Chloride Channel ClC-Kb Research Articles

Clinical Findings and Genetic Analysis of Nine Mexican Families with Bartter Syndrome

Bartter's syndrome (BS) is a group of salt-wasting tubulopathies characterized by hypokalemia, metabolic alkalosis, hypercalciuria, secondary hyperaldosteronism, and low or normal blood pressure. Loss-of-function variants in genes encoding for five proteins expressed in the thick ascending limb of Henle in the nephron, produced different genetic types of BS. Clinical and genetic analysis of families with Antenatal Bartter syndrome (ABS) and with Classic Bartter syndrome (CBS). Nine patients from unrelated non-consanguineous Mexican families were studied. Massive parallel sequencing of a gene panel or whole-exome sequencing was used to identify the causative gene. Proband 1 was homozygous for the pathogenic variant p.Arg302Gln in the SLC12A1 gene encoding for the sodium-potassium-chloride NKCC2 cotransporter. Proband 3 was homozygous for the nonsense variant p.Cys308* in the KCNJ1 gene encoding for the ROMK potassium channel. Probands 7, 8, and 9 showed variants in the CLCKNB gene encoding the chloride channel ClC-Kb: proband 7 was compound heterozygous for the deletion of the entire gene and the missense change p.Arg438Cys; proband 8 presented a homozygous deletion of the whole gene and proband 9 was homozygous for the nonsense mutation p.Arg595*. A heterozygous variant of unknown significance was detected in the SLC12A1 gene in proband 2, and no variants were found in SLC12A1, KCNJ1, BSND, CLCNKA, CLCNKB, and MAGED2 genes in probands 4, 5, and 6. Genetic analysis identified loss-of-function variants in the SLC12A1, KCNJ1, and CLCNKB genes in four patients with ABS and in the CLCNKB gene in two patients with CBS.

Bartter's syndrome: clinical findings, genetic causes and therapeutic approach.

Bartter's syndrome (BS) is a rare group of salt losing tubulopathies due to the impairment of transport mechanisms at the thick ascending limb of the Henle's loop. Literature reviews and original research articles were collected from database, including PubMed and Scopus. According to the time of onset and symptoms, BS can be classified into antenatal and classic BS. Molecular studies have identified different subtypes of BS. BS types I, II and III are caused by mutations on genes encoding the luminal Na+-K+-2Cl- co-transporter, the luminal K+ channel ROMK, and the basolateral chloride channel ClC-Kb (CLCNKB), respectively. Loss-of-function mutations of Barttin CLCNK type accessory beta subunit cause BS type IVa. Simultaneous mutations of CLCNKB and CLCNKA cause BS type IVb. BS type V consists in a novel transient form characterized by antenatal presentation due to mutations in the MAGE family member D2. Severe gain-of-function mutations of the extracellular calcium sensing receptor gene can result in an autosomal dominant condition of BS. Main clinical and biochemical alterations in BS include polyuria, dehydration, hypokalemia, hypochloremic metabolic alkalosis, hyperreninemia, high levels of prostaglandins, normal or low blood pressure, hypercalciuria and failure to thrive. Treatment focuses mainly at correcting dehydration and electrolyte disturbances and in measures to reduce polyuria, including the use of nonsteroidal anti-inflammatory medications to control excessive renal prostaglandin E2 production. Early diagnosis and treatment of BS may prevent long-term consequences such as growth failure, nephrocalcinosis and end-stage renal disease.

Functional Study of Novel Bartter's Syndrome Mutations in ClC-Kb and Rescue by the Accessory Subunit Barttin Toward Personalized Medicine.

Type III and IV Bartter syndromes (BS) are rare kidney tubulopathies caused by loss-of-function mutations in the CLCNKB and BSND genes coding respectively for the ClC-Kb chloride channels and accessory subunit barttin. ClC-K channels are expressed in the Henle’s loop, distal convoluted tubule, and cortical collecting ducts of the kidney and contribute to chloride absorption and urine concentration. In our Italian cohort, we identified two new mutations in CLCNKB, G167V and G289R, in children affected by BS and previously reported genetic variants, A242E, a chimeric gene and the deletion of the whole CLCNKB. All the patients had hypokalemia and metabolic alkalosis, increased serum renin and aldosterone levels and were treated with a symptomatic therapy. In order to define the molecular mechanisms responsible for BS, we co-expressed ClC-Kb wild type and channels with point mutations with barttin in HEK 293 cells and characterized chloride currents through the patch-clamp technique. In addition, we attempted to revert the functional defect caused by BS mutations through barttin overexpression. G167V and A242E channels showed a drastic current reduction compared to wild type, likely suggesting compromised expression of mutant channels at the plasma membrane. Conversely, G289R channel was similar to wild type raising the doubt that an additional mutation in another gene or other mechanisms could account for the clinical phenotype. Interestingly, increasing ClC-K/barttin ratio augmented G167V and A242E mutants’ chloride current amplitudes towards wild type levels. These results confirm a genotype-phenotype correlation in BS and represent a preliminary proof of concept that molecules functioning as molecular chaperones can restore channel function in expression-defective ClC-Kb mutants.

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

ObjectiveType 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.MethodsThe 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.ResultsOur 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.ConclusionOur 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.

In silico model of the human ClC-Kb chloride channel: pore mapping, biostructural pathology and drug screening.

The human ClC-Kb channel plays a key role in exporting chloride ions from the cytosol and is known to be involved in Bartter syndrome type 3 when its permeation capacity is decreased. The ClC-Kb channel has been recently proposed as a potential therapeutic target to treat hypertension. In order to gain new insights into the sequence-structure-function relationships of this channel, to investigate possible impacts of amino-acid substitutions, and to design novel inhibitors, we first built a structural model of the human ClC-Kb channel using comparative modeling strategies. We combined in silico and in vitro techniques to analyze amino acids involved in the chloride ion pathway as well as to rationalize the possible role of several clinically observed mutations leading to the Bartter syndrome type 3. Virtual screening and drug repositioning computations were then carried out. We identified six novel molecules, including 2 approved drugs, diflusinal and loperamide, with Kd values in the low micromolar range, that block the human ClC-Kb channel and that could be used as starting point to design novel chemical probes for this potential therapeutic target.

Clinical and Genetic Spectrum of Bartter Syndrome Type 3

Bartter syndrome type 3 is a clinically heterogeneous hereditary salt-losing tubulopathy caused by mutations of the chloride voltage-gated channel Kb gene (CLCNKB), which encodes the ClC-Kb chloride channel involved in NaCl reabsorption in the renal tubule. To study phenotype/genotype correlations, we performed genetic analyses by direct sequencing and multiplex ligation-dependent probe amplification and retrospectively analyzed medical charts for 115 patients with CLCNKB mutations. Functional analyses were performed in Xenopus laevis oocytes for eight missense and two nonsense mutations. We detected 60 mutations, including 27 previously unreported mutations. Among patients, 29.5% had a phenotype of ante/neonatal Bartter syndrome (polyhydramnios or diagnosis in the first month of life), 44.5% had classic Bartter syndrome (diagnosis during childhood, hypercalciuria, and/or polyuria), and 26.0% had Gitelman-like syndrome (fortuitous discovery of hypokalemia with hypomagnesemia and/or hypocalciuria in childhood or adulthood). Nine of the ten mutations expressed in vitro decreased or abolished chloride conductance. Severe (large deletions, frameshift, nonsense, and essential splicing) and missense mutations resulting in poor residual conductance were associated with younger age at diagnosis. Electrolyte supplements and indomethacin were used frequently to induce catch-up growth, with few adverse effects. After a median follow-up of 8 (range, 1-41) years in 77 patients, chronic renal failure was detected in 19 patients (25%): one required hemodialysis and four underwent renal transplant. In summary, we report a genotype/phenotype correlation for Bartter syndrome type 3: complete loss-of-function mutations associated with younger age at diagnosis, and CKD was observed in all phenotypes.

Salt-losing nephropathy in mice with a null mutation of the Clcnk2 gene.

The basolateral chloride channel ClC-Kb facilitates Cl reabsorption in the distal nephron of the human kidney. Functional mutations in CLCNKB are associated with Bartter's syndrome type 3, a hereditary salt-losing nephropathy. To address the function of ClC-K2 invivo, we generated ClC-K2-deficient mice. ClC-K2-deficient mice were generated using TALEN technology. ClC-K2-deficient mice were viable and born in a Mendelian ratio. ClC-K2-/- mice showed no gross anatomical abnormalities, but they were growth retarded. The 24-h urine volume was increased in ClC-K2-/- mice (4.4±0.6 compared with 0.9±0.2mL per 24h in wild-type littermates; P=0.001). Accordingly, ambient urine osmolarity was markedly reduced (590±39 vs. 2216±132mosmolL-1 in wild types; P<0.0001). During water restriction (24h), urinary osmolarity increased to 1633±153 and 3769±129mosmolL-1 in ClC-K2-/- and wild-type mice (n=12; P<0.0001), accompanied by a loss of body weight of 12±0.4 and 8±0.2% respectively (P<0.0001). ClC-K2-/- mice showed an increased renal sodium excretion and compromised salt conservation during a salt-restricted diet. The salt-losing phenotype of ClC-K2-/- mice was associated with a reduced plasma volume, hypotension, a slightly reduced glomerular filtration rate, an increased renal prostaglandin E2 generation and a massively stimulated renin-angiotensin system. Clckb-/- mice showed a reduced sensitivity to furosemide and were completely resistant to thiazides. Loss of ClC-K2 compromises TAL function and abolishes salt reabsorption in the distal convoluted tubule. Our data suggest that ClC-K2 is crucial for renal salt reabsorption and concentrating ability. ClC-K2-deficient mice in most aspects mimic patients with Bartter's syndrome type 3.

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.

Association of the CLC-Kb T481S polymorphism with childhood hypertension

Essential hypertension is a difficult diagnosis in children and the gene of the renal-epithelial chloride channel ClC-Kb is potentially predisposing. In vitro studies have shown that a common ClC-Kb threonine481serine (T481S) polymorphism leads to enhanced chloride channel activity and may predispose for hypertension (HT). We therefore analysed children at risk for HT for the T481S polymorphism and associated genotype with blood pressure (BP) status. A total of 48 children with essential hypertension (mean age 14.4 ± 2.7 years, 26 male; 22 female; mean BP 143.4 ± 7.5/88 ± 5.8 mmHg) were compared with 78 children with white-coat HT (WCHT), who showed occasionally hypertensive BP values, which were not confirmed by ambulatory blood pressure monitoring (mean age 13.7 ± 2.5 years, 49 male, 29 female; mean BP 122.4 ± 4.3/68.2 ± 3.5 mmHg). Other causes of HT were excluded. Allelic frequencies of hypertensive patients were not significantly different from those with WCHT (HT: A 0.84; T 0.16 vs. WCHT: A 0.85; T 0.15). However, the T-allele was observed more frequently in WCHT subjects with systolic and diastolic BP exceeding the 90th percentile (A 0.71; T 0.29, n = 34, p < 0.05, considered as borderline hypertensive). The preliminary data suggest that children with WCHT carry the ClC-Kb T481S polymorphism more often and that this variant may predispose for development of arterial HT.

Identification and functional analysis of novel mutations of the CLCNKB gene in Chinese patients with classic Bartter syndrome

Mutations in the gene CLCNKB encoding the ClC-Kb chloride channel causes classic Bartter syndrome, which is characterized by hypokalaemic metabolic alkalosis, renal salt loss, hyper-reninaemic hyperaldosteronism and normal blood pressure. We aimed to investigate the underlying mutations in CLCNKB in two Chinese patients with classic Bartter syndrome and then test the effect of the mutations on ClC-Kb chloride channel activity. Mutation analysis of CLCNKB was performed by polymerase chain reaction (PCR) direct sequencing. Expression of the wild-type and mutant ClC-Kb was heterologous in Xenopus laevis oocytes. We identified three novel CLCNKB gene mutations, including one homozygous missense mutation (R351W) in one patient and two compound heterozygous mutations (R30X and A210V) in the other. As determined by two-electrode voltage-clamp analysis of ClC-Kb channel activity, R30X abolished the current amplitude; A210V and R351W significantly reduced the current amplitude. A210V was almost as sensitive as the wild type to extracellular pH and calcium, whereas R351W removed extracellular calcium activation and markedly reduced alkaline pH activation of ClC-Kb. The three novel CLCNKB mutations we identified in two Chinese patients with classic Bartter syndrome have a role in altering the functional properties of ClC-Kb channels.

Endocochlear potential depends on Cl− channels: mechanism underlying deafness in Bartter syndrome IV

Human Bartter syndrome IV is an autosomal recessive disorder characterized by congenital deafness and severe renal salt and fluid loss. It is caused by mutations in BSND, which encodes barttin, a beta-subunit of ClC-Ka and ClC-Kb chloride channels. Inner-ear-specific disruption of Bsnd in mice now reveals that the positive potential, but not the high potassium concentration, of the scala media depends on the presence of these channels in the epithelium of the stria vascularis. The reduced driving force for K(+)-entry through mechanosensitive channels into sensory hair cells entails a profound congenital hearing loss and subtle vestibular symptoms. Although retaining all cell types and intact tight junctions, the thickness of the stria is reduced early on. Cochlear outer hair cells degenerate over several months. A collapse of endolymphatic space was seen when mice had additionally renal salt and fluid loss due to partial barttin deletion in the kidney. Bsnd(-/-) mice thus demonstrate a novel function of Cl(-) channels in generating the endocochlear potential and reveal the mechanism leading to deafness in human Bartter syndrome IV.

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.

Gitelman syndrome

Gitelman syndrome (GS), also referred to as familial hypokalemia-hypomagnesemia, is characterized by hypokalemic metabolic alkalosis in combination with significant hypomagnesemia and low urinary calcium excretion. The prevalence is estimated at approximately 1:40,000 and accordingly, the prevalence of heterozygotes is approximately 1% in Caucasian populations, making it one of the most frequent inherited renal tubular disorders. In the majority of cases, symptoms do not appear before the age of six years and the disease is usually diagnosed during adolescence or adulthood. Transient periods of muscle weakness and tetany, sometimes accompanied by abdominal pain, vomiting and fever are often seen in GS patients. Paresthesias, especially in the face, frequently occur. Remarkably, some patients are completely asymptomatic except for the appearance at adult age of chondrocalcinosis that causes swelling, local heat, and tenderness over the affected joints. Blood pressure is lower than that in the general population. Sudden cardiac arrest has been reported occasionally. In general, growth is normal but can be delayed in those GS patients with severe hypokalemia and hypomagnesemia.GS is transmitted as an autosomal recessive trait. Mutations in the solute carrier family12, member 3 gene, SLC12A3, which encodes the thiazide-sensitive NaCl cotransporter (NCC), are found in the majority of GS patients. At present, more than 140 different NCC mutations throughout the whole protein have been identified. In a small minority of GS patients, mutations in the CLCNKB gene, encoding the chloride channel ClC-Kb have been identified.Diagnosis is based on the clinical symptoms and biochemical abnormalities (hypokalemia, metabolic alkalosis, hypomagnesemia and hypocalciuria). Bartter syndrome (especially type III) is the most important genetic disorder to consider in the differential diagnosis of GS. Genetic counseling is important. Antenatal diagnosis for GS is technically feasible but not advised because of the good prognosis in the majority of patients.Most asymptomatic patients with GS remain untreated and undergo ambulatory monitoring, once a year, generally by nephrologists. Lifelong supplementation of magnesium (magnesium-oxide and magnesium-sulfate) is recommended. Cardiac work-up should be offered to screen for risk factors of cardiac arrhythmias. All GS patients are encouraged to maintain a high-sodium and high potassium diet. In general, the long-term prognosis of GS is excellent.

Ion Channels

Ion Channels

Functional BSND Variants in Essential Hypertension

Defects in the handling of renal salt reabsorption may contribute to interindividual differences in blood-pressure regulation and susceptibility to hypertension. Sodium chloride reabsorption in the thick ascending limb (TAL) is dependent in part on the chloride channel, ClC-Kb (encoded by CLCNKB), and its accessory subunit, barttin (encoded by BSND). We investigated genetic variations in BSND in a screening population, and genotyped a homogenous cohort of normotensive and hypertensive Ghanaian subjects, in addition to four ethnically defined control populations. Functional consequences of the identified BSND variants were examined using a heterologous expression system. Three novel, nonsynonymous coding-sequence single-nucleotide polymorphisms were identified (V43I, E255Q, and G284D) in the screening population. BSND-V43I was identified in African American, Asian, and Hispanic subjects, with minor allele frequencies of 0.14, 0.18, and 0.01, respectively, but it was absent in the Caucasian population. BSND-E225Q and BSND-G284D were rare variants. Two of these variants (V43I and G284D) exhibited partial loss-of-function phenotypes when heterologously expressed with ClC-Kb chloride channels in cultured cells. In logistic regression analyses, we observed no association between hypertension and BSND-I43 in our study population. However, we did observe significant deviation from Hardy-Weinberg equilibrium in the normotensive population. We conclude that BSND-V43I, a common variant conferring partial loss of function, exhibits significant deviation from Hardy-Weinberg equilibrium in the Ghanaian normotensive control population. However, it does not independently confer protection against hypertension.

Influence of gain of function epithelial chloride channel ClC-Kb mutation on hearing thresholds

Hearing depends on functional ClC-K-type chloride channels composed of barttin with ClC-Ka or ClC-Kb. Loss-of-function mutations of the barttin gene BSND or of both, the ClC-Ka gene CLNKA and the ClC-Kb gene CLNKB lead to congenital deafness and renal salt wasting. Recently, we identified the gain-of-function mutation ClC-Kb T481S which is associated with increased blood pressure. To explore the impact of ClC-Kb T481S on hearing, healthy volunteers ( n = 329) and individuals suffering from tinnitus ( n = 246) volunteered for hearing tests ( n = 348) and genetic analysis ( n = 575). 19.1% of the individuals were heterozygote (ClC-Kb T481S/ClC-Kb) and 1.7% homozygote carriers. Pure tone average hearing threshold (PTAt) for air conduction was significantly ( p < 0.033) lower in ClC-Kb T481S carriers (13.2 ± 1.2 dB) than in wild-type individuals (17.1 ± 0.9 dB). The prevalence of ClC-Kb T481S carriers was significantly increased (29.7%) in individuals with PTAt < 15 dB ( p < 0.05) and significantly decreased (13.2%) in individuals with PTAt > 30 dB ( p < 0.017). The difference was largely due to the female population. Bone conduction was less affected pointing to an effect of the mutation on middle ear function. Tinnitus tended to be more frequent in ClC-Kb T481S carriers, a difference, however, not statistically significant. In conclusion, hearing thresholds are slightly lower in carriers of ClC-Kb T481S, i.e., the gain-of-function polymorphism ClC-Kb T481S exerts a subtle but significant protective effect against hearing loss.

Molecular Analysis of the CLCNKB Gene in Japanese Patients with Classic Bartter Syndrome

Deletions or mutations in the gene encoding the basolateral chloride channel CLC-Kb (CLCNKB) cause classic Bartter syndrome (MIM 602023), which is characterized by hypokalemic metabolic alkalosis, hyperreninemic hyperaldosteronism and hypercalciura. These patients are usually diagnosed during infancy or childhood due to failure to thrive and growth retardation. The purpose of this study was to investigate the underlying mutations in Japanese patients with classic Bartter syndrome. Seven Japanese patients from seven different families diagnosed as having classic Bartter syndrome were studied. Analysis of CLCNKB demonstrated a large deletion in two patients, a partial deletion in one patient and two mutations (DeltaL130 in exon 4 and W610X in exon 16) in the remaining four patients. DeltaL130 is a novel mutation, but W610X was previously reported in three unrelated Japanese patients. Six out of the seven patients were diagnosed due to typical characteristics of classic Bartter syndrome such as failure to thrive and poor weight gain however, one patient was asymptomatic with mild hypokalemia. In conclusion, we identified a novel mutation of the CLCNKB gene, DeltaL130. We did not determine whether the W610X mutation in our patients was from a common ancestor or if this mutation is frequent in Japan.

Dissecting Hypertension by Obesity Identifies a Locus at 1p36

The elevated arterial pressure that defines essential hypertension is regarded as the manifestation of a diverse array of interacting genetic and environmental causes. As such, hypertension is a heterogeneous disorder in which multiple contributing factors are responsible for the overarching phenotype of high blood pressure that is the primary clinical manifestation observed. Such heterogeneity has undoubtedly hampered efforts to elucidate the genetic basis of essential hypertension. Overweight and obesity are well known to increase the risk of essential hypertension. Yet, there are many hypertensive individuals whose weight is normal. So does obesity hypertension have a different underlying genetic cause than lean hypertension? In the current issue, Pausova et al report finding a locus with suggestive linkage to hypertension and then reanalyzed their data after dividing their families into lean and obese.1 Of considerable interest, the significance of the linkage peak increased for the obese families but disappeared in the case of the families with hypertension who were not obese. The striking contrast in results for each category of hypertension lends strong support to the idea that hypertension of obesity has a different genetic basis than other categories of hypertension. Not only is this finding important in discovery of the genetic basis of obesity hypertension, but it has obvious implications for approaches that might be used to identify the basis for other “intermediate phenotypes” of hypertension. The study by Pausova et al involved 55 extended families from the geographically remote French-Canadian Saguenay/Lac-St-Jean region of Quebec. This relatively small, isolated population has been spared the level of genetic “noise” present in populations elsewhere. As a result, the degree of genetic homogeneity should be elevated and likely contributed to the …

A founder mutation in the CLCNKB gene causes Bartter syndrome type III in Spain

The term "Bartter syndrome" encompasses a group of closely related inherited tubulopathies characterized by markedly reduced NaCl transport by the distal nephron. At present, five different genetic variants have been demonstrated. The majority of patients with so-called classic Bartter syndrome carry inactivating mutations of the CLCNKB gene encoding the basolateral ClC-Kb chloride channel (Bartter syndrome type III). The purpose of this study was to investigate the underlying mutation in cases of classic Bartter syndrome followed at our center. Ten patients, including two sisters, with clinical and biochemical features of classic Bartter syndrome were included in the mutational analysis. They originated from different regions of Spain with either Basque or Spanish ancestry. There was no history of consanguineous marriage in any of the kindreds. The parents and siblings of each patient, as well as a population of 300 healthy control adult subjects, were also analyzed. All ten patients were found to be homozygous for an identical missense mutation in the CLCNKB gene, substituting a threonine for an alanine at codon 204 (A204T) in the putative fifth transmembrane domain of the protein. None of the 300 control subjects were homozygous for the A204T allele. Overall, the A204T mutation was detected on 2/600 control chromosomes. Despite sharing a common mutation, the clinical manifestations of the syndrome in the patients varied from lack of symptoms to severe growth retardation. Demonstration of a point mutation within the CLCNKB gene as the apparently unique cause of Bartter syndrome type III in Spain is highly suggestive of a founder effect. Our results also support the lack of correlation between genotype and phenotype in this disease.

Renal cysts and nephrocalcinosis in a patient with Bartter syndrome type III

Chronic hypokalemia is known to induce renal cyst formation in some diseases including primary aldosteronism, distal renal tubular acidosis, Liddle disease and apparent mineralocorticoid excess syndrome. Although chronic hypokalemia is the main clinical feature of Bartter syndrome, renal cyst formation in this disease has never been reported. We describe a patient with classic Bartter syndrome who exhibited renal cysts and nephrocalcinosis. Direct sequencing analysis of the chloride channel CLC-Kb gene identified a heterozygous nonsense mutation (W610X) in exon 16 indicating a diagnosis of Bartter syndrome type III. Although the precise mechanism underlying the development of renal cysts in our patient remains unclear, chronic hypokalemia and nephrocalcinosis may contribute to cyst development.