Alkaptonuria (AKU) is a rare genetic disorder that affects the metabolism of a particular amino acid called tyrosine. It is caused by a mutation in the HGD gene, which encodes an enzyme called homogentisate 1,2-dioxygenase. This enzyme is responsible for breaking down homogentisic acid (HGA), a byproduct of tyrosine metabolism. In people with AKU, the HGD gene mutation causes a deficiency of this enzyme, leading to the accumulation of HGA in the body. This can cause a range of symptoms, including joint problems, dark urine, and skin pigmentation changes.


AKU is an autosomal recessive disorder, which means that an affected individual has inherited two copies of the mutated HGD gene, one from each parent. Carriers of the mutation, who have only one copy of the mutated gene, do not usually develop symptoms of the condition.

The HGD gene is located on chromosome 3, and over 100 different mutations have been identified in this gene that can cause AKU. The most common mutation is known as p. Gly101Ser, which accounts for around 50% of cases in Europe and North America. However, the frequency of different mutations varies depending on the geographical location, with different mutations being more common in different populations.


The symptoms of AKU typically begin to appear in early adulthood, although they can sometimes be present from birth. One of the most common symptoms is the darkening of urine, which occurs due to the presence of HGA. This darkening is more pronounced after exposure to air, and can cause staining of clothes and bed linen.

Another common symptom of AKU is joint problems, which can lead to severe pain and stiffness. This occurs because HGA can accumulate in the joints, causing damage to the cartilage and other tissues. The most commonly affected joints are the spine, hips, and knees, although other joints can also be affected.

In addition to joint problems and dark urine, AKU can also cause a range of other symptoms. These can include skin pigmentation changes, particularly on the ears, forehead, and cheeks, and the formation of small lumps under the skin. In some cases, AKU can also affect the heart and kidneys, although this is less common.

Treatment Options

Currently, there is no cure for AKU. However, there are several treatment options available that can help to manage the symptoms of the condition and improve quality of life.

One of the main treatment options for AKU is the use of pain relief medication. This can include nonsteroidal anti-inflammatory drugs (NSAIDs) and opioid-based painkillers. Physical therapy and exercise can also be beneficial for managing joint pain and stiffness.

In addition to pain relief, there are also several medications that can help to reduce the levels of HGA in the body. These include nitisinone, which works by blocking an enzyme called 4-hydroxyphenylpyruvate dioxygenase (HPPD) that is involved in HGA production. Another medication called ascorbic acid (vitamin C) has also been shown to be effective in reducing HGA levels, although it is less potent than nitisinone.

Finally, surgical interventions may be necessary in some cases of AKU. Joint replacement surgery can help to alleviate severe joint pain and restore mobility, while other surgical procedures may be necessary to treat complications such as heart valve disease or kidney stones.


Alkaptonuria is a rare genetic disorder that can cause a range of symptoms, including joint problems, dark urine, and skin pigmentation changes. While there is currently no cure

for AKU, there are several treatment options available that can help to manage the symptoms and improve quality of life. These can include pain relief medication, physical therapy, medications to reduce HGA levels, and surgical interventions.

It is important for individuals with AKU to work closely with their healthcare providers to develop a treatment plan that is tailored to their specific needs. Regular monitoring of symptoms and HGA levels is also important, as this can help to identify any changes in the condition and adjust treatment as necessary.

Genetic counseling is also recommended for individuals with AKU and their families. This can help to identify carriers of the HGD gene mutation and provide information on the risk of passing on the condition to future generations. In some cases, prenatal testing may also be available to identify the presence of AKU in a developing fetus.

While AKU can be a challenging condition to manage, with appropriate treatment and support, individuals with this condition can lead fulfilling and productive lives. Ongoing research into the causes and treatment of AKU is also important, as this can help to improve our understanding of the condition and develop new treatment options that may one day lead to a cure.

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