Severity of phenotype in cystinosis varies with mutations in the CTNS gene: predicted effect on the model of cystinosin

Perspectives from cystinosis: access to healthcare may be a confounding factor for variant classification

Genetic variability persists across diverse populations, and it may impact the characterization of heritable diseases in different ancestral groups. Cystinosis is a metabolic disease caused by pathogenic variants in the CTNS gene causing the cellular accumulation of cystine. We attempted to assess the currently poorly characterized prevalence of cystinosis by employing a population genetics methodology. However, we encountered a significant challenge due to genetic variations across different populations, and the consideration of potential disparities in access to healthcare made our results inconclusive. Pathogenic CTNS variants were identified in a representative global population cohort using The Human Gene Mutation Database (HGMD) and the 1000 Genomes (1 KG) database. The c.124G>A (p.Val42Ile) variant was reported to be pathogenic based on an observation in the white population presenting with atypical phenotypes, but it would be reclassified as benign in the African ancestral group if applying the ACMG allele frequency guideline due to its high allele frequency specifically in this population. Inclusion or exclusion of this c.124G>A (p.Val42Ile) variant results in a significant change in estimated disease prevalence, which can impact the diagnosis and treatment of affected patients with a broad range of phenotypic presentations. This observation led us to postulate that pathogenic manifestations of the disease may be underdiagnosed due to variable expressivity and systemic inequities in access to care, specifically in the African subpopulation. We call for a more cautious and inclusive approach to achieve more equitable care across diverse populations.

Disulfidptosis, A Novel Cell Death Pathway: Molecular Landscape and Therapeutic Implications

Programmed cell death is pivotal for several physiological processes, including immune defense. Further, it has been implicated in the pathogenesis of developmental disorders and the onset of numerous diseases. Multiple modes of programmed cell death, including apoptosis, pyroptosis, necroptosis, and ferroptosis, have been identified, each with their own unique characteristics and biological implications. In February 2023, Liu Xiaoguang and his team discovered "disulfidptosis," a novel pathway of programmed cell death. Their findings demonstrated that disulfidptosis is triggered in glucose-starved cells exhibiting high expression of a protein called SLC7A11. Furthermore, disulfidptosis is marked by a drastic imbalance in the NADPH/NADP+ ratio and the abnormal accumulation of disulfides like cystine. These changes ultimately lead to the destabilization of the F-actin network, causing cell death. Given that high SLC7A11 expression is a key feature of certain cancers, these findings indicate that disulfidptosis could serve as the basis of innovative anti-cancer therapies. Hence, this review delves into the discovery of disulfidptosis, its underlying molecular mechanisms and metabolic regulation, and its prospective applications in disease treatment.

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.

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).

Exposure to SARS-CoV-2 and Infantile Diseases

Background and Aim  Immune response against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in newborns and children after prophylactic immunization is currently a relevant research topic. The present study analyzes the issue by examining the possibility that the anti-SARS-CoV-2 immune responses are not uniquely directed against the virus but can-via molecular mimicry and the consequent cross-reactivity-also hit human proteins involved in infantile diseases. Methods  Human proteins that-if altered-associate with infantile disorders were searched for minimal immune pentapeptide determinants shared with SARS-CoV-2 spike glycoprotein (gp). Then, the shared pentapeptides were analyzed for immunologic potential and immunologic imprinting phenomena. Results  Comparative sequence analysis shows that: (1) numerous pentapeptides (namely, 54) are common to SARS-CoV-2 spike gp and human proteins that, when altered, are linked to infantile diseases; (2) all the shared peptides have an immunologic potential since they are present in experimentally validated SARS-CoV-2 spike gp-derived epitopes; and (3) many of the shared peptides are also hosted in infectious pathogens to which children can have already been exposed, thus making immunologic imprint phenomena feasible. Conclusion  Molecular mimicry and the consequent cross-reactivity can represent the mechanism that connects exposure to SARS-CoV-2 and various pediatric diseases, with a fundamental role of the immunologic memory and the history of the child's infections in determining and specifying the immune response and the pathologic autoimmune sequela.

Clinical and genetic characteristics of Tunisian children with infantile nephropathic cystinosis

BACKGROUND

Nephropathic cystinosis is an autosomal recessive disease caused by a mutation in the CTNS gene which encodes cystinosin, a lysosomal cystine transporter. The spectrum of mutations in the CTNS gene is not well defined in the North African population. Here, we investigated twelve patients with nephropathic cystinosis belonging to eight Tunisian families in order to analyze the clinical and genetic characteristics of Tunisian children with infantile nephropathic cystinosis.

METHODS

Clinical data were collected retrospectively. Molecular analysis of the CTNS gene was performed by Sanger sequencing.

RESULTS

We describe a new splicing mutation c.971-1G > C in the homozygous state in 6/12 patients which seems to be a founder mutation. The reported deletion of 23nt c.771_793 Del (p.Gly258Serfs*30) was detected in a homozygous state in one patient and in a heterozygous compound state with the c.971-1G > C mutation in 3/12 patients. Two of 12 patients have a deletion of exons 4 and 5 of the CTNS gene. None of our patients had the most common 57-kb deletion.

CONCLUSIONS

The mutational spectrum in the Tunisian population is different from previously described populations. Thus, a molecular diagnostic strategy must be implemented in Tunisia, by targeting as a priority the common mutations described in this country. Such a strategy will allow a cost-effective diagnosis confirmation as well as early administration of treatment with oral cysteamine. A higher resolution version of the Graphical abstract is available as Supplementary information.

Structural basis for proton coupled cystine transport by cystinosin

Amino acid transporters play a key role controlling the flow of nutrients across the lysosomal membrane and regulating metabolism in the cell. Mutations in the gene encoding the transporter cystinosin result in cystinosis, an autosomal recessive metabolic disorder characterised by the accumulation of cystine crystals in the lysosome. Cystinosin is a member of the PQ-loop family of solute carrier (SLC) transporters and uses the proton gradient to drive cystine export into the cytoplasm. However, the molecular basis for cystinosin function remains elusive, hampering efforts to develop novel treatments for cystinosis and understand the mechanisms of ion driven transport in the PQ-loop family. To address these questions, we present the crystal structures of cystinosin from Arabidopsis thaliana in both apo and cystine bound states. Using a combination of in vitro and in vivo based assays, we establish a mechanism for cystine recognition and proton coupled transport. Mutational mapping and functional characterisation of human cystinosin further provide a framework for understanding the molecular impact of disease-causing mutations.

Mutations in CTNS, the lysosomal cystine-proton symporter, cause cystinosis. Here authors report crystal structures of CTNS from Arabidopsis thaliana in complex with cystine, and establish the mode of ligand recognition and mechanism for proton-coupled cystine export from the lysosome.

Molecular characterization of CTNS mutations in Tunisian patients with ocular cystinosis

Background

Ocular cystinosis is a rare autosomal recessive disorder characterized by intralysosomal cystine accumulation in renal, ophthalmic (cornea, conjunctiva), and other organ abnormalities. Patients with ocular cystinosis are mostly asymptomatic and typically experience mild photophobia due to cystine crystals in the cornea observed accidently during a routine ocular examination. The ocular cystinosis is associated with different mutations in CTNS gene. Cysteamine therapy mostly corrects the organ abnormalities.

Methods

This study was performed in collaboration with the department of ophthalmology of Farhat Hached Hospital. The Optical Coherence Tomography (OCT) of the cornea and retinal photography were used to search cystine crystals within the corneas and conjunctiva in eight Tunisian patients. Screening for the common 57-kb deletion was performed by standard multiplex PCR, followed by direct sequencing of the entire CTNS gene.

Results

The studied patients were found to have cystine crystal limited anterior corneal stroma and the conjunctiva associated with retinal crystals accumulation. CTNS gene sequencing disclosed 7 mutations: three missense mutations (G308R, p.Q88K, and p.S139Y); one duplication (C.829dup), one framshift mutation (p.G258f), one splice site mutation (c.681 + 7delC) and a large deletion (20,327-bp deletion). Crystallographic structure analysis suggests that the novel mutation p.S139Y is buried in a first transmembrane helix closed to the lipid bilayer polar region, introducing a difference in hydrophobicity which could affect the hydrophobic interactions with the membrane lipids. The second novel mutation p.Q88K which is located in the lysosomal lumen close to the lipid membrane polar head region, introduced a basic amino acid in a region which tolerate only uncharged residue. The third missense mutation introduces a positive change in nonpolar tail region of the phospholipid bilayer membrane affecting the folding and stability of the protein in the lipid bilayer.

Conclusions

Our data demonstrate that impaired transport of cystine out of lysosomes is the most common, which is obviously associated with the mutations of transmembrane domains of cystinosine resulting from a total loss of its activity.

Cystinosis and two rare mutations in CTNS gene: two case reports

Background

Cystinosis is an autosomal recessive disorder characterized by an accumulation of the amino acid cystine in lysosomes throughout the body. Cystinosis is an inherited disease resulting from the failure of lysosomal cystine transport. The responsible gene, Cystinosin, Lysosomal Cystine Transporter (CTNS), encodes the lysosomal cystine carrier cystinosin.

Case presentation

In this case report, we reviewed the genetic basis of cystinosis and investigated two Iranian cases affected by cystinosis, one of which revealed a rare mutation in the CTNS gene. Two patients, 9-year-old (patient A) and 11-year-old (patient B) symptomatic Iranian females with renal insufficiency, were diagnosed with cystinosis on the basis of their clinical features and laboratory tests. After genetic counseling, blood samples were obtained from the patients and their parents. Genomic Deoxyribonucleic Acid (DNA) was extracted from whole blood, and mutation analysis was performed using polymerase chain reaction and sequencing methods for all exons of the CTNS gene. At least 148 different pathogenic and deleterious mutations in the CTNS gene have been reported to date. Owing to our patient’s prominent clinical features of cystinosis, we carried out a targeted search for mutations in the CTNS gene.

Conclusions

This led us to confirm the existence of a homozygous DNA variation c.257_258deletionCT (p.Ser86PhefsTer38) in exon 6 of the gene in patient A and another homozygous DNA variation, c.323delA (p.Q108RfsTer10), in the same exon in patient B. As expected, the mentioned mutation existed in both her parents in a heterozygous state. Variations c.257_258delCT and c.323delA reported in three Iranian patients in the CTNS gene are frameshifts, and truncating mutations that affect product function result in relatively mild symptoms of cystinosis. The present finding confirms previous research and proves the importance of the association of this gene rare mutations with cystinosis. Since reported mutations are rare, their previous reports in Iranian patients indicate the high frequency of these mutations in our region.

Atypical manifestations of infantile-onset nephropathic cystinosis: a diagnostic challenge

A 7-month-old male infant was referred to us for evaluation of hypercalcemia and failure to thrive. He was the second-born child to third-degree consanguineous parents with a birth weight of 3.5 kg. The index child was severely underweight. Initial laboratory investigations showed hypercalcemia (13.6 mg/dL), hypophosphatemia, hyponatremia, hypokalemia and hypochloremia. The initial serum bicarbonate level was 20.9 mEq/L. The urine calcium: creatinine ratio (0.05) was normal. He was noted to have polyuria (6 mL/kg/hr) and required intravenous fluids to maintain intravascular volume and manage hypercalcemia, along with potassium chloride supplements. The serum calcium decreased to 9.7 mg/dL after hydration for 48 h. At this juncture, the child was noted to exhibit metabolic acidosis (serum bicarbonate 16 mEq/L) for the first time. Thereafter, fractional excretion of bicarbonate was estimated to be 16.5% while the tubular threshold maximum for phosphorus per glomerular filtration rate was 1.2 mg/dL; indicating bicarbonaturia and phosphaturia, respectively. Glycosuria with aminoaciduria were also noted. Clinical exome sequencing revealed a NM_004937.3:c.809_811del in exon 10 of the CTNS gene that resulted in in-frame deletion of amino acids [NP_004928.2:p.Ser270del] at the protein level. The child is now growing well on oral potassium citrate, neutral phosphate and sodium bicarbonate supplements. This case was notable for absence of metabolic acidosis at admission. Instead, severe hypercalcemia was a striking presenting manifestation, that has not been reported previously in literature. Cystinosis has been earlier described in association with metabolic acidosis, hypocalcemia and hypomagnesemia. However, typical features like metabolic acidosis were masked in early stages of the disease in our case posing a diagnostic challenge. This atypical initial presentation adds to the constellation of clinical features in this condition.

Next-Generation Sequencing-Based Genetic Diagnostic Strategies of Inherited Kidney Diseases

BACKGROUND

At least 10% of adults and most of the children who receive renal replacement therapy have inherited kidney diseases. These disorders substantially decrease their life quality and have a large effect on the health-care system. Multisystem complications, with typical challenges for rare disorders, including variable phenotypes and fragmented clinical and biological data, make genetic diagnosis of inherited kidney disorders difficult. In current clinical practice, genetic diagnosis is important for clinical management, estimating disease development, and applying personal treatment for patients.

SUMMARY

Inherited kidney diseases comprise hundreds of different disorders. Here, we have summarized various monogenic kidney disorders. These disorders are caused by mutations in genes coding for a wide range of proteins including receptors, channels/transporters, enzymes, transcription factors, and structural components that might also have a role in extrarenal organs (bone, eyes, brain, skin, ear, etc.). With the development of next-generation sequencing technologies, genetic testing and analysis become more accessible, promoting our understanding of the pathophysiologic mechanisms of inherited kidney diseases. However, challenges exist in interpreting the significance of genetic variants and translating them to guide clinical managements. Alport syndrome is chosen as an example to introduce the practical application of genetic testing and diagnosis on inherited kidney diseases, considering its clinical features, genetic backgrounds, and genetic testing for making a genetic diagnosis.

KEY MESSAGES

Recent advances in genomics have highlighted the complexity of Mendelian disorders, which is due to allelic heterogeneity (distinct mutations in the same gene produce distinct phenotypes), locus heterogeneity (mutations in distinct genes result in similar phenotypes), reduced penetrance, variable expressivity, modifier genes, and/or environmental factors. Implementation of precision medicine in clinical nephrology can improve the clinical diagnostic rate and treatment efficiency of kidney diseases, which requires a good understanding of genetics for nephrologists.

Testicular function in males with infantile nephropathic cystinosis

STUDY QUESTION

Do males with the rare lysosomal storage disease infantile nephropathic cystinosis (INC) have a chance of biological fatherhood?

SUMMARY ANSWER

Cryostorage of semen could be an option for approximately 20% of young males with INC, with surgical sperm retrieval from the centre of the testes providing additional opportunities for fatherhood.

WHAT IS KNOWN ALREADY

Biallelic mutations in the cystinosin (CTNS) gene in INC cause dysfunction in cystine transport across lysosomal membranes and cystine accumulation throughout the body. Spontaneous paternity in cystinosis has not been described, despite the availability of cysteamine treatment. Azoospermia has been diagnosed in small case series of males with INC. ART using ICSI requires few spermatozoa, either from semen or extracted surgically from the testes of azoospermic men. However, there is limited evidence to suggest this could be successful in INC.

STUDY DESIGN, SIZE, DURATION

In this prospective cohort study performed between 2018 and 2019, we performed a cross-sectional investigation of 18 male patients with INC to delineate endocrine and spermatogenic testicular function.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Serum hormone levels, semen samples (according to World Health Organization 2010 standards), and testicular ultrasound images were analysed in 18 male patients aged 15.4–40.5 years. Surgical sperm extraction was performed in two, and their testicular biopsies were investigated by light and electron microscopy. Past adherence to cysteamine treatment was assessed from medical record information, using a composite scoring system.

MAIN RESULTS AND THE ROLE OF CHANCE

Adherence to cysteamine treatment was high in most patients. Testicular volumes and testosterone levels were in the normal ranges, with the exception of two and three older patients, respectively. Serum LH levels were above the normal range in all subjects aged ≥20 years. FSH levels were elevated in all but four males: three with spermatozoa in semen and one adolescent. Inhibin B levels were shown to be lower in older men. Testicular ultrasound revealed signs of obstruction in 67% of patients. Reduced fructose and zinc seminal markers were found in 33%, including two patients with azoospermia who underwent successful surgical sperm retrieval. Histology identified fully preserved spermatogenesis in the centre of their testes, but also tubular atrophy and lysosomal overload in Sertoli and Leydig cells of the testicular periphery.

LIMITATIONS, REASONS FOR CAUTION

Limitations of this study are the small number of assessed patients and the heterogeneity of their dysfunction in cystine transport across lysosomal membranes.

WIDER IMPLICATIONS OF THE FINDINGS

This study suggests that testicular degeneration in cystinosis results from the lysosomal overload of Sertoli and Leydig cells of the testicular periphery, and that this can possibly be delayed, but not prevented, by good adherence to cysteamine treatment. Endocrine testicular function in INC may remain compensated until the fourth decade of life; however, azoospermia may occur during adolescence. Cryostorage of semen could be an option for approximately 20% of young males with INC, with surgical sperm retrieval providing additional opportunities for biological fatherhood.

STUDY FUNDING/COMPETING INTEREST(S)

This work was supported by the Cystinosis Foundation Germany. The authors have no competing interests to declare.

TRIAL REGISTRATION NUMBER

n/a.

Molecular Mechanisms and Treatment Options of Nephropathic Cystinosis

Nephropathic cystinosis is a severe, monogenic systemic disorder that presents early in life and leads to progressive organ damage, particularly affecting the kidneys. It is caused by mutations in the CTNS gene, which encodes the lysosomal transporter cystinosin, resulting in intralysosomal accumulation of cystine. Recent studies demonstrated that the loss of cystinosin is associated with disrupted autophagy dynamics, accumulation of distorted mitochondria, and increased oxidative stress, leading to abnormal proliferation and dysfunction of kidney cells. We discuss these molecular mechanisms driving nephropathic cystinosis. Further, we consider how unravelling molecular mechanisms supports the identification and development of new strategies for cystinosis by the use of small molecules, biologicals, and genetic rescue of the disease in vitro and in vivo.

Potential use of stem cells as a therapy for cystinosis

Cystinosis is an autosomal recessive metabolic disease that belongs to the family of lysosomal storage disorders (LSDs). Initial symptoms of cystinosis correspond to the renal Fanconi syndrome. Patients then develop chronic kidney disease and multi-organ failure due to accumulation of cystine in all tissue compartments. LSDs are commonly characterized by a defective activity of lysosomal enzymes. Hematopoietic stem and progenitor cell (HSPC) transplantation is a treatment option for several LSDs based on the premise that their progeny will integrate in the affected tissues and secrete the functional enzyme, which will be recaptured by the surrounding deficient cells and restore physiological activity. However, in the case of cystinosis, the defective protein is a transmembrane lysosomal protein, cystinosin. Thus, cystinosin cannot be secreted, and yet, we showed that HSPC transplantation can rescue disease phenotype in the mouse model of cystinosis. In this review, we are describing a different mechanism by which HSPC-derived cells provide cystinosin to diseased cells within tissues, and how HSPC transplantation could be an effective one-time treatment to treat cystinosis but also other LSDs associated with a lysosomal transmembrane protein dysfunction.

Ocular manifestations of intermediate cystinosis: To treat or not to treat?

PURPOSE

To report a case of intermediate cystinosis with an atypical presentation in which a delayed non-treatment decision has been proved right over a large period of time.

METHODS

This is a unique case report of a patient who underwent full ophthalmologic evaluations including anterior chamber optical coherence tomography on a regular basis during a 7-year follow-up period.

RESULTS

A 30-year-old woman with photophobia was being studied by the Department of Nephrology with a suspicion of Alport syndrome. Slit-lamp examination showed iridescent deposits throughout the corneal anterior stroma and the inferior tarsal conjunctiva bilaterally. Anterior chamber optical coherence tomography showed stromal hyperreflectivity. CTNS gene was found to be positive for c.416C>T (Ser139Phe) mutation. The patient was offered oral and topical cysteamine which was refused. After a period of 5 years of follow-up, general health status remained stable, corneal disease showed no progression and photophobia complaints diminished. However, the patient was advised to start systemic and topical cysteamine because of the unknown development of the disease.

CONCLUSION

In this reported case, a delayed non-treatment decision has been proved right contrary to published evidence of active treatment of photophobia. The decision whether to treat or not to treat corneal involvement of the disease is not straightforward. Besides biomicroscopic evaluations, patients' complaints and expectations should be taken into account.

A case of ocular cystinosis associated with two potentially severe CTNS mutations

BACKGROUND

Ocular cystinosis is a rare autosomal recessive disorder caused by one severe and one mild mutation in the CTNS gene. It is characterised by cystine deposition within the cornea and conjunctiva however, the kidneys are not affected. We report a case of ocular cystinosis caused by two potentially severe CTNS mutations and discuss the possible mechanism of renal sparing.

METHODS

This is an observational case report of the proband and her unaffected relatives. All subjects underwent ophthalmic examination, whilst in the proband, In vivo laser scanning confocal microscopy was used to demonstrate cystine crystals within her corneas and conjunctiva. Genetic diagnosis was confirmed by DNA sequencing of the proband and the segregation of the mutations was established in her relatives. RT-PCR of leukocyte RNA was undertaken to determine if aberrant splicing of the CTNS gene was taking place Results: The proband was found to have cystine crystals limited to the anterior corneal stroma and the conjunctiva. Sequencing of the proband's CTNS gene found her to be a compound heterozygote for a 27bp deletion in exon8/intron 8 (c.559_561 + 24del) and a novel c.635C>T variant in exon 9 that is predicted be pathogenic and to result in the substitution of alanine with valine at amino acid position 212 (p.Ala212Val), which is within the 3^rd transmembrane spanning domain of the CTNS protein. Examination of the proband's leukocyte RNA failed to demonstrate any aberrant CTNS gene splicing.

CONCLUSION

We present a case of ocular cystinosis caused by two potentially severe CTNS gene mutations. The lack of renal involvement may be due to localised (ocular) aberrant CTNS RNA splicing.

A deletion in the Ctns gene causes renal tubular dysfunction and cystine accumulation in LEA/Tohm rats

The Long-Evans Agouti (LEA/Tohm) rat has recently been established as a new rat model of type 2 diabetes. The onset of diabetes mellitus was observed only in male LEA/Tohm rats; however, urinary glucose appeared before the onset of diabetes. To clarify the genetic basis of urinary glucose, we performed genetic linkage analysis using (BN × LEA) F₂ intercross progeny. A recessively acting locus responsible for urinary glucose excretion (ugl) was mapped to a 7.9 Mb region of chromosome 10, which contains the cystinosin (Ctns) gene. The Ctns gene encodes the cystine transporter, which transports cystine out of the lysosome and is responsible for nephropathic cystinosis in humans. Sequence analysis identified a 13-bp deletion in the Ctns gene, leading to a truncated and loss-of-function protein, which cause cystine accumulation in various tissues. We also developed a novel congenic rat strain harboring the Ctns^ugl mutation on the F344 genetic background. Phenotypic analysis of F344-Ctns^ugl rats indicated that the incidence of urinary glucose was 100% in both males and females at around 40 weeks of age, and marked cystine accumulation was observed in the tissues, as well as remarkable renal lesions and cystine crystals in the lysosomes of the renal cortex. Furthermore, treatment with cysteamine depleted the cystine contents in F344-Ctns^ugl rat embryonic fibroblasts. These results indicated that the F344-Ctns^ugl rat provides a novel rat model of cystinosis, which allows not only a better understanding of the pathogenesis and pathophysiology of cystinosis but will also contribute to the development of new therapies.

Atypical onset of nephropathic infantile cystinosis in a Russian patient with rare CTNS mutation

We report a Russian patient with atypical onset of infantile nephropathic cystinosis. The disease debuted with vomiting and loss of weight and motor skills. Nephropathic changes appeared 6 months after onset of disease. Exome sequencing can be useful for diagnosing cystinosis in patients with neurological abnormalities before onset of nephropathic symptoms.

Amino acid transporters revisited: New views in health and disease

Amino acid transporters (AATs) are membrane-bound transport proteins that mediate transfer of amino acids into and out of cells or cellular organelles. AATs have diverse functional roles ranging from neurotransmission to acid-base balance, intracellular energy metabolism, and anabolic and catabolic reactions. In cancer cells and diabetes, dysregulation of AATs leads to metabolic reprogramming, which changes intracellular amino acid levels, contributing to the pathogenesis of cancer, obesity and diabetes. Indeed, the neutral amino acid transporters (NATs) SLC7A5/LAT1 and SLC1A5/ASCT2 are likely involved in several human malignancies. However, a clinical therapy that directly targets AATs has not yet been developed. The purpose of this review is to highlight the structural and functional diversity of AATs, their diverse physiological roles in different tissues and organs, their wide-ranging implications in human diseases and the emerging strategies and tools that will be necessary to target AATs therapeutically.

Novel applications of array comparative genomic hybridization in molecular diagnostics

INTRODUCTION

In 2004, the implementation of array comparative genomic hybridization (array comparative genome hybridization [CGH]) into clinical practice marked a new milestone for genetic diagnosis. Array CGH and single-nucleotide polymorphism (SNP) arrays enable genome-wide detection of copy number changes in a high resolution, and therefore microarray has been recognized as the first-tier test for patients with intellectual disability or multiple congenital anomalies, and has also been applied prenatally for detection of clinically relevant copy number variations in the fetus. Area covered: In this review, the authors summarize the evolution of array CGH technology from their diagnostic laboratory, highlighting exonic SNP arrays developed in the past decade which detect small intragenic copy number changes as well as large DNA segments for the region of heterozygosity. The applications of array CGH to human diseases with different modes of inheritance with the emphasis on autosomal recessive disorders are discussed. Expert commentary: An exonic array is a powerful and most efficient clinical tool in detecting genome wide small copy number variants in both dominant and recessive disorders. However, whole-genome sequencing may become the single integrated platform for detection of copy number changes, single-nucleotide changes as well as balanced chromosomal rearrangements in the near future.

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.

Nephropathic Cystinosis: Symptoms, Treatment, and Perspectives of a Systemic Disease

Cystinosis is a rare autosomal recessive lysosomal storage disorder caused by mutations in the CTNS gene. Main dysfunction is a defective clearance of cystine from lysosomes that leads to accumulation of cystine crystals in every tissue of the body. There are three different forms: infantile nephropathic cystinosis, which is the most common form, juvenile nephropatic, and non-nephropathic cystinosis. Mostly, first symptom in infantile nephropathic cystinosis is renal Fanconi syndrome that occurs within the first year of life. Another prominent symptom is photophobia due to corneal crystal deposition. Cystine depletion therapy with cysteamine delays end-stage renal failure but does not stop progression of the disease. A new cysteamine formulation with delayed-release simplifies the administration schedule but still does not cure cystinosis. Even long-term depletion treatment resulting in bypassing the defective lysosomal transporter cannot reverse Fanconi syndrome. A future perspective offering a curative therapy may be transplantation of CTNS-carrying stem cells that has successfully been performed in mice.

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.

Exacerbating and reversing lysosomal storage diseases: from yeast to humans

Lysosomal storage diseases (LSDs) arise from monogenic deficiencies in lysosomal proteins and pathways and are characterized by a tissue-wide accumulation of a vast variety of macromolecules, normally specific to each genetic lesion. Strategies for treatment of LSDs commonly depend on reduction of the offending metabolite(s) by substrate depletion or enzyme replacement. However, at least 44 of the ~50 LSDs are currently recalcitrant to intervention. Murine models have provided significant insights into our understanding of many LSD mechanisms; however, these systems do not readily permit phenotypic screening of compound libraries, or the establishment of genetic or gene-environment interaction networks. Many of the genes causing LSDs are evolutionarily conserved, thus facilitating the application of models system to provide additional insight into LSDs. Here, we review the utility of yeast models of 3 LSDs: Batten disease, cystinosis, and Niemann-Pick type C disease. We will focus on the translation of research from yeast models into human patients suffering from these LSDs. We will also discuss the use of yeast models to investigate the penetrance of LSDs, such as Niemann-Pick type C disease, into more prevalent syndromes including viral infection and obesity.

Infantile Nephropathic Cystinosis: A Novel CTNS Mutation

Cystinosis is a rare autosomal recessive metabolic disorder characterized by the accumulation of cystine in lysosomes, which results from defects in the carrier-mediated transport protein encoded by the CTNS gene. Infantile nephropathic cystinosis (INC) is one of the major complications of cystinosis. It is characterized by findings of Fanconi's syndrome within the first year of life. Here we report two patients with INC presenting with signs of Fanconi's syndrome and describe a novel CTNS mutation.

Cystinosis (ctns) zebrafish mutant shows pronephric glomerular and tubular dysfunction

The human ubiquitous protein cystinosin is responsible for transporting the disulphide amino acid cystine from the lysosomal compartment into the cytosol. In humans, Pathogenic mutations of CTNS lead to defective cystinosin function, intralysosomal cystine accumulation and the development of cystinosis. Kidneys are initially affected with generalized proximal tubular dysfunction (renal Fanconi syndrome), then the disease rapidly affects glomeruli and progresses towards end stage renal failure and multiple organ dysfunction. Animal models of cystinosis are limited, with only a Ctns knockout mouse reported, showing cystine accumulation and late signs of tubular dysfunction but lacking the glomerular phenotype. We established and characterized a mutant zebrafish model with a homozygous nonsense mutation (c.706 C > T; p.Q236X) in exon 8 of ctns. Cystinotic mutant larvae showed cystine accumulation, delayed development, and signs of pronephric glomerular and tubular dysfunction mimicking the early phenotype of human cystinotic patients. Furthermore, cystinotic larvae showed a significantly increased rate of apoptosis that could be ameliorated with cysteamine, the human cystine depleting therapy. Our data demonstrate that, ctns gene is essential for zebrafish pronephric podocyte and proximal tubular function and that the ctns-mutant can be used for studying the disease pathogenic mechanisms and for testing novel therapies for cystinosis.

Impact of Cystinosin Glycosylation on Protein Stability by Differential Dynamic Stable Isotope Labeling by Amino Acids in Cell Culture (SILAC)

Cystinosis is a rare autosomal recessive lysosomal storage disorder characterized by intralysosomal accumulation of cystine. The causative gene for cystinosis is CTNS, which encodes the protein cystinosin, a lysosomal proton-driven cystine transporter. Over 100 mutations have been reported, leading to varying disease severity, often in correlation with residual cystinosin activity as a transporter and with maintenance of its protein-protein interactions. In this study, we focus on the ΔITILELP mutation, the only mutation reported that sometimes leads to severe forms, inconsistent with its residual transported activity. ΔITILELP is a deletion that eliminates a consensus site on N66, one of the protein's seven glycosylation sites. Our hypothesis was that the ΔITILELP mutant is less stable and undergoes faster degradation. Our dynamic stable isotope labeling by amino acids in cell culture (SILAC) study clearly showed that wild-type cystinosin is very stable, whereas ΔITILELP is degraded three times more rapidly. Additional lysosome inhibition experiments confirmed ΔITILELP instability and showed that the degradation was mainly lysosomal. We observed that in the lysosome, ΔITILELP is still capable of interacting with the V-ATPase complex and some members of the mTOR pathway, similar to the wild-type protein. Intriguingly, our interactomic and immunofluorescence studies showed that ΔITILELP is partially retained at the endoplasmic reticulum (ER). We proposed that the ΔITILELP mutation causes protein misfolding, ER retention and inability to be processed in the Golgi apparatus, and we demonstrated that ΔITILELP carries high-mannose glycans on all six of its remaining glycosylation sites. We found that the high turnover of ΔITILELP, because of its immature glycosylation state in combination with low transport activity, might be responsible for the phenotype observed in some patients.

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.

Exome Sequencing Discerns Syndromes in Patients from Consanguineous Families with Congenital Anomalies of the Kidneys and Urinary Tract

Congenital anomalies of the kidneys and urinary tract (CAKUT) are the leading cause of CKD in children, featuring a broad variety of malformations. A monogenic cause can be detected in around 12% of patients. However, the morphologic clinical phenotype of CAKUT frequently does not indicate specific genes to be examined. To determine the likelihood of detecting causative recessive mutations by whole-exome sequencing (WES), we analyzed individuals with CAKUT from 33 different consanguineous families. Using homozygosity mapping and WES, we identified the causative mutations in nine of the 33 families studied (27%). We detected recessive mutations in nine known disease-causing genes: ZBTB24, WFS1, HPSE2, ATRX, ASPH, AGXT, AQP2, CTNS, and PKHD1 Notably, when mutated, these genes cause multiorgan syndromes that may include CAKUT as a feature (syndromic CAKUT) or cause renal diseases that may manifest as phenocopies of CAKUT. None of the above monogenic disease-causing genes were suspected on clinical grounds before this study. Follow-up clinical characterization of those patients allowed us to revise and detect relevant new clinical features in a more appropriate pathogenetic context. Thus, applying WES to the diagnostic approach in CAKUT provides opportunities for an accurate and early etiology-based diagnosis and improved clinical management.

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.

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.

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.

Lysosome dysfunction in the pathogenesis of kidney diseases

The lysosome, an organelle central to macromolecule degradation and recycling, plays a pivotal role in normal cell processes, ranging from autophagy to redox regulation. Not surprisingly, lysosomes are an integral part of the renal epithelial molecular machinery that facilitates normal renal physiology. Two inherited diseases that manifest as kidney dysfunction are Fabry's disease and cystinosis, each of which is caused by a primary biochemical defect at the lysosome resulting from loss-of-function mutations in genes that encode lysosomal proteins. The functions of the lysosomes in the kidney and how lysosomal dysfunction might contribute to Fabry's disease and cystinosis are discussed. Unlike most other pediatric renal diseases, therapies are available for Fabry's disease and cystinosis, but require early diagnosis. Recent analysis of ceroid neuronal lipofuscinosis type 3 (Cln3) null mice, a mouse model of lysosomal disease that is primarily associated with neurological deficits, revealed renal functional abnormalities. As current and future therapeutics increase the life-span of those suffering from diseases like neuronal ceroid lipofuscinosis, it remains a distinct possibility that many more lysosomal disorders that primarily manifest as infant and juvenile neurodegenerative diseases may also include renal disease phenotypes.

Treatment of corneal cystine crystal accumulation in patients with cystinosis

Cystinosis is a rare autosomal recessive disorder characterized by the accumulation of cystine within the cells of different organs. Infantile nephropathic cystinosis is the most common and severe phenotype. With the success of renal transplantation, these patients are now living longer and thus more long-term complications within different organs are becoming apparent. Ophthalmic manifestations range from corneal deposits of cystine crystals to pigmentary retinopathy. With increasing age, more severe ocular complications have been reported. Photophobia is a prominent symptom for patients. With prolonged survival and increasing age, this symptom, along with corneal erosions and blepharospasm, can become debilitating. This review revisits the basic pathogenesis of cystinosis, the ocular manifestations of the disease, and the treatment of corneal crystals.

Inhibition of intracellular clusterin attenuates cell death in nephropathic cystinosis

Nephropathic cystinosis, characterized by accumulation of cystine in the lysosomes, is caused by mutations in CTNS. The molecular and cellular mechanisms underlying proximal tubular dysfunction and progressive renal failure in nephropathic cystinosis are largely unclear, and increasing evidence supports the notion that cystine accumulation alone is not responsible for the end organ injury in cystinosis. We previously identified clusterin as potentially involved in nephropathic cystinosis. Here, we studied the expression of clusterin in renal proximal tubular epithelial cells obtained from patients with nephropathic cystinosis. The cytoprotective secretory form of clusterin, as evaluated by Western blot analysis, was low or absent in cystinosis cells compared with normal primary cells. Confocal microscopy revealed elevated levels of intracellular clusterin in cystinosis cells. Clusterin in cystinosis cells localized to the nucleus and cytoplasm and showed a filamentous and punctate aggresome-like pattern compared with diffuse cytoplasmic staining in normal cells. In kidney biopsy samples from patients with nephropathic cystinosis, clusterin protein expression was mainly limited to the proximal tubular cells. Furthermore, expression of clusterin overlapped with the expression of apoptotic proteins (apoptosis-inducing factor and cleaved caspase-3) and autophagy proteins (LC3 II and p62). Silencing of the clusterin gene resulted in a significant increase in cell viability and attenuation of apoptosis in cystinosis cells. Results of this study identify clusterin as a pivotal factor in the cell injury mechanism of nephropathic cystinosis and provide evidence linking cellular stress and injury to Fanconi syndrome and progressive renal injury in nephropathic cystinosis.

Mutational Spectrum of the CTNS Gene in Egyptian Patients with Nephropathic Cystinosis

BACKGROUND

Nephropathic cystinosis is a rare autosomal recessive disorder caused by mutations in the CTNS gene, encoding for cystinosin, a carrier protein transporting cystine out of lysosomes. Its deficiency leads to cystine accumulation and cell damage in multiple organs, especially in the kidney. In this study, we aimed to provide the first report describing the mutational spectrum of Egyptian patients with nephropathic cystinosis and their genotype-phenotype correlation.

METHODS

Fifteen Egyptian patients from 13 unrelated families with infantile nephropathic cystinosis were evaluated clinically, biochemically, and genetically. Screening for the common 57-kb deletion was performed by standard multiplex PCR, followed by direct sequencing of the ten coding exons, exon-intron interfaces, and promoter region.

RESULTS

None of the 15 Egyptian patients had the 57-kb deletion. Twenty-seven mutant alleles and 12 pathogenic mutations were detected including six novel mutations: two frameshift (c.260_261delTT; p.F87SfsX36, c.1032delCinsTG; p.F345CfsX19), one nonsense (c.734G>A; p.W245fsX), two missense (c.1084G>A; pG362R, c.560A>G; p.K187R), and one intronic splicing mutation (IVS3+5g>t). A novel promoter region mutation (1-593-41C>T) seemed to be detected but was excluded as a pathogenic mutation by quantitative real-time PCR analysis.

CONCLUSIONS

This study could be the basis for future genetic counseling and prenatal diagnosis of patients with nephropathic cystinosis in Egyptian and surrounding populations. The screening for the 57-kb deletion is not recommended anymore outside its geographical distribution, especially in the region of the Middle East. A common Middle Eastern mutation (c.681G>A; E227E) was pointed out and discussed.

Cystinosis as a lysosomal storage disease with multiple mutant alleles: Phenotypic-genotypic correlations

Cystinosis is an autosomal recessive lysosomal storage disease with an unclear enzymatic defect causing lysosomal cystine accumulation with no corresponding elevation of plasma cystine levels leading to multisystemic dysfunction. The systemic manifestations include a proximal renal tubular defect (Fanconi-like), endocrinal disturbances, eye involvements, with corneal, conjunctival and retinal depositions, and neurological manifestations in the form of brain and muscle dysfunction. Most of the long-term ill effects of cystinosis are observed particularly in patients with long survival as a result of a renal transplant. Its responsible CTNS gene that encodes the lysosomal cystine carrier protein (cystinosin) has been mapped on the short arm of chromosome 17 (Ch17 p13). There are three clinical forms based on the onset of main symptoms: nephropathic infantile form, nephropathic juvenile form and non-nephropathic adult form with predominant ocular manifestations. Avoidance of eye damage from sun exposure, use of cystine chelators (cysteamine) and finally renal transplantation are the main treatment lines. Pre-implantation genetic diagnosis for carrier parents is pivotal in the prevention of recurrence.

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.

Two novel CTNS mutations in cystinosis patients in Thailand

Cystinosis 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. We performed mutation analysis of CTNS in six cystinosis patients from four families in Thailand. Using PCR sequencing of the entire coding regions, we identified all eight mutant alleles, including two mutations, p.G309D and p.Q284X, that have not been previously reported. This study expands the mutational and population spectrum of nephropathic cystinosis.

Mechanism of proton/substrate coupling in the heptahelical lysosomal transporter cystinosin

Secondary active transporters use electrochemical gradients provided by primary ion pumps to translocate metabolites or drugs "uphill" across membranes. Here we report the ion-coupling mechanism of cystinosin, an unusual eukaryotic, proton-driven transporter distantly related to the proton pump bacteriorhodopsin. In humans, cystinosin exports the proteolysis-derived dimeric amino acid cystine from lysosomes and is impaired in cystinosis. Using voltage-dependence analysis of steady-state and transient currents elicited by cystine and neutralization-scanning mutagenesis of conserved protonatable residues, we show that cystine binding is coupled to protonation of a clinically relevant aspartate buried in the membrane. Deuterium isotope substitution experiments are consistent with an access of this aspartate from the lysosomal lumen through a deep proton channel. This aspartate lies in one of the two PQ-loop motifs shared by cystinosin with a set of eukaryotic membrane proteins of unknown function and is conserved in about half of them, thus suggesting that other PQ-loop proteins may translocate protons.

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.

Cysteamine therapy delays the progression of nephropathic cystinosis in late adolescents and adults

Nephropathic cystinosis is a multisystem autosomal recessive disease caused by cystine accumulation, which is usually treated by oral cysteamine. In order to determine long-term effects of this therapy, we enrolled 86 adult patients (mean age 26.7 years) diagnosed with nephropathic cystinosis, 75 of whom received cysteamine. Therapy was initiated at a mean age of 9.9 years with a mean duration of 17.4 years. By last follow-up, 78 patients had end-stage renal disease (mean age 11.1 years), 62 had hypothyroidism (mean age 13.4), 48 developed diabetes (mean age 17.1 years), and 32 had neuromuscular disorders (mean age 23.3 years). Initiating cysteamine therapy before 5 years of age significantly decreased the incidence and delayed the onset of end-stage renal disease, and significantly delayed the onset of hypothyroidism, diabetes, and neuromuscular disorders. The development of diabetes and hypothyroidism was still significantly delayed, however, in patients in whom therapy was initiated after 5 years of age, compared with untreated patients. The life expectancy was significantly improved in cysteamine-treated versus untreated patients. Thus, cysteamine decreases and delays the onset of complications and improves life expectancy in cystinosis. Hence, cysteamine therapy should be introduced as early as possible during childhood and maintained lifelong.

Natural history of adolescent-onset cystinosis

Cystinosis is a rare autosomal recessive disease caused by mutations of the CTNS gene in which cystine accumulates throughout the body as a result of a defective efflux of cystine from lysosomes. Three phenotypic forms have been described according to the age of onset and the severity of the clinical symptoms: infantile, intermediate, and ocular non-nephropathic cystinosis. Here we report the natural history of cystinosis in a 55-year-old man with intermediate nephropathic cystinosis diagnosed at 9 years of age. Although tubulopathy was unnoticed in the early years, he required transplantation at age 16. Sequencing analysis of all the CTNS exons revealed that the proband is homozygous for a 21-bp in-frame deletion in exon 5 (c. 198_218del21), resulting in an in-frame deletion of 7 amino acids from the N-terminal domain of the cystinosin protein. Our patient has had relatively mild extra-renal disease despite lack of early cysteamine therapy. He has been able to attend university and pursue a professional career into the 6th decade.

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.

Pregnancy-associated cardiomyopathy occurring in a young patient with nephropathic cystinosis

Cystinosis is a rare autosomal recessive metabolic disorder characterised by an intracellular accumulation of cystine leading to severe organ dysfunction. It affects renal function, has extra-renal complications but has rarely been associated with cardiac disease. Renal transplantation and cysteamine have dramatically improved the prognosis in the nephropathic form. We present the case of a young adult Caucasian female diagnosed with nephropathic cystinosis and receiving haemodialysis who subsequently developed dilated cardiomyopathy. She presented with acute cardiac failure occurring early after stillbirth following an unplanned pregnancy when her cysteamine had been stopped. Transthoracic echocardiography showed typical features of dilated cardiomyopathy which was absent on pre-pregnancy scans. Investigations failed to identify an underlying cause for her cardiomyopathy. She responded to conventional treatment and currently has had full recovery of her cardiac function confirmed on follow-up echocardiography. As cardiomyopathy rarely co-exists with cystinosis, we believe that this case represents pregnancy-associated cardiomyopathy rather than direct involvement by her cystinosis, particularly as a minority of pregnant patients with associated cardiomyopathy develop heart failure early before the conventional period for peripartum cardiomyopathy. Patient characteristics and maternal outcomes are similar, albeit with higher risk of premature delivery suggesting the same underlying pathological process.

Characterization of CTNS mutations in Arab patients with cystinosis

BACKGROUND

Cystinosis is an autosomal recessive disease characterized by impaired transport of free cystine out of lysosomes with resulting renal and ophthalmic manifestations. Mutations in CTNS, encoding cystinosin, are the only known cause of this autosomal recessive disorder with more than 85 different mutations described so far.

PURPOSE

To identify CTNS mutations in Arab cystinosis patients.

METHODS

In this study, we have analyzed the mutational spectrum of CTNS in a population of 21 patients from 13 families of Arab origin. The entire coding region and flanking intronic regions of CTNS were analyzed by direct sequencing.

RESULTS

Eight mutations were identified, four of which are novel (c.530A>G, c.681G>A, 1013T>G, and c.1018_1041del).

CONCLUSION

These alleles will provide the basis for routine molecular diagnosis of cystinosis in the region.