Tests of three new gene therapies to treat inherited, blindness-causing diseases such as retinitis pigmentosa and age-related macular degeneration are showing much promise, according to a UCLA professor and leading authority in retinal diseases.
At least 150 faulty genes cause inherited retinal diseases. But scientists are taking advantage of new viral vectors and the retina's unique structure to place genes directly into the eye, where they take over from the faulty, or "bad," genes.
Speaking April 4 at the Experimental Biology 2005 meeting in San Diego, Dr. Dean Bok, professor of neurobiology at the David Geffen School of Medicine at UCLA and professor of ophthalmology at the Jules Stein Eye Institute at UCLA, outlined three therapies now under study that fight retinal diseases by targeting the right genes in the right places in the eye. Bok spoke at a scientific session of the American Association of Anatomists.
Though most of the studies are still in the preclinical testing stage in animals, the tests have led to at least one human trial whose results the National Eye Institute will announce later in April.
The three gene therapies are:
Ciliary Neurotrophic Factor (CNTF)
In retinitis pigmentosa, the photoreceptor cells in the eye slowly die, usually with a vision loss of roughly 5 percent per year until the person is completely blind. A drug called ciliary neurotrophic factor (CNTF) has long been known to stop the degeneration of the photoreceptor cells, although exactly how it accomplishes this is not well understood. The bigger question has been how to get it into the eye. Bok and his collaborators were among the first scientists to incorporate a CNTF mini-gene into the DNA of a virus.
When this genetically modified virus was placed in the vicinity of the retinal pigment epithelium in the eye of mice with a form of retinitis pigmentosa, the epithelial cells gobbled it up along with the CNTF gene. Since the incorporated genes were stable, the retinal pigment epithelium acted like a perpetual slow release capsule — the epithelial cells began to release CNTF, which bathed the retinal cells and stopped the degeneration of the photoreceptor cells. The mice's lost vision was not restored, but it was stabilized.
Scientists were reluctant to put a virus that could not be retrieved into the human eye. Then, a few years ago, Neurotech, a small company based in France and Rhode Island, developed an encapsulated cell technology — a little capsule that could be placed in the eye and, if need be, taken out.
Scientists used existing immortal retinal pigment epithelial cells, engineered them to secrete CNTF, and placed them in these small capsules, which they then hung inside the eyes of dogs with a retinitis pigmentosa-like disease. The capsule is designed to allow oxygen and nutrients in to sustain the epithelial cells, and to allow CNTF to leak out where it can reach the photoreceptor cells. But the tiny pores of the capsule prevent the modified epithelial cells from escaping and being attacked and destroyed by the body's immune system.
The National Eye Institute will release results from a Phase I clinical trial in humans later in April, Bok said.
Bok collaborated with the research team that first showed a gene called RPE-65 is essential for vision. Without well-functioning RPE-65 genes, the eye is unable to convert dietary vitamin A into a form that can be used for vision. When that gene is not working properly or at all, a replacement gene is delivered in a virus, which is injected into a specific location in the retina. The virus then invades cells around it and those cells pick up the replacement gene.
Humans, dogs and other animals with a mutation in this gene are born blind or nearly blind. Preclinical studies in mice and dogs have found that early treatment establishes functional vision. The first human trials are expected to begin in 2006, although they will take place in people 18 years or older who can give legal consent, Bok said. If these Phase I safety trials prove successful, Bok expects work to begin in babies soon after. The clinical trial, which is funded by the National Eye Institute, will be a multi-institutional effort at Cornell University, the University of Pennsylvania and the University of Florida.
Seeking and destroying mutant RNA
"This is the most challenging, I think, of all the types of diseases that confront us," Bok said.
This is because doctors have to compromise or completely destroy the product of the dominant mutant gene, by destroying the RNA caused by the mutant gene. The tool to accomplish this is a ribozyme, which is an enzymatic form of RNA. After an engineered virus enters a cell, it makes a specific ribozyme that selectively seeks out and destroys mutant RNA, Bok said. This relieves the cell of the effects from the mutant RNA. Then it's possible to add the replacement "good" RNA.
"In this case you're introducing two genes into the virus — one destroys the RNA, the other replaces the mutant RNA, so the cell functions appropriately," he said. This work is in collaboration with investigators at the University of California, San Francisco, and the University of Florida.
This is good news for people with diseases like certain forms of early-onset retinitis pigmentosa caused by a dominant gene, Bok said. He said preclinical studies in rats genetically engineered to have one of the 100 rhodoposin mutations found in humans have been very successful.
In these and other studies, scientists working with gene therapy in the eye have a great advantage, Bok said. "All animals have vision and vision research has the largest collection of animal models in any biomedical field, from zebra fish to dogs, some of which experience retinal problems similar to those of humans. That has helped us move toward these new therapies relatively quickly.
"The bottom line is that we are very optimistic about gene-based therapy in inherited retinal diseases," he said.
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