ADAR1 Regulation of Innate RNA Sensing in Immune Disease

Abstract

Detection of nucleic acids and production of type I interferons (IFNs) are principal elements of antiviral defense, but can cause autoimmune disease if dysregulated. Loss of function mutations in the human ADAR gene cause Aicardi-Goutières Syndrome (AGS), a rare and severe autoimmune disease that resembles congenitally acquired viral infection. Our lab and others defined ADAR1 as an essential negative regulator of an RNA-sensing pathway. Specifically, accumulation of endogenous ADAR1 RNA substrates within cells triggers type I IFN production through the anti-viral MDA5/MAVS pathway, highlighting the connection between innate antiviral responses and autoimmunity, with important implications for the treatment of AGS and related diseases. However, the mechanisms of MDA5-dependent disease pathogenesis in vivo remain unknown. Here, we introduce a knockin mouse that models the most common ADAR AGS mutation in humans. In defining this model we confirm that the unique z-alpha domain of ADAR1 is required, along with the deaminase domain, for MDA5 regulation. We establish that it is haploinsufficiency paired with an otherwise non-deleterious allele that drives disease, and may explain the dominance of this allele amongst the broader population. We used this new Adar-mutant mouse model to confirm several putative effectors of disease that results from ADAR1 loss of function. These mice develop lethal disease that requires MDA5, the RIG-I-like receptor LGP2, type I interferons, and the eIF2α kinase PKR. We additionally show that a small molecule inhibitor of the integrated stress response (ISR) that acts downstream of eIF2α phosphorylation prevents immunopathology, and rescues the mice from mortality. We also determined that haploinsufficiency is essential to the progression of disease, and demonstrate increased MDA5 signaling in p150 and Adar1 heterozygous mice. We describe a new set of mice that will allow in vitro and in vivo interrogation of the RNAs that activate MDA5 in the absence of sufficient ADAR1 editing. Our findings place PKR and the ISR as central components of immunopathology in vivo and identify new therapeutic targets for treatment of human diseases associated with the ADAR1-MDA5 axis, and shed light on how decreased ADAR1 activity may be leveraged for cancer treatments.