Serendipitous Science and a Key Conclusion
By Susan E. Johnson
NEI Freelance Writer
In 1989, NEI scientist T. Michael Redmond, Ph.D., received a hand-me-down project from another NEI group--a serendipitous event that shaped his next 20 years of research. This second-hand work led his team to discover a crucial protein involved in vision, which served as the basis of a recent groundbreaking gene therapy trial for a blinding eye condition.
Today, Redmond's group in the NEI Laboratory of Retinal Cell and Molecular Biology continues to study the protein, known as RPE65. In a recent interview, Redmond talks about the combination of luck and persistence that drives the scientific process and ultimately leads to medical advances.
What aspect of vision does your research involve?
My research relates to the visual cycle and the role of vitamin A in maintaining vision. During the visual process, biochemical reactions in the eye convert vitamin A from the dietary form to the form required for vision. This pathway is known as the visual cycle. We found that a gene called RPE65, which makes a protein of the same name, is very important in the visual cycle. We also found that the RPE65 gene is mutated in many people who have an inherited blinding disease called Leber congenital amaurosis (LCA).
How does RPE65 work?
We're still trying to understand the mechanism. We know that the protein changes the form of vitamin A, but exactly how it functions is still unclear. It's a work in progress, really.
Why did your lab focus on RPE65 rather than some other aspect of the visual cycle?
It was a bit of an accident, which is fairly common in science. An immunology laboratory here at the NEI developed an antibody to target a layer of the retina called the retinal pigment epithelium (RPE). The RPE is the site where dietary vitamin A is metabolized to a form that is essential for the light-sensitivity of the retina. But NEI immunologists weren't interested in continuing their work with this antibody, so they basically handed it over to us. We discovered that the antibody was targeting a particular protein in the RPE, which we called RPE65--a name we based on the molecule's size. We also found the DNA sequence which codes for that protein.
T. Michael Redmond, Ph.D.
Chief, Laboratory of Retinal Cell and Molecular Biology
National Eye Institute
What were your next steps in studying RPE65?
We decided to make a knock-out mouse--a mouse in which a gene of interest is 'knocked out,' or deleted. This would help us figure out what happens when the gene is gone. I think the knock-out mouse was what really set the field in motion.
How did the mice help?
In a pretty short amount of time, about 6 months, we figured out that the reason why a lack of RPE65 was so devastating to the eye was that it completely shut off production of the form of vitamin A needed for vision. No other pathway could fill in, so vision was lost from the beginning. In a normal mouse eye, the retina is orange-pink in color. But in the knock-out mouse, the retinal tissue is gray. The converted form of vitamin A makes the pigment that gives the retina its color and its light sensitivity, and it just wasn't there in the knock-out mice. Of course, we still had to do all the biochemistry studies to support this, but it was a night and day difference just looking at those two retinas. At about the same time, we were looking for any possible role of RPE65 in human retinal diseases.
How did you know where to look?
Based on our initial biochemical research, we made a best guess as to what to look for--a severe, blinding condition that affects people early in life, which is why we looked at LCA. This was initially just a hunch. But, in collaboration with my former NEI postdoctoral fellow who now works in France, we performed genetic screenings on 13 or 14 families with LCA there. In 1997, we identified a brother and sister with LCA who had mutations in both copies of the RPE65 gene. That was when we officially matched RPE65 mutations with a particular disease. Another group had similar findings implicating RPE65 in LCA, and the field grew from there.
Others have used your initial RPE65 work to begin phase 1 safety trials on LCA gene therapy, in which a functional copy of the RPE65 gene is injected into the eye. How did your work lead to this?
The knock-out mouse has provided a very, very good model for the human disease, and I think that's part of the reason why the research went so quickly. We just gave out these mice to any researchers who wanted them. This allowed a variety of researchers to do extensive work to understand the biochemistry of the visual cycle. Researchers also identified a breed of dogs that had an RPE65 mutation, causing LCA. These dogs were successfully treated with gene therapy in 2001. So, the scientists who were developing gene therapy for humans could cut straight to the chase.
What's next for your lab?
Most of what we know about the RPE65 form of LCA involves the extreme form, in which the RPE65 protein is completely missing. However, a large proportion of patients have intermediate forms of LCA, with low levels of RPE65 activity. One project we're doing at the moment is making mouse models in which we're not totally deleting the gene, but making mutations that reduce the activity of the RPE65 protein. Then, we can better understand the full spectrum of RPE65 disease and its impact on vision. And, though RPE65 is essential, it's only part of the larger picture of vitamin A metabolism in the eye, so we're investigating how this entire process is regulated.