Choroideremia

Category:
Genetic Disorder

Prevalence:
1 out of 50,000

Resources:
NORD

Genetics Home Reference

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Choroideremia is a genetic disorder of sight that usually affects males. Female carriers may have mild symptoms without loss of vision. Major symptoms are difficulty seeing in the dark leading to progressive loss of peripheral vision, followed by tunnel vision. The rate and degree of vision loss differs among individuals. Night blindness, usually the first noticeable symptom, generally occurs during childhood.

Choroideremia is characterized by extensive loss of all retinal layers in the eyes. This disorder usually begins during childhood with wasting (atrophy) of the pigmented retinal epithelium, retina, and choroid. The retina is the light sensitive, most internal layer, consisting of many nerve containing layers. A layer of single pigmented cells is the layer outside of the retina. The choroid is the next layer located between the retina and the “white” section of the eye (the sclera); this layer contains small blood vessel). Degeneration of the blood vessels of the choroid is followed by damage to the retina, which usually leads to loss of peripheral vision that can progress to eventual blindness. Central vision is usually preserved until late in life. The symptoms of choroideremia may vary greatly between affected individuals. Female carriers usually have very mild symptoms with night blindness or sensitivity to glare occurring late in life.

Choroideremia can be caused by many different mutations in the CHM gene. The CHM gene encodes REP1 (RAB escort protein 1), a protein that takes part in targetomg vesicles (small sacs of substances) in and out of cells.

Choroideremia is an X linked recessive genetic condition. These disorders are caused by an abnormal gene on the X chromosome and manifest mostly in males. Females who have an altered gene on one of their X chromosomes are carriers for that disorder. Carrier females usually do not display symptoms, because females have two X chromosomes and only one carries the altered gene. Males have one X chromosome that is inherited from their mothers; if a male inherits an X chromosome that contains the altered gene, he will develop the disease.

Female carriers of an X linked disorder have a 25% chance with each pregnancy to have a carrier daughter like themselves, a 25% chance to have a non carrier daughter, a 25% chance to have a son affected with the disease and a 25% chance to have an unaffected son.

If a male with an X linked disorder has a child, he will pass the altered gene to all of his daughters, who will be carriers. A male cannot pass an X linked gene to his sons because males always pass their Y chromosomes instead of their X chromosome to male offspring.

There is currently no cure for Choroideremia, but a Gene Therapy treatment is now in Human Clinical Trials in the United States, the UK, and in Canada. Individuals wishing to participate in these Clinical Trials will need to have their diagnosis of CHM verified with a Genetic Test. The early results from these Clinical Trials have been very positive, and may soon offer individuals a first ever treatment for CHM.

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