Mutational specificities of environmental carcinogens in the lacl gene of Escherichia coli. II: A host-mediated approach to N-nitroso-N,N-dimethylamine and endogenous mutagenesis in vivo

Risk assessment of N-nitrosamines in food

EFSA was asked for a scientific opinion on the risks to public health related to the presence of N-nitrosamines (N-NAs) in food. The risk assessment was confined to those 10 carcinogenic N-NAs occurring in food (TCNAs), i.e. NDMA, NMEA, NDEA, NDPA, NDBA, NMA, NSAR, NMOR, NPIP and NPYR. N-NAs are genotoxic and induce liver tumours in rodents. The in vivo data available to derive potency factors are limited, and therefore, equal potency of TCNAs was assumed. The lower confidence limit of the benchmark dose at 10% (BMDL10) was 10 μg/kg body weight (bw) per day, derived from the incidence of rat liver tumours (benign and malignant) induced by NDEA and used in a margin of exposure (MOE) approach. Analytical results on the occurrence of N-NAs were extracted from the EFSA occurrence database (n = 2,817) and the literature (n = 4,003). Occurrence data were available for five food categories across TCNAs. Dietary exposure was assessed for two scenarios, excluding (scenario 1) and including (scenario 2) cooked unprocessed meat and fish. TCNAs exposure ranged from 0 to 208.9 ng/kg bw per day across surveys, age groups and scenarios. 'Meat and meat products' is the main food category contributing to TCNA exposure. MOEs ranged from 3,337 to 48 at the P95 exposure excluding some infant surveys with P95 exposure equal to zero. Two major uncertainties were (i) the high number of left censored data and (ii) the lack of data on important food categories. The CONTAM Panel concluded that the MOE for TCNAs at the P95 exposure is highly likely (98-100% certain) to be less than 10,000 for all age groups, which raises a health concern.

Context Matters: Contribution of Specific DNA Adducts to the Genotoxic Properties of the Tobacco-Specific Nitrosamine NNK

The tobacco-specific nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) is a potent pulmonary carcinogen in laboratory animals. It is classified as a Group 1 human carcinogen by the International Agency for Cancer Research. NNK is bioactivated upon cytochrome P450 catalyzed hydroxylation of the carbon atoms adjacent to the nitrosamino group to both methylating and pyridyloxobutylating agents. Both pathways generate a spectrum of DNA damage that contributes to the overall mutagenic and toxic properties of this compound. NNK is also reduced to form 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL), which is also carcinogenic. Like NNK, NNAL requires metabolic activation to DNA alkylating agents. Methyl hydroxylation of NNAL generates pyridylhydroxybutyl DNA adducts, and methylene hydroxylation leads to DNA methyl adducts. The consequence of this complex metabolism is that NNK generates a vast spectrum of DNA damage, any form of which can contribute to the overall carcinogenic properties of this potent pulmonary carcinogen. This Perspective reviews the chemistry and genotoxic properties of the collection of DNA adducts formed from NNK. In addition, it provides evidence that multiple adducts contribute to the overall carcinogenic properties of this chemical. The adduct that contributes to the genotoxic effects of NNK depends on the context, such as the relative amounts of each DNA alkylating pathway occurring in the model system, the levels and genetic variants of key repair enzymes, and the gene targeted for mutation.

Protection by dietary compounds against mutation in a transgenic rodent

One of the most relevant biomarkers of genotoxicity and, potentially, carcinogenesis is the occurrence of mutations. Data indicate that carcinogens are highly specific with regard to their target tissue in inducing both tumors and mutations. This specificity may reflect the dependence on tissue-specific metabolic activation, the organ-specific environment or both. Ideally, therefore, mutation should be determined in a real animal rather than in a cell culture system. The lacI transgenic rodent model provides such a system. We have used this model to investigate tissue, species and sex specificity of mutation induced by selected dietary carcinogens and to examine how some compounds may alter the induction of mutation. We have studied mutation using several chemicals, including the dietary heterocyclic amine 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), the environmentally important aromatic hydrocarbon benzo[a]pyrene and the food contaminant aflatoxin B1. We have shown that the mutagenic potency of these chemicals can be modulated by other dietary compounds, including green tea and conjugated linoleic acid, and the dioxin 2,3,7,8-tetrachlorodibenzo[b,e][1,4]dioxin (TCDD). These results demonstrate that the lacI transgenic rodent is a useful model for the study of chemoprevention in vivo.

Mutant frequencies and mutation spectra of dimethylnitrosamine (DMN) at the lacI and cII loci in the livers of Big Blue transgenic mice

The lacI gene in Big Blue transgenic rodents has traditionally been used as a surrogate gene for in vivo mutations. Recently, a more efficient and less expensive assay involving direct selection in the smaller lambda cII gene has been developed. Little is known, however, about the comparative sensitivity of the two loci or their influence on the recovered mutation spectrum following mutagen treatment. We have compared the mutation frequency (MF) and mutational spectrum (MS) of lacI and cII from the same DNA samples isolated from the liver of control and dimethylnitrosamine (DMN)-treated mice. A three-fold (p<0.01) increase in the MF was observed at both loci in the DMN-treated group compared to the corresponding control groups. While the DMN-induced mutation spectrum at lacI was significantly different from its corresponding spontaneous mutation spectrum (p<0.001), the mutation spectrum at cII (p>0.28) was not. The mutation spectra at the two loci from the DMN-treated mice resembled each other but the 4, 2.5 and 12-fold increase in the mutation frequency of A:T>T:A transversions, single base deletions and deletions of more than four base pairs, respectively, at lacI, altered the spectra significantly (p<0.007). The number of mutations of these classes at cII was also increased, but the fractions were lower than at lacI. The spontaneous mutation spectra at the cII and lacI loci resembled each other except for the seven-fold increase in G:C<C:G transversions in the cII spectrum resulting in a significant difference (p<0.0001) between the spectra. Our initial data indicates that although cII is as sensitive to mutation induction as lacI, fewer sites are available for certain classes of mutations to be manifest resulting in an apparent lack in change in the mutation spectrum.

von Hippel-Lindau gene mutations in N-nitrosodimethylamine-induced rat renal epithelial tumors

BACKGROUND

Mutations in the von Hippel-Lindau (VHL) gene are common in human clear cell kidney cancers. Carcinogens in cigarette smoke, especially nitrosamines, are known to induce kidney tumors of a variety of histologic types in rodents--but with no evidence of VHL mutations; however, none of these tumors resembled human clear cell carcinomas. We examined N-nitrosodimethylamine-induced kidney tumors of the clear or mixed clear/granular cell type in Wistar rats to assess the presence of VHL mutations.

METHODS

Sections of eight clear or mixed clear/granular cell kidney tumors that had been formalin fixed and paraffin embedded were microdissected. DNA was extracted from the microdissected tissue, and exons 1-3 of the rat VHL gene were examined by use of polymerase chain reaction and cycle sequencing techniques.

RESULTS

Four VHL gene mutations (three G:C to A:T and one A:T to G:C) were detected in three of the tumors in contrast to no mutations in 40 previously reported rat kidney tumors of other histologic types (three of eight tumors versus none of 40; two-sided Fisher's exact test; P=.003). Only tumors showing prominent swollen clear cell cytology with a signet-ring appearance had VHL mutations.

CONCLUSIONS

To our knowledge, this is the first report of VHL mutations in kidney tumors after direct chemical exposure and provides a possible molecular pathway linking tobacco smoking to kidney cancer.

Mutational spectra induced under distinct excision repair conditions by the 3 methylating agents N-methyl-N-nitrosourea, N-methyl-N'-nitro-N-nitrosoguanidine and N-nitrosodimethylamine in postmeiotic male germ cells of Drosophila

This paper describes the analysis of mutations induced at the vermilion locus in postmeiotic male germ cell stages of Drosophila exposed to 3 different N-methyl-N-nitroso compounds: N-methyl-N-nitrosourea (MNU); N-methyl-N'-nitro-N-nitrosoguanidine (MNNG); and N-nitrosodimethylamine (DMN). With MNU and DMN, the impact of DNA nucleotide excision repair (NER) on the spectra of mutations was studied. Mutants were isolated from F1 (mutations fixed before the first mitotic replication after fertilization) and F2 (mutations fixed following one or more mitotic replications; mosaics in F1) generations. The vermilion system enables the analysis of both intra- and inter-locus DNA changes for which several techniques have been adapted: (1) amplification of the vermilion gene by PCR, cloning of the fragment and sequence analysis of ssDNA; (2) Southern blot hybridization; and (3) cytological analysis of polytene chromosomes. In total, 49 MNU (26 from the exr+ genotype and 23 from the exr- genotype), 47 DMN (28 from the exr+ genotype and 19 from the exr- genotype) and 16 MNNG-induced mutations were characterized. The F1 spectra of all 3 agents contained base-pair changes and deletions (intra- and multi-locus) in a ratio of roughly 1 to 1, indicating a significant contribution of nitrogen DNA adducts to the spectra. In all F2 spectra the levels of base-pair changes were significantly higher compared to those in the F1 spectra, a finding also made for methyl methanesulfonate-induced mutations in earlier studies. There is an increase of mutations of, especially, the transversion types of mutations under exr- conditions in comparison to the exr+ situation. The induced transversions, clearly present in all spectra (exr+ and exr-), are presumably caused by N-methyl DNA adducts, which upon release from the DNA backbone lead to apurinic sites in a time-related process. Regarding the occurrence of transitions, it turned out for all 3 mutagens that the AT-->GC type strongly dominated the GC-->AT transitions. This suggest that O6-methylguanine is efficiently repaired, in contrast to O4-methylthymine. Based on the data obtained in the vermilion system with ENU, we propose, in addition, that the Drosophila alkyltransferase system repairs O6-methylguanine more efficiently than O6-ethylguanine.

Mutational specificities of environmental carcinogens in the lac1 gene of Escherichia coli, VII: The host-mediated assay and its comparison with in vitro mutagenesis induced by 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone

To investigate the influence of different types of metabolic activation (9,000 x g supernatant (S9) activation vs. a host-mediated approach) on 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK)-induced mutational specificity, we determined by DNA sequencing the distribution of forward mutations recovered in the N-terminal region of the lac1 gene of Escherichia coli. After activation with the S9 liver fraction from rats treated with Aroclor 1254, a diverse spectrum of mutations was recovered, with 55% of the events being G:C-->A:T transitions. In contrast, after the host-mediated assay in mice, G:C-->A:T transitions accounted for over 94% of the mutations recovered. Generally, NNK metabolism can proceed through two distinct pathways, involving either alpha-methyl or methylene hydroxylation. These two pathways produce different distributions of DNA damage. The difference in the mutational spectra we observed thus likely reflects the difference in the contributions of each pathway under the two different treatment conditions.

Mutational specificity of N-nitrosodimethylamine: comparison between in vivo and in vitro assays

We have determined the DNA alterations recovered after treatment with Aroclor 1254-treated rat S9-activated N-nitrosodimethylamine (NDMA) in the N-terminal region of the lacI gene of E. coli. A total of 125 independent LacI-d mutants of E. coli were characterized by DNA sequencing. Consistent with the known methylating ability of this compound, the predominant mutation was the G:C-->A:T transition, which accounted for 90% of all the mutations recovered. Non G:C-->A:T events include 2 G:C-->T:A, 2 G:C-->C:G, 2 A:T-->G:C, 3 A:T-->T:A and 3 frameshifts. Contingency analysis reveals that NDMA-induced mutations recovered after in vitro activation (S9) have a spectrum very similar to that previously obtained after in vivo activation employing a mouse host-mediated assay. In both systems, G:C-->A:T events clearly dominate and their distribution reveals similar site-specificity. Moreover, the proportion and kind of non-G:C-->A:T events are also similar.

Multivariate statistical analysis of mutational spectra of alkylating agents

A series of multivariate statistical methods were used to explore the current knowledge on the mutational spectra of alkylating agents (AA) in bacterial and mammalian cells. The data relative to lac I and gpt genes of Escherichia coli were considered. The analysis focused on the distribution of GC to AT transitions, which account for the majority of AA-induced mutations. The statistical analysis of 15 different mutational spectra obtained by various laboratories pointed to a number of biological factors involved in the mutational process. First of all, factor and cluster analyses demonstrated that the mutational profiles obtained in mammalian cells form a homogeneous cluster different from the cluster formed by the bacterial cell mutational spectra. SN1-type AAs give rise to classes of mutational spectra statistically different from the spectra induced by the SN2-type AAs. The analysis of the mutated sequences of both genes pointed to a correlation between mutation induction by SN1 AAs, which react through a positively charged alkylating intermediate, and the occurrence of mutations at guanines preceded 5' by a purine. Moreover, our statistical analysis showed that the distribution of AA-induced mutations is not affected by the transcriptional activity of the target gene, but is strongly determined by the sequence specificity of AA-induced mutagenesis and by the structure of the target proteins. The agreement of our results with the findings of previous studies indicates that the multivariate data analysis methods are a sensitive and reliable tool for exploring the mechanisms underlying complex biological processes. The novelty of the present results lies in their quantitative character, and in the clarity of the graphical displays. We propose the use of this methodological approach to explore the large bulk of information available on mutational spectra.

Mutational specificities of environmental carcinogens in the lacl gene of Escherichia coli H. V: DNA sequence analysis of mutations in bacteria recovered from the liver of Swiss mice exposed to 1,2-dimethylhydrazine, azoxymethane, and methylazoxymethanolacetate

The host-mediated assay (HMA) was used to determine the spectra of mutations induced in the lacl gene of Escherichia coli cells recovered from the livers of Swiss mice exposed to the carcinogens 1,2-dimethylhydrazine (SDMH), azoxymethane (AOM), and methylazoxymethanolacetate (MAMA). These spectra were further compared with changes induced by dimethylnitrosamine (DMNA) in the HMA methodology. A total of 177 independent lacl mutations arising in the HMA following exposure to SDMH, AOM, and MAMA were analyzed. Single-base substitutions accounted for 97% of all mutations analyzed. The vast majority of the single-base substitutions consisted of G:C----A:T transitions (94% of all mutations). The remaining mutations consisted of A:T----G:C transitions (3% of all mutations) while non-base substitutions accounted for only 3% of the total mutagenesis. The latter mutations consisted of one frameshift mutation and four lacO deletions. The distribution of G:C----A:T transitions induced by the three chemicals in the first 200 bp of the lacl gene was not random, but rather clustered at sites where a target guanine was flanked at the 5' site by a purine residue.

Mutagenesis by N-nitroso compounds: relationships to DNA adducts, DNA repair, and mutational efficiencies

The relationships between DNA alkylation, DNA repair and mutagenesis by N-nitroso compounds in Salmonella were examined. DNA adducts formed by treatment of the bacteria with N-nitroso compounds were monitored. Critical to the study was establishing which adducts led to mutations. Two methods were employed. In one, correlations in the dose-responses for adducts and mutagenesis were sought. For instance O6-methyl- and -ethyl-guanine, in contrast to other adducts, exhibited thresholds in their accumulation in Salmonella DNA, and mutagenesis at GC base pairs also exhibited the same threshold, suggesting a dependence of mutagenesis on the O6-alkylguanines. In the second method, mutagenesis induced by different mutagens with overlapping adduct spectra was compared. For example, EMS and ENU generate similar ratios of adenine adducts, but only ENU produces thymine adducts, and only ENU induced AT-GC and AT-CG base changes. These observations suggested that ethylthymines led to these mutations. Furthermore, it was found that these mutations were largely dependent on the presence of the plasmid, pKM101, indicating that error-prone repair activity contributes importantly in their processing to mutations. When DNA adducts by N-nitrosopyrrolidine were examined it was found that only one major adduct was detected in an excision-repair-deficient strain, and that this adduct was not present in a repair-proficient strain. Mutagenesis was also greatly reduced in the proficient strain, suggesting that mutagenesis was dependent on this adduct. From the relationships between premutagenic adduct levels and mutagenesis it was possible to calculate estimated values for the mutational efficiencies for several adducts. This calculation assumed an average distribution of adducts and mutations and required knowledge of the target size and the types of mutations that could lead to phenotypic changes. For the unrepaired O6-methyl- and -ethyl-guanines, and the O-ethylthymines the mutational efficiencies were high (ca. 30-70%), but for the N-nitrosopyrrolidine adduct it was low (ca. 1%). Initial studies were carried out on the mutational specificities of two higher homologue N-nitroso compounds (the N-nitroso-N-propyl- and N-butyl-nitroguanidines) in uvrB/pKM101 strains. This class of nitroso compounds is known to form similar DNA adducts as ENU. Their specificities were similar to that of N-nitroso-N-ethylurea at a high dose except the fraction of mutations at AT base pairs was reduced. The fraction of GC-CG transversions was although low, increased. The mutational specificities of N-nitroso-N-methylurea and N-nitrosopyrrolidine were significantly different from the specificity of E