Medical progress: Cystinosis

Transport of cystine and cysteine in mammalian cells.

The activity of a cystine transport system in lysosomes prepared from the leukocytes of patients with cystinosis was found to be deficient. In normal subjects, this system was resistant to N-ethylmaleimide and demonstrated saturation kinetics. Lysosomes from individuals heterozygous for cystinosis demonstrated a reduced maximum velocity for cystine egress from lysosomes. The rate of cystine escape from normal lysosomes was enhanced by adenosine triphosphate. The availability of normal and mutant lysosomes provides a means of investigating mechanisms of amino acid transport across lysosomal membranes.

Cystine transport is defective in isolated leukocyte lysosomes from patients with cystinosis

We treated 93 children with nephropathic cystinosis with oral cysteamine (mean dose, 51.3 mg per kilogram of body weight per day) for up to 73 months. This agent is known to be effective in depleting cells of cystine. In our study, the mean cystine depletion from leukocytes was 82 percent. A historical control group of 55 children received either ascorbic acid (27 children) or placebo (28). At age six, 2 of 17 controls had a serum creatinine level less than 1.0 mg per deciliter, as compared with 17 of 27 patients treated with cysteamine for at least one year (odds ratio, 12.8; 95 percent confidence interval, 2.1 to 33.9). At the end of the study, creatinine clearance was higher in the cysteamine group than in the control group (38.5 vs. 29.7 ml per minute per 1.73 m2; 95 percent confidence limits on the difference, 1.8 and 15.8), even though the cysteamine group was on average 1.4 years older than the control group. Cysteamine also improved growth; those in the cysteamine group between two and three years of age grew at 93 percent of the normal velocity, as compared with 54 percent in the control group. Fourteen percent of the patients could not tolerate the taste and smell of cysteamine. Concurrent controls treated in a blinded fashion with a placebo were not included in this study. With this limitation in mind, we conclude that oral cysteamine, by depleting cells of cystine, helps maintain renal glomerular function, improves growth, and constitutes the current treatment of choice for nephropathic cystinosis.

Cysteamine Therapy for Children with Nephropathic Cystinosis

Cystinosis is an inherited lysosomal storage disease characterized by defective transport of cystine out of lysosomes. However, the causative gene, CTNS, encodes a seven transmembrane domain lysosomal protein, cystinosin, unrelated to known transporters. To investigate the molecular function of cystinosin, the protein was redirected from lysosomes to the plasma membrane by deletion of its C-terminal GYDQL sorting motif (cystinosin-DeltaGYDQL), thereby exposing the intralysosomal side of cystinosin to the extracellular medium. COS cells expressing cystinosin-DeltaGYDQL selectively take up L-cystine from the extracellular medium at acidic pH. Disruption of the transmembrane pH gradient or incubation of the cells at neutral pH strongly inhibits the uptake. Cystinosin-DeltaGYDQL is directly involved in the observed cystine transport, since this activity is highly reduced when the GYDQL motif is restored and is abolished upon introduction of a point mutation inducing early-onset cystinosis. We conclude that cystinosin represents a novel H(+)-driven transporter that is responsible for cystine export from lysosomes, and propose that cystinosin homologues, such as mammalian SL15/Lec35 and Saccharomyces cerevisiae ERS1, may perform similar transport processes at other cellular membranes.

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Cystinosin, the protein defective in cystinosis, is a H+-driven lysosomal cystine transporter

ATP-dependent lysosomal cystine efflux is defective in cystinosis.

Proton-translocating ATPase and lysosomal cystine transport.

Cystinotic fibroblasts contain approximately 100 times more cystine than do normal control fibroblasts. When cystinotic fibroblasts were placed in the presence of 30 mM cysteine-glutathione mixed disulfide (CSSG) for 24 hr, their cystine content increased about 3-fold. Similar treatment of normal fibroblasts and fibroblasts from patients heterozygous for cystinosis resulted in a 6- to 7-fold increase in cystine content. In all three cell types, the intracellular free cystine is located within lysosomes. When placed in cystine-free medium after 24 hr in CSSG-containing medium, the normal and heterozygous fibroblasts rapidly lost their lysosomal cystine (t 1/2 = 20 min), but the cystine content of the cystinotic cells remained stable for over 90 min. In contrast to the findings in intact fibroblasts, cystine loss could not be demonstrated from isolated, cystine-loaded lysosomes from any of the three cell types.

Adam J. Jonas

Papers

Cysteamine Therapy for Children with Nephropathic Cystinosis

ATP-dependent lysosomal cystine efflux is defective in cystinosis.

Proton-translocating ATPase and lysosomal cystine transport.

Cystine accumulation and loss in normal, heterozygous, and cystinotic fibroblasts

A simple, rapid assay for cysteamine and other thiols.