Cystinuria

Cystinuria is an inherited disorder of amino-acid transport in the kidneys and intestine. Excess cystine in urine can crystallize, causing recurrent kidney stones.

SLC3A1: SLC3A1 encodes a subunit of the transporter responsible for reabsorbing cystine and dibasic amino acids. Variants in this gene account for many cases by disrupting the shared transport system.
Urinalysis: Urinalysis is laboratory examination of urine for cells, chemicals, crystals, and other findings. Hexagonal cystine crystals or elevated cystine can support diagnosis.
Autosomal recessive inheritance: Autosomal recessive inheritance occurs when disease develops after pathogenic variants affect both copies of a gene. This pattern commonly describes cystinuria caused by SLC3A1 variants.
Nephrolithiasis: Nephrolithiasis is the formation or presence of stones within the kidneys or urinary tract. Cystinuria is a major inherited cause of recurrent nephrolithiasis.
Cystinosis: Cystinosis is a lysosomal storage disease caused by cystine accumulation inside cells. Despite the shared name and cystine involvement, it is a different disorder from urinary cystine transport failure.
SLC7A9: SLC7A9 encodes the light subunit of a transporter that reabsorbs cystine and dibasic amino acids. Its variants provide another genetic route to the same urinary transport defect.
Nitroprusside test: The nitroprusside test detects cystine or related sulfhydryl compounds through a color reaction. It offers a rapid screen for excess urinary cystine.
Autosomal dominant inheritance: Autosomal dominant inheritance occurs when one pathogenic gene variant can cause a trait or disorder. Some SLC7A9-related cases show dominant transmission with incomplete penetrance, unlike the classic recessive pattern.
Renal colic: Renal colic is severe, episodic pain caused by obstruction of the urinary tract, often by a stone. Cystine stones can trigger painful obstruction as they move through the urinary tract.
Hartnup disease: Hartnup disease is an inherited defect in neutral amino-acid transport, especially affecting tryptophan handling. It also causes amino-acid transport abnormalities but does not chiefly produce cystine stones.

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