A simple, non-invasive gene therapy restores sight
June 14, 2013
UC Berkeley researchers have developed an new method for inserting genes into retina cells that is easier and more effective, It could greatly expand gene therapy to help restore sight to patients with blinding diseases ranging from inherited defects like retinitis pigmentosa to degenerative illnesses of old age, such as macular degeneration.
Unlike current treatments, the new procedure delivers genes to hard-to-reach cells throughout the entire retina, and is relatively non-invasive, compared to current procedures.
Over the last six years, three groups of researchers have successfully restored some sight to more than a dozen people with a rare inherited eye disease called Leber’s congenital amaurosis, which leads to complete loss of vision in early adulthood. by injecting a virus with a normal gene directly into the retina of an eye with a defective gene.
They achieved this by inserting this corrective gene into adeno-associated viruses (AAV), and injecting these common but benign respiratory viruses directly into the retina.
The photoreceptor cells take up the viruses and incorporate the functional gene into their
chromosomes nucleus to make a critical protein that the defective gene could not, rescuing the photoreceptors and restoring sight.
However, even with the invasive process, the virus and associated gene were not capable of reaching all the retinal cells that needed fixing. The technique cannot be applied to most blinding diseases because the needle often causes retinal detachment, making the situation worse.
Also, the standard AAV used in eye and other types of gene therapy cannot penetrate into tissue to reach the photoreceptors and other cells, such as retinal pigment epithelium, that need to be fixed. The retina is about 100,000 times thicker than the diameter of AAV, which is about 20 nanometers.
In addition, “sticking a needle through the retina and injecting the engineered virus behind the retina is a risky surgical procedure,” said David Schaffer, professor of chemical and biomolecular engineering and director of the Berkeley Stem Cell Center at UC Berkeley.
“But doctors have no choice, because none of the gene delivery viruses can travel all the way through the back of the eye to reach the photoreceptors – the light sensitive cells that need the therapeutic gene.”
A new virus for simple, safe gene delivery
So years ago, Schaffer set out to find a way to “evolve” AAV to penetrate tissues, including eye and liver, as a way to deliver genes to specific cells.
To date, he has generated 100 million variants of AAV – each carrying slightly different proteins on its coat — from which he and his colleagues selected five that were effective in penetrating the retina.
“Building upon 14 years of research, we have now created a virus that you just inject into the liquid vitreous humor inside the eye, and it delivers genes to a very difficult-to-reach population of delicate retinal cells in a way that is surgically non-invasive and safe. “It’s a 15-minute procedure, and you can likely go home that day.”
When injected into the eye of a normal monkey, the AAV delivers the corrective gene to all areas of the retina and restores retinal cells nearly to normal.
The viruses penetrate cells spottily across the retina, but almost completely in the very important fine-vision area called the fovea. (Current viruses do not penetrate foveal cells at all.)
The engineered virus works far better than current therapies in rodent models of two human degenerative eye diseases, and can penetrate photoreceptor cells in monkeys’ eyes, which are like those of humans.
Schaffer said he and his team are now collaborating with physicians to identify the patients most likely to benefit from this gene delivery technique and, after some preclinical development, hope soon to head into clinical trials.
Schaffer predicts that the viruses can be used not only to insert genes that restore function to non-working genes, but can knock out genes or halt processes that are actively killing retina cells, which may be the case in age-related macular degeneration.
“When I first got here 14 years ago, I really had the idea or the goal that I wanted to work on problems that would have direct impact on human health, and we are now getting there,” Schaffer said.
The work was supported by the Nanomedicine Development Center for the Optical Control of Biological Function of the National Institutes of Health and the Foundation Fighting Blindness.