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

We have generated mice deficient in O-6-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 Mgm t null mice displayed very low O-6-methylguanine DNA methyltransferase acti vity, suggesting that Mgmt constitutes the major, if not the only, O-6-meth ylguanine DNA methyltransferase. Primary mouse embryo fibroblasts and bone marrow cells from Mgmt -/- mice were significantly more sensitive to the to xic effects of the chemotherapeutic alkylating agents 1,3-bis (2-chloroethy l)-1-nitrosourea, streptozotocin and temozolomide than those from Mgmt wild -type mice. As expected, Mgmt-deficient fibroblasts and bone marrow cells w ere 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 t han those for Mgmt +/+ mice for 1,3-bis(2-chloroethyl)-1-nitrosourea, N-met hyl-N-nitrosourea and streptozotocin; similar 50% lethal doses were observe d for mitomycin C, Necropsies of both wild-type and Mgmt -/- mice following drug treatment revealed histological evidence of significant ablation of h ematopoietic tissues, but such ablation occurred at much lower doses for th e Mgmt -/- mice. These results demonstrate the critical importance of O-6-m ethylguanine DNA methyltransferase in protecting cells and animals against the toxic effects of alkylating agents used for cancer chemotherapy.