Genotoxic evaluation of chronic fluoride exposure: micronucleus and sperm morphology studies

Could fluoride be considered a genotoxic chemical agent in vivo? A systematic review with meta-analysis

The goal of this study was to perform systematic review (SR) to investigate the scientific literature regarding the genotoxicity effects of fluoride exposure (FE). The search of databases used for this study was PubMed/Medline, SCOPUS and Web of Science. The quality of included studies was assessed using the EPHPP (Effective Public Health Practice Project). A total of 20 potentially relevant studies were selected for evaluating the genotoxicity induced by fluoride. Few studies have revealed that FE induces genotoxicity. A total of 14 studies demonstrated negative results whereas 6 studies did not. After reviewing the twenty studies, 1 was classified as weak, 10 were considered moderate and 9 were considered strong, according to the EPHPP. Taken together, it has been established that genotoxicity of fluoride is limited.

In Vivo Evaluation of Micronucleus Frequencies in Buccal Mucosal Cells of Orthodontic Patients with and Without Fluoride Use

Introduction:

Fluoride agents to prevent white spot lesions are used often during orthodontic treatment. The beneficial effects of fluoride, when consumed within permissible limits on dental structures, are well known. Their implications on underlying biological tissues, however, are unknown. Mouthwashes and dentifrices with fluorides are associated with metal ion release into the mouth with possible cell genotoxicity. Since these cariostatic agents are frequently used during orthodontic therapy, a deeper understanding of the effects of fluoride on oral tissues was considered necessary.

Methodology:

Three groups of patients (30 each)—group 1 (untreated controls), group 2 (non-fluoridated), and group 3 (Fluoridated) were analyzed. Patients in groups 2 and 3 were bonded with the same bracket prescription and treated with similar archwire sequences. Buccal mucosal cells at 4 specific time periods (before treatment, 1 week, 30 days, and 6 months) were collected, using a wooden tongue depressor, and assessed for any nuclear abnormalities. Comparisons of changes were made with an untreated control group and also between the non-fluoridated and fluoridated groups. Relevant conclusions were drawn after analysis of the results.

Results:

Greater number of nuclei were observed at the 30-day time interval in the fluoridated group, which was statistically significant at P < .001.

Conclusion:

Use of fluoridated oral hygiene products in patients undergoing fixed orthodontic treatment with NiTi archwires could increase the risk of micronuclei formation in buccal mucosal cells.

Genotoxic and mutagenic studies of teratogens in developing rat and mouse

In this review, genotoxic and mutagenic effects of teratogenic chemical agents in both rat and mouse have been reviewed. Of these chemicals, 97 are drugs and 33 are pesticides or belong to other groups. Large literature searches were conducted to determine the effects of chemicals on chromosome abnormalities, sister chromatid exchanges, and micronucleus formation in experimental animals such as rats and mice. In addition, studies that include unscheduled DNA synthesis, DNA adduct formations, and gene mutations, which help to determine the genotoxicity or mutagenicity of chemicals, have been reviewed. It has been estimated that 46.87% of teratogenic drugs and 48.48% of teratogenic pesticides are positive in all tests. So, all of the teratogens involved in this group have genotoxic and mutagenic effects. On the other hand, 36.45% of the drugs and 21.21% of the pesticides have been found to give negative results in at least one test, with the majority of the tests giving positive results. However, only 4.16% of the drugs and 18.18% of the pesticides were determined to give negative results in the majority of the tests. Among tests with major negative results, 12.50% of the teratogenic drugs and 12.12% of the teratogenic pesticides were negative in all conducted tests.

Fluoride-induced histopathology and synthesis of stress protein in liver and kidney of mice

Selective low (15 mg sodium fluoride (NaF)/L) and relatively high (150 mg NaF/L) doses of in vivo fluoride (F) treatment to Swiss albino mice through drinking water elicited organ-specific toxicological response. All the F-exposed groups showed severe alterations in both liver and kidney architectures, but there was no significant change in the rate of water consumption and body weight. Vacuolar degeneration, micronecrotic foci in the hepatocytes, and hepatocellular hypertrophy were evident in the mice exposed to low dose (15 mg NaF/L for 30 days) while sinusoidal dilation with enlarged central vein surrounded by deep-blue erythrocytes were preponderant when treated with the same dose for a period of 90 days. Blood filled spaces, disintegration of tubular epithelium, and atrophy of glomeruli were also recorded in the kidney of the same treatment group. Change in reduced glutathione level (GSH), glutathione-s-transferase (GST) activity, malondialdehyde (MDA) production in both liver and kidney, disturbances in liver function, induction of heat shock protein 70 (Hsp 70) expression in kidney and its down regulation in liver were positively correlated with histopathological lesion.

Fluoride-induced disruption of reproductive hormones in men

Fluoride-induced reproductive effects have been reported in experimental models and in humans. However, these effects were found in heavily exposed scenarios. Therefore, in this work our objective was to study reproductive parameters in a population exposed to fluoride at doses of 3-27 mg/day (high-fluoride-exposed group-HFEG). Urinary fluoride levels, semen parameters, and reproductive hormones in serum (LH, FSH, estradiol, prolactin, inhibin-B, free and total testosterone) were measured. Results were compared with a group of individuals exposed to fluoride at lower doses: 2-13 mg/day (low-fluoride-exposed group-LFEG). A significant increase in FSH (P<0.05) and a reduction of inhibin-B, free testosterone, and prolactin in serum (P<0.05) were noticed in the HFEG. When HFEG was compared to LFEG, a decreased sensitivity was found in the FSH response to inhibin-B (P<0.05). A significant negative partial correlation was observed between urinary fluoride and serum levels of inhibin-B (r=-0.333, P=0.028) in LFEG. Furthermore, a significant partial correlation was observed between a chronic exposure index for fluoride and the serum concentrations of inhibin-B (r=-0.163, P=0.037) in HFEG. No abnormalities were found in the semen parameters studied in the present work, neither in the HFEG, nor in the LFEG. The results obtained indicate that a fluoride exposure of 3-27 mg/day induces a subclinical reproductive effect that can be explained by a fluoride-induced toxic effect in both Sertoli cells and gonadotrophs.

Long-term exposure to fluoride in drinking water and sister chromatid exchange frequency in human blood lymphocytes

The genetic toxicity of fluoride has been investigated extensively by various test systems. However, results obtained have been inconsistent. Fluoride has been reported to be non-genotoxic, genotoxic, and synergistic or antagonistic with certain mutagens. To date, there are no published human studies on the genotoxicity of fluoride. The purpose of this investigation was to determine genotoxic risks of long-term exposure to various concentrations of fluoride in drinking water in humans with normal or inadequate nutrition. Six groups of subjects with either normal or inadequate nutritional intakes were selected from areas of approximately 0.2, 1.0, or 4.8 ppm (10.5, 52.6, or 252.6 mumol/L) fluoride in water. The subjects had been continuous residents in the area for at least 35 years. Samples of drinking water, plasma, and urine were analyzed for fluoride content. Blood lymphocytes were examined to determine the frequency of sister chromatid exchange (SCE). Blood chemistry and electrolytes were also analyzed. The results showed that average daily fluoride intake as well as urine and plasma fluoride levels increased with increase in the fluoride content of the drinking water. The blood chemistry and electrolyte values were within the normal range for all populations, but several parameters were significantly different. While the numerical differences were small, the subjects with low fluoride in the water (0.11 and 0.23 ppm or 5.8 and 12.1 mumol/L) had significantly higher SCE frequencies than those with higher fluoride exposures.(ABSTRACT TRUNCATED AT 250 WORDS)

The genotoxic and cytotoxic activities of inorganic fluoride in cultured rat bone marrow cells

The effects of sodium and potassium fluoride (NaF and KF) at concentrations ranging from 10(-7) to 10(-2) M for 12, 24, or 36 h on cultured rat bone marrow cells have been studied with respect to cytotoxicity and induction of sister-chromatid exchanges (SCE). At the three exposure times, cell survival progressively decreased with increasing concentrations. Treatment with 10(-2) M fluoride resulted in a statistically significant death (62-65%) of cells. Similarly, no dividing cells were encountered at concentrations of 10(-3) M and 10(-2) M, and significant reductions in mitotic index (MI) were calculated at 10(-4) M. In contrast, cell kinetics, expressed as cell proliferation index (CPI), revealed no significant inhibitory effect of fluoride on cell proliferation. Furthermore, the mean SCE score reached a maximum (7.64 SCE/cell) in the 24-h-treated cultures. This value was not significantly different from that observed in sodium chloride (NaCl) at 10(-2) M (5.42 SCE/cell) and distilled water (4.86 SCE/cell) controls. In comparison, mitomycin-C (MMC, positive control) at 5 x 10(-8) M caused an average of 22.13 SCE/cell. These results indicated an inhibition of cell division and death of cells with high doses of fluoride with no effect on SCE frequencies.

Bone marrow micronucleus assay: a review of the mouse stocks used and their published mean spontaneous micronucleus frequencies

We have examined published negative control data from 581 papers on micronucleated bone marrow polychromatic erythrocytes (mnPCE) for differences in mean frequency and the frequency distribution profile among the mouse stocks used with the bone marrow micronucleus assay. For the 55 mouse stocks with published micronucleus assay data, the overall mean frequency is 1.95 mnPCE/1,000 PCE (1.95 mnPCE/1,000); for the 13 stocks most commonly used in the assay, it is 1.88 mnPCE/1,000. During the last 5 years, the mnPCE rate for these 13 major stocks has been 1.74 mnPCE/1,000. This current mean frequency is a substantial decrease from the mean of 3.07 mnPCE/1,000 observed for these 13 stocks for data published prior to 1981. Of the major stocks, the highest mean mnPCE negative control frequencies were observed for MS/Ae > BALB/c > C57Bl/6, and the lowest for CD-1 < Swiss Webster. We note that hybrid mouse stocks appear to have lower and less variable negative control frequencies than either of their parent strains and that the negative control frequency for some progeny stocks have diverged significantly from that of the parent stocks. Overall mean negative control frequencies appear to be correlated with breadth of the frequency distribution profile of published mean negative control values. Furthermore, a possible correlation between negative control frequency in the micronucleus assay and sensitivity to clastogens of different mouse strains may be indicated. The databases generated here allow us to define a range of norms for both the historical mean frequency and individual experimental mean frequencies for most stocks, but in particular, for the more commonly used mouse stocks. Our analysis, for the most part, bears out the recommendation of the first Gene-Tox Report on the micronucleus assay that the historical negative control frequency for a mouse stock should fall between 1 and 3 mnPCE/1,000. Eighty-six percent of the most commonly used mouse stocks have historical mean frequencies within this range. Though individual experimental mean values would not necessarily be expected to fall within the 1-3.00 mnPCE/1,000 range, 65.3% of the 2,327 published negative control values do, and 83.5% are < 3 mnPCE/1,000. The frequency with which an individual experimental mean value lies outside the 1.00 to 3.00 mnPCE/1,000 range differs among stocks and appears related to the mouse mean frequency. We suggest that the recommended range for historical mean frequency be extended slightly, to approximately 3.4 mnPCE/1,000, to accommodate some commonly used strains with overall mean negative control frequencies just above 3.00 mnPCE/1,000.(ABSTRACT TRUNCATED AT 400 WORDS)

Genetic toxicity of fluoride

F- is not mutagenic in standard bacterial systems, but produces chromosome aberrations and gene mutations in cultured mammalian cells. Although there is disagreement in the literature concerning the ability of F- to induce chromosome aberrations in cultured human and rodent cells, the weight of the evidence leads to the conclusion that F- exposure results in increased chromosome aberrations in these test systems. NaF induced primarily chromatid gaps and chromatid breaks, indicating that the rodent cells are responsive in the G2 stage of the cell cycle. In contrast, studies with synchronized human cells indicated that the S phase was the most sensitive. If F- does have a cell cycle-specific effect, it could be expected that differences in the cell treatment and harvest protocols could lead to conflicting results for the induction of chromosome aberrations. Gene mutations were produced in cultured rodent and human cells in the majority of the studies. Unfortunately, a number of the in vitro and in vivo cytogenetic studies are of questionable utility because of the protocols used, the quality of the responses reported, or the interpretations of the data. The conflicting results in the in vivo cytogenetic studies are difficult to reconcile. There are reports of increased chromosome aberrations in rat bone marrow and testes, but other studies, using similar protocols and dose ranges, have reported no induced chromosome damage. Although some of the studies were performed at toxic levels of F-, other studies, including those that showed positive results, were at F- concentrations (1-5 ppm) equivalent to human exposure levels. In the majority of studies that were reported to be positive, there were high background frequencies, or the investigators reported categories of nuclear or chromosome damage that are difficult to interpret. Interestingly, many of the positive results were obtained when anaphase cells were scored, whereas similar treatment protocols in other laboratories yielded negative results when metaphase cells were the only cell type examined. It is difficult, without additional data, to determine the reasons for finding chromosome breaks in anaphase, but not metaphase, cells. Other reports have presented insufficient information to allow adequate evaluations. Therefore, at this time, the question of whether F- produces chromosome damage in vivo should be considered unresolved.

Analysis of clastogenic effect of Porto Alegre drinking water supplies on mouse bone marrow cells

Studies were conducted to evaluate the clastogenic activity of drinking water from Porto Alegre and Guaíba (Rio Grande do Sul, Brazil) estuarine waters. Mouse bone marrow was the target organ. C57B1/6 male and female mice received the water samples as their only liquid supply. Bone marrow cells were collected on the 16th day after the beginning of treatment. The analysis of metaphases demonstrated that the water supplies did not increase the structural chromosome aberration frequencies compared to the control groups. Concerning numerical alterations, only one treated female group showed a significant difference (loss of one chromosome) when compared to the control group, but this result is not considered relevant.

Genotoxic evaluation of chronic fluoride exposure: sister-chromatid exchange study

As part of a continuing investigation, this study was conducted to examine the genotoxic effects of chronic exposure to sodium fluoride (NaF) in drinking water on the frequency of sister-chromatid exchange (SCE) in the bone-marrow cells of male Chinese hamsters. Animals at about 3 weeks of age were randomly assigned to 6 groups, each with at least 3 hamsters, and were maintained on a low fluoride diet (less than 0.2 ppm F) throughout the experiment. At 4 weeks of age, the animals in groups I-V began to receive drinking water containing fluoride at concentrations of 0, 1, 10, 50 and 75 ppm, respectively. Group VI was treated with cyclophosphamide and served as the positive control. The animals were sacrificed at 24 weeks of age by cervical dislocation. The humeri and plasma were analyzed for fluoride content, which was found to increase with the increase in fluoride concentration in drinking water. Slides of chromosomes from bone-marrow cells were prepared and blindly examined for the frequency of SCE. The mean scores of SCE for the hamsters receiving drinking water containing F concentrations up to 75 ppm for 21 weeks ranged from 4.28 to 6.28 per cell, and were not significantly different from those of the negative controls (4.60-5.44/cell). The results indicated that chronic fluoride exposure had no effect on the frequency of SCE in Chinese hamster bone-marrow cells under the conditions of the present investigation.