Duke Study Finds More Genes May Be Affected By Toxic Cadmium

November 24, 1998

DURHAM, N.C. -- Duke University researchers studying primitive worms have found that as many as 31 different genes may react to the metal cadmium, an industrial additive and contaminant that is toxic to humans and animals. Twenty-two are unknown genes with no previous links to cadmium exposure.

The findings, by assistant professor Jonathan Freedman and doctoral student Vivian Hsiu-Chuan Liao of Duke's Nicholas School of Environment, are published in the Nov. 27 issue of the Journal of Biological Chemistry. Their work was funded by the Nicholas School's Marine/Freshwater Biomedical Center.

"The important thing is that these new genes serve as starting points for studies in higher organisms," Freedman said in an interview. "They can serve as a model to start investigating how cadmium can be a carcinogen in humans. We can define brand new pathways in terms of toxicology that have never been seen before.

"They can be used to define brand new regulatory pathways in organisms. For example, how does cadmium get picked up by a cell and then turn on all these different genes? These genes can serve as tools to define those pathways."

Another option is to engineer living "biomarkers" that could be used to monitor for cadmium contamination by testing for responses by certain genes, he added.

Cadmium is ranked seventh on the U.S. Environmental Protection Agency's and Agency for Toxic Substances' "Top 20 Hazardous Substances Priority List," the authors' article notes. Cadmium is used in metal coatings, nickel-cadmium batteries and pigments, and is released into the atmosphere from ore smelting and fossil fuel combustion.

Humans exposed to cadmium continually accumulate the metal in their livers, lungs and kidneys, and cadmium exposure has been linked to kidney damage, respiratory diseases and neurological disorders. It is also known to cause various cancers in rodents, and population studies suggest it causes human tumors, too.

Previous studies, some of them led by Freedman, had identified less than 15 genes that respond to cadmium exposure by triggering the synthesis of elevated amounts of proteins, he said in an interview. Those protein products include HSP70, a "heat shock" protein generated in response to stress; pyruvate carboxylase, a liver enzyme that converts pyruvate to the sugar glucose; and metallothione, which binds to the metal before it can damage cells.

"When I originally set out doing this work, it just didn't seem to me that people had discovered all the genes that could be affected by cadmium," Freedman said. "It seemed to me there should be more."

To continue that search, Freedman and Liao drew on his laboratory's expertise in C. elegans, a microscopic soil nematode that in its adult form contains exactly 959 cells. Its simplicity aids in pinpointing universal genetic traits.

The C. elegans genome -- the DNA sequences that make up its gene-bearing chromosomes -- has also been almost entirely sequenced. And many of the roundworm's genes and proteins have been found to perform identical functions in higher animals, including humans.

C. elegans' simplicity, its readily available DNA data, and its shared traits with the human genome made it an ideal test animal to hunt for additional cadmium-responsive genes. "C. elegans and humans share between 3,000 and 5,000 genes," noted Freedman, whose specialty is environmental toxicology.

The researchers identified the new genes by looking for their genetic outputs and creating laboratory copies of their DNA.

Liao first exposed some of the roundworms to elevated levels of cadmium while leaving others unexposed. She then isolated the total pools of initial gene products, called messenger RNAs (mRNAs), from both exposed and unexposed animals. Messenger RNA is the molecule copied from genomic DNA in the cell's nucleus and carried to the site of protein synthesis.

After repeating the experiments to eliminate false positives, Liao made complementary DNA (cDNA) copies of portions of those mRNAs. She did so because DNAs are easier to work with in the laboratory than RNAs, Freedman said.

Using those links of cDNA, known as expressed sequence tags (ESTs), the authors were able to determine which genes produced which mRNAs by matching the ESTs with available genomic C. elegans sequence data. That record of efforts to sequence all of the roundworm's genes is available on the World Wide Web at www.sanger.ac.uk/Projects/C_elegans/.

Since they had collected mRNAs from both cadmium exposed and unexposed roundworms, the authors were able to compare EST data from those two groups to pinpoint genes that were switched on by the toxic metal.

They also used an additional on-line website www.ncbi.nlm.nih.gov/BLAST/ to determine which sequences of amino acids -- the building blocks of proteins -- would be produced from their mRNA sequences, and how those compared with known proteins.

These procedures identified 31 "unique C. elegans gene products whose levels of expression (mRNA production) increased following cadmium exposure," the pair wrote in their Journal of Biological Chemistry report. Twenty-two of these mRNAs were found to encode "novel" proteins never before identified, they added.

The genes expressing those proteins are also novel in the sense that their ties to cadmium or any other purpose was previously unknown, even though all have been sequenced in the C. elegans genome project, Freedman said in his interview.

In the article, Freedman hypothesized why some of the additional genes might respond to cadmium exposure, but learning more will take more research, he said. "All we've done at this point is show that there may be 22 novel genes whose levels of expression increase in response to cadmium," he added.

Because different genes are activated at different times in an animal's life, the researchers were careful to select roundworms representing the entire C. elegans life cycle. Freedman and Liao thus avoided the age bias that they inadvertently introduced in a previous set of experiments by exposing entire roundworm colonies for too long a time.

They ended up with too many roundworms from the early larval stage of life because the cadmium was gradually killing off the adults, Freedman said.

Duke University Medical Center

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