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Retinitis Pigmentosa
Retinitis Pigmentosa (RP) describes a group of inherited retinal degenerations (IRDs) affecting the photoreceptor (light-sensing) cells which line the back of the eye, called the retina. People living with RP experience a gradual decline in their vision because the two photoreceptor cell types – rod and cone cells – degenerate and die.
Rod cells are responsible for vision in low light (scotopic vision), while also having an important role in peripheral vision. Cone cells are most concentrated in the central region of the retina (macula) and have a prominent role in central vision (reading). Cone cells perceive bright light (photopic vision) and are also necessary for colour vision. During RP onset, rod cells stop working first which impairs night vision, followed by blind spots which develop in our peripheral vision. Deterioration of cone cells follows, but many people living with RP maintain reasonable central vision well into middle age.
What are the symptoms of RP?
Due to rod cells degenerating first, the initial signs and symptoms of RP are night blindness and difficulty transitioning between low and bright light. As peripheral vision progressively deteriorates, tunnel vision ensues and only central vision is maintained.
RP has a variable age of onset, whereby some people are diagnosed with the condition in early childhood, while others might not be diagnosed until they are adults. The rate of RP progression and degree of vision loss also varies between individuals. It is therefore very difficult to predict what a person’s vision will be like at specific times in the future. Both eyes are usually affected in a similar manner.
What Causes RP?
RP is one of the most complex genetic conditions, known to be caused by more than 50 genes and affects approximately 1 in 3000 to 1 in 4,000 individuals worldwide1. The nature of RP inheritance also contributes to this complexity, whereby RP-causing mutations can be passed from parents to children in an autosomal dominant, autosomal recessive and X-linked pattern.
Autosomal dominant inheritance describes how one faulty copy of a gene is sufficient to cause the disease. The person will have received this mutated copy from an affected individual and has a 50% chance of passing this copy to their children. The autosomal form of RP tends to be milder than other forms with many people preserving vision into middle age. Autosomal recessive inheritance explains that two malfunctioning copies of a particular gene are required in order for the disease to develop, whereby one mutated copy comes from each parent.
In X-linked inheritance, the mutated gene is located on the X chromosome. For X-linked recessive conditions, males are primarily affected as they only have one X chromosome and one Y chromosome. One copy of the mutated gene on the X chromosome is sufficient to cause RP to develop. In contrast, females require both of their X chromosomes to have the mutated gene. X-linked recessive RP is a very severe form of the disease which primarily affects males and results in only central remaining by the third decade of life. Due to the phenomenon of X-chromosome Inactivation, female carriers can sometimes express symptoms of the condition and in extreme cases, where females may be severely affected.
RP is a genetic condition, but there are cases with no family history and if a family is diagnosed with RP, we strongly advise that other family members have an eye exam with an eye doctor (ophthalmologist). In addition to developing independently as non-syndromic RP, RP can occur as part of other complex syndromes, displaying a variety of symptoms with varying levels of severity, such as in usher syndrome and bardet-biedl syndrome.
What treatments are available?
There is currently only one treatment for children and adults living with a rare form of autosomal recessive RP caused by mutations in both copies of the RPE65 gene. Luxturna (voretigene neparvovec) is a gene therapy which works by providing a normal copy of the RPE65 gene and replaces the mutated copy to restore normal function and while not suitable for treating all forms of RP, its approval marks great progress in search for cures for other forms of RP and other IRDs. This gene therapy is delivered as a one-time subretinal injection to the back of the eye and is authorised for use in the United States and all 28 member states of the EU, as well as Iceland, Liechtenstein and Norway.
There are no other approved therapies for other forms of RP, but there are many treatments and devices being investigated at clinical trial stage, which you can check out on the ClinicalTrials.gov website. These clinical trials are essential for investigating the safety and effectiveness of these treatments and ensures they achieve a desired therapeutic effect. It’s important to be aware that you can only participate in a clinical trial or access potential treatments for your condition once you have the results of your genetic test. Learn more about genetic testing on our Genetic Testing Services Resource.
As RP is characterised by progressive degeneration of the light-sensing rod and cone cells of the retina, a promising field of study is stem-cell biology. This research focuses on growing two cell types in particular, photoreceptor cells and retinal pigment epithelium (RPE) cells. RPE cells support the photoreceptor cells but are not directly responsible for vision and light perception. It is hoped that by replacing RPE cells while the retina is still working, it will support better retina function, visual acuity will be maintained and surviving retinal cells can be nourished.
Retinal implants are a type of biomedical technology currently being developed for a many retinal diseases, including retinitis pigmentosa. A number of these implants have shown success in delivering a form of artificial vision to individuals, even after complete degeneration and death of the photoreceptor cells, although none restore natural vision. These microchips relay an electronic signal to the remaining retinal cells, which pass the signal down the optic nerve and is processed, to form a visual image in the brain.
Some bionic devices, such as the Argus® II Retinal Prosthesis System, IRIS®II and Alpha AMS have been developed which facilitate people living with retinal diseases to regain their independence and have received approval for sale in Europe by satisfying safety and reliability standards (CE Mark). These technologies involved surgical subretinal implantation of the prosthesis as well as other non-invasive components, such as glasses. They work by functioning similar to the degenerated photoreceptor cells and electrically stimulating the retinal tissue, but don’t yet restore natural vision.
Other designs which show great promise are retinal implants composed of semi-conductive polymers, which are flexible and can bend similar to normal tissue2. These structures have shown their ability to achieve electric impulses similar to what is seen from a rod or cone cell and are being investigated in live animal models, with some success in replicating normal eye activity in response to exposure to light i.e. pupils dilating appropriately. Although these inventions are still being refined before entering clinical trial, they show great promise for the future.
Despite the lack of current treatments for RP, it is still very important to have regular eye check-ups. This is because people with RP are still at risk for other kinds of eye complications that can affect the general population, and may be treatable. RP patients tend to develop cataracts at an earlier age than the non-RP population and can do very well from cataract surgery, although the visual outcome obviously depends on the severity of the retinal degeneration. Regular visits to your eye doctor can also make you aware of current advances as we learn more about RP.
References
1. National Organization for Rare Disorders. Available at https://rarediseases.org/rare-diseases/retinitis-pigmentosa/. Accessed March 2020.
2. Maya-Vetencourt JF, Ghezzi D, Antognazza MR, et al. A fully organic retinal prosthesis restores vision in a rat model of degenerative blindness. Nat Mater2017; 16: 681–689