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. We now know that Alkaptonuria is a recessive disorder, caused by a single gene defect, 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 alkaptonuria. Many of these mutations change 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 inactivate the enzyme by changing its structure.
HGD is a vital enzyme in tyrosine metabolism. The graphic below shows a general overview of this pathway. With a malfunctioning or inactive HGD enzyme, AKU patients are unable to convert Homogentisic Acid (HGA) into Maleylacetoacetic acid. Therefore instead of the natural condition of eliminating excess tyrosine from the body; AKU patients end up converting excess tyrosine ultimately to HGA.
Therefore in the body of an AKU patient, HGA accumulates at more than 2000 times the normal rate. Some of this is excreted in the urine, but a significant quantity remains. A characteristic of HGA is that it will turn black upon oxidation. Indeed, this gives the black urine and occasional black sweat indicative of AKU. However a large proportion of patients will not have these symptoms.
Historically, much of the research into AKU focussed on the urine. It was discovered that the darkening was delayed in an acidic solution and rapid in an alkaline solution. This affinity for alkali led Boedeker in 1859 to describe the urine as containing an alcapton – 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. Not only reducing the copper reagent to an orange precipitate, but also darkening the solution due to its alkalinity. The net effect results in orange particles suspended in a muddy-brown solution.
Clinical Presentation of AKU
AKU has several characteristic symptoms; urine that darkens on standing, ochronosis in certain tissues, and, the most severe symptom, degenerative arthropathy resulting from ochronosis in joint tissues. The joints most affected are those of the thigh, hips and knees, whereas the ankles and wrists are usually much less involved.
Ochronotic arthopathies have a large number of presentations including limitations of movement to the shoulder, hip and knee. The intervertebral discs often show degeneration and can cause pain including sciatica, lordosis and kyphosis. Cardiovascular symptoms include effects on the mitral and aortic valves, which can harden and need replacing. Ochronosis can also increase arteriosclerotic plaques.
Overall, patients suffering from Alkaptonuria-induced ochronosis can experience a lot of pain, incapacity and disability The oldest recorded sufferer is 99 years of age and there have been examples of first diagnosis as late as 77 years, following bronchoscopy.
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, and benzoquinoneacetic acid has been identified in the in vitro environment as an intermediate in the oxidation of HGA. However intracellular granules are also present in chondrocytes of patients with ochronosis and it is still not known whether the pigment deposition and binding to ECM components occurs primarily at the intracellular or extracellular level.
The need for a treatment
There is still no effective treatment for Alkaptonuria. Dietary modifications to restrict intake of tyrosine and phenylalanine have met with limited success owing to the difficulty in maintaining such a restrictive diet for life. There is some indication that ascorbic acid (Vitamin C) might have a protective effect but results have been variable.
More recently nitisinone, a potent inhibitor of the enzyme responsible for converting hydroxyphenylpyruvate to HGA, has also been evaluated. A study by the NIH was inconclusive, but the drug still looks hopeful as a future treatment.
