Human Mus81 and FANCB independently contribute to repair of DNA damage during replication

Recent studies suggest a crucial role for homologous recombination (HR) in repairing replication-associated DNA lesions. In mammals, the Mus81 endonuclease and the Fanconi anemia (FA) pathway have been implicated in HR repair; however, their functional relationship has remained unexplored. Here, we knockout the genes for Mus81 and FANCB, a component of the FA core complex, in the human Nalm-6 cell line. We show that Mus81 plays an important role in cell proliferation to suppress cell death when FANCB is missing, indicating a functional linkage between Mus81 and the FA pathway. In DNA cross-link repair, roles for Mus81 and the FA pathway appear to have an overlapping function. Intriguingly, Mus81 and FANCB act independently in surviving exposure to camptothecin (CPT). Although CPT-induced FANCD2 and Mus81 foci co-localize with Rad51, loss of Mus81, but not FANCB, results in significantly decreased levels of spontaneous and CPT-induced sister chromatid exchanges (SCEs). In addition, Mus81, unlike FANCB, has no significant role in gene targeting as well as in repairing hydroxyurea (HU)-induced stalls of replication forks. Collectively, our results provide the first evidence for differential functions of Mus81 and the FA pathway in repair of DNA damage during replication in human cells.