Mutagenic bypass of uracil-derived abasic sites underlies the SBS17 mutational signature

Understanding how mutations arise is central to predicting and preventing cancer, yet most of the mutational signatures found in cancer genomes still have no known mechanistic cause. Among these, single-base-substitution signature 17 (SBS17) is the dominant mutational process in gastric and oesophageal adenocarcinoma. SBS17 has been linked to oxidative damage and to the chemotherapeutic agent 5-fluorouracil (5-FU), but the molecular events that generate it are unknown. Here we demonstrate that SBS17 causes driver mutations in gastrointestinal cancer. We then combine genetics in cell lines and organoids with whole-genome sequencing to define the mechanistic aetiology of SBS17 mutagenesis. We disprove the prevailing oxidative damage hypothesis and instead show that SBS17 arises through the misincorporation of dUTP during DNA replication, followed by uracil excision by the glycosylase UNG and mutagenic bypass of the resulting abasic (AP) sites by the translesion synthesis machinery. Importantly, we reveal that the same mechanism underlies both spontaneous and chemotherapy-induced mutations. Together, our results resolve the origin of gastric mutagenesis, opening therapeutic avenues for curbing ongoing mutagenesis, tumour evolution and drug resistance in gastrointestinal cancers.