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Introduction: Activation of the focal adhesion kinase (FAK) in podocytes is involved in the pathogenesis of minimal change disease (MCD), but the pathway leading to its activation in this disease is unknown. Here, we tested whether podocyte β1 integrin is the upstream modulator of FAK activation and podocyte injury in experimental models of MCD-like injury., Methods: We used lipopolysaccharide (LPS) and MCD sera to induce MCD-like changes in vivo and in cultured human podocytes, respectively. We performed functional studies using specific β1 integrin inhibitors in vivo and in vitro, and integrated histological analysis, western blotting, and immunofluorescence to assess for morphological and molecular changes in podocytes. By ELISA, we measured serum LPS levels in 35 children with MCD or presumed MCD (idiopathic nephrotic syndrome [INS]) and in 18 healthy controls., Results: LPS-injected mice showed morphological (foot process effacement, and normal appearing glomeruli on light microscopy) and molecular features (synaptopodin loss, nephrin mislocalization, FAK phosphorylation) characteristic of human MCD. Administration of a β1 integrin inhibitor to mice abrogated FAK phosphorylation, and ameliorated proteinuria and podocyte injury following LPS. Children with MCD/INS in relapse had higher serum LPS levels than controls. In cultured human podocytes, β1 integrin blockade prevented cytoskeletal rearrangements following exposure to MCD sera in relapse., Conclusions: Podocyte β1 integrin activation is an upstream mediator of FAK phosphorylation and podocyte injury in models of MCD-like injury., (Copyright © 2022 Sociedad Española de Nefrología. Published by Elsevier España, S.L.U. All rights reserved.)

Autosomal dominant loss-of-function (LoF) variants in cytotoxic T-lymphocyte associated protein 4 ( CTLA4 ) cause immune dysregulation with autoimmunity, immunodeficiency and lymphoproliferation (IDAIL). Incomplete penetrance and variable expressivity are characteristic of IDAIL caused by CTLA-4 haploinsufficiency (CTLA-4h), pointing to a role for genetic modifiers. Here, we describe an IDAIL proband carrying a maternally inherited pathogenic CTLA4 variant and a paternally inherited rare LoF missense variant in CLEC7A, which encodes for the β-glucan pattern recognition receptor DECTIN-1. The CLEC7A variant led to a loss of DECTIN-1 dimerization and surface expression. Notably, DECTIN-1 stimulation promoted human and mouse regulatory T cell (T reg ) differentiation from naïve αβ and γδ T cells, even in the absence of transforming growth factor-β. Consistent with DECTIN-1's T reg -boosting ability, partial DECTIN-1 deficiency exacerbated the T reg defect conferred by CTL4-4h. DECTIN-1/ CLEC7A emerges as a modifier gene in CTLA-4h, increasing expressivity of CTLA4 variants and acting in functional epistasis with CTLA-4 to maintain immune homeostasis and tolerance.

Purpose: To determine whether hypercapnia is associated with risk of hospital readmission related to anorexia nervosa (AN) in children and adolescents., Methods: We performed a prospective study of patients ≤ 18 years old admitted due to AN decompensation from November 2018 to October 2019. Both subtypes of AN, restricting subtype (AN-R) and binge-eating/purging subtype (AN-BP), were included. Study participants were evaluated upon admission, at discharge and six months after discharge. T-tests or Mann-Whitney U tests was used to compare means values. Pearson or Spearman correlations were used to measure the association between two variables. Logistic regression models were developed to evaluate the relationship between scoring methods and readmission., Results: Of the 154 persons admitted during the study period, 131 met the inclusion criteria. Median age was 15.1 years. At admission, 71% of participants were malnourished and 33 (25%) had been previously admitted. We observed a marked decrease in venous pH and stable pCO 2 elevation during follow-up period. Hypercapnia at discharge was associated with a twofold increased likelihood of readmission and the odds of readmission increased as discharge pCO 2 rose. These findings did not depend on AN subtype or participant sex. Electrolytes persisted within the normal range., Conclusion: Hypercapnia and respiratory acidosis are common alterations in children and adolescents hospitalized due to AN decompensation. Hypercapnia persists for at least 6 months after discharge despite clinical improvement and is associated with higher odds of readmission. This is the first study to identify an abnormal laboratory finding as a potential predictor of readmission in AN., Level of Evidence: IV: Multiple time series without intervention., (© 2023. The Author(s).)

Background: X-linked hypophosphatemia (XLH) is a rare, hereditary, progressive, renal phosphate-wasting disorder characterized by a pathological increase in FGF23 concentration and activity. Due to its rarity, diagnosis may be delayed, which can adversely affect outcomes. As a chronic disease resulting in progressive accumulation of musculoskeletal manifestations, it is important to understand the natural history of XLH over the patient's lifetime and the impact of drug treatments and other interventions. This multicentre, international patient registry (International XLH Registry) was established to address the paucity of these data. Here we present the findings of the first interim analysis of the registry., Results: The International XLH Registry was initiated in August 2017 and includes participants of all ages diagnosed with XLH, regardless of their treatment and management. At the database lock for this first interim analysis (29 March 2021), 579 participants had entered the registry before 30 November 2020 and are included in the analysis (360 children [62.2%], 217 adults [37.5%] and 2 whose ages were not recorded [0.3%]; 64.2% were female). Family history data were available for 319/345 (92.5%) children and 145/187 (77.5%) adults; 62.1% had biological parents affected by XLH. Genetic testing data were available for 341 (94.7%) children and 203 (93.5%) adults; 370/546 (67.8%) had genetic test results; 331/370 (89.5%) had a confirmed PHEX mutation. A notably longer time to diagnosis was observed in adults ≥ 50 years of age (mean [median] duration 9.4 [2.0] years) versus all adults (3.7 [0.1] years) and children (1.0 [0.2] years). Participants presented with normal weight, shorter length or height and elevated body mass index (approximately - 2 and + 2 Z-scores, respectively) versus the general population. Clinical histories were collected for 349 participants (239 children and 110 adults). General data trends for prevalence of bone, dental, renal and joint conditions in all participants were aligned with expectations for a typical population of people with XLH., Conclusion: The data collected within the International XLH Registry, the largest XLH registry to date, provide substantial information to address the paucity of natural history data, starting with demographic, family history, genetic testing, diagnosis, auxology and baseline data on clinical presentation., (© 2023. Institut National de la Santé et de la Recherche Médicale (INSERM).)

Given the relatively recent introduction of burosumab in the management of X-linked hypophosphatemia (XLH), there is limited real-world data to guide its use in clinical practice. As a group of European physicians experienced with burosumab treatment in clinical practice, we convened with the objective of sharing these practice-based insights on the use of burosumab in children and adolescents with XLH. We attended two virtual meetings, then discussed key questions via Within3, a virtual online platform. Points of discussion related to patient selection criteria, burosumab starting dose, dose titration and treatment monitoring. Our discussions revealed that criteria for selecting children with XLH varied across Europe from all children above 1 year to only children with overt rickets despite conventional treatment being eligible. We initiated burosumab dosing according to guidance in the Summary of Product Characteristics, an international consensus statement from 2019 and local country guidelines. Dose titration was primarily guided by serum phosphate levels, with some centers also using the ratio of tubular maximum reabsorption of phosphate to glomerular filtration rate (TmP/GFR). We monitored response to burosumab treatment clinically (growth, deformities, bone pain and physical functioning), radiologically (rickets and deformities) and biochemically (serum phosphate, alkaline phosphatase, 1,25-dihydroxyvitamin D, 25-hydroxyvitamin D, urine calcium-creatinine ratio and TmP/GFR). Key suggestions made by our group were initiation of burosumab treatment in children as early as possible, from the age of 1 year, particularly in those with profound rickets, and a need for clinical studies on continuation of burosumab throughout adolescence and into adulthood., Competing Interests: MZM, received honoraria from Kyowa Kirin International for attending advisory boards and for educational lectures/webinars. GIB, received honoraria from Kyowa Kirin International for attending advisory boards and webinars. CdLC, received honoraria from Kyowa Kirin International for attending advisory boards and for educational lectures. AL, an investigator for clinical trials for Kyowa Kirin International and has received research grant support from Kyowa Kirin International. AM, received an honorarium from Kyowa Kirin International for participation in the Expert Practice Exchange meeting. AR, received honoraria from Kyowa Kirin International for consultancy and for lectures. FS, received honoraria from Kyowa Kirin International for teaching activities and scientific consultancy. DS, received honoraria from Kyowa Kirin Germany for scientific consultancy. NS, received an honorarium from Kyowa Kirin International for participation in the Expert Practice Exchange meeting. ON, received honoraria from Kyowa Kirin International for attending advisory boards and for educational lectures/webinars, as well as speakers’ honoraria from Abbott and Biomarin. The authors declare that this study received funding from Kyowa Kirin International. The funder had a role in organising meetings upon which this manuscript is based. The medical writing support was also funded by Kyowa Kirin International., (Copyright © 2023 Mughal, Baroncelli, de Lucas-Collantes, Linglart, Magnolato, Raimann, Santos, Schnabel, Shaw and Nilsson.)

Background: Fanconi syndrome (FS) can be of primary or secondary origin. Some cases of FS secondary to the use of sodium valproate (VPA) have been described, mostly in children with severe psychomotor retardation who are fed by feeding device. The objetive of this study was to describe patients treated for this entity in our center, comparing them against the published literature., Methods: Descriptive study of our patients and those found in the literature. Epidemiologic and clinical data were collected., Results: We describe seven patients (three to 17 years old) with severe psychomotor retardation and undergoing treatment with VPA. Four presented pathologic fractures before the diagnosis of FS, and in three patients the diagnosis was reached due to abnormal laboratory findings. A review of the published cases was carried out and, including our sample, a total of 42 patients were studied: 51.3% were male, and the median age at diagnosis of FS was 6 years. Severe psychomotor retardation was found in 92.8% of patients, 78% carried a feeding device, and 77.5% received treatment with several antiepileptic drugs. The mean duration of VPA treatment was 5.7 years (range 2 to 7.5 years). Fifteen patients (37.5%) had bone complications. The resolution time of FS after discontinuation of drug therapy ranged from two to 19 months (median 4 months)., Conclusions: FS related to VPA is a rare complication, but it should be considered in patients with epilepsy, especially if they have severe psychomotor retardation, are users of feeding devices, and receive other antiepileptic treatments in addition to VPA., (Copyright © 2022 Elsevier Inc. All rights reserved.)

Although circumstantial evidence supports enhanced Toll-like receptor 7 (TLR7) signalling as a mechanism of human systemic autoimmune disease 1-7 , evidence of lupus-causing TLR7 gene variants is lacking. Here we describe human systemic lupus erythematosus caused by a TLR7 gain-of-function variant. TLR7 is a sensor of viral RNA 8 , 9 and binds to guanosine 10 - 12 . We identified a de novo, previously undescribed missense TLR7 Y264H variant in a child with severe lupus and additional variants in other patients with lupus. The TLR7 Y264H variant selectively increased sensing of guanosine and 2',3'-cGMP 10-12 , and was sufficient to cause lupus when introduced into mice. We show that enhanced TLR7 signalling drives aberrant survival of B cell receptor (BCR)-activated B cells, and in a cell-intrinsic manner, accumulation of CD11c + age-associated B cells and germinal centre B cells. Follicular and extrafollicular helper T cells were also increased but these phenotypes were cell-extrinsic. Deficiency of MyD88 (an adaptor protein downstream of TLR7) rescued autoimmunity, aberrant B cell survival, and all cellular and serological phenotypes. Despite prominent spontaneous germinal-centre formation in Tlr7 Y264H mice, autoimmunity was not ameliorated by germinal-centre deficiency, suggesting an extrafollicular origin of pathogenic B cells. We establish the importance of TLR7 and guanosine-containing self-ligands for human lupus pathogenesis, which paves the way for therapeutic TLR7 or MyD88 inhibition., (© 2022. The Author(s).)

Minimal change disease (MCD) is the most common type of idiopathic nephrotic syndrome in childhood and represents about 15% cases in adults. It is characterized by massive proteinuria, edema, hypoalbuminemia, and podocyte foot process effacement on electron microscopy. Clinical and experimental studies have shown an association between MCD and immune dysregulation. Given the lack of inflammatory changes or immunocomplex deposits in the kidney tissue, MCD has been traditionally thought to be mediated by an unknown circulating factor(s), probably released by T cells that directly target podocytes leading to podocyte ultrastructural changes and proteinuria. Not surprisingly, research efforts have focused on the role of T cells and podocytes in the disease process. Nevertheless, the pathogenesis of the disease remains a mystery. More recently, B cells have been postulated as an important player in the disease either by activating T cells or by releasing circulating autoantibodies against podocyte targets. There are also few reports of endothelial injury in MCD, but whether glomerular endothelial cells play a role in the disease remains unexplored. Genome-wide association studies are providing insights into the genetic susceptibility to develop the disease and found a link between MCD and certain human haplotype antigen variants. Altogether, these findings emphasize the complex interplay between the immune system, glomerular cells, and the genome, raising the possibility of distinct underlying triggers and/or mechanisms of proteinuria among patients with MCD. The heterogeneity of the disease and the lack of good animal models of MCD remain major obstacles in the understanding of MCD. In this study, we will review the most relevant candidate mediators and mechanisms of proteinuria involved in MCD and the current models of MCD-like injury., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2021 Purohit, Piani, Ordoñez, de Lucas-Collantes, Bauer and Cara-Fuentes.)

Introduction: Minimal change disease (MCD) is considered a podocyte disorder triggered by unknown circulating factors. Here, we hypothesized that the endothelial cell (EC) is also involved in MCD., Methods: We studied 45 children with idiopathic nephrotic syndrome (44 had steroid sensitive nephrotic syndrome [SSNS], and 12 had biopsy-proven MCD), 21 adults with MCD, and 38 healthy controls (30 children, 8 adults). In circulation, we measured products of endothelial glycocalyx (EG) degradation (syndecan-1, heparan sulfate [HS] fragments), HS proteoglycan cleaving enzymes (matrix metalloprotease-2 [MMP-2], heparanase activity), and markers of endothelial activation (von Willebrand factor [vWF], thrombomodulin) by enzyme-linked immunosorbent assay (ELISA) and mass spectrometry. In human kidney tissue, we assessed glomerular EC (GEnC) activation by immunofluorescence of caveolin-1 ( n = 11 MCD, n = 5 controls). In vitro , we cultured immortalized human GEnC with sera from control subjects and patients with MCD/SSNS sera in relapse ( n = 5 per group) and performed Western blotting of thrombomodulin of cell lysates as surrogate marker of endothelial activation., Results: In circulation, median concentrations of all endothelial markers were higher in patients with active disease compared with controls and remained high in some patients during remission. In the MCD glomerulus, caveolin-1 expression was higher, in an endothelial-specific pattern, compared with controls. In cultured human GEnC, sera from children with MCD/SSNS in relapse increased thrombomodulin expression compared with control sera., Conclusion: Our data show that alterations involving the systemic and glomerular endothelium are nearly universal in patients with MCD and SSNS, and that GEnC can be directly activated by circulating factors present in the MCD/SSNS sera during relapse., (© 2021 International Society of Nephrology. Published by Elsevier Inc.)

Introduction: Tuberous sclerosis complex (TSC) displays great phenotypic variability. Increasingly early diagnosis, including prenatal identification, entails the need for the paediatrician and neuropaediatrician to establish early suspicion and identification of factors that may influence prognosis and treatment., Aim: To determine the clinical criteria for early diagnosis, initial complementary tests, actions and treatments to prevent different comorbidities, so as to improve the prognosis of these patients., Patients and Methods: Descriptive, retrospective study of = 18-year-olds with a definitive diagnosis of TSC in a tertiary hospital from 1998 to 2019. We collected variables referring to epidemiological data, multisystem involvement, complementary tests and genetics., Results: Ninety-four patients were analysed. The main diagnostic reasons were epilepsy and rhabdomyomas. The frequency of occurrence of clinical criteria was determined, and neuropathological findings were the main findings, followed by cutaneous stigmata, rhabdomyomas and renal lesions. Statistical relationships were found between clinical, radiological and genetic aspects, the influence of preventive activities on the occurrence of epilepsy and the relevance of everolimus use were tested., Conclusions: Rhabdomyomas and skin stigmata in patients and parents are major diagnostic signs in infants. Tubers and subependymal nodules are statistically associated with the development of epilepsy. Early epileptic spasms, refractory to treatment in the first months, increase the risk of cognitive deficits and autism spectrum disorder. Epileptic abnormalities need to be closely monitored in the first year of life. Everolimus is an alternative treatment for several comorbidities, but its early use (< 3 years) requires further study.

Introduction: Lupus nephritis is an early manifestation in the development of systemic lupus erythematosus that worsens the morbidity and mortality of these patients., Objective: To study the form of presentation in patients with lupus nephritis, the clinical and immunological characteristics, and their relationship with renal histology., Patients and Method: Retrospective study in children under 18 years of age, with lupus nephritis, in follow-up in a third level children's hospital in Madrid, between January 2012 and May 2020. We recorded demographic, clinical, and laboratory data (blood count, renal function, liver function, protein, ionogram, blood glucose, uric acid, lactate dehydrogenase, coagulation, and urine analysis), as well as immunological data (immunoglobulins, antinuclear antibodies, comple ment, and lupus anticoagulant), and histological classification data. Descriptive analysis and analysis of associations between variables was performed, with a significant p < 0.05., Results: 16 patients (11 women) were included, the median age at presentation was 10.6 ± 2.3 years (5.7-15.3). The median time between symptoms onset and renal involvement was 6.3 months ± 10.5 (range 0 - 33.6). Renal involvement was the initial manifestation in 37.5% of patients. 50% had arthralgias or arthritis prior to diagnosis, and 25% had fever and constitutional symptoms (asthenia, anorexia, and/or weight loss). The most frequent form of renal involvement was microhematuria associated with proteinuria in non-nephrotic range. In the renal anatomo-pathological study, according to the ISN/RPS 2003 classification, grades III (46.6%) and IV (33.3%) predominated., Conclusions: Six patients presented renal involvement at baseline with musculoskeletal involvement being more frequent. Most patients (86.6%) presented advanced lupus nephritis in the histological study at diagnosis. Immunologic in volvement was the only marker that correlated with systemic involvement.

Background: X-linked hypophosphatemia (XLH) is a hereditary rare disease caused by loss-of-function mutations in PHEX gene leading tohypophosphatemia and high renal loss of phosphate. Rickets and growth retardation are the major manifestations of XLH in children, but there is a broad phenotypic variability. Few publications have reported large series of patients. Current data on the clinical spectrum of the disease, the correlation with the underlying gene mutations, and the long-term outcome of patients on conventional treatment are needed, particularly because of the recent availability of new specific medications to treat XLH., Results: The RenalTube database was used to retrospectively analyze 48 Spanish patients (15 men) from 39 different families, ranging from 3 months to 8 years and 2 months of age at the time of diagnosis (median age of 2.0 years), and with XLH confirmed by genetic analysis. Bone deformities, radiological signs of active rickets and growth retardation were the most common findings at diagnosis. Mean (± SEM) height was - 1.89 ± 0.19 SDS and 55% (22/40) of patients had height SDS below-2. All cases had hypophosphatemia, serum phosphate being - 2.81 ± 0.11 SDS. Clinical manifestations and severity of the disease were similar in both genders. No genotype-phenotype correlation was found. Conventional treatment did not attenuate growth retardation after a median follow up of 7.42 years (IQR = 11.26; n = 26 patients) and failed to normalize serum concentrations of phosphate. Eleven patients had mild hyperparathyroidism and 8 patients nephrocalcinosis., Conclusions: This study shows that growth retardation and rickets were the most prevalent clinical manifestations at diagnosis in a large series of Spanish pediatric patients with XLH confirmed by mutations in the PHEX gene. Traditional treatment with phosphate and vitamin D supplements did not improve height or corrected hypophosphatemia and was associated with a risk of hyperparathyroidism and nephrocalcinosis. The severity of the disease was similar in males and females.

Thrombotic thrombocytopaenic purpura (TTP) is a life-threatening thrombotic microangiopathy characterised by microangiopathic haemolytic anaemia, thrombocytopaenia and organ ischaemia. TTP is caused by a severe functional deficiency of ADAMTS13 activity. We describe a 10-year-old girl presenting anaemia and thrombocytopaenia with schistocytes. Urine protein to creatinine ratio was within nephrotic range. ADAMTS13 activity was 0%, and no anti-ADAMTS13 antibodies were found. A renal biopsy showed deposits of IgG, C3 and C1q in the capillary membrane, compatible with class V lupus nephritis. Therapeutic plasma exchange (TPE) was performed in conjunction with therapy consisting of steroids and mycophenolate mofetil. After 11 months of follow-up, the patient remains in remission with normal ADAMTS13 activity. Although acquired TTP is a rare finding in children, differential diagnosis of thrombotic microangiopathy should include ADAMTS13 and the assay should be performed early. TTP treatment is based on TPE, although the underlying disease must be ruled out to optimise treatment and prevent relapse., Competing Interests: Competing interests: None declared., (© BMJ Publishing Group Limited 2020. No commercial re-use. See rights and permissions. Published by BMJ.)

Systemic lupus erythematosus (SLE) is the prototypic systemic autoimmune disease. It is thought that many common variant gene loci of weak effect act additively to predispose to common autoimmune diseases, while the contribution of rare variants remains unclear. Here we describe that rare coding variants in lupus-risk genes are present in most SLE patients and healthy controls. We demonstrate the functional consequences of rare and low frequency missense variants in the interacting proteins BLK and BANK1, which are present alone, or in combination, in a substantial proportion of lupus patients. The rare variants found in patients, but not those found exclusively in controls, impair suppression of IRF5 and type-I IFN in human B cell lines and increase pathogenic lymphocytes in lupus-prone mice. Thus, rare gene variants are common in SLE and likely contribute to genetic risk.

Background & Aims: Iodine deficiency inhibits the normal development of human beings and is the leading cause of preventable mental retardation. Our study aims to update the urinary iodine concentrations and the intake of iodized salt in children in Madrid (Spain)., Methods: A cross-sectional study was designed where 217 children aged 3-14 years old were studied. A nutritional survey including the intake of iodized salt and other iodine-rich foods was performed. In addition, the urinary concentration of iodine was determined in each patient., Results: Near 60% of the surveyed households routinely used iodized salt. Significant differences in age, sex, country of birth, or country of birth and parents educational levels and iodized salt consumption were not found. The median of the urinary iodine level (120 μg/L; interquartile range 80-184) was significantly higher in boys than girls and more elevated in younger children. Iodized salt and milk consumption significantly increased the concentration of urinary iodine. Children who drank less than two glasses of milk per day and did not consume iodized salt have four times the risk of iodine deficiency compared to children who daily drank at least two glasses of milk and consumed iodized salt (P < 0.001)., Conclusions: The nutritional level of iodine in the children studied is appropriate despite the low consumption of iodized salt. This is due to the consumption of dairy products and milk. The younger the child, the better his/her iodine nutritional level. Teenagers studied are at higher risk of iodine deficiency., (Copyright © 2019 European Society for Clinical Nutrition and Metabolism. Published by Elsevier Ltd. All rights reserved.)

Background: X-linked hypophosphatemia (XLH) is an inherited disease of phosphate metabolism in which inactivating mutations of the Phosphate Regulating Endopeptidase Homolog, X-Linked (PHEX) gene lead to local and systemic effects including impaired growth, rickets, osteomalacia, bone abnormalities, bone pain, spontaneous dental abscesses, hearing difficulties, enthesopathy, osteoarthritis, and muscular dysfunction. Patients with XLH present with elevated levels of fibroblast growth factor 23 (FGF23), which is thought to mediate many of the aforementioned manifestations of the disease. Elevated FGF23 has also been observed in many other diseases of hypophosphatemia, and a range of animal models have been developed to study these diseases, yet the role of FGF23 in the pathophysiology of XLH is incompletely understood., Methods: The role of FGF23 in the pathophysiology of XLH is here reviewed by describing what is known about phenotypes associated with various PHEX mutations, animal models of XLH, and non-nutritional diseases of hypophosphatemia, and by presenting molecular pathways that have been proposed to contribute to manifestations of XLH., Results: The pathophysiology of XLH is complex, involving a range of molecular pathways that variously contribute to different manifestations of the disease. Hypophosphatemia due to elevated FGF23 is the most obvious contributor, however localised fluctuations in tissue non-specific alkaline phosphatase (TNAP), pyrophosphate, calcitriol and direct effects of FGF23 have been observed to be associated with certain manifestations., Conclusions: By describing what is known about these pathways, this review highlights key areas for future research that would contribute to the understanding and clinical treatment of non-nutritional diseases of hypophosphatemia, particularly XLH.

Mitochondrial diseases (MD) are a heterogeneous group of clinical syndromes characterized by the involvement of different organ systems. They constitute the most prevalent hereditary metabolic disease group., Objective: To review the importance of the kidney in MD from the nephrologist's perspective within the setting of a pediatric tertiary reference center., Study Design: Retrospective study of children (<18 years) with MD followed between 2000 and 2016 at a tertiary Spanish center., Results: 52 patients were included. The mean age at the time of the study was 10 years (SD ± 5.1). The mean follow-up time was 6.1 years (SD ± 4.7). The median age at diagnosis was 2.5 years (0.3-13.5).The median number of affected systems was two (range 1-6). The nervous system was the most affected system, with 51 patients (~98%) presenting with neurological involvement. 20 patients (~40%) presented with endocrinological manifestations, 18 (~35%) with vision problems, 16 (~30%) with gastrointestinal symptoms, 5 (~10%) patients developed hearing impairment, and 6 (~10%) cardiac disease.We detected renal involvement in 13 patients (25%). Eight patients had tubular disease, most frequently hypercalciuria with hypouricemia and five patients had glomerular involvement, with proteinuria and/or decreased glomerular filtration rate as the most frequent symptoms. Only 21 patients (~40%) had been seen by a pediatric nephrologist., Conclusions: Renal disease was a common occurrence in patients with mitochondrial disease, present in our study in 25% of patients. A regular screening of renal function parameters and the involvement of a nephrologist as part of the multidisciplinary approach to mitochondrial disease appears warranted.