The novel coronavirus SARS-CoV-2, the cause of the COVID-19 pandemic, has inspired one of the most efficient vaccine development campaigns in human history. A key aspect of COVID-19 mRNA vaccines is the use of the modified nucleobase N1-methylpseudouridine (m1Ψ) to increase their effectiveness. In this Outlook, we summarize the development and function of m1Ψ in synthetic mRNAs. By demystifying how a novel element within these medicines works, we aim to foster understanding and highlight future opportunities for chemical innovation.

Modifications in an Emergency: The Role of N1-Methylpseudouridine in COVID-19 Vaccines

While Nature harnesses RNA and DNA to store, read and write genetic information, the inherent programmability, synthetic accessibility and wide functionality of these nucleic acids make them attractive tools for use in a vast array of applications. In medicine, antisense oligonucleotides (ASOs), siRNAs, and therapeutic aptamers are explored as potent targeted treatment and diagnostic modalities, while in the technological field oligonucleotides have found use in new materials, catalysis, and data storage. The use of natural oligonucleotides limits the possible chemical functionality of resulting technologies while inherent shortcomings, such as susceptibility to nuclease degradation, provide obstacles to their application. Modified oligonucleotides, at the level of the nucleobase, sugar and/or phosphate backbone, are widely used to overcome these limitations. This review provides the reader with an overview of non-native modifications and the challenges faced in the design, synthesis, application and outlook of novel modified oligonucleotides.

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Recent progress in non-native nucleic acid modifications.

Synthetic biology aims to improve human health and the environment by repurposing biological enzymes for use in practical applications. However, natural enzymes often function with suboptimal activity when engineered into biological pathways or challenged to recognize unnatural substrates. Overcoming this problem requires efficient directed evolution methods for discovering new enzyme variants that function with a desired activity. Here, we describe the construction, validation, and application of a fluorescence-activated droplet sorting (FADS) instrument that was established to evolve enzymes for synthesizing and modifying artificial genetic polymers (XNAs). The microfluidic system enables droplet sorting at ∼2-3 kHz using fluorescent sensors that are responsive to enzymatic activity. The ability to evolve nucleic acid enzymes with customized properties will uniquely drive emerging applications in synthetic biology, biotechnology, and healthcare.

Fluorescence-Activated Droplet Sorting for Single-Cell Directed Evolution.

The term "xeno-nucleic acids", abbreviated XNA, has grown in popularity to the point that it has become a catch-all phrase for almost any unnatural nucleic acid, raising the question: what is XNA and how does it differ from chemically modified DNA?

What Is XNA

Modified nucleic acids, also called xeno nucleic acids (XNAs), offer a variety of advantages for biotechnological applications and address some of the limitations of first-generation nucleic acid therapeutics. Indeed, several therapeutics based on modified nucleic acids have recently been approved and many more are under clinical evaluation. XNAs can provide increased biostability and furthermore are now increasingly amenable to in vitro evolution, accelerating lead discovery. Here, we review the most recent discoveries in this dynamic field with a focus on progress in the enzymatic replication and functional exploration of XNAs.

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Modified nucleic acids: replication, evolution, and next-generation therapeutics.

The isolation of synthetic genetic polymers (XNAs) with catalytic activity demonstrates that catalysis is not limited to natural biopolymers, but it remains unknown whether such systems can achieve robust catalysis with Michaelis-Menten kinetics. Here, we describe an efficient RNA-cleaving 2'-fluoroarabino nucleic acid enzyme (FANAzyme) that functions with a rate enhancement of >106-fold over the uncatalyzed reaction and exhibits substrate saturation kinetics typical of most natural enzymes. The FANAzyme was generated by in vitro evolution using natural polymerases that were found to recognize FANA substrates with high fidelity. The enzyme comprises a small 25 nucleotide catalytic domain flanked by substrate-binding arms that can be engineered to recognize diverse RNA targets. Substrate cleavage occurs at a specific phosphodiester bond located between an unpaired guanine and a paired uracil in the substrate recognition arm. Our results expand the chemical space of nucleic acid enzymes to include nuclease-resistant scaffolds with strong catalytic activity.

https://escholarship.org/content/qt3xd7j4zf/qt3xd7j4zf.pdf?t=pnl80i

Evolution of a General RNA-Cleaving FANA Enzyme

Natural and modified nucleoside triphosphates impact nearly every major aspect of healthcare research from DNA sequencing to drug discovery. However, a scalable synthetic route to these molecules has long been hindered by the need for purification by high performance liquid chromatography (HPLC). Here, we describe a fundamentally different approach that uses a novel P(V) pyrene pyrophosphate reagent to generate derivatives that are purified by silica gel chromatography and converted to the desired compounds on scales vastly exceeding those achievable by HPLC. The power of this approach is demonstrated through the synthesis of a broad range of natural and unnatural nucleoside triphosphates (dNTPs and xNTPs) using protocols that are efficient, inexpensive, and operationally straightforward.

P(V) Reagents for the Scalable Synthesis of Natural and Modified Nucleoside Triphosphates

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.

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

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.

Arlene K. Ngor

Papers

Evolution of a General RNA-Cleaving FANA Enzyme

P(V) Reagents for the Scalable Synthesis of Natural and Modified Nucleoside Triphosphates

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

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