| The human XRCC9 gene was first cloned by its ability to partially rescue hypersensitivity to mitomycin C (MMC) in the Chinese hamster ovary (CHO) mutant, UV40. Subsequent to the initial characterization of this gene, XRCC9 was found to complement MMC sensitivity in Fanconi anemia complementation group G (FA-G) cells, and inactivating mutations were identified within this gene in FA-G patients. Fanconi's anemia (FA) is a rare autosomal recessive disorder characterized by a diverse array of developmental and cellular abnormalities. FA cells exhibit genomic instability, cell cycle defects, and sensitivity to oxidative damage, interferon-γ and TNFα. Additionally, these cells are hypersensitive to the cytotoxic and genotoxic effects of cross-linking agents such as MMC and diepoxybutane (DEB). However, cross-sensitivity to other genotoxic agents is not typically associated with FA cells, although it is a feature of the CHO mutants UV40 and NM3. The cellular defect that underlies the pleiotropic FA phenotype is a matter of great debate, and has been ascribed to either defects in DNA repair, or in the response to the accumulation of endogenous reactive oxygen intermediates (ROI). This work conclusively demonstrate that CHO cells lacking FancG/Xrcc9 protein (such as NM3) are sensitive to multiple genotoxic agents, and that sensitivity is complemented to wild-type levels by the introduction of the hamster FancG/Xrcc9 gene. Furthermore, over-expression of FancG/Xrcc9 in NM3 cells fails to restore full resistance to the genotoxic agents MMC and MMS, suggesting that cellular levels of FancG/Xrcc9 are tightly regulated. The same appears to hold true for FANCG expression in human cells. FancG/Xrcc9 protein levels remain constant throughout the cell cycle, in both subcellular compartments, and do not change in response to treatment with MMC. Taken together, these data suggest a pivotal role for FancG/Xrcc9 in responding to multiple genotoxic agents to maintain cellular homeostasis, possibly as a downstream effector of a redox-sensitive cellular stress-response pathway. |
