Retinitis pigmentosa is a group of inherited retinal degenerations that affects more than 1.5 million people worldwide. The disease progressively destroys the retina’s photoreceptors, the rods and cones that normally capture light and convert it into electrical signals for the brain. Patients typically first notice difficulty seeing at night, followed by a narrowing of peripheral vision that worsens over years, ultimately leading to severe vision loss or blindness. Because the retina is part of the central nervous system, its capacity for self-repair is extremely limited, and once photoreceptors are lost they are not naturally replaced.

One of the central obstacles to treating the disease is its sheer genetic diversity. Retinitis pigmentosa can arise from mutations in any one of more than 100 distinct genes, which makes a mutation-by-mutation strategy impractical for most patients. Gene therapies that correct specific defects have succeeded for other blinding conditions, but building and validating a separate treatment for every causative gene is proving tremendously difficult and costly. The Pittsburgh-led team, under the direction of José-Alain Sahel, chair of ophthalmology at Pitt and director of the UPMC Vision Institute, pursued a fundamentally different logic: rather than fixing the underlying mutation, the therapy aims to make surviving retinal cells light-sensitive again, regardless of which gene caused the degeneration.

The technical approach relies on optogenetics, in which cells are genetically altered to produce light-sensing proteins borrowed from other organisms. In this trial, the researchers delivered a gene encoding a protein called ChrimsonR into surviving retinal ganglion cells through a single injection into the eye. Retinal ganglion cells are the output neurons of the retina; in a healthy eye they relay signals from photoreceptors to the brain, and in advanced retinitis pigmentosa many of them remain viable even after the light-sensing cells above them have died. By equipping these surviving cells with an engineered light-sensitive protein, the therapy effectively converts the retina’s output neurons into a substitute photoreceptor layer.

ChrimsonR alone is not enough, however. The protein responds to specific wavelengths of amber-red light, and ordinary ambient illumination is not structured in a way the modified cells can interpret usefully. To bridge that gap, participants wear specialized goggles with a built-in camera that captures the visual scene and sends the information to a portable processor. The processor converts the image into patterns of light at wavelengths designed to activate ChrimsonR, and a projector inside the goggles projects those patterns back onto the retina. The system can, for example, transform the image of a hand into a simplified pattern of light that stimulates the treated cells, allowing the brain to receive visual information through a completely rewired pathway.

On safety, the early-stage trial delivered encouraging results. Most eye-related side effects were mild or moderate, consisting mainly of temporary inflammation and short-lived increases in pressure inside the eye. No findings reported in the release suggested the therapy itself caused lasting harm, an important benchmark for a treatment that involves permanently introducing a new light-sensing protein into neural tissue. The researchers emphasize that this was a small, early-stage study, and that the treatment did not restore normal vision or the ability to read. Even so, some participants became better able to detect when an object was present, determine where it was located, and reach toward it accurately while using the goggles, functional abilities that can translate into greater independence in daily life.

The study went beyond behavioral testing to ask whether the new visual signals were actually reaching the brain. Using electroencephalography, a team led by Marlene Behrmann, professor of ophthalmology at Pitt, found evidence that visual signals reached and were processed by the visual cortex when participants viewed objects. That finding is scientifically striking, because it suggests that in people with profound, long-standing vision loss, the brain’s visual processing machinery remains intact and capable of interpreting input delivered through an entirely novel channel. Four participants showed consistent improvements across multiple real-world visual tasks over months to years of testing, indicating that the gains were not fleeting artifacts of a single laboratory session.

Sahel noted that the results demonstrate the visual system retains a remarkable capacity to process new information even after profound vision loss. He also pointed to the approach’s broader potential: because the strategy targets surviving retinal ganglion cells rather than any specific mutation, it could in principle help patients with other blinding diseases in which light-sensing cells have been lost but ganglion cells remain viable, a group that currently has few or no treatment options. That mutation-agnostic design is what distinguishes optogenetic therapy from the growing but still narrow portfolio of gene-specific treatments.

The new findings build directly on a landmark 2021 study published in Nature Medicine, which reported the first partial recovery of visual function in a blind patient following optogenetic therapy, the first clinical application of optogenetics in medicine. That work, carried out with longtime collaborator Botond Roska of the Institute of Molecular and Clinical Ophthalmology Basel, who is also a corresponding author of the current study, earned the two researchers the António Champalimaud Vision Award, the largest prize in the field, last month. The current trial was conducted as part of the PIONEER clinical study with support from GenSight Biologics, and included collaborators from Sorbonne Université, Institut de la Vision, and L’Hôpital Fondation Adolphe de Rothschild in Paris, the Institute of Molecular and Clinical Ophthalmology Basel and University of Basel in Switzerland, Streetlab in Paris, and Moorfields Eye Hospital in London.

The optogenetic program is one of several complementary strategies the Pittsburgh-led team is pursuing for end-stage retinal disease. Last year, Sahel was senior author of a New England Journal of Medicine paper describing vision restoration using an implanted prosthetic device, known as PRIMA, in patients with end-stage age-related macular degeneration. That technology has since received European Union authorization to be marketed for clinical use in many European countries. Together, the two approaches illustrate a shifting landscape in which blindness once considered irreversible is being reclassified as a engineering problem: if light-sensing cells are gone, the task becomes finding surviving neural tissue that can be repurposed to carry visual signals, whether by genetically reprogramming ganglion cells or by implanting a photovoltaic prosthetic. For the retinitis pigmentosa community, the new trial offers the strongest evidence yet that such repurposing can work safely in humans, while underscoring how much work remains before the therapy could move from a small trial toward approved, widely available treatment.

Subject of Research: Optogenetic gene therapy for restoring visual function in advanced retinitis pigmentosa

Article Title: Optogenetic therapy safe, shows signs of vision restoration in patients with advanced retinitis pigmentosa, according to international study led by Pitt scientists

Article References: Optogenetic therapy safe, shows signs of vision restoration in patients with advanced retinitis pigmentosa, according to international study led by Pitt scientists. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: optogenetics, retinitis pigmentosa, gene therapy, retinal ganglion cells, ChrimsonR, visual prosthesis, New England Journal of Medicine, University of Pittsburgh, vision restoration, clinical trial, retinal degeneration, visual cortex