Alkaptonuria is a rare inherited genetic disease which is caused by a mutation in the HGD gene for the enzyme homogentisate 1,2-dioxygenase (EC 1.13.11.5); if a person inherits an abnormal copy from both parents (it is a recessive condition), the body accumulates an intermediate substance called homogentisic acid in the blood and tissues. Homogentisic acid and its oxidized form alkapton are excreted in the urine, giving it an unusually dark color. The accumulating homogentisic acid causes damage to cartilage (ochronosis, leading to osteoarthritis) and heart valves, as well as precipitating as kidney stones and stones in other organs. Symptoms usually develop in people over 30 years old, although the dark discoloration of the urine is present from birth. Apart from treatment of the complications (such as pain relief and joint replacement for the cartilage damage), the drug nitisinone has been found to suppress homogentisic acid production, and research is ongoing as to whether it can improve symptoms. Alkaptonuria is a rare disease; it occurs in one in 250,000 people, but is more common in Slovakia and the Dominican Republic. Signs and symptoms
Patients with alkaptonuria are asymptomatic as children or young adults, but their urine may turn brown or even inky black if collected and left exposed to open air. Pigmentation may be noted in the cartilage of the ear and other cartilage, and the sclera and corneal limbus of the eye.After the age of 30, people begin to develop pain in the weight-bearing joints of the spine, hips, and knees.
This process is called ochronosis (as the tissue looks ochre); ochronotic tissue is stiffened and unusually brittle, impairing its normal function and causing damage. Diagnosis
If the diagnosis of alkaptonuria is suspected, it can be confirmed or excluded by collecting urine for 24 hours and determining the amount of homogentisic acid by means of chromatography. No assay of HGA in blood has been validated. The Genetic Testing Registry  is used for maintaining information about the genetic test for alkaptonuria.The severity of the symptoms and response to treatment can be quantified through a validated questionnaire titled the AKU Severity Score Index. This  assigns scores to the presence of particular symptoms and features, such as the presence of eye and skin pigmentation, joint pain, heart problems, and organ stones. Treatment
No treatment modality has been unequivocally demonstrated to reduce the complications of alkaptonuria. Main treatment attempts have focused on preventing ochronosis through the reduction of accumulating homogentisic acid. Such commonly recommended treatments include large doses of ascorbic acid (vitamin C) or dietary restriction of amino acids phenylalanine and tyrosine. However, vitamin C treatment does not have definitively proven effectiveness and protein restriction (which can be difficult to adhere to) has not shown to be effective in clinical studies.Several studies have suggested that the herbicide nitisinone may be effective in the treatment of alkaptonuria. Nitisinone inhibits the enzyme 4-hydroxyphenylpyruvate dioxygenase, responsible for converting tyrosine to homogentisic acid, thereby blocking the production and accumulation of HGA.