A Novel Splicing Mutation in SLC12A3 Associated With Gitelman Syndrome and Idiopathic Intracranial Hypertension

R158Q and G212S, novel pathogenic compound heterozygous variants in SLC12A3 of Gitelman syndrome

The dysfunction of Na⁺-Cl^- cotransporter (NCC) caused by mutations in solute carrier family12, member 3 gene (SLC12A3) primarily causes Gitelman syndrome (GS). In identifying the pathogenicity of R158Q and G212S variants of SLC12A3, we evaluated the pathogenicity by bioinformatic, expression, and localization analysis of two variants from a patient in our cohort. The prediction of mutant protein showed that p.R158Q and p.G212S could alter protein's three-dimensional structure. Western blot showed a decrease of mutant Ncc. Immunofluorescence of the two mutations revealed a diffuse positive staining below the plasma membrane. Meanwhile, we conducted a compound heterozygous model-Ncc R156Q/G210S mice corresponding to human NCC R158Q/G212S. Ncc^R156Q/G210S mice clearly exhibited typical GS features, including hypokalemia, hypomagnesemia, and increased fractional excretion of K⁺ and Mg²⁺ with a normal blood pressure level, which made Ncc^R156Q/G210S mice an optimal mouse model for further study of GS. A dramatic decrease and abnormal localization of the mutant Ncc in distal convoluted tubules contributed to the phenotype. The hydrochlorothiazide test showed a loss of function of mutant Ncc in Ncc^R156Q/G210S mice. These findings indicated that R158Q and G212S variants of SLC12A3 were pathogenic variants of GS.

Diagnosis of idiopathic intracranial hypertension - the importance of excluding secondary causes: A systematic review

BACKGROUND

Idiopathic intracranial hypertension is characterized by increased intracranial pressure without any pathological findings on neuroimaging, except for signs of high intracranial pressure. Before diagnosing idiopathic intracranial hypertension secondary causes of increased intracranial pressure should be excluded.

OBJECTIVE

to characterize the phenotype of patients with secondary intracranial hypertension and to identify possible risk factors for secondary intracranial hypertension.

METHODS

We have systematically searched the PubMed database. The publications were analyzed according to the patient phenotype, age, gender, comorbidities, body mass index/weight status, and additional medication. The results are summarized in four categories: medication, infection, hormonal induced intracranial hypertension and miscellaneous groups of diseases related to sIH.

RESULTS

We identified 105 eligible papers which included 272 cases. There were 49.6% pediatric cases. Among the adult group,70.9% were women. A total of 40.4% of all cases were obese or overweight, 27% among adults and 13.4% among pediatric cases. Increased BMI and recent weight gain, anemia, renal diseases and hypertension were the most frequent comorbidities related to sIH.

CONCLUSION

Among sIH patients, 40.4% were obese or overweight; two thirds were women. We recommend that even patients with a typical IIH phenotype should be screened for secondary causes.

Genes causing congenital hydrocephalus: Their chromosomal characteristics of telomere proximity and DNA compositions

Congenital hydrocephalus (CH) is caused by genetic mutations, but whether factors impacting human genetic mutations are disease-specific remains elusive. Given two factors associated with high mutation rates, we reviewed how many disease-susceptible genes match with (i) proximity to telomeres or (ii) high adenine and thymine (A + T) content in human CH as compared to other disorders of the central nervous system (CNS). We extracted genomic information using a genome data viewer. Importantly, 98 of 108 genes causing CH satisfied (i) or (ii), resulting in >90% matching rate. However, such a high accordance no longer sustained as we checked two factors in Alzheimer's disease (AD) and/or familial Parkinson's disease (fPD), resulting in 84% and 59% matching, respectively. A disease-specific matching of telomere proximity or high A + T content predicts causative genes of CH much better than neurodegenerative diseases and other CNS conditions, likely due to sufficient number of known causative genes (n = 108) and precise determination and classification of the genotype and phenotype. Our analysis suggests a need for identifying genetic basis of both factors before human clinical studies, to prioritize putative genes found in preclinical models into the likely (meeting at least one) and more likely candidate (meeting both), which predisposes human genes to mutations.

Learning Physiology From Inherited Kidney Disorders

The identification of genes causing inherited kidney diseases yielded crucial insights in the molecular basis of disease and improved our understanding of physiological processes that operate in the kidney. Monogenic kidney disorders are caused by mutations in genes coding for a large variety of proteins including receptors, channels and transporters, enzymes, transcription factors, and structural components, operating in specialized cell types that perform highly regulated homeostatic functions. Common variants in some of these genes are also associated with complex traits, as evidenced by genome-wide association studies in the general population. In this review, we discuss how the molecular genetics of inherited disorders affecting different tubular segments of the nephron improved our understanding of various transport processes and of their involvement in homeostasis, while providing novel therapeutic targets. These include inherited disorders causing a dysfunction of the proximal tubule (renal Fanconi syndrome), with emphasis on epithelial differentiation and receptor-mediated endocytosis, or affecting the reabsorption of glucose, the handling of uric acid, and the reabsorption of sodium, calcium, and magnesium along the kidney tubule.

Characteristics and Follow-Up of 13 pedigrees with Gitelman syndrome

CONTEXT

Gitelman syndrome (GS) is clinically heterogeneous. The genotype and phenotype correlation has not been well established. Though the long-term prognosis is considered to be favorable, hypokalemia is difficult to cure.

OBJECTIVE

To analyze the clinical and genetic characteristics and treatment of all members of 13 GS pedigrees.

METHODS

Thirteen pedigrees (86 members, 17 GS patients) were enrolled. Symptoms and management, laboratory findings, and genotype-phenotype associations among all the members were analyzed.

RESULTS

The average ages at onset and diagnosis were 27.6 ± 10.2 years and 37.9 ± 11.6 years, respectively. Males were an average of 10 years younger and exhibited more profound hypokalemia than females. Eighteen mutations were detected. Two novel mutations (p.W939X, p.G212S) were predicted to be pathogenic by bioinformatic analysis. GS patients exhibited the lowest blood pressure, serum K⁺, Mg²⁺, and 24-h urinary Ca²⁺ levels. Although blood pressure, serum K⁺ and Mg²⁺ levels were normal in heterozygous carriers, 24-h urinary Na⁺ excretion was significantly increased. During follow-up, only 41.2% of patients reached a normal serum K⁺ level. Over 80% of patients achieved a normal Mg²⁺ level. Patients were taking 2-3 medications at higher doses than usual prescription to stabilize their K⁺ levels. Six patients were taking spironolactone simultaneously, but no significant elevation in the serum K⁺ level was observed.

CONCLUSION

The phenotypic variability of GS and therapeutic strategies deserve further research to improve GS diagnosis and prognosis. Even heterozygous carriers exhibited increased 24-h Na⁺ urine excretion, which may make them more susceptible to diuretic-induced hypokalemia.

Gitelman Syndrome: A Rare Cause of Seizure Disorder and a Systematic Review

Gitelman syndrome is one of the few inherited causes of metabolic alkalosis due to salt losing tubulopathy. It is caused by tubular defects at the level of distal convoluted tubules, mimicking a thiazide-like tumor. It usually presents in late childhood or in teenage as nonspecific weakness, fatigability, polyuria, and polydipsia but very rarely with seizures. It is classically associated with hypokalemia, hypomagnesemia, hypocalciuria, hyperreninemia, and hyperaldosteronism. However, less frequently, it can present with normal magnesium levels. It is even rarer to find normomagnesemic patients of GS who develop seizures as the main complication since hypomagnesemia is considered the principal etiology of abnormal foci of seizure-related brain activity in GS cases. Interestingly, patients with GS are oftentimes diagnosed during pregnancy when the classic electrolyte pattern consistent with GS is noticed. Our case presents GS with normal serum magnesium in a patient, with seizures being the main clinical presentation. We also did a comprehensive literature review of 122 reported cases to show the prevalence of normal magnesium in GS cases and an overview of clinical and biochemical variability in GS. We suggest that further studies and in-depth analysis are required to understand the pathophysiology of seizures in GS patients with both normal and low magnesium levels.

Gitelman syndrome: consensus and guidance from a Kidney Disease: Improving Global Outcomes (KDIGO) Controversies Conference

Gitelman syndrome (GS) is a rare, salt-losing tubulopathy characterized by hypokalemic metabolic alkalosis with hypomagnesemia and hypocalciuria. The disease is recessively inherited, caused by inactivating mutations in the SLC12A3 gene that encodes the thiazide-sensitive sodium-chloride cotransporter (NCC). GS is usually detected during adolescence or adulthood, either fortuitously or in association with mild or nonspecific symptoms or both. The disease is characterized by high phenotypic variability and a significant reduction in the quality of life, and it may be associated with severe manifestations. GS is usually managed by a liberal salt intake together with oral magnesium and potassium supplements. A general problem in rare diseases is the lack of high quality evidence to inform diagnosis, prognosis, and management. We report here on the current state of knowledge related to the diagnostic evaluation, follow-up, management, and treatment of GS; identify knowledge gaps; and propose a research agenda to substantiate a number of issues related to GS. This expert consensus statement aims to establish an initial framework to enable clinical auditing and thus improve quality control of care.

Gitelman Syndrome in a School Boy Who Presented with Generalized Convulsion and Had a R642H/R642W Mutation in the SLC12A3 Gene

An 8-year-old Japanese boy presented with a generalized convulsion. He had hypokalemia (serum K 2.4 mEq/L), hypomagnesemia, and metabolic alkalosis (BE 5.7 mmol/L). In addition, his plasma renin activity was elevated. He was tentatively diagnosed with epilepsy on the basis of the electroencephalogram findings and was treated by potassium L-aspartate and carbamazepine to control the hypokalemia and seizure, respectively. However, a year later, the patient continued to have similar abnormal laboratory data. A presumptive diagnosis of Gitelman syndrome (GS) was then made and the patient's peripheral blood mononuclear cells were subjected to sequence analysis of the SLC12A3 gene, which encodes a thiazide-sensitive sodium-chloride cotransporter. The patient was found to have compound heterozygous mutations, namely, R642H inherited from his father and R642W inherited from his mother. Thus, if a patient shows persistent hypokalemia and metabolic alkalosis, GS must be considered, even if the patient exhibits atypical clinical symptoms.

Idiopathic intracranial hypertension associated with either primary or secondary aldosteronism

BACKGROUND

Idiopathic intracranial hypertension (IIH) is a syndrome consisting of headache, visual field defects and papilledema of uncertain etiology. The prospect was raised previously as to an association between aldosteronism and increased intracranial pressure in 2 middle-aged women with IIH and primary aldosteronism (PAL). Since then, 2 additional adults were identified and 2 other cases were reported from the United Kingdom, whereas 6 cases of IIH and secondary aldosteronism (SAL) in children have been reported in the English literature worldwide.

METHODS

A retrospective analysis of cases from author institutions and published literature comparing clinical features, laboratory findings and therapeutic interventions in these 12 cases.

RESULTS

The female-to-male ratio was 10:2. The mean age of the PAL patients was 49 ± 3 years-all hypertensive, with adrenal pathology in most. The mean age of the SAL patients was 11 ± 2 years-mostly normotensive, with 3 having Bartter's and 2 Gitelman syndromes, and 1 renal congenital hypoplasia. Plasma aldosterone levels were elevated (31 ± 5 ng/dL) in PAL and SAL, whereas plasma renin activity was suppressed in PAL. Hypokalemia (3.2 ± 0.2 mmol/L), hypomagnesemia (1.6 ± 0.3 mg/dL) and a putative metabolic alkalosis (serum HCO3 30 ± 1 mmol/L) were observed. IIH symptoms were controlled by spironolactone in 5, amiloride in 1, correction of hypokalemia and hypomagnesemia in 2, discontinuation of nonsteroidal anti-inflammatory drugs in 2, and reduction of body weight in 1. One patient required serial lumbar punctures, 2 a ventriculoperitoneal shunt, whereas all 3 patients with adrenal adenoma underwent surgical resection.

CONCLUSIONS

An association between IIH and PAL occurs in hypertensive middle-aged women, whereas normotensive girls having an inherited renal tubular defect may have IIH with SAL. Patients with IIH should be evaluated for aldosteronism and considered for spironolactone therapy.

A case of Gitelman syndrome associated with idiopathic intracranial hypertension

An 18-year-old woman with Gitelman syndrome (GS) associated with idiopathic intracranial hypertension (IIH) is described. She was obese and showed a 10 kg gain in body weight over a period of 8 months. She presented with headache, vomiting, and diplopia. She had bilateral papilledema, and right abducens palsy. CSF examination demonstrated high pressure (over 320 mmH(2)O) with normal cytochemistry. Brain MRI was normal. She showed mild alkalosis, hypokalemia, hypomagnesemia, increased plasma renin activity, and normal blood pressure. Two heterozygous mutations in the SLC12A3 gene were identified. Therefore, she was diagnosed as GS with IIH. We should keep in mind the possible occurrence of IIH in GS.

Diagnosis of diseases of steroid hormone production, metabolism and action

Biochemical tests have been the basis for investigations of disorders affecting steroid hormones. In recent years it has been possible however to study the genes that determine functional enzymes, cofactors, receptors, transcription factors and signaling systems that are involved in the process. Analyses of mutations are available as a diagnostic service for only a few of these genes although research laboratories may be able to provide a service. Both biochemical and genetic research have brought to light new disorders. Some genes for transcription factors involved in the development of the endocrine organs have also been identified and patients with defects in these processes have been found. This paper will review general aspects of adrenal disorders with emphasis on clinical and laboratory findings. As with all endocrine investigations there are few single measurements that provide a definitive answer to a diagnosis. Timing of samples in relation to age, gender and time of day needs to be considered.

Transcriptional and functional analyses of SLC12A3 mutations: new clues for the pathogenesis of Gitelman syndrome

Gitelman syndrome (GS) is a recessive salt-losing tubulopathy that is caused by mutations in the SLC12A3 gene that encodes the sodium-chloride co-transporter (NCC). GS is characterized by significant inter- and intrafamilial phenotype variability, with early onset and/or severe clinical manifestations in some patients. No correlations between the disease variability and the position/nature of SLC12A3 mutations have been investigated thus far. In this study, extensive mutational analyses of SLC12A3 were performed in 27 patients with GS, including genomic DNA sequencing, multiplex ligation-dependent probe amplification, cDNA analysis, and quantification of allele-specific transcripts, in parallel with functional analyses in Xenopus laevis oocytes and detailed phenotyping. Twenty-six SLC12A3 mutations were identified in 25 patients with GS, including eight novel (detection rate 80%). Transcript analysis demonstrated that splicing mutations of SLC12A3 lead to frameshifted mRNA subject to degradation by nonsense-mediated decay. Heterologous expression documented a novel class of NCC mutants with defective intrinsic transport activity. A subgroup of patients presented with early onset, growth retardation, and/or detrimental manifestations, confirming the potential severity of GS. The mutations that were associated with a severe presentation were the combination at least for one allele of a missplicing resulting in a truncated transcript that was downregulated by nonsense-mediated decay or a nonfunctional, cell surface-absent mutant. The most recurrent mutation on the second allele was a newly described NCC mutant that affected the functional properties of the co-transporter. These data suggest that the nature/position of SLC12A3 mutation, combined with male gender, is a determinant factor in the severity of GS and provide new insights in the underlying pathogenic mechanisms of the disease.