neuroscientists have overcome a major obstacle in gene therapy research. They've devised a method that will safely deliver and regulate expression of therapeutic genes introduced into the central nervous system to treat Parkinson's disease and other neurodegenerative diseases.
The method, developed by Martha C. Bohn and colleagues, is described in the June issue of the journal Gene Therapy. Bohn is Medical Research Institute Council Professor of Pediatrics at the Children's Memorial Institute for Education and Research and professor of pediatrics and of molecular pharmacology and biological chemistry at Northwestern University Feinberg School of Medicine.
Jiang Lixin, a post-doctoral fellow in Bohn's laboratory, created three different viral vectors -- carrier molecules -- that used human fluorescent green protein to track gene delivery and expression in cells. The vectors, made with the harmless adeno-associated virus (AAV), carried the "tet-off" system, in which the introduced gene is continually expressed or "on" but can be temporarily "turned off" when a small dose of the tetracycline antibiotic derivative doxycycline is administered.
One vector, known as rAAVS3, displayed particularly tighter regulation in neurons when gene expression was measured at the protein and molecular RNA levels.
To assess regulation in the brain, the researchers injected the vector into the striatum of rats, the area in the brain where the neurotransmitter dopamine activates the nerve cells that control motor coordination.
In their experiments, Bohn and co-researchers found that up to 99 percent of the vector-introduced gene was turned off when the rats were given small doses of doxycycline. In Parkinson's disease, dopamine-producing neurons degenerate, resulting in gait problems, muscle rigidity and tremors .
Several years ago Bohn's laboratory group discovered that glial cells in the embryonic brain stem secrete factors, or proteins, that promote survival and differentiation of dopamine neurons.
One of these proteins, called glial cell line-derived neurotrophic factor (GDNF), is a potent factor that promotes growth of not only dopamine neurons, but also motor neurons and several other types of neurons. GDNF may have therapeutic potential for several neurodegenerative diseases, including Parkinson's disease and Lou Gehrig's disease.
Bohn's laboratory was the first to show that introduction of a GDNF gene in a rodent model of Parkinson's disease halts the disease process.
"GDNF gene therapy has exciting potential to 'cure' Parkinson's disease, but since putting a gene into the brain may lead to expression and increased levels of GDNF protein for years, it will be important to have some way to turn off gene expression to arrest unanticipated side effects," Bohn said.
Bohn and her colleagues have been developing viral vectors that offer a safe means to deliver GDNF, as well as other therapeutic genes. The AAV vector that the researchers used in these experiments is safe and approved for use in several clinical trials in the brain of humans; however, no vector in which the gene can be turned off is yet approved for use in clinical trials.
"A crucial piece of our research is related to safety," Bohn said. "We were excited to find the right mechanism to deliver the gene into the nervous system and tightly control its expression using doxycycline, a drug already approved by the Food and Drug Administration and found to have no side effects."
Bohn cautioned that thorough safety and toxicity studies of the new vector are needed and that her laboratory group is not ready to assess its use in humans.