Editing-independent effects of Drosophila Adar on heterochromatin silencing

ADAR RNA editing enzymes deaminate selected adenosines to inosines in dsRNA. In Drosophila, inosine in dsRNA inhibits cleavage by Dcr2, and some ADAR proteins contribute an additional, editing-independent inhibition. The Drosophila AdarG isoform, in particular, has been proposed to inhibit HP1-mediated heterochromatin silencing of repetitive sequences initiated by specific dsRNAs. To address the functions of AdarG, we overexpressed it from new UAS-Adar lines, under the control of a temperature-regulated Act5C^ts-GAL4 driver. Overexpression of the adult AdarG isoform or catalytically inactive AdarE374A led to larval lethality with some escaper pupae that show an ecdysone-related, head eversion defect. This indicates an editing-independent effect of high Adar expression. Pupae show aberrantly elevated innate immune and early ecdysone gene transcript expression, and no flies eclose. RNAi knockdown of ecdysone receptor A (EcRA) or increased expression of the histone H3K9me2,3-associated HP1 protein partially rescues AdarG overexpression defects and normalizes gene expression in rescued progeny flies. In other reports, Drosophila mutants with reduced HP1, or egg (SetDB1), Su(var)3-9 double mutants with reduced histone H3K9me2,3 also produce larvae with ecdysone-related and innate immune defects. We show that overexpressed AdarG inhibits histone H3K9me-mediated epigenetic silencing through an editing-independent effect, most likely at the dsRNA/Dcr2/Ago2 initiation stage.