Xpf and not the Fanconi anaemia proteins or Rev3 accounts for the extreme resistance to cisplatin in Dictyostelium discoideum

Organisms like Dictyostelium discoideum, often referred to as DNA damage “extremophiles”, can survive exposure to extremely high doses of radiation and DNA crosslinking agents. These agents form highly toxic DNA crosslinks that cause extensive DNA damage. However, little is known about how Dictyostelium and the other “extremophiles” can tolerate and repair such large numbers of DNA crosslinks. Here we describe a comprehensive genetic analysis of crosslink repair in Dictyostelium discoideum. We analyse three gene groups that are crucial for a replication-coupled repair process that removes DNA crosslinks in higher eukarya: The Fanconi anaemia pathway (FA), translesion synthesis (TLS), and nucleotide excision repair. Gene disruption studies unexpectedly reveal that the FA genes and the TLS enzyme Rev3 play minor roles in tolerance to crosslinks in Dictyostelium. However, disruption of the Xpf nuclease subcomponent results in striking hypersensitivity to crosslinks. Genetic interaction studies reveal that although Xpf functions with FA and TLS gene products, most Xpf mediated repair is independent of these two gene groups. These results suggest that Dictyostelium utilises a distinct Xpf nuclease-mediated repair process to remove crosslinked DNA. Other DNA damage–resistant organisms and chemoresistant cancer cells might adopt a similar strategy to develop resistance to DNA crosslinking agents.

Organisms are constantly exposed to environmental and endogenous molecules that chemically modify the DNA in their genomes. A particularly pernicious chemical modification is when the two strands of DNA are crosslinked. These crosslinks must be removed so that genomes can be copied, and the damage caused by their persistence is often exploited in cancer chemotherapy. It is also no surprise that all organisms have developed effective means to remove these lesions, and work in prokaryotes and eukaryotes has shown that crosslinks are removed by the concerted action of certain DNA repair pathways. Whilst the obvious route of accumulating crosslinks is by exposure to anti-cancer drugs, these lesions may also arise spontaneously in DNA. This could be why inherited inactivation of one of the crosslink repair pathways results in the catastrophic human illness Fanconi anaemia. Here we determine how the social amoeba Dictyostelium discoideum, an organism that is unusually resistant to DNA-damaging agents, removes crosslinks. Our results indicate that this organism has evolved a distinct strategy to remove these lesions. More specifically, we discover that a particular nuclease subcomponent removes the crosslinks by a distinct repair process. We postulate that this strategy to remove crosslinks could be used by other DNA damage–resistant organisms and also by cancer cells that have developed resistance to chemotherapy.