U01.01.121 Cystinuria

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Learning Objectives

  • Identify the COLA transporter defect located in the renal PCT and intestines.
  • Recognize the characteristic hexagonal shape of cystine stones.
  • Explain the chemical structure of Cystine and its relationship to cysteine.
  • Understand treatment strategies, including urinary alkalinization and chelating agents.

1. Pathophysiology and Genetics

Cystinuria is a common autosomal recessive disorder (1:7000) caused by a hereditary defect in the renal proximal convoluted tubule (PCT) and intestinal amino acid transporter.

This transporter is responsible for the reabsorption of four specific amino acids, remembered by the acronym COLA:

  • C: Cystine (two cysteines connected by a disulfide bond)
  • O: Ornithine
  • L: Lysine
  • A: Arginine

While all four are lost in the urine, only Cystine is poorly soluble, leading to its precipitation and the formation of renal calculi (stones).

2. Clinical Findings and Diagnosis

Patients typically present with symptoms of nephrolithiasis, such as flank pain and hematuria, often starting in childhood or young adulthood.

  • Cystine Stones: These are characterized by a unique hexagonal shape under microscopy. They are often radiolucent or faintly radiopaque.
  • Diagnostic Tests:
    • Urinary Sodium-Cyanide Nitroprusside Test: A positive test result shows a purple-red color, indicating the presence of cystine.
    • NMR Spectroscopy: Used for detailed analysis of urine composition.

Activity

Memory Hook: “COLA” amino acids are lost, but “SIX-tine” (Cystine) makes Six-sided (hexagonal) stones.

3. Management and Treatment

The primary goal of treatment is to increase the solubility of cystine to prevent stone formation.

Treatment Strategy Mechanism / Goal
Urinary Alkalinization Potassium citrate or Acetazolamide; ↑ pH makes cystine more soluble.
Chelating Agents Penicillamine forms a more soluble complex with cystine.
Hydration and Diet ↑ Fluid intake; diet low in methionine (precursor to cysteine).

Activity

Clinical Notes & Corrections:

  • Stone Recurrence: Cystinuria is a leading cause of recurrent kidney stones in children. Any child with a history of stones should be screened for this condition.
  • Solubility: Cystine is most soluble at a pH > 7.5. This is why alkalinizing the urine is a cornerstone of therapy.

Activity