Salt-losing Tubulopathy Research Articles

Gitelman syndrome combined with complete growth hormone deficiency.

Gitelman syndrome is a rare autosomal recessive hereditary salt-losing tubulopathy, that manifests as hypokalemic metabolic alkalosis, hypomagnesemia, and hypocalciuria. It is caused by mutations in the solute carrier family 12(sodium/chloride transporters), member 3 (SLC12A3) gene encoding the thiazide-sensitive sodium chloride cotransporter channel (NCCT) in the distal convoluted tubule of the kidney. It is associated with muscle weakness, cramps, tetany, vomiting, diarrhea, abdominal pain, and growth retardation. The incidence of growth retardation, the exact cause of which is unknown, is lower than that of Bartter syndrome. Herein, we discuss the case of an overweight 12.9-year-old girl of short stature presenting with hypokalemic metabolic alkalosis. The patient, on the basis of detection of a heterozygous mutation in the SLC12A3 gene and poor growth hormone (GH) responses in two provocative tests, was diagnosed with Gitelman syndrome combined with complete GH deficiency. GH treatment accompanied by magnesium oxide and potassium replacement was associated with a good clinical response.

Generation and validation of a zebrafish model of EAST (Epilepsy, ataxia, sensorineural deafness and tubulopathy) syndrome

SUMMARYRecessive mutations in KCNJ10, which encodes an inwardly rectifying potassium channel, were recently identified as the cause of EAST syndrome, a severe and disabling multi-organ disorder consisting of epilepsy, ataxia, sensorineural deafness and tubulopathy that becomes clinically apparent with seizures in infancy. A Kcnj10 knockout mouse shows postnatal mortality and is therefore not suitable for drug discovery. Because zebrafish are ideal for in vivo screening for potential therapeutics, we tested whether kcnj10 knockdown in zebrafish would fill this need. We cloned zebrafish kcnj10 and demonstrated that its function is equivalent to that of human KCNJ10. We next injected splice- and translation-blocking kcnj10 antisense morpholino oligonucleotides and reproduced the cardinal symptoms of EAST syndrome – ataxia, epilepsy and renal tubular defects. Several of these phenotypes could be assayed in an automated manner. We could rescue the morphant phenotype with complementary RNA (cRNA) encoding human wild-type KCNJ10, but not with cRNA encoding a KCNJ10 mutation identified in individuals with EAST syndrome. Our results suggest that zebrafish will be a valuable tool to screen for compounds that are potentially therapeutic for EAST syndrome or its individual symptoms. Knockdown of kcnj10 represents the first zebrafish model of a salt-losing tubulopathy, which has relevance for blood pressure control.

Novel missense mutations of WNK1 in patients with hypokalemic salt‐losing tubulopathies

Clinical and genetic studies have suggested that a loss of function and gain of function mutation in the same gene can cause different diseases. The aim of this study was to test the hypothesis that inactivating mutations in WNK1 (with no K (lysine) protein kinase-1) or WNK4 could be a new candidate for causing hypokalemic salt-losing tubulopathy (SLT) in those patients with unknown genetic defects because SLT is the opposite phenotype to pseudohypoaldosteronism type II (PHAII). We screened 44 SLTs patients and found that 33 (75%) cases had homozygous or compound heterozygous mutations in CLCNKB or SLC12A3. Two novel missense mutations were identified in WNK1, but not in WNK4, in 2 of the remaining 11 patients. The WNK1 mutations occurred in the protein C-terminus domain, de novo and inherited, respectively. One of these WNK1 mutations was shown to reduce NCC protein membrane expression in vitro because of impairing the suppressive effect of WNK4-mediated inhibition. Taken together, our findings suggest that inactivating mutations in WNK1 may cause SLT, a phenotype opposite to that of PHAII caused by WNK1 intronic deletion.

Genotype, Phenotype, and Follow-Up in Taiwanese Patients with Salt-Losing Tubulopathy Associated withSLC12A3Mutation

Genotype, phenotype, and follow-up analysis is rarely performed in a large number of patients with Gitelman's syndrome (GS) caused by mutations in SLC12A3 encoding the thiazide-sensitive NaCl cotransporter. One hundred seventeen Taiwanese GS patients (70 males and 47 females, age 24 ± 10 yr) with SLC12A3 mutations belonging to 92 unrelated families were investigated. Genomic DNA and/or cDNA from blood leukocytes were analyzed for SLC12A3 mutations and haplotype analysis with intragenic and extragenic markers for recurrent SLC12A3 mutations. Clinical symptoms and biochemical studies at the first presentation as well as follow-up were examined. Forty different SLC12A3 mutations were identified. Approximately 12% of patients had triple SLC12A3 mutations. Fourteen mutations were recurrent with only one founder effect (S710X). Typical hypocalciuria and hypomagnesemia were not found in seven and nine patients, respectively. In addition to male patients having an earlier age of onset, more severe hypokalemia, and significantly lower serum aldosterone concentration, patients with homozygous and deep mutations in intron 13 (c.1670-191C → T) had more severe phenotype. Seven and five patients had chronic kidney disease (stage III-IV) and type 2 diabetes at the follow-up, respectively. Screening of recurrent hot spot SLC12A3 mutations may provide an early diagnosis of GS. Besides gender effect, the nature of homozygous and deep intronic mutations may influence the phenotype. Patients with GS may be at increased risk for the development of chronic kidney disease and type 2 diabetes.

Aquaporin-2: new mutations responsible for autosomal-recessive nephrogenic diabetes insipidus-update and epidemiology.

It is clinically useful to distinguish between two types of hereditary nephrogenic diabetes insipidus (NDI): a ‘pure’ type characterized by loss of water only and a complex type characterized by loss of water and ions. Patients with congenital NDI bearing mutations in the vasopressin 2 receptor gene, AVPR2, or in the aquaporin-2 gene, AQP2, have a pure NDI phenotype with loss of water but normal conservation of sodium, potassium, chloride and calcium. Patients with hereditary hypokalemic salt-losing tubulopathies have a complex phenotype with loss of water and ions. They have polyhydramnios, hypercalciuria and hypo- or isosthenuria and were found to bear KCNJ1 (ROMK) and SLC12A1 (NKCC2) mutations. Patients with polyhydramnios, profound polyuria, hyponatremia, hypochloremia, metabolic alkalosis and sensorineural deafness were found to bear BSND mutations. These clinical phenotypes demonstrate the critical importance of the proteins ROMK, NKCC2 and Barttin to transfer NaCl in the medullary interstitium and thereby to generate, together with urea, a hypertonic milieu. This editorial describes two new developments: (i) the genomic information provided by the sequencing of the AQP2 gene is key to the routine care of these patients, and, as in other genetic diseases, reduces health costs and provides psychological benefits to patients and families and (ii) the expression of AQP2 mutants in Xenopus oocytes and in polarized renal tubular cells recapitulates the clinical phenotypes and reveals a continuum from severe loss of function with urinary osmolalities <150 mOsm/kg H2O to milder defects with urine osmolalities >200 mOsm/kg H2O.

Status epilepticus as the only presentation of the neonatal Bartter syndrome

Bartter syndrome is a rare hereditary (autosomal recessive) salt-losing tubulopathy characterized by hypokalemia, hypochloremia, metabolic alkalosis, and normal blood pressure with hyperreninemia, The underlying renal abnormality results in excessive urinary losses of sodium, chloride, and potassium. We report a case of a four-month-old infant with neonatal Bartter syndrome, who presented only with status epilepticus. To the best of our present knowledge, there is no reported case of Bartter syndrome who presented with status epilepticus.

Gitelman's syndrome: a pathophysiological and clinical update

Gitelman's syndrome (GS), also known as familial hypokalemic hypomagnesemia, is a rare autosomal recessive hereditary salt-losing tubulopathy, characterized by hypokalemic metabolic alkalosis, hypomagnesemia, and hypocalciuria, which is usually caused by mutations in the SLC12A3 gene encoding the thiazide-sensitive sodium chloride contrasporter. Because 18-40% of suspected GS patients carry only one SLC12A3 mutant allele, large genomic rearrangements must account for unidentified mutations. The clinical manifestations of GS are highly variable in terms of age at presentation, severity of symptoms, and biochemical abnormalities. Molecular analysis in our sibling's patients revealed compound heterozygous mutations in the coding region of SLC12A3 as underlying their disease. Such compound heterozygosity can result in disease phenotype for such loss of function mutations in the absence of homozygosis through consanguineous inheritance of mutant alleles, identical by descent. Missense mutations account for approximately 70% of the mutations in GS, and there is a predisposition to large rearrangements caused by the presence of repeated sequences within the SLC12A3. We report two adult male siblings of Jewish origin with late onset GS, who presented in their fifth decade of life with muscle weakness, hypokalemia, hypomagnesaemia, and metabolic alkalosis. Rapid clinical and biochemical improvement was achieved by replacement therapy with potassium and magnesium.

Bartter- and Gitelman-like syndromes: salt-losing tubulopathies with loop or DCT defects

Salt-losing tubulopathies with secondary hyperaldosteronism (SLT) comprise a set of well-defined inherited tubular disorders. Two segments along the distal nephron are primarily involved in the pathogenesis of SLTs: the thick ascending limb of Henle’s loop, and the distal convoluted tubule (DCT). The functions of these pre- and postmacula densa segments are quite distinct, and this has a major impact on the clinical presentation of loop and DCT disorders – the Bartter- and Gitelman-like syndromes. Defects in the water-impermeable thick ascending limb, with its greater salt reabsorption capacity, lead to major salt and water losses similar to the effect of loop diuretics. In contrast, defects in the DCT, with its minor capacity of salt reabsorption and its crucial role in fine-tuning of urinary calcium and magnesium excretion, provoke more chronic solute imbalances similar to the effects of chronic treatment with thiazides. The most severe disorder is a combination of a loop and DCT disorder similar to the enhanced diuretic effect of a co-medication of loop diuretics with thiazides. Besides salt and water supplementation, prostaglandin E2-synthase inhibition is the most effective therapeutic option in polyuric loop disorders (e.g., pure furosemide and mixed furosemide–amiloride type), especially in preterm infants with severe volume depletion. In DCT disorders (e.g., pure thiazide and mixed thiazide–furosemide type), renin–angiotensin–aldosterone system (RAAS) blockers might be indicated after salt, potassium, and magnesium supplementation are deemed insufficient. It appears that in most patients with SLT, a combination of solute supplementation with some drug treatment (e.g., indomethacin) is needed for a lifetime.

Improving Genetic Testing for Gitelman’s Syndrome

Gitelman’s syndrome (GS) is a rare, autosomal recessive, salt-losing tubulopathy caused by mutations in the SLC12A3 gene, which encodes the thiazide-sensitive NaCl cotransporter (NCC). Because 18 to 40% of suspected GS patients carry only one SLC12A3 mutant allele, large genomic rearrangements may account for unidentified mutations. Here, we directly sequenced genomic DNA from a large cohort of 448 unrelated patients suspected of having GS. We found 172 distinct mutations, of which 100 were unreported previously. In 315 patients (70%), we identified two mutations; in 81 patients (18%), we identified one; and in 52 patients (12%), we did not detect a mutation. In 88 patients, we performed a search for large rearrangements by multiplex ligation-dependent probe amplification (MLPA) and found nine deletions and two duplications in 24 of the 51 heterozygous patients. A second technique confirmed each rearrangement. Based on the breakpoints of seven deletions, nonallelic homologous recombination by Alu sequences and nonhomologous end-joining probably favor these intragenic deletions. In summary, missense mutations account for approximately 59% of the mutations in Gitelman’s syndrome, and there is a predisposition to large rearrangements (6% of our cases) caused by the presence of repeated sequences within the SLC12A3 gene.

Spectrum of Mutations in Gitelman Syndrome

Gitelman's syndrome (GS) is a rare, autosomal recessive, salt-losing tubulopathy caused by mutations in the SLC12A3 gene, which encodes the thiazide-sensitive NaCl cotransporter (NCC). Because 18 to 40% of suspected GS patients carry only one SLC12A3 mutant allele, large genomic rearrangements may account for unidentified mutations. Here, we directly sequenced genomic DNA from a large cohort of 448 unrelated patients suspected of having GS. We found 172 distinct mutations, of which 100 were unreported previously. In 315 patients (70%), we identified two mutations; in 81 patients (18%), we identified one; and in 52 patients (12%), we did not detect a mutation. In 88 patients, we performed a search for large rearrangements by multiplex ligation-dependent probe amplification (MLPA) and found nine deletions and two duplications in 24 of the 51 heterozygous patients. A second technique confirmed each rearrangement. Based on the breakpoints of seven deletions, nonallelic homologous recombination by Alu sequences and nonhomologous end-joining probably favor these intragenic deletions. In summary, missense mutations account for approximately 59% of the mutations in Gitelman's syndrome, and there is a predisposition to large rearrangements (6% of our cases) caused by the presence of repeated sequences within the SLC12A3 gene.

The Pharmacological Characteristics of Molecular-Based Inherited Salt-Losing Tubulopathies

Our understanding of inherited salt-losing tubulopathies has improved with recent advances in molecular genetics. However, the terminology of Bartter syndrome and Gitelman syndrome does not always accurately reflect their pathophysiological basis or clinical presentation, and some patients are difficult to diagnose from their clinical presentations. In the present study, we conducted molecular analysis and diuretic tests for patients with inherited salt-losing tubulopathies to clarify the pharmacological characteristics of these disorders. We detected mutations and subsequently conducted diuretic tests using furosemide and thiazide for 16 patients with salt-losing tubulopathies (two with SLC12A1; two with KCNJ1; nine with CLCNKB; and three with SLC12A3). Patients with SLC12A1 mutations showed no response to furosemide, whereas those with SLC12A3 mutations showed no response to thiazide. However, patients with CLCNKB mutations showed no response to thiazide and a normal response to furosemide, and those with KCNJ1 mutations showed a good response to both diuretics. This study revealed the following characteristics of these disorders: 1) subjects with CLCNKB mutations showed one or more biochemical features of Gitelman syndrome (including hypomagnesemia, hypocalciuria, and fractional chloride excretion insensitivity to thiazide administration); and 2) subjects with KCNJ1 mutations appeared to show normal fractional chloride excretion sensitivity to furosemide and thiazide administration. These results indicate that these disorders are difficult to distinguish in some patients, even when using diuretic challenge. This clinical report provides important findings that can improve our understanding of inherited salt-losing tubulopathies and renal tubular physiology.

Gitelman syndrome: pathophysiological and clinical aspects

Giltelman syndrome (GS) is a recessive salt-losing tubulopathy of children or young adults caused by a mutation of genes encoding the human sodium chloride cotransporters and magnesium channels in the thiazide-sensitive segments of the distal convoluted tubule. The plasma biochemical picture is characterized by hypokalemia, hypomagnesemia, hypocalciuria, metabolic alkalosis and hypereninemic hyperaldosteronism. However, patients with GS present some clinical and biochemical alterations resembling that observed in thiazide diuretics abuse. On the pathophysiological point of view, GS represents a useful and interesting human model to better understand the clinical consequences of plasma hydro-electrolytes and acid-base derangements, associated with multiple hormonal alterations. The impact of this complex disorder involves cardiovascular, muscle-skeletal and some other physiological functions, adversely affecting the patient's quality of life. This review tries to summarize and better explain the linkage between the electrolytes, neurohormonal derangements and clinical picture. Moreover, the differential diagnosis between other similar electrolyte-induced clinical disorders and GS is also discussed.

Cardiac arrhythmias and rhabdomyolysis in Bartter–Gitelman patients

Recent data demonstrate that patients affected with hypokalemic salt-losing tubulopathies are prone to acute cardiac arrhythmias and rhabdomyolysis. The tendency to these potentially fatal complications is especially high if chronic hypokalemia is severe, in patients with diarrhea, vomiting or a prolonged QT interval on standard electrocardiography, in patients on drug management with compounds prolonging the electrocardiographic QT interval (including antiarrhythmic agents, some antihistamines, macrolides, antifungals, psychotropics, beta2-adrenergic agonists or cisapride), following acute alcohol abuse and during exercise. Cardiac arrhythmias and rhabdomyolysis occur with sufficient frequency in hypokalemic salt-losing tubulopathies to merit wider awareness of their presence and the preparation of specific prevention and management recommendations.

In vivo and in vitro splicing assay of SLC12A1 in an antenatal salt-losing tubulopathy patient with an intronic mutation

Type I Bartter syndrome (BS), an inherited salt-losing tubulopathy, is caused by mutations of the SLC12A1 gene. While several intronic nucleotide changes in this gene have been detected, transcriptional analysis had not been conducted because mRNA analysis is possible only when renal biopsy specimens can be obtained or occasionally when mRNA is expressed in the leukocytes. This report concerns a type I BS patient due to compound heterozygosity for the SLC12A1 gene. Genomic DNA sequencing disclosed the presence of two novel heterozygous mutations of c.724 + 4A > G in intron 5 and c.2095delG in intron 16, but it remains to be determined whether the former would be likely to influence the transcription. In this report, we conducted both in vivo assay of RT-PCR analysis using RNA extracted from the proband's urinary sediments and in vitro functional splicing study by minigene construction, and obtained evidence that this intronic mutation leads to complete exon 5 skipping. To the best of our knowledge, this is the first study to use non-invasive methods for both an in vivo assay and an in vitro functional splicing assay of inherited kidney disease. These analytical assays could be adapted for all inherited kidney diseases.

Epilepsy, Ataxia, Sensorineural Deafness, Tubulopathy, andKCNJ10Mutations

Five children from two consanguineous families presented with epilepsy beginning in infancy and severe ataxia, moderate sensorineural deafness, and a renal salt-losing tubulopathy with normotensive hypokalemic metabolic alkalosis. We investigated the genetic basis of this autosomal recessive disease, which we call the EAST syndrome (the presence of epilepsy, ataxia, sensorineural deafness, and tubulopathy). Whole-genome linkage analysis was performed in the four affected children in one of the families. Newly identified mutations in a potassium-channel gene were evaluated with the use of a heterologous expression system. Protein expression and function were further investigated in genetically modified mice. Linkage analysis identified a single significant locus on chromosome 1q23.2 with a lod score of 4.98. This region contained the KCNJ10 gene, which encodes a potassium channel expressed in the brain, inner ear, and kidney. Sequencing of this candidate gene revealed homozygous missense mutations in affected persons in both families. These mutations, when expressed heterologously in xenopus oocytes, caused significant and specific decreases in potassium currents. Mice with Kcnj10 deletions became dehydrated, with definitive evidence of renal salt wasting. Mutations in KCNJ10 cause a specific disorder, consisting of epilepsy, ataxia, sensorineural deafness, and tubulopathy. Our findings indicate that KCNJ10 plays a major role in renal salt handling and, hence, possibly also in blood-pressure maintenance and its regulation.

Antenatal Bartter′s syndrome with sensorineural deafness

Bartter's syndrome is a group of inherited, salt-losing tubulopathies presenting as metabolic alkalosis with normotensive hyperreninemia and hyperaldosteronism. We report here the first case of a neonate with bilateral, sensorineural deafness, a variant of antenatal Bartter's syndrome from an Indian community.

Phenotype–genotype correlation in antenatal and neonatal variants of Bartter syndrome

Ante/neonatal Bartter syndrome (BS) is a hereditary salt-losing tubulopathy due to mutations in genes encoding proteins involved in NaCl reabsorption in the thick ascending limb of Henle's loop. Our aim was to study the frequency, clinical characteristics and outcome of each genetic subtype. Charts of 42 children with mutations in KCNJ1 (n = 19), SLC12A1 (n = 13) CLCNKB (n = 6) or BSND (n = 4) were retrospectively analysed. The median follow-up was 8.3 [0.4-18.0] years. We describe 24 new mutations: 10 in KCNJ1, 11 in SLC12A1 and 3 in CLCNKB. The onset of polyhydramnios, birth term, height and weight were similar for all groups; three patients had no history of polyhydramnios or premature birth and had CLCNKB mutations according to a less severe renal sodium wasting. Contrasting with these data, patients with CLCNKB had the lowest potassium (P = 0.006 versus KCNJ1 and P = 0.034 versus SLC12A1) and chloride plasma concentrations (P = 0.039 versus KCNJ1 and P = 0.024 versus SLC12A1) and the highest bicarbonataemia (P = 0.026 versus KCNJ1 and P = 0.014 versus SLC12A1). Deafness at diagnosis was constant in patients with BSND mutations; transient neonatal hyperkalaemia was present in two-thirds of the children with KCNJ1 mutations. Nephrocalcinosis was constant in KCNJ1 and SLC12A1 but not in BSND and CLCNKB patients. In most cases, water/electrolyte supplementation + indomethacin led to catch-up growth. Three patients developed chronic renal failure: one with KCNJ1 mutations during the second decade of age and two with CLCNKB and BSND mutations and without nephrocalcinosis during the first year of life. We confirmed in a large cohort of ante/ neonatal BS that deafness, transient hyperkalaemia and severe hypokalaemic hypochloraemic alkalosis orientate molecular investigations to BSND, KCNJ1 and CLCNKB genes, respectively. Chronic renal failure is a rare event, associated in this cohort with three genotypes and not always associated with nephrocalcinosis.

An improved terminology and classification of Bartter-like syndromes

This Review outlines a terminology and classification of Bartter-like syndromes that is based on the underlying causes of these inherited salt-losing tubulopathies and is, therefore, more clinically relevant than the classical definition. Three major types of salt-losing tubulopathy can be defined: distal convoluted tubule dysfunction leading to hypokalemia (currently known as Gitelman or Bartter syndrome), the more-severe condition of polyuric loop dysfunction (often referred to as antenatal Bartter or hyperprostaglandin E syndrome), and the most-severe condition of combined loop and distal convoluted tubule dysfunction (antenatal Bartter or hyperprostaglandin E syndrome with sensorineural deafness). These three subtypes can each be further subdivided according to the identity of the defective ion transporter or channel: the sodium-chloride cotransporter NCCT or the chloride channel ClC-Kb in distal convoluted tubule dysfunction; the sodium-potassium-chloride cotransporter NKCC2 or the renal outer medullary potassium channel in loop dysfunction; and the chloride channels ClC-Ka and ClC-Kb or their beta-subunit Barttin in combined distal convoluted tubule and loop dysfunction. This new classification should help clinicians to better understand the pathophysiology of these syndromes and choose the most appropriate treatment for affected patients, while avoiding potentially harmful diagnostic and therapeutic approaches.

Evaluating PVALB as a candidate gene for SLC12A3-negative cases of Gitelman's syndrome

Loss-of-function mutations in SLC12A3 coding for the thiazide-sensitive NaCl cotransporter (NCC) cause Gitelman's syndrome (GS), a recessively inherited salt-losing tubulopathy. Most GS patients are compound heterozygous. However, up to 30% of GS patients carry only a single mutant allele, and a normal SLC12A3 screening is also observed in a small subset of patients. Locus heterogeneity could explain the lack of detection of mutant SLC12A3 alleles in GS patients. The renal phenotype of the parvalbumin knockout mice pointed to PVALB as a candidate gene for GS for SLC12A3-negative cases. PCR and direct sequencing of PVALB was performed in 132 GS patients in whom only one or no (N = 79) mutant SLC12A3 allele was found. The possible interference of biallelic SNPs (single nucleotide polymorphisms) on normal transcription or normal splicing was investigated. Genotyping of 110 anonymous blood donors was performed to determine the allelic frequency in the normal population. No sequence variants resulting in amino acid substitution or truncated protein within the PVALB gene were found in the 264 chromosomes tested. Ten biallelic SNPs, including six novel polymorphisms, were identified: five in the 5' UTR, none of them affecting predicted regulatory elements; three in the coding region, without alteration of the consensus splice sites, and two in the 3' UTR. The observed allelic frequencies did not differ significantly between GS patients and controls. Our results strongly suggest that mutations in the PVALB gene are not involved in GS patients who harbour a single or no mutant SLC12A3 allele.

Mechanisms of Disease: the kidney-specific chloride channels ClCKA and ClCKB, the Barttin subunit, and their clinical relevance

Rodent ClC-K1 and ClC-K2, and their respective human orthologs ClCKA and ClCKB, are chloride channels specific to the kidney (and inner ear); Barttin is their functionally important subunit. ClC-K1 is predominantly localized to the thin ascending limb of the loop of Henle. ClC-K2 is expressed more broadly in the distal nephron; expression levels are highest along the thick ascending limb of the loop of Henle and distal convoluted tubule. Expression of ClC-K1 is upregulated by dehydration and downregulated by the diuretic furosemide, whereas expression of ClC-K2 is upregulated by furosemide and downregulated by high salt levels. ClCKA is important for maintenance of the corticomedullary osmotic gradient and the kidney's capacity to concentrate urine. If its ortholog, ClC-K1, is nonfunctional in mice, renal diabetes insipidus develops. ClCKB is a key determinant of tubular reabsorption of chloride and electrolytes along the distal tubule. A severe salt-losing tubulopathy (Bartter syndrome type III) develops if ClCKB is nonfunctional, whereas a common genetic variant of the CLCNKB gene that leads to increased activity of ClCKB results in salt-dependent hypertension. Disruption of the gene encoding Barttin, BSND, results in a 'double knockout' of the functions of both ClCKA and ClCKB, manifesting as Bartter syndrome type IV with sensorineural deafness and an especially severe salt-losing phenotype.