RNA editing is the essential function of ADAR1 and, in the absence of MDA5, is dispensable for normal adult homeostasis [mmPCR-seq]

Adenosine-to-Inosine (A-to-I) editing of dsRNA by ADAR proteins is a pervasive feature of the epitranscriptome. There are estimated to be over 100 million potential A-to-I editing sites in humans and A-to-I editing can have varying consequences for gene expression. Whilst editing resulting in protein recoding defines the role of ADAR2, ADAR1 has been proposed to have both editing-dependent and -independent functions. The relative contribution of these putative functions to ADAR1 biology is unclear. We demonstrate that the absence of ADAR1-mediated editing is well tolerated when the cytosolic dsRNA sensor MDA5 is deleted. These mice have normal hematopoiesis, tissue patterning and life span. A direct comparison of the complete deletion of ADAR1 and the specific loss of A-to-I editing activity demonstrates that RNA editing is the only essential function of ADAR1 in adult mice. Therefore, preventing MDA5 substrate formation by endogenous RNA is the essential in vivo function of ADAR1-mediated editing. Microfluidics-based multiplex PCR and deep sequencing (mmPCR-seq) identification of A-to-I editing sites in 8 tissues from 12 week old mice in a E861A point mutant of ADAR on a MDA5 knockout background