DNA Damage and Repair after Low Doses of UV-C Radiation; Comparable Rates of Repair in Rodent and Human Cells

Abstract

The UV endonuclease assay for cyclobutane pyrimidine dimers in the DNA of UV-C-irradiated mammalian cells has been modified by replacing alkaline sucrose sedimentation with alkaline unwinding and hydroxylapatite chromatography to determine the number of DNA breaks introduced by the endonuclease. Dimers induced by doses as low as 0·25 Jm⁻² can be detected and the assay has been used to examine the capacity of human, hamster and mouse cells to remove damage inflicted by sublethal doses of UV-C. In addition, incision activity has been measured by incubating cells with DNA synthesis inhibitors after irradiation with UV-C. In rodent and human cells, given a dose of UV-C of 1 Jm⁻² about half of the endonuclease-sensitive sites are lost in 5–6 h. The incision capacity of these cells corresponds well with the extent of removal of dimers. Thus, although rodent cells are normally considered to be relatively deficient in nucleotide excision repair, we find that rodent and human cells have comparable capacities to deal with low levels of UV-C-induced damage.