Homology-directed Fanconi anemia pathway crosslink repair is dependent on DNA replication

Homologous recombination (also termed homology-directed repair, HDR) is a major pathway for the repair of DNA interstrand crosslinks (ICLs) in mammalian cells. Cells from Fanconi anemia (FA) patients are characterized by extreme ICL sensitivity, but their reported defect in HDR is mild. Here, we examined ICL-induced HDR using a GFP reporter and observed a profound defect in ICL-induced HDR in FA cells, but only when the reporter could replicate. FA is a rare recessive disease characterized by developmental abnormalities, bone marrow failure and cancer predisposition1. Diagnosis of FA is based on increased levels of chromosome abnormalities after treatment of patient-derived cells with agents that cause ICLs1. To date, FA patients have been found to have mutations in any of thirteen genes (FANC genes), the majority of which encode FA core complex members required for the monoubiquitylation of FANCI–FANCD2; the remaining FANC genes include the breast cancer susceptibility gene BRCA2 (FANCD1) and genes that interact with BRCA1 and BRCA22. ICLs impede DNA replication, and recent studies with Xenopus cell free extracts have described replication fork dynamics at an ICL in molecular detail3. These studies hav