Gene therapy experts discuss targeting ALS-affected cells
The 11th annual meeting of the American Society of Gene Therapy (ASGT) took place in Boston May 28-June 1. Among the dozens of sessions was an MDA-sponsored program on gene therapy for neurodegeneration.
Kenneth Fischbeck from the National Institute of Neurological Diseases and Stroke at the National Institutes of Health in Bethesda, Md., chaired the special session, which focused on identifying cellular targets and evaluating strategies for reaching them.
Like other neurodegenerative disorders, ALS primarily affects the central nervous system (CNS), the brain and spinal cord. Surrounding these tissues are protective membranes known as the blood-brain and blood-spinal cord barriers. These barriers prevent entry of many viruses and bacteria but pose a daunting challenge to gene therapists, because all ALS gene-therapy strategies require entry into the CNS, either directly or via muscle fibers.
Once these challenges are overcome, Fischbeck said, ALS gene therapy can attempt a variety of strategies, such as minimizing potential toxicity from glutamate; controlling damage to the mitochondria, which produce energy for cells; stopping abnormal clumping of proteins; blocking the effects of harmful genetic mutations; or protecting surviving motor neurons, the cells most affected in ALS.
|Some researchers have used lentiviral vectors to deliver VEGF genes to nerve cells via muscle fibers.
Brian Kaspar, an MDA grantee at Nationwide Children’s Hospital in Columbus, Ohio, discussed how ALS is accompanied by a massive increase in the number of nervous-system support cells known as astrocytes. Early in the disease, astrocytes seem to help ailing motor neurons. But their proliferation also seems to activate another type of cell, the microglia, which can kill motor neurons.
The proteins VEGF and IGF1 delay the activation of microglia, and Kaspar’s group would like to deliver them to the ALS battlefront.
VEGF and IGF1 genes can be inserted into the shells of adeno-associated viruses (AAVs), a common method of transporting genes to muscle tissue. But getting these transporters into the CNS is challenging.
Some researchers have used transporters made from lentiviruses with success in mice. Lentiviral transporters (vectors) carrying genes for VEGF have entered muscle cells and traveled up nerve fibers to nerve-cell bodies, prolonging the lives of mice with an ALS-like disease caused by mutated SOD1 genes, Kaspar noted.
In addition, Kaspar said, cells deep in the cerebellar area at the back of the brain have been fitted with biological pumps that effectively deliver IGF1 to the nervous system, at least in mice.
Type 4 AAV vectors, Kaspar noted, can target special cells that line the ventricles (open spaces) in the brain, and they too could be used to deliver therapeutic proteins to the surrounding areas.
|Jude Samulski says gene therapists are moving toward laboratory-engineered viral vectors.
But gone are the days of settling for what nature has provided in terms of AAV transporters, noted Kaspar and others during the meeting. Now it’s possible to manipulate surface proteins on AAV transporters in order to create new vehicles that can enter the central nervous system.
In trying some of these new transporters in mice, Kaspar has found therapeutic genes can enter 50 percent to 70 percent of spinal-cord motor neurons in mice after being injected into a facial vein, and that astrocytes can be targeted after an injection into a tail vein.
Jude Samulski, who has MDA funding at the University of North Carolina at Chapel Hill, said several gene-therapy clinical trials using AAV transporters already are being conducted in neurological diseases other than ALS. The limitations, he said, are “resources, not science.”
Samulski said each AAV type has different tissue targets and that gene therapists can’t assume they’ll all behave the same way in the body.
He noted there are nine basic types of natural AAVs, with new ones being designed in the laboratory. In his MDA-supported Duchenne muscular dystrophy gene-therapy trial, Samulski and colleagues are using a transporter known as AAV2.5. It’s a natural AAV2 viral shell with a laboratory-engineered change in five amino acids.
“The field is moving toward shuffled viruses,” Samulski said, referring to engineered viral shells like AAV2.5. He recommends doing high-through-put screening for AAV types that are good at getting past barriers and into nerve cells.
Echoing the views of several other presenters, Samulski said the immune response question has not been settled yet.
“It’s early in our understanding,” he said, noting that so far they haven’t seen any serious immunologic rejection of the viral shells in the muscular dystrophy gene-therapy trial. However, he noted, “You can’t infer too much from one study.”