REPAIR OF ULTRAVIOLET-B AND SINGLET OXYGEN-INDUCED DNA-DAMAGE IN XERODERMA-PIGMENTOSUM CELLS

Ultraviolet B (UVB) (290-320 nm) is capable of damaging the DNA molecu le directly by generating predominantly pyrimidine dimers, UVA (320-40 0 mm) does not alter the DNA molecule directly, However, when it is ab sorbed by cellular photosensitizers, it can damage the DNA molecule in directly, e.g., by mediation of singlet oxygen, generating predominant ly 8-hydroxyguanine, These indirect effects have been implicated in th e mutagenic, genotoxic, and carcinogenic effects of UVA, To study the processing of directly and indirectly UV-induced DNA damage in intact, DNA-repair-proficient and -deficient human cells, we used the replica ting plasmid pRSVcat, either irradiated with up to 10 kJ/m(2) UVB or t reated with the photosensitizer methylene blue plus visible light (whi ch generates singlet oxygen). These treated plasmids were introduced i nto lymphoblast Lines from normal donors or from patients with xeroder ma pigmentosum (XP) complementation groups A, C, D, E, and variant. DN A repair was assessed by measuring activity of reactivated chloramphen icol-acetyl-tuansferase enzyme, encoded by the plasmid's cat gene, in cell extracts after 3 d. As expected, the repair of UVB-induced DNA da mage was reduced in all XP cell lines, and the degree varied with the complementation group. XP-A, -D, -E, and -variant cells were normally efficient in the repair of singlet oxygen-induced DNA damage. Only thr ee of four XP-C cell lines showed a markedly reduced repair of these l esions. This indicates differential DNA-repair pathways for directly a nd indirectly UV-induced DNA damage in human cells and suggests that b oth may be affected in XP-C.