Removal and persistence of SCE-inducing damage in human lymphocytes in vitro

The SCE frequency was studied in PHA-stimulated human lymphocytes exposed to various SCE-inducing agents in different stages of the cell cycle. Melphalan, HN2, MMS, and UV light were found to induce a higher SCE frequency in late G1 (18-24 hr after PHA stimulation) than in early G1 (1-6 hr after PHA) or G0 (before PHA stimulation). In contrast, CCNU induced more SCEs in early G1 than in late G1, and the adriamycin-induced SCE frequency was about the same after treatment in early and late G1. These results suggest that SCE-inducing lesions are being removed at different rates in human G1 lymphocytes. The removal of SCE-inducing HN2 lesions was found to be about 10 times more rapid in late G1 than in early G1, indicating the activation of a cross-link repair mechanism prior to DNA replication in human lymphocytes. Cells treated with MMS in the second G1 (after cultivation for about 55 hr in the presence of PHA and BrdUrd) showed a higher SCE frequency than cells treated with the same dose of MMS in the first G1. This result indicates that some type of interaction occurs between MMS damage and BrdUrd lesions in the DNA during replication, which leads to an enhanced induction of SCE. Analysis of SCEs induced during the 2 first vs. the third cell cycle in third-generation metaphases showed that most of the SCE-inducing damage caused by treatment with HN2 and melphalan in G1 of the first cell cycle are removed before the S phase of the third cell cycle, whereas damage caused by MMS and adriamycin seem to be more persistent. These observations suggest that the rate by which different types of SCE-inducing damage are removed or modified in resting (G0) of PHA-stimulated human lymphocytes can have a great influence on the SCE frequency. This is of practical importance in studies using SCE analysis to evaluate human exposure to suspected genotoxic agents in the environment.