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 Alkaptonuria is a rare genetic disorder that causes one’s body to produce black sweat and makes urine turn black if it is exposed to air. It is caused by a defect in the homogentisate 1,2-dioxygenase (HGD) gene, which impairs an enzyme that breaks down a substance called homogentisic acid. This acid is a by-product from the digestion of two amino acids, phenylalanine and tyrosine. In alkaptonuria patients, the homogentisic acid is excreted through sweat and urine and turns dark when exposed to air. When the body cannot excrete this acid as fast as it is produced, it is deposited in the joints and connective tissues. This causes cartilage and areas of skin exposed to the sun, such as the cheeks, forehead and armpits, to darken. Over time, a build-up of this substance in the joints leads to arthritis.
This condition affects one in 250,000 people and is more common in Slovakia and the Dominican Republic, said Dr Wilson Wang, who heads the division of hip and knee surgery at the National University Hospital (NUH). NUH has two patients with this condition, including Mr Anthony Adalia. It is an autosomal recessive disease, which means both parents must have the faulty gene, and each parent must pass a copy of it to their child for the child to inherit the condition.
Since the homogentisic acid is not toxic to cells, the disorder fortunately does not reduce one’s life expectancy. However, alkaptonuria patients may be at risk of heart disease if the black pigment is deposited in the aorta and coronary arteries and clogs them up, said Dr James Yip, senior consultant at the National University Heart Centre, Singapore. The black pigment may also be deposited in the aortic valve, causing tightness of the heart valve. This can be picked up as a heart murmur in the early stage, said Dr Yip.
Warning signs of severe tightening of the valves include chest pains, breathlessness and fainting spells. Open-heart surgery may be needed to replace the diseased valve. If the condition is detected early, preventive measures include taking a medication called nitisinonen to reduce the amount of the pigment produced. Few people suffer from the side effects of taking nitisinonen, which include low blood count, liver problems and redness of the eye, said Dr Yip. Nothing can be done to stop the pigment from being deposited in the cartilage. Non-weight-bearing exercises of moderate intensity, such as swimming, may help a patient maintain the range of motion in his joints. Younger alkaptonuria patients should take up non-contact and lower-impact sports.
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