Preferential methylation of the Ha-ras proto-oncogene by methylnitrosourea in rat mammary glands

Molecular criteria for mutagenesis by DNA methylation: Some computational elucidations

Alkylating agents and N-nitroso compounds are well-known mutagens and carcinogens which act by alkylating DNA at the nucleobase moieties. Criteria for mutagenicity through DNA alkylation include (a) absence of the Watson-Crick (N1-guanine and N3-thymine) protons, (b) rotation of the alkyl group away from the H-bonding zone, (c) configuration of the alkylated base pair close to the Watson-Crick type. This computational study brings together these three molecular criteria for the first time. Three methylated DNA bases-N7-methylguanine, O6-methylguanine and O4-methylthymine-are studied using computational chemical methods. Watson-Crick proton loss is predicted more feasible for the mutagenic O6-methylguanine and O4-methylthymine than for the non-mutagenic N7-methylguanine in agreement with the observed trend for pKa values. Attainment of a conformer conducive to mutagenesis is more feasible for O6-methylguanine than for O4-methylthymine, though the latter is more mutagenic. These methylated bases yield 9 H-bonded pairs with normal DNA bases. At biological pH, O6-methylguanine and O4-methylthymine would yield stable mutagenic pairs having Watson-Crick type configuration by H-bonded pairing with thymine and guanine respectively, while N7-methylguanine would yield a non-mutagenic pair with cytosine. The three criteria thus well differentiate the non-mutagenic N7-methylguanine from the mutagenic O6-methylguanine and O4-methylthymine in good accord with experimental observations.

Increased H-ras mutation frequency in mammary tumors of rats initiated with N-methyl-N-nitrosourea (MNU) and treated with acrylamide

We recently demonstrated the incidence and multiplicity of N-methyl-N-nitrosourea (MNU)-induced mammary tumors to be increased by administration of acrylamide (AA) in post-initiation in rats. In the present study, to clarify the mechanisms of enhancement, H-ras gene mutations in mammary tumors induced in MNU-initiated rats with or without subsequent AA administration were investigated. Frequencies of mutations in codon 12 from GGA to GAA were significantly (p < 0.05) higher in rats with AA administration (82%, 23 out of 28 tumors) as compared to those without AA (50%, 9 out of 18 tumors), but the latency and volume of H-ras mutation-harboring tumors were similar to those of the mutation-lacking tumors. No mutations in codons 13 or 61 were detected in either treatment groups. The results thus indicate that H-ras gene mutations in codon 12 play a pivotal role in initiation of carcinogenesis and it appears possible that AA administration may selectively co-stimulate and/or maintain initiated cells via other genomic or non-genomic events in MNU-treated rats.

Coexposure to ethanol with N-nitrosodimethylamine or 4-(Methylnitrosamino)-1-(3-pyridyl)-1-butanone during lactation of rats: marked increase in O(6)-methylguanine-DNA adducts in maternal mammary gland and in suckling lung and kidney

Use of alcoholic beverages increases risk of cancer at several target sites, including the breast. Of several possible mechanisms for this effect, competitive inhibition by ethanol of hepatic clearance of nitrosamines, resulting in increased dose delivery to posthepatic tissues, gives the quantitatively most pronounced enhancement. We investigated whether this effect would pertain to the mammary gland, and to ethanol and nitrosamines delivered translactationally to sucklings. Ethanol (1.6 g/kg) was administered by gavage to nursing Sprague-Dawley rats 10 min before 5 mg/kg N-nitrosodimethylamine (NDMA) or 50 mg/kg 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK); treatment was on postnatal days 1, 7, or 14. Tissues taken 4 h later for analysis of O(6)-methylguanine in DNA were liver, blood, and mammary glands from the mothers, and liver, lung, kidney, and blood from the sucklings. Ethanol cotreatment resulted in a marked, 10-fold increase in O(6)-methylguanine adducts from NDMA in mammary gland, as well as smaller but significant increases in this tissue from NNK and in maternal blood cells from both chemicals; adducts in maternal liver decreased slightly. In the sucklings, ethanol cotreatment also lowered adducts in liver after NDMA or NNK treatment. After NDMA, adducts were also detected in suckling lung and kidney and were increased five- to 10-fold after ethanol coexposure. Adducts from either chemical, with or without ethanol, decreased markedly in all suckling tissues with development from postnatal day 1 to day 14. Thus ethanol coexposure with nitrosamines increases O(6)-methylguanine DNA adducts in mammary gland and strongly influences adduct formation in suckling tissues after translactational delivery.

Fast repair of O6-ethylguanine, but not O6-methylguanine, in transcribed genes prevents mutation of H-ras in rat mammary tumorigenesis induced by ethylnitrosourea in place of methylnitrosourea

Differential repair of structurally distinct mutagenic lesions in critical genes may influence the cellular risk of malignant conversion. We have investigated rat mammary tumorigenesis induced by N-ethyl-N-nitrosourea (EtNU) versus N-methyl-N-nitrosourea (MeNU) with respect to tumor incidence, ras gene mutation, and gene-specific repair. Both carcinogens induced mammary adenocarcinomas at high yield. In mammary epithelia (very low expression of O6-alkylguanine-DNA alkyltransferase, MGMT), O6-methylguanine (O6-MeGua) was eliminated from transcribed (H-ras and beta-actin) and inactive genes (IgE heavy chain) at the same slow rate as determined for bulk genomic DNA. The persistence of O6-MeGua in DNA correlated with a high frequency of G:C --> A:T transition mutations at codon 12 of the H-ras gene in MeNU-induced tumors. Repair of O6-ethylguanine (O6-EtGua), too, was slow in the IgE heavy chain gene as in bulk DNA. Contrasting with O6-MeGua, however, O6-EtGua was removed approximately 20 times faster from the active H-ras and beta-actin genes via MGMT-independent mechanism(s). Accordingly, no H-ras codon 12 mutations were found in EtNU-induced tumors, and 5- to 8-fold surplus alkyltransferase activity of the mammary epithelia-via a bacterial ada transgene-did not significantly counteract tumorigenesis in EtNU-exposed contrary to MeNU-treated animals. Neither MeNU- nor EtNU-induced tumors exhibited mutations at codons 13 and 61 of H-ras or codons 12, 13, and 61 of K-ras. Fast repair of O6-EtGua, but not O6-MeGua, in transcribed genes thus prevents mutational activation of H-ras when rat mammary carcinogenesis is initiated by EtNU in place of MeNU.

Brief communication, P53 accumulation in N-methyl-N-nitrosourea-induced rat mammary tumors

P53 aberrant protein expression and mutation is a component of many human tumors but less information is available regarding involvement in relevant animal models. We have examined invasive mammary epithelial neoplasms for p53 aberrant protein expression from Sprague-Dawley rats induced by two intrajugular injections of N-methyl-N-nitrosourea (NMU, 50 mg/kg), 7 days apart, beginning at 44-49 days of age. Positive nuclear staining was observed in 8/10 mammary tumor frozen sections using PAb 421, a mouse monoclonal antibody, compared to a mixed mouse IgG negative control antibody. Paraffin sections from two additional sets of similarly induced rat mammary tumors also showed positive nuclear staining (22/37 tumors) with CM-5, a rabbit polyclonal antibody. Our results indicate that elevated cellular content of p53 is a common event in invasive palpable mammary tumors induced by NMU in this dual-injection model system.