Identification and detection of the common 65-kb deletion breakpoint in the nephropathic cystinosis gene (CTNS)

Residual Cystine Transport Activity for Specific Infantile and Juvenile CTNS Mutations in a PTEC-Based Addback Model

Cystinosis is a rare, autosomal recessive, lysosomal storage disease caused by mutations in the gene CTNS, leading to cystine accumulation in the lysosomes. While cysteamine lowers the cystine levels, it does not cure the disease, suggesting that CTNS exerts additional functions besides cystine transport. This study investigated the impact of infantile and juvenile CTNS mutations with discrepant genotype/phenotype correlations on CTNS expression, and subcellular localisation and function in clinically relevant cystinosis cell models to better understand the link between genotype and CTNS function. Using CTNS-depleted proximal tubule epithelial cells and patient-derived fibroblasts, we expressed a selection of CTNSmutants under various promoters. EF1a-driven expression led to substantial overexpression, resulting in CTNS protein levels that localised to the lysosomal compartment. All CTNSmutants tested also reversed cystine accumulation, indicating that CTNSmutants still exert transport activity, possibly due to the overexpression conditions. Surprisingly, even CTNSmutants expression driven by the less potent CTNS and EFS promoters reversed the cystine accumulation, contrary to the CTNSG339R missense mutant. Taken together, our findings shed new light on CTNS mutations, highlighting the need for robust assessment methodologies in clinically relevant cellular models and thus paving the way for better stratification of cystinosis patients, and advocating for the development of more personalized therapy.

CTNS Mutations Causing Autosomal Recessive Cystinosis in a Subset of Iranian Population: Report of Two New Variants

Background

Nephropathic cystinosis (NC) is an uncommon autosomal recessive disease with abnormality in lysosomal storage that appearances in patients with mutations in the CTNS gene encoding a lysosomal transporter cystinosin. Disrupted function of this transporter is followed by accumulation of cysteine crystals in cells of many various organs. This study aimed to investigate the mutations of the CTNS gene in 20 Iranian patients suffering from NC.

Materials and Methods

Twenty Iranian cystinosis patients referring to Imam Hossein Hospital of Isfahan were employed in this case-series study. After extraction of genomic DNA, the promoter and entire coding regions of CTNS were analysed using sanger sequencing in all patients. Gap-Polymerase Chain Reaction was used to detect 57 kb deletion in the CTNS gene. In silico study was performed to analyse variants.

Results

The large deletion was not seen in any NC patients. Molecular analysis which conducted to screen the CTNS gene of patients, identified eight different mutations, including two new mutations, c.971_972insC and c.956_956delA, which have not been reported before, and c.681G>A mutation, which was identified as a frequently founded mutation in the Middle East and was observed in 35% of patients. In this study, five other mutations including c.1015G>A, c.922G>A, c.323_323delA, c.433C>T, and c.18_21delGACT were also observed, which have been reported in previous studies.

Conclusion

The mutational spectrum in the Iranian patients is the same as previously reported mutations except that two new mutations were found. The present findings will present suggestions for regular molecular diagnosis of cystinosis in Iran.

Studies with Human-Induced Pluripotent Stem Cells Reveal That CTNS Mutations Can Alter Renal Proximal Tubule Differentiation

Cystinosis is an autosomal recessive disease resulting from mutations in ctns, which encodes for cystinosin, a proton-coupled cystine transporter that exports cystine from lysosomes. The major clinical form, infantile cystinosis, is associated with renal failure due to the malfunctioning of the renal proximal tubule (RPT). To examine the hypothesis that the malfunctioning of the cystinotic RPT arises from defective differentiation, human-induced pluripotent stem cells (hiPSCs) were generated from human dermal fibroblasts from an individual with infantile cystinosis, as well as a normal individual. The results indicate that both the cystinotic and normal hiPSCs are pluripotent and can form embryoid bodies (EBs) with the three primordial germ layers. When the normal hiPSCs were subjected to a differentiation regime that induces RPT formation, organoids containing tubules with lumens emerged that expressed distinctive RPT proteins, including villin, the Na+/H+ Exchanger (NHE) isoform 3 (NHE3), and the NHE Regulatory Factor 1 (NHERF1). The formation of tubules with lumens was less pronounced in organoids derived from cystinotic hiPSCs, although the organoids expressed villin, NHE3, and NHERF1. These observations can be attributed to an impairment in differentiation and/or by other defects which cause cystinotic RPTs to have an increased propensity to undergo apoptosis or other types of programmed cell death.

Next generation sequencing as second-tier test in high-throughput newborn screening for nephropathic cystinosis

Nephropathic cystinosis is a rare autosomal recessive lysosomal storage disorder, which causes loss of renal proximal tubular function and progressive loss of glomerular function, finally leading to end stage renal failure at school age. In the course of the disease most patients will need kidney transplantation if treatment has not been started before clinical manifestation. With an effective treatment available, a newborn screening assay is highly demanded. Since newborns with cystinosis usually do not show symptoms within the first months of life and no biochemical markers are easily detectable, a DNA-based method seems to be an obvious tool for early diagnosis. Screening was performed using high-throughput nucleic acid extraction followed by 384-well qPCR and melting analysis for the three most frequent variants (57 kb deletion NC_000017.11:g.3600934_3658165del (GRCh38); c.18_21del GACT; c.926dupG) responsible for the defective lysosomal membrane protein cystinosin (CTNS). To increase sensitivity, all heterozygous samples identified in qPCR assay were verified and screened for additional variants by applying next generation sequencing. From January 2018 to July 2019 nearly 292,000 newborns were successfully screened. We identified two newborns with a homozygous 57 kb deletion and a second one with heterozygous 57 kb deletion and a G>C substitution at position c.-512 on the second allele. Cystinosis is an example for diseases caused by a limited number of high prevalence and a high number of low prevalence variants. We have shown that qPCR combined with NGS can be used as a high throughput, cost effective tool in newborn screening for such diseases.

Hierarchical processing of visual stimuli in nephropathic cystinosis

Previous studies have shown that individuals with cystinosis may exhibit difficulty with visuospatial tasks. Global and local (hierarchical) processing are specific types of visuospatial tasks mediated by the right and left parietal lobes respectively. The study objective was to determine whether individuals with cystinosis and carriers of the cystinosin gene mutation show deficits in global and/or local processing. The study included 48 children (32 controls, 16 cystinosis) and 56 adults (25 controls, 25 carriers, and 6 cystinosis). Participants were instructed to replicate 10 hierarchical stimuli to assess global-local processing. The primary outcome measure was mean global and local performance on the hierarchical stimuli task between subgroups. Error subtypes were included to further assess each image. Compared to the control subgroups, adult and child cystinosis patients as well adult carriers demonstrated significant deficits in the global processing of a hierarchical stimulus against a background of unimpaired local processing. Child cystinosis patients performed significantly more poorly than controls on all error subtypes except local shape distortion. Adult cystinosis patients and carriers made significantly more global shape distortion errors than the controls. Our study shows that the cognitive profile documented in cystinosis patients and carriers includes significant difficulties with the global processing of an image. Results of the carrier population are important since they suggest that the cognitive impairments observed in patients cannot be attributed to symptomatic manifestations of the disease. Instead, the global processing deficits observed provide insight into the potential role of the cystinosin gene mutation on neurodevelopmental differences seen in these individuals.

A 57 kB Genomic Deletion Causing CTNS Loss of Function Contributes to the CTNS Mutational Spectrum in the Middle East

Background: Nephropathic Cystinosis, the most common cause of renal Fanconi syndrome, is a lysosomal transport disorder with an autosomal recessive inheritance pattern. A large number of mutations in CTNS have been identified as causative to date. A 57 kb deletion encompassing parts of CTNS is most commonly identified in Caucasians but this allele has not been identified in individuals of Eastern Mediterranean, Middle Eastern, Persian, or Arab origin to date. Methods and Results: Implementing whole exome sequencing (WES) in a consanguineous Iranian family, we identified this large deletion affecting CTNS in a patient initially presenting with hypokalemic metabolic alkalosis symptoms and considerable proteinuria. Conclusion: We show WES is a cost and time efficient genetic diagnostics modality to identify the underlying molecular pathology in Cystinosis individuals and provide a summary of all previously reported CTNS alleles in the Middle east population. Our work also highlights the importance to consider the 57-kb deletion as underlying genetic cause in non-European populations, including the Middle East. Limited diagnostic modalities for Cystinosis in developing countries could account for the lack of previously reported cases in these populations carrying this allele. Further, our findings emphasize the utility of WES to define genetic causes in clinically poorly defined phenotypes and demonstrate the requirement of Copy number variation (CNV) analysis of WES data.

Clinical implementation of gene panel testing for lysosomal storage diseases

Background

The diagnostic workup in patients with a clinical suspicion of lysosomal storage diseases (LSD) is often difficult due to the variability in the clinical phenotype. The gold standard for diagnosis of LSDs consists of enzymatic testing. However, due to the sequential nature of this methodology and inconsistent genotype–phenotype correlations of certain LSDs, finding a diagnosis can be challenging.

Method

We developed and clinically implemented a gene panel covering 50 genes known to cause LSDs when mutated. Over a period of 18 months, we analyzed 150 patients who were referred for LSD testing and compared these results with the data of patients who were previously enrolled in a scheme of classical biochemical testing.

Results

Our panel was able to determine the molecular cause of the disease in 22 cases (15%), representing an increase in diagnostic yield compared to biochemical tests developed for 21 LSDs (4.6%). We were furthermore able to redirect the diagnosis of a mucolipidosis patient who was initially suspected to be affected with galactosialidosis. Several patients were identified as being affected with neuronal ceroid lipofuscinosis, which cannot readily be detected by enzyme testing. Finally, several carriers of pathogenic mutations in LSD genes related to the disease phenotype were identified as well, thus potentially increasing the diagnostic yield of the panel as heterozygous deletions cannot be detected.

Conclusion

We show that the implementation of a gene panel for LSD diagnostics results in an increased yield in comparison to classical biochemical testing. As the panel is able to cover a wider range of diseases, we propose to implement this methodology as a first‐tier test in cases of an aspecific LSD presentation, while enzymatic testing remains the first choice in patients with a more distinctive clinical presentation. Positive panel results should however still be enzymatically confirmed whenever possible.

Slow progression of renal failure in a child with infantile cystinosis

Cystinosis is a rare autosomal recessive lysosomal transport disorder, characterized by the accumulation of the aminoacid cystine and progressive dysfunction of several organs. Kidneys are severely affected, and the most frequent form, infantile nephropathic cystinosis, presents with growth failure in infancy, renal Fanconi syndrome and end-stage renal disease by the first decade of life. We report of a girl with infantile nephropathic cystinosis that has reached adolescence without the need of renal replacement therapy and without extrarenal manifestations despite her delayed diagnosis and treatment initiation. The girl with this intermediate phenotype was found to have compound heterozygosity of one known (1015G > A) and one novel (587_588insA) mutation in CTNS gene. Our case points to the wide clinical presentation of infantile nephropathic cystinosis and suggest that long-term outcome is not always ominous as generally thought.

Cysteamine hydrochloride eye drop solution for the treatment of corneal cystine crystal deposits in patients with cystinosis: an evidence-based review

Cystinosis is a rare, autosomal recessive disorder leading to defective transport of cystine out of lysosomes. Subsequent cystine crystal accumulation can occur in various tissues, including the ocular surface. This review explores the efficacy of cysteamine hydrochloride eye drops in the treatment of corneal cystine crystal accumulation and its safety profile.

Mutation analysis of the CTNS gene in Iranian patients with infantile nephropathic cystinosis: identification of two novel mutations

Nephropathic cystinosis is an inherited lysosomal transport disorder caused by mutations in the CTNS gene that encodes for a lysosomal membrane transporter, cystinosin. Dysfunction in this protein leads to cystine accumulation in the cells of different organs. The accumulation of cystine in the kidneys becomes apparent with renal tubular Fanconi syndrome between 6 and 12 months of age and leads to renal failure in the first decade of life. The aim of this study was to analyze the CTNS mutations in 20 Iranian patients, from 20 unrelated families, all of whom were afflicted with infantile nephropathic cystinosis. In these patients, seven different mutant alleles were found, including two new mutations, c.517T>C; p.Y173H and c.492_515del, that have not been previously reported. In addition, we observed that c.681G>A, the common Middle Eastern mutation, was the most common mutation in our patients. Moreover, a new minisatellite or variable number of tandem repeat marker (KX499495) was identified at the CTNS gene. Seven different alleles were found for this marker, and its allele frequency and heterozygosity degree were calculated in cystinosis patients and healthy individuals.

CTNS molecular genetics profile in a Persian nephropathic cystinosis population

PURPOSE

In this report, we document the CTNS gene mutations of 28 Iranian patients with nephropathic cystinosis age 1-17 years. All presented initially with severe failure to thrive, polyuria, and polydipsia.

METHODS

Cystinosis was primarily diagnosed by a pediatric nephrologist and then referred to the Iran University of Medical Sciences genetics clinic for consultation and molecular analysis, which involved polymerase chain reaction (PCR) amplification to determine the presence or absence of the 57-kb founder deletion in CTNS, followed by direct sequencing of the coding exons of CTNS.

RESULTS

The common 57-kb deletion was not observed in any of the 28 Iranian patients. In 14 of 28 patients (50%), mutations were observed in exons 6 and 7. No mutation was detected in exon 5, and only one (3.6%) patient with cystinosis showed a previously reported 4-bp deletion in exon 3 of CTNS. Four patients (14.3%) had a previously reported mutation (c.969C>A; p.N323K) in exon 11, and five (18%) had novel homozygous deletions in exon 6 leading to premature truncation of the protein. These deletions included c.323delA; p.Q108RfsX10 in three individuals and c.257-258delCT; p.S86FfsX37 in two cases. Other frame-shift mutations were all novel homozygous single base pair deletion/insertions including one in CTNS exon 9 (c.661insT; p.V221CfsX6), and four (14.3%) in exon 4, i.e., c.92insG; p.V31GfsX28 in two and c.120delC; p.T40TfsX10 in two. In total, we identified eight previously reported mutations and eight novel mutations in our patients. The only detected splice site mutation (IVS3-2A>C) was associated with the insertion mutation in the exon 9.

CONCLUSION

This study, the first molecular genetic analysis of non-ethnic-specific Iranian nephropathic cystinosis patients, may provide guidance for molecular diagnostics of cystinosis in Iran.

The renal Fanconi syndrome in cystinosis: pathogenic insights and therapeutic perspectives

Cystinosis is an autosomal recessive metabolic disease that belongs to the family of lysosomal storage disorders. It is caused by a defect in the lysosomal cystine transporter, cystinosin, which results in an accumulation of cystine in all organs. Despite the ubiquitous expression of cystinosin, a renal Fanconi syndrome is often the first manifestation of cystinosis, usually presenting within the first year of life and characterized by the early and severe dysfunction of proximal tubule cells, highlighting the unique vulnerability of this cell type. The current therapy for cystinosis, cysteamine, facilitates lysosomal cystine clearance and greatly delays progression to kidney failure but is unable to correct the Fanconi syndrome. This Review summarizes decades of studies that have fostered a better understanding of the pathogenesis of the renal Fanconi syndrome associated with cystinosis. These studies have unraveled some of the early molecular changes that occur before the onset of tubular atrophy and identified a role for cystinosin beyond cystine transport, in endolysosomal trafficking and proteolysis, lysosomal clearance, autophagy and the regulation of energy balance. These studies have also led to the identification of new potential therapeutic targets and here, we outline the potential role of stem cell therapy for cystinosis and provide insights into the mechanism of haematopoietic stem cell-mediated kidney protection.

Cystinosis in Eastern Turkey

BACKGROUND

This study was conducted to investigate CTNS (cystinosin, lysosomal cystine transporter) gene mutations and the clinical spectrum of nephropathic cystinosis among patients diagnosed with the disease in a single center in Turkey.

METHODS

Patients' clinical and laboratory data were extracted from an electronic health registry. Molecular CTNS gene analysis was performed using either next-generation sequencing or Sanger sequencing.

RESULTS

Eleven patients (age range: 1.5-12 years) from nine families were identified. The presenting complaint was growth retardation in seven patients; polydipsia and polyuria in three patients; and vomiting in two patients. At presentation, electrolyte loss was noted in all patients, of which eight patients presented with metabolic acidosis, and three patients presented with metabolic alkalosis. All patients also presented with proteinuria and glucosuria, and four patients developed varying degrees of renal insufficiency, for which peritoneal dialysis was initiated in one patient. Cystine crystals were detected via ocular examination in one patient at presentation. No cystine crystals were detected among patients who underwent bone marrow aspiration. In the CTNS gene, a p.T7FX7 (c.18-21del4bp) mutation was detected in three patients, whereas a p.E227E (c.681 G>A) (homozygous) mutation was detected in eight patients.

CONCLUSIONS

We detected two distinct mutations, p.T7FX7 (c.18-21del4bp) and p.E227E (c.681 G>A) (homozygous), in the CTNS gene in 11 patients with cystinosis from the East Anatolian region of Turkey. Patients with a homozygous c.681 G>A (p.E227E) mutation are more likely to develop chronic renal failure and should be monitored closely, whereas patients with a p.T7FX7 (c.18-21del4bp) mutation have a milder phenotype. Additionally, metabolic alkalosis does not exclude cystinosis, although cystinosis is a cause of proximal renal tubular acidosis.

Diagnosis and Monitoring of Cystinosis Using Immunomagnetically Purified Granulocytes

BACKGROUND

Cystine determination is a critical biochemical test for the diagnosis and therapeutic monitoring of the lysosomal storage disease cystinosis. The classical mixed-leukocyte cystine assay requires prompt specialized recovery/isolation following blood drawing, providing cystine concentrations normalized to total protein from assorted types of white blood cells, each with varying cystine content.

METHODS

We present a new workflow for cystine determination using immunomagnetic granulocyte purification, and new reference ranges established from 47 patient and 27 obligate heterozygote samples assayed. Samples were collected in acid-citrate dextrose tubes and their stability was proven to allow for overnight shipping before analysis. Cystine was quantified by LC-MS/MS.

RESULTS

The new method was reproducible (<15% root mean square error) and specific, assaying purified granulocytes from blood samples that no longer required immediate preparation and therefore allowing for up to 30 h before processing. There was a nearly a 2-fold increase in the therapeutic target (1.9 nmol half-cystine/mg protein) range, established using distributions of patient, obligate heterozygote, and control samples. The 2.5-97.5 percentile ranges (-2 SD to +2 SD around mean) for these cohorts were 0.67-6.05 nmol/mg protein for patients, 0.33-1.35 nmol/mg protein for obligate heterozygotes, and 0.09-0.35 nmol/mg protein for controls.

CONCLUSIONS

The intracellular cystine determination method using immunopurified granulocytes followed by LC-MS/MS analysis improves the inherent variability of mixed leukocyte analysis and eliminates the need for immediate sample preparation following blood draw.

CTNS mutations in publicly-available human cystinosis cell lines

Patient samples play an important role in the study of inherited metabolic disorders. Open-access biorepositories distribute such samples. Unfortunately, not all clinically-characterized samples come with reliable genotype information. During studies directed toward population frequency assessments of cystinosis, a rare heritable disorder, we sequenced the CTNS gene from 14 cystinosis-related samples obtained from the Coriell Cell Repository. As a result, the disease genotypes of 7 samples were determined for the first time. The reported disease genotypes of 2 additional samples were found to be incorrect. Furthermore, we identified and experimentally confirmed a novel mutation, c.225 + 5G > A, which causes skipping of the 5th exon and is associated with infantile nephropathic cystinosis.

Excellent long-term outcome of renal transplantation in cystinosis patients

Background

Cystinosis is a rare lysosomal disorder leading to end stage renal disease in more than 90 % of patients before 20 years of age. Data about safety and efficiency of renal transplantation in patients with cystinosis is scarce. We evaluated long-term outcomes of renal transplantation in adult patients with cystinosis.

Methods

Data of renal transplantation (n = 31) in 30 adult patients with cystinosis in 5 French university transplant centers between 1980 and 2013 were retrospectively analyzed. A control cohort of 93 patients was matched for age, graft date, living/deceased donor status and transplant center.

Results

Median age at transplantation was 20.4 years (7–36.5). At transplantation, all patients with cystinosis had corneal cystine deposits, 3 had diabetes and 7 had hypothyroidism. Graft survival was better in patients with cystinosis than in control patients (p = 0.013). Multivariate analysis confirmed that cystinosis was an independent protective factor for graft survival (Hazard Ratio (HR) 0.11; CI95 [0.02-0.61]). Specific complications of cystinosis occurred during follow up: diabetes mellitus (n = 4), hypothyroidism (n = 1), liver involvement (n = 1), neurologic involvement (n = 2). Proportion of post-transplant diabetes mellitus (PTDM) was not statistically different in cystinosis group compared to control group: 4 (13.0 %) compared to 5 (5.0 %), respectively (p = 0.25), with no differences regarding calcineurin inhibitors and steroids treatments during follow-up.

Conclusions

Renal transplantation appears to be safe with excellent long-term outcomes in patients with cystinosis. These patients may receive standard immunosuppressive regimens with steroids and calcineurin inhibitors.

Genetic basis of cystinosis in Tunisian patients: Identification of novel mutation in CTNS gene

Nephropathic cystinosis (NC) is an autosomal recessive disorder characterized by defective transport of cystine across the lysosomal membrane and resulting in renal, ophthalmic, and other organ abnormalities. Mutations in the CTNS gene cause a deficiency of the transport protein, cystinosin. This study was performed to investigate mutations of the CTNS gene in three Tunisian families with NC. Polymerase chain reaction (PCR), ARMS multiplex PCR and direct sequencing were performed for molecular characterization of the CTNS gene in 3 unrelated Tunisian patients and their parents. Based on family history, prenatal diagnosis (PND) was performed in fetal DNA isolated from chorionic villi obtained at 10-12  weeks of gestation. None of the patients showed the most common 57-kb deletion in heterozygous or homozygous status. One patient was homozygous for the previously reported mutation c.1515G > A (p.G308R). One patient presented the novel gross deletion of 20,327 bp. One was homozygote for the previously reported mutation c.771_793del (p.Gly258Serfs*30). In addition, eight polymorphisms were identified in the 3 patients and their parents. The prenatal diagnosis in one family showed that the fetus DNA was heterozygous for the c.771_793del (p.Gly258Serfs*30) mutation. This study expands the mutational and population spectrum of NC, representing the first molecular diagnosis of NC in Tunisian population. The mutation screening of the CTNS gene was used for prenatal diagnosis to prevent and/or limit this inheritable disease in our country where the families are particularly large and have a high rate of consanguinity.

Hirschsprung's disease with infantile nephropathic cystinosis

The case of a 3-year-old boy diagnosed to have Hirschsprung's disease with infantile nephropathic cystinosis is being reported. Both these conditions are etiologically and genetically different as per current understanding and available information. The association is incidental and has not reported before in the English literature.

Common mutation causes cystinosis in the majority of black South African patients

BACKGROUND

The mutations responsible for cystinosis in South African patients are currently unknown. A pertinent question is whether they are similar to those described elsewhere in the world.

METHODS

Children who were being managed for cystinosis in the Western Cape Province of South Africa between 2002 and 2013 were studied. All underwent molecular analysis to detect sequence variations in the cystinosis gene.

RESULTS

This cohort study included 20 patients, 13 of whom were Xhosa-speaking black South Africans and seven were Cape Coloureds (mixed race); none were Caucasian. All had nephropathic infantile-type cystinosis with evidence of proximal tubulopathy, with glycosuria and renal phosphate wasting. Diagnosis was confirmed in 19 cases by demonstrating an elevated cystine concentration in leukocytes. Molecular analysis of the cystinosin gene revealed that 19 patients had a G > A mutation in intron 11 (CTNS-c.971-12G > A p.D324AfsX44) which caused an out-of-frame 10-bp insertion. Of these 19 patients, 16 were homozygous for this mutation, which was the most frequent mutation identified in the alleles of the black South African and Cape Coloured patients (96 and 71 %, respectively).

CONCLUSION

We recommend that black South African and Cape Coloured patients presenting with cystinosis be tested for CTNS-c.971-12G > A in the first instance, with the possibility of prenatal testing being offered to at-risk families.

Cystinosis in adult and adolescent patients: Recommendations for the comprehensive care of cystinosis

INTRODUCTION

Cystinosis is a rare lysosomal systemic disease that mainly affects the kidney and the eye. Patients with cystinosis begin renal replacement therapy during the first decade of life in absence of treatment. Prognosis of cystinosis depends on early diagnosis, and prompt starting and good compliance with cysteamine treatment. Kidney disease progression, extra-renal complications and shorter life expectancy are more pronounced in those patients that do not follow treatment. The objective of this work was to elaborate recommendations for the comprehensive care of cystinosis and the facilitation of patient transition from paediatric to adult treatment, based on clinical experience. The goal is to reduce the impact of the disease, and to improve patient quality of life and prognosis.

METHODS

Bibliographic research and consensus meetings among a multidisciplinary professional team of experts in the clinical practice, with cystinotic patients (T-CiS.bcn group) from 5 hospitals located in Barcelona.

RESULTS

This document gathers specific recommendations for diagnosis, treatment and multidisciplinary follow-up of cystinotic patients in the following areas: nephrology, dialysis,renal transplant, ophthalmology, endocrinology, neurology, laboratory, genetic counselling,nursing and pharmacy.

CONCLUSIONS

A reference document for the comprehensive care of cystinosis represents a support tool for health professionals who take care of these patients. It is based on the following main pillars: (a) a multi-disciplinary approach, (b) appropriate disease monitoring and control of intracellular cystine levels in leukocytes, (c) the importance of adherence to treatment with cysteamine, and (d) the promotion of patient self-care by means of disease education programmes. All these recommendations will lead us, in a second phase, to create a coordinated transition model between paediatric and adult care services which will contemplate the specific needs of cystinosis.

Cystinosis: the evolution of a treatable disease

Cystinosis is a rare autosomal recessive disorder involving lysosomal storage of the amino acid cystine due to a defect in the membrane transport protein, cystinosin. Since the introduction of kidney transplants and the availability of cystine-depleting medical therapy, this previously fatal disease was transformed into a treatable disorder. Renal allografts and medical therapy targeting the basic metabolic defect have altered the natural hisotry of cystinosis so drastically that patients have a life expectancy extending past 50 years. Consequently, early diagnosis and appropriate therapy are critically important. In this article, we offer a review of the manifestations of cystinosis, including the proximal tubular dysfunction of renal Fanconi syndrome, and discuss the prevention and treatment of the disorder's systemic complications. We focus on the nephropathic forms of cystinosis, aiming to assist nephrologists and other physicians to develop early recognition and appropriate management of cystinosis patients.

Stem cell microvesicles transfer cystinosin to human cystinotic cells and reduce cystine accumulation in vitro

Cystinosis is a rare disease caused by homozygous mutations of the CTNS gene, encoding a cystine efflux channel in the lysosomal membrane. In Ctns knockout mice, the pathologic intralysosomal accumulation of cystine that drives progressive organ damage can be reversed by infusion of wildtype bone marrow-derived stem cells, but the mechanism involved is unclear since the exogeneous stem cells are rarely integrated into renal tubules. Here we show that human mesenchymal stem cells, from amniotic fluid or bone marrow, reduce pathologic cystine accumulation in co-cultured CTNS mutant fibroblasts or proximal tubular cells from cystinosis patients. This paracrine effect is associated with release into the culture medium of stem cell microvesicles (100–400 nm diameter) containing wildtype cystinosin protein and CTNS mRNA. Isolated stem cell microvesicles reduce target cell cystine accumulation in a dose-dependent, Annexin V-sensitive manner. Microvesicles from stem cells expressing CTNS^Red transfer tagged CTNS protein to the lysosome/endosome compartment of cystinotic fibroblasts. Our observations suggest that exogenous stem cells may reprogram the biology of mutant tissues by direct microvesicle transfer of membrane-associated wildtype molecules.

Cognition in nephropathic cystinosis: pattern of expression in heterozygous carriers

Individuals with cystinosis exhibit specific cognitive deficits in visual spatial function. The purpose of the current study was to examine if obligate heterozygotes of the CTNS mutation have the same pattern of cognitive functioning seen in homozygotes, namely aberrant visual-spatial functioning against a background of relatively intact visual-perceptual functioning and overall cognitive ability. Study participants were 254 adults (100 heterozygotes and 154 controls), ages 17 years 10 months through 74 years 9 months. Tests of intelligence, visual perceptual, and visual spatial functioning were administered. Our results showed that cystinosis heterozygotes demonstrated intelligence within the normal range, and performed similarly to controls on tests of visual-perceptual ability. In contrast, the heterozygotes performed significantly more poorly on each of the visual-spatial tests when compared to controls. Obligate heterozygotes for the CTNS mutation display a similar pattern of visual processing decrements as do individuals with cystinosis. Namely, carriers demonstrate relative weaknesses in visual-spatial processing, while maintaining normal visual perceptual ability and intelligence in the normal range. The visual spatial decrements in heterozygotes were not as marked as those found in individuals with cystinosis, suggesting a gene dosing effect. This study provides an impetus for other studies of gene-behavior relationships in recessive disorders, and may stimulate further interest in the role of aberrant genes on "individual differences" in behavior.

Genetic basis of cystinosis in Turkish patients: a single-center experience

We report the molecular findings for the CTNS gene in 12 Turkish cystinosis patients aged 7-29 years. All presented initially with severe failure to thrive, polyuria, and polydipsia. Cystinosis was diagnosed at age 1 month to 9 years. Seven patients reached end-stage renal failure at ages ranging from 6.5 to 15 years. Whereas three of the remaining five have renal Fanconi syndrome with proteinuria, two have had kidney failure of varying degrees. Molecular analyses involved an initial multiplex polymerase chain reaction (PCR) to determine the presence or absence of the 57-kb northern European founder deletion in CTNS, followed by sequencing of the ten coding exons of CTNS. Comprehensive mutation analysis verified that none of the 12 patients carried the common 57-kb deletion. We identified four previously reported nucleotide variations associated with cystinosis and five new variants: a 10-kb deletion, three missense variants, and a nucleotide substitution in a potential branch point site of intron 4. This study is the first molecular analysis of Turkish cystinosis patients and provides guidance for the molecular diagnosis of cystinosis in this population.

The pathogenesis of cystinosis: mechanisms beyond cystine accumulation

Renal proximal tubules are highly sensitive to ischemic and toxic insults and are affected in diverse genetic disorders, of which nephropathic cystinosis is the most common. The disease is caused by mutations in the CTNS gene, encoding the lysosomal cystine transporter cystinosin, and is characterized by accumulation of cystine in the lysosomes throughout the body. In the majority of the patients, this leads to generalized proximal tubular dysfunction (also called DeToni-Debré-Fanconi syndrome) in the first year and progressive renal failure during the first decade. Extrarenal organs are affected by cystinosis as well, with clinical symptoms manifesting mostly after 10 yr of age. The cystine-depleting agent cysteamine significantly improves life expectancy of patients with cystinosis, but offers no cure, pointing to the complexity of the disease mechanism. In this review, current knowledge on the pathogenesis of cystinosis is described and placed in perspective of future research.

An Indian boy with nephropathic cystinosis: a case report and molecular analysis of CTNS mutation

Cystinosis is a rare autosomal recessive lysosomal storage disorder characterized by excessive accumulation of cystine within the lysosome. Cystinosis is caused by mutations in the lysosomal cystine transporter, cystinosin (CTNS). The CTNS gene consists of 12 exons and encodes for an integral lysosomal membrane protein with seven transmembrane domains. A majority of cystinotic patients are of European descents, and only a few cases have been reported from other ethnic groups. Here we report a case of nephropathic cystinosis in an Indian boy born to consanguineous parents. Major symptoms of the patient include weight loss, vomiting, dehydration, and cystine crystals in the cornea. Ichthyosis on the arms and legs is also observed. Sequencing analysis of all the CTNS exons revealed that the proband is homozygous for a 3-bp in-frame deletion in exon 10 (c.809_811delCCT), resulting in the loss of a conserved p.Ser270del within the fifth transmembrane domain of CTNS. His parents are both heterozygous for the same mutation. This work represents the first molecular characterization of cystinotic patients from India. Interestingly, a p.Ser270del resulting from c.809_811delCCT in CTNS had been identified in a European patient. Therefore, it appears that this mutation arose independently in the two different continents.

Analysis of the CTNS gene in nephropathic cystinosis Mexican patients: report of four novel mutations and identification of a false positive 57-kb deletion genotype with LDM-2/exon 4 multiplex PCR assay

OBJECTIVE

Identify CTNS gene mutations in nephropathic cystinosis Mexican patients.

SUBJECTS AND METHODS

Eleven patients were included, nine presenting infantile nephropathic cystinosis and two siblings with the juvenile phenotype. The common 57-kb deletion was detected by multiplex PCR using large deletion marker-2 (LDM-2)/exon 4 set primers. Those alleles negative for 57-kb deletion were screened by single strand confirmation polymorphism (SSCP) and subsequent direct sequencing.

RESULTS

In our sample, five mutations previously reported are identified: 57-kb deletion, EX4_EX5del, c.985_986insA, c.357_360delGACT, and c.537_557del. We detect a false assignation of 57-kb deletion homozygous genotype by using the LDM-2/exon 4 primers. In addition, four novel and severe mutations are identified: c.379delC, c.1090_1093delACCAinsCG, c.986C>G (p.T216R), and c.400+5G>A.

CONCLUSIONS

Our sample of Mexican patients display allelic heterogeneity as compared to European or North American cystinosis cases. The identification of novel mutations might suggest the presence of exclusive American CTNS alleles in Mexican population. In order to prevent the false positive assignation of 57-kb deletion genotype, as caused by the presence of another type of intragenic CTNS gross deletion, we propose to analyze a different control CTNS exon to those originally reported in both LDM multiplex PCR assays, especially when parental DNA samples are not available.

Nodular regenerative hyperplasia and severe portal hypertension in cystinosis

BACKGROUND & AIMS

Cystinosis is a rare autosomal-recessive disorder characterized by the intralysosomal accumulation of cystine, which is responsible for widespread tissue destruction. Liver biopsy specimens of patients with cystinosis show cystine crystal formation in Kupffer cells. However, significant liver disease and portal hypertension is not a common complication of cystinosis. We report the case histories of 2 young men with poorly treated nephropathic cystinosis who developed noncirrhotic portal hypertension with evidence of nodular regenerative hyperplasia (NRH).

METHODS

Liver biopsy examinations, upper and lower endoscopy with biopsy examination, imaging studies, venous pressure measurements, and laboratory investigations were used to evaluate the causes of the liver disease and portal hypertension.

RESULTS

Histologic examination of liver biopsy specimens from both patients showed changes characteristic of NRH with portal hypertension documented by measurement of pressure gradients. In addition, endoscopy in the first patient showed varices and portal hypertensive gastropathy.

CONCLUSIONS

NRH was confirmed by histologic examination of the liver in both patients and is the likely cause of their portal hypertension. NRH may represent a rare, late complication of cystinosis, although the mechanism remains undefined.

Swallowing dysfunction in 101 patients with nephropathic cystinosis: benefit of long-term cysteamine therapy

Nephropathic cystinosis is a rare, autosomal recessive lysosomal storage disorder caused by mutations in the CTNS gene that codes for a cystine transporter in the lysosomal membrane. Affected patients store 50-100 times the normal amounts of cystine in their cells, and suffer renal tubular and glomerular disease, growth retardation, photophobia, and other systemic complications, including a myopathy and swallowing dysfunction. Using videofluoroscopy and ultrasound examinations, we assessed the swallowing function of 101 patients with nephropathic cystinosis on their most recent admission to the National Institutes of Health Clinical Center between 1987 and 2004. These patients ranged in age from 6 to 45 years; more than half had significant complaints of swallowing difficulty. On examination of barium swallow, the oral, pharyngeal, and esophageal phases of swallowing were abnormal in 24%, 51%, and 73% of patients, respectively. The frequency of dysfunction increased with age for each phase of swallowing. Both the Swallowing Severity Score (a measure of dysfunction on barium swallow) and the Oral Muscle Composite Score (a reflection of vocal strength, oral-facial movement, and tongue and lip function) increased (that is, worsened) with the number of years that a patient was not receiving treatment with cysteamine, the cystine-depleting agent of choice in cystinosis. The severity scores decreased with the number of years on cysteamine therapy. The Swallowing Severity Score varied directly with the severity of muscle disease, but was not correlated with the presence or absence of the 57-kb CTNS deletion that commonly occurs in nephropathic cystinosis patients. We conclude that swallowing dysfunction in cystinosis presents a risk of fatal aspiration, correlates with the presence of muscle atrophy, and, based on cross-sectional data, increases in frequency with age and number of years without cysteamine treatment. Cystine-depleting therapy with cysteamine should be considered the treatment of choice for both pre- and posttransplant cystinosis patients.

Cellular, molecular and clinical characterization of patients with Hermansky-Pudlak syndrome type 5

Hermansky-Pudlak syndrome (HPS) is a disorder of lysosome-related organelles such as melanosomes and platelet dense granules. Seven genes are now associated with HPS in humans. An accurate diagnosis of each HPS subtype has important prognostic and treatment implications. Here we describe the cellular, molecular, and clinical aspects of the recently identified HPS-5 subtype. We first analyzed the genomic organization and the RNA expression pattern of HPS5, located on chromosome 11p14, and demonstrated tissue-specific expression of at least three alternatively spliced HPS5 mRNA transcripts, coding for HPS5A and HPS5B proteins, that differ at their 5'-ends. Genetic screening of 15 unassigned HPS patients yielded six new HPS5 mutations in four patients. Clinically, our HPS-5 patients exhibited iris transillumination, variable hair and skin pigmentation, and absent platelet dense bodies, but not pulmonary fibrosis or granulomatous colitis. In two patients with homozygous missense mutations, hemizygosity was ruled out by gene-dosage multiplex polymerase chain reaction, and immunocytochemical analyses of their fibroblasts supported the HPS-5 diagnosis. Specifically, LAMP-3 distribution was restricted to the perinuclear region in HPS-5 fibroblasts, in contrast to the normal LAMP-3 distribution, which extended to the periphery. This specific intracellular vesicle distribution in fibroblasts, in combination with the clinical features, will improve the characterization of the HPS-5 subtype.

A new mutation in two siblings with cystinosis presenting with Bartter syndrome

Nephropathic cystinosis is a severe autosomal recessive inherited metabolic disease characterized by accumulation of free cystine in lysosomes. Cystinosis can lead to renal failure and multiorgan impairment. Only five cases of cystinosis with associated Bartter syndrome are reported in the literature, and no genetic evaluation has been reported. We describe two siblings with nephropathic cystinosis presenting with features of Bartter syndrome and their genetic pattern.

Pharmacological treatment of nephropathic cystinosis with cysteamine

Cystinosis, clinically recognised since 1903, is an autosomal recessive lysosomal storage disease caused by mutations in CTNS. This gene codes for a lysosomal cystine transporter, whose absence leads to intracellular cystine crystals, widespread cellular destruction, renal Fanconi syndrome in infancy, renal glomerular failure in later childhood and other systemic complications. Before the availability of kidney transplantation, patients affected with cystinosis uniformly died during childhood. After solid organ transplantations became successful in the 1960s, cystinosis patients survived, but eventually developed life-threatening consequences of the disease (e.g., swallowing disorders). Since the introduction of cysteamine into the pharmacological management of cystinosis, well-treated adolescent and young adult patients have experienced normal growth and maintenance of renal glomerular function. Oral cysteamine therapy is given at doses of 60 - 90 mg/kg/day q.i.d. every 6 h, and generally achieves approximately 90% depletion of cellular cystine, as measured in circulating leucocytes. Cysteamine (and kidney transplantation) have commuted the death sentence of cystinosis into a nearly normal life with a chronic disease. Because treatment with oral cysteamine can prevent, or significantly delay, the complications of cystinosis, early and accurate diagnosis, as well as proper treatment, is critical.

Long-term follow-up of well-treated nephropathic cystinosis patients

We report the excellent clinical outcomes of siblings with nephropathic cystinosis treated diligently with cysteamine starting at 20 months and 2 months of age. Now 15 and 8 years old, they have glomerular filtration rates of 78 and 105 mL/min/1.73m 2 , respectively. These cases illustrate the critical importance of early diagnosis and treatment.

FISH diagnosis of the common 57-kb deletion in CTNS causing cystinosis

Cystinosis is an autosomal recessive lysosomal storage disease caused by mutations in CTNS. The most prevalent CTNS mutation, a 57-kb deletion, occurs in approximately 60% of patients in the United States and northern Europe and removes exons 1-9, most of exon 10, the CTNS promoter region, and all of an adjacent gene of unknown function called CARKL. CTNS codes for the lysosomal cystine transporter, whose absence leads to intracellular cystine accumulation, widespread cellular destruction, renal Fanconi syndrome in infancy, renal glomerular failure in later childhood, and other systemic complications. Because treatment with oral cysteamine can prevent or delay these complications significantly, early and accurate diagnosis is critical. This study describes the generation of fluorescence in situ hybridization (FISH) probes for the 57-kb deletion in CTNS, enabling cytogenetics laboratories to test for this common mutation. The probes would also be able to detect a less frequent 11.7-kb deletion. A blinded study was performed using multiplex PCR analysis as the gold standard to determine the presence or absence of the 57-kb deletion. The FISH probes, evaluated on 12 lymphoblastoid cell lines from singly deleted, doubly deleted, and nondeleted patients, made the correct diagnosis in every case. This appears to be the first FISH-based diagnostic method described for any lysosomal storage disorder. It can assist in the antenatal and perinatal diagnosis of cystinosis and promote earlier salutary therapy with cysteamine.

Early oral cysteamine therapy for nephropathic cystinosis

Nephropathic cystinosis is an autosomal recessive lysosomal storage disorder in which intracellular cystine accumulates due to impaired transport out of lysosomes. The clinical manifestations include renal tubular Fanconi syndrome in the 1st year of life, with hypophosphatemic rickets, hypokalemia, polyuria, dehydration and acidosis, growth retardation, hypothyroidism, photophobia, renal glomerular deterioration by 10 years of age, and late complications such as myopathy, pancreatic insufficiency, and retinal blindnesss. The cystinosis gene, CTNS, codes for cystinosin, a 367 amino acid protein with seven transmembrane domains. More than 50 CTNSmutations have been identified, but approximately 50% of Northern European patients have a 57257-bp deletion which removes the first nine exons of CTNS. The mainstay of cystinosis therapy is oral cysteamine (Cystagon). This aminothiol can lower intracellular cystine content by 95%, and has proven efficacy in delaying renal glomerular deterioration, enhancing growth, preventing hypothyroidism, and lowering muscle cystine content. Its early and diligent use is critical; in one study, for every month of treatment prior to 3 years of age, 14 months' worth of later renal function were preserved. Several examples of individual patients treated early and having preserved renal function and normal growth are available. Newborn screening using a chip containing cDNA to detect common CTNSmutations may allow diagnosis and treatment in the first weeks of life.

CONCLUSIONS

Early diagnosis and treatment of nephropathic cystinosis can change the course of this disease.

Mutational spectrum of the CTNS gene in Italy

Classic nephropathic or infantile cystinosis (NC) is an autosomal recessive disorder; the gene coding for the integral membrane protein cystinosin, which is responsible for membrane transport of cystine (CTNS), was cloned. Mutation analysis of the CTNS gene of Caucasian patients revealed a common 57-kb deletion, and several other mutations spread throughout the entire gene. In the present study, we report the CTNS mutations identified in 42 of 46 Italian families with NC. The percentage of mutations characterized in this study is 86%. The mutational spectrum of the Italian population is different from that of populations of North European origin: the 57-kb deletion is present in a lower percentage, while the splicing mutations represent 30% of mutation detected in our sample. In all, six novel mutations have been identified, and the origin of one recurrent mutation has been traced.

New aspects of the pathogenesis of cystinosis

Cystinosis is a lysosomal transport disorder characterized by an intra-lysosomal accumulation of cystine, the disulfide of the amino acid cysteine. It is the most common inherited cause of the renal Fanconi syndrome. There are various clinical forms, infantile, juvenile, and ocular, based on age of onset and severity of symptoms. The first clinical description appeared in the early 1900s, but it was not until 1998 that the causative gene, CTNS, was identified. CTNS encodes cystinosin, a novel seven transmembrane domain (TM) protein. Cystinosin is a lysosomal membrane protein that requires two lysosomal targeting signals: a classic GYDQL motif in its C-terminal tail and a novel conformational motif, the core of which is YFPQA, situated in the fifth inter-TM loop. Cystinosin is the lysosomal cystine transporter and its activity is H(+)-driven. A mouse model of cystinosis was recently generated and Ctns(-/-) mice accumulate cystine in all tissues. A high level of cystine accumulates in the kidney, but these mice do not present with proximal tubulopathy or renal dysfunction. The Ctns(-/-) mouse model may provide clues to the cause of the Fanconi syndrome associated with cystinosis, the origin of which remains poorly understood.

CTNS mutations in African American patients with cystinosis

Cystinosis, an autosomal recessive lysosomal storage disorder, is rarely diagnosed in African Americans. The disease results from mutations in the gene CTNS; at least 55 such mutations have been reported. By far the most common is a 57,257-bp deletion of Northern European origin encompassing most of the CTNS gene. We performed mutation analysis on DNA from four African American patients with cystinosis. In one individual with classical, nephropathic cystinosis, we identified a new molecular defect, i.e., a homozygous GT-->CC substitution at the +5 position of IVS 5 of CTNS (IVS 5+5 GT-->CC). The out-of-frame splicing of exon 5 creates a null allele consistent with the patient's severe phenotype. Two patients were heterozygous and one homozygous for the common 57-kb deletion allele, reflecting the admixture of African and Northern European gene pools in North America. The two African Americans heterozygous for the 57-kb deletion were also hemizygous for a 928G-->A change, associated with ocular or nonnephropathic cystinosis. These two individuals are the only known African Americans with ocular cystinosis. We conclude that the diagnosis of cystinosis should be entertained in African Americans with symptoms of the disease, and that mutation analysis for the 57-kb deletion should be considered in this group of patients.

Characterization of a putative founder mutation that accounts for the high incidence of cystinosis in Brittany

Cystinosis is an autosomal recessive disorder, characterized by an accumulation of intralysosomal cystine, with an incidence of 1 in 100,000 to 200,000 live births. A higher incidence of cystinosis, 1 in 26,000 live births, has been reported in the western French province of Brittany. PCR amplification and sequencing has identified a 27-bp deletion starting 3 bp before the end of exon 8 and continuing into intron 8, 898-900+24del27, which has only been detected in families from this region. Reverse transcription-PCR amplification of RNA from an affected individual has shown that this mutation is indeed a splice-site mutation and results in the production of aberrant transcripts. These transcripts are predicted to either severely truncate cystinosin or alter its topology, thus accounting for the severe phenotype of these individuals. The mutation 898-900+24del27 has been identified in 7 of 18 alleles studied. This mutation is likely to be a founder mutation and would account for the higher incidence of cystinosis in Brittany.(1)

The promoter of a lysosomal membrane transporter gene, CTNS, binds Sp-1, shares sequences with the promoter of an adjacent gene, CARKL, and causes cystinosis if mutated in a critical region

Although >55 CTNS mutations occur in patients with the lysosomal storage disorder cystinosis, no regulatory mutations have been reported, because the promoter has not been defined. Using CAT reporter constructs of sequences 5' to the CTNS coding sequence, we identified the CTNS promoter as the region encompassing nucleotides -316 to +1 with respect to the transcription start site. This region contains an Sp-1 regulatory element (GGCGGCG) at positions -299 to -293, which binds authentic Sp-1, as shown by electrophoretic-mobility-shift assays. Three patients exhibited mutations in the CTNS promoter. One patient with nephropathic cystinosis carried a -295 G-->C substitution disrupting the Sp-1 motif, whereas two patients with ocular cystinosis displayed a -303 G-->T substitution in one case and a -303 T insertion in the other case. Each mutation drastically reduced CAT activity when inserted into a reporter construct. Moreover, each failed either to cause a mobility shift when exposed to nuclear extract or to compete with the normal oligonucleotide's mobility shift. The CTNS promoter region shares 41 nucleotides with the promoter region of an adjacent gene of unknown function, CARKL, whose start site is 501 bp from the CTNS start site. However, the patients' CTNS promoter mutations have no effect on CARKL promoter activity. These findings suggest that the CTNS promoter region should be examined in patients with cystinosis who have fewer than two coding-sequence mutations.

The molecular basis of Dutch infantile nephropathic cystinosis

Infantile nephropathic cystinosis, an inborn error of metabolism with an autosomal recessive inheritance pattern, is characterized by lysosomal storage of the amino acid cystine due to an impaired transport of cystine out of the lysosomes. Initial clinical features consist of the renal Fanconi syndrome and crystals in the cornea. Oral therapy with cysteamine lowers the intracellular cystine content. Recently, the gene coding for the integral membrane protein cystinosin, which is responsible for membrane transport of cystine (CTNS), was cloned. Mutation analysis of the CTNS gene of Caucasian patients revealed a common 57-kb deletion, and several other mutations spread throughout the entire gene. In the present study, we developed an improved screening method for the detection of the common 57-kb deletion. By use of this method we detected the 57-kb deletion in 59% of the examined Dutch alleles. The remaining alleles were screened for other mutations by genomic sequencing of the different exons, revealing three previously described mutations. Furthermore, we studied a possible genotype-phenotype relation of the homozygous deleted patients, which could not be demonstrated in our study population. Next to biochemical determination of cystine in leukocytes or fibroblasts, molecular genetic analysis enables prenatal diagnosis and facilitates identification of carriers.

Pulmonary dysfunction in adults with nephropathic cystinosis

OBJECTIVE

To characterize the pulmonary dysfunction in patients with nephropathic cystinosis after renal transplantation.

DESIGN

Cross-sectional analysis of consecutive adult patients.

PATIENTS

Twelve adult, nephropathic cystinosis patients and 3 adult, ocular, nonnephropathic cystinosis patients admitted to the National Institutes of Health Clinical Center.

RESULTS

The 12 nephropathic cystinosis patients (age range, 21 to 40 years) showed an extraparenchymal pattern of restrictive lung disease, with inspiratory and expiratory dysfunction. Specifically, the mean FVC was 58% of predicted, the mean FEV(1) was 57% of predicted, and the mean total lung capacity was 66% of predicted, while the mean residual volume was normal. Furthermore, the mean maximal inspiratory pressure for the eight patients tested was 40% of predicted, and the mean maximal expiratory pressure was 26% of predicted. Two patients died of respiratory insufficiency. All the patients had lived at least 17 years, while lacking compliant cystine-depleting therapy with oral cysteamine. Seven patients had a conical chest, restricting excursion, and 10 of the 12 patients had evidence of the myopathy that typifies late cystinosis. In fact, the severity of pulmonary disease correlated directly with the severity of myopathy in our group of 12 patients. In contrast, the lung parenchyma was essentially normal, as gauged by chest radiographs and CT scans of the lung. The three patients with nonnephropathic cystinosis displayed entirely normal pulmonary function.

CONCLUSION

The distal myopathy characteristic of nephropathic cystinosis results in an extraparenchymal pattern of restrictive lung disease in adults who have not received long-term cystine depletion. Whether or not oral cysteamine therapy can prevent this complication remains to be determined.

Corneal crystals in nephropathic cystinosis: natural history and treatment with cysteamine eyedrops

Although renal disease is the most prominent feature of the lysosomal storage disease cystinosis, corneal cystine crystal formation remains a major complication, leading to photophobia, corneal erosions, and keratopathies. Moreover, the extent of corneal crystal accumulation reflects the course and severity of the disease itself, and the cornea is accessible to direct examination. Therefore, we employed a scoring system, based on a library of slit-lamp photographs of corneas with increasing crystal densities (0.00-3.00), to assess the degree of crystal accumulation in 170 patients with nephropathic cystinosis examined at the National Institutes of Health between 1976 and 2000. None of the patients had received topical cystine-depleting therapy at the time of the evaluation. In this natural history study, infants in the first year of life had absent or minimal corneal crystals, i.e., a corneal cystine crystal score (CCCS) of 0 or 0.25. However, the CCCS increased linearly with age, such that every patient had visible crystals by 16 months of age, and plateaued at approximately 3.00 by early adolescence. Longitudinal studies in representative patients support the cross-sectional results. Individuals homozygous for the common 57-kb deletion involving the cystinosis gene (CTNS) displayed the same course of corneal crystal accumulation as did individuals not bearing the large deletion. Patients with ocular or nonnephropathic cystinosis had CCCSs that were, in general, half those expected for patients with nephropathic cystinosis of the same age. Administration of 0.55% cysteamine eyedrops, given 6 to 12 times per day, dissolved corneal cystine crystals in 10 representative patients with nephropathic cystinosis aged 1 to 32 years within 8 to 41 months.

Lysosomal transport disorders

In the group of lysosomal storage diseases, transport disorders occupy a special place because they represent rare examples of inborn errors of metabolism caused by a defect of an intracellular membrane transporter. In particular, two disorders are caused by a proven defect in carrier-mediated transport of metabolites: cystinosis and the group of sialic acid storage disorders (SASD). The recent identification of the gene mutations for both disorders will improve patient diagnosis and shed light on new physiological mechanisms of intracellular trafficking.