Choroideremia is a rare genetic disorder that causes vision loss and leads to eventual blindness in all cases, usually over the course of several decades. The disease involves a mutation in the CHM gene, which is located on the X chromosome, one of the two sex chromosomes—males generally have two kinds of sex chromosomes (X and Y) while females have two of the same kind (XX). The disorder mainly affects men: those with XY chromosomes only have one copy of the gene located on the single X chromosome. As a result, a mutation of the single CHM gene will lead to the disease. Females with XX chromosomes, on the other hand, have an additional copy of CHM on the additional X chromosome: in this case, both copies of the gene must be mutated for the condition to manifest, which is significantly less likely. It is estimated that the disease affects 1 in 50,000 to 1 in 100,000 individuals (again, mostly men), though it is possible that it is under-diagnosed as a result of its similarity to other disorders.
Before its identification as a distinct disease, choroideremia was believed to be a form of retinitis pigmentosa (RP). With choroideremia, the gradual loss of photoreceptors, the retinal cells responsible for converting light into visual signals for our brains, is the main driver of vision loss. This is the case with RP as well. In both diseases, childhood onset is typical and the experience of gradual vision loss is quite similar: rod photoreceptors die off first, leading to an initial loss of peripheral and night vision. This is followed by the degeneration of cone photoreceptors, resulting in the loss of the central and high-acuity vision they are responsible for. The overall experience is often likened to a progressively worsening form of tunnel vision.
The underlying genetic and molecular causes of photoreceptor death are unique in choroideremia, however. The CHM gene that is mutated in choroideremia is responsible for producing a protein called REP-1, which serves the important function of “escorting” other proteins to their appropriate places within cells so that they can do their work. With an absence of REP-1, vital cell maintenance and production is inhibited in the retina, which is what triggers the death of photoreceptors. Other areas of the eye are damaged due to a lack of REP-1 as well: the choroid, a network of blood vessels that supply nutrients and oxygen to the retina, as well as the retinal pigment epithelium (RPE), a cell layer below the retina that nourishes the photoreceptors.
Content on this page was written by Dr. Chad Andrews and Dr. Mary Sunderland, and was edited and approved by Dr. Ian MacDonald. The information was most recently updated on August 23, 2018
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