Cystinosis is an inborn error of metabolism characterized by an abnormal accumulation of the amino acid cystine in the cells of the body.
Also called cystine storage disease, cystine accumulates in the lysosomes of cells and forms crystals in various tissues such as the bone marrow, the cornea, the kidney and the liver.
The most severe from of the disease, nephropathic cystinosis, often develops early on in childhood and causes rickets, liver enlargement, and kidney malfunction. The cystine crystal deposits in the kidney eventually cause Fanconi syndrome which describes defective reabsorption of amino acids, salts, sugar and water in the kidneys. Children who are not treated for the condition often die from kidney failure by the age of 10.
Prenatal diagnostic testing is available for families known to be at risk of giving birth to an affected child. Chorionic villus sampling may be performed at around 8 or 9 weeks of gestation and amniocentesis at around 14 to 16 weeks of gestation.
Diagnosing cystinosis and starting treatment as early on as possible can help to prolong lifespan in people with the condition. The two approaches to cystinosis treatment are symptomatic treatment and disease specific treatment.
Symptomatic therapy involves managing the Fanconi syndrome. The affected kidneys fail to reabsorb essential nutrients as well as water, calcium, sodium, chloride, potassium, bicarbonate, amino acids, glucose and carnitine. The excessive salt and water loss causes intense thirst and cravings for salt-rich foods. Symptomatic therapy involves replacing these lost fluids, salts, and minerals.
Oral solutions of sodium bicarbonate or sodium–potassium citrate are usually prescribed. Oral calcium is given to prevent bone damage and the development of rickets. Sodium phosphate and Vitamin D are prescribed to aid bone health and growth. Underactive thyroid gland requires thyroxine supplements and renal damage may require regular hemodialysis and even kidney transplant.
A drug called cysteamine has been approved as the standard treatment for cystinosis. This agents depletes the intracellular content of cystine. Administered on a regular basis, this reduces the level of cystine stored in the lysosomes and starts to improve renal function and growth. The early administration of this drug has allowed many patients to survive into their third decade of life without needing a kidney transplant.
Cysteamine works by entering the lysosomes of cells via a specific transporter and forming a cysteamine–cystine complex that is capable of exiting the lysosome. In the cytoplasm, the cystine and the cysteamine undergo reduction by glutathione. The cystine is then removed from the cell and the cysteamine is recycled and goes on to remove more cystine from the lysosomes. With each cycle, 1 mole of cysteamine removes 1 mole of half-cystine.