Mice over-expressing human O6 alkylguanine-DNA alkyltransferase selectively reduce O6 methylguanine mediated carcinogenic mutations to threshold levels after N-methyl-N-nitrosourea

• Published in: Oncogene Vol. 18; no. 25; p. 3783
• Main Authors: Allay, E, Veigl, M, Gerson, S L
• Format: Journal Article
• Language: English
• Published: England 24-06-1999
• Subjects: Alkylating Agents - pharmacology; Alkylating Agents - toxicity; Animals; Carcinogens - pharmacology; Carcinogens - toxicity; Cell Transformation, Neoplastic - chemically induced; Cell Transformation, Neoplastic - genetics; Codon - genetics; DNA Adducts - metabolism; DNA Damage; DNA Methylation; Genes, ras - drug effects; Guanine - analogs & derivatives; Guanine - metabolism; Humans; Lac Operon - drug effects; Lymphoma - chemically induced; Lymphoma - genetics; Lymphoma - prevention & control; Methylnitrosourea - pharmacology; Methylnitrosourea - toxicity; Mice; Mice, Transgenic; Mutagenesis; Mutagens - pharmacology; Mutagens - toxicity; O-Methylguanine-DNA Methyltransferase - biosynthesis; O-Methylguanine-DNA Methyltransferase - genetics; Organ Specificity; Point Mutation; Spleen - enzymology; Thymus Gland - enzymology; Thymus Neoplasms - chemically induced; Thymus Neoplasms - genetics; Thymus Neoplasms - prevention & control; Transgenes - drug effects
• ISSN: 0950-9232

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.