DNA repair in a UV resistant Chinese hamster ovary cell line

We have established an ultraviolet (UV) resistant Chinese hamster ovary (CHO) cell line B11UVres by repetitive UV exposure of the CHO cell line B11. We have characterized the resistant cell line with respect to growth, sensitivity to various DNA damaging agents, and the repair of UV induced DNA lesions. When examining sensitivity to UV in clonogenic survival studies, we find that the ID50 is increased 2.3-fold in the resistant cell line B11UVres compared to the parental cell line B11. Although the doubling time of the resistant cell line is greater than that of the parental cell line, there is no difference in the rate of replication after UV irradiation. When measuring repair of UV induced DNA lesions in the overall genome we find no significant difference between the two cell lines. However, at early times after UV, there is a significant increase in the rate of repair of cyclobutane pyramidine dimers (CPDs) in the transcribed strand of the dihydrofolate reductase (DHFR) gene in the B11UVres cells compared to the B11 cells. There is a small increase of steady state transcription of the DHFR gene in the UV resistant cells, but hardly enough to account for the repair increase. The UV resistant cell line B11UVres is not cross-resistant to the cross-linking agents mitomycin C or cisplatin, but shows increased sensitivity to these compounds.