Mice over-expressing human O6 alkylguanine-DNA alkyltransferase selectively reduce O6 methylguanine mediated carcinogenic mutations to threshold levels after N-methyl-N-nitrosourea.
While it is well known that MNU induces thymic lymphomas in the mouse, it remains unclear which pre-mutagenic lesions are responsible for lymphomagenic transformation. One lesion thought to play a critical role is O6methylguanine[O6mG]which initiates G: C to A:T transition mutations in K-ras and other oncogenes. O6alkylguanine-DNA alkyltransferase (AGT), encoded by the methylguanine methyltransferase gene [MGMT], removes the methyl group thereby preventing the mutation from occurring. When overexpressed in the thymus, MGMT protects mice from MNU-induced thymic lymphomas. To determine whether MGMT overexpression reduced G: C to A: T mutation frequency after MNU, Big Blue lacI and MGMT+/Big Blue mice were treated with MNU and analysed for mutations in the lacI and K-ras genes. The incidence of MNU-induced lymphomas was 84% in Big Blue lacI mice compared to 14% in MGMT+Big Blue lacI mice. Sixty-two per cent of the lymphomas had a GGT to GAT activating mutation in codon 12 of K-ras consistent with O6mG adduct-mediated point mutagenesis. LacI mutation frequency in thymus of MNU treated Big Blue mice was 45-fold above background whereas it was 11-fold above background in MNU treated MGMT+/Big Blue mice. Most lacI mutations were G:C to A:T transitions, implicating O6mG even in the MGMT+mice. No mutations were attributable to chromosomal aberrations or rearrangements. Thus, O6mG adducts account for the carcinogenic effect of MNU and MGMT overexpression is selectively able to reduce O6methylguanine adducts below a carcinogenic threshold. Other adducts are mutagenic but appear to contribute much less to malignant transformation or oncogene activation.
Division of Hematology/Oncology and Ireland Cancer Center at Case Western Reserve University and University Hospital of Cleveland, Ohio 44106, USA.
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Gerson
D000477:Alkylating Agents / Q000494:pharmacology* / Q000633:toxicity; D000818:Animals; D002273:Carcinogens / Q000494:pharmacology* / Q000633:toxicity; D002471:Cell Transformation, Neoplastic / Q000139:chemically induced / Q000235:genetics; D003062:Codon / Q000235:genetics; D018736:DNA Adducts / Q000378:metabolism*; D004249:DNA Damage; D019175:DNA Methylation; D011905:Genes, ras / Q000187:drug effects*; D006147:Guanine / Q000031:analogs & derivatives* / Q000378:metabolism; D006801:Humans; D007763:Lac Operon / Q000187:drug effects*; D008223:Lymphoma / Q000139:chemically induced / Q000235:genetics / Q000517:prevention & control*; D008770:Methylnitrosourea / Q000494:pharmacology* / Q000633:toxicity; D051379:Mice; D008822:Mice, Transgenic; D016296:Mutagenesis; D009153:Mutagens / Q000494:pharmacology* / Q000633:toxicity; D019853:O(6)-Methylguanine-DNA Methyltransferase / Q000096:biosynthesis* / Q000235:genetics; D009928:Organ Specificity; D017354:Point Mutation; D013154:Spleen / Q000201:enzymology; D013950:Thymus Gland / Q000201:enzymology; D013953:Thymus Neoplasms / Q000139:chemically induced / Q000235:genetics / Q000517:prevention & control*; D019076:Transgenes / Q000187:drug effects*
D000477:Alkylating Agents; D002273:Carcinogens; D003062:Codon; D018736:DNA Adducts; D009153:Mutagens; D006147:Guanine; D008770:Methylnitrosourea; C008449:O-(6)-methylguanine; D019853:O(6)-Methylguanine-DNA Methyltransferase