NAT10 and N4-Acetylcytidine restrain R-loop levels and related inflammatory responses
N4-acetylcytidine (ac4C) is an evolutionarily conserved RNA modification that is deposited on diverse RNAs by N-acetyltransferase 10 (NAT10), a protein with high biological significance for aging and cancer. Here, we performed a comprehensive survey of ac4C using metabolic labeling, sodium cyanoborohydride chemical treatment coupled to next generation sequencing (NGS) and ac4C antibody-based cell and molecular biology techniques. Our NGS analysis confirms robust ac4C modification of rRNA and specific tRNA species in a NAT10-dependent manner, but suggests low or spurious ac4C acetylation in mRNA. Analysis of RNA-seq data also revealed an induction of inflammatory responses as well as mutagenesis at transcriptionally active sites in NAT10-KO cells. This finding led us to further explore the role of NAT10 in R-loops, which have recently been shown to induce APOBEC3B-mediated mutagenesis. Our analysis revealed that R-loops are modified with ac4C in a NAT10-dependent manner. Furthermore, NAT10 restrains the levels of R-loops at a subset of differentially expressed genes in a manner dependent on its catalytic activity. Together with cellular biology data showing ac4C containing RNA in endosomal structures, we propose that increased levels of ac4C-unmodified RNAs, likely derived from R-loops, in endosomal structures induce inflammatory responses. Overall design: NAT10 knockout and NaCNBH3 treatment were applied individually and jointly to determine their effects on ac4C and gene expression. RNA was profiled from two replicates in each of four total conditions, and either processed for normal RNA-seq or further extracted to retrieve tRNAs. These were also sequenced for a total of 16 samples.