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

Navigating the multifaceted intricacies of the Na+-Cl- cotransporter, a highly regulated key effector in the control of hydromineral homeostasis

The Na+-Cl- cotransporter (NCC; SLC12A3) is a highly regulated integral membrane protein that is known to exist as three splice variants in primates. Its primary role in the kidney is to mediate the cosymport of Na+ and Cl- across the apical membrane of the distal convoluted tubule. Through this role and the involvement of other ion transport systems, NCC allows the systemic circulation to reclaim a fraction of the ultrafiltered Na+, K+, Cl-, and Mg+ loads in exchange for Ca2+ and [Formula: see text]. The physiological relevance of the Na+-Cl- cotransport mechanism in humans is illustrated by several abnormalities that result from NCC inactivation through the administration of thiazides or in the setting of hereditary disorders. The purpose of the present review is to discuss the molecular mechanisms and overall roles of Na+-Cl- cotransport as the main topics of interest. On reading the narrative proposed, one will realize that the knowledge gained in regard to these themes will continue to progress unrelentingly no matter how refined it has now become.

Causal relationship of gut microbiota with diabetic nephropathy: a Mendelian randomization analysis

Background

Patients with DN (diabetic nephropathy) show remarkable variations in their gut microbiota composition. However, to date, no study has shown whether a causal relationship exists between gut microbiota composition and DN.

Methods

Here, we performed a two-sample Mendelian randomization (MR) investigation for identifying causal associations of gut microbiota with DN. Gut microbiota genetic data were gathered from the recent genome-wide association study pooled data of the MiBioGen consortium, which included 24 cohorts and 18,340 individuals.

Results

IVW(Inverse variance weighting) revealed that Verrucomicrobia [odds ratio (OR) = 1.390; 95% confidence interval (CI) = 1.10-1.75; p = 0.005], Peptostreptococcaceae (OR = 1.284; 95% CI = 1.03-1.59; p = 0.012), Verrucomicrobiaceae (OR = 1.390; 95% CI = 1.10-1.75; p = 0.005), Akkermansia (OR = 1.390; 95% CI = 1.10-1.75; p = 0.005), Butyricimonas (OR = 1.261; 95% CI = 1.02-1.55; p = 0.031), Catenibacterium (OR = 1.278; 95% CI = 1.02-1.59; p = 0.030).

Conclusion

Two-sample MR analysis identified 12 microbial taxa in gut microbiota (one of which is yet to be officially named) that showed significant causal associations with DN; 8 of these taxa significantly increased the risk of DN, while the remaining 4 taxa (including the one without an official name) reduced the risk of DN. The precise mechanisms influencing the interactions of gut microbiota with DN occurrence remain unclear; hence, additional investigations should be conducted to clarify these mechanisms.

Failure to thrive in children with tubulopathies increases the risk of overweight later in life

Failure to thrive (FTT) is an inadequate growth in young children. It can increase the risk of overweight or obesity later in life. Patients with renal tubulopathies can present FTT due to solute losses in the urine. We aimed to test our hypothesis that children with tubulopathies have an increased risk of overweight and obesity due to rebound following FTT that could complicate these conditions. We enrolled 26 patients with tubulopathies and evaluated for the first time within the first 12 months of life (mean age: 4.8 months ± 2.6 SDS). FTT was evident in 17 out of 26 patients (65.4%). The mean age at the last follow-up was 14.1 years ± 5.5 SDS. The mean age at overweight/obesity onset was 9.0 years ± 3.6 SDS. The prevalence of overweight/obesity was 73.1% (19/26). Among the patients with FTT, 15 (88.2%) developed overweight/obesity compared to 4 out of the 9 patients (44.4%) without FFT (p = 0.028). The presence of FTT determined an OR for obesity/overweight of 9.4 (95% CI: 1.3-67.6; p = 0.026). FTT continued to be significantly associated with obesity/overweight also after adjustment for preterm birth and birth weight <10th percentile (OR = 23.3; 95% CI: 1.95-279.4; p = 0.01). In conclusion, in our series, patients with tubulopathies presented an increased risk of overweight/obesity due to the FTT that can complicate these conditions.

Genome mining yields putative disease-associated ROMK variants with distinct defects

Bartter syndrome is a group of rare genetic disorders that compromise kidney function by impairing electrolyte reabsorption. Left untreated, the resulting hyponatremia, hypokalemia, and dehydration can be fatal, and there is currently no cure. Bartter syndrome type II specifically arises from mutations in KCNJ1, which encodes the renal outer medullary potassium channel, ROMK. Over 40 Bartter syndrome-associated mutations in KCNJ1 have been identified, yet their molecular defects are mostly uncharacterized. Nevertheless, a subset of disease-linked mutations compromise ROMK folding in the endoplasmic reticulum (ER), which in turn results in premature degradation via the ER associated degradation (ERAD) pathway. To identify uncharacterized human variants that might similarly lead to premature degradation and thus disease, we mined three genomic databases. First, phenotypic data in the UK Biobank were analyzed using a recently developed computational platform to identify individuals carrying KCNJ1 variants with clinical features consistent with Bartter syndrome type II. In parallel, we examined genomic data in both the NIH TOPMed and ClinVar databases with the aid of Rhapsody, a verified computational algorithm that predicts mutation pathogenicity and disease severity. Subsequent phenotypic studies using a yeast screen to assess ROMK function-and analyses of ROMK biogenesis in yeast and human cells-identified four previously uncharacterized mutations. Among these, one mutation uncovered from the two parallel approaches (G228E) destabilized ROMK and targeted it for ERAD, resulting in reduced cell surface expression. Another mutation (T300R) was ERAD-resistant, but defects in channel activity were apparent based on two-electrode voltage clamp measurements in X. laevis oocytes. Together, our results outline a new computational and experimental pipeline that can be applied to identify disease-associated alleles linked to a range of other potassium channels, and further our understanding of the ROMK structure-function relationship that may aid future therapeutic strategies to advance precision medicine.

SLC12A cryo-EM: analysis of relevant ion binding sites, structural domains, and amino acids

The solute carrier family 12A (SLC12A) superfamily of membrane transporters modulates the movement of cations coupled with chloride across the membrane. In doing so, these cotransporters are involved in numerous aspects of human physiology: cell volume regulation, ion homeostasis, blood pressure regulation, and neurological action potential via intracellular chloride concentration modulation. Their physiological characterization has been largely studied; however, understanding the mechanics of their function and the relevance of structural domains or specific amino acids has been a pending task. In recent years, single-particle cryogenic electron microscopy (cryo-EM) has been successfully applied to members of the SLC12A family including all K+:Cl- cotransporters (KCCs), Na+:K+:2Cl- cotransporter NKCC1, and recently Na+:Cl- cotransporter (NCC); revealing structural elements that play key roles in their function. The present review analyzes the data provided by these cryo-EM reports focusing on structural domains and specific amino acids involved in ion binding, domain interactions, and other important SCL12A structural elements. A comparison of cryo-EM data from NKCC1 and KCCs is presented in the light of the two recent NCC cryo-EM studies, to propose insight into structural elements that might also be found in NCC and are necessary for its proper function. In the final sections, the importance of key coordination residues for substrate specificity and their implication on various pathophysiological conditions and genetic disorders is reviewed, as this could provide the basis to correlate structural elements with the development of novel and selective treatments, as well as mechanistic insight into the function and regulation of cation-coupled chloride cotransporters (CCCs).

A Case of a Novel MAGED2 Mutation Resulting in Non-transient Bartter's Syndrome in an Adult Female

Bartter's syndrome (BS) is a disorder caused by a group of rare mutations that result in defective salt reabsorption in the thick ascending loop of Henle. BS is characterized by salt wasting, hypokalemia, and metabolic alkalosis, among other abnormalities. A MAGE-D2 mutation results in an X-linked form of BS. It results in a transient antenatal presentation that is observed to completely resolve by early infancy, usually occurring in males. We present a case of an adult female with intermittent recurrence of symptoms and metabolic derangements consistent with BS. She also has a family history of polyhydramnios and renal disease. Genetic testing later confirmed a novel MAGE-D2 mutation. Her atypical presentation emphasizes the heterogenous presentation of the different mutations and raises the possibility of persistence of abnormalities beyond infancy in mutations of the MAGE-D2 gene.

Research progress on renal calculus associate with inborn error of metabolism

Renal calculus is a common disease with complex etiology and high recurrence rate. Recent studies have revealed that gene mutations may lead to metabolic defects which are associated with the formation of renal calculus, and single gene mutation is involved in relative high proportion of renal calculus. Gene mutations cause changes in enzyme function, metabolic pathway, ion transport, and receptor sensitivity, causing defects in oxalic acid metabolism, cystine metabolism, calcium ion metabolism, or purine metabolism, which may lead to the formation of renal calculus. The hereditary conditions associated with renal calculus include primary hyperoxaluria, cystinuria, Dent disease, familial hypomagnesemia with hypercalciuria and nephrocalcinosis, Bartter syndrome, primary distal renal tubular acidosis, infant hypercalcemia, hereditary hypophosphatemic rickets with hypercalciuria, adenine phosphoribosyltransferase deficiency, hypoxanthine-guanine phosphoribosyltransferase deficiency, and hereditary xanthinuria. This article reviews the research progress on renal calculus associated with inborn error of metabolism, to provide reference for early screening, diagnosis, treatment, prevention and recurrence of renal calculus.

A Rare Presentation of Adult-Onset Bartter Syndrome: A Case Report

Bartter syndrome is a rare, salt-wasting tubulopathy with impaired ion reabsorption in the ascending limb of the loop of Henle, which results in hypokalemia, hypochloremia, and hypercalciuria. It usually presents in neonates, with vomiting, dehydration, and failure to thrive. It results from mutations in several genes, including KCNJ1, CLCNKB, CLCNKA, BSND, and ROMK, which encode ion transporters. We report a rare presentation of adult-onset Bartter syndrome. In this case, a 27-year-old man presented to the hospital with upper and lower limb weakness. Bartter syndrome was suspected based on serum electrolytes assessment and arterial blood gas analysis. The patient was initiated on potassium chloride (KCL) infusion and potassium chloride syrup to correct hypokalemia.

Bartter syndrome type III with glomerular dysplasia and chronic kidney disease: A case report

Background

Bartter syndrome (BS) type III is a rare autosomal recessive genetic disease. Its clinical features are polyuria, hypokalemia, hypochloremia, metabolic alkalosis, and hyperreninaemia. A few BS type III can be complicated with chronic kidney disease.

Case presentation

We report a 14-year-old boy with Bartter syndrome caused by a c.1792C > T (p.Q598*) mutation in the CLCNKB gene. He was a no deafness and full-term baby, and he had renal dysplasia and chronic kidney disease (CKD). In addition, we summarize all cases of BS type III complicated with CKD.

Conclusions

We report a case of Bartter syndrome complicated by chronic kidney disease caused by a new mutation of CLCNKB. As we all know, BS type IV is usually combined with chronic kidney disease, and BS type III can also integrate with CKD. We don't find BS type III with glomerular dysplasia in the literature. So renal damage in BS type III is not only FSGS; clinicians must also be aware of glomerular dysplasia.

Genetic diagnosis of Bartter syndrome in Iranian patients and detection of a novel homozygous CLCNKB mutation

An Iranian girl with clinical symptoms of Bartter syndrome like hypokalemia, polyuria, polydipsia, hyponatremia, and hypochloremic alkalosis was referred to us in whom the CLCNKB gene was genetically evaluated using Sanger sequencing. A homozygous pathogenic variant of c.1332_1335delCTCT was detected in this patient.

Autosomal Dominant Hypocalcemia Type 1: A Systematic Review

Autosomal dominant hypocalcemia type 1 (ADH1) is a rare form of hypoparathyroidism due to activating variants of the calcium-sensing receptor gene (CASR). Inherited or de novo activating variants of the CASR alter the set point for extracellular calcium, resulting in inadequate parathyroid hormone (PTH) secretion and inappropriate renal calcium excretion leading to hypocalcemia and hypercalciuria. Conventional therapy includes calcium and activated vitamin D, which can worsen hypercalciuria, resulting in renal complications. A systematic literature review, using published reports from 1994 to 2021, was conducted to catalog CASR variants, to define the ADH1 clinical spectrum, and to determine the effect of treatment on patients with ADH1. There were 113 unique CASR variants reported, with a general lack of genotype/phenotype correlation. Clinical data were available in 191 patients; 27% lacked symptoms, 32% had mild/moderate symptoms, and 41% had severe symptoms. Seizures, the most frequent clinical presentation, occurred in 39% of patients. In patients with blood and urine chemistries available at the time of diagnosis (n = 91), hypocalcemia (99%), hyperphosphatemia (59%), low PTH levels (57%), and hypercalciuria (34%) were observed. Blood calcium levels were significantly lower in patients with severe symptoms compared with asymptomatic patients (6.8 ± 0.7 versus 7.6 ± 0.7 mg/dL [mean ± SD]; p < 0.0001), and the age of presentation was significantly lower in severely symptomatic patients (9.1 ± 15.0 versus 19.3 ± 19.4 years; p < 0.01). Assessments for complications including nephrocalcinosis, nephrolithiasis, renal impairment, and brain calcifications in 57 patients on conventional therapy showed that 75% had at least one complication. Hypercalciuria was associated with nephrocalcinosis, nephrolithiasis, renal impairment, or brain calcifications (odds ratio [OR] = 9.3; 95% confidence interval [CI] 2.4-37.2; p < 0.01). In 27 patients with urine calcium measures before and after starting conventional therapy, the incidence of hypercalciuria increased by 91% (p < 0.05) after therapy initiation. ADH1 is a condition often associated with severe symptomatology at presentation with an increase in the risk of renal complications after initiation of conventional therapy. © 2022 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Balancing Benefits and Risks of Indomethacin in the Management of Antenatal Bartter Syndrome: A Case Report

Background

Bartter syndrome, a very rare inherited renal tubular disorder, characterized by urinary salt wastage, hypokalemia, polyuria, and metabolic alkalosis, may manifest antenatally as severe isolated polyhydramnios. Indomethacin is known to reduce salt wastage and subsequent polyhydramnios during pregnancy; however, it reduces the Ductus Arteriosus diameter among other potential complications, such as inhibition of gastrointestinal perfusion and increasing the risk of renal toxicity.

Case

A 36-year-old multigravida presented with severe isolated polyhydramnios at 30 weeks of gestation. Based on a history of a previous pregnancy affected with Bartter syndrome, indomethacin was initiated. Amniotic fluid volume and Ductus Arteriosus diameter were monitored. As evidence lacks on optimal dose and duration of indomethacin, multiple-dose adjustments were made to reduce the amniotic fluid volume while maintaining normal Ductus Arteriosus diameter. Progressive polyhydramnios led to Cesarean section at 34+ weeks of gestation resulting in a healthy fetus diagnosed with Bartter syndrome in the early neonatal period.

Conclusion

We share our experience in the adjustment of the dose and duration of Indomethacin therapy in the treatment of severe polyhydramnios associated with antenatal Bartter syndrome. Amniotic fluid index, Ductus Arteriosus diameter, and umbilical artery doppler work together as key indicators to guide the success and safety of the therapy.

Late-Onset Bartter Syndrome Type II Due to a Novel Compound Heterozygous Mutation in KCNJ1 Gene: A Case Report and Literature Review

Background

Bartter syndrome (BS) type II is a rare autosomal recessive renal tubular disorder caused by mutations in the KCNJ1 gene, which encodes the apical renal outer medullary potassium (ROMK) channel in the thick ascending limb (TAL) of Henle’s loop. BS type II is typically considered as a disorder of infancy and seldom seen in adults.

Case Presentation

A 34-year-old woman was admitted with generalized body numbness and hand convulsions, without growth retardation. Laboratory tests revealed hypokalemic metabolic alkalosis, hyperreninemic hyperaldosteronism, and nephrocalcinosis. She was misdiagnosed during the initial diagnosis process and was finally diagnosed with late-onset BS type II via genetic testing through next-generation sequencing combined with Sanger sequencing. A novel compound heterozygous p.Leu207Ile/p. Cys308Arg variant in exon 5 of the KCNJ1 gene from her parents was identified and speculated to be a potential pathogenic gene variation.

Conclusion

We report a case of late-onset BS type II with a novel compound heterozygous mutation in KCNJ1. Both variants are novel and have never been reported. Our report will have a significant impact on the diagnosis of BS in other patients without typical clinical presentations and emphasizes the importance of genetic investigation.

Genetic evaluation of paediatric nephrocalcinosis: phenotype-driven genetic panels reveal a rare diagnosis

Monogenic causes of paediatric nephrocalcinosis are associated with extensive phenotypic variability. We report a 14-year-old male who presented at 8 years of age with incidentally identified nephrocalcinosis alongside growth impairment and dental anomalies. Extensive genetic investigation confirmed a molecular diagnosis of Bartter syndrome type II. This is exceptional in both late presentation and the presence of amelogenesis imperfecta, a very rare association of inherited tubulopathies. Details of the nephrocalcinosis gene panel analysed and associated phenotypes are presented to highlight the utility of a phenotype-driven genetic panel in resolving an atypical presentation of nephrocalcinosis, allowing precise diagnosis, tailored therapy and prognostication.

A Rare Cause of Refractory Severe Polyhydramnios: Antenatal Bartter Syndrome

Background: Antenatal Bartter syndrome is an autosomal recessive disorder causing severe polyuria that leads to severe polyhydramnios and preterm labor. Prenatal diagnosis of antenatal Bartter syndrome is difficult because the genetic diagnosis can only be confirmed following a clinical diagnosis in infants. Reports of prenatal diagnosis and treatment of antenatal Bartter syndrome are limited. Case Presentation: We present the case of a 33-year-old pregnant woman with refractory polyhydramnios at 31 weeks of gestation. There were no structural anomalies or placental problems on ultrasonography; therefore, antenatal Bartter syndrome was suspected. With repeated amniocentesis and indomethacin therapy, the pregnancy continued to 36 weeks of gestation. The clinical features of the infant and subsequent genetic testing confirmed the diagnosis of antenatal Bartter syndrome. The baby was in good clinical condition at the 3-month follow-up visit. Conclusions: For pregnant women with early onset and refractory severe polyhydramnios without morphological anomalies, antenatal Bartter syndrome should be highly suspected.

Calcium Transport in the Kidney and Disease Processes

Calcium is a key ion involved in cardiac and skeletal muscle contractility, nerve function, and skeletal structure. Global calcium balance is affected by parathyroid hormone and vitamin D, and calcium is shuttled between the extracellular space and the bone matrix compartment dynamically. The kidney plays an important role in whole-body calcium balance. Abnormalities in the kidney transport proteins alter the renal excretion of calcium. Various hormonal and regulatory pathways have evolved that regulate the renal handling of calcium to maintain the serum calcium within defined limits despite dynamic changes in dietary calcium intake. Dysregulation of renal calcium transport can occur pharmacologically, hormonally, and via genetic mutations in key proteins in various nephron segments resulting in several disease processes. This review focuses on the regulation transport of calcium in the nephron. Genetic diseases affecting the renal handling of calcium that can potentially lead to changes in the serum calcium concentration are reviewed.

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.