Effects of thymidine kinase and methyltransferase deficiency on mutagenesis in a human lymphoblastoid cell line

FISH comets show that the salvage enzyme TK1 contributes to gene-specific DNA repair

Thymidine kinase 1 (TK1) is a salvage enzyme that phosphorylates thymidine, imported from surrounding fluids, to create dTMP, which is further phosphorylated to the DNA precursor dTTP. TK1 deficiency has for a long time been known to cause increased cellular sensitivity to DNA damage. We have examined preferential strand break repair of DNA domains in TK1(+) and TK1(-) clones of the Raji cell line, by the Comet-FISH technique, in bulk DNA and in the actively transcribed tumor suppressor (TP53) and human telomerase reverse transcriptase (hTERT) gene regions, over 1 h after 5Gy γ-irradiation. Results showed that repair of the TP53 and hTERT gene regions was more efficient in TK1(+) compared to TK1(-) cells, a trend also reflected to a lesser degree in genomic DNA repair between the cell-lines. The targeted gene-specific repair in TK(+) cells occurred rapidly, mainly over the first 15 min repair-period. Therefore, TK1 is needed for preferential repair of actively transcribed regions, through a previously unsuspected mechanism. In principle, TK1 could exert its protective effects through supply of a supplementary dTTP pool for accurate repair of damaged genes; but Raji TK1(+) cells in thymidine free media still show preferential repair of transcribed regions. TK1 therefore does not exert its protective effects through dTTP pools, but through another unidentified mechanism, which affects sensitivity to and mutagenicity by DNA damaging agents.

Effect of antioxidant vitamin supplementation on DNA damage and repair in human lymphoblastoid cells

In this study the possible protective effects of ascorbic acid and alpha-tocopherol (singly and in combination) on Raji lymphoblastoid cells exposed to various doses of X-rays or hydrogen peroxide (H2O2) are investigated. DNA strand breaks and alkali-labile sites were measured using the alkaline comet assay. Survival and hypoxanthine guanine phosphoribosyl transferase mutant frequency were measured using the colony-forming assay. Ascorbic acid (60 microM) and alpha-tocopherol (30 microM) were added singly or together to cell culture medium 24 hours before treatment and were present during treatment. After the 24-hour supplementation period with ascorbic acid alone, alpha-tocopherol alone and ascorbic acid + alpha-tocopherol, the level of endogenous DNA damage was significantly lower (p < 0.05) than in the nonsupplemented culture, as assessed by the comet assay. By use of the comet assay, it was observed that ascorbic acid exhibited an overall protective effect against DNA damage induced after X-ray treatment, whereas alpha-tocopherol exhibited an overall protective effect against DNA damage induced after H2O2 treatment. Significant increases were observed in the percent survival after 1-Gy X-rays and 5 and 20 microM H2O2 in those cultures supplemented with ascorbic acid alone and alpha-tocopherol alone relative to the nonsupplemented cultures. The endogenous level of mutant frequency was also significantly decreased in the presence of ascorbic acid relative to the nonsupplemented culture. These findings are consistent with the concept that ascorbic acid and alpha-tocopherol can, under certain conditions, protect against oxidatively induced DNA damage.