APOBEC3A, not APOBEC3B, drives deaminase mutagenesis in human gastric epithelium

Cancer genomes frequently carry APOBEC (apolipoprotein B mRNA editing catalytic polypeptide-like)-associated DNA mutations, suggesting APOBEC enzymes as innate mutagens during cancer initiation and evolution. However, the pure mutagenic impacts of the specific enzymes among this family that are responsible for APOBEC-associated mutagenesis remain unclear in human normal cell lineage, particularly the comparative mutagenic activities of APOBEC3A and APOBEC3B. Here, we investigated the mutagenic contributions of these enzymes through whole-genome sequencing of human normal gastric organoid lines carrying doxycycline-inducible APOBEC3A or APOBEC3B cassettes. Our findings demonstrated that transcriptional APOBEC3A upregulation led to the acquisition of a massive number of genomic mutations in a few cell cycles. By contrast, APOBEC3B upregulation did not generate a substantial number of mutations in gastric epithelium. APOBEC3B-associated mutagenesis remained insignificant even after a combined inactivation of TP53. Based on the spectrum of acquired mutations after APOBEC3A upregulation, we further analyzed APOBEC3A-associated mutational signatures, encompassing indels mainly composed of 1bp deletions, characteristics of clustered mutations, and selective pressures operative on cells carrying the mutations. Our observations provide a clear foundation for understanding the mutational impact of APOBEC enzymes in human cells.