DNA repair methyltransferase (Mgmt) knockout mice are sensitive to the lethal effects of chemotherapeutic alkylating agents

Abstract

We have generated mice deficient in O6-methylguanine DNA methyltransferase activity encoded by the murine Mgmt gene using homologous recombination to delete the region encoding the Mgmt active site cysteine. Tissues from Mgmt null mice displayed very low O6-methylguanine DNA methyltransferase activity, suggesting that Mgmt constitutes the major, if not the only, O6-methylguanine DNA methyltransferase. Primary mouse embryo fibroblasts and bone marrow cells from Mgmt -/- mice were significantly more sensitive to the toxic effects of the chemotherapeutic alkylating agents 1,3-bis(2-chloroethyl)-1-nitrosourea, streptozotocin and temozolomide than those from Mgmt wild-type mice. As expected, Mgmt-deficient fibroblasts and bone marrow cells were not sensitive to UV light or to the crosslinking agent mitomycin C. In addition, the 50% lethal doses for Mgmt -/- mice were 2- to 10-fold lower than those for Mgmt +/+ mice for 1,3-bis(2chloroethyl)-1-nitrosourea, N-methyl-N-nitrosourea and streptozotocin; similar 50% lethal doses were observed for mitomycin C. Necropsies of both wild-type and Mgmt -/mice following drug treatment revealed histological evidence of significant ablation of hematopoietic tissues, but such ablation occurred at much lower doses for the Mgmt -/- mice. These results demonstrate the critical importance of O6-methylguanine DNA methyltransferase in protecting cells and animals against the toxic effects of alkylating agents used for cancer chemotherapy.

The relationship between aging and carcinogenesis: a critical appraisal

“…The enhanced tumor resistance in p53 '/m mice dependent on wild-type p53, supporting a model in which the m-allele product requires wild-type p53 to promote tumor suppression. The authors claim that the association of early aging and tumor resistance in mice consistent with the idea that senescence is a mechanisms of tumor suppression [134,238,248,249]. The paradox that overactive p53 suppresses cancer but accelerates aging can be explained by the fact that cancer results from the malfunctioning of p53 in single cells, whereas aging involves a tissuewide process [250].…”

“…Long-live mutant Ames dwarf mice and knockout p66 she(/( mice were less vulnerable to oxidative damage than wild-type controls [228,235], whereas senescence-prone strain, SAMP, had increased production of ROS [207], DNA damages and somatic mutation as compared with senescence-resistant SAMR strain [236]. MGMT-overexpressed mice are more resistant to alkylating agents [199,237], whereas deficient in DNA repair MGMT (/( and Parp (/( mice are more susceptible to effect of alkylating chemicals and ionizing radiation [218,238]. No significant differences were found in the mutation spectra and the mutation incidence between p53 (/( and p53 '/' mice [239,240], whereas the incidence of spontaneous tumors in p53 (/( mice were increased as compared with wild-type control [241,242].…”

Role of O6-alkylguanine-DNA alkyltransferase in the cytotoxic activity of cloretazine

“…Thus, BCNU alkylates the N 7 position of DNA guanine to a greater extent than the O 6 position (9). The difference in the LD 50 values for BCNU in wild-type and AGT knockout mice is <2-fold (34). In an in vitro DNA cross-linking assay, BCNU caused a relatively large number of DNA strand breaks due to depurination resulting from the alkylation of the N 7 position of guanine, whereas cloretazine did not (11).…”

Gene silencing in phenomena related to DNA repair

“…Thus, methyltransferase has a vital role in protecting these organs from toxic effects of alkylating agents. It is notable that Mgmt 7/7 mice are considerably more sensitive to chemotherapeutic alkylating drugs, presently in clinical use, than are wild type mice (Glassner et al, 1999;Shiraishi et al, 2000).…”