Controversies and research agenda in nephropathic cystinosis: conclusions from a “Kidney Disease: Improving Global Outcomes” (KDIGO) Controversies Conference

Development of the Dutch translational knowledge agenda for inherited metabolic diseases

Background

Inherited metabolic diseases (IMDs) may have considerable implications for patients and their families. Despite their individual rarity, covering a spectrum of over 1800 distinct diseases, the diseases collectively exert a significant impact, with often lifelong disabilities. The United for Metabolic Diseases consortium was established to catalyze research with translation into the best possible care.

Aim

To generate a translational knowledge agenda, which identifies and prioritizes research questions, directly relevant to patient care or for IMD patients and their families.

Methods and Results

Following a process established by the Knowledge Institute of the Dutch Association of Medical Specialists, we generated a comprehensive translational knowledge agenda for IMDs. A multidisciplinary steering committee, composed of 12 diverse metabolic experts collected research questions through an online questionnaire using snowballing. The 462 proposed questions were categorized and prioritized during a meeting attended by 22 representatives of all stakeholder groups. The resulting top 10 research questions cover multiple themes, i.e. prediction of disease progression, development of novel tools, mechanistic insights, improved diagnostics, therapeutic integration of multi-omics techniques, assessment of impact on daily life, expanding treatment avenues, optimal study designs, effect of lifestyle interventions, and data utilization using FAIR principles.

Discussion

This collective endeavor reflects the collaborative spirit needed for rare disease research. This knowledge agenda will guide funding directions and applications but will also boost interdisciplinary collaboration to push the field of IMDs research forward in a renewed UMD consortium. Patient engagement, transparency, and a comprehensive approach make this knowledge agenda a pivotal step toward addressing the pressing research needs and priorities in this domain.

Quality of life and mental health status in caregivers of pediatric patients with nephropathic cystinosis

There are few studies assessing psychological burden and quality of life (QoL) in caregivers of pediatric patients with nephropathic cystinosis, a severe chronic disease. This observational, single-center study aimed to explore the levels of anxiety, depression, care burden, and QoL status in caregivers of patients with nephropathic cystinosis. The Hospital Anxiety and Depression Scale (HADS), the Zarit Caregiver Burden Scale, and the Short Form-36 (SF-36) were administered to caregivers of pediatric patients with nephropathic cystinosis. Nine caregivers of pediatric patients with nephropathic cystinosis participated in the study (6 boys and 3 girls; mean age, 12.6 ± 4.2 years). All participating caregivers were the patient's mothers. Of the 9 caregivers, 6 showed anxiety/depression and 4 severe care burden. Overall, SF-36 QoL domains with a worse perception by caregivers were 'general health' and 'health change over time'. Mothers without depression/anxiety and low care burden had better QoL perception (p = 0.02). All caregivers with high care burden showed anxiety/depression. In our study cohort, caregivers of pediatric patients with nephropathic cystinosis showed high levels of anxiety/depression, high care burden, and impaired QoL, highlighting the importance of detecting psycho-social issues to implement strategies that relieve family stress and improve coping strategies.

Gastrointestinal challenges in nephropathic cystinosis: clinical perspectives

Gastrointestinal (GI) sequelae, such as vomiting, hyperacidity, dysphagia, dysmotility, and diarrhea, are nearly universal among patients with nephropathic cystinosis. These complications result from disease processes (e.g., kidney disease, cystine crystal accumulation in the GI tract) and side effects of treatments (e.g., cysteamine, immunosuppressive therapy). GI involvement can negatively impact patient well-being and jeopardize disease outcomes by compromising drug absorption and patient adherence to the strict treatment regimen required to manage cystinosis. Given improved life expectancy due to advances in kidney transplantation and the transformative impact of cystine-depleting therapy, nephrologists are increasingly focused on addressing extra-renal complications and quality of life in patients with cystinosis. However, there is a lack of clinical data and guidance to inform GI-related monitoring, interventions, and referrals by nephrologists. Various publications have examined the prevalence and pathophysiology of selected GI complications in cystinosis, but none have summarized the full picture or provided guidance based on the literature and expert experience. We aim to comprehensively review GI sequelae associated with cystinosis and its treatments and to discuss approaches for monitoring and managing these complications, including the involvement of gastroenterology and other disciplines.

Ocular Involvement in Patients with Infantile Nephropathic Cystinosis

Cystinosis is a rare autosomal recessive lysosomal storage disease associated with high mortality and morbidity rates. The most distinctive ocular manifestations of cystinosis are photophobia, tearing, and blurred vision. Herein, we assessed the ocular involvement of four patients from two families diagnosed with infantile nephropathic cystinosis using optical coherence tomography (OCT) and in vivo confocal microscopy (IVCM). Anterior segment OCT demonstrated multiple hyperreflective punctate deposits, and IVCM revealed needle-shaped bright crystal deposits in the corneal stroma in all patients. Three patients also had crystal deposits in the epithelium, where epithelial cell disruption was observed. Crystal deposits around the subepithelial nerve plexus were noted in some sections. In one patient, round and needle-shaped bright deposits along with inflammatory cells were observed in the limbal region of the conjunctiva. Infrared fundus images of two female siblings revealed hyperreflective crystal-like deposits around the optic disc, macula, and peripheral retina, and enhanced depth imaging OCT showed accumulation of crystals in all layers of the retina.

Intermediate cystinosis: a case report of 10-year treatment with cysteamine

BACKGROUND

Cystinosis is a lysosomal storage disorder characterized by an autosomal recessive phenotype. Intermediate cystinosis, which progresses slowly and causes renal failure, accounts for approximately 5% of all cystinosis cases. Patients with intermediate cystinosis may not exhibit the typical symptoms of cystinosis, such as Fanconi syndrome and ocular symptoms. Because of its diverse clinical presentation and rarity, intermediate cystinosis can be difficult to diagnose. Additionally, few patients can tolerate cystine-depleting drugs, such as cysteamine, because of their complicated administration schedules and side effects. We report a case of intermediate cystinosis that was treated with cysteamine for 10 years.

CASE PRESENTATION

Urinary abnormalities were first diagnosed when the patient was 3 years of age during a health examination specifically for 3-year-old children, which is unique to Japan. Cystinosis was diagnosed when the patient was 12 years of age. Cysteamine therapy was initiated and regular cystine concentration measurements were performed. Although proteinuria persisted, the patient's renal function progressed slowly. Two renal biopsies were performed, and multinucleated podocytes and cystine crystals without focal segmental glomerulosclerosis lesions were observed in the biopsy specimens. The patient's renal function remained stable.

CONCLUSIONS

This case of intermediate cystinosis was treated with cysteamine over the course of 10 years. Intermediate cystinosis requires an appropriate diagnosis and long-term treatment.

Worldwide disparities in access to treatment and investigations for nephropathic cystinosis: a 2023 perspective

BACKGROUND

Nephropathic cystinosis (NC) is a rare lysosomal disease, leading to early kidney failure and extra-renal comorbidities. Its prognosis strongly relies on early diagnosis and treatment by cysteamine. Developing economies (DEing) face many challenges when treating patients for rare and chronic diseases. The aim here is to evaluate the access to investigations and treatment in DEing, and to assess for potential inequalities with Developed Economies (DEed).

METHODS

In this international cross-sectional study, a questionnaire on access, price and reimbursement of genetic, biological analyses, and treatment was sent to nephrology centers worldwide during 2022.

RESULTS

A total of 109 centers responded, coming from 49 countries and managing 741 patients: 43 centers from 30 DEing and Economies in transition (TrE), and 66 from 19 DEed. In 2022, genetics availability was 63% in DEing and 100% in DEed, whereas intra leukocytes cystine levels (IL-CL) were available for 30% of DEing patients, and 94% of DEed patients, both increasing over the last decade, as has access to immediate release cysteamine and to cysteamine eye drops in DEing. However, delayed released cysteamine can be delivered to only 7% vs. 74% of patients from DEing and DEed, respectively, and is still poorly reimbursed in DEing.

CONCLUSIONS

Over the last decade, access to investigations (namely genetics and IL-CL) and to cysteamine have improved in DEing and TrE. However, discrepancies remain with DEed: access to delayed released cysteamine is limited, and reimbursement is still profoundly insufficient, therefore limiting their current use.

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.

Event-related potential (ERP) evidence of early visual processing differences in cystinosis

Cystinosis, a rare lysosomal storage disease, is characterized by cystine crystallization and accumulation within tissues and organs, including the kidneys and brain. Its impact on neural function appears mild relative to its effects on other organs, but therapeutic advances have led to substantially increased life expectancy, necessitating deeper understanding of its impact on neurocognitive function. Behavioral difficulties have been reported in cystinosis in the visual and visual-processing domain. Very little is known, however, about how the brains of people living with cystinosis process visual information, although cysteamine accumulation in the retina is a prominent feature of cystinosis. Here, electrophysiology was recorded during a Go/No-Go task to investigate early visual processing in cystinosis, compared to an age-matched control group. Analyses focused on early stages of cortical visual processing. The groups differed in their initial cortical response, with individuals with cystinosis exhibiting a significantly larger visual evoked potential (VEP) in the 130 to 150 ms time window. The timing and topography of this response suggested an enhanced P1 in cystinosis that 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. 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.

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.

Pregnancy in cystinosis patients with chronic kidney disease: A European case series

Cystinosis is a rare autosomal recessive disease leading to end-stage renal disease within the second or third decade of life. Since the era of specific treatment with cysteamine, prognosis has substantially improved and pregnancy becomes an increasing concern. Pregnancy data in patients with cystinosis were collected through an anonymized survey. We collected data for 19 pregnancies in 12 women. Seventeen patients were transplanted, 1 was on hemodialysis and 1 had chronic kidney disease (CKD) stage 4. These 19 pregnancies resulted in 13 live births (68.4%): 3 spontaneous early miscarriages, 1 ectopic pregnancy, 1 early pre-eclampsia (at 21 weeks), and 1 preterm birth with neonatal death at 24 weeks were reported. After exclusion of early miscarriage or termination, pregnancy success rate was 86.7%. In successful pregnancies, median gestational age at delivery was 34 weeks (24-37). Preeclampsia occurred in seven pregnancies (7/15, 46.7%). A cesarean section was performed in all pregnancies. Median baby weight at delivery was 2175 g (620-3374 g). After pregnancy, one patient reached end-stage renal disease, but she already had advanced CKD before pregnancy (creatinine 239 μmol/L, eGFR 23 ml/min/1.73 m² ). In three other patients, there was a decrease of eGFR of 8, 20, and 53 ml/min/1.73 m² , respectively. The majority of pregnancies were successful, but severe antenatal and post-natal complications may occur, in particular preeclampsia that was noticed in nearly half of patients and fetal loss in one-third of them. These results may help pre-pregnancy counseling and pregnancy management.

Biomarkers in Nephropathic Cystinosis: Current and Future Perspectives

Early diagnosis and effective therapy are essential for improving the overall prognosis and quality of life of patients with nephropathic cystinosis. The severity of kidney dysfunction and the multi-organ involvement as a consequence of the increased intracellular concentration of cystine highlight the necessity of accurate monitoring of intracellular cystine to guarantee effective treatment of the disease. Cystine depletion is the only available treatment, which should begin immediately after diagnosis, and not discontinued, to significantly slow progression of renal and extra-renal organ damage. This review aims to discuss the importance of the close monitoring of intracellular cystine concentration to optimize cystine depletion therapy. In addition, the role of new biomarkers in the management of the disease, from timely diagnosis to implementing treatment during follow-up, is overviewed.

Expert guidance on the multidisciplinary management of cystinosis in adolescent and adult patients

Cystinosis, a rare autosomal recessive lysosomal storage disorder, results in an abnormal accumulation of the amino acid cystine in multiple organs and tissues of the body. Renal symptoms typically develop in the first few months of life, with extra-renal manifestations becoming apparent over the next 10–20 years, which require coordinated multidisciplinary care. Here, we describe a consensus-based guidance to support the management of adolescents and adults living with cystinosis. The programme was led by a Steering Committee (SC) of six experts in the management of patients with cystinosis, who identified a list of 15 key questions reflecting the multi-organ effects of cystinosis. An Extended Faculty (EF) of eight additional specialists was invited to answer the questions via an online digital platform using a quasi-Delphi approach. The consolidated answers were summarized into recommendations. Where evidence was lacking, recommendations were developed using collective expert consensus. The EF was asked to agree/disagree with the clinical recommendations. The expert-agreed clinical recommendations provide guidance that considers both renal and extra-renal systems. The topics covered are advice on fertility and family planning, consideration of the nervous, muscular, ophthalmic, cardio-respiratory, endocrine, dermatological and gastrointestinal systems, as well as guidance on dental care, diet, lifestyle, and improving quality of life and psychological well-being. In summary, this work outlines recommendations and a checklist for clinicians with a vision for improving and standardizing the multidisciplinary care for patients with cystinosis.

Newborn Screening: Review of its Impact for Cystinosis

Newborn screening (NBS) programmes are considered to be one of the most successful secondary prevention measures in childhood to prevent or reduce morbidity and/or mortality via early disease identification and subsequent initiation of therapy. However, while many rare diseases can now be detected at an early stage using appropriate diagnostics, the introduction of a new target disease requires a detailed analysis of the entire screening process, including a robust scientific background, analytics, information technology, and logistics. In addition, ethics, financing, and the required medical measures need to be considered to allow the benefits of screening to be evaluated at a higher level than its potential harm. Infantile nephropathic cystinosis (INC) is a very rare lysosomal metabolic disorder. With the introduction of cysteamine therapy in the early 1980s and the possibility of renal replacement therapy in infancy, patients with cystinosis can now reach adulthood. Early diagnosis of cystinosis remains important as this enables initiation of cysteamine at the earliest opportunity to support renal and patient survival. Using molecular technologies, the feasibility of screening for cystinosis has been demonstrated in a pilot project. This review aims to provide insight into NBS and discuss its importance for nephropathic cystinosis using molecular technologies.

Cysteamine bitartrate delayed-release capsules control leukocyte cystine levels and promote statural growth and kidney health in an open-label study of treatment-naïve patients <6 years of age with nephropathic cystinosis

Nephropathic cystinosis is a rare autosomal recessive lysosomal storage disease that is characterized by accumulation of cysteine and formation of crystals within cells of different organs and tissues causing systemic manifestations in childhood that include poor linear growth, ocular involvement, hypothyroidism, and progressive kidney disease. This study was a long-term, prospective open-label evaluation of twice-daily delayed release (DR) cysteamine capsules in cystinosis patients <6 years of age who were naïve to any form of cysteamine treatment. Fifteen treatment-naïve patients <6 years old (mean age 2.2 ± 1.0 years, 53% male, 73% White) were enrolled and treated with DR-cysteamine capsules for up to 18 months. Patients had clinically meaningful decreases in WBC cysteine concentration during treatment (3.2 ± 3.0 nmol ½ cystine/mg protein at Day 1 to 0.8 ± 0.8 nmol ½ cystine/mg protein at study exit), and anthropometric data improvements were consistently observed in height, weight and body surface area. Additionally, estimated glomerular filtration rate increased from 55.93 ± 22.43 ml/min/1.73 m2 at baseline to 63.79 ± 21.44 ml/min/1.73 m2 at study exit. Pharmacokinetic/Pharmacodynamic results support the use of the same starting, escalation, and maintenance doses according to body surface for children aged <6 years that are currently recommended in adults and older children. All patients experienced ≥1 adverse event(s) with vomiting (80%) and upper respiratory tract infection (53%) most frequently reported. Based on our study, patients <6 years of age with nephropathic cystinosis without prior treatment can safely and effectively initiate treatment with DR-cysteamine, a delayed-release form of cysteamine bitartrate that can be given every 12 h.

Limitations and Challenges in the Stability of Cysteamine Eye Drop Compounded Formulations

Accumulation of cystine crystals in the cornea of patients suffering from cystinosis is considered pathognomonic and can lead to severe ocular complications. Cysteamine eye drop compounded formulations, commonly prepared by hospital pharmacy services, are meant to diminish the build-up of corneal cystine crystals. The objective of this work was to analyze whether the shelf life proposed for six formulations prepared following different protocols used in hospital pharmacies is adequate to guarantee the quality and efficacy of cysteamine eye drops. The long-term and in-use stabilities of these preparations were studied using different parameters: content of cysteamine and its main degradation product cystamine; appearance, color and odor; pH and viscosity; and microbiological analysis. The results obtained show that degradation of cysteamine was between 20% and 50% after one month of storage in the long-term stability study and between 35% and 60% in the in-use study. These data confirm that cysteamine is a very unstable molecule in aqueous solution, the presence of oxygen being the main degradation factor. Saturation with nitrogen gas of the solutions offers a means of reducing cysteamine degradation. Overall, all the formulae studied presented high instability at the end of their shelf life, suggesting that their clinical efficacy might be dramatically compromised.

Bartter syndrome and hypothyroidism masquerading cystinosis in a 3-year-old girl: rare manifestation of a rare disease

Cystinosis is a multisystem disorder with varied presentations secondary to deposition of cystine crystals in different organ systems. Children with cystinosis typically present with renal tubular acidosis and failure to thrive. We report a 3-year-old girl, born to a third-degree consanguineous couple, who presented with failure to thrive and polyuria. Laboratory investigations showed metabolic alkalosis suggestive of a Bartter-like syndrome and acquired hypothyroidism. Although metabolic alkalosis is a rare manifestation of cystinosis, the presence of renal tubular dysfunction and hypothyroidism prompted consideration of a probable diagnosis of cystinosis in the index child. Slit-lamp examination revealed cystine crystals in the cornea and genetic analysis showed a mutation in exon 9 of the CTNS (cystinosin, lysosomal cystine transporter) gene on chromosome 17. We highlight the importance of considering cystinosis as a differential diagnosis for Bartter syndrome and hypothyroidism.

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.

KDIGO Clinical Practice Guideline on the Evaluation and Management of Candidates for Kidney Transplantation

The 2020 Kidney Disease: Improving Global Outcomes (KDIGO) Clinical Practice Guideline on the Evaluation and Management of Candidates for Kidney Transplantation is intended to assist health care professionals worldwide who evaluate and manage potential candidates for deceased or living donor kidney transplantation. This guideline addresses general candidacy issues such as access to transplantation, patient demographic and health status factors, and immunological and psychosocial assessment. The roles of various risk factors and comorbid conditions governing an individual's suitability for transplantation such as adherence, tobacco use, diabetes, obesity, perioperative issues, causes of kidney failure, infections, malignancy, pulmonary disease, cardiac and peripheral arterial disease, neurologic disease, gastrointestinal and liver disease, hematologic disease, and bone and mineral disorder are also addressed. This guideline provides recommendations for evaluation of individual aspects of a candidate's profile such that each risk factor and comorbidity are considered separately. The goal is to assist the clinical team to assimilate all data relevant to an individual, consider this within their local health context, and make an overall judgment on candidacy for transplantation. The guideline development process followed the Grades of Recommendation Assessment, Development, and Evaluation (GRADE) approach. Guideline recommendations are primarily based on systematic reviews of relevant studies and our assessment of the quality of that evidence, and the strengths of recommendations are provided. Limitations of the evidence are discussed with differences from previous guidelines noted and suggestions for future research are also provided.

Adult complications of nephropathic cystinosis: a systematic review

While nephropathic cystinosis is classically thought of as a childhood disease, with improved treatments, patients are more commonly living into adulthood. We performed a systematic review of the literature available on what complications this population faces as it ages. Nearly every organ system is affected in cystinosis, either from the disease itself or from sequelae of kidney transplantation. While cysteamine is known to delay the onset of end-stage kidney disease, its effects on other complications of cystinosis are less well determined. More common adult-onset complications include myopathy, diabetes, and hypothyroidism. Some less common complications, such as neurologic dysfunction, can still have a profound impact on those with cystinosis. Areas for further research in this area include additional study of the impact of cysteamine on the nonrenal manifestations of cystinosis, as well as possible avenues for new and novel treatments.

Inherited disorders of lysosomal membrane transporters

Disorders caused by defects in lysosomal membrane transporters form a distinct subgroup of lysosomal storage disorders (LSDs). To date, defects in only 10 lysosomal membrane transporters have been associated with inherited disorders. The clinical presentations of these diseases resemble the phenotypes of other LSDs; they are heterogeneous and often present in children with neurodegenerative manifestations. However, for pathomechanistic and therapeutic studies, lysosomal membrane transport defects should be distinguished from LSDs caused by defective hydrolytic enzymes. The involved proteins differ in function, localization, and lysosomal targeting, and the diseases themselves differ in their stored material and therapeutic approaches. We provide an overview of the small group of disorders of lysosomal membrane transporters, emphasizing discovery, pathomechanism, clinical features, diagnostic methods and therapeutic aspects. We discuss common aspects of lysosomal membrane transporter defects that can provide the basis for preclinical research into these disorders.

Clinical trial readiness study of distal myopathy and dysphagia in nephropathic cystinosis

BACKGROUND

Nephropathic cystinosis is a lysosomal storage disorder with late-onset systemic complications, such as myopathy and dysphagia. Currently employed outcome measures lack sensitivity and responsiveness for dysphagia and myopathy, a limitation to clinical trial readiness.

METHODS

We evaluated 20 patients with nephropathic cystinosis in two visits over the course of a year to identify outcomes sensitive to detect changes over time. Patients also underwent an expiratory muscle strength training program to assess any effects on aspiration and dysphagia.

RESULTS

There were significant differences in the Timed Up and Go Test (TUG) and Timed 25-Foot Walk (25-FW) between baseline and 1-y follow-up (P < .05). Maximum expiratory pressure (MEP) and peak cough flow (PCF) significantly improved following respiratory training (P < .05).

CONCLUSIONS

Improved respiratory outcomes may enhance patients ability to expel aspirated material from the airway, stave off pulmonary sequelae associated with chronic aspiration, and yield an overall improvement in physical health and well-being.

Oh cystinosin: let me count the ways!

Nephropathic cystinosis is the most common genetic cause of a renal Fanconi syndrome and results from dysfunction of the lysosomal cystine-transporter protein cystinosin. The multiple organ dysfunctions of affected patients were thought to be related to the defective protein, with cystine crystal formation. However, such crystals were not always present when looked for. More recently, study of the biology of cystinosis has expanded to include many other cellular processes that may be pathogenic in the disease, and now galectin-3 can be added to those identified.

Chitotriosidase as a Novel Biomarker for Therapeutic Monitoring of Nephropathic Cystinosis

BACKGROUND

Nephropathic cystinosis, a hereditary lysosomal storage disorder caused by dysfunction of the lysosomal cotransporter cystinosin, leads to cystine accumulation and cellular damage in various organs, particularly in the kidney. Close therapeutic monitoring of cysteamine, the only available disease-modifying treatment, is recommended. White blood cell cystine concentration is the current gold standard for therapeutic monitoring, but the assay is technically demanding and is available only on a limited basis. Because macrophage-mediated inflammation plays an important role in the pathogenesis of cystinosis, biomarkers of macrophage activation could have potential for the therapeutic monitoring of cystinosis.

METHODS

We conducted a 2-year prospective, longitudinal study in which 61 patients with cystinosis who were receiving cysteamine therapy were recruited from three European reference centers. Each regular care visit included measuring four biomarkers of macrophage activation: IL-1β, IL-6, IL-18, and chitotriosidase enzyme activity.

RESULTS

A multivariate linear regression analysis of the longitudinal data for 57 analyzable patients found chitotriosidase enzyme activity and IL-6 to be significant independent predictors for white blood cell cystine levels in patients of all ages with cystinosis; a receiver operating characteristic analysis ranked chitotriosidase as superior to IL-6 in distinguishing good from poor therapeutic control (on the basis of white blood cell cystine levels of <2 nmol 1/2 cystine/mg protein or ≥2 nmol 1/2 cystine/mg protein, respectively). Moreover, in patients with at least one extrarenal complication, chitotriosidase significantly correlated with the number of extrarenal complications and was superior to white blood cell cystine levels in predicting the presence of multiple extrarenal complications.

CONCLUSIONS

Chitotriosidase enzyme activity holds promise as a biomarker for use in therapeutic monitoring of nephropathic cystinosis.

Trends in kidney transplant outcomes in children and young adults with cystinosis

Temporal changes in kidney transplant outcomes for cystinosis are unknown. We used the SRTR to identify all kidney transplants performed for cystinosis in patients younger than 31 years between 1987 and 2017. We divided time into three equal eras (1987-1997, 1998-2007, and 2008-2017) to assess changes in outcomes using Cox proportional and linear regression models. We examined 441 transplants in 362 patients. Age at ESRD progressively increased (12.1 vs 13.3 vs 13.4; P = .046). Eras 2 and 3 had lower risk of acute rejection (aHR 2 vs 1:0.45; P < .001) (aHR 3 vs 1:0.26; P < .001) and higher 5-year mean GFR (difference 2 vs 1:9.2 mL/min/1.73 m² ; P = .005) (difference 3 vs 1:12.9 mL/min/1.73 m² ; P = .002) compared with era 1. Five-year graft survival was similar across eras, but 5-year patient survival was higher for era 2 (aHR: 0.25; P = .01). Seventy-nine patients underwent retransplantation. Five-year patient (94.2% vs 92.5%; P = .57) and graft survival (79.1% vs 74.1%; P = .52) were similar between primary and subsequent transplants. Age at ESRD, acute rejection, GFR at 5 years, and patient survival improved over time. Kidney retransplantation is associated with excellent outcomes in children and young adults with cystinosis.

Clinical myopathy in patients with nephropathic cystinosis

BACKGROUND

Nephropathic cystinosis is a lysosomal storage disorder. Patient survival years after renal transplantation has revealed systemic complications including distal myopathy and dysphagia.

METHODS

We evaluated 20 adult patients with nephropathic cystinosis using patient-reported and clinical outcome measures. Standard motor measures, video fluoroscopy swallow studies, and tests of respiratory function were performed. We also used Rasch analysis of an initial survey to design a 16-item survey focused on upper and lower extremity function, which was completed by 31 additional patients.

RESULTS

Distal myopathy and dysphagia were common in patients with nephropathic cystinosis. Muscle weakness ranges from mild involvement of intrinsic hand muscles to prominent distal greater than proximal weakness and contractures.

CONCLUSIONS

In addition to further characterization of underlying dysphagia and muscle weakness, we propose a new psychometrically devised, disease specific, functional outcome measures for distal myopathy in patients with nephropathic cystinosis.

Proximal renal tubular acidosis with and without Fanconi syndrome

Proximal renal tubular acidosis (RTA) is caused by a defect in bicarbonate (HCO₃⁻) reabsorption in the kidney proximal convoluted tubule. It usually manifests as normal anion-gap metabolic acidosis due to HCO₃⁻ wastage. In a normal kidney, the thick ascending limb of Henle’s loop and more distal nephron segments reclaim all of the HCO₃⁻ not absorbed by the proximal tubule. Bicarbonate wastage seen in type II RTA indicates that the proximal tubular defect is severe enough to overwhelm the capacity for HCO₃⁻ reabsorption beyond the proximal tubule. Proximal RTA can occur as an isolated syndrome or with other impairments in proximal tubular functions under the spectrum of Fanconi syndrome. Fanconi syndrome, which is characterized by a defect in proximal tubular reabsorption of glucose, amino acids, uric acid, phosphate, and HCO₃⁻, can occur due to inherited or acquired causes. Primary inherited Fanconi syndrome is caused by a mutation in the sodium-phosphate cotransporter (NaP_i-II) in the proximal tubule. Recent studies have identified new causes of Fanconi syndrome due to mutations in the EHHADH and the HNF4A genes. Fanconi syndrome can also be one of many manifestations of various inherited systemic diseases, such as cystinosis. Many of the acquired causes of Fanconi syndrome with or without proximal RTA are drug-induced, with the list of causative agents increasing as newer drugs are introduced for clinical use, mainly in the oncology field.

Learning Physiology From Inherited Kidney Disorders

The identification of genes causing inherited kidney diseases yielded crucial insights in the molecular basis of disease and improved our understanding of physiological processes that operate in the kidney. Monogenic kidney disorders are caused by mutations in genes coding for a large variety of proteins including receptors, channels and transporters, enzymes, transcription factors, and structural components, operating in specialized cell types that perform highly regulated homeostatic functions. Common variants in some of these genes are also associated with complex traits, as evidenced by genome-wide association studies in the general population. In this review, we discuss how the molecular genetics of inherited disorders affecting different tubular segments of the nephron improved our understanding of various transport processes and of their involvement in homeostasis, while providing novel therapeutic targets. These include inherited disorders causing a dysfunction of the proximal tubule (renal Fanconi syndrome), with emphasis on epithelial differentiation and receptor-mediated endocytosis, or affecting the reabsorption of glucose, the handling of uric acid, and the reabsorption of sodium, calcium, and magnesium along the kidney tubule.

Vigier R, Zustin J, Haffner D. Management of bone disease in cystinosis: Statement from an international conference

Cystinosis is an autosomal recessive storage disease due to impaired transport of cystine out of lysosomes. Since the accumulation of intracellular cystine affects all organs and tissues, the management of cystinosis requires a specialized multidisciplinary team consisting of pediatricians, nephrologists, nutritionists, ophthalmologists, endocrinologists, neurologists' geneticists, and orthopedic surgeons. Treatment with cysteamine can delay or prevent most clinical manifestations of cystinosis, except the renal Fanconi syndrome. Virtually all individuals with classical, nephropathic cystinosis suffer from cystinosis metabolic bone disease (CMBD), related to the renal Fanconi syndrome in infancy and progressive chronic kidney disease (CKD) later in life. Manifestations of CMBD include hypophosphatemic rickets in infancy, and renal osteodystrophy associated with CKD resulting in bone deformities, osteomalacia, osteoporosis, fractures, and short stature. Assessment of CMBD involves monitoring growth, leg deformities, blood levels of phosphate, electrolytes, bicarbonate, calcium, and alkaline phosphatase, periodically obtaining bone radiographs, determining levels of critical hormones and vitamins, such as thyroid hormone, parathyroid hormone, 25(OH) vitamin D, and testosterone in males, and surveillance for nonrenal complications of cystinosis such as myopathy. Treatment includes replacement of urinary losses, cystine depletion with oral cysteamine, vitamin D, hormone replacement, physical therapy, and corrective orthopedic surgery. The recommendations in this article came from an expert meeting on CMBD that took place in Salzburg, Austria, in December 2016.

Swallowing dysfunction in patients with nephropathic cystinosis

INTRODUCTION

Nephropathic cystinosis is a rare autosomal recessive lysosomal storage disorder caused by mutations in the CTNS gene. Patients with nephropathic cystinosis suffer not only from renal disease but have also other systemic complications like myopathy and swallowing dysfunction. Dysphagia for solid food is mentioned in patients with cystinosis, but in clinical practice swallowing investigations are only performed when the patient has complaints. The aim of this study was to explore the swallowing function in patients with cystinosis by use of the Test of Mastication and Swallowing Solids (TOMASS), and to compare their performance with patients with myotonic dystrophy type 1 - a neuromuscular disease in which dysphagia for solid food is a known problem.

METHODS

Twenty adult patients with cystinosis (11 men and 9 women, range 19-51 years) and 10 patients with myotonic dystrophy type 1 (5 men and 5 women, range 20-60 years) were included. All cystinosis patients were treated with cysteamine. Data of the two groups were compared with normative data using independent-samples t-tests. In case the variables were not normally distributed, the non-parametric Mann-Whitney U test was used.

RESULTS

There was a significant difference in the number of bites, masticatory cycles, swallows and total time between the normal values and cystinosis patients. The results of the cystinosis patients were comparable to those of the patients with myotonic dystrophy.

DISCUSSION AND CONCLUSION

Adult patients with cystinosis have significant dysphagia for solid food. Clinicians treating these patients should be aware of this fact. The TOMASS can be performed easily in clinical practice to investigate whether patients with cystinosis have swallowing dysfunction. The swallowing dysfunction can now be diagnosed by use of a non-invasive, very simple, non-harmful test. It can be discussed whether this should be added to the regular care scheme of cystinosis patients in order to regularly follow-up swallowing function.

Effects of long-term cysteamine treatment in patients with cystinosis

Cystinosis is a rare autosomal-recessive lysosomal storage disease with high morbidity and mortality. It is caused by mutations in the CTNS gene that encodes the cystine transporter, cystinosin, which leads to lysosomal cystine accumulation. Patients with infantile nephropathic cystinosis, the most common and most severe clinical form of cystinosis, commonly present with renal Fanconi syndrome by 6-12 months of age, and without specific treatment, almost all will develop end-stage renal disease (ESRD) by 10-12 years of age. Early corneal cystine crystal deposition is a hallmark of the disease. Cystinosis also presents with gastrointestinal symptoms (e.g., vomiting, decreased appetite, and feeding difficulties) and severe growth retardation and may affect several other organs over time, including the eye, thyroid gland, gonads, pancreas, muscles, bone marrow, liver, nervous system, lungs, and bones. Cystine-depleting therapy with cysteamine orally is the only specific targeted therapy available for managing cystinosis and needs to be combined with cysteamine eye drops for corneal disease involvement. In patients with early treatment initiation and good compliance to therapy, long-term cysteamine treatment delays progression to ESRD, significantly improves growth, decreases the frequency and severity of extrarenal complications, and is associated with extended life expectancy. Therefore, early diagnosis of cystinosis and adequate life-long treatment with cysteamine are essential for preventing end-organ damage and improving the overall prognosis in these patients.

Multidisciplinary approach for patients with nephropathic cystinosis: model for care in a rare and chronic renal disease

Care for patients with chronic and rare diseases is complex, especially considering the lack of knowledge about the disease, which makes early and precise diagnosis difficult, as well as the need for specific tests, sometimes of high complexity and cost. Added to these factors are difficulties in obtaining adequate treatment when available, in raising patient and family awareness about the disease and treatment compliance. Nephropathic cystinosis is among these diseases. After more than 20 years as a care center for these patients, the authors propose a follow-up protocol, which has been used with improvement in the quality of care and consists of a multidisciplinary approach, including care provided by a physician, nurse, psychologist, nutritionist and social worker. In this paper, each field objectively exposes how to address points that involve the stages of diagnosis and its communication with the patient and their relatives or guardians, covering the particularities of the disease and the treatment, the impact on the lives of patients and families, the approach to psychological and social issues and guidelines on medications and diets. This protocol could be adapted to the follow-up of patients with other rare diseases, including those with renal involvement. This proposal is expected to reach the largest number of professionals involved in the follow-up of these patients, strengthening the bases for the creation of a national protocol, observing the particularities of each case.

Research priority setting in childhood chronic disease: a systematic review

OBJECTIVE

To evaluate research priority setting approaches in childhood chronic diseases and to describe the priorities of stakeholders including patients, caregivers/families and health professionals.

DESIGN

We conducted a systematic review of MEDLINE, Embase, PsycINFO and CINAHL from inception to 16 October 2016. Studies that elicited stakeholder priorities for paediatric chronic disease research were eligible for inclusion. Data on the prioritisation process were extracted using an appraisal checklist. Generated priorities were collated into common topic areas.

RESULTS

We identified 83 studies (n=15 722). Twenty (24%) studies involved parents/caregivers and four (5%) children. The top three health areas were cancer (11%), neurology (8%) and endocrine/metabolism (8%). Priority topic areas were treatment (78%), disease trajectory (48%), quality of life/psychosocial impact (48%), disease onset/prevention (43%), knowledge/self-management (33%), prevalence (30%), diagnostic methods (28%), access to healthcare (25%) and transition to adulthood (12%). The methods included workshops, Delphi techniques, surveys and focus groups/interviews. Specific methods for collecting and prioritising research topics were described in only 60% of studies. Most reviewed studies were conducted in high-income nations.

CONCLUSIONS

Research priority setting activities in paediatric chronic disease cover many discipline areas and have elicited a broad range of topics. However, child/caregiver involvement is uncommon, and the methods often lack clarity. A systematic and explicit process that involves patients and families in partnership may help to inform a more patient and family-relevant research agenda in paediatric chronic disease.

Clinical Approach to Proximal Renal Tubular Acidosis in Children

Proximal renal tubular acidosis (pRTA) is an inherited or acquired clinical syndrome in which there is a decreased bicarbonate reclamation in the proximal tubule resulting in normal anion gap hyperchloremic metabolic acidosis. In children, pRTA may be isolated but is often associated with a general proximal tubular dysfunction known as Fanconi syndrome which frequently heralds an underlying systemic disorder from which it arises. When accompanied by Fanconi syndrome, pRTA is characterized by additional renal wasting of phosphate, glucose, uric acid, and amino acids. The most common cause of inherited Fanconi syndrome in the pediatric age group is cystinosis, a disease with therapeutic implications. In this article, we summarize the clinical presentation and differential diagnosis of pRTA and Fanconi syndrome and provide a practical approach to their evaluation in children.

[Cystinosis : Diagnosis, cystine-depleting therapy, and transition]

This article presents a case of cystinosis in a young man. Diagnosis of the disease and the problem of transition to adult care are described. Cystinosis is a rare lysosomal storage disease with first manifestation in early childhood presenting as renal Fanconi syndrome. Without treatment, the disease leads to severe health impairment. Due to the rarity of the disease, a correct diagnosis is often delayed. Without treatment, cystinosis often leads to end-stage renal failure, blindness, hypothyroidism, diabetes mellitus, and rickets. Cystine-depleting therapy with cysteamine significantly improves mortality and quality of life.

Common Elements in Rare Kidney Diseases: Conclusions from a Kidney Disease: Improving Global Outcomes (KDIGO) Controversies Conference

Rare kidney diseases encompass at least 150 different conditions, most of which are inherited. Although individual rare kidney diseases raise specific issues, as a group these rare diseases can have overlapping challenges in diagnosis and treatment. These challenges include small numbers of affected patients, unidentified causes of disease, lack of biomarkers for monitoring disease progression, and need for complex care. To address common clinical and patient issues among rare kidney diseases, the KDIGO Controversies Conference entitled, Common Elements in Rare Kidney Diseases, brought together a panel of multidisciplinary clinical providers and patient advocates to address five central issues for rare kidney diseases. These issues encompassed diagnostic challenges, management of kidney functional decline and progression of chronic kidney disease, challenges in clinical study design, translation of advances in research to clinical care, and provision of practical and integrated patient support. Thus, by a process of consensus, guidance for addressing these challenges was developed and is presented here.

Worldwide view of nephropathic cystinosis: results from a survey from 30 countries

BACKGROUND

Nephropathic cystinosis is a rare inherited metabolic disorder leading to progressive renal failure and extra-renal comorbidity. The prognosis relies on early adherence to cysteamine treatment and symptomatic therapies. Developing nations [DiN] experience many challenges for management of cystinosis. The aim of this study was to assess the management characteristics in DiN compared with developed nations [DeN].

METHODS

A questionnaire was sent between April 2010 and May 2011 to 87 members of the International Pediatric Nephrology Association, in 50 countries.

RESULTS

A total of 213 patients were included from 41 centres in 30 nations (109 from 17 DiN and 104 from 13 DeN). 7% of DiN patients died at a median age of 5 years whereas no death was observed in DeN. DiN patients were older at the time of diagnosis. In DiN, leukocyte cystine measurement was only available in selected cases for diagnosis but never for continuous monitoring. More patients had reached end-stage renal disease in DiN (53.2 vs. 37.9%, p = 0.03), within a shorter time of evolution (8 vs. 10 yrs., p = 0.0008). The earlier the cysteamine treatment, the better the renal outcome, since the median renal survival increased up to 16.1 [12.5-/] yrs. in patients from DeN treated before the age of 2.5 years of age (p = 0.0001). However, the renal survival was not statistically different between DeN and DiN when patients initiated cysteamine after 2.5 years of age. The number of transplantations and the time from onset of ESRD to transplantation were not different in DeN and DiN. More patients were kept under maintenance dialysis in DiN (26% vs.19%, p = 0.02); 79% of patients from DiN vs. 45% in DeN underwent peritoneal dialysis.

CONCLUSIONS

Major discrepancies between DiN and DeN in the management of nephropathic cystinosis remain a current concern for many patients living in countries with limited financial resources.

Nephropathic cystinosis: an update

PURPOSE OF REVIEW

Over the past few decades, cystinosis, a rare lysosomal storage disorder, has evolved into a treatable metabolic disease. The increasing understanding of its pathophysiology has made cystinosis a prototype disease, delivering new insights into several fundamental biochemical and cellular processes.

RECENT FINDINGS

In this review, we aim to provide an overview of the latest advances in the pathogenetic, clinical, and therapeutic aspects of cystinosis.

SUMMARY

The development of alternative therapeutic monitoring strategies and new systemic and ocular cysteamine formulations might improve outcome of cystinosis patients in the near future. With the dawn of stem cell based therapy and new emerging gene-editing technologies, novel tools have become available in the search for a cure for cystinosis.

Orphan drug policies and use in pediatric nephrology

Orphan drugs designed to treat rare diseases are often overpriced per patient. Novel treatments are sometimes even more expensive for patients with ultra-rare diseases, in part due to the limited number of patients. Pharmaceutical companies that develop a patented life-saving drug are in a position to charge a very high price, which, at best, may enable these companies to further develop drugs for use in rare disease. However, is there a limit to how much a life-saving drug should cost annually per patient? Government interventions and regulations may opt to withhold a life-saving drug solely due to its high price and cost-effectiveness. Processes related to drug pricing, reimbursement, and thereby availability, vary between countries, thus having implications on patient care. These processes are discussed, with specific focus on three drugs used in pediatric nephrology: agalsidase beta (for Fabry disease), eculizumab (for atypical hemolytic uremic syndrome), and cysteamine bitartrate (for cystinosis). Access to and costs of orphan drugs have most profound implications for patients, but also for their physicians, hospitals, insurance policies, and society at large, particularly from financial and ethical standpoints.

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.