To edit or not to edit: regulation of ADAR editing specificity and efficiency

A probe-based capture enrichment method for detection of A-to-I editing in low abundance transcripts

Exactly two decades ago, the ability to use high-throughput RNA sequencing technology to identify sites of editing by ADARs was employed for the first time. Since that time, RNA sequencing has become a standard tool for researchers studying RNA biology and led to the discovery of RNA editing sites present in a multitude of organisms, across tissue types, and in disease. However, transcriptome-wide sequencing is not without limitations. Most notably, RNA sequencing depth of a given transcript is correlated with expression, and sequencing depth impacts the ability to robustly detect RNA editing events. This chapter focuses on a method for enrichment of low-abundance transcripts that can facilitate more efficient sequencing and detection of RNA editing events. An important note is that while we describe aspects of the protocol important for capturing intron-containing transcripts, this probe-based enrichment method could be easily modified to assess editing within any low-abundance transcript. We also provide some perspectives on the current limitations as well as important future directions for expanding this technology to gain more insights into how RNA editing can impact transcript diversity.

Dynamics of diversified A-to-I editing in Streptococcus pyogenes is governed by changes in mRNA stability

Adenosine-to-inosine (A-to-I) RNA editing plays an important role in the post-transcriptional regulation of eukaryotic cell physiology. However, our understanding of the occurrence, function and regulation of A-to-I editing in bacteria remains limited. Bacterial mRNA editing is catalysed by the deaminase TadA, which was originally described to modify a single tRNA in Escherichia coli. Intriguingly, several bacterial species appear to perform A-to-I editing on more than one tRNA. Here, we provide evidence that in the human pathogen Streptococcus pyogenes, tRNA editing has expanded to an additional tRNA substrate. Using RNA sequencing, we identified more than 27 editing sites in the transcriptome of S. pyogenes SF370 and demonstrate that the adaptation of S. pyogenes TadA to a second tRNA substrate has also diversified the sequence context and recoding scope of mRNA editing. Based on the observation that editing is dynamically regulated in response to several infection-relevant stimuli, such as oxidative stress, we further investigated the underlying determinants of editing dynamics and identified mRNA stability as a key modulator of A-to-I editing. Overall, our findings reveal the presence and diversification of A-to-I editing in S. pyogenes and provide novel insights into the plasticity of the editome and its regulation in bacteria.

Translation Inhibition Mediated by Interferon-Stimulated Genes during Viral Infections

Viruses often pose a significant threat to the host through the exploitation of cellular machineries for their own benefit. In the context of immune responses, myriad host factors are deployed to target viral RNAs and inhibit viral protein translation, ultimately hampering viral replication. Understanding how "non-self" RNAs interact with the host translation machinery and trigger immune responses would help in the development of treatment strategies for viral infections. In this review, we explore how interferon-stimulated gene products interact with viral RNA and the translation machinery in order to induce either global or targeted translation inhibition.

RNA binding proteins as mediators of pathological cardiac remodeling

RNA binding proteins (RBPs) play a central in the post-transcriptional regulation of gene expression, which can account for up to 50% of all variations in protein expression within a cell. Following their binding to target RNAs, RBPs most typically confer changes in gene expression through modulation of alternative spicing, RNA stabilization/degradation, or ribosome loading/translation rate. All of these post-transcriptional regulatory processes have been shown to play a functional role in pathological cardiac remodeling, and a growing body of evidence is beginning to identify the mechanistic contribution of individual RBPs and their cardiac RNA targets. This review highlights the mechanisms of RBP-dependent post-transcriptional gene regulation in cardiomyocytes and fibroblasts and our current understanding of how RNA binding proteins functionally contribute to pathological cardiac remodeling.

Revealing the hidden RBP-RNA interactions with RNA modification enzyme-based strategies

RNA-binding proteins (RBPs) are powerful and versatile regulators in living creatures, playing fundamental roles in organismal development, metabolism, and various diseases by the regulation of gene expression at multiple levels. The requirements of deep research on RBP function have promoted the rapid development of RBP-RNA interplay detection methods. Recently, the detection method of fusing RNA modification enzymes (RME) with RBP of interest has become a hot topic. Here, we reviewed RNA modification enzymes in adenosine deaminases that act on RNA (ADAR), terminal nucleotidyl transferase (TENT), and activation-induced cytosine deaminase/ApoB mRNA editing enzyme catalytic polypeptide-like (AID/APOBEC) protein family, regarding the biological function, biochemical activity, and substrate specificity originated from enzyme selves, their domains and partner proteins. In addition, we discussed the RME activity screening system, and the RME mutations with engineered enzyme activity. Furthermore, we provided a systematic overview of the basic principles, advantages, disadvantages, and applications of the RME-based and cross-linking and immunopurification (CLIP)-based RBP target profiling strategies, including targets of RNA-binding proteins identified by editing (TRIBE), RNA tagging, surveying targets by APOBEC-mediated profiling (STAMP), CLIP-seq, and their derivative technology. This article is categorized under: RNA Interactions with Proteins and Other Molecules > Protein-RNA Recognition RNA Processing > RNA Editing and Modification.

Adenosine deaminases that act on RNA, then and now

In this article, I recount my memories of key experiments that led to my entry into the RNA editing/modification field. I highlight initial observations made by the pioneers in the ADAR field, and how they fit into our current understanding of this family of enzymes. I discuss early mysteries that have now been solved, as well as those that still linger. Finally, I discuss important, outstanding questions and acknowledge my hope for the future of the RNA editing/modification field.

REDH: A database of RNA editome in hematopoietic differentiation and malignancy

BACKGROUND

The conversion of adenosine (A) to inosine (I) through deamination is the prevailing form of RNA editing, impacting numerous nuclear and cytoplasmic transcripts across various eukaryotic species. Millions of high-confidence RNA editing sites have been identified and integrated into various RNA databases, providing a convenient platform for the rapid identification of key drivers of cancer and potential therapeutic targets. However, the available database for integration of RNA editing in hematopoietic cells and hematopoietic malignancies is still lacking.

METHODS

We downloaded RNA sequencing (RNA-seq) data of 29 leukemia patients and 19 healthy donors from National Center for Biotechnology Information (NCBI) Gene Expression Omnibus (GEO) database, and RNA-seq data of 12 mouse hematopoietic cell populations obtained from our previous research were also used. We performed sequence alignment, identified RNA editing sites, and obtained characteristic editing sites related to normal hematopoietic development and abnormal editing sites associated with hematologic diseases.

RESULTS

We established a new database, "REDH", represents RNA editome in hematopoietic differentiation and malignancy. REDH is a curated database of associations between RNA editome and hematopoiesis. REDH integrates 30,796 editing sites from 12 murine adult hematopoietic cell populations and systematically characterizes more than 400,000 edited events in malignant hematopoietic samples from 48 cohorts (human). Through the Differentiation, Disease, Enrichment, and knowledge modules, each A-to-I editing site is systematically integrated, including its distribution throughout the genome, its clinical information (human sample), and functional editing sites under physiological and pathological conditions. Furthermore, REDH compares the similarities and differences of editing sites between different hematologic malignancies and healthy control.

CONCLUSIONS

REDH is accessible at http://www.redhdatabase.com/ . This user-friendly database would aid in understanding the mechanisms of RNA editing in hematopoietic differentiation and malignancies. It provides a set of data related to the maintenance of hematopoietic homeostasis and identifying potential therapeutic targets in malignancies.

Alterations in RNA editing in skeletal muscle following exercise training in individuals with Parkinson's disease

Parkinson's Disease (PD) is the second most common neurodegenerative disease behind Alzheimer's Disease, currently affecting more than 10 million people worldwide and 1.5 times more males than females. The progression of PD results in the loss of function due to neurodegeneration and neuroinflammation. The etiology of PD is multifactorial, including both genetic and environmental origins. Here we explored changes in RNA editing, specifically editing through the actions of the Adenosine Deaminases Acting on RNA (ADARs), in the progression of PD. Analysis of ADAR editing of skeletal muscle transcriptomes from PD patients and controls, including those that engaged in a rehabilitative exercise training program revealed significant differences in ADAR editing patterns based on age, disease status, and following rehabilitative exercise. Further, deleterious editing events in protein coding regions were identified in multiple genes with known associations to PD pathogenesis. Our findings of differential ADAR editing complement findings of changes in transcriptional networks identified by a recent study and offer insights into dynamic ADAR editing changes associated with PD pathogenesis.

Changes in ADAR RNA editing patterns in CMV and ZIKV congenital infections

BACKGROUND

RNA editing is a process that increases transcriptome diversity, often through Adenosine Deaminases Acting on RNA (ADARs) that catalyze the deamination of adenosine to inosine. ADAR editing plays an important role in regulating brain function and immune activation, and is dynamically regulated during brain development. Additionally, the ADAR1 p150 isoform is induced by interferons in viral infection and plays a role in antiviral immune response. However, the question of how virus-induced ADAR expression affects host transcriptome editing remains largely unanswered. This question is particularly relevant in the context of congenital infections, given the dynamic regulation of ADAR editing during brain development, the importance of this editing for brain function, and subsequent neurological symptoms of such infections, including microcephaly, sensory issues, and other neurodevelopmental abnormalities. Here, we begin to address this question, examining ADAR expression in publicly available datasets of congenital infections of human cytomegalovirus (HCMV) microarray expression data, as well as mouse cytomegalovirus (MCMV) and mouse/ human induced pluripotent neuroprogenitor stem cell (hiNPC) Zika virus (ZIKV) RNA-seq data.

RESULTS

We found that in all three datasets, ADAR1 was overexpressed in infected samples compared to uninfected samples. In the RNA-seq datasets, editing rates were also analyzed. In all mouse infections cases, the number of editing sites was significantly increased in infected samples, albeit this was not the case for hiNPC ZIKV samples. Mouse ZIKV samples showed altered editing of well-established protein-recoding sites such as Gria3, Grik5, and Nova1, as well as editing sites that may impact miRNA binding.

CONCLUSIONS

Our findings provide evidence for changes in ADAR expression and subsequent dysregulation of ADAR editing of host transcriptomes in congenital infections. These changes in editing patterns of key neural genes have potential significance in the development of neurological symptoms, thus contributing to neurodevelopmental abnormalities. Further experiments should be performed to explore the full range of editing changes that occur in different congenital infections, and to confirm the specific functional consequences of these editing changes.

RNA Editing in Cancer Progression

Coding and noncoding RNA molecules play their roles in ensuring cell function and tissue homeostasis in an ordered and systematic fashion. RNA chemical modifications can occur both at bases and ribose sugar, and, similarly to DNA and histone modifications, can be written, erased, and recognized by the corresponding enzymes, thus modulating RNA activities and fine-tuning gene expression programs. RNA editing is one of the most prevalent and abundant forms of post-transcriptional RNA modification in normal physiological processes. By altering the sequences of mRNAs, it makes them different from the corresponding genomic template. Hence, edited mRNAs can produce protein isoforms that are functionally different from the corresponding genome-encoded variants. Abnormalities in regulatory enzymes and changes in RNA-modification patterns are closely associated with the occurrence and development of various human diseases, including cancer. To date, the roles played by RNA modifications in cancer are gathering increasing interest. In this review, we focus on the role of RNA editing in cancer transformation and provide a new perspective on its impact on tumorigenesis, by regulating cell proliferation, differentiation, invasion, migration, stemness, metabolism, and drug resistance.

Lessons from discovery of true ADAR RNA editing sites in a human cell line

BACKGROUND

Conversion or editing of adenosine (A) into inosine (I) catalyzed by specialized cellular enzymes represents one of the most common post-transcriptional RNA modifications with emerging connection to disease. A-to-I conversions can happen at specific sites and lead to increase in proteome diversity and changes in RNA stability, splicing, and regulation. Such sites can be detected as adenine-to-guanine sequence changes by next-generation RNA sequencing which resulted in millions reported sites from multiple genome-wide surveys. Nonetheless, the lack of extensive independent validation in such endeavors, which is critical considering the relatively high error rate of next-generation sequencing, leads to lingering questions about the validity of the current compendiums of the editing sites and conclusions based on them.

RESULTS

Strikingly, we found that the current analytical methods suffer from very high false positive rates and that a significant fraction of sites in the public databases cannot be validated. In this work, we present potential solutions to these problems and provide a comprehensive and extensively validated list of A-to-I editing sites in a human cancer cell line. Our findings demonstrate that most of true A-to-I editing sites in a human cancer cell line are located in the non-coding transcripts, the so-called RNA 'dark matter'. On the other hand, many ADAR editing events occurring in exons of human protein-coding mRNAs, including those that can recode the transcriptome, represent false positives and need to be interpreted with caution. Nonetheless, yet undiscovered authentic ADAR sites that increase the diversity of human proteome exist and warrant further identification.

CONCLUSIONS

Accurate identification of human ADAR sites remains a challenging problem, particularly for the sites in exons of protein-coding mRNAs. As a result, genome-wide surveys of ADAR editome must still be accompanied by extensive Sanger validation efforts. However, given the vast number of unknown human ADAR sites, there is a need for further developments of the analytical techniques, potentially those that are based on deep learning solutions, in order to provide a quick and reliable identification of the editome in any sample.

The cellular and KSHV A-to-I RNA editome in primary effusion lymphoma and its role in the viral lifecycle

Adenosine-to-inosine RNA editing is a major contributor to transcriptome diversity in animals with far-reaching biological consequences. Kaposi's sarcoma-associated herpesvirus (KSHV) is the etiological agent of several human malignancies including primary effusion lymphoma (PEL). The extent of RNA editing within the KSHV transcriptome is unclear as is its contribution to the viral lifecycle. Here, we leverage a combination of biochemical and genomic approaches to determine the RNA editing landscape in host- and KSHV transcriptomes during both latent and lytic replication in PEL. Analysis of RNA editomes reveals it is dynamic, with increased editing upon reactivation and the potential to deregulate pathways critical for latency and tumorigenesis. In addition, we identify conserved RNA editing events within a viral microRNA and discover their role in miRNA biogenesis as well as viral infection. Together, these results describe the editome of PEL cells as well as a critical role for A-to-I editing in the KSHV lifecycle.

Uncovering Cis-Regulatory Elements Important for A-to-I RNA Editing in Fusarium graminearum

Adenosine-to-inosine (A-to-I) RNA editing independent of adenosine deaminase acting on RNA (ADAR) enzymes was discovered in fungi recently, and shown to be crucial for sexual reproduction. However, the underlying mechanism for editing is unknown. Here, we combine genome-wide comparisons, proof-of-concept experiments, and machine learning to decipher cis-regulatory elements of A-to-I editing in Fusarium graminearum. We identified plenty of RNA primary sequences and secondary structural features that affect editing specificity and efficiency. Although hairpin loop structures contribute importantly to editing, unlike in animals, the primary sequences have more profound influences on editing than secondary structures. Nucleotide preferences at adjacent positions of editing sites are the most important features, especially preferences at the -1 position. Unexpectedly, besides the number of positions with preferred nucleotides, the combination of preferred nucleotides with depleted ones at different positions are also important for editing. Some cis-sequence features have distinct importance for editing specificity and efficiency. Machine learning models built from diverse sequence and secondary structural features can accurately predict genome-wide editing sites but not editing levels, indicating that the cis-regulatory principle of editing efficiency is more complex than that of editing specificity. Nevertheless, our model interpretation provides insights into the quantitative contribution of each feature to the prediction of both editing sites and levels. We found that efficient editing of FG3G34330 transcripts depended on the full-length RNA molecule, suggesting that additional RNA structural elements may also contribute to editing efficiency. Our work uncovers multidimensional cis-regulatory elements important for A-to-I RNA editing in F. graminearum, helping to elucidate the fungal editing mechanism. IMPORTANCE A-to-I RNA editing is a new epigenetic phenomenon that is crucial for sexual reproduction in fungi. Deciphering cis-regulatory elements of A-to-I RNA editing can help us elucidate the editing mechanism and develop a model that accurately predicts RNA editing. In this study, we discovered multiple RNA sequence and secondary structure features important for A-to-I editing in Fusarium graminearum. We also identified the cis-sequence features with distinct importance for editing specificity and efficiency. The potential importance of full-length RNA molecules for editing efficiency is also revealed. This study represents the first comprehensive investigation of the cis-regulatory principles of A-to-I RNA editing in fungi.

Multiple Levels of Triggered Factors and the Obligated Requirement of Cell-to-Cell Movement in the Mutation Repair of Cucumber Mosaic Virus with Defects in the tRNA-like Structure

Simple Summary

Based on analysis of the tRNA-like structure (TLS) mutation in cucumber mosaic virus (CMV), mutation repair is correlated with several levels of triggered factors, including the dose of inoculation of virus mutants, the quantity effect on corresponding viral RNA, and the quality effect on corresponding viral RNA. All types of TLS mutation in different RNAs of CMV can be repaired at a low dose around the dilution end-point. At a high dose of inoculation, TLS mutations in RNA2 and RNA3, but not RNA1, can be repaired, which correlates with the relative quantity defect of RNA2 or the genome size defect of RNA3. In addition, all the above types of mutation repair necessarily require cell-to-cell movement, which presents the obligated effect of cell-to-cell movement on mutation repair.

Abstract

Some debilitating mutations in RNA viruses are repairable; however, the triggering factors of mutation repair remain largely unknown. In this study, multiple triggering factors of mutation repair are identified based on genetic damage to the TLS in CMV. TLS mutations in different RNAs distinctively impact viral pathogenicity and present different types of mutation repair. RNA2 relative reduction level or RNA3 sequence change resulting from TLS mutation is correlated with a high rate of mutation repair, and the TLS mutation of RNA1 fails to be repaired at the high inoculum dose. However, the TLS mutation of RNA1 can be repaired at a low dose of inoculation, particularly around the dilution end-point or in the mixed inoculation with RNA2 having a pre-termination mutation of the 2b gene, an RNAi suppressor. Taken together, TLS mutations resulting in quality or quantity defects of the viral genome or TLS mutations at low doses around the dilution end-point are likely to be repaired. Different levels of TLS mutation repair necessarily require cell-to-cell movement, therefore implying its obligated effect on the evolution of low-fitness viruses and providing a new insight into Muller’s ratchet. This study provides important information on virus evolution and the application of mild viral vaccines.

Reduction of A-to-I RNA editing in the failing human heart regulates formation of circular RNAs

Alterations of RNA editing that affect the secondary structure of RNAs can cause human diseases. We therefore studied RNA editing in failing human hearts. Transcriptome sequencing showed that adenosine-to-inosine (A-to-I) RNA editing was responsible for 80% of the editing events in the myocardium. Failing human hearts were characterized by reduced RNA editing. This was primarily attributable to Alu elements in introns of protein-coding genes. In the failing left ventricle, 166 circRNAs were upregulated and 7 circRNAs were downregulated compared to non-failing controls. Most of the upregulated circRNAs were associated with reduced RNA editing in the host gene. ADAR2, which binds to RNA regions that are edited from A-to-I, was decreased in failing human hearts. In vitro, reduction of ADAR2 increased circRNA levels suggesting a causal effect of reduced ADAR2 levels on increased circRNAs in the failing human heart. To gain mechanistic insight, one of the identified upregulated circRNAs with a high reduction of editing in heart failure, AKAP13, was further characterized. ADAR2 reduced the formation of double-stranded structures in AKAP13 pre-mRNA, thereby reducing the stability of Alu elements and the circularization of the resulting circRNA. Overexpression of circAKAP13 impaired the sarcomere regularity of human induced pluripotent stem cell-derived cardiomyocytes. These data show that ADAR2 mediates A-to-I RNA editing in the human heart. A-to-I RNA editing represses the formation of dsRNA structures of Alu elements favoring canonical linear mRNA splicing and inhibiting the formation of circRNAs. The findings are relevant to diseases with reduced RNA editing and increased circRNA levels and provide insights into the human-specific regulation of circRNA formation.

A Systematic Review of Common and Brain-Disease-Specific RNA Editing Alterations Providing Novel Insights into Neurological and Neurodegenerative Disease Manifestations

RNA editing contributes to transcriptome diversification through RNA modifications in relation to genome-encoded information (RNA–DNA differences, RDDs). The deamination of Adenosine (A) to Inosine (I) or Cytidine (C) to Uridine (U) is the most common type of mammalian RNA editing. It occurs as a nuclear co- and/or post-transcriptional event catalyzed by ADARs (Adenosine deaminases acting on RNA) and APOBECs (apolipoprotein B mRNA editing enzyme catalytic polypeptide-like genes). RNA editing may modify the structure, stability, and processing of a transcript. This review focuses on RNA editing in psychiatric, neurological, neurodegenerative (NDs), and autoimmune brain disorders in humans and rodent models. We discuss targeted studies that focus on RNA editing in specific neuron-enriched transcripts with well-established functions in neuronal activity, and transcriptome-wide studies, enabled by recent technological advances. We provide comparative editome analyses between human disease and corresponding animal models. Data suggest RNA editing to be an emerging mechanism in disease development, displaying common and disease-specific patterns. Commonly edited RNAs represent potential disease-associated targets for therapeutic and diagnostic values. Currently available data are primarily descriptive, calling for additional research to expand global editing profiles and to provide disease mechanistic insights. The potential use of RNA editing events as disease biomarkers and available tools for RNA editing identification, classification, ranking, and functional characterization that are being developed will enable comprehensive analyses for a better understanding of disease(s) pathogenesis and potential cures.

Inosine and its methyl derivatives: Occurrence, biogenesis, and function in RNA

Inosine is one of the most common post-transcriptional modifications. Since its discovery, it has been noted for its ability to contribute to non-Watson-Crick interactions within RNA. Rapidly accumulating evidence points to the widespread generation of inosine through hydrolytic deamination of adenosine to inosine by different classes of adenosine deaminases. Three naturally occurring methyl derivatives of inosine, i.e., 1-methylinosine, 2'-O-methylinosine and 1,2'-O-dimethylinosine are currently reported in RNA modification databases. These modifications are expected to lead to changes in the structure, folding, dynamics, stability and functions of RNA. The importance of the modifications is indicated by the strong conservation of the modifying enzymes across organisms. The structure, binding and catalytic mechanism of the adenosine deaminases have been well-studied, but the underlying mechanism of the catalytic reaction is not very clear yet. Here we extensively review the existing data on the occurrence, biogenesis and functions of inosine and its methyl derivatives in RNA. We also included the structural and thermodynamic aspects of these modifications in our review to provide a detailed and integrated discussion on the consequences of A-to-I editing in RNA and the contribution of different structural and thermodynamic studies in understanding its role in RNA. We also highlight the importance of further studies for a better understanding of the mechanisms of the different classes of deamination reactions. Further investigation of the structural and thermodynamic consequences and functions of these modifications in RNA should provide more useful information about their role in different diseases.

ADAR Editing in Viruses: An Evolutionary Force to Reckon with

Adenosine Deaminases that Act on RNA (ADARs) are RNA editing enzymes that play a dynamic and nuanced role in regulating transcriptome and proteome diversity. This editing can be highly selective, affecting a specific site within a transcript, or nonselective, resulting in hyperediting. ADAR editing is important for regulating neural functions and autoimmunity, and has a key role in the innate immune response to viral infections, where editing can have a range of pro- or antiviral effects and can contribute to viral evolution. Here we examine the role of ADAR editing across a broad range of viral groups. We propose that the effect of ADAR editing on viral replication, whether pro- or antiviral, is better viewed as an axis rather than a binary, and that the specific position of a given virus on this axis is highly dependent on virus- and host-specific factors, and can change over the course of infection. However, more research needs to be devoted to understanding these dynamic factors and how they affect virus-ADAR interactions and viral evolution. Another area that warrants significant attention is the effect of virus-ADAR interactions on host-ADAR interactions, particularly in light of the crucial role of ADAR in regulating neural functions. Answering these questions will be essential to developing our understanding of the relationship between ADAR editing and viral infection. In turn, this will further our understanding of the effects of viruses such as SARS-CoV-2, as well as many others, and thereby influence our approach to treating these deadly diseases.

Comparative genomic analysis of a naturally born serpentized pig reveals putative mutations related to limb and bone development

BACKGROUND

It is believed that natural selection acts on the phenotypical changes caused by mutations. Phenotypically, from fishes to amphibians to reptiles, the emergence of limbs greatly facilitates the landing of ancient vertebrates, but the causal mutations and evolutionary trajectory of this process remain unclear.

RESULTS

We serendipitously obtained a pig of limbless phenotype. Mutations specific to this handicapped pig were identified using genome re-sequencing and comparative genomic analysis. We narrowed down the causal mutations to particular chromosomes and even several candidate genes and sites, such like a mutation-containing codon in gene BMP7 (bone morphogenetic protein) which was conserved in mammals but variable in lower vertebrates.

CONCLUSIONS

We parsed the limbless-related mutations in the light of evolution. The limbless pig shows phenocopy of the clades before legs were evolved. Our findings might help deduce the emergence of limbs during vertebrate evolution and should be appealing to the broad community of human genetics and evolutionary biology.

RNA editing signatures identify melanoma patients who respond to Pembrolizumab or Nivolumab treatment

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Global RNA editing levels or ADAR expression do not correlate with response to immunotherapy in melanoma patients.

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RNA editing signatures within genes can segregate patients that will respond to immunotherapy across patient cohorts.

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Recurrent RNA editing sites, those that are shared between melanoma patients, provide accurate prognostic predictions.

Immunotherapy has improved the prognosis for many melanoma patients; however, our capacity to predict patient responses and to understand the biological differences between patients who will or will not respond is limited. Gene expression profiling of tumors from patients who respond to immunotherapy has focused on deriving primarily immune-related signatures; however, these have shown limited predictive power. Recent studies have highlighted the role of RNA editing in modulating resistance to immunotherapy. To evaluate the utility of RNA editing activity as a discriminative tool in predicting immunotherapy response, we conducted a retrospective analysis of RNA-sequencing data from melanoma patients treated with Pembrolizumab or Nivolumab. Here, we developed RNA editing signatures that can identify patients who will respond to immunotherapy with very high accuracy and confidence. Our analysis demonstrates that RNA editing is a strong discriminative tool for examining sensitivity of melanoma patients to immunotherapy.

Mutation profiling of a limbless pig reveals genome-wide regulation of RNA processing related to bone development

Mutation is the basis of phenotypic changes and serves as the source of natural selection. The development of limbs has been the milestone in vertebrate evolution. Several limb and bone-related genes were verified experimentally, but other indirect and regulatory factors of limb development remained untested, especially very few cases were observed in natural environment. We report a naturally born serpentized pig without hindlimbs. Whole genome sequencing followed by comparative genomic analysis revealed multiple interesting patterns on the handicapped pig-specific mutations. Although the bone-related genes are not directly subjected to mutations, other regulatory factors such as the RNA deaminase genes Adar are damaged in the handicapped pig, leading to the abolished A-to-I deamination in many functional, conserved genes as well as the bone-related genes. This is a precious case that the limbless phenotype is observed in naturally born non-model organisms. Our study broadened the generality of the limbless phenotype across mammals and extended the regulation of hindlimb development to other non-bone-related genes. Our knowledge of limb and bone-related mutations and regulation would also contribute to human genetics.

The Regulation of RNA Modification Systems: The Next Frontier in Epitranscriptomics?

RNA modifications, long considered to be molecular curiosities embellishing just abundant and non-coding RNAs, have now moved into the focus of both academic and applied research. Dedicated research efforts (epitranscriptomics) aim at deciphering the underlying principles by determining RNA modification landscapes and investigating the molecular mechanisms that establish, interpret and modulate the information potential of RNA beyond the combination of four canonical nucleotides. This has resulted in mapping various epitranscriptomes at high resolution and in cataloguing the effects caused by aberrant RNA modification circuitry. While the scope of the obtained insights has been complex and exciting, most of current epitranscriptomics appears to be stuck in the process of producing data, with very few efforts to disentangle cause from consequence when studying a specific RNA modification system. This article discusses various knowledge gaps in this field with the aim to raise one specific question: how are the enzymes regulated that dynamically install and modify RNA modifications? Furthermore, various technologies will be highlighted whose development and use might allow identifying specific and context-dependent regulators of epitranscriptomic mechanisms. Given the complexity of individual epitranscriptomes, determining their regulatory principles will become crucially important, especially when aiming at modifying specific aspects of an epitranscriptome both for experimental and, potentially, therapeutic purposes.

Central 5-HTR2C in the Control of Metabolic Homeostasis

The 5-hydroxytryptamine 2C receptor (5-HTR2C) is a class G protein-coupled receptor (GPCR) enriched in the hypothalamus and the brain stem, where it has been shown to regulate energy homeostasis, including feeding and glucose metabolism. Accordingly, 5-HTR2C has been the target of several anti-obesity drugs, though the associated side effects greatly curbed their clinical applications. Dissecting the specific neural circuits of 5-HTR2C-expressing neurons and the detailed molecular pathways of 5-HTR2C signaling in metabolic regulation will help to develop better therapeutic strategies towards metabolic disorders. In this review, we introduced the regulatory role of 5-HTR2C in feeding behavior and glucose metabolism, with particular focus on the molecular pathways, neural network, and its interaction with other metabolic hormones, such as leptin, ghrelin, insulin, and estrogens. Moreover, the latest progress in the clinical research on 5-HTR2C agonists was also discussed.

To protect and modify double-stranded RNA - the critical roles of ADARs in development, immunity and oncogenesis

Adenosine deaminases that act on RNA (ADARs) are present in all animals and function to both bind double-stranded RNA (dsRNA) and catalyze the deamination of adenosine (A) to inosine (I). As inosine is a biological mimic of guanosine, deamination by ADARs changes the genetic information in the RNA sequence and is commonly referred to as RNA editing. Millions of A-to-I editing events have been reported for metazoan transcriptomes, indicating that RNA editing is a widespread mechanism used to generate molecular and phenotypic diversity. Loss of ADARs results in lethality in mice and behavioral phenotypes in worm and fly model systems. Furthermore, alterations in RNA editing occur in over 35 human pathologies, including several neurological disorders, metabolic diseases, and cancers. In this review, a basic introduction to ADAR structure and target recognition will be provided before summarizing how ADARs affect the fate of cellular RNAs and how researchers are using this knowledge to engineer ADARs for personalized medicine. In addition, we will highlight the important roles of ADARs and RNA editing in innate immunity and cancer biology.

Profiling neural editomes reveals a molecular mechanism to regulate RNA editing during development

Adenosine (A) to inosine (I) RNA editing contributes to transcript diversity and modulates gene expression in a dynamic, cell type–specific manner. During mammalian brain development, editing of specific adenosines increases, whereas the expression of A-to-I editing enzymes remains unchanged, suggesting molecular mechanisms that mediate spatiotemporal regulation of RNA editing exist. Herein, by using a combination of biochemical and genomic approaches, we uncover a molecular mechanism that regulates RNA editing in a neural- and development-specific manner. Comparing editomes during development led to the identification of neural transcripts that were edited only in one life stage. The stage-specific editing is largely regulated by differential gene expression during neural development. Proper expression of nearly one-third of the neurodevelopmentally regulated genes is dependent on adr-2, the sole A-to-I editing enzyme in C. elegans. However, we also identified a subset of neural transcripts that are edited and expressed throughout development. Despite a neural-specific down-regulation of adr-2 during development, the majority of these sites show increased editing in adult neural cells. Biochemical data suggest that ADR-1, a deaminase-deficient member of the adenosine deaminase acting on RNA (ADAR) family, is competing with ADR-2 for binding to specific transcripts early in development. Our data suggest a model in which during neural development, ADR-2 levels overcome ADR-1 repression, resulting in increased ADR-2 binding and editing of specific transcripts. Together, our findings reveal tissue- and development-specific regulation of RNA editing and identify a molecular mechanism that regulates ADAR substrate recognition and editing efficiency.

The ADAR Family in Amphioxus: RNA Editing and Conserved Orthologous Site Predictions

RNA editing is a relatively unexplored process in which transcribed RNA is modified at specific nucleotides before translation, adding another level of regulation of gene expression. Cephalopods use it extensively to increase the regulatory complexity of their nervous systems, and mammals use it too, but less prominently. Nevertheless, little is known about the specifics of RNA editing in most of the other clades and the relevance of RNA editing from an evolutionary perspective remains unknown. Here we analyze a key element of the editing machinery, the ADAR (adenosine deaminase acting on RNA) gene family, in an animal with a key phylogenetic position at the root of chordates: the cephalochordate amphioxus. We show, that as in cephalopods, ADAR genes in amphioxus are predominantly expressed in the nervous system; we identify a number of RNA editing events in amphioxus; and we provide a newly developed method to identify RNA editing events in highly polymorphic genomes using orthology as a guide. Overall, our work lays the foundations for future comparative analysis of RNA-editing events across the metazoan tree.

RNA editing in mesothelioma: a look forward

RNA editing is a post-transcriptional process increasing transcript diversity, thereby regulating different biological processes. We recently observed that mutations resulting from RNA editing due to hydrolytic deamination of adenosine increase during the development of mesothelioma, a rare cancer linked to chronic exposure to asbestos. This review gathers information from the published literature and public data mining to explore several aspects of RNA editing and their possible implications for cancer growth and therapy. We address possible links between RNA editing and particular types of mesothelioma genetic and epigenetic alterations and discuss the relevance of an edited substrate in the context of current chemotherapy or immunotherapy.

High-throughput mutagenesis reveals unique structural features of human ADAR1

Adenosine Deaminases that act on RNA (ADARs) are enzymes that catalyze adenosine to inosine conversion in dsRNA, a common form of RNA editing. Mutations in the human ADAR1 gene are known to cause disease and recent studies have identified ADAR1 as a potential therapeutic target for a subset of cancers. However, efforts to define the mechanistic effects for disease associated ADAR1 mutations and the rational design of ADAR1 inhibitors are limited by a lack of structural information. Here, we describe the combination of high throughput mutagenesis screening studies, biochemical characterization and Rosetta-based structure modeling to identify unique features of ADAR1. Importantly, these studies reveal a previously unknown zinc-binding site on the surface of the ADAR1 deaminase domain which is important for ADAR1 editing activity. Furthermore, we present structural models that explain known properties of this enzyme and make predictions about the role of specific residues in a surface loop unique to ADAR1.

Chemical Profiling of A-to-I RNA Editing Using a Click-Compatible Phenylacrylamide

Straightforward methods for detecting adenosine-to-inosine (A-to-I) RNA editing are key to a better understanding of its regulation, function, and connection with disease. We address this need by developing a novel reagent, N-(4-ethynylphenyl)acrylamide (EPhAA), and illustrating its ability to selectively label inosine in RNA. EPhAA is synthesized in a single step, reacts rapidly with inosine, and is "click"-compatible, enabling flexible attachment of fluorescent probes at editing sites. We first validate EPhAA reactivity and selectivity for inosine in both ribonucleosides and RNA substrates, and then apply our approach to directly monitor in vitro A-to-I RNA editing activity using recombinant ADAR enzymes. This method improves upon existing inosine chemical-labeling techniques and provides a cost-effective, rapid, and non-radioactive approach for detecting inosine formation in RNA. We envision this method will improve the study of A-to-I editing and enable better characterization of RNA modification patterns in different settings.

Bioinformatics approaches for deciphering the epitranscriptome: Recent progress and emerging topics

Post-transcriptional RNA modification occurs on all types of RNA and plays a vital role in regulating every aspect of RNA function. Thanks to the development of high-throughput sequencing technologies, transcriptome-wide profiling of RNA modifications has been made possible. With the accumulation of a large number of high-throughput datasets, bioinformatics approaches have become increasing critical for unraveling the epitranscriptome. We review here the recent progress in bioinformatics approaches for deciphering the epitranscriptomes, including epitranscriptome data analysis techniques, RNA modification databases, disease-association inference, general functional annotation, and studies on RNA modification site prediction. We also discuss the limitations of existing approaches and offer some future perspectives.

Suppression of adenosine-to-inosine (A-to-I) RNA editome by death associated protein 3 (DAP3) promotes cancer progression

RNA editing introduces nucleotide changes in RNA sequences. Recent studies have reported that aberrant A-to-I RNA editing profiles are implicated in cancers. Albeit changes in expression and activity of ADAR genes are thought to have been responsible for the dysregulated RNA editome in diseases, they are not always correlated, indicating the involvement of secondary regulators. Here, we uncover DAP3 as a potent repressor of editing and a strong oncogene in cancer. DAP3 mainly interacts with the deaminase domain of ADAR2 and represses editing via disrupting association of ADAR2 with its target transcripts. PDZD7, an exemplary DAP3-repressed editing target, undergoes a protein recoding editing at stop codon [Stop →Trp (W)]. Because of editing suppression by DAP3, the unedited PDZD7WT, which is more tumorigenic than edited PDZD7Stop518W, is accumulated in tumors. In sum, cancer cells may acquire malignant properties for their survival advantage through suppressing RNA editome by DAP3.

ADAR expression and copy number variation in patients with advanced gastric cancer

Background

Gastric cancer (GC) is a world health problem and it is the third leading cause of cancer deaths worldwide. The current practice for prognosis assessment in GC is based on radiological and pathological criteria and they may not result in an accurate prognosis. The aim of this study is to evaluate expression and copy number variation of the ADAR gene in advanced GC and clarify its correlation with survival and histopathological characteristics.

Methods

Forty two patients with stage III and IV GC were included in this study. ADAR gene expression and copy number variation were measured by real-time PCR and Quantitative multiplex fluorescent-PCR, respectively. Survival analysis performed based on the Kaplan–Meier method and Mantel–Cox test.

Results

ADAR mRNA was significantly overexpressed in the tumor tissues when compared to the adjacent normal tissues (p < 0.01). Also, ADAR expression level in stage IV was higher than stage III. 40% of patients showed amplification in ADAR gene and there was a positive correlation between ADAR copy number and expression. Increased ADAR expression was clearly correlated with poorer survival outcomes and Mantel–Cox test showed statistically significant differences between low and high expression groups (p < 0.0001). ADAR overexpression and amplification were significantly associated with metastasis, size and stage of tumor.

Conclusions

Together, our data indicate that amplification leads to over expression of ADAR and it could be used as a prognostic biomarker for disease progression, especially for the metastatic process in GC.

Editome Disease Knowledgebase (EDK): a curated knowledgebase of editome-disease associations in human

RNA editing, as an essential co-/post-transcriptional RNA modification type, plays critical roles in many biological processes and involves with a variety of human diseases. Although several databases have been developed to collect RNA editing data in both model and non-model animals, there still lacks a resource integrating associations between editome and human disease. In this study, we present Editome-Disease Knowledgebase (EDK; http://bigd.big.ac.cn/edk), an integrated knowledgebase of RNA editome-disease associations manually curated from published literatures. In the current version, EDK incorporates 61 diseases associated with 248 experimentally validated abnormal editing events located in 32 mRNAs, 16 miRNAs, 1 lncRNA and 11 viruses, and 44 aberrant activities involved with 6 editing enzymes, which together are curated from more than 200 publications. In addition, to facilitate standardization of editome-disease knowledge integration, we propose a data curation model in EDK, factoring an abundance of relevant information to fully capture the context of editome-disease associations. Taken together, EDK is a comprehensive collection of editome-disease associations and bears the great utility in aid of better understanding the RNA editing machinery and complex molecular mechanisms associated with human diseases.

Slo2 potassium channel function depends on RNA editing-regulated expression of a SCYL1 protein

Slo2 potassium channels play important roles in neuronal function, and their mutations in humans may cause epilepsies and cognitive defects. However, it is largely unknown how Slo2 is regulated by other proteins. Here we show that the function of C. elegans Slo2 (SLO-2) depends on adr-1, a gene important to RNA editing. ADR-1 promotes SLO-2 function not by editing the transcripts of slo-2 but those of scyl-1, which encodes an orthologue of mammalian SCYL1. Transcripts of scyl-1 are greatly decreased in adr-1 mutants due to deficient RNA editing at a single adenosine in their 3’-UTR. SCYL-1 physically interacts with SLO-2 in neurons. Single-channel open probability (P_o) of neuronal SLO-2 is ~50% lower in scyl-1 knockout mutant than wild type. Moreover, human Slo2.2/Slack P_o is doubled by SCYL1 in a heterologous expression system. These results suggest that SCYL-1/SCYL1 is an evolutionarily conserved regulator of Slo2 channels.

Deciphering miRNAs' Action through miRNA Editing

MicroRNAs (miRNAs) are small non-coding RNAs with the capability of modulating gene expression at the post-transcriptional level either by inhibiting messenger RNA (mRNA) translation or by promoting mRNA degradation. The outcome of a myriad of physiological processes and pathologies, including cancer, cardiovascular and metabolic diseases, relies highly on miRNAs. However, deciphering the precise roles of specific miRNAs in these pathophysiological contexts is challenging due to the high levels of complexity of their actions. Indeed, regulation of mRNA expression by miRNAs is frequently cell/organ specific; highly dependent on the stress and metabolic status of the organism; and often poorly correlated with miRNA expression levels. Such biological features of miRNAs suggest that various regulatory mechanisms control not only their expression, but also their activity and/or bioavailability. Several mechanisms have been described to modulate miRNA action, including genetic polymorphisms, methylation of miRNA promoters, asymmetric miRNA strand selection, interactions with RNA-binding proteins (RBPs) or other coding/non-coding RNAs. Moreover, nucleotide modifications (A-to-I or C-to-U) within the miRNA sequences at different stages of their maturation are also critical for their functionality. This regulatory mechanism called "RNA editing" involves specific enzymes of the adenosine/cytidine deaminase family, which trigger single nucleotide changes in primary miRNAs. These nucleotide modifications greatly influence a miRNA's stability, maturation and activity by changing its specificity towards target mRNAs. Understanding how editing events impact miRNA's ability to regulate stress responses in cells and organs, or the development of specific pathologies, e.g., metabolic diseases or cancer, should not only deepen our knowledge of molecular mechanisms underlying complex diseases, but can also facilitate the design of new therapeutic approaches based on miRNA targeting. Herein, we will discuss the current knowledge on miRNA editing and how this mechanism regulates miRNA biogenesis and activity.

A protein-protein interaction underlies the molecular basis for substrate recognition by an adenosine-to-inosine RNA-editing enzyme

Adenosine deaminases that act on RNA (ADARs) convert adenosine to inosine within double-stranded regions of RNA, resulting in increased transcriptomic diversity, as well as protection of cellular double-stranded RNA (dsRNA) from silencing and improper immune activation. The presence of dsRNA-binding domains (dsRBDs) in all ADARs suggests these domains are important for substrate recognition; however, the role of dsRBDs in vivo remains largely unknown. Herein, our studies indicate the Caenorhabditis elegans ADAR enzyme, ADR-2, has low affinity for dsRNA, but interacts with ADR-1, an editing-deficient member of the ADAR family, which has a 100-fold higher affinity for dsRNA. ADR-1 uses one dsRBD to physically interact with ADR-2 and a second dsRBD to bind to dsRNAs, thereby tethering ADR-2 to substrates. ADR-2 interacts with >1200 transcripts in vivo, and ADR-1 is required for 80% of these interactions. Our results identify a novel mode of substrate recognition for ADAR enzymes and indicate that protein-protein interactions can guide substrate recognition for RNA editors.

A rADAR defense against RNAi

This Outlook by Pasquinelli discusses the study by Reich et al. in this issue of Genes & Development. They demonstrate that in C. elegans, A-to-I editing in double-stranded regions of protein-coding transcripts protects these RNAs from targeting by the RNAi pathway. Editing of cellular dsRNA by adenosine deaminases that act on RNA (ADARs) helps prevent host RNA silencing and inadvertent antiviral activity.

Adenosine deaminases that act on RNA (ADARs) convert adenosines (A) to inosines (I) in stretches of dsRNA. The biological purpose of these editing events for the vast majority of ADAR substrates is largely unknown. In this issue of Genes & Development, Reich and colleagues (pp. 271–282) demonstrate that in Caenorhabditis elegans, A-to-I editing in double-stranded regions of protein-coding transcripts protects these RNAs from targeting by the RNAi pathway. Disruption of this safeguard through loss of ADAR activity coupled with enhanced RNAi results in developmental abnormalities and profound changes in gene expression that suggest aberrant induction of an antiviral response. Thus, editing of cellular dsRNA by ADAR helps prevent host RNA silencing and inadvertent antiviral activity.

Explaining Pathogenicity of Congenital Zika and Guillain-Barré Syndromes: Does Dysregulation of RNA Editing Play a Role?

Previous studies of Zika virus (ZIKV) pathogenesis have focused primarily on virus-driven pathology and neurotoxicity, as well as host-related changes in cell proliferation, autophagy, immunity, and uterine function. It is now hypothesized that ZIKV pathogenesis arises instead as an (unintended) consequence of host innate immunity, specifically, as the side effect of an otherwise well-functioning machine. The hypothesis presented here suggests a new way of thinking about the role of host immune mechanisms in disease pathogenesis, focusing on dysregulation of post-transcriptional RNA editing as a candidate driver of a broad range of observed neurodevelopmental defects and neurodegenerative clinical symptoms in both infants and adults linked with ZIKV infections. The authors collect and synthesize existing evidence of ZIKV-mediated changes in the expression of adenosine deaminases acting on RNA (ADARs), known links between abnormal RNA editing and pathogenesis, as well as ideas for future research directions, including potential treatment strategies.

Mapping the dsRNA World

Long double-stranded RNAs (dsRNAs) are abundantly expressed in animals, in which they frequently occur in introns and 3' untranslated regions of mRNAs. Functions of long, cellular dsRNAs are poorly understood, although deficiencies in adenosine deaminases that act on RNA, or ADARs, promote their recognition as viral dsRNA and an aberrant immune response. Diverse dsRNA-binding proteins bind cellular dsRNAs, hinting at additional roles. Understanding these roles is facilitated by mapping the genomic locations that express dsRNA in various tissues and organisms. ADAR editing provides a signature of dsRNA structure in cellular transcripts. In this review, we detail approaches to map ADAR editing sites and dsRNAs genome-wide, with particular focus on high-throughput sequencing methods and considerations for their successful application to the detection of editing sites and dsRNAs.

A-to-I mRNA editing in fungi: occurrence, function, and evolution

A-to-I RNA editing is an important post-transcriptional modification that converts adenosine (A) to inosine (I) in RNA molecules via hydrolytic deamination. Although editing of mRNAs catalyzed by adenosine deaminases acting on RNA (ADARs) is an evolutionarily conserved mechanism in metazoans, organisms outside the animal kingdom lacking ADAR orthologs were thought to lack A-to-I mRNA editing. However, recent discoveries of genome-wide A-to-I mRNA editing during the sexual stage of the wheat scab fungus Fusarium graminearum, model filamentous fungus Neurospora crassa, Sordaria macrospora, and an early diverging filamentous ascomycete Pyronema confluens indicated that A-to-I mRNA editing is likely an evolutionarily conserved feature in filamentous ascomycetes. More importantly, A-to-I mRNA editing has been demonstrated to play crucial roles in different sexual developmental processes and display distinct tissue- or development-specific regulation. Contrary to that in animals, the majority of fungal RNA editing events are non-synonymous editing, which were shown to be generally advantageous and favored by positive selection. Many non-synonymous editing sites are conserved among different fungi and have potential functional and evolutionary importance. Here, we review the recent findings about the occurrence, regulation, function, and evolution of A-to-I mRNA editing in fungi.

Efficient and precise editing of endogenous transcripts with SNAP-tagged ADARs

Molecular tools that target RNA at specific sites allow recoding of RNA information and processing. SNAP-tagged deaminases guided by a chemically stabilized guide RNA can edit targeted adenosine to inosine in several endogenous transcripts simultaneously, with high efficiency (up to 90%), high potency, sufficient editing duration, and high precision. We used adenosine deaminases acting on RNA (ADARs) fused to SNAP-tag for the efficient and concurrent editing of two disease-relevant signaling transcripts, KRAS and STAT1. We also demonstrate improved performance compared with that of the recently described Cas13b-ADAR.

Pre-reproductive stress and fluoxetine treatment in rats affect offspring A-to-I RNA editing, gene expression and social behavior

Adenosine to inosine RNA editing is an epigenetic process that entails site-specific modifications in double-stranded RNA molecules, catalyzed by adenosine deaminases acting on RNA (ADARs). Using the multiplex microfluidic PCR and deep sequencing technique, we recently showed that exposing adolescent female rats to chronic unpredictable stress before reproduction affects editing in the prefrontal cortex and amygdala of their newborn offspring, particularly at the serotonin receptor 5-HT2c (encoded by Htr2c). Here, we used the same technique to determine whether post-stress, pre-reproductive maternal treatment with fluoxetine (5 mg/kg, 7 days) reverses the effects of stress on editing. We also examined the mRNA expression of ADAR enzymes in these regions, and asked whether social behavior in adult offspring would be altered by maternal exposure to stress and/or fluoxetine. Maternal treatment with fluoxetine altered Htr2c editing in offspring amygdala at birth, enhanced the expression of Htr2c mRNA and RNA editing enzymes in the prefrontal cortex, and reversed the effects of pre-reproductive stress on Htr2c editing in this region. Furthermore, maternal fluoxetine treatment enhanced differences in editing of glutamate receptors between offspring of control and stress-exposed rats, and led to enhanced social preference in adult offspring. Our findings indicate that pre-gestational fluoxetine treatment affects patterns of RNA editing and editing enzyme expression in neonatal offspring brain in a region-specific manner, in interaction with pre-reproductive stress. Overall, these findings imply that fluoxetine treatment affects serotonergic signaling in offspring brain even when treatment is discontinued before gestation, and its effects may depend upon prior exposure to stress.

Emergency Services of Viral RNAs: Repair and Remodeling

Reproduction of RNA viruses is typically error-prone due to the infidelity of their replicative machinery and the usual lack of proofreading mechanisms. The error rates may be close to those that kill the virus. Consequently, populations of RNA viruses are represented by heterogeneous sets of genomes with various levels of fitness. This is especially consequential when viruses encounter various bottlenecks and new infections are initiated by a single or few deviating genomes. Nevertheless, RNA viruses are able to maintain their identity by conservation of major functional elements. This conservatism stems from genetic robustness or mutational tolerance, which is largely due to the functional degeneracy of many protein and RNA elements as well as to negative selection. Another relevant mechanism is the capacity to restore fitness after genetic damages, also based on replicative infidelity. Conversely, error-prone replication is a major tool that ensures viral evolvability. The potential for changes in debilitated genomes is much higher in small populations, because in the absence of stronger competitors low-fit genomes have a choice of various trajectories to wander along fitness landscapes. Thus, low-fit populations are inherently unstable, and it may be said that to run ahead it is useful to stumble. In this report, focusing on picornaviruses and also considering data from other RNA viruses, we review the biological relevance and mechanisms of various alterations of viral RNA genomes as well as pathways and mechanisms of rehabilitation after loss of fitness. The relationships among mutational robustness, resilience, and evolvability of viral RNA genomes are discussed.

C. elegans ADARs antagonize silencing of cellular dsRNAs by the antiviral RNAi pathway

In this study, Reich et al. researched the functions of Caenorhabditis elegans adenosine deaminases that act on RNA (ADARs), which catalyze A-to-I RNA editing in dsRNA. Using dsRNA immunoprecipitation (dsRIP) and RNA-seq, they identified 1523 regions of clustered A-to-I editing, termed editing-enriched regions (EERs), in four stages of C. elegans development, often with highest expression in embryos.

Cellular dsRNAs are edited by adenosine deaminases that act on RNA (ADARs). While editing can alter mRNA-coding potential, most editing occurs in noncoding sequences, the function of which is poorly understood. Using dsRNA immunoprecipitation (dsRIP) and RNA sequencing (RNA-seq), we identified 1523 regions of clustered A-to-I editing, termed editing-enriched regions (EERs), in four stages of Caenorhabditis elegans development, often with highest expression in embryos. Analyses of small RNA-seq data revealed 22- to 23-nucleotide (nt) siRNAs, reminiscent of viral siRNAs, that mapped to EERs and were abundant in adr-1;adr-2 mutant animals. Consistent with roles for these siRNAs in silencing, EER-associated genes (EAGs) were down-regulated in adr-1;adr-2 embryos, and this was dependent on associated EERs and the RNAi factor RDE-4. We observed that ADARs genetically interact with the 26G endogenous siRNA (endo-siRNA) pathway, which likely competes for RNAi components; deletion of factors required for this pathway (rrf-3 or ergo-1) in adr-1;adr-2 mutant strains caused a synthetic phenotype that was rescued by deleting antiviral RNAi factors. Poly(A)⁺ RNA-seq revealed EAG down-regulation and antiviral gene induction in adr-1;adr-2;rrf-3 embryos, and these expression changes were dependent on rde-1 and rde-4. Our data suggest that ADARs restrict antiviral silencing of cellular dsRNAs.

Selective Recognition of RNA Substrates by ADAR Deaminase Domains

Adenosine deamination is one of the most prevalent post-transcriptional modifications in mRNA and is catalyzed by ADAR1 and ADAR2 in humans. ADAR1 and ADAR2 have different substrate selectivity, which is believed to mainly originate from the proteins' deaminase domains (hADAR1d and hADAR2d, respectively). RNA-seq of the Saccharomyces cerevisiae transcriptome subjected to ADAR-catalyzed RNA editing identified substrates with common secondary structure features preferentially edited by hADAR1d over hADAR2d. The relatively small size and efficient reaction of one of these substrates suggested it could be useful for further study of the hADAR1d reaction. Indeed, a short hairpin stem from the S. cerevisiae HER1 mRNA was efficiently deaminated by hADAR1d and used to generate an hADAR1d-specific fluorescent reporter of editing activity. Using substrates preferred by either hADAR1d or hADAR2d in vitro, we found that a chimeric protein bearing an RNA-binding loop from hADAR2d grafted onto hADAR1d showed ADAR2-like selectivity. Finally, a high-throughput mutagenesis analysis (Sat-FACS-Seq) of conserved residues in an RNA-binding loop of hADAR1d revealed essential amino acids for function, advancing our understanding of RNA recognition by this domain.

Species specific RNA A-to-I editing of mosquito RDL modulates GABA potency and influences agonistic, potentiating and antagonistic actions of ivermectin

The insect GABA receptor, RDL, is the target of several classes of pesticides. The peptide sequences of RDL are generally highly conserved between diverse insects. However, RNA A-to-I editing can effectively alter amino acid residues of RDL in a species specific manner, which can affect the potency of GABA and possibly insecticides. We report here that RNA A-to-I editing alters the gene products of Rdl in three mosquito disease vectors, recoding five amino acid residues in RDL of Aedes aegypti and six residues in RDLs of Anopheles gambiae and Culex pipiens, which is the highest extent of editing in RDL observed to date. Analysis of An. gambiae Rdl cDNA sequences identified 24 editing isoforms demonstrating a considerable increase in gene product diversity. RNA editing influenced the potency of the neurotransmitter, GABA, on An. gambiae RDL editing isoforms expressed in Xenopus laevis oocytes, as demonstrated by EC₅₀s ranging from 5 ± 1 to 246 ± 41 μM. Fipronil showed similar potency on different editing isoforms, with IC₅₀s ranging from 0.18 ± 0.08 to 0.43 ± 0.09 μM. In contrast, editing of An. gambiae RDL affected the activating, potentiating and inhibiting actions of ivermectin. For example, ivermectin potentiated currents induced by GABA at the EC₂₀ concentration in the unedited isoform but not in the fully edited variant. Editing of a residue in the first transmembrane domain or the cys-loop influenced this potentiation, highlighting residues involved in the allosteric mechanisms of cys-loop ligand-gated ion channels. Understanding the interactions of ivermectin with molecular targets may have relevance to mosquito control in areas where people are administered with ivermectin to treat parasitic diseases.

A-to-I RNA editing in the rat brain is age-dependent, region-specific and sensitive to environmental stress across generations

BACKGROUND

Adenosine-to-inosine (A-to-I) RNA editing is an epigenetic modification catalyzed by adenosine deaminases acting on RNA (ADARs), and is especially prevalent in the brain. We used the highly accurate microfluidics-based multiplex PCR sequencing (mmPCR-seq) technique to assess the effects of development and environmental stress on A-to-I editing at 146 pre-selected, conserved sites in the rat prefrontal cortex and amygdala. Furthermore, we asked whether changes in editing can be observed in offspring of stress-exposed rats. In parallel, we assessed changes in ADARs expression levels.

RESULTS

In agreement with previous studies, we found editing to be generally higher in adult compared to neonatal rat brain. At birth, editing was generally lower in prefrontal cortex than in amygdala. Stress affected editing at the serotonin receptor 2c (Htr2c), and editing at this site was significantly altered in offspring of rats exposed to prereproductive stress across two generations. Stress-induced changes in Htr2c editing measured with mmPCR-seq were comparable to changes measured with Sanger and Illumina sequencing. Developmental and stress-induced changes in Adar and Adarb1 mRNA expression were observed but did not correlate with editing changes.

CONCLUSIONS

Our findings indicate that mmPCR-seq can accurately detect A-to-I RNA editing in rat brain samples, and confirm previous accounts of a developmental increase in RNA editing rates. Our findings also point to stress in adolescence as an environmental factor that alters RNA editing patterns several generations forward, joining a growing body of literature describing the transgenerational effects of stress.

How Complex Can Resistance to Dieldrin, the Insect γ-Aminobutyric Acid Receptor, Get?

Recently, Taylor-Wells et al published evidence that the γ-aminobutyric acid (GABA) receptor, resistance to dieldrin (RDL), from mosquitoes undergoes RNA A-to-I editing to generate an extraordinarily large range of isoforms. This editing was found to affect GABA receptor pharmacology, as it influenced the potency of GABA and ivermectin. This highlights RNA editing as a species-specific mechanism to fine-tune receptor function as well as possibly increase tolerance of mosquitoes to certain insecticides. This commentary also considers novel findings from analysis of Rdl transcripts from individual mosquitoes taken from different geographical areas.

Stem-loop structure preference for site-specific RNA editing by APOBEC3A and APOBEC3G

APOBEC3A and APOBEC3G cytidine deaminases inhibit viruses and endogenous retrotransposons. We recently demonstrated the novel cellular C-to-U RNA editing function of APOBEC3A and APOBEC3G. Both enzymes deaminate single-stranded DNAs at multiple TC or CC nucleotide sequences, but edit only a select set of RNAs, often at a single TC or CC nucleotide sequence. To examine the specific site preference for APOBEC3A and -3G-mediated RNA editing, we performed mutagenesis studies of the endogenous cellular RNA substrates of both proteins. We demonstrate that both enzymes prefer RNA substrates that have a predicted stem-loop with the reactive C at the 3′-end of the loop. The size of the loop, the nucleotides immediately 5′ to the target cytosine and stability of the stem have a major impact on the level of RNA editing. Our findings show that both sequence and secondary structure are preferred for RNA editing by APOBEC3A and -3G, and suggest an explanation for substrate and site-specificity of RNA editing by APOBEC3A and -3G enzymes.