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

Abstract

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.

To “Z” or not to “Z”: Z-RNA, self-recognition, and the MDA5 helicase

“…The further analysis of p150 biology in mice with Zα loss of function variants was confounded by the presence of p110 that edits many substrates in common with p150 as expected from their shared dsRNA and deaminase domains [37][38][39]46]. In a technical tour de force, Kim and colleagues succeeded in creating a mouse that expressed only p150, but with no detectable p110 [47]. They did this by knocking out the p110 constitutive promoters and the associated exons 1b and 1c, while leaving the p150 exon 1a and the p150 interferon promoter intact.…”

The Simple Biology of Flipons and Condensates Enhances the Evolution of Complexity

“…With ADAR1, Zα and another structure-specific, double-stranded RNA (dsRNA)-specific binding motif target the deaminase domain to modify adenosines in dsRNA, forming inosine that is treated by the downstream processes as the equivalent of guanosine. This process produces non-synonymous codon changes in a limited number of human substrates [ 47 ].…”

Special Issue: A, B and Z: The Structure, Function and Genetics of Z-DNA and Z-RNA

“…A number of themes emerged from the presentations and follow-up discussions. Based on presentations by Yukio Kawahara [ 11 ], Alan Herbert [ 12 ], Tony Sun [ 13 ] and Qingde Wang [ 14 ], the key role of ADAR1 p150, which contains the Z-DNA and Z-RNA binding Zα domain in regulating type I interferon responses in both human and mice, is now beyond doubt. The Kawahara lab has constructed a mouse model that expresses the ADAR p150 isoform that contains the Zα domain, but not the shorter p110 isoform.…”