Induction of bobbed (BB) mutations by polycyclic aromatic carcinogens in Drosophila

Mutagenicity of 7,12-dimethylbenz[a]anthracene and some other aromatic mutagens in Drosophila melanogaster

The optimal conditions for mutagenesis studies with DMBA and some other aromatic carcinogens in Drosophila were investigated in detail. The results presented in this paper indicate the following. The mutagenic effectiveness of DMBA is dependent on the route of administration, injection being far more effective when compared with feeding. The choice of the solvent is a crucial experimental condition. DMBA, when dissolved in oil/DMF, is ineffective whereas a special fat emulsion of DMBA gives high mutation frequencies. There appears to be an extreme strain dependence in the mutagenicity of DMBA. Mutagenic effectiveness in strain Berlin-K was rather low, whereas Oregon-K and Karsnäs-60 proved to be very susceptible to DMBA. Under the conditions of test, DMBA did not induce loss of a ring-X chromosome and did not produce recessive lethal mutations in such a chromosome. DMBA did not produce 2-3 translocations to any significant extent. An increase in DMBA-induced recessive lethal mutations was found upon storage of treated sperm with an optimal storage time of 4-10 days. DMBA is efficient in the production of delayed recessive lethal mutations in strain Berlin-K. Twice as many lethals were recovered with the F3 generation as compared with those in F2. In strain Oregon-K, where the frequency of F2 lethals was much higher than in strain Berlin-K, the ratio of F3/F2 lethals was clearly lower. Enzyme induction with phenobarbital reduces the mutagenic effectiveness of DMBA. Whereas TMBA was not mutagenic in Berlin-K, considerable mutagenicity was observed in Oregon-K and Karsnäs-60. Injection of carcinogenic polycyclic aromatic hydrocarbons and aromatic amines, when dissolved in special fat emulsions, enhances the mutagenic effectiveness of some compounds (DMBA, TMBA, DA and AcO-AAF), but this procedure does not always solve the problems pertinent to these classes of promutagens in Drosophila.

Mutagenic selectivity of carcinogenic nitroso compounds: III. N,alpha-acetoxymethyl-N-methylnitrosamine

A comparative genetic study was undertaken on the testicular tissue of Drosophila with N-alpha-acetoxymethol-N-METHYLNITROSAMINE (AcODMN) and its unsubstituted parent N,N-dimethylnitrosamine (DMN), to assess the role of intracellular metabolism on their mutagenicities. The relative genetic potencies of the two compounds were deduced from regression studies of the dose effect on the metabolically inert sperm and the metabolizing early germ cells (spermatocytes and spermatogonia) with respect to the induction of the non-specific X-chromosome recessives (lethals and visibles) and the specific effects on representatives of the RNA genes, especially rDNA. Genetic activity per unit molar dose was invariably higher for the acetoxy derivative as compared to the parent amine, but the differential in this respect varied significantly for various mutational classes and as a function of the metabolic level in the target cells. The induction of point-mutations (X-recessives) increased with the level of intracellular metabolism with bothe compounds and this was more pronounced with the parent amine, which was in accordance with the DNA methylation mechanism. In contrast, the yield of the specific rDNA deletions was not markedly enhanced with the increased metabolic activity in the early germ cells, especially with the acetoxy derivative. The induction of these deletions could not, therefore, be explained on the basis of DNA methylation, but was reconcilable with the posible generation of a nitroso-aldehydic metabolite, which could effect DNA-protein cross-linkage within the genic nucleoproteins. The two test compounds gave comparable frequencies of mosaicism among corresponding mutations and the same rDNA selectivity index, which indicated identical molecular mechanisms of mutagenesis. The higher genetic potency of the acetoxy derivative as compared to the parent amine would thus be indicative of its greater yield of the same mutagenic metabolites. Carcinogenicity studies with the two compounds paralleled the mutagenicity results, which whould suggest that the same molecular mechanisms could well be responsible for the initiation of both phenomena.

Mutagenic selectivity of carcinogenic nitroso compounds. II. N,N-dimethylnitrosamine

The genetic properties in the hepatocarcinogen N,N-dimethylnitrosamine (DMN) were examined in Drosophila for the assessment of the role of dose, cellular metabolism and genic target in its mutagenicity. Genetic activity was assayed with respect to the induction of the non-specific X-chromosome recessives (lethals and visibles) relative to the effects on specific genic sites, especially rDNA, which yields bobbed (bb) mutations. Dosses and germ cell types, which indicated that DMN induced at least some multiple-hit mutagenic events. The genetic activity of DMN was favoured by cellular metabolism for all mutational classes, as was indicated by the progressive increase in mutational classes, as was indicated by the progressive increase in mutation yield during spermatogenesis--from the metabolically inert mature sperm to the actively metabolizing spermatocyte and spermatogonia. The role of DNA methylation in the mutagenicity of DMN was deduced from quantitative assays for its genetic activity relative to the methylating nitrosamide--N-methyl-N-nitrosourethane (MNUr)--over the same dose range (1-10 mM) and on identical cell types and genic targets. In the metabolically inert cells (mature sperm), the two compounds were equally active with respect to the non-specific effects (X-recessives), but MNUr, but the two compounds were equally effective on rDNA. These results could not be entirely interpreted by the methylation hypothesis and indicated that a DMN aldehydic metabolite, structurally analogous to MNUr, might be responsible for the induction of the rDNA mutations. The rDNA selectivity index of DMN was significantly lower than for MNUr, which paralleled their relative carcinogenic verstilities. However, DMN was comparatively more effective on the tRNA genes, a feature which might be associated with its oncogenic specificity.

Some aspects of the detection of potential mutagenic agents in Drosophila

The Drosophila system is a valuable test for detecting and characterizing mutagenic agents. Tester strains are available or can be synthesized for determining almost all types of genetical change ranging from gene mutations to chromosome rearrangements in a great variety of cell types of both sexes. Metabolic activation of all groups of indirect mutagens tested so far (aryldialkyltriazenes, cyclophosphamides, nitrosamines, azo-, hydrazo- and azoxyalkanes, aflatoxins, and polycyclic hydrocarbons; about 35 representatives in all), gives strong although indirect support for the considerable metabolizing ability of Drosophila. This capability would be expected from comprehensive biochemical data on bioactivation of foreign compounds in other insects. From a comparison of which types of genetical change are induced at high, low and threshold concentrations, it appears that lethal tests remain the most reliable method for any screening program. Mutagenic agents such as diethylnitrosamine, hycanthone and certain triazenes, which are highly efficient in the induction of recessive lethals (gene mutations and/or deficiencies), would not have been detected in Drosophila if chromosome breakage were the only indicator for mutagenic activity. Moreover, for several mono- and polyfunctional agents, the lowest dose which is still genetically active was definitely lowest for recessive lethals when compared with dominant lethals, chromosome rearrangements or loss. If a new mutagen is discovered by a screening procedure using Drosophila, an accurate picture of its ability to cause either or both gene mutations and chromosome aberrations can be drawn. Such work will be valuable in helping to clarify similar problems in mammalian systems. For instance, it was important to learn that mutagens of the nitrosamine type apparently fail to produce breakage events in Drosophila. Similarly, three cyclophosphamides appeared not to have chromosome breaking ability. However, from a more detailed study, in which a series of concentrations was used, it became obvious that a penetration effect or, more likely, a rate-limiting factor in bioactivation, was the cause of the negative results obtained with these agents.

Mutagenic selectivity of carcinogenic nitroso compounds 2. n-methyl-N-nitrosourethane

The mutagenicity of the carcinogen methylnitrosourethane (MNUr) was examined in Drosophila with a view to the determination of its activity on heterochromatic loci (especially rDNA) relative to those in the euchromatin. Assays were made of the yield of rDNA mutations (bobbed: bb) relative to other X-chromosome recessive lethals and visibles [X(l + v)] in the same male germ cells after treatment with different doses (1-10 mM) and at various stages in spermatogenesis. Dose dependence followed the same pattern for all genic loci and germ cell stages. In all instances, the regression of mutation frequency on injected molar dose was approximately linear, but could better be described by a quadratic dose curve. In contrast, the mutagenicity pattern during spermatogenesis varied according to the target genes. The response of the euchromatic loci reached a peak among the earlier germ cells (probably the spermatocytes), whereas that for the heterochromatic sites (including rDNA) was maximal in mature sperm. Mutagenic selectivity for rDNA with MNUr, as indicated by the percentage bb/X-mutations, was among the highest for the intrinsically reactive carcinogens (alkylating and arylating agents). This correlates with the strong carcinogenicity of MNUr and adds further support to the concept that rDNA mutations might well be a crucial step in cancer initiation.