Adenosine Deaminase Acting On RNA 1 Research Articles

ADAR1 ablation decreases bone mass by impairing osteoblast function in mice

Bone mass is controlled through a delicate balance between osteoblast-mediated bone formation and osteoclast-mediated bone resorption. We show here that RNA editing enzyme adenosine deaminase acting on RNA 1 (ADAR1) is critical for proper control of bone mass. Postnatal conditional knockout of Adar1 (the gene encoding ADAR1) resulted in a severe osteopenic phenotype. Ablation of the Adar1 gene significantly suppressed osteoblast differentiation without affecting osteoclast differentiation in bone. In vitro deletion of the Adar1 gene decreased expression of osteoblast-specific osteocalcin and bone sialoprotein genes, alkaline phosphatase activity, and mineralization, suggesting a direct intrinsic role of ADAR1 in osteoblasts. ADAR1 regulates osteoblast differentiation by, at least in part, modulation of osterix expression, which is essential for bone formation. Further, ablation of the Adar1 gene decreased the proliferation and survival of bone marrow stromal cells and inhibited the differentiation of mesenchymal stem cells towards osteoblast lineage. Finally, shRNA knockdown of the Adar1 gene in MC-4 pre-osteoblasts reduced cyclin D1 and cyclin A1 expression and cell growth. Our results identify ADAR1 as a new key regulator of bone mass and suggest that ADAR1 functions in this process mainly through modulation of the intrinsic properties of osteoblasts (i.e., proliferation, survival and differentiation).

Deletion of the RNA‐editing enzyme ADAR1 causes regression of established chronic myelogenous leukemia in mice

Patients with chronic myelogenous leukemia (CML) respond well to tyrosine kinase inhibitors (TKIs) of the Bcr-Abl oncoprotein. However, intolerance and resistance to these agents remains a challenge, and TKIs are unable to eradicate rare leukemia-initiating cells. Leukemia treatment would benefit from a better understanding of molecular signals that are necessary for the survival of leukemia-initiating cells but dispensable for normal hematopoietic stem cells. Leukemia-initiating cells in CML can arise from myeloid progenitor cells, a population that we have reported in normal hematopoiesis to depend on the RNA-editing enzyme adenosine deaminase acting on RNA-1 (ADAR1). We now report that Bcr-Abl transformed leukemic cells were ADAR1-dependent in a conditional ADAR1 knockout mouse model. ADAR1 deletion reversed leukocytosis and splenomegaly, and preferentially depleted primitive Lin-Sca+Kit+ (LSK) leukemic cells but not LSK cells lacking the leukemic oncoprotein. ADAR1 deletion ultimately normalized the peripheral white blood count, eliminating leukemic cells as assessed by PCR. These results uncover a novel requirement for ADAR1 in myeloid leukemic cells and indicate that ADAR1 may comprise a new molecular target for CML-directed therapeutics.

Dyschromatosis symmetrica hereditaria

Dyschromatosis symmetrica hereditaria (DSH) is a rare pigmentary genodermatosis, which is acquired by autosomal dominant inheritance with high penetrance. Most cases of this condition have been reported from East Asian countries, including Japan, China and Taiwan. Its symptoms are mixed hyper- and hypopigmented macules on the dorsal aspect of the hands and feet and freckle-like macules on the face. The gene responsible for DSH has been identified as adenosine deaminase acting on RNA1 (ADAR1). The ADAR1 protein catalyzes the transformation of adenosine to inosine in dsRNA substrates (so-called A-to-I editing) and is involved in various activities, such as viral inactivation, structural change of the protein and the resultant cell survival. However, its function in the skin and role in the development of DSH are still unknown. To date, more than 100 mutations of ADAR1 have been reported in patients with DSH, and the catalytic domain deaminase is believed to be crucial to the activities of this gene. Some complications of DSH have been reported and, intriguingly, several patients have been reported to develop neurological symptoms, such as dystonia and mental deterioration. Because ADAR1 plays various important roles in human tissue, we believe that a clarification of the pathogenesis of DSH will promote the understanding of the physiological functions of ADAR1, which will have significant scientific implications.

Adenosine Deaminase Acting on RNA 1 (ADAR1) Suppresses the Induction of Interferon by Measles Virus

ADAR1, the interferon (IFN)-inducible adenosine deaminase acting on RNA, catalyzes the C-6 deamination of adenosine (A) to produce inosine (I) in RNA substrates with a double-stranded character. Because double-stranded RNA is a known inducer of IFN, we tested the role of ADAR1 in IFN induction following virus infection. HeLa cells made stably deficient in ADAR1 (ADAR1(kd)) were compared to vector control (CON(kd)) and protein kinase PKR-deficient (PKR(kd)) cells for IFN-β induction following infection with either parental (wild-type [WT]) recombinant Moraten vaccine strain measles virus (MV) or isogenic knockout mutants deficient for either V (V(ko)) or C (C(ko)) protein expression. We observed potent IFN-β transcript induction in ADAR1(kd) cells by all three viruses; in contrast, in ADAR1-sufficient CON(kd) cells, only the C(ko) mutant virus was an effective inducer and the IFN-β RNA induction was amplified by PKR. The enhanced IFN-β transcript-inducing capacity of the WT and V(ko) viruses seen in ADAR1-deficient cells correlated with the enhanced activation of PKR, IFN regulatory factor IRF3, and activator of transcription ATF2, reaching levels similar to those seen in C(ko) virus-infected cells. However, the level of IFN-β protein produced was not proportional to the level of IFN-β RNA but rather correlated inversely with the level of activated PKR. These results suggest that ADAR1 functions as an important suppressor of MV-mediated responses, including the activation of PKR and IRF3 and the induction of IFN-β RNA. Our findings further implicate a balanced interplay between PKR and ADAR1 in modulating IFN-β protein production following virus infection.

Inosine-Containing RNA Is a Novel Innate Immune Recognition Element and Reduces RSV Infection

During viral infections, single- and double-stranded RNA (ssRNA and dsRNA) are recognized by the host and induce innate immune responses. The cellular enzyme ADAR-1 (adenosine deaminase acting on RNA-1) activation in virally infected cells leads to presence of inosine-containing RNA (Ino-RNA). Here we report that ss-Ino-RNA is a novel viral recognition element. We synthesized unmodified ssRNA and ssRNA that had 6% to16% inosine residues. The results showed that in primary human cells, or in mice, 10% ss-Ino-RNA rapidly and potently induced a significant increase in inflammatory cytokines, such as interferon (IFN)-β (35 fold), tumor necrosis factor (TNF)-α (9.7 fold), and interleukin (IL)-6 (11.3 fold) (p<0.01). Flow cytometry data revealed a corresponding 4-fold increase in influx of neutrophils into the lungs by ss-Ino-RNA treatment. In our in vitro experiments, treatment of epithelial cells with ss-Ino-RNA reduced replication of respiratory syncytial virus (RSV). Interestingly, RNA structural analysis showed that ss-Ino-RNA had increased formation of secondary structures. Our data further revealed that extracellular ss-Ino-RNA was taken up by scavenger receptor class-A (SR-A) which activated downstream MAP Kinase pathways through Toll-like receptor 3 (TLR3) and dsRNA-activated protein kinase (PKR). Our data suggests that ss-Ino-RNA is an as yet undescribed virus-associated innate immune stimulus.

Identification of two novel splice mutations of the ADAR1 gene in two Chinese families with dyschromatosis symmetrica hereditaria

Dyschromatosis symmetrica hereditaria (DSH) is a rare, autosomal dominant dermatosis, characterized by a mixture of hyperpigmented and hypopigmented macules on the dorsa of the hands and feet. The DSH locus has been mapped to chromosome 1q21, and in 2003, pathogenic mutations were identified in the ADAR1 (adenosine deaminase acting on RNA1) gene. In this study, we performed mutation detection of the ADAR1 gene in two Chinese families with DSH. PCR and direct sequencing of the ADAR1 gene were used to identify and confirm the mutations in the two families. Furthermore, we analysed the RNA transcripts by reverse transcriptase (RT)-PCR. Two aberrant splice products were confirmed with RT-PCR and DNA direct sequence analysis. These novel findings further extend our understanding of the role of ADAR1 in DSH.

Increased cortical expression of an RNA editing enzyme occurs in major depressive suicide victims.

RNA editing is a posttranscriptional process which critically modulates the function of several neurotransmitter receptors regulating mood, anxiety, learning, and memory. Data from several postmortem studies have shown increased 5-hydroxytryptamine-2C receptor RNA editing in mood disorders and suicide, and therefore the 5-hydroxytryptamine-2C receptor might be expected to have reduced signal transduction in these patients. In this study, we have tested the hypothesis that the expression levels of the enzymes which catalyze RNA editing, adenosine deaminase acting on RNA 1 (ADAR1) and ADAR2, are also abnormal in suicide. Gene expression was measured in the dorsolateral prefrontal cortex of individuals from the Stanley Consortium Brain series, which includes patients with schizophrenia (n=15), major depression (n=15), bipolar disorder (n=15), and a comparison group (n=14). Of the psychiatric patients, 20 were suicide victims. ADAR1 expression was found to be significantly increased in major depressive suicide victims compared with patients who did not commit suicide. Neither ADAR1 nor ADAR2 expression was altered in any of the other diagnostic groups. These data indicate that ADAR1 could play a role in the pathophysiology of suicide in patients with major depression.

The c.3463C>T mutation of the ADAR1 gene in patients with dyschromatosis symmetrical hereditaria

To analyze the mutation of the adenosine deaminase acting on RNA 1 (ADAR1) gene in a pedigree with dyschromatosis symmetrical hereditaria (DSH). Mutation analysis of the ADAR1 gene was carried out by PCR and direct DNA sequencing in the DSH family, as well as in 50 unrelated healthy controls. A missense mutation of c.3463C>T, which results in p.R1155W in the ADAR1 protein, was found in the 2 patients, but was absent in the 2 healthy members in the family and 50 unrelated individuals. A missense mutation of c.3463C>T in the ADAR1 gene was detected in the DSH family, which is likely responsible for the pathogenesis of the disease.

Conversion of Adenines to Inosines in RNA During Virus Infections Is an Innate Immune Stimulus (89.10)

Abstract Inflammatory responses that are generated during viral infections are critical for protective anti-viral immune responses. The exact viral-derived structures (pathogen-associated molecular patterns, PAMPs) that activate uninfected cells to induce pro-inflammatory signals are not completely characterized. To date, the viral PAMPs that are identified include double-stranded RNA (dsRNA) and 5'-triphosphate-containing single-stranded RNA (ssRNA). During viral infections, the cellular enzyme adenosine deaminase acting on RNA-1 (ADAR-1) catalyzes conversion of adenines to inosines, which is a pathway for blocking viral replication. ADAR-1 activation in the infected cells leads to the presence of inosine-containing viral and cellular RNA (Ino-RNA). Here, we report that Ino-RNA from infected cells is a stimulus for inflammatory innate immune response. Treatment of primary human bronchial epithelial (PHBE) cells, primary human alveolar macrophages or C57BL/J6 mice with in vitro synthesized Ino-RNA rapidly and potently induced several markers of innate immune response such as IFN-β, TNF-α, IL-6 and RANTES. We further identified increased secondary structures in ssRNA upon conversion of adenines to inosines. The increased secondary structures activated the dsRNA-specific binding protein PKR. In addition, Ino-RNA activated the MAP Kinase pathways, which is downstream of PKR. Our results suggest that Ino-RNA is a novel viral innate immune recognition element.

Conversion of Adenines to Inosines in RNA During Virus Infections Activates Innate Immune Responses

Innate immune responses generated during viral infections are critical for protective anti‐viral immunity. The exact viral‐derived structures (also known as pathogen‐associated molecular patterns, PAMPs) that activate uninfected cells to induce activation signals are not completely characterized. To date, the viral PAMPs that are identified include dsRNA and 5′ triphosphate ssRNA, which are recognized by several cellular pathways such as SR‐A, PKR, RIG‐I, TLR‐3 and MAVS. During viral infections, the cellular enzyme ADAR‐1 (adenosine deaminase acting on RNA‐1) catalyzes conversion of adenines to inosines (A‐I editing), which is a pathway for blocking viral replication. ADAR‐1 activation in the infected cells leads to the presence of inosine‐containing viral and cellular RNA (Ino‐RNA). Here, we provide in vitro and in vivo data to affirm our hypothesis that the release of Ino‐RNA from infected cells and its interaction with surrounding uninfected cells is a potent stimulus for inflammatory innate immune responses. The innate response was confirmes by rapidly and potently induction of IFN‐b, TNF‐a, IL‐6 and RANTES. The A‐I editing increased secondary structures of RNA which resulted in activation of the dsRNA‐specific protein kinase (PKR). Our data uncover a novel strategy for uninfected host cells to detect viral infections in the surrounding tissue.

Mutational spectrum of the ADAR1 gene in dyschromatosis symmetrica hereditaria

Dyschromatosis symmetrica hereditaria (DSH) is a rare autosomal dominant cutaneous disorder characterized by a mixture of hyperpigmented and hypopigmented macules of various sizes on the extremities and caused by the mutations of adenosine deaminase acting on RNA1 (ADAR1) gene. We screened 14 unrelated families or sporadic cases for mutation in the full coding sequence of this gene. Eight novel heterozygous mutations of ADAR1 and four known mutations were identified, including four missense mutations (p.R26K, p.Y1192D, p.R916Q, p.R1155W), six frameshift mutations (p.N205fsX217, p.V211fsX217, p.V404fsX417, p.I914fsX927, p.L1053fsX1076, p.L1070fs1092), and two nonsense mutations (p.R474X, p.R1096X). Interestingly, we failed to detect any mutations of ADAR1 in one family. Including our data, there are now 93 different mutations reported in 105 independent patients that we have tabulated. From the review of clinical features in these reports, we found that the same mutation could lead to different phenotypes even in the same family and did not establish a clear correlation between genotypes and phenotypes. Finally this study is useful for functional studies of the protein and to define a diagnostic strategy for mutation screening of the ADAR1 gene.

A missense mutation c.G2747A (p.R916Q) of ADAR1 gene in dyschromatosis symmetrica hereditaria is not a novel mutation

Dyschromatosis symmetrica hereditaria (DSH: OMIM no. 127400) is a rare pigmentary genodermatosis predominantly seen in Asia, including Japan, Taiwan and China. It is inherited in autosomal dominant manner and has high penetrance. Its clinical manifestations are intermingled hypoand hyperpigmented macules on the dorsal aspects of hands, feet and the face, and they appear in infancy or early childhood [3]. We have shown that adenosine deaminase acting on RNA1 (ADAR1), also known as the double-stranded RNAspeciWc adenosine deaminase (DSRAD), is the responsible gene for DSH [1] and around 90 of various mutations have been reported. We have been making the database of ADAR1 gene mutations reported to date. Figure 1 shows the schematic mutations. We have read an article described by Xu et al. [4] that they reported two “novel” mutations of ADAR1, c.G2747A leading to p.R916Q in exon 9 and c.C3124T leading to p.R1042C in exon 12, respectively. The latter c.C3124T (p.R1042C) is certainly a novel mutation; however, former mutation c.G2747A (p.R916Q) has already been reported by our group [2]. We would be very grateful if the editor will check them and let the author correct the statement. We really appreciate your kindly cooperation. Although many mutations of ADAR1 have been detected, the pathogenesis and mechanism of onset of DSH have not been elucidated yet. Further investigations are needed to clarify them.

An Essential Role of the RNA Editing Enzyme ADAR1 in T Cell Development.

Abstract 917Post-transcriptional regulation such as RNA editing in hematopoiesis and lymphopoiesis is poorly understood. ADAR1 (adenosine deaminase acting on RNA-1) is a RNA editing enzyme essential for embryonic development. Disruption of the ADAR1 gene was shown to cause defective embryonic hematopoiesis (Wang Q et al, Science 2000). Moreover, we have recently obtained direct evidence for the preferential effect of ADAR1 deletion on adult hematopoietic progenitor cells as opposed to the more primitive cells via a RNA-editing dependent mechanism by different conditional gene deletion strategies (Xufeng R et al PNAS 2009, in press). To further determine the role of ADAR1 in T cell development, we generated a mouse model in which ADAR1 was deleted specifically in T lymphocytes by interbreeding ADAR1lox/lox mice with Lck-Cre transgenic mice. In our current study, we report that ADAR1 is essential for T cell differentiation at the late progenitor stage in the thymus, coincident with T cell receptor-α/β expression. In ADAR1lox/loxLck-Cre mice, mature T cells decreased dramatically in peripheral blood, spleen and lymph nodes in comparison to littermate controls. In the thymus, the production of CD4+/CD8+ double positive cells was severely impaired and massive cell death was observed in pre-T cell populations. Within the pro-T cells, ADAR1 deletion resulted in a significant decrease of late progenitor cells but not early progenitor subsets. In both pro-T and pre-T cell stages, defective T cell development preferentially occurred in the beta chain positive cells, but was not apparent in gamma/delta T cells. Our data demonstrated an indispensable role of ADAR1 in early T cell differentiation that correlated with T cell receptor beta chain expression, thereby indicating that RNA editing by ADAR1 is an essential event in T cell development. Disclosures:No relevant conflicts of interest to declare.

ADAR1 is required for hematopoietic progenitor cell survival via RNA editing

Adenosine Deaminase Acting on RNA 1 (ADAR1) is an RNA-editing enzyme that converts adenosine to inosine, following RNA transcription. ADAR1's essential role in embryonic development, especially within the hematopoietic lineage, has been demonstrated in knock-out mice. However, a specific role for ADAR1 in adult hematopoietic progenitor cells (HPCs) remains elusive. In this report, we show that ADAR1 is required for survival of differentiating HPCs as opposed to more primitive cells in adult mice by multiple strategies targeting floxed ADAR1 for deletion by Cre recombinase. As a consequence, ADAR1-deficient hematopoietic stem cells (HSCs) were incapable of reconstituting irradiated recipients although being phenotypically present in the recipient bone marrow. While an effect on HSCs cannot be completely ruled out, the preferential effect of ADAR1 absence on HPCs over more primitive hematopoietic cells was consistent with the increased expression of ADAR1 within HPCs, as well as the inability of ADAR1-deficient HPCs to form differentiated colonies and increased apoptotic fraction during ex vivo culture. Moreover, we have obtained direct evidence that ADAR1 functions in HPCs via an RNA-editing dependent mechanism. Therefore, ADAR1 plays an essential role in adult hematopoiesis through its RNA editing activity in HPCs.

Inosine-Containing ssRNA Is a Novel Viral-Associated Immune Recognition Element (43.3)

Abstract Inflammatory responses that are generated during viral infections are critical for protective anti-viral immune responses. The exact viral-derived structures (also known as pathogen-associated molecular patterns, PAMPs) that activate uninfected cells to induce pro-inflammatory signals are not completely characterized. To date, the viral PAMPs that are identified include dsRNA and 5' triphosphate ssRNA, which are recognized by several cellular pathways such as SR-A, PKR, RIG-I, TLR-3 and MAVS. During viral infections, the cellular enzyme ADAR-1 (adenosine deaminase acting on RNA-1) catalyzes conversion of adenines to inosines, which is a pathway for blocking viral replication. ADAR-1 activation in the infected cells leads to the presence of inosine-containing viral and cellular RNA (Ino-RNA). Here, we provide in vitro and in vivo data to affirm our hypothesis that the release of Ino-RNA from infected cells and its interaction with surrounding uninfected cells is a potent stimulus for inflammatory innate immune responses. Treatment of primary human bronchial epithelial (PHBE) cells or C57BL/J6 with in vitro synthesized Ino-RNA rapidly and potently induced several markers of innate immune response such as IFN-β, TNF-a, IL-6 and RANTES. Our data uncovers a novel strategy for uninfected host cells to detect viral infections in the surrounding tissue.

Six novel mutations of the ADAR1 gene in Chinese patients with dyschromatosis symmetrica hereditaria

Dyschromatosis symmetrica hereditaria (DSH) is a rare autosomal dominantly inherited dermatosis and characterized by a mixture of hyperpigmented and hypopigmented macules on the back of hands and feet. The DSH locus was mapped to chromosome 1q21 and subsequently pathogenic mutations were identified in the adenosine deaminase acting on RNA1 (ADAR1) gene in 2003. In this study, we performed a mutation analysis of the ADAR1 gene in eight Chinese families and one sporadic patient with typical DSH. PCR and direct sequencing of the ADAR1 gene were performed to identify and confirm the mutations in the eight families and the sporadic patient. Six novel and one known mutations were identified, including four missense mutations (p.K1105N, p.G1047R, p.F1099L, p.G1068R), two frameshift mutations (p.Q779fs-792x, p.P441fs-463x) and one nonsense mutation (p.R1096x). Six novel mutations were found in five unrelated families and one sporadic case, which have further improved our understanding on the role of ADAR1 in DSH. Interestingly, we failed to detect any mutations of ADAR1 in two families.

ADENOSINE DEAMINASE ACTING ON RNA 1 ACCELERATES CELL CYCLE THROUGH INCREASED TRANSLATION AND ACTIVITY OF CYCLIN-DEPENDENT KINASE 2

Adenosine-to-inosine RNA editing has been recently implicated in the pathogenesis of inflammation through the upregulation of the editase adenosine deaminase acting on RNA 1 (ADAR1). Because cell proliferation is a key feature of the inflammatory process, the present study tested the hypothesis that overexpression of ADAR1 accelerates cell cycle. To that end, human embryonic kidney 293 cells were transiently transfected with ADAR1 or vector, and cell cycle was evaluated by fluorescence-activated cell sorter. Overexpression of wild-type ADAR1 decreased the proportion of G0-G1 cells (-19%, P<0.01, n=3), increased the percentage of S phase cells (+19%, P<0.01, n=3), and did not change the ratio of cells residing in the G2-M phase (n=3). This finding was supported by three observations. First, there was a parallel production in ADAR1-transfected cells of cyclin-dependent kinase (Cdk) 2 and cyclin A, a pivotal protein complex upregulated at the G1-S phase checkpoint, and of [p]-Histone H1, a marker of Cdk2 activity (+102%, P<0.01, n=3). Second, ADAR1-transfected cells displayed higher activity of the proliferation marker, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide. Third, using anti-ADAR1 antibody, direct binding of ADAR1 to Cdk2 messenger RNA was demonstrated in ADAR1-transfected cells by protein-RNA cross-linking and immunoprecipitation (+974%, P<0.01, n=3). Finally, causal relationships between ADAR1 and Cdk2 were confirmed by a study with the Cdk2 inhibitor, kenpaullone, which prevented the ADAR1-induced shift from the G0-G1 to the S phase. Taken together, these data show that ADAR1 increases cell cycle by shifting cells from the G0-G1 to the S phase through the upregulation of Cdk2.

Identification of Novel Ligands for the Z-DNA Binding Protein by Structure-Based Virtual Screening

We describe the first discovery of small molecules that bind to the Z-DNA binding domain of human ADAR1 (Adenosine Deaminase Acting on RNA 1) by structure-based virtual screening of chemical database. These molecules bind to Z-DNA binding domain to inhibit the interaction with the Z-DNA. Many viruses have Z-DNA binding proteins, which are structurally similar to Z-DNA binding domain of human ADAR1, and the ability of Z-DNA binding protein to bind the Z-DNA is essential for their pathogenicity. Therefore, the molecules identified in this study may serve as novel leads for the design of agents that inhibit biological functions of those pathogenic viruses.

The large form of ADAR 1 is responsible for enhanced hepatitis delta virus RNA editing in interferon‐α‐stimulated host cells

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.

Liver Disintegration in the Mouse Embryo Caused by Deficiency in the RNA-editing Enzyme ADAR1

ADAR1 (adenosine deaminase acting on RNA-1) is widely expressed in mammals, but its biological role is unknown. We show here by gene targeting that ADAR1 selectively edits in vivo two of five closely spaced adenosines in the serotonin 5-hydroxytryptamine subtype 2C receptor pre-mRNA of nervous tissue; and hence, site-selective adenosine-to-inosine editing is indeed a function of ADAR1. Remarkably, homozygosity for two different null alleles of ADAR1 caused a consistent embryonic phenotype appearing early at embryonic day 11 and leading to death between embryonic days 11.5 and 12.5. This phenotype manifests a rapidly disintegrating liver structure, along with severe defects in definitive hematopoiesis, encompassing both erythroid and myeloid/granuloid progenitors as well as spleen colony-forming activity from the aorta-gonad-mesonephros region and fetal liver. Probably as a consequence of these developmental impairments, ADAR1-deficient embryonic stem cells failed to contribute to liver, bone marrow, spleen, thymus, and blood in adult chimeric mice. Thus, ADAR1 subserves critical steps in developing non-nervous tissue, which are likely to include transcript editing.