Alkaptonuria

Alkaptonuria is a rare condition that affects about 1 in 250,000 – 1 million people worldwide.

It is a condition that is inherited and causes urine to turn black when exposed to air. There is also a build-up of a dark pigment known as ochronosis in cartilage, skin and other connective tissues. This pigmentation, which is blue-black, starts to show after about the age of 30.

Alkaptonuria is more common in areas of Slovakia (affecting about 1 in 19,000 people) and in the Dominican Republic.

Those with the condition will usually develop arthritis which begins in early adulthood. This usually affects the spine and large joints. There can be other results of the condition including heart problems, kidney stones and prostate stones.

The Genes Related to Alkaptonuria

The gene that causes alkaptonuria is the HGD gene when it becomes mutated. This gene is what causes the production of an enzyme known as homogentisate oxidase. This is what helps break down certain amino acids that are important parts of proteins. When the HGD gene is mutated the enzyme is impaired, meaning that homogentisic acid begins to accumulate.

Excess homogentisic acid is deposited into connective tissues causing the cartilage and skin to darken. This is also what leads to the other problems associated with alkaptonuria.

This condition in inherited recessively, where both parents would each carry one copy of the gene leading to the child inheriting two copies of the mutated gene. The parents themselves do not usually show any signs or symptoms of the condition.