Recent work has highlighted interplay between components of the Fanconi anemia (FA) pathway, an inherited genome instability syndrome characterized by hypersensitivity to DNA interstrand cross-links (ICLs), and the breast cancer susceptibility proteins BRCA1 and BRCA2/FANCD1. Is has also been suggested that certain defects within FANCD2, which is the central factor in the FA pathway, may lead to an increased risk of sporadic breast cancer.

Mass spectrometry and candidate western blotting analyses were carried out on FCD-2 immunoprecipitates from untreated and cisplatin-treated whole worm extracts.

Using the nematode worm as a model system, we have identified the circadian protein CLK-2 and ATL-1 (C. elegans ATR) as factors that coimmunoprecipitate with C. elegans FANCD2 (FCD-2) following ICL damage. C. elegans atl-1 and clk-2 mutants and siRNA depletion of human hCLK2 (KIAA00693) compromises FCD-2/FANCD2 recruitment to blocked replication forks and confers ICL sensitivity, a hallmark of FA. Cells deficient for hCLK2 are also defective for damage-induced mono-ubiquitylation of FANCD2 and exhibit radio-resistant DNA synthesis indicative of an S-phase checkpoint defect. ATR activation leading to BRCA1-mediated ubiquitylation remains intact in hCLK2 depleted cells, yet ATR-dependent phosphorylation of Chk1 and Claspin is severely attenuated following S-phase insults. Finally, recruitment of the homologous recombination factor RAD51 is also impaired in cells depleted of hCLK2, which leads to a reduced homologous recombination frequency at sites of DNA damage.

These data indicate that the novel factor hCLK2 is an essential component of the mammalian S-phase checkpoint required to coordinate both FA and HR-mediated repair responses following replication stress.