Cystinosis

cystinosis, also called cystine storage disease, inborn error of metabolism resulting in the deposition of crystals of the amino acid cystine in various body tissues. The tissues that typically are affected include the bone marrow, the liver, the cornea (where the crystals can be seen), and the kidney. There are three distinct forms of cystinosis—nephropathic (infantile), intermediate (adolescent), and nonnephropathic (benign, or ocular)—which differ with respect to clinical presentation, progression, and severity.

The nephropathic type begins in infancy and is characterized by very high intracellular concentrations of cystine, particularly in white blood cells and fibroblasts. When they are about six to nine months old, affected infants refuse to eat, grow poorly, and urinate excessively; these symptoms are related to more severe aspects of the disorder, which include rickets, enlargement of the liver, stunted growth, and kidney malfunction. The crystal deposits in the kidney tubules lead eventually to a full-blown manifestation of Fanconi syndrome, a variation of de Toni-Fanconi syndrome that is characterized by a generalized defect in the reabsorption of all amino acids, sugar, salts, and water. Children with nephropathic cystinosis who are not treated for their condition typically experience complete kidney failure by about age 10. By comparison, nonnephropathic cystinosis is much less severe, being characterized mainly by the accumulation of cystine crystals in the cornea, which can result in photophobia (abnormal visual sensitivity to bright light). Intermediate cystinosis is similar to the nephropathic form but has a later onset, typically in adolescence, with complete kidney failure occurring usually between ages 15 and 25.

All three forms of cystinosis are associated with variations in a gene known as CTNS, which encodes cystinosin, a protein that normally transports cystine out of cellular organelles called lysosomes. When the gene is mutated, however, it produces a dysfunctional form of cystinosin. The extent to which the protein’s functional capacity is affected depends on the specific mutation involved; for example, severe truncating mutations and partial gene deletions that cause loss of cystinosin function have been identified in patients affected by the nephropathic form.

Genetic testing may be used to confirm some cases of cystinosis. Diagnosis of the disorder is otherwise based on symptoms of Fanconi syndrome (e.g., increased urinary excretion of amino acids, sugar, salts, and water), the detection of cystine crystals in the cornea, and the detection of increased cystine content in cells.

Treatment usually is with the cystine-depleting agent cysteamine, the effectiveness of which can be checked through regular monitoring of cellular cystine levels. Because the renal damage associated with cystinosis is irreversible, severely affected patients often require dialysis and eventual kidney transplantation.