What causes cystinosis in Fanconi syndrome?

Updated: Feb 09, 2018
  • Author: Sahar Fathallah-Shaykh, MD; Chief Editor: Craig B Langman, MD  more...
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Answer

Cystinosis is caused by the accumulation of cystine in lysosomes, probably as a result of a defect in efflux. The gene for cystinosis (CTNS) was mapped to band 17p13. The gene encodes for cystinosin, a 7-transmembrane-domain protein made up of 367 amino acids. The phenotype severity in cystinosis appears to vary with the mutations in the CTNS gene.

Benign, or adult, cystinosis is characterized by the deposition of relatively low amounts of cystine in the cornea and bone marrow. The kidneys are spared, and the renal manifestations are absent.

Infantile or nephropathic cystinosis is characterized by the presence of large amounts of cystine in all cells, including the kidneys. Incidence is approximately 1 case per 200,000 live births. Children with cystinosis usually have a fair complexion and blond hair, although the syndrome is also described in African Americans.

The first signs are polyuria and polydipsia, followed by episodes of dehydration, anorexia, and failure to thrive. The metabolic and renal features are detectable after the first few months of life. Nephrocalcinosis becomes evident shortly thereafter. Photophobia, which is caused by the deposition of cystine crystals in the cornea, usually appears in children aged 3-6 years. Retinopathy is a later finding. In the absence of treatment, the disease leads to chronic renal failure by the end of the first decade of life. Dialysis and transplantation can be successfully performed in these children.


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