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Choroideremia
Choroideremia (alternatively spelled choroideraemia) is a rare and inherited retinal degeneration (IRD) which causes progressive vision loss, due to degeneration of specialised light-sensing photoreceptor cells in the retina.
Vision loss due to Choroideremia becomes more severe as time progresses, and can eventually lead to blindness.
What are the symptoms of Choroideremia?
Choroideremia causes damage to the network of blood vessels behind the retina that are known as the choroid. The choroid supplies oxygen and nutrients to support and nourish the retinal pigment epithelial (RPE) cells and the photoreceptor (rod and cone) cells. One of the earliest symptoms of the condition is difficulty seeing at night time, caused by the death of rod cells when there is a lack of nourishment from the choroid. Over time, the visual field narrows and progresses to tunnel vision and blindness commonly occurs in late adulthood.
There are a number of diagnostic tests available for Choroideremia, such as electroretinograms and optical coherence tomography (OCT). Electroretinograms assess the electrical responses of the retina to light, by testing the rod and cone photoreceptor cells. Although both cell-types are affected, rod cells are mainly affected first and recognising this helps to diagnose Choroideremia. OCT imaging uses light to assess the integrity of the various layers of the retina at high resolution. However, because some of the symptoms associated with Choroideremia are also common in other retinal conditions, for example Retinitis Pigmentosa (RP), understanding your family’s eye history and getting a genetic diagnosis are extremely important to ensure the correct diagnosis.
What Causes Choroideremia?
Choroideremia is caused by a mutation in the CHM gene, which is located on the X chromosome. The CHM gene makes an essential protein called REP-1, which is involved in escorting essential nutrients between cells in the back of the eye. Mutations in this gene lead to impaired cellular activities, causing cell death. Choroideremia is therefore passed down through generations via an X-linked pattern of inheritance. Males are primarily affected as they have only one X chromosome (X Y), while females have two X chromosomes (X X), inheriting one from each parent. Affected males cannot pass on the disease to their sons, because they pass on their Y chromosome. Men with choroideremia must pass on the disease gene to all of their daughters, who then become carriers of the gene.
Females who are carriers (one normal CHM gene copy and one faulty CHM gene copy) have 50% of the retinal cells working on the altered CHM gene and the other 50% working on the normal copy of the CHM gene. These women will have very subtle, if any, symptoms of the disease. However, in some cases, X chromosome inactivation in some women may be skewed in favour of either the normal or the altered CHM gene copy. If more than 50% of the normal CHM copy is inactivated, the carrier female will have more symptoms. In rare, extreme cases of skewed inactivation the carrier female might be almost as severely affected as a male.
Learn more about inheritance patterns in our Genetics and Inheritance section.
What treatments are available?
Currently, there are no proven therapies available for treating Choroideremia. However, great progress has been made in Choroideremia research and a number of clinical trials are assessing the safety and effectiveness of certain treatment options and there is great hope that a treatment will emerge in the future. You can read more about ongoing clinical trials on the ClinicalTrials.gov website.
Maintaining remaining vision and preventing further vision loss is essential while no treatments are available. There are a variety of low vision aids, including telescopic and magnifying lenses as well as a wide range of assistive technologies for people with visual impairments.
General eye check-ups are also very important for people living with Choroideremia, as these individuals may still be at risk of developing other kinds of eye problems that affect the general population, some of which are treatable.
References
1. National Institute of Health. Available at https://ghr.nlm.nih.gov/condition/choroideremia#statistics. Accessed April 2020.