Cystinuria is an inheritable disorder of
amino acid transport, transmitted as an autosomal recessive trait,
Cystinuria is caused by defective transport of
cystine and
dibasic amino acids through the epithelial cells of the renal tubule and intestinal tract.
Cystine stones are caused by the excessive renal excretion of
cystine as a result of its low solubility in urine. Recently, a human kidney
cDNA, named rBAT (also D2: the gene now designated SLC3A1), which elicits the transport for
cystine and
dibasic amino acids, has been reported. The 90-kd Type II
glycoprotein stimulated
cystine and dibasic and neutral
amino acid uptake into oocytes with kinetics similar to the renal brush border transporter. The human gene for this
protein resides on chromosome 2. The most frequent mutations found involved substitution of the
threonine for
methionine 467. Eight additional mutations in SLC3A1 have been found.
Cystine stones frequently occur in the second or third decade of life, with an occasional occurrence in infancy and in old age. Urinary
cystine excretion exceeding 250 mg/g
creatinine is usually diagnostic of homozygous
cystinuria. The goal of treatment is to reduce the urinary
cystine concentration below its solubility limit (250 mg/L).