Superfund Research Program
Studies Examine the Mechanisms of Chromium Mutagenesis
Found on approximately one-third of the country's Superfund hazardous waste sites, chromium is often a significant environmental contaminant in older industrial areas that have a history of manufacturing activity in leather tanning, chrome plating, wood preserving, or dye and pigment production. These industries generate large quantities of chromium-laden wastes. While the disposal of chromium wastes is now controlled, in the past these wastes were disposed in ways that resulted in significant chromium contamination of soil, groundwater, and surface water in many industrial areas of the United States. Once released into the environment, chromium persists because as a metallic element it is non-degradable.
One of the most useful forms of chromium to industry has been the hexavalent form, chromium(VI), which combines readily with other elements to form compounds such as copper dichromate (a wood preservative) and cadmium chromate (a pigment). Much of the chromium(VI) found in the environment is a result of industrial discharges. Although useful in the production of synthetic compounds, chromium(VI) is associated with most of the adverse health effects of chromium.
Epidemiological studies have established that inhalation of chromium(VI) compounds can increase a person's risk of lung cancer. Chromium(VI) compounds are also carcinogenic in animals, as well as mutagenic in bacterial and mammalian cell systems. As a "mutagenic" environmental carcinogen, chromium(VI) has the ability to alter the DNA base sequence within a cell. However, little is known about the steps that chromium(VI) takes inside a cell to produce mutations in DNA. There is also limited knowledge about the links between chromium's activity as a mutagen and a carcinogen.
To improve understanding in these areas, researchers in the University of Cincinnati Superfund Basic Research Program are studying the mechanisms by which chromium causes mutations in cells. In recent studies, the researchers determined a mutational mechanism that may contribute to chromium-induced lung cancer in humans. Their studies showed that the mutations induced by chromium(VI) in three different mutagenesis systems are very similar to the mutations induced by agents that cause oxidative damage to DNA, results that suggest a role for reactive oxygen species in chromium mutagenesis. These important findings bring the scientists closer to their ultimate goal of discovering the biological basis for the carcinogenic effects of chromium compounds.
The three mutagenesis systems used in these studies helped the scientists elucidate specific aspects of chromium mutagenesis. A yeast system was used to characterize premutagenic DNA damage, while a mammalian cell system was used to characterize the mutational spectra or patterns of DNA base substitutions that result from chromium exposure. A genetically engineered mouse system, the "big blue transgenic mouse," was used to relate the results from the cultured cell systems to chromium mutagenesis in the lung, which is the target organ relevant to human disease. These three systems are powerful tools for exploring the molecular events associated with DNA mutations, and when used together they allow for a comprehensive analysis of chromium mutagenesis.
In the yeast system, the researchers determined that chromium(VI) mutagenesis was highest in a strain of yeast that is sensitive to agents that cause oxidative DNA damage. The specific types of DNA damage observed include DNA base damage and DNA strand breaks, while the specific types of mutations observed include deletions, insertions, and base substitution mutations.
In the mammalian system, research was conducted using a shuttle vector, which is a type of recombinant DNA molecule commonly used in mutational studies. The specific shuttle vector used in these studies is known as pZ189, and it was inserted into, propagated in, and harvested from primate host cells. The shuttle vector contains a "mutagenesis target," which is a sequence of DNA that is highly mutable and allows for measurement of mutations in a predetermined, easily isolated region of DNA. In one line of experiments, pZ189 was exposed to chromium(VI) prior to transfecting the cells; in a second set of experiments, pZ189 was exposed after being inserted into the primate cells. The mutational spectra obtained from both sets of experiments were very similar and revealed a pattern of chromium(VI)-induced DNA base mutations similar to the spectra induced by agents that cause oxidative damage to DNA (hydrogen peroxide, for example). This mutagenic activity was dependent on the chemical reduction of chromium(VI) by glutathione, a cellular reducing agent, suggesting the role of reactive oxygen intermediates in chromium-induced DNA damage.
In the big blue transgenic mouse system, the mutagenesis target is an inactive beta-galactosidase reporter gene that has been inserted into the mouse genome and becomes active upon exposure to mutagenic chemicals. Mutations are identified in this system by isolating DNA from various mouse tissues, transferring it to bacterial hosts, and scoring the activity of beta-galactosidase. The researchers treated the lungs of the mice with chromium(VI) and recovered the reporter gene after one to four weeks. These studies demonstrated that chromium(VI) is mutagenic in the lung, confirming that chromium has the potential to act as a genotoxic carcinogen in the whole organism. Mutagenicity was time- and dose-dependent, and the results were consistent with the hypothesis that chromium(VI) causes mutations through oxidative damage. These findings are significant because this is the first demonstration of chromium(VI)-induced mutations in an experimental animal.
While everyone is exposed to minute amounts of chromium from environmental sources and the food supply, people who work in or live near industries that process or use chromium compounds can be exposed to higher-than-normal levels of the metal, as it is still used in large quantities in industry. The toxicity of the hexavalent form of chromium, the presence of this metal on many hazardous waste sites, and the widespread use of the element in the production of synthetic compounds have led to increased concern about chromium exposures.
These studies, which systematically characterized the mutagenicity of chromium(VI), are significant for increasing our understanding of the links between chromium exposure, DNA damage, mutagenesis, and carcinogenesis, knowledge that will help clarify the health risks associated with exposure to chromium. The findings from the yeast, mammalian cell, and transgenic mouse mutagenesis systems together indicate that the mutagenicity of chromium(VI) may occur by a mechanism involving oxidative damage of DNA. The results also support the hypothesis that chromium mutagenesis can be induced by the generation of reactive oxygen species during the reduction of chromium(VI) in the cell.
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To learn more about this research, please refer to the following sources:
- Cheng L, Liu S, Dixon K. 1998. Analysis of repair and mutagenesis of chromium-induced DNA damage in yeast, mammalian cells, and transgenic mice. Environ Health Perspect 106(Suppl.4):1027-1032. PMID:9703488
- Liu S, Dixon K. 1996. Induction of mutagenic DNA damage by chromium (VI) and glutathione. Environ Mol Mutagen 28(2):71-79. PMID:8844987
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