Systematic Analysis of A-to-I RNA Editing Upon Releasing of ADAR from the Nucleolus

Adenosine-to-inosine (A-to-I) RNA editing, catalyzed by two isoforms of ADAR (p110 and p150) and ADARB1, is a critical regulatory step in gene expression. Intriguingly, ADAR p110 and ADARB1 are conspicuously enriched in the nucleolus, though the biological effects remain unclear. To investigate a putative role of nucleolar enrichment of ADAR, we progressively released it from the nucleolus into the nucleoplasm, by treating cells briefly with low doses of actinomycin D known to disassemble the nucleolus. Deep sequencing of the transcriptome revealed that as ADAR dissociated from the nucleolus, RNA editing increased significantly, with sharp rises in both the number of edited sites and editing frequency. This co-transcriptional editing, predominantly in intronic regions, disrupted pre-mRNA splicing, causing exon skipping and retention that remodeled gene expression. These findings suggest that nucleolar localization of ADAR serves to restrain its activity, preventing excessive editing that could lead to splicing errors and cellular dysfunction.