Function and detection of 5-methylcytosine in eukaryotic RNA

Chemistry of installing epitranscriptomic 5-modified cytidines in RNA oligomers

Studies of 5-hydroxymethylcytidine (hm5C), 5-formylcytidine (f5C) and 5-carboxycytidine (ca5C) modifications as products of the 5-methylcytidine (m5C) oxidative demethylation pathway in cellular mRNAs constitute an important element of the new epitranscriptomic field of research. The dynamic process of m5C conversion and final turnover to the parent cytidine is considered a post-transcriptional layer of gene-expression regulation. However, the regulatory mechanism associated with epitranscriptomic cytidine modifications remains largely unknown. Therefore, oligonucleotides containing m5C oxidation products are of great value for the next generation of biochemical, biophysical, and structural studies on their function, metabolism, and contribution to human diseases. Herein, we summarize the synthetic strategies developed for the incorporation of hm5C, f5C and ca5C into RNA oligomers by phosphoramidite chemistry, including post-synthetic C5-cytidine functionalization and enzymatic methods.

Epitranscriptomic Regulations in the Heart

RNA modifications affect key stages of the RNA life cycle, including splicing, export, decay, and translation. Epitranscriptomic regulations therefore significantly influence cellular physiology and pathophysiology. Here, we selected some of the most abundant modifications and reviewed their roles in the heart and in cardiovascular diseases: N6-methyladenosine (m6A), N6,2'-O-dimethyladenosine (m6Am), N1-methyladenosine (m1A), pseudouridine (?), 5 methylcytidine (m5C), and inosine (I). Dysregulation of epitranscriptomic machinery affecting these modifications vastly changes the cardiac phenotype and is linked with many cardiovascular diseases such as myocardial infarction, cardiomyopathies, or heart failure. Thus, a deeper understanding of these epitranscriptomic changes and their regulatory mechanisms can enhance our knowledge of the molecular underpinnings of prevalent cardiac diseases, potentially paving the way for novel therapeutic strategies. Keywords: Epitranscriptomics, RNA modifications, Epigenetics, m6A, RNA, Heart.

Chemical Proteomic Discovery of Isotype-Selective Covalent Inhibitors of the RNA Methyltransferase NSUN2

5-Methylcytosine (m5 C) is an RNA modification prevalent on tRNAs, where it can protect tRNAs from endonucleolytic cleavage to maintain protein synthesis. The NSUN family (NSUN1-7 in humans) of RNA methyltransferases are capable of installing the methyl group onto the C5 position of cytosines in RNA. NSUNs are implicated in a wide range of (patho)physiological processes, but selective and cell-active inhibitors of these enzymes are lacking. Here, we use cysteine-directed activity-based protein profiling (ABPP) to discover azetidine acrylamides that act as stereoselective covalent inhibitors of human NSUN2. Despite targeting a conserved catalytic cysteine in the NSUN family, the NSUN2 inhibitors show negligible cross-reactivity with other human NSUNs and exhibit good proteome-wide selectivity. We verify that the azetidine acrylamides inhibit the catalytic activity of recombinant NSUN2, but not NSUN6, and demonstrate that these compounds stereoselectively disrupt NSUN2-tRNA interactions in cancer cells, leading to a global reduction in tRNA m5 C content. Our findings thus highlight the potential to create isotype-selective and cell-active inhibitors of NSUN2 with covalent chemistry targeting a conserved catalytic cysteine.

Role of methylation in vernalization and photoperiod pathway: a potential flowering regulator?

Recognized as a pivotal developmental transition, flowering marks the continuation of a plant's life cycle. Vernalization and photoperiod are two major flowering pathways orchestrating numerous florigenic signals. Methylation, including histone, DNA and RNA methylation, is one of the recent foci in plant development. Considerable studies reveal that methylation seems to show an increasing potential regulatory role in plant flowering via altering relevant gene expression without altering the genetic basis. However, little has been reviewed about whether and how methylation acts on vernalization- and photoperiod-induced flowering before and after FLOWERING LOCUS C (FLC) reactivation, what role RNA methylation plays in vernalization- and photoperiod-induced flowering, how methylation participates simultaneously in both vernalization- and photoperiod-induced flowering, the heritability of methylation memory under the vernalization/photoperiod pathway, and whether and how methylation replaces vernalization/photoinduction to regulate flowering. Our review provides insight about the crosstalk among the genetic control of the flowering gene network, methylation (methyltransferases/demethylases) and external signals (cold, light, sRNA and phytohormones) in vernalization and photoperiod pathways. The existing evidence that RNA methylation may play a potential regulatory role in vernalization- and photoperiod-induced flowering has been gathered and represented for the first time. This review speculates about and discusses the possibility of substituting methylation for vernalization and photoinduction to promote flowering. Current evidence is utilized to discuss the possibility of future methylation reagents becoming flowering regulators at the molecular level.

Vital roles of m5C RNA modification in cancer and immune cell biology

RNA modification plays an important role in epigenetics at the posttranscriptional level, and 5-methylcytosine (m5C) has attracted increasing attention in recent years due to the improvement in RNA m5C site detection methods. By influencing transcription, transportation and translation, m5C modification of mRNA, tRNA, rRNA, lncRNA and other RNAs has been proven to affect gene expression and metabolism and is associated with a wide range of diseases, including malignant cancers. RNA m5C modifications also substantially impact the tumor microenvironment (TME) by targeting different groups of immune cells, including B cells, T cells, macrophages, granulocytes, NK cells, dendritic cells and mast cells. Alterations in immune cell expression, infiltration and activation are highly linked to tumor malignancy and patient prognosis. This review provides a novel and holistic examination of m5C-mediated cancer development by examining the exact mechanisms underlying the oncogenicity of m5C RNA modification and summarizing the biological effects of m5C RNA modification on tumor cells as well as immune cells. Understanding methylation-related tumorigenesis can provide useful insights for the diagnosis as well as the treatment of cancer.

Chemical modifications to mRNA nucleobases impact translation elongation and termination

Messenger RNAs (mRNAs) serve as blueprints for protein synthesis by the molecular machine the ribosome. The ribosome relies on hydrogen bonding interactions between adaptor aminoacyl-transfer RNA molecules and mRNAs to ensure the rapid and faithful translation of the genetic code into protein. There is a growing body of evidence suggesting that chemical modifications to mRNA nucleosides impact the speed and accuracy of protein synthesis by the ribosome. Modulations in translation rates have downstream effects beyond protein production, influencing protein folding and mRNA stability. Given the prevalence of such modifications in mRNA coding regions, it is imperative to understand the consequences of individual modifications on translation. In this review we present the current state of our knowledge regarding how individual mRNA modifications influence ribosome function. Our comprehensive comparison of the impacts of 16 different mRNA modifications on translation reveals that most modifications can alter the elongation step in the protein synthesis pathway. Additionally, we discuss the context dependence of these effects, highlighting the necessity of further study to uncover the rules that govern how any given chemical modification in an mRNA codon is read by the ribosome.

Biological roles of RNA m5C modification and its implications in Cancer immunotherapy

Epigenetics including DNA and RNA modifications have always been the hotspot field of life sciences in the post-genome era. Since the first mapping of N6-methyladenosine (m6A) and the discovery of its widespread presence in mRNA, there are at least 160-170 RNA modifications have been discovered. These methylations occur in different RNA types, and their distribution is species-specific. 5-methylcytosine (m5C) has been found in mRNA, rRNA and tRNA of representative organisms from all kinds of species. As reversible epigenetic modifications, m5C modifications of RNA affect the fate of the modified RNA molecules and play important roles in various biological processes including RNA stability control, protein synthesis, and transcriptional regulation. Furthermore, accumulative evidence also implicates the role of RNA m5C in tumorigenesis. Here, we review the latest progresses in the biological roles of m5C modifications and how it is regulated by corresponding "writers", "readers" and "erasers" proteins, as well as the potential molecular mechanism in tumorigenesis and cancer immunotherapy.

Epigenetic interaction of microbes with their mammalian hosts

The interaction of microbiota with its host has the ability to alter the cellular functions of both, through several mechanisms. Recent work, from many laboratories including our own, has shown that epigenetic mechanisms play an important role in the alteration of these cellular functions. Epigenetics broadly refers to change in the phenotype without a corresponding change in the DNA sequence. This change is usually brought by epigenetic modifications of the DNA itself, the histone proteins associated with the DNA in the chromatin, non-coding RNA or the modifications of the transcribed RNA. These modifications, also known as epigenetic code, do not change the DNA sequence but alter the expression level of specific genes. Microorganisms seem to have learned how to modify the host epigenetic code and modulate the host transcriptome in their favour. In this review, we explore the literature that describes the epigenetic interaction of bacteria, fungi and viruses, with their mammalian hosts.

Protein-based molecular recognition tools for detecting and profiling RNA modifications

RNA undergoes extensive biochemical modification following transcription. In addition to RNA splicing, transcripts are processed by a suite of enzymes that alter the chemical structure of different nucleobases. Broadly termed as 'RNA editing,' these modifications impart significant functional changes to translation, localization, and stability of individual transcripts within the cell. These changes are dynamic and required for a number of critical cellular processes, and dysregulation of these pathways is responsible for several disease states. Accurately detecting, measuring, and mapping different RNA modifications across the transcriptome is vital to understanding their broader functions as well as leveraging these events as diagnostic biomarkers. Here, we review recent advances in profiling several types of RNA modifications, with particular emphasis on adenosine-to-inosine (A-to-I) and N6-methyladenosine (m6A) RNA editing. We especially highlight approaches that utilize proteins to detect or enrich modified RNA transcripts before sequencing, and we summarize recent insights yielded from these techniques.

Changes of the tRNA Modification Pattern during the Development of Dictyostelium discoideum

Dictyostelium discoideum is a social amoeba, which on starvation develops from a single-cell state to a multicellular fruiting body. This developmental process is accompanied by massive changes in gene expression, which also affect non-coding RNAs. Here, we investigate how tRNAs as key regulators of the translation process are affected by this transition. To this end, we used LOTTE-seq to sequence the tRNA pool of D. discoideum at different developmental time points and analyzed both tRNA composition and tRNA modification patterns. We developed a workflow for the specific detection of modifications from reverse transcriptase signatures in chemically untreated RNA-seq data at single-nucleotide resolution. It avoids the comparison of treated and untreated RNA-seq data using reverse transcription arrest patterns at nucleotides in the neighborhood of a putative modification site as internal control. We find that nucleotide modification sites in D. discoideum tRNAs largely conform to the modification patterns observed throughout the eukaroytes. However, there are also previously undescribed modification sites. We observe substantial dynamic changes of both expression levels and modification patterns of certain tRNA types during fruiting body development. Beyond the specific application to D. discoideum our results demonstrate that the developmental variability of tRNA expression and modification can be traced efficiently with LOTTE-seq.

Deciphering Epitranscriptome: Modification of mRNA Bases Provides a New Perspective for Post-transcriptional Regulation of Gene Expression

Gene regulation depends on dynamic and reversibly modifiable biological and chemical information in the epigenome/epitranscriptome. Accumulating evidence suggests that messenger RNAs (mRNAs) are generated in flashing bursts in the cells in a precisely regulated manner. However, the different aspects of the underlying mechanisms are not fully understood. Cellular RNAs are post-transcriptionally modified at the base level, which alters the metabolism of mRNA. The current understanding of epitranscriptome in the animal system is far ahead of that in plants. The accumulating evidence indicates that the epitranscriptomic changes play vital roles in developmental processes and stress responses. Besides being non-genetically encoded, they can be of reversible nature and involved in fine-tuning the expression of gene. However, different aspects of base modifications in mRNAs are far from adequate to assign the molecular basis/functions to the epitranscriptomic changes. Advances in the chemogenetic RNA-labeling and high-throughput next-generation sequencing techniques are enabling functional analysis of the epitranscriptomic modifications to reveal their roles in mRNA biology. Mapping of the common mRNA modifications, including N 6-methyladenosine (m6A), and 5-methylcytidine (m5C), have enabled the identification of other types of modifications, such as N 1-methyladenosine. Methylation of bases in a transcript dynamically regulates the processing, cellular export, translation, and stability of the mRNA; thereby influence the important biological and physiological processes. Here, we summarize the findings in the field of mRNA base modifications with special emphasis on m6A, m5C, and their roles in growth, development, and stress tolerance, which provide a new perspective for the regulation of gene expression through post-transcriptional modification. This review also addresses some of the scientific and technical issues in epitranscriptomic study, put forward the viewpoints to resolve the issues, and discusses the future perspectives of the research in this area.

Epigenetic interaction of microbes with their mammalian hosts

The interaction of microbiota with its host has the ability to alter the cellular functions of both, through several mechanisms. Recent work, from many laboratories including our own, has shown that epigenetic mechanisms play an important role in the alteration of these cellular functions. Epigenetics broadly refers to change in the phenotype without a corresponding change in the DNA sequence. This change is usually brought by epigenetic modifications of the DNA itself, the histone proteins associated with the DNA in the chromatin, non-coding RNA or the modifications of the transcribed RNA. These modifications, also known as epigenetic code, do not change the DNA sequence but alter the expression level of specific genes. Microorganisms seem to have learned how to modify the host epigenetic code and modulate the host transcriptome in their favour. In this review, we explore the literature that describes the epigenetic interaction of bacteria, fungi and viruses, with their mammalian hosts.

TET-mediated 5-methylcytosine oxidation in tRNA promotes translation

Oxidation of 5-methylcytosine (5mC) in DNA by the ten-eleven translocation (TET) family of enzymes is indispensable for gene regulation in mammals. More recently, evidence has emerged to support a biological function for TET-mediated m5C oxidation in messenger RNA. Here, we describe a previously uncharacterized role of TET-mediated m5C oxidation in transfer RNA (tRNA). We found that the TET-mediated oxidation product 5-hydroxylmethylcytosine (hm5C) is specifically enriched in tRNA inside cells and that the oxidation activity of TET2 on m5C in tRNAs can be readily observed in vitro. We further observed that hm5C levels in tRNA were significantly decreased in Tet2 KO mouse embryonic stem cells (mESCs) in comparison with wild-type mESCs. Reciprocally, induced expression of the catalytic domain of TET2 led to an obvious increase in hm5C and a decrease in m5C in tRNAs relative to uninduced cells. Strikingly, we also show that TET2-mediated m5C oxidation in tRNA promotes translation in vitro. These results suggest TET2 may influence translation through impacting tRNA methylation and reveal an unexpected role for TET enzymes in regulating multiple nodes of the central dogma.

Advances in RNA cytosine-5 methylation: detection, regulatory mechanisms, biological functions and links to cancer

As an important posttranscriptional modification of RNA, 5-methylcytosine (m⁵C) has attracted increasing interest recently, with accumulating evidence suggesting the involvement of RNA m⁵C modification in multiple cellular processes as well as tumorigenesis. Cooperatively, advances in m⁵C detection techniques have enabled transcriptome mapping of RNA methylation at single-nucleotide resolution, thus stimulating m⁵C-based investigations. In this review, we summarize currently available approaches for detecting m⁵C distribution in RNA as well as the advantages and disadvantages of these techniques. Moreover, we elucidate the regulatory mechanisms of RNA m⁵C modification by introducing the molecular structure, catalytic substrates, cellular distributions and biological functions of RNA m⁵C regulators. The functional consequences of m⁵C modification on mRNAs, tRNAs, rRNAs and other RNA species, including viral RNAs and vault RNAs, are also discussed. Finally, we review the role of RNA m⁵C modification in cancer pathogenesis and progression, in hopes of providing new insights into cancer treatment.

Naturally occurring modified ribonucleosides

The chemical identity of RNA molecules beyond the four standard ribonucleosides has fascinated scientists since pseudouridine was characterized as the "fifth" ribonucleotide in 1951. Since then, the ever-increasing number and complexity of modified ribonucleosides have been found in viruses and throughout all three domains of life. Such modifications can be as simple as methylations, hydroxylations, or thiolations, complex as ring closures, glycosylations, acylations, or aminoacylations, or unusual as the incorporation of selenium. While initially found in transfer and ribosomal RNAs, modifications also exist in messenger RNAs and noncoding RNAs. Modifications have profound cellular outcomes at various levels, such as altering RNA structure or being essential for cell survival or organism viability. The aberrant presence or absence of RNA modifications can lead to human disease, ranging from cancer to various metabolic and developmental illnesses such as Hoyeraal-Hreidarsson syndrome, Bowen-Conradi syndrome, or Williams-Beuren syndrome. In this review article, we summarize the characterization of all 143 currently known modified ribonucleosides by describing their taxonomic distributions, the enzymes that generate the modifications, and any implications in cellular processes, RNA structure, and disease. We also highlight areas of active research, such as specific RNAs that contain a particular type of modification as well as methodologies used to identify novel RNA modifications. This article is categorized under: RNA Processing > RNA Editing and Modification.

Transcriptome-Wide 5-Methylcytosine Functional Profiling of Long Non-Coding RNA in Hepatocellular Carcinoma

Background

Growing evidence indicates that methylation status is associated with the pathogenesis of numerous types of cancers. Among these, hepatocellular carcinoma (HCC) is a deadly disease threatening global human health. Although 5-methylcytosine (m5C) has been identified as an important regulatory modification, its distribution in solid tumors, including HCC, remains unclear. The present study aimed to explore the distribution of m5C in HCC.

Materials and Methods

Six pairs of human HCC tissues and adjacent non-tumor tissues were collected to analyze the transcriptome-wide m5C methylation of long non-coding RNA (lncRNA). RNA MeRIP-seq was performed to identify m5C peaks on lncRNA and differences in m5C distribution between HCC and adjacent tissues. Gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment pathway analyses were explored to predict the possible roles of m5C.

Results

Using m5C peak sequencing, we observed that a sequence motif was necessary for m5C methylation in HCC lncRNA. Unsupervised hierarchical cluster analysis confirmed that lncRNA m5C methylation occurred more frequently in HCC than adjacent non-tumor tissues. RNA sequencing data demonstrated that more genes were up-regulated by methylation in HCC, while methylation down-regulated more genes in adjacent non-tumor tissues. GO and KEGG pathway analyses revealed that genes having a significant correlation with m5C sites in lncRNA were involved in HCC signaling pathways.

Conclusion

Our results revealed the substantially different amounts and distributions of m5C in HCC compared to adjacent non-tumor tissue. We further predicted the cellular functions in HCC that m5C may participate in to provide evidence implicating m5C lncRNA epigenetic regulation in the tumorigenesis and progression in HCC.

Dynamic methylome of internal mRNA N7-methylguanosine and its regulatory role in translation

Over 150 types of RNA modifications are identified in RNA molecules. Transcriptome profiling is one of the key steps in decoding the epitranscriptomic panorama of these chemical modifications and their potential functions. N⁷-methylguanosine (m⁷G) is one of the most abundant modifications present in tRNA, rRNA and mRNA 5'cap, and has critical roles in regulating RNA processing, metabolism and function. Besides its presence at the cap position in mRNAs, m⁷G is also identified in internal mRNA regions. However, its transcriptome-wide distribution and dynamic regulation within internal mRNA regions remain unknown. Here, we have established m⁷G individual-nucleotide-resolution cross-linking and immunoprecipitation with sequencing (m⁷G miCLIP-seq) to specifically detect internal mRNA m⁷G modification. Using this approach, we revealed that m⁷G is enriched at the 5'UTR region and AG-rich contexts, a feature that is well-conserved across different human/mouse cell lines and mouse tissues. Strikingly, the internal m⁷G modification is dynamically regulated under both H₂O₂ and heat shock treatments, with remarkable accumulations in the CDS and 3'UTR regions, and functions in promoting mRNA translation efficiency. Consistently, a PCNA 3'UTR minigene reporter harboring the native m⁷G modification site displays both enriched m⁷G modification and increased mRNA translation upon H₂O₂ treatment compared to the m⁷G site-mutated minigene reporter (G to A). Taken together, our findings unravel the dynamic profiles of internal mRNA m⁷G methylome and highlight m⁷G as a novel epitranscriptomic marker with regulatory roles in translation.

Archaeal NSUN6 catalyzes m5C72 modification on a wide-range of specific tRNAs

Human NOL1/NOP2/Sun RNA methyltransferase family member 6 (hNSun6) generates 5-methylcytosine (m5C) at C72 of four specific tRNAs, and its homologs are present only in higher eukaryotes and hyperthermophilic archaea. Archaeal NSun6 homologs possess conserved catalytic residues, but have distinct differences in their RNA recognition motifs from eukaryotic NSun6s. Until now, the biochemical properties and functions of archaeal NSun6 homologs were unknown. In archaeon Pyrococcus horikoshii OT3, the gene encoding the NSun6 homolog is PH1991. We demonstrated that the PH1991 protein could catalyze m5C72 formation on some specific PhtRNAs in vitro and was thus named as PhNSun6. Remarkably, PhNSun6 has a much wider range of tRNA substrates than hNSun6, which was attributed to its tRNA substrate specificity. The mechanism was further elucidated using biochemical and crystallographic experiments. Structurally, the binding pocket for nucleotide 73 in PhNSun6 is specific to accommodate U73 or G73-containing PhtRNAs. Furthermore, PhNSun6 lacks the eukaryotic NSun6-specific Lys-rich loop, resulting in the non-recognition of D-stem region by PhNSun6. Functionally, the m5C72 modification could slightly promote the thermal stability of PhtRNAs, but did not affect the amino acid accepting activity of PhtRNAs.

Carbon Nanotube- and Asbestos-Induced DNA and RNA Methylation Changes in Bronchial Epithelial Cells

Carbon nanotubes (CNTs) are nanoscale tube-shaped carbon materials used in many industrial areas. Their fiber shape has caused concerns about their toxicity given their structural similarity with asbestos. The aim here was to elucidate the effect of CNTs and asbestos exposure on global DNA and RNA methylation and the methylation of genes associated with cell cycle, inflammation, and DNA damage processes in human lung cells. Human bronchial epithelial cells (16HBE14o-) were exposed for 24 h to 25 and 100 μg/mL CNTs (single-walled CNTs [SWCNTs] and multiwalled CNTs [MWCNTs]) and 2.5 μg/mL asbestos (chrysotile, amosite, and crocidolite). Global DNA and RNA (hydroxy)methylation to cytosines was measured by a validated liquid chromatography tandem-mass spectrometry method. Global RNA methylation to adenines was measured by a colorimetric ELISA-like assay. Gene-specific DNA methylation status at certain cytosine-phosphate-guanine (CpG) sites of cyclin-dependent kinase inhibitor 1A ( CDKN1A), serine/threonine kinase ( ATM), and TNF receptor-associated factor 2 ( TRAF2) were analyzed by using bisulfite pyrosequencing technology. Only MWCNT-exposed cells showed significant global DNA hypomethylation of cytosine and global RNA hypomethylation of adenosine. SWCNT, MWCNT, and amosite exposure decreased DNA methylation of CDKN1A. ATM methylation was affected by chrysotile, SWCNT, and MWCNT. However, SWCNT exposure led to DNA hypermethylation of TRAF2. These findings contribute to further understanding of the effect of CNTs on different carcinogenic pathways.

Electron Microscope Detection of 5-Methylcytosine on DNA and RNA

5-Methylcytosine is the major epigenetic modification occurring on DNA. It is known to be involved not only in gene expression regulation but also in the control of chromatin structure. However, this modification is also found on different types of RNA, including mRNA. Generally, biomolecular techniques are applied for studying the epigenetic profile of nucleic acids. Here, we describe the ultrastructural detection of 5-methylcytosine as an unusual approach to localize this modification on chromatin regions and/or RNA single molecules. This tool requires a careful sample preparation to preserve antigen epitopes that will be revealed immunocytochemically by a specific anti-5-methylcytosine antibody. The multiple staining procedures that can be adopted allow the identification of both DNA or RNA. A semiquantitative analysis can also be carried out.

Differentiating Positional Isomers of Nucleoside Modifications by Higher-Energy Collisional Dissociation Mass Spectrometry (HCD MS)

The analytical identification of positional isomers (e.g., 3-, N⁴-, 5-methylcytidine) within the > 160 different post-transcriptional modifications found in RNA can be challenging. Conventional liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS) approaches rely on chromatographic separation for accurate identification because the collision-induced dissociation (CID) mass spectra of these isomers nearly exclusively yield identical nucleobase ions (BH₂⁺) from the same molecular ion (MH⁺). Here, we have explored higher-energy collisional dissociation (HCD) as an alternative fragmentation technique to generate more informative product ions that can be used to differentiate positional isomers. LC-MS/MS of modified nucleosides characterized using HCD led to the creation of structure- and HCD energy-specific fragmentation patterns that generated unique fingerprints, which can be used to identify individual positional isomers even when they cannot be separated chromatographically. While particularly useful for identifying positional isomers, the fingerprinting capabilities enabled by HCD also offer the potential to generate HPLC-independent spectral libraries for the rapid analysis of modified ribonucleosides. Graphical Abstract ᅟ.

Emerging role of dynamic RNA modifications during animal development

The central dogma of molecular biology statically says that the information flows from DNA to messenger RNA to protein. But the recent advances in mass spectrometry and high throughput technology have helped the scientists to view RNA as little more than a courier of genetic information encoded in the DNA. The dynamics of RNA modifications in coding and non-coding RNAs are just emerging as a carrier of non-genetic information, uncovering a new layer of complexity in the regulation of gene expression and protein translation. In this review, we summarize about the current knowledge of N6-methyladenosine (m6A), N1-methyladenosine (m1A), 5-methylcytosine (m5C) and pseudouridine (Ψ) modifications in RNA, and described how these RNA modifications are implicated in early animal development and in several human diseases.

Synthesis, Thermodynamic Properties, and Crystal Structure of RNA Oligonucleotides Containing 5-Hydroxymethylcytosine

5-Hydroxymethylcytosine (hm⁵C) is an RNA modification that has attracted significant interest because of the finding that RNA hydroxymethylation can favor mRNA translation. For insight into the mechanistic details of hm⁵C function to be obtained, the availability of RNAs containing this modification at defined positions that can be used for in vitro studies is highly desirable. In this work, we present an eight-step route to 5-hydroxymethylcytidine (hm⁵rC) phosphoramidite for solid-phase synthesis of modified RNA oligonucleotides. Furthermore, we examined the effects of hm⁵rC on RNA duplex stability and its impact on structure formation using the sarcin-ricin loop (SRL) motif. Thermal denaturation experiments revealed that hm⁵rC increases RNA duplex stability. By contrast, when cytosine within an UNCG tetraloop motif was replaced by hm⁵rC, the thermodynamic stability of the corresponding hairpin fold was attenuated. Importantly, incorporation of hm⁵rC into the SRL motif resulted in an RNA crystal structure at 0.85 Å resolution. Besides changes in the hydration pattern at the site of modification, a slight opening of the hm⁵rC-G pair compared to the unmodified C-G in the native structure was revealed.

Epitranscriptome sequencing technologies: decoding RNA modifications

In recent years, major breakthroughs in RNA-modification-mediated regulation of gene expression have been made, leading to the emerging field of epitranscriptomics.Our understanding of the distribution, regulation and function of these dynamic RNA modifications is based on sequencing technologies. In this Review, we focus on the major mRNA modifications in the transcriptome of eukaryotic cells: N6-methyladenosine, N6, 2'-O-dimethyladenosine, 5-methylcytidine, 5-hydroxylmethylcytidine, inosine, pseudouridine and N¹-methyladenosine. We discuss the sequencing technologies used to profile these epitranscriptomic marks, including scale, resolution, quantitative feature, pre-enrichment capability and the corresponding bioinformatics tools. We also discuss the challenges of epitranscriptome profiling and highlight the prospect of future detection tools. We aim to guide the choice of different detection methods and inspire new ideas in RNA biology.

Chemical Modifications to RNA: A New Layer of Gene Expression Regulation

The first chemical modification to RNA was discovered nearly 60 years ago; to date, more than 100 chemically distinct modifications have been identified in cellular RNA. With the recent development of novel chemical and/or biochemical methods, dynamic modifications to RNA have been identified in the transcriptome, including N⁶-methyladenosine (m⁶A), inosine (I), 5-methylcytosine (m⁵C), pseudouridine (Ψ), 5-hydroxymethylcytosine (hm⁵C), and N¹-methyladenosine (m¹A). Collectively, the multitude of RNA modifications are termed epitranscriptome, leading to the emerging field of epitranscriptomics. In this review, we primarily focus on recently reported chemical modifications to mRNA; we discuss their chemical properties, biological functions, and mechanisms with an emphasis on their high-throughput detection methods. We also envision that future tools, particularly novel chemical biology methods, could further facilitate and enable studies in the field of epitranscriptomics.

Environmentally sensitive molecular probes reveal mutations and epigenetic 5-methyl cytosine in human oncogenes

There is currently an unmet need for reliable tools that allow for direct detection and quantification of modifications in genomic DNA.

Simultaneous Quantification of Methylated Cytidine and Adenosine in Cellular and Tissue RNA by Nano-Flow Liquid Chromatography-Tandem Mass Spectrometry Coupled with the Stable Isotope-Dilution Method

The rising interest in understanding the functions, regulation, and maintenance of the epitranscriptome calls for robust and accurate analytical methods for the identification and quantification of post-transcriptionally modified nucleosides in RNA. Monomethylations of cytidine and adenosine are common post-transcriptional modifications in RNA. Herein, we developed an LC-MS/MS/MS coupled with the stable isotope-dilution method for the sensitive and accurate quantifications of 5-methylcytidine (m(5)C), 2'-O-methylcytidine (Cm), N(6)-methyladenosine (m(6)A), and 2'-O-methyladenosine (Am) in RNA isolated from mammalian cells and tissues. Our results showed that the distributions of m(5)C, Cm and Am are tissue-specific. In addition, the 2'-O-methylated ribonucleosides (Cm and Am) are present at higher levels than the corresponding methylated nucleobase products (m(5)C and m(6)A) in total RNA isolated from mouse brain, pancreas, and spleen but not mouse heart. We also found that the levels of m(5)C, Cm, and Am are significantly lower (by 6.5-43-fold) in mRNA than in total RNA isolated from HEK293T cells, whereas the level of m(6)A was slightly higher (by 1.6-fold) in mRNA than in total RNA. The availability of this analytical method, in combination with genetic manipulation, may facilitate the future discovery of proteins involved in the maintenance and regulation of these RNA modifications.

Transcriptome-wide mapping of 5-methylcytidine RNA modifications in bacteria, archaea, and yeast reveals m5C within archaeal mRNAs

The presence of 5-methylcytidine (m⁵C) in tRNA and rRNA molecules of a wide variety of organisms was first observed more than 40 years ago. However, detection of this modification was limited to specific, abundant, RNA species, due to the usage of low-throughput methods. To obtain a high resolution, systematic, and comprehensive transcriptome-wide overview of m⁵C across the three domains of life, we used bisulfite treatment on total RNA from both gram positive (B. subtilis) and gram negative (E. coli) bacteria, an archaeon (S. solfataricus) and a eukaryote (S. cerevisiae), followed by massively parallel sequencing. We were able to recover most previously documented m⁵C sites on rRNA in the four organisms, and identified several novel sites in yeast and archaeal rRNAs. Our analyses also allowed quantification of methylated m⁵C positions in 64 tRNAs in yeast and archaea, revealing stoichiometric differences between the methylation patterns of these organisms. Molecules of tRNAs in which m⁵C was absent were also discovered. Intriguingly, we detected m⁵C sites within archaeal mRNAs, and identified a consensus motif of AUCGANGU that directs methylation in S. solfataricus. Our results, which were validated using m⁵C-specific RNA immunoprecipitation, provide the first evidence for mRNA modifications in archaea, suggesting that this mode of post-transcriptional regulation extends beyond the eukaryotic domain.

Ribonucleic acids are universally used to express genetic information in the form of gene transcripts. Although we envision RNA as a mere copy of the DNA four-base code, modification of specific RNA bases can expand the information code. Such modifications are abundant in transfer RNA (tRNA) and ribosomal RNA (rRNA), where they contribute to translation fidelity and ribosome assembly. Recent studies in eukaryotes have shown that mRNA modifications such as RNA-editing (conversion of an adenosine base to inosine), N6-adenine methylation (m⁶A), and 5-methylcytidine (m⁵C) can change the coding sequence, alter splicing patterns, or change RNA stability. However, no mRNA modifications in bacteria or archaea have been documented to date. We have used an approach that enables mapping of the m⁵C modifications across all expressed genes in a given organism. Applying this approach on model bacterial, archaeal, and fungal microorganisms enabled us to reveal the modified RNA bases in these organisms, and to provide an accurate and sensitive map of these modifications. In archaea, we documented multiple genes whose mRNAs are subject to RNA modification, suggesting that similar to eukaryotes, these organisms may utilize mRNA modifications as a mechanism for gene regulation.

Mapping and significance of the mRNA methylome

Internal methylation of eukaryotic mRNAs in the form of N6-methyladenosine (m(6)A) and 5-methylcytidine (m(5)C) has long been known to exist, but progress in understanding its role was hampered by difficulties in identifying individual sites. This was recently overcome by high-throughput sequencing-based methods that mapped thousands of sites for both modifications throughout mammalian transcriptomes, with most sites found in mRNAs. The topology of m(6)A in mouse and human revealed both conserved and variable sites as well as plasticity in response to extracellular cues. Within mRNAs, m(5)C and m(6)A sites were relatively depleted in coding sequences and enriched in untranslated regions, suggesting functional interactions with post-transcriptional gene control. Finer distribution analyses and preexisting literature point toward roles in the regulation of mRNA splicing, translation, or decay, through an interplay with RNA-binding proteins and microRNAs. The methyltransferase (MTase) METTL3 'writes' m(6)A marks on mRNA, whereas the demethylase FTO can 'erase' them. The RNA:m(5)C MTases NSUN2 and TRDMT1 have roles in tRNA methylation but they also act on mRNA. Proper functioning of these enzymes is important in development and there are clear links to human disease. For instance, a common variant of FTO is a risk allele for obesity carried by 1 billion people worldwide and mutations cause a lethal syndrome with growth retardation and brain deficits. NSUN2 is linked to cancer and stem cell biology and mutations cause intellectual disability. In this review, we summarize the advances, open questions, and intriguing possibilities in this emerging field that might be called RNA modomics or epitranscriptomics.

Identification of differentially methylated regions during vernalization revealed a role for RNA methyltransferases in bolting

Sugar beet (Beta vulgaris altissima) is a biennial root crop with an absolute requirement for cold exposure to bolt and flower, a process called vernalization. Global DNA methylation variations have been reported during vernalization in several plants. However, few genes targeted by DNA methylation during vernalization have been described. The objectives of this study were to identify differentially methylated regions and to study their involvement in bolting induction and tolerance. Restriction landmark genome scanning was applied to DNA from shoot apical meristems of sugar beet genotypes, providing a direct quantitative epigenetic assessment of several CG methylated genes without prior knowledge of gene sequence. Several differentially methylated regions exhibiting variations of gene-body DNA methylation and expression during cold exposure and/or between genotypes were identified, including an AROGENATE DEHYDRATASE and two RNA METHYLCYTOSINE TRANSFERASE sequences. One RNA METHYLCYTOSINE TRANSFERASE sequence displayed gene-body hypermethylation and activation of expression, while the other was hypomethylated and inhibited by cold exposure. Global RNA methylation and phenolic compound levels changed during cold exposure in a genotype-dependent way. The use of methyl RNA immunoprecipitation of total RNA and reverse transcription-PCR analysis revealed mRNA methylation in a vernalized bolting-resistant genotype for the FLOWERING LOCUS 1 gene, a repressor of flowering. Finally, Arabidopsis mutants for RNA METHYLCYTOSINE TRANSFERASE and AROGENATE DEHYDRATASE were shown to exhibit, under different environmental conditions, early or late bolting phenotypes, respectively. Overall, the data identified functional targets of DNA methylation during vernalization in sugar beet, and it is proposed that RNA methylation and phenolic compounds play a role in the floral transition.

The human tRNA m (5) C methyltransferase Misu is multisite-specific

The human tRNA m ( 5) C methyltransferase Misu is a novel downstream target of the proto-oncogene Myc that participates in controlling cell division and proliferation. Misu catalyzes the transfer of a methyl group from S-adenosyl-L-methionine to carbon 5 of cytosines in tRNAs. It was previously shown to catalyze in vitro the intron-dependent formation of m ( 5) C at the first position of the anticodon (position 34) within the human pre-tRNA (Leu) (CAA). In addition, it was recently reported that C48 and C49 are methylated in vivo by Misu. We report here the expression of hMisu in Escherichia coli and its purification to homogeneity. We show that this enzyme methylates position 48 in tRNA (Leu) (CAA) with or without intron and positions 48, 49 and 50 in tRNA (Gly2) (GCC) in vitro. Therefore, hMisu is the enzyme responsible for the methylation of at least four cytosines in human tRNAs. By comparison, the orthologous yeast enzyme Trm4 catalyzes the methylation of carbon 5 of cytosine at positions 34, 40, 48 or 49 depending on the tRNAs.

From cis-regulatory elements to complex RNPs and back

Messenger RNAs (mRNAs), the templates for translation, have evolved to harbor abundant cis-acting sequences that affect their posttranscriptional fates. These elements are frequently located in the untranslated regions and serve as binding sites for trans-acting factors, RNA-binding proteins, and/or small non-coding RNAs. This article provides a systematic synopsis of cis-acting elements, trans-acting factors, and the mechanisms by which they affect translation. It also highlights recent technical advances that have ushered in the era of transcriptome-wide studies of the ribonucleoprotein complexes formed by mRNAs and their trans-acting factors.

Widespread occurrence of 5-methylcytosine in human coding and non-coding RNA

The modified base 5-methylcytosine (m⁵C) is well studied in DNA, but investigations of its prevalence in cellular RNA have been largely confined to tRNA and rRNA. In animals, the two m⁵C methyltransferases NSUN2 and TRDMT1 are known to modify specific tRNAs and have roles in the control of cell growth and differentiation. To map modified cytosine sites across a human transcriptome, we coupled bisulfite conversion of cellular RNA with next-generation sequencing. We confirmed 21 of the 28 previously known m⁵C sites in human tRNAs and identified 234 novel tRNA candidate sites, mostly in anticipated structural positions. Surprisingly, we discovered 10 275 sites in mRNAs and other non-coding RNAs. We observed that distribution of modified cytosines between RNA types was not random; within mRNAs they were enriched in the untranslated regions and near Argonaute binding regions. We also identified five new sites modified by NSUN2, broadening its known substrate range to another tRNA, the RPPH1 subunit of RNase P and two mRNAs. Our data demonstrates the widespread presence of modified cytosines throughout coding and non-coding sequences in a transcriptome, suggesting a broader role of this modification in the post-transcriptional control of cellular RNA function.