Gene Research Sheds Light on Cure for Colorblindness

Colorblindness can likely be cured by gene therapy, according to Medical College of Wisconsin researchers who are conducting a treatment trial designed to discover what happens when a specialized set of normal genes is injected directly into the eye.

"I'm sure that, ultimately, gene therapy will cure colorblindness," said Jay Neitz, PhD, Medical College of Wisconsin Professor of Cellular Biology, Neurobiology and Anatomy. "It's not a matter of whether it will or not, it's a matter of when. I think the probability is extremely high."

Usually a hereditary condition that is present at birth, colorblindness afflicts more than nine million people in the US. Normal eyes distinguish color through three different photoreceptors located in the retina, some for red, some for blue and some for green. When one set of these many photoreceptors is missing due to genetic mutation, color-vision deficiency occurs to varying degrees depending on the severity of the mutation and which color receptors have been lost.

Dr. Neitz is collaborating on the gene treatment trial with his wife, Maureen Neitz, PhD, who is on the Eye Institute staff and a Medical College Professor of Ophthalmology, and Eye Institute surgeon Thomas B. Connor, MD, Medical College Associate Professor of Ophthalmology. (Quotes in this article are from Dr. Jay Neitz.)

The project is focused on the cone-shaped photoreceptors in the eye, which are responsible for defining color and visual acuity in normal light. The eye employs other photoreceptors, shaped like rods, for vision in dim light.

"If something happens with one of the cone photoreceptors, you have colorblindness," said Dr. Neitz. "If something happens with two out of the three, if they're the red and green ones - the blue cones are so rare - you end up with a blinding condition. So the blue cones are just as important for color vision but they're not very important for the rest of our vision. You can lose your green cones, too, and maybe those would influence your vision. That person's vision may not be 100% normal, but pretty close."

Same Genes Affect AMD
The research into gene therapy for colorblindness may also have applications in diagnosing and treating age-related macular degeneration (AMD), the leading cause of blindness in older populations in the US, because the same cells are involved. In colorblindness, genetic mutation kills off certain photoreceptors at a very young age in the life of those cells. Photoreceptor cells are killed much later on in AMD, but it is believed that they're also doomed by genetic factors.

"The same cells that create color vision are the ones that give us vision, said Dr. Neitz. "It's not like they're different. The very same cells that are affected in colorblindness are the ones that are affected in age-related macular degeneration. If certain kinds of things go wrong it gives you colorblindness. Other things go wrong, and you get age-related macular degeneration. In both cases, it's the loss of function in those cones that give you a loss of vision."

Images in Dr. Neitz' office, looking very much like photographs of stars in the universe, show the array of photoreceptors in the retina. Bright areas are living photoreceptors while missing photoreceptors are represented by dark "holes".

"Age-related macular degeneration also has a very large component, we believe, where the same gene that causes that (colorblind) person to lose his cones at a certain age is the same gene that cause people to lose their cones when they're 65 years old," said Dr. Neitz. "In other words, that person had a set of green cones at one time. If he hadn't had them, there would be no holes left behind.

"We believe that there is a gene mutation that caused that cell to die. So it's originally born and it lasts for some period of time. There are different kinds of mutations. Some of them are more deadly than others. That particular mutation is a very, very deadly one. Once you have it, the life span of the cone might be only two or three years. There are other mutations that are only mildly toxic, so you can go through life and your vision lasted for fifty, sixty or seventy years. Ultimately, the cone will die just as those cones did (in the colorblind person whose cones died very young)."

Putting Normal Genes Into the Eye
In the colorblindness trial, normal genes are being injected directly into the retinas of animals to take the place of the missing genes and, it is hoped, confer normal color vision.

"Just as there are three different kinds of cone photoreceptors in the eye - red, green and blue - each one of those is encoded by a different gene," said Dr. Neitz. "There's a red photo-pigment gene, a green one, and a blue one. Those are then expressed in those different cones to give you the red, green and blue cones.

"In general, we can take a blood sample from someone, and since every one of your cells contain all of your DNA, we can look at the cone photo-pigment genes and see if there are mutations. In the case of colorblindness, we have been able to identify the kinds of mutations in the cone photo-pigment genes that underlie colorblindness. Basically, we have outlined the genetic basis for colorblindness."

"For our particular gene we have already piloted the whole retrovirus that we're using and already put it into rat eyes," said Dr. Neitz. "We know that it will attack the photoreceptors and turn on inside the photoreceptors. The question is, can we get it to work in this monkey model and if it does work, does it confer color vision?"

The researchers are using a retrovirus to take advantage of cellular mechanics in the same way genetic therapy is used for other diseases, Dr. Neitz said, adding that the eye has the advantage of being a closed vessel so gene therapy can be targeted very directly. Photoreceptors are located in the retina, the back layer of the eye. To introduce the normal genes, the retina is clinically detached very briefly from a nutritive eye layer directly in front of it, creating a space in which a fluid can bathe all of the photoreceptors before the layers come back together.

Retrovirus "One-Ups" Virus
"What's in this fluid is the missing gene," said Dr. Neitz. "The problem is to get it into the genome so that it can be expressed. To do that, the gene is being carried by a retrovirus. Normally, what a virus does is attach itself to a cell. Since it doesn't have any of the machinery for copying its DNA, it uses the cellular machinery. The virus injects its own DNA into the cell, and that 'hijacks' the cell's machinery in order to replicate the DNA.

"In gene therapy we do the opposite. We one-up the virus, hijacking the mechanism that attaches itself and injects its DNA. The normal DNA sequences that are responsible for replicating the virus have been removed, because once they get into the cell they will replicate over and over again and burst the cell.

"In its place we've put the DNA that makes the human cone photo-pigment. The virus now will attach to the cell, putting the human cone photo-pigment gene in there. By mechanisms that we don't really understand that gene will ultimately integrate into the genome and become functional."

Dr. Neitz noted that this is the first trial ever of gene therapy in any animal involving the cone photoreceptors as a target. "I feel really confident about this, and since all the pieces are in place I'm hoping that it will work in a monkey within two years, he said. "If it works in a monkey we have to go to human trials and have to get FDA approval and so forth, so it could be about five years to get to the point where we're able to use the technique in humans.

"If you're interested in people's vision, it's really cone vision that's important. In the long run we're hopeful that this kind of gene therapy will become a way to treat a wide variety of blinding disorders that affect the cone photoreceptors - they're really the big target in both colorblindness and in AMD."

Dan Ullrich
HealthLink Contributing Writer

MCW Health News presents up-to-date information on patient care and medical research by the physicians of the Medical College of Wisconsin.