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Retinitis Pigmentosa
Retinitis pigmentosa (RP) refers to a group of inherited diseases that lead to retinal degeneration. The retina is responsible for capturing images from the visual field. People with RP experience a gradual decline in their vision because photoreceptor cells (rods and cones) die.
Forms of RP and related diseases include Usher syndrome, Leber’s congenital amaurosis, rod-cone disease, Bardet-Biedl syndrome, and Refsum disease, among others.
Symptoms
Symptoms depend on whether rods or cones are initially involved. In most forms of RP, rods are affected first. Because rods are concentrated in the outer portions of the retina and are triggered by dim light, their degeneration affects peripheral and night vision. When the more centrally located cones, which responsible for color and sharp central vision, become involved, the loss is in color perception and central vision.
Night blindness is one of the earliest and most frequent symptoms of RP. People with mainly cone degeneration, however, first experience decreased central vision and ability to discriminate color.
RP is typically diagnosed in adolescents and young adults. It is a progressive disorder. The rate of progression and degree of visual loss varies from person to person. Most people with RP are legally blind by age 40, with a central visual field of less than 20 degrees in diameter. It is a genetic disorder and, therefore, is almost always inherited.
How is RP inherited?
An estimated 100,000 people in the U.S. have RP, mainly caused by mutated genes inherited from one or both parents. Mutated genes give the wrong instructions to photoreceptor cells, telling them to make an incorrect protein, or too little or too much protein. (Cells need the proper amount of particular proteins in order to function properly.) Many different gene mutations exist in RP. In Usher syndrome, for example, at least 14 disease-causing genes have been identified.
Genetic mutations can be passed from parent to offspring through one of three genetic inheritance patterns — autosomal recessive, autosomal dominant, or X-linked. In autosomal recessive RP, parents who carry the gene but have no symptoms themselves could have some children who are affected and others who are not. Similarly, in autosomal dominant RP, an affected parent could have affected and unaffected children. In families with X-linked RP, only males are affected; females carry the genetic trait but do not experience serious vision loss.
Novel Small Molecule Neuroprotectants
Mimetogen has developed a number of small molecule neurotrophin mimetics that demonstrate strong protective properties, rescuing retinal pigment epithelial cells from oxidative stress-induced apoptosis.
The retinal pigment epithelium (RPE) is the pigmented cell layer just outside the neurosensory retina that nourishes retinal visual cells, and is firmly attached to the underlying choroid and overlying retinal visual cells. The retinal pigment epithelium is involved in the phagocytosis of the outer segment of photoreceptor cells and it is also involved in the vitamin A cycle where it isomerizes all trans retinol to 11-cis retinal.
The retinal pigment epithelium also serves as the limiting transport factor that maintains the retinal environment by supplying small molecules such as amina acid, ascorbic acid and D-glucose while remaining a tight barrier to choroidal blood borne substances. Homeostasis of the ionic environment is maintained by a delicate transport exchange system. Dysfunction of the RPE is found in Age-Related Macular Degeneration and retinitis pigmentosa.
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