Disease relevance of Mgmt

• Eight weeks after exposure to this dose of MNU, 40% of MNU-treated Mgmt(-/-) Mlh1(+/-) mice had thymic lymphomas and there were no tumors in those mice not given the treatment [1].
• These studies provide strong evidence that inactivation of AGT both in vivo and in vitro by O6-bG is species selective and impacts O6-bG-mediated enhancement of BCNU toxicity [2].
• These MGMT-/- mice were most sensitive to the alkylating carcinogen, methylnitrosourea; when varied doses of methylnitrosourea were administered to 6-week-old mice and survivals at the 30th day were determined, LD50s of MGMT-/- and MGMT+/+ mice were 20 and 240 mg/kg of body weight, respectively [3].
• A large number of thymic lymphomas, as well as lung adenomas, occurred in MGMT-/- mice exposed to methylnitrosourea at a dose of 2.5 mg/kg of body weight [3].
• cDNA for mouse O6-methylguanine-DNA methyltransferase was expressed in methyltransferase-deficient Escherichia coli mutant cells, and the overproduced mouse enzyme was purified to a homogeneous state [4].

High impact information on Mgmt

• The enzyme O6-methylguanine-DNA methyltransferase repairs O6-methylguanine as well as a minor methylated base, O4-methylthymine, in DNA [5].
• Mice with mutations in both alleles of the MGMT gene and one of the mismatch repair genes, MLH1, are as resistant to methylnitrosourea (MNU) as are wild-type mice, in terms of survival, but do have numerous tumors after receiving MNU [5].
• We selected for G156A MGMT (triangle upMGMT) transduced LTRC present in 5 x 10(4) to 100 x 10(4) marrow cells infused into nonmyeloablated mice by the administration of O(6)-benzylguanine (BG) and BCNU every 3 to 4 weeks [6].
• We have been testing the hypothesis that the pyridyloxobutylation pathway increases the mutagenic activity of the DNA methylation pathway by preventing the repair of O(6)-mG by O(6)-alkylguanine-DNA alkyltransferase (AGT) [7].
• MGMT+/- mice were as resistant as MGMT+/+ mice, but some difference in survival time was noted when the two genotypes of mice were exposed to a relatively high dose of methylnitrosourea [3].

Chemical compound and disease context of Mgmt

• The goal was to determine whether environmental mutagens shorten the latency or increase the incidence of LMO1 + lymphomas and whether mice transgenic for both LMO1 and MGMT, and thereby able to repair O6-methylguanine DNA adducts induced by MNU, would be protected [8].
• The comparatively low level of MGMT in papillomas may contribute to their ease of conversion to squamous cell carcinomas by N-ethyl-N-nitrosourea or n-methyl-N'-nitro-N-nitrosoguanidine [9].
• To circumvent this, O6-benzylguanine, an inactivator of AGT, has been developed and is currently in clinical development with BCNU; however, the potential long-term toxicities associated with this treatment are unknown [10].
• These data indicate that alkylating agent hypersensitivity of MGMT(-/-) mice results from hematopoietic damage at the stem level [11].

Biological context of Mgmt

• Although mutations stemming from O6-guanine alkylations would be predicted to be cumulative, we found no evidence of an Mgmt-dependent alteration in mutation spectrum in DNA samples from 12 month-old mice [12].
• Mutational-reporter transgenes rescued from mice lacking either Mgmt, or both Mgmt and Msh6 suggest that O6-alkylguanine-induced miscoding does not contribute to the spontaneous mutational spectrum [12].
• O6-methylguanine methyltransferase, Mgmt, constitutes the first line of defense against O6-alkylguanine, which can result in G : C to A : T transitions upon DNA replication [12].
• Mice with mutations in both alleles of the Mgmt and the Mlh1 gene, the former encoding a DNA repair methyltransferase and the latter a protein functioning at an early step of mismatch repair, are as resistant to the killing action of alkylating agents as are wild-type mice [1].
• The 50% inhibitory concentration of O6-bG for inactivation of mouse AGT was >10-fold higher than for the human protein in MGMT-transfected Chinese hamster ovary (CHO) cells [2].

Anatomical context of Mgmt

• As expected, Mgmt-deficient fibroblasts and bone marrow cells were not sensitive to UV light or to the crosslinking agent mitomycin C [13].
• A dose of O6-bG, which inactivated human AGT, markedly sensitized human MGMT-transfected CHO cells to 1,3-bis (2-chloroethyl)-1-nitrosourea (BCNU), whereas mouse MGMT-transfected CHO cells were much more resistant [2].
• Recent reports dispute whether murine AGT, in cell-free systems, is less sensitive to O6-bG than the human AGT protein, raising the possibility that efficacy seen in the mouse host may not predict the therapeutic index observed in clinical trials [2].
• However, despite successful depletion of MGMT activity in wild-type astrocytes by O(6)-benzylguanine (BG), resistance to BCNU persisted unchanged [14].
• Repression of MLH1 and MGMT genes in colon mucosa adjacent to implanted cancer in athymic mouse [15].

Associations of Mgmt with chemical compounds

• 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 [13].
• 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 [13].
• O6-benzlguanine (O6-bG) potentiates nitrosourea cytotoxicity of human tumor xenografts in nude mice by inactivating O6-alkylguanine-DNA alkyltransferase (AGT) [2].
• MGMT(-/-) mice were considerably more sensitive to dacarbazine, a monofunctional triazene, than were wild-type mice, in terms of survival [16].
• On the other hand, there was no difference in survival of MGMT(+/+) and MGMT(-/-) mice exposed to cyclophosphamide, a bifunctional nitrogen mustard [16].
• Acrolein and chloroacetaldehyde, metabolites of cyclophosphamide, were not mutagenic in Mgmt+/+ and Mgmt-/- mice [17].

Regulatory relationships of Mgmt

• The results give evidence for involvement of p53 in DNA damage-induced MGMT promoter activation [18].

Other interactions of Mgmt

• Mice defective in Mgmt and/or Mlh1 gave clear evidence of the significant roles of these proteins in carcinogenesis [19].
• To optimize our ability to detect mutations resulting from O6-alkylguanine-induced G : T mismatches, mice with combined deficiencies of Mgmt and the DNA mismatch repair molecule, Msh6, were analysed [12].
• DNA repair methyltransferase (Mgmt) knockout mice are sensitive to the lethal effects of chemotherapeutic alkylating agents [13].
• A mutation of GGT to AGT at K-ras codon 12 was found by SSCP in one adenocarcinoma from a total of 19 specimens including 5 adenocarcinomas, 9 adenomatous hyperplastic regions, 1 squamous cell carcinoma and 4 normal-like stomach regions from 4 mice [20].
• Thus, transfection mediated high level expression of p53 has inhibitory effect both on basal MGMT promoter activity and its activation by IR [18].

Analytical, diagnostic and therapeutic context of Mgmt

• Gene targeting was used to obtain mice defective in the MGMT gene, encoding O6-methylguanine-DNA methyltransferase [Tsuzuki et al., Carcinogenesis (Lond.), 17: 1215-1220, 1996] [3].
• 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 [13].
• In the hyperplastic mucosa adjacent to KM12SM tumors in the cecum of athymic mice, reductions in the levels of the mutL homologue 1 (MLH1) and O6-methylguanine-DNA methyltransferase (MGMT) proteins were detected by immunohistochemistry and immunoblotting [15].
• We show here that haematopoietic progenitors co-expressing mutant MGMT with an ABC-transporter exhibit resistance to combination chemotherapy in vitro [21].
• VACO 8MGMT (V8MGMT) sublines possessed high levels of AGT activity in cell culture and were > 10-fold resistant to the CENU 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) [22].

References