Repair rate in human fibroblasts measured by thymine dimer excorporation

Summary

The UV photoproduct, thymine dimer ($\overline {TT}$), is excorporated with a remarkably low rate from the DNA of human fibroblasts grown in cell culture. An UV dose of 18 J/m² creates 0.045% $\overline {TT}$ (related to thymine). Within the first two days of repair logarithmically growing and quiescent fibroblasts exhibit the same repair rates; thereafter, the proportion of $\overline {TT} /T$ is lower in growing cells due to recovery of DNA replication. Only about 50% of the lesions are excised within 24 h. In quiescent cells, 13% of the thymine dimers originally present can be detected as late as a week after UV-irradiation. Two distinct first-order rate constants indicate that approximately half of the dimers are less accessible to repair. Repair measured by the nucleoid decondensation technique corresponds to the faster repair rate, whereas the slow repair rate cannot be detected by this method. Saturation of repair is found beyond 27 J/m². The remarkably slow rate of excision indicates that thymine dimers are not lethal lesions in human fibroblasts.