Requirement of the RNA-editing Enzyme ADAR2 for Normal Physiology in Mice

Abstract

ADAR2, an RNA editing enzyme that converts specific adenosines to inosines in certain pre-mRNAs, often leading to amino acid substitutions in the encoded proteins, is mainly expressed in brain. Of all ADAR2-mediated edits, a single one in the pre-mRNA of the AMPA receptor subunit GluA2 is essential for survival. Hence, early postnatal death of mice lacking ADAR2 is averted when the critical edit is engineered into both GluA2 encoding Gria2 alleles. Adar2 ؊/؊ /Gria2 R/R mice display normal appearance and life span, but the general phenotypic effects of global lack of ADAR2 have remained unexplored. Here we have employed the Adar2 ؊/؊ /Gria2 R/R mouse line, and Gria2 R/R mice as controls, to study the phenotypic consequences of loss of all ADAR2-mediated edits except the critical one in GluA2. Our extended phenotypic analysis covering ϳ320 parameters identified significant changes related to absence of ADAR2 in behavior, hearing ability, allergy parameters and transcript profiles of brain.The most common RNA modification in higher organisms is the enzymatic deamination of specific adenosines to inosine (A-to-I editing), wrought by members of the ADAR protein family (1). ADARs, short for adenosine deaminases acting on RNA, were discovered by their ability to convert in vitro in double-stranded RNA ϳ50% of the adenosines to inosines, thus destabilizing the double-stranded structure (2). Occurring in worms, flies and mammals, these enigmatic enzymes are now known to switch specific adenosines to inosine in mostly primary nuclear transcripts (3-6). A-to-I editing often recodes amino acids within the gene-encoded protein, based on reading inosine as guanosine by the translational machinery, and can remove or create splice acceptor sites (7), thus altering the pattern of RNA splicing. Moreover, editing in 5Ј and 3ЈUTRs and intronic sequence may lead to altered translational efficiency,

RNA editing of protein sequences: A rare event in human transcriptomes

“…Accordingly, known ADAR2 target sites, such as CYFIP2 (Nishimoto et al 2008;Riedmann et al 2008;Horsch et al 2011) or KCNA1 (Horsch et al 2011), showed no evidence of editing. Given the high expression level of ADAR, we expect A-to-G changes to occur frequently in our samples.…”

Reduced levels of protein recoding by A-to-I RNA editing in Alzheimer's disease

“…Three ADAR gene family members (ADAR1-3) have been identified in mammals. ADAR (ADAR1) and ADARB1 (ADAR2) can be detected in many tissues, while ADARB2 (ADAR3) seems to be restricted to the brain (Jacobs et al 2009;Horsch et al 2011;Savva et al 2012). It is believed that ADAR and ADARB1 are the main catalytic enzymes that are accountable for all A-to-I editing activity, while the third member, ADARB2 (ADAR3), lacks a catalytic domain and its function is yet unclear .…”

Single-cell transcriptomics reveals specific RNA editing signatures in the human brain

“…A-to-I deamination is catalyzed by members of the adenosine deaminase (ADAR) family of enzymes that act on dsRNA and occurs mainly in the primate-specific Alu repetitive elements (Nishikura 2016). ADARs are extremely important for the maintenance of cell homeostasis as mouse null mutants develop epileptic seizures and die several weeks after birth (Higuchi et al 2000;Horsch et al 2011). In addition, dysregulated RNA editing levels at specific recoding sites have been linked with a variety of diseases including neurological or psychiatric disorders and cancer (Chen et al 2013;Behm and Öhman 2016;Khermesh et al 2016).…”