Stargardt's Disease: A Genetic Eye Disorder
As an adult, one can expect to be subjected to vision loss. However, one does not expect as a child to go from having normal vision to serious visual impairment; this is what Stargardt’s Macular Degeneration does. Stargardt’s is a rapid progression of vision loss typically occurring in adolescence and early adulthood. The affected area of this disease is the retina and more specifically, the macula, which is a small area in the back of the retina. It is here where cell death occurs and how Stargardt develops. People with Stargardt’s can have 20/200 vision or worse (normal vision is 20/20). Though there are different types of juvenile macular degeneration, Stargardt’s is the most common with a prevalence of about 1 in every 10,000.
To understand Stargardt’s, one needs a brief understanding of the structure of an eye. The retina is a fine layer of tissue that is located at the back of the eye near the optic nerve. Towards the front of your eye is the lens. The retina’s job is to receive the light that was focused by the lens and convert it into neural signals. These neural signals are able to be translated by the brain for visual recognition. The retina is able to complete this process because it contains a layer of photoreceptor cells, where being photoreceptive means that the cells are light sensitive. These specialized cells also have special names: rods and cones. Lastly, the macula is at the center of the retina, where its main function is to process sharp and straight-ahead vision. Though rods and cones are both photoreceptive, they have different responsibilities. Rods lie just outside the macula and are sensitive to processing dark and low levels of light. Cones, however, are found in the macula and process fine visual detail. Unfortunately, during the onset of Stargardt’s, cones are more affected than the rods, which can explain why fine visual detail diminishes so quickly, though it is not known why rods are less affected.
Now knowing a brief structure of the eye, how cell death occurs in the retina can be discussed. When one is diagnosed with Stargardt macular degeneration, it is most commonly due to a mutation in the ABCA4 gene. This gene has the code to make specific protein transporters. These proteins have the responsibility of transporting out any and all toxic byproducts from the photoreceptor cells. These byproducts can result from a chemical reaction inside the cell. For example, a chemical reaction takes place to convert Vitamin A into light sensitive molecules that will be used in the rods and cones. As a result of this reaction, byproducts are produced that need to be cleaned out. The mutation in the ABCA4 gene prevents the protein transports from being made, causing a buildup of toxic products and lipofuscin, a fatty deposit, within the photoreceptors. As lipofuscin build up increases around the macula, central vision becomes seriously impaired, and eventually cell death of the photoreceptors takes place.
Though Stargardt is the most common macular degeneration, macular degeneration itself is a rare disease. So how can one develop it? The answer is genetics. As discussed, most of Stargardt’s is caused by an autosomal recessive mutation in the ABCA4 gene. To be able to develop this disease from this genetic mutation, one needs to have two parents that are both carriers of the recessive allele. If these two carriers have a child, there is a: 25% chance of a child with the disease, 25% chance of having a child who neither carries the gene or has the disease, and 50% chance of having a child who is a carrier of the gene but does not have the disease. Looking at these numbers, it is no wonder why macular degeneration is pretty rare. However, just because it is rare does not mean that it can not happen. It is devastating news to be diagnosed with Stargardt’s especially at a young age. To know that one will have serious vision impairment by age 20 can be emotionally draining. But, there is light at the end of the tunnel. Recently, there has been new research to find a cure. Some promising ones include gene therapy, which can potentially repair the mutation in the ABCA4 gene, and stem cell research, where new photoreceptor cells can be developed to replace the dead ones. Moreover, in a highly technological society, there are a vast number of applications and devices that one can use to assist with everyday visual tasks. Scientists are optimistic that one day they can reverse this vision loss so the world can become clear again.
References:
https://www.nei.nih.gov/learn-about-eye-health/eye-conditions-and-diseases/stargardt-disease
https://www.brightfocus.org/macular/infographic/normal-macula
https://medlineplus.gov/genetics/condition/stargardt-macular-degeneration/#causes
