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. As the kidneys filter blood to create urine, cystine is normally absorbed back into the bloodstream. People with cystinuria cannot properly reabsorb cystine into their bloodstream and the amino acid accumulates in their urine.

As urine becomes more concentrated in the kidneys, the excess cystine 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 molecules in the kidneys to form larger stones. These crystals and stones can create blockages in the urinary tract and reduce the ability of the kidneys to eliminate waste 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 making the two parts (subunits) of a transporter protein complex that is made primarily in the kidneys. Normally this protein complex controls the reabsorption of certain amino acids, including cystine, into the blood from the filtered fluid that will become urine. Mutations in either the SLC3A1 gene or SLC7A9 gene disrupt the ability of the transporter protein complex to reabsorb amino acids, which causes them to become concentrated in the urine. As the levels of cystine in the urine increase, the crystals typical of cystinuria form. The other amino acids that are reabsorbed by the transporter protein complex do not create crystals when they accumulate in the 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.