Increasing the functional density of threose nucleic acid.

Chemical strategies that augment genetic polymers with amino acid residues that are overrepresented on the paratope surface of an antibody offer a promising route for enhancing the binding properties of nucleic acid aptamers. Here, we describe the chemical synthesis of α-l-threofuranosyl cytidine nucleoside triphosphate (tCTP) carrying either a benzyl or phenylpropyl side chain at the pyrimidine C-5 position. Polymerase recognition studies indicate that both substrates are readily incorporated into a full-length α-l-threofuranosyl nucleic acid (TNA) product by extension of a DNA primer-template duplex with an engineered TNA polymerase. Similar primer extension reactions performed using nucleoside triphosphate mixtures containing both C-5 modified tCTP and C-5 modified tUTP substrates enable the production of doubly modified TNA strands for a panel of 20 chemotype combinations. Kinetic measurements reveal faster on-rates (kon) and tighter binding affinity constants (Kd) for engineered versions of TNA aptamers carrying chemotypes at both pyrimidine positions as compared to their singly modified counterparts. These findings expand the chemical space of evolvable non-natural genetic polymers by offering a path for improving the quality of biologically stable TNA aptamers for future clinical applications.

Functionally Enhanced XNA Aptamers Discovered by Parallelized Library Screening.

In vitro evolution strategies have been used for >30 years to generate nucleic acid aptamers against therapeutic targets of interest, including disease-associated proteins. However, this process requires many iterative cycles of selection and amplification, which severely restricts the number of target and library design combinations that can be explored in parallel. Here, we describe a single-round screening approach to aptamer discovery that relies on function-enhancing chemotypes to increase the distribution of high-affinity sequences in a random-sequence library. We demonstrate the success of de novo discovery by affinity selection of threomers against the receptor binding domain of the S1 protein from SARS-CoV-2. Detailed biochemical characterization of the enriched population identified threomers with binding affinity values that are comparable to aptamers produced by conventional SELEX. This work establishes a highly parallelizable path for querying diverse chemical repertoires and may offer a viable route for accelerating the discovery of therapeutic aptamers.

Engineering TNA polymerases through iterative cycles of directed evolution.

DNA polymerases are important tools for biotechnology, synthetic biology, and chemical biology as they are routinely used to amplify and edit genetic information. However, natural polymerases do not recognize artificial genetic polymers (also known as xeno-nucleic acids or XNAs) with unique sugar-phosphate backbone structures. Directed evolution offers a possible solution to this problem by facilitating the discovery of engineered versions of natural polymerases that can copy genetic information back and forth between DNA and XNA. Here we report a directed evolution strategy for discovering polymerases that can synthesize threose nucleic acid (TNA) on DNA templates. The workflow involves library generation and expression in E. coli, high-throughput microfluidics-based screening of uniform water-in-oil droplets, plasmid recovery, secondary screening, and library regeneration. This technique is sufficiently general that it could be applied to a wide range of problems involving DNA modifying enzymes.

Parameterizing the Binding Properties of XNA Aptamers Isolated from a Low Stringency Selection.

Machine learning offers a guided approach to aptamer discovery, but more information is needed to develop algorithms that can intelligently identify high-performing aptamers to a broad array of targets. Critical to this effort is the need to experimentally parameterize the difference between low and high affinity binders to a given target. Although classical selection experiments help define the upper limit by converging on a small number of tight binding sequences, very little is known about the lower limit of binding that defines the boundary between binders and nonbinders. Here, we apply a quantitative approach to explore the diversity of aptamers isolated from two identical in vitro selections performed under low stringency conditions. Starting from a library of 1 trillion unique threose nucleic acid (TNA) sequences, 7 rounds of selection were performed to enrich binders to a known aptagenic target. High density sequencing of each round of selection followed by a detailed kinetic analysis of 136 TNA aptamers yielded a narrow range of equilibrium dissociation constants (KD = ∼ 1-15 nM) that were consistent between two experimental replicates. These findings offer insights into the lower limit of binding that may be expected for aptamers generated against aptagenic targets and could provide useful constraints for evaluating the results of experimental and computational approaches.

Shorter Is Better: The α-(l)-Threofuranosyl Nucleic Acid Modification Improves Stability, Potency, Safety, and Ago2 Binding and Mitigates Off-Target Effects of Small Interfering RNAs.

Chemical modifications are necessary to ensure the metabolic stability and efficacy of oligonucleotide-based therapeutics. Here, we describe analyses of the α-(l)-threofuranosyl nucleic acid (TNA) modification, which has a shorter 3'-2' internucleotide linkage than the natural DNA and RNA, in the context of small interfering RNAs (siRNAs). The TNA modification enhanced nuclease resistance more than 2'-O-methyl or 2'-fluoro ribose modifications. TNA-containing siRNAs were prepared as triantennary N-acetylgalactosamine conjugates and were tested in cultured cells and mice. With the exceptions of position 2 of the antisense strand and position 11 of the sense strand, the TNA modification did not inhibit the activity of the RNA interference machinery. In a rat toxicology study, TNA placed at position 7 of the antisense strand of the siRNA mitigated off-target effects, likely due to the decrease in the thermodynamic binding affinity relative to the 2'-O-methyl residue. Analysis of the crystal structure of an RNA octamer with a single TNA on each strand showed that the tetrose sugar adopts a C4'-exo pucker. Computational models of siRNA antisense strands containing TNA bound to Argonaute 2 suggest that TNA is well accommodated in the region kinked by the enzyme. The combined data indicate that the TNA nucleotides are promising modifications expected to increase the potency, duration of action, and safety of siRNAs.

Stability and mechanism of threose nucleic acid toward acid-mediated degradation.

Xeno-nucleic acids (XNAs) have gained significant interest as synthetic genetic polymers for practical applications in biomedicine, but very little is known about their biophysical properties. Here, we compare the stability and mechanism of acid-mediated degradation of α-l-threose nucleic acid (TNA) to that of natural DNA and RNA. Under acidic conditions and elevated temperature (pH 3.3 at 90°C), TNA was found to be significantly more resistant to acid-mediated degradation than DNA and RNA. Mechanistic insights gained by reverse-phase HPLC and mass spectrometry indicate that the resilience of TNA toward low pH environments is due to a slower rate of depurination caused by induction of the 2'-phosphodiester linkage. Similar results observed for 2',5'-linked DNA and 2'-O-methoxy-RNA implicate the position of the phosphodiester group as a key factor in destabilizing the formation of the oxocarbenium intermediate responsible for depurination and strand cleavage of TNA. Biochemical analysis indicates that strand cleavage occurs by ß-elimination of the 2'-phosphodiester linkage to produce an upstream cleavage product with a 2'-threose sugar and a downstream cleavage product with a 3' terminal phosphate. This work highlights the unique physicochemical properties available to evolvable non-natural genetic polymers currently in development for biomedical applications.

Highly Parallelized Screening of Functionally Enhanced XNA Aptamers in Uniform Hydrogel Particles.

Xeno-nucleic acid (XNA) aptamers based on evolvable non-natural genetic polymers hold enormous potential as future diagnostic and therapeutic agents. However, time-consuming and costly procedures requiring the purification of individual XNA sequences produced by large-scale polymerase-mediated primer extension reactions pose a major bottleneck to the discovery of highly active XNA motifs for biomedical applications. Here, we describe a straightforward approach for rapidly surveying the binding properties of XNA aptamers identified by in vitro selection. Our strategy involves preparing XNA aptamer particles in which many copies of the same aptamer sequence are distributed throughout the gel matrix of a polyacrylamide-encapsulated magnetic particle. Aptamer particles are then screened by flow cytometry to assess target binding affinity and deduce structure-activity relationships. This generalizable and highly parallel assay dramatically accelerates the pace of secondary screening by allowing a single researcher to evaluate 48-96 sequences per day.

Chemical evolution of an autonomous DNAzyme with allele-specific gene silencing activity.

Low activity has been the primary obstacle impeding the use of DNA enzymes (DNAzymes) as gene silencing agents in clinical applications. Here we describe the chemical evolution of a DNAzyme with strong catalytic activity under near physiological conditions. The enzyme achieves ~65 turnovers in 30 minutes, a feat only previously witnessed by the unmodified parent sequence under forcing conditions of elevated Mg2+ and pH. Structural constraints imposed by the chemical modifications drive catalysis toward a highly preferred UGUD motif (cut site underlined) that was validated by positive and negative predictions. Biochemical assays support an autonomous RNA cleavage mechanism independent of RNase H1 engagement. Consistent with its strong catalytic activity, the enzyme exhibits persistent allele-specific knock-down of an endogenous mRNA encoding an undruggable oncogenic KRAS target. Together, these results demonstrate that chemical evolution offers a powerful approach for discovering new chemotype combinations that can imbue DNAzymes with the physicochemical properties necessary to support therapeutic applications.

Amplification-Free COVID-19 Detection by Digital Droplet REVEALR.

The COVID-19 pandemic, caused by the SARS-CoV-2 virus, exposed a pressing need for new public health tools for pathogen detection, disease diagnosis, and viral genotyping. REVEALR (RNA-encoded viral nucleic acid analyte reporter) is an isothermal DNAzyme-based point-of-care diagnostic that functions with a detection limit of ∼10 copies/µL when coupled with a preamplification step and can be utilized for viral genotyping of SARS-CoV-2 variants of concern through base pair mismatch recognition in a competitive binding format. Here, we describe an advanced REVEALR platform, termed digital droplet REVEALR (ddREVEALR), that can achieve direct viral detection and absolute sample quantitation utilizing a signal amplification strategy that relies on chemical modifications, DNAzyme multiplexing, and volume compression. Using an AI-assisted image-based readout, ddREVEALR was found to achieve 95% positive predictive agreement from a set of 20 nasal pharyngeal swabs collected at UCI Medical Center in Orange, California. We propose that the combination of amplification-free and protein-free analysis makes ddREVEALR a promising application for direct viral RNA detection of clinical samples.

Crystallographic analysis of engineered polymerases synthesizing phosphonomethylthreosyl nucleic acid.

Xeno-nucleic acids (XNAs) are synthetic genetic polymers with backbone structures composed of non-ribose or non-deoxyribose sugars. Phosphonomethylthreosyl nucleic acid (pTNA), a type of XNA that does not base pair with DNA or RNA, has been suggested as a possible genetic material for storing synthetic biology information in cells. A critical step in this process is the synthesis of XNA episomes using laboratory-evolved polymerases to copy DNA information into XNA. Here, we investigate the polymerase recognition of pTNA nucleotides using X-ray crystallography to capture the post-catalytic complex of engineered polymerases following the sequential addition of two pTNA nucleotides onto the 3'-end of a DNA primer. High-resolution crystal structures reveal that the polymerase mediates Watson-Crick base pairing between the extended pTNA adducts and the DNA template. Comparative analysis studies demonstrate that the sugar conformation and backbone position of pTNA are structurally more similar to threose nucleic acid than DNA even though pTNA and DNA share the same six-atom backbone repeat length. Collectively, these findings provide new insight into the structural determinants that guide the enzymatic synthesis of an orthogonal genetic polymer, and may lead to the discovery of new variants that function with enhanced activity.

REVEALR-Based Genotyping of SARS-CoV-2 Variants of Concern in Clinical Samples.

The SARS-CoV-2 virus has evolved into new strains that increase viral transmissibility and reduce vaccine protection. The rapid circulation of these more harmful strains across the globe has created a pressing need for alternative public health screening tools. REVEALR (RNA-encoded viral nucleic acid analytic reporter), a rapid and highly sensitive DNAzyme-based detection system, functions with perfect accuracy against patient-derived clinical samples. Here, we design REVEALR into a novel genotyping assay that detects single-base mismatches corresponding to each of the major SARS-CoV-2 strains found in the United States. Of 34 sequence-verified patient samples collected in early, mid, and late 2021 at the UCI Medical Center in Orange, California, REVEALR identified the correct variant [Wuhan-Hu-1, alpha (B.1.1.7), gamma (P.1), epsilon (B.1.427/9), delta (B.1.617.2), and omicron (B.1.1.529)] with 100% accuracy. The assay, which is programmable and amenable to multiplexing, offers an important new approach to personalized diagnostics.

Evolution of Functionally Enhanced α-l-Threofuranosyl Nucleic Acid Aptamers.

Synthetic genetic polymers (xeno-nucleic acids, XNAs) have the potential to transition aptamers from laboratory tools to therapeutic agents, but additional functionality is needed to compete with antibodies. Here, we describe the evolution of a biologically stable artificial genetic system composed of α-l-threofuranosyl nucleic acid (TNA) that facilitates the production of backbone- and base-modified aptamers termed "threomers" that function as high quality protein capture reagents. Threomers were discovered against two prototypical protein targets implicated in human diseases through a combination of in vitro selection and next-generation sequencing using uracil nucleotides that are uniformly equipped with aromatic side chains commonly found in the paratope of antibody-antigen crystal structures. Kinetic measurements reveal that the side chain modifications are critical for generating threomers with slow off-rate binding kinetics. These findings expand the chemical space of evolvable non-natural genetic systems to include functional groups that enhance protein target binding by mimicking the structural properties of traditional antibodies.

Transliteration of synthetic genetic enzymes.

Functional nucleic acids lose activity when their sequence is prepared in the backbone architecture of a different genetic polymer. The only known exception to this rule is a subset of aptamers whose binding mechanism involves G-quadruplex formation. We refer to such examples as transliteration-a synthetic biology concept describing cases in which the phenotype of a nucleic acid molecule is retained when the genotype is written in a different genetic language. Here, we extend the concept of transliteration to include nucleic acid enzymes (XNAzymes) that mediate site-specific cleavage of an RNA substrate. We show that an in vitro selected 2'-fluoroarabino nucleic acid (FANA) enzyme retains catalytic activity when its sequence is prepared as α-l-threofuranosyl nucleic acid (TNA), and vice versa, a TNA enzyme that remains functional when its sequence is prepared as FANA. Structure probing with DMS supports the hypothesis that FANA and TNA enzymes having the same primary sequence can adopt similarly folded tertiary structures. These findings provide new insight into the sequence-structure-function paradigm governing biopolymer folding.

Synthesis and Polymerase Recognition of Threose Nucleic Acid Triphosphates Equipped with Diverse Chemical Functionalities.

Expanding the chemical space of evolvable non-natural genetic polymers (XNAs) to include functional groups that enhance protein target binding affinity offers a promising route to therapeutic aptamers with high biological stability. Here we describe the chemical synthesis and polymerase recognition of 10 chemically diverse functional groups introduced at the C-5 position of α-l-threofuranosyl uridine nucleoside triphosphate (tUTP). We show that the set of tUTP substrates is universally recognized by the laboratory-evolved polymerase Kod-RSGA. Insights into the mechanism of TNA synthesis were obtained from a high-resolution X-ray crystal structure of the postcatalytic complex bound to the primer-template duplex. A structural analysis reveals a large cavity in the enzyme active site that can accommodate the side chain of C-5-modified tUTP substrates. Our findings expand the chemical space of evolvable nucleic acid systems by providing a synthetic route to artificial genetic polymers that are uniformly modified with diversity-enhancing functional groups.

REVEALR: A Multicomponent XNAzyme-Based Nucleic Acid Detection System for SARS-CoV-2.

Isothermal amplification strategies capable of rapid, inexpensive, and accurate nucleic acid detection provide new options for large-scale pathogen detection, disease diagnosis, and genotyping. Here we report a highly sensitive multicomponent XNA-based nucleic acid detection platform that combines analyte preamplification with X10-23-mediated catalysis to detect the viral pathogen responsible for COVID-19. The platform, termed RNA-Encoded Viral Nucleic Acid Analyte Reporter (REVEALR), functions with a detection limit of ≤20 aM (∼10 copies/µL) using conventional fluorescence and paper-based lateral flow readout modalities. With a total assay time of 1 h, REVEALR provides a convenient nucleic acid alternative to equivalent CRISPR-based approaches, which have become popular methods for SARS-CoV-2 detection. The assay shows no cross-reactivity for other in vitro transcribed respiratory viral RNAs and functions with perfect accuracy against COVID-19 patient-derived clinical samples.

Following replicative DNA synthesis by time-resolved X-ray crystallography.

The mechanism of DNA synthesis has been inferred from static structures, but the absence of temporal information raises longstanding questions about the order of events in one of life's most central processes. Here we follow the reaction pathway of a replicative DNA polymerase using time-resolved X-ray crystallography to elucidate the order and transition between intermediates. In contrast to the canonical model, the structural changes observed in the time-lapsed images reveal a catalytic cycle in which translocation precedes catalysis. The translocation step appears to follow a push-pull mechanism where the O-O1 loop of the finger subdomain acts as a pawl to facilitate unidirectional movement along the template with conserved tyrosine residues 714 and 719 functioning as tandem gatekeepers of DNA synthesis. The structures capture the precise order of critical events that may be a general feature of enzymatic catalysis among replicative DNA polymerases.

Functional Comparison of Laboratory-Evolved XNA Polymerases for Synthetic Biology.

Artificial genetic polymers (XNAs) have enormous potential as new materials for synthetic biology, biotechnology, and molecular medicine; yet, very little is known about the biochemical properties of XNA polymerases that have been developed to synthesize and reverse-transcribe XNA polymers. Here, we compare the substrate specificity, thermal stability, reverse transcriptase activity, and fidelity of laboratory-evolved polymerases that were established to synthesize RNA, 2'-fluoroarabino nucleic acid (FANA), arabino nucleic acid (ANA), hexitol nucleic acid (HNA), threose nucleic acid (TNA), and phosphonomethylthreosyl nucleic acid (PMT). We find that the mutations acquired to facilitate XNA synthesis increase the tolerance of the enzymes for sugar-modified substrates with some sacrifice to protein-folding stability. Bst DNA polymerase was found to have weak reverse transcriptase activity on ANA and uncontrolled reverse transcriptase activity on HNA, differing from its known recognition of FANA and TNA templates. These data benchmark the activity of current XNA polymerases and provide opportunities for generating new polymerase variants that function with greater activity and substrate specificity.

A biologically stable DNAzyme that efficiently silences gene expression in cells.

Efforts to use RNA-cleaving DNA enzymes (DNAzymes) as gene-silencing agents in therapeutic applications have stalled due to their low efficacy in clinical trials. Here we report a xeno-nucleic-acid-modified version of the classic DNAzyme 10-23 that achieves multiple-turnover activity under cellular conditions and resists nuclease digestion. The new reagent, X10-23, overcomes the problem of product inhibition, which limited previous 10-23 designs, using molecular chemotypes with DNA, 2'-fluoroarabino nucleic acid and α-L-threofuranosyl nucleic acid backbone architectures that balance the effects of enhanced biological stability with RNA hybridization and divalent metal ion coordination. In cultured mammalian cells, X10-23 facilitates persistent gene silencing by efficiently degrading exogenous and endogenous messenger RNA transcripts. Together, these results demonstrate that new molecular chemotypes can improve the activity and stability of DNAzymes, and may provide a new route for nucleic acid enzymes to reach the clinic.

Allele-Specific RNA Knockdown with a Biologically Stable and Catalytically Efficient XNAzyme.

Therapeutic targeting of allele-specific single nucleotide mutations in RNA is a major challenge in biology and medicine. Herein, we describe the utility of the XNAzyme X10-23 to knock down allele-specific mRNA sequences in cells. We demonstrate the value of this approach by targeting the "undruggable" mutation G12V in oncogenic KRAS. Our results demonstrate how catalytic XNAs could be employed to suppress the expression of mRNAs carrying disease-causing mutations that are difficult to target at the protein level with small molecule therapeutics.