"While this work is a long way from clinical application in humans, it is a prime example of the potential of in-vivo gene therapy in the brain," says graduate student Derek Choi-Lundberg, who presented this work at the Society for Neuroscience meeting in November. Using gene therapy to spur the brain to produce vital substances suggests a promising, less invasive route for a variety of neurological disorders, including Parkinson's.
Parkinson's disease targets a group of 200,000 to 400,000 dopamine-producing neurons in a section of the brain known as the substantia nigra, a tiny region smaller than a pea deep within the brain stem that serves as the brain's motor control center. These neurons produce dopamine, a neurotransmitter that's particularly important in the initiation and control of movement. In Parkinson's patients these neurons degenerate and eventually die; no one knows exactly why. The disease afflicts about half a million people in the U.S. alone.
The team injected a modified version of the common cold virus, an adenovirus, directly into the brains of rats whose dopamine-producing neurons were exposed to a toxin, hydroxydopamine, that gradually kills the neurons. The virus, stripped of its ability to replicate, carried into cells the gene that encodes glial cell-derived neurotrophic factor, or GDNF, causing them to produce the substance directly. Over the next six weeks, neurons in untreated rats were three to four times more likely to die than neurons in the rats that received the GDNF gene (69 percent vs. 21 percent).
Over the last two decades scientists have discovered that neurons require a bath of proteins, called neurotrophic factors, to stay alive and healthy. About two years ago several research teams showed that one such factor, GDNF, can help keep alive the neurons that produce dopamine, the type of neuron that dies in patients with Parkinson's disease. Earlier this year, tests at the University of Kentucky College of Medicine showed that GDNF helped relieve Parkinson's symptoms in monkeys.
But getting GDNF into the brain is a problem because of its large size and short active life. In studies up to now, scientists have either injected GDNF protein directly into an animal's brain, or they have genetically engineered cells outside the body and then grafted them into the brain.
"Grafting cells into the brain, or repeating injections every few weeks or months, are fairly invasive procedures," says Martha Bohn, Ph.D., chief investigator and associate professor of neurobiology and anatomy at Rochester. "In our approach you simply inject a viral vector into a specific area of the brain once; you modify the area of the brain that is involved in the disease, and GDNF is secreted continuously in this area. That's what you need, since Parkinson's is a prolonged disease."
Adds Gene Redmond, director of the Neurotransplantation Program at Yale University: "There has been a great deal of work suggesting that GDNF might be useful in the brain; the critical limiting factor has been the lack of an appropriate long-lasting method of delivery. Packaging GDNF in a viral vector and inserting it directly into brain cells is quite ingenious."
Treatment for Parkinson's currently consists mainly of medication to boost the amount of dopamine in the brain, though a few patients opt for an experimental surgical procedure known as a pallidotomy. In most cases the disease progresses slowly over a period of years or decades, causing the tell-tale tremors, rigidity, and slow movement. Also underway are experimental clinical trials where surgeons transplant healthy fetal dopamine neurons into the brains of Parkinson's patients.
Scientists have conducted a few clinical trials involving neurotrophic factors. In two studies on patients with amyotrophic lateral sclerosis (Lou Gehrig's disease), therapy with a neurotrophic factor known as CNTF (ciliary neurotrophic factor) was not effective. Some patients even experienced side effects such as weight loss and coughing, which scientists believe were caused by the particular method of delivery.
"There's still interest in neurotrophic factors, but it seems clear that we must target their delivery specifically to the neurons that are dying," says Choi-Lundberg. "Gene therapy has the potential to do that." Bohn is beginning to experiment with different types of vectors in an effort to target specific cells in the brain more precisely.
Other authors of the paper are Qing Lin, M.D., and Hasan Mohajeri, Ph.D., of the University of Rochester; Beverly Davidson, Ph.D., of University of Iowa College of Medicine; and Yung-Nien Chang, Ph.D., Yawen L. Chiang, Ph.D., and Carl M. Hay, Ph.D., of Genetic Therapy, Inc., in Gaithersburg, MD.
This work was funded by the National Parkinson's Foundation and supported by Genetic Therapy, Inc., which supplied the viral vectors used to shuttle the GDNF gene into cells. Genetic Therapy is a pioneer gene therapy company now owned by Novartis; Genetic Therapy and the University have filed for a patent on the use of viral vectors in gene therapy for diseases like Parkinson's. In addition, the National Institutes of Health has recently awarded Bohn a five-year, $750,000 grant to continue this work.