Mutation Update of the CLCN5 Gene Responsible for Dent Disease 1

Clinical features and genetic analysis of 15 Chinese children with dent disease

OBJECTIVE

 The clinical characteristics, genetic mutation spectrum, treatment strategies and prognoses of 15 children with Dent disease were retrospectively analyzed to improve pediatricians' awareness of and attention to this disease.

METHODS

 We analyzed the clinical and laboratory data of 15 Chinese children with Dent disease who were diagnosed and treated at our hospital between January 2017 and May 2023 and evaluated the expression of the CLCN5 and OCRL1 genes.

RESULTS

 All 15 patients were male and complained of proteinuria, and the incidence of low-molecular-weight proteinuria (LMWP) was 100.0% in both Dent disease 1 (DD1) and Dent disease 2 (DD2) patients. The incidence of hypercalciuria was 58.3% (7/12) and 66.7% (2/3) in DD1 and DD2 patients, respectively. Nephrocalcinosis and nephrolithiasis were found in 16.7% (2/12) and 8.3% (1/12) of DD1 patients, respectively. Renal biopsy revealed focal segmental glomerulosclerosis (FSGS) in 1 patient, minimal change lesion in 5 patients, and small focal acute tubular injury in 1 patient. A total of 11 mutations in the CLCN5 gene were detected, including 3 missense mutations (25.0%, c.1756C > T, c.1166T > G, and c.1618G > A), 5 frameshift mutations (41.7%, c.407delT, c.1702_c.1703insC, c.137delC, c.665_666delGGinsC, and c.2200delG), and 3 nonsense mutations (25.0%, c.776G > A, c.1609C > T, and c.1152G > A). There was no significant difference in age or clinical phenotype among patients with different mutation types (p > 0.05). All three mutations in the OCRL1 gene were missense mutations (c.1477C > T, c.952C > T, and c.198A > G).

CONCLUSION

 Pediatric Dent disease is often misdiagnosed. Protein electrophoresis and genetic testing can help to provide an early and correct diagnosis.

A novel transgenic mouse model highlights molecular disruptions involved in the pathogenesis of Dent disease 1

Dent disease (DD) is a hereditary renal disorder characterized by low molecular weight (LMW) proteinuria and progressive renal failure. Inactivating mutations of the CLCN5 gene encoding the 2Cl-/H+exchanger ClC-5 have been identified in patients with DD type 1. ClC-5 is essentially expressed in proximal tubules (PT) where it is thought to play a role in maintaining an efficient endocytosis of LMW proteins. However, the exact pathological roles of ClC-5 in progressive dysfunctions observed in DD type 1 are still unclear. To address this issue, we designed a mouse model carrying the most representative type of ClC-5 missense mutations found in DD patients. These mice showed a characteristic DD type 1 phenotype accompanied by altered endo-lysosomal system and autophagy functions. With ageing, KI mice showed increased renal fibrosis, apoptosis and major changes in cell metabolic functions as already suggested in previous DD models. Furthermore, we made the interesting new discovery that the Lipocalin-2-24p3R pathway might be involved in the progression of the disease. These results suggest a crosstalk between the proximal and distal nephron in the pathogenesis mechanisms involved in DD with an initial PT impairment followed by the Lipocalin-2 internalisation and 24p3R overexpression in more distal segments of the nephron. This first animal model of DD carrying a pathogenic mutation of Clcn5 and our findings pave the way aimed at exploring therapeutic strategies to limit the consequences of ClC-5 disruption in patients with DD type 1 developing chronic kidney disease.

Gene therapy of Dent disease type 1 in newborn ClC-5 null mice for sustained transgene expression and gene therapy effects

Dent disease type 1 is caused by changes in the chloride voltage-gated channel 5 (CLCN5) gene on chromosome X, resulting in the lack or dysfunction of chloride channel ClC-5. Individuals affected by Dent disease type 1 show proteinuria and hypercalciuria. Previously we found that lentiviral vector-mediated hCLCN5 cDNA supplementary therapy in ClC-5 null mice was effective only for three months following gene delivery, and the therapeutic effects disappeared four months after treatment, most likely due to immune responses to the ClC-5 proteins expressed in the treated cells. Here we tried two strategies to reduce possible immune responses: 1) confining the expression of ClC-5 expression to the tubular cells with tubule-specific Npt2a and Sglt2 promoters, and 2) performing gene therapy in newborn mutant mice whose immune system has not fully developed. We found that although Npt2a and Sglt2 promoters successfully drove ClC-5 expression in the kidneys of the mutant mice, the treatment did not ameliorate the phenotypes. However, gene delivery to the kidneys of newborn Clcn5 mutant mice enabled long-term transgene expression and phenotype improvement. Our data suggest that performing gene therapy on Dent disease affected subjects soon after birth could be a promising strategy to attenuate immune responses in Dent disease type 1 gene therapy.

A Focus on the Proximal Tubule Dysfunction in Dent Disease Type 1

Dent disease type 1 is a rare X-linked recessive inherited renal disorder affecting mainly young males, generally leading to end-stage renal failure and for which there is no cure. It is caused by inactivating mutations in the gene encoding ClC-5, a 2Cl-/H+ exchanger found on endosomes in the renal proximal tubule. This transporter participates in reabsorbing all filtered plasma proteins, which justifies why proteinuria is commonly observed when ClC-5 is defective. In the context of Dent disease type 1, a proximal tubule dedifferentiation was shown to be accompanied by a dysfunctional cell metabolism. However, the exact mechanisms linking such alterations to chronic kidney disease are still unclear. In this review, we gather knowledge from several Dent disease type 1 models to summarize the current hypotheses generated to understand the progression of this disorder. We also highlight some urinary biomarkers for Dent disease type 1 suggested in different studies.

Structural basis of adenine nucleotides regulation and neurodegenerative pathology in ClC-3 exchanger

The ClC-3 chloride/proton exchanger is both physiologically and pathologically critical, as it is potentiated by ATP to detect metabolic energy level and point mutations in ClC-3 lead to severe neurodegenerative diseases in human. However, why this exchanger is differentially modulated by ATP, ADP or AMP and how mutations caused gain-of-function remains largely unknow. Here we determine the high-resolution structures of dimeric wildtype ClC-3 in the apo state and in complex with ATP, ADP and AMP, and the disease-causing I607T mutant in the apo and ATP-bounded state by cryo-electron microscopy. In combination with patch-clamp recordings and molecular dynamic simulations, we reveal how the adenine nucleotides binds to ClC-3 and changes in ion occupancy between apo and ATP-bounded state. We further observe I607T mutation induced conformational changes and augments in current. Therefore, our study not only lays the structural basis of adenine nucleotides regulation in ClC-3, but also clearly indicates the target region for drug discovery against ClC-3 mediated neurodegenerative diseases.

4-Phenylbutyric Acid Treatment Reduces Low-Molecular-Weight Proteinuria in a Clcn5 Knock-in Mouse Model for Dent Disease-1

Dent disease-1 (DD-1) is a rare X-linked tubular disorder characterized by low-molecular-weight proteinuria (LMWP), hypercalciuria, nephrolithiasis and nephrocalcinosis. This disease is caused by inactivating mutations in the CLCN5 gene which encodes the voltage-gated ClC-5 chloride/proton antiporter. Currently, the treatment of DD-1 is only supportive and focused on delaying the progression of the disease. Here, we generated and characterized a Clcn5 knock-in mouse model that carries a pathogenic CLCN5 variant, c. 1566_1568delTGT; p.Val523del, which has been previously detected in several DD-1 unrelated patients, and presents the main clinical manifestations of DD-1 such as high levels of urinary b2-microglobulin, phosphate and calcium. Mutation p.Val523del causes partial ClC-5 retention in the endoplasmic reticulum. Additionally, we assessed the ability of sodium 4-phenylbutyrate, a small chemical chaperone, to ameliorate DD-1 symptoms in this mouse model. The proposed model would be of significant value in the investigation of the fundamental pathological processes underlying DD-1 and in the development of effective therapeutic strategies for this rare condition.

Review of childhood genetic nephrolithiasis and nephrocalcinosis

Nephrolithiasis (NL) is a common condition worldwide. The incidence of NL and nephrocalcinosis (NC) has been increasing, along with their associated morbidity and economic burden. The etiology of NL and NC is multifactorial and includes both environmental components and genetic components, with multiple studies showing high heritability. Causative gene variants have been detected in up to 32% of children with NL and NC. Children with NL and NC are genotypically heterogenous, but often phenotypically relatively homogenous, and there are subsequently little data on the predictors of genetic childhood NL and NC. Most genetic diseases associated with NL and NC are secondary to hypercalciuria, including those secondary to hypercalcemia, renal phosphate wasting, renal magnesium wasting, distal renal tubular acidosis (RTA), proximal tubulopathies, mixed or variable tubulopathies, Bartter syndrome, hyperaldosteronism and pseudohyperaldosteronism, and hyperparathyroidism and hypoparathyroidism. The remaining minority of genetic diseases associated with NL and NC are secondary to hyperoxaluria, cystinuria, hyperuricosuria, xanthinuria, other metabolic disorders, and multifactorial etiologies. Genome-wide association studies (GWAS) in adults have identified multiple polygenic traits associated with NL and NC, often involving genes that are involved in calcium, phosphorus, magnesium, and vitamin D homeostasis. Compared to adults, there is a relative paucity of studies in children with NL and NC. This review aims to focus on the genetic component of NL and NC in children.

Drosophila ClC-c Is a Homolog of Human CLC-5 and a New Model for Dent Disease Type 1

Key Points

Drosophila can be a model for Dent Disease type 1. Drosophila Clc-C mutations function similar to human CLC-5 Dent 1 mutations.

Background

Drosophila serve as exceptional alternative models for in vivo and ex vivo research and may provide an avenue for in-depth investigation for human ClC-5 and Dent disease type 1 (DD1). The Drosophila ClC-c (CG5284) has sequence homology with human ClC-5 and is hypothesized to encompass similar functional and phenotypical roles with ClC-5 and variants that cause DD1.

Methods

Ion transport function and activity of Drosophila ClC-c and homologous DD1 variants were assessed by voltage clamp electrophysiology. Membrane localization was demonstrated in Drosophila expressing a GFP-labeled construct of ClC-c. Genetic expression of an RNAi against ClC-c mRNA was used to generate a knockdown fly that serves as a DD1 disease model. Tubule secretion of cations and protein were assessed, as well as the crystal formation in the Malpighian tubules.

Results

Voltage clamp experiments demonstrate that ClC-c is voltage-gated with Cl−-dependent and pH-sensitive currents. Inclusion of homologous DD1 mutations pathogenic variants (S393L, R494W, and Q777X) impairs ClC-c ion transport activity. In vivo expression of ClC-c-eGFP in Malpighian tubules reveals that the membrane transporter localizes to the apical membrane and nearby cytosolic regions. RNAi knockdown of ClC-c (48% decreased mRNA expression) causes increased secretion of both urinary protein and Ca2+ and increased occurrence of spontaneous tubule crystals.

Conclusions

Drosophila ClC-c shows orthologous function and localization to human ClC-5. Thus, Drosophila and ClC-c regulation may be useful for future investigations of Cl− transport, Ca2+ homeostasis, and urinary protein loss in DD1.

Dent disease 1-linked novel CLCN5 mutations result in aberrant location and reduced ion currents

Dent disease is a rare renal tubular disease with X-linked recessive inheritance characterized by low molecular weight proteinuria (LMWP), hypercalciuria, and nephrocalcinosis. Mutations disrupting the 2Cl-/1H+ exchange activity of chloride voltage-gated channel 5 (CLCN5) have been causally linked to the most common form, Dent disease 1 (DD1), although the pathophysiological mechanisms remain unclear. Here, we conducted the whole exome capture sequencing and bioinformatics analysis within our DD1 cohort to identify two novel causal mutations in CLCN5 (c.749 G > A, p. G250D, c.829 A > C, p. T277P). Molecular dynamics simulations of ClC-5 homology model suggested that these mutations potentially may induce structural changes, destabilizing ClC-5. Overexpression of variants in vitro revealed aberrant subcellular localization in the endoplasmic reticulum (ER), significant accumulation of insoluble aggregates, and disrupted ion transport function in voltage clamp recordings. Moreover, human kidney-2 (HK-2) cells overexpressing either G250D or T277P displayed higher cell-substrate adhesion, migration capability but reduced endocytic function, as well as substantially altered transcriptomic profiles with G250D resulting in stronger deleterious effects. These cumulative findings supported pathogenic role of these ClC-5 mutations in DD1 and suggested a cellular mechanism for disrupted renal function in Dent disease patients, as well as a potential target for diagnostic biomarker or therapeutic strategy development.

Characterization of pre-mRNA Splicing Defects Caused by CLCN5 and OCRL Mutations and Identification of Novel Variants Associated with Dent Disease

Dent disease (DD) is an X-linked renal tubulopathy characterized by low-molecular-weight proteinuria, hypercalciuria, nephrocalcinosis, nephrolithiasis and progressive renal failure. Two-thirds of cases are associated with inactivating variants in the CLCN5 gene (Dent disease 1, DD1) and a few present variants in the OCRL gene (Dent disease 2, DD2). The aim of the present study was to test the effect on the pre-mRNA splicing process of DD variants, described here or in the literature, and describe the clinical and genotypic features of thirteen unrelated patients with suspected DD. All patients presented tubular proteinuria, ten presented hypercalciuria and five had nephrolithiasis or nephrocalcinosis. CLCN5 and OCRL genes were analyzed by Sanger sequencing. Nine patients showed variants in CLCN5 and four in OCRL; eight of these were new. Bioinformatics tools were used to select fifteen variants with a potential effect on pre-mRNA splicing from our patients' group and from the literature, and were experimentally tested using minigene assays. Results showed that three exonic missense mutations and two intronic variants affect the mRNA splicing process. Our findings widen the genotypic spectrum of DD and provide insight into the impact of variants causing DD.

Identification of mutations in 15 nephrolithiasis-related genes leading to a molecular diagnosis in 85 Chinese pediatric patients

BACKGROUND

The aim of this study was to describe the genotypic and phenotypic characteristics of Chinese pediatric patients with hereditary nephrolithiasis.

METHODS

Whole-exome sequencing (WES) was performed on 218 Chinese pediatric patients with kidney stones, and genetic and clinical data were collected and analyzed retrospectively.

RESULTS

The median age at onset in our cohort was 2.5 years (age range, 0.3-13 years). We detected 79 causative mutations in 15 genes, leading to a molecular diagnosis in 38.99% (85/218) of all cases. Monogenic mutations were present in 80 cases, and digenic mutations were present in 5 cases; 34.18% (27/79) of mutations were not included in the databases. Six common mutant genes, i.e., HOGA1, AGXT, GRHPR, SLC3A1, SLC7A9, and SLC4A1, were found in 84.71% of the patients overall. Furthermore, three mutations (A278A, c.834_834 + 1GG > TT, and C257G) in HOGA1, two mutations (K12QfX156 and S275RfX28) in AGXT, and one mutation (C289DfX22) in GRHPR represented hotspot mutations. The patients with HOGA1 mutations had the earliest onset age (0.8 years), followed by those with SLC7A9 (1.8 years), SLC4A1 (2.7 years), AGXT (4.3 years), SLC3A1 (4.8 years), and GRHPR (8 years) mutations (p = 0.002). Nephrocalcinosis was most commonly observed in patients with AGXT gene mutations.

CONCLUSIONS

Fifteen causative genes were detected in 85 Chinese pediatric patients with kidney stone diseases. The most common mutant genes, novel mutations, hotspot mutations, and genotype-phenotype correlations were also found. This study contributes to the understanding of genetic profiles and clinical courses in pediatric patients with hereditary nephrolithiasis. A higher resolution version of the Graphical abstract is available as Supplementary information.

A novel likely pathogenic CLCN5 variant in Dent's disease

BACKGROUND

The majority of cases of Dent's disease are caused by pathogenic variants in the CLCN5 gene, which encodes a voltage-gated chloride ion channel (ClC-5), resulting in proximal tubular dysfunction. We present three members of the same family and one unrelated paediatric patient with the same insertion-deletion CLCN5 variant. The identification of these patients and positive familial segregation led to the re-classification of this variant from one of unknown significance to one of likely pathogenicity.

CASE PRESENTATION

A 41 year old male presented with end stage kidney failure, proteinuria and haematuria. Whole genome sequencing identified an insertion-deletion variant in CLCN5, resulting in a missense change (c.1744_1745delinsAA p.(Ala582Lys)). His brother and nephew, who both exhibited renal impairment, haematuria, proteinuria, glycosuria and nephrocalcinosis, were found to have the same variant. In addition, genetic testing of an unrelated paediatric patient who presented with proteinuria and hypercalciuria, demonstrated the same variant.

CONCLUSIONS

The identification of this novel variant in four individuals with features of Dent's disease, has led to the re-classification of the variant to one of likely pathogenicity. As a result, our patients and any future patients with the same variant can be offered a likely diagnosis, without the need for kidney biopsy, and their family members can be offered genetic screening.

Genotypic variability in patients with clinical diagnosis of Bartter syndrome type 3

Bartter syndrome (BS) is a salt-losing hereditary tubulopathy characterized by hypokalemic metabolic alkalosis with secondary hyperaldosteronism. Confirmatory molecular diagnosis may be difficult due to genetic heterogeneity and overlapping of clinical symptoms. The aim of our study was to describe the different molecular findings in patients with a clinical diagnosis of classic BS. We included 27 patients (26 families) with no identified pathogenic variants in CLCNKB. We used a customized Ion AmpliSeq Next-Generation Sequencing panel including 44 genes related to renal tubulopathies. We detected pathogenic or likely pathogenic variants in 12 patients (44%), reaching a conclusive genetic diagnosis. Variants in SLC12A3 were found in 6 (Gitelman syndrome). Median age at diagnosis was 14.6 years (range 0.1-31), with no history of prematurity or polyhydramnios. Serum magnesium level was low in 2 patients (33%) but urinary calcium excretion was normal or low in all, with no nephrocalcinosis. Variants in SLC12A1 were found in 3 (BS type 1); and in KCNJ1 in 1 (BS type 2). These patients had a history of polyhydramnios in 3 (75%), and the mean gestational age was 34.2 weeks (SD 1.7). The median age at diagnosis was 1.8 years (range 0.1-6). Chronic kidney disease and nephrocalcinosis were present in 1 (25%) and 3 (75%) patients, respectively. A variant in CLCN5 was found in one patient (Dent disease), and in NR3C2 in another patient (Geller syndrome). Genetic diagnosis of BS is heterogeneous as different tubulopathies can present with a similar clinical picture. The use of gene panels in these diseases becomes more efficient than the study gene by gene with Sanger sequencing.

The Site and Type of CLCN5 Genetic Variation Impact the Resulting Dent Disease-1 Phenotype

Introduction

Dent disease is an X-linked recessive disorder associated with low molecular weight proteinuria (LMWP), nephrocalcinosis, kidney stones, and kidney failure in the third to fifth decade of life. It consists of Dent disease 1 (DD1) (60% of patients) because of pathogenic variants in the CLCN5 gene and Dent disease 2 (DD2) with changes in OCRL.

Methods

Retrospective review of 162 patients from 121 different families with genetically confirmed DD1 (82 different pathogenic variants validated using American College of Medical Genetics [ACMG] guidelines). Clinical and genetic factors were compared using observational statistics.

Results

A total of 110 patients had 51 different truncating (nonsense, frameshifting, large deletions, and canonical splicing) variants, whereas 52 patients had 31 different nontruncating (missense, in-frame, noncanonical splicing, and stop-loss) changes. Sixteen newly described pathogenic variants were found in our cohort. Among patients with truncating variants, lifetime stone events positively correlated with chronic kidney disease (CKD) evolution. Patients with truncating changes also experienced stone events earlier in life and manifested a higher albumin excretion rate than the nontruncating group. Nevertheless, neither age of nephrocalcinosis nor CKD progression varied between the truncating versus nontruncating patients. A large majority of nontruncating changes (26/31; 84%) were clustered in the middle exons that encode the voltage ClC domain whereas truncating changes were spread across the protein. Variants associated with kidney failure were restricted to truncating (11/13 cases), plus a single missense variant previously shown to markedly reduce ClC-5 functional activity that was found in the other 2 individuals.

Conclusion

DD1 manifestations, including the risk of kidney stones and progression to kidney failure, may relate to the degree of residual ClC-5 function.

Impaired Endosome Maturation Mediates Tubular Proteinuria in Dent Disease Cell Culture and Mouse Models

SIGNIFICANCE STATEMENT

Loss of function of the 2Cl - /H + antiporter ClC-5 in Dent disease causes an unknown impairment in endocytic traffic, leading to tubular proteinuria. The authors integrated data from biochemical and quantitative imaging studies in proximal tubule cells into a mathematical model to determine that loss of ClC-5 impairs endosome acidification and delays early endosome maturation in proximal tubule cells, resulting in reduced megalin recycling, surface expression, and half-life. Studies in a Dent mouse model also revealed subsegment-specific differences in the effects of ClC-5 knockout on proximal tubule subsegments. The approach provides a template to dissect the effects of mutations or perturbations that alter tubular recovery of filtered proteins from the level of individual cells to the entire proximal tubule axis.

BACKGROUND

Loss of function of the 2Cl - /H + antiporter ClC-5 in Dent disease impairs the uptake of filtered proteins by the kidney proximal tubule, resulting in tubular proteinuria. Reduced posttranslational stability of megalin and cubilin, the receptors that bind to and recover filtered proteins, is believed to underlie the tubular defect. How loss of ClC-5 leads to reduced receptor expression remains unknown.

METHODS

We used biochemical and quantitative imaging data to adapt a mathematical model of megalin traffic in ClC-5 knockout and control cells. Studies in ClC-5 knockout mice were performed to describe the effect of ClC-5 knockout on megalin traffic in the S1 segment and along the proximal tubule axis.

RESULTS

The model predicts that ClC-5 knockout cells have reduced rates of exit from early endosomes, resulting in decreased megalin recycling, surface expression, and half-life. Early endosomes had lower [Cl - ] and higher pH. We observed more profound effects in ClC-5 knockout cells expressing the pathogenic ClC-5 E211G mutant. Alterations in the cellular distribution of megalin in ClC-5 knockout mice were consistent with delayed endosome maturation and reduced recycling. Greater reductions in megalin expression were observed in the proximal tubule S2 cells compared with S1, with consequences to the profile of protein retrieval along the proximal tubule axis.

CONCLUSIONS

Delayed early endosome maturation due to impaired acidification and reduced [Cl - ] accumulation is the primary mediator of reduced proximal tubule receptor expression and tubular proteinuria in Dent disease. Rapid endosome maturation in proximal tubule cells is critical for the efficient recovery of filtered proteins.

Dent disease manifesting as nephrotic syndrome

Dent disease is an X-linked recessive renal tubular disorder, which is mainly caused by mutations of the CLCN5 gene and OCRL gene. It is characterized by low molecular weight proteinuria, hypercalciuria, nephrocalcinosis or nephrolithiasis, and progressive renal failure. Nephrotic syndrome is a glomerular disorder characterized by massive proteinuria, hypoalbuminemia, edema, and hyperlipidemia. In this study, we report two cases of Dent disease manifesting as nephrotic syndrome. Two patients were initially diagnosed with nephrotic syndrome due to edema, nephrotic range proteinuria, hypoalbuminemia, and hyperlipidemia, and responded to prednisone and tacrolimus therapy. Genetic testing revealed mutations in the OCRL and CLCN5 genes. They were eventually diagnosed with Dent disease. Nephrotic syndrome is a rare and insidious phenotype of Dent disease, and its pathogenesis is not fully understood. Patients with nephrotic syndrome are recommended to routinely undergo urinary protein classification and urinary calcium testing, especially those with frequently recurrent nephrotic syndrome and poor response to steroid and immunosuppressive therapy. To date, there is no effective drug treatment for Dent disease. About 30% to 80% of patients progress to end-stage renal disease at the age of 30-50.

Pathogenic variant detection rate by whole exome sequencing in Thai patients with biopsy-proven focal segmental glomerulosclerosis

The spectra of underlying genetic variants for various clinical entities including focal segmental glomerulosclerosis (FSGS) vary among different populations. Here we described the clinical and genetic characteristics of biopsy-proven FSGS patients in Thailand. Patients with FSGS pathology, without secondary causes, were included in our study. Clinical laboratory and pathological data were collected. Whole-exome sequencing (WES) was subsequently performed. 53 unrelated FSGS patients were recruited. 35 patients were adults (66.0%), and 51 patients were sporadic cases (96.2%). Clinical diagnosis before kidney biopsy was steroid-resistant nephrotic syndrome (SRNS) in 58.5%, and proteinuric chronic kidney disease in 32.1%. Using WES, disease-associated pathogenic/likely pathogenic (P/LP) variants could be identified in six patients including the two familial cases, making the P/LP detection rate of 11.3% (6/53). Of these six patients, two patients harbored novel variants with one in the COL4A4 gene and one in the MAFB gene. Four other patients carried previously reported variants in the CLCN5, LMX1B, and COL4A4 genes. Four of these patients (4/6) received immunosuppressive medications as a treatment for primary FSGS before genetic diagnosis. All four did not respond to the medications, emphasizing the importance of genetic testing to avoid unnecessary treatment. Notably, the mutation detection rates in adult and pediatric patients were almost identical, at 11.4% and 11.1%, respectively. In conclusion, the overall P/LP variant detection rate by WES in biopsy-proven FSGS patients was 11.3%. The most identified variants were in COL4A4. In addition, three novel variants associated with FSGS were detected.

Dent disease presenting with nyctalopia and electroretinographic correlates of vitamin A deficiency

Purpose

To report a unique case of Dent Disease presenting with nyctalopia associated with vitamin A deficiency and abnormal electroretinogram findings without prior systemic symptomatology.

Observations

A 16-year-old male presented with a several month history of nyctalopia and peripheral vision deficits. Central visual acuity, anterior and posterior segment examinations, and macular optical coherence tomography were unremarkable. Electroretinogram (ERG) testing revealed a rod-cone dystrophic pattern, with further workup demonstrating serum vitamin A deficiency (VAD). Laboratory evaluation revealed renal dysfunction and proteinuria with a significantly elevated urinary retinol-binding protein (RBP). Kidney biopsy showed glomerular and tubular disease.Genetic screening for inherited renal disease was performed identifying a hemizygous pathogenic variant c.2152C>T (p.Arg718*) in the Chloride Voltage-Gated Channel 5 (CLCN5) gene, confirming the diagnosis of X-linked Dent Disease. Following vitamin A supplementation, our patient reported resolution of nyctalopia and reversal of abnormal ERG findings were demonstrated.

Conclusions and Importance

To our knowledge, this is the first case in the literature describing Dent disease solely presenting with ophthalmic symptoms of nyctalopia and abnormal electroretinogram findings that later reversed with vitamin A repletion. This case stresses the importance for clinicians to consider renal tubular disorders in the differential for VAD.

Dent Disease Type 1: Still an Under-Recognized Renal Proximal Tubulopathy: A Case Report

Dent disease is a rare renal tubular disorder that appears almost exclusively in males. The diagnosis is still challenging, and therefore Dent disease is occasionally misdiagnosed. We report a case of a 45-year-old man with Dent disease who developed renal failure. Since the age of 7 months, he persistently exhibited proteinuria. At the age of 24 years, he underwent kidney biopsy, which revealed focal segmental glomerulosclerosis. The patient’s brother was found to have proteinuria since he was 2 years old. At the age of 45 years, the patient was transferred to a tertiary care nephrologist, and Dent disease was suspected. Genetic testing revealed a CLCN5 mutation. We highlight the broad spectrum of clinical manifestations in Dent disease and the importance of having a high clinical suspicion to attain a definitive diagnosis. Furthermore, future research regarding the clinical course of the disease, prognosis, and effective treatment options is needed.

Renal Expression of CLC-5 and Megalin/Cubilin in Dent-1 Disease With Nonsense Mutations of CLCN5 Gene

The study aims to explore the clinicopathological features and whether the nonsense mutations of CLCN5 gene have effect on the renal expression of CLC-5 protein and megalin/cubilin complex in children with Dent-1 disease. The clinicopathological features and genetic examination of three patients with Dent-1 disease were investigated. The expression of CLC-5 and megalin/cubilin complex in renal tissues was detected by using immunohistochemistry method. Urinary albumin, α1-microglobulin, β2-microglobulin, retinol binding protein, and calcium levels were measured by immunonephelometry. Urinary calcium and low molecular weight proteinuria (LMWP) were enhanced in three patients, and two presented with nephrotic range proteinuria. Focal glomerular obsolescence, minor tubulointerstitial injury, and focal calcification in corticomedullary junction were found in one patient. Nonsense mutations of CLCN5 gene from their mothers were identified in all three patients with Dent-1 disease; however, the expression of CLC-5 protein was not decreased in renal tubular cells. As the receptor complex of albumin and LMWP reabsorption, the expression of megalin/cubilin in the brush border of proximal tubules was decreased in Dent-1 patients. Even if the renal CLC-5 protein is expressed normally, the reduced expression of megalin/cubilin in the brush border of renal proximal tubules may be helpful to understand the physiopathology of Dent-1 disease with nonsense mutations of CLCN5 gene.

The genetics of kidney stone disease and nephrocalcinosis

Kidney stones (also known as urinary stones or nephrolithiasis) are highly prevalent, affecting approximately 10% of adults worldwide, and the incidence of stone disease is increasing. Kidney stone formation results from an imbalance of inhibitors and promoters of crystallization, and calcium-containing calculi account for over 80% of stones. In most patients, the underlying aetiology is thought to be multifactorial, with environmental, dietary, hormonal and genetic components. The advent of high-throughput sequencing techniques has enabled a monogenic cause of kidney stones to be identified in up to 30% of children and 10% of adults who form stones, with ~35 different genes implicated. In addition, genome-wide association studies have implicated a series of genes involved in renal tubular handling of lithogenic substrates and of inhibitors of crystallization in stone disease in the general population. Such findings will likely lead to the identification of additional treatment targets involving underlying enzymatic or protein defects, including but not limited to those that alter urinary biochemistry.

A novel CLCN5 frame shift mutation responsible for Dent disease 1: Case report

BACKGROUND

Dent disease is a group of inherited X-linked recessive renal tubular disorders. This group of disorders is characterized by low molecular weight proteinuria (LMWP), nephrocalcinosis, hypercalciuria and renal failure.

CASE PRESENTATION

Here we report one 11-year-old Chinese boy (proband) and one 13-year-old Chinese boy who was proband's cousin, both presented with massive proteinuria. Further laboratory examinations revealed a lack of nephrocalcinosis, nor any other signs of tubular dysfunction, but only LMWP and hypercalciuria. There was no abnormality in growth, renal function or mineral density of the bones. A novel deletion (c.1448delG) in the CLCN5 gene was identified, resulting in a frame shift mutation (p.Gly483fs). The proband's and his cousin's mothers were found to be the carrier of this mutation.

CONCLUSIONS

In this study, we have found a novel frameshift mutation (c. 1448delG) at exon 11 of the CLCN5 gene which leads to Dent disease 1, expanding the spectrum of CLCN5 mutations.

Novel Dent disease 1 cellular models reveal biological processes underlying ClC-5 loss-of-function

Dent disease 1 (DD1) is a rare X-linked renal proximal tubulopathy characterized by low molecular weight proteinuria and variable degree of hypercalciuria, nephrocalcinosis and/or nephrolithiasis, progressing to chronic kidney disease. Although mutations in the electrogenic Cl-/H+ antiporter ClC-5, which impair endocytic uptake in proximal tubule cells, cause the disease, there is poor genotype-phenotype correlation and their contribution to proximal tubule dysfunction remains unclear. To further discover the mechanisms linking ClC-5 loss-of-function to proximal tubule dysfunction, we have generated novel DD1 cellular models depleted of ClC-5 and carrying ClC-5 mutants p.(Val523del), p.(Glu527Asp) and p.(Ile524Lys) using the human proximal tubule-derived RPTEC/TERT1 cell line. Our DD1 cellular models exhibit impaired albumin endocytosis, increased substrate adhesion and decreased collective migration, correlating with a less differentiated epithelial phenotype. Despite sharing functional features, these DD1 cell models exhibit different gene expression profiles, being p.(Val523del) ClC-5 the mutation showing the largest differences. Gene set enrichment analysis pointed to kidney development, anion homeostasis, organic acid transport, extracellular matrix organization and cell-migration biological processes as the most likely involved in DD1 pathophysiology. In conclusion, our results revealed the pathways linking ClC-5 mutations with tubular dysfunction and, importantly, provide new cellular models to further study DD1 pathophysiology.

Diversity of functional alterations of the ClC-5 exchanger in the region of the proton glutamate in patients with Dent disease 1

Mutations in the CLCN5 gene encoding the 2Cl^- /1H⁺ exchanger ClC-5 are associated with Dent disease 1, an inherited renal disorder characterized by low-molecular-weight (LMW) proteinuria and hypercalciuria. In the kidney, ClC-5 is mostly localized in proximal tubule cells, where it is thought to play a key role in the endocytosis of LMW proteins. Here, we investigated the consequences of eight previously reported pathogenic missense mutations of ClC-5 surrounding the "proton glutamate" that serves as a crucial H⁺ -binding site for the exchanger. A complete loss of function was observed for a group of mutants that were either retained in the endoplasmic reticulum of HEK293T cells or unstainable at plasma membrane due to proteasomal degradation. In contrast, the currents measured for the second group of mutations in Xenopus laevis oocytes were reduced. Molecular dynamics simulations performed on a ClC-5 homology model demonstrated that such mutations might alter ClC-5 protonation by interfering with the water pathway. Analysis of clinical data from patients harboring these mutations demonstrated no phenotype/genotype correlation. This study reveals that mutations clustered in a crucial region of ClC-5 have diverse molecular consequences in patients with Dent disease 1, ranging from altered expression to defects in transport.

Dent disease: classification, heterogeneity and diagnosis

BACKGROUND

Dent disease is a rare tubulopathy characterized by manifestations of proximal tubular dysfunction, which occurs almost exclusively in males. It mainly presents symptoms in early childhood and may progress to end-stage renal failure between the 3rd and 5th decades of human life. According to its various genetic basis and to clinical signs and symptoms, researchers define two forms of Dent disease (Dent diseases 1 and 2) and suggest that these forms are produced by mutations in the CLCN5 and OCRL genes, respectively. Dent diseases 1 and 2 account for 60% and 15% of all Dent disease cases, and their genetic cause is generally understood. However, the genetic cause of the remaining 25% of Dent disease cases remains unidentified.

DATA SOURCES

All relevant peer-reviewed original articles published thus far have been screened out from PubMed and have been referenced.

RESULTS

Genetic testing has been used greatly to identify mutation types of CLCN5 and OCRL gene, and next-generation sequencing also has been used to identify an increasing number of unknown genotypes. Gene therapy may bring new hope to the treatment of Dent disease. The abuse of hormones and immunosuppressive agents for the treatment of Dent disease should be avoided to prevent unnecessary harm to children.

CONCLUSIONS

The current research progress in classification, genetic heterogeneity, diagnosis, and treatment of Dent disease reviewed in this paper enables doctors and researchers to better understand Dent disease and provides a basis for improved prevention and treatment.

Clinical manifestation and genetic findings in three boys with low molecular Weight Proteinuria - three case reports for exploring Dent Disease and Fanconi syndrome

Background

Dent disease is an X-linked form of progressive renal disease. This rare disorder was characterized by hypercalciuria, low molecular weight (LMW) proteinuria and proximal tubular dysfunction, caused by pathogenic variants in CLCN5 (Dent disease 1) or OCRL (Dent disease 2) genes. Fanconi syndrome is a consequence of decreased water and solute resorption in the proximal tubule of the kidney. Fanconi syndrome caused by proximal tubular dysfunction such as Dent disease might occur in early stage of the disease.

Case presentation

Three cases reported in this study were 3-, 10- and 14-year-old boys, and proteinuria was the first impression in all the cases. All the boys presented with LMW proteinuria and elevated urine albumin-to-creatinine ratio (ACR). Case 1 revealed a pathogenic variant in exon 11 of CLCN5 gene [NM_001127899; c.1444delG] and a nonsense mutation at nucleotide 1509 [p.L503*], and he was diagnosed as Dent disease 1. Case 2 carried a deletion of exon 3 and 4 of OCRL1 gene [NM_000276.4; c.120-238delG^…A] and a nonsense mutation at nucleotide 171 in exon 5 [p.E57*], and this boy was diagnosed as Dent disease 2. Genetic analysis of Case 3 showed a missense mutation located in exon 2 of HNF4A gene [EF591040.1; c.253C > T; p.R85W] which is responsible for Fanconi syndrome. All of three pathogenic variants were not registered in GenBank.

Conclusions

Urine protein electrophoresis should be performed for patients with proteinuria. When patients have LMW proteinuria and/or hypercalciuria, definite diagnosis and identification of Dent disease and Fanconi syndrome requires further genetic analyses.

Bartter-Like Syndrome as the Initial Presentation of Dent Disease 1: A Case Report

Dent disease is a rare genetic disease characterized by low-molecular-weight proteinuria. Dent disease with Bartter-like syndrome is rare and can easily be misdiagnosed and mistreated. Herein, we report a case of Dent disease 1 with Bartter-like syndrome as the initial manifestation. The patient was admitted to The Second Xiangya Hospital of Central South University due to polydipsia, polyuria, and weakness of both lower limbs at 2 years of age. Laboratory tests showed that serum sodium, potassium and chlorine levels were low, while serum creatinine levels were normal. The calcium level in the urine was normal. The patient was initially diagnosed with Bartter syndrome, and despite medical interventions, he eventually developed chronic kidney disease stage 4 at 13 years of age. To determine the cause, the patient was recommended to undergo genetic testing, which showed a CLCN5 gene c. 941C > T mutation (p.S314L), and was finally diagnosed as Dent disease 1. The clinical manifestations of Dent disease are complex and diverse. For patients with atypical clinical manifestations or unsatisfactory therapeutic effects, genetic testing is recommended.

Comparison of clinical and genetic characteristics between Dent disease 1 and Dent disease 2

BACKGROUND

Dent disease is associated with low molecular weight proteinuria and hypercalciuria and caused by pathogenic variants in either of two genes: CLCN5 (Dent disease 1) and OCRL (Dent disease 2). It is generally not accompanied by extrarenal manifestations and it is difficult to distinguish Dent disease 1 from Dent disease 2 without gene testing. We retrospectively compared the characteristics of these two diseases using one of the largest cohorts to date.

METHODS

We performed gene testing for clinically suspected Dent disease, leading to the genetic diagnosis of 85 males: 72 with Dent disease 1 and 13 with Dent disease 2. A retrospective review of the clinical findings and laboratory data obtained from questionnaires submitted in association with the gene testing was conducted for these cases.

RESULTS

The following variables had significantly higher levels in Dent disease 2 than in Dent disease 1: height standard deviation score (height SDS), serum creatinine-based estimated GFR (Cr-eGFR) (median: 84 vs. 127 mL/min/1.73 m², p < 0.01), serum aspartate aminotransferase (AST), serum alanine aminotransferase (ALT), serum lactate dehydrogenase (LDH), serum creatine phosphokinase (CK), serum potassium, serum inorganic phosphorus, serum uric acid, urine protein/creatinine ratio (median: 3.5 vs. 1.6 mg/mg, p < 0.01), and urine calcium/creatinine ratio. There were no significant differences in serum sodium, serum calcium, alkaline phosphatase (ALP), urine β2-microglobulin, incidence of nephrocalcinosis, and prevalence of intellectual disability or autism spectrum disorder.

CONCLUSIONS

The clinical and laboratory features of Dent disease 1 and Dent disease 2 were shown in this study. Notably, patients with Dent disease 2 showed kidney dysfunction at a younger age, which should provide a clue for the differential diagnosis of these diseases.

Making a Dent in Dent Disease

Dent disease (DD) is a rare kidney disorder caused by mutations in the Cl^-/H⁺ exchanger ClC-5. Extensive physiologic characterization of the transporter has begun to illuminate its role in endosomal ion homeostasis. Nevertheless, we have yet to understand how loss of ClC-5 function in the kidney proximal tubule impairs membrane traffic of megalin and cubilin receptors to cause the low molecular weight proteinuria characteristic of DD. This review identifies open questions that remain to be answered, evaluates the current literature addressing these questions, and suggests new testable models that may link loss of ClC-5 function to tubular proteinuria in DD.

Genetics and phenotypic heterogeneity of Dent disease: the dark side of the moon

Dent disease is a rare genetic proximal tubulopathy which is under-recognized. Its phenotypic heterogeneity has led to several different classifications of the same disorder, but it is now widely accepted that the triad of symptoms low-molecular-weight proteinuria, hypercalciuria and nephrocalcinosis/nephrolithiasis are pathognomonic of Dent disease. Although mutations on the CLCN5 and OCRL genes are known to cause Dent disease, no such mutations are found in about 25–35% of cases, making diagnosis more challenging. This review outlines current knowledge regarding Dent disease from another perspective. Starting from the history of Dent disease, and reviewing the clinical details of patients with and without a genetic characterization, we discuss the phenotypic and genetic heterogeneity that typifies this disease. We focus particularly on all those confounding clinical signs and symptoms that can lead to a misdiagnosis. We also try to shed light on a concealed aspect of Dent disease. Although it is a proximal tubulopathy, its misdiagnosis may lead to patients undergoing kidney biopsy. In fact, some individuals with Dent disease have high-grade proteinuria, with or without hematuria, as in the clinical setting of glomerulopathy, or chronic kidney disease of uncertain origin. Although glomerular damage is frequently documented in Dent disease patients’ biopsies, there is currently no reliable evidence of renal biopsy being of either diagnostic or prognostic value. We review published histopathology reports of tubular and glomerular damage in these patients, and discuss current knowledge regarding the role of CLCN5 and OCRL genes in glomerular function.

From protein uptake to Dent disease: An overview of the CLCN5 gene

Proteinuria is a well-known risk factor, not only for renal disorders, but also for several other problems such as cardiovascular diseases and overall mortality. In the kidney, the chloride channel Cl^-/H⁺ exchanger ClC-5 encoded by the CLCN5 gene is actively involved in preventing protein loss. This action becomes evident in patients suffering from the rare proximal tubulopathy Dent disease because they carry a defective ClC-5 due to CLCN5 mutations. In fact, proteinuria is the distinctive clinical sign of Dent disease, and mainly involves the loss of low-molecular-weight proteins. The identification of CLCN5 disease-causing mutations has greatly improved our understanding of ClC-5 function and of the ClC-5-related physiological processes in the kidney. This review outlines current knowledge regarding the CLCN5 gene and its protein product, providing an update on ClC-5 function in tubular and glomerular cells, and focusing on its relationship with proteinuria and Dent disease.

Functional analysis of suspected splicing variants in CLCN5 gene in Dent disease 1

Background

In recent years, the elucidation of splicing abnormalities as a cause of hereditary diseases has progressed. However, there are no comprehensive reports of suspected splicing variants in the CLCN5 gene in Dent disease cases. We reproduced gene mutations by mutagenesis, inserted the mutated genes into minigene vectors, and investigated the pathogenicity and onset mechanisms of these variants.

Methods

We conducted functional splicing assays using a hybrid minigene for six suspected splicing variants (c.105G>A, c.105+5G>C, c.106−17T>G, c.393+4A>G, c.517−8A>G, c.517−3C>A) in CLCN5. We extracted information on these variants from the Human Gene Mutation Database. We reproduced minigene vectors with the insertion of relevant exons with suspected splicing variants. We then transfected these minigene vectors into cultured cells and extracted and analyzed the mRNA. In addition, we conducted in silico analysis to confirm our minigene assay results.

Results

We successfully determined that five of these six variants are pathogenic via the production of splicing abnormalities. One showed only normal transcript production and was thus suspected of not being pathogenic (c.106−17T>G).

Conclusion

We found that five CLCN5 variants disrupted the original splice site, resulting in aberrant splicing. It is sometimes difficult to obtain mRNA from patient samples because of the fragility of mRNA or its low expression level in peripheral leukocytes. Our in vitro system can be used as an alternative to in vivo assays to determine the pathogenicity of suspected splicing variants.

Electronic supplementary material

The online version of this article (10.1007/s10157-020-01876-x) contains supplementary material, which is available to authorized users.

Genetic Analyses in Dent Disease and Characterization of CLCN5 Mutations in Kidney Biopsies

Dent disease (DD), an X-linked renal tubulopathy, is mainly caused by loss-of-function mutations in CLCN5 (DD1) and OCRL genes. CLCN5 encodes the ClC-5 antiporter that in proximal tubules (PT) participates in the receptor-mediated endocytosis of low molecular weight proteins. Few studies have analyzed the PT expression of ClC-5 and of megalin and cubilin receptors in DD1 kidney biopsies. About 25% of DD cases lack mutations in either CLCN5 or OCRL genes (DD3), and no other disease genes have been discovered so far. Sanger sequencing was used for CLCN5 gene analysis in 158 unrelated males clinically suspected of having DD. The tubular expression of ClC-5, megalin, and cubilin was assessed by immunolabeling in 10 DD1 kidney biopsies. Whole exome sequencing (WES) was performed in eight DD3 patients. Twenty-three novel CLCN5 mutations were identified. ClC-5, megalin, and cubilin were significantly lower in DD1 than in control biopsies. The tubular expression of ClC-5 when detected was irrespective of the type of mutation. In four DD3 patients, WES revealed 12 potentially pathogenic variants in three novel genes (SLC17A1, SLC9A3, and PDZK1), and in three genes known to be associated with monogenic forms of renal proximal tubulopathies (SLC3A, LRP2, and CUBN). The supposed third Dent disease-causing gene was not discovered.

Multicenter study of the clinical features and mutation gene spectrum of Chinese children with Dent disease

Dent disease is a rare X-linked recessive inherited tubular disease. In this multicenter study, the clinical presentation and genetic background of Chinese children with Dent disease are studied to improve the cognition and diagnostic ability of pediatricians. In this prospective cohort, we described the genotype and phenotype of a national cohort composed of 45 pediatric probands with Dent disease belonging to 45 families from 12 different regions of China recruited from 2014 to 2018 by building up the multicenter registration system. The CLCN5 gene from 32 affected families revealed 28 different mutations. The OCRL gene from 13 affected families revealed 13 different mutations. The incidence of low-molecular-weight proteinuria (LMWP) in both Dent disease type 1 populations and Dent disease type 2 populations was 100.0%; however, the incidence of other manifestations was not high, which was similar to previously reported data. Therefore, LMWP is a key clinical feature that should alert clinicians to the possibility of Dent disease. A high amount of LMWP combined with positive gene test results can be used as the diagnostic criteria for this disease. The diagnostic criteria are helpful in reducing the missed diagnosis of this disease and are beneficial for protecting the renal function of these patients through early diagnosis and early intervention.

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.

A Novel CLCN5 Splice Site Mutation in a Boy with Incomplete Phenotype of Dent Disease

Dent disease is a rare X-linked renal proximal tubulopathy presenting with low-molecular-weight proteinuria (LMWP), hypercalciuria, and nephrocalcinosis, other signs of incomplete renal Fanconi syndrome, and renal failure. Early identification of patients who harbor disease-associated mutations is important for effective medical care and avoidance of unnecessary interventions. We report the case of an asymptomatic 9-year-old boy who presented with proteinuria in routine examination. Further investigation revealed the presence of nephrotic range proteinuria, mostly LMWP and mild hypercalciuria without nephrocalcinosis, or other features of tubular dysfunction. Renal function, growth, and bone mineral density were within regular limits. The male gender and the presence of LMWP and hypercalciuria even in the absence of other findings prompted us to genetic investigation for Dent disease. A novel splice site mutation (c.416-2A > G) of the chloride voltage-gated channel 5 ( CLCN5 ) gene, responsible for Dent disease type 1 was identified. In silico analysis revealed that this mutation interferes with the mating of exons 4 and 5. Due to early molecular diagnosis, our patient did not undergo a renal biopsy, neither required aggressive pharmacological interventions. This case underscores the diversity and complexity of CLCN5 mutations and highlights the importance of early molecular testing in male patients with incomplete phenotype of Dent disease.

ClC-5 alleviates renal fibrosis in unilateral ureteral obstruction mice

Renal fibrosis is the major feature of end-stage renal disease with high mortality. Chloride (Cl^-) moving along Cl^- channels has been suggested to play to an important role in renal function. This study aims to investigate the role of ClC-5 in renal fibrosis in unilateral ureteral occlusion (UUO) mice. C57BL/6 mice received UUO surgery followed by delivery of adeno-associated virus encoding ClC-5 cDNA (AAVClC-5). Western blotting, real-time PCR and histological analysis were used to investigate the effects of ClC-5 on renal fibrosis and underlying mechanisms. The expression of ClC-5 was significantly decreased in renal cortex of UUO mice and transforming growth factor-β1 (TGF-β1)-stimulated HK2 cells. Overexpression of ClC-5 in vivo markedly ameliorated UUO-induced renal injury and fibrosis. The increased expressions of plasminogen activator inhibitor type 1, connective tissue growth factor, collagen III and collagen IV were also inhibited by ClC-5 upregulation. Moreover, UUO-induced immune cell infiltration and inflammatory cytokines release were attenuated in mice infected with AAVClC-5. In addition, the in vivo and in vitro results showed that ClC-5 overexpression prevented epithelial-to-mesenchymal transition (EMT), concomitantly with a restoration of E-cadherin expression and a decrease of vimentin, α-SMA and S100A4 expressions. Furthermore, ClC-5 overexpression inhibited UUO- or TGF-β1-induced increase in nuclear factor kappa B (NF-κB) acetylation and matrix metalloproteinases-9 (MMP-9) expression. However, downregulation of ClC-5 in HK2 cells further potentiated TGF-β1-induced EMT and increase in NF-κB acetylation and MMP-9 expression. ClC-5 upregulation ameliorates renal fibrosis via inhibiting NF-κB/MMP-9 pathway signaling activation, suggesting that ClC-5 may be a novel therapeutic target for treating renal fibrosis and chronic kidney disease.

Molecular Control of Phosphorus Homeostasis and Precision Treatment of Hypophosphatemic Disorders

Purpose of Review

Serum phosphorus is maintained in a narrow range by balancing dietary phosphate absorption, influx and efflux of phosphorus from bone and intracellular stores, and renal reabsorption of filtered phosphate. Acute hypophosphatemia, typically caused by transient increases in cellular uptake, can lead to severe complications such as cardiopulmonary dysfunction and rhabdomyolysis that can warrant parenteral phosphate repletion. Chronic hypophosphatemia, however, generally represents true phosphate deficiency and may result in long-term metabolic and skeletal complications, particularly in children due to the critical importance of phosphorus to skeletal mineralization and longitudinal growth.

Recent Findings

In addition to the well characterized roles of vitamin D and parathyroid hormone (PTH), a new bone-kidney axis has been discovered that regulates phosphate homeostasis through the bone-derived hormone Fibroblast Growth Factor 23 (FGF23) and its phosphaturic actions that are mediated by activation of fibroblast growth factor receptors (FGFRs) complexed with α-Klotho in renal tubules. Chronic hypophosphatemia can now be classified as FGF23 dependent or independent.

Summary

In cases of FGF23 dependent hypophosphatemia, traditional non-specific treatments with elemental phosphorus and 1,25(OH)₂ vitamin D (calcitriol) can now be replaced with a targeted approach by using an FGF-23 blocking antibody (Burosumab).

Update on Dent Disease

Dent disease is an X-linked form of chronic kidney disease characterized by hypercalciuria, low molecular weight proteinuria, nephrocalcinosis, and proximal tubular dysfunction. Clinical presentation is highly variable. Male patients may present with early-onset rickets, recurrent nephrolithiasis, or insidiously with asymptomatic proteinuria or chronic kidney disease. Mutations in both the CLCN5 and OCRL1 genes have been associated with the Dent phenotype and are now classified as Dent-1 and Dent-2, respectively. This article describes the clinical presentation, laboratory evaluation, genetics, pathophysiology, management, and future therapies of Dent disease.

Next-Generation Sequencing in Early Diagnosis of Dent Disease 1: Two Case Reports

Dent disease 1 is a rare X-linked recessive inherited disease, caused by pathogenic variants in the chloride voltage-gated channel 5 (CLCN5) gene. Dent disease 1 is characterized by low molecular weight (LMW) proteinuria, hypercalciuria, nephrocalcinosis, and chronic kidney disease. Infants may manifest only asymptomatic LMW proteinuria, which increases the difficulty of early diagnosis. We describe two male infants presenting only with nephrotic-range LMW proteinuria observed on examination using urine protein electrophoresis. Hereditary renal tubular diseases were highly suspected based on early onset age and LMW proteinuria. Thus, next-generation sequencing (NGS) was performed and pathogenic mutations in CLCN5 were identified in both patients. A diagnosis of Dent disease 1 was established based on the above informations. The two patients developed hypercalciuria during late follow-up, which verified the diagnosis. These two cases highlight the importance of next-generation sequencing in the early diagnosis of Dent disease 1 with only LMW proteinuria.

Barttin Regulates the Subcellular Localization and Posttranslational Modification of Human Cl-/H+ Antiporter ClC-5

Dent disease 1 (DD1) is a renal salt-wasting tubulopathy associated with mutations in the Cl-/H+ antiporter ClC-5. The disease typically manifests with proteinuria, hypercalciuria, nephrocalcinosis, and nephrolithiasis but is characterized by large phenotypic variability of no clear origin. Several DD1 cases have been reported lately with additional atypical hypokalemic metabolic alkalosis and hyperaldosteronism, symptoms usually associated with another renal disease termed Bartter syndrome (BS). Expression of the Bartter-like DD1 mutant ClC-5 G261E in HEK293T cells showed that it is retained in the ER and lacks the complex glycosylation typical for ClC-5 WT. Accordingly, the mutant abolished CLC ionic transport. Such phenotype is not unusual and is often observed also in DD1 ClC-5 mutants not associated with Bartter like phenotype. We noticed, therefore, that one type of BS is associated with mutations in the protein barttin that serves as an accessory subunit regulating the function and subcellular localization of ClC-K channels. The overlapping symptomatology of DD1 and BS, together with the homology between the proteins of the CLC family, led us to investigate whether barttin might also regulate ClC-5 transport. In HEK293T cells, we found that barttin cotransfection impairs the complex glycosylation and arrests ClC-5 in the endoplasmic reticulum. As barttin and ClC-5 are both expressed in the thin and thick ascending limbs of the Henle's loop and the collecting duct, interactions between the two proteins could potentially contribute to the phenotypic variability of DD1. Pathologic barttin mutants differentially regulated trafficking and processing of ClC-5, suggesting that the interaction between the two proteins might be relevant also for the pathophysiology of BS. Our findings show that barttin regulates the subcellular localization not only of kidney ClC-K channels but also of the ClC-5 transporter, and suggest that ClC-5 might potentially play a role not only in kidney proximal tubules but also in tubular kidney segments expressing barttin. In addition, they demonstrate that the spectrum of clinical, genetic and molecular pathophysiology investigation of DD1 should be extended.

[Genetic and biochemical features of the monogenic hereditary urolithiasis]

Urolithiasis is a common urological problem. In most cases, this multifactorial pathology develops due to the combination of inherited low-penetrance gene variants and environment factors such as urinary tract infections and unbalanced diet. However, some cases are monogenic. These hereditary forms of urolithiasis manifest in childhood, and are characterized by multiple, bilateral and recurrent kidney stones and progress to chronic renal failure relatively early. Due to widening acceptance of exome and gene panel sequencing, substantially larger percentages of urolithiasis cases are now attributed to hereditary causes, up to 20% among patients of 18 years old or younger. Here we review genetic and biochemical mechanisms of urolithiasis, with an emphasis on its hereditary forms, including fermentopathies (primary hyperoxaluria, adenine phosphorobosyltransferase deficiency, phosphoribosyl-pyrophosphate-synthetase deficiency, xanthinuria, Lesch-Nihan syndrome) and these caused by membrane transport alterations (Dent's disease, familial hypomagnesia with hypercalciuria and nephrocalcinosis, hypophosphatemic urolithiasis, distal tubular acidosis, cystinuria, Bartter's syndrome). We suggest a comprehensive gene panel for NGS diagnostics of the hereditary urolithiasis. It is expected that accurate and timely diagnosis of hereditary forms of urolithiasis would enable the counselling of the carriers in affected families, and ensure personalized management of the patients with these conditions.

CLC Chloride Channels and Transporters: Structure, Function, Physiology, and Disease

CLC anion transporters are found in all phyla and form a gene family of eight members in mammals. Two CLC proteins, each of which completely contains an ion translocation parthway, assemble to homo- or heteromeric dimers that sometimes require accessory β-subunits for function. CLC proteins come in two flavors: anion channels and anion/proton exchangers. Structures of these two CLC protein classes are surprisingly similar. Extensive structure-function analysis identified residues involved in ion permeation, anion-proton coupling and gating and led to attractive biophysical models. In mammals, ClC-1, -2, -Ka/-Kb are plasma membrane Cl−channels, whereas ClC-3 through ClC-7 are 2Cl−/H+-exchangers in endolysosomal membranes. Biological roles of CLCs were mostly studied in mammals, but also in plants and model organisms like yeast and Caenorhabditis elegans. CLC Cl−channels have roles in the control of electrical excitability, extra- and intracellular ion homeostasis, and transepithelial transport, whereas anion/proton exchangers influence vesicular ion composition and impinge on endocytosis and lysosomal function. The surprisingly diverse roles of CLCs are highlighted by human and mouse disorders elicited by mutations in their genes. These pathologies include neurodegeneration, leukodystrophy, mental retardation, deafness, blindness, myotonia, hyperaldosteronism, renal salt loss, proteinuria, kidney stones, male infertility, and osteopetrosis. In this review, emphasis is laid on biophysical structure-function analysis and on the cell biological and organismal roles of mammalian CLCs and their role in disease.

A novel CLCN5 pathogenic mutation supports Dent disease with normal endosomal acidification

Dent disease is an X-linked recessive renal tubular disorder characterized by low-molecular-weight proteinuria, hypercalciuria, nephrolithiasis, nephrocalcinosis, and progressive renal failure. Inactivating mutations of CLCN5, the gene encoding the 2Cl^- /H⁺ exchanger ClC-5, have been reported in patients with Dent disease 1. In vivo studies in mice harboring an artificial mutation in the "gating glutamate" of ClC-5 (c.632A > C, p.Glu211Ala) and mathematical modeling suggest that endosomal chloride concentration could be an important parameter in endocytosis, rather than acidification as earlier hypothesized. Here, we described a novel pathogenic mutation affecting the "gating glutamate" of ClC-5 (c.632A>G, p.Glu211Gly) and investigated its molecular consequences. In HEK293T cells, the p.Glu211Gly ClC-5 mutant displayed unaltered N-glycosylation and normal plasma membrane and early endosomes localizations. In Xenopus laevis oocytes and HEK293T cells, we found that contrasting with wild-type ClC-5, the mutation abolished the outward rectification, the sensitivity to extracellular H⁺ and converted ClC-5 into a Cl^- channel. Investigation of endosomal acidification in HEK293T cells using the pH-sensitive pHluorin2 probe showed that the luminal pH of cells expressing a wild-type or p.Glu211Gly ClC-5 was not significantly different. Our study further confirms that impaired acidification of endosomes is not the only parameter leading to defective endocytosis in Dent disease 1.

Genetic Analysis of Dent's Disease and Functional Research of CLCN5 Mutations

Dent's disease is an X-linked inherited renal disease. Patients with Dent's disease often carry mutations in genes encoding the Cl^-/H⁺ exchanger ClC-5 and/or inositol polyphosphate 5-phosphatase (OCRL1). However, the mutations involved and the biochemical effects of these mutations are not fully understood. To characterize genetic changes in Dent's disease patients, in this study, samples from nine Chinese patients were subjected to genetic analysis. Among the nine patients, six were classified as having Dent-1 disease, one had Dent-2 disease, and two could not be classified. Expression of ClC-5 carrying Dent's disease-associated mutations in HEK293 cells had varying effects: (1) no detectable expression of mutant protein; (2) retention of a truncated protein in the endoplasmic reticulum; or (3) diminished protein expression with normal distribution in early endosomes. Dent's disease patients showed genetic heterogeneity and over 20% of patients did not have CLCN5 or OCRL1 mutations, suggesting the existence of other genetic factors. Using next-generation sequencing, we identified possible modifier genes that have not been previously reported in Dent's disease patients. Heterozygous variants in CFTR, SCNN1A, and SCNN1B genes associated with cystic fibrosis (CF) or CF-like disease were detected in four of our nine patients. These results may form the basis for future characterization of Dent's disease and genetic counseling approaches.

Dent disease in Poland: what we have learned so far?

PURPOSE

Dent disease (DD) is a rare tubulopathy characterized by proximal tubular dysfunction leading to chronic kidney disease (CKD). The aim of the study was to characterize patients with DD in Poland.

METHODS

A retrospective analysis of a national cohort with genetically confirmed diagnosis.

RESULTS

Of 24 males, all patients except one carried mutations in the CLCN5 gene; in one patient a mutation in the OCRL gene was disclosed. Molecular diagnosis was delayed 1 year on average (range 0-21 years). The most common features were tubular proteinuria (100%), hypercalciuria (87%), and nephrocalcinosis (56%). CKD (≤stage II) and growth deficiency were found in 45 and 22% of patients, respectively. Over time, a progression of CKD and persistence of growth impairment was noted. Subnephrotic and nephrotic proteinuria (20%) was found in most patients, but tubular proteinuria was assessed in only 67% of patients. In one family steroid-resistant nephrotic syndrome prompted a genetic testing, and reverse phenotyping. Five children (20%) underwent kidney biopsy, and two of them were treated with immunosuppressants. Hydrochlorothiazide and angiotensin-converting enzyme inhibitors were prescribed for a significant proportion of patients (42 and 37.5%, respectively), while supplemental therapy with phosphate, potassium, vitamin D (12.5% each), and alkali (4.2%) was insufficient, when compared to the percentages of patients requiring repletion.

CONCLUSIONS

We found CLCN5 mutations in the vast majority of Polish patients with DD. Proteinuria was the most constant finding; however, tubular proteins were not assessed commonly, likely leading to delayed molecular diagnosis and misdiagnosis in some patients. More consideration should be given to optimize the therapy.

Hepatocyte Nuclear Factor-1β Controls Mitochondrial Respiration in Renal Tubular Cells

AKI is a frequent condition that involves renal microcirculation impairment, infiltration of inflammatory cells with local production of proinflammatory cytokines, and subsequent epithelial disorders and mitochondrial dysfunction. Peroxisome proliferator-activated receptor γ coactivator 1-α (PPARGC1A), a coactivator of the transcription factor PPAR-γ that controls mitochondrial biogenesis and function, has a pivotal role in the early dysfunction of the proximal tubule and the subsequent renal repair. Here, we evaluated the potential role of hepatocyte nuclear factor-1β (HNF-1β) in regulating PPARGC1A expression in AKI. In mice, endotoxin injection to induce AKI also induced early and transient inflammation and PPARGC1A inhibition, which overlapped with downregulation of the HNF-1β transcriptional network. In vitro, exposure of proximal tubule cells to the inflammatory cytokines IFN-γ and TNF-α led to inhibition of HNF-1β transcriptional activity. Moreover, inhibition of HNF-1β significantly reduced PPARGC1A expression and altered mitochondrial morphology and respiration in proximal tubule cells. Chromatin immunoprecipitation assays and PCR analysis confirmed HNF-1β binding to the Ppargc1a promoter in mouse kidneys. We also demonstrated downregulation of renal PPARGC1A expression in a patient with an HNF1B germinal mutation. Thus, we propose that HNF-1β links extracellular inflammatory signals to mitochondrial dysfunction during AKI partly via PPARGC1A signaling. Our findings further strengthen the view of HNF1B-related nephropathy as a mitochondrial disorder in adulthood.