Cystinuria is an inherited disorder characterized by the impaired reabsorption of
cystine in the proximal tubule of the nephron and the gastrointestinal epithelium. The only clinically significant manifestation is recurrent
nephrolithiasis secondary to the poor solubility of
cystine in urine. Although
cystinuria is a relatively common disorder, it accounts for no more than 1% of all
urinary tract stones. Thus far, mutations in 2 genes, SLC3A1 and SLC7A9, have been identified as being responsible for most cases of
cystinuria by encoding defective subunits of the
cystine transporter. With the discovery of mutated genes, the classification of patients with
cystinuria has been changed from one based on phenotypes (I, II, III) to one based on the affected genes (I and non-type I; or A and B). Most often this classification can be used without gene sequencing by determining whether the affected individual's parents have abnormal urinary
cystine excretion. Clinically, insoluble
cystine precipitates into hexagonal crystals that can coalesce into larger, recurrent
calculi. Prevention of stone formation is the primary goal of management and includes hydration,
dietary restriction of
salt and animal
protein, urinary alkalinization, and
cystine-binding
thiol drugs.