What else is it called?
- Hyperornithinemia with gyrate atrophy of choroid and retina
- OAT deficiency
- OKT deficiency
- Ornithine aminotransferase deficiency
- Ornithine-delta-aminotransferase deficiency
- Ornithine keto acid aminotransferase deficiency
- Ornithinemia with gyrate atrophy
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What causes it?
Gyrate atrophy is believed to be caused by faults in the OAT gene. This gene provides the instructions to produce an enzyme called ornithine aminotransferase, which helps to break down a molecule called ornithine. Ornithine is a molecule which has a multitude of roles in the body. It is primarily involved in the urea cycle [how urea is broken down to urine] and helps to remove ammonia which is produced when our bodies break down dietary protein.. It is believed that ornithine also contributes to maintaining a balance of amino acids. These are important as they arethe building blocks which make up proteins. ] Ornithine can convert into another molecule called pyrroline-5-carboxylate [P5C]. This molecule can then be converted into another two amino acids called glutamate and proline.
If you have this condition, your body cannot produce ornithine aminotransferase at all, or you may only be able to produce it in small quantities. This can thereby result in an increased amount of ornithine in the blood. This can also halt production of P5C due to a lack of functioning ornithine aminotransferase. It is also believed that the gene defect plays a role in reducing the amount of creatine that is produced. Creatine is a molecule which is stored as energy in muscles and is utilised by the muscles and the nervous system when muscles are contracted. Currently, it is not fully understood how this condition affects the way our body uses creatine.
How common is it?
To date there have been less than 150 individuals diagnosed with this condition worldwide. Gyrate atrophy has been identified as being more prominent in areas of Finland. It has also been reported in other countries such as Germany, Italy and Israel. Due to the low number of people diagnosed with this condition it is not yet clear if this condition is more common amongst males or females or within specific ethnicities.
What are the signs and symptoms?
Gyrate atrophy is commonly associated with progressive vision loss, this is due to deterioration of the retina and choroid in the eye. Other signs and symptoms of this condition may include:
- Near sightedness
- Difficulty seeing in a low light
- Loss of peripheral vision [out of the side of the eyes] resulting in tunnel vision
- Increased pigmentation of the fundus [the interior surface of the eye]
- Night blindness
As this condition affects the urea cycle, newborn babies have a high level of ammonia in the blood (neonatal hyperammonaemia). Most often hyperammonaemia and vision loss are the only symptoms but in rare cases learning disabilities, neurological problems and/or muscle weakess in the upper arms and legs may also occur.
Symptoms are most likely to occur by the age of 50 i however diagnosis is generally quite varied and can be between the ages of 1- 44 years. Symptoms will become more obvious with age, as this is a progressive condition. Night blindness [nyctalopia] occurs within the first 25 years of their life.
How is it diagnosed?
This condition is diagnosed through clinical examinations and a detailed history of symptoms. Tests will include a range of specialist eye examinations and blood tests to measure the level of ornithine in the blood. Genetic testing can confirm diagnosis.
Early detection of this condition is important as there is an opportunity to slow the progression of the symptoms and the rate of irreversible vision loss.
Can it be treated?
Treatment for this condition is usually aimed at managing symptoms as they occur. This condition will be managed by a multidisciplinary team of specialists who will work together to develop the best treatment plan for you. Diets which restrict arginine [an amino acid] have shown promise, however this has been stated as a difficult diet to maintain as there is a lot of foods that contain this amino acid. A specialist dietitian will advise you on any dietary changes needed and help you to manage your diet carefully..
Treatment with vitamin B6 supplementation has showed promise in reducing the amount of ornithine in the blood. There is no evidence to suggest that this has any long-term promise is this has only shown to reduce levels of ornithine. However, this method of treatment is easier to manage then removing arginine from the diet.
Creatine supplementation may also be useful to assist with muscle weakness. t
Cataract surgery may also be required with individuals who develop cataracts.
There is no current research into this condition for further treatment, gene replacement therapy is a promising method which may be trialled in the future, this would aim to replace the enzyme which is mutated with this condition so that levels of orthenine are maintained at the correct threshold value.
Do my family need to be tested?
Gyrate atrophy can only be passed on to a child if both parents have a copy of the faulty gene. This is called autosomal recessive inheritance. A person who has a copy of the faulty gene is known as a carrier.
If both parents are carriers, their child has a one in four (25%) chance of inheriting the disorder, and a one in two chance (50%) of being a carrier. This is the same for each child the parents have.
If the child only inherits one copy of the faulty gene, they will be a carrier but will not have the disorder. In some rare cases, carriers have had mild symptoms of the disorder for which they carry the faulty gene.
Once you are diagnosed, you can speak to a genetic counsellor. They can explain how you may have inherited Gyrate atrophy. They can also tell you about genetic testing for the rest of your family. They can provide advice and support if you go on to have children of your own.
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