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Gene Therapy in Ophthalmology Update 10: Gene Therapy Research in Dogs Cures X-Linked Retinitis Pigmentosa – Paves the Way

Posted Mar 14 2012 6:15pm
Researchers at several universities and laboratories collaborated to treat dogs afflicted with the x-linked form of retinitis pigmentosa, to deliver the therapeutic RPGR gene specifically to the diseased rods and cones, which led to functional and structural recovery. This is the first proof that this condition is treatable in an animal model and the researchers feel the results are promising and relevant for translation to humans afflicted with this disease.

The results have been published in the journal, Proceedings of the National Academy of Sciences.

Here are twin news releases from the University of Pennsylvania and the University of Florida, that participated in the study:

Gene therapy research cures retinitis pigmentosa in dogs January 23, 2012
Members of a University of Pennsylvania research team have shown that they can prevent, or even reverse, a blinding retinal disease, X-linked Retinitis Pigmentosa, or XLRP, in dogs.
The disease in humans and dogs is caused by defects in the RPGR gene and results in early, severe and progressive vision loss. It is one of the most common inherited forms of retinal degeneration in man.
"Every single abnormal feature that defines the disease in the dogs was corrected following treatment," said lead author William Beltran, assistant professor of ophthalmology at Penn's School of Veterinary Medicine.
"We were thrilled," said senior author Gustavo Aguirre, professor of medical genetics and ophthalmology at Penn Vet. "The treated cells were completely normal, and this effect resulted from introducing the normal version of the human gene into the diseased photoreceptor cells."
The similarities between humans and dogs, in terms of both eye anatomy, physiology, disease characteristics and positive response to this gene therapy, raise hope for a clear path to human therapies.
Beltran and Aguirre collaborated with Artur Cideciyan and Samuel Jacobson at the Scheie Eye Institute, part of the University of Pennsylvania's Perelman School of Medicine. This achievement results from more than 10 years of close collaboration between the scientists at Penn's Veterinary and Medical Schools and the University of Florida.
In addition to others at Penn Vet, Scheie, and Florida, researchers at the Universities of Michigan and Massachusetts and the National Eye Institute at the National Institutes of Health contributed to the research.
The study was published in the journal Proceedings of the National Academy of Sciences .
The gene therapy approach used takes advantage of a viral vector - a genetically modified virus that doesn't cause disease and is unable to divide -- to deliver the therapeutic RPGR gene specifically to diseased rods and cones. In the absence of treatment, these cells malfunction and progressively die.
The research team has previously successfully applied a similar approach to two other heritable vision disorders that occur in both humans and dogs: Leber congenital amaurosis (LCA) and achromatopsia. The present study was more challenging, as it was necessary to target both main classes of photoreceptor cells.
While the exact disease mechanism of the RPGR form of XLRP is still unknown, the researchers were able to successfully treat dogs with two different RPGR mutations. The mutations disrupt photoreceptors in different ways, but both ultimately cause them to become useless for vision. While this form of blindness is rare in dogs, it is common in humans. Patients with XLRP usually begin to lose night vision as children and become almost totally blind by middle age.
This is the first proof that this condition is treatable in an animal model; a single subretinal injection administered to the diseased dogs led to functional and structural recovery. The dogs' recovery was assessed using a variety of methods that are used clinically in patients, such as electroretinography and optical coherence tomography.
The researchers feel the results are promising and relevant for translation to the clinic.
"We are intervening to treat both classes of photoreceptor cells, rods and cones, and that has never been done before in a large animal model," Beltran said. "And not only can we prevent the disease onset but also restore the remaining photoreceptors cells to normal once the disease is ongoing."
While the ability to repair both rods and cones was itself a first, the research team went further, showing that its treatment also repaired the photoreceptor connections to other retinal neurons that eventually send visual signals to the brain, another first.
"This not only provides hope for reversing XLRP but potentially for any form of photoreceptor degeneration," Aguirre said. "Altered inner retinal wiring is a common feature for these diseases that has been considered irreversible.
"The study required a combination of genetic tools and surgical technique to make sure the therapy targeted only the diseased cells. The viral vector had to be injected in the sub-retinal space so as to be in close proximity to the photoreceptors. Likewise, you need to ultimately deliver the therapy to the right location of the retina," Aguirre said.
"In the human disease, careful characterization of the areas of the retina that need to be treated is going to be critical for therapy to succeed in the clinic," Cideciyan said.
The genetic aspect of the viral vector used in this study involved a double safeguard. The first safety feature was to use a viral vector that is known to predominantly target both rods and cones but not other cells. The second safeguard involved attaching the healthy RPGR gene to a "promoter," a piece of genetic code that would "switch on" the gene only if the virus penetrated the correct cell.
Selecting the right promoter was critical; the lead researchers at the University of Florida, William W. Hauswirth and Alfred S. Lewin, had to find one that that would be turned on exclusively in rods and cones. This way, even if the virus made its way to a non-photoreceptor cell, that cell would not start activating the RPGR gene.
That both the promoter and the RPGR gene it activates are taken from humans is a strong sign that the treatment may be translatable to patients.
"While there is still much work to do to assess long-term efficiency and safety with this approach, there is hope that this vector and knowledge could be used in a few years to treat the many patients losing vision from XLRP," Jacobson said.
Provided by University of Pennsylvania

Researchers develop gene therapy that could correct a common form of blindness January 23, 2012
A new gene therapy method developed by University of Florida researchers has the potential to treat a common form of blindness that strikes both youngsters and adults. The technique works by replacing a malfunctioning gene in the eye with a normal working copy that supplies a protein necessary for light-sensitive cells in the eye to function. The findings are published today (Monday, Jan. 23) in the Proceedings of the National Academy of Sciences online.
Several complex and costly steps remain before the gene therapy technique can be used in humans, but once at that stage, it has great potential to change lives.
"Imagine that you can't see or can just barely see, and that could be changed to function at some levels so that you could read, navigate, maybe even drive - it would change your life considerably," said study co-author William W. Hauswirth, Ph.D., the Rybaczki-Bullard professor of ophthalmology in the UF College of Medicine and a professor and eminent scholar in department of molecular genetics and microbiology and the UF Genetics Institute. "Providing the gene that's missing is one of the ultimate ways of treating disease and restoring significant visual function."
The researchers tackled a condition called X-linked retinitis pigmentosa, a genetic defect that is passed from mothers to sons. Girls carry the trait, but do not have the kind of vision loss seen among boys. About 100,000 people in the U.S. have a form of retinitis pigmentosa, which is characterized by initial loss of peripheral vision and night vision, which eventually progresses to tunnel vision, then blindness. In some cases, loss of sight coincides with the appearance of dark-colored areas on the usually orange-colored retina.
The UF researchers previously had success pioneering the use of gene therapy in clinical trials to reverse a form of blindness known as Leber's congenital amaurosis. About 5 percent of people who have retinitis pigmentosa have this form, which affects the eye's inner lining.
"That was a great advance, which showed that gene therapy is safe and lasts for years in humans, but this new study has the potential for a bigger impact, because it is treating a form of the disease that affects many more people," said John G. Flannery, Ph.D., a professor of neurobiology at the University of California, Berkeley who is an expert in the design of viruses for delivering replacement genes. Flannery was not involved in the current study.
The X-linked form of retinitis pigmentosa addressed in the new study is the most common, and is caused by degeneration of light-sensitive cells in the eyes known as photoreceptor cells. It starts early in life, so though affected children are often born seeing, they gradually lose their vision.
"These children often go blind in the second decade of life, which is a very crucial period," said co-author Alfred S. Lewin, Ph.D., a professor in the UF College of Medicine department of molecular genetics and microbiology and a member of the UF Genetics Institute. "This is a compelling reason to try to develop a therapy, because this disease hinders people's ability to fully experience their world."
Both Lewin and Hauswirth are members of UF's Powell Gene Therapy Center.
The UF researchers and colleagues at the University of Pennsylvania performed the technically challenging task of cloning a working copy of the affected gene into a virus that served as a delivery vehicle to transport it to the appropriate part of the eye. They also cloned a genetic "switch" that would turn on the gene once it was in place, so it could start producing a protein needed for the damaged eye cells to function.
After laboratory tests proved successful, the researchers expanded their NIH-funded studies and were able to cure animals in which X-linked retinitis pigmentosa occurs naturally. The injected genes made their way only to the spot where they were needed, and not to any other places in the body. The study gave a good approximation of how the gene therapy might work in humans.
"The results are encouraging and the rescue of the damaged photoreceptor cells is quite convincing," said Flannery, who is on the scientific advisory board of the Foundation Fighting Blindness, which provided some funding for the study. "Since this type of study is often the step before applying a treatment to human patients, showing that it works is critical."
The researchers plan to repeat their studies on a larger scale over a longer term, and make a version of the virus that proves to be safe in humans. Once that is achieved, a pharmaceutical grade of the virus would have to be produced and tested before moving into clinical trials in humans. The researchers will be able to use much of the technology they have already developed and used successfully to restore vision.
Provided by University of Florida

Editor’s Note: The lead author of the article about the study, Dr. William Beltran, said privately that much more pre-clinical work needs to be done before entertaining thoughts on beginning human clinical trials.
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