Mechanism Of Mutation Induction Research Articles

Elevated Minisatellite Mutation Rate in the Post-Chernobyl Families from Ukraine

Germline mutation at eight human minisatellite loci has been studied among families from rural areas of the Kiev and Zhitomir regions of Ukraine, which were heavily contaminated by radionuclides after the Chernobyl accident. The control and exposed groups were composed of families containing children conceived before and after the Chernobyl accident, respectively. The groups were matched by ethnicity, maternal age, parental occupation, and smoking habits, and they differed only slightly by paternal age. A statistically significant 1.6-fold increase in mutation rate was found in the germline of exposed fathers, whereas the maternal germline mutation rate in the exposed families was not elevated. These data, together with the results of our previous analysis of the exposed families from Belarus, suggest that the elevated minisatellite mutation rate can be attributed to post-Chernobyl radioactive exposure. The mechanisms of mutation induction at human minisatellite loci are discussed.

Spontaneous and induced minisatellite instability in the human genome.

1. Minisatellites, originally developed for forensic DNA fingerprinting and profiling, have also provided extremely informative systems for analysing processes of tandem repeat DNA turnover in the human genome. 2. Minisatellite instability involves distinct mutation processes in somatic and germline cells. In sperm, tandem repeat instability most likely arises at meiosis as a by-product of high-frequency meiotic recombination events occurring in and near minisatellites. Instability is not an intrinsic property of minisatellite repeat DNA, but instead appears to be controlled by flanking recombination initiation elements. 3. Minisatellites not only show high-frequency spontaneous mutation in the germline, but also appear to be very sensitive to mutation induction by ionizing radiation, both in experimentally irradiated mice and in human populations exposed following the Chernobyl disaster; the mechanisms of mutation induction by radiation remain enigmatic.

The mechanism of mutation induction by a hydrogen bond ambivalent, bicyclic N4-oxy-2'-deoxycytidine in Escherichia coli.

The triphosphate of the nucleoside deoxyribosyl dihydropyrimido[4,5-c][1,2]oxazin-7-one (dP) is known to be incorporated into DNA efficiently by Taq polymerase and is a useful tool for polymerase-mediated in vitro mutagenesis. It is shown here that dP is a potent mutagen in Escherichia coli and Salmonella typhimurium . In E.coli , this deoxycytidine analog induces both GC-->AT and AT-->GC transitions. No induced transversions are observed. It is highly mutagenic in wild-type E.coli, but this is much reduced in a strain lacking thymidine kinase. Mutagenesis induced by dP is efficiently inhibited by the addition of thymidine. Partially purified thymidine kinase from E.coli catalyzes phosphorylation of dP to its 5'-monophosphate. When E.coli was grown in the presence of dP, the nucleoside analog was incorporated into its DNA. The content of dP in DNA was dependent on the concentration of dP added to the medium. The incorporation characteristics of the 5'-triphosphate of dP (dPTP) were also studied using E.coli DNA polymerase I large fragment. The results confirm that this triphosphate can be incorporated opposite A and G in the template with similar efficiencies. This indicates that dP is metabolized as a thymidine analog and that the resulting triphosphate induces a high rate of mutagenesis through replicational errors.

Spontaneous and induced minisatellite instability.

Minisatellites provide not only the basis for DNA fingerprinting and DNA profiling but also extremely informative systems for analysing processes of tandem repeat turnover in the human genome. Minisatellite instability appears to involve distinct mutation processes in somatic and germline cells; in the germline, mutation is frequently dominated by inter-allelic conversion-like events most likely occurring at meiosis and apparently regulated by cis-acting mutation initiator elements. Attempts to define these initiators in transgenic mice have so far been thwarted by what appears to be a major human/mouse barrier to the inter-species transfer of repeat instability. Minisatellites not only show high frequency spontaneous mutation in the germline, but also appear to be very sensitive to mutation induction by ionizing radiation, both in experimentally irradiated mice and in human populations exposed following the Chernobyl disaster; the mechanisms of mutation induction by radiation remain enigmatic.

Genetic effects from exposure to hazardous agents.

Mammalian germ cell stages exhibit differences in DNA synthesis activity, capability to repair DNA damage, and chromosome-associated proteins. The sensitivity to mutation induction may be influenced by such factors as the accessibility of DNA to chemical mutagens, the interval between DNA damage induction and the next round of DNA replication, and the repair of DNA damage. Such qualitative and quantitative differences indicate the complexities of mutation induction in vivo and emphasize that no single in vitro test system can adequately represent the in vivo situation. Therefore, germ-cell mutagenesis in humans can most adequately be represented by an in vivo mammalian germ-cell test system. Information regarding the mechanisms of mutation induction in germ cells of the mouse, appropriate mutation test systems available in the mouse, as well as principles of chemical mutagenesis in the mouse and their implications for an adequate human genetic risk estimation will be discussed.

Mutagenesis Induced by Radiation

We are irradiated with cosmic rays at about 280 uSv a year as natural background on the earth. It has been calculated on trial that the irradiation dose by cosmic rays presumably increase to be about 40 mSv when we stay in space station for 6 months, which is higher than 50 mSv a year, the dose limit for radiation workers. Radiation produces a wide range of different types of damages of DNA and induces mutation. Mutation is the result of error in the process of repair and replication of DNA. Mechanism of mutation induction has been studied in detail in Escherichia coli. Genes such as uvrA, B and C, and umuC/D express in the cells inducing SOS response, resulting in enhancement of repair capacity and of mutagenesis. Although enhancement of the capacity of DNA repair is observed in UV- or radiation-irradiated mammalian cells, it dose not presumably associate enhancement of mutagenesis. Mechanism of mutation induction in mammalian cells may be different from that in bacterial cells. Base substitution mutations are predominantly detected in plasmid or phage DNA replcating in E. coli. GC to AT transition is most frequent base substitution. Large deletion of DNA fragment in addition to base substitution and frameshift are observed in irradiated mammalian cell. Further studies should be performed on radiation-induced mutagenesis at molecular level.

Mutagenicity of nifurtimox in Escherichia coli

The effects of nifurtimox, a nitrofuran derivative, on killing and mutation induction in 3 Escherichia coli strains having different DNA-repair systems for UV lesion were studied and compared with the effects of furylfuramide. Nifurtimox induces mutations at a high frequency in both Hs30R ( uvrA −) and H/r30R (radiation-resistant), although no significant killing effect is detected with Hs30R. No significantly induced mutation frequency could be detected with NG30 ( recA −), which is very sensitive to killing by nifurtimox. The characteristics of lesions of DNA induced by nifurtimox and the mechanism of mutation induction in Hs30R are discussed.

Mutagenicity and mutagenic specificity of metronidazole and niridazole inneurospora crassa

Mutagenicity and mutagenic specificity of niridazole and metronidazole, two chemotherapeutic agents used in the treatment of human parasitic diseases, were studied with the ad-3 test system of Neurospora crassa. The results show that neither compound is mutagenic in resting conidia. In growing vegetative cells, however, both compounds are mutagenic in N. crassa. Genetic analysis of the mutants indicated that niridazole induces predominantly base-pair substitution mutations. None of the niridazole-induced mutants resulted from multilocus deletions. The spectra of genetic alterations induced by metronidazole are similar to those induced by the mono-functional alkylating agents ethyleneimine (EI), ethylmethanesulfonate (EMS), and ICR-177. It is therefore suggested that the mechanism of mutation induction by metronidazole in Neurospora is similar to that of monofunctional alkylating agents.

Mutation induction by difunctional alkylating agents in Neurospora crassa.

The genetic characterization of ad-3 mutants of Neurospora crassa induced by two carcinogenic difunctional akylating agents, 1,2,4,5-diepoxypentane (DEP) and 1,2,7,8-diepoxyoctane (DEO), has shown that point mutations at the ad-3B locus have similar complementation patterns. In addition to the induction of point mutations, DEP induces a low frequency (7.5%) of multilocus deletions, whereas DEO induces an extremely high frequency (42.0%). The distribution of the different classes of ad-3 mutants and the frequency of multilocus deletion mutants among DEP-induced mutants are not significantly different from those induced by the monofunctional alkylating agents EI, EMS and ICR-177 at comparable forward-mutation frequencies. Moreover, the frequencies of DEP-induced ad-3B mutants showing allelic completion or having nonpolarized complementation patterns are similar to those of ad-3B mutants induced by monofunctional agents. It is suggested, therefore, that the mechanism of mutation-induction by DEP in N. crassa is similar to that of monofunctional alkylating agents. Mutation-induction by DEO probably results both from the mechanism of action of monofunctional alkylating agents and from inter-strand cross-linkage of the DNA molecular by the two functional epoxy groups.

Mutagenic activities of metal compounds in bacteria

Environmental contaminations by certain metal compounds are bringing about serious problems to human health, including genetic hazards. It has been reported that some compounds of iron, manganese and mercury induce point mutations in microorganisms. Also it has been observed that those of aluminum, antimony, arsenic, cadmium, lead and tellurium cause chromosome aberrations in plants, insects and cultured human cells. The mechanism of mutation induction by these metals remains, however, still obscure. For screening of chemical mutagens, Kada et al, recently developed a simple and efficient method named rec-assay by observing differential growth sensitivities to drugs in wild and recombination-deficient strains of Bacillus subtilis. When a chemical is more inhibitory for Rec/sup -/ than for Rec/sup +/ cells, it is reasonable to suspect mutagenicity based on its DNA-damaging capacity. In the present report, 56 metal compounds were tested by the rec-assay. Compounds showing positive results in the assay such as potassium dichromate (K/sub 2/Cr/sub 2/O/sub 7/), ammonium molybdate ((NH/sub 4/)/sub 6/Mo/sub 7/O/sub 24/) and sodium arsenite (NaAsO/sub 2/) were then examined as to their capacities to induce reversions in E. coli Trp/sup -/ strains possessing different DNA repair pathways. 11 references, 3 tables.