Mouse embryonic stem cells are hypersensitive to apoptosis triggered by the DNA damage O6-methylguanine due to high E2F1 regulated mismatch repair

Exposure of stem cells to genotoxins may lead to embryonic lethality or teratogenic effects. This can be prevented by efficient DNA repair or by eliminating genetically damaged cells. Using undifferentiated mouse embryonic stem (ES) cells as a pluripotent model system, we compared ES cells with differentiated cells, with regard to apoptosis induction by alkylating agents forming the highly mutagenic and killing DNA adduct O6-methylguanine. Upon treatment with N-methyl-N′-nitro-N-nitrosoguanidine (MNNG), ES cells undergo apoptosis at much higher frequency than differentiated cells, although they express a high level of the repair protein O6-methylguanine-DNA methyltransferase (MGMT). Apoptosis induced by MNNG is due to O6-methylguanine DNA adducts, since inhibition of MGMT sensitized ES cells. The high sensitivity of ES cells to O6-methylating agents is due to high expression of the mismatch repair proteins MSH2 and MSH6 (MutSα), which declines during differentiation. High MutSα expression in ES cells was related to a high hyperphosphorylated retinoblastoma (ppRb) level and E2F1 activity that upregulates MSH2, causing, in turn, stabilization of MSH6. Non-repaired O6-methylguanine adducts were shown to cause DNA double-stranded breaks, stabilization of p53 and upregulation of Fas/CD95/Apo-1 at significantly higher level in ES cells than in fibroblasts. The high apoptotic response of ES cells to O6-methylguanine adducts may contribute to reduction of the mutational load in the progenitor population.