Unscheduled DNA synthesis and HPRT mutation in fetal Syrian hamster and human respiratory epithelial cells exposed to ethylnitrosourea.

We previously reported that the chromosomes of fetal Syrian hamster respiratory epithelial cells were less stable toward ethylnitrosourea (ENU) than those of comparable human cells. Following this, we compared the sensitivity of genetic materials of the same cell systems to the same mutagen in terms of unscheduled DNA synthesis (UDS) and mutation at HPRT locus (HPRT-). UDS occurred 5 (with 0.1 mg ENU/ml) to 7 (with 0.4 mg ENU/ml) times more frequently in the hamster cells than in the human cells. This much lower UDS frequency in human cells cannot be solely explained by the fact that the human cells possess only a moderately larger (1.6 to 2.9 times) size of intracellular deoxythymidine triphosphate (dTTP) pool than the hamster cells. This finding would thus indicate that the hamster cells actually carry out DNA repair, whether correct or aberrant, more often than the human cells. Moreover, HPRT- was also 5 (at 0.4 mg/ml) to 26 (at 0.8 mg/ml) times more frequent in the hamster cells than in the human cells. Therefore, the current results suggest that the DNA repair mechanisms of the hamster cells are less accurate and more unstable than those of the human cells. Our previous findings with regard to the chromosomal stability give support to this hypothesis.