UF researchers reverse metabolic disorder in mice

May 30, 2001

GAINESVILLE, Fla. – Several hundred babies are born in the United States each year with an inherited disorder known as PKU, which requires adherence to an unpleasant protein-free diet to thwart development of mental retardation.

Now University of Florida researchers report a small victory on the road to a cure: In male laboratory mice, they have effectively reversed the condition with a single treatment of gene therapy. The findings raise the hope that as the science progresses in coming years, people with the disorder will no longer have to shun so much of the typical human diet, including meat, fish, poultry, nuts and dairy products.

“Some people mistakenly believe that there already is a cure for PKU, but cure should be in quotation marks,” said Philip J. Laipis, who is presenting his research Saturday (6/2) at the American Society of Gene Therapy meeting in Seattle. “The ‘cure’ is that you eat an artificial diet of medical food. It’s a liquid and doesn’t taste very good, and there’s not much you can do to disguise the taste.

“We’re interested in advancing gene therapy because the potential is that a single treatment would last a lifetime or at least a significant number of years,” said Laipis, a professor of biochemistry and molecular biology who is affiliated with UF’s Genetics Institute and Powell Gene Therapy Center.

Laipis estimated that human trials of the experimental approach are several years away. PKU, which stands for phenylketonuria, results from a deficiency or complete absence of a specific enzyme that is necessary to convert the amino acid phenylalanine – found in all types of protein – into another amino acid, tyrosine. A buildup of phenylalanine is toxic to the central nervous system, particularly the brain.

Because infants with the disease face an extremely high risk of mental retardation if not treated, all babies in the United States and many other countries are screened for the disorder shortly after birth. Retardation rates have declined as a result.

In years past, it was assumed that strict dietary control was not necessary for adolescents and adults, but researchers now suggest that it’s important to keep phenylalanine levels in check throughout life to avoid other problems such as learning disabilities, hyperactivity, tremors and personality disorders.

Dietary control is especially important for women with PKU who would like to have children. High phenylalanine levels during pregnancy can result in children born with serious birth defects. Studies have shown that 85 percent of women with PKU who become pregnant do not have the condition under control at the time of conception.

Laipis said he began his research after meeting a patient of UF faculty member Dr. Roberto T. Zori. The patient, who has the disease, asked him point blank several years ago: “Why don’t you try gene therapy for PKU?”

Laipis and his colleagues recognized that the disease, caused by a single gene mutation, was a good target for an approach involving the delivery of corrective genes to the cells of selected tissues.

For the experiment, researchers in UF’s colleges of Medicine and Veterinary Medicine genetically modified the apparently harmless adeno-associated virus to contain a healthy copy of the gene, known as PAH. The scientists injected the modified virus into mice that have a gene defect similar to that found in people with PKU.

In treated male mice, the gene therapy resulted in sharp reductions of phenylalanine, to levels considered close to normal.

“Basically we’ve cured males,” Laipis said. “We know what the effective dose is, and there don’t seem to be any adverse effects related to the treatment. Unfortunately, the story with the female mice is more complicated. We initially found that the females did not respond to the treatment.”

The researchers are exploring several options for adjusting the treatment for females.

“There may be reasons specifically related to how testosterone influences enzyme activity in mice that would explain why there was a difference in response to the treatment,” Laipis said. “We don’t know if this will be true in people.

“This is an important question for us to address in our ongoing studies,” Laipis said. “Before we begin testing in people, we also have a lot of work to do to understand the distribution of the corrective genes in the cells and the stability of the treatment and to determine the best age to treat.”

UF’s research was supported in part by funding from the Children’s Miracle Network, a nonprofit organization that raises funds and awareness for 170 children’s hospitals throughout North America, the Powell Gene Therapy Center and the National Institutes of Health.