Sensitivity of nucleotide excision repair-deficient human cells to ionizing radiation and cyclophosphamide

Nucleotide excision repair (NER)-deficient rodent and human cells (such as those derived from patients with xeroderma pigmentosum, XP) are hypersensitive to UV light. Some of these cell lines, specifically certain rodent mutants with severe defects in the ERCC1 and XPF genes, are dramatically sensitive to crosslinking agents such as phosphoramide mustard (PM). These crosslink-sensitive rodent mutants also exhibit sensitization to gamma-rays under hypoxic (but not under aerated) conditions. Like their rodent counterparts, human XP cells are highly sensitive to UV light; however, none of the human XP lines, even XPF, displays extreme hypersensitivity to crosslinking agents. Studying XP cells, therefore, allows us to further assess the extent to which the phenotypic characteristic of hypoxia-specific radiosensitization of mammalian cells tracks with defects in crosslink repair (as opposed to NER). The sensitivity to PM and gamma-rays of normal human fibroblasts and human XP fibroblasts from two complementation groups, XPA and XPF, was assessed using a clonogenic survival assay. Compared with normal cells, XPA cells were not appreciably hypersensitive to PM or to gamma-rays under either aerated or hypoxic conditions. XPF cells were modestly (approximately 1.75-fold) sensitive to PM but showed no significant radiosensitization under either aerated or hypoxic conditions. Thus, although the phenotype of human XPF cells is quite different from that of "severe" rodent XPF mutants such as UV41, the characteristic of hypoxia-specific radiosensitization consistently tracks with extreme hypersensitivity to crosslinking agents and is separable from UV sensitivity (and thus from defects in NER).