Gene Summary

Gene:ADARB1; adenosine deaminase, RNA-specific, B1
Aliases: RED1, ADAR2, DRABA2, DRADA2
Summary:This gene encodes the enzyme responsible for pre-mRNA editing of the glutamate receptor subunit B by site-specific deamination of adenosines. Studies in rat found that this enzyme acted on its own pre-mRNA molecules to convert an AA dinucleotide to an AI dinucleotide which resulted in a new splice site. Alternative splicing of this gene results in several transcript variants, some of which have been characterized by the presence or absence of an ALU cassette insert and a short or long C-terminal region. [provided by RefSeq, Jul 2008]
Databases:OMIM, VEGA, HGNC, Ensembl, GeneCard, Gene
Protein:double-stranded RNA-specific editase 1
Source:NCBIAccessed: 20 August, 2015


What does this gene/protein do?
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Cancer Overview

Research Indicators

Publications Per Year (1990-2015)
Graph generated 20 August 2015 using data from PubMed using criteria.

Literature Analysis

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Tag cloud generated 20 August, 2015 using data from PubMed, MeSH and CancerIndex

Specific Cancers (4)

Data table showing topics related to specific cancers and associated disorders. Scope includes mutations and abnormal protein expression.

Note: list is not exhaustive. Number of papers are based on searches of PubMed (click on topic title for arbitrary criteria used).

Latest Publications: ADARB1 (cancer-related)

Wei J, Li Z, Du C, et al.
Abnormal expression of an ADAR2 alternative splicing variant in gliomas downregulates adenosine-to-inosine RNA editing.
Acta Neurochir (Wien). 2014; 156(6):1135-42 [PubMed] Free Access to Full Article Related Publications
BACKGROUND: RNA editing is catalyzed by adenosine deaminases acting on RNA (ADARs). ADAR2 is the main enzyme responsible for recoding editing in humans. Adenosine-to-inosine (A-to-I) editing at the Q/R site is reported to be decreased in gliomas; however, the expression of ADAR2 mRNA was not greatly affected.
METHODS: We determined ADAR2 mRNA expression in human glioblastoma cell lines and in normal human glial cells by real-time RT-PCR. We also determined ADAR2 mRNA expression in 44 glioma tissues and normal white matter. After identifying an alternative splicing variant (ASV) of ADAR2 in gliomas, we performed sequencing. We then classified glioblastomas based on the presence (+) or absence (-) of the ASV to determine the correlations between ASV + and malignant features of glioblastomas, such as invasion, peritumoral brain edema, and survival time.
RESULTS: There were no significant differences in ADAR2 mRNA expression among human glioblastoma cell lines or in gliomas compared with normal white matter (all p > 0.05). The ASV, which contained a 47-nucleotide insertion in the ADAR2 mRNA transcript, was detected in the U251 and BT325 cell lines, and in some glioma tissues. The expression rate of ASV differed among gliomas of different grades. ASV + glioblastomas were more malignant than ASV - glioblastomas.
CONCLUSIONS: ADAR2 is a family of enzymes in which ASVs result in differences in enzymatic activity. The ADAR2 ASV may be correlated with the invasiveness of gliomas. Identification of the mechanistic characterization of ADAR2 ASV may have future potential for individualized molecular targeted-therapy for glioma.

Liu WH, Chen CH, Yeh KH, et al.
ADAR2-mediated editing of miR-214 and miR-122 precursor and antisense RNA transcripts in liver cancers.
PLoS One. 2013; 8(12):e81922 [PubMed] Free Access to Full Article Related Publications
A growing list of microRNAs (miRNAs) show aberrant expression patterns in hepatocellular carcinoma (HCC), but the regulatory mechanisms largely remain unclear. RNA editing catalyzed by members of the adenosine deaminase acting on the RNA (ADAR) family could target the miRNA precursors and affect the biogenesis process. Therefore, we investigate whether RNA editing could be one mechanism contributing to the deregulation of specific miRNAs in HCC. By overexpression of individual ADARs in hepatoma cells, RNA editing on the precursors of 16 miRNAs frequently deregulated in HCC was screened by a sensitive high-resolution melting platform. The results identified RNA precursors of miR-214 and miR-122 as potential targets edited by ADAR2. A subset of HCC showing elevated ADAR2 verified the major editings identified in ARAR2 overexpressed hepatoma cells, either with A-to-I or U-to-C changes. The unusual U-to-C editing at specific residues was demonstrated as being attributed to the A-to-I editing on the RNA transcripts complementary to the pri-miRNAs. The editing event caused a decrease of the RNA transcript complementary to pri-miR-214, which led to the decrease of pri-miR-214 and miR-214 and resulted in the increased protein level of its novel target gene Rab15. In conclusion, the current study discovered ADAR2-mediated editing of the complementary antisense transcripts as a novel mechanism for regulating the biogenesis of specific miRNAs during hepatocarcinogenesis.

Qin YR, Qiao JJ, Chan TH, et al.
Adenosine-to-inosine RNA editing mediated by ADARs in esophageal squamous cell carcinoma.
Cancer Res. 2014; 74(3):840-51 [PubMed] Related Publications
Esophageal squamous cell carcinoma (ESCC), the major histologic form of esophageal cancer, is a heterogeneous tumor displaying a complex variety of genetic and epigenetic changes. Aberrant RNA editing of adenosine-to-inosine (A-to-I), as it is catalyzed by adenosine deaminases acting on RNA (ADAR), represents a common posttranscriptional modification in certain human diseases. In this study, we investigated the status and role of ADARs and altered A-to-I RNA editing in ESCC tumorigenesis. Among the three ADAR enzymes expressed in human cells, only ADAR1 was overexpressed in primary ESCC tumors. ADAR1 overexpression was due to gene amplification. Patients with ESCC with tumoral overexpression of ADAR1 displayed a poor prognosis. In vitro and in vivo functional assays established that ADAR1 functions as an oncogene during ESCC progression. Differential expression of ADAR1 resulted in altered gene-specific editing activities, as reflected by hyperediting of FLNB and AZIN1 messages in primary ESCC. Notably, the edited form of AZIN1 conferred a gain-of-function phenotype associated with aggressive tumor behavior. Our findings reveal that altered gene-specific A-to-I editing events mediated by ADAR1 drive the development of ESCC, with potential implications in diagnosis, prognosis, and treatment of this disease.

Wang Q, Hui H, Guo Z, et al.
ADAR1 regulates ARHGAP26 gene expression through RNA editing by disrupting miR-30b-3p and miR-573 binding.
RNA. 2013; 19(11):1525-36 [PubMed] Free Access to Full Article Related Publications
Rho GTPase activating protein 26 (ARHGAP26) is a negative regulator of the Rho family that converts the small G proteins RhoA and Cdc42 to their inactive GDP-bound forms. It is essential for the CLIC/GEEC endocytic pathway, cell spreading, and muscle development. The present study shows that ARHGAP26 mRNA undergoes extensive A-to-I RNA editing in the 3' UTR that is specifically catalyzed by ADAR1. Furthermore, the mRNA and protein levels of ARHGAP26 were decreased in cells in which ADAR1 was knocked down. Conversely, ADAR1 overexpression increased the abundance of ARHGAP26 mRNA and protein. In addition, we found that both miR-30b-3p and miR-573 target the ARHGAP26 gene and that RNA editing of ARHGAP26 mediated by ADAR1 abolished the repression of its expression by miR-30b-3p or miR-573. When ADAR1 was overexpressed, the reduced abundance of ARHGAP26 protein mediated by miR-30b-3p or miR-573 was rescued. Importantly, we also found that knocking down ADAR1 elevated RhoA activity, which was consistent with the reduced level of ARHGAP26. Conversely, when ADAR1 was overexpressed, the amount of RhoA-GTP decreased. The similar expression patterns of ARHGAP26 and ADAR1 in human tissue samples further confirmed our findings. Taken together, our results suggest that ADAR1 regulates the expression of ARHGAP26 through A-to-I RNA editing by disrupting the binding of miR-30b-3p and miR-573 within the 3' UTR of ARHGAP26. This study provides a novel insight into the mechanism by which ADAR1 and its RNA editing function regulate microRNA-mediated modulation of target genes.

Chan TH, Lin CH, Qi L, et al.
A disrupted RNA editing balance mediated by ADARs (Adenosine DeAminases that act on RNA) in human hepatocellular carcinoma.
Gut. 2014; 63(5):832-43 [PubMed] Free Access to Full Article Related Publications
OBJECTIVE: Hepatocellular carcinoma (HCC) is a heterogeneous tumour displaying a complex variety of genetic and epigenetic changes. In human cancers, aberrant post-transcriptional modifications, such as alternative splicing and RNA editing, may lead to tumour specific transcriptome diversity.
DESIGN: By utilising large scale transcriptome sequencing of three paired HCC clinical specimens and their adjacent non-tumour (NT) tissue counterparts at depth, we discovered an average of 20 007 inferred A to I (adenosine to inosine) RNA editing events in transcripts. The roles of the double stranded RNA specific ADAR (Adenosine DeAminase that act on RNA) family members (ADARs) and the altered gene specific editing patterns were investigated in clinical specimens, cell models and mice.
RESULTS: HCC displays a severely disrupted A to I RNA editing balance. ADAR1 and ADAR2 manipulate the A to I imbalance of HCC via their differential expression in HCC compared with NT liver tissues. Patients with ADAR1 overexpression and ADAR2 downregulation in tumours demonstrated an increased risk of liver cirrhosis and postoperative recurrence and had poor prognoses. Due to the differentially expressed ADAR1 and ADAR2 in tumours, the altered gene specific editing activities, which was reflected by the hyper-editing of FLNB (filamin B, β) and the hypo-editing of COPA (coatomer protein complex, subunit α), are closely associated with HCC pathogenesis. In vitro and in vivo functional assays prove that ADAR1 functions as an oncogene while ADAR2 has tumour suppressive ability in HCC.
CONCLUSIONS: These findings highlight the fact that the differentially expressed ADARs in tumours, which are responsible for an A to I editing imbalance, has great prognostic value and diagnostic potential for HCC.

Nemlich Y, Greenberg E, Ortenberg R, et al.
MicroRNA-mediated loss of ADAR1 in metastatic melanoma promotes tumor growth.
J Clin Invest. 2013; 123(6):2703-18 [PubMed] Free Access to Full Article Related Publications
Some solid tumors have reduced posttranscriptional RNA editing by adenosine deaminase acting on RNA (ADAR) enzymes, but the functional significance of this alteration has been unclear. Here, we found the primary RNA-editing enzyme ADAR1 is frequently reduced in metastatic melanomas. In situ analysis of melanoma samples using progression tissue microarrays indicated a substantial downregulation of ADAR1 during the metastatic transition. Further, ADAR1 knockdown altered cell morphology, promoted in vitro proliferation, and markedly enhanced the tumorigenicity in vivo. A comparative whole genome expression microarray analysis revealed that ADAR1 controls the expression of more than 100 microRNAs (miRNAs) that regulate many genes associated with the observed phenotypes. Importantly, we discovered that ADAR1 fundamentally regulates miRNA processing in an RNA binding–dependent, yet RNA editing–independent manner by regulating Dicer expression at the translational level via let-7. In addition, ADAR1 formed a complex with DGCR8 that was mutually exclusive with the DGCR8-Drosha complex that processes pri-miRNAs in the nucleus. We found that cancer cells silence ADAR1 by overexpressing miR-17 and miR-432, which both directly target the ADAR1 transcript. We further demonstrated that the genes encoding miR-17 and miR-432 are frequently amplified in melanoma and that aberrant hypomethylation of the imprinted DLK1-DIO3 region in chromosome 14 can also drive miR-432 overexpression.

Hochberg M, Gilead L, Markel G, et al.
Insulin-like growth factor-binding protein-7 (IGFBP7) transcript: A-to-I editing events in normal and cancerous human keratinocytes.
Arch Dermatol Res. 2013; 305(6):519-28 [PubMed] Related Publications
Non-melanoma skin cancers (NMSC) are the most common malignancies in caucasians worldwide. Insulin-like growth factor-binding protein-7 (IGFBP7) was suggested to function as a tumor suppressor gene in several cancers, and to play a role in the proliferation of keratinocytes. A-to-I RNA editing is a post-transcriptional mechanism frequently used to expand and diversify transcriptome and proteome repertoire in eukaryotic cells. A-to-I RNA editing can alter codons, substitute amino acids and affect protein sequence, structure, and function. Two editing sites were identified within the IGFBP7 transcript. To evaluate the expression and editing of IGFBP7 mRNA in NMSC compared to normal epidermis. We examined the expression and mRNA editing level of IGFBP7 in 22 basal cell carcinoma (BCC), 15 squamous cell carcinoma (SCC), and 18 normal epidermis samples that were surgically removed from patients by the Mohs Micrographic Surgery procedure. We studied the effect of IGFBP7 editing on an immortalized HaCaT keratinocyte cell model. IGFBP7 mRNA is over expressed in BCC and SCC compared to normal epidermis. Moreover, the IGFBP7 transcript is highly edited in normal epidermis, but its editing is significantly reduced in BCC and SCC. The edited form of IGFBP7 can inhibit proliferation and induce senescence in cultured keratinocytes. This study describes for the first time A-to-I editing in the coding sequence of a tumor suppressor gene in humans, and suggests that IGFBP7 editing serves as a fine-tuning mechanism to maintain the equilibrium between proliferation and senescence in normal skin.

Shimokawa T, Rahman MF, Tostar U, et al.
RNA editing of the GLI1 transcription factor modulates the output of Hedgehog signaling.
RNA Biol. 2013; 10(2):321-33 [PubMed] Free Access to Full Article Related Publications
The Hedgehog (HH) signaling pathway has important roles in tumorigenesis and in embryonal patterning. The Glioma-associated oncogene 1 (GLI1) is a key molecule in HH signaling, acting as a transcriptional effector and, moreover, is considered to be a potential therapeutic target for several types of cancer. To extend our previous focus on the implications of alternative splicing for HH signal transduction, we now report on an additional post-transcriptional mechanism with an impact on GLI1 activity, namely RNA editing. The GLI1 mRNA is highly edited at nucleotide 2179 by adenosine deamination in normal cerebellum, but the extent of this modification is reduced in cell lines from the cerebellar tumor medulloblastoma. Additionally, basal cell carcinoma tumor samples exhibit decreased GLI1 editing compared with normal skin. Interestingly, knocking down of either ADAR1 or ADAR2 reduces RNA editing of GLI1. This adenosine to inosine substitution leads to a change from Arginine to Glycine at position 701 that influences not only GLI1 transcriptional activity, but also GLI1-dependent cellular proliferation. Specifically, the edited GLI1, GLI1-701G, has a higher capacity to activate most of the transcriptional targets tested and is less susceptible to inhibition by the negative regulator of HH signaling suppressor of fused. However, the Dyrk1a kinase, implicated in cellular proliferation, is more effective in increasing the transcriptional activity of the non-edited GLI1. Finally, introduction of GLI1-701G into medulloblastoma cells confers a smaller increase in cellular growth relative to GLI1. In conclusion, our findings indicate that RNA editing of GLI1 is a regulatory mechanism that modulates the output of the HH signaling pathway.

Jiang Q, Crews LA, Barrett CL, et al.
ADAR1 promotes malignant progenitor reprogramming in chronic myeloid leukemia.
Proc Natl Acad Sci U S A. 2013; 110(3):1041-6 [PubMed] Free Access to Full Article Related Publications
The molecular etiology of human progenitor reprogramming into self-renewing leukemia stem cells (LSC) has remained elusive. Although DNA sequencing has uncovered spliceosome gene mutations that promote alternative splicing and portend leukemic transformation, isoform diversity also may be generated by RNA editing mediated by adenosine deaminase acting on RNA (ADAR) enzymes that regulate stem cell maintenance. In this study, whole-transcriptome sequencing of normal, chronic phase, and serially transplantable blast crisis chronic myeloid leukemia (CML) progenitors revealed increased IFN-γ pathway gene expression in concert with BCR-ABL amplification, enhanced expression of the IFN-responsive ADAR1 p150 isoform, and a propensity for increased adenosine-to-inosine RNA editing during CML progression. Lentiviral overexpression experiments demonstrate that ADAR1 p150 promotes expression of the myeloid transcription factor PU.1 and induces malignant reprogramming of myeloid progenitors. Moreover, enforced ADAR1 p150 expression was associated with production of a misspliced form of GSK3β implicated in LSC self-renewal. Finally, functional serial transplantation and shRNA studies demonstrate that ADAR1 knockdown impaired in vivo self-renewal capacity of blast crisis CML progenitors. Together these data provide a compelling rationale for developing ADAR1-based LSC detection and eradication strategies.

Bianchetti L, Kieffer D, Féderkeil R, Poch O
Increased frequency of single base substitutions in a population of transcripts expressed in cancer cells.
BMC Cancer. 2012; 12:509 [PubMed] Free Access to Full Article Related Publications
BACKGROUND: Single Base Substitutions (SBS) that alter transcripts expressed in cancer originate from somatic mutations. However, recent studies report SBS in transcripts that are not supported by the genomic DNA of tumor cells.
METHODS: We used sequence based whole genome expression profiling, namely Long-SAGE (L-SAGE) and Tag-seq (a combination of L-SAGE and deep sequencing), and computational methods to identify transcripts with greater SBS frequencies in cancer. Millions of tags produced by 40 healthy and 47 cancer L-SAGE experiments were compared to 1,959 Reference Tags (RT), i.e. tags matching the human genome exactly once. Similarly, tens of millions of tags produced by 7 healthy and 8 cancer Tag-seq experiments were compared to 8,572 RT. For each transcript, SBS frequencies in healthy and cancer cells were statistically tested for equality.
RESULTS: In the L-SAGE and Tag-seq experiments, 372 and 4,289 transcripts respectively, showed greater SBS frequencies in cancer. Increased SBS frequencies could not be attributed to known Single Nucleotide Polymorphisms (SNP), catalogued somatic mutations or RNA-editing enzymes. Hypothesizing that Single Tags (ST), i.e. tags sequenced only once, were indicators of SBS, we observed that ST proportions were heterogeneously distributed across Embryonic Stem Cells (ESC), healthy differentiated and cancer cells. ESC had the lowest ST proportions, whereas cancer cells had the greatest. Finally, in a series of experiments carried out on a single patient at 1 healthy and 3 consecutive tumor stages, we could show that SBS frequencies increased during cancer progression.
CONCLUSION: If the mechanisms generating the base substitutions could be known, increased SBS frequency in transcripts would be a new useful biomarker of cancer. With the reduction of sequencing cost, sequence based whole genome expression profiling could be used to characterize increased SBS frequency in patient's tumor and aid diagnostic.

Shaikhibrahim Z, Lindstrot A, Ochsenfahrt J, et al.
Epigenetics-related genes in prostate cancer: expression profile in prostate cancer tissues, androgen-sensitive and -insensitive cell lines.
Int J Mol Med. 2013; 31(1):21-5 [PubMed] Free Access to Full Article Related Publications
Epigenetic changes have been suggested to drive prostate cancer (PCa) development and progression. Therefore, in this study, we aimed to identify novel epigenetics-related genes in PCa tissues, and to examine their expression in metastatic PCa cell lines. We analyzed the expression of epigenetics-related genes via a clustering analysis based on gene function in moderately and poorly differentiated PCa glands compared to normal glands of the peripheral zone (prostate proper) from PCa patients using Whole Human Genome Oligo Microarrays. Our analysis identified 12 epigenetics-related genes with a more than 2-fold increase or decrease in expression and a p-value <0.01. In modera-tely differentiated tumors compared to normal glands of the peripheral zone, we found the genes, TDRD1, IGF2, DICER1, ADARB1, HILS1, GLMN and TRIM27, to be upregulated, whereas TNRC6A and DGCR8 were found to be downregulated. In poorly differentiated tumors, we found TDRD1, ADARB and RBM3 to be upregulated, whereas DGCR8, PIWIL2 and BC069781 were downregulated. Our analysis of the expression level for each gene in the metastatic androgen-sensitive VCaP and LNCaP, and -insensitive PC3 and DU-145 PCa cell lines revealed differences in expression among the cell lines which may reflect the different biological properties of each cell line, and the potential role of each gene at different metastatic sites. The novel epigenetics-related genes that we identified in primary PCa tissues may provide further insight into the role that epigenetic changes play in PCa. Moreover, some of the genes that we identified may play important roles in primary PCa and metastasis, in primary PCa only, or in metastasis only. Follow-up studies are required to investigate the functional role and the role that the expression of these genes play in the outcome and progression of PCa using tissue microarrays.

Valles I, Pajares MJ, Segura V, et al.
Identification of novel deregulated RNA metabolism-related genes in non-small cell lung cancer.
PLoS One. 2012; 7(8):e42086 [PubMed] Free Access to Full Article Related Publications
Lung cancer is a leading cause of cancer death worldwide. Several alterations in RNA metabolism have been found in lung cancer cells; this suggests that RNA metabolism-related molecules are involved in the development of this pathology. In this study, we searched for RNA metabolism-related genes that exhibit different expression levels between normal and tumor lung tissues. We identified eight genes differentially expressed in lung adenocarcinoma microarray datasets. Of these, seven were up-regulated whereas one was down-regulated. Interestingly, most of these genes had not previously been associated with lung cancer. These genes play diverse roles in mRNA metabolism: three are associated with the spliceosome (ASCL3L1, SNRPB and SNRPE), whereas others participate in RNA-related processes such as translation (MARS and MRPL3), mRNA stability (PCBPC1), mRNA transport (RAE), or mRNA editing (ADAR2, also known as ADARB1). Moreover, we found a high incidence of loss of heterozygosity at chromosome 21q22.3, where the ADAR2 locus is located, in NSCLC cell lines and primary tissues, suggesting that the downregulation of ADAR2 in lung cancer is associated with specific genetic losses. Finally, in a series of adenocarcinoma patients, the expression of five of the deregulated genes (ADAR2, MARS, RAE, SNRPB and SNRPE) correlated with prognosis. Taken together, these results support the hypothesis that changes in RNA metabolism are involved in the pathogenesis of lung cancer, and identify new potential targets for the treatment of this disease.

Galeano F, Rossetti C, Tomaselli S, et al.
ADAR2-editing activity inhibits glioblastoma growth through the modulation of the CDC14B/Skp2/p21/p27 axis.
Oncogene. 2013; 32(8):998-1009 [PubMed] Free Access to Full Article Related Publications
Grade IV astrocytoma or glioblastoma multiforme (GBM) is one of the most aggressive and lethal tumors affecting humans. ADAR2-mediated A-to-I RNA editing, an essential post-transcriptional modification event in brain, is impaired in GBMs and astrocytoma cell lines. However, the role of ADAR2 editing in astrocytomas remains to be defined. Here, we show that ADAR2 editing rescue in astrocytomas prevents tumor growth in vivo and modulates an important cell cycle pathway involving the Skp2/p21/p27 proteins, often altered in glioblastoma. We demonstrate that ADAR2 deaminase activity is essential to inhibit tumor growth. Indeed, we identify the phosphatase CDC14B, which acts upstream of the Skp2/p21/p27 pathway, as a novel and critical ADAR2 target gene involved in glioblastoma growth. Specifically, ADAR2-mediated editing on CDC14B pre-mRNA increases its expression with a consequent reduction of the Skp2 target protein, as shown both in vitro and in vivo. We found that, compared to normal brain, both CDC14B editing and expression are progressively impaired in astrocytomas from grade I to IV, being very low in GBMs. These findings (1) demonstrate that post-transcriptional A-to-I RNA editing might be crucial for glioblastoma pathogenesis, (2) identify ADAR2-editing enzyme as a novel candidate tumor suppressor gene and (3) provide proof of principle that ADAR2 or its substrates may represent a suitable target(s) for possible novel, more effective and less toxic approaches to the treatment of GBMs.

Chen CP, Lin SP, Chen M, et al.
Mosaic supernumerary r(1)(p13.2q23.3) in a 10-year-old girl with epilepsy facial asymmetry psychomotor retardation kyphoscoliosis dermatofibrosarcoma and multiple exostoses.
Genet Couns. 2011; 22(3):273-80 [PubMed] Related Publications
We report molecular cytogenetic characterization of mosaic supernumerary r(1)(p13.2q23.3) in a 10-year-old girl with epilepsy, facial asymmetry, psychomotor retardation, kyphoscoliosis, dermatofibrosarcoma and multiple exostoses. The supernumerary r(1) is associated with gene dosage increase of CHRNB2, ADAR and KCNJ10 in the pericentromeric area of 1q, and a breakpoint within CTTNBP2NL at 1p13.2. We speculate that the gene dosage increase of CHRNB2, ADAR and KCNJ10 is most likely responsible for epilepsy, and the breakpoint at 1p13.2 in the supernumerary r(1) is most likely responsible for the development of multiple exostoses and osteochondroma in this patient.

Gallo A, Locatelli F
ADARs: allies or enemies? The importance of A-to-I RNA editing in human disease: from cancer to HIV-1.
Biol Rev Camb Philos Soc. 2012; 87(1):95-110 [PubMed] Related Publications
Adenosine deaminases acting on RNA (ADARs) are enzymes that convert adenosine (A) to inosine (I) in nuclear-encoded RNAs and viral RNAs. The activity of ADARs has been demonstrated to be essential in mammals and serves to fine-tune different proteins and modulate many molecular pathways. Recent findings have shown that ADAR activity is altered in many pathological tissues. Moreover, it has been shown that modulation of RNA editing is important for cell proliferation and migration, and has a protective effect on ischaemic insults. This review summarises available recent knowledge on A-to-I RNA editing and ADAR enzymes, with particular attention given to the emerging role played by these enzymes in cancer, some infectious diseases and immune-mediated disorders.

Galeano F, Leroy A, Rossetti C, et al.
Human BLCAP transcript: new editing events in normal and cancerous tissues.
Int J Cancer. 2010; 127(1):127-37 [PubMed] Free Access to Full Article Related Publications
Bladder cancer-associated protein (BLCAP) is a highly conserved protein among species, and it is considered a novel candidate tumor suppressor gene originally identified from human bladder carcinoma. However, little is known about the regulation or the function of this protein. Here, we show that the human BLCAP transcript undergoes multiple A-to-I editing events. Some of the new editing events alter the highly conserved amino terminus of the protein creating alternative protein isoforms by changing the genetically coded amino acids. We found that both ADAR1 and ADAR2-editing enzymes cooperate to edit this transcript and that different tissues displayed distinctive ratios of edited and unedited BLCAP transcripts. Moreover, we observed a general decrease in BLCAP-editing level in astrocytomas, bladder cancer and colorectal cancer when compared with the related normal tissues. The newly identified editing events, found to be downregulated in cancers, could be useful for future studies as a diagnostic tool to distinguish malignancies or epigenetic changes in different tumors.

Skarda J, Amariglio N, Rechavi G
RNA editing in human cancer: review.
APMIS. 2009; 117(8):551-7 [PubMed] Related Publications
In eukaryotes mRNA transcripts are extensively processed by different post-transcriptional events such as alternative splicing and RNA editing in order to generate many different mRNAs from the same gene, increasing the transcriptome and then the proteome diversity. The most frequent RNA editing mechanism in mammals involves the conversion of specific adenosines into inosines by the ADAR family of enzymes. This editing event can alter the sequence and the secondary structure of RNA molecules, with consequences for final proteins and regulatory RNAs. Alteration in RNA editing has been connected to tumor progression and many other important human diseases. Analysis of many editing sites in various cancer types is expected to provide new diagnostic and prognostic markers and might contribute to early detection of cancer, the monitoring of response to therapy, and to the detection of minimal residual disease.

Flanagan JM, Funes JM, Henderson S, et al.
Genomics screen in transformed stem cells reveals RNASEH2A, PPAP2C, and ADARB1 as putative anticancer drug targets.
Mol Cancer Ther. 2009; 8(1):249-60 [PubMed] Related Publications
Since the sequencing of the human genome, recent efforts in cancer drug target discovery have focused more on the identification of novel functions of known genes and the development of more appropriate tumor models. In the present study, we investigated in vitro transformed human adult mesenchymal stem cells (MSC) to identify novel candidate cancer drug targets by analyzing the transcriptional profile of known enzymes compared with non-transformed MSC. The identified enzymes were compared with published cancer gene expression data sets. Surprisingly, the majority of up-regulated enzymes are already known cancer drug targets or act within known druggable pathways. Only three enzymes (RNASEH2A, ADARB1, and PPAP2C) are potentially novel targets that are up-regulated in transformed MSC and expressed in numerous carcinomas and sarcomas. We confirmed the overexpression of RNASEH2A, PPAP2C, and ADARB1 in transformed MSC, transformed fibroblasts, and cancer cell lines MCF7, SK-LMS1, MG63, and U2OS. In functional assays, we show that small interfering RNA knockdown of RNASEH2A inhibits anchorage-independent growth but does not alter in vitro proliferation of cancer cell lines, normal MSC, or normal fibroblasts. Knockdown of PPAP2C impaired anchorage-dependent in vitro growth of cancer cell lines and impaired the in vitro growth of primary MSC but not differentiated human fibroblasts. We show that the knockdown of PPAP2C decreases cell proliferation by delaying entry into S phase of the cell cycle and is transcriptionally regulated by p53. These in vitro data validate PPAP2C and RNASEH2A as putative cancer targets and endorse this in silico approach for identifying novel candidates.

Gallo A, Galardi S
A-to-I RNA editing and cancer: from pathology to basic science.
RNA Biol. 2008 Jul-Sep; 5(3):135-9 [PubMed] Related Publications
In eukaryotes mRNA transcripts are extensively processed by different post-transcriptional events such as alternative splicing and RNA editing in order to generate many different mRNAs from the same gene, increasing the transcriptome and then the proteome. The most frequent RNA editing mechanism in mammals involves the conversion of specific adenosines into inosines by the ADAR family of enzymes. This editing event can change both the sequence and the secondary structure of RNA molecules, with important consequences on both the final proteins and regulatory RNAs. Alteration in RNA editing has been connected to numerous human pathologies and recent studies have demonstrated its importance in tumor progression.

Martinez HD, Jasavala RJ, Hinkson I, et al.
RNA editing of androgen receptor gene transcripts in prostate cancer cells.
J Biol Chem. 2008; 283(44):29938-49 [PubMed] Free Access to Full Article Related Publications
Reactivation of the androgen receptor (AR) signaling pathway represents a critical step in the growth and survival of androgen-independent (AI) prostate cancer (CaP). In this study we show the DU145 and PC3 AI human CaP cell lines respond to androgens and require AR expression for optimal proliferation in vitro. Interestingly, AR gene transcripts in DU145 and PC3 cells harbored a large number of single base pair nucleotide transitions that resulted in missense mutations in selected AR codons. The most notable lesion detected in AR gene transcripts included the oncogenic codon 877T-->A gain-of-function mutation. Surprisingly, AR gene transcript nucleotide transitions were not genome-encoded substitutions, but instead the mutations co-localized to putative A-to-I, U-to-C, C-to-U, and G-to-A RNA editing sites, suggesting the lesions were mediated through RNA editing mechanisms. Higher levels of mRNA encoding the A-to-I RNA editing enzymes ADAR1 and ADARB1 were observed in DU145 and PC3 cells relative to the androgen-responsive LNCaP and 22Rv1 human CaP cell lines, which correlated with higher levels of AR gene transcript A-to-I editing detected in DU145 and PC3 cells. Our results suggest that AR gene transcripts are targeted by different RNA editing enzymes in DU145 and PC3 cells. Thus RNA editing of AR gene transcripts may contribute to the etiology of hormone-refractory phenotypes in advanced stage AI CaP.

Cenci C, Barzotti R, Galeano F, et al.
Down-regulation of RNA editing in pediatric astrocytomas: ADAR2 editing activity inhibits cell migration and proliferation.
J Biol Chem. 2008; 283(11):7251-60 [PubMed] Related Publications
Since alterations in post-transcriptional events can contribute to the appearance and/or progression of cancer, we investigated whether RNA editing, catalyzed by the ADAR (adenosine deaminases that act on RNA) enzymes, is altered in pediatric astrocytomas. We find a decrease in ADAR2 editing activity that seems to correlate with the grade of malignancy in children. Despite the loss of ADAR2 editing activity in tumor tissues, the high grade astrocytomas do not exhibit alterations in ADAR2 expression when compared with their specific control tissues. However, high expression levels of ADAR1 and ADAR3 were found in tumors when compared with normal tissues dissected in the same area of the brain. We reintroduced either ADAR2 or the inactive version of ADAR2 in three astrocytoma cell lines (U118, A172, U87). The "reverted" editing status is necessary and sufficient for a significant decrease in cell malignant behavior as measured by proliferation, cell cycle, and migration assays. We show that elevated levels of ADAR1, as found in astrocytomas, do indeed interfere with ADAR2 specific editing activity. Furthermore, we show that the endogenous ADAR1 can form heterodimers with ADAR2 in astrocytes.

Paz N, Levanon EY, Amariglio N, et al.
Altered adenosine-to-inosine RNA editing in human cancer.
Genome Res. 2007; 17(11):1586-95 [PubMed] Free Access to Full Article Related Publications
Adenosine-to-inosine (A-to-I) RNA editing was recently shown to be abundant in the human transcriptome, affecting thousands of genes. Employing a bioinformatic approach, we identified significant global hypoediting of Alu repetitive elements in brain, prostate, lung, kidney, and testis tumors. Experimental validation confirmed this finding, showing significantly reduced editing in Alu sequences within MED13 transcripts in brain tissues. Looking at editing of specific recoding and noncoding sites, including in cancer-related genes, a more complex picture emerged, with a gene-specific editing pattern in tumors vs. normal tissues. Additionally, we found reduced RNA levels of all three editing mediating enzymes, ADAR, ADARB1, and ADARB2, in brain tumors. The reduction of ADARB2 correlated with the grade of malignancy of glioblastoma multiforme, the most aggressive of brain tumors, displaying a 99% decrease in ADARB2 RNA levels. Consistently, overexpression of ADAR and ADARB1 in the U87 glioblastoma multiforme cell line resulted in decreased proliferation rate, suggesting that reduced A-to-I editing in brain tumors is involved in the pathogenesis of cancer. Altered epigenetic control was recently shown to play a central role in oncogenesis. We suggest that A-to-I RNA editing may serve as an additional epigenetic mechanism relevant to cancer development and progression.

Langenau DM, Keefe MD, Storer NY, et al.
Effects of RAS on the genesis of embryonal rhabdomyosarcoma.
Genes Dev. 2007; 21(11):1382-95 [PubMed] Free Access to Full Article Related Publications
Embryonal rhabdomyosarcoma (ERMS) is a devastating cancer with specific features of muscle differentiation that can result from mutational activation of RAS family members. However, to date, RAS pathway activation has not been reported in a majority of ERMS patients. Here, we have created a zebrafish model of RAS-induced ERMS, in which animals develop externally visible tumors by 10 d of life. Microarray analysis and cross-species comparisons identified two conserved gene signatures found in both zebrafish and human ERMS, one associated with tumor-specific and tissue-restricted gene expression in rhabdomyosarcoma and a second comprising a novel RAS-induced gene signature. Remarkably, our analysis uncovered that RAS pathway activation is exceedingly common in human RMS. We also created a new transgenic coinjection methodology to fluorescently label distinct subpopulations of tumor cells based on muscle differentiation status. In conjunction with fluorescent activated cell sorting, cell transplantation, and limiting dilution analysis, we were able to identify the cancer stem cell in zebrafish ERMS. When coupled with gene expression studies of this cell population, we propose that the zebrafish RMS cancer stem cell shares similar self-renewal programs as those found in activated satellite cells.

Hartwig D, Schütte C, Warnecke J, et al.
The large form of ADAR 1 is responsible for enhanced hepatitis delta virus RNA editing in interferon-alpha-stimulated host cells.
J Viral Hepat. 2006; 13(3):150-7 [PubMed] Related Publications
Hepatitis delta virus (HDV) RNA editing controls the formation of hepatitis-delta-antigen-S and -L and therefore indirectly regulates HDV replication. Editing is thought to be catalysed by the adenosine deaminase acting on RNA1 (ADAR1) of which two different forms exist, interferon (IFN)-alpha-inducible ADAR1-L and constitutively expressed ADAR1-S. ADAR1-L is hypothesized to be a part of the innate cellular immune system, responsible for deaminating adenosines in viral dsRNAs. We examined the influence of both forms on HDV RNA editing in IFN-alpha-stimulated and unstimulated hepatoma cells. For gene silencing, an antisense oligodeoxyribonucleotide against a common sequence of both forms of ADAR1 and another one specific for ADAR1-L alone were used. IFN-alpha treatment of host cells led to approximately twofold increase of RNA editing compared with unstimulated controls. If ADAR1-L expression was inhibited, this substantial increase in editing could no longer be observed. In unstimulated cells, ADAR1-L suppression had only minor effects on editing. Inhibition of both forms of ADAR1 simultaneously led to a substantial decrease of edited RNA independently of IFN-alpha-stimulation. In conclusion, the two forms of ADAR1 are responsible almost alone for HDV editing. In unstimulated cells, ADAR1-S is the main editing activity. The increase of edited RNA under IFN-alpha-stimulation is because of induction of ADAR1-L, showing for the first time that this IFN-inducible protein is involved in the base modification of replicating HDV RNA. Thus, induction of ADAR1-L may at least partially cause the antiviral effect of IFN-alpha in natural immune response to HDV as well as in case of therapeutic administration of IFN.

Yang W, Wang Q, Kanes SJ, et al.
Altered RNA editing of serotonin 5-HT2C receptor induced by interferon: implications for depression associated with cytokine therapy.
Brain Res Mol Brain Res. 2004; 124(1):70-8 [PubMed] Related Publications
Members of the ADAR (adenosine deaminases acting on RNA) gene family are involved in one type of RNA editing that converts adenosine residues to inosine. The A-to-I editing of serotonin receptor subtype 2C (5-HT(2C)R) mRNA leads to replacement of three amino acid residues located within the intracellular loop II domain, resulting in dramatic alterations in G-protein coupling functions of the receptor. It has been speculated that RNA editing may play a role in several pharmacological and behavioral processes where the serotonergic plasticity is mediated through 5-HT(2C)R. Interferon-alpha (IFN-alpha) often causes severe depression in patients treated for chronic viral hepatitis and certain malignancies. In this study, we examined the effects of IFN-alpha on RNA editing in human glioblastoma cell lines, which express 5-HT(2C)R mRNAs. ADAR1 expression and the pattern of the 5-HT(2C)R mRNA editing rapidly changed in response to IFN-alpha, leading to the dominant expression of the 5-HT(2C)R-VSI isoform predicted to have reduced G-protein coupling functions. Our results support the hypothesis that 5-HT(2C)R mRNA editing has causative relevance in the pathophysiology of depression associated with cytokine therapy.

Barbon A, Vallini I, La Via L, et al.
Glutamate receptor RNA editing: a molecular analysis of GluR2, GluR5 and GluR6 in human brain tissues and in NT2 cells following in vitro neural differentiation.
Brain Res Mol Brain Res. 2003; 117(2):168-78 [PubMed] Related Publications
The properties of some glutamate receptors are modified by RNA editing. This post-transcriptional mechanism involves the enzymatic deamination of specific adenosines in the pre-mRNA of the glutamate receptors, performed by specific RNA adenosine deaminases (ADARs). This event gives rise to the substitution of a gene-encoded amino acid with a different one that modifies the physiological properties of the ion channel. Here we report an analysis of the editing levels of AMPA GluR2, and kainate GluR5 and GluR6 in a human teratocarcinoma cell line (NT2) during in vitro neural differentiation, in conjunction with an analysis of the expression levels of GluR and ADAR genes. The editing levels were analysed using a specific standardised assay based on sequence analysis. This assay can be performed on all editing sites with a high level of sensitivity and reproducibility. Whereas GluR gene expression increased during NT2 neural differentiation, the expression of ADAR genes may be detected at comparable levels even in undifferentiated NT2 cells, remaining relatively stable during the differentiation process. Furthermore, most of the glutamate receptor editing sites increased their editing levels during NT2 neural differentiation, suggesting that the level of ADAR mRNAs is not closely related to the variable editing levels detected in the GluRs analysed. In human brain tissues, the editing levels appeared finely modulated in the different areas, indicating the possible formation of ion channels with different functional properties, thus generating a complex tissue-specific regulation of receptors and modulation of excitatory stimuli.

Gao B, Kunos G
Cell type-specific transcriptional activation and suppression of the alpha1B adrenergic receptor gene middle promoter by nuclear factor 1.
J Biol Chem. 1998; 273(48):31784-7 [PubMed] Related Publications
Nuclear factor 1 (NF1) has been reported to be a transcriptional activator for some genes and a transcriptional silencer for others. Here we report that in Hep3B cells, cotransfection of NF1/L, NF1/Red1, or NF1/X with the alpha1B adrenergic receptor (alpha1BAR) gene middle (P2) promoter increases P2 activity to more or less the same degree, whereas in DDT1 MF-2 cells cotransfection of NF1/L or NF1/Red1 causes a small but statistically significant decrease in the P2 promoter activity, and NF1/X causes a greater, 70% inhibition. Further experiments using truncated NF1/X mutants indicate that NF1/X contains both positive and negative regulatory domains. The positive domain, located between amino acids 416 and 505, is active in Hep3B cells, whereas the negative domain, located between amino acids 243 and 416, is active in DDT1 MF-2 cells. These functional domains are also capable of regulating transcription when isolated from their natural context and fused into the GAL4 binding domain. Furthermore, NF1 affinity purified from rat liver nuclear extracts copurified with a non-DNA binding protein, which can bind to the P2 promoter of the alpha1BAR gene via interacting with NF1. Taken together, these findings indicate that NF1/X contains both activation and suppression domains that may be recognized and modulated by cell type-specific cofactors. This may be one of the mechanisms whereby NF1 can activate or suppress the expression of different genes, and it may also underlie the tissue-specific regulation of the alpha1B AR gene.

Niswender CM
Recent advances in mammalian RNA editing.
Cell Mol Life Sci. 1998; 54(9):946-64 [PubMed] Related Publications
RNA editing is a posttranscriptional modification that results in the generation of nucleotides within an RNA transcript that do not match the bases present within the genome. Mammalian RNA editing events, often represented by cytidine-to-uridine and adenosine-to-inosine conversions, are predominantly mediated by base deamination. In the past decade, important advances have been made in the understanding of editing mechanisms, the identification of RNA sequences and structures necessary for editing regulation, and the cloning and characterization of editing enzymes. It has also recently been appreciated that RNA editing within mammalian substrates can have profound functional consequences in protein function, implicating this posttranscriptional modification as important in the production of molecular diversity.

Lai F, Chen CX, Lee VM, Nishikura K
Dramatic increase of the RNA editing for glutamate receptor subunits during terminal differentiation of clonal human neurons.
J Neurochem. 1997; 69(1):43-52 [PubMed] Related Publications
RNA editing plays an important role in determining physiological characteristics of certain glutamate-gated receptor (GluR) channels such as Ca2+ permeability and desensitization kinetics. In one case, the editing changes a gene-encoded glutamine (Q) to an arginine (R) codon located in the channel-forming domain of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor subunit GluR-B and also the kainate receptor subunits GluR5 and GluR6. Another case of RNA editing alters an arginine (R) to a glycine (G) codon at a position termed the "R/G" site of AMPA subunits GluR-B, C, and D. Double-stranded RNA-specific adenosine deaminases (DRADA) have been implicated as agents involved in the editing. By using a human teratocarcinoma cell line, NT2, we investigated the change of the RNA editing of GluR subunits in conjunction with the expression of two DRADA members, DRADA1 and DRADA2 genes, during neuronal differentiation. Whereas Q/R and R/G site RNA editing both become progressively activated in differentiating NT2 cells, the expression of the two DRADA genes can already be detected even in the undifferentiated NT2 cells. Development of the editing machinery appears to require, in addition to DRADA enzymes, a currently unidentified mechanism(s) that may become activated during neuronal differentiation.

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