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Fort Lauderdale, FL—A potential new strategy has been proposed to treat optic neuritis and multiple sclerosis, namely, delivery of the superoxide dismutase (Sod) 2 gene. Intravitreal Sod2 injections in mice suppressed myelin fiber injury and rescued the retinal ganglion cells (RGCs) for up to 1 year after the inoculation, according to John Guy, MD.
"Optic neuritis is considered an inflammatory disorder of demyelination that usually has a good prognosis," Dr. Guy said at the annual meeting of the Association for Research in Vision and Ophthalmology. "There are, however, no other options for patients who have 20/40 or worse vision that persists for 6 months or longer and in whom the disease continues to recur or progress despite currently available treatments."
He and his colleagues conducted a study in which they investigated the role of oxidative stress to the mitochondria in the pathogenesis of injury to the optic nerve. They obtained specimens of the optic nerve from patients with optic neuritis and from animals in which experimental autoimmune optic nerve encephalomyelitis (EAE) was induced. They identified a reactive oxygen species in the optic nerves that contributes to permanent loss of visual function by destroying myelin in animal and human specimens.
He described two patients with severe demyelination and loss of neurons in which he and his colleagues found evidence of the reaction of nitric oxide and superoxide that forms peroxynitrite. In this patient, most axons and neurons had been destroyed, noted Dr. Guy, the Dean's Distinguished Professor in Neuro-Ophthalmology, department of ophthalmology, University of Florida, Gainesville.
Strategy to suppress injury
Dr. Guy and coworkers devised a strategy to suppress oxidative injury to mitochondria and to find a protective effect on acute optic neuritis by injecting adeno-associated virus (AAV)-Sod2 or recombinant AAV-Sod1 into the vitreous of mice with EAE. The control mice received recombinant AAV without a therapeutic gene.
The effects of this treatment approach were evaluated 1 month later. Sod1 and Sod2 were also tested to determine if they provided any long-term protective effect in EAE at 3 and 12 months after injection induction of EAE. Serial magnetic resonance imaging (MRI) was used to follow the mice sensitized for EAE. Dr. Guy reported that the MRIs initially showed contrast enhancement and swelling of the optic nerves at 2 to 4 weeks and reductions in the diameters of the optic nerves that occurred between 3 months and 1 year after induction of EAE. Injection of Sod2 suppressed these MRI findings.
Western blot testing showed presence of mitochondrial proteins 3 days after antigenic sensitization and before the infiltration of inflammatory cells into the optic nerve.
Mass spectroscopy identified the nitrated proteins to be mHsp70, according to Dr. Guy, which he described as critical to the transport of nuclear-encoded mitochondrial proteins from the cytosol and proteins encoded by mtDNA, cytochrome c oxidase subunit IV, and NADPH-ubiquinone oxidoreductase. Reactive oxygen species were seen in the optic nerves specimen, some in the mitochondria.
Sod2 was more effective as a therapy than Sod1, in that the Sod2 levels increased five-fold, suppressing myelin fiber injury by 31% up to 1 year after one gene inoculation. However, surprisingly, Sod1 increased the myelin fiber injury. Sod2 suppressed degenerating axons, some with an accumulation of mitochondria, hydropic degeneration, and loss of cristae at 1 year, providing a four-fold rescue of RGCs, he said.
Based on these results, Sod2 gene delivery may be a therapeutic strategy for optic neuritis and multiple sclerosis that is worth pursuing.