Restoring vision in mouse models of retinal degeneration using human rod opsin.

Degeneration of the retina is a common cause of blindness. In many cases, this degeneration is predominantly a feature of the rod and cone photoreceptors, leaving the channel of communication between the eye and brain (the optic nerve) intact. In patients with such conditions then, the retina is still present but lacks the ability to respond to light. If one could restore photosensitivity to the retina of these people, one could hope to recreate vision. This indeed has been achieved in some patients using electronic prostheses. This places the emphasis on identifying the best possible way of regenerating visual responses in the degenerate retina. There are a number of exciting technologies in development to achieve this goal. It is likely that each will have strengths and limitations and that different patient groups benefit from different approaches. We are therefore at a stage at which we need to explore a variety of strategies for restoring photosensitivity to the degenerate retina in pre-clinical studies. We recently, trialled a very simple approach to this problem - introducing the protein that makes human rods photoreceptive (rod opsin) to surviving neurones in the retina of a mouse model of blindness. We found that rod opsin could make these neurones photoreceptive, and that the treated mice were able to recognise patterns and natural movies presented with standard computer monitors. If similar results were achieved in human patients with similar conditions, we could hope to allow patients currently completely blind to recognise some objects and use vision for spatial navigation in ordinary indoor lighting. As the intervention involves introducing a native human protein into the part of the body it is ordinarily expressed using methods already in clinical use, safety and ethical barriers to trialling in humans are minimised. However, before moving to patients we need a better appreciation of how this new therapy could work. Here we propose a series of experiments in mice that will tell us how safe and effective this potentially exciting new therapy could be.

Degeneration of rod and cone photoreceptors is a major cause of blindness. In patients with this condition, inner retinal neurones survive and we know from work with electronic prostheses that if they can be presented with visual information the machinery exists to transmit it to the brain where it can be interpreted. This proposal is based around a great deal of preliminary data showing that intra-vitreal injection of AAV2/2 driving expression of human rod opsin in surviving ON bipolar cells in a mouse model of advanced retinal degeneration (rd1) can render them photoreceptive and support visual discrimination. The quality of vision produced by this simple approach compares very favourably with that provided by alternative strategies. This proposal builds upon that breakthrough, and comprises a thorough investigation of the safety and efficacy of the rod opsin based intervention in mice using behavioural and electrophysiological assays established in the applicants' laboratories. We aim to determine the best visual performance achievable with this approach by measuring its spatial and temporal resolution over a range of background light intensities and visual contrasts; ask how stable the restored vision is over time; whether it is effective for other forms of retinal dystrophy; and whether it would augment or interfere with low vision at early stages of degeneration. In preparation for clinical application we also ask whether the restored vision can be assayed using clinically relevant electrophysiology (electroretinograms and visual evoked potentials) and can be produced by sub-retinal delivery of virus. Our experiments will also address the question of safety by revealing what impact (if any) exogenous expression of rod opsin has on function or viability of the inner retina. We propose making a transgenic mouse line in which human rod opsin is expressed in all ON bipolar cells to augment our established viral gene delivery methods in addressing these questions.

This research is focussed upon the translation of our discoveries towards providing a treatment that will partially restore sight in patients who are blind from hereditary retinal degenerations, a common cause of blindness representing 5.5% of registrations for severe sight impairment in the UK. Therefore, ultimately, intended beneficiaries of this research are patients with severe visual disabilities from hereditary retinal degenerations, and because this approach could restore vision irrespective of the specific underlying gene defect, it may be widely applicable to this group of patients. Not only would it benefit the patients, it would benefit their carers and reduce burden on others who provide support including social services, other governmental departments and charities (by reducing costs and allowing re-allocation of resources).

There will be many beneficiaries along the translational pathway. The immediate outputs from this research will benefit academics and clinicians working in the fields of hereditary retinal degenerations, optogenetics, retinal physiology and visual neurosciences. The MRC and University of Manchester will benefit from the kudos associated with high quality research outputs from this research. Researchers working on the project will gain expertise in optogenetics and techniques involved in measuring visual function in the retina and the brain. Translation into a novel therapeutic will involve engagement with industry and potential economic benefit to industry and government.