Alkaptonuria (AKU) was the first condition noted as following Mendelian Inheritance by Sir Archibald Garrod in 1902. Since then, more work has been done to understand its genetic mechanism. AKU is now understood to be a recessive disorder, caused by a single gene defect. It is mapped to Chromosome 3 between regions 3q21-q23, the site of the homogentisate 1,2-dioxygenase (HGD) gene.
More than 80 mutations in the HGD gene have been identified in people with the disease. Many of these mutations lead to changes in single amino acids in the homogentisate oxidase protein. A substitution of the amino acid valine for methionine at position 368 is the most common HGD mutation in European populations. Mutations in the HGD gene probably deactivate the enzyme by changing its structure.
HGD is a vital enzyme in tyrosine metabolism. The graphic below shows a general overview of the pathway. With a malfunctioning or inactive HGD enzyme, alkaptonuria patients are unable to convert homogentisic acid (HGA) into maleylacetoacetic acid. Instead of the natural condition, in which excess tyrosine is eliminated from the body, AKU patients end up converting excess tyrosine into HGA.
In the body of an AKU patient, HGA accumulates at more than 2,000 times the normal rate. Some of this is excreted in the urine, but a significant quantity remains. HGA turns black on oxidation, leading to black urine and occasional black sweat which can be indicative of alkaptonuria. A small proportion of patients, however, will not develop these symptoms.
Much of the research into AKU has focused on the urine. It was discovered that the darkening was delayed in an acidic solution and accelerated in alkaline conditions. This affinity for alkali led Boedeker, in 1859, to describe the urine as containing an alkapton - later leading to the name alkaptonuria.
Through a simple test with Benedict's sugar reagent, it was noted that the urine was a powerful reducing agent. This not only reduces the copper reagent to an orange precipitate, but also darkens the solution due to its alkalinity. The net effect is orange particles suspended in a muddy-brown solution.
Alkaptonuria has several characteristic symptoms: urine that darkens on standing, ochronosis in certain tissues and degenerative arthropathy resulting from ochronosis in joint tissues. Arthropathy is the most severe symptom of AKU. The joints most affected are those of the spine, hips and knees. This variability in arthropathy may be related to load-bearing. In addition, it is notable that pigmentation of the pinna of the ear can vary from left to right. This has led to some speculation that mechanical loading may be an important factor in pigment deposition.
The ochronosis of tissues results from the deposition of oxidised and polymerised HGA, in the form of benzoquinones, in the extracellular matrices of connective tissues.
Although there have been many publications on AKU, there is little understanding of the mechanism of ochronosis. Mammalian cartilages contain polyphenyl oxidases, which can catalyse the oxidation of HGA into pigment. Benzoquinone acetic acid has been identified in vitro as an intermediate in the oxidation of HGA. However, intracellular granules are also present in chondrocytes of patients with ochronosis. It is still not known whether the pigment deposition and binding to extra-cellular matrix (ECM) components occurs primarily at the intracellular or extracellular level.
There is still no approved, effective treatment for alkaptonuria. Dietary modifications to restrict the intake of tyrosine and phenylalanine have met with limited success because of the difficulty in maintaining such restrictive diets over time. There is some indication that ascorbic acid (vitamin C) may have a protective effect but results of research in this area have been variable.
Recently, nitisinone has been evaluated. It is an inhibitor of the enzyme responsible for converting hydroxyphenylpyruvate to HGA. A study of nitisinone conducted by the US National Institutes of Health (NIH) was inconclusive, but the NIH published its results so that other researchers could benefit from insights generated by this preliminary work.
Further research is being done in Phase III clinical trial of nitisinone. The series of clinical trials, called DevelopAKUre, will assess the right dose, the right age at which to administer the drug and the safety and efficacy of treating AKU patients with nitisinone. If you have a patient who may be interested in the trials or you wish to find out more, contact Ciarn Scott, by emailing [email protected]
The National AKU Centre in Liverpool gives English and Scottish patients access to the drug nitisinone. It also provides patients with a full health assessment and advice from specialists in AKU. To find out more, contact Lesley Harrison, by emailing [email protected] .
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