Breaking the central dogma by RNA editing

Bowhead NEIL1: molecular cloning, characterization, and enzymatic properties

Nei Like DNA Glycosylase 1 (NEIL1) is a DNA glycosylase, which specifically processes oxidative DNA damage by initiating base excision repair. NEIL1 recognizes and removes bases, primarily oxidized pyrimidines, which have been damaged by endogenous oxidation or exogenous mutagenic agents. NEIL1 functions through a combined glycosylase/AP (apurinic/apyrimidinic)-lyase activity, whereby it cleaves the N-glycosylic bond between the DNA backbone and the damaged base via its glycosylase activity and hydrolysis of the DNA backbone through beta-delta elimination due to its AP-lyase activity. In our study we investigated our hypothesis proposing that the cancer resistance of the bowhead whale can be associated with a better DNA repair with NEIL1 being upregulated or more active. Here, we report the molecular cloning and characterization of three transcript variants of bowhead whale NEIL1 of which two were homologous to human transcripts. In addition, a novel NEIL1 transcript variant was found. A differential expression of NEIL mRNA was detected in bowhead eye, liver, kidney, and muscle. The A-to-I editing of NEIL1 mRNA was shown to be conserved in the bowhead and two adenosines in the 242Lys codon were subjected to editing. A mass spectroscopy analysis of liver and eye tissue failed to demonstrate the existence of a NEIL1 isoform originating from RNA editing. Recombinant bowhead and human NEIL1 were expressed in E. coli and assayed for enzymatic activity. Both bowhead and human recombinant NEIL1 catalyzed, with similar efficiency, the removal of a 5-hydroxyuracil lesion in a DNA bubble structure. Hence, these results do not support our hypothesis but do not refute the hypothesis either.

NEIL1 Recoding due to RNA Editing Impacts Lesion-Specific Recognition and Excision

A-to-I RNA editing is widespread in human cells but is uncommon in the coding regions of proteins outside the nervous system. An unusual target for recoding by the adenosine deaminase ADAR1 is the pre-mRNA of the base excision DNA repair enzyme NEIL1 that results in the conversion of a lysine (K) to arginine (R) within the lesion recognition loop and alters substrate specificity. Differences in base removal by unedited (UE, K242) vs edited (Ed, R242) NEIL1 were evaluated using a series of oxidatively modified DNA bases to provide insight into the chemical and structural features of the lesion base that impact isoform-specific repair. We find that UE NEIL1 exhibits higher activity than Ed NEIL1 toward the removal of oxidized pyrimidines, such as thymine glycol, uracil glycol, 5-hydroxyuracil, and 5-hydroxymethyluracil. Gas-phase calculations indicate that the relative rates in excision track with the more stable lactim tautomer and the proton affinity of N3 of the base lesion. These trends support the contribution of tautomerization and N3 protonation in NEIL1 excision catalysis of these pyrimidine base lesions. Structurally similar but distinct substrate lesions, 5-hydroxycytosine and guanidinohydantoin, are more efficiently removed by the Ed NEIL1 isoform, consistent with the inherent differences in tautomerization, proton affinities, and lability. We also observed biphasic kinetic profiles and lack of complete base removal with specific combinations of the lesion and NEIL1 isoform, suggestive of multiple lesion binding modes. The complexity of NEIL1 isoform activity implies multiple roles for NEIL1 in safeguarding accurate repair and as an epigenetic regulator.

Identification of the genetic central dogma in osteogenic differentiation of MSCs by osteoinductive medium from transcriptional data sets

Background

The genetic central dogma (GCD) has been demonstrated its essential function in many biological processes and diseases. However, its roles in the process of osteogenic differentiation of mesenchymal stem cells (MSCs) remain unclear.

Methods

In this project, we analyzed an online database of osteogenic differentiation of MSCs after 14 days and 28 days by osteoinductive medium (GSE83770). The differentially expressed genes were screened by GEO2R, with further conducting of KEGG pathways using DAVID. In addition, protein–protein interactions of the enriched pathways were performed using STRING with marked hub genes measured by the CytoHubba. Hub genes were verified by quantitative reverse‐transcription polymerase chain reaction.

Results

Results showed that six pathways related to GCD, including DNA replication, Aminoacyl‐tRNA biosynthesis, Mismatch repair, Ribosome, Spliceosome, and RNA degradation pathways enriched in the early stage (14 days vs. undifferentiated MSCs) of osteogenesis. The Lysosome pathway was highly enriched in the late stage (28 vs. 14 days) of osteogenesis, and Ribosome pathway plays a key role throughout the entire process (28 days vs. undifferentiated MSCs) of osteogenesis.

Conclusion

Both DNA replication and protein translation were functionally worked in the early stage of osteogenesis, whereas the Lysosome pathway was the only GCD‐related one in the late stage of osteogenesis. The GCD‐related Ribosome pathway occupied the entire process of osteogenesis.

We identified six pathways associated with genetic central dogma (DNA replication, Aminoacyl‐tRNA biosynthesis, Mismatch repair, Ribosome, Spliceosome, and RNA degradation pathways), which participated in the early stage of osteogenesis (0−14 days).

The Lysosome pathway plays a crucial role in the late stage of osteogenesis (14–28 days), whereas the Ribosome pathway participates in the entire process (28 days vs. undifferentiated mesenchymal stem cells) of osteogenesis.

Artificial RNA Editing with ADAR for Gene Therapy

Editing mutated genes is a potential way for the treatment of genetic diseases. G-to-A mutations are common in mammals and can be treated by adenosine-to-inosine (A-to-I) editing, a type of substitutional RNA editing. The molecular mechanism of A-to-I editing involves the hydrolytic deamination of adenosine to an inosine base; this reaction is mediated by RNA-specific deaminases, adenosine deaminases acting on RNA (ADARs), family protein. Here, we review recent findings regarding the application of ADARs to restoring the genetic code along with different approaches involved in the process of artificial RNA editing by ADAR. We have also addressed comparative studies of various isoforms of ADARs. Therefore, we will try to provide a detailed overview of the artificial RNA editing and the role of ADAR with a focus on the enzymatic site directed A-to-I editing.

Role of m6A in osteoporosis, arthritis and osteosarcoma (Review)

RNA modification is a type of post-transcriptional modification that regulates important cellular pathways, such as the processing and metabolism of RNA. The most abundant form of methylation modification is RNA N6-methyladenine (m6A), which plays various post-transcriptional regulatory roles in cellular biological functions, including cell differentiation, embryonic development and disease occurrence. Bones play a pivotal role in the skeletal system as they support and protect muscles and other organs, facilitate movement and ensure haematopoiesis. The development and remodelling of bones require a delicate and accurate regulation of gene expression by epigenetic mechanisms that involve modifications of histone, DNA and RNA. The present review discusses the enzymes and proteins involved in mRNA m6A methylation modification and summarises current research progress and the mechanisms of mRNA m6A methylation in common orthopaedic diseases, including osteoporosis, arthritis and osteosarcoma.

RNA editing at a limited number of sites is sufficient to prevent MDA5 activation in the mouse brain

Adenosine deaminase acting on RNA 1 (ADAR1), an enzyme responsible for adenosine-to-inosine RNA editing, is composed of two isoforms: nuclear p110 and cytoplasmic p150. Deletion of Adar1 or Adar1 p150 genes in mice results in embryonic lethality with overexpression of interferon-stimulating genes (ISGs), caused by the aberrant recognition of unedited endogenous transcripts by melanoma differentiation-associated protein 5 (MDA5). However, among numerous RNA editing sites, how many RNA sites require editing, especially by ADAR1 p150, to avoid MDA5 activation and whether ADAR1 p110 contributes to this function remains elusive. In particular, ADAR1 p110 is abundant in the mouse brain where a subtle amount of ADAR1 p150 is expressed, whereas ADAR1 mutations cause Aicardi-Goutières syndrome, in which the brain is one of the most affected organs accompanied by the elevated expression of ISGs. Therefore, understanding RNA editing-mediated prevention of MDA5 activation in the brain is especially important. Here, we established Adar1 p110-specific knockout mice, in which the upregulated expression of ISGs was not observed. This result suggests that ADAR1 p150-mediated RNA editing is enough to suppress MDA5 activation. Therefore, we further created Adar1 p110/Adar2 double knockout mice to identify ADAR1 p150-mediated editing sites. This analysis demonstrated that although the elevated expression of ISGs was not observed, only less than 2% of editing sites were preserved in the brains of Adar1 p110/Adar2 double knockout mice. Of note, we found that some sites were highly edited, which was comparable to those found in wild-type mice, indicating the presence of ADAR1 p150-specific sites. These data suggest that RNA editing at a very limited sites, which is mediated by a subtle amount of ADAR1 p150, is sufficient to prevents MDA5 activation, at least in the mouse brain.

RNA Editing of the Human DNA Glycosylase NEIL1 Alters Its Removal of 5-Hydroxyuracil Lesions in DNA

Editing of the pre-mRNA of the DNA repair glycosylase NEIL1 results in substitution of a Lys with Arg in the lesion recognition loop of the enzyme. Unedited (UE, Lys242) NEIL1 removes thymine glycol lesions in DNA ∼30 times faster than edited (Ed, Arg242) NEIL1. Herein, we evaluated recognition and excision mediated by UE and Ed NEIL1 of 5-hydroxyuracil (5-OHU), a highly mutagenic lesion formed via oxidation of cytosine. Both NEIL1 isoforms catalyzed low levels of 5-OHU excision in single-stranded DNA, bubble and bulge DNA contexts and in duplex DNA base paired with A. Removal of 5-OHU in base pairs with G, T, and C was found to be faster and proceed to a higher overall extent with UE than with Ed NEIL1. In addition, the presence of mismatches adjacent to 5-OHU magnified the hampered activity of the Ed isoform. However, Ed NEIL1 was found to exhibit higher affinity for 5-OHU:G and 5-OHU:C duplexes than UE NEIL1. These results suggest that NEIL1 plays an important role in detecting and capturing 5-OHU lesions in inappropriate contexts, in a manner that does not lead to excision, to prevent mutations and strand breaks. Indeed, inefficient removal of 5-OHU by NEIL1 from 5-OHU:A base pairs formed during replication would thwart mutagenesis. Notably, nonproductive engagement of 5-OHU by Ed NEIL1 suggests the extent of 5-OHU repair will be reduced under cellular conditions, such as inflammation, that increase the extent of NEIL1 RNA editing. Tipping the balance between the two NEIL1 isoforms may be a significant factor leading to genome instability.

Recognition of DNA adducts by edited and unedited forms of DNA glycosylase NEIL1

Pre-mRNA encoding human NEIL1 undergoes editing by adenosine deaminase ADAR1 that converts a single adenosine to inosine, and this conversion results in an amino acid change of lysine 242 to arginine. Previous investigations of the catalytic efficiencies of the two forms of the enzyme revealed differential release of thymine glycol (ThyGly) from synthetic oligodeoxynucleotides, with the unedited form, NEIL1 K242 being ≈30-fold more efficient than the edited NEIL1 K242R. In contrast, when these enzymes were reacted with oligodeoxynucleotides containing guanidinohydantoin or spiroiminohydantoin, the edited K242R form was ≈3-fold more efficient than the unedited NEIL1. However, no prior studies have investigated the efficiencies of these two forms of NEIL1 on either high-molecular weight DNA containing multiple oxidatively-induced base damages, or oligodeoxynucleotides containing a bulky alkylated formamidopyrimidine. To understand the extent of changes in substrate recognition, γ-irradiated calf thymus DNA was treated with either edited or unedited NEIL1 and the released DNA base lesions analyzed by gas chromatography-tandem mass spectrometry. Of all the measured DNA lesions, imidazole ring-opened 4,6-diamino-5-formamidopyrimidine (FapyAde) and 2,6-diamino-4-hydroxy-5-formamidopyrimidine (FapyGua) were preferentially released by both NEIL1 enzymes with K242R being ≈1.3 and 1.2-fold more efficient than K242 on excision of FapyAde and FapyGua, respectively. Consistent with the prior literature, large differences (≈7.5 to 12-fold) were measured in the excision of ThyGly from genomic DNA by the unedited versus edited NEIL1. In contrast, the edited NEIL1 was more efficient (≈3 to 5-fold) on release of 5-hydroxycytosine. Excision kinetics on DNA containing a site-specific aflatoxin B₁-FapyGua adduct revealed an ≈1.4-fold higher rate by the unedited NEIL1. Molecular modeling provides insight into these differential substrate specificities. The results of this study and in particular, the comparison of substrate specificities of unedited and edited NEIL1 using biologically and clinically important base lesions, are critical for defining its role in preservation of genomic integrity.

Targeting the 5-HT2C Receptor in Biological Context and the Current State of 5-HT2C Receptor Ligand Development

Serotonin (5-HT) 5-HT2C receptor (5-HT2CR) is recognized as a critical mediator of diseaserelated pathways and behaviors based upon actions in the central nervous system (CNS). Since 5-HT2CR is a class A G protein-coupled receptor (GPCR), drug discovery efforts have traditionally pursued the activation of the receptor through synthetic ligands with agonists proposed for the treatment of obesity, substance use disorders and impulse control disorders while antagonists may add value for the treatment of anxiety, depression and schizophrenia. The most significant agonist discovery to date is the FDAapproved anti-obesity medication lorcaserin. In recent years, efforts towards developing other mechanisms to enhance receptor function have resulted in the discovery of Positive Allosteric Modulators (PAMs) for the 5-HT2CR, with several molecule series now reported. The biological significance and context for signaling and function of the 5-HT2CR, and the current status of 5-HT2CR agonists and PAMs are discussed in this review.

Direct Biomimetic Synthesis of β-Carboline Alkaloids from Two Amino Acids

The increasing importance of enzyme mimics in organic synthesis inspired us to design a novel biomimetic synthesis of β-carboline alkaloids directly from tryptophan and a second amino acid. This novel one-pot protocol utilizes abundant and readily available starting materials and thus presents a green and user-friendly alternative to conventional methods that rely on stepwise synthesis. Driven by molecular iodine and TFA, decarboxylation, deamination, Pictet-Spengler reaction, and oxidation reactions proceeded sequentially, transforming biomass amino acids into value-added alkaloid motifs.

SPRINT: an SNP-free toolkit for identifying RNA editing sites

Motivation

RNA editing generates post-transcriptional sequence alterations. Detection of RNA editing sites (RESs) typically requires the filtering of SNVs called from RNA-seq data using an SNP database, an obstacle that is difficult to overcome for most organisms.

Results

Here, we present a novel method named SPRINT that identifies RESs without the need to filter out SNPs. SPRINT also integrates the detection of hyper RESs from remapped reads, and has been fully automated to any RNA-seq data with reference genome sequence available. We have rigorously validated SPRINT’s effectiveness in detecting RESs using RNA-seq data of samples in which genes encoding RNA editing enzymes are knock down or over-expressed, and have also demonstrated its superiority over current methods. We have applied SPRINT to investigate RNA editing across tissues and species, and also in the development of mouse embryonic central nervous system. A web resource (http://sprint.tianlab.cn) of RESs identified by SPRINT has been constructed.

Availability and implementation

The software and related data are available at http://sprint.tianlab.cn.

Supplementary information

Supplementary data are available at Bioinformatics online.

Going global: the new era of mapping modifications in RNA

The post-transcriptional modification of RNA by the addition of one or more chemical groups has been known for over 50 years. These chemical modifications, once thought to be static, are now being discovered to play key regulatory roles in gene expression. The advent of massive parallel sequencing of RNA (RNA-seq) now allows us to probe the complexity of cellular RNA and how chemically altering RNA structure expands the RNA vocabulary. Here we present an overview of the various strategies and technologies that are available to profile RNA chemical modifications at the cellular level. These strategies can be characterized as targeted and untargeted approaches: targeted strategies are developed for one single chemical modification while untargeted strategies are more broadly applicable to a range of such chemical changes. Key for all of these approaches is the ability to locate modifications within the RNA sequence. While most of these methods are built upon an RNA-Seq pipeline, alternative approaches based on mass spectrometry or conventional DNA sequencing retain value in the overall analysis process. We also look forward toward future opportunities and technologies that may expand the types of modifications that can be globally profiled. Given the ever increasing recognition that these RNA chemical modifications play important biological roles, a variety of methods, preferably orthogonal approaches, will be required to globally identify, validate and quantify RNA chemical modifications found in the transcriptome. WIREs RNA 2017, 8:e1367. doi: 10.1002/wrna.1367 For further resources related to this article, please visit the WIREs website.

Triplex-forming peptide nucleic acid modified with 2-aminopyridine as a new tool for detection of A-to-I editing

RNA editing from adenosine to inosine (A-to-I editing) is one of the mechanisms that regulate and diversify the transcriptome. Here, a triplex-forming peptide nucleic acid (PNA) modified with a 2-aminopyridine nucleobase was applied for the recognition of the A-to-I editing event in double-stranded RNAs. The triplex-forming PNA enabled sequence-specific detection of single nucleobase editing at sub-nanomolar concentration.

Therapeutic Strategies and New Intervention Points in Chronic Hepatitis Delta Virus Infection

Chronic hepatitis delta virus infection (CHD) is a condition arising from super-infection of hepatitis B virus (HBV)-infected patients, resulting in a more rapid advance in liver pathology and hepatocellular carcinoma than is observed for HBV mono-infection. Although hepatitis delta virus (HDV) is structurally simple, its life cycle involves the complex participation of host enzymes, HBV-derived surface antigen (HBsAg), and HDV-auto-ribozyme and hepatitis delta antigen (HDAg) activities. Unsatisfactory clinical trial results with interferon-based therapies are motivating researchers to adjust and redirect the approach to CHD drug development. This new effort will likely require additional structural and functional studies of the viral and cellular/host components involved in the HDV replication cycle. This review highlights recent work aimed at new drug interventions for CHD, with interpretation of key pre-clinical- and clinical trial outcomes and a discussion of promising new technological approaches to antiviral drug design.

Changing blue fluorescent protein to green fluorescent protein using chemical RNA editing as a novel strategy in genetic restoration

Using the transition from cytosine of BFP (blue fluorescent protein) gene to uridine of GFP (green fluorescent protein) gene at position 199 as a model, we successfully controlled photochemical RNA editing to effect site-directed deamination of cytidine (C) to uridine (U). Oligodeoxynucleotides (ODNs) containing 5'-carboxyvinyl-2'-deoxyuridine ((CV) U) were used for reversible photoligation, and single-stranded 100-nt BFP DNA and in vitro-transcribed full-length BFP mRNA were the targets. Photo-cross-linking with the responsive ODNs was performed using UV (366 nm) irradiation, which was followed by heat treatment, and the cross-linked nucleotide was cleaved through photosplitting (UV, 312 nm). The products were analyzed using restriction fragment length polymorphism (RFLP) and fluorescence measurements. Western blotting and fluorescence-analysis results revealed that in vitro-translated proteins were synthesized from mRNAs after site-directed RNA editing. We detected substantial amounts of the target-base-substituted fragment using RFLP and observed highly reproducible spectra of the transition-GFP signal using fluorescence spectroscopy, which indicated protein stability. ODNc restored approximately 10% of the C-to-U transition. Thus, we successfully used non-enzymatic site-directed deamination for genetic restoration in vitro. In the near future, in vivo studies that include cultured cells and model animals will be conducted to treat genetic disorders.

8-Azapurines as isosteric purine fluorescent probes for nucleic acid and enzymatic research

The 8-azapurines, and their 7-deaza and 9-deaza congeners, represent a unique class of isosteric (isomorphic) analogues of the natural purines, frequently capable of substituting for the latter in many biochemical processes. Particularly interesting is their propensity to exhibit pH-dependent room-temperature fluorescence in aqueous medium, and in non-polar media. We herein review the physico-chemical properties of this class of compounds, with particular emphasis on the fluorescence emission properties of their neutral and/or ionic species, which has led to their widespread use as fluorescent probes in enzymology, including enzymes involved in purine metabolism, agonists/antagonists of adenosine receptors, mechanisms of catalytic RNAs, RNA editing, etc. They are also exceptionally useful fluorescent probes for analytical and clinical applications in crude cell homogenates.

Enzymatic interconversion of isomorphic fluorescent nucleosides: adenosine deaminase transforms an adenosine analogue into an inosine analogue

Adenosine deaminase, a major enzyme involved in purine metabolism, converts an isomorphic fluorescent analogue of adenosine (thA) to an isomorphic inosine analogue (thI), which possesses distinct spectral features, allowing one to monitor the enzyme-catalyzed reaction and its inhibition in real time. The utility of this sensitive fluorescently-monitored transformation for the high throughput detection and analysis of ADA inhibitors is demonstrated.

Global regulation of alternative splicing by adenosine deaminase acting on RNA (ADAR)

Alternative mRNA splicing is a major mechanism for gene regulation and transcriptome diversity. Despite the extent of the phenomenon, the regulation and specificity of the splicing machinery are only partially understood. Adenosine-to-inosine (A-to-I) RNA editing of pre-mRNA by ADAR enzymes has been linked to splicing regulation in several cases. Here we used bioinformatics approaches, RNA-seq and exon-specific microarray of ADAR knockdown cells to globally examine how ADAR and its A-to-I RNA editing activity influence alternative mRNA splicing. Although A-to-I RNA editing only rarely targets canonical splicing acceptor, donor, and branch sites, it was found to affect splicing regulatory elements (SREs) within exons. Cassette exons were found to be significantly enriched with A-to-I RNA editing sites compared with constitutive exons. RNA-seq and exon-specific microarray revealed that ADAR knockdown in hepatocarcinoma and myelogenous leukemia cell lines leads to global changes in gene expression, with hundreds of genes changing their splicing patterns in both cell lines. This global change in splicing pattern cannot be explained by putative editing sites alone. Genes showing significant changes in their splicing pattern are frequently involved in RNA processing and splicing activity. Analysis of recently published RNA-seq data from glioblastoma cell lines showed similar results. Our global analysis reveals that ADAR plays a major role in splicing regulation. Although direct editing of the splicing motifs does occur, we suggest it is not likely to be the primary mechanism for ADAR-mediated regulation of alternative splicing. Rather, this regulation is achieved by modulating trans-acting factors involved in the splicing machinery.

Analysis of miR-376 RNA cluster members in the mouse inner ear

Mutations in phosphoribosyl pyrophosphate synthetase 1 (PRPS1) are associated with a spectrum of non-syndromic to syndromic hearing loss. PRPS1 transcript levels have been shown to be regulated by the microRNA-376 genes. The long primary RNA transcript of the miR-376 RNA cluster members undergo extensive and simultaneous A → I editing at one or both of two specific sites (+4 and +44) in particular human and mouse tissues. The PRPS1 gene, which contains target sites for the edited version of miR-376a-5p within its 3'UTR, has been shown to be repressed in a tissue-specific manner. To investigate whether the transcription of Prps1 is regulated by miR-376 cluster members in the mouse inner ear, we first quantified the expression of the mature miR-376 RNAs by quantitative real-time-PCR. The spatio-temporal patterns of miR-376 expression were assessed by in situ hybridization. Finally, we examined whether A →I editing of pri-miR-376 RNAs occurs in mouse inner ear by direct sequencing. Our data showed that the miR-376a-3p, b-3p, c-3p are present in mouse embryonic inner ears and intensive expression of miR-376a-3p/b-3p was detected in the sensory epithelia and ganglia of both auditory and vestibular portions of the inner ear. In adult inner ear, the expression of miR-376a-3p/b-3p is restricted within ganglion neurons of auditory and vestibular systems as well as the cells in the stria vascularis. Only unedited pri-miR-376 RNAs were detected in the cochlea suggesting that the activity of PRPS1 in the inner ear may not be regulated through the editing of miR-376 cluster.

Using PCR to Target Misconceptions about Gene Expression

We present a PCR-based laboratory exercise that can be used with first- or second-year biology students to help overcome common misconceptions about gene expression. Biology students typically do not have a clear understanding of the difference between genes (DNA) and gene expression (mRNA/protein) and often believe that genes exist in an organism or cell only when they are expressed. This laboratory exercise allows students to carry out a PCR-based experiment designed to challenge their misunderstanding of the difference between genes and gene expression. Students first transform E. coli with an inducible GFP gene containing plasmid and observe induced and un-induced colonies. The following exercise creates cognitive dissonance when actual PCR results contradict their initial (incorrect) predictions of the presence of the GFP gene in transformed cells. Field testing of this laboratory exercise resulted in learning gains on both knowledge and application questions on concepts related to genes and gene expression.

A screening protocol for identification of functional mutants of RNA editing adenosine deaminases

Genetic screens can be used to evaluate a spectrum of mutations and thereby infer the function of particular residues within a protein. The Adenosine Deaminase Acting on RNA (ADAR) family of RNA-editing enzymes selectively deaminate adenosines (A) in double-helical RNA, generating inosine (I). The protocol described here exploits the editing activity of ADAR2 in a yeast-based screen by inserting an editing substrate sequence with a stop codon incorporated at the editing site upstream from the sequence encoding the reporter α-galactosidase. A-to-I editing changes the stop codon to a tryptophan codon, allowing normal expression of the reporter. This technique is particularly well-suited for screening ADAR and ADAR substrate mutant libraries for editing activity. Curr. Protoc. Chem. Biol. 4:357-369 © 2012 by John Wiley & Sons, Inc.

Darned in 2013: inclusion of model organisms and linking with Wikipedia

DARNED (DAtabase of RNa EDiting, available at http://darned.ucc.ie) is a centralized repository of reference genome coordinates corresponding to RNA nucleotides having altered templated identities in the process of RNA editing. The data in DARNED are derived from published datasets of RNA editing events. RNA editing instances have been identified with various methods, such as bioinformatics screenings, deep sequencing and/or biochemical techniques. Here we report our current progress in the development and expansion of the DARNED. In addition to novel database features the DARNED update describes inclusion of Drosophila melanogaster and Mus musculus RNA editing events and the launch of a community-based annotation in the RNA WikiProject.

The finger of an angel: memory return with epigenetic manipulation

Scientists have been trying to crack the memory code for hundreds of years; however, centuries later, even the simplest elements of memory formation are still not fully understood. Recent studies in epigenetics indicate neuronal activity can induce transient reprogramming of epigenetic codes required for long-term memory consolidation. This suggests epigenetics as a basic mechanism in the regulation of long-term memory; and highlights the possibility that epigenetic modifications, as well as environmental factors, can change certain gene expression of brain neurons to restore the ability to remember, even with an aging brain or innate mental deficits. This article is an overview of basic knowledge and current research on epigenetic regulation of long-term memory, and prospects for future research.

Posttranscriptional recoding by RNA editing

The posttranscriptional recoding of nuclear RNA transcripts has emerged as an important regulatory mechanism during eukaryotic gene expression. In particular the deamination of adenosine to inosine (interpreted by the translational machinery as a guanosine) is a frequent event that can recode the meaning of amino acid codons in translated exons, lead to structural changes in the RNA fold, or may affect splice consensus or regulatory sequence sites in noncoding exons or introns and modulate the biogenesis of small RNAs. The molecular mechanism of how the RNA editing machinery and its substrates recognize and interact with each other is not understood well enough to allow for the ab initio delineation of bona fide RNA editing sites. However, progress in the identification of various physiological modification sites and their characterization has given important insights regarding molecular features and events critical for productive RNA editing reactions. In addition, structural studies using components of the RNA editing machinery and together with editing competent substrate molecules have provided information on the chemical mechanism of adenosine deamination within the context of RNA molecules. Here, I give an overview of the process of adenosine deamination RNA editing and describe its relationship to other RNA processing events and its currently established roles in gene regulation.

SNP-based large-scale identification of allele-specific gene expression in human B cells

Polymorphism and variations in gene expression provide the genetic basis for human variation. Allelic variation of gene expression, in particular, may play a crucial role in phenotypic variation and disease susceptibility. To identify genes with allelic expression in human cells, we genotyped genomic DNA and cDNA isolated from 31 immortalized B cell lines from three Centre d'Etude du Polymorphisme Humain (CEPH) families using high-density single-nucleotide polymorphism (SNP) chips containing 13,900 exonic SNPs. We identified seven SNPs in five genes with monoallelic expression, 146 SNPs in 125 genes with allelic imbalance in expression with preferentially higher expression of one allele in a heterozygous individual. The monoallelically expressed genes (ERAP2, MDGA1, LOC644422, SDCCAG3P1 and CLTCL1) were regulated by cis-acting, non-imprinted differential allelic control. In addition, all monoallelic gene expression patterns and allelic imbalances in gene expression in B cells were transmitted from parents to offspring in the pedigree, indicating genetic transmission of allelic gene expression. Furthermore, frequent allele substitution, probably due to RNA editing, was also observed in 21 genes in 23 SNPs as well as in 48 SNPs located in regions containing no known genes. In this study, we demonstrated that allelic gene expression is frequently observed in human B cells, and SNP chips are very useful tools for detecting allelic gene expression. Overall, our data provide a valuable framework for better understanding allelic gene expression in human B cells.

Accurate identification of A-to-I RNA editing in human by transcriptome sequencing

RNA editing enhances the diversity of gene products at the post-transcriptional level. Approaches for genome-wide identification of RNA editing face two main challenges: separating true editing sites from false discoveries and accurate estimation of editing levels. We developed an approach to analyze transcriptome sequencing data (RNA-seq) for global identification of RNA editing in cells for which whole-genome sequencing data are available. We applied the method to analyze RNA-seq data of a human glioblastoma cell line, U87MG. Around 10,000 DNA-RNA differences were identified, the majority being putative A-to-I editing sites. These predicted A-to-I events were associated with a low false-discovery rate (∼5%). Moreover, the estimated editing levels from RNA-seq correlated well with those based on traditional clonal sequencing. Our results further facilitated unbiased characterization of the sequence and evolutionary features flanking predicted A-to-I editing sites and discovery of a conserved RNA structural motif that may be functionally relevant to editing. Genes with predicted A-to-I editing were significantly enriched with those known to be involved in cancer, supporting the potential importance of cancer-specific RNA editing. A similar profile of DNA-RNA differences as in U87MG was predicted for another RNA-seq data set obtained from primary breast cancer samples. Remarkably, significant overlap exists between the putative editing sites of the two transcriptomes despite their difference in cell type, cancer type, and genomic backgrounds. Our approach enabled de novo identification of the RNA editome, which sets the stage for further mechanistic studies of this important step of post-transcriptional regulation.

RNA editing changes the lesion specificity for the DNA repair enzyme NEIL1

Editing of the pre-mRNA for the DNA repair enzyme NEIL1 causes a lysine to arginine change in the lesion recognition loop of the protein. The two forms of NEIL1 are shown here to have distinct enzymatic properties. The edited form removes thymine glycol from duplex DNA 30 times more slowly than the form encoded in the genome, whereas editing enhances repair of the guanidinohydantoin lesion by NEIL1. In addition, we show that the NEIL1 recoding site is a preferred editing site for the RNA editing adenosine deaminase ADAR1. The edited adenosine resides in an A-C mismatch in a hairpin stem formed by pairing of exon 6 to the immediate upstream intron 5 sequence. As expected for an ADAR1 site, editing at this position is increased in human cells treated with interferon α. These results suggest a unique regulatory mechanism for DNA repair and extend our understanding of the impact of RNA editing.

Increased A-to-I RNA editing of the transcript for GABAA receptor subunit α3 during chick retinal development

Adenosine-to-inosine (A-to-I) RNA editing is a cotranscriptional or posttranscriptional gene regulatory mechanism that increases the diversity of the proteome in the nervous system. Recently, the transcript for GABA type A receptor subunit α3 was found to be subjected to RNA editing. The aim of this study was to determine if editing of the chicken α3 subunit transcript occurs in the retina and if the editing is temporally regulated during development. We also raised the question if editing of the α3 transcript was temporally associated with the suggested developmental shift from excitation to inhibition in the GABA system. The editing frequency was studied by using Sanger and Pyrosequencing, and to monitor the temporal aspects, we studied the messenger RNA expression of the GABAA receptor subunits and chloride pumps, known to be involved in the switch. The results showed that the chick α3 subunit was subjected to RNA editing, and its expression was restricted to cells in the inner nuclear and ganglion cell layer in the retina. The extent of editing increased during development (after embryonic days 8-9) concomitantly with an increase of expression of the chloride pump KCC2. Expression of several GABAA receptor subunits known to mediate synaptic GABA actions was upregulated at this time. We conclude that editing of the chick GABAA subunit α3 transcript in chick retina gives rise to an amino acid change that may be of importance in the switch from excitatory to inhibitory receptors.

One hundred years of pleiotropy: a retrospective

Pleiotropy is defined as the phenomenon in which a single locus affects two or more distinct phenotypic traits. The term was formally introduced into the literature by the German geneticist Ludwig Plate in 1910, 100 years ago. Pleiotropy has had an important influence on the fields of physiological and medical genetics as well as on evolutionary biology. Different approaches to the study of pleiotropy have led to incongruence in the way that it is perceived and discussed among researchers in these fields. Furthermore, our understanding of the term has changed quite a bit since 1910, particularly in light of modern molecular data. This review traces the history of the term "pleiotropy" and reevaluates its current place in the field of genetics.

Molecular diversity through RNA editing: a balancing act

RNA editing by adenosine deamination fuels the generation of RNA and protein diversity in eukaryotes, particularly in higher organisms. This includes the recoding of translated exons, widespread editing of retrotransposon-derived repeat elements and sequence modification of microRNA (miRNA) transcripts. Such changes can bring about specific amino acid substitutions, alternative splicing and changes in gene expression levels. Although the overall prevalence of adenosine-to-inosine (A-to-I) editing and its specific functional impact on many of the affected genes is not yet known, the importance of balancing RNA modification levels across time and space is becoming increasingly evident. In particular, transcriptome instabilities in the form of too much or too little RNA editing activity, or misguided editing, manifest in several human disease phenotypes and can disrupt that balance.

Covalent hydration energies for purine analogs by quantum chemical methods

In this work, covalent hydration energies for a variety of azanaphthalenes and purine analogs have been calculated using a variety of quantum chemical methods. On the basis of these results, we recommend the CPCM(UA0)-B3LYP/6-31+G(d,p) level for rapid prediction of covalent hydration energies. However, we caution the use of this methodology for computing covalent hydration energies for fluorine-containing compounds.

ADAR editing in double-stranded UTRs and other noncoding RNA sequences

ADARs are a family of enzymes, present in all animals, that convert adenosine to inosine within double-stranded RNA (dsRNA). Inosine and adenosine have different base-pairing properties, and thus, editing alters RNA structure, coding potential and splicing patterns. The first ADAR substrates identified were edited in codons, and ADARs were presumed to function primarily in proteome diversification. Although this is an important function of ADARs, especially in the nervous system, editing in coding sequences is rare compared to editing in noncoding sequences. Introns and untranslated regions of mRNA are the primary noncoding targets, but editing also occurs in small RNAs, such as miRNAs. Although the role of editing in noncoding sequences remains unclear, ongoing research suggests functions in the regulation of a variety of post-transcriptional processes.

A computational screen for site selective A-to-I editing detects novel sites in neuron specific Hu proteins

Background

Several bioinformatic approaches have previously been used to find novel sites of ADAR mediated A-to-I RNA editing in human. These studies have discovered thousands of genes that are hyper-edited in their non-coding intronic regions, especially in alu retrotransposable elements, but very few substrates that are site-selectively edited in coding regions. Known RNA edited substrates suggest, however, that site selective A-to-I editing is particularly important for normal brain development in mammals.

Results

We have compiled a screen that enables the identification of new sites of site-selective editing, primarily in coding sequences. To avoid hyper-edited repeat regions, we applied our screen to the alu-free mouse genome. Focusing on the mouse also facilitated better experimental verification. To identify candidate sites of RNA editing, we first performed an explorative screen based on RNA structure and genomic sequence conservation. We further evaluated the results of the explorative screen by determining which transcripts were enriched for A-G mismatches between the genomic template and the expressed sequence since the editing product, inosine (I), is read as guanosine (G) by the translational machinery. For expressed sequences, we only considered coding regions to focus entirely on re-coding events. Lastly, we refined the results from the explorative screen using a novel scoring scheme based on characteristics for known A-to-I edited sites. The extent of editing in the final candidate genes was verified using total RNA from mouse brain and 454 sequencing.

Conclusions

Using this method, we identified and confirmed efficient editing at one site in the Gabra3 gene. Editing was also verified at several other novel sites within candidates predicted to be edited. Five of these sites are situated in genes coding for the neuron-specific RNA binding proteins HuB and HuD.

MiRNA editing

RNA editing by A-to-I modification is a widespread mechanism in complex organisms that leads to the posttranscriptional alteration of protein coding as well as noncoding sequences. MiRNA transcripts have been recognized as a major target for RNA editing enzymes, and single-nucleotide changes through editing can impact the biogenesis of mature miRNAs, as well as the target specificity of the regulatory RNA. Bona fide A-to-I RNA editing events are validated experimentally through parallel analysis of genomic DNA and transcribed sequences of miRNA genes isolated from the same specimen through gene-specific amplification and sequencing of endogenous transcripts.

Matching active site and substrate structures for an RNA editing reaction

The RNA-editing adenosine deaminases (ADARs) catalyze deamination of adenosine to inosine in a double-stranded structure found in various RNA substrates, including mRNAs. Here we present recent efforts to define structure/activity relationships for the ADAR reaction. We describe the synthesis of new phosphoramidites for the incorporation of 7-substituted-8-aza-7-deazaadenosine derivatives into RNA. These reagents were used to introduce the analogues into mimics of the R/G-editing site found in the pre-mRNA for the human glutamate receptor B subunit (GluR B). Analysis of the kinetics of the ADAR2 reaction with analogue-containing RNAs indicated 8-aza-7-deazaadenosine is an excellent substrate for this enzyme with a deamination rate eight times greater than that for adenosine. However, replacing the C7 hydrogen in this analogue with bromine, iodine, or propargyl alcohol failed to increase the deamination rate further but rather decreased the rate. Modeling of nucleotide binding in the enzyme active site suggested amino acid residues that may be involved in nucleotide recognition. We carried out a functional screen of a library of ADAR2 mutants expressed in S. cerevisiae that varied the identity of these residues to identify active deaminases with altered active sites. One of these mutants (ADAR2 R455A) was able to substantially overcome the inhibitory effect of the bulky C7 substituents (-Br, -I, propargyl alcohol). These results advance our understanding of the importance of functional groups found in the edited nucleotide and the role of specific active site residues of ADAR2.

RNA editing: a driving force for adaptive evolution?

Genetic variability is considered a key to the evolvability of species. The conversion of an adenosine (A) to inosine (I) in primary RNA transcripts can result in an amino acid change in the encoded protein, a change in secondary structure of the RNA, creation or destruction of a splice consensus site, or otherwise alter RNA fate. Substantial transcriptome and proteome variability is generated by A-to-I RNA editing through site-selective post-transcriptional recoding of single nucleotides. We posit that this epigenetic source of phenotypic variation is an unrecognized mechanism of adaptive evolution. The genetic variation introduced through editing occurs at low evolutionary cost since predominant production of the wild-type protein is retained. This property even allows exploration of sequence space that is inaccessible through mutation, leading to increased phenotypic plasticity and provides an evolutionary advantage for acclimatization as well as long-term adaptation. Furthermore, continuous probing for novel RNA editing sites throughout the transcriptome is an intrinsic property of the editing machinery and represents the molecular basis for increased adaptability. We propose that higher organisms have therefore evolved to systems with increasing RNA editing activity and, as a result, to more complex systems.

Identification of a selective nuclear import signal in adenosine deaminases acting on RNA

The adenosine deaminases acting on RNA (ADARs) comprise a family of RNA editing enzymes that selectively modify single codons within RNA primary transcripts with often profound impact on protein function. Little is known about the mechanisms that regulate nuclear RNA editing activity. Editing levels show cell-type specific and developmental modulation that does not strictly coincide with observed expression levels of ADARs. Here, we provide evidence for a molecular mechanism that might control nuclear import of specific ADARs and, in turn, nuclear RNA editing. We identify an in vivo ADAR3 interaction partner, importin alpha 1 (KPNA2) that specifically recognizes an arginine-rich ADAR3 sequence motif and show that it acts as a functional nuclear localization sequence. Furthermore, whereas KPNA2, but not KPNA1 or KNPA3, recognizes the ADAR3 NLS, we observe the converse binding specificity with ADAR2. Interestingly, alternative splicing of ADAR2 pre-mRNA introduces an ADAR3-like NLS that alters the interaction profile with the importins. Thus, in vivo RNA editing might be regulated, in part, through controlled subcellular localization of ADARs, which in turn is governed by the coordinated local expression of importin α proteins and ADAR protein variants.

An innovative real-time PCR method to measure changes in RNA editing of the serotonin 2C receptor (5-HT(2C)R) in brain

The serotonin 2C receptor (5-HT(2C)R) plays a significant role in psychiatric disorders (e.g., depression) and is a target for pharmacotherapy. The 5-HT(2C)R is widely expressed in brain and spinal cord and is the only G-protein coupled receptor currently known to undergo mRNA editing, a post-transcriptional modification that results in translation of distinct, though closely related, protein isoforms. The 5-HT(2C)R RNA can be edited at five sites to alter up to three amino acids resulting in modulation of receptor:G-protein coupling and constitutive activity. To rapidly quantify changes ex vivo in individual 5-HT(2C)R isoform levels in response to treatment, we adapted quantitative (real-time) reverse transcription polymerase chain reaction (qRT-PCR) utilizing TaqMan probes modified with a minor groove binder (MGB). Probes were developed for four 5-HT(2C)R RNA isoforms and their sensitivity and specificity were validated systematically using standard templates. Relative expression of the four isoforms was measured in cDNAs from whole brain extracted from 129S6 and C57BL/6J mice. Rank order derived from this qRT-PCR analysis matched that derived from DNA sequencing. In mutant mice solely expressing either non-edited or fully edited 5-HT(2C)R transcripts, only expected transcripts were detected. These data suggest this qRT-PCR method is a precise and rapid means to detect closely related mRNA sequences ex vivo without the necessity of characterizing the entire 5-HT(2C)R profile. Implementation of this technique will expand and expedite studies of specific brain 5-HT(2C)R mRNA isoforms in response to pharmacological, behavioral and genetic manipulation, particularly in ex vivo studies which require rapid collection of data on large numbers of samples.

Subcellular localization and expression of bamboo mosaic virus satellite RNA-encoded protein

The satellite RNA of bamboo mosaic virus (satBaMV) has a single open reading frame encoding a non-structural protein, P20, which facilitates long-distance movement of satBaMV in BaMV and satBaMV co-infected plants. Immunohistochemistry and immunoelectron microscopy revealed that the P20 protein accumulated in the cytoplasm and nuclei in co-infected cells. P20 and the helper virus coat protein (CP) were highly similar in their subcellular localization, except that aggregates of BaMV virions were not labelled with anti-P20 serum. The BaMV CP protein was fairly abundant in mesophyll cells, whilst P20 was more frequently detected in mesophyll cells and vascular tissues. The expression kinetics of the P20 protein was similar to but slightly earlier than that of CP in co-infected Bambusa oldhamii protoplasts and Nicotiana benthamiana leaves. However, satBaMV-encoded protein levels declined rapidly in the late phase of co-infection. During co-infection, in addition to the intact P20, a low-molecular-mass polypeptide of 16 kDa was identified as a P20 C-terminally truncated product; the possible method of generation of the truncated protein is discussed.

Novel exon of mammalian ADAR2 extends open reading frame

BACKGROUND

The post-transcriptional processing of pre-mRNAs by RNA editing contributes significantly to the complexity of the mammalian transcriptome. RNA editing by site-selective A-to-I modification also regulates protein function through recoding of genomically specified sequences. The adenosine deaminase ADAR2 is the main enzyme responsible for recoding editing and loss of ADAR2 function in mice leads to a phenotype of epilepsy and premature death. Although A-to-I RNA editing is known to be subject to developmental and cell-type specific regulation, there is little knowledge regarding the mechanisms that regulate RNA editing in vivo. Therefore, the characterization of ADAR expression and identification of alternative ADAR variants is an important prerequisite for understanding the mechanisms for regulation of RNA editing and the causes for deregulation in disease.

METHODOLOGY/PRINCIPAL FINDINGS

Here we present evidence for a new ADAR2 splice variant that extends the open reading frame of ADAR2 by 49 amino acids through the utilization of an exon located 18 kilobases upstream of the previously annotated first coding exon and driven by a candidate alternative promoter. Interestingly, the 49 amino acid extension harbors a sequence motif that is closely related to the R-domain of ADAR3 where it has been shown to function as a basic, single-stranded RNA binding domain. Quantitative expression analysis shows that expression of the novel ADAR2 splice variant is tissue specific being highest in the cerebellum.

CONCLUSIONS/SIGNIFICANCE

The strong sequence conservation of the ADAR2 R-domain between human, mouse and rat ADAR2 genes suggests a conserved function for this isoform of the RNA editing enzyme.

Alternative splicing of the mouse embryonic poly(A) binding protein (Epab) mRNA is regulated by an exonic splicing enhancer: a model for post-transcriptional control of gene expression in the oocyte

Embryonic poly(A) binding protein (EPAB), expressed in oocytes and early embryos, binds and stabilizes maternal mRNAs, and mediates initiation of their translation. We identified an alternatively spliced form of Epab lacking exon 10 (c.Ex10del) and investigated the regulation of Epab mRNA alternative splicing as a model for alternative splicing in oocytes and early preimplantation embryos. Specifically, we evaluated the following mechanisms: imprinting; RNA editing and exonic splicing enhancers (ESEs). Sequence analysis led to the identification of two single nucleotide polymorphisms (SNPs): one was detected in exon 9 (rs55858A/G), and served as a marker for the parental origin of the alternatively spliced form, and the other was found in exon 10 (rs56574G/C), and co-segregated with the exon 9 SNP. We found that the presence of rs56574G in exon 10 led to the formation of an ESE, leading to efficient exclusion of exon 10. Real-time RT–PCR results revealed a 5-fold increase in the expression of the c.Ex10del alternative splicing variant in animals carrying rs56574G/G in exon 10 compared with rs56574C/C at the same locus. Our findings suggest that SNPs may alter the ratio between alternative splicing variants of oocyte-specific proteins. The role that these subtle differences play in determining individual reproductive outcome remains to be determined.

Prediction of C-to-U RNA editing sites in higher plant mitochondria using only nucleotide sequence features

RNA editing is a class of post-transcriptional processing which contributes to the organism complexity. C-to-U RNA editing is commonly observed in higher plant mitochondria. The in vivo mechanism of recognizing C-to-U RNA editing sites is still unknown. In recent years, many efforts have been made to computationally predict C-to-U RNA editing sites. But all existing methods require using knowledge other than an RNA sequence. In the present work, we propose the first method for predicting C-to-U RNA editing sites using only nucleotide sequence features. This method was developed based on the SVM algorithm combined with a triplet scoring model. Our method can achieve 84% overall accuracy which is comparable to other methods. We also computationally found that several triplets never appear upstream near an edited cytidine, indicating that these triplets may protect a cytidine from being edited. This discovery suggests the need for further experimental research and may be helpful in understanding the editing site recognition mechanism.