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Choroideremia (CHM) is a rare inherited disorder that causes progressive loss of vision due to degeneration of the choroid and retina. It occurs almost exclusively in males. In childhood, night blindness is the most common first symptom. As the disease progresses, there is loss of vision, frequently starting as an irregular ring that gradually expands both in toward central vision and out toward the extreme periphery.
Progression of the disease continues throughout the individual's life. Both the rate of change and the degree of visual loss are variable among those affected, even within the same family.
The actual vision loss is caused by degeneration of several layers of cells that are essential to sight. These layers, which line the inside of the back of the eye, are called the choroid, the retinal pigment epithelium (RPE), and the retina. The choroid is a network of blood vessels located between the retina and the sclera, the "white of the eye." Choroidal vessels provide oxygen and nutrients to both the RPE and the retina's photoreceptor cells.
The RPE, directly beneath the retina, supports the function of photoreceptor cells. Photoreceptors convert light into the electrical impulses that transfer messages to the brain where "seeing" actually occurs. In the early stages of Choroideremia, the choroid and the retinal pigment epithelium initially deteriorate. Eventually, photoreceptor cells also degenerate. As a result, vision is lost.
Choroideremia is caused by the deletion of the Rab escort protein 1 (REP1). Rab escort protein 2 (REP2) is 75% identical and can almost compensate for the loss of REP1. Though the eye does express the REP2 protein (no cell could survive without some REP activity) evidently, in the eye, this is not enough. The REPs are essential for the prenylation of Rab proteins. Studies have shown that there is a build up of unprenylated Rab27 in lymphoblasts from Choroideremia patients. The link between the build up of unprenylated Rabs and blindness is not known.
Generally, only men show symptoms of this disease. Initially a person suffering from choroideremia has night blindness, which begins in youth. As the disease progresses, a CHM sufferer loses their peripheral vision and depth perception, eventually losing all sight by middle age. In some cases, a severe loss of acuity and color perception become evident as the disease progresses.
The link between the loss of REP1 and the build up of unprenylated Rab27 and the degeneration of the eye is unknown as yet.
There is a genetic blood test to diagnose Choroideremia. It was created by Dr. Ian MacDonald at the University of Alberta. Free genetic testing is available for US and Canadian Residents through the eyeGENE project which is coordinated by the National Eye Institute at the US National Institutes of Health. Preclinical trial work is underway at the Imperial College of London under the direction of Dr. Miguel Seabra and at Moorfields Eye Hospital in London under the direction of Dr. Robert MacLaren. In the United States preclinical trial work is underway at the University of Pennsylvania under the direction of Dr. Jean Bennett and Dr. Albert Maguire. Dr. Ian MacDonald is also pursuing clinical trials at the University of Alberta in Canada. The Choroideremia Research Foundation, an international non-profit organization that for over ten years has been dedicated to raising awareness and securing funding for choroideremia research, is currently funding pre-clinical trial work for Dr. Seabra and Dr. Bennett. Dr. Ian MacDonald also serves on the board of directors for the CRF and receives funding from CRF-Canada. Human clinical trials could start as early as 2012. CRF-Canada also supports Dr. Seabra's and Dr. MacLaren's work as does Fight for Sight.
In a procedure first attempted at Moorfields Eye Hospital in London, researchers at the John Radcliffe Hospital in Oxford have used gene therapy to attempt to curtail the condition which is caused by a faulty gene labelled REP1 and causes the light sensitive cells in the eye to gradually die. The procedure will halt the cell death by introducing functioning copies of the gene into the eye.
- Foundation Fighting Blindness
- Choroideremia Research Foundation
- Dr. Jean Bennett news on Leber Congenital Amaurosis Gene Therapy