Retinitis pigmentosa can be caused by genetic mutations in over 60 genes. More than 20 genes are associated with the autosomal dominant mode of inheritance. Of these 20 genes, the rhodopsin gene, RHO, is the most prevalent to cause autosomal dominant retinitis pigmentosa, and more than 45 mutations in RHO have been identified (Sung et al, 1994). Because there are so many genes that encode various structures in the retina, it is difficult to delegate a single gene to the disease. However, by studying families that show symptoms, researchers are able to locate the mutation, contribute to the scientific community and hopefully provide more explanations that will lead to treatments and cures.
The RHO gene provides instructions for making rhodopsin. Rhodopsin is the photopigment in rods that absorbs photons of light and converts it into an action potential cascade. It is made up of cis-retinal and is important for low-light conditions. When light hits rhodopsin, it begins to decompose, cis-retinal is converted to trans-retinal, and the membrane conductance for Na+ in the outer segment of the rod decreases. The electrical signals are transmitted to the brain. The fovea contains a high density of cones and is located in the macula of the retina. Here, the retinal layers are spread apart to allow light to get to the photoreceptors with minimal interference. This allows for improved resolution and higher acquity for central vision. There is a higher ratio of rods to cones around the fovea that are more responsible for peripheral vision. Since retinitis pigmentosa involves the degeneration of rods and cones, the disease can affect both peripheral, central vision, night blindness and a variety of vision disruptions in low-dimness and bright settings.
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