Choroideremia (CHM) is a rare inherited disorder that causes progressive loss of vision due to degeneration of the choroid and retina which is caused by a lack of RAB Escort Protein-1 (REP-1). Choroideremia occurs almost exclusively in males. The first symptom, most commonly noticed in childhood, is night blindness. 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. Loss of acuity, depth perception, color perception and an increase in the severity of night blindness may also occur during this progression. 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 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, and as a result, vision is lost.
Choroideremia is genetically passed through families by an X-linked pattern of inheritance. In this type of inheritance, the gene for Choroideremia is located on the X chromosome. Females have two X chromosomes, but generally, only one of the X chromosomes will carry the defective gene. Because females have a healthy version of the gene on their other X chromosome that produces REP-1, they normally do not suffer the full effects of CHM. In many cases, female carriers suffer mild symptoms of CHM later in life, such as night blindness and glare sensitivity. In rare cases, female carriers even suffer the full effects of CHM.
Males have only one X chromosome (paired with one Y chromosome) and are therefore genetically susceptible to inherit X-linked diseases. Males cannot be carriers of X-linked diseases, but they will pass the gene on their X chromosome to their daughters, who then become carriers. Affected males never pass an X-linked disease gene to their sons because fathers pass only the Y chromosome to their sons. Female carriers have a 50 percent chance (or 1 chance in 2) of passing the X-linked disease gene to their daughters, who as a result will become carriers and will have a 50 percent chance of passing the gene to their sons, who would then acquire the disease.
Basic description of the REP-1 protein – Dr. Ian MacDonald
The choroideremia gene product is called REP-1 (for Rab escort protein-1). This protein functions to bring other small proteins (thinking of them as signals) into association with an enzyme that adds 20-carbon long chains to the small signals. These signal proteins can then fit into the lipid membrane that surrounds the cell. The small signals are thought to play a role in allowing nutrients to pass across cells. Imagine that this process occurs constantly at the back of the eye as special nutrients are required to keep the biochemical pathways of vision operating at capacity while our eyes are open. Apparently there is another protein called REP-2 in all cells that allows normal cell function if REP-1 is not present. Scientists suggest that the amount of REP-2 may not be sufficient to allow the normal cell processes to occur in some cells such those of the eye. Unfortunately, the male patient with choroideremia makes a defective REP-1 protein that is rapidly lost from the eye and REP-2 is not able to replace its function. You recall that the gene is on the X chromosome. Males only have one X and so only one copy of the gene that makes the protein. If the gene copy is changed, there is no other normal copy around to mask the effect of not having the normal protein available. Research is trying hard to find ways of getting the cells in the eye to make the normal protein.
At this time, there is no treatment or cure for this disease. However, what we have is hope. In our short history, we have already seen incredible advances made in CHM research. Now, it is no longer a question of if a cure will be developed; it is a question of when!
The Choroideremia Research Foundation, Inc. is dedicated to helping researchers find that cure or treatment. By donating to the Foundation, or by becoming an active member, you will help us accomplish our goal of restoring sight to CHMers worldwide!
(Thanks to the Foundation Fighting Blindness and Dr. Ian MacDonald for some of this material)