Alkaptonuria

Definition

Definition of Alkaptonuria
Alkaptonuria is a rare inherited genetic disorder of phenylalanine and tyrosine metabolism. This is an autosomal recessive condition that is due to a defect in the enzyme homogentisate 1,2-dioxygenase (EC 1.13.11.5), which participates in the degradation of tyrosine. As a result, a toxic tyrosine byproduct called homogentisic acid (or alkapton) accumulates in the blood and is excreted in urine in large amounts (hence -uria). Excessive homogentisic acid causes damage to cartilage (ochronosis, leading to osteoarthritis) and heart valves as well as precipitating as kidney stones. Treatment with nitisinone, which suppresses homogentisic acid production, is being studied. Alkaptonuria is more common in Slovakia and the Dominican Republic than in other countries.

Symptoms

Symptoms of Alkaptonuria
Alkaptonuria is often asymptomatic, but the sclera of the eyes may be pigmented (often only at a later age), and the skin may be darkened in sun-exposed areas and around sweat glands; sweat may be coloured brown. Urine may turn brown if collected and left exposed to open air, especially when left standing for a period of time. Kidney stones and stone formation in the prostate (in men) are common and may occur in more than a quarter of cases.

The main symptoms of alkaptonuria are due to the accumulation of homogentisic acid in tissues. In the joints this leads to cartilage damage, specifically in the spine, leading to low back pain at a young age in most cases. Cartilage damage may also occur in the hip and shoulder. Joint replacement surgery (hip and shoulder) is often necessary at a relatively young age.

Causes

Causes of Alkaptonuria
A defect in the HGD gene causes alkaptonuria.

The gene defect makes the body unable to properly break down certain amino acids (tyrosine and phenylalanine). As a result, a substance called homogentisic acid builds up in the skin and other body tissues. The acid leaves the body through the urine. The urine turns brownish-black when it mixes with air.

Alkaptonuria is inherited, which means it is passed down from parents to their children. To get this disease, each of your parents must pass you a copy of the faulty HGD gene.

Diagnosis

Diagnosis of Alkaptonuria
The diagnosis of alkaptonuria needs to be suspected before diagnostic testing can be performed using paper chromatography and thin layer chromatography. Both blood plasma and urine can be used for diagnosis. In healthy subjects, homogentisic acid is absent in both blood plasma and urine. In alkaptonuria, plasma levels are 6.6 micrograms/ml on average, and urine levels are on average 3.12 mmol/mmol of creatinine.

Prognosis

Prognosis of Alkaptonuria
The outcome is expected to be good.

Treatment

Treatment of Alkaptonuria
No treatment modality has been unequivocally demonstrated to reduce the complications of alkaptonuria. Commonly recommended treatments include large doses of ascorbic acid (vitamin C)and dietary restriction of phenylalanine and tyrosine. Dietary restriction may be effective in children, but benefits in adults have not been demonstrated.

The insecticide nitisinone inhibits 4-hydroxyphenylpyruvate dioxygenase, the enzyme that generates homogentisic acid from 4-hydroxyphenylpyruvic acid. This reduces homogentisic acid. The main side-effect is irritation of the cornea, and there is a concern that it will cause the symptoms of hereditary tyrosinaemia type III because of the possible accumulation of tyrosine or other intermediaries. Further studies are being conducted.