Cystinosis

Cystinosis is a rare autosomal recessive disease characterized by a lysosomal storage disorder. The gene associated with cystinosis is CTNS which codes for cystinosin, a membrane protein that transports cystine from within the lysosome to the cytoplasm. As a consequence of several pathogenic variants in CTNS, altered cystine accumulates in the form of cystine crystals that damage tissues and organs.

The incidence in Europe and the United States is estimated at 1 in 100,000-200,000 individuals, although it may be higher in certain geographic areas such as Brittany (France) and Quebec (Canada).

Cystinosis is classified into three types: infantile, juvenile and ocular, depending on the age of onset and severity of symptoms. The most frequent (90% of cases) and severe form is infantile, in which there is progressive dysfunction of the renal tubules or renal Fanconi syndrome and loss of function of the renal glomeruli leading eventually to renal failure. Cystine accumulates in other organs causing hypothyroidism, insulin-dependent diabetes, enlarged liver and spleen, retinal abnormalities, and muscle and brain involvement.

The first symptoms of juvenile cystinosis usually appear around the age of 8 years, with an intermediate clinical picture leading to the onset of terminal nephropathy after the age of 15 years.

Finally, the ocular form is seen in adults, usually asymptomatic, in whom cystine accumulates in the cornea and who may present with photophobia only.

Treatment consists of the administration of electrolytes and vitamin supplements, indomethacin (improves the patient's overall condition and growth), and cysteamine, which decreases cystine concentration and thus slows progression to renal failure.

With cysteamine treatment and kidney transplantation, the life expectancy of patients with cystinosis can be prolonged; however, cysteamine treatment does not prevent renal failure.