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Cystinosis is a metabolic disease characterized by an accumulation of cystine inside the lysosomes, causing damage in different organs and tissues, particularly in the kidneys and eyes. Three clinical forms have been described: nephropathic infantile, nephropathic juvenile and ocular (see these terms).
The incidence of cystinosis is estimated at around 1/100,000- 1/200,000 live births.
In the infantile form (the most common) the first clinical signs appear between 3 and 6 months of age, with a polyuric-polydipsic syndrome and marked growth delay secondary to a generalized impaired proximal tubular reabsorptive capacity with severe fluid-electrolyte balance alterations (Fanconi syndrome ; see this term). Hypophosphatemic rickets is also observed that causes bone deformities. Ocular involvement caused by cystine deposits in the cornea and conjunctiva is responsible for photophobia which usually appears after 3 years of age. Cystine deposits in various organs progressively lead to hypothyroidism, insulin-dependent diabetes, hepatosplenomegaly with portal hypertension, and muscle and cerebral involvement. In the absence of specific treatment, the disease progresses to end stage renal failure before the age of 10. The first symptoms of juvenile cystinosis (< 5% of patients), typically appear around 6-8 years of age with a milder form of proximal tubulopathy and/or proteinuria with nephrotic syndrome. Progression to renal failure occurs later than in the infantile form. Finally, the ocular form is seen in adults who are generally asymptomatic and may suffer only from photophobia.
Cystinosis is due to a defect in cystine transport out of lysosomes. The causative gene, CTNS (17p13), encodes cystinosin, a lysosomal membrane protein. Mutations in this gene have been detected for all 3 forms of the disease, with a 57-kb deletion detected in 60%-70% of patients from Northern Europe with the infantile form.
The diagnosis is based on blood and urine analysis showing features of Fanconi syndrome (metabolic acidosis, hypokalemia, hyperaminoaciduria, glycosuria), detection of cystine crystals in the cornea and determination of elevated cystine levels in leucocytes. It is confirmed by CTNS gene analysis.
A prenatal diagnosis can be obtained by genetic analysis in families with a previously affected child.
Transmission is autosomal recessive.
Management and treatment
Treatment consists of administering electrolyte and vitamin supplements; indomethacin, which improves the general status and growth; and cysteamine (at a dose of 1.3 to 1.9 g/m2 per day, given in 4 doses every 6 hours), which lowers the cystine content in the lysosomes, thereby slowing or even stopping the progression to renal failure and the development of extra-renal manifestations. The side effects of cysteamine include gastrointestinal symptoms, bad breath, sweat odor and allergic reactions. A delayed-released formulation has been developed and approved in the USA for the infantile form, which allows patients to receive cysteamine only twice a day, thus improving compliance and quality of life. Topical cysteamine eye drops are also needed as systemic cysteamine has no effect on cystine corneal deposits.
Life expectancy has significantly improved with therapy. Cysteamine delays the need for renal replacement therapy. The disease does not recur in the graft after renal transplantation but continues to progress in other organs and may cause complications (swallowing dysfunction, lung disease, cardiomyopathy) that may worsen the prognosis.