Characterization of Modified RNA by Top-Down Mass Spectrometry

A High-Throughput Workflow for Mass Spectrometry Analysis of Nucleic Acids by Nanoflow Desalting

Broad interest in nucleic acids, both their therapeutic capabilities and understanding the nuances of their structure and resulting function, has increased in recent years. Post-transcriptional modifications, in particular, have become an important analysis target, as these covalent modifications to the sugars, nitrogenous bases, and phosphate backbone impart differential functionality to synthetic and biological nucleic acids. Characterizing these post-transcriptional modifications can be difficult with traditional sequencing workflows; however, advancements in top-down mass spectrometry address these challenges. Online desalting platforms have enabled facile sample cleanup and reliable ionization of increasingly large (100 nt) oligonucleotides, and application of existing tandem mass spectrometry techniques has yielded information-rich spectra which can be used to interrogate primary sequences. To extend the capabilities of top-down MS and its analysis of nucleic acids, we have developed a nanoflow desalting platform for high-throughput and low sample-use desalting coupled with collision-induced dissociation (CID), 213 nm ultraviolet photodissociation (UVPD), and activated-ion electron photodetachment dissociation (a-EPD) to yield high-quality MS/MS spectra. Fragments identified using an m/z-domain isotope matching strategy yielded high sequence coverage (>70%) of a yeast phenylalanine tRNA.

The multifaceted roles of mass spectrometric analysis in nucleic acids drug discovery and development

The deceptively simple concepts of mass determination and fragment analysis are the basis for the application of mass spectrometry (MS) to a boundless range of analytes, including fundamental components and polymeric forms of nucleic acids (NAs). This platform affords the intrinsic ability to observe first-hand the effects of NA-active drugs on the chemical structure, composition, and conformation of their targets, which might affect their ability to interact with cognate NAs, proteins, and other biomolecules present in a natural environment. The possibility of interfacing with high-performance separation techniques represents a multiplying factor that extends these capabilities to cover complex sample mixtures obtained from organisms that were exposed to NA-active drugs. This report provides a brief overview of these capabilities in the context of the analysis of the products of NA-drug activity and NA therapeutics. The selected examples offer proof-of-principle of the applicability of this platform to all phases of the journey undertaken by any successful NA drug from laboratory to bedside, and provide the rationale for its rapid expansion outside traditional laboratory settings in support to ever growing manufacturing operations.

Studying the Key Intermediate of RNA Autohydrolysis by Cryogenic Gas-Phase Infrared Spectroscopy

Over the course of the COVID-19 pandemic, mRNA-based vaccines have gained tremendous importance. The development and analysis of modified RNA molecules benefit from advanced mass spectrometry and require sufficient understanding of fragmentation processes. Analogous to the degradation of RNA in solution by autohydrolysis, backbone cleavage of RNA strands was equally observed in the gas phase; however, the fragmentation mechanism remained elusive. In this work, autohydrolysis-like intermediates were generated from isolated RNA dinucleotides in the gas phase and investigated using cryogenic infrared spectroscopy in helium nanodroplets. Data from both experiment and density functional theory provide evidence for the formation of a five-membered cyclic phosphate intermediate and rule out linear or six-membered structures. Furthermore, the experiments show that another prominent condensed-phase reaction of RNA nucleotides can be induced in the gas phase: the tautomerization of cytosine. Both observed reactions are therefore highly universal and intrinsic properties of the investigated molecules.

Regulatory guidelines and preclinical tools to study the biodistribution of RNA therapeutics

The success of the messenger RNA-based COVID-19 vaccines of Moderna and Pfizer/BioNTech marks the beginning of a new chapter in modern medicine. However, the rapid rise of mRNA therapeutics has resulted in a regulatory framework that is somewhat lagging. The current guidelines either do not apply, do not mention RNA therapeutics, or do not have widely accepted definitions. This review describes the guidelines for preclinical biodistribution studies of mRNA/siRNA therapeutics and highlights the relevant differences for mRNA vaccines. We also discuss the role of in vivo RNA imaging techniques and other assays to fulfill and/or complement the regulatory requirements. Specifically, quantitative whole-body autoradiography, microautoradiography, mass spectrometry-based assays, hybridization techniques (FISH, bDNA), PCR-based methods, in vivo fluorescence imaging, and in vivo bioluminescence imaging, are discussed. We conclude that this new and rapidly evolving class of medicines demands a multi-layered approach to fully understand its biodistribution and in vivo characteristics.

THERAPEUTIC OLIGONUCLEOTIDES, IMPURITIES, DEGRADANTS, AND THEIR CHARACTERIZATION BY MASS SPECTROMETRY

Oligonucleotides are an emerging class of drugs that are manufactured by solid-phase synthesis. As a chemical class, they have unique product-related impurities and degradants, characterization of which is an essential step in drug development. The synthesis cycle, impurities produced during the synthesis and degradation products are presented and discussed. The use of liquid chromatography combined with mass spectrometry for characterization and quantification of product-related impurities and degradants is reviewed. In addition, sequence determination of oligonucleotides by gas-phase fragmentation and indirect mass spectrometric methods is discussed. © 2019 John Wiley & Sons Ltd. Mass Spec Rev.

Radical Transfer Dissociation for De Novo Characterization of Modified Ribonucleic Acids by Mass Spectrometry

Mass spectrometry (MS) can reliably detect and localize all mass-altering modifications of ribonucleic acids (RNA), but current MS approaches that allow for simultaneous de novo sequencing and modification analysis generally require specialized instrumentation. Here we report a novel RNA dissociation technique, radical transfer dissociation (RTD), that can be used for the comprehensive de novo characterization of ribonucleic acids and their posttranscriptional or synthetic modifications. We demonstrate full sequence coverage for RNA consisting of up to 39 nucleotides and show that RTD is especially useful for RNA with highly labile modifications such as 5-hydroxymethylcytidine and 5-formylcytidine.

Enhancing the Mass Spectrometry Sensitivity for Oligonucleotide Detection by Organic Vapor Assisted Electrospray

There are two challenges in oligonucleotide detection by liquid chromatography coupled with mass spectrometry (LC-MS), the serious ion suppression effects caused by ion-pair reagents and the low detection sensitivity in positive mode MS. In this study, highly concentrated alcohol vapors were introduced into an enclosed electrospray ionization chamber, and oligonucleotides could be well detected in negative mode MS even with 100 mM triethylammonium acetate (TEAA) as an ion-pair reagent. The MS signal intensity was improved 600-fold (for standard oligonucleotide dT15) by the isopropanol vapor assisted electrospray, and effective ion-pair LC separation was feasibly coupled with high-sensitive MS detection. Then, oligonucleotides were successfully detected in positive mode MS with few adducts by propanoic acid vapor assisted electrospray. The signal intensity was enhanced more than 10-fold on average compared with adding acids into the electrospray solution. Finally, oligonucleotides and peptides or histones were simultaneously detected in MS with little interference with each other. Our strategy provides a useful alternative for investigating the biological functions of oligonucleotides.

Native Top-Down Mass Spectrometry of TAR RNA in Complexes with a Wild-Type tat Peptide for Binding Site Mapping

Ribonucleic acids (RNA) frequently associate with proteins in many biological processes to form more or less stable complex structures. The characterization of RNA-protein complex structures and binding interfaces by nuclear magnetic resonance (NMR) spectroscopy, X-ray crystallography, or strategies based on chemical crosslinking, however, can be quite challenging. Herein, we have explored the use of an alternative method, native top-down mass spectrometry (MS), for probing of complex stoichiometry and protein binding sites at the single-residue level of RNA. Our data show that the electrostatic interactions between HIV-1 TAR RNA and a peptide comprising the arginine-rich binding region of tat protein are sufficiently strong in the gas phase to survive phosphodiester backbone cleavage of RNA by collisionally activated dissociation (CAD), thus allowing its use for probing tat binding sites in TAR RNA by top-down MS. Moreover, the MS data reveal time-dependent 1:2 and 1:1 stoichiometries of the TAR-tat complexes and suggest structural rearrangements of TAR RNA induced by binding of tat peptide.