title: Mismatch repair provokes chromosome aberrations in hamster cells treated with methylating agents or 6-thioguanine, but not with ethylating agents.
abstract: O6-Methylguanine (O6MeG) is important in induction of chromosome aberrations (abs), with the unusual property that new abs are produced in the second cycle after treatment; cells lacking repair by O6-alkylguanine DNA-alkyltransferase (AGT) have more abs at the second division (M2) than at the first (M1). These second-cycle abs are likely caused by attempted correction by mismatch repair (MMR) of O6MeG:T mispairs, since we previously showed that MMR-deficient human cells (MT1 lymphoblasts) treated with SN-1 methylating agents do not produce new abs at M2 and are resistant to killing. Here we used MMR-deficient rodent cells to examine ab induction by alkylators and by incorporated 6-thioguanine (6-tG) which produces mispairs. BrdUrd labeling was used to identify cells at first, second and third metaphase after treatment (M1, M2 and M3). MMR-deficient Chinese hamster Clone B cells were 10-fold more resistant to ab induction by methyl nitrosourea and 1-methyl-3-nitro-1-nitrosoguanidine compared to their MMR-proficient parent cells, CHO MT+. Both cell lines express AGT and can remove the methyl group from O6MeG. Clone B has twice the AGT activity of CHO MT+, but inhibition of AGT with O6-benzylguanine did not change ab induction, indicating that methylation tolerance of Clone B cells was due to defective MMR and not to increased repair of O6MeG. Confirming the importance of O6MeG in inducing abs, even when it is a minor component of the adducts induced, Clone B cells were 2-fold more resistant to ab induction by methyl methanesulfonate and dimethylsulfate, whereas they had normal sensitivity to ethyl nitrosourea and 1-ethyl-3-nitro-1-nitrosoguanidine. Clone B cells are also resistant to killing by 6-tG, and 6-tG induced few abs in MMR-deficient Clone B (6-fold lower than CHO MT+ cells). Since mispairs do not occur until the cell cycle following incorporation of 6-tG, new abs in MMR-proficient cells are expected one cell cycle later than with the methylators, i.e., at M3. As expected, in normal CHO MT+, high ab levels were seen at M3, but there was also ab induction at M2. Similarly, with methylating agents we saw higher levels of abs at M1 in the MMR-proficient CHO MT+ cells than in Clone B cells, suggesting that in the rodent cells, MMR is involved in ab formation from mispairs or modified base pairs induced in the first S-phase, such as O6MeG:C. These rodent cells thus differ from human MT1 lymphoblasts which had similar ab levels to their normal parent cells at the first metaphase after treatment with methylators.
authors.affiliation: Merck Research Laboratories, Department of Genetic and Cellular Toxicology, West Point, PA 19486, USA.
authors.forename: M J
authors.initials: MJ
authors.lastname: Armstrong
authors.forename: S M
authors.initials: SM
authors.lastname: Galloway
mesh_terms: D000477:Alkylating Agents / Q000494:pharmacology*; D000818:Animals; D016466:CHO Cells; D002455:Cell Division; D002869:Chromosome Aberrations; D006224:Cricetinae; D004260:DNA Repair; D004791:Enzyme Inhibitors / Q000494:pharmacology*; D006147:Guanine / Q000031:analogs & derivatives / Q000494:pharmacology; D008769:Methylnitronitrosoguanidine / Q000494:pharmacology; D008770:Methylnitrosourea / Q000494:pharmacology; D013866:Thioguanine / Q000494:pharmacology*
chemical_list: D000477:Alkylating Agents; D004791:Enzyme Inhibitors; C064976:O(6)-benzylguanine; D008769:Methylnitronitrosoguanidine; D006147:Guanine; D008770:Methylnitrosourea; C008449:O-(6)-methylguanine; D013866:Thioguanine