Cystine diathesis

Cystine Diathesis

Cystine diathesis is a rare metabolic disorder characterized by the excessive excretion of cystine in the urine, leading to the formation of cystine stones in the kidneys, bladder, and urinary tract. This condition is a type of cystinuria, which is a genetic disorder affecting the transport of certain amino acids in the kidneys.

Pathophysiology

Cystine diathesis results from a defect in the renal tubular reabsorption of cystine, an amino acid that is relatively insoluble in urine. Under normal circumstances, cystine is reabsorbed in the proximal tubules of the kidneys. However, in individuals with cystine diathesis, this reabsorption is impaired, leading to high concentrations of cystine in the urine. When the concentration of cystine exceeds its solubility, it precipitates and forms crystals, which can aggregate into stones.

Genetics

Cystine diathesis is inherited in an autosomal recessive manner. The condition is caused by mutations in the SLC3A1 or SLC7A9 genes, which encode components of a transporter responsible for the reabsorption of cystine and other dibasic amino acids in the kidneys. Individuals with mutations in these genes have impaired transport, leading to increased urinary excretion of cystine.

Clinical Presentation

Patients with cystine diathesis typically present with symptoms related to kidney stones, such as:

• Flank pain
• Hematuria (blood in urine)
• Recurrent urinary tract infections
• Obstruction of the urinary tract

The age of onset can vary, but symptoms often begin in childhood or early adulthood.

Diagnosis

Diagnosis of cystine diathesis involves:

• Urinalysis: Detection of hexagonal cystine crystals in the urine.
• Cyanide-nitroprusside test: A qualitative test that detects elevated levels of cystine in the urine.
• Genetic testing: Identification of mutations in the SLC3A1 or SLC7A9 genes.

Treatment

Management of cystine diathesis focuses on preventing stone formation and includes:

• Increased fluid intake: To dilute the urine and reduce cystine concentration.
• Alkalinization of urine: Using medications such as potassium citrate to increase cystine solubility.
• Cystine-binding thiol drugs: Such as tiopronin or penicillamine, which form soluble complexes with cystine.
• Dietary modifications: Reducing sodium and protein intake to decrease cystine excretion.

Prognosis

With appropriate management, individuals with cystine diathesis can reduce the frequency of stone formation and maintain kidney function. However, the condition requires lifelong monitoring and treatment to prevent complications.

See Also

• Cystinuria
• Kidney stones
• Metabolic disorders

NIH genetic and rare disease info

• NIH info on Cystine diathesis

Cystine diathesis is a rare disease.