GAINESVILLE, Fla. --- Scientists report this week they have demonstrated that the injection of two corrective genes into a specific brain region generated significant restoration of normal limb movement in rats with a chemical-induced form of Parkinson’s disease. The findings – by a team of researchers from the University of Florida in Gainesville and Lund University in Lund, Sweden – are published in the current online version of the journal of the Proceedings of the National Academy of Sciences. Neuroscientists Anders Bjorklund of Sweden and Ronald Mandel with UF said the strategy that proved effective in the rodents is not a cure for Parkinson’s disease, but is expected to lead to a better method for delaying and controlling symptoms of the progressively disabling condition. About 1 million Americans are affected by Parkinson’s disease, which occurs most often between the ages of 65 and 90.
"We found that the simultaneous delivery of two selected genes, coupled with a powerful gene-activating agent, works like a pump to prime the production of L-dopa, which is then converted into dopamine by appropriate nerve cells in the brain," said Mandel, a professor of neuroscience with UF’s Evelyn F. and William L. McKnight Brain Institute and the UF Genetics Institute. Dopamine is a neurotransmitter chemical that plays a lead role in coordinating limb movements.
Limb impairments were completely reversed in rats that had near-total Parkinsonian lesions on only one side of the brain, meaning that some of their dopamine-producing cells remained intact. These partial lesions mimic the kind of damage (progressive death of L-dopa-producing cells) found in people with the disease, according to the scientists. Even in the rats with complete destruction of dopamine-producing cells, the delivery of gene therapy resulted in a limited amount of restored motor function. "Quite frankly, I was surprised by this successful outcome, since our previous experiments in the same animal model failed to result in restored motor function," said Mandel.
One key to success, he said, was to figure out how much L-dopa the corrective genes need to generate to produce long-lasting functional effects. "Our team was first to determine this critical threshold," he said. The experiment that helped the rats regain motor function involved a single injection of two different genes, each packaged separately along with the selected gene promoter in a gene-transport molecule (vector). Both genes code for enzymes essential to triggering production of L-dopa. Based on prior discoveries by Dr. Bjorklund, the researchers injected these genes into the striatal region of the forebrain, the destination point of dopamine-producing cells. When the injected vectors land in this area, they deposit their payload of genes, which are "turned on" by the gene-promoter (a selected segment of DNA) to initiate L-dopa production.
Mandel said videotapes of the rats moving about in tall clear-glass cylinders provide the most visible demonstration of the spontaneous behavior regained after gene therapy. Initial tapes show healthy rats standing up and exploring the sides of the cylinder with their front paws while walking around on their hind legs. In the next tapes, taken after the induction of Parkinsonian lesions, the rats can be seen dragging one limp and rigid front paw while walking around and exploring their environment with the normal front limb. Then, in videos taken after gene therapy, the same rats demonstrate normal function of both front paws.
The vector used for the studies was the apparently harmless adeno-associated virus, or AAV, developed at UF as a gene carrier in the early 1980s by Nicholas Muzyczka, an eminent scholar with the UF Genetics Institute and a co-researcher in the Parkinson’s study, and Dr. Kenneth Berns, molecular biologist. UF holds the patent on use of the AAV vector in the brain.
Mandel said the effectiveness of gene therapy in the Parkinsonian rats generates hope that the therapy can eventually be applied in humans, with the potential to double the time a person with Parkinson’s disease will respond well to standard medications. Typically a patient responds well to medication for three to five years, but the beneficial effects gradually diminish because side effects begin to reduce the therapeutic value of the drug.
"Before the U.S. Food and Drug Administration may approve this type of therapy for human testing, we have to solve a critically important problem," said Muzyczka, who also is co-director of the Powell Gene Therapy Center at UF. "The gene promoter we’re now using stays turned on continually, resulting in continuous dopamine production unlike the way our bodies adjust this production to meet our needs. We need a gene regulator that functions more like a rheostat to turn dopamine production on and off as needed to sustain normal limb movement."
UF researchers now are investigating several promising candidate gene promoters. Muzyczka said he is optimistic that human application may occur in about five years.
"We anticipate gene therapy will offer a way to help patients with Parkinson’s disease live many years longer free of disabling symptoms," Mandel said. "There is no reason why gene therapy couldn’t also be given in addition to medication and surgical procedures such as pallidotomy, or deep-brain stimulation."
The research reported in the journal results from more than four years of studies involving nearly 200 rats. Funding came from the Swedish Medical Research Council, and the Knut and Alice Wallenberg Foundation.
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