Differential hypersensitivity of xeroderma pigmentosum lymphoblastoid cell lines to ultraviolet light mutagenesis

Survival and mutation after u.v. light irradiation were examined in four human lymphoblastoid cell lines; one cell line with normal excision-repair capacity (HH4), two excision-repair-deficient xeroderma pigmentosum (XP) cell lines from patient XP3BE (complementation group C) and XP7NI (group A), and one cell line from an XP heterozygote (XPF7NI, father of XP7NI). Relative to HH4 and XPF7NI, both XP3BE and XP7NI were more sensitive to the cytotoxic effect of u.v. by virtue of a diminished shoulder and a steeper slope in the survival curve determined by growth curve extrapolations. The mutagenesis for 6-thioguanine resistance (TGr) was measured by the limiting dilution technique using 96 well microtiter plates. Even at non-cytotoxic fluences, u.v. light induced TGr mutations in HH4 and XPF7NI with no apparent threshold, and the mutagenic response of XPF7NI was no different than that of HH4. Both XP cell lines were more sensitive than normal cells to u.v.-induced mutation. However, XP7NI cells were extremely hypermutable by u.v.; i.e. 3.3 J/m2 induced an approximately 500-fold increase of mutant fraction with the background of 2 X 10(-6). XP7NI cells remained much more mutable by u.v. even when plotted against survival, whereas the mutant fractions for XP3BE and HH4 followed the same line. These results imply that the mechanism of u.v.-induced mutation in XP7NI cells may intrinsically be different from that in XP3BE, XP heterozygote or normal cells, or that potentially mutagenic lesions are repaired much less efficiently in XP7NI cells than are potentially lethal lesions, as compared with XP3BE, XPF7NI and HH4 cells.