Gene therapy lets blind mice see
A gene therapy technique that can help to repair the retina and restore lost vision has been pioneered at the University of Oxford. The retina is the light-sensitive sheet of tissue at the back of the eye where rod and cone cells convert light waves into electrical signals that the brain can understand. But in diseases like retinitis pigmentosa, these cells die off, leaving patients unable to see. Now Samantha De Silva and her colleagues have developed a way to use a harmless virus to deliver genetic instructions that can make other, surviving healthy cells in the retina become light sensitive and make blind mice see…
Samantha - In these conditions there’s a gradual loss of light-sensing cells at the back of the eye, so those are the rods and cones, and, over time, the rods and cones eventually die resulting in blindness. Despite the loss of rods and cones, the rest of the cells in the retina actually remain largely structurally intact, along with the connections between those cells and the connections to the brain. So what we aim to do is see if there’s any way of stimulating these remaining retinal cells to try and restore light sensitivity and even vision.
Chris - So essentially, you’re turning cells in the back of the eye that wouldn’t normally function, like rods and cones, into cells that can take up some of the light sensing functions of the rods and cones so you can get signals into the brain again?
Samantha - Exactly. Initial work done in 2005 by Professor Mark Hankins, who’s one of the senior authors on this paper, showed that if you take a light sensing protein, and one of those is called melanopsin which is already present in the human eye, and put it in a cell which is not light sensitive, that cell then becomes light sensing. Based on that work we tried to see if we could express melanopsin, so human protein, in these remaining retinal cells and make them light sensitive in the absence of rods and cones.
Chris - How do you get the instruction to make that light sensing chemical melanopsin into the cells that now need it then?
Samantha - This is where the gene therapy comes in. Essentially, what we do is we take a virus called an adeno-associated virus which is harmless to humans, and we genetically engineer that virus to get rid of all the genes and proteins that it normally expresses and make it express proteins that we want it to express, and that’s the ultimate concept of gene therapy. What we did is we engineered these viruses to express melanopsin and then injected them underneath the retina of mice with retinitis pigmentosa to see whether the virus was taken up, and whether it would be effective.
Chris - Does the virus go into these cells that are surviving and add the genes to them?
Samantha - Yeah. The first finding of our paper really is that this virus was effective. It was able to get the melanopsin into the remaining cells and that expression was initially seen after a few weeks but persisted after 15 months after a single injection. So it essentially lasted the whole lifetime of that mouse.
Chris - Was there any inflammation because, obviously, you’re putting a lot of something foreign into a part of the body where it would never normally be seen - does the immune system react?
Samantha - That's partly the beauty of using the subretinal approach in that the space underneath the retina is what we call immune privileged, so there is not normally an immune response mounted to anything being in that area. The eye lends itself very well to gene therapy for that reason so we didn’t see any immune response in any of the mice that were injected or treated.
Chris - How did you know you had made a difference to the vision in these mice?
Samantha - Once we were able to establish that we had got the virus working and there was melanopsin in the cells, we used electrodes to record from the treated retinas and that showed that when we shone a light on the retinas they generated electrical signals that could be transmitted to the brain.
Chris - And then, how did you prove that the brain was actually interpreting this information because it’s one thing for the retina to be a little bit more electrically active but that doesn’t mean the signals are going to the brain?
Samantha - Exactly. So the next step, we looked at a number of different things - one was the pupil light response. Normally, when you shine light into someone’s eye their pupil constricts and that’s a marker of the appropriate circuits within the brain being activated. In blind mice, that pupil response is severely attenuated because of the lack of detection of light. Whereas in the treated mice, the pupil light reflex was restored both at 2 months and then going on to look at mice at 13 months.
In terms of the light responses and vision responses we found, we found that the mice in our study were able to detect a change in their visual environment and that would probably equate to a completely blind person being able to recognise their environment, so possibly where a door is, where a window is, where an obstacle in the road or something like that is. So we’re not talking about rapidly dynamic vision because I don’t think melanopsin would be able to restore that but, obviously, if you’ve got nothing at all to start with then that’s a significant improvement.
Chris - Is the next step to now do a clinical trial? Are you at the stage where you could safely do that?
Samantha - Essentially, all the groundwork has been done and to take it forward to a clinical trial there are further steps such as to make a virus which is suitable for use in humans rather just in the lab. All that, and the regulatory work takes a couple of years so we’d hope to try and get it to patients within the next few years.