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Key Points Modified nucleotides are present in many RNA species and have multiple roles in the global regulation and fine-tuning of gene expression. Despite growing interest in the discovery and analysis of RNA modification profiles and their dynamics, conventional methods for transcriptome-wide profiling are laborious and time-consuming. Next-generation sequencing brings new perspectives for global analysis of RNA modification in different cells and tissues under various physiological conditions. Current methods and technologies applied for RNA modification analysis at the transcriptome level are discussed. Most of these techniques use high-throughput DNA sequencing, coupled to the use of specific chemical reagents or specific antibodies to reveal modified RNA nucleotides. An epitranscriptome analysis should be followed by thorough validation of obtained candidate sites by complementary orthogonal approaches. Experimental and bioinformatic approaches and challenges for global analysis of RNA modification are discussed. Although it has been known for decades that RNA is subjected to numerous covalent modifications, there has been a recent surge in interest driven by sequencing-based transcriptome-wide detection methods and the realization that RNA modifications have important roles in diverse biological processes. This Review describes the range of detection strategies for RNA modifications, their particular strengths and limitations, and how responsible and complementary application of these techniques will be required to ensure the quality and interpretability of the rapidly accumulating data sets. RNA modifications are emerging players in the field of post-transcriptional regulation of gene expression, and are attracting a comparable degree of research interest to DNA and histone modifications in the field of epigenetics. We now know of more than 150 RNA modifications and the true potential of a few of these is currently emerging as the consequence of a leap in detection technology, principally associated with high-throughput sequencing. This Review outlines the major developments in this field through a structured discussion of detection principles, lays out advantages and drawbacks of new high-throughput methods and presents conventional biophysical identification of modifications as meaningful ways for validation.

The ability of chemical modifications of single nucleotides to alter the electrostatic charge, hydrophobic surface and base pairing of RNA molecules is exploited for the clinical use of stable artificial RNAs such as mRNA vaccines and synthetic small RNA molecules — to increase or decrease the expression of therapeutic proteins. Furthermore, naturally occurring biochemical modifications of nucleotides regulate RNA metabolism and function to modulate crucial cellular processes. Studies showing the mechanisms by which RNA modifications regulate basic cell functions in higher organisms have led to greater understanding of how aberrant RNA modification profiles can cause disease in humans. Together, these basic science discoveries have unravelled the molecular and cellular functions of RNA modifications, have provided new prospects for therapeutic manipulation and have led to a range of innovative clinical approaches.Native nucleotide modifications regulate RNA function and metabolism, the study of which has revealed disease mechanisms, offers therapeutic potential and enables innovative clinical strategies. Chemical modifications in RNA are harnessed for clinical use in stable artificial RNAs such as mRNA vaccines and synthetic small RNA molecules.

New analytics of post-transcriptional RNA modifications have paved the way for a tremendous upswing of the biological and biomedical research in this field. This especially applies to methods that included RNA-Seq techniques, and which typically result in what is termed global scale modification mapping. In this process, positions inside a cell’s transcriptome are receiving a status of potential modification sites (so called modification calling), typically based on a score of some kind that issues from the particular method applied. The resulting data are thought to represent information that goes beyond what is contained in typical transcriptome data, and hence the field has taken to use the term “epitranscriptome”. Due to the high rate of newly published mapping techniques, a significant number of chemically distinct RNA modifications have become amenable to mapping, albeit with variegated accuracy and precision, depending on the nature of the technique. This review gives a brief overview of known techniques, and how they were applied to modification calling.

A controversial discussion on the occurrence of the RNA modification m 1 A in mRNA takes a new turn, as an antibody with a central role in modification mapping was shown to also bind mRNA cap structures.

Although cytosine-C5 methylation is a prominent modification of tRNAs, its functional significance has been unclear. Mice that lack both the Dnmt2 and NSun2 tRNA methyltransferases showed developmental and cellular differentiation defects, and loss of Dnmt2 and NSun2 was further associated with tRNA degradation and reduced rates of protein synthesis, suggesting that this modification promotes mouse development by supporting protein synthesis. The function of cytosine-C5 methylation, a widespread modification of tRNAs, has remained obscure, particularly in mammals. We have now developed a mouse strain defective in cytosine-C5 tRNA methylation, by disrupting both the Dnmt2 and the NSun2 tRNA methyltransferases. Although the lack of either enzyme alone has no detectable effects on mouse viability, double mutants showed a synthetic lethal interaction, with an underdeveloped phenotype and impaired cellular differentiation. tRNA methylation analysis of the double-knockout mice demonstrated complementary target-site specificities for Dnmt2 and NSun2 and a complete loss of cytosine-C5 tRNA methylation. Steady-state levels of unmethylated tRNAs were substantially reduced, and loss of Dnmt2 and NSun2 was further associated with reduced rates of overall protein synthesis. These results establish a biologically important function for cytosine-C5 tRNA methylation in mammals and suggest that this modification promotes mouse development by supporting protein synthesis.

N 6 -methyladenosine (m 6 A) is the most abundant internal modification on mRNA which influences most steps of mRNA metabolism and is involved in several biological functions. The E3 ubiquitin ligase Hakai was previously found in complex with components of the m 6 A methylation machinery in plants and mammalian cells but its precise function remained to be investigated. Here we show that Hakai is a conserved component of the methyltransferase complex in Drosophila and human cells. In Drosophila , its depletion results in reduced m 6 A levels and altered m 6 A-dependent functions including sex determination. We show that its ubiquitination domain is required for dimerization and interaction with other members of the m 6 A machinery, while its catalytic activity is dispensable. Finally, we demonstrate that the loss of Hakai destabilizes several subunits of the methyltransferase complex, resulting in impaired m 6 A deposition. Our work adds functional and molecular insights into the mechanism of the m 6 A mRNA writer complex. The E3 ligase Hakai can interact with the m 6 A methylation machinery but its function is still unclear. Here, the authors show that Hakai is a conserved component of the m 6 A methyltransferase complex and provide functional and molecular insights into its role in regulating m 6 A levels in Drosophila .

In the absence of extensive transcription control mechanisms the pathogenic parasite Trypanosoma brucei crucially depends on translation regulation to orchestrate gene expression. However, molecular insight into regulating protein biosynthesis is sparse. Here we analyze the small non-coding RNA (ncRNA) interactome of ribosomes in T. brucei during different growth conditions and life stages. Ribosome-associated ncRNAs have recently been recognized as unprecedented regulators of ribosome functions. Our data show that the tRNA Thr 3´half is produced during nutrient deprivation and becomes one of the most abundant tRNA-derived RNA fragments (tdRs). tRNA Thr halves associate with ribosomes and polysomes and stimulate translation by facilitating mRNA loading during stress recovery once starvation conditions ceased. Blocking or depleting the endogenous tRNA Thr halves mitigates this stimulatory effect both in vivo and in vitro . T. brucei and its close relatives lack the well-described mammalian enzymes for tRNA half processing, thus hinting at a unique tdR biogenesis in these parasites. Trypanosoma brucei mainly relies on translational regulation to adjust gene expression, but details are unclear. Here the authors show that, under stress conditions, tRNA Thr half level increases, associates with ribosomes and polysomes, and stimulates protein synthesis by facilitating mRNA loading.

Self/foreign discrimination by the innate immune system depends on receptors that identify molecular patterns as associated to pathogens. Among others, this group includes endosomal Toll-like receptors, among which Toll-like receptors (TLR) 3, 7, 8, and 13 recognize and discriminate mammalian from microbial, potentially pathogen-associated, RNA. One of the discriminatory principles is the recognition of endogenous RNA modifications. Previous work has identified a couple of RNA modifications that impede activation of TLR signaling when incorporated in synthetic RNA molecules. Of note, work that is more recent has now shown that RNA modifications in their naturally occurring context can have immune-modulatory functions: Gm, a naturally occurring ribose-methylation within tRNA resulted in a lack of TLR7 stimulation and within a defined sequence context acted as antagonist. Additional RNA modifications with immune-modulatory functions have now been identified and recent work also indicates that RNA modifications within the context of whole prokaryotic or eukaryotic cells are indeed used for immune-modulation. This review will discuss new findings and developments in the field of immune-modulatory RNA modifications.

Newly growing evidence highlights the essential role that epitranscriptomic marks play in the development of many cancers; however, little is known about the role and implications of altered epitranscriptome deposition in prostate cancer. Here, we show that the transfer RNA N 7 -methylguanosine (m 7 G) transferase METTL1 is highly expressed in primary and advanced prostate tumours. Mechanistically, we find that METTL1 depletion causes the loss of m 7 G tRNA methylation and promotes the biogenesis of a novel class of small non-coding RNAs derived from 5'tRNA fragments. 5'tRNA-derived small RNAs steer translation control to favour the synthesis of key regulators of tumour growth suppression, interferon pathway, and immune effectors. Knockdown of Mettl1 in prostate cancer preclinical models increases intratumoural infiltration of pro-inflammatory immune cells and enhances responses to immunotherapy. Collectively, our findings reveal a therapeutically actionable role of METTL1-directed m 7 G tRNA methylation in cancer cell translation control and tumour biology.

Nanocarrier-based drug delivery is a promising therapeutic approach that offers unique possibilities for the treatment of various diseases. However, inside the blood stream, nanocarriers’ properties may change significantly due to interactions with proteins, aggregation, decomposition or premature loss of cargo. Thus, a method for precise, in situ characterization of drug nanocarriers in blood is needed. Here we show how the fluorescence correlation spectroscopy that is a well-established method for measuring the size, loading efficiency and stability of drug nanocarriers in aqueous solutions can be used to directly characterize drug nanocarriers in flowing blood. As the blood is not transparent for visible light and densely crowded with cells, we label the nanocarriers or their cargo with near-infrared fluorescent dyes and fit the experimental autocorrelation functions with an analytical model accounting for the presence of blood cells. The developed methodology contributes towards quantitative understanding of the in vivo behavior of nanocarrier-based therapeutics. While nanocarrier-based drug delivery is a promising therapeutic approach, in situ characterization of drug nanocarriers in blood remains difficult. Here, the authors demonstrate how the fluorescence correlation spectroscopy can be used to directly characterize drug nanocarriers in flowing blood.