title: Evidence that DNA excision-repair in xeroderma pigmentosum group A is limited but biologically significant.
abstract: The loss of pyrimidine dimers in nondividing populations of an excision-repair deficient xeroderma pigmentosum group A strain (XP12BE) was measured throughout long periods (up to 5 months) following exposure to low doses of ultraviolet light (UV, 254 nm) using a UV endonuclease-alkaline sedimentation assay. Excision of about 90% of the dimers induced by 1 J/m2 occurred during the first 50 days. The rate curve has some similarities with that of normal excision-repair proficient cultures that may not be coincidental. Rate curves for both XP12BE and normal cultures are characterized by a fast and slow component, with both rate constants for the XP12BE cultures (0.15 day-1 and 0.025 day-1) a factor of 10 smaller than those observed for the respective components of normal cell cultures. The slow components for both XP12BE and normal cultures extrapolate to about 30% of the initial number of dimers. No further excision was detected throughout an additional 90-day period even though the cultures were capable of excision-repair of other newly-introduced pyrimidine dimers. We conclude that nondividing XP12BE cells in addition to having a slower repair rate, cannot repair some of the UV-induced DNA damage. The repair in XP12BE is shown to have biological significance as detected by a cell-survival assay and dose-fractionation techniques. Nondividing XP12BE cells are more resistant to UV when irradiated chronically than when irradiated acutely with the same total dose.
mesh_terms: D002470:Cell Survival / Q000528:radiation effects; D002478:Cells, Cultured; D004260:DNA Repair / Q000528:radiation effects; D004262:DNA Restriction Enzymes / Q000378:metabolism; D004273:DNA, Neoplasm / Q000378:metabolism; D005347:Fibroblasts / Q000378:metabolism; D006801:Humans; D014466:Ultraviolet Rays; D014983:Xeroderma Pigmentosum / Q000235:genetics*
chemical_list: D004273:DNA, Neoplasm; D004262:DNA Restriction Enzymes