Cystinosis

At a glance

An inherited lysosomal storage disorder resulting in the accumulation of cystine in various organs of the body (eg, kidneys, eyes, liver, muscles, pancreas, brain, leukocytes). Untreated, the infantile nephropathic form will result in end-stage renal failure before the age of 10 years.

Synonyms

Abderhalden-Kaufmann-Lignac Syndrome; Lignac-Fanconi Syndrome.

Incidence

Estimated to be in the range of 0.5-1:100,000 live births, although a wide geographic variability has been reported (eg, incidence of 1:26,000 in Brittany, France and 1:62,500 in parts of Quebec, Canada). Males are slightly more often affected (M:F = 1.5:1). Cystinosis is the most common inherited cause of ☞De Toni-Debré-Fanconi Syndrome (renal Fanconi Syndrome).

Genetic inheritance

Autosomal recessive for all types of Cystinosis. The defect affects the CTNS (Cystinosin) gene that has been mapped to chromosome 17p13. Cystinosin is a lysosomal cystine-proton cotransporter and its mutations result in the intralysosomal accumulation of cystine. To date, more than 100 different CTNS mutations have been described.

Pathophysiology

Cystine is a degradation product of cysteine and originates from lysosomal hydrolysis of ingested proteins. Cystinosis is a lysosomal storage disease caused by impaired transport of cystine out of the lysosomes into the cytoplasm. This energy-dependent transport relies on cystinosin, an integral membrane protein that acts as the lysosomal cystine transporter. In the nephropathic form of Cystinosis (see below), this results in cystine deposits within the cells of various organs where they affect the cellular energy metabolism. Cystine crystal deposits in the proximal tubule cells of the kidney result in early renal involvement and eventually renal Fanconi Syndrome (generalized proximal renal tubular dysfunction). It is characterized by a generalized transport defect in the renal proximal tubules with renal losses of glucose, phosphate, calcium, uric acid, amino acids, and bicarbonates, eventually leading to short stature, osteomalacia, and renal failure.

Diagnosis

Based on the clinical and laboratory findings. The diagnosis is confirmed by molecular analysis of the cystinosin gene. Three forms of Cystinosis can be distinguished: Infantile Nephropathic Cystinosis, Juvenile Nephropathic Cystinosis, and Adult Non-Nephropathic Cystinosis.

Infantile Nephropathic Cystinosis (Synonyms: Classic Onset Cystinosis; Early Onset Cystinosis): This is not only the most severe, but also the most common form of Cystinosis, accounting for 95% of all cases. The diagnosis is based on postnatal growth retardation becoming obvious after the first 6 months of life and the features of renal Fanconi Syndrome before 1 year of age with progressive deterioration of renal function toward end-stage renal disease (ESRD) usually before the age of 10 years if left untreated. Measurement of cystine content in leukocytes (polymorphonuclear leukocytes) is used to confirm the diagnosis. Cystine crystals can be detected in the cornea and conjunctiva (but may not be detectable before 16 months of age) and in biopsies ...