This rare genetic disorder is characterized by the patient’s urine becoming a dark brownish-black color when the urine is exposed to air. A genetic defect results in the body not being able to properly break down phenylalanine and tyrosine, two amino acids. This results in homogentisic acid, a substance in the body, building up in body tissues and the skin. This acid exits the body via the urine.
How Common is this Genetic Disorder?
Alkaptonuria affects one in 250,000 to one-million people throughout the world. This condition is more common in the Dominican Republic and it affects about one in 19,000 in certain areas of Slovakia.
Genetics of Alkaptonuria
HGD gene mutations are responsible for causing this disorder. When this gene is mutated, it impairs the homogentisate oxidase enzyme’s ability to break down tyrosine and phenylalanine, resulting in homogentisic acid accumulating in the body. This disorder is inherited in an autosomal recessive pattern.
Symptoms of Alkaptonuria
After several hours, the urine in an infant’s diaper can become an almost black color. However, many people with this disorder are not aware they have it until they are around 40 years old when other problems, such as joint problems begin to occur. Symptoms can include:
- Arthritis, specifically affecting the spine, that worsens with time
- Dark spots on the cornea and white of the eye
- Darkening of the ear
Those suffering from alkaptonuria can develop arthritis in adulthood. Approximately 50 percent of adults with this disorder will develop arthritis due to homogentisic acid building up in the cartilage.
Other possible complications include:
- Possibly needing a valve replacement if too much homogentisic acid builds up on the heart valves. The mitral valve is most often affected.
- Those with this disorder are at a higher risk of developing coronary artery disease earlier in life.
- Prostate stones and kidney stones are more common in those with this disorder.
Diagnosing this Genetic Disorder
Alkaptonuria is diagnosed with a urine test, referred to as urinalysis. When ferric chloride is added to the urine being tested, it becomes black in color in patients with this disorder.
How is Alkaptonuria Treated?
There is no cure for this disorder, but there are effective treatments. Patients can benefit from high doses of vitamin C. High doses of vitamin C have been shown to help reduce how much brown pigment builds up in the cartilage. It may also help to slow down the development of arthritis. This genetic disorder, though it can cause symptoms or result in complications and can be bothersome, does have a good prognosis for most patients.
Medline Plus. (2009). Alkaptonuria. Retrieved on June 24, 2010 from Medline Plus: https://www.nlm.nih.gov/medlineplus/ency/article/001200.htm
Genetics Home Reference. (2007). Alkaptonuria. Retrieved on June 24, 2010 from Genetics Home Reference: https://ghr.nlm.nih.gov/condition=alkaptonuria
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