Development of AAV gene therapy for blindness caused by cone-rod dystrophy

Cone-rod dystrophies (CRDs) are common genetic (inherited) causes of irreversible and incurable blindness. Individuals with cone-rod dystrophies are born with normally functioning eyes but go on to develop progressive loss of central vision, loss of colour vision, and later, loss of peripheral and night vision. CRDs are highly variable, with different ages of onset, severity and rates of progression and are associated with different genetic mutations ('mistakes' within the genetic code). Such disorders often become manifest in middle-age, and can progress rapidly with severe physical, financial, social and psychological harm to the individual and their families.

One particular mutation that is associated with CRDs is in a gene called cadherin-1 (CDHR1). This gene codes for a protein that regulates the structure of part of the cone and rod photoreceptors - the photosensitive cells in the retina that detect light. This gene has been implicated in the development of a specific form of severe CRD that has been described in the Faroe Islands, the United Kingdom and elsewhere with onset in late childhood. However, new data suggest that mutations in the CDHR1 gene are a more common cause of CRD that previously considered.

The aim of this PhD is to develop a new treatment strategy for CRD associated with the CDHR1 mutation. The suggested approach involves inserting healthy cadherin-1 genes into eyes of mice who are deficient for this gene using a specially engineered virus (AAV) containing the healthy human cadherin-1 gene. It is anticipated that through this gene transfer, that photoreceptor cells should have a normal amount of the healthy protein, thus preserving cone and rod photoreceptors and maintaining vision. The virus, which has no capacity to replicate and in itself cause disease, will be delivered to cone photoreceptor cells through an injection underneath the retina. This strategy has previously proved successful in restoring vision in a number of mouse models of retinal degenerative disease and in early human clinical trials for other retinal diseases.

In my PhD project, I will aim to characterize the mouse which has had the cadherin gene removed, develop a new AAV viral vector which carries the healthy cadherin-1 gene, inject the virus under the retina of such mice and prove that the harmful effects of the gene mutation to the mouse retina have been reversed. I will optimize the efficiency of the viral vector to ensure that fewer viral particles are delivered (thus minimising any potential side-effects), whilst allowing for the normal levels of healthy cadherin-1 are produced to rescue the degenerating retina in cone-rod dystrophy.

In addition, I will study human patients with cone-rod dystrophy associated with CDHR1 gene mutations identified from genetic registries. I will characterize the nature of the genetic mutations in such patients, and describe the clinical features of this form of cone-rod dystrophy. In particular, we hope to identify patients with a newly described mutation in CDHR1, which is not currently detected by a common form of genetic testing. This clinical data will inform and refine the laboratory-based gene therapy experiments to allow us greater insights into the human disease that we seek to cure. It will also allow us to identify the optimal time in the course of the disease to treat these patients with gene therapy in the future.

It is my hope that this work will form the basis for a clinical trial in human subjects with cone-rod dystrophy and that I can be active in overseeing this transition in the years following my PhD. The ultimate goal of this project is thus to develop a genetic technology which may slow or even halt the progression of this hitherto untreatable and blinding retinal disease. It has the potential to impart a significant improvement in quality of life and ability to work for many thousands of individuals with cone-rod dystrophy across the world.

Cone-rod dystrophies (CRD) are an important genetic cause of irreversible, central visual loss. Gene therapy represents a logical therapeutic strategy to prevent or delay the onset of visual loss in patients with CRD. Next generation sequencing has identified CDHR1 mutations as a more common cause of CRD than previously considered (Charbel Issa, unpublished data). There are patients in the UK with CDHR1-associated CRD who may benefit from gene therapy.

Proof of principle studies of gene therapy in other forms of inherited eye disease have demonstrated safety and efficacy in preserving the central vision in patients with other retinal degenerations. Gene therapy as a cure for CDHR1-related CRD has not been attempted, yet it is an ideal model to study gene therapy for CRDs in that: (1) the associated CRD has a relatively late onset with rapid decline and wide therapeutic window, (2) the relatively small size of CDHR1 (2,580 base pairs) allows AAV encoding of the transgene, (3) a CHDR1-/- mouse is available for the development of gene therapy. This has never been attempted; all experiments suggest here will be novel.

I intend to phenotype the CHDR1-/- mouse using a variety of imaging, electrophysiological and histological techniques. I will then construct an adeno-associated viral vector (AAV-8) to restore visual function in CDRH1-/- mice using the identified outcome measures. In parallel, I intend to characterize the human phenotype in patients with CDHR1-associated CRDs identified from next generation sequencing databases. This will assist in identifying the optimal time in the course of the disease to undertake gene therapy in future clinical trials. The laboratory study and clinical phenotyping will be complementary in the targeted development of gene therapy for CDHR1-related CRD.

This project will enable a greater understanding of human CDHR1-related CRD, and attempt to develop a gene therapy treatment to cure a currently blinding disease of the retina.

1. Quality of life of patients currently afflicted with incurable forms of blindness.
2. Cost to the NHS of patients currently afflicted with incurable forms of blindness.
3. Cost to the economy of patients currently afflicted with incurable forms of blindness.


Benefit to patients
The proposed technology is directly relevant to preserving the sight of patients who are gradually going blind from photoreceptor loss caused by cone-rod dystrophies. Affecting more than 1 in 40,000 individuals, cone-rod dystrophies result in progressive loss of central vision, colour vision, followed late by night-blindness and peripheral retinal degeneration. Cone-rod dystrophies often present in middle-age and are consequently financially, socially and psychologically devastating both to patients and their families. Currently, there is no treatment available for any form of cone-rod dystrophy. It is highly likely that patients with cone-rod dystrophies will benefit directly through the development of a clinical trial at the end of this project. Professor Robert MacLaren has a strong track record of developing laboratory research into new treatments for patients who are blind with incurable retinal disease. Patients with CDHR-related cone-rod dystrophy would find news of this research encouraging.

Benefit to the NHS
The cost of blindness is significant and is largely borne by the government in the UK. The cost of a guide dog is approximately £50,000 for 12 years and the cost of an electronic retinal implant is in the region of £100,000 based on the Second Sight device. A gene therapy which halts or slows down photoreceptor loss may not only be more effective in the long run but it is possible that it may become more affordable if the technology is more widely adopted.

Benefit to the economy
The United Kingdom already occupies a leading international position in biotechnology research. Successful results in NHS funded clinical trials led by Professor Robert MacLaren have been reported around the world and have further highlighted the important role of UK research in developing new treatments for blindness. The high-profile research in Oxford further underlies the reputation the UK holds as a centre of excellence for biomedical research.