Chromosome aberrations: persistence of alkylation damage and modulation by O6-alkylguanine-DNA alkyltransferase

Abstract Alkylating agents produce a spectrum of DNA lesions alkylated at different sites on the molecule. These lesions differ in their propensities to cause effects such as cytotoxicity, mutations and sister-chromatid exchanges. We have used our observations that some methylating agents produce increasing levels of chromosome aberrations (abs) through successive cell cycles in Chinese hamster ovary cells, but not in normal human cells, to begin a study of which alkylated products are most likely to lead to chromosome abs, and in particular which adducts persist in DNA and cause abs after the first cell cycle. We previously observed increasing yields of abs with successive cell cycles in CHO-WBL cells treated with dimethyl nitrosamine (DMN), e.g., at 10 mM DMN, 8.8% cells with abs at first metaphase (M1) and 26.0% and third metaphase (M3) after treatment. Here we tested 4 methylating agents and their ethyl analogs in CHO cells, normal human fibroblasts (L136), and human lymphocytes. We sampled cells at several times after treating for 3 h (CHO and lymphocytes) or 4.5 h (L136). S9 metabolic activation was used for DMN and diethyl nitrosomine. BrdUrd labeling was used to identify cells in M1, M2 and M3. The methylating agents were more potent aberration (ab) inducers than ethylating agents, on a molar basis. In CHO cells, yields of abs were maintained or increased through up to 3 cell cycles after treatment with DMN, methyl methanesulfonate, methyl nitrosourea and 1-methyl-3-nitro-1-nitrosoguanidine (MNNG). With ethylating agents the ab yields in CHO cells were similar or lower in second and third cycles. In contrast, there was no evidence for persistence of lesions leading to abs in either human cell type; ab yields were markedly decreased with subsequent cell cycles for all agents. Normal human cells are proficient in repair of alkylation at the O 6 site of guanine by O 6 -alkylguanine-DNA alkyltransferase (AGT), whereas CHO cells lack AGT activity. To explore the role of repair by AGT, on the lesions involved in production of abs, we studied L136 cells, with and without O 6 -benzylguanine (BZG), a specific inhibitor of AGT. With MNNG, inhibition of AGT resulted in higher ab yield and production of abs through later cell cycles, so that human fibroblsts now behaved similarly to CHO cells. Preliminary data from the reciprocal experiment in CHO cells engineered to express high levels of AGT revealed a greatly decreased ab response to MNNG. In addition, the low ab yields observed were similar through later cycles or increased only slightly. Although multiple types of lesions and mechanisms of repair may be involved in ab production, O 6 -methylguanine is an important lesion, particularly in secondary ab production in later cycles. The differences between CHO and normal human cells in response to methylating agents may be due largely to inherent levels of AGT.