Eos CSO Alan Horsager Interviewed by New Scientist
Genes from algae allow blind mice to see
"The idea is to develop a treatment for blindness," says Alan Horsager, a neuroscientist at the Institute of Genetic Medicine at the University of Southern California, Los Angeles, who leads the research. "We introduce a gene that encodes a light-sensitive protein, and we target the expression of that gene to a subset of retinal cells."
Some 15 million people worldwide have some form of blindness, such as retinitis pigmentosa (RP) or age-related macular degeneration (AMD). In people with these conditions the photoreceptors, which transform light hitting the eye into electrical impulses, are damaged, preventing the brain from receiving image information.
As the global population ages, it is thought that the number of people affected will increase. There are experimental attempts to develop electronic implants and to use stem cells to grow new retinal tissues to restore sight, but there is currently no commercial treatment available.
Horsager hopes his work will change that. His team's approach is based on gene therapy, where a "tame" virus is harnessed to transfer a gene into target cells in the recipient. In this case the gene of interest is one that makes Channelrhodopsin-2 (ChR2), a photosensitive protein used by unicellular algae to help them move towards light. The target cells are bipolar cells in the retina.
The retina contains three cellular layers that work together to detect and transmit light signals to the brain (see diagram). The first layer contains the photoreceptors - the rods and cones that detect light. The second layer is made of bipolar cells that act as a conduit between the photoreceptor and the third type of cell, the ganglion, which transmits the light signals to the brain.
In people with RP and AMD, the photoreceptors have been damaged and lost, so the ganglion cells do not receive signals and the brain cannot produce an image. The idea behind the gene therapy is to make the bipolar cells function as photoreceptors by producing ChR2. The modified bipolar cell would then be able to sense light and transmit a signal to the ganglion.
Horsager's team tested their technique using three groups of mice: one with normal vision, and two groups of mouse strains that naturally become blind with age in a similar way to people with RP and AMD. One blind group was treated with the gene therapy, while the other two groups were not.
Treated mice received a sub-retinal injection of the virus containing the algal gene. Ten weeks after the injection, the team dissected some of the mice and used immunolabelling to see whether ChR2 was being expressed in the retina. They found that the protein was being made by the bipolar cells.
But the strongest evidence of the treatment's success came when treated mice were put in the centre of a water maze with six possible corridors, only one of which led to a ledge that the mice could clamber out of the water onto. With a guiding light shining at the end of the corridor which contained the ledge, the gene-therapy mice were able to find the escape platform 2.5 times faster, on average, than the untreated blind mice. The work will appear in Molecular Therapy.
Repeating the test 10 months later, the team found that the treated mice were still showing significant improvements in vision compared with the untreated blind mice. "Our expectation is that this would be a one-time treatment that is permanent or semi-permanent," says Horsager.
Concerns have been raised about the safety of gene therapy in the past, not least about links between the viruses used to transfer the genes and disease. Horsager says the algal genes were only expressed in the target cells, and that there is no evidence of an immune response in the mice, suggesting that the transfer of the foreign gene has been restricted to the bipolar cells.
However, small amounts of ChR2 DNA were found in other tissues. "Regulatory agencies would be very concerned that ChR2 DNA was found in tissues outside of the treated eye," says Robert Lanza, of Advanced Cell Technology in Worcester, Massachusetts. Horsager's team believe the rogue DNA is due to cross-contamination during the analysis process.
"It's a good paper, and it's clear that they are heading towards a clinical trial with the information they are gathering," says Pete Coffey of the department of ophthalmology at University College London. But he points out that although there is a statistical difference between the performance of the treated and untreated mice, that difference is small.
Full story can be found here.