An improved terminology and classification of Bartter-like syndromes

Molecular complexity analysis of the diagnosis of Gitelman syndrome in China

Gitelman syndrome (GS) is an autosomal recessive renal tubal disease characterized by hypomagnesemia, hypokalemia, and hypocalciuria. The disease is caused by defects in the SLC12A3 gene, which encodes the thiazide diuretic-sensitive sodium chloride cotransporter (NCCT). In this study, a 20-year-old female patient with recurrent hypokalemia was tested for a hypokalemia-related panel using Next Generation Sequencing. Pedigree analysis was performed on her parents (non-consanguineous) and sister using Sanger sequencing. The results revealed that the patient carried compound heterozygous variants of the SLC12A3 gene: c.179C > T (p.T60M) and c.1001G > A (p.R334Q). Furthermore, her asymptomatic 6-year-old sister also carried both mutations. While the p.T60M mutation had been reported previously, the p.R334Q mutation was novel, and amino acid position 334 was identified as a mutation hotspot. Our findings provide an accurate molecular diagnosis that is essential for the diagnosis, counseling, and management of not only the symptomatic patient but also her asymptomatic sister. This study contributes to our understanding of the GS, which has a prevalence of approximately 1 in 40,000 and a heterozygous mutation carrier rate of 1% in Caucasians. Specifically, we observed a compound heterozygous mutation of the SLC12A3 gene in a 20-year-old female patient presenting with clinical symptoms consistent with GS.

Bartter Syndrome-Related Variants Distribution: Brazilian Data and Its Comparison with Worldwide Cohorts

BACKGROUND

Genetic testing is recommended for accurate diagnosis of Bartter syndrome (BS) and serves as a basis for implementing specific target therapies. However, populations other than Europeans and North Americans are underrepresented in most databases and there are uncertainties in the genotype-phenotype correlation. We studied Brazilian BS patients, an admixed population with diverse ancestry.

METHODS

We evaluated the clinical and mutational profile of this cohort and performed a systematic review of BS mutations from worldwide cohorts.

RESULTS

Twenty-two patients were included; Gitelman syndrome was diagnosed in 2 siblings with antenatal BS and congenital chloride diarrhea in 1 girl. BS was confirmed in 19 patients: BS type 1 in 1 boy (antenatal BS); BS type 4a in 1 girl and BS type 4b in 1 girl, both of them with antenatal BS and neurosensorial deafness; BS type 3 (CLCNKB mutations): 16 cases. The deletion of the entire CLCNKB (1-20 del) was the most frequent variant. Patients carrying the 1-20 del presented earlier manifestations than those with other CLCNKB-mutations and the presence of homozygous 1-20 del was correlated with progressive chronic kidney disease. The prevalence of the 1-20 del in this BS Brazilian cohort was similar to that of Chinese cohorts and individuals of African and Middle Eastern descent from other cohorts.

CONCLUSION

This study expands the genetic spectrum of BS patients with different ethnics, reveals some genotype/phenotype correlations, compares the findings with other cohorts, and provides a systematic review of the literature on the distribution of BS-related variants worldwide.

Machine Learning to Identify Genetic Salt-Losing Tubulopathies in Hypokalemic Patients

Introduction

Clinically distinguishing patients with the inherited salt-losing tubulopathies (SLTs), Gitelman or Bartter syndrome (GS or BS) from other causes of hypokalemia (LK) patients is difficult, and genotyping is costly. We decided to identify clinical characteristics that differentiate SLTs from LK.

Methods

A total of 66 hypokalemic patients with possible SLTs were recruited to a prospective observational cohort study at the University College London Renal Tubular Clinic, London. All patients were genotyped for pathogenic variants in genes which cause SLTs; 39 patients had pathogenic variants in genes causing SLTs. We obtained similar data sets from cohorts in Taipei and Kobe, as follows: the combined data set comprised 419 patients; 291 had genetically confirmed SLT. London and Taipei data sets were combined to train machine learning (ML) algorithms, which were then tested on the Kobe data set.

Results

Single biochemical variables (e.g., plasma renin) were significantly, but inconsistently, different between SLTs and LK in all cohorts. A decision table algorithm using serum bicarbonate and urinary sodium excretion (FE_Na) achieved a classification accuracy of 74%. This was superior to all the single biochemical variables identified previously.

Conclusion

ML algorithms can differentiate true SLT in the context of a specialist clinic with some accuracy. However, based on routine biochemistry, the accuracy is insufficient to make genotyping redundant.

Gitelman's Syndrome in Pregnancy With Adverse Foetal Outcome: A Case Report

Gitelman's syndrome (GS) is a disorder characterized by hypokalaemia, hypomagnesemia, hypocalciuric and metabolic alkalosis. Despite the fact that it affects women of child-bearing age, only limited information is available regarding its impact on maternal and foetal outcome. We present the case of an un-booked and un-investigated 23-year-old primigravida who presented with chief complaints of vomiting and loose stools. The patient also complained of absent foetal movements in the last 12 hours. Investigations revealed hypokalaemia and hypomagnesemia and ultrasound revealed intra-uterine foetal demise. The patient was symptomatically relieved after electrolyte correction. Scarce reports on Gitelman's syndrome in pregnancy have been documented with the majority of cases showing positive outcomes for the foetus. We hereby present a report of a primigravida with Gitelman's syndrome and foetal loss which is considered uncommon.

Modus operandi of ClC-K2 Cl- Channel in the Collecting Duct Intercalated Cells

The renal collecting duct is known to play a critical role in many physiological processes, including systemic water-electrolyte homeostasis, acid-base balance, and the salt sensitivity of blood pressure. ClC-K2 (ClC-Kb in humans) is a Cl^--permeable channel expressed on the basolateral membrane of several segments of the renal tubule, including the collecting duct intercalated cells. ClC-Kb mutations are causative for Bartters' syndrome type 3 manifested as hypotension, urinary salt wasting, and metabolic alkalosis. However, little is known about the significance of the channel in the collecting duct with respect to the normal physiology and pathology of Bartters' syndrome. In this review, we summarize the available experimental evidence about the signaling determinants of ClC-K2 function and the regulation by systemic and local factors as well as critically discuss the recent advances in understanding the collecting-duct-specific roles of ClC-K2 in adaptations to changes in dietary Cl^- intake and maintaining systemic acid-base homeostasis.

Bartter Syndrome: Perspectives of a Pediatric Nephrologist

Bartter syndrome (BS) is one of the most well-known hereditary tubular disorders, characterized by hypokalemic, hypochloremic metabolic alkalosis, and polyuria/polydipsia. This disease usually presents before or during infancy, and adult nephrologists often inherit the patients from pediatric nephrologists since this is a life-long condition. Here, a few case scenarios will be presented to recount how they first got diagnosed and how their clinical courses were during childhood until adulthood, in addition to a brief review of the disease and its treatment.

A novel CLCNKB variant in a Chinese family with classic Bartter syndrome and prenatal genetic diagnosis

Background

Type III Bartter syndrome (BS), often known as classic Bartter syndrome is caused by variants in CLCNKB gene, which encoding the basolateral chloride channel protein ClC‐Kb, and is characterized by renal salt wasting, hypokalemia, metabolic alkalosis, increased renin, and aldosterone levels.

Methods

A 2‐year‐old boy presented severe malnutrition, severe metabolic alkalosis and severe hypokalemia and was clinically diagnosed with BS. The trio exome sequencing (ES) was performed to discover the genetic cause of this patient, followed by validation using Sanger sequencing and quantitative polymerase chain reaction subsequently.

Results

The genetic analysis indicated that this patient with a compound heterozygous variants of CLCNKB gene including a novel nonsense variant c.876 T > A and a whole‐gene deletion. The two variants were inherited from his parents, respectively. Subsequently, target sequencing of CLCNKB gene was performed for next pregnancy, and prenatal genetic diagnosis was provided for the family.

Conclusions

The results of current study identified the compound heterozygous variants in a patient with classic BS. The novel variant expands the spectrum of CLCNKB variants in BS. Our study also indicates that ES is an alternative tool to simultaneously detect single‐nucleotide variants and copy‐number variants.

The Bartter-Gitelman Spectrum: Fifty Year Follow-up with Revision of Diagnosis after Whole Genome Sequencing

Bartter syndrome (BS) and Gitelman syndrome (GS) are renal tubular disorders affecting sodium, potassium, and chloride reabsorption. Clinical features include muscle cramps and weakness, in association with hypokalemia, hypochloremic metabolic alkalosis, and hyperreninemic hyperaldosteronism. Hypomagnesemia and hypocalciuria are typical of GS, while juxtaglomerular hyperplasia is characteristic of BS. GS is due to SLC12A3 variants, whereas BS is due to variants in SLC12A1, KCNJ1, CLCNKA, CLCNKB, BSND, MAGED2, or CASR. We had the opportunity to follow up one of the first reported cases of a salt-wasting tubulopathy, who based on clinical features was diagnosed with GS. The patient had presented at age 10 years with tetany precipitated by vomiting or diarrhea. She had hypokalemia, a hypochloremic metabolic alkalosis, hyponatremia, mild hypercalcemia, and normomagnesemia, and subsequently developed hypocalciuria and hypomagnesemia. A renal biopsy showed no evidence for juxtaglomerular hyperplasia. She developed chronic kidney failure at age 55 years, and ocular sclerochoroidal calcification, associated with BS and GS, at older than 65 years. Our aim was therefore to establish the genetic diagnosis in this patient using whole-genome sequencing (WGS). Leukocyte DNA was used for WGS analysis, and this revealed a homozygous c.226C > T (p.Arg76Ter) nonsense CLCNKB mutation, thereby establishing a diagnosis of BS type-3. WGS also identified 2 greater than 5-Mb regions of homozygosity that suggested likely mutational heterozygosity in her parents, who originated from a Greek island with fewer than 1500 inhabitants and may therefore have shared a common ancestor. Our results demonstrate the utility of WGS in establishing the correct diagnosis in renal tubular disorders with overlapping phenotypes.

Gitelman syndrome and ectopic calcification in the retina and joints

Gitelman syndrome is a rare inherited renal tubular disorder with features that resemble thiazide use, including a hypokalemic metabolic alkalosis, hypomagnesemia, hypocalciuria and a low or normal blood pressure, hyperreninemia and hyperaldosteronism. Treatment is primarily correction of the potassium and magnesium levels. The diagnosis is confirmed with genetic testing but Gitelman syndrome is often not suspected. However, the association with ectopic calcification in the retina, blood vessels and chondrocalcinosis in the joints is a useful pointer to this diagnosis. Bilateral symmetrical whitish deposits of calcium pyrophosphate are visible superotemporally on ophthalmoscopy and retinal photography but are actually located beneath the retina in the sclerochoroid. Optical coherence tomography is even more sensitive for their detection. These deposits increase in size with time, but the rate of progression slows with long-term correction of the hypomagnesemia. Calcification may be complicated by atrophy of the overlying retina and visual loss. The deposits often correlate with ectopic calcification in the aorta and coronary and cerebral vessels. Chondrocalcinosis occurs in the large joints such as the knees. Ectopic calcification in Gitelman syndrome indicates the need for more aggressive management of Mg levels. Calcification is much less common in Bartter syndrome, which itself is rarer and associated less often with hypomagnesemia.

Contradiction between genetic analysis and diuretic loading test in type I Bartter syndrome: a case report

BACKGROUND

In typical cases of Bartter syndrome (BS), assessing response to diuretics (furosemide and thiazide), hereinafter referred to as diuretic loading test, may be used to diagnose the type by detecting which part of the kidney tubule is not functioning correctly. However, the diuretic loading test may not always agree with the results of genetic analyses.

CASE PRESENTATION

A 5-year-old boy was admitted due to lower extremity weakness and abnormal gait. He had a recurrent episode of muscle weakness and laboratory results showed severe hypokalemia. The direct genomic sequencing of the case revealed a new mutation in the SLC12A1 gene, which is associated with type I Bartter syndrome. Because there was the difference between the phenotype and genotype, we conducted a diuretic loading test to confirm the diagnosis. However, the results showed a clear increase in urine excretion of Na and Cl. These results were not consistent with typical type I BS, but consistent with the patient's phenotype.

CONCLUSION

The diuretic loading test has limited utility for diagnosis especially in atypical cases. On the other hand, this test, which allows assessment of channel function, is useful for better understanding of the genotype-phenotype correlation.

Gitelman Syndrome in Pregnancy: A Clinical Challenge

PURPOSE

 Disease progress may be affected by pregnancy-related changes, and underlying conditions may also affekt pregnancy outcomes in women with Gitelman syndrome (GS). Case presentation A 35-year-old woman with GS (gravida 2 para 1) was referred to our hospital to start routine antenatal care follow-up at 6 weeks of gestation. At the age of 31, she had been diagnosed with GS after her first uneventful pregnancy. Upon early admission, her serum Mg+level was 0.51 mmol/L and her serum K+level 2.7 mmol/L with normal kidney function tests. She was already taking oral combined potassium citrate and potassium bicarbonate supplementation once a day before pregnancy. At the eighth gestational week, the medication was changed to an oral potassium color sachet of 1.5 gram per day until labor because of the insufficient dosage to maintain optimum potassium levels. She was also taking 365 milligrams of oral magnesium oxide twice a day before and during pregnancy. In the third trimester of the pregnancy, her serum Mg+level was 0.48 mmol/L and serum K+level 2.8 mmol/L. Because of the previous uterine surgery history, she underwent an elective cesarean operation at 39 weeks' gestation under spinal anesthesia and delivered a healthy 3090-gram female infant.

CONCLUSION

 Increased need for potassium and magnesium supplementation should be the critical considerations when managing pregnant patients with GS.

Cystic Fibrosis Presenting as Pseudo-Bartter Syndrome: An Important Diagnosis that is Missed!

Cystic fibrosis (CF), an autosomal recessive disorder, occurs due to mutations in CFTR gene resulting in impaired cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel function in various epithelia. In addition to the well-known pulmonary and pancreatic morbidities, CF is characterized by electrolyte and acid-base abnormalities- hypochloremia, hyponatremia, hypokalemia and metabolic alkalosis. These are collectively known as Pseudo-Bartter syndrome, as similar abnormalities are seen in Bartter syndrome- an inherited tubulopathy affecting thick ascending limb of loop of Henle. There may be a significant clinical overlap between the Classic Bartter syndrome, Gitelman syndrome and CF presenting as Pseudo-Bartter syndrome, especially in early childhood. This review focuses on Pseudo-Bartter syndrome in CF, its pathogenesis and differentiation from Bartter/Gitelman syndrome. Other causes of metabolic abnormalities resembling Bartter syndrome are also highlighted.

Bartter syndrome with long-term follow-up: a case report

Bartter syndrome is a rare inherited disease caused by CLCNKB mutation, which results in inactivation of the chloride channel Kb protein. Bartter syndrome is characterized by extreme hypokalemia, hypochloremia, metabolic alkalosis, hyperrenin-induced angiotensinemia, hyperaldosteronemia, and normal blood pressure. We herein report a case of Bartter syndrome that manifested as vomiting, hypokalemia, metabolic alkalosis, normal blood pressure, and significant hyperrenin-induced angiotensinemia. The patient, a 5-month-old girl, carried two known heterozygous pathogenic mutations: c.88C > T (p.Arg30*), which she had inherited from her father, and c.1313G > A (p.Arg438His), which she had inherited from her mother. Treatment with indomethacin, a nonsteroidal anti-inflammatory drug, led to rapid improvement of the hypokalemia, and treatment was continued for 14 years. The indomethacin also induced a sustainable reduction in the hypokalemia and metabolic alkalosis.

Inherited salt-losing tubulopathy: An old condition but a new category of tubulopathy

Bartter syndrome (BS) and Gitelman syndrome (GS) are syndromes associated with congenital tubular dysfunction, characterized by hypokalemia and metabolic alkalosis. Clinically, BS is classified into two types: the severe antenatal/neonatal type, which develops during the fetal period with polyhydramnios and preterm delivery; and the relatively mild classic type, which is usually found during infancy with failure to thrive. GS can be clinically differentiated from BS by its age at onset, usually after school age, or laboratory findings of hypomagnesemia and hypocalciuria. Recent advances in molecular biology have shown that these diseases can be genetically classified into type 1 to 5 BS and GS. As a result, it has become clear that the clinical classification of antenatal/neonatal BS, classic BS, and GS does not always correspond to the clinical symptoms associated with the genotypes in a one-to-one manner; and there is clinically no clear differential border between type 3 BS and GS. This has caused confusion among clinicians in the diagnosis of these diseases. It has been proposed that the disease name "inherited salt-losing tubulopathy" can be used for cases of tubulopathies accompanied by hypokalemia and metabolic alkalosis. It is reasonable to use this term prior to genetic typing into type 1-5 BS or GS, to avoid confusion in a clinical setting. In this article, we review causative genes and phenotypic correlations, diagnosis, and treatment strategies for salt-losing tubulopathy as well as the clinical characteristics of pseudo-BS/GS, which can also be called a "salt-losing disorder".

Isolated nephrocalcinosis due to compound heterozygous mutations in renal outer medullary potassium channel

Identification of a monogenic etiology is possible in a proportion of patients with childhood-onset nephrolithiasis or nephrocalcinosis. Bartter syndrome (BS), a hereditary tubulopathy characterized by polyuria, hypokalemic alkalosis and growth retardation that rarely presents with isolated nephrocalcinosis. Patients with defect in renal outer medullary potassium channel, encoded by the KCNJ1 gene causing BS type 2, typically present during the neonatal period. We describe a 14-year-old girl with mild late-onset BS type 2 with reported pathogenic compound heterozygous variations in exon 2 of KCNJ1 (c.146G > A and c.657C > G). This patient presented with isolated medullary nephrocalcinosis due to hypercalciuria; absence of hypokalemia and metabolic alkalosis was unique. This case highlights the importance of screening the KCNJ1 gene in patients with hypercalciuria and nephrocalcinosis, even in older children.

Splicing Characterization of CLCNKB Variants in Four Patients With Type III Bartter Syndrome

Objective

Type III Bartter syndrome (BS) is caused by loss-of-function mutations in the gene encoding basolateral chloride channel ClC-Kb (CLCNKB), and is characterized by hypokalemic metabolic alkalosis and hyperreninemic hyperaldosteronism. Here, we investigated the molecular defects in four Chinese children with clinical manifestations of Bartter syndrome.

Methods

The genomic DNA of the four patients was screened for gene variations using whole-exome sequencing (WES). The candidate variants were validated by direct Sanger sequencing. Quantitative PCR (qPCR) was subsequently performed to confirm the whole CLCNK gene deletion mutation. A minigene assay and reverse transcription PCR (RT-PCR) were performed to analyze the effect of splice variants in vitro.

Results

Our patients showed early onset age with hyponatremia, hypokalemia, hypochloremia, repeated vomiting and growth retardation, suggesting Bartter syndrome. Genetic analysis revealed that all patients carried compound heterozygous or homozygous truncating variants in the CLCNKB gene. In particular, we identified a novel nonsense variant c.239G > A (p.(Trp80*)), two splice site variants (c.1053-1 G > A and c.1228-2A > G), a whole gene deletion, and a novel synonymous variant c.228A > C (p.(Arg76Arg)) which located -2 bp from the 5′ splice donor site in exon 3. Furthermore, our in vitro minigene analysis revealed c.228A > C, c.1053-1G > A, and c.1228-2A > G cause the skipping of exon 3, exon 12, and exon 13, respectively.

Conclusion

Our results support that the whole CLCNKB gene deletion is the most common mutation in Chinese patients with type III BS, and truncating and whole gene deletion variants may account for a more severe phenotype of patients. We verified the pathogenic effect of three splicing variants (c.228A > C, c.1053-1G > A, and c.1228-2A > G) which disturbed the normal mRNA splicing, suggesting that splice variants play an important role in the molecular basis of type III BS, and careful molecular profiling of these patients will be essential for future effective personalized treatment options.

Bartter and Gitelman syndromes: Questions of class

Bartter and Gitelman syndromes are rare inherited tubulopathies characterized by hypokalaemic, hypochloraemic metabolic alkalosis. They are caused by mutations in at least 7 genes involved in the reabsorption of sodium in the thick ascending limb (TAL) of the loop of Henle and/or the distal convoluted tubule (DCT). Different subtypes can be distinguished and various classifications have been proposed based on clinical symptoms and/or the underlying genetic cause. Yet, the clinical phenotype can show remarkable variability, leading to potential divergences between classifications. These problems mostly relate to uncertainties over the role of the basolateral chloride exit channel CLCNKB, expressed in both TAL and DCT and to what degree the closely related paralogue CLCNKA can compensate for the loss of CLCNKB function. Here, we review what is known about the physiology of the transport proteins involved in these disorders. We also review the various proposed classifications and explain why a gene-based classification constitutes a pragmatic solution.

Dent disease: A window into calcium and phosphate transport

This review examines calcium and phosphate transport in the kidney through the lens of the rare X-linked genetic disorder Dent disease. Dent disease type 1 (DD1) is caused by mutations in the CLCN5 gene encoding ClC-5, a Cl^- /H⁺ antiporter localized to early endosomes of the proximal tubule (PT). Phenotypic features commonly include low molecular weight proteinuria (LMWP), hypercalciuria, focal global sclerosis and chronic kidney disease; calcium nephrolithiasis, nephrocalcinosis and hypophosphatemic rickets are less commonly observed. Although it is not surprising that abnormal endosomal function and recycling in the PT could result in LMWP, it is less clear how ClC-5 dysfunction disturbs calcium and phosphate metabolism. It is known that the majority of calcium and phosphate transport occurs in PT cells, and PT endocytosis is essential for calcium and phosphorus reabsorption in this nephron segment. Evidence from ClC-5 KO models suggests that ClC-5 mediates parathormone endocytosis from tubular fluid. In addition, ClC-5 dysfunction alters expression of the sodium/proton exchanger NHE3 on the PT apical surface thus altering transcellular sodium movement and hence paracellular calcium reabsorption. A potential role for NHE3 dysfunction in the DD1 phenotype has never been investigated, either in DD models or in patients with DD1, even though patients with DD1 exhibit renal sodium and potassium wasting, especially when exposed to even a low dose of thiazide diuretic. Thus, insights from the rare disease DD1 may inform possible underlying mechanisms for the phenotype of hypercalciuria and idiopathic calcium stones.

A novel compound heterozygous KCNJ1 gene mutation presenting as late-onset Bartter syndrome: Case report

RATIONALE

Bartter syndrome is an autosomal-recessive inherited disease in which patients present with hypokalemia and metabolic alkalosis. We present 1 case with Bartter syndrome, due to a novel compound heterozygous mutation in the KCNJ1 gene encoding the ATP-sensitive inward rectifier potassium channel in the thick ascending limb of the loop of Henle.

PATIENT CONCERNS

A patient was admitted to our hospital because of weakness, polyuria, and polydipsia. At presentation to our hospital, the female Chinese patient was 34 years old and her physical examination was normal. Laboratory studies revealed hypokalemia, metabolic alkalosis, hypercalciuria, hyperparathyroidemia, and hyper-reninemia. In addition, urinary potassium was obviously higher. Computer tomography scan confirmed the patient had the bilateral medullary nephrocalcinosis.

DIAGNOSIS

Blood samples were received from the patient and her parents, and deoxyribonucleic acid was extracted. The genetic analysis of SLC12A1, SLC12A3, KCNJ1, CLCNKB, BSND, and CASR was performed. The compound heterozygous KCNJ1 gene mutation was validated using conventional Sanger sequencing methods.

INTERVENTIONS

The patient was treated with potassium supplementation. Her blood and urine chemistries improved over the next week. Serum potassium normalized with improvement in polyuria and polydipsia over the next month.

OUTCOMES

Our patient was compound heterozygous for Thr234Ile and Thr71Met in the KCNJ1 gene. The c.701C>T variant predicted a change from a threonine codon to an isoleucine codon (p.Thr234Ile). The c.212C>T variant predicted a change from a threonine codon to a methionine codon (p.Thr71Met). The unaffected mother was heterozygous for the Thr234Ile mutation, whereas unaffected father was heterozygous for the Thr71Met mutation.

LESSONS

The phenotypes of the patient were similar to other patients with Bartter syndrome. The phenotypes of the patient could eventually be explained by the presence of the novel compound heterozygous p.Thr234Ile/p.Thr71Met variants in the KCNJ1 gene.

The utility of next generation sequencing in the correct diagnosis of congenital hypochloremic hypokalemic metabolic alkalosis

Persistent hypokalemic hypochloremic metabolic alkalosis represents a heterogeneous group of genetic disorders of which the most common is Bartter syndrome (BS). BS is an inherited renal tubulopathy caused by defective salt reabsorption in the thick ascending loop of Henle, which results in persistent hypokalemic hypochloremic metabolic alkalosis. Here we report a 10-year-old girl of a consanguineous family. She presented prenatally with severe polyhydramnios and distended bowel loops. Thereafter, she displayed failure to thrive and had recurrent admissions due to dehydration episodes associated with diarrhea, and characterized by hypokalemia, hypochloremia and metabolic alkalosis. BS was considered her working diagnosis for several years despite negative genetic analysis of the known genes associated with BS. Whole exome sequencing identified a novel homozygous c.1652delT deleterious frameshift mutation in the SLC26A3 gene, which confirmed the diagnosis of congenital chloride diarrhea (CCD), a rare autosomal recessive disease that mimics biochemically BS. A review of twelve additional reported cases of CCD that were initially misdiagnosed as BS, emphasizes CCD in the differential diagnosis of BS, and highlights the clinical discrepancies between these two entities. Taken together, our report further emphasizes the typical clinical features of CCD, and the importance of next generation sequencing in the diagnosis of syndromes with genetic heterogeneity. We suggest including SLC26A3 in the extended BS targeted gene panels.

Bartter syndrome: causes, diagnosis, and treatment

Bartter syndrome is an inherited renal tubular disorder caused by a defective salt reabsorption in the thick ascending limb of loop of Henle, resulting in salt wasting, hypokalemia, and metabolic alkalosis. Mutations of several genes encoding the transporters and channels involved in salt reabsorption in the thick ascending limb cause different types of Bartter syndrome. A poor phenotype-genotype relationship due to the interaction with other cotransporters and different degrees of compensation through alternative pathways is currently reported. However, phenotypic identification still remains the first step to guide the suspicion of Bartter syndrome. Given the rarity of the syndrome, and the lack of genetic characterization in most cases, limited clinical evidence for treatment is available and the therapy is based mainly on the comprehension of renal physiology and relies on the physician's personal experiences. A better understanding of the mutated channels and transporters could possibly generate targets for specific treatment in the future, also encompassing drugs aiming to correct deficiencies in folding or plasma membrane expression of the mutated proteins.

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.

Activation of renal ClC-K chloride channels depends on an intact N terminus of their accessory subunit barttin

ClC-K channels belong to the CLC family of chloride channels and chloride/proton antiporters. They contribute to sodium chloride reabsorption in Henle's loop of the kidney and to potassium secretion into the endolymph by the stria vascularis of the inner ear. Their accessory subunit barttin stabilizes the ClC-K/barttin complex, promotes its insertion into the surface membrane, and turns the pore-forming subunits into a conductive state. Barttin mutations cause Bartter syndrome type IV, a salt-wasting nephropathy with sensorineural deafness. Here, studying ClC-K/barttin channels heterologously expressed in MDCK-II and HEK293T cells with confocal imaging and patch-clamp recordings, we demonstrate that the eight-amino-acids-long barttin N terminus is required for channel trafficking and activation. Deletion of the complete N terminus (Δ2-8 barttin) retained barttin and human hClC-Ka channels in intracellular compartments. Partial N-terminal deletions did not compromise subcellular hClC-Ka trafficking but drastically reduced current amplitudes. Sequence deletions encompassing Thr-6, Phe-7, or Arg-8 in barttin completely failed to activate hClC-Ka. Analyses of protein expression and whole-cell current noise revealed that inactive channels reside in the plasma membrane. Substituting the deleted N terminus with a polyalanine sequence was insufficient for recovering chloride currents, and single amino acid substitutions highlighted that the correct sequence is required for proper function. Fast and slow gate activation curves obtained from rat V166E rClC-K1/barttin channels indicated that mutant barttin fails to constitutively open the slow gate. Increasing expression of barttin over that of ClC-K partially recovered this insufficiency, indicating that N-terminal modifications of barttin alter both binding affinities and gating properties.

Pathophysiology of antenatal Bartter's syndrome

PURPOSE OF REVIEW

Antenatal Bartter syndrome (aBS) is a heterogenous disease resulting from defective ion transport in the thick ascending limb of the loop of Henle. Novel insights into the pathophysiology, as well as the recent identification of a novel genetic cause of aBS, merit an update on this topic.

RECENT FINDINGS

In aBS, severe salt losing is further aggravated by defective salt sensing in the macula densa, where a reduced tubular salt concentration is perceived and glomerular filtration is increased instead of decreased. As patients with aBS come of age, there is an increased incidence of proteinuria and impaired renal function.Moreover, we recently reported a new form of aBS. Indeed, we described a series of nine families in whom pregnancies with male fetuses where complicated by acute polyhydramnios, preterm delivery and with severe but transient polyuria. We identified mutations in melanoma-associated antigen D2 in all study participants and showed, in vivo and in vitro, reduced expression of the furosemide and thiazide sensitive transporters sodium-potassium-2-chloride cotransporter and sodium chloride cotransporter, respectively.

SUMMARY

Genetic studies revealed the complexity of ion transport in the thick ascending limb of the loop of Henle and will help to clarify the pathophysiology, which is essential to design new therapies.

Salt-Losing Tubulopathies in Children: What's New, What's Controversial?

Renal tubulopathies provide insights into the inner workings of the kidney, yet also pose therapeutic challenges. Because of the central nature of sodium in tubular transport physiology, disorders of sodium handling may affect virtually all aspects of the homeostatic functions of the kidney. Yet, owing to the rarity of these disorders, little clinical evidence regarding treatment exists. Consequently, treatment can vary widely between individual physicians and centers and is based mainly on understanding of renal physiology, reported clinical observations, and individual experiences. Salt-losing tubulopathies can affect all tubular segments, from the proximal tubule to the collecting duct. But the more frequently observed disorders are Bartter and Gitelman syndrome, which affect salt transport in the thick ascending limb of Henle's loop and/or the distal convoluted tubule, and these disorders generate the greatest controversies regarding management. Here, we review clinical and molecular aspects of salt-losing tubulopathies and discuss novel insights provided mainly by genetic investigations and retrospective clinical reviews. Additionally, we discuss controversial topics in the management of these disorders to highlight areas of importance for future clinical trials. International collaboration will be required to perform clinical studies to inform the treatment of these rare disorders.

Mutation spectrum of Chinese patients with Bartter syndrome

OBJECTIVE

Bartter syndrome (BS) has been rarely reported in Chinese population except for a few case reports. This investigation was aimed to analyze the mutations of the causal genes in sixteen Chinese patients with BS, and review their followup and treatment.

METHODS

Identify mutations by the next generation sequencing and the multiplex ligation-dependent probe amplification (MLPA). Clinical characteristics and biochemical findings at the first presentation as well as follow-up were reviewed.

RESULTS

15 different CLCNKB gene mutations were identified in fourteen patients with BS, including 11 novel ones. A novel missense mutation and a novel small deletion were found from SLC12A1 gene. A novel gross deletion was found in CLCNKA gene. A recurrent missense mutation was identified from BSND gene. We found that the whole gene deletion mutation of CLCNKB gene was the most frequent mutation (32%), and the rate of gross deletion was up to 50 percent in this group of Chinese patients.

CONCLUSION

The present study has found 19 mutations, including 14 novel ones, which would enrich the human gene mutation database (HGMD) and provide valuable references to the genetic counseling and diagnosis of the Chinese population.

Genetics of Magnesium Disorders

Background

Magnesium (Mg²⁺), the second most abundant cation in the cell, is woven into a multitude of cellular functions. Dysmagnesemia is associated with multiple diseases and, when severe, can be life-threatening.

Summary

This review discusses Mg²⁺ homeostasis and function with specific focus on renal Mg²⁺ handling. Intrarenal channels and transporters related to Mg²⁺ absorption are discussed. Unraveling the rare genetic diseases with manifestations of dysmagnesemia has greatly increased our understanding of the complex and intricate regulatory network in the kidney, specifically, functions of tight junction proteins including claudin-14, -16, -19, and -10; apical ion channels including: TRPM6, K_v1.1, and ROMK; small regulatory proteins including AC3 and ANK3; and basolateral proteins including EGF receptor, γ-subunit (FXYD2) of Na-K-ATPase, K_ir4.1, CaSR, CNNM2, and SLC41A. Although our understanding of Mg²⁺ handling of the kidney has expanded considerably in the last two decades, many questions remain. Future studies are needed to elucidate a multitude of unknown aspects of Mg²⁺ handling in the kidney.

Key Message

Understanding rare and genetic diseases of Mg²⁺ dysregulation has expanded our knowledge and furthers the development of strategies for preventing and managing dysmagnesemia.

Rectal Cancer in a Patient with Bartter Syndrome: A Case Report

A woman with rectal cancer was scheduled for surgery. However, she also had hypokalemia, hyperreninemia, and hyperaldosteronism in the absence of any known predisposing factors or endocrine tumors. She was given intravenous potassium, and her blood abnormalities stabilized after tumor resection. Genetic analysis revealed mutations in several genes associated with Bartter syndrome (BS) and Gitelman syndrome, including SLC12A1, CLCNKB, CASR, SLC26A3, and SLC12A3. Prostaglandin E2 (PGE2) plays an important role in BS and worsens electrolyte abnormalities. The PGE2 level is reportedly increased in colorectal cancer, and in the present case, immunohistochemical examination revealed an increased PGE2 level in the tumor. We concluded that the tumor-related PGE2 elevation had worsened the patient’s BS, which became more manageable after tumor resection.

A novel variant in the SLC12A1 gene in two families with antenatal Bartter syndrome

AIM

Bartter syndrome is an autosomal-recessive inherited disease in which patients present with hypokalaemia and metabolic alkalosis. We present two apparently nonrelated cases with antenatal Bartter syndrome type I, due to a novel variant in the SLC12A1 gene encoding the bumetanide-sensitive sodium-(potassium)-chloride cotransporter 2 in the thick ascending limb of the loop of Henle.

METHODS

Blood samples were received from the two cases and 19 of their relatives, and deoxyribonucleic acid was extracted. The coding regions of the SLC12A1 gene were amplified using polymerase chain reaction, followed by bidirectional direct deoxyribonucleic acid sequencing.

RESULTS

Each affected child in the two families was homozygous for a novel inherited variant in the SLC12A1gene, c.1614T>A. The variant predicts a change from a tyrosine codon to a stop codon (p.Tyr538Ter). The two cases presented antenatally and at six months of age, respectively.

CONCLUSION

The two cases were homozygous for the same variant in the SLC12A1 gene, but presented clinically at different ages. This could eventually be explained by the presence of other gene variants or environmental factors modifying the phenotypes. The phenotypes of the patients were similar to other patients with antenatal Bartter syndrome.

Genetic heterogeneity in patients with Bartter syndrome type 1

Bartter syndrome (BS) type 1 is an autosomal recessive kidney disorder caused by loss-of-function mutations in the solute carrier family 12 member 1 (SLC12A1) gene. To date, 72 BS type 1 patients harboring SLC12A1 mutations have been documented. Of these 144 alleles studied, 68 different disease-causing mutations have been detected in 129 alleles, and no mutation was detected in the remaining 15 alleles. The mutation types included missense/nonsense mutations, splicing mutations and small insertions and deletions ranging from 1 to 4 nucleotides. A large deletion encompassing a whole exon in the SLC12A1 gene has not yet been reported. The current study initially identified an undocumented homozygous frameshift mutation (c.1833delT) by Sanger sequencing analysis of a single infant with BS type 1. However, in a subsequent analysis, the mutation was detected only in the father's DNA. Upon further investigation using a next-generation sequencing approach, a deletion in exons 14 and 15 in both the patient and patient's mother was detected. The deletion was subsequently confirmed by use of a long-range polymerase chain reaction and was determined to be 3.16 kb in size based on sequencing of the junction fragment. The results of the present study demonstrated that pathogenic variants of SLC12A1 are heterogeneous. Large deletions appear to serve an etiological role in BS type 1, and may be more prevalent than previously thought.

Cryptic exon activation in SLC12A3 in Gitelman syndrome

Gitelman syndrome (GS) is an autosomal recessive renal tubulopathy characterized by hypokalemic metabolic alkalosis with hypocalciuria and hypomagnesemia. GS clinical symptoms range from mild weakness to muscular cramps, paralysis or even sudden death as a result of cardiac arrhythmia. GS is caused by loss-of-function mutations in the solute carrier family 12 member 3 (SLC12A3) gene, but molecular mechanisms underlying such a wide range of symptoms are poorly understood. Here we report cryptic exon activation in SLC12A3 intron 12 in a clinically asymptomatic GS, resulting from an intronic mutation c.1669+297 T>G that created a new acceptor splice site. The cryptic exon was sandwiched between the L3 transposon upstream and a mammalian interspersed repeat downstream, possibly contributing to inclusion of the cryptic exon in mature transcripts. The mutation was identified by targeted next-generation sequencing of candidate genes in GS patients with missing pathogenic SLC12A3 alleles. Taken together, this work illustrates the power of next-generation sequencing to identify causal mutations in intronic regions in asymptomatic individuals at risk of developing potentially fatal disease complications, improving clinical management of these cases.

Accentuated hyperparathyroidism in type II Bartter syndrome

BACKGROUND

Bartter syndrome (BS) may be associated with different degrees of hypercalciuria, but marked parathyroid hormone (PTH) abnormalities have not been described.

METHODS

We compared clinical and laboratory data of patients with either ROMK-deficient type II BS (n = 14) or Barttin-deficient type IV BS (n = 20).

RESULTS

Only BS-IV patients remained mildly hypokalemic in spite of a higher need for potassium supplementation. Estimated glomerular filtration rate (eGFR) was mildly decreased in only four BS-IV patients. Average PTH values were significantly higher in BS-II (160.6 ± 85.8 vs. 92.5 ± 48 pg/ml in BS-IV, p = 0.006). In both groups, there was a positive correlation between age and log(PTH). Levels of 25(OH) vitamin D were not different. Total serum calcium was lower (within normal limits) and age-related serum phosphate (Pi)-SDS was increased in BS-II (1.19 ± 0.71 vs. 0.01 ± 1.04 in BS-IV, p < 0.001). The GFR threshold for Pi reabsorption was higher in BS-II (5.63 ± 1.25 vs. 4.36 ± 0.98, p = 0.002). Spot urine calcium/creatinine ratio and nephrocalcinosis rate (100 vs. 16 %) were higher in the BS-II group.

CONCLUSIONS

PTH, serum Pi levels, and urinary threshold for Pi reabsorption are significantly elevated in type II vs. type IV BS, suggesting a PTH resistance state. This may be a response to more severe long-standing hypercalciuria, leading to a higher rate of nephrocalcinosis in BS-II.

New perspective of ClC-Kb/2 Cl- channel physiology in the distal renal tubule

Since its identification as the underlying molecular cause of Bartter's syndrome type 3, ClC-Kb (ClC-K2 in rodents, henceforth it will be referred as ClC-Kb/2) is proposed to play an important role in systemic electrolyte balance and blood pressure regulation by controlling basolateral Cl(-) exit in the distal renal tubular segments from the cortical thick ascending limb to the outer medullary collecting duct. Considerable experimental and clinical effort has been devoted to the identification and characterization of disease-causing mutations as well as control of the channel by its cofactor, barttin. However, we have only begun to unravel the role of ClC-Kb/2 in different tubular segments and to reveal the regulators of its expression and function, e.g., insulin and IGF-1. In this review we discuss recent experimental evidence in this regard and highlight unexplored questions critical to understanding ClC-Kb/2 physiology in the kidney.

Pathophysiology and clinical presentations of salt-losing tubulopathies

At least three renal tubular segments are involved in the pathophysiology of salt-losing tubulopathies (SLTs). Whether the pathogenesis starts either in the thick ascending limb of the loop of Henle (TAL) or in the distal convoluted tubule (DCT), it is the function of the downstream-localized aldosterone sensitive distal tubule (ASDT) to contribute to the adaptation process. In isolated TAL defects (loop disorders) ASDT adaptation is supported by upregulation of DCT, whereas in DCT disorders the ASDT is complemented by upregulation of TAL function. This upregulation has a major impact on the clinical presentation of SLT patients. Taking into account both the symptoms and signs of primary tubular defect and of the secondary reactions of adaptation, a clinical diagnosis can be made that eventually leads to an appropriate therapy. In addition to salt wasting, as occurs in all SLTs, characteristic features of loop disorders are hypo- or isosthenuric polyuria and hypercalciuria, whereas characteristics of DCT disorders are hypokalemia and (symptomatic) hypomagnesemia. In both SLT categories, replacement of urinary losses is the primary goal of treatment. In loop disorders COX inhibitors are also recommended to mitigate polyuria, and in DCT disorders magnesium supplementation is essential for effective treatment. Of note, the combination of a salt- and potassium-rich diet together with an adequate fluid intake is always the basis of long-term treatment in all SLTs.

Identification of two novel mutations in SLC12A3 gene in two Chinese pedigrees with Gitelman syndrome and review of literature

OBJECTIVE

Gitelman syndrome (GS) is one of the most common causes of inherited hypokalaemia. As it was caused by mutations in the SLC12A3 gene, GS is a highly heterogeneous disease. Here, we aimed to investigate the clinical and genetic characteristics of two Chinese pedigrees and summarize the advance in GS genetics, diagnosis and management.

SUBJECTS AND METHODS

Two three-generation families with GS were identified and screened for mutations in the SLC12A3 gene. Genotype-phenotype correlations were analysed.

RESULTS

The two probands (A and B) were characterized by hypokalaemia, hypomagnesaemia and hypocalciuria without hypertension. Complete DNA sequencing of the SLC12A3 gene revealed two novel compound heterozygous mutations (c.179C>T and c.234delG; c.486-490delTACGGinsA and c.1925G>A), which are predicted to drastically affect normal protein structure. The female members of the pedigrees showed mild-to-no phenotype, although they carried the same mutations as the probands. Moreover, proband B presented with more severe symptoms than did proband A, which might be related to a lower serum magnesium level. During the 1-year follow-up, both probands showed satisfactory symptom improvement following the use of potassium and magnesium supplements.

CONCLUSION

Our findings strongly suggested that the two novel mutations in the SLC12A3 gene are the causative agents of GS, which may provide further insights into the function of this gene and help clinicians better understand this disorder.

Developmental changes in renal tubular transport-an overview

The adult kidney maintains a constant volume and composition of extracellular fluid despite changes in water and salt intake. The neonate is born with a kidney that has a small fraction of the glomerular filtration rate of the adult and immature tubules that function at a lower capacity than that of the mature animal. Nonetheless, the neonate is also able to maintain a constant extracellular fluid volume and composition. Postnatal renal tubular development was once thought to be due to an increase in the transporter abundance to meet the developmental increase in glomerular filtration rate. However, postnatal renal development of each nephron segment is quite complex. There are isoform changes of several transporters as well as developmental changes in signal transduction that affect the capacity of renal tubules to reabsorb solutes and water. This review will discuss neonatal tubular function with an emphasis on the differences that have been found between the neonate and adult. We will also discuss some of the factors that are responsible for the maturational changes in tubular transport that occur during postnatal renal development.

Bartter and Gitelman syndromes: Spectrum of clinical manifestations caused by different mutations

Bartter and Gitelman syndromes (BS and GS) are inherited disorders resulting in defects in renal tubular handling of sodium, potassium and chloride. Previously considered as genotypic and phenotypic heterogeneous diseases, recent evidence suggests that they constitute a spectrum of disease caused by different genetic mutations with the molecular defects of chloride reabsorption originating at different sites of the nephron in each condition. Although they share some characteristic metabolic abnormalities such as hypokalemia, metabolic alkalosis, hyperplasia of the juxtaglomerular apparatus with hyperreninemia, hyperaldosteronism, the clinical and laboratory manifestations may not always allow distinction between them. Diuretics tests, measuring the changes in urinary fractional excretion of chloride from baseline after administration of either hydrochlorothiazide or furosemide show very little change (< 2.3%) in the fractional excretion of chloride from baseline in GS when compared with BS, except when BS is associated with KCNJ1 mutations where a good response to both diuretics exists. The diuretic test is not recommended for infants or young children with suspected BS because of a higher risk of volume depletion in such children. Clinical symptoms and biochemical markers of GS and classic form of BS (type III) may overlap and thus genetic analysis may specify the real cause of symptoms. However, although genetic analysis is available, its use remains limited because of limited availability, large gene dimensions, lack of hot-spot mutations, heavy workup time and costs involved. Furthermore, considerable overlap exists between the different genotypes and phenotypes. Although BS and GS usually have distinct presentations and are associated with specific gene mutations, there remains considerable overlap between their phenotypes and genotypes. Thus, they are better described as a spectrum of clinical manifestations caused by different gene mutations.

ClC-K chloride channels: emerging pathophysiology of Bartter syndrome type 3

The mutations in the CLCNKB gene encoding the ClC-Kb chloride channel are responsible for Bartter syndrome type 3, one of the four variants of Bartter syndrome in the genetically based nomenclature. All forms of Bartter syndrome are characterized by hypokalemia, metabolic alkalosis, and secondary hyperaldosteronism, but Bartter syndrome type 3 has the most heterogeneous presentation, extending from severe to very mild. A relatively large number of CLCNKB mutations have been reported, including gene deletions and nonsense or missense mutations. However, only 20 CLCNKB mutations have been functionally analyzed, due to technical difficulties regarding ClC-Kb functional expression in heterologous systems. This review provides an overview of recent progress in the functional consequences of CLCNKB mutations on ClC-Kb chloride channel activity. It has been observed that 1) all ClC-Kb mutants have an impaired expression at the membrane; and 2) a minority of the mutants combines reduced membrane expression with altered pH-dependent channel gating. Although further investigation is needed to fully characterize disease pathogenesis, Bartter syndrome type 3 probably belongs to the large family of conformational diseases, in which the mutations destabilize channel structure, inducing ClC-Kb retention in the endoplasmic reticulum and accelerated channel degradation.

Molecular pathophysiology of Bartter's and Gitelman's syndromes

BACKGROUND

In the last two decades, progress in cytogenetic and genome research has enabled investigators to unravel the underlying molecular mechanisms of inherited tubulopathies such as Bartter's and Gitelman's syndromes and helped physicians to better understand not only these two pathologic entities but also renal pathophysiology and salt sensitive hypertension.

DATA SOURCES

Articles collected from PubMed and open access journals included original articles, research articles, and comprehensive reviews. They were evaluated by the authors with an special emphasis on originality and up to date information about molecular pathophysiology.

RESULTS

Bartter's and Gitelman's syndromes are two different inherited salt loosing tubulopathies. They are characterized by various inability of distal nephron to reabsorb sodium chloride with resultant extarcellular volume contraction and increased activity of the renin angiotensin aldosterone system. Hypokalemic metabolic alkalosis is a common feature of these two forms of tubulopathies. Hypercalciuria characterizes the majority of Bartter's syndrome, and hypomagnesemia with hypocalciuria characterizes Gitelman's syndrome. Low blood pressure is a common feature among patients who suffered from these tubulopathies. Bartter's syndromes encompass a heterogeneous group of ion channels defects localized at the thick ascending limp of Henle's loop with resultant loss of function of sodium-potassium-2 chloride cotransporter. These defects result in the impairment of the countercurrent multiplication system of the kidney as well as calcium, potassium and acid base disturbances which in the majority of cases are proved lethal especially in the antenatal and/or immediate postnatal life period. The underlying pathology in Gitelman's syndrome is defined to the distal convoluted tubule and is related to loss of function of the sodium-chloride cotransporter. The results of this defect encompass the inability of extracellular volume homeostasis, magnesium and potassium conservation, and acid base disturbances which are generally mild and in the majority of cases are not life-threatening.

CONCLUSIONS

Recent advances in molecular pathophysiology of Bartter's and Gitelman's syndromes have helped physicians to better understand the underlying mechanisms of these pathologic entities which remain obscure. Data collected from experiments among genetically manipulated animals enable us to better understand the pathophysiology of mammalian kidney and the underlying mechanisms of salt sensitive hypertension and to lay a foundation for the future development of new drugs, especially diuretics and antihypertensive drugs.

Clinical salt deficits

Salt retention or salt deficit has a bearing on the body fluid volume. Both states are clinically difficult to recognize and quantitate. Salt deficit is particularly cumbersome in that regard since orthostatic blood pressure, heart rate changes, and simple physical inspection are inaccurate and unreliable. Salt deficit can be acute such as after hemorrhage or massive diarrhea, or more chronic as observed in Addison's disease, failure of renal sodium chloride transporters, drug-related effects, or distal nephron disease. Molecular genetics has given us important new insights into salt deficit syndromes. Recent recognition of a novel sodium storage compartment involving sodium binding to proteoglycans adds to the overall complexity of these syndromes.

Genetic analysis in Bartter syndrome from India

Bartter syndrome is a group of inherited, salt-losing tubulopathies presenting as hypokalemic metabolic alkalosis with normotensive hyperreninemia and hyperaldosteronism. Around 150 cases have been reported in literature till now. Mutations leading to salt losing tubulopathies are not routinely tested in Indian population. The authors have done the genetic analysis for the first time in the Bartter syndrome on two cases from India. First case was antenatal Bartter syndrome presenting with massive polyuria and hyperkalemia. Mutational analysis revealed compound heterozygous mutations in KCNJ1(ROMK) gene [p(Leu220Phe), p(Thr191Pro)]. Second case had a phenotypic presentation of classical Bartter syndrome however, genetic analysis revealed only heterozygous novel mutation in SLC12A gene p(Ala232Thr). Bartter syndrome is a clinical diagnosis and genetic analysis is recommended for prognostication and genetic counseling.

The renal channelopathies

Specific channels permit movement of selected ions through cellular membranes, and are of vital importance in a number of physiological processes, particularly in excitable tissues such as nerve and muscle, but also in endocrine organs and in epithelial biology. Disorders of channel proteins are termed channelopathies, and their importance is increasingly recognised within medicine. In the kidney, ion channels have critical roles enabling sodium and potassium reuptake or excretion along the nephron, in magnesium homeostasis, in the control of water reabsorption in the collecting duct, and in determining glomerular permeability. In this review, we assess the channelopathies encountered in each nephron segment, and see how their molecular and genetic characterisation in the past 20–30 years has furthered our understanding of normal kidney physiology and disease processes, aids correct diagnosis and promises future therapeutic opportunities.

Mixed Bartter-Gitelman syndrome: an inbred family with a heterogeneous phenotype expression of a novel variant in the CLCNKB gene

Patients with renal diseases associated with salt-losing tubulopathies categorized as Gitelman and classic form of Bartter syndrome have undergone genetic screening for possible mutation capture in two different genes: SLC12A3 and CLCNKB. Clinical symptoms of these two diseases may overlap.

Bartter syndrome and Gitelman syndrome are autosomal recessive salt-losing tubulopathies with hypokalemia, metabolic alkalosis, hyperreninemia, hyperplasia of the juxtaglomerular apparatus, hyperaldosteronism, and, in some patients, hypomagnesemia.

Here we describe four patients from an inbred family with a novel missense variant in the CLCNKB gene. All of patients are asymptomatic; yet they have the typical metabolic abnormality of salt losing tubulopathies. One of those patients had hypomagnesaemia while others not. Clinical and laboratory data of all patients was described. All 4 patients have a homozygous c.490G > T missense variant in exon 5 of the CLCNKB gene. This variant alters a glycine into a cysteine on amino acid position 164 of the resulting protein (p.Gly164Cys). The c.490G > T variant is a novel variant not previously described in other patients nor controls. Polyphen analysis predicts the variation to be possibly damaging. Analysis of SLC12A3 was normal.

Here in we are describing a novel homozygous c.490G > T missense variation was identified in exon 5 of the CLCNKB gene was identified in an Emirati patients with a mild manifestation of Bartter - Gitelman syndrome.

Cell biology and physiology of CLC chloride channels and transporters

Proteins of the CLC gene family assemble to homo- or sometimes heterodimers and either function as Cl(-) channels or as Cl(-)/H(+)-exchangers. CLC proteins are present in all phyla. Detailed structural information is available from crystal structures of bacterial and algal CLCs. Mammals express nine CLC genes, four of which encode Cl(-) channels and five 2Cl(-)/H(+)-exchangers. Two accessory β-subunits are known: (1) barttin and (2) Ostm1. ClC-Ka and ClC-Kb Cl(-) channels need barttin, whereas Ostm1 is required for the function of the lysosomal ClC-7 2Cl(-)/H(+)-exchanger. ClC-1, -2, -Ka and -Kb Cl(-) channels reside in the plasma membrane and function in the control of electrical excitability of muscles or neurons, in extra- and intracellular ion homeostasis, and in transepithelial transport. The mainly endosomal/lysosomal Cl(-)/H(+)-exchangers ClC-3 to ClC-7 may facilitate vesicular acidification by shunting currents of proton pumps and increase vesicular Cl(-) concentration. ClC-3 is also present on synaptic vesicles, whereas ClC-4 and -5 can reach the plasma membrane to some extent. ClC-7/Ostm1 is coinserted with the vesicular H(+)-ATPase into the acid-secreting ruffled border membrane of osteoclasts. Mice or humans lacking ClC-7 or Ostm1 display osteopetrosis and lysosomal storage disease. Disruption of the endosomal ClC-5 Cl(-)/H(+)-exchanger leads to proteinuria and Dent's disease. Mouse models in which ClC-5 or ClC-7 is converted to uncoupled Cl(-) conductors suggest an important role of vesicular Cl(-) accumulation in these pathologies. The important functions of CLC Cl(-) channels were also revealed by human diseases and mouse models, with phenotypes including myotonia, renal loss of salt and water, deafness, blindness, leukodystrophy, and male infertility.

Severe alkalosis and hypokalemia with stanozolol misuse

Stanozolol is a popular androgenic anabolic steroid, used by body builders and athletes for physical performance enhancement. There are few data on its potential adverse effects and no documented cases of it causing severe electrolyte imbalance. Here, we report a patient presenting to a tertiary care emergency department with reduced conscious level, profound hypokalemia, and severe metabolic alkalosis, resulting from stanozolol misuse. This is the first such case reported.

Bartter syndrome: presentation in an extremely premature neonate

Reports of Bartter syndrome in premature neonates are rare. We describe the presentation and clinical course of a neonate born at 25.6 weeks estimated gestational age with polyuria, hyponatremia, hypokalemia and hypercalciuria ,who was diagnosed with neonatal Bartter syndrome. The evaluation, diagnosis and management of neonatal Bartter syndrome in this premature neonate are discussed.