In clinical trials aiming to treat rare forms of genetic blindness, patients’ vision has already been restored by reintroducing the deficient gene in their retina. Today, researchers are developing a similar gene therapy protocol that has the potential to be used in treating various kinds of blindness independently of the defective gene. For this, they use a coding gene for an opsin, a light-sensitive protein.
Gene therapy consists of introducing a drug gene in target cells through a vector, which is usually a modified virus. This approach has already been successfully tested in a few cases of rare genetic blindness, such as Leber’s congenital amaurosis, a form of extremely rapid retinal degeneration. However, extending this therapy to other retinal pathologies remains limited by the diversity of mutations that can be involved, a vast number of which remain unknown to date. This is the case for retinitis pigmentosa, a group of progressive diseases characterized by a gradual loss of vision—night vision first, then during the day—related to the death of specialized nerve cells, the rods, followed by cones.
Due to the highly technical nature and high cost of developing gene therapies, it is important to develop approaches that achieve the goal of treating as many patients as possible. To this aim, optogenetics represents a valuable solution: introducing an opsin gene in the retinal cells allows this light-sensitive protein to convert the light energy to electric activity, which can then be transmitted to the optical nerve to restore vision. An Inserm team* has just published an article describing the success of such an approach. The team conducted conclusive experiments in animals and on human tissue samples. Today, the approach needs to be perfected.
To achieve this result, Deniz Dalkara’s team accomplished several steps: the first was developing viral vectors that were as efficient as possible. In concrete terms, they had to determine which type of virus could reach the target cells in the retina as easily as possible, in such a manner as to obtain a strong, specific expression of the opsin gene in these target cells. The team developed two viral vectors based on two adeno-associated viruses (AAVs). According to the researcher: “the first can be administered rather easily via injection in the vitreous body. It then moves on to the cells in the fovea, the retinal area with the highest cone density. But AAVs are very common viruses in the environment, with more than 70% of the population having antibodies directed against this AAV. In this case, it is not possible to inject the treatment in the vitreous body, since it contains antibodies that would neutralize the virus. The sub-retinal route must be considered under these circumstances.” This operation, which is much more delicate, causes temporary retinal detachment that can sometimes lead to complications, particularly in the fovea, which it can durably alter. “We have developed a second vector from another type of AAV, which presents the advantage of being injectable beneath the retina at a short distance from the fovea. It then migrates to the fovea with no risk of damaging it,” explains Dalkara. To ensure the efficacy of the optogenetic approach, the researchers used animal models and human tissues (post-mortem retinas and mini-retinas developed in vitro from stem cells reprogrammed to become retinal cells).
This model could be tested in clinical trials, but it does need to be perfected. “Microbial opsin is much less sensitive to light than our cones. Consequently, an approach to gene therapy such as this one will rely on the use of special eyeglasses equipped with cameras that capture light stimuli and intensify them for the retina,” says the researcher. Furthermore, “in retinitis pigmentosa, the cones that perceive colors and day vision die. This is caused by the prior death of the rods that usually provide them with a trophic factor. Even though opsin can restore vision, it cannot prevent the gradual death of the cones. As a result, we are seeking to improve the treatment through a simultaneous expression of the opsin-coding gene and of the gene coding for the trophic factor, which would make it possible to preserve and restore vision in the longer term.”
*Inserm/CNRS/Université Pierre et Marie Curie Unit 968, Vision Institute, Gene therapies and animal models for neurodegenerative illnesses team, Paris