Mutagenic activity of 6-azido deoxyhexoses and azido alcohols in Salmonella typhimurium and its inhibition by a structure-similar carbon source in the medium

Potent mutagenicity of an azide, 3-azido-1,2-propanediol, in human TK6 cells

Sodium azide is a strong mutagen that has been successfully employed in mutation breeding of crop plants. In biological systems, it is metabolically converted to the proximate mutagen azidoalanine, which requires further bioactivation to a putative ultimate mutagen that remains elusive. The nature of the DNA modifications induced by azides leading to mutations is also unknown. Other mutagenic organic azido compounds seem to share the same bioactivation pathway to the ultimate mutagenic species as they induce point mutations dependent on the same DNA repair pathways. We investigated mutations induced by the representative mutagen 3-azido-1,2-propanediol (azidoglycerol, AZG) in the human TK6 cell line. Until now, azides have been considered to be non-mutagens and non-carcinogens in mammals, including humans, as judged only by the conventional clastogenicity chromosomal aberration types of bioassays. Here, we show the potent mutagenicity of AZG in cultured human cells, comparable to alkylating agents such as methyl methanesulfonate at concentrations with similar lethality. The potent ability of an organic azide to induce base substitutions in a mammalian system raises an alert with respect to human exposure to organic and inorganic azido compounds.

Effect of episomally encoded DNA polymerases on chemically induced mutagenesis at the hisG46 target in Ames test

BACKGROUND

The standard Ames test strains owe their high sensitivity to chemical and physical mutagens to the episomal Y-family DNA polymerase RI encoded by the mucAB operon. The S. typhimurium test strains carry also another related samAB operon on a 60-kDa cryptic plasmid. In contrast to the chromosomally encoded Y-family DNA polymerases V and IV, these plasmid born polymerase genes have no direct counterpart in mammalian cells. By replicating damaged templates, DNA polymerases play a central role in mutagenesis and genome stability. It is therefore imperative to investigate their specificity to understand differences in mutagenesis between the prokaryotic versus eukaryotic (mammalian) systems. To this end we have isolated and separately expressed the DNA polymerase subunits encoded by the mucAB and samAB operons. After demonstrating how these enzymes control chemical and UV mutagenesis at the standard hisD3052 and hisG428 Ames test targets, we are now adding the third Ames test target hisG46 to the trilogy.

RESULTS

Four new Ames tester strains based on the hisG46 target have been constructed expressing the activated DNA polymerase MucA' and SamA' accessory subunits combined with the MucB and SamB catalytical subunits under the control of lac promoter. These polymerase assemblies were substituted for the endogenous PolRI, PolV and SamAB polymerases present in the standard TA100 strain and tested for their abilities to promote chemically induced mutagenesis. SamA' + SamB has been able to promote mutagenesis induced by AF-2 and 1,8-DNP to higher extent than SamA' + MucB. The MucA' + MucB (PolRI*) more efficiently promoted MMS as well as spontaneous mutagenesis than its wild type counterpart but was less efficient for other mutagens including AFB1. Strikingly azide mutagenesis was inhibited by PolRI and also SamA'B.

CONCLUSION

A new system for SOS-independent overexpression of the activated DNA polymerases RI and SamA'B and their chimeras in the hisG46 Ames test background has been established and validated with several representative mutagens. Overall, the TA100 strain showed the highest sensitivity towards most tested mutagens. The observed inhibition of azide mutagenesis by PolRI* suggests that this type of Y-family DNA polymerases can perform also "corrective" error free replication on a damaged DNA.

Exclusive induction of G:C to A:T transitions by 3-azido-1,2-propanediol in yeast

Sodium azide is a strong mutagen which has been successfully employed in mutation breeding of crop plants. In biological systems, it is metabolized to azidoalanine, but further bioactivation to a putative ultimate mutagen as well as the nature of the induced DNA modifications leading to mutations remain elusive. In this study, mutations induced in the CAN1 gene of yeast Saccharomyces cerevisiae by the representative mutagen 3-azido-1,2-propanediol (azidoglycerol, AZG) have been sequenced. Analysis of the forward mutation spectrum to canavanine resistance revealed that AZG induced nearly exclusively G:C to A:T transitions. AZG also induced reversions to tryptophan prototrophy by base-pair substitutions in a dose-dependent manner. This unusual mutational specificity may be shared by other organic azido compounds.

Sodium azide induces mitotic recombination in Drosophila melanogaster larvae

Sodium azide (NaN3), a potent mutagen for bacteria and barley, was tested for somatic mutation and mitotic recombination induction in wing imaginal disc cells of Drosophila melanogaster. Comparisons were made among inversion-free flr3/mwh, inversion-heterozygous TM3, Ser/mwh, and inversion-free, high bioactivation OR(R), flr3/mwh flies. Third instar larvae were exposed chronically for 48 h to sodium azide at 0.5, 0.63, 0.75, 0.88 and 1.0 mM. The frequencies of spots per wing obtained in the three kinds of progeny scored were compared. In inversion-free flies, sodium azide induced large single and total spots at all concentrations tested, and small single and twin spots at 0.75 mM and higher concentrations. In contrast, it failed to increase the frequency of small and large single spots in inversion-heterozygous flies. In high bioactivation flies (which are inversion-free), sodium azide increased the frequency of large single spots at 0.63, 0.88 and 1.0 mM and the frequency of total spots at 0.63 mM. From the absence of genotoxic activity observed in inversion-heterozygous flies it is concluded that sodium azide induces exclusively mitotic recombination in wing somatic cells of Drosophila melanogaster larvae after chronic exposure. This recombinogenic activity is reduced in the presence of high bioactivation capacity.

Comparative mutagenicity of chemicals selected for test in the International Program on Chemical Safety's collaborative study on plant systems for the detection of environmental mutagens

A review has been made for the four compounds (maleic hydrazide, methyl nitrosourea, sodium azide, azidoglycerol) tested in the International Program on Chemical Safety's collaborative study on plant systems. Maleic hydrazide (MH) is a weak cytotoxic/mutagenic chemical in mammalian tissues and is classified as a class 4 chemical. In contrast, with few exceptions such as Arabidopsis, MH is a potent mutagen/clastogen in plant systems. The difference in its response between plant and animal tissue is likely due to differences in the way MH is metabolized. MH appears to be noncarcinogenic and has been given a negative NCI/NTP carcinogen rating. Methyl nitrosourea (MNU) is a toxic, mutagenic, radiomimetic, carcinogenic, and teratogenic chemical. It has been shown to be a mutagen in bacteria, fungi, Drosophila, higher plants, and animal cells both in vitro and in vivo. MNU is a clastogen in both animal and human cell cultures, plant root tips and cell cultures inducing both chromosome and chromatid aberrations as well as sister-chromatid exchanges. Carcinogenicity has been confirmed in numerous studies and involves the nervous system, intestine, kidney, stomach, bladder and uterus, in the rat, mouse, and hamster. MNU produces stage-specific teratogenic effects and also interferes with embryonic development. The experimental evidence that strongly indicates the mutagenic effects of MNU underlines the possible hazard of this compound to human beings. The experimental evidence for the stringent handling of this compound is clear. Sodium azide (NaN3) is cytotoxic in several animal and plant systems and functions by inhibiting protein synthesis and replicative DNA synthesis at low dosages. It is mutagenic in bacteria, higher plants and human cells and has been used as a positive control in some systems. In general, tests for clastogenicity have been negative or weakly positive. No evidence of carcinogenicity has been reported in a 2-year study seeking carcinogenic activity in male and female rats. Its advantages in comparison to other efficient mutagens are claimed to be a high production of gene mutations accompanied by a low frequency of chromosomal rearrangements and safer handling because of its nonclastogenic and noncarcinogenic action on humans. Azidoglycerol (AG) is a very potent mutagen in bacteria, yeast and higher plants including Arabidopsis and Tradescantia; however, it only slightly enhances the frequencies of recessive lethals in Drosophila. AG is at best a weak clastogen and is without effect in inducing chromosomal aberrations and SCEs in human peripheral lymphocytes in vitro. In microbial and plant systems, AG is considerably more potent than sodium azide in the maximal frequencies of mutation induced. In particular, in Saccharomyces cerevisae, AG is 3000-fold more mutagenic than sodium azide. Its carcinogenic and teratogenic properties are unknown.

Effects of deuterium labeling on azido amino acid mutagenicity in Salmonella typhimurium

The mutagenic effects of azide (N3-) anion in bacterial test systems require the formation of the novel mutagenic metabolite, 3-azido-L-alanine (AZAL). Although the mechanism of AZAL-induced mutagenicity is unknown, subsequent bioactivation of this metabolite appears likely. Earlier studies have shown that other azide-containing amino acids are mutagenic as well. In fact, the mutagenic potency of the synthetic AZAL homologue, L-2-amino-4-azidobutanoic acid (HomoAZAL), was several times that of AZAL. To gain insight into the biochemical processing and mutagenicity of azido amino acids in Salmonella typhimurium, several specifically deuterium-labeled azido amino acids have been prepared and tested for mutagenic potency. In addition, the effect of (aminooxy)acetic acid (AOA) (a potent inhibitor of pyridoxal-dependent processes) on AZAL-induced mutagenesis was examined. The results showed that 2-deuterium substitution of AZAL resulted in a slight increase in mutagenic potency, while AOA treatment resulted in no change in AZAL potency. Taken together these findings did not support the involvement of pyridoxal-dependent processes in AZAL bioactivation. In contrast, deuterium substitution adjacent to the azide group in HomoAZAL and 5-azido-L-norvaline (N3-Norval) resulted in a large decrease in mutagenic potency when compared to the non-deuterium labeled compounds. These observations are consistent with a bioactivation mechanism involving rate-limiting C-H bond cleavage in the formation of the ultimate mutagen. Moreover, this effect of deuterium labeling points to processing of the azide-containing side chain as a key feature in the mutagenic activation mechanism.

Mutagenicity of 3-azido-1,2-propanediol and 9-(3-azido-2-hydroxypropyl)-adenine in repair deficient strains of Escherichia coli

The mutagenicity of two non-aromatic organic azido compounds, 3-azido-1,2-propanediol (AG) and 9-(3-azido-2-hydroxypropyl)-adenine (AHPA), was studied in E. coli repair deficient strains uvrA6, uvrA6 + umuC36, uvrA6+ umuC122::Tn5, polA1, tagA1+ alkA1, ada and dam3. The mutagenicity of both agents was markedly enhanced by defects of UvrABC excinuclease (uvrA6) and was independent of umuC function of the SOS error-prone pathway. Neither azido compound promoted umuDC operon expression. The mutagenicity of AG in tag A1, alkA1 and ada mutants does not differ from that found in the wild-type strain. The expression of both ada and alkA genes was not elevated by AG. Experiments on polA1 and dam3 mutants suggest that DNA polymerase I as well as the mutHLS mismatch repair pathway does not contribute to the removal of putative DNA lesions induced by AG.

Sodium azide mutagenesis: preferential generation of A.T-->G.C transitions in the barley Ant18 gene

The molecular basis for the absence of anthocyanins and proanthocyanidins in four independent sodium azide-induced ant18 mutants of barley was examined by sequencing the gene encoding dihydroflavonol 4-reductase in these mutants. Sodium azide generated 21 base substitutions, which corresponds to 0.17% of the 12,704 nucleotides sequenced. Of the substitutions, 86% were nucleotide transitions, and 14% were transversions. A.T-->G.C base pair transitions were about 3 times more frequent than G.C-->A.T transitions. No deletions or mutation hot spots were found. The absence of dihydroflavonol 4-reductase activity in ant18-159, ant18-162, and ant18-164 plants is caused by missense mutations in the respective genes. By using microprojectile bombardment, a plasmid harboring the wild-type Ant18 gene was introduced into ant18-161 mutant cells and resulted in the development of anthocyanin pigmentation, which demonstrates that the mutation is corrected by expression of the introduced gene. On the other hand, a plasmid derivative with the two ant18-161-specific base transitions at the 5' splice site of intron 3 prevented complementation. It is concluded that the absence of detectable mRNA for dihydroflavonol 4-reductase in ant18-161 cells is due to the mutations in the pre-mRNA splice donor site.