Science + Technology

Gene Therapy for Retinal Diseases Advances With New Viral and Capsule Mechanisms to Place Genes Directly in Eye

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Tests of three new gene therapies totreat inherited, blindness-causing diseases such as retinitispigmentosa and age-related macular degeneration areshowing much promise, according to a UCLA professor and leading authority inretinal diseases.

At least 150 faulty genes causeinherited retinal diseases. But scientists are taking advantage of new viralvectors and the retina's unique structure to place genes directly into the eye,where they take over from the faulty, or "bad," genes.

SpeakingApril 4 at the Experimental Biology 2005 meeting in San Diego, Dr. Dean Bok, professor of neurobiology at the David Geffen Schoolof Medicine at UCLA and professor of ophthalmology at the Jules Stein EyeInstitute at UCLA, outlined three therapies now under study that fight retinaldiseases by targeting the right genes in the right places in the eye. Bok spoke at a scientific session of the AmericanAssociation of Anatomists.

Thoughmost of the studies are still in the preclinicaltesting stage in animals, the tests have led to at least one human trial whoseresults the National Eye Institute will announce later in April.

Thethree gene therapies are:

Ciliary Neurotrophic Factor (CNTF)

In retinitispigmentosa, the photoreceptor cells in the eye slowlydie, usually with a vision loss of roughly 5 percent per year until the personis completely blind. A drug called ciliary neurotrophic factor (CNTF) has long been known to stop thedegeneration of the photoreceptor cells, although exactly how it accomplishesthis is not well understood. The bigger question has been how to get it intothe eye. Bok and his collaborators were among thefirst scientists to incorporate a CNTF mini-gene into the DNA of a virus.

When this genetically modifiedvirus was placed in the vicinity of the retinal pigment epithelium in the eyeof mice with a form of retinitis pigmentosa,the epithelial cells gobbled it up along with the CNTF gene. Since theincorporated genes were stable, the retinalpigment epithelium acted like a perpetual slow release capsule — the epithelialcells began to release CNTF, which bathed the retinal cells and stopped thedegeneration of the photoreceptor cells. The mice's lost vision was notrestored, but it was stabilized.

Scientists were reluctant toput a virus that could not be retrieved into the human eye. Then, a few yearsago, Neurotech, a small company based in France andRhode Island, developed an encapsulated cell technology — a little capsule thatcould be placed in the eye and, if need be, taken out.

Scientists used existingimmortal retinal pigment epithelial cells, engineered them to secrete CNTF, andplaced them in these small capsules, which they then hung inside the eyes ofdogs with a retinitis pigmentosa-likedisease. The capsule is designed to allow oxygen and nutrients in to sustainthe epithelial cells, and to allow CNTF to leak out where it can reach thephotoreceptor cells. But the tiny pores of the capsule prevent the modifiedepithelial cells from escaping and being attacked and destroyed by the body'simmune system.

The National Eye Institute willrelease results from a Phase I clinical trial in humans later in April, Bok said.

RPE-65

Bokcollaborated with the research team that first showed a gene called RPE-65 isessential for vision. Without well-functioning RPE-65 genes, the eye is unableto convert dietary vitamin A into a form that can be used for vision. When thatgene is not working properly or at all, a replacement gene is delivered in avirus, which is injected into a specific location in the retina. The virus theninvades 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 nearlyblind. Preclinical studies in mice and dogs havefound that early treatment establishes functional vision. The first humantrials are expected to begin in 2006, although they will take place in people18 years or older who can give legal consent, Boksaid. If these Phase I safety trials prove successful, Bokexpects work to begin in babies soon after. Theclinical trial, which is funded by the National Eye Institute, will be amulti-institutional effort at Cornell University, the University ofPennsylvania and the University of Florida.

Seeking and destroying mutant RNA

"This is the mostchallenging, I think, of all the types of diseases that confront us," Bok said.

This is because doctors have tocompromise or completely destroy the product of the dominant mutant gene, by destroying the RNA caused by themutant gene. The tool to accomplish this is a ribozyme,which is an enzymatic form of RNA. After an engineered virus enters a cell, itmakes a specific ribozyme that selectively seeks outand destroys mutant RNA, Bok said. This relieves thecell of the effects from the mutant RNA. Then it's possible to add thereplacement "good" RNA.

"In this case you'reintroducing two genes into the virus — one destroys the RNA, the other replacesthe mutant RNA, so the cell functions appropriately," he said. This work is in collaboration with investigators at theUniversity of California, San Francisco, and the University of Florida.

This is good news for peoplewith diseases like certain forms of early-onset retinitispigmentosa caused by a dominant gene, Bok said. He said preclinical studiesin rats genetically engineered to have one of the 100 rhodoposin mutationsfound 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 hasthe largest collection of animal models in any biomedical field, from zebrafish to dogs, some of which experience retinal problems similar to those ofhumans. That has helped us move toward these new therapies relatively quickly.

"The bottom line is that we arevery optimistic about gene-based therapy in inherited retinal diseases," hesaid.

-UCLA-

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