pediatrics
Cystinuria
Introduction
Cystinuria is an inherited defect of the renal tubules in which reabsorption of cystine (the homodimer of the amino acidcysteine) is impaired, urinary excretion is increased, and cystine stones form in the urinary tract. Symptoms are colic caused by stones and perhaps urinary infection or the sequela of chronic kidney disease. Diagnosis is by measurement of cystine excretion in the urine. Treatment is with increased fluid intake and alkalinization of the urine. Cystinuria was originally classified according to urinary excretion of cystine and dibasic amino acids in obligate carriers. In this classification, parents of affected children were assessed as having either normal (type I), moderate (type III), or significant (type II) increases in cystine excretion. A newer classification is based on genotype: Type A patients have homozygous mutations in the geneSLC3A1and type B patients have homozygous mutations inSLC7A9. These genes encode proteins that together form a heterodimer responsible for cystine and dibasic amino acid transport in the proximal tubule. Cystinuria should not be confused with cystinosis ( see Hereditary Fanconi syndrome ). 