Volume 11, Number 1
New York City
Knowledge of the cellular and molecular basis of neuron death after acute neurologic insult has increased enormously in recent years, sparking interest in the possibility of delivering neuroprotective genes to the central nervous system with viral vectors. The aim of such gene therapy, said Robert M. Sapolsky, PhD, is to reduce neuronal injury at the time of neurologic insult.
“During the last decade, we have examined the neuroprotective potential of about 25 different genes,” Dr. Sapolsky related in his address to the 127th Annual Meeting of the American Neurological Association. The data on these genes have all come from in vitro and animal studies, he added, but many neurologists are convinced that gene therapy will eventually become a relevant part of clinical neurology. Dr. Sapolsky is a Professor of Biological Sciences and Neurology at Stanford University in California.
THE BEST VECTOR
Herpes simplex virus has become one of the preferred viral vectors for neuroprotective gene therapy because it has a natural tropism for neurons and also a large genome that permits the insertion of multiple exogenous therapeutic genes. Furthermore, the herpes simplex virus can efficiently infect inactive and dividing cells and can do so asymptomatically for the life of the host.
There are two main methods of altering herpes simplex virus for use as a vector, explained Dr. Sapolsky. One is the removal of just enough genetic material from the virus so that it is nonreplicating and reasonably safe. The second method involves stripping the virus to the point that it is completely benign. Intracerebral delivery of the altered virus occurs after the addition of neuroprotective genes.
Previously, other researchers had found that delivery of the anti-apoptotic gene Bcl-2 prevented significant neuronal loss in the striatum of rats with focal ischemia due to occlusion of the middle cerebral artery. Bcl-2 delivery occurred about 1.5 hours after ischemia onset, making Dr. Sapolsky’s study one of the first to show gene therapy’s potential for postinsult neuroprotection.
Subsequent rat models have produced similar findings for other herpes simplex virus–mediated gene therapies, he related. The anti-apoptotic genes p35 and gamma 34.5 were shown to prevent hippocampal damage after excitotoxicity, for example, and the 72-kD inducible heat shock protein appeared to attenuate cerebral ischemic injury. The glucose transporter (GLUT-1) and the calcium buffer calbindin D28k have demonstrated substantial postinsult neuroprotection as well.
FACTORS INFLUENCING TREATMENT OUTCOMES
Studies indicate that the timing of delivery plays a key role in the outcome of neuroprotective gene therapy. “If you intervene very late in the cascade of steps leading to neuron death … it is possible to save the neuron from death without sparing function,” Dr. Sapolsky explained. Functional sparing, he added, might also depend on insult severity and the type of neuron affected by the insult.
The importance of early therapy was apparent in a study of rats treated with calbindin D28k by herpes simplex virus vector immediately or one hour after excitotoxic insult. The delayed gene therapy group took significantly longer to regain the ability to perform a hippocampal-dependent task (navigating a maze), even though the amount and pattern of posttherapy neuron sparing were equal in all of the study animals.
Another recent study showed that the choice of gene therapy may influence outcomes, particularly posttreatment neuron function. Researchers used herpes simplex virus to overexpress GLUT-1 or Bcl-2 after excitotoxic insult in vitro and in rats. The two genes were similarly effective for preventing hippocampal neuron loss, but only GLUT-1 preserved hippocampal function, which the researchers also assessed by observing the rats’ ability to navigate a maze.
The superior performance of GLUT-1 was likely related to its mechanism of action—the attenuation of early, energy-dependent facets of neuron death and the prevention of oxygen radical accumulation, according to Dr. Sapolsky. In contrast, Bcl-2 blocked events that occurred much later in the process leading to neuron death.
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