Cystinuria is an inherited metabolic disorder characterised by formation of stones in the kidneys, bladder, and urethra. In those affected, reabsorption of amino acid cystine is impaired due to defective renal tubules resulting in high concentration of cystine in the urine and stone formation in the urinary system. Prevalence of cystinuria is approximately 1:10,000. Cystinuria can occurs at any age, but renal colic due to cystine stone appears generally within the first two decades, with a median onset age of 15-years. Diagnostic tests for cystinuria include physical examination, detection of cystine stones and assay of excreted cystine in urine. Treatment is aimed at preventing stones from forming and may include high hydration to reduce urinary cystine osmolality, urinary alkalinisation to increase cystine solubility, and pharmacological cystine-binding medications.
Among kidney stone diseases, cystinuria is the only disorder known to be associated with genetic mutations. Two genes have been identified causing cystinuria; SLC3A1 and SLC7A9 genes. Both genes are expressed in the renal proximal tubules and the intestinal tract and encode subunits of trans-epithelial transporters for the dibasic amino acids cystine, ornithine, lysine and arginine. Mutations in the SLC3A1 gene are generally associated with an autosomal recessive pattern of inheritance, whereas SLC7A9 variants result in a broad clinical variability even within the same family.
Al-Hermi et al. (2002) described three cases with cystinuria from Bahrain. The first case was a 13 year-old boy, who had staghorn stone in his left kidney at the age of 6-years. The second case was a 10-year-old boy, brother of the first patient. He had bilateral tiny renal stones, and his urine was positive for cystine crystals and urine amino acid assay showed high levels of cystine, ornithine, lysine, and arginine. The third case was a 4-year-old girl, who presented at the age of one and half years with failure to thrive and recurrent urinary tract infection. She had bilateral ureteral stones, and her urine testing for amino acids showed high levels of cystine and dibasic aminoaciduria.
Weinberger et al. (1974) demonstrated an unusually high frequency of type II or III cystinuria among Jews of Libyan origin.