The cytidine deaminase APOBEC3A is required for large ribosomal subunit biogenesis

Cancer initiates as a consequence of genomic mutations, and its subsequent progression relies on increased production of ribosomes to maintain high levels of protein synthesis for unchecked cell growth. Recently, cytidine deaminases have been uncovered as sources of mutagenesis in cancer. To form more established connections between these two cancer driving processes, we interrogated the cytidine deaminase family of proteins for potential roles in human ribosome biogenesis. We identified and validated APOBEC3A and APOBEC4 as novel ribosome biogenesis factors through our laboratorys established screening platform for the discovery of regulators of nucleolar function in MCF10A cells. We show that APOBEC3A is required for cell cycle progression and global protein synthesis. More specifically, we highlight APOBEC3As role within the processing and maturation steps that form the large subunit 5.8S and 28S ribosomal (r)RNAs. Through an innovative nuclear RNA sequencing methodology, we identify candidate APOBEC3A C-to-U editing sites on the pre-rRNA and pre-mRNAs for the first time. Our work reveals the exciting possibility that the pre-rRNA can be edited during its maturation. More broadly, we found an additional function of APOBEC3A in cancer pathology, expanding its relevance as a target for cancer therapeutics.