Naturewatch briefing on the Three Rs
Could the nematode worm replace rats and mice in animal experiments?
A primitive roundworm called Caenorhabditis elegans (C. elegans) is being evaluated in a university laboratory in North America as a cheaper and quicker alternative to rats and mice in testing chemicals for several kinds of toxicity. We look at how this may be achieved.
In its natural environment, C. elegans spends its brief life dining on microbes in the soil. But Jonathan Freedman of Duke University's Nicholas School of the Environment and Earth Sciences envisions that, in a laboratory setting, these simple animals could substantially reduce, and in some cases perhaps eventually replace, the need for expensive, large-scale rodent studies.
The tiny roundworm has long been a favorite among molecular biologists and health researchers since its 959 cells contain many genes and proteins that function similarly to those of higher animals, including humans. Freedman, an associate professor of environmental toxicology who has worked with C. elegans since graduate school, now seeks to apply this extensive knowledge about the roundworm's biology to answer questions involving toxicology.
"The idea is to quickly screen chemicals with C. elegans so you don’t have to do so many mega-rat studies," Freedman said in an interview. "If Company X thinks it has a chemical that may be a nerve toxin or cause cancer, we will put it through our system to help find out. What we've done is save that company millions of dollars because it no longer has to do as large a rat study. It can cost a company $10 million and it may have to go through 100,000 rats over a year or two just to do a complete study on one chemical. With our worms, I envision we’ll be able to get the whole thing done in a couple of weeks to maybe a month."
Freedman has a $2.4 million three-year contract with the National Toxicology Program (NTP) to evaluate the feasibility of such a "high-throughput" C. elegans toxicity testing system that will use robotic equipment to mix chemicals and sort worms efficiently. During this evaluation, his group plans to expose developing roundworms to 200 different chemicals, in collaboration with researchers at the NTP and Environmental Protection Agency. The group will also evaluate how chemicals affect a roundworm’s neural systems at various stages of life.
Freedman foresees several advantages if researchers use nematodes instead of the rats and mice that are now the laboratory standards for such work, especially during the expensive initial stages that typically involve large-scale screening. For example, it takes just 3½ days for the roundworm to develop from egg to adult. Since each adult worm is only 1 millimeter in length, large numbers can be maintained and tested in small spaces. Because the roundworms are transparent, researchers can also directly monitor chemicals effects on the worms' developing internal organs.
Toxicity screening during development evaluates how pre-selected amounts of chemicals affect groups of animals as they grow. Even if the animals do not sicken or die as they mature in the presence of toxicants, the chemicals may affect their organs in ways that can be investigated - normally through surgery or necropsy.
As with laboratory rats and mice, Freedman and his workers can even produce "knockout" varieties of C. elegans to evaluate how the animal's physiology changes if specific genes are excluded from its genome through biochemical manipulation. Unlike genetically engineered rodents, the roundworms themselves are not engineered. Instead, their bacterial food is simply spiked with "antisense" DNA designed to block the function of the gene. "To knock out one mouse gene can cost $100,000 to do a genetics study that takes a year," Freedman said. "Whereas in C. elegans we just feed the roundworms a strain of bacterium and the gene is knocked out."
In addition, strains of the roundworms have been genetically engineered in Freedman's lab and others to make various cells change color or emit a fluorescent glow in the presence of a toxic chemical.
"We know basically everything about the development of this particular organism through its entire lifespan," said Christopher Portier, associate director of NTP which is funding the program. "Since we're interested in finding something that, while it won't be definitive, can warn us about chemicals that could potentially affect development, this is an extremely good model for doing that." Using standard rats and mice, "we can only do so many assays is a given year. It’s just a matter of resources, space and availability of animals. So the idea is to develop an inexpensive, fairly sensitive screen that can give us guidance on what to test. Potentially, if we get enough information put together and we feel comfortable enough with C. elegans, it may in fact replace the rodent. But for now what we're looking for is something that helps us set priorities on what to test in rodents."
- News report based on a press release from Duke University in North Carolina, USA, 27 OCtober 2004.