Mutation frequency in Salmonella exposed to weak 100-Hz magnetic fields

Do extremely low frequency magnetic fields enhance the effects of environmental carcinogens? A meta-analysis of experimental studies

PURPOSE

This paper is a meta-analysis of data from in vitro studies and short-term animal studies that have combined extremely low frequency magnetic fields with known carcinogens or other toxic physical or chemical agents.

MATERIALS AND METHODS

The data was analyzed by systematic comparison of study characteristics between positive and negative studies to reveal possible consistent patterns.

RESULTS

The majority of the studies reviewed were positive, suggesting that magnetic fields do interact with other chemical and physical exposures. Publication bias is unlikely to explain the findings. Interestingly, a nonlinear 'dose-response' was found, showing a minimum percentage of positive studies at fields between 1 and 3 mT. The radical pair mechanism (magnetic field effects on recombination of radical pairs) is a good candidate mechanism for explaining the biphasic dose-response seen in the present analysis.

CONCLUSIONS

Most of the studies reviewed used magnetic fields of 100 microT or higher, so the findings are not directly relevant for explaining the epidemiological findings suggesting increased risk of childhood leukemia above 0.4 microT. However, confirmed adverse effects even at 100 microT would have implications for risk assessment and management, including the need to reconsider the exposure limits for magnetic fields. There is an obvious need for further studies on combined effects with magnetic fields.

Intermediate frequency magnetic fields do not have mutagenic, co-mutagenic or gene conversion potentials in microbial genotoxicity tests

We used bacterial mutation and yeast genotoxicity tests to evaluate the effects of intermediate frequency (IF; 2 kHz, 20 kHz and 60 kHz) magnetic fields (MFs) on mutagenicity, co-mutagenicity and gene conversion. We constructed a Helmholtz type exposure system that generated vertical and sinusoidal IF MFs, such as 0.91 mT at 2 kHz, 1.1 mT at 20 kHz and 0.11 mT at 60 kHz. Mutagenicity, co-mutagenicity and gene conversion assays were performed for each of the three MF exposure conditions. Mutagenicity testing was performed in four strains of Salmonella typhimurium (TA98, TA100, TA1535 and TA1537) and two strains of Escherichia coli (WP2 uvrA and WP2 uvrA/pKM101) to cover a wide spectrum of point mutations. For co-mutagenicity tests, we used four sensitive test strains (TA98, TA100, WP2 uvrA and WP2 uvrA/pKM101) with five chemical mutagens (t-butyl hydroperoxide (BH, a hydroxyl free radical precursor), 2-(2-furyl)-3-(5-nitro-2-furyl) acrylamide (AF2) and N-ethyl-N'-nitro-N-nitrosoguanidine (ENNG, DNA reactive reagents), benz[a]pyrene (BaP) and 2-aminoanthracene (2AA, DNA reactive promutagens). Gene conversion testing was performed in the yeast test strain, Saccharomyces cerevisiae XD83. We also examined the effects on the repair process of DNA damage by UV irradiation. No statistically significant effects were observed between exposed and control groups in any of the genotoxicity tests, indicating that the IF MFs (0.91 mT at 2 kHz, 1.1 mT at 20 kHz or 0.11 mT at 60 kHz) do not have mutagenic or co-mutagenic potentials for the chemical mutagens tested under these experimental conditions. Our findings also indicate that these IF MFs do not induce gene conversion or affect the repair process of DNA damage in eukaryotic cells.

A 50 Hz, 14 mT magnetic field is not mutagenic or co-mutagenic in bacterial mutation assays

We used bacterial mutation assays to assess the mutagenic and co-mutagenic effects of power frequency magnetic fields (MF). For the former, we exposed four strains of Salmonella typhimurium (TA98, TA100, TA1535, TA1537) and two strains of Escherichia coli (WP2 uvrA, WP2 uvrA/pKM101) to 50Hz, 14mT circularly polarized MF for 48h. All results were negative. For the latter, we treated S. typhimurium (TA98, TA100) and E. coli (WP2 uvrA, WP2 uvrA/pKM101) cells with eight model mutagens (N-ethyl-N'-nitro-N-nitrosoguanidine, 2-(2-furyl)-3-(5-nitro-2-furyl) acrylamide, 4-nitroquinoline-N-oxide, 2-aminoanthracene, N(4)-aminocytidine, t-butyl hydroperoxide, cumen hydroperoxide, and acridine orange) with and without the MF. The MF induced no significant, reproducible enhancement of mutagenicity. We also investigated the effect of MF on mutagenicity and co-mutagenicity of fluorescent light (ca. 900lx for 30min) with and without acridine orange on the most sensitive tester strain, E. coli WP2 uvrA/pKM101. Again, we observed no significant difference between the mutation rates induced with and without MF. Thus, a 50Hz, 14mT circularly polarized MF had no detectable mutagenic or co-mutagenic potential in bacterial tester strains under our experimental conditions. Nevertheless, some evidence supporting a mutagenic effect for power frequency MFs does exist; we discuss the potential mechanisms of such an effect in light of the present study and studies done by others.

Magnetic field exposure stimulates transposition through the induction of DnaK/J synthesis

Like some naturally occurring environmental stress factors such as heat shock and UV irradiation, magnetic field exposure is also stimulatory to transposition activity. This feature could be illustrated by a bacterial conjugation study using an Escherichia coli strain that carries the transposable element Tn5 as the donor. When the donor cultures were exposed to a low-frequency (50 Hz) magnetic field of 1.2 mT, Tn5 located on the bacterial chromosome was stimulated to transpose and settled on the extrachromosomal episome, and eventually transferred to the recipient cell through conjugation. Such transposition activity stimulation was mediated by the induced synthesis and accumulation of the heat shock proteins DnaK/J.

Mutagenicity and co-mutagenicity of static magnetic fields detected by bacterial mutation assay

Possible mutagenic and co-mutagenic effects of strong static magnetic fields were estimated using bacterial mutagenicity test. Mutagenic potential of static magnetic fields up to 5T (T:1T=10,000 G) was not detected by the bacterial mutagenicity test using four strains of Salmonella typhimurium (TA98, TA100, TA1535 and TA1537) and Escherichia coli WP2 uvrA either in the pre-incubation method or in the plate incorporation method. In the co-mutagenicity test, E. coli WP2 uvrA cells were treated with various chemical mutagens and were simultaneously exposed to a 2T or a 5T static magnetic field. Mutation rate in the exposed group was significantly higher than that in the non-exposed group when cells were treated with N-ethyl-N'-nitro-N-nitrosoguanidine (ENNG), N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), ethylmethanesulfonate (EMS), 4-nitroquinoline-N-oxide (4-NQO), 2-amino-3-methyl-3H-imidazo[4,5-f]quinoline (IQ) or 2-(2-furyl)-3-(5-nitro-2-furyl) acrylamide (AF-2). The mutagenicity of 2-aminoanthracene (2-AA), 9-aminoacridine (9-AA), N4-aminocytidine and 2-acetoamidofluorene (2-AAF) was not affected by the magnetic field exposure. Possible mechanisms of the co-mutagenicity of magnetic fields are discussed.

Reproductive and teratologic effects of low-frequency electromagnetic fields: a review of in vivo and in vitro studies using animal models

In order to evaluate the reproductive risks of low-frequency electromagnetic fields (EMF), it is important to include epidemiological and animal studies in the evaluation, as well as the appropriate basic science information in developmental biology and teratology. This review presents a critical review of in vivo animal studies and in vitro tests, as well as the biological plausibility of the allegations of reproductive risks. In vitro or in vivo studies in nonhuman species can be used to study mechanisms and the effects that have been suggested by human investigations. Only well designed whole-animal teratology studies are appropriate when the epidemiologists and clinical teratologists are uncertain about the environmental risks. Even the inference of teratogenesis cannot be drawn from culture experiments, because the investigator is not in a position to know whether any of his observations will be manifested in living organisms at term. Other aspects of reproductive failure such as abortion, infertility, stillbirth, and prematurity, cannot be addressed by in vitro or culture experiments. In fact, they are very difficult to design and interpret in nonprimate in vivo models. The biological plausibility some of the basic mechanisms involved in reproductive pathology were evaluated, concentrating primarily on the mechanisms involved in the production of birth defects. The studies dealing with mutagenesis, cell death and cell proliferation using in vitro systems do not indicate that EMFs have the potential for deleteriously affecting proliferating and differentiating embryonic cells at the exposures to which populations are usually exposed. Of course, there is no environmental agent that has no effect, deleterious or not, at very high exposures. The animal and in vitro studies dealing with the reproductive effects of EMF exposure are extensive. There are >70 EMF research projects that deal with some aspect of reproduction and growth. Unfortunately, a large proportion of the embryology studies used the chick embryo and evaluated the presence or absence of teratogenesis after 48-52 h of development. This is not a stage of development at which an investigator could determine whether teratogenesis occurred. The presence of clinically relevant teratogenesis can only be determined at the end of the gestational period. The chick embryo studies are also of little assistance to the epidemiologist or clinician in determining whether EMF represents a hazard to the human embryo, and the results are, in any event, inconsistent. On the other hand, the studies involving nonhuman mammalian organisms dealing with fetal growth, congenital malformations, embryonic loss, and neurobehavioral development were predominantly negative and are therefore not supportive of the hypothesis that low-frequency EMF exposures result in reproductive toxicity.

The genotoxic potential of electric and magnetic fields: an update

We review 23 studies on the potential genotoxicity of electric and magnetic fields that have appeared in the published literature since our 1993 review of 55 published studies (McCann et al., Mutat. Res. 297 (1993) 61-95) and six additional studies published prior to 1993, which were not previously reviewed. As in our previous review, internal electric fields present in media (for in vitro experiments) and in the torso (for in vivo experiments) were estimated. Individual experiments are evaluated using basic data quality criteria. The potential for genotoxicity of electric and magnetic fields is discussed in light of the significant body of genotoxicity data that now exists. Three unsuccessful attempts to replicate previously reported positive results have appeared since our previous review. We conclude that, in spite of the 34 studies reviewed in this and our previous publication that report positive genotoxic effects, none satisfy all of three basic conditions: independent reproducibility, consistency with the scientific knowledge base, and completeness according to basic data quality criteria. As we discuss, these criteria are satisfied for several groups of negative studies in several exposure categories (ELF magnetic fields, 150 microT-5 mT, combined ELF electric and ELF magnetic fields, approx. 0.2 mT, 240 mV/m, and static magnetic fields, 1-3.7 T). The evidence reviewed here strengthens the conclusion of our previous review, that the preponderance of evidence suggests that ELF electric or magnetic fields do not have genotoxic potential. Nevertheless, a pool of positive results remains, which have not yet been tested by independent replication. Among the 12 studies reviewed here, which report statistically significant or suggestive positive results, we point particularly to results from five laboratories [J. Miyakoshi, N. Yamagishi, S. Ohtsu, K. Mohri, H. Takebe, Increase in hypoxanthine-guanine phosphoribosyl transferase gene mutations by exposure to high-density 50-Hz magnetic fields, Mutat. Res. 349 (1996) 109-114; J. Miyakoshi, K. Kitagawa, H. Takebe, Mutation induction by high-density, 50-Hz magnetic fields in human MeWo cells exposed in the DNA synthesis phase, Int. J. Radiat. Biol. 71 (1997) 75-79; H. Lai. N.P. Singh, Acute exposure to a 60-Hz magnetic field increases DNA strand breaks in rat brain cells, Bioelectromagnetics, 18 (1997) 156-165; H. Lai, N.P. Singh, Melatonin and N-tert-butyl-alpha-phenylnitrone block 60-Hz magnetic field-induced DNA single and double strand breaks in rat brain cells, J. Pineal Res. 22 (1997) 152-162; T. Koana, M. Ikehata, M. Nakagawa, Estimation of genetic effects of a static magnetic field by a somatic cell test using mutagen-sensitive mutants of Drosophila melanogaster, Bioelectrochem. Bioenergetics 36 (1995) 95-100; F.L. Tabrah, H.F. Mower, S. Batkin, P.B. Greenwood, Enhanced mutagenic effect of a 60-Hz time-varying magnetic field on numbers of azide-induced TA100 revertant colonies, Bioelectromagnetics 15 (1994) 85-93; S. Tofani, A. Ferrara, L. Anglesio, G. Gilli, Evidence for genotoxic effects of resonant ELF magnetic fields, Bioelectrochem. Bioenergetics, 36 (1995) 9-13], which satisfy most basic data quality criteria and may be of interest.

Lack of an EMF-induced genotoxic effect in the Ames assay

A few epidemiological studies have linked exposure to electromagnetic fields (EMF) and the incidence of cancer. Since many carcinogens are mutagens in the Ames assay, the purpose of this study was to determine if exposure of four tester strains of Salmonella typhimurium (TA97a, TA98, TA100, and TA102) to EMF would increase their rate of mutation. Parallel plate electrodes and Helmholtz coils were used to create uniform field properties (300 V/in., 0.3 mT). Separate and combined alternating electric and magnetic fields effects were studied at a combined field frequency of 60, 600, and 6000 Hz at room temperature. These fields did not elevate the temperature of the culture plates above room temperature, Petri dishes containing each tester strain in top agar were exposed to an electric field (E), magnetic field (M), combined electric and magnetic field (EM), or no additional field above ambient conditions in the lab (control). Four plates containing each strain were exposed in each condition: two plates had the appropriate positive-control mutagen for each strain included in the top agar and two plates did not. Plates were exposed to either E, M, EM, or control conditions at room temperature for 48 hr. and then incubated an additional 24 hr. at 37 deg. C. The plates containing mutagen in the top agar showed an increased number of colonies consistent with mutagenesis. However, the rate of mutation in the S. typhimurium strains TA97a, TA98, TA100, and TA102 in either the presence or absence of mutagen was not affected by 48 hr. exposure at room temperature to E, M, or EM fields at 60, 600, or 6000 Hz.

Reproductive and teratologic effects of electromagnetic fields

The reproductive risks of electromagnetic fields (EMF) were evaluated based on an extensive review of the scientific literature pertaining to human epidemiologic studies, secular trend data, in vivo animal studies and in vitro studies, and biologic plausibility. The epidemiologic studies involving the reproductive effects of EMF exposures to human populations have included populations exposed to: (1) video display terminals (VDTs), and (2) power lines and household appliances. The clinical use of diagnostic MRI (magnetic resonance imaging) has been increasing, but there are few reports or studies of pregnant women or individuals of reproductive age who have been exposed to MRI, and whose reproductive performance has been evaluated. The population that has been studied most frequently are women exposed to VDTs, but their EMF exposures are extremely low and frequently are at the level of the ambient EMF in a house or office. The results of epidemiologic studies involving VDTs are generally negative for the reproductive effects that have been studied. Based on the number of studies, the exposure levels, and the fairly consistent results, it can be argued that VDT epidemiologic studies should no longer be given priority. There have been fewer studies concerned with the reproductive risks of power lines, electric substations, and home appliances. In some publications, positive findings for reproductive risks were reported, but the more consistent findings indicate that EMF, even at these higher exposures, do not generate a measurable increase in reproductive failures in the human population. When compared to other fields of human epidemiology, it is obvious that these studies have many difficulties. Exposures are rarely determined. Studies frequently involve small sample sizes and the investigators rarely have a combined expertise in EMF physics, engineering, and reproductive biology. Because of the allegation that there may be particular windows of frequency, wave shape, and intensity that may be deleterious, it is impossible to disregard low frequency EMF exposures as having no deleterious reproductive effects. Yet the epidemiologic data that are available would point in that direction. Secular trend data analysis of birth defect incidence data indicate that increasing generation of electric power during this century is not associated with a concomitant rise in the incidence of birth defects. There are over 70 EMF research projects dealing with animal and in vitro studies that are concerned with some aspect of reproduction and growth. Unfortunately, a large proportion of the embryology studies utilized the chick embryo and evaluated the presence or absence of teratogenesis after 48 to 52 hours of development.(ABSTRACT TRUNCATED AT 400 WORDS)

A critical review of the genotoxic potential of electric and magnetic fields

55 published articles were identified which reported results of tests of ELF (extremely low frequency) or static electric or magnetic fields for genotoxic effects. The biological assays used spanned a wide range, including microbial systems, plants, Drosophila, mammalian and human cells in vitro and in vivo. Experimental results were grouped into four exposure categories: ELF Electric; ELF Magnetic; Static Electric; and Static Magnetic. The internal electric fields present in media (for in vitro experiments) and in the torso and extremities (for in vivo experiments) were estimated, providing an index of comparison. All experiments were critically analyzed with respect to basic data quality criteria. Experiments within each exposure category were then compared to determine if results reinforced or contradicted one another. The preponderance of evidence suggests that neither ELF nor static electric or magnetic fields have a clearly demonstrated potential to cause genotoxic effects. However, there may be genotoxic activity from exposure under conditions where phenomena auxiliary to an electric field, such as spark discharges, electrical shocks, or corona can occur. In addition, two unconfirmed reports suggest the genotoxic potential of certain chemical mutagens or ionizing radiation may be affected by co-exposure to electric or magnetic fields. Certain exposure categories are not represented or are under-represented by tests in some genotoxicity test systems that are usually included in minimal test batteries as specified by EPA for chemicals. It is suggested that consideration be given to whether additional genotoxicity testing is warranted to fill these gaps.

International Commission for Protection Against Environmental Mutagens and Carcinogens. Power frequency electric and magnetic fields: a review of genetic toxicology

Epidemiologic studies have reported a modestly increased risk of childhood leukemia associated with certain electric power wire configurations. Since cancer likely involves DNA damage, this review discusses the evidence of direct and indirect genetic toxicity effects for both electric and magnetic fields at 50- and 60-Hz and miscellaneous pulsed exposures. Exposure conditions vary greatly among different end points measured, making comparisons and conclusions among experiments difficult. Although most of the available evidence does not suggest that electric and/or magnetic fields cause DNA damage, the existence of some positive findings and limitations in the set of studies carried out suggest a need for additional work.

Effect of 60-Hz magnetic fields on ultraviolet light-induced mutation and mitotic recombination in Saccharomyces cerevisiae

The purpose of this study was to examine the effect of extremely low frequency (ELF) magnetic fields on the induction of genetic damage. In general, mutational studies involving ELF magnetic fields have proven negative. However, studies examining sister-chromatid exchange and chromosome aberrations have yielded conflicting results. In this study, we have examined whether 60-Hz magnetic fields are capable of inducing mutation or mitotic recombination in the yeast Saccharomyces cerevisiae. In addition we determined whether magnetic fields were capable of altering the genetic response of S. cerevisiae to UV (254 nm). We measured the frequencies of induced mutation, gene conversion and reciprocal mitotic crossing-over for exposures to magnetic fields alone (1 mT) or in combination with various UV exposures (2-50 J/m2). These experiments were performed using a repair-proficient strain (RAD+), as well as a strain of yeast (rad3) which is incapable of excising UV-induced thymine dimers. Magnetic field exposures did not induce mutation, gene conversion or reciprocal mitotic crossing-over in either of these strains, nor did the fields influence the frequencies of UV-induced genetic events.

Cancer promotion in a mouse-skin model by a 60-Hz magnetic field: II. Tumor development and immune response

This paper describes preliminary findings on the influence of 60-Hz (2-mT) magnetic fields on tumor promotion and co-promotion in the skins of mice. The effect of magnetic fields on natural killer (NK) cell activity in spleen and blood was also examined. Groups of 32 juvenile female mice were exposed to the magnetic field as described in part I. The dorsal skin of all animals was treated with a subthreshold dose of the carcinogen 7,12-dimethyl-benz(a)anthracene (DMBA). One week after the treatment, two groups were sham exposed (group A) or field exposed at 2 mT (group B) 6 h/day for 21 weeks, to test whether the field would act as a tumor promoter. No tumors developed in these two groups of mice. To test whether the magnetic field would modify tumor development by directly affecting tumor growth or by suppressing immune surveillance, two additional groups of mice were treated weekly with the tumor promoter 12-0-tetradecanoylphorbol-13-acetate (TPA) and then either sham exposed (group C) or field exposed (group D). The time to appearance of tumors was shorter (but not statistically so) in the group exposed to magnetic fields and TPA. Some differences in NK cell activity and spleen size were observed between the sham- and field-exposed groups.