Cystinuria

Cystinuria is a condition characterized by the buildup of cystine crystals or stones in the kidneys and bladder. Cystine is an amino acid, one of the building blocks of proteins. Normally, cystine is reabsorbed back into the bloodstream after blood entering the kidneys is filtered to create urine. Only small amounts of cystine can stay dissolved in urine. People with cystinuria cannot properly reabsorb cystine into their bloodstream. As urine becomes more concentrated in the kidneys, the cystine cannot stay dissolved and forms crystals. As these crystals become larger, they form stones that may lodge in the kidneys or in the bladder. Sometimes cystine crystals combine with calcium in the kidneys to form large stones. Blockages in the urinary tract by these crystals and stones reduce the ability of the kidneys to eliminate wastes through urine. The stones also provide sites where bacteria may cause infections.

Cystinuria affects approximately 1 in 10,000 people.

Mutations in the SLC3A1 and SLC7A9 genes cause cystinuria.

The SLC3A1 and SLC7A9 genes provide instructions for producing the two parts of a transporter protein that is made primarily in the kidneys. Normally this protein allows certain amino acids, including cystine, to be reabsorbed into the blood from the filtered fluid that will become urine. Mutations in either of these genes disrupt the ability of this transporter protein to reabsorb these amino acids, allowing them to become concentrated in the urine. As the levels of cystine in the urine increase, the crystals typical of cystinuria are able to form. The other amino acids that are not reabsorbed do not create crystals in urine.

Read more about the SLC3A1 and SLC7A9 genes.

This condition is inherited in an autosomal recessive pattern, which means both copies of the gene in each cell have mutations. The parents of an individual with an autosomal recessive condition each carry one copy of the mutated gene, but they typically do not show signs and symptoms of the condition.