Genetic On-Off Switch Found At Purdue

Gunter Kohlhaw, a recently retired Purdue biochemistry professor, envisions a time when gene therapy will alleviate suffering from diabetes mellitus, familial gout, hypercholesterolemia or any of the other 400-plus diseases caused by an underlying genetic deficiency. But in many cases, before gene therapy can work efficiently, doctors must find genetic switches to turn therapeutic genes on and off. Such switches let them regulate levels of compounds like insulin in diabetics.

Kohlhaw and his co-workers have found one switch that looks promising.

"We have a system that is in its infancy, but which conceivably could be useful in mammals," Kohlhaw says. "We know it works perfectly in cultured mouse cells."

Other on-off gene switches already exist, but they depend on hormones or the antibiotic tetracycline. People have to eat or get an injection of a hormone or antibiotic to turn the therapeutic genes on or off, and some people can't or don't want to risk side effects associated with those compounds.

Because the compounds that make up Kohlhaw's gene switch are natural components of yeast, they shouldn't affect human health, he says, although researchers need to test them to be sure. Both compounds are part of the yeast's system for producing leucine, an amino acid essential to human health.

"What's really unique about the system is that it's not present in humans, so it can be genetically engineered in human cells and completely controlled from the outside," says Michael Hampsey, a professor in the Department of Biochemistry at the University of Medicine and Dentistry of New Jersey. "It could be used in gene therapy and other systems where tight control over gene expression is critical."

The on-off switch discovered by Kohlhaw and his co-workers at Purdue consists of two main parts. First is a regulator protein (Leu3p) that, by itself, inhibits the action of any gene under its control. Second is the inducer (isopropylmalate), a chemical that turns the gene-inhibiting regulator protein into a gene-activating protein – which turns on the gene. The Purdue researchers describe some details of how Leu3p works in a recent issue of the Journal of Biological Chemistry.

In operation, Kohlhaw's system acts more like a dimmer switch on a light than like a plain on-off switch. Without the inducer, the switch is off. Add a little inducer and you turn it on. Add more inducer and you "turn up the light."

Kohlhaw and research associate Hui Guo put this genetic dimmer switch in a circle of DNA inserted in cultured mouse cells. In a second DNA circle, they placed a gene that responded to the dimmer switch by producing luciferase, the enzyme that causes fireflies to light up. Without the inducer, the cells made very little luciferase. As Kohlhaw added more and more inducer, cells began to churn out more and more of the luciferase enzyme.

If further research shows that the genetic dimmer switch works as well in humans as in mouse cells and if isopropylmalate is indeed nontoxic, people with diabetes, for example, might control their insulin levels by eating the right amounts of isopropylmalate, Kohlhaw says.

"The genes could be turned on or off at will," he says. "The therapeutic gene would be controlled by the ingestion of the inducer."

Kohlhaw says industries also have expressed an interest in putting the gene switch in plants, where it might be used to turn on or off the production of economically important compounds.