- Open Access
Rewriting the transcriptome: adenosine-to-inosine RNA editing by ADARs
Genome Biology volume 18, Article number: 205 (2017)
One of the most prevalent forms of post-transcritpional RNA modification is the conversion of adenosine nucleosides to inosine (A-to-I), mediated by the ADAR family of enzymes. The functional requirement and regulatory landscape for the majority of A-to-I editing events are, at present, uncertain. Recent studies have identified key in vivo functions of ADAR enzymes, informing our understanding of the biological importance of A-to-I editing. Large-scale studies have revealed how editing is regulated both in cis and in trans. This review will explore these recent studies and how they broaden our understanding of the functions and regulation of ADAR-mediated RNA editing.
The post-transcriptional modification of RNA is a key process controlling the output of the genome, shaping the transcriptional landscape and ultimately cellular and organismal fate. Many types of RNA regulation have been identified, from differential splicing and isoform usage through to disctinct classes of chemical modification . There are greater than 100 known distinct modifications that can occur on and to RNA, highlighting the higher order regulation that can be layered onto RNA . Of the modifications described to date, a highly pervasive and prevalent form is the direct enzymatic deamination of adenosine nucleosides in RNA, resulting in their conversion to inosine, a process termed A-to-I editing [3,4,5].
A-to-I editing was initially identified as an activity causing the unwinding of transfected RNA duplexes in Xenopus eggs [6, 7]. It was subsequently identified that this unwinding activity was the result of the covalent modification of the RNA, and that the activity was specific to double-stranded RNA (dsRNA) [8, 9]. This activity was found in a range of species, including mammals. The specific characteristic of this modification was identified based on the analysis of the sequence differences between the genomic DNA and mRNA sequences of the GluA2 glutamate receptor (gene name Gria2) . In this example, there was a change in transcript and protein sequence from that predicted by the genomic DNA, with an arginine codon (CGG) in the mRNA in place of the genomically encoded glutamine codon (CAG). Inosine is resolved as guanosine upon sequencing and also by the translational machinery, meaning that A-to-I editing is identified as A-to-G variations in the sequence traces (either Sanger or RNA-seq) compared with the genomic sequence [10,11,12,13,14].
A-to-I editing is performed by the adenosine deaminase acting on RNA (ADAR) family of proteins [15,16,17,18]. ADARs catalyze the deamination of adenosine to inosine, through the hydrolytic deamination of the 6-position of adenosine . Inosine preferentially base pairs with cytidine. The editing of adenosines can result in a decrease or an increase in base pairing of the dsRNA substrate depending upon the sequence context. While conceptually the identification of an edited adenosine should be relatively straightforward by comparing the transcript sequence to the genome, this has not proven to be the case [20,21,22,23,24]. Several factors need to be considered to accurately define A-to-I editing: that editing occurs generally at low frequency (the majority of editing occurs at less than 20% frequency); that errors can be introduced by random hexamers used to generate the samples and by the sequencing technology; and that paralogs and closely related sequences (including SNPs) need to be able to be distinguished so that the events can be assigned accurately within the genome . The later issue becomes more relevant when editing of repeat regions, such as Alu and retrotransposons, is assessed due to the high level of sequence similarity present in these regions.
Our knowledge of editing was largely confined to a select few well-studied targets, until the parallel advances in computational methods and sequencing approaches that generate significantly greater transcriptome coverage converged to allow the accurate identification of editing in many different species. Since the identification of this modification and with the relatively recent advances in sequencing methods, the number of known sites that can be subjected to A-to-I editing has grown exponentially, with current estimates of up to 100 million possible editing events in the human genome [25, 26].
Types of editing
Two primary types of A-to-I editing have been defined. The first is site-selective editing [25, 27,28,29]. This type of editing refers to the deamination of a specific adenosine in an RNA . This can occur in isolation with no editing detected at neighboring adenosines or in short clustered regions within a given transcript (see  for an example). The efficiency of site-selective editing of a given base varies widely, from near 100% for the canonical example of Gria2 to less than 0.1%, with the majority of editing occurring at a frequency of less than 20% when assessed genome-wide . An adenosine subjected to editing in one tissue or subregion of a tissue may be differentially edited in other tissues or regions of the same tissue, suggesting that regulation of editing occurs and that it does not represent an “all or none” phenomenon . There are many additional examples of highly specific A-to-I editing events [10, 14, 32,33,34]. Site-selective editing is best associated with transcript recoding, where the editing causes a change in the protein sequence and subsequent function . Despite the capacity for protein recoding arising from A-to-I editing, the proportion of editing events that result in this outcome are a very small minority of those now described in mammalian genomes, and the degree of conservation of these is generally low [26, 35]. The consequences of recoding can vary, from the introduction of silent mutations with no discernable consequence for protein function through to mutations that alter the function of the protein dramatically, with the GluA2 Q/R site defining this latter paradigm [12,13,14, 32, 33].
The second and distinctive type of A-to-I editing is hyper-editing [36, 37], which refers to a similar phenomenon as editing enriched regions (EERs) [38, 39]. Hyper-editing is indicated by the editing of a large or excessive proportion of adenosines in close proximity to each other within the same transcript [40,41,42]. In mammals, this class of editing is mostly associated with regions of repetitive sequence where high levels of homology arise from the base pairing of inverted repeats, resulting in the editing of a high proportion of adenosines in a short region of several hundred base pairs . This primarily occurs in intronic regions and 3′ UTRs in the mammalian context. In humans and primates this includes Alu elements and other types of repetitive regions [26, 28, 35, 43]. This type of extensive editing has also been observed in viral sequences, where the viral dsRNA can be subjected to extensive editing in the infected cell [9, 44,45,46,47].
Expansion of RNA editing sites
The initial identification of A-to-I editing sites was largely based on serendipitous discoveries stemming from the detailed assessment of a single transcript [10, 48]. Evidence for hyper-editing first arose from virology, where it was noted that the dsRNA of certain types of virus could be heavily modified [9, 49]. Methods were developed, and more recently adapted for use with high-throughput sequencing, to allow identification of inosine-containing transcripts. These approaches rely on either the preferential cleavage of inosine-containing transcripts by enzymes such as RNAse T1, or upon the chemical conversion of inosine by cyanoethylation, to allow identification of edited sites [50,51,52].
Methodologies to systematically map A-to-I editing have primarily utilized the in silico analysis of expressed sequence tag databases or, more recently, the analysis of large RNA-seq datasets [26,27,28, 31, 35, 43, 53,54,55]. With the advent of high-throughput sequencing technologies, which have enabled base resolution analysis of most of the genome and the rapid cost per base reductions in their utilization, the numbers of editing sites catalogued has dramatically expanded [25, 26, 35, 56]. Targetted approaches, such as microfluidic multiplex PCR and sequencing (mmPCR-seq), allowing the highly accurate sampling of editing at a significant number of known editing sites across a range of tissues/samples at low cost, have added significantly to our ability to profile editing across tissues of an organism . These approaches have also made possible the comparison of editing among species and phyla, providing important understanding of its prevalence and clues to its function in different contexts [42, 58]. The analysis of editing across and within species has been highly informative to our understanding of the extent and consequences of A-to-I editing over evolution [56, 59,60,61]. The inclusion of genetically modified cells and organisms, such as tissues from the various ADAR knockout animals and cell lines with reduced ADAR expression/function, has enabled the experimental validation of large numbers of the sites that have been identified computationally in addition to the discovery of additional sites [40, 41, 62, 63]. These complementary approaches have provided important validation of the methods and have been extended to begin to understand the differential effects and site preferences of the individual ADAR proteins.
Intuitively, RNA editing sites can be identified by finding genetic variants (A-to-G transitions on the forward strand, T-to-C on the reverse strand) present in the RNA-seq data but absent in the matched whole genome sequence from the same individual or species [28, 64]. More recently, methods have evolved and a number of rigorous methods have been established to identify RNA editing sites, including those that can use RNA-seq alone rather than a reference genome [29, 65, 66]. Furthermore, special techniques have been developed to identify hyper-editing sites that often escape from the conventional approaches . This has been necessary due to the excessive numbers of edited bases in regions of hyper-editing which can impact on genomic alignment of these regions, making differentiation of these regions from sequencing errors of “bad reads” imperative. A historical view of the development of methods to reliably identify RNA editing sites is summarized in detail in a recent review (see reference  for a detailed perspective on this topic). Several databases are publically available to assess and query RNA-editing sites across species, including RADAR , DARNED [67, 68], and REDIbd/REDItools .
The numbers and conservation of ADARs varies across species. Mammals have three proteins: ADAR1 (ADAR), ADAR2 (ADARB1), and ADAR3 (ADARB2); Drosophila melanogaster has a single Adar (phenotypically most similar to mammalian ADAR2 [70, 71]); and Caenorhabditis elegans has two genes, adr-1 and adr-2 (phenotypically most similar to ADAR3 and ADAR2, respectively ). Each ADAR has dsRNA binding regions and a highly conserved carboxy terminal catalytic domain, distantly related to the bacterial cytidine deaminases [17, 73]. Mammalian ADAR1 and ADAR2 have demonstrated catalytic activity and participate in A-to-I editing; in contrast, no editing activity has been detected with ADAR3 on known subtrates and it appears to be catalytically inactive [74, 75]. Unlike ADAR1 and ADAR2, ADAR3 does not appear to homodimerize and this may be an important contributor to its lack of activity [17, 74]. Similarly, in C. elegans adr-2 is capable of A-to-I editing while adr-1, akin to mammalian ADAR3, does not display editing activity .
The expression of each of the ADARs varies across development and tissues in mammals . ADAR1 is widely expressed throughout the body and is the most highly expressed ADAR outside the central nervous system (CNS). A unique feature of ADAR1 is that it can be expressed as two distinct editing competent isoforms, and increasing evidence supports that these may have both overlapping and distinctive functions [18, 30, 77, 78]. ADAR1 is expressed as a consititutive p110 kDa isoform (ADAR1 p110), which localizes primarily to the nucleus, and an inducibile ADAR1 p150 isoform . The larger isoform can be induced by activation of the interferon and innate immune sensing system and localizes to the cytoplasm . ADAR2 and ADAR3 are most highly expressed in the brain and CNS, with expression more restricted in other tissues. ADAR2 contributes significantly to editing in the testis in the mouse . The completion of detailed body maps and single cell studies of gene expression will enable a significantly refined understanding of when and how different ADARs are expressed throughout the body.
The phenotypes associated with loss of function of ADARs differ between species. In C. elegans deletion of adr-1 or adr-2 resulted in defects in chemotaxis , phenotypes that are consistent with a role in neuronal function. Interestingly, the chemotaxis defect could be rescued by concurrent deletion of components of the RNAi pathway, including rde-1 and rde-4, implicating an interaction between RNA editing and RNAi pathways . Very recently, the chemotactic defect in adr-2-deficient C. elegans has been determined to be an editing-dependent effect . The normal expression of the mRNA of clec-41, a predicted C-type lectin protein, was dependent upon editing by ADR-2. In ADR-2-deficient cells, the expression of clec-41 was significantly reduced. When clec-41 expression was restored in adr-2-deficient neural cells, the chemotactic defect could be rescued, providing direct evidence that neuronal/chemotactic phenotypes of adr-2 mutants can be attributed to altered gene expression of an edited transcript .
Deletion of the single ADAR in Drosophila resulted in behavioral and locomotion abnormalities with brain lesions upon aging [70, 84, 85]. More recently, hypomorphic alleles have been established in Drosophila which have defects in sleep patterns , with evidence for a conserved disruption of circadian rhythm in Adar2 –/–- mice . In both C. elegans and Drosophila, the germline deletion of ADARs is compatibile with life and the mutants are viable but phenotypic . Phylogenetic analysis demonstrated that mammalian ADAR2 could rescue Drosophila Adar null mutants, but that mammalian ADAR1 could not . This result, coupled with evolutionary analysis, suggested that ADAR1 and ADAR2 evolved separately and have conserved, but specialized, functions. Analysis of mammalian mutant models has now confirmed this.
In mice, deletion of Adar2 resulted in the fully penetrant development of postnatal seizures that ultimately result in death by 20–25 days of age . This phenotype was rescued by the substitution of a single adenine to guanine in the Q/R position of the Gria2 gene, mimicking constitutive editing at this site [11, 13]. The rescued Adar2 –/– Gria2 R/R animals have a normal lifespan, are fertile, but have some subtle phenotypes that were revealed by broad-based phenotyping . This elegant model of rescue of lethality by a single A-to-I site substitution within a single RNA substrate illustrated definitively the paradigm of ADAR-mediated editing resulting in protein recoding as an essential consequence of A-to-I editing. Retrospectively, this result was also confounding as it suggested that editing of a large range of sites that have been subsequently defined was of limited biological relevance. Alternatively, it hinted that most editing may be required for “fine tuning” rather than being essential for homeostasis in mammals, and so may require specific contexts or settings for phenotypes to be revealed. However, as we now appreciate, the levels of redundancy and overlap of editing substrates between ADAR1 and ADAR2 are important considerations when interpreting the in vivo results.
In contrast to the Adar2 –/–- phenotype, the deletion of Adar1 (Adar1 –/–, both p110 and p150 isoforms [30, 90]), the deletion of the p150 isoform specifically (Adar1p150 –/–- ), or the specific inactivation of the editing activity/catalytic domain (Adar1 E861A/E861A, both p110 and p150 are editing deficient ) resulted in embryonic lethality between E11.5 and E13.5. These animals are characterized by a failure in fetal hematopoiesis and liver disintegration, marked by high levels of cell death. Subsequent studies identified the profound deregulation of transcripts related to the innate immune sensing (interferon) response upon deletion or mutation of ADAR1 . Using genetic intercrosses of the Adar1 mutants it has been identified by several groups including our own that a key in vivo function of ADAR1 is to modify endogenous RNA, via editing, to prevent activation of the cytosolic dsRNA sensing pathway centred on MDA5 and its downstream effector MAVS (Table 1) [41, 78, 92]. A number of genetic pathways have been tested by crossing to the Adar1 mutants and assessing rescue of viability. Of the pathways tested in vivo, to date the only significant rescue has been achieved with the deletion of MDA5 and MAVS [41, 78, 92]. This function is unique to ADAR1, and is not shared by other mammalian ADARs. It was recently reported in human cell lines that deletion of RNaseL could rescue the viability of ADAR1 –/– cell lines, in a comparable manner to deletion of MAVS . It is not presently clear whether the requirement for RNaseL is downstream of MDA5/MAVS signaling or can be initiated independently of this axis and whether the effect is physiologically relevant in vivo.
A question that has not been definitively resolved is the extent to which the phenotypes seen in the different mutant mouse models are due to editing-dependent or editing-independent functions. This is reasonably clear for the Adar2 –/– animals, with the profound rescue of the phenotype in these mice by the Gria2 R/R allele demonstrating that the physiologically most important function of ADAR2 is A-to-I editing. The Adar2 –/– Gria2 R/R animals do have additional subtle phenotypes that were revealed after a comprehensive phenotypic analysis and testing suggesting that there are specific requirements for ADAR2 outside of Gria2 editing; however, whether these reflect the lack of editing of specific substrates or editing-independent functions is not clear . In the case of ADAR1, a number of editing-independent functions have been proposed and phenotypes observed in rescued mice that were interpreted as independent of the editing activity of ADAR1. These range from roles in miRNA biogenesis [94,95,96,97,98,99,100], affecting mRNA stability [100,101,102], alternative 3′ UTR usage , and altering RNA splicing [103, 104] and the rates and efficiency of translation . In vivo, the small numbers of Adar1 –/– Mavs –/– and Adar1p150 –/– Mavs –/– rescued mice that survived past 10 days of age had developmental defects in the kidney, small intestine, and lymph node and a failure of B lymphopoiesis . In contrast to these reported roles for editing-independent activities of ADAR1, we found that an Adar1 editing-deficient allele (Adar1 E861A) demonstrated highly comparable phenotypes in both a germline-deficient or acute adult somatic deletion model to ADAR1 null alleles [40, 41, 106]. That the specific absence of editing, with a protein still being expressed, and the complete absence of the protein are so similar argues strongly that there are limited additional in vivo functions for the protein beyond editing. These genetic results do not exclude context-specific functions of ADAR1 independent of editing that were either not assessed or not active in the cell types assessed (primarily hematopoietic cells). At an organismal level A-to-I editing is the most essential function of ADAR1 and this function is required to prevent inappropriate activation of the innate immune system by endogenous RNA species.
No editing activity has been demonstrated by ADAR3. The role of mammalian ADAR3 is less clear, but data are accumulating from both C. elegans and mammalian models that ADAR3 may act to reduce the availability of substrates for ADAR1 or ADAR2, resulting in a net overall inhibitory effect on editing levels [72, 75, 76]. No phenotypes similar to those identified in the Adar1 and Adar2 mutants have been reported for Adar3 –/– animals to date. Therefore, it is the combination of expression patterns of the different ADAR isoforms that can determine the nature and extent of editing in a given cell and tissue, with ADAR3 providing a counterpoint to ADAR1 and ADAR2 .
These genetics studies have refined our understanding of the functions of A-to-I editing and of the individual roles that ADAR isoforms fulfill in vivo: ADAR2 is key to site-selective editing, especially in the CNS, whereas ADAR1-mediated editing has an essential role in the prevention of activation of the cytosolic dsRNA innate immune sensing system by endogenous RNA (Fig. 1).
Dynamic regulation of editing
While our appreciation of the numbers and extent of editing has rapidly expanded, it is less well understood how this process is physiologically regulated. For example, it is established that the same RNA transcript in different regions of the brain is subjected to variable levels of editing [28, 31]. Studies have now described A-to-I editing from very early development in single cells to the analysis of a specific brain region over a cohort spanning a large proportion of the lifespan of humans [107, 108]. Such studies have identified the dynamic regulation of A-to-I editing, both temporally and developmentally, indicating a process modulated at multiple levels.
A key contributor to the difference is the distinct patterns of expression of the ADAR proteins. Our recent work analysing thousands of human RNA-seq data sets from the GTEx project revealed that the expression of ADARs partially, but not fully, accounts for the variation of RNA editing levels . Different ADARs appear to play distinct roles. Specifically, ADAR1 and ADAR2 expression can explain about 20 and 2.8%, respectively, of the variation in overall editing of repetitive sites. In constrast, for non-repetitive protein-coding sites, ADAR1 and ADAR2 expression can explain 6 and 25% of the variation, respectively. Intriguingly, ADAR3, which is enzymatically inactive, negatively affects RNA editing, possibly by competing with ADAR1 and ADAR2 to bind the editing substrates, a finding consistent with observations in model organisms . These findings suggested important roles of ADARs in regulating RNA editing, but also prompt searches for additional regulators and modifiers of RNA editing to better account for the editing variation. These include the influence of the structure of the dsRNA containing the targeted adenosine, the neighboring bases to the editing site and the influence of other RNA binding proteins or modifers of ADAR function. Collectively these factors combine to result in the observed level of editing for a given site.
Cis regulation of A-to-I RNA editing
Both cis and trans effects contribute to the regulation of RNA editing. Cis regulation refers to the primary RNA sequence and secondary dsRNA structure as the substrate for editing. Trans regulation indicates that trans-acting factors, such as ADARs and other regulators, alter the editing efficiency observed at a given locus (Fig. 2). We have recently generated two independent lines of evidence suggesting that RNA editing is mainly regulated in cis. First, when we compare RNA editing of conserved sites in multiple tissues from human, primate, and mouse, the samples are clustered by species types, rather than by tissue types . This is very similar to the findings that RNA splicing regulation is also mainly cis directed [109, 110]. Second, using closely related Drosophila species, D. melanogaster and D. sechellia, and their F1 hybrids, we differentiated the effects of cis sequences from trans regulators by comparing species-specific editing levels in F1 hybrids and their parents. We found that cis sequence differences are largely responsible for editing level differences between these two Drosophila species, whereas trans regulators are likely only responsible for subtle changes . These data prompt us to better understand the underlying rules of RNA editing cis regulation.
How ADARs target a specific A-to-I RNA editing site is a long-standing question that is not well addressed. Both the primary sequence and secondary structure (i.e., cis-acting regulatory elements) surrounding the editing site guide the preference and selectivity of ADARs. ADAR has a preferred sequence motif neighboring the targeted adenosine, in particular the 5′ and 3′ nearest neighboring positions to the editing site, with the depletion and enrichment of G upstream and downstream of the editing site, respectively [50, 112, 113]. Recent analysis of crystal structures of human ADAR2 deaminase domain bound to substrate RNA now provide a basis for the nearest neighbor preference of ADARs . These structures demonstrated the 5′-neighbor preference for a U or A, as when this base is a G or C there is a destabilizing interaction with the backbone of the ADAR protein which reduces, but does not abolish, the interaction and thus impacts on editing efficiency. Additionally, adenosines edited in a dsRNA are affected by mismatches, bulges, and loops both positively and negatively, implicating complex structural contributions to editing specificity [112, 115]. While these specific examples are informative, they prompt systematic studies to more completely decipher the cis regulatory code of RNA editing.
We and others recently applied a quantitative trait locus (QTL) mapping approach to identify genetic variants associated with variability in RNA editing [116,117,118]. With accurate measurement of RNA editing levels at 789 sites in 131 D. melanogaster strains, we identified 545 editing QTLs (edQTLs) associated with differences in RNA editing . We demonstrated that many edQTLs can act through changes in the local secondary structure for edited dsRNAs. Furthermore, we found that edQTLs located outside of the edited dsRNA duplex are enriched in secondary structure . While these studies are unprecedented, future studies are needed to systematically understand the features of RNA sequence and structure to enable deciphering of the cis regulatory code of RNA editing. Consistent with these results, an assessment of editing across 21 diverse organisms concluded that editing is enriched in regions of putative double-strandedness and is relatively rare in coding regions . This analysis further confirmed the near unique requirement for editing in cephalopods (octopus/squid), where there is a profoundly elevated level of A-to-I editing [42, 56, 60].
An additional finding from the analysis of the crystal structures of the human ADAR2 deaminase domain bound to an RNA substrate was that differences between the ADAR proteins themselves may affect substrate specificity . It was identified that both ADAR2 and ADAR1 share homology for a previously unrecognized side chain (R510 in ADAR2) which is absent in ADAR3. This residue interacts with the RNA substrate and mutation of the R510 residue in hADAR2 to either a glutamine or an alanine reduced the deaminase activity by an order of magnitude . This difference may be an important contributor to the inability of ADAR3 to edit. It was also reported that there are differences between the RNA-binding loops of ADAR2 and ADAR1. These differences may be important in substrate selection and editing efficiency of a given substrate by ADAR1 and ADAR2. Therefore, the collective effect of RNA substrate structure, the sequence context surrounding the adenosine, and which ADAR protein binds all contribute to the efficiency of editing at a given adenosine.
Trans regulators and modifiers of ADARs and editing efficiency
Beyond ADAR editing enzymes themselves only a handful of proteins have been identified that modulate RNA editing, despite speculation about the existence of additional trans regulators involved in the RNA editing machinery. In Drosophila, the fragile X protein FMR1 biochemically and genetically interacts with ADAR to influence editing levels , the RNA helicase maleless controls the editing of one transcript through regulating its splicing , and the transcription factor period is thought to modulate editing at a small number of sites through an unknown mechanism . However, these regulators combined explain editing level regulation at fewer than 1% of known editing sites in Drosophila, underscoring the need for additional efforts to identify editing regulators with broader effects.
In mammals, two proteins are known to regulate ADAR2’s global activity through post-translational modifications. Pin1 promotes editing by binding ADAR2 in a phosphorylation-dependent manner, while WWP2 decreases editing by targeting ADAR2 for ubiquitination . By taking advange of the large GTEx dataset, we recently identified AIMP2 as a novel negative regulator of RNA editing because its expression is negatively correlated with overall editing levels across thousands of samples. Further experimental validation demonstrated that AIMP2 acts to inhibit RNA editing, at least partially, through lowering the protein level of ADARs . Additionally, a genetic screen in yeast expressing mammalian ADAR2 identified a handful of mammalian enhancers and suppressors of ADAR2 editing, mostly RNA binding proteins, which appear to regulate a small number of sites [123, 124]. There is a clear need for systematic searches of novel RNA editing regulators in mammals to better explain the dynamic regulation patterns that have been observed.
ADARs, editing, and disease: what happens when editing goes awry?
The available data suggest a more pronounced separation of biological function between ADAR1 and ADAR2 than was previously expected. Mutations in ADAR2 have not been reported to be associated with human disease. In contrast, mutations of ADAR are associated with the human diseases dyschromatosis symmetrica hereditaria (DSH) [125, 126] and Aicardi–Goutières syndrome (AGS) [127,128,129]. Over 100 heterozygous ADAR mutations have been reported in DSH and are associated with altered pigmentation (areas of hypo- and hyperpigmentation) on the face and dorsal aspects of the extremities that first appear in infancy/early childhood. This condition is not fatal and the symptoms appear to be largely restricted to the skin.
More recently, and contrasting with the phenotypes of DSH, Crow, Rice, and colleagues identified biallelic ADAR mutations as one of the genetic causes of AGS . AGS has some clinical features that are similar to congenital viral infections. AGS patients, including those with ADAR mutations, develop a severe neurodevelopmental disorder characterized by intracranial calcifications and motor disorders, and have evidence of an activated innate immune/interferon response (“interferonopathy”) in their peripheral blood, consistent with the results from murine mutants . Mutations in eight genes are associated with AGS, with a clustering of genes involved in cytosolic DNA metabolism (TREX1, RNASEH2B, RNASEH2C, RNASEH2A, SAMHD1) and those regulating cytosolic RNA metabolism (ADAR and IFIH1) [129, 131]. In AGS, unlike DSH, biallelic mutations of ADAR are seen in affected patients and are predicted to be significantly more detrimental to the RNA editing/interacting potential of the mutant proteins. Interestingly, despite the significantly different numbers of repetitive elements between the species (Alu repeats are primate restricted), the transcriptional response to ADAR1 deficiency is conserved between mouse and human, as is the specific requirement for MDA5 in this response . These results, corroborated by evidence from murine models, demonstrate that significant reductions in the activity of ADAR1 are poorly tolerated in vivo. In contrast to the deleterious consequences of reduced ADAR1 function in human kindreds, germline mutations in ADAR2 or ADAR3 have not yet been clearly described or associated with human disease.
A range of different human diseases are associated with altered editing and ADAR activity. In these cases, the direct mutation of the ADAR genes does not cause this association, as is seen in AGS. There is a significant body of work demonstrating reductions in editing, principally ascribed to ADAR2, in a range of neuronal and CNS disorders, including Alzheimer’s disease and amyotrophic lateral sclerosis [132,133,134,135,136]. In the majority of cases, these studies have reported reduced editing of specific targets in these disease settings when compared with normal tissue or non-affected samples. To date there has not been a clear association of reduced ADAR1 function with diseases of the CNS, outside of the germline diseases noted above. This contrasts with the clinical phenotypes of AGS, when profound changes in the CNS are observed in patients with biallelic mutation in ADAR.
Our appreciation of the extent and characteristics of A-to-I editing have rapidly expanded, paralleling the technological advancements in sequencing methods. This has been particularly informative in the context of cancer, where large datasets from diverse human cancers have been harnessed to identify links between altered A-to-I editing levels and a range of different cancer types. Initial reports described changes, generally reductions, of ADAR2-mediated editing at selected targets in tumors of the CNS such as glioblastoma and astrocytoma [137, 138]. Recent studies utilizing large RNA-seq datasets from human cancers have identified a trend of increased overall editing and ADAR1 expression in cancer types ranging from leukemias to solid tumors [33, 95, 139,140,141,142,143,144,145]. Reasons for the increased ADAR1 expression have been associated with both copy number gains at chromosome 1, where the ADAR gene resides, and the activation of interferon/innate immune sensing responses in tumors leading to an increase in ADAR1 expression. The biological consequences of increased ADAR1 and an increased level of overall editing in tumors is only beginning to be explored. In some specific examples, such as in melanoma, reduced editing efficiency has been proposed to be important in the pathogenesis of these tumors [146, 147], although this appears to be less common than increased expression of ADAR1 and higher overall editing levels. Our understanding of the consequences of changes in A-to-I editing on cancer initiation and maintenance, both at the level of its effect on specific transcripts and also on the global transcriptomome of the cancer cells, is only beginning to be explored, and how this contributes to tumor evolution requires further study.
Our understanding of the landscape of A-to-I editing has rapidly expanded over the past decade. The efforts of many investigators have enabled us to catalogue editing across the transcriptomes of many species. The ability to identify editing with high confidence at the genome scale has enabled a better understanding of how editing contributes to genome diversity in a range of contexts: evolutionarily, developmentally, and pathogenically. Paralleling the identification of A-to-I editing events, studies using genetically modified organisms have greatly enhanced our understanding of the in vivo roles and functions of ADARs. These studies have established that ADAR1 serves a unique function in the regulation of the innate immune response to self-RNA, while ADAR2 principally contributes to editing in a more site-selective manner, and ADAR3 competes with ADAR1/2 for substrates, but does not edit them directly. Further studies have broadened our understanding of factors contributing to A-to-I editing efficiency of a given substrate, principally the cis regulation of RNA sequence and structure surrounding the edited adenosine and, to a lesser extent, the trans regulation of ADAR protein activity/levels by other cellular proteins. At the cellular level, how altered A-to-I editing, both increased and decreased, impacts cell fate is only beginning to be explored. This is particularly relevant in disease contexts, where evidence has solidified that there is altered activity of ADAR proteins. In inherited disorders such as AGS the loss/reduction of ADAR1 activity has a profound impact on normal functioning and is ultimately lethal. In cancer, where elevated ADAR1 expression and activity have been frequently reported, it remains to be shown if these reflect a function in driving tumor initiation and maintenance or reflect the physiological function of ADAR1, to edit endogenous dsRNA to prevent activation of the innate immune system. Many of the tools developed to allow our present understanding of the physiological roles of ADARs can be applied to understand these pathogenic roles.
Modifications of RNA, outside of A-to-I editing, are increasingly being defined as key regulators of transcriptional output and more than 100 distinct types of modifications have been identified to date [1, 2]. This raises many important questions about how these modifications are co-ordinated and interact with/influence each other, ultimately impacting the fate of the given RNA and cell. Such conceptual models have been established and experimentally defined for the interactions of modifcations impacting DNA and chromatin. As an example of an RNA modification, N(6)-methyladenosine (m6A) is the most frequent internal modification of mRNA . There are many parallels between the roles identified for m6A and those of A-to-I editing, including roles in the viral life cycle [149, 150] and in the regulation of cell fate determination [151,152,153] and cancer [154,155,156]. Given their respective prevalence across the transcriptome, how m6A and A-to-I editing interact and alter the fate of the targeted RNA transcripts is at present unclear . It may be that these are distinct epitranscriptomic processes that individually impact the fate of a given RNA, or that there is a level of interaction that occurs between these highly prevelant modifications. This will be relevant to normal cell function but also in pathogenic settings. As we understand more about the biological functions of the distinct modifications and the cell types that co-express the enzymes capable of writing, reading and erasing these marks, we will begin to understand the cartography of RNA modifications and how they reshape transcriptome output.
Adenosine deaminanse acting on RNA
Central nervous system
Dyschromatosis symmetrica hereditaria
Editing quantitative trait locus
Quantitative trait locus
Gilbert WV, Bell TA, Schaening C. Messenger RNA modifications: Form, distribution, and function. Science. 2016;352:1408–12.
Zhao BS, Roundtree IA, He C. Post-transcriptional gene regulation by mRNA modifications. Nat Rev Mol Cell Biol. 2017;18:31–42.
Bass BL. RNA Editing by adenosine deaminases that act on RNA. Annu Rev Biochem. 2002;71:817–46.
Hogg M, Paro S, Keegan LP, O’Connell MA. RNA editing by mammalian ADARs. Adv Genet. 2011;73:87–120.
Nishikura K. Functions and regulation of RNA editing by ADAR deaminases. Annu Rev Biochem. 2010;79:321–49.
Bass BL, Weintraub H. A developmentally regulated activity that unwinds RNA duplexes. Cell. 1987;48:607–13.
Rebagliati MR, Melton DA. Antisense RNA injections in fertilized frog eggs reveal an RNA duplex unwinding activity. Cell. 1987;48:599–605.
Bass BL, Weintraub H. An unwinding activity that covalently modifies its double-stranded RNA substrate. Cell. 1988;55:1089–98.
Bass BL, Weintraub H, Cattaneo R, Billeter MA. Biased hypermutation of viral RNA genomes could be due to unwinding/modification of double-stranded RNA. Cell. 1989;56:331.
Sommer B, Kohler M, Sprengel R, Seeburg PH. RNA editing in brain controls a determinant of ion flow in glutamate-gated channels. Cell. 1991;67:11–9.
Higuchi M, Single FN, Kohler M, Sommer B, Sprengel R, Seeburg PH. RNA editing of AMPA receptor subunit GluR-B: a base-paired intron–exon structure determines position and efficiency. Cell. 1993;75:1361–70.
Brusa R, Zimmermann F, Koh DS, Feldmeyer D, Gass P, Seeburg PH, et al. Early-onset epilepsy and postnatal lethality associated with an editing-deficient GluR-B allele in mice. Science. 1995;270:1677–80.
Higuchi M, Maas S, Single FN, Hartner J, Rozov A, Burnashev N, et al. Point mutation in an AMPA receptor gene rescues lethality in mice deficient in the RNA-editing enzyme ADAR2. Nature. 2000;406:78–81.
Lomeli H, Mosbacher J, Melcher T, Hoger T, Geiger JR, Kuner T, et al. Control of kinetic properties of AMPA receptor channels by nuclear RNA editing. Science. 1994;266:1709–13.
O’Connell MA, Krause S, Higuchi M, Hsuan JJ, Totty NF, Jenny A, et al. Cloning of cDNAs encoding mammalian double-stranded RNA-specific adenosine deaminase. Mol Cell Biol. 1995;15:1389–97.
O’Connell MA, Keller W. Purification and properties of double-stranded RNA-specific adenosine deaminase from calf thymus. Proc Natl Acad Sci U S A. 1994;91:10596–600.
Melcher T, Maas S, Herb A, Sprengel R, Higuchi M, Seeburg PH. RED2, a brain-specific member of the RNA-specific adenosine deaminase family. J Biol Chem. 1996;271:31795–8.
Patterson JB, Samuel CE. Expression and regulation by interferon of a double-stranded-RNA-specific adenosine deaminase from human cells: evidence for two forms of the deaminase. Mol Cell Biol. 1995;15:5376–88.
Polson AG, Crain PF, Pomerantz SC, McCloskey JA, Bass BL. The mechanism of adenosine to inosine conversion by the double-stranded RNA unwinding/modifying activity: a high-performance liquid chromatography–mass spectrometry analysis. Biochemistry. 1991;30:11507–14.
Li M, Wang IX, Li Y, Bruzel A, Richards AL, Toung JM, et al. Widespread RNA and DNA sequence differences in the human transcriptome. Science. 2011;333:53–8.
Piskol R, Peng Z, Wang J, Li JB. Lack of evidence for existence of noncanonical RNA editing. Nat Biotechnol. 2013;31:19–20.
Kleinman CL, Majewski J. Comment on “Widespread RNA and DNA sequence differences in the human transcriptome”. Science. 2012;335:1302. author reply 1302.
Lin W, Piskol R, Tan MH, Li JB. Comment on “Widespread RNA and DNA sequence differences in the human transcriptome”. Science. 2012;335:1302. author reply 1302.
Pickrell JK, Gilad Y, Pritchard JK. Comment on “Widespread RNA and DNA sequence differences in the human transcriptome”. Science. 2012;335:1302. author reply 1302.
Bazak L, Haviv A, Barak M, Jacob-Hirsch J, Deng P, Zhang R, et al. A-to-I RNA editing occurs at over a hundred million genomic sites, located in a majority of human genes. Genome Res. 2014;24:365–76.
Ramaswami G, Li JB. Identification of human RNA editing sites: A historical perspective. Methods. 2016;107:42–7.
Levanon EY, Eisenberg E, Yelin R, Nemzer S, Hallegger M, Shemesh R, et al. Systematic identification of abundant A-to-I editing sites in the human transcriptome. Nat Biotechnol. 2004;22:1001–5.
Ramaswami G, Lin W, Piskol R, Tan MH, Davis C, Li JB. Accurate identification of human Alu and non-Alu RNA editing sites. Nat Methods. 2012;9:579–81.
Ramaswami G, Zhang R, Piskol R, Keegan LP, Deng P, O’Connell MA, et al. Identifying RNA editing sites using RNA sequencing data alone. Nat Methods. 2013;10:128–32.
Hartner JC, Schmittwolf C, Kispert A, Muller AM, Higuchi M, Seeburg PH. Liver disintegration in the mouse embryo caused by deficiency in the RNA-editing enzyme ADAR1. J Biol Chem. 2004;279:4894–902.
Li JB, Levanon EY, Yoon JK, Aach J, Xie B, Leproust E, et al. Genome-wide identification of human RNA editing sites by parallel DNA capturing and sequencing. Science. 2009;324:1210–3.
Yeo J, Goodman RA, Schirle NT, David SS, Beal PA. RNA editing changes the lesion specificity for the DNA repair enzyme NEIL1. Proc Natl Acad Sci U S A. 2010;107:20715–9.
Chen L, Li Y, Lin CH, Chan TH, Chow RK, Song Y, et al. Recoding RNA editing of AZIN1 predisposes to hepatocellular carcinoma. Nat Med. 2013;19:209–16.
Feng Y, Sansam CL, Singh M, Emeson RB. Altered RNA editing in mice lacking ADAR2 autoregulation. Mol Cell Biol. 2006;26:480–8.
Ramaswami G, Li JB. RADAR: a rigorously annotated database of A-to-I RNA editing. Nucleic Acids Res. 2014;42:D109–13.
Porath HT, Carmi S, Levanon EY. A genome-wide map of hyper-edited RNA reveals numerous new sites. Nat Commun. 2014;5:4726.
Carmi S, Borukhov I, Levanon EY. Identification of widespread ultra-edited human RNAs. PLoS Genet. 2011;7:1–11.
Blango MG, Bass BL. Identification of the long, edited dsRNAome of LPS-stimulated immune cells. Genome Res. 2016;26:852–62.
Whipple JM, Youssef OA, Aruscavage PJ, Nix DA, Hong C, Johnson WE, et al. Genome-wide profiling of the C. elegans dsRNAome. RNA. 2015;21:786–800.
Liddicoat BJ, Hartner JC, Piskol R, Ramaswami G, Chalk AM, Kingsley PD, et al. Adenosine-to-inosine RNA editing by ADAR1 is essential for normal murine erythropoiesis. Exp Hematol. 2016;44:947–63.
Liddicoat BJ, Piskol R, Chalk AM, Ramaswami G, Higuchi M, Hartner JC, et al. RNA editing by ADAR1 prevents MDA5 sensing of endogenous dsRNA as nonself. Science. 2015;349:1115–20.
Porath HT, Knisbacher BA, Eisenberg E, Levanon EY. Massive A-to-I RNA editing is common across the Metazoa and correlates with dsRNA abundance. Genome Biol. 2017;18:185.
Neeman Y, Levanon EY, Jantsch MF, Eisenberg E. RNA editing level in the mouse is determined by the genomic repeat repertoire. RNA. 2006;12:1802–9.
George CX, Gan Z, Liu Y, Samuel CE. Adenosine deaminases acting on RNA, RNA editing, and interferon action. J Interferon Cytokine Res. 2010;31:99–117.
Figueroa T, Boumart I, Coupeau D, Rasschaert D. Hyperediting by ADAR1 of a new herpesvirus lncRNA during the lytic phase of the oncogenic Marek’s disease virus. J Gen Virol. 2016;97:2973–88.
Ko NL, Birlouez E, Wain-Hobson S, Mahieux R, Vartanian JP. Hyperediting of human T-cell leukemia virus type 2 and simian T-cell leukemia virus type 3 by the dsRNA adenosine deaminase ADAR-1. J Gen Virol. 2012;93:2646–51.
Kumar M, Carmichael GG. Nuclear antisense RNA induces extensive adenosine modifications and nuclear retention of target transcripts. Proc Natl Acad Sci U S A. 1997;94:3542–7.
Wagner RW, Smith JE, Cooperman BS, Nishikura K. A double-stranded RNA unwinding activity introduces structural alterations by means of adenosine to inosine conversions in mammalian cells and Xenopus eggs. Proc Natl Acad Sci U S A. 1989;86:2647–51.
Cattaneo R, Schmid A, Eschle D, Baczko K, ter Meulen V, Billeter MA. Biased hypermutation and other genetic changes in defective measles viruses in human brain infections. Cell. 1988;55:255–65.
Polson AG, Bass BL. Preferential selection of adenosines for modification by double-stranded RNA adenosine deaminase. EMBO J. 1994;13:5701–11.
Cattenoz PB, Taft RJ, Westhof E, Mattick JS. Transcriptome-wide identification of A > I RNA editing sites by inosine specific cleavage. RNA. 2013;19:257–70.
Sakurai M, Yano T, Kawabata H, Ueda H, Suzuki T. Inosine cyanoethylation identifies A-to-I RNA editing sites in the human transcriptome. Nat Chem Biol. 2010;6:733–40.
Eisenberg E, Adamsky K, Cohen L, Amariglio N, Hirshberg A, Rechavi G, et al. Identification of RNA editing sites in the SNP database. Nucleic Acids Res. 2005;33:4612–7.
Eisenberg E, Li JB, Levanon EY. Sequence based identification of RNA editing sites. RNA Biol. 2010;7:248–52.
Alon S, Mor E, Vigneault F, Church GM, Locatelli F, Galeano F, et al. Systematic identification of edited microRNAs in the human brain. Genome Res. 2012;22:1533–40.
Alon S, Garrett SC, Levanon EY, Olson S, Graveley BR, Rosenthal JJ, et al. The majority of transcripts in the squid nervous system are extensively recoded by A-to-I RNA editing. Elife. 2015;4:ᅟ. doi:10.7554/eLife.05198.
Zhang R, Li X, Ramaswami G, Smith KS, Turecki G, Montgomery SB, Li JB. Quantifying RNA allelic ratios by microfluidic multiplex PCR and sequencing. Nat Methods. 2014;11:51–4.
Zhang R, Deng P, Jacobson D, Li JB. Evolutionary analysis reveals regulatory and functional landscape of coding and non-coding RNA editing. PLoS Genet. 2017;13:e1006563.
Paz-Yaacov N, Levanon EY, Nevo E, Kinar Y, Harmelin A, Jacob-Hirsch J, et al. Adenosine-to-inosine RNA editing shapes transcriptome diversity in primates. Proc Natl Acad Sci U S A. 2010;107:12174–9.
Liscovitch-Brauer N, Alon S, Porath HT, Elstein B, Unger R, Ziv T, et al. Trade-off between transcriptome plasticity and genome evolution in cephalopods. Cell. 2017;169:191–202.e11.
Lev-Maor G, Sorek R, Levanon EY, Paz N, Eisenberg E, Ast G. RNA-editing-mediated exon evolution. Genome Biol. 2007;8:R29.
Ben-Shoshan SO, Kagan P, Sultan M, Barabash Z, Dor C, Jacob-Hirsch J, et al. ADAR1 deletion induces NFkappaB and interferon signaling dependent liver inflammation and fibrosis. RNA Biol. 2017;14:587–602.
George CX, Ramaswami G, Li JB, Samuel CE. Editing of cellular self-RNAs by adenosine deaminase ADAR1 suppresses innate immune stress responses. J Biol Chem. 2016;291:6158–68.
Peng Z, Cheng Y, Tan BC, Kang L, Tian Z, Zhu Y, et al. Comprehensive analysis of RNA-Seq data reveals extensive RNA editing in a human transcriptome. Nat Biotechnol. 2012;30:253–60.
Zhu S, Xiang JF, Chen T, Chen LL, Yang L. Prediction of constitutive A-to-I editing sites from human transcriptomes in the absence of genomic sequences. BMC Genomics. 2013;14:206.
Bahn JH, Lee JH, Li G, Greer C, Peng G, Xiao X. Accurate identification of A-to-I RNA editing in human by transcriptome sequencing. Genome Res. 2012;22:142–50.
Kiran A, Baranov PV. DARNED: a DAtabase of RNa EDiting in humans. Bioinformatics. 2010;26:1772–6.
Kiran AM, O’Mahony JJ, Sanjeev K, Baranov PV. Darned in 2013: inclusion of model organisms and linking with Wikipedia. Nucleic Acids Res. 2013;41:D258–61.
Picardi E, Pesole G. REDItools: high-throughput RNA editing detection made easy. Bioinformatics. 2013;29:1813–4.
Palladino MJ, Keegan LP, O’Connell MA, Reenan RA. A-to-I pre-mRNA editing in Drosophila is primarily involved in adult nervous system function and integrity. Cell. 2000;102:437–49.
Keegan LP, McGurk L, Palavicini JP, Brindle J, Paro S, Li X, et al. Functional conservation in human and Drosophila of Metazoan ADAR2 involved in RNA editing: loss of ADAR1 in insects. Nucleic Acids Res. 2011;39:7249–62.
Washburn MC, Kakaradov B, Sundararaman B, Wheeler E, Hoon S, Yeo GW, et al. The dsRBP and inactive editor ADR-1 utilizes dsRNA binding to regulate A-to-I RNA editing across the C. elegans transcriptome. Cell Rep. 2014;6:599–607.
Mittaz L, Scott HS, Rossier C, Seeburg PH, Higuchi M, Antonarakis SE. Cloning of a human RNA editing deaminase (ADARB1) of glutamate receptors that maps to chromosome 21q22.3. Genomics. 1997;41:210–7.
Chen CX, Cho DS, Wang Q, Lai F, Carter KC, Nishikura K. A third member of the RNA-specific adenosine deaminase gene family, ADAR3, contains both single- and double-stranded RNA binding domains. RNA. 2000;6:755–67.
Oakes E, Anderson A, Cohen-Gadol A, Hundley HA. Adenosine deaminase that acts on RNA 3 (ADAR3) binding to glutamate receptor subunit B pre-mRNA inhibits RNA editing in glioblastoma. J Biol Chem. 2017;292:4326–35.
Tan MH, Li Q, Shanmugam R, Piskol R, Kohler J, Young AN, et al. Dynamic landscape and regulation of RNA editing in mammals. Nature. 2017;550:249–54.
Ward SV, George CX, Welch MJ, Liou LY, Hahm B, Lewicki H, et al. RNA editing enzyme adenosine deaminase is a restriction factor for controlling measles virus replication that also is required for embryogenesis. Proc Natl Acad Sci U S A. 2011;108:331–6.
Pestal K, Funk CC, Snyder JM, Price ND, Treuting PM, Stetson DB. Isoforms of RNA-editing enzyme ADAR1 independently control nucleic acid sensor MDA5-driven autoimmunity and multi-organ development. Immunity. 2015;43:933–44.
George CX, Samuel CE. Human RNA-specific adenosine deaminase ADAR1 transcripts possess alternative exon 1 structures that initiate from different promoters, one constitutively active and the other interferon inducible. Proc Natl Acad Sci U S A. 1999;96:4621–6.
Snyder EM, Licht K, Braun RE. Testicular adenosine to inosine RNA editing in the mouse is mediated by ADARB1. Biol Reprod. 2017;96:244–53.
Tonkin LA, Saccomanno L, Morse DP, Brodigan T, Krause M, Bass BL. RNA editing by ADARs is important for normal behavior in Caenorhabditis elegans. EMBO J. 2002;21:6025–35.
Tonkin LA, Bass BL. Mutations in RNAi rescue aberrant chemotaxis of ADAR mutants. Science. 2003;302:1725.
Deffit SN, Yee BA, Manning AC, Rajendren S, Vadlamani P, Wheeler EC, et al. The C. elegans neural editome reveals an ADAR target mRNA required for proper chemotaxis. Elife. 2017;6:e28625.
Palladino MJ, Keegan LP, O’Connell MA, Reenan RA. dADAR, a Drosophila double-stranded RNA-specific adenosine deaminase is highly developmentally regulated and is itself a target for RNA editing. RNA. 2000;6:1004–18.
Jepson JE, Savva YA, Yokose C, Sugden AU, Sahin A, Reenan RA. Engineered alterations in RNA editing modulate complex behavior in Drosophila: regulatory diversity of adenosine deaminase acting on RNA (ADAR) targets. J Biol Chem. 2011;286:8325–37.
Robinson JE, Paluch J, Dickman DK, Joiner WJ. ADAR-mediated RNA editing suppresses sleep by acting as a brake on glutamatergic synaptic plasticity. Nat Commun. 2016;7:10512.
Terajima H, Yoshitane H, Ozaki H, Suzuki Y, Shimba S, Kuroda S, et al. ADARB1 catalyzes circadian A-to-I editing and regulates RNA rhythm. Nat Genet. 2017;49:146–51.
Keegan LP, Khan A, Vukic D, O’Connell MA. ADAR RNA editing below the backbone. RNA. 2017;23(9):1317–28. doi:10.1261/rna.060921.117.
Horsch M, Seeburg PH, Adler T, Aguilar-Pimentel JA, Becker L, Calzada-Wack J, et al. Requirement of the RNA-editing enzyme ADAR2 for normal physiology in mice. J Biol Chem. 2011;286:18614–22.
Wang Q, Miyakoda M, Yang W, Khillan J, Stachura DL, Weiss MJ, et al. Stress-induced apoptosis associated with null mutation of ADAR1 RNA editing deaminase gene. J Biol Chem. 2004;279:4952–61.
Hartner JC, Walkley CR, Lu J, Orkin SH. ADAR1 is essential for the maintenance of hematopoiesis and suppression of interferon signaling. Nat Immunol. 2009;10:109–15.
Mannion NM, Greenwood SM, Young R, Cox S, Brindle J, Read D, et al. The RNA-editing enzyme ADAR1 controls innate immune responses to RNA. Cell Rep. 2014;9:1482–94.
Li Y, Banerjee S, Goldstein SA, Dong B, Gaughan C, Rath S, et al. Ribonuclease L mediates the cell-lethal phenotype of double-stranded RNA editing enzyme ADAR1 deficiency in a human cell line. Elife. 2017;6:e25687.
Ota H, Sakurai M, Gupta R, Valente L, Wulff BE, Ariyoshi K, et al. ADAR1 forms a complex with Dicer to promote microRNA processing and RNA-induced gene silencing. Cell. 2013;153:575–89.
Zipeto MA, Court AC, Sadarangani A, Delos Santos NP, Balaian L, et al. ADAR1 activation drives leukemia stem cell self-renewal by impairing Let-7 biogenesis. Cell Stem Cell. 2016;19:177–91.
Chen T, Xiang JF, Zhu S, Chen S, Yin QF, Zhang XO, et al. ADAR1 is required for differentiation and neural induction by regulating microRNA processing in a catalytically independent manner. Cell Res. 2015;25:459–76.
Bahn JH, Ahn J, Lin X, Zhang Q, Lee JH, Civelek M, et al. Genomic analysis of ADAR1 binding and its involvement in multiple RNA processing pathways. Nat Commun. 2015;6:6355.
Yang W, Chendrimada TP, Wang Q, Higuchi M, Seeburg PH, Shiekhattar R, et al. Modulation of microRNA processing and expression through RNA editing by ADAR deaminases. Nat Struct Mol Biol. 2006;13:13–21.
Kawahara Y, Zinshteyn B, Sethupathy P, Iizasa H, Hatzigeorgiou AG, Nishikura K. Redirection of silencing targets by adenosine-to-inosine editing of miRNAs. Science. 2007;315:1137–40.
Qi L, Song Y, Chan THM, Yang H, Lin CH, Tay DJT, et al. An RNA editing/dsRNA binding-independent gene regulatory mechanism of ADARs and its clinical implication in cancer. Nucleic Acids Res. 2017. doi:10.1093/nar/gkx667.
Stellos K, Gatsiou A, Stamatelopoulos K, Perisic Matic L, John D, Lunella FF, et al. Adenosine-to-inosine RNA editing controls cathepsin S expression in atherosclerosis by enabling HuR-mediated post-transcriptional regulation. Nat Med. 2016;22:1140–50.
Wang Q, Hui H, Guo Z, Zhang W, Hu Y, He T, et al. ADAR1 regulates ARHGAP26 gene expression through RNA editing by disrupting miR-30b-3p and miR-573 binding. RNA. 2013;19:1525–36.
Rueter SM, Dawson TR, Emeson RB. Regulation of alternative splicing by RNA editing. Nature. 1999;399:75–80.
Schoft VK, Schopoff S, Jantsch MF. Regulation of glutamate receptor B pre-mRNA splicing by RNA editing. Nucleic Acids Res. 2007;35:3723–32.
Feng S, Li H, Zhao J, Pervushin K, Lowenhaupt K, Schwartz TU, et al. Alternate rRNA secondary structures as regulators of translation. Nat Struct Mol Biol. 2011;18:169–76.
Heraud-Farlow JE, Chalk AM, Linder SE, Li Q, Taylor S, White JM, et al. Protein recoding by ADAR1-mediated RNA editing is not essential for normal development and homeostasis. Genome Biol. 2017;18:166.
Hwang T, Park CK, Leung AK, Gao Y, Hyde TM, Kleinman JE, et al. Dynamic regulation of RNA editing in human brain development and disease. Nat Neurosci. 2016;19:1093–9.
Picardi E, Horner DS, Pesole G. Single-cell transcriptomics reveals specific RNA editing signatures in the human brain. RNA. 2017;23:860–5.
Merkin J, Russell C, Chen P, Burge CB. Evolutionary dynamics of gene and isoform regulation in mammalian tissues. Science. 2012;338:1593–9.
Barbosa-Morais NL, Irimia M, Pan Q, Xiong HY, Gueroussov S, Lee LJ, et al. The evolutionary landscape of alternative splicing in vertebrate species. Science. 2012;338:1587–93.
Sapiro AL, Deng P, Zhang R, Li JB. Cis regulatory effects on A-to-I RNA editing in related Drosophila species. Cell Rep. 2015;11:697–703.
Eggington JM, Greene T, Bass BL. Predicting sites of ADAR editing in double-stranded RNA. Nat Commun. 2011;2:319.
Lehmann KA, Bass BL. Double-stranded RNA adenosine deaminases ADAR1 and ADAR2 have overlapping specificities. Biochemistry. 2000;39:12875–84.
Matthews MM, Thomas JM, Zheng Y, Tran K, Phelps KJ, Scott AI, et al. Structures of human ADAR2 bound to dsRNA reveal base-flipping mechanism and basis for site selectivity. Nat Struct Mol Biol. 2016;23:426–33.
Lehmann KA, Bass BL. The importance of internal loops within RNA substrates of ADAR1. J Mol Biol. 1999;291:1–13.
Park E, Guo J, Shen S, Demirdjian L, Wu YN, Lin L, et al. Population and allelic variation of A-to-I RNA editing in human transcriptomes. Genome Biol. 2017;18:143.
Ramaswami G, Deng P, Zhang R, Anna Carbone M, Mackay TF, Li JB. Genetic mapping uncovers cis-regulatory landscape of RNA editing. Nat Commun. 2015;6:8194.
Kurmangaliyev YZ, Ali S, Nuzhdin SV. Genetic determinants of RNA editing levels of ADAR targets in Drosophila melanogaster. G3 (Bethesda). 2015;6:391–6.
Bhogal B, Jepson JE, Savva YA, Pepper AS, Reenan RA, Jongens TA. Modulation of dADAR-dependent RNA editing by the Drosophila fragile X mental retardation protein. Nat Neurosci. 2011;14:1517–24.
Reenan RA, Hanrahan CJ, Ganetzky B. The mle(napts) RNA helicase mutation in Drosophila results in a splicing catastrophe of the para Na+ channel transcript in a region of RNA editing. Neuron. 2000;25:139–49.
Hughes ME, Grant GR, Paquin C, Qian J, Nitabach MN. Deep sequencing the circadian and diurnal transcriptome of Drosophila brain. Genome Res. 2012;22:1266–81.
Marcucci R, Brindle J, Paro S, Casadio A, Hempel S, Morrice N, et al. Pin1 and WWP2 regulate GluR2 Q/R site RNA editing by ADAR2 with opposing effects. EMBO J. 2011;30:4211–22.
Garncarz W, Tariq A, Handl C, Pusch O, Jantsch MF. A high-throughput screen to identify enhancers of ADAR-mediated RNA-editing. RNA Biol. 2013;10:192–204.
Tariq A, Garncarz W, Handl C, Balik A, Pusch O, Jantsch MF. RNA-interacting proteins act as site-specific repressors of ADAR2-mediated RNA editing and fluctuate upon neuronal stimulation. Nucleic Acids Res. 2013;41:2581–93.
Zhang XJ, He PP, Li M, He CD, Yan KL, Cui Y, et al. Seven novel mutations of the ADAR gene in Chinese families and sporadic patients with dyschromatosis symmetrica hereditaria (DSH). Hum Mutat. 2004;23:629–30.
Miyamura Y, Suzuki T, Kono M, Inagaki K, Ito S, Suzuki N, et al. Mutations of the RNA-specific adenosine deaminase gene (DSRAD) are involved in dyschromatosis symmetrica hereditaria. Am J Hum Genet. 2003;73:693–9.
Rice GI, Kasher PR, Forte GM, Mannion NM, Greenwood SM, Szynkiewicz M, et al. Mutations in ADAR1 cause Aicardi–Goutières syndrome associated with a type I interferon signature. Nat Genet. 2012;44:1243–8.
Crow YJ. Type I, interferonopathies: mendelian type I interferon up-regulation. Curr Opin Immunol. 2015;32:7–12.
Crow YJ, Chase DS, Lowenstein Schmidt J, Szynkiewicz M, Forte GM, Gornall HL, et al. Characterization of human disease phenotypes associated with mutations in TREX1, RNASEH2A, RNASEH2B, RNASEH2C, SAMHD1, ADAR, and IFIH1. Am J Med Genet A. 2015;167A:296–312.
Rice GI, Kitabayashi N, Barth M, Briggs TA, Burton ACE, Carpanelli ML, et al. Genetic, Phenotypic, and interferon biomarker status in ADAR1-related neurological disease. Neuropediatrics. 2017;48:166–84.
Crow YJ, Manel N. Aicardi–Goutières syndrome and the type I interferonopathies. Nat Rev Immunol. 2015;15:429–40.
Gaisler-Salomon I, Kravitz E, Feiler Y, Safran M, Biegon A, Amariglio N, et al. Hippocampus-specific deficiency in RNA editing of GluA2 in Alzheimer’s disease. Neurobiol Aging. 2014;35:1785–91.
Khermesh K, D’Erchia AM, Barak M, Annese A, Wachtel C, Levanon EY, et al. Reduced levels of protein recoding by A-to-I RNA editing in Alzheimer’s disease. RNA. 2016;22:290–302.
Kwak S, Kawahara Y. Deficient RNA editing of GluR2 and neuronal death in amyotropic lateral sclerosis. J Mol Med. 2005;83:110–20.
Krestel H, Raffel S, von Lehe M, Jagella C, Moskau-Hartmann S, Becker A, et al. Differences between RNA and DNA due to RNA editing in temporal lobe epilepsy. Neurobiol Dis. 2013;56:66–73.
Rosenthal JJ, Seeburg PH. A-to-I RNA editing: effects on proteins key to neural excitability. Neuron. 2012;74:432–9.
Cenci C, Barzotti R, Galeano F, Corbelli S, Rota R, Massimi L, et al. Down-regulation of RNA editing in pediatric astrocytomas: ADAR2 editing activity inhibits cell migration and proliferation. J Biol Chem. 2008;283:7251–60.
Galeano F, Rossetti C, Tomaselli S, Cifaldi L, Lezzerini M, Pezzullo M, et al. ADAR2-editing activity inhibits glioblastoma growth through the modulation of the CDC14B/Skp2/p21/p27 axis. Oncogene. 2013;32:998–1009.
Han L, Diao L, Yu S, Xu X, Li J, Zhang R, et al. The genomic landscape and clinical relevance of A-to-I RNA editing in human cancers. Cancer Cell. 2015;28:515–28.
Fumagalli D, Gacquer D, Rothe F, Lefort A, Libert F, Brown D, et al. Principles governing A-to-I RNA editing in the breast cancer transcriptome. Cell Rep. 2015;13:277–89.
Paz-Yaacov N, Bazak L, Buchumenski I, Porath HT, Danan-Gotthold M, Knisbacher BA, et al. Elevated RNA editing activity is a major contributor to transcriptomic diversity in tumors. Cell Rep. 2015;13:267–76.
Shah SP, Morin RD, Khattra J, Prentice L, Pugh T, Burleigh A, et al. Mutational evolution in a lobular breast tumour profiled at single nucleotide resolution. Nature. 2009;461:809–13.
Chan TH, Lin CH, Qi L, Fei J, Li Y, Yong KJ, et al. A disrupted RNA editing balance mediated by ADARs (Adenosine DeAminases that act on RNA) in human hepatocellular carcinoma. Gut. 2014;63:832–43.
Qin YR, Qiao JJ, Chan TH, Zhu YH, Li FF, Liu H, et al. Adenosine-to-inosine RNA editing mediated by ADARs in esophageal squamous cell carcinoma. Cancer Res. 2014;74:840–51.
Chan TH, Qamra A, Tan KT, Guo J, Yang H, Qi L, et al. ADAR-Mediated RNA editing predicts progression and prognosis of gastric cancer. Gastroenterology. 2016;151:637–50.e610.
Shoshan E, Mobley AK, Braeuer RR, Kamiya T, Huang L, Vasquez ME, et al. Reduced adenosine-to-inosine miR-455-5p editing promotes melanoma growth and metastasis. Nat Cell Biol. 2015;17:311–21.
Nemlich Y, Greenberg E, Ortenberg R, Besser MJ, Barshack I, Jacob-Hirsch J, et al. MicroRNA-mediated loss of ADAR1 in metastatic melanoma promotes tumor growth. J Clin Invest. 2013;123:2703–18.
Dominissini D, Moshitch-Moshkovitz S, Schwartz S, Salmon-Divon M, Ungar L, Osenberg S, et al. Topology of the human and mouse m6A RNA methylomes revealed by m6A-seq. Nature. 2012;485:201–6.
Kane SE, Beemon K. Precise localization of m6A in Rous sarcoma virus RNA reveals clustering of methylation sites: implications for RNA processing. Mol Cell Biol. 1985;5:2298–306.
Finkel D, Groner Y. Methylations of adenosine residues (m6A) in pre-mRNA are important for formation of late simian virus 40 mRNAs. Virology. 1983;131:409–25.
Geula S, Moshitch-Moshkovitz S, Dominissini D, Mansour AA, Kol N, Salmon-Divon M, et al. Stem cells. m6A mRNA methylation facilitates resolution of naive pluripotency toward differentiation. Science. 2015;347:1002–6.
Lence T, Akhtar J, Bayer M, Schmid K, Spindler L, Ho CH, et al. m6A modulates neuronal functions and sex determination in Drosophila. Nature. 2016;540:242–7.
Haussmann IU, Bodi Z, Sanchez-Moran E, Mongan NP, Archer N, Fray RG, et al. m6A potentiates Sxl alternative pre-mRNA splicing for robust Drosophila sex determination. Nature. 2016;540:301–4.
Zhang S, Zhao BS, Zhou A, Lin K, Zheng S, Lu Z, et al. m6A Demethylase ALKBH5 maintains tumorigenicity of glioblastoma stem-like cells by sustaining FOXM1 expression and cell proliferation program. Cancer Cell. 2017;31:591–606.e596.
Xiang Y, Laurent B, Hsu CH, Nachtergaele S, Lu Z, Sheng W, et al. RNA m6A methylation regulates the ultraviolet-induced DNA damage response. Nature. 2017;543:573–6.
Cui Q, Shi H, Ye P, Li L, Qu Q, Sun G, et al. m6A RNA methylation regulates the self-renewal and tumorigenesis of glioblastoma stem cells. Cell Rep. 2017;18:2622–34.
Dominissini D. Genomics and proteomics. roadmap to the epitranscriptome. Science. 2014;346:1192.
Work in CRW’s laboratory is supported by grants from the NHMRC (1102006), Cancer Council Victoria, and a Victorian Cancer Agency Mid Career Research Fellowship, in part by the Victorian State Government OIS (to St Vincent’s Institute). Work in JBL’s laboratory is supported by the NIH (R01GM102484, R01GM104215, and R01MH115080), Ellison Medical Foundation, and Stanford University Department of Genetics.
The authors declare that they have no competing interests.
About this article
Cite this article
Walkley, C.R., Li, J.B. Rewriting the transcriptome: adenosine-to-inosine RNA editing by ADARs. Genome Biol 18, 205 (2017). https://doi.org/10.1186/s13059-017-1347-3
- Adenosine Deaminase Acting On RNA (ADAR)
- ADAR Family
- ADAR Enzymes
- Editing Events
- ADAR Proteins