Structural Elucidation of Bisulfite Adducts to Pseudouridine That Result in Deletion Signatures during Reverse Transcription of RNA

Recent developments, opportunities, and challenges in the study of mRNA pseudouridylation

Pseudouridine is an abundant mRNA modification found in diverse organisms ranging from bacteria and viruses to multicellular plants and humans. New developments in pseudouridine profiling provide quantitative tools to map mRNA pseudouridylation sites. Sparse biochemical studies establish the potential for mRNA pseudouridylation to affect most stages of the mRNA life cycle from birth to death. This recent progress sets the stage for deeper investigations into the molecular and cellular functions of specific mRNA pseudouridines, including in disease.

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.

RNA pseudouridine modification in plants

Pseudouridine is one of the well-known chemical modifications in various RNA species. Current advances to detect pseudouridine show that the pseudouridine landscape is dynamic and affects multiple cellular processes. Although our understanding of this post-transcriptional modification mainly depends on yeast and human models, the recent findings provide strong evidence for the critical role of pseudouridine in plants. Here, we review the current knowledge of pseudouridine in plant RNAs, including its synthesis, degradation, regulatory mechanisms, and functions. Moreover, we propose future areas of research on pseudouridine modification in plants.

Quantifying m6A and Ψ Modifications in the Transcriptome via Chemical-Assisted Approaches

Since the discovery of the first chemically modified RNA nucleotide in 1951, more than 170 types of chemical modifications have been characterized in RNA so far. Since the discovery of the reversible and dynamic nature of N6-methyladenosine (m6A) in mRNA modification, researchers have identified about ten modifications in eukaryotic mRNA; together with modifications on the noncoding RNAs, the term "epitranscriptome" has been coined to describe the ensemble of various chemical RNA modifications. The past decade has witnessed the discovery of many novel molecular functions of mRNA modifications, demonstrating their crucial roles in gene expression regulation. As the most abundant modifications in mRNA, the study of m6A and Ψ has been facilitated by innovative high-throughput sequencing technologies, which can be based on antibodies, enzymes, or novel chemistry. Among them, chemical-assisted methods utilize selective chemistry that can discriminate modified versus unmodified nucleotides, enabling the transcriptome-wide mapping of m6A and Ψ modifications and functional studies.Our group has developed several sequencing technologies to investigate these epitranscriptomic marks including m6A, Ψ, m1A, and m6Am. Among them, we have recently developed two methods for absolute quantification of m6A and Ψ in the transcriptome based on chemical reactivity to distinguish and measure the two modifications. In GLORI, we used glyoxal and nitrite to mediate efficient deamination of regular adenosine, while m6A remained unaffected, thereby enabling efficient and unbiased detection of single-base resolution and absolute quantification of m6A modification. In CeU-seq and PRAISE, we used different chemistry to achieve selective labeling and detection of transcriptome-wide Ψ. CeU-seq is based on an azido-derivatized carbodiimide compound, while PRAISE utilizes the unique activity of bisulfite to Ψ. PRAISE results in the formation of ring-opening Ψ-bisulfite adduct and quantitatively detects Ψ as 1-2 nt deletion signatures during sequencing. The resulting base-resolution and stoichiometric information expanded our understanding to the profiles of RNA modifications in the transcriptome. In particular, the quantitative information on RNA methylome is critical for characterizing the dynamic and reversible nature of RNA modifications, for instance, under environmental stress or during development. Additionally, base-resolution and stoichiometric information can greatly facilitate the analysis and characterization of functional modification sites that are important for gene expression regulation, especially when one modification type may have multiple or even opposing functions within a specific transcript. Together, the quantitative profiling methods provide the modification stoichiometry information, which is critical to study the regulatory roles of RNA modifications.In this Account, we will focus on the quantitative sequencing technologies of m6A and Ψ developed in our group, review recent advances in chemical-assisted reactions for m6A and Ψ detection, and discuss the challenges and future opportunities of transcriptome-wide mapping technologies for RNA modifications.

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.

Nucleoside Analogs: A Review of Its Source and Separation Processes

Nucleoside analogs play a crucial role in the production of high-value antitumor and antimicrobial drugs. Currently, nucleoside analogs are mainly obtained through nucleic acid degradation, chemical synthesis, and biotransformation. However, these methods face several challenges, such as low concentration of the main product, the presence of complex matrices, and the generation of numerous by-products that significantly limit the development of new drugs and their pharmacological studies. Therefore, this work aims to summarize the universal separation methods of nucleoside analogs, including crystallization, high-performance liquid chromatography (HPLC), column chromatography, solvent extraction, and adsorption. The review also explores the application of molecular imprinting techniques (MITs) in enhancing the identification of the separation process. It compares existing studies reported on adsorbents of molecularly imprinted polymers (MIPs) for the separation of nucleoside analogs. The development of new methods for selective separation and purification of nucleosides is vital to improving the efficiency and quality of nucleoside production. It enables us to obtain nucleoside products that are essential for the development of antitumor and antiviral drugs. Additionally, these methods possess immense potential in the prevention and control of serious diseases, offering significant economic, social, and scientific benefits to the fields of environment, biomedical research, and clinical therapeutics.

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.

Mapping epigenetic modifications by sequencing technologies

The "epigenetics" concept was first described in 1942. Thus far, chemical modifications on histones, DNA, and RNA have emerged as three important building blocks of epigenetic modifications. Many epigenetic modifications have been intensively studied and found to be involved in most essential biological processes as well as human diseases, including cancer. Precisely and quantitatively mapping over 100 [1], 17 [2], and 160 [3] different known types of epigenetic modifications in histone, DNA, and RNA is the key to understanding the role of epigenetic modifications in gene regulation in diverse biological processes. With the rapid development of sequencing technologies, scientists are able to detect specific epigenetic modifications with various quantitative, high-resolution, whole-genome/transcriptome approaches. Here, we summarize recent advances in epigenetic modification sequencing technologies, focusing on major histone, DNA, and RNA modifications in mammalian cells.

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.

Quantitative sequencing using BID-seq uncovers abundant pseudouridines in mammalian mRNA at base resolution

Functional characterization of pseudouridine (Ψ) in mammalian mRNA has been hampered by the lack of a quantitative method that maps Ψ in the whole transcriptome. We report bisulfite-induced deletion sequencing (BID-seq), which uses a bisulfite-mediated reaction to convert pseudouridine stoichiometrically into deletion upon reverse transcription without cytosine deamination. BID-seq enables detection of abundant Ψ sites with stoichiometry information in several human cell lines and 12 different mouse tissues using 10-20 ng input RNA. We uncover consensus sequences for Ψ in mammalian mRNA and assign different 'writer' proteins to individual Ψ deposition. Our results reveal a transcript stabilization role of Ψ sites installed by TRUB1 in human cancer cells. We also detect the presence of Ψ within stop codons of mammalian mRNA and confirm the role of Ψ in promoting stop codon readthrough in vivo. BID-seq will enable future investigations of the roles of Ψ in diverse biological processes.

BID-seq: The Quantitative and Base-Resolution Sequencing Method for RNA Pseudouridine

Quantitative and base-resolution sequencing methods are critical to investigations of the biological functions of diverse RNA modifications. These methods may also be employed for clinical studies and clinical applications in the future. In this In Focus article, we introduce and discuss the development of Bisulfite-Induced Deletion sequencing (BID-seq) for quantitatively detecting mRNA pseudouridine (Ψ) modifications at base resolution.

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

In RNA, pseudouridine (Ψ) and 5-methylcytidine (m5C) are located by their differential reactions with NaHSO3 at pH 5. The pyrimidines were allowed to react with NaHSO3, NaN3, NaCN, or NaSCN at pH 5 to find that NaHSO3 was unique in achieving quantitative yields. Pseudouridine reaction selectivity with NaHSO3 was found at pH 7 supported by the reaction rate constants. The Ψ derivative N1-methylpseudouridine found in mRNA vaccines reacts similarly with bisulfite to yield ribose adducts.

Nanopore Dwell Time Analysis Permits Sequencing and Conformational Assignment of Pseudouridine in SARS-CoV-2

Direct RNA sequencing for the epitranscriptomic modification pseudouridine (Ψ), an isomer of uridine (U), was conducted with a protein nanopore sensor using a helicase brake to slowly feed the RNA into the sensor. Synthetic RNAs with 100% Ψ or U in 20 different known human sequence contexts identified differences during sequencing in the base-calling, ionic current, and dwell time in the nanopore sensor; however, the signals were found to have a dependency on the context that would result in biases when sequencing unknown samples. A solution to the challenge was the identification that the passage of Ψ through the helicase brake produced a long-range dwell time impact with less context bias that was used for modification identification. The data analysis approach was employed to analyze publicly available direct RNA sequencing data for SARS-CoV-2 RNA taken from cell culture to locate five conserved Ψ sites in the genome. Two sites were found to be substrates for pseudouridine synthase 1 and 7 in an in vitro assay, providing validation of the analysis. Utilization of the helicase as an additional sensor in direct RNA nanopore sequencing provides greater confidence in calling RNA modifications.

Arrow pushing in RNA modification sequencing

Methods to accurately determine the location and abundance of RNA modifications are critical to understanding their functional role. In this review, we describe recent efforts in which chemical reactivity and next-generation sequencing have been integrated to detect modified nucleotides in RNA. For eleven exemplary modifications, we detail chemical, enzymatic, and metabolic labeling protocols that can be used to differentiate them from canonical nucleobases. By emphasizing the molecular rationale underlying these detection methods, our survey highlights new opportunities for chemistry to define the role of RNA modifications in disease.

Regulation and Function of RNA Pseudouridylation in Human Cells

Recent advances in pseudouridine detection reveal a complex pseudouridine landscape that includes messenger RNA and diverse classes of noncoding RNA in human cells. The known molecular functions of pseudouridine, which include stabilizing RNA conformations and destabilizing interactions with varied RNA-binding proteins, suggest that RNA pseudouridylation could have widespread effects on RNA metabolism and gene expression. Here, we emphasize how much remains to be learned about the RNA targets of human pseudouridine synthases, their basis for recognizing distinct RNA sequences, and the mechanisms responsible for regulated RNA pseudouridylation. We also examine the roles of noncoding RNA pseudouridylation in splicing and translation and point out the potential effects of mRNA pseudouridylation on protein production, including in the context of therapeutic mRNAs.

Thioguanosine Conversion Enables mRNA-Lifetime Evaluation by RNA Sequencing Using Double Metabolic Labeling (TUC-seq DUAL)

Temporal information about cellular RNA populations is essential to understand the functional roles of RNA. We have developed the hydrazine/NH₄ Cl/OsO₄ -based conversion of 6-thioguanosine (6sG) into A', where A' constitutes a 6-hydrazino purine derivative. A' retains the Watson-Crick base-pair mode and is efficiently decoded as adenosine in primer extension assays and in RNA sequencing. Because 6sG is applicable to metabolic labeling of freshly synthesized RNA and because the conversion chemistry is fully compatible with the conversion of the frequently used metabolic label 4-thiouridine (4sU) into C, the combination of both modified nucleosides in dual-labeling setups enables high accuracy measurements of RNA decay. This approach, termed TUC-seq DUAL, uses the two modified nucleosides in subsequent pulses and their simultaneous detection, enabling mRNA-lifetime evaluation with unprecedented precision.