Adar contributes to genome integrity by regulating R-loop homeostasis in Drosophila.

Adenosine deaminase acting on RNA (Adar) is a critical enzyme involved in post-transcriptional epigenetic regulation through adenosine-to-inosine (A-to-I) RNA editing. However, the biological role and regulatory mechanisms of Adar remain largely unknown. Using Drosophila as a model, we found that loss of Adar leads to spontaneous genome instability characterized by DNA damage and mitotic defects. Genome-wide ssDRIP-seq revealed global R-loop accumulation in Adar mutants, particularly at gene promoters, introns, and repetitive regions including telomeric retrotransposons. Notably, overexpression of RNase H1 (RNH1) suppressed R-loop accumulation and rescued genome instability in Adar-deficient flies. Strikingly, a catalytically inactive Adar mutant (E374A), which lacks A-to-I editing activity, retained its ability to suppress R-loop accumulation and preserve genome integrity. Our findings identify a novel editing-independent role of Adar in maintaining genome stability via regulation of R-loop homeostasis. This work highlights the evolutionarily conserved functions of Adar beyond RNA editing and establishes Drosophila as a valuable model to study R-loop-mediated genomic instability.