July 27, 2000
La Jolla, CA – A genetically engineered mouse, equipped with a human gene that senses potentially toxic substances in the body, including drugs, has been created by scientists at The Salk Institute.
The transgenic or so-called “humanized” mouse should become a fundamental tool used by the pharmaceutical industry to test for potential drug-drug interaction and tolerance in a human-like system that’s built into an animal. Until now, the industry has had no other effective way of testing for potentially harmful reactions, except in patients.
“It provides the first opportunity to really do an in vivo analysis for drug safety and drug development,” said Ronald M. Evans, professor and director of the Gene Expression Laboratory at Salk and principal author of a paper published in the current issue of Nature.
“By transferring this human receptor gene into the rodent, we have created an ideal system that should respond to human drugs,” said Wen Xei, a postdoctoral researcher in Evans’ lab and the paper’s lead author.
The transferred gene, called SXR (for steroid and xenobiotic receptor), was originally isolated in Evans’ laboratory in 1993. The discovery came while researchers were searching for the human counterpart, or homolog, for a frog gene called BXR.
Over the next few years, the Evans team subjected SXR to thousands of tests. Among other things, the group found that SXR serves as a biological sentry that not only detects the presence of potentially harmful substances, it also triggers a metabolic garbage disposal that grinds them up and flushes away the residue.
They also determined that the sensor resides primarily in the liver and intestine, two organs known to be responsible for degrading and eliminating foreign substances and toxins from the body. The group further discovered that the sensor controls a family of enzymes called cytochrome P450s, which are known to break down a wide spectrum of natural and synthetic compounds.
These earlier studies determined that SXR is activated by a large range of foreign substances, or xenobiotics, including steroids such as DHEA, allergens and certain prescription drugs.
Among the most potent activators of SXR was rifampicin, an antibiotic commonly used to treat tuberculosis and other particularly stubborn infections. This was especially intriguing since rifampicin is considered the “gold standard” for triggering the xenobiotic reaction.
It also turns out that women on birth control pills who take rifampicin may be the recipients of surprising news.
“They get so-called ‘miracle babies,’ because when you activate the xenobiotic system, it not only eliminates the drug such as rifampicin, it may also eliminate other drugs you are taking,” said Evans, the March of Dimes Chair of Molecular Developmental Biology at Salk.
“So this is how you provoke a drug-drug interaction,” he continued. “One drug activates the system and suddenly you become resistant to a second or third drug. And you can have all sorts of consequences, almost all of which are not good.
“For example, your HIV medicine, your antibiotics and your birth control pills become useless.”
Likewise, it’s been recently reported that the popular herbal anti-depressant St. John’s wort stimulates the xenobiotic response, which effectively neutralizes the action of other medications, including birth control pills.
“What we found out in this study is that women who are taking contraceptive pills and then take St. John’s wort become pregnant,” said Evans. “This is because the drug induces metabolism that not only eliminates the herb, but also the contraceptive pill.”
For the most part, the drug industry has relied primarily on normal mice and rats as models to test for xenobiotic reactions. But many studies demonstrate the unreliability of the rodent model.
For example, while the antibiotic rifampicin activates human SXR, it has no effect at all in either rats or mice.
But now the Salk scientists have created a mouse that’s been genetically fitted with a human SXR receptor, while its own rodent version of the xenobiotic sensor has been removed or “knocked out.”
In their Nature paper, the Evans-led team compared the action of a popular anesthetic on a normal mouse with a transgenic SXR mouse whose xenobiotic response is constantly turned on so it’s now resistant to the effects of most drugs. While all the normal mice slept for at least a half-hour following administration of the anesthetic, the transgenic mice remained awake.
“So, this shows you how activation of a genetic network, in this case the xenobiotic response network, confers resistance to drugs,” said Evans.
The study also underscores how this receptor has evolved from species to species, the researchers said.
“Because a rodent diet may include different classes of foreign and potentially toxic substances, the rodent receptor is somewhat different than the human receptor,” said Evans.
“We view the differences as a hallmark of the kinds of evolutionary niches that animals have occupied,” he continued, “and the kind of unique environmental toxins that are encountered because of their diets and their environment.”
Also participating in the study were Bruce Blumberg, now with the University of California, Irvine; Joyce L. Barwick and Philip S. Guzelian, of the University of Colorado Health Sciences Center in Boulder, Colorado; Brent A. Neuschwander-Tetri and Elizabeth M. Brunt, at the St. Louis University School of Medicine in St. Louis, Mo.; and Michael Downes and Cynthia M. Simon, both at Salk.
Research for the studies was supported by grants from the National Institutes of Health and the G. Harold and Leila Y. Mathers Charitable Foundation.
The Salk Institute for Biological Studies, located in La Jolla, Calif., is an independent nonprofit institution dedicated to fundamental discoveries in the life sciences, the improvement of human health and conditions, and the training of future generations of researchers. The Institute was founded in 1960 by Jonas Salk, M.D., with a gift of land from the City of San Diego and the financial support of the March of Dimes Birth Defects.