Pseudouridine and N1-Methylpseudouridine Display pH-Independent Reaction Rates with Bisulfite Yielding Ribose Adducts

Quantitative detection of pseudouridine in RNA by mass spectrometry

Pseudouridine (Ψ) is one of the most prevalent and dynamic modification in RNA, and was shown to evade the host immune response in mRNA vaccines. Despite its significance, the biological role of Ψ remains poorly understood as certain key limitations and challenges in the detection of Ψ are yet to be overcome. In account of this, we report the usage of a chemical labelling strategy for the first quantitative detection of Ψ by mass spectrometry. We demonstrate a labelling efficiency exceeding 99% in isolated yeast tRNAs hosting multiple Ψs. LC-MS/MS analysis enables precise mapping of Ψ at single-base resolution, while simultaneously capturing a wide array of additional post-transcriptional modifications, which is not achieved with current sequencing technologies. This advancement may help unravel the dynamics and biological implications of Ψ, shedding light on its interplay with other modifications and deepening our understanding of its functional role.

BID-seq for transcriptome-wide quantitative sequencing of mRNA pseudouridine at base resolution

Pseudouridine (Ψ) is an abundant RNA modification that is present in and affects the functions of diverse non-coding RNA species, including rRNA, tRNA and small nuclear RNA. Ψ also exists in mammalian mRNA and probably exhibits functional roles; however, functional investigations of mRNA Ψ modifications in mammals have been hampered by the lack of a quantitative method that detects Ψ at base precision. We have recently developed bisulfite-induced deletion sequencing (BID-seq), which provides the community with a quantitative method to map RNA Ψ distribution transcriptome-wide at single-base resolution. Here, we describe an optimized BID-seq protocol for mapping Ψ distribution across cellular mRNAs, which includes fast steps in both library preparation and data analysis. This protocol generates highly reproducible results by inducing high deletion ratios at Ψ modification within diverse sequence contexts, and meanwhile displayed almost zero background deletions at unmodified uridines. When used for transcriptome-wide Ψ profiling in mouse embryonic stem cells, the current protocol uncovered 8,407 Ψ sites from as little as 10 ng of polyA+ RNA input. This optimized BID-seq workflow takes 5 days to complete and includes four main sections: RNA preparation, library construction, next-generation sequencing (NGS) and data analysis. Library construction can be completed by researchers who have basic knowledge and skills in molecular biology and genetics. In addition to the experimental protocol, we provide BID-pipe ( https://github.com/y9c/pseudoU-BIDseq ), a user-friendly data analysis pipeline for Ψ site detection and modification stoichiometry quantification, requiring only basic bioinformatic and computational skills to uncover Ψ signatures from BID-seq data.

Base-Resolution Sequencing Methods for Whole-Transcriptome Quantification of mRNA Modifications

RNA molecules are not merely a combination of four bases of A, C, G, and U. Chemical modifications occur in almost all RNA species and play diverse roles in gene expression regulation. The abundant cellular RNAs, such as ribosomal RNA (rRNA) and transfer RNA (tRNA), are known to have the highest density of RNA modifications, which exert critical functions in rRNA and tRNA biogenesis, stability, and subsequent translation. In recent years, modifications on low-abundance RNA species in mammalian cells, such as messenger RNA (mRNA), regulatory noncoding RNA (ncRNA), and chromatin-associated RNA (caRNA), have been shown to contain multiple different chemical modifications with functional significance. As the most abundant mRNA modification in mammals, N6-methyladenosine (m6A) affects nearly every stage of mRNA processing and metabolism, with the antibody-based m6A-MeRIP-seq (methylated RNA immunoprecipitation sequencing) followed by high-throughput sequencing widely employed in mapping m6A distribution transcriptome-wide in diverse biological systems. In addition to m6A, other chemical modifications such as pseudouridine (Ψ), 2'-O-methylation (Nm), 5-methylcytidine (m5C), internal N7-methylguanosine (m7G), N1-methyladenosine (m1A), N4-acetylcytidine (ac4C), etc. also exist in polyA-tailed RNA in mammalian cells, requiring effective mapping approaches for whole-transcriptome profiling of these non-m6A mRNA modifications. Like m6A, the antibody-based enrichment followed by sequencing has been the primary method to study distributions of these modifications. Methods to more quantitatively map these modifications would dramatically improve our understanding of distributions and modification density of these chemical marks on RNA, thereby bettering informing functional implications. In this Account, aimed at both single-base resolution and modification fraction quantification, we summarize our recent advances in developing a series of chemistry- or biochemistry-based methods to quantitatively map RNA modifications, including m6A, Ψ, m5C, m1A, 2'-O-methylation (Nm), and internal m7G, in mammalian mRNA at base resolution. These new methods, including m6A-SAC-seq, eTAM-seq, BID-seq, UBS-seq, DAMM-seq, m1A-quant-seq, Nm-Mut-seq, and m7G-quant-seq, promise to conduct base-resolution mapping of most major mRNA modifications with low RNA input and uncover dynamic changes in modification stoichiometry during biological and physiological processes, facilitating future investigations on these RNA modifications in regulating cellular gene expression and as potential biomarkers for clinical diagnosis and prognosis. These quantitative sequencing methods allow the mapping of most mRNA modifications with limited input sample requirements. The same modifications on diverse RNA species, such as caRNA, ncRNA, nuclear nascent RNA, mitochondrial RNA, cell-free RNA (cfRNA), etc., could be sequenced using the same methods.

Advantages and challenges associated with bisulfite-assisted nanopore direct RNA sequencing for modifications

Nanopore direct RNA sequencing is a technology that allows sequencing for epitranscriptomic modifications with the possibility of a quantitative assessment. In the present work, pseudouridine (Ψ) was sequenced with the nanopore before and after the pH 7 bisulfite reaction that yields stable ribose adducts at C1' of Ψ. The adducted sites produced greater base call errors in the form of deletion signatures compared to Ψ. Sequencing studies on E. coli rRNA and tmRNA before and after the pH 7 bisulfite reaction demonstrated that using chemically-assisted nanopore sequencing has distinct advantages for minimization of false positives and false negatives in the data. The rRNA from E. coli has 19 known U/C sequence variations that give similar base call signatures as Ψ, and therefore, are false positives when inspecting base call data; however, these sites are refractory to reacting with bisulfite as is easily observed in nanopore data. The E. coli tmRNA has a low occupancy Ψ in a pyrimidine-rich sequence context that is called a U representing a false negative; partial occupancy by Ψ is revealed after the bisulfite reaction. In a final study, 5-methylcytidine (m5C) in RNA can readily be observed after the pH 5 bisulfite reaction in which the parent C deaminates to U and the modified site does not react. This locates m5C when using bisulfite-assisted nanopore direct RNA sequencing, which is otherwise challenging to observe. The advantages and challenges of the overall approach are discussed.

Bisulfite and Nanopore Sequencing for Pseudouridine in RNA

Nucleophilic addition of bisulfite to pyrimidine bases has been known for a half century, and the reaction has been in use for at least a quarter of a century for identifying 5-methylcytidine in DNA. This account focuses on the chemistry of bisulfite with pseudouridine, an isomer of the RNA nucleoside uridine in which the uracil base is connected to C1' of ribose via C5 instead of N1. Pseudouridine, Ψ, is the most common nucleotide modification found in cellular RNA overall, in part due to its abundance in rRNAs and tRNAs. It has a stabilizing influence on RNA structure because N1 is now available for additional hydrogen bonding and because the heterocycle is slightly better at π stacking. The isomerization of U to Ψ in RNA strands is catalyzed by 13 different enzymes in humans and 11 in E. coli; some of these enzymes are implicated in disease states which is testament to the biological importance of pseudouridine in cells. Recently, pseudouridine came into the limelight as the key modification that, after N1 methylation, enables mRNA vaccines to be delivered efficiently into human tissue with minimal generation of a deleterious immunogenic response. Here we describe the bisulfite reaction with pseudouridine which gives rise to a chemical sequencing method to map the modified base in the epitranscriptome. Unlike the reaction with cytidine, the addition of bisulfite to Ψ leads irreversibly to form an adduct that is bypassed during cDNA synthesis by reverse transcriptases yielding a characteristic deletion signature. Although there were hints to the structure of the bisulfite adduct(s) 30 to 50 years ago, it took modern spectroscopic and computational methods to solve the mystery. Raman spectroscopy along with extensive NMR, ECD, and computational work led to the assignment of the major product as the (R) diastereomer of an oxygen adduct at C1' of a ring-opened pseudouridine. Mechanistically, this arose from a succession of conjugate addition, E2 elimination, and a [2,3] sigmatropic rearrangement, all of which are stereodefined reactions. A minor reaction with excess bisulfite led to the (S) isomer of a S-adducted SO3- group. Understanding structure and mechanism aided the design of a Ψ-specific sequencing reaction and guided attempts to improve the utility and specificity of the method. Separately, we have been investigating the use of nanopore direct RNA sequencing, a single-molecule method that directly analyzes RNA strands isolated from cells after end-ligation of adaptor sequences. By combining the electrical current and base-calling data from the nanopore with dwell-time analysis from the helicase employed to deliver RNA to the nanopore, we were able to map Ψ sites in nearly all sequence contexts. This analysis was employed to find Ψ residues in the SARS-CoV-2 vRNA, to analyze the sequence context effects of mRNA vaccine synthesis via in vitro transcription, and to evaluate the impact of stress on chemical modifications in the E. coli ribosome. Most recently, we found that bisulfite treatment of RNA leading to Ψ adducts could modulate the nanopore signal to help in mapping modifications of low occupancy.

Quantitative base-resolution sequencing technology for mapping pseudouridines in mammalian mRNA

Posttranscriptional RNA modifications occur in almost all types of RNA in all life forms. As an abundant RNA modification in mammals, pseudouridine (Ψ) regulates diverse biological functions of different RNA species such as ribosomal RNA (rRNA), transfer RNA (tRNA), small nuclear RNA (snRNA), etc. However, the functional investigation of mRNA pseudouridine (Ψ) has been hampered by the lack of a quantitative method that can efficiently map Ψ transcriptome-wide. We developed bisulfite-induced deletion sequencing (BID-seq), with the optimized bisulfite-based chemical reaction to convert pseudouridine selectively and completely into Ψ-BS adduct without cytosine deamination. The Ψ-BS adduct can be further read out as deletion signatures during reverse transcription. The deletion ratios induced by Ψ sites were used for estimating the modification stoichiometry at each modified site. BID-seq starts with 10-20 ng polyA+ RNA and detects thousands of mRNA Ψ sites with stoichiometry information in cell lines and tissues. We uncovered consensus motifs for Ψ in mammalian mRNA and assigned specific 'writer' proteins to individual Ψ deposition. BID-seq also confirmed the presence of Ψ within stop codons of mammalian mRNA. BID-seq set the stage for future investigations of Ψ functions in diverse biological processes.

Modified mRNA as a Treatment for Myocardial Infarction

Myocardial infarction (MI) is a severe disease with high mortality worldwide. However, regenerative approaches remain limited and with poor efficacy. The major difficulty during MI is the substantial loss of cardiomyocytes (CMs) with limited capacity to regenerate. As a result, for decades, researchers have been engaged in developing useful therapies for myocardial regeneration. Gene therapy is an emerging approach for promoting myocardial regeneration. Modified mRNA (modRNA) is a highly potential delivery vector for gene transfer with its properties of efficiency, non-immunogenicity, transiency, and relative safety. Here, we discuss the optimization of modRNA-based therapy, including gene modification and delivery vectors of modRNA. Moreover, the effective of modRNA in animal MI treatment is also discussed. We conclude that modRNA-based therapy with appropriate therapeutical genes can potentially treat MI by directly promoting proliferation and differentiation, inhibiting apoptosis of CMs, as well as enhancing paracrine effects in terms of promoting angiogenesis and inhibiting fibrosis in heart milieu. Finally, we summarize the current challenges of modRNA-based cardiac treatment and look forward to the future direction of such treatment for MI. Further advanced clinical trials incorporating more MI patients should be conducted in order for modRNA therapy to become practical and feasible in real-world treatment.