Mismatch repair provokes chromosome aberrations in hamster cells treated with methylating agents or 6-thioguanine, but not with ethylating agents

O-6-Methylguanine (O(6)MeG) 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 O-6-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 O(6)MeG: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 O(6)MeG. Clone B has twice the AGT activity of CHO MT+, but inhibition of AGT with O-6-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 O(6)MeG. Confirming the importance of O(6)MeG 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 O(6)MeG: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.