A drug already on the market for a completely unrelated condition could be used to treat a form of mental retardation linked to autism—if the results of a study in mice hold up, researchers report.

Scientists used rapamycin—a medication doctors prescribe to patients who have had transplants to prevent their bodies from rejecting the new organs—to treat learning disorders associated with a disease called tuberous sclerosis complex (TSC) in mice. TSC is a rare genetic disorder that causes brain tumors, seizures, learning disabilities, skin lesions and kidney tumors in the 50,000 Americans and one million people worldwide who have the disease.

Half of those with TSC are autistic, and as many as one in five people with the condition also suffer from mental retardation, so the hope is that rapamycin may be used to treat learning disabilities and short-term memory deficits in all kinds of autism as well, says neurobiologist and co-author of a study in Nature Medicine, Alcino Silva of the David Geffen School of Medicine at the University of California, Los Angeles.

Silva and his colleagues created mice with TSC by removing one copy of the gene TSC2. (If researchers delete both copies of the gene, the resulting mice die shortly after birth.) When the both copies of the gene are turned on in either mice and humans, they produce and regulate proteins that help strengthen connections between nerve cells, which the brain needs to remember and learn.

The TSC mice performed poorly in various learning tests, such as recalling where a platform was in a pool of water and distinguishing between cages based on what was inside them. In the mice, Silva says, "learning and memory are disrupted just like they [are] affected in most patients with tuberous sclerosis."

So why try rapamycin? The team got the idea, Silva says, after they realized the drug regulates one of the proteins that the TSC gene does, just in different parts of the body. When they tried the experiment in animals three to six months of age—well into adulthood for mice, according to Silva—rapamycin leveled the playing field between normal and TSC mice in as little as three days.

"What was surprising is that we could give rapamycin to adult mice and reverse their condition," Silva explains. "We did not know…that this drug would be equally effective for the learning disabilities" as it is for tissue rejection.

Rapamycin costs about $1,000 per month for transplant recipients. It suppresses the immune system in the body—which is necessary to thwart tissue rejection. There are, however, the expected side effects from a drug that suppresses the immune system: impaired wound healing, infections, mouth sores and, in rare instances, skin cancer.

In addition to the learning deficits, Silva says his team has "early, positive signs" from mouse models that rapamycin may also be able to treat the kidney tumors, skin lesions, brain tumors and epilepsy associated with TSC. David Franz, director of the Tuberous Sclerosis Clinic at Cincinnati Children's Hospital Medical Center, adds that the drug reduced kidney and brain tumors in small clinical trials he has conducted.

The results of the new work are similar to findings in two other diseases related to autismfragile X syndrome and Rett syndrome. Scientists were able to reverse mental retardation in mice suffering both of those illnesses, as well. Put together, Silva says, these results suggest that researchers are beginning to find brain malfunctions that cause autism—and may be possible to reverse.

These studies "suggest that we're about to have a paradigm shift in how we look at developmental disorders, like autism," he explains. These illnesses should no longer be viewed as something a person is born with, according to Silva, who believes these disorders can be eliminated by altering the brain's biochemistry.

Franz agrees that rapamycin can profoundly modify TSC—but, he notes that the mice in this study only show some of the learning disabilities seen in the disease. He thinks, however, that Silva may be overreaching in extending its benefit to all autism sufferers. "You might make them better," he says, "but I don't think you're going to normalize them."

The next step, Silva says, is a clinical trial of rapamycin in humans with TSC. That study is already underway at the University of Cambridge in England.