Differential response of two cell lines sequentially irradiated with low X-ray doses

An experiment was designed to compare the effect of repeated low doses of X-rays in two different cell lines: one transformed, epithelial like and aneuploid Chinese hamster ovary K-1 (CHO-K1); the other originated from a human primary culture, fibroblast, diploid and non-transformed, MRC-5. CHO and MRC-5 cells were cultured for 14 or eight passages, respectively. Irradiation was performed once per passage when cells were in the quiescent state (90 - 95% in G1/G0). Cells were exposed to 10.0 mSv X-ray doses. Ionizing radiation did not induce apoptosis or necrosis in the exposed CHO cell population. Significant increases of low-level damaged cells (degrees 1 and 2) were found for the 14 cycles of radiation when compared with controls, except for the first irradiation cycle. No significant increases in the frequency of cells with severe damage were observed. The frequency of MRC-5 cells with low-level damage increased significantly when compared with controls for radiation cycles seven and eight. Significant increases of apoptosis, necrosis and severe damage were found only for the highest dose. Transformed and non-transformed cell types responded differently to direct and indirect damage using low-dose repeat exposures to ionizing radiation. Though more investigation is needed to understand the mechanisms of radiation effects in chronic low-dose-exposed cell populations, cellular type should be taken into account in the design of in vitro experiments for understanding low-dose-irradiation effects.

DNA damage in Chinese hamster cells repeatedly exposed to low doses of X-rays

In a recent paper we reported the results of an experiment carried out by analysing chromosomal damage in Chinese hamster (CHO) cells exposed to low doses of X-rays. The present investigation was undertaken in order to validate those results using a different approach, the single cell gel electrophoresis assay (comet assay) immediately after irradiation. Cells were cultured during 14 cycles, irradiation treatment was performed once per cycle when the cells were at 90-95% of confluence. Doses of 2.5, 5.0 and 10.0 mSv were used. Sequential irradiation of CHO cells induced a decrease of cells without migration and an increase of cells showing DNA damage with the three doses employed. Significant increases of low-level damaged cells (p < 0.001) were found for the 14 exposures when compared to controls except for the first irradiations with 2.5 and 10 mSv, respectively. No significant increase of the frequency of cells with severe damage was observed in any case. These findings could be explained by assuming a complex interactive process of cell recovery, DNA damage and repair together with the induction of genomic instability, the incidence of bystander effects as well as some kind of radioadaptative response of the cells. If these phenomena are limited to the cell line employed deserves further investigation.

Malsegregation as a possible mechanism of aneuploidy induction by metal salts in MRC-5 human cells

Many aneugenic compounds are known to affect one or more components of the mitotic apparatus leading to an erroneous migration of chromosomes. Malsegregation occurs when a chromosome (or a chromatid) fails to migrate and remains at the metaphase plate. Nondisjunction implies the lack of dissociation between sister chromatids and the migration of both together to the same pole. The aim of the present study was to provide evidence that the aneugenic effect of some metal salts is the consequence of malsegregation at anaphase and that it is not caused by nondisjunction mechanisms. The frequencies of lagging chromosomes at anaphase-telophase of mitosis, hypoploid metaphases, and kinetochore-positive micronuclei induced by cadmium chloride, potassium dichromate, and cacodilic acid (dimethylarsinic acid) in MRC-5 human cells were compared. The data indicate that all the tested compounds are able to induce aneuploidy in MRC-5 human cells. Positive, statistically significant correlations were found when kinetochore-positive micronuclei, hypoploidy, and lagging chromosome frequencies were compared. The results suggest that malsegregation is the main mechanism involved in the induction of aneuploidy by metal salts in MRC-5 cells.

Genotoxic ability of cadmium, chromium and nickel salts studied by kinetochore staining in the cytokinesis-blocked micronucleus assay

The aneugenic and clastogenic ability of cadmium chloride(II), cadmium sulfate(II), nickel chloride(II), nickel sulfate(II), chromium chloride(III) and potassium dichromate(IV) have been evaluated through kinetochore-stained micronucleus test. Traditional genotoxicity assays evaluate DNA damage, gene mutations and chromosome breakage. However, these tests are not adequate to detect aneugenic agents that do not act directly on DNA. Staining kinetochores in the cytokinesis-blocked micronucleus assay is a useful way to discriminate between clastogens and aneuploidogens and may allow a rapid identification of aneuploidy-inducing environmental compounds. Human diploid fibroblasts (MRC-5) were employed. All compounds increased micronuclei frequency in a statistically significant way. However, increases in kinetochore-positive micronuclei frequencies were higher than in kinetochore-negative ones. The present work demonstrates the genotoxic ability of the cadmium and chromium salts studied. Aneugenic as well as clastogenic ability could be observed with this assay. Nickel salts, as it was expected because of their known weak mutagenicity, showed lower genotoxic effects than the other metal salts studied. As the test employed only allows the detection of malsegregation, it is proposed that this mechanism is at least one of those by which the tested metal salts induced aneuploidy. On the other hand, visualization of kinetochores in all experiments suggests that the compounds studied did not act by damaging these structures.

Chromosomal aberrations in peripheral blood lymphocytes from native Andean women and children from northwestern Argentina exposed to arsenic in drinking water

For conducting an adequate human cancer risk assessment of inorganic arsenic (As) in the low-dose region, it is important to establish its mode of action. In this context, the nature of genotoxic effects induced by this agent is of considerable interest. However, the results from such investigations in human have been conflicting. In an attempt to resolve this issue, the clastogenic and aneugenic potential of As was investigated in women and children from native population exposed to high levels (around 0.2 mg/l) of natural As via drinking water in San Antonio de los Corbes in the Andean region of Salta, Northwestern Argentina. The water did not contain elevated levels of heavy metals, such as lead or cadmium, nor was the investigated population exposed to significant industrial pollution or to pesticides. An ethnically similar control group from Rosario de Lerma, Salta, where only extremely low concentration of arsenic in drinking water could be detected, was used as a control. To evaluate the genotoxic effects in peripheral blood lymphocytes, micronuclei (MN) in binucleated cells, sister-chromatid exchanges (SCEs) and the fluorescence in situ hybridization technique (FISH) in combination with chromosome specific DNA libraries were employed. The data obtained clearly indicate a highly significant increase in the frequency of MN and of trisomy in lymphocytes from exposed children and women in comparison with controls, but no notable effects were found on the frequencies of SCEs, specific translocations, or on cell cycle progression. As supported by FISH analysis, at least a proportion of MN appears to originate from whole chromosome loss. An additional finding was the unusually low background levels of MN in unexposed individuals from this ethnic group as compared to other populations, e.g., Caucasians.