A novel epithelial sodium channel gamma-subunit de novo frameshift mutation leads to Liddle syndrome

Liddle syndrome misdiagnosed as primary aldosteronism is caused by inaccurate aldosterone-rennin detection while a novel SCNN1G mutation is discovered

PURPOSE

Through describing the confusing misdiagnosis process of Liddle syndrome, we try to reveal the importance of accurate aldosterone-renin detection and a genetic test for Liddle syndrome.

METHODS

We found a family of hypertension and hypokalaemia with the proband of a 21-year-old female who had been misdiagnosed as primary aldosteronism (PA). She presented with high aldosterone and low renin levels. Aldosterone is not suppressed in the saline infusion test and captopril challenge test. However, treatment with a standard dose of spironolactone has no blood pressure improvement effect. A heterozygous variant of SCNN1G was found with whole exome sequencing and Liddle syndrome is indicated. Treatment with amiloride was effective. We rechecked aldosterone-renin levels with two different aldosterone and renin test kits. Clinical features and the mutant gene SCNN1G of each family member were determined by the Sanger method.

RESULTS

The two kits had nearly opposite results. Among those Liddle syndrome patients confirmed by a genetic test, for Test kit A all ARR were screened positive while for test kit B negative. It seems Test kit B is consistent with the diagnosis while test kit A misleads the diagnosis. A novel SCNN1G mutation, c.1729 C > T, was found in this family, which introduce a premature stop codon in the γ subunit in the epithelial Na⁺ channel (ENaC) and resulted in a deletion of 72 amino acids at the carboxyl end.

CONCLUSION

inaccurate ARR detection might misdiagnose Liddle syndrome. A Gene test is an important method for the diagnosis of Liddle syndrome. A novel SCNN1G missense mutation, c.1729 C > T, is found in a Chinese family.

ANKRD36 Is Involved in Hypertension by Altering Expression of ENaC Genes

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Pediatric Liddle Syndrome Caused by a Novel SCNN1G Variant in a Chinese Family and Characterized by Early-Onset Hypertension

BACKGROUND

Liddle syndrome (LS), an autosomal dominant disorder, is a common monogenic hypertension in pediatrics. In this study, we reported a novel SCNN1G variant in a Chinese family with pediatric LS, and conduct a systematic review of epithelial sodium channel (ENaC)-gene-positive LS cases to conclude the clinical genetic features of LS in childhood.

METHODS

Next-generation sequencing and in silico analysis were performed in the proband to discover candidate variants. Sanger sequencing was used to identify the predicted likely pathogenic variant. LS patients in this family were treated with amiloride. The Medline database was searched to summarize clinical features of pediatric LS cases whose age at genetic diagnosis was not more than 18 years.

RESULTS

Genetic analysis identified a novel SCNN1G missense variant (c.1874C>T, p.Pro625Leu) in the proband with LS in childhood. In silico analysis revealed this heterozygous variant was highly conserved and deleterious. A total of 38 publications described pediatric LS associated with 25 pathogenic variants in SCNN1B and SCNN1G in 54 children. Despite the phenotypic heterogeneity, early-onset hypertension is the most common feature. All LS patients in this family or the reviewed cases showed significantly improvements in hypertension and hypokalemia after treatment with ENaC inhibitors.

CONCLUSIONS

This study identified a novel SCNN1G missense variant in a patient with pediatric LS, expanding the genetic spectrum of SCNN1G and demonstrating the PY motif of γ-ENaC as a potential mutant region. Early identification and specific management of LS in children and adolescents are important to prevent the development of hypertensive end-organ disease.

Liddle syndrome due to a novel mutation in the γ subunit of the epithelial sodium channel (ENaC) in family from Russia: a case report

Background

Liddle syndrome is a monogenic disease with autosomal dominant inheritance. Basic characteristics of this disease are hypertension, reduced concentration of aldosterone and renin activity, as well as increased excretion of potassium leading to low level of potassium in serum and metabolic alkalosis. The cause of Liddle syndrome is missense or frameshift mutations in SCNN1A, SCNN1B, or SCNN1G genes that encode epithelial sodium channel subunits.

Case presentation

We describe a family with Liddle syndrome from Russia. 15-year-old proband has arterial hypertension, hypokalemia, hyporeninemia, metabolic alkalosis, but aldosterone level is within the normal range. At 12 years of age, arterial hypertension was noticed for the first time. We identified novel frameshift mutation c.1769delG (p.Gly590Alafs) in SCNN1G, which encodes the γ subunit of ENaC in vertebrates. The father and younger sister also harbor this heterozygous deletion. Treatment with amiloride of proband and his sister did not normalize the blood pressure, but normalized level of plasma renin activity.

Conclusions

Our results expand the mutational spectrum of Liddle syndrome and provide further proof that the conserved PY motif is crucial to control of ENaC activity. Genetic analysis has implications for the management of hypertension, specific treatment with amiloride and counselling in families with Liddle syndrome.

A Novel Frameshift Mutation of SCNN1G Causing Liddle Syndrome with Normokalemia

BACKGROUND

Liddle syndrome (LS) is an autosomal dominant disorder caused by single-gene mutations of the epithelial sodium channel (ENaC). It is characterized by early-onset hypertension, spontaneous hypokalemia and low plasma renin and aldosterone concentrations. In this study, we reported an LS pedigree with normokalemia resulting from a novel SCNN1G frameshift mutation.

METHODS

Peripheral blood samples were collected from the proband and eight family members for DNA extraction. Next-generation sequencing and Sanger sequencing were performed to identify the SCNN1G mutation. Clinical examinations were used to comprehensively evaluate the phenotypes of two patients.

RESULTS

Genetic analysis identified a novel SCNN1G frameshift mutation, p.Arg586Valfs*598, in the proband with LS. This heterozygous frameshift mutation generated a premature stop codon and deleted the vital PY motif of ENaC. The same mutation was present in his elder brother with LS, and his mother without any LS symptoms. Biochemical examination showed normokalemia in the three mutation carriers. The mutation identified was not found in any other family members, 100 hypertensives, or 100 healthy controls.

CONCLUSIONS

Our study identified a novel SCNN1G frameshift mutation in a Chinese family with LS, expanding the genetic spectrum of SCNN1G. Genetic testing helped us identify LS with a pathogenic mutation when the genotypes and phenotype were not completely consistent because of the hypokalemia. This case emphasizes that once a proband is diagnosed with LS by genetic testing, family genetic sequencing is necessary for early diagnosis and intervention for other family members, to protect against severe cardiovascular complications.

A family with Liddle's syndrome caused by a new c.1721 deletion mutation in the epithelial sodium channel β-subunit

A 19-year-old male with early refractory hypertension, hypokalemia, serum potassium level of 3.4 mmol/l and hypoaldosteronemia was indicated in the present study. According to the results of laboratory tests and examinations, the patient was suspected of having Liddle's syndrome (LS). Genetic analysis of SCNN1B revealed a deletion mutation (c.1721delC). This mutation caused a length extension of SCNN1B coding sequence, which resulted in p.Pro574HisfsX675. A total of 34 family members were enrolled in the study and 29 of these family members underwent genetic testing. A total of 10 family members were clinically diagnosed with hypertension. Notably, 5 family members shared the same gene mutation as the proband and all cases with the mutation had hypertension. Blood pressure of the gene mutation carriers was well controlled by tailored treatment. In conclusion, a patient with early onset and refractory hypertension, hypokalemia and hypoaldosteronemia was diagnosed clinically and genetically with LS. Notably, a novel mutation (c.1721delC) was identified by DNA analysis. The present findings indicate that genetic analysis is useful, not only in the diagnosis of LS, but also in designing a tailored treatment.

Liddle Syndrome: Review of the Literature and Description of a New Case

Liddle syndrome is an inherited form of low-renin hypertension, transmitted with an autosomal dominant pattern. The molecular basis of Liddle syndrome resides in germline mutations of the SCNN1A, SCNN1B and SCNN1G genes, encoding the α, β, and γ-subunits of the epithelial Na⁺ channel (ENaC), respectively. To date, 31 different causative mutations have been reported in 72 families from four continents. The majority of the substitutions cause an increased expression of the channel at the distal nephron apical membrane, with subsequent enhanced renal sodium reabsorption. The most common clinical presentation of the disease is early onset hypertension, hypokalemia, metabolic alkalosis, suppressed plasma renin activity and low plasma aldosterone. Consequently, treatment of Liddle syndrome is based on the administration of ENaC blockers, amiloride and triamterene. Herein, we discuss the genetic basis, clinical presentation, diagnosis and treatment of Liddle syndrome. Finally, we report a new case in an Italian family, caused by a SCNN1B p.Pro618Leu substitution.

A novel frameshift mutation of epithelial sodium channel β-subunit leads to Liddle syndrome in an isolated case

OBJECTIVE

Liddle syndrome, an autosomal dominant form of monogenic hypertension, is attributed to mutations in the genes encoding β and γ subunits (SCNN1B and SCNN1G) of the epithelial sodium channel (ENaC). The aim of this study was to search for pathogenic mutations of SCNN1B and SCNN1G in an adolescent under the impression of Liddle syndrome and no family history of hypertension.

DESIGN AND PATIENTS

We screened the C-terminus of SCNN1B and SCNN1G in an adolescent with poorly controlled hypertension who was clinically diagnosed as having Liddle syndrome. We also screened for the mutation in his parents, 100 hypertensive patients and 100 controls.

RESULTS

Genetic analysis of SCNN1B revealed a frameshift mutation induced by insertion of an additional cytosine into a string of six located between codons 617 and 618, which is predicted to introduce a new termination codon at position 621 and produce a protein truncated by 20 amino acids. This frameshift mutation was not detected in the patient's parents, the 100 hypertensive patients or the 100 controls, indicating that this is a de novo mutation and not a common genetic polymorphism. There was no mutation of SCNN1G in any of the individuals examined.

CONCLUSION

Based on direct DNA sequencing, we identified a novel frameshift mutation in the βENaC gene in an isolated case of Liddle syndrome. Confirmation of the diagnosis and effective tailored treatment in the patient were achieved, implying that genetic testing is a useful tool to diagnose Liddle syndrome.

Association of epithelial sodium channel β-subunit common polymorphism with essential hypertension families in a Chinese population

The purpose of this study was to investigate whether common polymorphisms in the C-terminus of SCNN1B gene encoding the β-subunit of epithelial sodium channel are associated with essential hypertension (EH) in Chinese hypertensive families. A total of 433 subjects from 102 EH families were recruited. Biochemical and anthropometric indices and systematic screening of the C-terminus of SCNN1B were performed. Four single nucleotide polymorphisms (SNPs) were found. Homozygotes for the common A allele at rs3743966 had on average a 12.06 mmHg higher SBP and a 7.43 mmHg higher DBP than homozygotes for the rarer T allele. AA + AT genotype of rs3743966 was also found to maybe a risk factor of hypertension by logistic regression and transmission/disequilibrium test. AA + AT genotype of rs3743966 maybe a risk factor of EH. In conclusion, there was a significant association between the rs3743966 SNP in intron 12 and EH in Chinese hypertensive families.

Molecular genetics of Liddle's syndrome

Liddle's syndrome, an autosomal dominant form of monogenic hypertension, is characterized by salt-sensitive hypertension with early penetrance, hypokalemia, metabolic alkalosis, suppression of plasma rennin activity and aldosterone secretion, and a clear-cut response to epithelial sodium channel (ENaC) blockers but not spironolactone therapy. Our understanding of ENaCs and Na(+) transport defects has expanded greatly over the past two decades and provides detailed insight into the molecular basis of Liddle's syndrome. In this review, we offer an overview of recent advances in understanding the molecular genetics of Liddle's syndrome, involving mutation analysis, molecular mechanisms and genetic testing. The ENaC in the distal nephron is composed of α, β and γ subunits that share similar structures. Mutations associated with Liddle's syndrome are positioned in either β or γ subunits and disturb or truncate a conserved proline-rich sequence (i.e., PY motif), leading to constitutive activation of the ENaC. Genetic testing has made it possible to make accurate diagnoses and develop tailored therapies for mutation carriers.

Phenotype-genotype analysis in two Chinese families with Liddle syndrome

The families with Liddle syndrome show marked phenotypic variation in blood pressure, serum potassium and other clinical manifestations. Here we analyzed the correlation of genotype-phenotype in two Chinese families with Liddle syndrome. The sequence of C-terminus of SCNN1B and SCNN1G were screened in the two families with likely Liddle syndrome. In addition to hypertension and hypokalemia, one of the two pedigrees had sudden death in their family members, so the exons of 428 reported genes-related to cardiovascular diseases were screened as well in the family. A heterozygous βR566X nonsense mutation was found in the proband-1 in the first pedigree, and the proband's sister and father. They showed mild phenotype with hypertension under control. In contrast, two of the four previous studies report that the mutation causes severe phenotype. A heterozygous βR597PfrX607 frameshift mutation was identified in the proband-2 in the second pedigree, showing malignant phenotype including resistant hypertension, hypokalemia, higher PRA and plasma angiotensin II levels. Both the proband-2 and the proband-2's father had sudden death in their twenties, but no meaningful mutations were found by screening of the exons in 428 cardiovascular disease-related genes. However, the same mutation has been related to moderate phenotype in previous studies. Our results confirmed that the phenotypes of Liddle syndrome are varied significantly even with the same mutation. The mechanisms why the same mutation causes very different phenotype need to be explored because intervention of these modifiers may change the disease course and prognosis accordingly.

Status quo of annotation of human disease variants

BACKGROUND

The ever on-going technical developments in Next Generation Sequencing have led to an increase in detected disease related mutations. Many bioinformatics approaches exist to analyse these variants, and of those the methods that use 3D structure information generally outperform those that do not use this information. 3D structure information today is available for about twenty percent of the human exome, and homology modelling can double that fraction. This percentage is rapidly increasing so that we can expect to analyse the majority of all human exome variants in the near future using protein structure information.

RESULTS

We collected a test dataset of well-described mutations in proteins for which 3D-structure information is available. This test dataset was used to analyse the possibilities and the limitations of methods based on sequence information alone, hybrid methods, machine learning based methods, and structure based methods.

CONCLUSIONS

Our analysis shows that the use of structural features improves the classification of mutations. This study suggests strategies for future analyses of disease causing mutations, and it suggests which bioinformatics approaches should be developed to make progress in this field.

A family with Liddle syndrome caused by a novel missense mutation in the PY motif of the beta-subunit of the epithelial sodium channel

OBJECTIVE

To identify the gene mutation in β and γ subunits of the epithelial sodium channel (ENaC) in an adolescent and family members with Liddle syndrome, an autosomal dominant form of secondary hypertension.

STUDY DESIGN

We screened an adolescent with severe hypertension who was clinically diagnosed with Liddle syndrome for mutations in the C-terminus of the SCNN1B and SCNN1G genes. We also screened for these mutations in his family members, in 100 hypertensive patients, and in 100 controls.

RESULTS

The index case, a 14-year-old boy, was diagnosed with Liddle syndrome by the identification of a novel missense mutation, P614L, in the PY motif of the β subunit of the ENaC. Testing of relatives considered at risk revealed 6 subjects heterozygous for the mutation. All genetically affected subjects had a history of severe hypertension as well as hypokalemia. No other variants in the β or γ subunits of the ENaC were detected.

CONCLUSION

Based on direct DNA sequencing, we have detected a novel mutation that causes Liddle syndrome. This confirms the diagnosis and helps guide effective therapy for this adolescent and his affected relatives. These findings provide further evidence that the conserved PY motif is critical to regulation of ENaC activity.

Liddle syndrome in a Serbian family and literature review of underlying mutations

Severe and reproducible low-renin hypertension responsive to salt restriction and amiloride-thiazide therapy in a 13-year-old otherwise asymptomatic boy suggested Liddle syndrome. This assumption was strengthened by a positive family history of hypertension poorly responsive to conventional treatment or sudden deaths under 40 years of age in four generations. DNA analysis of the beta and gamma subunits of the epithelial sodium channel revealed a heterozygous mutation c.C1852T (p.Pro618Ser) in the SCNN1B gene in the patient and in both his hypertensive mother and uncle. A PubMed search revealed 21 different disease-causing mutations reported to date, all but two clustering in the cytoplasmic C-terminal regions of either beta (16 mutations) or gamma (5) subunit, leading to a three- to eightfold increase in the amiloride-sensitive sodium current. Inter- and intrafamilial variability in both hypertension and hypokalemia were disclosed, which may not be obligatory among the subjects carrying a Liddle mutation.

CONCLUSION

Liddle syndrome should be considered as a cause of hypertension in children or adolescents particularly with suppressed renin activity. Early diagnosis and appropriately tailored treatment avoid complications of long-term unrecognized or inappropriately managed hypertension.

A clinical phenotype mimicking essential hypertension in a newly discovered family with Liddle's syndrome

BACKGROUND

Liddle's syndrome (LS) is a monogenic form of hypertension simulating a mineralocorticoid excess, and is currently suspected in young hypokalemic hypertensives. The aims of the study were: (i) to evaluate the clinical phenotype of LS in a newly identified Italian family of Sicilian origin carrying a gain-of-function mutation of the β subunit of the epithelial sodium channel (ENaC) (P617L) previously reported by our group in an apparently unrelated Sicilian patient presenting the typical phenotype of LS including hypokalemia; (ii) to determine whether an unknown biological relationship exists between the newly identified family and the family of the proband previously reported.

METHODS

Genetic analysis was performed in the present family, in the individual in which the βP617L mutation was first observed, and in his relatives.

RESULTS

βP617L mutation was identified in the proband and in three maternal relatives. None of them showed hypokalemia. Mild to severe early onset hypertension and left ventricular hypertrophy were present in all of them. Analysis of mitochondrial DNA (mtDNA) and Y chromosome profiles in the present family and in the proband's family previously reported showed the absence of a relationship between them. The availability of only one carrier of the mutation in one of the two families meant that a genetic analysis able to assess a founder effect was not feasible.

CONCLUSIONS

LS should be considered in all cases of early onset hypertension, independently of the plasma potassium concentration. The incidence of LS may be greater than is currently thought, because hypokalemia is not invariably present.

Role of the epithelial sodium channel in salt-sensitive hypertension

The epithelial sodium channel (ENaC) is a heteromeric channel composed of three similar but distinct subunits, α, β and γ. This channel is an end-effector in the rennin-angiotensin-aldosterone system and resides in the apical plasma membrane of the renal cortical collecting ducts, where reabsorption of Na(+) through ENaC is the final renal adjustment step for Na(+) balance. Because of its regulation and function, the ENaC plays a critical role in modulating the homeostasis of Na(+) and thus chronic blood pressure. The development of most forms of hypertension requires an increase in Na(+) and water retention. The role of ENaC in developing high blood pressure is exemplified in the gain-of-function mutations in ENaC that cause Liddle's syndrome, a severe but rare form of inheritable hypertension. The evidence obtained from studies using animal models and in human patients indicates that improper Na(+) retention by the kidney elevates blood pressure and induces salt-sensitive hypertension.

Inherited forms of mineralocorticoid hypertension

PURPOSE OF REVIEW

Inherited forms of mineralocorticoid hypertension are a group of monogenic disorders that, although rare, have enlightened our understanding of normal physiology, and subsequent processes implicated in the pathogenesis of 'essential' hypertension. They often present in early life and can be a cause of major morbidity and mortality that can be effectively treated with simple but targeted pharmacological therapy. Interestingly, all the conditions centre on the regulation of sodium transport through its epithelial channel, either directly or through mediators that act via the mineralocorticoid receptor.

RECENT FINDINGS

In recent years, molecular mechanisms of these conditions and their functional consequences have been elucidated. Diagnosis has been facilitated by plasma and urinary biomarkers.

SUMMARY

We provide an overview and diagnostic approach to apparent mineralocorticoid excess, glucocorticoid remediable aldosteronism, familial hyperaldosteronism type 2, Liddle's syndrome, Gordon's syndrome, activating mutations of the mineralocorticoid receptor, generalized glucocorticoid resistance and hypertensive forms of congenital adrenal hyperplasia.

Truncated beta epithelial sodium channel (ENaC) subunits responsible for multi-system pseudohypoaldosteronism support partial activity of ENaC

Aldosterone regulated epithelial sodium channels (ENaC) are constructed of three homologous subunits. Mutations in the alpha-, beta- and gamma-ENaC subunit genes (SCNN1A, SCNN1B and SCNN1G) are associated with multi-system pseudohypoaldosteronism (PHA), and mutations in the PY motif of carboxy-terminal region of beta and gamma subunits are associated with Liddle syndrome of hereditary hypertension. In this study we identified two frameshift mutations in the SCNN1B alleles of a female infant diagnosed with multi-system PHA inherited from her parents. This is the first case of PHA in an Ashkenazi family in Israel. The p.Glu217fs (c.648dupA in exon 4) and p.Tyr306fs (c.915delC in exon 6) mutations produce shortened beta-ENaC subunits with 253 and 317 residues respectively instead of the 640 residues present in beta-ENaC subunit. Expression of cRNAs carrying these mutations in Xenopus oocytes showed that the mutations drastically reduce but do not eliminate ENaC activity. The findings reveal that truncated beta-ENaC subunits are capable of partially supporting intracellular transport of the other two subunits to the membrane and the final assembly of a weakly active channel together with normal alpha- and gamma-ENaC subunits. Moreover, these results enhance our understanding of the long-term consequences of these types of mutations in PHA patients.