In 2008, three independent research groups reported that patients with the rare genetic retinal disease Leber's congenital amaurosis had been successfully treated using gene therapy with adeno-associated virus (AAV).
These results were widely seen as a success in the gene therapy field, and have generated excitement and momentum for AAV-mediated applications in retinal disease.
The great advantage in using adeno-associated virus for the gene therapy is that it poses minimal immune responses and mediates long-term transgene expression in a variety of retinal cell types.
For example, tight junctions that form the blood-retina barrier, separate subretinal space from the blood supply, providing protection from microbes and decreasing most immune-mediated damages.
[6] Following the successful clinical trials in LCA, researchers have been developing similar treatments using adeno-associated virus for age-related macular degeneration (AMD).
Whereas wet AMD is currently treated using frequent injections of recombinant protein into the eyeball, the goal of these treatments is long-term disease management following a single administration.
[9] Delivered as a single intravitreal injection, Ixo-vec enables sustained release of aflibercept, an anti-VEGF protein that helps control abnormal blood vessel growth and fluid leakage, which are key in AMD progression.
[10] Results from the OPTIC and LUNA trials demonstrate Ixo-vec’s effectiveness in significantly reducing the need for regular injections over extended periods.
Visual acuity remained stable, and anatomical outcomes, like reductions in central subfield thickness (CST), were achieved.
[12] Dr. Robert MacLaren of the University of Oxford, who lead the trial, co-developed the treatment with Dr. Miguel Seabra of the Imperial College, London.
Since the cells of the retina are non-dividing, AAV continues to persist and provide expression of the therapeutic DNA sequence over a long time period that can last several years.
Using the subretinal route, AAV is injected underneath the retina, taking advantage of the potential space between the photoreceptors and RPE layer, in a short surgical procedure.
[20] One example of an engineered variant has recently been described that efficiently transduces Muller glia following intravitreal injection, and has been used to rescue an animal model of aggressive, autosomal-dominant retinitis pigmentosa.
[29][30] When induction occurs by oral doxycycline, this system shows tight regulation of gene expression in both photoreceptor and RPE cells.
One study that was done by Royal College of Surgeons (RCS) in rat model shows that a recessive mutation in a receptor tyrosine kinase gene, mertk results in a premature stop codon and impaired phagocytosis function by RPE cells.
The model organism with this disease received a subretinal injection of AAV serotype 2 carrying a mouse Mertk cDNA under the control of either the CMV or RPE65 promoters.
[29] Specifically the AAV 5 vector containing the wild-type human RSI cDNA driven by a mouse opsin promoter showed long-term retinal functional and structural recovery.
Also the retinal structural reliability improved greatly after the treatment, characterized by an increase in the outer nuclear layer thickness.
[29] Retinitis pigmentosa is an inherited disease which leads to progressive night blindness and loss of peripheral vision as a result of photoreceptor cell death.
[29] Another mutation in the rhodopsin structural protein, specifically peripherin 2 which is a membrane glycoprotein involved in the formation of photoreceptor outersegment disk, can lead to recessive RP and macular degeneration in human[33] (19).
In a mouse experiment, AAV2 carrying a wild-type peripherin 2 gene driven by a rhodopsin promoter was delivered to the mice by subretinal injection.
One group of researchers injected the rod derived cone viability factor (RdCVF) protein (encoded for by the Nxnl1 (Txnl6) gene) into the eye of the most commonly occurring dominant RP mutation rat models.
[29][37] In looking at an animal model, the opsin transgene contains a truncated protein lacking the last 15 amino acids of the C terminus, which causes alteration in rhodopsin transport to the outer segment and leads to retinal degeneration.
[29] In normal tissues VEGF stimulates endothelial cell proliferation in a dose dependent manner, but such activity is lost with other angiogenic factors.
One clinical finding shows that the levels of PEDF in aqueous humor of human are decreased with increasing age, indicating that the reduction may lead to the development of AMD.
[29][44] In animal model, an AAV with human PEDF cDNA under the control of the CMV promoter prevented choroidal and retinal NV.