Clinical and Genetic Characterization of Patients with Bartter and Gitelman Syndrome

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.

Genetic background of neonatal hypokalemia

Neonatal hypokalemia (defined as a serum potassium level <3.5 mEq/L) is the most common electrolyte disorder encountered in clinical practice. In addition to common secondary causes, primary genetic etiologies are also closely associated with hypokalemia. Currently, a systematic characterization of these genetic disorders is lacking, making early recognition challenging and clinical management uncertain. This review will aid clinicians by summarizing the genetic background of neonatal hypokalemia from two aspects: (1) increased excretion of K+, whereby genetic factors primarily lead to increased renal Na+ influx, decreased H+ efflux, or reduced Cl- influx, ultimately resulting in increased K+ efflux; and (2) decreased extracellular distribution of K+, whereby genetic factors result in abnormalities in transmembrane ion channels, reducing outward potassium currents or generating inward cation leak currents. We describe over ten genetic diseases associated with neonatal hypokalemia, which involve pathogenic variants in dozens of genes and affect multiple target organs, including the kidneys, intestines, and skeletal muscle. For example, in the renal tubules, pathogenic variants in the SLC12A1 gene encoding the Na+-K+-2Cl- cotransporter lead to renal K+ loss, causing Bartter syndrome type I; in intestinal epithelial cells, pathogenic variants in the SLC26A3 gene result in a defective Cl⁻-HCO₃⁻ exchanger, causing congenital chloride diarrhea; and in skeletal muscle, pathogenic variants in the CACNA1S gene impact membrane calcium ion channels resulting in hypokalemic periodic paralysis. Given the wide variety of organs and genetic alterations that can contribute to neonatal hypokalemia, we believe this review will provide valuable insights for clinical diagnosis and treatment.

Utility of Genetic Testing in Adults with CKD: A Systematic Review and Meta-Analysis

Key Points

Diagnostic yield of genetic testing in adults with CKD is 40%. Risk factors including positive family history and extra-kidney features associate with higher diagnostic yield, although young age at testing did not. Seventeen percent of patients who received a genetic diagnosis were reclassified into a different phenotype after testing.

Background

Clinical and pathological confirmation of the diagnosis for CKD has limitations, with up to one third of individuals remaining without a formal diagnosis. Increasingly, data suggest that these limitations can be overcome by genetic testing. The objective of this study was to estimate the diagnostic yield of genetic testing in adults with CKD.

Methods

Cohort studies that report diagnostic yield of genetic testing in adults with CKD published in PubMed or Embase between January 1, 2005, and December 31, 2023, were included. The Joanna Briggs Institute critical appraisal tool for prevalence studies was used to assess bias. Duplicate independent data extraction and a meta-analysis of proportions using generalized linear mixed models were completed.

Results

We included 60 studies with 10,107 adults with CKD who underwent genetic testing. We found a diagnostic yield of 40% (95% confidence interval, 33 to 46); yield varied by CKD subtype with the highest yield of 62% (95% confidence interval, 57 to 68) in cystic kidney disease. Positive family history and presence of extra-kidney features were associated with higher diagnostic yield. Reclassification of the before testing diagnosis after a positive genetic testing result occurred in 17% of the solved cohort. Six studies showed the clinical benefits of genetic testing including cascade testing for family members and treatment changes.

Conclusions

Overall, we show that genetic testing is informative in a high proportion of clinically selected adults with CKD. The study was limited by heterogeneity in reporting, testing technologies, and cohort characteristics.

Clinical Trial registry name and registration number:

International prospective register of systematic reviews (CRD42023386880).

A narrative review of monogenic disorders causing nephrolithiasis and chronic kidney disease

The incidence and prevalence of genetic diseases associated with chronic kidney disease (CKD) have been increasing over the last decades. To identify the monogenic causes of kidney stone disorders linked to CKD, a comprehensive literature review was conducted to identify monogenic genes causing nephrolithiasis and CKD. Among 41 identifiable monogenic causes of nephrolithiasis the following diseases primary hyperoxaluria familial hypomagnesemia with hypercalciuria and nephrocalcinosis, cystinuria; Dent disease, adenine phosphoribosyltransferase deficiency, Lesch-Nyhan syndrome, and idiopathic Infantile hypercalciuria have been linked to CKD and may progress to end-stage kidney disease (ESKD). Autosomal dominant hypocalcemia and Bartter syndrome (BS) are also hereditary kidney diseases that can cause urolithiasis but are rarely associated with CKD. A few BS type III can be complicated with CKD. A substantial number of kidney stone patients with genetic disorders progress to CKD. Genetic diagnosis should be initiated in all children presenting with kidney stones.

Sodium Chloride Cotransporter in Hypertension

The sodium chloride cotransporter (NCC) is essential for electrolyte balance, blood pressure regulation, and pathophysiology of hypertension as it mediates the reabsorption of ultrafiltered sodium in the renal distal convoluted tubule. Given its pivotal role in the maintenance of extracellular fluid volume, the NCC is regulated by a complex network of cellular pathways, which eventually results in either its phosphorylation, enhancing sodium and chloride ion absorption from urines, or dephosphorylation and ubiquitination, which conversely decrease NCC activity. Several factors could influence NCC function, including genetic alterations, hormonal stimuli, and pharmacological treatments. The NCC's central role is also highlighted by several abnormalities resulting from genetic mutations in its gene and consequently in its structure, leading to dysregulation of blood pressure control. In the last decade, among other improvements, the acquisition of knowledge on the NCC and other renal ion channels has been favored by studies on extracellular vesicles (EVs). Dietary sodium and potassium intake are also implicated in the tuning of NCC activity. In this narrative review, we present the main cornerstones and recent evidence related to NCC control, focusing on the context of blood pressure pathophysiology, and promising new therapeutical approaches.

Clinical, genetic characteristics and outcome of four Chinese patients with Bartter syndrome type 3: Further insight into a genotype-phenotype correlation

Aim

To investigate the characteristics of 4 Chinese patients with Bartter syndrome type 3 (BS Type 3).

Methods

The clinical data, genetic analysis, and outcome of four cases with Bartter syndrome type 3 were retrospectively summarised.

Results

Gene sequencing analysis showed that all children carried a compound heterozygous mutation in the CLCNKB gene and were diagnosed with BS type 3. All types of mutations were detected, including two missense mutations, one nonsense mutation, one small fragment deletion mutation, two large deletion mutations and one splice-site mutation. The splice-site mutation c.100 + 1 (IVS2) C > T was novel. Two cases carried large deletion mutations. The patients presented as classic BS with modest manifestations. The most common sign was growth retardation. There was no polyhydramnios or preterm delivery. All cases were treated with potassium chloride supplementation and indomethacin. During long-term follow-up, clinical symptoms and growth retardation improved significantly. Nephrocalcinosis or renal dysfunction was not observed.

Conclusion

The clinical manifestations of BS type 3 are mostly presented as cBS. Growth retardation is a common sign. BS type 3 had a good long-term prognosis. There were various types of mutations in the CLCNKB gene. Large deletions were the most common.

Tubular Diseases and Stones Seen From Pediatric and Adult Nephrology Perspectives

The tubular system of the kidneys is a complex series of morphologic and functional units orchestrating the content of tubular fluid as it flows along the nephron and collecting ducts. Renal tubules maintain body water, regulate electrolytes and acid-base balance, reabsorb precious organic solutes, and eliminate specific metabolites, toxins, and drugs. In addition, decisive mechanisms to adjust blood pressure are governed by the renal tubules. Genetic as well as acquired disorders of these tubular functions may cause serious diseases that manifest both in childhood and adulthood. This article addresses a selection of tubulopathies and the underlying pathomechanisms, while highlighting the important differences in pediatric and adult nephrology care. These range from rare monogenic conditions such as nephrogenic diabetes insipidus, cystinosis, and Bartter syndrome that present in childhood, to the genetic and acquired tubular pathologies causing hypertension or nephrolithiasis that are more prevalent in adults. Both pediatric and adult nephrologists must be aware of these conditions and the age-dependent manifestations that warrant close interaction between the two subspecialties.

A Clinical Workflow for Cost-Saving High-Rate Diagnosis of Genetic Kidney Diseases

SIGNIFICANCE STATEMENT

To optimize the diagnosis of genetic kidney disorders in a cost-effective manner, we developed a workflow based on referral criteria for in-person evaluation at a tertiary center, whole-exome sequencing, reverse phenotyping, and multidisciplinary board analysis. This workflow reached a diagnostic rate of 67%, with 48% confirming and 19% modifying the suspected clinical diagnosis. We obtained a genetic diagnosis in 64% of children and 70% of adults. A modeled cost analysis demonstrated that early genetic testing saves 20% of costs per patient. Real cost analysis on a representative sample of 66 patients demonstrated an actual cost reduction of 41%. This workflow demonstrates feasibility, performance, and economic effect for the diagnosis of genetic kidney diseases in a real-world setting.

BACKGROUND

Whole-exome sequencing (WES) increases the diagnostic rate of genetic kidney disorders, but accessibility, interpretation of results, and costs limit use in daily practice.

METHODS

Univariable analysis of a historical cohort of 392 patients who underwent WES for kidney diseases showed that resistance to treatments, familial history of kidney disease, extrarenal involvement, congenital abnormalities of the kidney and urinary tract and CKD stage ≥G2, two or more cysts per kidney on ultrasound, persistent hyperechoic kidneys or nephrocalcinosis on ultrasound, and persistent metabolic abnormalities were most predictive for genetic diagnosis. We prospectively applied these criteria to select patients in a network of nephrology centers, followed by centralized genetic diagnosis by WES, reverse phenotyping, and multidisciplinary board discussion.

RESULTS

We applied this multistep workflow to 476 patients with eight clinical categories (podocytopathies, collagenopathies, CKD of unknown origin, tubulopathies, ciliopathies, congenital anomalies of the kidney and urinary tract, syndromic CKD, metabolic kidney disorders), obtaining genetic diagnosis for 319 of 476 patients (67.0%) (95% in 21 patients with disease onset during the fetal period or at birth, 64% in 298 pediatric patients, and 70% in 156 adult patients). The suspected clinical diagnosis was confirmed in 48% of the 476 patients and modified in 19%. A modeled cost analysis showed that application of this workflow saved 20% of costs per patient when performed at the beginning of the diagnostic process. Real cost analysis of 66 patients randomly selected from all categories showed actual cost reduction of 41%.

CONCLUSIONS

A diagnostic workflow for genetic kidney diseases that includes WES is cost-saving, especially if implemented early, and is feasible in a real-world setting.

Bartter Syndrome-Related Variants Distribution: Brazilian Data and Its Comparison with Worldwide Cohorts

BACKGROUND

Genetic testing is recommended for accurate diagnosis of Bartter syndrome (BS) and serves as a basis for implementing specific target therapies. However, populations other than Europeans and North Americans are underrepresented in most databases and there are uncertainties in the genotype-phenotype correlation. We studied Brazilian BS patients, an admixed population with diverse ancestry.

METHODS

We evaluated the clinical and mutational profile of this cohort and performed a systematic review of BS mutations from worldwide cohorts.

RESULTS

Twenty-two patients were included; Gitelman syndrome was diagnosed in 2 siblings with antenatal BS and congenital chloride diarrhea in 1 girl. BS was confirmed in 19 patients: BS type 1 in 1 boy (antenatal BS); BS type 4a in 1 girl and BS type 4b in 1 girl, both of them with antenatal BS and neurosensorial deafness; BS type 3 (CLCNKB mutations): 16 cases. The deletion of the entire CLCNKB (1-20 del) was the most frequent variant. Patients carrying the 1-20 del presented earlier manifestations than those with other CLCNKB-mutations and the presence of homozygous 1-20 del was correlated with progressive chronic kidney disease. The prevalence of the 1-20 del in this BS Brazilian cohort was similar to that of Chinese cohorts and individuals of African and Middle Eastern descent from other cohorts.

CONCLUSION

This study expands the genetic spectrum of BS patients with different ethnics, reveals some genotype/phenotype correlations, compares the findings with other cohorts, and provides a systematic review of the literature on the distribution of BS-related variants worldwide.