Photoreceptor transplantation is a potential future treatment for blindness caused by retinal degeneration. Photoreceptor transplantation restores visual responses in end-stage retinal degeneration, but has also been assessed in non-degenerate retinas. In the latter scenario, subretinal transplantation places donor cells beneath an intact host outer nuclear layer (ONL) containing host photoreceptors. Here we show that host cells are labelled with the donor marker through cytoplasmic transfer—94±4.1% of apparently well-integrated donor cells containing both donor and host markers. We detect the occurrence of Cre-Lox recombination between donor and host photoreceptors, and we confirm the findings through FISH analysis of X and Y chromosomes in sex-discordant transplants. We do not find evidence of nuclear fusion of donor and host cells. The artefactual appearance of integrated donor cells in host retinas following transplantation is most commonly due to material transfer from donor cells. Understanding this novel mechanism may provide alternate therapeutic strategies at earlier stages of retinal degeneration.

Stargardt disease, an ATP-binding cassette, subfamily A, member 4 (ABCA4)-related retinopathy, is a genetic condition characterized by the accelerated accumulation of lipofuscin in the retinal pigment epithelium, degeneration of the neuroretina, and loss of vision. No approved treatment exists. Here, using a murine model of Stargardt disease, we show that the propensity of vitamin A to dimerize is responsible for triggering the formation of the majority of lipofuscin and transcriptional dysregulation of genes associated with inflammation. Data further demonstrate that replacing vitamin A with vitamin A deuterated at the carbon 20 position (C20-D₃-vitamin A) impedes the dimerization rate of vitamin A—by approximately fivefold for the vitamin A dimer A2E—and subsequent lipofuscinogenesis and normalizes the aberrant transcription of complement genes without impairing retinal function. Phenotypic rescue by C20-D₃-vitamin A was also observed noninvasively by quantitative autofluorescence, an imaging technique used clinically, in as little as 3 months after the initiation of treatment, whereas upon interruption of treatment, the age-related increase in autofluorescence resumed. Data suggest that C20-D₃-vitamin A is a clinically amiable tool to inhibit vitamin A dimerization, which can be used to determine whether slowing the dimerization of vitamin A can prevent vision loss caused by Stargardt disease and other retinopathies associated with the accumulation of lipofuscin in the retina. Significance Accumulation of lipofuscin in the retinal pigment epithelium precedes retinal degenerations and dystrophies responsible for blindness-causing retinal diseases. The mechanism behind lipofuscin formation in the retina or in any tissue is poorly understood. Here we show in mice that the dimerization of vitamin A is responsible for triggering the formation of more than 50% of ocular lipofuscin. Replacing three hydrogen atoms on vitamin A with deuterium inhibits vitamin A dimerization, resulting in reduced lipofuscin and transcriptional normalization of genes associated with inflammation without compromising retinal function. Thus, vitamin A deuterated at the carbon 20 position provides a clinically amiable tool to prevent vitamin A dimerization in humans to assess whether impeding such dimerization might prevent retinal degenerations such as Stargardt disease and age-related macular degeneration.

Optogenetic strategies to restore vision in patients who are blind from end-stage retinal degenerations aim to render remaining retinal cells light sensitive once photoreceptors are lost. Here, we assessed long-term functional outcomes following subretinal delivery of the human melanopsin gene (OPN4) in the rd1 mouse model of retinal degeneration using an adeno-associated viral vector. Ectopic expression of OPN4 using a ubiquitous promoter resulted in cellular depolarization and ganglion cell action potential firing. Restoration of the pupil light reflex, behavioral light avoidance, and the ability to perform a task requiring basic image recognition were restored up to 13 mo following injection. These data suggest that melanopsin gene therapy via a subretinal route may be a viable and stable therapeutic option for the treatment of end-stage retinal degeneration in humans. Significance Inherited retinal degenerations may result in blindness due to a progressive loss of photoreceptor cells. We assess subretinal delivery of human melanopsin using an adeno-associated viral vector to remaining retinal cells in a model of end-stage retinal degeneration. Human melanopsin, being already present in the eye, is unlikely to generate an immune response when introduced via gene therapy. Furthermore, this method of delivery has been proven to be safe in clinical trials and may be more effective at delivering vector in primates than the alternative method of intravitreal injection. We demonstrate long-term vector expression and restoration of visual function, indicating that this therapy could be stable and efficacious in the treatment of patients with end-stage retinal degenerations.

RNA editing aims to treat genetic disease through altering gene expression at the transcript level. Pairing site-directed RNA-targeting mechanisms with engineered deaminase enzymes allows for the programmable correction of G>A and T>C mutations in RNA. This offers a promising therapeutic approach for a range of genetic diseases. For inherited retinal degenerations caused by point mutations in large genes not amenable to single-adeno-associated viral (AAV) gene therapy such as USH2A and ABCA4, correcting RNA offers an alternative to gene replacement. Genome editing of RNA rather than DNA may offer an improved safety profile, due to the transient and potentially reversible nature of edits made to RNA. This review considers the current site-directing RNA editing systems, and the potential to translate these to the clinic for the treatment of inherited retinal degeneration.