ADAR1 Za domain P195A mutation activates the MDA-5 dependent RNA-sensing signaling pathway in the brain enhanced by haploinsufficiency without overall RNA editing level decrease.
The A-to-I RNA editing enzyme ADAR1 critically regulates the cellular RNA sensing signaling pathway, as the RNA-sensing signaling pathway recognizes unedited RNAs as “non-self” to activate the innate immune response. Mutations of ADAR1 cause severe inflammatory tissue injury, as shown in Aicardi-Goutières syndrome (AGS), in which severe inflammation occurs in the brain. The most frequent ADAR1 mutation in AGS is the P193A mutation in the Za domain, which exists in the AGS families coupling with an additional ADAR1 mutation. How this mutation alters the cellular RNAs leading to innate immune activation, has not been fully understood. This study analyzed RNA editing status and innate immune activation in the brains of a mouse model that carries the human ADAR1 P193A-equivalent mouse P195A mutation. We found that this mutation alone can cause excessive ISG expression in the brains, especially in the periventricular areas reflecting the pathologic feature of AGS. Furthermore, we found that ADAR1 P195A mutation did not affect the RNA editing activity in the overall RNA editing level; however, it leads to ISG expression in a dose-dependent manner, causing severer innate immune activation in haploinsufficiency status, whereas wildtype ADAR1 is redundant to suppress innate immune activation. The finding from this study indicated that the intact Z-RNA binding activity of ADAR1 plays a critical role in suppressing self-cellular RNA sensing, and innate immune homeostasis requires a minimum Z-RNA binding capacity of ADAR1. Overall design: WT_WT, WT_d, 195M_195M, 195M_d, with three repeats each