Nlrp2 deletion ameliorates kidney damage in a mouse model of cystinosis

Cystinosis is a rare autosomal recessive disorder caused by mutations in the CTNS gene that encodes cystinosin, a ubiquitous lysosomal cystine/H+ antiporter. The hallmark of the disease is progressive accumulation of cystine and cystine crystals in virtually all tissues. At the kidney level, human cystinosis is characterized by the development of renal Fanconi syndrome and progressive glomerular and interstitial damage leading to end-stage kidney disease in the second or third decade of life. The exact molecular mechanisms involved in the pathogenesis of renal disease in cystinosis are incompletely elucidated. We have previously shown upregulation of NLRP2 in human cystinotic proximal tubular epithelial cells and its role in promoting inflammatory and profibrotic responses. Herein, we have investigated the role of NLRP2 in vivo using a mouse model of cystinosis in which we have confirmed upregulation of Nlrp2 in the renal parenchyma. Our studies show that double knock out Ctns-/- Nlrp2-/- animals exhibit delayed development of Fanconi syndrome and kidney tissue damage. Specifically, we observed at 4-6 months of age that animals had less glucosuria and calciuria and markedly preserved renal tissue, as assessed by significantly lower levels of inflammatory cell infiltration, tubular atrophy, and interstitial fibrosis. Also, the mRNA expression of some inflammatory mediators (Cxcl1 and Saa1) and the rate of apoptosis were significantly decreased in 4-6-month old kidneys harvested from Ctns-/- Nlrp2-/- mice compared to those obtained from Ctns-/-mice. At 12-14 months of age, renal histological was markedly altered in both genetic models, although double KO animals had lower degree of polyuria and low molecular weight proteinuria and decreased mRNA expression levels of Il6 and Mcp1. Altogether, these data indicate that Nlrp2 is a potential pharmacological target for delaying progression of kidney disease in cystinosis.

Functional analysis of the CTNS gene exonic variants predicted to affect splicing

Cystinosis is a severe, monogenic systemic disease caused by variants in CTNS gene. Currently, there is growing evidence that exonic variants in many diseases can affect pre-mRNA splicing. The impact of CTNS gene exonic variants on splicing regulation may be underestimated due to the lack of routine studies at the RNA level. Here, we analyzed 59 exonic variants in the CTNS gene using bioinformatics tools and identified candidate variants that may induce splicing alterations by minigene assays. We identified six exonic variants that induce splicing alterations by disrupting the ratio of exonic splicing enhancers/exonic splicing silencers (ESEs/ESSs) or by interfering with the recognition of classical splice sites, or both. Our results help in the correct molecular characterization of variants in cystinosis and inform emerging therapies. Furthermore, our work suggests that the combination of in silico and in vitro assays facilitates to assess the effects of DNA variants driving rare genetic diseases on splicing regulation and will enhance the clinical utility of variant functional annotation.

Neurologic involvement in cystinosis: Focus on brain lesions and new evidence of four-repeat (4R-) Tau immunoreactivity

Nephropathic cystinosis is a rare autosomal recessive storage disorder caused by CTNS gene mutations, leading to autophagy-lysosomal pathway impairment and cystine crystals accumulation. Neurologic involvement is highly variable and includes both neurodevelopmental and neurodegenerative disturbances, as well as focal neurologic deficits. By presenting longitudinal data of a 28-year-old patient with a large infratentorial lesion, we summarized the pathology, clinical and imaging features of neurological involvement in cystinosis patients. Brain damage in form of cystinosis-related cerebral lesions occurs in advanced disease phases and is characterized by the accumulation of cystine crystals, subsequent inflammation with vasculitis-like features, necrosis, and calcification. Epilepsy is a frequent comorbidity in affected individuals. Steroids might play a role in the symptomatic treatment of "stroke-like" episodes due to edematous-inflammatory lesions, but probably do not change the overall prognosis. Lifelong compliance to depleting therapy with cysteamine still represents the main therapeutic option. However, consequences of CTNS gene defects are not restricted to cystine accumulation. New evidence of four-repeat (4R-) Tau immunoreactivity suggests concurrent progressive neurodegeneration in cystinosis patients, highlighting the need of innovative therapeutic strategies, and shedding light on the crosstalk between proteinopathies and autophagy-lysosomal system defects. Eventually, emerging easily accessible biomarkers such as serum neurofilament light chains (NfL) might detect subclinical neurologic involvement in cystinosis patients.

Omic Studies on In Vitro Cystinosis Model: siRNA-Mediated CTNS Gene Silencing in HK-2 Cells

Cystinosis is an autosomal recessive disease caused by mutations in the CTNS gene encoding a protein called cystinosine, which is a lysosomal cystine transporter. Disease-causing mutations lead to accumulation of cystine crystals in the lysosomes, thereby causing dysfunction of vital organs. Determination of the increased leukocyte cystine level is one of the most used methods for diagnosis. However, this method is expensive, difficult to perform, and may yield different results in different laboratories. In this study, a disease model was created with CTNS gene-silenced HK2 cells, which can mimic cystinosis in cell culture, and multiomics methods (ie, proteomics, metabolomics, and fluxomics) were implemented at this cell culture to investigate new biomarkers for the diagnosis. CTNS-silenced cell line exhibited distinct metabolic profiles compared with the control cell line. Pathway analysis highlighted significant alterations in various metabolic pathways, including alanine, aspartate, and glutamate metabolism; glutathione metabolism; aminoacyl-tRNA biosynthesis; arginine and proline metabolism; beta-alanine metabolism; ascorbate and aldarate metabolism; and histidine metabolism upon CTNS silencing. Fluxomics analysis revealed increased cycle rates of Krebs cycle intermediates such as fumarate, malate, and citrate, accompanied by enhanced activation of inorganic phosphate and ATP production. Furthermore, proteomic analysis unveiled differential expression levels of key proteins involved in crucial cellular processes. Notably, peptidyl-prolyl cis-trans isomerase A, translation elongation factor 1-beta (EF-1beta), and 60S acidic ribosomal protein decreased in CTNS-silenced cells. Additionally, levels of P0 and tubulin α-1A chain were reduced, whereas levels of 40S ribosomal protein S8 and Midasin increased. Overall, our study, through the utilization of an in vitro cystinosis model and comprehensive multiomics approach, led to the way toward the identification of potential new biomarkers while offering valuable insights into the pathogenesis of cystinosis.

Event-related potential (ERP) evidence for visual processing differences in children and adults with cystinosis (CTNS gene mutations)

BACKGROUND

Cystinosis, a rare lysosomal storage disease caused by mutations in the CTNS gene, is characterized by cystine crystallization and accumulation within multiple tissues, including kidney and brain. Its impact on neural function appears mild relative to its effects on other organs during early disease, but since therapeutic advances have led to substantially increased life expectancy, neurological implications are of increasing interest, necessitating deeper understanding of the impact of cystinosis on neurocognitive function. Behavioral difficulties have been reported in cystinosis in the visual domain. Very little is known, however, about how the brains of people living with cystinosis process visual information. This is especially interesting given that cystine accumulation in the cornea and posterior ocular structures is a hallmark of cystinosis.

METHODS

Here, high-density scalp electrophysiology was recorded to visual stimuli (during a Go/No-Go task) to investigate visual processing in individuals with cystinosis, compared to age-matched controls. Analyses focused on early stages of cortical visual processing.

RESULTS

The groups differed in their initial cortical response, with individuals with cystinosis exhibiting a significantly larger visual evoked potential (VEP) in the 130-150 ms time window. The groups also differed in the associations between neural responses and verbal abilities: While controls with higher IQ scores presented larger neural responses, that relationship was not observed in cystinosis.

CONCLUSIONS

The enlarged VEP in cystinosis could be the result of cortical hyperexcitability and/or differences in attentional engagement and explain, at least partially, the visual and visual-spatial difficulties described in this population.

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.

Evaluation of the efficacy of cystinosin supplementation through CTNS mRNA delivery in experimental models for cystinosis

Messenger RNA (mRNA) therapies are emerging in different disease areas, but have not yet reached the kidney field. Our aim was to study the feasibility to treat the genetic defect in cystinosis using synthetic mRNA in cell models and ctns-/- zebrafish embryos. Cystinosis is a prototype lysosomal storage disorder caused by mutations in the CTNS gene, encoding the lysosomal cystine-H+ symporter cystinosin, and leading to cystine accumulation in all cells of the body. The kidneys are the first and the most severely affected organs, presenting glomerular and proximal tubular dysfunction, progressing to end-stage kidney failure. The current therapeutic standard cysteamine, reduces cystine levels, but has many side effects and does not restore kidney function. Here, we show that synthetic mRNA can restore lysosomal cystinosin expression following lipofection into CTNS-/- kidney cells and injection into ctns-/- zebrafish. A single CTNS mRNA administration decreases cellular cystine accumulation for up to 14 days in vitro. In the ctns-/- zebrafish, CTNS mRNA therapy improves proximal tubular reabsorption, reduces proteinuria, and restores brush border expression of the multi-ligand receptor megalin. Therefore, this proof-of-principle study takes the first steps in establishing an mRNA-based therapy to restore cystinosin expression, resulting in cystine reduction in vitro and in the ctns-/- larvae, and restoration of the zebrafish pronephros function.

ER-associated degradation in cystinosis pathogenesis and the prospects of precision medicine

Cystinosis is a lysosomal storage disease that is characterized by the accumulation of dipeptide cystine within the lumen. It is caused by mutations in the cystine exporter, cystinosin. Most of the clinically reported mutations are due to the loss of transporter function. In this study, we identified a rapidly degrading disease variant, referred to as cystinosin(7Δ). We demonstrated that this mutant is retained in the ER and degraded via the ER-associated degradation (ERAD) pathway. Using genetic and chemical inhibition methods, we elucidated the roles of HRD1, p97, EDEMs, and the proteasome complex in cystinosin(7Δ) degradation pathway. Having understood the degradation mechanisms, we tested some chemical chaperones previously used for treating CFTR F508Δ and demonstrated that they could facilitate the folding and trafficking of cystinosin(7Δ). Strikingly, chemical chaperone treatment can reduce the lumenal cystine level by approximately 70%. We believe that our study conclusively establishes the connection between ERAD and cystinosis pathogenesis and demonstrates the possibility of using chemical chaperones to treat cystinosin(7Δ).

Genistein improves renal disease in a mouse model of nephropathic cystinosis: a comparison study with cysteamine

Cysteamine is currently the only therapy for nephropathic cystinosis. It significantly improves life expectancy and delays progression to end-stage kidney disease; however, it cannot prevent it. Unfortunately, compliance to therapy is often weak, particularly during adolescence. Therefore, finding better treatments is a priority in the field of cystinosis. Previously, we found that genistein, an isoflavone particularly enriched in soy, can revert part of the cystinotic cellular phenotype that is not sensitive to cysteamine in vitro. To test the effects of genistein in vivo, we fed 2-month-old wild-type and Ctns-/- female mice with either a control diet, a genistein-containing diet or a cysteamine-containing diet for 14 months. Genistein (160 mg/kg/day) did not affect the growth of the mice or hepatic functionality. Compared with untreated mice at 16 months, Ctns-/- mice fed with genistein had lower cystine concentrations in their kidneys, reduced formation of cystine crystals, a smaller number of LAMP1-positive structures and an overall better-preserved parenchymal architecture. Cysteamine (400 mg/kg/day) was efficient in reverting the lysosomal phenotype and in preventing the development of renal lesions. These preclinical data indicate that genistein ameliorates kidney injury resulting from cystinosis with no side effects. Genistein therapy represents a potential treatment to improve the outcome for patients with cystinosis.

[Cystinosis: From the gene identification to the first gene therapy clinical trial]

Cystinosis is an autosomal recessive metabolic disease characterized by lysosomal accumulation of cystine in all the cells of the body. Infantile cystinosis begins in infancy by a renal Fanconi syndrome and eventually leads to multi-organ failure, including the kidney, eye, thyroid, muscle, and pancreas, eventually causing premature death in early adulthood. The current treatment is the drug cysteamine that only delays the progression of the disease. We identified the gene involved, CTNS, and showed that the encoded protein, cystinosin, is a proton-driven cystine transporter. We generated a mouse model of cystinosis, the Ctns-/- mice, that recapitulates the main disease complications. The goal was next to develop a gene therapy approach for cystinosis. We used bone marrow stem cells as a vehicle to bring the healthy CTNS gene to tissues, and we showed that wild-type hematopoietic stem and progenitor cell (HSPC) transplantation led to abundant tissue integration of bone marrow-derived cells, significant decrease of tissue cystine accumulation and long-term kidney, eye and thyroid preservation. We then developed an autologous transplantation approach of HSPCs modified ex vivo using a lentiviral vector to introduce a functional CTNS cDNA, and showed its efficacy in Ctns-/- mice. We conducted the pharmacology/toxicology studies, developed the manufacturing process using human CD34+ cells, and design the clinical trial. We received Food and Drug Administration (FDA)-clearance to start a phase 1/2 clinical trial for cystinosis in December 2018. Six patients have been treated so far. In this review, we describe the path to go from the gene to a gene therapy approach for cystinosis.

The Pitfall of White Blood Cell Cystine Measurement to Diagnose Juvenile Cystinosis

Cystinosis is an autosomal recessive lysosomal storage disease, caused by mutations in the CTNS gene, resulting in multi-organ cystine accumulation. Three forms of cystinosis are distinguished: infantile and juvenile nephropathic cystinosis affecting kidneys and other organs such as the eyes, endocrine system, muscles, and brain, and adult ocular cystinosis affecting only the eyes. Currently, elevated white blood cell (WBC) cystine content is the gold standard for the diagnosis of cystinosis. We present a patient with proteinuria at adolescent age and corneal cystine crystals, but only slightly elevated WBC cystine levels (1.31 ½ cystine/mg protein), precluding the diagnosis of nephropathic cystinosis. We demonstrate increased levels of cystine in skin fibroblasts and urine-derived kidney cells (proximal tubular epithelial cells and podocytes), that were higher than the values observed in the WBC and healthy control. CTNS gene analysis shows the presence of a homozygous missense mutation (c.590 A > G; p.Asn177Ser), previously described in the Arab population. Our observation underlines that low WBC cystine levels can be observed in patients with juvenile cystinosis, which may delay the diagnosis and timely administration of cysteamine. In such patients, the diagnosis can be confirmed by cystine measurement in slow-dividing cells and by molecular analysis of the CTNS gene.

Cystinosis: Status of research and treatment in India and the world

Cystinosis is an autosomally inherited rare genetic disorder in which cystine accumulates in the lysosome. The defect arises from a mutation in the lysosomal efflux pump, cystinosin (or CTNS). Despite the disease being known for more than a century, research, diagnosis, and treatment in India have been very minimal. In recent years, however, some research on cystinosis has been carried out on understanding the pathophysiology and in the development of a humanized yeast model for interrogating the CTNS protein. There has also been a greater awareness of the disease that has been facilitated by the formation of the Cystinosis Foundation of India just over a decade ago. Awareness among primary physicians is critical for early diagnosis, which in turn is critical for proper treatment. Eight different mutations have been observed in cystinosis patients in India, and the mutation spectrum seems different to what has been seen in the US and Europe. Despite these positive developments, there are immense hurdles still to be surmounted. This includes ensuring that the diagnosis is done sooner, making cysteamine more easily available, and, also for the future, to make accessible the promise of gene therapy to cystinosis patients.

Outcome of infantile nephropathic cystinosis depends on early intervention, not genotype: A multicenter sibling cohort study

Infantile nephropathic cystinosis (INC) is an inheritable lysosomal storage disorder characterized by lysosomal cystine accumulation, progressive kidney disease, and multiple extrarenal complications (ERCs). Cysteamine postpones the onset of end-stage kidney disease (ESKD) and reduces the incidence of ERCs; however, cysteamine is generally initiated upon establishment of the renal Fanconi syndrome (FS) and partial loss of kidney function, whereas data on long-term effects of cysteamine administered from neonatal age are lacking. An international multicenter retrospective cohort study of siblings with INC was set up to investigate the outcome in relation to age at initiation of cysteamine versus CTNS genotype, with attention to patients treated with cysteamine from neonatal age. None of the siblings treated from neonatal age (n = 9; age 10 ± 6 years) had reached ESKD, while 22% of their index counterparts (n = 9; age 14 ± 5 years) had commenced renal replacement therapy. Siblings treated with cysteamine from the onset of symptoms at a younger age compared with their index counterparts, reached ESKD at a significant older age (13 ± 3 vs. 10 ± 3 years, p = 0.002). In contrast, no significant difference in ERCs was observed between sibling and index patients, independently from the age at initiation of cysteamine. The CTNS genotype had no impact on the overall outcome in this cohort. In INC, presymptomatic treatment with cysteamine results in a better renal outcome in comparison to treatment initiated from the onset of symptoms. This justifies including cystinosis into newborn screening programs. SYNOPSIS: In infantile nephropathic cystinosis, presymptomatic treatment with cysteamine improves the renal outcome which justifies the inclusion of cystinosis into newborn screening programs.

OUP accepted manuscript

Recessive mutations in the CTNS gene encoding the lysosomal transporter cystinosin cause cystinosis, a lysosomal storage disease leading to kidney failure and multisystem manifestations. A Ctns knockout mouse model recapitulates features of cystinosis, but the delayed onset of kidney manifestations, phenotype variability and strain effects limit its use for mechanistic and drug development studies. To provide a better model for cystinosis, we generated a Ctns knockout rat model using CRISPR/Cas9 technology. The Ctns^−/− rats display progressive cystine accumulation and crystal formation in multiple tissues including kidney, liver and thyroid. They show an early onset and progressive loss of urinary solutes, indicating generalized proximal tubule dysfunction, with development of typical swan-neck lesions, tubulointerstitial fibrosis and kidney failure, and decreased survival. The Ctns^−/− rats also present crystals in the cornea, and bone and liver defects, as observed in patients. Mechanistically, the loss of cystinosin induces a phenotype switch associating abnormal proliferation and dedifferentiation, loss of apical receptors and transporters, and defective lysosomal activity and autophagy in the cells. Primary cultures of proximal tubule cells derived from the Ctns^−/− rat kidneys confirmed the key changes caused by cystine overload, including reduced endocytic uptake, increased proliferation and defective lysosomal dynamics and autophagy. The novel Ctns^−/− rat model and derived proximal tubule cell system provide invaluable tools to investigate the pathogenesis of cystinosis and to accelerate drug discovery.

In Vitro and In Vivo Models to Study Nephropathic Cystinosis

The development over the past 50 years of a variety of cell lines and animal models has provided valuable tools to understand the pathophysiology of nephropathic cystinosis. Primary cultures from patient biopsies have been instrumental in determining the primary cause of cystine accumulation in the lysosomes. Immortalised cell lines have been established using different gene constructs and have revealed a wealth of knowledge concerning the molecular mechanisms that underlie cystinosis. More recently, the generation of induced pluripotent stem cells, kidney organoids and tubuloids have helped bridge the gap between in vitro and in vivo model systems. The development of genetically modified mice and rats have made it possible to explore the cystinotic phenotype in an in vivo setting. All of these models have helped shape our understanding of cystinosis and have led to the conclusion that cystine accumulation is not the only pathology that needs targeting in this multisystemic disease. This review provides an overview of the in vitro and in vivo models available to study cystinosis, how well they recapitulate the disease phenotype, and their limitations.

Cystinosis induced by CTNS gene mutation: a rare disease study

A boy, aged 1 year and 6 months, was found to have persistent positive urine glucose at the age of 4 months, with polydipsia, polyuria, and growth retardation. Laboratory examinations suggested that the boy had low specific weight urine, anemia, hypokalemia, hyponatremia, hypomagnesemia, metabolic acidosis, glycosuria, acidaminuria, increased fractional excretion of potassium, and decreased tubular reabsorption of phosphate. X-ray examinations of the head, thorax, and right hand showed changes of renal rickets. The slit-lamp examination showed a large number of cystine crystals in the cornea. The genetic testing showed a suspected pathogenic homozygous mutation of the CTNS gene, C.922g>A(p.Gly308Arg). The boy was finally diagnosed with cystinosis. At the beginning of treatment, symptomatic supportive treatment was given to maintain the stability of the internal environment, and cysteamine tartaric acid capsules were used after diagnosis to remove cysteine. This article reported a case of cystinosis caused by CTNS gene mutation and summarized the etiology, clinical features, diagnosis, and treatment of this disease, which can provide a reference for the early diagnosis, treatment, and subsequent study of the disease.

Drug Repurposing in Rare Diseases: An Integrative Study of Drug Screening and Transcriptomic Analysis in Nephropathic Cystinosis

Diagnosis and cure for rare diseases represent a great challenge for the scientific community who often comes up against the complexity and heterogeneity of clinical picture associated to a high cost and time-consuming drug development processes. Here we show a drug repurposing strategy applied to nephropathic cystinosis, a rare inherited disorder belonging to the lysosomal storage diseases. This approach consists in combining mechanism-based and cell-based screenings, coupled with an affordable computational analysis, which could result very useful to predict therapeutic responses at both molecular and system levels. Then, we identified potential drugs and metabolic pathways relevant for the pathophysiology of nephropathic cystinosis by comparing gene-expression signature of drugs that share common mechanisms of action or that involve similar pathways with the disease gene-expression signature achieved with RNA-seq.

Hematopoietic Stem Cell Gene Therapy for Cystinosis: From Bench-to-Bedside

Cystinosis is an autosomal recessive metabolic disease that belongs to the family of lysosomal storage disorders. The gene involved is the CTNS gene that encodes cystinosin, a seven-transmembrane domain lysosomal protein, which is a proton-driven cystine transporter. Cystinosis is characterized by the lysosomal accumulation of cystine, a dimer of cysteine, in all the cells of the body leading to multi-organ failure, including the failure of the kidney, eye, thyroid, muscle, and pancreas, and eventually causing premature death in early adulthood. The current treatment is the drug cysteamine, which is onerous and expensive, and only delays the progression of the disease. Employing the mouse model of cystinosis, using Ctns-/- mice, we first showed that the transplantation of syngeneic wild-type murine hematopoietic stem and progenitor cells (HSPCs) led to abundant tissue integration of bone marrow-derived cells, a significant decrease in tissue cystine accumulation, and long-term kidney, eye and thyroid preservation. To translate this result to a potential human therapeutic treatment, given the risks of mortality and morbidity associated with allogeneic HSPC transplantation, we developed an autologous transplantation approach of HSPCs modified ex vivo using a self-inactivated lentiviral vector to introduce a functional version of the CTNS cDNA, pCCL-CTNS, and showed its efficacy in Ctns-/- mice. Based on these promising results, we held a pre-IND meeting with the Food and Drug Administration (FDA) to carry out the FDA agreed-upon pharmacological and toxicological studies for our therapeutic candidate, manufacturing development, production of the GMP lentiviral vector, design Phase 1/2 of the clinical trial, and filing of an IND application. Our IND was cleared by the FDA on 19 December 2018, to proceed to the clinical trial using CD34+ HSPCs from the G-CSF/plerixafor-mobilized peripheral blood stem cells of patients with cystinosis, modified by ex vivo transduction using the pCCL-CTNS vector (investigational product name: CTNS-RD-04). The clinical trial evaluated the safety and efficacy of CTNS-RD-04 and takes place at the University of California, San Diego (UCSD) and will include up to six patients affected with cystinosis. Following leukapheresis and cell manufacturing, the subjects undergo myeloablation before HSPC infusion. Patients also undergo comprehensive assessments before and after treatment to evaluate the impact of CTNS-RD-04 on the clinical outcomes and cystine and cystine crystal levels in the blood and tissues for 2 years. If successful, this treatment could be a one-time therapy that may eliminate or reduce renal deterioration as well as the long-term complications associated with cystinosis. In this review, we will describe the long path from bench-to-bedside for autologous HSPC gene therapy used to treat cystinosis.

An international cohort study spanning five decades assessed outcomes of nephropathic cystinosis

Nephropathic cystinosis is a rare disease secondary to recessive mutations of the CTNS gene encoding the lysosomal cystine transporter cystinosin, causing accumulation of cystine in multiple organs. Over the years, the disease has evolved from being a fatal condition during early childhood into a treatable condition, with patients surviving into adulthood. Data on cystinosis are limited by the rarity of the disease. Here, we have investigated factors associated with kidney and growth outcome in a very large cohort of 453 patients born between 1964 and 2016 and followed in Belgium, Germany, Austria, France, Italy, Spain, The Netherlands, Turkey and United Kingdom. From the 1970s to the 1990s, the median increase in kidney survival was 9.1 years. During these years, cysteamine, a cystine-depleting agent, was introduced for the treatment of cystinosis. Significant risk factors associated with early progression to end-stage kidney disease assessed by Cox proportional multivariable analysis included delayed initiation of cysteamine therapy and higher mean leucocyte cystine levels. No significant effect on kidney function was observed for gender, pathogenic variant of the CTNS gene, and the prescription of indomethacin or renin angiotensin system blockers. Significantly improved linear growth was associated with early use of cysteamine and lower leukocyte cystine levels. Thus, our study provides strong evidence in favor of early diagnosis and optimization of cystine depletion therapy in nephropathic cystinosis.

MFSD12 mediates the import of cysteine into melanosomes and lysosomes

Dozens of genes contribute to the wide variation in human pigmentation. Many of these genes encode proteins that localize to the melanosome-the organelle, related to the lysosome, that synthesizes pigment-but have unclear functions1,2. Here we describe MelanoIP, a method for rapidly isolating melanosomes and profiling their labile metabolite contents. We use this method to study MFSD12, a transmembrane protein of unknown molecular function that, when suppressed, causes darker pigmentation in mice and humans3,4. We find that MFSD12 is required to maintain normal levels of cystine-the oxidized dimer of cysteine-in melanosomes, and to produce cysteinyldopas, the precursors of pheomelanin synthesis made in melanosomes via cysteine oxidation5,6. Tracing and biochemical analyses show that MFSD12 is necessary for the import of cysteine into melanosomes and, in non-pigmented cells, lysosomes. Indeed, loss of MFSD12 reduced the accumulation of cystine in lysosomes of fibroblasts from patients with cystinosis, a lysosomal-storage disease caused by inactivation of the lysosomal cystine exporter cystinosin7-9. Thus, MFSD12 is an essential component of the cysteine importer for melanosomes and lysosomes.

Bone Disease in Nephropathic Cystinosis: Beyond Renal Osteodystrophy

Patients with chronic kidney disease (CKD) display significant mineral and bone disorders (CKD-MBD) that induce significant cardiovascular, growth and bone comorbidities. Nephropathic cystinosis is an inherited metabolic disorder caused by the lysosomal accumulation of cystine due to mutations in the CTNS gene encoding cystinosin, and leads to end-stage renal disease within the second decade. The cornerstone of management relies on cysteamine therapy to decrease lysosomal cystine accumulation in target organs. However, despite cysteamine therapy, patients display severe bone symptoms, and the concept of “cystinosis metabolic bone disease” is currently emerging. Even though its exact pathophysiology remains unclear, at least five distinct but complementary entities can explain bone impairment in addition to CKD-MBD: long-term consequences of renal Fanconi syndrome, malnutrition and copper deficiency, hormonal disturbances, myopathy, and intrinsic/iatrogenic bone defects. Direct effects of both CTNS mutation and cysteamine on osteoblasts and osteoclasts are described. Thus, the main objective of this manuscript is not only to provide a clinical update on bone disease in cystinosis, but also to summarize the current experimental evidence demonstrating a functional impairment of bone cells in this disease and to discuss new working hypotheses that deserve future research in the field.

Mitochondrial Dynamics of Proximal Tubular Epithelial Cells in Nephropathic Cystinosis

Nephropathic cystinosis is a rare lysosomal storage disorder caused by mutations in CTNS gene leading to Fanconi syndrome. Independent studies reported defective clearance of damaged mitochondria and mitochondrial fragmentation in cystinosis. Proteins involved in the mitochondrial dynamics and the mitochondrial ultrastructure were analyzed in CTNS-/- cells treated with cysteamine, the only drug currently used in the therapy for cystinosis but ineffective to treat Fanconi syndrome. CTNS-/- cells showed an overexpression of parkin associated with deregulation of ubiquitination of mitofusin 2 and fission 1 proteins, an altered proteolytic processing of optic atrophy 1 (OPA1), and a decreased OPA1 oligomerization. According to molecular findings, the analysis of electron microscopy images showed a decrease of mitochondrial cristae number and an increase of cristae lumen and cristae junction width. Cysteamine treatment restored the fission 1 ubiquitination, the mitochondrial size, number and lumen of cristae, but had no effect on cristae junction width, making CTNS-/- tubular cells more susceptible to apoptotic stimuli.

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.

Nephropathic Cystinosis Mimicking Bartter Syndrome: a Novel Mutation

Cystinosis is a rare autosomal recessive disorder resulting from defective lysosomal transport of cystine due to mutations in the cystinosin lysosomal cystine transporter (CTNS) gene. The clinical phenotype of nephropathic cystinosis is characterized by renal tubular Fanconi syndrome and development of end-stage renal disease during the first decade. Although metabolic acidosis is the classically prominent finding of the disease, a few cases may present with hypokalemic metabolic alkalosis mimicking Bartter syndrome. Bartter-like presentation may lead to delay in diagnosis and initiation of specific treatment for cystinosis. We report a case of a 6-year-old girl initially presenting with the features of Bartter syndrome that was diagnosed 2 years later with nephropathic cystinosis and a novel CTNS mutation.

Cysteamine revisited: repair of arginine to cysteine mutations

Cysteamine is a small aminothiol endogenously derived from coenzyme A degradation. For some decades, synthetic cysteamine has been employed for the treatment of cystinosis, and new uses of the drug continue to emerge. In this review, we discuss the role of cysteamine in cellular and extracellular homeostasis and focus on the potential use of aminothiols to reconstitute the function of proteins harboring arginine (Arg) to cysteine (Cys) mutations, via repair of the Cys residue into a moiety that introduces an amino group, as seen in basic amino acid residues Lys and Arg. Cysteamine has been utilized in vitro and ex vivo in four different genetic disorders, and thus provides "proof of principle" that aminothiols can modify Cys residues. Other aminothiols such as mercaptoethylguanidine (MEG) with closer structural resemblance to the guanidinium moiety of Arg are under examination for their predicted enhanced capacity to reconstitute loss of function. Although the use of aminothiols holds clinical potential, more studies are required to refine specificity and treatment design. The efficacy of aminothiols to target proteins may vary substantially depending on their specific extracellular and intracellular locations. Redox potential, pH, and specific aminothiol abundance in each physiological compartment are expected to influence the reactivity and turnover of cysteamine and analogous drugs. Upcoming research will require the use of suitable cell and animal models featuring Arg to Cys mutations. Since, in general, Arg to Cys changes comprise about 8% of missense mutations, repair of this specific mutation may provide promising avenues for many genetic diseases.

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.

Hematopoietic Stem Cells Transplantation Can Normalize Thyroid Function in a Cystinosis Mouse Model

Hypothyroidism is the most frequent and earliest endocrine complication in cystinosis, a multisystemic lysosomal storage disease caused by defective transmembrane cystine transporter, cystinosin (CTNS gene). We recently demonstrated in Ctns(-/-) mice that altered thyroglobulin biosynthesis associated with endoplasmic reticulum stress, combined with defective lysosomal processing, caused hypothyroidism. In Ctns(-/-) kidney, hematopoietic stem cell (HSC) transplantation provides long-term functional and structural protection. Tissue repair involves transfer of cystinosin-bearing lysosomes from HSCs differentiated as F4/80 macrophages into deficient kidney tubular cells, via tunneling nanotubes that cross basement laminae. Here we evaluated the benefit of HSC transplantation for cystinotic thyroid and investigated the underlying mechanisms. HSC engraftment in Ctns(-/-) thyroid drastically decreased cystine accumulation, normalized the TSH level, and corrected the structure of a large fraction of thyrocytes. In the thyroid microenvironment, HSCs differentiated into a distinct, mixed macrophage/dendritic cell lineage expressing CD45 and major histocompatibility complex II but low CD11b and F4/80. Grafted HSCs closely apposed to follicles and produced tunneling nanotube-like extensions that crossed follicular basement laminae. HSCs themselves further squeezed into follicles, allowing extensive contact with thyrocytes, but did not transdifferentiate into Nkx2.1-expressing cells. Our observations revealed significant differences of basement lamina porosity between the thyroid and kidney and/or intrinsic macrophage invasive properties once in the thyroid microenvironment. The contrast between extensive thyrocyte protection and low HSC abundance at steady state suggests multiple sequential encounters and/or remanent impact. This is the first report demonstrating the potential of HSC transplantation to correct thyroid disease and supports a major multisystemic benefit of stem cell therapy for cystinosis.

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.

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.

Studying nonobstructive azoospermia in cystinosis: histologic examination of testes and epididymis and sperm analysis in a Ctns⁻/⁻ mouse model

OBJECTIVE

To study the pathogenesis of male infertility in cystinosis due to nonobstructive azoospermia, using a Ctns(-/-) mouse model.

DESIGN

Observational case-control study.

SETTING

Academic research laboratory.

ANIMAL(S)

Male C57BL/6 Ctns(-/-) mice were compared with C57BL/6 wild-type (wt) mice.

INTERVENTION(S)

None.

MAIN OUTCOME MEASURE(S)

Fertility was studied using litter size (n = 3 vs. n = 2). After animals were sacrificed, testes, epididymis, and vas deferens were removed for testicular cystine measurements (n = 5 vs. n = 6), histologic studies (n = 3 vs. n = 3), and sperm analysis (n = 3 vs. n = 3).

RESULT(S)

Mean testicular cystine content was significantly higher in Ctns(-/-) mice compared with wt mice (26.6 ± 1.22 vs. 0.1 ± 0.01 nmol cystine/mg protein). Testes of Ctns(-/-) mice had lower weight compared with wt mice (0.096 ± 0.009 g vs. 0.112 ± 0.004 g), but mice fertility was similar (litter size 6.6 ± 1.4 vs. 6.3 ± 2.6 pups). Neither histologic nor sperm abnormalities were found.

CONCLUSION(S)

The Ctns(-/-) mouse model generated on C57BL/6 background is not suitable for clarifying the pathogenesis of male infertility in cystinosis. The etiology of nonobstructive azoospermia in these patients remains unclear.

Distribution of cystinosin-LKG in human tissues

Nephropathic cystinosis is multisystemic progressive disorder caused by mutations of CTNS gene that encodes for the lysosomal cystine co-transporter cystinosin, and for a less abundant isoform termed cystinosin-LKG, which is expressed in not only lysosomes but also other cell compartments. To overcome the absence of high-quality antibodies against cystinosin, we have obtained a rabbit antiserum against cystinosin-LKG and have analyzed in human tissues the expression of the two known cystinosin isoforms by RT-PCR, and the expression of cystinosin-LKG by immunohistochemistry. In most tissues, CTNS-LKG represents 5-20 % of CTNS transcripts, with the exception of the testis that expresses both isoforms in equal proportions. Cystinosin-LKG was found to be highly expressed in renal tubular cells, pancreatic islets of Langerhans, Leydig cells of the testis, mucoserous glands of the bronchial wall, melanocytes and keratinocytes. These results are parallel with many features of cystinosis, such as early onset Fanconi syndrome, male infertility, diabetes mellitus and hypopigmentation. Intermediate expression levels were of the LKG isoform observed in the gastro-intestinal tract and thyroid glands; low levels of expression were observed in the brain, skeletal and cardiac muscles.

Elevated concentrations of sedoheptulose in bloodspots of patients with cystinosis caused by the 57-kb deletion: implications for diagnostics and neonatal screening

Cystinosis is an autosomal recessive lysosomal storage disease caused by mutations in CTNS. The most prevalent CTNS mutation is a homozygous 57-kb deletion that also includes an adjacent gene named SHPK (CARKL), encoding sedoheptulokinase. Patients with this deletion have elevated urinary concentrations of sedoheptulose. Using derivatisation with pentafluorobenzyl hydroxylamine and liquid chromatography-tandem mass spectrometry (LC-MS/MS), we developed a new sensitive method for the quantification of sedoheptulose in dried blood spots. This method can be utilized as a quick screening test to detect cystinosis patients homozygous for the 57-kb deletion in CTNS; which is the most common mutation of cystinosis. Sedoheptulose concentrations in the deleted patients were 6 to 23 times above the upper limit for controls. The assessment of sedoheptulose in a bloodspot from a known cystinosis patient homozygous for the 57-kb deletion retrieved from the Dutch neonatal screening program showed that sedoheptulose was already elevated in the neonatal period. There was no overlap in sedoheptulose levels between cystinosis patients homozygous for the 57-kb deletion and cystinosis patients not homozygous for this deletion. Our presented method can be used prior to mutation analysis to detect cystinosis patients homozygous for the 57-kb deletion. We feel that the presented method enables fast (pre)-symptomatic detection of cystinosis patients homozygous for the 57-kb deletion, allowing early treatment.

Insights into novel cellular injury mechanisms by gene expression profiling in nephropathic cystinosis

Nephropathic cystinosis is a rare, inherited metabolic disease caused by functional defects of cystinosin associated with mutations in the CTNS gene. The mechanisms underlying the phenotypic alterations associated with this disease are not well known. In this study, gene expression profiles in peripheral blood of nephropathic cystinosis patients (N = 7) were compared with controls (N = 7) using microarray technology. In unsupervised hierarchical clustering analysis, cystinosis samples co-clustered, and 1,604 genes were significantly differentially expressed between both groups. Gene ontology analysis revealed that differentially expressed genes in cystinosis were enriched in cell organelles such as mitochondria, lysosomes, and endoplasmic reticulum (p ≤ 0.030). The majority of the differentially regulated genes were involved in oxidative phosphorylation, apoptosis, mitochondrial dysfunction, endoplasmic reticulum stress, antigen processing and presentation, B-cell-receptor signaling, and oxidative stress (p ≤ 0.003). Validation of selected genes involved in apoptosis and oxidative phosphorylation was performed by quantitative real-time polymerase chain reaction (PCR). Electron microscopy and confocal imaging of cystinotic renal proximal tubular epithelial cells further confirmed anomalies in the cellular organelles and pathways identified by microarray analysis. Further analysis of these genes and pathways may offer critical insights into the clinical spectrum of cystinosis patients and ultimately lead to novel links for targeted therapy.

Modulation of CTNS gene expression by intracellular thiols

The cysteine/cystine (Cys/CySS) couple represents one of the major cell thiol/disulfide systems and is involved in the regulation of several metabolic pathways and the cell redox state. Nephropathic cystinosis (NC) is an autosomal recessive disease characterized by renal cellular dysfunction due to mutations in the CTNS gene, which encodes cystinosin, a CySS lysosomal transporter. To analyze the mechanisms involved in cell damage in NC, we have investigated the effects of CTNS gene overexpression or inhibition on cell thiol/disulfide systems and vice versa. Overexpression of the CTNS gene had no remarkable effect on intracellular Cys/CySS and GSH/GSSG redox state. Silencing the CTNS gene increased cell CySS and Cys and decreased cell GSH and GSSG and increased mildly the redox state of the Cys/CySS-couple. Extracellular CySS and Cys deprivation for 48 h caused an oxidation of the Cys/CySS (73 mV) and GSH/GSSG (100 mV) redox couples and increased CTNS mRNA levels by 1.9+/-0.2-fold (p<0.001). Conversely, a reduced cell environment associated with a GSH/GSSG reduction from -250.1+/-3.10 to -330.6+/-4.70 mV (p<0.001) and a Cys/CySS reduction from -167.0+/-11.30 to -240.0+/-8.17 mV (p<0.005) was associated with a 40% decrease in CTNS mRNA levels (p<0.05). By regression analysis, CTNS gene expression was correlated with intracellular Cys level and with Cys/CySS redox state.

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.

Nephropathic cystinosis in children: An overlooked disease

Nephropathic cystinosis is rare genetic disease characterized by defective lysosomal cystine transport and increased lysosomal cystine. Corneal Cystine Crystal Scoring (CCCS) for diagnosis of nephropathic cystinosis was studied in all suspected children with renal Fanconi syndrome and siblings of diagnosed cases over a two year period. In addition to oral cysteamine, cysteamine eye drops were provided to all diagnosed patients and CCCS was followed up on a quarterly basis. Of 33 screened cases, 14 had corneal cystine crystals. Crystals were absent in two cystinotic patients under the age of 20 months. The mean age at diagnosis was 52.7 months and five patients had ERSD. After six months of treatment, the mean CCCS did not increase from the initial value of 1.81; associated with a decrease of 0.5 in two cases and a similar increase in two others. Scores decreased in two other patients after 12 months. Compliance was generally inadequate due to the high frequency of administration and the need for multi-drug regimen. CCCS is a simple and reasonably sensitive method for diagnosis of nephropathic cystinosis above two years of age. Topical treatment with cysteamine eye drops prevents progression of deposits and may decrease it with adequate compliance. Further follow up is still recommended to monitor long term effects of both systemic and topical cysteamine therapy.

Specific cognitive deficits in young children with cystinosis: evidence for an early effect of the cystinosin gene on neural function

OBJECTIVES

Infantile nephropathic cystinosis is associated with a specific cognitive deficit in visual spatial processing in older children and adults. The cause of this deficit is unknown. This study was designed to determine whether the cognitive deficit is present in young children with cystinosis, suggesting an early effect of the genetic disorder on brain development.

STUDY DESIGN

Young children (n = 25; age, 3-8 years) with cystinosis and 25 matched control subjects underwent cognitive testing, including tests of intelligence, visual perceptual, visual spatial, and visual motor functions.

RESULTS

Children with cystinosis performed significantly more poorly on tests of visual spatial and visual motor function than did control subjects. Visual perceptual abilities were equivalent in the 2 groups.

CONCLUSION

The same pattern of visual spatial deficit is present in young children with cystinosis as has previously been demonstrated in older children and adults, which suggests that there may be an influence of the cystinosis gene on brain development, rather than an adverse effect of prolonged cystine accumulation in the brain during childhood.

The ocular anomalies in a cystinosis animal model mimic disease pathogenesis

Cystinosis is a lysosomal storage disorder characterized by abnormal accumulation of cystine, which forms crystals at high concentrations. The causative gene CTNS encodes cystinosin, the lysosomal cystine transporter. The eye is one of the first organs affected (corneal lesions and photophobia in the first and visual impairment in the second decade of life). We characterized the ocular anomalies of Ctns-/- mice to determine whether they mimic those of patients. The most dramatic cystine accumulation was seen in the iris, ciliary body, and cornea of Ctns-/- mice. Consistently, Ctns-/- mice had a low intraocular pressure (IOP) and seemed mildly photophobic. Retinal cystine levels were elevated but increased less dramatically with age. Consistently, the retina was intact and electroretinogram (ERG) profiles were normal in mice younger than 19 mo; beyond this age, retinal crystals and lesions appeared. Finally, the lens contained the lowest cystine levels and crystals were not seen. The temporospatial pattern of cystine accumulation in Ctns-/- mice parallels that of patients and validates the mice as a model for the ocular anomalies of cystinosis. This work is a prerequisite step to the testing of novel ocular cystine-depleting therapies.

Cystine dimethylester model of cystinosis: still reliable?

The ability of cystine dimethylester (CDME) to load lysosomes with cystine has been used to establish the basic defect in cystinosis: defective cystine exodus from lysosomes. Using CDME loading, it has been postulated that cystine accumulation in cystinosis affects mitochondrial ATP production, resulting in defective renal tubular reabsorption. Recent studies in cystinotic fibroblasts, however, show normal adenosine triphosphate (ATP) generation capacity. To investigate the effect of CDME in more detail, mitochondrial ATP generation, reactive oxygen species production, and viability are compared in fibroblasts loaded with CDME with those of cystinotic cells with a defective cystine transporter. Intracellular cystine levels were comparable in fibroblasts loaded with CDME (1 mM, 30 min) and cystinotic fibroblasts. Intracellular ATP levels and mitochondrial ATP production were decreased in fibroblasts loaded with CDME, but normal in cystinotic fibroblasts. Superoxide production was increased with 300% after CDME loading, whereas no changes were observed in cystinotic fibroblasts. Exposure to CDME led to cell death in a time- and concentration-dependent manner. Our data demonstrate that CDME has a toxic effect on mitochondrial ATP production and cell viability. These effects are not observed in cystinotic cells, indicating that a more appropriate model is required for studying the pathogenesis of cystinosis.

Promotion of oxidative stress in kidney of rats loaded with cystine dimethyl ester

Cystinosis is a systemic genetic disease caused by a lysosomal transport deficiency accumulating cystine in most tissues. Although tissue damage might depend on cystine accumulation, the mechanisms of tissue damage are not fully understood. Studies performed in fibroblasts of cystinotic patients and in kidney cells loaded with cystine dimethyl ester (CDME) suggest that apoptosis is enhanced in this disease. Considering that oxidative stress is a known apoptosis inducer, our main objective was to investigate the effects of CDME loading on several parameters of oxidative stress in the kidney of young rats. Animals were injected twice a day with 1.6 micromol/g body weight CDME and/or 0.26 micromol/g body weight cysteamine (CSH) from the 16th to the 20th postpartum day and killed after 1 or 12 h. CDME induced lipoperoxidation and protein carbonylation and stimulated superoxide dismutase, glutathione peroxidase (GPx), and catalase activities, probably through the formation of superoxide anions, hydrogen peroxide, and hydroxyl free radicals. Coadministration of CSH, the drug used to treat cystinotic patients, prevented, at least in part, those effects, possibly acting as a scavenger of free radicals. These results suggest that the induction of oxidative stress might be one of the mechanisms leading to tissue damage in cystinotic patients.

Non-verbal deficits in young children with a genetic metabolic disorder: WPPSI-III performance in cystinosis

Cystinosis is a recessive genetic metabolic disorder in which the amino acid cystine accumulates in various organs of the body. Previous studies have demonstrated visuospatial dysfunction in children and adults with this disorder. It is not known whether this is a result of the genetic alteration or an accumulation of cystine in the brain over time. This study investigated patterns of performance in 20 young children with cystinosis (4-7 years) and 20 matched controls on the Wechsler Preschool and Primary Scale of Intelligence-Third Edition (WPPSI-III). The children with cystinosis had a mean Full Scale IQ at the low end of the average range. Their overall cognitive functioning was comprised of average verbal abilities, low average non-verbal abilities, and low average processing speed. Multivariate analyses indicated that the cystinosis and control groups were not significantly different on the verbal subtests. In contrast, the cystinosis group performed significantly more poorly than controls on the performance and processing speed subtests. Although overall intellectual function was in the normal range, young children with cystinosis demonstrated a discrepancy such that non-verbal abilities were poorer relative to verbal abilities. This pattern resembles the cognitive profile found previously in older individuals with cystinosis and indicates that the specific cognitive profile emerges early in development. These findings suggest that the cognitive dysfunction in cystinosis is not merely the result of cystine accumulation over time but may be related to differences in brain development as a consequence of alterations or deletions of the cystinosin gene.

Potential role of apoptosis in development of the cystinotic phenotype

Much still remains unclear about the proximal biochemical effects of mutations on development of the phenotype in inborn errors of metabolism. Cystinosis is an example of this phenomenon. We have recently shown that cystinotic cells undergo apoptosis at a two- to fourfold higher rate than controls. Cystinotic cells pre-treated with cysteamine, normalizing cystine content, display a four- to fivefold decrease in apoptosis, while normal cells pre-treated with cystine dimethylester, increasing lysosomal cystine, exhibit a fivefold increase in apoptosis. We speculate that cystine exits the lysosomal compartment during early apoptosis and affects apoptotic proteins in the cytosol, causing an inappropriate commitment to proceed to cell death. The resulting chronic hypocellularity could account for all the characteristics of the nephropathic cystinotic phenotype. The milder variants of cystinosis may result from modifying mutations within an apoptotic protein, ablating the proapoptotic effects of cystine. Failure of the mouse knockout for cystinosis to show renal involvement may be the result of differences in apoptotic processes between man and mouse. Apoptosis is a major final common pathway for many disease states. Therefore, a better understanding of the effect of lysosomal cystine on apoptosis may help to clarify development of other diseases.

Lysosomal cystine augments apoptosis and causes the phenotype in cystinosis

Nephropathic cystinosis is a lethal inborn error of metabolism that destroys kidney function by age 10 years. It is characterized by lysosomal cystine accumulation. How the cystine causes the phenotype is an open question. We propose that during apoptosis, permeablized lysosomes permit cystine to reach the cytosol where mixed disulfide formation occurs, augmenting apoptosis by interaction with a variety of pro-apoptotic proteins.

[Cystinosis: an infrequent cause of distal myopathy]

INTRODUCTION

Cystinosis is a hereditary disease with clinical symptoms that are caused by the accumulation of cystine crystals in different tissues. Distal vacuolar myopathy has been reported as one of its later complications.

CASE REPORT

Here, we present the case of a 20-year-old male diagnosed with cystinosis at the age of 2 years, with severe renal involvement that required a transplant. The patient gradually developed weakness and atrophy of the muscles in his hands. Neurophysiological and histological studies enabled a diagnosis of distal vacuolar myopathy to be established, and electron microscopy revealed deposits of cystine crystals.

CONCLUSIONS

Cystinosis must be included within the differential diagnosis of distal myopathies. Timely treatment with cysteamine could prevent the development of this complication.

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.

Glutathione precursors replenish decreased glutathione pool in cystinotic cell lines

Cystinosis is an inherited disorder due to mutations in the CTNS gene which encodes cystinosin, a lysosomal transmembrane protein involved in cystine export to the cytosol. Both accumulation of cystine in the lysosome and decreased cystine in the cytosol may participate in the pathogenic mechanism underlying the disease. We observed that cystinotic cell lines have moderate decrease of glutathione content during exponential growth phase. This resulted in increased solicitation of oxidative defences of the cell denoted by concurrent superoxide dismutase induction, although without major oxidative insult under our experimental conditions. Finally, decreased glutathione content in cystinotic cell lines could be counterbalanced by a series of exogenous precursors of cysteine, denoting that lysosomal cystine export is a natural source of cellular cysteine in the studied cell lines.