Resistance to Insulin in Patients with Gitelman Syndrome and a Subtle Intermediate Phenotype in Heterozygous Carriers: A Cross-Sectional Study

Hypomagnesemia induces impaired glucose metabolism and insulin resistance in patients with Gitelman syndrome

AIMS

To investigate the contributing factors of impaired glucose metabolism in patients with Gitelman syndrome (GS).

METHODS

This study collected clinical data and conducted oral glucose tolerance tests (OGTT) on 44 GS patients, with 60 non-functioning adrenal incidentaloma (NFAI) patients serving as controls.

RESULTS

Compared to NFAI patients, GS patients exhibited a significantly higher prevalence of impaired glucose metabolism (P < 0.001), with markedly higher homeostasis model assessment of insulin resistance (HOMA-IR), lower quantitative insulin sensitivity check index, and lower Matsuda index compared to NFAI patients (all P < 0.001). The homeostasis model assessment for β cells was elevated (P = 0.003) and the insulin secretion sensitivity index-2 was reduced (P = 0.007) in GS patients relative to NFAI patients. Logistic regression identified hypomagnesemia (P = 0.042) and hypokalemia (P = 0.046) as risk factors for dysregulated glucose metabolism in GS patients. Additionally, higher body mass index (BMI) (P = 0.016) and hypomagnesemia (P = 0.045) were significant contributors to IR. Notably, GS patients had a steeper linear regression slope between BMI and HOMA-IR compared to NFAI patients (P = 0.016). A negative linear correlation between plasma magnesium and BMI (R = 0.54, P < 0.001) was found in GS patients.

CONCLUSIONS

Hypomagnesemia may contribute to increased BMI, exacerbating impaired glucose metabolism and IR in GS patients.

The role of SLC12A3 gene variant c.1964G > A in co-existing Gitelman syndrome and unilateral limb paralysis: a case report and literature review

We report a Gitelman syndrome (GS) pedigree from a Chinese family. The proband, a middle-aged man, presented with hypokalemia, hypomagnesemia, and unilateral limb paralysis. After a comprehensive evaluation, peripheral neuropathy and the cranial or spinal cord disorders were ruled out. Genetic testing identified a homozygous c.1964G > A variant in the SLC12A3 gene. Despite potassium and magnesium supplementation, the patient's clinical symptoms persisted. Additionally, 13 heterozygous family members, including his parents, showed no typical GS manifestations. However, the proband's two brothers, who also carried the same homozygous mutation and exhibited hypokalemia and hypomagnesemia, did not develop unilateral limb paralysis. This case suggests that the c.1964G > A variant may be associated with a severe GS phenotype, including unilateral limb paralysis. Clinicians should be aware of the diagnostic challenges and therapeutic limitations in managing GS, particularly in patients with severe manifestations. Genetic testing is essential for accurate diagnosis, and ongoing monitoring and symptomatic management are critical to improving the quality of life for affected individuals.

Functional evaluation of novel compound heterozygous variants in SLC12A3 of Gitelman syndrome

BACKGROUND

Gitelman syndrome (GS) is an inherited renal tubular disorder characterized by hypokalemic alkalosis and hypomagnesemia, due to biallelic pathogenic variants in the solute carrier family 12 member 3 (SLC12A3) gene encoding a sodium-chloride (Na-Cl) cotransporter (NCC). This work aimed at identifying SLC12A3 variants in the GS pedigree and reveal the effect of the mutations on protein structure and function.

METHODS

Whole-exome sequencing (WES) and Sanger sequencing were performed in the pedigree. Configuration prediction of two mutant NCC proteins were achieved using SWISS-MODEL. The SLC12A3 missense mutants were generated by site-specific mutagenesis, and the protein expression, location and Na+ uptake activity were assessed by using the HEK293T cell line.

RESULTS

Genetic analysis identified novel compound heterozygous SLC12A3 variants (c.718G > A/p.E240K and c.2675T > C/p.L892P) in the patient with typical GS phenotype. Both of her parents, elder brother and her son carried the heterozygous p.L892P variant, but only the elder brother exhibited mild hypokalemia. Bioinformatics tools predicted that both mutations were highly species conserved and pathogenic. The prediction of mutant protein indicated that p.E240K and p.L892P altered protein's secondary and three-dimensional (3D) structure and stability. Functional experiments revealed decreased protein expression and Na+ uptake activity caused by these two variants, especially the p.L892P variant.

CONCLUSION

Our study presents the genetic and functional evidence for the novel compound heterozygous loss-of-function variants in SLC12A3 that may synergistically cuase GS, and expands the mutation spectrum of SLC12A3 variants in patients with GS.

Cardiovascular and arrhythmic manifestations of Bartter's and Gitelman's syndromes: do not forget the heart. A narrative literature review

Bartter's and Gitelman's syndromes (BS/GS) are genetically determined kidney tubulopathies leading to electrolyte and neurohormonal abnormalities. Although considered benign entities, major adverse cardiovascular events may complicate both syndromes, in form of ventricular arrhythmias leading to palpitations, syncope or sudden cardiac death, microvascular cardiac dysfunction and exercise-induced myocardial contractile deficit. The mechanisms leading to cardiovascular complications are not only driven by chronic electrolyte abnormalities, i.e. chronic hypokalemia and hypomagnesemia, but also by neurohormonal alterations that can impair vascular tone and myocardial contractility. In presence of triggering factors, BS/GS patients may experience a spectrum of cardiac arrhythmias necessitating prompt diagnosis and treatment. The aim of this review is to explore the pathophysiological mechanisms of BS and GS, highlighting those responsible for cardiovascular involvement, and to analyze the spectrum of associated cardiovascular complications. This highlights the importance of an integrated shared management of GS/BS patients between Nephrologist and Cardiologist.

Genotype-phenotype correlations in children with Gitelman syndrome

BACKGROUND

This study aimed to analyze genotype-phenotype correlations in children with Gitelman syndrome (GS).

METHODS

This multicenter retrospective study included 50 Korean children diagnosed with SLC12A3 variants in one or both alleles and the typical laboratory findings of GS. Genetic testing was performed using the Sanger sequencing except for one patient.

RESULTS

The median age at the diagnosis was 10.5 years (interquartile range, 6.8;14.1), and 41 patients were followed up for a median duration of 5.4 years (interquartile range, 4.1;9.6). A total of 30 different SLC12A3 variants were identified. Of the patients, 34 (68%) had biallelic variants, and 16 (32%) had monoallelic variants on examination. Among the patients with biallelic variants, those (n = 12) with the truncating variants in one or both alleles had lower serum chloride levels (92.2 ± 3.2 vs. 96.5 ± 3.8 mMol/L, P = 0.002) at onset, as well as lower serum potassium levels (3.0 ± 0.4 vs. 3.4 ± 0.3 mMol/L, P = 0.016), and lower serum chloride levels (96.1 ± 1.9 vs. 98.3 ± 3.0 mMol/L, P = 0.049) during follow-up than those without truncating variants (n = 22). Patients with monoallelic variants on examination showed similar phenotypes and treatment responsiveness to those with biallelic variants.

CONCLUSIONS

Patients with GS who had truncating variants in one or both alleles had more severe electrolyte abnormalities than those without truncating variants. Patients with GS who had monoallelic SLC12A3 variants on examination had almost the same phenotypes, response to treatment, and long-term prognosis as those with biallelic variants.

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.

Effects of thiazides and new findings on kidney stones and dysglycemic side effects

Thiazide and thiazide-like diuretics (thiazides) belong to the most frequently prescribed drugs worldwide. By virtue of their natriuretic and vasodilating properties, thiazides effectively lower blood pressure and prevent adverse cardiovascular outcomes. In addition, through their unique characteristic of reducing urine calcium, thiazides are also widely employed for the prevention of kidney stone recurrence and reduction of bone fracture risk. Since their introduction into clinical medicine in the early 1960s, thiazides have been recognized for their association with metabolic side effects, particularly impaired glucose tolerance, and new-onset diabetes mellitus. Numerous hypotheses have been advanced to explain thiazide-induced glucose intolerance, yet underlying mechanisms remain poorly defined. Regrettably, the lack of understanding and unpredictability of these side effects has prompted numerous physicians to refrain from prescribing these effective, inexpensive, and widely accessible drugs. In this review, we outline the pharmacology and mechanism of action of thiazides, highlight recent advances in the understanding of thiazide-induced glucose intolerance, and provide an up-to-date discussion on the role of thiazides in kidney stone prevention.

The first compound heterozygous mutations in SLC12A3 and PDX1 genes: a unique presentation of Gitelman syndrome with distinct insulin resistance and familial diabetes insights

Background

Gitelman Syndrome (GS) patients frequently exhibit disrupted glucose metabolism, attributed to hypokalemia, hypomagnesemia and heightened aldosterone. This study delved into the genetic underpinnings linked to insulin resistance and diabetes in a GS patient, contextualized within his family history.

Methods

The hydrochlorothiazide and furosemide loading test were performed to ascertain the presence of GS. Oral glucose tolerance test (OGTT) evaluated glucose metabolism and insulin sensitivity. Whole-exome sequencing, validated by Sanger sequencing, was employed to confirm gene mutations, which were then tracked among the patient's relatives.

Results

Symptoms and laboratory examination confirmed the clinical diagnosis of GS. Comprehensive whole-exome sequencing, augmented by Sanger sequencing validation, revealed a compound heterozygous mutation within the SLC12A3 gene (c.1108G>C in exon 9, c.676G>A in exon 5 and c.2398G>A in exon 20) in the patient. The OGTT affirmed diabetes and heightened insulin resistance, distinct from previous patients with GS we evaluated. Further genetic analysis identified a missense heterozygous mutation (c.97C>G in exon 1) within the PDX1 gene, inherited from the patient's diabetic mother without GS. Furthermore, the patient's brother, with impaired glucose tolerance but regular potassium levels, also bore this mutation, hinting at additional impacts of the PDX1 gene mutation on glucose metabolism regulation beyond the known impacts of GS.

Conclusion

This study unveils unprecedented compound heterozygous mutations in the SLC12A3 and PDX1 genes in a GS patient. These findings illuminate the potential complex genetic factors influencing glucose metabolism disruptions in GS.

Take-home message

This research uncovers a novel combination of SLC12A3 and PDX1 gene mutations in a Gitelman Syndrome patient, revealing intricate genetic factors that potentially disrupt glucose metabolism and shedding light on familial diabetes links.

Gitelman syndrome combined with diabetes mellitus: A case report and literature review

RATIONALE

Gitelman syndrome (GS) is an uncommon autosomal recessive tubulopathy resulting from a functional deletion mutation in the SLC12A3 gene. Its onset is typically insidious and challenging to discern, and it is characterized by hypokalemia, metabolic alkalosis, and reduced urinary calcium excretion. There is limited literature on the diagnosis and management of GS in individuals with concomitant diabetes.

PATIENT CONCERNS

A 36-year-old male patient with a longstanding history of diabetes exhibited suboptimal glycemic control. Additionally, he presented with concurrent findings of hypokalemia, hypomagnesemia, hypocalciuria, and metabolic alkalosis.

DIAGNOSIS

Building upon the patient's clinical manifestations and extensive laboratory evaluations, we conducted thorough genetic testing, leading to the identification of a compound heterozygous mutation within the SLC12A3 gene. This definitive finding confirmed the diagnosis of GS.

INTERVENTIONS

We have formulated a detailed medication regimen for patients, encompassing personalized selection of hypoglycemic medications and targeted electrolyte supplementation.

OUTCOMES

Following 1 week of comprehensive therapeutic intervention, the patient's serum potassium level effectively normalized to 3.79 mmol/L, blood glucose parameters stabilized, and there was significant alleviation of clinical symptoms.

LESSONS

GS has a hidden onset and requires early diagnosis and intervention based on patient related symptoms and laboratory indicators in clinical practice, and personalized medication plans need to be provided according to the specific situation of the patient.

Novel SLC12A3 gene mutations and clinical characteristics in two pedigrees with Gitelman syndrome

OBJECTIVE

Gitelman syndrome (GS) is an autosomal recessive tubulopathy resulting from inactivating mutations in the SLC12A3 gene that encodes the thiazide-sensitive sodium-chloride cotransporter (NCC). To date, more than 500 mutations have been identified in the SLC12A3 gene. In this study, we identified two new mutations in the SLC12A3 gene in two Chinese GS pedigrees.

DESIGN, PATIENTS AND MEASUREMENTS

The clinical characteristics and laboratory examination of two suspected GS patients in our hospital were analyzed. In addition, two pedigrees including 11 members and 2 patients underwent SLC12A3 gene analysis.

RESULTS

Both patients were middle-aged women with characteristics of hypokalemic metabolic alkalosis, hypomagnesemia, low level of urinary calcium and the elevated levels of renin-angiotensin-aldosterone system. So, they were clinically diagnosed as GS. Patient 2 also had type 2 diabetes and Graves' disease. Both patients were found to carry two mutations of SLC12A3 gene by Sanger direct sequencing, which were all compound heterozygous mutations. We identified three mutations in these two Chinese GS pedigrees, one of which was c.179C>T (Thr60Met). The novel c.2159G>T (p. Gly720Val) and c.2675T>C (p. Leu892Pro) mutations were strongly predicted to be pathogenic using four network programs-Polyphen-2, SIFT, Mutation Taster and LRT.

CONCLUSIONS

We identified two novel SLC12A3 genetic variant [c.2159G>T (p.Gly720Val) and c.2675T>C (p.Leu892Pro)] in two Chinese GS pedigrees. The discovery of new mutations has enriched the spectrum of SLC12A3 genotypes.

Thiazides Attenuate Insulin Secretion Through Inhibition of Mitochondrial Carbonic Anhydrase 5b in β -Islet Cells in Mice

SIGNIFICANCE STATEMENT

Thiazide diuretics (thiazides) are among the most widely prescribed drugs worldwide, but their use is associated with glucose intolerance and new-onset diabetes mellitus. The molecular mechanisms remain elusive. Our study reveals that thiazides attenuate insulin secretion through inhibition of the mitochondrial carbonic anhydrase isoform 5b (CA5b) in pancreatic β cells. We furthermore discovered that pancreatic β cells express only one functional carbonic anhydrase isoform, CA5b, which is critical in replenishing oxaloacetate in the mitochondrial tricarboxylic acid (TCA) cycle (anaplerosis). These findings explain the mechanism for thiazide-induced glucose intolerance and reveal a fundamental role of CA5b in TCA cycle anaplerosis and insulin secretion in β cells.

BACKGROUND

Thiazide diuretics are associated with glucose intolerance and new-onset diabetes mellitus. Previous studies demonstrated that thiazides attenuate insulin secretion, but the molecular mechanisms remain elusive. We hypothesized that thiazides attenuate insulin secretion via one of the known molecular thiazide targets in β cells.

METHODS

We performed static insulin secretion experiments with islets of wild-type, Sodium/chloride co-transporter (NCC) (SLC12A3), and sodium-driven chloride/bicarbonate exchanger (NDCBE) (SLC4A8) knock-out (KO) mice and with murine Min6 cells with individual knockdown of carbonic anhydrase (CA) isoforms to identify the molecular target of thiazides in β cells. CA isoform 5b (CA5b) KO mice were then used to assess the role of the putative thiazide target CA5b in β -cell function and in mediating thiazide sensitivity in vitro and in vivo .

RESULTS

Thiazides inhibited glucose- and sulfonylurea-stimulated insulin secretion in islets and Min6 cells at pharmacologically relevant concentrations. Inhibition of insulin secretion by thiazides was CO 2 /HCO 3- -dependent, not additive to unselective CA inhibition with acetazolamide, and independent of extracellular potassium. By contrast, insulin secretion was unaltered in islets of mice lacking the known molecular thiazide targets NCC or NDCBE. CA expression profiling with subsequent knockdown of individual CA isoforms suggested mitochondrial CA5b as a molecular target. In support of these findings, thiazides significantly attenuated Krebs cycle anaplerosis through reduction of mitochondrial oxaloacetate synthesis. CA5b KO mice were resistant to thiazide-induced glucose intolerance, and thiazides did not alter insulin secretion in CA5b KO islets.

CONCLUSIONS

Thiazides attenuate insulin secretion via inhibition of the mitochondrial CA5b isoform in β cells of mice.

Two Brothers from Macedonia with Gitelman Syndrome

Gitelman syndrome (GS) is a rare renal tubulopathy with an autosomal recessive mode of inheritance, caused by biallelic pathogenic variants in the SLC12A3 gene. The clinical features may overlap with other disorders, such as Bartter syndrome type 3, HNF1B nephropathy or even mitochondrial disease, but can be distinguished by molecular genetic analysis. Here we report on two preschool brothers, who presented with a several months' history of episodes of carpopedal spasms and muscle aches. The biochemical analyses revealed hypokalemia and hypomagnesemia without metabolic alkalosis. A 24-h urine sample demonstrated hypocalciuria. The molecular analyses showed that both patients were heterozygous for 3 (likely) pathogenic variants in SLC12A3: c.1805_1806del; p. (Tyr602Cysfs*31), c.2660+1G>A and c.2944 A>T; p. (Ile982Phe). Analysis of the parents showed that the mother was heterozygous for the c.2944 A>T p.(Ile982Phe) variant, and the father carried the other 2 variants (c.1805_1806del and c.2660+1G>A). Herein we present two children in a family from N. Macedonia with clinical manifestations and electrolyte imbalances suggestive of GS. The results of the tubulopathy next generation sequencing (NGS) panel confirmed the diagnosis. The boys are treated with a high salt diet and oral potassium and magnesium supplements.

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

Introduction

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

Methods

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

Results

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

Conclusion

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

Tracing angiotensin II's yin-yang effects on cardiovascular-renal pathophysiology

Adverse changes in cardiovascular and renal systems are major contributors to overall morbidity and mortality. Human cardiovascular and renal systems exhibit a complex network of positive and negative feedback that is reflected in the control of vascular tone via angiotensin II (Ang II) based signaling. This review will examine in some depth, the multiple components and processes that control the status and reflect the health of these various cardiovascular and renal systems, such as pathways associated to monomeric G proteins, RhoA/Rho kinase system and ERK, oxidative stress and NO balance. It will specifically emphasize the "yin-yang" nature of Ang II signaling by comparing and contrasting the effects and activity of various systems, pathways and components found in hypertension to those found in Gitelman's and Bartter's syndromes (GS/BS), two rare autosomal recessive tubulopathies characterized by electrolytic imbalance, metabolic alkalosis, sodium wasting and prominent activation of the renin-angiotensin-aldosterone system. Notwithstanding the activation of the renin-angiotensin-aldosterone system, GS/BS are normo-hypotensive and protected from cardiovascular-renal remodeling and therefore can be considered the mirror image, the opposite of hypertension.

Long-Read Sequencing Identifies Novel Pathogenic Intronic Variants in Gitelman Syndrome

BACKGROUND

Gitelman syndrome is a salt-losing tubulopathy characterized by hypokalemic alkalosis and hypomagnesemia. It is caused by homozygous recessive or compound heterozygous pathogenic variants in SLC12A3 , which encodes the Na + -Cl - cotransporter (NCC). In up to 10% of patients with Gitelman syndrome, current genetic techniques detect only one specific pathogenic variant. This study aimed to identify a second pathogenic variant in introns, splice sites, or promoters to increase the diagnostic yield.

METHODS

Long-read sequencing of SLC12A3 was performed in 67 DNA samples from individuals with suspected Gitelman syndrome in whom a single likely pathogenic or pathogenic variant was previously detected. In addition, we sequenced DNA samples from 28 individuals with one variant of uncertain significance or no candidate variant. Midigene splice assays assessed the pathogenicity of novel intronic variants.

RESULTS

A second likely pathogenic/pathogenic variant was identified in 45 (67%) patients. Those with two likely pathogenic/pathogenic variants had a more severe electrolyte phenotype than other patients. Of the 45 patients, 16 had intronic variants outside of canonic splice sites (nine variants, mostly deep intronic, six novel), whereas 29 patients had an exonic variant or canonic splice site variant. Midigene splice assays of the previously known c.1670-191C>T variant and intronic candidate variants demonstrated aberrant splicing patterns.

CONCLUSION

Intronic pathogenic variants explain an important part of the missing heritability in Gitelman syndrome. Long-read sequencing should be considered in diagnostic workflows for Gitelman syndrome.

Loss of Slc12a2 specifically in pancreatic β-cells drives metabolic syndrome in mice

The risk of type-2 diabetes and cardiovascular disease is higher in subjects with metabolic syndrome, a cluster of clinical conditions characterized by obesity, impaired glucose metabolism, hyperinsulinemia, hyperlipidemia and hypertension. Diuretics are frequently used to treat hypertension in these patients, however, their use has long been associated with poor metabolic outcomes which cannot be fully explained by their diuretic effects. Here, we show that mice lacking the diuretic-sensitive Na+K+2Cl-cotransporter-1 Nkcc1 (Slc12a2) in insulin-secreting β-cells of the pancreatic islet (Nkcc1βKO) have reduced in vitro insulin responses to glucose. This is associated with islet hypoplasia at the expense of fewer and smaller β-cells. Remarkably, Nkcc1βKO mice excessively gain weight and progressive metabolic syndrome when fed a standard chow diet ad libitum. This is characterized by impaired hepatic insulin receptor activation and altered lipid metabolism. Indeed, overweight Nkcc1βKO but not lean mice had fasting and fed hyperglycemia, hypertriglyceridemia and non-alcoholic steatohepatitis. Notably, fasting hyperinsulinemia was detected earlier than hyperglycemia, insulin resistance, glucose intolerance and increased hepatic de novo gluconeogenesis. Therefore, our data provide evidence supporting the novel hypothesis that primary β-cell defects related to Nkcc1-regulated intracellular Cl-homeostasis and β-cell growth can result in the development of metabolic syndrome shedding light into additional potential mechanisms whereby chronic diuretic use may have adverse effects on metabolic homeostasis in susceptible individuals.

Parathyroid hormone and phosphate homeostasis in patients with Bartter and Gitelman syndrome: an international cross-sectional study

BACKGROUND

Small cohort studies have reported high parathyroid hormone (PTH) levels in patients with Bartter syndrome and lower serum phosphate levels have anecdotally been reported in patients with Gitelman syndrome. In this cross-sectional study, we assessed PTH and phosphate homeostasis in a large cohort of patients with salt-losing tubulopathies.

METHODS

Clinical and laboratory data of 589 patients with Bartter and Gitelman syndrome were provided by members of the European Rare Kidney Diseases Reference Network (ERKNet) and the European Society for Paediatric Nephrology (ESPN).

RESULTS

A total of 285 patients with Bartter syndrome and 304 patients with Gitelman syndrome were included for analysis. Patients with Bartter syndrome type I and II had the highest median PTH level (7.5 pmol/L) and 56% had hyperparathyroidism (PTH >7.0 pmol/L). Serum calcium was slightly lower in Bartter syndrome type I and II patients with hyperparathyroidism (2.42 versus 2.49 mmol/L; P = .038) compared to those with normal PTH levels and correlated inversely with PTH (rs -0.253; P = .009). Serum phosphate and urinary phosphate excretion did not correlate with PTH. Overall, 22% of patients had low serum phosphate levels (phosphate-standard deviation score < -2), with the highest prevalence in patients with Bartter syndrome type III (32%). Serum phosphate correlated with tubular maximum reabsorption of phosphate/glomerular filtration rate (TmP/GFR) (rs 0.699; P < .001), suggesting renal phosphate wasting.

CONCLUSIONS

Hyperparathyroidism is frequent in patients with Bartter syndrome type I and II. Low serum phosphate is observed in a significant number of patients with Bartter and Gitelman syndrome and appears associated with renal phosphate wasting.

The genetic spectrum of Gitelman(-like) syndromes

PURPOSE OF REVIEW

Gitelman syndrome is a recessive salt-wasting disorder characterized by hypomagnesemia, hypokalemia, metabolic alkalosis and hypocalciuria. The majority of patients are explained by mutations and deletions in the SLC12A3 gene, encoding the Na+-Cl--co-transporter (NCC). Recently, additional genetic causes of Gitelman-like syndromes have been identified that should be considered in genetic screening. This review aims to provide a comprehensive overview of the clinical, genetic and mechanistic aspects of Gitelman(-like) syndromes.

RECENT FINDINGS

Disturbed Na+ reabsorption in the distal convoluted tubule (DCT) is associated with hypomagnesemia and hypokalemic alkalosis. In Gitelman syndrome, loss-of-function mutations in SLC12A3 cause impaired NCC-mediated Na+ reabsorption. In addition, patients with mutations in CLCKNB, KCNJ10, FXYD2 or HNF1B may present with a similar phenotype, as these mutations indirectly reduce NCC activity. Furthermore, genetic investigations of patients with Na+-wasting tubulopathy have resulted in the identification of pathogenic variants in MT-TI, MT-TF, KCNJ16 and ATP1A1. These novel findings highlight the importance of cell metabolism and basolateral membrane potential for Na+ reabsorption in the DCT.

SUMMARY

Altogether, these findings extend the genetic spectrum of Gitelman-like electrolyte alterations. Genetic testing of patients with hypomagnesemia and hypokalemia should cover a panel of genes involved in Gitelman-like syndromes, including the mitochondrial genome.

METFORMIN-AND GLICLAZIDE-BASED DIABETES TREATMENT EXPERIENCE IN A PATIENT WITH GITELMAN SYNDROME

INTRODUCTION

Gitelman Syndrome (GS) is a genetic tubulopathy frequently linked with insulin resistance. The possibility of developing Diabetes Mellitus (DM) in GS increases with the causes of insulin resistance. Hypokalemia is one of the most common electrolyte disorders in GS, and most diabetic drugs can cause hypokalemia. Considering this dilemma, we presented a DM treatment experience in a GS case.

CASE PRESENTATION

A 47-year-old male GS patient with a potassium-rich diet complained of weight loss and dry mouth for 2-3 months. The laboratory tests revealed a higher HbA1c level, as high as 11.8%. The remaining abnormal laboratory test results (hypokalemia, hypomagnesemia, metabolic alkalosis) indicated a patient with GS. The patient was placed on a metformin+gliclazide-based treatment. Oral potassium and magnesium supplementation were started for the patient whose hypokalemia increased in the first control, and the potassium dose was doubled in the third control. In the first-month follow-up visit, it was observed that the blood potassium level was improved, and hyperglycemia was optimized.

CONCLUSION

In brief, any treatment for DM can be selected in GS patients with DM by performing frequent electrolyte monitoring. Like our case, oral potassium supplementation was adequate for the metformin + gliclazide combination-coincidence hypokalemia.

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.

Novel mutations of the SLC12A3 gene in patients with Gitelman syndrome

Mutations in the SLC12A3 gene have been reported to cause Gitelman syndrome (GS). This study aimed to investigate the genetic mutations and clinical features of patients with GS. Four pedigrees (4 GS patients and 14 family members) were enrolled. The symptoms, laboratory results, management, and genotypes were analyzed. Genomic DNA was screened for gene variations using Sanger sequencing. DNA sequences were compared with reference sequences. The effects of the mutations were predicted using prediction tools (Mutation Taster, PolyPhen-2, SIFT, and PROVEAN). Genetic analysis revealed six genetic variants of SLC12A3, including three novel heterozygous mutations (c.2T > C, c.1609C > T, c.3055G > A) and three previously characterized mutations (c.1456G > A, c.2542G > A, c.1077C > G). These mutations were predicted to exert a damaging effect based on predictive in silico tools. GS patients had low blood pressure and low levels of serum K⁺, serum Mg²⁺, and 24-h urinary Ca²⁺ but high levels of 24-h urinary K⁺. These clinical manifestations and genotypes were consistent with the diagnostic criteria of GS. The study described the phenotypes and genotypes of 4 pedigrees involving GS patients, demonstrating the importance of SLC12A3 gene screening for GS.

Gitelman syndrome with a novel frameshift variant in SLC12A3 gene accompanied by chronic kidney disease and type 2 diabetes mellitus

Gitelman syndrome is an autosomal recessive genetic disease caused by pathogenic variants in SLC12A3 resulting in the loss of function of the Na-Cl co-transporter (NCC) in the distal tubules. Hypokalemia and diuretic effects can cause secondary type 2 diabetes and renal function decline. Here, we present the case of a 49-year-old male patient with chronic persistent treatment-resistant hypokalemia for the past 13 years who had been receiving treatment for type 2 diabetes mellitus for 6 years. He was referred to our department due to the presence of urinary protein, impaired renal function, high renin activity, and hyperaldosteronism. Laboratory test results showed hypokalemia, hypomagnesemia, hypocalciuria, and metabolic alkalosis. Using next-generation and Sanger sequencing, we identified a novel stop-gain variant (NM_000339.3:c.137del [p.His47fs]) and a missense variant (NM_000339.3:c.2927C > T [p.Ser976Phe]) in the SLC12A3 gene. This novel pathogenic variant was located at the intracellular N-terminus of the NCC. Based on these findings, the patient was diagnosed with Gitelman syndrome. The use of next-generation sequencing facilitated the exclusion of diseases with similar clinical symptoms.

Allele-specific RT-PCR for the rapid detection of recurrent SLC12A3 mutations for Gitelman syndrome

Recurrent mutations in the SLC12A3 gene responsible for autosomal recessive Gitelman syndrome (GS) are frequently reported, but the exact prevalence is unknown. The rapid detection of recurrent SLC12A3 mutations may help in the early diagnosis of GS. This study was aimed to investigate the prevalence of recurrent SLC12A3 mutations in a Taiwan cohort of GS families and develop a simple and rapid method to detect recurrent SLC12A3 mutations. One hundred and thirty independent Taiwan families with genetically confirmed GS were consecutively enrolled to define recurrent SLC12A3 mutations and determine their prevalence. Using TaqMan probe-based real-time polymerase chain reaction, we designed a mutation detection plate with all recurrent mutations. We validated this mutation detection plate and tested its feasibility in newly diagnosed GS patients. A total of 57 mutations in the SLC12A3 gene were identified and 22 including 2 deep intronic mutations were recurrent mutations consisting of 87.1% (242/278, 18 triple) of all allelic mutations. The recurrent mutation-based TaqMan assays were fully validated with excellent sensitivity and specificity in genetically diagnosed GS patients and healthy subjects. In clinical validation, recurrent mutations were recognized in 92.0% of allelic mutations from 12 GS patients within 4 h and all were confirmed by direct sequencing. Recurrent SLC12A3 mutations are very common in Taiwan GS patients and can be rapidly identified by this recurrent mutation-based SLC12A3 mutation plate.

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.

A novel compound heterozygous variant of the SLC12A3 gene in Gitelman syndrome with diabetes and the choices of the appropriate hypoglycemic drugs: a case report

BACKGROUND

Gitelman syndrome (GS) is an autosomal recessive tubulopathy caused by mutations of the SLC12A3 gene. It is characterized by hypokalemic metabolic alkalosis, hypomagnesemia and hypocalciuria. It is universally known that both hypokalemia and hypomagnesemia can influence insulin secretion and insulin resistance, but the exact mechanisms require further study. We identified a novel deletion variant of the SLC12A3 gene and discussed the appropriate hypoglycemic drugs in Gitelman syndrome (GS) patients with type 2 diabetes.

CASE PRESENTATION

A 55-year-old diabetic female patient was hospitalized for evaluation because of paroxysmal general weakness and numbness of extremities for one year. We suspected that she was suffering from GS by initial estimation. Direct Sanger sequencing was used to analyze the causative gene SLC12A3 of GS. Oral glucose tolerance test (OGTT) was carried out to assess the glucose metabolism and insulin resistance status. Genetic analysis revealed that she was a compound heterozygote for a recurrent missense mutation c.179C > T and a novel deletion c.1740delC in SLC12A3, thus her diagnosis of GS was confirmed. The patient was treated with potassium chloride (3.0 g/d) and magnesium chloride (element magnesium 350 mg/d) on the basis of initial treatment of diabetes with hypoglycemic drug (Repaglinide, 3.0 mg/day). However, she developed frequent hypoglycemia after one week. OGTT showed that her glucose metabolism and insulin resistance much improved after potassium and magnesium supplemental therapy. Then we changed the hypoglycemic agent to a dipeptidyl peptidase-4 (DPP-4) inhibitor (Trajenta 5 mg/d), since then her blood glucose level remained normal during two-year of follow-up.

CONCLUSION

We have identified a novel deletion of the SLC12A3 gene and discussed the appropriate hypoglycemic drugs in Gitelman syndrome (GS) patients with type 2 diabetes. We suggested that attention need to be paid to blood glucose monitoring in GS patients, especially when hypokalemia and hypomagnesemia are corrected. Besides, the insufficient blood volume and serum electrolyte disturbance should also be taken into consideration in the selecting hypoglycemic drugs for GS patients.

Heterozygosity for a Pathogenic Variant in SLC12A3 That Causes Autosomal Recessive Gitelman Syndrome Is Associated with Lower Serum Potassium

BACKGROUND

Potassium levels regulate multiple physiologic processes. The heritability of serum potassium level is moderate, with published estimates varying from 17% to 60%, suggesting genetic influences. However, the genetic determinants of potassium levels are not generally known.

METHODS

A whole-exome sequencing association study of serum potassium levels in 5812 subjects of the Old Order Amish was performed. A dietary salt intervention in 533 Amish subjects estimated interaction between p.R642G and sodium intake.

RESULTS

A cluster of variants, spanning approximately 537 kb on chromosome 16q13, was significantly associated with serum potassium levels. Among the associated variants, a known pathogenic variant of autosomal recessive Gitelman syndrome (p.R642G SLC12A3) was most likely causal; there were no homozygotes in our sample. Heterozygosity for p.R642G was also associated with lower chloride levels, but not with sodium levels. Notably, p.R642G showed a novel association with lower serum BUN levels. Heterozygotes for p.R642G had a two-fold higher rate of self-reported bone fractures and had higher resting heart rates on a low-salt diet compared with noncarriers.

CONCLUSIONS

This study provides evidence that heterozygosity for a pathogenic variant in SLC12A3 causing Gitelman syndrome, a canonically recessive disorder, contributes to serum potassium concentration. The findings provide insights into SLC12A3 biology and the effects of heterozygosity on electrolyte homeostasis and related subclinical phenotypes that may have implications for personalized medicine and nutrition.

Mechanisms coupling sodium and magnesium reabsorption in the distal convoluted tubule of the kidney

Hypomagnesaemia is a common feature of renal Na+ wasting disorders such as Gitelman and EAST/SeSAME syndrome. These genetic defects specifically affect Na+ reabsorption in the distal convoluted tubule, where Mg2+ reabsorption is tightly regulated. Apical uptake via TRPM6 Mg2+ channels and basolateral Mg2+ extrusion via a putative Na+ -Mg2+ exchanger determines Mg2+ reabsorption in the distal convoluted tubule. However, the mechanisms that explain the high incidence of hypomagnesaemia in patients with Na+ wasting disorders of the distal convoluted tubule are largely unknown. In this review, we describe three potential mechanisms by which Mg2+ reabsorption in the distal convoluted tubule is linked to Na+ reabsorption. First, decreased activity of the thiazide-sensitive Na+ /Cl- cotransporter (NCC) results in shortening of the segment, reducing the Mg2+ reabsorption capacity. Second, the activity of TRPM6 and NCC are determined by common regulatory pathways. Secondary effects of NCC dysregulation such as hormonal imbalance, therefore, might disturb TRPM6 expression. Third, the basolateral membrane potential, maintained by the K+ permeability and Na+ -K+ -ATPase activity, provides the driving force for Na+ and Mg2+ extrusion. Depolarisation of the basolateral membrane potential in Na+ wasting disorders of the distal convoluted tubule may therefore lead to reduced activity of the putative Na+ -Mg2+ exchanger SLC41A1. Elucidating the interconnections between Mg2+ and Na+ transport in the distal convoluted tubule is hampered by the currently available models. Our analysis indicates that the coupling of Na+ and Mg2+ reabsorption may be multifactorial and that advanced experimental models are required to study the molecular mechanisms.

Investigation of Vestibular Function in Adult Patients with Gitelman Syndrome: Results of an Observational Study

Gitelman syndrome (GS) is a rare salt-losing tubulopathy caused by an inactivating mutation in the SLC12A3 gene, encoding the thiazide-sensitive sodium chloride cotransporter (NCC). Patients with GS frequently complain of vertigo, usually attributed to hypovolemia. Because NCC is also located in the endolymphatic sac, we hypothesized that patients with GS might have vestibular dysfunction. Between April 2013 and September 2016, 20 (22%) out of 90 patients followed at the reference center complained of vertigo in the absence of orthostatic hypotension. Sixteen of them were referred to an otology department for investigation of vestibular function. The vertigo was of short duration and triggered in half of them by head rotation. Seven patients (44%) had a vestibular syndrome. Vestibular syndrome was defined: (1) clinically, as nystagmus triggered by the head shaking test (n = 5); and/or (2) paraclinically, as an abnormal video head impulse test (n = 0), abnormal kinetic test (n = 4) and/or abnormal bithermal caloric test (n = 3). Five patients had associated auditory signs (tinnitus, aural fullness or hearing loss). In conclusion, we found a high frequency of vestibular disorder in GS patients suffering from vertigo, suggesting a role of NCC in the inner ear. Referent physicians of these patients should be aware of this extrarenal manifestation that requires specific investigations and treatment.

Type 2 diabetes mellitus caused by Gitelman syndrome-related hypokalemia: A case report

INTRODUCTION

Gitelman syndrome (GS) is an autosomal-recessive disease caused by SLC12A3 gene mutations. It is characterized by hypokalemic metabolic alkalosis in combination with hypomagnesemia and hypocalciuria. Recently, patients with GS are found at an increased risk for developing type 2 diabetes mellitus (T2DM). However, diagnosis of hyperglycemia in GS patients has not been thoroughly investigated, and family studies on SLC12A3 mutations and glucose metabolism are rare. Whether treatment including potassium and magnesium supplements, and spironolactone can ameliorate impaired glucose tolerance in GS patients, also needs to be investigated.

PATIENT CONCERNS

We examined a 55-year-old Chinese male with intermittent fatigue and persistent hypokalemia for 17 years.

DIAGNOSES

Based on the results of the clinical data, including electrolytes, oral glucose tolerance test (OGTT), and genetic analysis of the SLC12A3 gene, GS and T2DM were newly diagnosed in the patient. Two mutations of the SLC12A3 gene were found in the patient, one was a missense mutation p.N359K in exon 8, and the other was a novel insert mutation p.I262delinsIIGVVSV in exon 6. SLC12A3 genetic analysis and OGTT of 9 other family members within 3 generations were also performed. Older brother, youngest sister, and son of the patient carried the p.N359K mutation in exon 8. The older brother and the youngest sister were diagnosed with T2DM and impaired glucose tolerance by OGTT, respectively.

INTERVENTIONS

The patient was prescribed potassium and magnesium (potassium magnesium aspartate, potassium chloride) oral supplements and spironolactone. The patient was also suggested to maintain a high potassium diet. Acarbose was used to maintain the blood glucose levels.

OUTCOMES

The electrolyte imbalance including hypokalemia and hypomagnesemia, and hyperglycemia were improved with a remission of the clinical manifestations.

CONCLUSION

GS is one of the causes for manifestation of hypokalemia. SLC12A3 genetic analysis plays an important role in diagnosis of GS. Chinese male GS patients characterized with heterozygous SLC12A3 mutation should be careful toward occurrence of T2DM. Moreover, the patients with only 1 SLC12A3 mutant allele should pay regular attention to blood potassium and glucose levels. GS treatment with potassium and magnesium supplements, and spironolactone can improve impaired glucose metabolism.