Synergism between U.V. and thymidine treatments in the induction of cytogenetic damage in wild-type Friend erythroleukaemia cells

International Commission for Protection Against Environmental Mutagens and Carcinogens. Deoxyribonucleoside triphosphate levels: a critical factor in the maintenance of genetic stability

DNA precursor pool imbalances can elicit a variety of genetic effects and modulate the genotoxicity of certain DNA-damaging agents. These and other observations indicate that the control of DNA precursor concentrations is essential for the maintenance of genetic stability, and suggest that factors which offset this control may contribute to environmental mutagenesis and carcinogenesis. In this article, we review the biochemical and genetic mechanisms responsible for regulating the production and relative amounts of intracellular DNA precursors, describe the many outcomes of perturbations in DNA precursor levels, and discuss implications of such imbalances for sensitivity to DNA-damaging agents, population monitoring, and human diseases.

Nucleotide pools and mutagenic effects of alkylating agents in wild-type and APRT-deficient Friend erythroleukaemia cells

Wild-type Friend mouse erythroleukaemia cells (clone 707) were compared with adenine phosphoribosyltransferase (APRT)-deficient mutant subclones (707DAP8 and 707DAP10) for sensitivity to cell killing and mutagenesis by ethyl methanesulphonate (EMS) and methyl methanesulphonate (MMS). Cells were exposed to 0-300 micrograms/ml EMS and to 0-20 micrograms/ml MMS for a period of 16 h. A slight difference was found between wild-type cells and the two APRT-deficient subclones in terms of sensitivity to cell killing by both mutagens. The APRT-deficient subclones were, however, significantly more sensitive than wild-type cells to mutagenesis to 5-bromo-2-deoxyuridine resistance and 6-thioguanine resistance by EMS and MMS. The APRT-deficient subclones were found to have significantly decreased levels of dATP and dTTP nucleotides and decreased levels of all four ribonucleoside triphosphates (ATP, GTP, CTP and UTP) relative to wild-type cells. Wild-type Friend cells were found to have insignificant levels O6-methylguanine-DNA methyl transferase and it is suggested that the increased mutagen sensitivity of APRT-deficient cells may be due to imbalance of deoxyribonucleoside triphosphate pools during DNA excision-repair processes, or more probably due to deficiency of ATP for ATP-dependent DNA excision-repair enzymes.

The effects of thymidine on deoxyribonucleotide pool levels, cytotoxicity and mutation induction in Friend mouse erythroleukaemia cells

The ability of excess thymidine (10(-6)-10(-3) M) to enhance the frequency of 6-thioguanine (6-TG) resistant cell mutants and 2,6-diaminopurine (DAP) resistant cell mutants in Friend mouse erythroleukaemia cells, clone 707, was investigated. A significant increase in mutant frequency for both markers was observed at the higher (10(-4) and 10(-3) M) thymidine treatments. Measurements of deoxyribonucleoside triphosphate pool sizes in the cells revealed a dramatic elevation of the deoxythymidine triphosphate and deoxyguanosine triphosphate pools, an increase in the deoxyadenosine triphosphate pool and an almost complete disappearance of the deoxycytidine triphosphate pool at the higher thymidine treatments. This complemented the mutagenesis data. These results support the view that increases in mutant frequency may take place following perturbations in DNA precursor pools through a resultant decrease in the fidelity of DNA synthesis. Measurements of deoxyribonucleoside triphosphate pools were also carried out on clone 707 Friend cells and a thymidine kinase-deficient subclone, 707 BUF. The thymidine kinase-deficient subclone had significantly reduced deoxythymidine triphosphate and deoxyguanosine triphosphate pools relative to those observed in-clone 707 cells. The previously observed mutagen hypersensitivity in thymidine kinase-deficient Friend cells may result through pool imbalance rendering DNA excision repair error prone.

Sensitivity to cell killing and the induction of cytogenetic damage following gamma irradiation in wild-type and thymidine kinase-deficient Friend mouse erythroleukaemia cells

Wild-type Friend erythroleukaemia (clone 707) cells and 2 thymidine kinase-deficient subclones, 707BUE and 707BUF, having thymidine kinase activities of 1.4% and 0.7% that of clone 707, were compared for sensitivity to killing and the induction of cytogenetic damage following gamma irradiation. Three doses of gamma irradiation were used (150, 300 and 450 cGy), and cells were harvested for metaphase spreads after 4, 8, 12, 15, 29 and 43 h. G2 delay was evident at 4 h following gamma irradiation in the 3 cell clones examined, and recovery of mitosis was observed to be dose-dependent. G2 delay was found to be most prolonged in subclone 707BUE and most prompt in clone 707. Increased sensitivity to the induction of cytogenetic aberrations at all three doses was apparent in the 2 thymidine kinase-deficient subclones (as compared to wild-type cells) at 15, 29 and 43 h. Th thymidine kinase-deficient subclones also showed increased sensitivity to gamma radiation-induced cell killing. Furthermore, subclone 707BUE consistently exhibited greater resistance to gamma irradiation than did the subclone with lower thymidine kinase activity, 707BUF. The importance of thymidine kinase levels and extended G2 delay for DNA repair processes is discussed.