Researchers, led by Paola Leone, Ph.D. assistant professor of neurosurgery and neurosurgeon Andrew Freese, M.D., Ph.D., associate professor of neurosurgery and director of neurosurgery research, both of Jefferson Medical College of Thomas Jefferson University in Philadelphia, recently introduced 90 billion copies of a healthy gene into the brains of three children - a six-and-a-half-year-old girl from Connecticut, a three-and-a-half-year-old boy from Illinois and a five-year-old boy. Their brains lacked the gene, which makes an enzyme needed to break down an acid substance in the brain.
The scientists hope to "overexpress" the gene, resulting in high concentrations of the enzyme in the brain. As a result, the acid substance should decrease and allow the brain to begin to develop more normally and help the formation of the brain's myelin, which insulates nerve fibers that send messages to and from the brain.
The Phase I trial, which is solely to test the safety of the procedure, involves the first use of an adeno-associated virus (AAV) in the human brain, which Dr. Leone says, is much superior to the fat molecule-based delivery system used in a previous trial. The virus acts as a delivery truck to carry the healthy genes inside affected brain cells. It is also the first viral gene therapy given for a neurodegenerative disorder, she notes.
"This Phase I safety study will be a useful reference for all future trials using viral vectors in the human brain," she says.
"The new system entails using very safe, tiny, non-pathogenic viruses that are extremely efficient to express genes in the brain," she explains. The virus and gene are injected directly into the brain, rather than the ventricles, which was done in an earlier gene therapy trial. By doing so, the researchers say, the new genes should reach millions of brain cells, many times more than in the earlier trial.
Canavan disease is an inherited neurological disorder characterized by spongy degeneration of the brain. It primarily affects children of Eastern European or Ashkenazi Jewish background. It is one of a group of genetic disorders called leukodystrophies that affect the growth of the myelin sheath surrounding and insulating nerve fibers in the brain. A genetic flaw in which an enzyme fails to be produced causes the disease. It is incurable, resulting in the over-production of a toxic compound in the brain, N-acetyl-aspartate (NAA).
Symptoms of Canavan disease, which appear in early infancy and progress quickly, may include mental retardation, loss of previously acquired motor skills, difficulty feeding, abnormal muscle tone, poor head control, and an abnormally enlarged head. As time progresses, those with the disease also may become paralyzed, blind, and lose hearing and their interaction with the outside world.
"Children fail to meet normal developmental milestones," Dr. Freese explains. "Within a few years, they cannot feed themselves, walk, or see well." Dr. Freese says that the disease is similar to Tay-Sachs disease in that both are metabolic disorders causing the buildup of toxic compounds that stunts normal brain development.
Six years ago, Drs. Leone, Freese and Matthew During, M.D., professor of neurosurgery at Jefferson Medical College, then all at Yale University, were contacted by families with children with Canavan disease to see about the possibility of developing a therapy for the disease. Drs. During, Leone and Freese developed a gene delivery system based on liposomes, or fatty molecules, and polymers, rather than viruses, to deliver the genetic information for the enzyme into the brain.
Dr. During originally showed in preliminary studies in New Zealand that the missing gene could effectively be delivered into the brains of two children with Canavan disease. In 1998, the team conducted a multicenter Phase I trial at Jefferson and Yale University in which they introduced genes using a non-viral delivery system into the brains of 14 children with the disease.
The Jefferson researchers are collaborating with specialists at Children's Hospital of Philadelphia to quantitatively monitor myelin using magnetic resonance imaging techniques as well as by analyzing the brain metabolism with magnetic resonance spectroscopy.
Several photographs taken by Dr. Leone showing patients, families, doctors and researchers participating in the earlier trial currently are on display at The Genomic Revolution exhibit at the American Museum of Natural History in New York until January 1, 2002.
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