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

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.

Chemotherapy-induced tubulopathy: a case report series

Acquired tubulopathies are frequently underdiagnosed. They can be characterized by the renal loss of specific electrolytes or organic solutes, suggesting the location of dysfunction. These tubulopathies phenotypically can resemble Bartter or Gitelman syndrome). These syndromes are infrequent, they may present salt loss resembling the effect of thiazides (Gitelman) or loop diuretics (Bartter). They are characterized by potentially severe hypokalemia, associated with metabolic alkalosis, secondary hyperaldosteronism, and often hypomagnesemia. Tubular dysfunction has been described as nephrotoxic effects of platinum-based chemotherapy. We present 4 cases with biochemical signs of tubular dysfunction (Bartter-like/Gitelman-like phenotype) related to chemotherapy.

Recurrent transient severe hypocalcaemia in two siblings with type 1 Bartter syndrome

Type 1 Bartter syndrome causes hypokalaemia and metabolic alkalosis owing to mutation in the SLC12A1 gene. Meanwhile, hypocalcaemia is rare in Bartter syndrome, except in type 5 Bartter syndrome. Herein, we describe two siblings with type 1 Bartter syndrome with recurrent transient severe hypocalcaemia. They each visited our hospital several times with chief complaints of numbness in the limbs, shortness of breath and tetany after stresses such as exercise or fever. Severe hypocalcaemia was also observed with a serum calcium level of approximately 6.0 mg/dL at each visit. The clinical symptoms and abnormalities in laboratory findings quickly improved with rest and intravenous treatment. In a steady state, no severe hypocalcaemia was evident, but serum intact parathyroid hormone (PTH) levels were high. In recent years, a large-scale study has revealed that type 1 and type 2 Bartter syndrome have high PTH values. In addition, there are reports that these patients develop hypocalcaemia due to PTH resistance. Therefore, our patient was also in a PTH-resistant state, and hypocalcaemia was thought to be exacerbated by physical stress. It is not well known that Bartter syndrome patients other than those with type 5 suffer from hypocalcaemia. And hypocalcaemia was not detected in normal examinations under steady-state conditions. Therefore, in patients with type 1 and type 2 Bartter syndrome, severe hypocalcaemia may occur, but may go unnoticed. When following up these patients, the attending physician must keep in mind that such patients are in a PTH-resistant state and that physical stress can cause severe hypocalcaemia.

A case of pseudo-Bartter/Gitelman syndrome caused by long-term laxative abuse, leading to end-stage kidney disease

Pseudo-Bartter/Gitelman syndrome (PBS/PGS) is a disorder that presents with hypokalemia and metabolic alkalosis resembling Gitelman syndrome (GS) due to secondary factors, such as lifestyle and /or medicines. Notably, PBS/PGS is more likely to cause renal dysfunction than GS. We report the first case of PBS/PGS due to long-term laxative abuse leading to end-stage kidney disease (ESKD). The patient was a 49-year-old woman with a history of constipation since school, who had used excessive doses of laxatives on her own judgment for nine years at least from 22 years of age. Two years later, blood tests revealed hypokalemia (serum K 3.1 mEq/L), and nine years later, the patient's renal function began to deteriorate (Cr-eGFR 48.7 mL/min/1.73 m2). Since abuse of laxatives was suspected as the cause, it was changed to the proper dosage of laxatives. At 33 years, the patient developed acute kidney injury (AKI), due to a urinary tract infection, and required intensive treatment, including hemodialysis. Although the patient was eventually weaned off dialysis, the renal function did not recover to pre-AKI levels. In suspected GS, comprehensive genetic testing for renal disease-related genes was performed; however, no obvious pathogenic variants were identified. Thereafter, despite decreasing the laxative doses and potassium supplementation, her renal function continued to decline. At 49 years, the patient developed ESKD and was started on maintenance hemodialysis. PBS/PGS is a disease that can lead to ESKD. An early diagnosis of PBS/PGS is crucial to prevent renal function deterioration, and the underlying causes should be removed immediately.

Pseudo-Gitelman Syndrome Presenting with Hypokalemic Metabolic Alkalosis and Hypocalciuria

Pseudo-Bartter syndrome is a well-known differential diagnosis that needs to be excluded in cases of normotensive hypokalemic metabolic alkalosis. Pseudo-Bartter syndrome and pseudo-Gitelman syndrome are often collectively referred to as pseudo-Bartter/Gitelman syndrome; however, pseudo-Gitelman syndrome should be considered as a separate entity because Gitelman syndrome is characterized by hypocalciuria and hypomagnesemia, while Bartter syndrome is usually associated with hypercalciuria. Herein, we report the cases of two young adult female patients who presented with severe hypokalemic metabolic alkalosis, hypocalciuria, and hypomagnesemia. Diuretic or laxative abuse and self-induced vomiting were absent, and a chloride deficit and remarkable bicarbonaturia were observed. Initial sequencing studies for SLC12A3, CLCKNB, and KCNJ10 revealed no mutations, and whole-exome sequencing revealed no pathogenic variants. The metabolic alkalosis was saline-responsive in one case, and steroid therapy was necessary in the other to relieve chronic tubulointerstitial nephritis, which was diagnosed with kidney biopsy. A new category of pseudo-Gitelman syndrome should be defined, and various etiologies should be investigated.

Heart's Dangerous Symphony: Torsade De Pointes Unleashed by Gitelman Syndrome-Induced Hypomagnesemia

Gitelman syndrome (GS) is a rare autosomal recessive salt-losing renal tubular disorder associated with a mutation of SLC12A3 or CLCNKB genes which encodes the thiazide-sensitive sodium-chloride co-transporter (NCCT) in the distal renal tubule. It is inherited as an autosomal recessive disorder. Hypokalemia, metabolic alkalosis, hypomagnesemia, hypocalciuria, and renin-angiotensin-aldosterone system (RAAS) activation are characteristics of GS. GS is often misdiagnosed or underdiagnosed owing to its low incidence and lack of awareness. Its prevalence is estimated to be around 1-10 per 40,000 people. We report a case of cardiac arrest secondary to torsade de pointes (TdP) because of GS-induced hypomagnesemia. Our case highlights the importance of clinicians being aware of the potential electrolyte abnormalities and complications associated with GS, as it can lead to catastrophic consequences if not identified and corrected earlier.

Gitelman syndrome with Graves' disease leading to rhabdomyolysis: a case report and literature review

A 14-year-old male patient who suffered from limb numbness, fatigue, and hypokalemia was considered Graves' disease (GD) complicated with thyrotoxic periodic paralysis (TPP) at the first diagnosis. Although with the treatment of antithyroid drugs, he developed severe hypokalemia and rhabdomyolysis (RM). Further laboratory tests revealed hypomagnesemia, hypocalciuria, metabolic alkalosis, hyperrenin, and hyperaldosteronemia. Genetic testing revealed compound heterozygous mutations in the SLC12A3 gene (c.506-1G > A, c.1456G > A) encoding the thiazide-sensitive sodium-chloride cotransporter, which presented a definitive diagnosis of Gitelman syndrome (GS). Moreover, gene analysis revealed his mother diagnosed with subclinical hypothyroidism due to Hashimoto's thyroiditis carried the c.506-1G > A heterozygous mutation in the SLC12A3 gene and his father carried the c.1456G > A heterozygous mutation in the SLC12A3 gene. His younger sister who had hypokalemia and hypomagnesemia carried the same compound heterozygous mutations as the proband and was diagnosed with GS as well, but with a much milder clinical presentation and better treatment outcome. This case suggested the potential relationship between GS and GD, clinicians should strengthen the differential diagnosis to avoid missed diagnosis.

Detecting pathogenic deep intronic variants in Gitelman syndrome

Gitelman syndrome (GS) is a rare, autosomal recessive, salt-losing tubulopathy caused by loss of function in the SLC12A3 gene (NM_000339.2), which encodes the natrium chloride cotransporter. The detection of homozygous or compound heterozygous SLC12A3 variants is expected in GS, but 18%-40% of patients with clinical GS carry only one mutant allele. Previous reports identified some pathogenic deep intronic variants in SLC12A3. Here, we report the screening of SLC12A3 deep intronic variants in 13 patients with suspected GS carrying one mutated SLC12A3 allele. Variant screening used the HaloPlex Target Enrichment System Kit capturing whole introns and the promotor region of SLC12A3, followed by SureCall variant analysis. Rare intronic variants (<1% frequency) were identified, and pathogenicity evaluated by the minigene system. Deep intronic variant screening detected seven rare SLC12A3 variants from six patients. Only one variant showed pathogenicity in the minigene system (c.602-16G>A, intron 4) through activation of a cryptic acceptor site. No variants were detected in the promotor region. Deep intronic screening identified only one pathogenic variant in patients with suspected GS carrying monoallelic SLC12A3 variants. Our results suggest that deep intronic variants partially explain the cause of monoallelic variants in patients with GS.

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.

Molecular Basis, Diagnostic Challenges and Therapeutic Approaches of Bartter and Gitelman Syndromes: A Primer for Clinicians

Gitelman and Bartter syndromes are rare inherited diseases that belong to the category of renal tubulopathies. The genes associated with these pathologies encode electrolyte transport proteins located in the nephron, particularly in the Distal Convoluted Tubule and Ascending Loop of Henle. Therefore, both syndromes are characterized by alterations in the secretion and reabsorption processes that occur in these regions. Patients suffer from deficiencies in the concentration of electrolytes in the blood and urine, which leads to different systemic consequences related to these salt-wasting processes. The main clinical features of both syndromes are hypokalemia, hypochloremia, metabolic alkalosis, hyperreninemia and hyperaldosteronism. Despite having a different molecular etiology, Gitelman and Bartter syndromes share a relevant number of clinical symptoms, and they have similar therapeutic approaches. The main basis of their treatment consists of electrolytes supplements accompanied by dietary changes. Specifically for Bartter syndrome, the use of non-steroidal anti-inflammatory drugs is also strongly supported. This review aims to address the latest diagnostic challenges and therapeutic approaches, as well as relevant recent research on the biology of the proteins involved in disease. Finally, we highlight several objectives to continue advancing in the characterization of both etiologies.

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.

Examination of the predicted prevalence of Gitelman syndrome by ethnicity based on genome databases

Gitelman syndrome is an autosomal recessive inherited salt-losing tubulopathy. It has a prevalence of around 1 in 40,000 people, and heterozygous carriers are estimated at approximately 1%, although the exact prevalence is unknown. We estimated the predicted prevalence of Gitelman syndrome based on multiple genome databases, HGVD and jMorp for the Japanese population and gnomAD for other ethnicities, and included all 274 pathogenic missense or nonsense variants registered in HGMD Professional. The frequencies of all these alleles were summed to calculate the total variant allele frequency in SLC12A3. The carrier frequency and the disease prevalence were assumed to be twice and the square of the total allele frequency, respectively, according to the Hardy–Weinberg principle. In the Japanese population, the total carrier frequencies were 0.0948 (9.5%) and 0.0868 (8.7%) and the calculated prevalence was 0.00225 (2.3 in 1000 people) and 0.00188 (1.9 in 1000 people) in HGVD and jMorp, respectively. Other ethnicities showed a prevalence varying from 0.000012 to 0.00083. These findings indicate that the prevalence of Gitelman syndrome in the Japanese population is higher than expected and that some other ethnicities also have a higher prevalence than has previously been considered.

Type 3 antenatal Bartter syndrome presenting with mild polyuria

Bartter syndrome (BS) is a well-recognised inherited tubular dysfunction that causes polyuria, metabolic alkalosis and hypokalaemia. Among BS cases, antenatal/neonatal BS (ABS) usually shows distinct polyhydramnios prenatally and presents features of BS in the early neonatal period. We encountered a premature infant with type 3 ABS presenting with mild polyuria and discuss the pathogenesis of mild polyuria in type 3 ABS. A male infant was born at 31 weeks' gestation. His mother received amniocentesis because of polyhydramnios. Hyponatraemia and hypokalaemia appeared within 3 days after birth. Metabolic alkalosis, hyperreninaemia and hyperaldosteronism were also identified. Temporary polyuria developed at 1 month after birth; however, the mean urine output during hospitalisation was within the normal range. CLCNKB compound heterozygous mutations were confirmed. Polyuria of type 3 ABS may be less severe than in other types of ABS. Lower urine sodium loss may be a characteristic feature of type 3 ABS.

Prenatal biochemical diagnosis of two forms of congenital diarrheal disorders (congenital chloride diarrhea and congenital sodium diarrhea): A series of 12 cases

OBJECTIVE

Congenital diarrheal disorders (CDDs) are a group of rare diseases among which some present as inherited disorders of intestinal electrolyte transportation: congenital chloride diarrhea (CCD) and congenital sodium diarrhea (CSD) with prenatal manifestations, mainly polyhydramnios, leading to premature delivery. Affected neonates present with watery stools, sometimes mistaken as urine, leading to a misdiagnosis of Bartter syndrome. The aim of this study was to study the value of a prenatal biochemical pattern in the case of suspected CDD.

METHODS

We retrospectively studied 12 amniotic fluids of CDD-affected fetuses prenatally suspected and confirmed after birth. Digestive enzymes, proteins, and electrolytes were assayed and showed abnormal biochemical patterns.

RESULTS

The 12 infants (eight CCD- and four CSD-affected) were born prematurely with a normal birth weight. Electrolytes and the Bartter index were normal for all cases. Amniotic fluid enzyme patterns were abnormal: anal leakage for nine, as expected, but vomiting of bile was observed for three infants, for whom an occlusive syndrome required surgery, and thereafter severe complications appeared with a poor prognosis.

CONCLUSION

Amniotic fluid biochemical patterns differentiate CDD from Bartter syndrome. If a vomiting bile pattern is observed, postnatal management should take into account the hypothesis of a most severe complication.

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.

Urinary Extracellular Vesicles and Salt-Losing Tubulopathies: A Proteomic Approach

Renal tubular cells release urinary extracellular vesicles (uEV) that are considered a promising source of molecular markers for renal dysfunction and injury. We investigated uEV proteomes of patients with hereditary salt-losing tubulopathies (SLTs), focusing on those caused by Gitelman and Bartter (BS) syndromes, to provide potential markers for differential diagnosis. Second morning urine was collected from patients with genetically proven SLTs and uEV were isolated by the ultracentrifugation-based protocol. The uEV proteome was run through a diagonal bidimensional electrophoresis (16BAC/SDS-PAGE), to improve hydrophobic protein resolution. Sixteen differential spots from the proteome of two variants (BS2 and BS3) were analysed by nLC-ESI-MS/MS after in-gel tryptic digestion. A total of 167 protein species were identified from 7 BS2 spots and 9 BS3 spot. Most of these proteins were membrane-associated proteins, in particular transmembrane proteins, and were related to typical renal functions. The differential content of some uEV was then validated by immunoblotting. Our work suggests that uEV proteomics represents a promising strategy for the identification of differential SLT proteins. This could play a role in understanding the pathophysiological disease mechanisms and may support the recognition of different syndromes.