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

CryoEM structures of the human CLC-2 voltage-gated chloride channel reveal a ball-and-chain gating mechanism

CLC-2 is a voltage-gated chloride channel that contributes to electrical excitability and ion homeostasis in many different tissues. Among the nine mammalian CLC homologs, CLC-2 is uniquely activated by hyperpolarization, rather than depolarization, of the plasma membrane. The molecular basis for the divergence in polarity of voltage gating among closely related homologs has been a long-standing mystery, in part because few CLC channel structures are available. Here, we report cryoEM structures of human CLC-2 at 2.46 - 2.76 Å, in the presence and absence of the selective inhibitor AK-42. AK-42 binds within the extracellular entryway of the Cl--permeation pathway, occupying a pocket previously proposed through computational docking studies. In the apo structure, we observed two distinct conformations involving rotation of one of the cytoplasmic C-terminal domains (CTDs). In the absence of CTD rotation, an intracellular N-terminal 15-residue hairpin peptide nestles against the TM domain to physically occlude the Cl--permeation pathway. This peptide is highly conserved among species variants of CLC-2 but is not present in other CLC homologs. Previous studies suggested that the N-terminal domain of CLC-2 influences channel properties via a "ball-and-chain" gating mechanism, but conflicting data cast doubt on such a mechanism, and thus the structure of the N-terminal domain and its interaction with the channel has been uncertain. Through electrophysiological studies of an N-terminal deletion mutant lacking the 15-residue hairpin peptide, we support a model in which the N-terminal hairpin of CLC-2 stabilizes a closed state of the channel by blocking the cytoplasmic Cl--permeation pathway.

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.

A retrospective study of infantile-onset Takayasu arteritis: experience from a tertiary referral center in China

Objective

Takayasu artery (TAK) is a chronic inflammatory disease that mainly affects the aorta and its major branches and is rarely reported in infants. We aimed to summarize the clinical features of infant TA (I-TA) in a tertiary care center.

Methods

We performed a retrospective study involving 10 infants diagnosed with TAK. A comprehensive evaluation of clinical, laboratory, radiographic features, disease activity, treatment and outcomes was carried out.

Results

A consecutive cohort was composed of 8 girls and 2 boys, with an age at diagnosis of 11.1 (1.7-36) months. The median time to diagnosis and the average time to follow-up were 9.5 days (2-235 days) and 10.9 (1-21) months, respectively. The most common initial manifestations were malaise (80%), fever (70%), hypertension (50%) and rash (30%). The mean Pediatric Vasculitis Activity Score (PVAS), Takayasu Clinical Activity Score (ITAS-2010) and ITAS-A scores were 2.8/63, 2.6/51, and 5.6/54, respectively. All patients had aberrant laboratory parameters. The most common lesions were in the thoracic aorta (60%) and abdominal aorta (60%). Corticosteroids combined with cyclophosphamide followed by long-term mycophenolate mofetil were initiated in most cases (70%). Biologics were attempted in 5 cases. Mortality was 40%.

Conclusions

It is challenging to diagnose TAK in infants in a timely manner. Considering the more vessels involved, more severe inflammation and higher mortality, aggressive treatment is warranted in infants. GCs and CYC treatment seem to be effective.

Machine Learning-Based Integrated Analysis of PANoptosis Patterns in Acute Myeloid Leukemia Reveals a Signature Predicting Survival and Immunotherapy

Objective

We conducted a meticulous bioinformatics analysis leveraging expression data of 226 PANRGs obtained from previous studies, as well as clinical data from AML patients derived from the HOVON database.

Methods

Through meticulous data analysis and manipulation, we were able to categorize AML cases into two distinct PANRG clusters and subsequently identify differentially expressed genes (PRDEGs) with prognostic significance. Furthermore, we organized the patient data into two corresponding gene clusters, allowing us to investigate the intricate relationship between the risk score, patient prognosis, and the immune landscape.

Results

Our findings disclosed significant associations between the identified PANRGs, gene clusters, patient survival, immune system, and cancer-related biological processes and pathways. Importantly, we successfully constructed a prognostic signature comprising nineteen genes, enabling the stratification of patients into high-risk and low-risk groups based on individually calculated risk scores. Furthermore, we developed a robust and practical nomogram model, integrating the risk score and other pertinent clinical features, to facilitate accurate patient survival prediction. Our comprehensive analysis demonstrated that the high-risk group exhibited notably worse prognosis, with the risk score proving to be significantly correlated with infiltration of most immune cells. The qRT-PCR results revealed significant differential expression patterns of LGR5 and VSIG4 in normal and human leukemia cell lines (HL-60 and MV-4-11).

Conclusions

Our findings underscore the potential utility of PANoptosis-based molecular clustering and prognostic signatures as predictive tools for assessing patient survival in AML.

Prenatal diagnosis of dent disease type I with a nonsense pathogenic variant in CLCN5: a case study

INTRODUCTION

Dent disease type I is a rare X-linked recessive renal tubular disease resulting from pathogenic variants in the CLCN5 gene. Due to the rarity of Dent disease type I and the diversity of its phenotypes, its clinical diagnosis is complex and poses a challenge to clinicians.

METHODS

A foetus and a child from a 36-year-old pregnant woman with a birth history of abnormal children were enrolled in this study. Pregnant women undergo amniocentesis for prenatal diagnosis at the gestational age of 12+ 3 weeks. Chromosomal microarray (CMA) analysis and whole-exome sequencing (WES) were employed to investigate the chromosomal copy number and single gene variants. Literature retrieval and data analysis were performed for genotype and phenotype collection analysis.

RESULTS

No chromosomal abnormalities or CNVs were detected in the entire family through karyotype and familial CMA analyses. WES identified a nonsense pathogenic variant in CLCN5 of the X chromosome, c.1942 C > T (exon 11, NM_000084), which was inherited from his mother, who exhibited regular clinical features.

CONCLUSION

This study suggests that children with low-molecular-weight proteinuria and hypercalciuria should undergo prompt genetic testing to exclude Dent disease.

The Apical Endocytic-Lysosomal Apparatus in CLCN5 Mutations with Phenotypic-Genotypic Correlations in Three Cases

Dent disease type 1 is characterized by pathogenic CLCN5 gene variants and impaired receptor-mediated endocytosis in proximal tubules. However, mutation-related abnormalities in proximal tubules have not yet been described. Here, we present three patients with CLCN5 alterations and distinct morphological changes of the apical endocytic-lysosomal apparatus. The proximal tubular ultrastructure was investigated in kidney biopsy samples of three boys genotyped for non-nephrotic proteinuria. Controls: seven patients with nephrotic-range glomerular proteinuria. The genotyping findings revealed an already-known missense mutation in one patient and hitherto undescribed frameshift variants in two patients. Low-molecular-weight proteinuria, focal global glomerulosclerosis, proximal tubular changes, and tubular calcium deposits characterized each case. Three subsets of proximal tubular cells were observed: those without any abnormality, those with aplasia of apical endocytic-lysosomal apparatus and shrinkage of cells, and those with hypoplasia of apical endocytic apparatus, accumulation of proteinaceous substance in dysmorphic lysosomes, and dysmorphic mitochondria. The distribution of subsets varied from patient to patient. In one patient with a frameshift variant, an oxidative stress-like injury of proximal tubular cells and podocytes accompanied the above-mentioned alterations. Focal aplasia/hypoplasia of apical endocytic apparatus and subsequent changes in cytoplasmic organelles characterized proximal tubules in the CLCN5 pathogenic variants.

CryoEM structures of the human CLC-2 voltage gated chloride channel reveal a ball and chain gating mechanism

CLC-2 is a voltage-gated chloride channel that contributes to electrical excitability and ion homeostasis in many different mammalian tissues and cell types. Among the nine mammalian CLC homologs, CLC-2 is uniquely activated by hyperpolarization, rather than depolarization, of the plasma membrane. The molecular basis for the divergence in polarity of voltage gating mechanisms among closely related CLC homologs has been a long-standing mystery, in part because few CLC channel structures are available, and those that exist exhibit high conformational similarity. Here, we report cryoEM structures of human CLC-2 at 2.46 - 2.76 Å, in the presence and absence of the potent and selective inhibitor AK-42. AK-42 binds within the extracellular entryway of the Cl--permeation pathway, occupying a pocket previously proposed through computational docking studies. In the apo structure, we observed two distinct apo conformations of CLC-2 involving rotation of one of the cytoplasmic C-terminal domains (CTDs). In the absence of CTD rotation, an intracellular N-terminal 15-residue hairpin peptide nestles against the TM domain to physically occlude the Cl--permeation pathway from the intracellular side. This peptide is highly conserved among species variants of CLC-2 but is not present in any other CLC homologs. Previous studies suggested that the N-terminal domain of CLC-2 influences channel properties via a "ball-and-chain" gating mechanism, but conflicting data cast doubt on such a mechanism, and thus the structure of the N-terminal domain and its interaction with the channel has been uncertain. Through electrophysiological studies of an N-terminal deletion mutant lacking the 15-residue hairpin peptide, we show that loss of this short sequence increases the magnitude and decreases the rectification of CLC-2 currents expressed in mammalian cells. Furthermore, we show that with repetitive hyperpolarization WT CLC-2 currents increase in resemblance to the hairpin-deleted CLC-2 currents. These functional results combined with our structural data support a model in which the N-terminal hairpin of CLC-2 stabilizes a closed state of the channel by blocking the cytoplasmic Cl--permeation pathway.

Vesicular CLC chloride/proton exchangers in health and diseases

Chloride is one of the most abundant anions in the human body; it is implicated in several physiological processes such as the transmission of action potentials, transepithelial salt transport, maintenance of cellular homeostasis, regulation of osmotic pressure and intracellular pH, and synaptic transmission. The balance between the extracellular and intracellular chloride concentrations is controlled by the interplay of ion channels and transporters embedded in the cellular membranes. Vesicular members of the CLC chloride protein family (vCLCs) are chloride/proton exchangers expressed in the membrane of the intracellular organelles, where they control vesicular acidification and luminal chloride concentration. It is well known that mutations in CLCs cause bone, kidney, and lysosomal genetic diseases. However, the role of CLC exchangers in neurological disorders is only now emerging with the identification of pathogenic CLCN gene variants in patients with severe neuronal and intellectual dysfunctions. This review will provide an overview of the recent advances in understanding the role of the vesicular CLC chloride/proton exchangers in human pathophysiology.

Emerging roles of proximal tubular endocytosis in renal fibrosis

Endocytosis is a crucial component of many pathological conditions. The proximal tubules are responsible for reabsorbing the majority of filtered water and glucose, as well as all the proteins filtered through the glomerular barrier via endocytosis, indicating an essential role in kidney diseases. Genetic mutations or acquired insults could affect the proximal tubule endocytosis processes, by disturbing or overstressing the endolysosomal system and subsequently activating different pathways, orchestrating renal fibrosis. This paper will review recent studies on proximal tubular endocytosis affected by other diseases and factors. Endocytosis plays a vital role in the development of renal fibrosis, and renal fibrosis could also, in turn, affect tubular endocytosis.

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.

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.

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.

Single-Cell Gene Expression Analysis in Patients with Medullary Sponge Kidney and a Retrospective Study

OBJECTIVE

To establish better diagnosis thinking and provide advanced understanding of MSK, the CT imaging features, clinical characteristics, and the expression of suspected genes in the kidney spatiotemporal immune zonation and fetal renal development were investigated.

METHODS

17 patients with MSK hospitalized in our hospital were selected as our research subjects. Human Phenotype Ontology, MalaCards: The Human Disease Database, GeneCards: The Human Gene Database, Human Protein Atlas, and Single Cell Expression Atlas were used to analyze this disease.

RESULTS

In our 17 patients, the incidence of MSK tended to be the same in male and female, and the onset age of MSK was probably 31-50 years old. The top one related disease of MSK was nephrocalcinosis and the most frequent phenotype related to MSK was nephrolithiasis. In addition, the expression of HNF1B, CLCN5, GDNF, ATP6V0A4, ATP6V1B1, LAMA2, RET, ACAN, and ABCC8 has been implicated in both human kidney immune zonation and fetal kidney development.

CONCLUSIONS

HNF1B, CLCN5, GDNF, ATP6V0A4, ATP6V1B1, LAMA2, RET, ACAN, and ABCC8 could be independent indicators for the diagnosis and preventive intervention of MSK patients, and abnormal kidney development due to mutations in key genes was the underlying cause of MSK.

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.

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

BACKGROUND

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

SUMMARY

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

KEY MESSAGES

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

Establishment of an induced pluripotent stem cell line (NCKDi003-A) from a patient with X-linked Dent disease (X-Dent) carrying the hemizygote mutation p. T277P (c. 829A > C) in the CLCN5 gene

Dent disease (DD) is a rare X-linked proximal tubulopathy associated with low molecular weight proteinuria (LMWP), hypercalciuria, nephrolithiasis and phosphoruria, which may progress to chronic kidney disease (CKD). About 60% of cases are caused by the mutation in CLCN5 gene. Recently, we identified a mutation in the sequence of homodimer of CLCN5 gene in a patient with DD. The Peripheral Blood Mononuclear Cells (PBMCs) of the patient were obtained and a line of induced pluripotent stem cells (iPSCs) was successfully generated. The iPSC line will be useful for further study of the pathogenesis and drug screening for DD.

CLN3, at the crossroads of endocytic trafficking

The CLN3 gene was identified over two decades ago, but the primary function of the CLN3 protein remains unknown. Recessive inheritance of loss of function mutations in CLN3 are responsible for juvenile neuronal ceroid lipofuscinosis (Batten disease, or CLN3 disease), a fatal childhood onset neurodegenerative disease causing vision loss, seizures, progressive dementia, motor function loss and premature death. CLN3 is a multipass transmembrane protein that primarily localizes to endosomes and lysosomes. Defects in endocytosis, autophagy, and lysosomal function are common findings in CLN3-deficiency model systems. However, the molecular mechanisms underlying these defects have not yet been fully elucidated. In this mini-review, we will summarize the current understanding of the CLN3 protein interaction network and discuss how this knowledge is starting to delineate the molecular pathogenesis of CLN3 disease. Accumulating evidence strongly points towards CLN3 playing a role in regulation of the cytoskeleton and cytoskeletal associated proteins to tether cellular membranes, regulation of membrane complexes such as channels/transporters, and modulating the function of small GTPases to effectively mediate vesicular movement and membrane dynamics.

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.

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.

Downregulation of megalin, cubilin, ClC-5 and podocin in Fabry nephropathy: potential implications in the decreased effectiveness of enzyme replacement therapy

Fabry disease is an X-linked disorder due to mutations in α-galactosidase A, resulting in the accumulation of enzyme substrates and cell malfunction. Kidney involvement is frequent, affecting all native kidney cell types. Podocyte damage results in proteinuria and chronic kidney disease. End-stage kidney disease is the rule in middle-aged males and some females with the classic phenotype. In podocytes and kidney proximal tubular cells, megalin is one of the molecules involved in enzyme replacement therapy (ERT) cellular absorption. After podocyte damage, podocin concentration is decreased and contributes to progressive proteinuria. We report in a male and a female patient the decreased expression of megalin, cubilin, ClC-5 and podocin compared to controls and chronic kidney disease (CKD) biopsies. Moreover, the decrease in ClC-5, a molecule engaged in endosomal-lysosomal acidification, could also affect ERT. These findings may partially explain some of the dysfunctions described in Fabry nephropathy and could highlight possible alterations in the pharmacokinetics of the delivered enzyme.