Effective immunity requires the innate immune system to distinguish foreign nucleic acids from cellular ones. Cellular double-stranded RNAs (dsRNAs) are edited by the RNA-editing enzyme ADAR1 to evade being recognized as viral dsRNA by cytoplasmic dsRNA sensors, including MDA5 and PKR. The loss of ADAR1-mediated RNA editing of cellular dsRNA activates MDA5. Additional RNA-editing-independent functions of ADAR1 have been proposed, but a specific mechanism has not been delineated. We now demonstrate that the loss of ADAR1-mediated RNA editing specifically activates MDA5, whereas loss of the cytoplasmic ADAR1p150 isoform or its dsRNA-binding activity enabled PKR activation. Deleting both MDA5 and PKR resulted in complete rescue of the embryonic lethality of Adar1p150-/- mice to adulthood, contrasting with the limited or no rescue by removing MDA5 or PKR alone. Our findings demonstrate that MDA5 and PKR are the primary in vivo effectors of fatal autoinflammation following the loss of ADAR1p150. 

ADAR1p150 prevents MDA5 and PKR activation via distinct mechanisms to avert fatal autoinflammation.

The ability to sense and respond to infection is essential for life. Viral infection produces double-stranded RNAs (dsRNAs) that are sensed by proteins that recognize the structure of dsRNA. This structure-based recognition of viral dsRNA allows dsRNA sensors to recognize infection by many viruses, but it comes at a cost-the dsRNA sensors cannot always distinguish between "self" and "nonself" dsRNAs. "Self" RNAs often contain dsRNA regions, and not surprisingly, mechanisms have evolved to prevent aberrant activation of dsRNA sensors by "self" RNA. Here, we review current knowledge about the life of endogenous dsRNAs in mammals-the biosynthesis and processing of dsRNAs, the proteins they encounter, and their ultimate degradation. We highlight mechanisms that evolved to prevent aberrant dsRNA sensor activation and the importance of competition in the regulation of dsRNA sensors and other dsRNA-binding proteins. 

The competitive landscape of the dsRNA world.

Novel insights into double-stranded RNA-mediated immunopathology.

Adenosine deaminase acting on RNA 1 (ADAR1) converts adenosine to inosine in double-stranded RNA (dsRNA) molecules, a process known as A-to-I editing. ADAR1 deficiency in humans and mice results in profound inflammatory diseases characterised by the spontaneous induction of innate immunity. In cells lacking ADAR1, unedited RNAs activate RNA sensors. These include melanoma differentiation-associated gene 5 (MDA5) that induces the expression of cytokines, particularly type I interferons (IFNs), protein kinase R (PKR), oligoadenylate synthase (OAS), and Z-DNA/RNA binding protein 1 (ZBP1). Immunogenic RNAs 'defused' by ADAR1 may include transcripts from repetitive elements and other long duplex RNAs. Here, we review these recent fundamental discoveries and discuss implications for human diseases. Some tumours depend on ADAR1 to escape immune surveillance, opening the possibility of unleashing anticancer therapies with ADAR1 inhibitors. 

ADAR1: from basic mechanisms to inhibitors

Effective immunity requires the innate immune system to distinguish foreign (non-self) nucleic acids from cellular (self) nucleic acids. Cellular double-stranded RNAs (dsRNAs) are edited by the RNA editing enzyme ADAR1 to prevent their dsRNA structure pattern being recognized as viral dsRNA by cytoplasmic dsRNA sensors including MDA5, PKR and ZBP1. A loss of ADAR1-mediated RNA editing of cellular dsRNA activates MDA5. However, additional RNA editing-independent functions of ADAR1 have been proposed, but a specific mechanism has not been delineated. We now demonstrate that the loss of ADAR1-mediated RNA editing specifically activates MDA5, while loss of the cytoplasmic ADAR1p150 isoform or its dsRNA binding activity enabled PKR activation. Deleting both MDA5 and PKR resulted in complete rescue of the embryonic lethality of Adar1p150-/- mice to adulthood, contrasting with the limited or no rescue by removing MDA5, PKR or ZBP1 alone, demonstrating that this is a species conserved function of ADAR1p150. Our findings demonstrate that MDA5 and PKR are the primary in vivo effectors of fatal autoinflammation following the loss of ADAR1p150.

/pdf/adar1p150-prevents-mda5-and-pkr-activation-via-distinct-uiqqz7nn.pdf

ADAR1p150 Prevents MDA5 and PKR Activation via Distinct Mechanisms to Avert Fatal Autoinflammation

The innate immune system detects viral infection via pattern recognition receptors and induces defense reactions such as production of type I interferon1. One such receptor, MDA5, is activated upon the recognition of double-stranded RNAs (dsRNAs) that are often produced during viral replication2. Endogenous dsRNAs evade MDA5 activation through RNA editing by ADAR1, thus preventing autoimmunity3-5. Among the large number of endogenous dsRNAs, the key substrates whose editing is critical to evade MDA5 activation (termed as immunogenic dsRNAs) remain elusive. Here we reveal the identity of human immunogenic dsRNAs, a surprisingly small fraction of all cellular dsRNAs, to fill the gap in the ADAR1-dsRNA-MDA5 axis. We found that, in contrast to previous findings6,7, the immunogenic dsRNAs were highly enriched in mRNAs and depleted of introns, an expected indication of bona fide substrates of cytosolic MDA5. The immunogenic dsRNAs, in contrast to non-immunogenic dsRNAs, tended to have shorter loop between the stems, which may facilitate dsRNA formation. They also tended to be enriched at the GWAS signals of common inflammatory diseases, implying that they are truly immunogenic. We validated the MDA5-dependent immunogenicity of the dsRNAs, which was dampened following ADAR1-mediated RNA editing. We anticipate that a focused analysis of immunogenic dsRNAs will greatly facilitate the understanding and treatment of cancer and inflammatory diseases in which the important roles of dsRNA editing and sensing continue to be revealed8-13.

https://www.biorxiv.org/content/biorxiv/early/2022/08/29/2022.08.29.505707.full.pdf

A Small Subset of Cytosolic dsRNAs Must Be Edited by ADAR1 to Evade MDA5-Mediated Autoimmunity

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A Small Subset of Cytosolic dsRNAs Must Be Edited by ADAR1 to Evade MDA5-Mediated Autoimmunity