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Alkaptonuria is an inherited condition that causes urine to turn black when exposed to air. Ochronosis, a buildup of dark pigment in connective tissues such as cartilage and skin, is also characteristic of the disorder. People with alkaptonuria typically develop arthritis in adulthood, particularly in the spine and large joints.
How common is alkaptonuria?
The condition is rare, affecting 1 in 250,000 to 1 million people worldwide. Alkaptonuria is more common in certain areas of Slovakia (where it has an incidence of about 1 in 19,000 people) and in the Dominican Republic.
What genes are related to alkaptonuria?
Mutations in the HGD gene cause alkaptonuria.
The HGD gene makes an enzyme called homogentisate oxidase, which helps break down the amino acids phenylalanine and tyrosine (important building blocks of proteins) into smaller molecules. Mutations in the HGD gene prevent the enzyme from playing its role in this process. As a result, a substance called homogentisic acid builds up in the body. Excess homogentisic acid is deposited in connective tissues, which causes cartilage and skin to darken and leads to arthritis. Homogentisic acid is also in present in urine, which makes the urine turn dark when exposed to air.
How do people inherit alkaptonuria?
This condition is inherited in an autosomal recessive pattern, which means two copies of the gene in each cell are altered. Most often, the parents of an individual with an autosomal recessive disorder each carry one copy of the altered gene but do not show signs and symptoms of the disorder.
