Gyrate atrophy of the choroid and retina

Description

Gyrate atrophy of the choroid and retina (often simply called gyrate atrophy) is an inherited disorder that is characterized by vision loss that worsens over time (progressive vision loss). People with this disorder experience a gradual deterioration of cells (atrophy) in specific regions of the eyes. The specialized light-sensitive tissue that lines the back of the eye (retina) and a nearby tissue layer called the choroid are particularly affected. Gyrate atrophy typically occurs in both eyes.

During childhood, most people with gyrate atrophy begin experiencing nearsightedness (myopia), difficulty seeing in low light (night blindness), and a loss of vision at the edges of the visual field (peripheral vision). Over time, the field of vision continues to narrow, resulting in tunnel vision. Eventually, vision in the center of the visual field is impaired. Many people with gyrate atrophy also develop clouding of the lenses of the eyes (cataracts). These progressive vision changes can lead to blindness in adulthood. Medical management can slow the rate of vision loss in some people with gyrate atrophy.

Although gyrate atrophy primarily affects the eyes, some people have additional features. Occasionally, newborns with gyrate atrophy develop excess ammonia in the blood (hyperammonemia),  which may lead to poor feeding, vomiting, seizures, or coma. Hyperammonemia in infants with gyrate atrophy generally responds quickly to treatment and does not recur.

Gyrate atrophy usually does not affect intellectual abilities, but some people with this condition may have certain changes in the brain that can be seen with medical imaging (such as magnetic resonance imaging). Some people with gyrate atrophy may experience seizures. Additional signs and symptoms of gyrate atrophy can include muscle weakness and numbness, tingling, or pain in the hands or feet (peripheral neuropathy).

Frequency

More than 150 individuals with gyrate atrophy have been reported in the scientific literature. The condition appears to be most prevalent in Finland, where as many as 1 in 50,000 people may be affected.

Causes

Genetic changes that cause a disease are called pathogenic variants. Pathogenic variants in the OAT gene can cause gyrate atrophy. The OAT gene provides instructions for making an enzyme called ornithine aminotransferase (OAT). This enzyme helps break down a molecule called ornithine. Ornithine is involved in the urea cycle, which gets rid of the excess nitrogen (in the form of ammonia) that is made when proteins are broken down to provide energy.

In addition to its role in the urea cycle, ornithine helps ensure the proper balance of protein building blocks (amino acids) in the body. This balance is important because a specific sequence of amino acids is required to build each of the many different proteins needed for the body's functions. OAT allows ornithine to be converted into another molecule called pyrroline-5-carboxylate (P5C). P5C can be converted into two amino acids: proline and glutamate. Glutamate serves as an important chemical messenger (neurotransmitter).

The pathogenic variants that are associated with gyrate atrophy reduce the amount of functional OAT protein, which impairs the conversion of ornithine into P5C. As a result, excess ornithine can build up in the blood, in the fluid that surrounds the brain and spinal cord (cerebrospinal fluid), and in the clear fluid in the front of the eye (aqueous humor).

Although it is not clear exactly how these changes result in the specific signs and symptoms of gyrate atrophy, researchers have suggested that a buildup of ornithine may be toxic for cells in the retina. It has also been proposed that excess ornithine may reduce the production of a molecule called creatine. Creatine is needed for many tissues in the body to properly store and use energy. It helps provide energy for muscle contraction and is also important for nervous system functioning, a feature that may explain the peripheral neuropathy and muscle weakness seen in some people with gyrate atrophy.

Inheritance

Gyrate atrophy is inherited in an autosomal recessive pattern, which means both copies of the gene in each cell must have a pathogenic variant to cause the disorder. The parents of an individual with an autosomal recessive condition each carry one copy of the altered gene, but they typically do not show signs and symptoms of the condition.

Autosomal recessive inheritance. Credit: U.S. National Library of Medicine.

Other Names for This Condition

• Gyrate atrophy
• HOGA
• Hyperornithinemia
• Hyperornithinemia-gyrate atrophy of choroid and retina
• Ornithine aminotransferase deficiency

Where this page came from

This page was imported from MedlinePlus Genetics, National Library of Medicine. Courtesy of MedlinePlus from the National Library of Medicine; its genetics summaries and the illustrations credited to the library are in the public domain. Pictures credited to others are not copied.

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