability, and scalability, while demonstrating improved selectivity. Major efforts have focused on optimizing sensors based on DNA wrappings, which have intermediate properties that can be improved through synthetic modifications. Although SWCNT sensors have, to date, been mainly engineered using empirical approaches, herein we highlight alternative techniques based on iterative screening that offer a more guided approach to tuning sensor properties. These more rational techniques can yield new combinations that incorporate the advantages of the diverse nanotube wrappings available to create high performance optical sensors.

In vitro selection of an XNA aptamer capable of small-molecule recognition

Despite advances in XNA evolution, the binding capabilities of artificial genetic polymers are currently limited to protein targets. Here, we describe the expansion of in vitro evolution techniques to enable selection of threose nucleic acid (TNA) aptamers to ochratoxin A (OTA). This research establishes the first example of an XNA aptamer of any kind to be evolved having affinity to a small-molecule target. Selection experiments against OTA yielded aptamers having affinities in the mid nanomolar range; with the best binders possessing K_D values comparable to or better than those of the best previously reported DNA aptamer to OTA. Importantly, the TNA can be incubated in 50% human blood serum for seven days and retain binding to OTA with only a minor change in affinity, while the DNA aptamer is completely degraded and loses all capacity to bind the target. This not only establishes the remarkable biostability of the TNA aptamer, but also its high level of selectivity, as it is capable of binding OTA in a large background of competing biomolecules. Together, this research demonstrates that refining methods for in vitro evolution of XNA can enable the selection of aptamers to a broad range of increasingly challenging target molecules.

Targeting hormone refractory prostate cancer by in vivo selected DNA libraries in an orthotopic xenograft mouse model

The targeting of specific tissue is a major challenge for the effective use of therapeutics and agents mediating this targeting are strongly demanded. We report here on an in vivo selection technology that enables the de novo identification of pegylated DNA aptamers pursuing tissue sites harbouring a hormone refractory prostate tumour. To this end, two libraries, one of which bearing an 11 kDa polyethylene glycol (PEG) modification, were used in an orthotopic xenograft prostate tumour mouse model for the selection process. Next-generation sequencing revealed an in vivo enriched pegylated but not a naïve DNA aptamer recognising prostate cancer tissue implanted either subcutaneous or orthotopically in mice. This aptamer represents a valuable and cost-effective tool for the development of targeted therapies for prostate cancer. The described selection strategy and its analysis is not limited to prostate cancer but will be adaptable to various tissues, tumours, and metastases. This opens the path towards DNA aptamers being experimentally and clinically engaged as molecules for developing targeted therapy strategies.

Direct sequencing of 2'-deoxy-2'-fluoroarabinonucleic acid (FANA) using nanopore-induced phase-shift sequencing (NIPSS)

2'-deoxy-2'-fluoroarabinonucleic acid (FANA), which is one type of xeno-nucleic acid (XNA), has been intensively studied in molecular medicine and synthetic biology because of its superior gene-silencing and catalytic activities. Although urgently required, FANA cannot be directly sequenced by any existing platform. Nanopore sequencing, which identifies a single molecule analyte directly from its physical and chemical properties, shows promise for direct XNA sequencing. As a proof of concept, different FANA homopolymers show well-distinguished pore blockage signals in a Mycobacterium smegmatis porin A (MspA) nanopore. By ligating FANA with a DNA drive-strand, direct FANA sequencing has been demonstrated using phi29 DNA polymerase by Nanopore-Induced Phase Shift Sequencing (NIPSS). When bound with an FANA template, the phi29 DNA polymerase shows unexpected reverse transcriptase activity when monitored in a single molecule assay. Following further investigations into the ensemble, phi29 DNA polymerase is shown to be a previously unknown reverse transcriptase for FANA that operates at room temperature, and is potentially ideal for nanopore sequencing. These results represent the first direct sequencing of a sugar-modified XNA and suggest that phi29 DNA polymerase could act as a promising enzyme for sustained sequencing of a wide variety of XNAs.

Non canonical genetic material

To increase the scope of natural biosystem, nucleic acids have been intensively modified. One direction includes the development of a synthetic alternative to the native DNA and RNA, denoted Xenobiotic nucleic acids (XNAs) that are able to store and transfer genetic information either by base-modification or backbone-modification. Another line of research aims to develop alternative third base pair additional to natural A:T and G:C. These unnatural base pairs (UBPs) can store increased information content encoded in three base pairs. This review outlines the recent progress made towards XNA and UBP applications as new components of the genomic DNA as well as biostable aptamers. New achievements in the replacement of a bacterial genome by unnatural non-canonical nucleotides are also described.

Genetic Alphabet Expansion Provides Versatile Specificities and Activities of Unnatural-Base DNA Aptamers Targeting Cancer Cells

The potential of genetic alphabet expansion technologies using artificial extra base pairs (unnatural base pairs) has been rapidly expanding and increasing. We present that the hydrophobic unnatural base, 7-(2-thienyl)imidazo[4,5-b]pyridine (Ds), which acts as a fifth letter in a DNA library, provides a series of high-affinity DNA aptamers with versatile binding specificities and activities to cancer cells. These Ds-containing DNA aptamers were generated by a method called cell-ExSELEX to target three breast cancer cell lines: MCF7, MDA-MB-231, and T-47D. Aptamer 14A-MCF7, which targets MCF7 cells, specifically binds to MCF7 cells, but not other cancer cell lines. Aptamer 07-MB231, which targets MDA-MB-231 cells, binds to a series of metastatic bone and lung cancer cell lines. Aptamer 05-MB231 targets MDA-MB-231 cells, but it also binds to all of the cancer and leukemia cell lines that we examined. None of these aptamers bind to normal cell lines, such as MCF10A and HUVEC. In addition, aptamers 14A-MCF7 and 05-MB231 are internalized within the cancer cells, and aptamer 05-MB231 possesses anti-proliferative properties against most cancer cell lines that we examined. These aptamers and the generation method are broadly applicable to cancer cell imaging, biomarker discovery, cancer cell profiling, anti-cancer therapies, and drug delivery systems.

Unique Thermal Stability of Unnatural Hydrophobic Ds Bases in Double-Stranded DNAs

Genetic alphabet expansion technology, the introduction of unnatural bases or base pairs into replicable DNA, has rapidly advanced as a new synthetic biology area. A hydrophobic unnatural base pair between 7-(2-thienyl)imidazo[4,5-b]pyridine (Ds) and 2-nitro-4-propynylpyrrole (Px) exhibited high fidelity as a third base pair in PCR. SELEX methods using the Ds-Px pair enabled high-affinity DNA aptamer generation, and introducing a few Ds bases into DNA aptamers extremely augmented their affinities and selectivities to target proteins. Here, to further scrutinize the functions of this highly hydrophobic Ds base, the thermal stabilities of double-stranded DNAs (dsDNA) containing a noncognate Ds-Ds or G-Ds pair were examined. The thermal stability of the Ds-Ds self-pair was as high as that of the natural G-C pair, and apart from the generally higher stability of the G-C pair than that of the A-T pair, most of the 5'-pyrimidine-Ds-purine-3' sequences, such as CDsA and TDsA, exhibited higher stability than the 5'-purine-Ds-pyrimidine-3' sequences, such as GDsC and ADsC, in dsDNAs. This trait enabled the GC-content-independent control of the thermal stability of the designed dsDNA fragments. The melting temperatures of dsDNA fragments containing the Ds-Ds pair can be predicted from the nearest-neighbor parameters including the Ds base. In addition, the noncognate G-Ds pair can efficiently distinguish its neighboring cognate natural base pairs from noncognate pairs. We demonstrated that real-time PCR using primers containing Ds accurately detected a single-nucleotide mismatch in target DNAs. These unique properties of the Ds base that affect the stabilities of the neighboring base pairs could impart new functions to DNA molecules and technologies.

Challenging cancer targets for aptamer delivery

The extraordinary boost in the understanding of the genetic and epigenetic mechanisms underlying the development and progression of different types of cancer, is offering an unprecedented hope for the development of precise therapeutics able to interfere or replace the expression of target genes. In the last decade, the design of stable, safe and effective RNA-based therapeutics has been significantly improved increasing the number of molecules now in preclinical or in clinical trials for cancer gene therapy. However, with few exclusions as liver and hematological malignancies which are easy accessible to drugs, the development of effective systemic approaches for the delivery of RNA therapeutics to target cells is still unmet. To be effective, targeting carriers must be able to overcome both functional and physical barriers to safely carry and accumulate the therapeutic through the organism selectively to the tumor site, penetrate the target cancer mass, promote the uptake and localization in the appropriate intracellular compartment ultimately leading to the effective modulation of gene expression. Nucleic acid aptamers are folded single stranded oligonucleotides that bind at high affinity and high specificity their targets (proteins, lipids, small molecules etc), coupling the advantages of binding specificity proper of antibodies to the chemical nature of nucleic acids, sometimes also termed "nucleic acid antibodies". In several cases, aptamers targeting cell surface receptors are recycled into the cell together with the bound receptor enabling to drive conjugated therapeutics to cancer cells in a receptor-dependent manner. Therefore, besides other in vivo delivery strategies, the use of aptamers as precise and effective targeting moieties for anticancer RNA-based therapeutics has rapidly emerged and has been successfully addressed by several laboratories. In this Review, we will focus on the most recent and challenging progresses in the field that highlights the precision and flexibility of aptamer-based chimeras paving the way to the development of safe and effective carriers for cancer gene therapeutics.

DNA aptamer generation by ExSELEX using genetic alphabet expansion with a mini-hairpin DNA stabilization method

A novel aptamer generation method to greatly augment the affinity and stability of DNA aptamers was developed by genetic alphabet expansion combined with mini-hairpin DNA technology. The genetic alphabet expansion increases the physicochemical and structural diversities of DNA aptamers by introducing extra components, unnatural bases, as a fifth base, allowing for the enhancement of DNA aptamer affinities. Furthermore, the mini-hairpin DNA technology stabilizes DNA aptamers against nuclease digestion and thermal denaturation, by introducing an extraordinarily stable mini-hairpin DNA containing a GCGAAGC sequence. This novel method provides stabilized high-affinity DNA aptamers for diagnostic and therapeutic applications.

Selection platforms for directed evolution in synthetic biology

Life on Earth is incredibly diverse. Yet, underneath that diversity, there are a number of constants and highly conserved processes: all life is based on DNA and RNA; the genetic code is universal; biology is limited to a small subset of potential chemistries. A vast amount of knowledge has been accrued through describing and characterizing enzymes, biological processes and organisms. Nevertheless, much remains to be understood about the natural world. One of the goals in Synthetic Biology is to recapitulate biological complexity from simple systems made from biological molecules-gaining a deeper understanding of life in the process. Directed evolution is a powerful tool in Synthetic Biology, able to bypass gaps in knowledge and capable of engineering even the most highly conserved biological processes. It encompasses a range of methodologies to create variation in a population and to select individual variants with the desired function-be it a ligand, enzyme, pathway or even whole organisms. Here, we present some of the basic frameworks that underpin all evolution platforms and review some of the recent contributions from directed evolution to synthetic biology, in particular methods that have been used to engineer the Central Dogma and the genetic code.

Customised nucleic acid libraries for enhanced aptamer selection and performance

Aptamers are short single-stranded oligo(deoxy)nucleotides that are selected to bind to target molecules with high affinity and specificity. Because of their sophisticated characteristics and versatile applicability, aptamers are thought to become universal molecular probes in biotechnological and therapeutic applications. However, the variety of possible interactions with a putative target molecule is limited by the chemical repertoire of the natural nucleobases. Consequently, many desired targets are not addressable by aptamers. This obstacle is overcome by broadening the chemical diversity of aptamers, mainly achieved by nucleobase-modifications and the introduction of novel bases or base pairs. We discuss these achievements and the characteristics of the respective modified aptamers, reflected by SOMAmers (slow off-rate modified aptamers), clickmers, and aptamers bearing an expanded genetic alphabet.

The 5-chlorouracil:7-deazaadenine base pair as an alternative to the dT:dA base pair

The 5-Cl-dU:7-deaza-dA base pair can be a substitute for the dT:dA base pair in an enzymatic replication process of 2 kb DNA.

High-Affinity DNA Aptamer Generation Targeting von Willebrand Factor A1-Domain by Genetic Alphabet Expansion for Systematic Evolution of Ligands by Exponential Enrichment Using Two Types of Libraries Composed of Five Different Bases

The novel evolutionary engineering method ExSELEX (genetic alphabet expansion for systematic evolution of ligands by exponential enrichment) provides high-affinity DNA aptamers that specifically bind to target molecules, by introducing an artificial hydrophobic base analogue as a fifth component into DNA aptamers. Here, we present a newer version of ExSELEX, using a library with completely randomized sequences consisting of five components: four natural bases and one unnatural hydrophobic base, 7-(2-thienyl)imidazo[4,5-b]pyridine (Ds). In contrast to the limited number of Ds-containing sequence combinations in our previous library, the increased complexity of the new randomized library could improve the success rates of high-affinity aptamer generation. To this end, we developed a sequencing method for each clone in the enriched library after several rounds of selection. Using the improved library, we generated a Ds-containing DNA aptamer targeting von Willebrand factor A1-domain (vWF) with significantly higher affinity (K_D = 75 pM), relative to those generated by the initial version of ExSELEX, as well as that of the known DNA aptamer consisting of only the natural bases. In addition, the Ds-containing DNA aptamer was stabilized by introducing a mini-hairpin DNA resistant to nucleases, without any loss of affinity (K_D = 61 pM). This new version is expected to consistently produce high-affinity DNA aptamers.

Aptamer-Mediated Targeted Delivery of Therapeutics: An Update

The selective delivery of drugs in a cell- or tissue-specific manner represents the main challenge for medical research; in order to reduce the occurrence of unwanted off-target effects. In this regard, nucleic acid aptamers have emerged as an attractive class of carrier molecules due to their ability to bind with high affinity to specific ligands; their high chemical flexibility; as well as tissue penetration capability. To date, different aptamer-drug systems and aptamer-nanoparticles systems, in which nanoparticles function together with aptamers for the targeted delivery, have been successfully developed for a wide range of therapeutics, including toxins; peptides; chemotherapeutics and oligonucleotides. Therefore, aptamer-mediated drug delivery represents a powerful tool for the safe and effective treatment of different human pathologies, including cancer; neurological diseases; immunological diseases and so on. In this review, we will summarize recent progress in the field of aptamer-mediated drug delivery and we will discuss the advantages, the achieved objectives and the challenges to be still addressed in the near future, in order to improve the effectiveness of therapies.

Post-ExSELEX stabilization of an unnatural-base DNA aptamer targeting VEGF165 toward pharmaceutical applications

A new technology, genetic alphabet expansion using artificial bases (unnatural bases), has created high-affinity DNA ligands (aptamers) that specifically bind to target proteins by ExSELEX (genetic alphabet Expansion for Systematic Evolution of Ligands by EXponential enrichment). We recently found that the unnatural-base DNA aptamers can be stabilized against nucleases, by introducing an extraordinarily stable, unique hairpin DNA (mini-hairpin DNA) and by reinforcing the stem region with G–C pairs. Here, to establish this aptamer generation method, we examined the stabilization of a high-affinity anti-VEGF₁₆₅ unnatural-base DNA aptamer. The stabilized aptamers displayed significantly increased thermal and nuclease stabilities, and furthermore, exhibited higher affinity to the target. As compared to the well-known anti-VEGF₁₆₅ RNA aptamer, pegaptanib (Macugen), our aptamers did not require calcium ions for binding to VEGF₁₆₅. Biological experiments using cultured cells revealed that our stabilized aptamers efficiently inhibited the interaction between VEGF₁₆₅ and its receptor, with the same or slightly higher efficiency than that of the pegaptanib RNA aptamer. The development of cost-effective and calcium ion-independent high-affinity anti-VEGF₁₆₅ DNA aptamers encourages further progress in diagnostic and therapeutic applications. In addition, the stabilization process provided additional information about the key elements required for aptamer binding to VEGF₁₆₅.

Nanostructures from Synthetic Genetic Polymers

Nanoscale objects of increasing complexity can be constructed from DNA or RNA. However, the scope of potential applications could be enhanced by expanding beyond the moderate chemical diversity of natural nucleic acids. Here, we explore the construction of nano-objects made entirely from alternative building blocks: synthetic genetic polymers not found in nature, also called xeno nucleic acids (XNAs). Specifically, we describe assembly of 70 kDa tetrahedra elaborated in four different XNA chemistries (2'-fluro-2'-deoxy-ribofuranose nucleic acid (2'F-RNA), 2'-fluoroarabino nucleic acids (FANA), hexitol nucleic acids (HNA), and cyclohexene nucleic acids (CeNA)), as well as mixed designs, and a ∼600 kDa all-FANA octahedron, visualised by electron microscopy. Our results extend the chemical scope for programmable nanostructure assembly, with implications for the design of nano-objects and materials with an expanded range of structural and physicochemical properties, including enhanced biostability.

Effect of backbone chemistry on hybridization thermodynamics of oligonucleic acids: a coarse-grained molecular dynamics simulation study

In this paper we study how varying oligonucleic acid backbone chemistry affects the hybridization/melting thermodynamics of oligonucleic acids. We first describe the coarse-grained (CG) model with tunable parameters that we developed to enable the study of both naturally occurring oligonucleic acids, such as DNA, and their chemically-modified analogues, such as peptide nucleic acids (PNAs) and locked nucleic acids (LNAs). The DNA melting curves obtained using such a CG model and molecular dynamics simulations in an implicit solvent and with explicit ions match with the melting curves obtained using the empirical nearest-neighbor models. We use these CG simulations to then elucidate the effect of backbone flexibility, charge, and nucleobase spacing along the backbone on the melting curves, potential energy and conformational entropy change upon hybridization and base-pair hydrogen bond residence time. We find that increasing backbone flexibility decreases duplex thermal stability and melting temperature mainly due to increased conformational entropy loss upon hybridization. Removing charges from the backbone enhances duplex thermal stability due to the elimination of electrostatic repulsion and as a result a larger energetic gain upon hybridization. Lastly, increasing nucleobase spacing decreases duplex thermal stability due to decreasing stacking interactions that are important for duplex stability.

Synthetic genetic polymers: advances and applications

Advances and applications of synthetic genetic polymers (xeno-nucleic acids) are reviewed in this article. The types of synthetic genetic polymers are summarized. The basic properties of them are elaborated and their technical applications are presented. Challenges and prospects of synthetic genetic polymers are discussed.

Targeting CD44, ABCG2 and CD133 markers using aptamers: in silico analysis of CD133 extracellular domain 2 and its aptamer

Truncated CSC marker aptamers penetrate tumor spheres and inhibits cell proliferation; a bioinformatics approach to decipher their structural interactions.

A High-Fidelity Codon Set for the T4 DNA Ligase-Catalyzed Polymerization of Modified Oligonucleotides

In vitro selection of nucleic acid polymers can readily deliver highly specific receptors and catalysts for a variety of applications; however, it is suspected that the functional group deficit of nucleic acids has limited their potential with respect to proteinogenic polymers. This has stimulated research toward expanding their chemical diversity to bridge the functional gap between nucleic acids and proteins to develop a superior biopolymer. In this study, we investigate the effect of codon library size and composition on the sequence specificity of T4 DNA ligase in the DNA-templated polymerization of both unmodified and modified oligonucleotides. Using high-throughput DNA sequencing of duplex pairs, we have uncovered a 256-membered codon set that yields sequence-defined modified ssDNA polymers in high yield and with high fidelity.

Modern affinity reagents: Recombinant antibodies and aptamers

Affinity reagents are essential tools in both basic and applied research; however, there is a growing concern about the reproducibility of animal-derived monoclonal antibodies. The need for higher quality affinity reagents has prompted the development of methods that provide scientific, economic, and time-saving advantages and do not require the use of animals. This review describes two types of affinity reagents, recombinant antibodies and aptamers, which are non-animal technologies that can replace the use of animal-derived monoclonal antibodies. Recombinant antibodies are protein-based reagents, while aptamers are nucleic-acid-based. In light of the scientific advantages of these technologies, this review also discusses ways to gain momentum in the use of modern affinity reagents, including an update to the 1999 National Academy of Sciences monoclonal antibody production report and federal incentives for recombinant antibody and aptamer efforts. In the long-term, these efforts have the potential to improve the overall quality and decrease the cost of scientific research.

Directed evolution of artificial enzymes (XNAzymes) from diverse repertoires of synthetic genetic polymers

This protocol describes the directed evolution of artificial endonuclease and ligase enzymes composed of synthetic genetic polymers (XNAzymes), using 'cross-chemistry selective enrichment by exponential amplification' (X-SELEX). The protocol is analogous to (deoxy)ribozyme selections, but it enables the development of fully substituted catalysts. X-SELEX is initiated by the synthesis of diverse repertoires (here 10(14) different sequences), using xeno nucleic acid (XNA) polymerases, on DNA templates primed with DNA, RNA or XNA oligonucleotides that double as substrates, allowing selection for XNA-catalyzed cleavage or ligation. XNAzymes are reverse-transcribed into cDNA using XNA-dependent DNA polymerases, and then PCR-amplified to generate templates for subsequent rounds or deep sequencing. We describe methods developed for four XNA chemistries, arabino nucleic acids (ANAs), 2'-fluoroarabino nucleic acids (FANAs), hexitol nucleic acids (HNAs) and cyclohexene nucleic acids (CeNAs), which require ∼1 week per round, and typically 10-20 rounds; in principle, these methods are scalable and applicable to a wide range of novel XNAzyme chemistries, substrates and reactions.

Dual aptamer-immobilized surfaces for improved affinity through multiple target binding in potentiometric thrombin biosensing

We developed a label-free and reagent-less potentiometric biosensor with improved affinity for thrombin. Two different oligomeric DNA aptamers that can recognize different epitopes in thrombin were introduced in parallel or serial manners on the sensing surface to capture the target via multiple contacts as found in many biological systems. The spacer and linker in the aptamer probes were optimized for exerting the best performance in molecular recognition. To gain the specificity of the sensor to the target, an antifouling molecule, sulfobeaine-3-undecanethiol (SB), was introduced on the sensor to form a self-assembled monolayer (SAM). Surface characterization revealed that the aptamer probe density was comparable to the distance of the two epitopes in thrombin, while the backfilling SB SAM was tightly aligned on the surface to resist nonspecific adsorption. The apparent binding parameters were obtained by thrombin sensing in potentiometry using the 1:1 Langmuir adsorption model, showing the improved dissociation constants (Kd) with the limit of detection of 5.5 nM on the dual aptamer-immobilized surfaces compared with single aptamer-immobilized ones. A fine control of spacer and linker length in the aptamer ligand was essential to realize the multivalent binding of thrombin on the sensor surface. The findings reported herein are effective for improving the sensitivity of potentiometric biosensor in an affordable way towards detection of tiny amount of biomolecules.

Predicting the Uncertain Future of Aptamer-Based Diagnostics and Therapeutics

Despite the great promise of nucleic acid aptamers in the areas of diagnostics and therapeutics for their facile in vitro development, lack of immunogenicity and other desirable properties, few truly successful aptamer-based products exist in the clinical or other markets. Core reasons for these commercial deficiencies probably stem from industrial commitment to antibodies including a huge financial investment in humanized monoclonal antibodies and a general ignorance about aptamers and their performance among the research and development community. Given the early failures of some strong commercial efforts to gain government approval and bring aptamer-based products to market, it may seem that aptamers are doomed to take a backseat to antibodies forever. However, the key advantages of aptamers over antibodies coupled with niche market needs that only aptamers can fill and more recent published data still point to a bright commercial future for aptamers in areas such as infectious disease and cancer diagnostics and therapeutics. As more researchers and entrepreneurs become familiar with aptamers, it seems inevitable that aptamers will at least be considered for expanded roles in diagnostics and therapeutics. This review also examines new aptamer modifications and attempts to predict new aptamer applications that could revolutionize biomedical technology in the future and lead to marketed products.