Scientists from the University of California, Berkeley and Lawrence Berkeley National Laboratory, along with those of University of Pennsylvania, have demonstrated restoration of visual function in animal models that can help restore sight in blind patients afflicted by diseases such as Retinitis Pigmentosa (RP) and Leber Congenital Amaurosis (LCA).
A new genetic therapy not only helped blind mice regain enough light sensitivity to distinguish flashing from non-flashing lights, but also restored light response to the retinas of dogs, setting the stage for future clinical trials of the therapy in humans.
The therapy employs a virus to insert a gene for a common ion channel into normally blind cells of the retina that survive after the light-responsive rod and cone photoreceptor cells die as a result of diseases such as retinitis pigmentosa. Photoswitches – chemicals that change shape when hit with light – are then attached to the ion channels to make them open in response to light, activating the retinal cells and restoring light sensitivity.
In a paper appearing online this week in the early edition of the journal Proceedings of the National Academy of Sciences, University of California, Berkeley, scientists who invented the photoswitch therapy and vision researchers at the School of Veterinary Medicine of the University of Pennsylvania (PennVet) report that blind mice regained the ability to navigate a water maze as well as normal mice.
The treatment worked equally well to restore light responses to the degenerated retinas of mice and dogs, indicating that it may be feasible to restore some light sensitivity in blind humans.
Advantages over other gene therapies
The therapy has several advantages over other sight restoration therapies now under investigation. It uses a virus already approved by the Food and Drug Administration for other genetic therapies in the eye; it delivers an ion channel gene similar to one normally found in humans, unlike others that employ genes from other species; and it can easily be reversed or adjusted by supplying new chemical photoswitches. Dogs with the retinal degeneration provide a key test of the new therapy.
The dogs were chosen because they have inherited a genetic disease caused by the same gene defect as some people with retinitis pigmentosa. Several of them at PennVet were treated and are currently undergoing tests to determine what degree of light sensitivity they now have.
Hybrid chemical-genetic therapy
Genetic diseases like retinitis pigmentosa destroy the photosensitive cells of the eye, the photoreceptors, but often leave intact the other cells in the retina: the bipolar cells that the photoreceptors normally talk to, and the ganglion cells that are the retina’s output to the brain. UC Berkeley colleagues have developed several optogenetic techniques for restoring light-sensitivity to surviving retinal cells other than the photoreceptors. These involve using the adeno-associated virus – a common and harmless vector or carrier for gene therapy – to successfully carry a modified gene into these cells. The virus inserts the therapeutic gene into the cell’s DNA and uses its instructions to produce a receptor protein – a modified version of a common glutamate receptor ion channel – that they display on their surface.
The researchers then inject a chemical photoswitch into the eye, which anchors to the modified receptor and stuffs the glutamate into its docking site on the receptor when activated by light. The newest version of the photoswitch is fast enough to turn the activity of retinal neurons on and off at a rate that approaches video rate of 30 frames per second.
In mice, they can successfully insert the gene into almost every one of the million or so retinal ganglion cells. This, the researchers say, should restore useful vision.
In this studies, the treated mice were able to distinguish between steady light and flashing light. The next step for the scientists is to figure out how good they are at telling images apart.
One key question the researchers wanted to answer is whether it is best to insert photoswitches into ganglion cells or bipolar cells. Viruses can be made to target one or the other. Because activity flowing from upstream bipolar cells to the retina’s output ganglion cells undergoes a lot of processing in the retinal circuit, the researchers were hoping that this same processing would occur when bipolar cells were given a new function they never had before, light-sensitivity. The answer seems to be yes.
When the photoswitched channels are inserted into bipolar cells and the output of the ganglion cells recorded, complicated patterns similar to that in a normal retina is seen, compared to the on-off activity seen when the same photoswitch is put into a ganglion cell.
The therapy appears to work only for about a week after a single “charging” with the photoswitch because the protein and attached chemical get recycled by the cell. While the modified receptors are replaced continually, since the new gene remains forever in the DNA, the chemical photoswitch – maleimide-azobenzene-glutamate, or MAG – must be resupplied by injection into the eye. This means an injection every week or so may be required, with the future development of a slow release formulation that will reduce the frequency of injections.
The researchers continue to study the effects of treatment in both mice and dogs, improve the photoswitch, and develop ways of attaching the photoswitch to other receptors, including some that could amplify the signal and allow perception of fainter light, as occurs normally in rods and cones.
A new genetic therapy not only helped blind mice regain enough light sensitivity to distinguish flashing from non-flashing lights, but also restored light response to the retinas of dogs, setting the stage for future clinical trials of the therapy in humans.
The therapy employs a virus to insert a gene for a common ion channel into normally blind cells of the retina that survive after the light-responsive rod and cone photoreceptor cells die as a result of diseases such as retinitis pigmentosa. Photoswitches – chemicals that change shape when hit with light – are then attached to the ion channels to make them open in response to light, activating the retinal cells and restoring light sensitivity.
In a paper appearing online this week in the early edition of the journal Proceedings of the National Academy of Sciences, University of California, Berkeley, scientists who invented the photoswitch therapy and vision researchers at the School of Veterinary Medicine of the University of Pennsylvania (PennVet) report that blind mice regained the ability to navigate a water maze as well as normal mice.
The treatment worked equally well to restore light responses to the degenerated retinas of mice and dogs, indicating that it may be feasible to restore some light sensitivity in blind humans.
Advantages over other gene therapies
The therapy has several advantages over other sight restoration therapies now under investigation. It uses a virus already approved by the Food and Drug Administration for other genetic therapies in the eye; it delivers an ion channel gene similar to one normally found in humans, unlike others that employ genes from other species; and it can easily be reversed or adjusted by supplying new chemical photoswitches. Dogs with the retinal degeneration provide a key test of the new therapy.
The dogs were chosen because they have inherited a genetic disease caused by the same gene defect as some people with retinitis pigmentosa. Several of them at PennVet were treated and are currently undergoing tests to determine what degree of light sensitivity they now have.
Hybrid chemical-genetic therapy
Genetic diseases like retinitis pigmentosa destroy the photosensitive cells of the eye, the photoreceptors, but often leave intact the other cells in the retina: the bipolar cells that the photoreceptors normally talk to, and the ganglion cells that are the retina’s output to the brain. UC Berkeley colleagues have developed several optogenetic techniques for restoring light-sensitivity to surviving retinal cells other than the photoreceptors. These involve using the adeno-associated virus – a common and harmless vector or carrier for gene therapy – to successfully carry a modified gene into these cells. The virus inserts the therapeutic gene into the cell’s DNA and uses its instructions to produce a receptor protein – a modified version of a common glutamate receptor ion channel – that they display on their surface.
The researchers then inject a chemical photoswitch into the eye, which anchors to the modified receptor and stuffs the glutamate into its docking site on the receptor when activated by light. The newest version of the photoswitch is fast enough to turn the activity of retinal neurons on and off at a rate that approaches video rate of 30 frames per second.
In mice, they can successfully insert the gene into almost every one of the million or so retinal ganglion cells. This, the researchers say, should restore useful vision.
In this studies, the treated mice were able to distinguish between steady light and flashing light. The next step for the scientists is to figure out how good they are at telling images apart.
One key question the researchers wanted to answer is whether it is best to insert photoswitches into ganglion cells or bipolar cells. Viruses can be made to target one or the other. Because activity flowing from upstream bipolar cells to the retina’s output ganglion cells undergoes a lot of processing in the retinal circuit, the researchers were hoping that this same processing would occur when bipolar cells were given a new function they never had before, light-sensitivity. The answer seems to be yes.
When the photoswitched channels are inserted into bipolar cells and the output of the ganglion cells recorded, complicated patterns similar to that in a normal retina is seen, compared to the on-off activity seen when the same photoswitch is put into a ganglion cell.
The therapy appears to work only for about a week after a single “charging” with the photoswitch because the protein and attached chemical get recycled by the cell. While the modified receptors are replaced continually, since the new gene remains forever in the DNA, the chemical photoswitch – maleimide-azobenzene-glutamate, or MAG – must be resupplied by injection into the eye. This means an injection every week or so may be required, with the future development of a slow release formulation that will reduce the frequency of injections.
The researchers continue to study the effects of treatment in both mice and dogs, improve the photoswitch, and develop ways of attaching the photoswitch to other receptors, including some that could amplify the signal and allow perception of fainter light, as occurs normally in rods and cones.
No comments:
Post a Comment