Model-informed dose optimization of mycophenolic acid in pediatric kidney transplant patients.

PURPOSE: We aimed to develop and evaluate a population PK model of mycophenolic acid (MPA) in pediatric kidney transplant patients to aid MPA dose optimization. METHODS: Data were collected from pediatric kidney transplant recipients from a Dutch academic hospital (Radboudumc, the Netherlands). Pharmacokinetic model-building and model-validation analyses were performed using NONMEM. Subsequently, we externally evaluated the final model using data from another academic hospital. The final model was used to develop an optimized dosing regimen. RESULTS: Thirty pediatric patients were included of whom 266 measured MPA plasma concentrations, including 20 full pharmacokinetic (PK) curves and 24 limited sampling curves, were available. A two-compartment model with a transition compartment for Erlang-type absorption best described the data. The final population PK parameter estimates were Ktr (1.48 h-1; 95% CI, 1.15-1.84), CL/F (16.0 L h-1; 95% CI, 10.3-20.4), Vc/F (24.9 L; 95% CI, 93.0-6.71E25), Vp/F (1590 L; 95% CI, 651-2994), and Q/F (36.2 L h-1; 95% CI, 9.63-74.7). The performance of the PK model in the external population was adequate. An optimized initial dose scheme based on bodyweight was developed. With the licensed initial dose, 35% of patients were predicted to achieve the target AUC, compared to 42% using the optimized scheme. CONCLUSION: We have successfully developed a pharmacokinetic model for MPA in pediatric renal transplant patients. The optimized dosing regimen is expected to result in better target attainment early in treatment. It can be used in combination with model-informed follow-up dosing to further individualize the dose when PK samples become available.

Diagnosing, discarding, or de-VUSsing: A practical guide to (un)targeted metabolomics as variant-transcending functional tests.

PURPOSE: For patients with inherited metabolic disorders (IMDs), any diagnostic delay should be avoided because early initiation of personalized treatment could prevent irreversible health damage. To improve diagnostic interpretation of genetic data, gene function tests can be valuable assets. For IMDs, variant-transcending functional tests are readily available through (un)targeted metabolomics assays. To support the application of metabolomics for this purpose, we developed a gene-based guide to select functional tests to either confirm or exclude an IMD diagnosis. METHODS: Using information from a diagnostic IMD exome panel, Kyoto Encyclopedia of Genes and Genomes, and Inborn Errors of Metabolism Knowledgebase, we compiled a guide for metabolomics-based gene function tests. From our practical experience with this guide, we retrospectively selected illustrative cases for whom combined metabolomic/genomic testing improved diagnostic success and evaluated the effect hereof on clinical management. RESULTS: The guide contains 2047 metabolism-associated genes for which a validated or putative variant-transcending gene function test is available. We present 16 patients for whom metabolomic testing either confirmed or ruled out the presence of a second pathogenic variant, validated or ruled out pathogenicity of variants of uncertain significance, or identified a diagnosis initially missed by genetic analysis. CONCLUSION: Metabolomics-based gene function tests provide additional value in the diagnostic trajectory of patients with suspected IMD by enhancing and accelerating diagnostic success.

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.

Improve in-depth immunological risk assessment to optimize genetic-compatibility and clinical outcomes in child and adolescent recipients of parental donor kidney transplants: protocol for the INCEPTION study.

BACKGROUND: Parental donor kidney transplantation is the most common treatment option for children and adolescents with kidney failure. Emerging data from observational studies have reported improved short- and medium-term allograft outcomes in recipients of paternal compared to maternal donors. The INCEPTION study aims to identify potential differences in immunological compatibility between maternal and paternal donor kidneys and ascertain how this affects kidney allograft outcomes in children and adolescents with kidney failure. METHODS: This longitudinal observational study will recruit kidney transplant recipients aged ≤18 years who have received a parental donor kidney transplant across 4 countries (Australia, New Zealand, United Kingdom and the Netherlands) between 1990 and 2020. High resolution human leukocyte antigen (HLA) typing of both recipients and corresponding parental donors will be undertaken, to provide an in-depth assessment of immunological compatibility. The primary outcome is a composite of de novo donor-specific anti-HLA antibody (DSA), biopsy-proven acute rejection or allograft loss up to 60-months post-transplantation. Secondary outcomes are de novo DSA, biopsy-proven acute rejection, acute or chronic antibody mediated rejection or Chronic Allograft Damage Index (CADI) score of > 1 on allograft biopsy post-transplant, allograft function, proteinuria and allograft loss. Using principal component analysis and Cox proportional hazards regression modelling, we will determine the associations between defined sets of immunological and clinical parameters that may identify risk stratification for the primary and secondary outcome measures among young people accepting a parental donor kidney for transplantation. This study design will allow us to specifically investigate the relative importance of accepting a maternal compared to paternal donor, for families deciding on the best option for donation. DISCUSSION: The INCEPTION study findings will explore potentially differential immunological risks of maternal and paternal donor kidneys for transplantation among children and adolescents. Our study will provide the evidence base underpinning the selection of parental donor in order to achieve the best projected long-term kidney transplant and overall health outcomes for children and adolescents, a recognized vulnerable population. TRIAL REGISTRATION: The INCEPTION study has been registered with the Australian New Zealand Clinical Trials Registry, with the trial registration number of ACTRN12620000911998 (14th September 2020).

Perioperative anesthesia care for the pediatric patient undergoing a kidney transplantation: An educational review.

Living-donor kidney transplantation is the first choice therapy for children with end-stage renal disease and shows good long-term outcome. Etiology of renal failure, co-morbidities, and hemodynamic effects, due to donor-recipient size mismatch, differs significantly from those in adult patients. Despite the complexities related to both patient and surgery, there is a lack of evidence-based anesthesia guidelines for pediatric kidney transplantation. This educational review summarizes the pathophysiological changes to consider and suggests recommendations for perioperative anesthesia care, based on recent research papers.

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-1ß, 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.

Preparing for a kidney transplant: Medical nephrectomy in children with nephrotic syndrome.

Nephrotic syndrome is characterized by proteinuria, hypoalbuminemia, and general edema. These symptoms may persist in children who reach ESRD, which is unfavorable for the patient's allograft outcome. In addition, this may hamper early diagnosis of a relapse after transplantation. Surgical bilateral nephrectomy is often considered for that reason, but medical nephrectomy may be a less invasive alternative. In this retrospective single-center case series, we identified all children on dialysis with ESRD due to nephrotic syndrome in which a medical nephrectomy was attempted before kidney transplantation between 2013 and 2018. Outcome was measured by urine output and serum albumin levels. Eight patients with either congenital nephrotic syndrome or focal segmental glomerular sclerosis were included in the study. All patients received an ACE inhibitor as drug of first choice for medical nephrectomy, to which 5 patients responded with oligoanuria and a significant rise in serum albumin, and 3 patients responded insufficiently. In 1 of these 3 patients, diclofenac was added to the ACE inhibitor, with good result. In the other 2 patients, indomethacin was initiated without success, and surgical bilateral nephrectomy was performed. Overall, 6/8 patients had a successful medical nephrectomy and did not need surgical nephrectomy. No recurrence of nephrotic syndrome was found after kidney transplantation in all but one. Medical nephrectomy with ACE inhibitors and/or non-steroidal anti-inflammatory drugs is a safe and non-invasive therapy to minimize proteinuria in children with ESRD due to nephrotic syndrome before kidney transplantation. We suggest that this strategy should be considered as therapy before proceeding with surgical nephrectomy.

Oscillometric and intra-arterial blood pressure in children post-kidney transplantation: Is invasive blood pressure measurement always needed?

OBJECTIVE: Blood pressure (BP) monitoring in children immediately after kidney transplantation is ideally performed with an arterial line. Accurate measurement of BP is necessary for optimal management. However, during the first days postoperative, the arterial line is removed and BP measurement is switched to a non-invasive device. The aim of this study was to determine the accuracy and reliability of the automated oscillometric device compared to invasive arterial BP (IBP) monitoring in patients after renal transplantation in pediatric intensive care unit (PICU). METHOD: We analyzed all simultaneously measured BPs in children with a kidney transplant in the Amalia Children's Hospital Radboud University Medical Center between January 1, 2012, and January 1, 2016. BP measurements were performed according to the hospital protocol. Agreement between invasive and non-invasive methods was assessed using Bland-Altman plots. RESULTS: A total of 29 patients were included in this retrospective study. The majority of children were male (59%), and median age was 11 years (range 1-17 years). Totally, 80 BP measurements were recorded during the first days post-kidney transplantation. The correlation coefficients (R) of systolic, diastolic, and MAP of non-invasive (NIBP) and IBP measurements were 0.84, 0.76, and 0.77, respectively (P < 0.01). Overall, the average MAP (7.5 ± 1.2 mm Hg; P < 0.05) NIBP values were lower compared to IBP. In hypertensive patients, MAP (10.4 ± 10.0 mm Hg; P < 0.05) BP values were significantly lower using the NIBP device. Clinically relevant difference of >10 mm Hg was found in 51% (41/80) of measurements and mainly observed in hypertensive measurements. CONCLUSIONS: IBP measurement is considered the golden standard for monitoring BP in patients immediately after kidney transplantation. NIBP values showed a good agreement with invasive reading, but the variability of NIBP mainly in hypertensive patients is high as it is the number of clinically relevant differences to IBP. We conclude that IBP remains the golden standard to monitor BP in children directly postoperatively.

The potential impact of hematocrit correction on evaluation of tacrolimus target exposure in pediatric kidney transplant patients.

BACKGROUND: Tacrolimus is an important immunosuppressive agent with high intra- and inter-individual pharmacokinetic variability and a narrow therapeutic index. As tacrolimus extensively accumulates in erythrocytes, hematocrit is a key factor in the interpretation of tacrolimus whole blood concentrations. However, as hematocrit values in pediatric kidney transplant patients are highly variable after kidney transplantation, translating whole blood concentration targets without taking hematocrit into consideration is theoretically incorrect. The aim of this study is to evaluate the potential impact of hematocrit correction on tacrolimus target exposure in pediatric kidney transplant patients. METHODS: Data were obtained from 36 pediatric kidney transplant patients. Two hundred fifty-five tacrolimus whole blood samples were available, together responsible for 36 area under the concentration-time curves (AUCs) and trough concentrations. First, hematocrit corrected concentrations were derived using a formula describing the relationship between whole blood concentrations, hematocrit, and plasma concentrations. Subsequently, target exposure was evaluated using the converted plasma target concentrations. Ultimately, differences in interpretation of target exposure were identified and evaluated. RESULTS: In total, 92% of our patients had lower hematocrit (median 0.29) than the reference value of adult kidney transplant patients. A different evaluation of target exposure for either trough level, AUC, or both was defined in 42% of our patients, when applying hematocrit corrected concentrations. CONCLUSION: A critical role for hematocrit in therapeutic drug monitoring of tacrolimus in pediatric kidney transplant patients is suggested in this study. Therefore, we believe that hematocrit correction could be a step towards improvement of tacrolimus dose individualization.

A randomized clinical trial indicates that levamisole increases the time to relapse in children with steroid-sensitive idiopathic nephrotic syndrome.

Levamisole has been considered the least toxic and least expensive steroid-sparing drug for preventing relapses of steroid-sensitive idiopathic nephrotic syndrome (SSINS). However, evidence for this is limited as previous randomized clinical trials were found to have methodological limitations. Therefore, we conducted an international multicenter, placebo-controlled, double-blind, randomized clinical trial to reassess its usefulness in prevention of relapses in children with SSINS. The efficacy and safety of one year of levamisole treatment in children with SSINS and frequent relapses were evaluated. The primary analysis cohort consisted of 99 patients from 6 countries. Between 100 days and 12 months after the start of study medication, the time to relapse (primary endpoint) was significantly increased in the levamisole compared to the placebo group (hazard ratio 0.22 [95% confidence interval 0.11-0.43]). Significantly, after 12 months of treatment, six percent of placebo patients versus 26 percent of levamisole patients were still in remission. During this period, the most frequent serious adverse event (four of 50 patients) possibly related to levamisole was asymptomatic moderate neutropenia, which was reversible spontaneously or after treatment discontinuation. Thus, in children with SSINS and frequent relapses, levamisole prolonged the time to relapse and also prevented recurrence during one year of treatment compared to prednisone alone. However, regular blood controls are necessary for safety issues.

Mortality risk disparities in children receiving chronic renal replacement therapy for the treatment of end-stage renal disease across Europe: an ESPN-ERA/EDTA registry analysis.

BACKGROUND: We explored the variation in country mortality rates in the paediatric population receiving renal replacement therapy across Europe, and estimated how much of this variation could be explained by patient-level and country-level factors. METHODS: In this registry analysis, we extracted patient data from the European Society for Paediatric Nephrology/European Renal Association-European Dialysis and Transplant Association (ESPN/ERA-EDTA) Registry for 32 European countries. We included incident patients younger than 19 years receiving renal replacement therapy. Adjusted hazard ratios (aHR) and the explained variation were modelled for patient-level and country-level factors with multilevel Cox regression. The primary outcome studied was all-cause mortality while on renal replacement therapy. FINDINGS: Between Jan 1, 2000, and Dec 31, 2013, the overall 5 year renal replacement therapy mortality rate was 15·8 deaths per 1000 patient-years (IQR 6·4-16·4). France had a mortality rate (9·2) of more than 3 SDs better, and Russia (35·2), Poland (39·9), Romania (47·4), and Bulgaria (68·6) had mortality rates more than 3 SDs worse than the European average. Public health expenditure was inversely associated with mortality risk (per SD increase, aHR 0·69, 95% CI 0·52-0·91) and explained 67% of the variation in renal replacement therapy mortality rates between countries. Child mortality rates showed a significant association with renal replacement therapy mortality, albeit mediated by macroeconomics (eg, neonatal mortality reduced from 1·31 [95% CI 1·13-1·53], p=0·0005, to 1·21 [0·97-1·51], p=0·10). After accounting for country distributions of patient age, the variation in renal replacement therapy mortality rates between countries increased by 21%. INTERPRETATION: Substantial international variation exists in paediatric renal replacement therapy mortality rates across Europe, most of which was explained by disparities in public health expenditure, which seems to limit the availability and quality of paediatric renal care. Differences between countries in their ability to accept and treat the youngest patients, who are the most complex and costly to treat, form an important source of disparity within this population. Our findings can be used by policy makers and health-care providers to explore potential strategies to help reduce these health disparities. FUNDING: ERA-EDTA and ESPN.

Effect and Process Evaluation of e-Powered Parents, a Web-Based Support Program for Parents of Children With a Chronic Kidney Disease: Feasibility Randomized Controlled Trial.

BACKGROUND: Parents of children with chronic kidney disease (CKD) experience high levels of stress in the daily management of their child's illness. Parents need continuously available support and information, yet online support programs are lacking. e-Powered Parents was developed to fill this gap; it is an online program consisting of (1) medical information, (2) an interactive part, and (3) four training modules (stress management, setting limits, communication, and coping). Prior to a large-scale evaluation, we conducted a feasibility study that consisted of an effect study and a process evaluation. OBJECTIVE: The objectives of our study were to (1) identify the outcome measures that are most likely to capture the potential benefit, (2) evaluate the potential effectiveness and effect size, and (3) evaluate recruitment, reach, the dose received, and context. METHODS: We conducted a feasibility study with a two-armed, wait-list randomized controlled trial (RCT). Prior to baseline, parents (n=146) were randomly allocated to group 1 or group 2. After completing the baseline questionnaire, parents in group 1 were given access to e-Powered Parents, while those in group 2 received usual care. At the 6-month follow-up (T1), all parents received a questionnaire and parents in group 2 were given access to e-Powered Parents as well. After 1.5 years, through an extra measurement (T2), we evaluated the effect of long-term exposure. Outcomes were the child's quality of life (Child Vulnerability Scale), parental stress (Pediatric Inventory for Parents) and fatigue (Multidimensional Fatigue Inventory), self-efficacy in communication with health care professionals (Perceived Efficacy in Patient-Physician Interactions, PEPPI-5), and parental perceptions of family management (Family Management Measure). Floor and ceiling effects and percentage of parents showing no change in scores were calculated. We used linear mixed models to evaluate the potential effectiveness and effect sizes using the intention-to-treat and per-protocol analyses. In the process evaluation, we evaluated recruitment, reach, the dose received, and context using a questionnaire sent to the parents, log-in data, and a focus group interview with health care professionals. RESULTS: At T1 (n=86) and T2 (n=51), no significant effects were found on any of the five outcomes. The PEPPI-5 showed ceiling effects and high percentages of parents showing no change between the measurement times. The information and interactive part of the intervention were used by 84% (57/68) of the parents in group 1 and 49% (32/65) of the parents in group 2. The information pages were visited most often. Overall, 64% (85/133) of the parents logged in to the training platform and 31% (26/85) actually used the training modules. CONCLUSIONS: We did not observe any significant effect on any of the outcomes. This could possibly be explained by the minimal use of the intervention and by parents' heterogeneity. For continued participation, we recommend a tailored intervention and further studies to find out whether and how online programs could be used to support parents in the management of their child's CKD. TRIAL REGISTRATION: Netherlands Trial Registry NTR4808; http://www.trialregister.nl/trialreg/admin/rctview.asp?TC=4808 (Archived by WebCite at http://www.webcitation.org/719rCicvW).

A Novel Hypokalemic-Alkalotic Salt-Losing Tubulopathy in Patients with CLDN10 Mutations.

Mice lacking distal tubular expression of CLDN10, the gene encoding the tight junction protein Claudin-10, show enhanced paracellular magnesium and calcium permeability and reduced sodium permeability in the thick ascending limb (TAL), leading to a urine concentrating defect. However, the function of renal Claudin-10 in humans remains undetermined. We identified and characterized CLDN10 mutations in two patients with a hypokalemic-alkalotic salt-losing nephropathy. The first patient was diagnosed with Bartter syndrome (BS) >30 years ago. At re-evaluation, we observed hypocalciuria and hypercalcemia, suggesting Gitelman syndrome (GS). However, serum magnesium was in the upper normal to hypermagnesemic range, thiazide responsiveness was not blunted, and genetic analyses did not show mutations in genes associated with GS or BS. Whole-exome sequencing revealed compound heterozygous CLDN10 sequence variants [c.446C>G (p.Pro149Arg) and c.465-1G>A (p.Glu157_Tyr192del)]. The patient had reduced urinary concentrating ability, with a preserved aquaporin-2 response to desmopressin and an intact response to furosemide. These findings were not in line with any other known salt-losing nephropathy. Subsequently, we identified a second unrelated patient showing a similar phenotype, in whom we detected compound heterozygous CLDN10 sequence variants [c.446C>G (p.(Pro149Arg) and c.217G>A (p.Asp73Asn)]. Cell surface biotinylation and immunofluorescence experiments in cells expressing the encoded mutants showed that only one mutation caused significant differences in Claudin-10 membrane localization and tight junction strand formation, indicating that these alterations do not fully explain the phenotype. These data suggest that pathogenic CLDN10 mutations affect TAL paracellular ion transport and cause a novel tight junction disease characterized by a non-BS, non-GS autosomal recessive hypokalemic-alkalotic salt-losing phenotype.

Dried Blood Spot Sampling for Tacrolimus and Mycophenolic Acid in Children: Analytical and Clinical Validation.

BACKGROUND: Tacrolimus and mycophenolic acid (MPA) are the backbone of immunosuppressive therapy after pediatric kidney transplantation. Dosing of these drugs is individualized by therapeutic drug monitoring. Dried blood spot (DBS) sampling may prove beneficial over conventional venous sampling. We aimed to develop and clinically validate a DBS method for tacrolimus and MPA in children. METHODS: A joint DBS liquid chromatography-mass spectrometry assay for tacrolimus and MPA was developed. DBS-specific items included the hematocrit effect and influence of spot volume. Subsequently, a clinical validation study among children aged 2-18 years was performed to assess the agreement between observed and DBS-predicted venous concentrations. Agreement of the methods was assessed with Passing-Bablok regression, Bland-Altman plots, and quantification of the DBS predictive performance in terms of bias (median percentage prediction error) and precision (median absolute percentage prediction error), both should be <15%. RESULTS: A total of 40 tacrolimus and 32 MPA samples were available from 28 children. Conversion factors were used to predict venous concentrations from DBS. For tacrolimus, 95% of the individual ratios of predicted and observed concentrations were within a range of 0.74-1.28, with 85% of these ratios between 0.80 and 1.20 (Bland-Altman plots). For MPA, the 95% limits of agreement represented a broader range of 0.49-1.49%, and 72% of individual ratios were between the 0.80 and 1.20 limits. Median percentage prediction error and median absolute percentage prediction error were less than 15% for both drugs. CONCLUSIONS: A DBS assay was developed for tacrolimus and MPA. Tacrolimus venous concentrations could be adequately predicted from DBS. DBS analysis of MPA seemed to be a semiquantitative measurement at the most when compared with conventional plasma analysis, considering the high variability between observed and predicted concentrations. Next, home-based DBS sampling of tacrolimus for the purpose of therapeutic drug monitoring will be implemented into routine clinical care.

Autosomal-Recessive Mutations in SLC34A1 Encoding Sodium-Phosphate Cotransporter 2A Cause Idiopathic Infantile Hypercalcemia.

Idiopathic infantile hypercalcemia (IIH) is characterized by severe hypercalcemia with failure to thrive, vomiting, dehydration, and nephrocalcinosis. Recently, mutations in the vitamin D catabolizing enzyme 25-hydroxyvitamin D3-24-hydroxylase (CYP24A1) were described that lead to increased sensitivity to vitamin D due to accumulation of the active metabolite 1,25-(OH)2D3. In a subgroup of patients who presented in early infancy with renal phosphate wasting and symptomatic hypercalcemia, mutations in CYP24A1 were excluded. Four patients from families with parental consanguinity were subjected to homozygosity mapping that identified a second IIH gene locus on chromosome 5q35 with a maximum logarithm of odds (LOD) score of 6.79. The sequence analysis of the most promising candidate gene, SLC34A1 encoding renal sodium-phosphate cotransporter 2A (NaPi-IIa), revealed autosomal-recessive mutations in the four index cases and in 12 patients with sporadic IIH. Functional studies of mutant NaPi-IIa in Xenopus oocytes and opossum kidney (OK) cells demonstrated disturbed trafficking to the plasma membrane and loss of phosphate transport activity. Analysis of calcium and phosphate metabolism in Slc34a1-knockout mice highlighted the effect of phosphate depletion and fibroblast growth factor-23 suppression on the development of the IIH phenotype. The human and mice data together demonstrate that primary renal phosphate wasting caused by defective NaPi-IIa function induces inappropriate production of 1,25-(OH)2D3 with subsequent symptomatic hypercalcemia. Clinical and laboratory findings persist despite cessation of vitamin D prophylaxis but rapidly respond to phosphate supplementation. Therefore, early differentiation between SLC34A1 (NaPi-IIa) and CYP24A1 (24-hydroxylase) defects appears critical for targeted therapy in patients with IIH.

Cystinosin deficiency causes podocyte damage and loss associated with increased cell motility.

The involvement of the glomerulus in the pathogenesis of cystinosis, caused by loss-of-function mutations in cystinosin (CTNS, 17p13), is a matter of controversy. Although patients with cystinosis demonstrate glomerular lesions and high-molecular-weight proteinuria starting from an early age, a mouse model of cystinosis develops only signs of proximal tubular dysfunction. Here we studied podocyte damage in patients with cystinosis by analyzing urinary podocyte excretion and by in vitro studies of podocytes deficient in cystinosin. Urine from patients with cystinosis presented a significantly higher amount of podocytes compared with controls. In culture, cystinotic podocytes accumulated cystine compatible with cystinosin deficiency. The expression of podocyte specific genes CD2AP, podocalyxin, and synaptopodin and of the WT1 protein was evident in all cell lines. Conditionally immortalized podocyte lines of 2 patients with different CTNS mutations had altered cytoskeleton, impaired cell adhesion sites, and increased individual cell motility. Moreover, these cells showed enhanced phosphorylation of both Akt1 and Akt2 (isoforms of protein kinase B). Inhibition of Akt by a specific inhibitor (Akti inhibitor 1/2) resulted in normalization of the hypermotile phenotype. Thus, our study extends the list of genetic disorders causing podocyte damage and provides the evidence of altered cell signaling cascades resulting in impaired cell adhesion and enhanced cell motility in cystinosis.

Prioritization and burden analysis of rare variants in 208 candidate genes suggest they do not play a major role in CAKUT.

The leading cause of end-stage renal disease in children is attributed to congenital anomalies of the kidney and urinary tract (CAKUT). Familial clustering and mouse models support the presence of monogenic causes. Genetic testing is insufficient as it mainly focuses on HNF1B and PAX2 mutations that are thought to explain CAKUT in 5­15% of patients. To identify novel, potentially pathogenic variants in additional genes, we designed a panel of genes identified from studies on familial forms of isolated or syndromic CAKUT and genes suggested by in vitro and in vivo CAKUT models. The coding exons of 208 genes were analyzed in 453 patients with CAKUT using next-generation sequencing. Rare truncating, splice-site variants, and non-synonymous variants, predicted to be deleterious and conserved, were prioritized as the most promising variants to have an effect on CAKUT. Previously reported disease-causing mutations were detected, but only five were fully penetrant causal mutations that improved diagnosis. We prioritized 148 candidate variants in 151 patients, found in 82 genes, for follow-up studies. Using a burden test, no significant excess of rare variants in any of the genes in our cohort compared with controls was found. Thus, in a study representing the largest set of genes analyzed in CAKUT patients to date, the contribution of previously implicated genes to CAKUT risk was significantly smaller than expected, and the disease may be more complex than previously assumed.

Racial Disparities in Access to and Outcomes of Kidney Transplantation in Children, Adolescents, and Young Adults: Results From the ESPN/ERA-EDTA (European Society of Pediatric Nephrology/European Renal Association-European Dialysis and Transplant Association) Registry.

BACKGROUND: Racial disparities in kidney transplantation in children have been found in the United States, but have not been studied before in Europe. STUDY DESIGN: Cohort study. SETTING & PARTICIPANTS: Data were derived from the ESPN/ERA-EDTA Registry, an international pediatric renal registry collecting data from 36 European countries. This analysis included 1,134 young patients (aged ≤19 years) from 8 medium- to high-income countries who initiated renal replacement therapy (RRT) in 2006 to 2012. FACTOR: Racial background. OUTCOMES & MEASUREMENTS: Differences between racial groups in access to kidney transplantation, transplant survival, and overall survival on RRT were examined using Cox regression analysis while adjusting for age at RRT initiation, sex, and country of residence. RESULTS: 868 (76.5%) patients were white; 59 (5.2%), black; 116 (10.2%), Asian; and 91 (8.0%), from other racial groups. After a median follow-up of 2.8 (range, 0.1-3.0) years, we found that black (HR, 0.49; 95% CI, 0.34-0.72) and Asian (HR, 0.54; 95% CI, 0.41-0.71) patients were less likely to receive a kidney transplant than white patients. These disparities persisted after adjustment for primary renal disease. Transplant survival rates were similar across racial groups. Asian patients had higher overall mortality risk on RRT compared with white patients (HR, 2.50; 95% CI, 1.14-5.49). Adjustment for primary kidney disease reduced the effect of Asian background, suggesting that part of the association may be explained by differences in the underlying kidney disease between racial groups. LIMITATIONS: No data for socioeconomic status, blood group, and HLA profile. CONCLUSIONS: We believe this is the first study examining racial differences in access to and outcomes of kidney transplantation in a large European population. We found important differences with less favorable outcomes for black and Asian patients. Further research is required to address the barriers to optimal treatment among racial minority groups.