A new gene
therapy method developed by University of Florida researchers,
William W. Hauswirth, PhD
Alfred S. Lewin, PhD
, has the potential to reverse X-linked retinitis pigmentosa. 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 were published in the
Jan. 23 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 Dr. Hauswirth, the Rybaczki-Bullard professor of ophthalmology in the
UF College of Medicine
and a professor and eminent scholar in the 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 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 United States 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.
The UF researchers previously had success pioneering the use of gene therapy in clinical trials to reverse 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, PhD
, a professor of neurobiology at the University of California, Berkeley who is an expert in the design of viruses for delivering replacement genes. Dr. 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 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 Dr. Lewin, 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.”
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 Dr. 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.
Flexible Adult Stem Cells Reside Within the RPE
In the future, patients
in need of perfectly matched neural stem cells may not need to look any farther than their own eyes. Researchers reporting in the
January issue of Cell Stem Cell
have identified adult stem cells of the central nervous system in the retinal pigment epithelium.
The new study shows that, in addition to underlying and supporting photoreceptors, the RPE also harbors self-renewing stem cells that can wake up to produce actively growing cultures when placed under the right conditions. They can also be coaxed into forming other cell types.
“You can get these cells from a 99-year-old,” said
Sally Temple, PhD
, scientific director of the
Neural Stem Cell Institute
in Rensselaer, N.Y. “These cells are laid down in the embryo and can remain dormant for 100 years. Yet you can pull them out and put them in culture and they begin dividing. It is kind of mind boggling.”
Dr. Temple’s group got the RPE-derived stem cells they describe from the eyes of donors in the hours immediately after their deaths. But the cells can also be isolated from the vitreous, which means they are accessible in living people as well. By comparison, access to most other neural stem cell populations would require major surgery.
Dr. Temple said they were curious about the proliferative potential of the RPE given that the tissue is known to be capable of regenerating entire retinas in salamanders. But that plasticity in adulthood had seemed to be lost in mice and chicks. Still, “given the evolutionary evidence, we thought it was worth revisiting,” she said.
They placed RPE tissue taken from 22-year-old to 99-year-old cadavers into many culture conditions to see what they could make the cells do. They found one set of conditions that got the cells dividing. Not all of the RPE cells have this regenerative potential; perhaps 10 percent of them do.
Further work showed that the cells are multipotent, which means that they can form different cell types, though the researchers admit there is more to do to fully explore the cells’ differentiation capacity.
There are other implications as well. For example, these cells may explain diseases in which other tissue types show up in the eye. Their presence also suggests that there might be some way to stimulate controlled repair of the eye in the millions of people who suffer from age-related macular degeneration.
Fingers Protein in TM Blockage
In certain cases of glaucoma, blockage of the trabecular meshwork results from a buildup of the protein myocilin.
Raquel Lieberman, PhD
, an assistant professor of chemistry and biochemistry at
, focused on examining the structural properties of these myocilin deposits.
“We were surprised to discover that both genetically defected as well as normal, or wild-type, myocilin are readily triggered to produce very stable fibrous residue containing a pathogenic material called amyloid,” said Dr. Lieberman.
Amyloid formation, in which a protein is converted from its normal form into fibers, is recognized as a major contributor to numerous non-ocular disorders, including Alzheimer’s, certain forms of diabetes and mad cow disease (in cattle). Scientists are currently studying ways to destroy amyloid fibrils as an option for treating these diseases. Further research, based on Dr. Lieberman’s findings, could potentially result in drugs that prevent or stop myocilin amyloid formation or destroy existing fibrils in glaucoma patients.
Until this point, amyloids linked to glaucoma had been restricted to the retinal area. In those cases, amyloids kill retina cells, leading to vision loss, but don’t affect intraocular pressure.
“The amyloid-containing myocilin deposits we discovered kill cells that maintain the integrity of TM tissue,” said Dr. Lieberman. “In addition to debris from dead cells, the fibrils themselves may also form an obstruction in the TM tissue. Together, these mechanisms may hasten the increase of intraocular pressure that impairs vision.”
Less Treatment for
Trachoma May Be Just as Effective
The researchers conducted a cluster-randomized trial in Ethiopia in 24 communities and randomized the two treatments, six months and 12 months. At baseline, 40 to 50 percent of the children in these communities had this condition. “They are the most susceptible and it can quickly spread from person to person by direct or even indirect contact,” Dr. Gaynor said.
Researchers tracked both groups and found the prevalence of infection decreased dramatically. “We found that from as high as 40 percent, the prevalence of trachoma went way down, even eliminated in some villages regardless of whether it was treated in an annual way or a biannual way,” Dr. Gaynor said. “You can genuinely get the same with less.”
Their finding is significant because of how easily the disease spreads. Trachoma can be transmitted through touching one’s eyes or nose after being in close contact with someone who is infected. It can also be spread through a towel or an article of clothing from a person who has trachoma. Even flies can transmit the disease.
Approximately 41 million people are infected with trachoma globally, and 8 million go blind because of lack of access to treatment. More than 150 million doses of azithromycin have been given out worldwide to treat this disease. Unlike other antibiotics, resistance to azithromycin has not been found in Chlamydia trachomatis, the bacteria that causes trachoma.
major finding give hope to Africa, Asia, the Middle East, and parts of Latin America and Australia, where trachoma is still a major problem.
“We will now be able to reach more people and make the treatment go twice as far as before,” Dr. Gaynor said. “This will make a huge impact in slowing down trachoma-related blindness globally.”