Incision‐independent Repair of a Psoralen‐interstrand Crosslink

Abstract

DNA interstrand crosslinks (ICLs) are extremely cytotoxic because they prevent strand separation and thus interfere with DNA replication and transcription. ICLs are caused by endogenous metabolites, as well as chemotherapeutics such as cisplatin, nitrogen mustards, and psoralen. A major ICL repair pathway in metazoans is coupled with DNA replication and requires the Fanconi anemia pathway. Using Xenopus egg extracts, our lab previously described the mechanism of replication‐coupled cisplatin‐ICL repair. In this process, two DNA replication forks first converge at the ICL. Following CMG helicase dissociation, the ubiquitylated FANCI‐FANCD2 complex stimulates dual incisions on both sides of the ICL to unhook the lesion, creating a double‐strand break (DSB). Translesion DNA synthesis bypasses the lesion on one sister and the DSB in the other sister is repaired by inter‐sister recombination. Here, we investigated the repair of a psoralen‐ICL formed between two thymines. Strikingly, repair of this ICL does not require the FANCI‐FANCD2 complex and does not involve incision of the parental DNA surrounding the ICL. Instead, the ICL appears to be unhooked via cleavage of the glycosidic bond of one of the adducted thymines by an unknown glycosylase. This generates an abasic site on one parental strand while leaving a psoralen adduct on the other. No DSB is generated. Next, translesion DNA synthesis bypasses the lesions on both daughter molecules. As seen for cisplatin ICLs, the repair of psoralen ICLs requires replication fork convergence. Our data identify a novel pathway that uses DNA glycosylase activity to resolve an ICL in the absence of DSB formation. We are currently investigating which DNA glycosylase is involved in this reaction.