Diagnosis of Liddle syndrome by genetic analysis of beta and gamma subunits of epithelial sodium channel--a report of five affected family members

Liddle's syndrome mechanisms, diagnosis and management

Liddle's syndrome is a genetic disorder characterized by hypertension with hypokalemic metabolic alkalosis, hyporeninemia and suppressed aldosterone secretion that often appears early in life. It results from inappropriately elevated sodium reabsorption in the distal nephron. Liddle's syndrome is caused by mutations to subunits of the Epithelial Sodium Channel (ENaC). Among other mechanisms, such mutations typically prevent ubiquitination of these subunits, slowing the rate at which they are internalized from the membrane, resulting in an elevation of channel activity. A minority of Liddle's syndrome mutations, though, result in a complementary effect that also elevates activity by increasing the probability that ENaC channels within the membrane are open. Potassium-sparing diuretics such as amiloride and triamterene reduce ENaC activity, and in combination with a reduced sodium diet can restore normotension and electrolyte imbalance in Liddle's syndrome patients and animal models. Liddle's syndrome can be diagnosed clinically by phenotype and confirmed through genetic testing. This review examines the clinical features of Liddle's syndrome, the differential diagnosis of Liddle's syndrome and differentiation from other genetic diseases with similar phenotype, and what is currently known about the population-level prevalence of Liddle's syndrome. This review gives special focus to the molecular mechanisms of Liddle's syndrome.

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.

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.

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.

The R563Q mutation of the epithelial sodium channel beta-subunit is associated with hypertension

BACKGROUND

A high prevalence of the R563Q mutation of the epithelial sodium channel β-subunit has been reported in South African hypertensives compared with unrelated normotensive controls. To delineate the effects of this mutation against a more uniform genetic background, this study investigated the association of the mutation with hypertension within affected kindreds.

METHODS

Forty-five index patients and members of their kindreds were studied. Blood pressure, serum potassium and the presence of the R563Q mutation were determined.

RESULTS

Of the 136 individuals studied, 89 were heterozygous for the R563Q mutation and 47 homozygous RR. The mean arterial pressure was significantly higher in the R563Q heterozygous group (p = 0.005) after adjusting for gender, race, age and kindred membership. Of the R563Q heterozygous subjects, 71 (80%) had hypertension, while 17 (36%) of the R563Q homozygous RR subjects were hypertensive. Six R563Q heterozygous subjects had hypokalaemia and one R563Q homozygous RR subject had hypokalaemia, but the difference was not statistically significant. Two heterozygous patients had Liddle's syndrome, both occurring during pregnancy.

CONCLUSION

The R563Q mutation of β-ENaC is associated with hypertension within affected kindreds, but does not usually cause the full Liddle's syndrome phenotype.

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

OBJECTIVE

Liddle syndrome is a rare autosomal-dominant monogenic form of hypertension caused by mutations in the C-termini of the epithelial sodium channel beta- or gamma-subunit encoded by SCNN1B and SCNN1G, respectively, and often presenting with a familial history of hypertension. The purpose of this study was to determine whether mutations of SCNN1B or SCNN1G were present in a patient clinically suspected to have Liddle syndrome with no familial history of hypertension.

DESIGN AND PATIENTS

We screened the C-terminus of SCNN1B and SCNN1G in the patient, and also screened for the mutation in his parents, 50 hypertensive patients and 50 controls.

RESULTS

In this patient, no mutations were found in the C-terminus of SCNN1B. However, we found a frameshift mutation caused by an 'AGCTC' deletion at the 583 codon in SCNN1G. The frameshift resulted in a new termination site at the 585 codon of the gamma-subunit and the deletion of its PY motif. Neither his parents nor 50 randomly selected patients with hypertension nor 50 controls have the mutation, indicating that this is a de novo mutation and not a common genetic polymorphism.

CONCLUSION

The de novo mutation is the first reported frameshift of the gamma-subunit causing Liddle syndrome. These data imply that a familial history of hypertension is not an essential criterion for the diagnosis of Liddle syndrome.

Rab4 GTP/GDP modulates amiloride-sensitive sodium channel (ENaC) function in colonic epithelia

The sodium-selective amiloride-sensitive epithelial sodium channel (ENaC) mediates electrogenic sodium re-absorption in tight epithelia. ENaC expression at the plasma membrane requires regulated transport, processing, and macromolecular assembly of subunit proteins in a defined and highly compartmentalized manner. Ras-related Rab GTPases monitor these processes in a highly regulated sequence of events. In order to evaluate the role of Rab proteins in ENaC function, Rab4 wild-type (WT), the GTPase-deficient mutant Rab4Q67L, and the dominant negative GDP-locked mutant Rab4S22N were over-expressed in the colon cancer cell line, HT-29 and amiloride-sensitive currents were recorded. Rab4 over-expression inhibited amiloride-sensitive currents. The effect was reversed by introducing Rab4-neutralizing antibody and Rab4 specific SiRNA. The GDP-locked Rab4 mutant inhibited, while GTPase-deficient mutant moderately stimulated amiloride-sensitive currents. Active status of Rab4 was confirmed by GTP overlay assay, while its expression was verified by Western blotting. Immunoprecipitation and pull-down assay suggest protein-protein interaction between Rab4 and ENaC. In addition, the functional modulation coincides with concomitant changes in ENaC expression at the cell surface and in intracellular pool. We propose that Rab4 is a critical element that regulates ENaC function by mechanisms that include GTP-GDP status, recycling, and expression level. Our observations imply that channel expression in apical membranes of epithelial cell system incorporates RabGTPase as an essential determinant of channel function and adds an exciting paradigm to ENaC therapeutics.

Impact of Nedd4 proteins and serum and glucocorticoid-induced kinases on epithelial Na+ transport in the distal nephron

The precise control of BP occurs via Na(+) homeostasis and involves the precise regulation of the epithelial Na(+) channel (ENaC) in the aldosterone-sensitive distal nephron. This has been corroborated by the linkage of mutations in the genes encoding ENaC subunits and Liddle's syndrome, a heritable form of human hypertension. Mapping of these mutations on ENaC indicated that inactivation of PY motifs is responsible and leads to the proposition that the channel interacts via its PY motifs with the WW domains of the Nedd4/Nedd4-like ubiquitin-protein ligase family. It is now well established that the cell surface expression of ENaC is controlled via ubiquitylation by this protein family and that this ubiquitylation is regulated by the aldosterone-induced protein serum and glucocorticoid induced kinase 1.

A novel epithelial sodium channel beta-subunit mutation associated with hypertensive Liddle syndrome

Low-renin hypertension responsive to amiloride-thiazide therapy in a 4-year-old Afro-Haitian girl suggested Liddle syndrome. Urine steroid profiling substantiated the diagnosis and DNA analysis of the epithelial sodium channel (ENaC) revealed a novel heterozygous beta ENaC mutation in the patient and in her hypertensive father. Liddle syndrome should be considered as a cause of hypertension in young children particularly with suppressed renin activity.

A diagnostic approach for the child with hypertension

Hypertension during childhood is not rare, with an estimated prevalence of between 1% and 2%, although it is often an underrecognized clinical entity. Elevated blood pressure may be a sign of underlying disease or it may represent early onset of essential hypertension. In recent years the measurement of blood pressure has been emphasized as an important component of the routine pediatric physical examination that enables early detection of children with hypertension. In the evaluation of the child with documented blood pressure elevation, confirmation of truly and persistently elevated blood pressure is of the utmost importance. In addition, a thorough history and a full clinical examination are essential. These are followed by appropriate investigations, which are tailored to the age of the child and to the severity of the blood pressure elevation. Investigations should not only focus on a search for the underlying cause, but also on establishing effects on target organs, complications or additional diseases and on assessment of the total cardiovascular risk to the individual patient. An algorithm, which is a valuable diagnostic tool for the diagnosis and management of the child with hypertension, is presented. All children with confirmed hypertension need long-term follow-up, counseling and treatment. In those cases where an underlying cause of the hypertension is detected, the established diagnosis then determines the specific therapy and management.

A new mutation, R563Q, of the beta subunit of the epithelial sodium channel associated with low-renin, low-aldosterone hypertension

OBJECTIVE

To determine the relationship between R563Q, a mutation of the renal epithelial sodium channel, and hypertension.

METHODS

Hypertensive patients with low renin and aldosterone, hypokalemia or resistant hypertension were selected for DNA analysis. Genomic DNA encoding the C-terminal domain of the epithelial sodium channel beta subunit from hypertensives and controls was amplified by polymerase chain reaction and screened for the R563Q mutation by digestion with Sfc1 restriction enzyme, or sequenced.

RESULTS

A previously undescribed mutation, R563Q, of the beta epithelial sodium channel was found in 10 of 139 black hypertensives, but was not present in any of 103 black normotensives, a significant (P = 0.0058) difference in frequency. The frequency of the mutation in the subgroup of black low-renin, low-aldosterone hypertensives (four of 14) was significantly (P = 0.0001) greater than in normotensives, and was also greater (P = 0.041) than in normal-high renin hypertensives, suggesting that R563Q is an activating mutation of the epithelial sodium channel. R563Q was also found in seven out of 250 mixed ancestry hypertensives, and was significantly (P = 0.017) associated with low-renin, low-aldosterone hypertension in this population group. The mutation was found in one of 100 mixed ancestry normotensives but not in any of 136 white hypertensives. Of the 18 R563Q patients, 11 had severe hypertension, leading to renal failure in two cases, while only two had hypokalaemia.

CONCLUSIONS

R563Q, a new variant of the beta epithelial sodium channel, is associated with low-renin, low-aldosterone hypertension, in South African black and mixed-ancestry patients. Only a minority of individuals with the R563Q allelle fully express the Liddle's syndrome phenotype.

A frameshift mutation of beta subunit of epithelial sodium channel in a case of isolated Liddle syndrome

BACKGROUND

Liddle syndrome is an autosomal dominant form of salt-sensitive hypertension caused by mutations in the epithelial sodium channel expressed in the distal nephron playing an essential role in Na absorption. All reported mutations in Liddle syndrome are either missense mutations or frameshift mutations destroying the PY motif closer to the C-terminus of the beta or gamma subunits causing the situation that the epithelial sodium channels are not degraded and sodium is pooled and thus hypertension and hypokalemia are caused.

METHODS

We sequenced the C-terminus of the beta or gamma subunits of the epithelial sodium channel in a Japanese family of a patient clinically diagnosed as having Liddle syndrome.

RESULTS

As a result, we found in the proband, a frameshift mutation of the beta subunit caused by a single cytosine insertion at the codon 595, introducing a new stop codon at 605 and deleting the last 34 amino acids from the normally encoded protein.

CONCLUSION

This mutation is carried by neither parent (with paternity proven) and hence confirms this has occurred as a event within this family.

Concerted action of ENaC, Nedd4-2, and Sgk1 in transepithelial Na(+) transport

The epithelial Na(+) channel (ENaC), located in the apical membrane of renal aldosterone-responsive epithelia, plays an essential role in controlling the Na(+) balance of extracellular fluids and hence blood pressure. As of now, ENaC is the only Na(+) transport protein for which genetic evidence exists for its involvement in the genesis of both hypertension (Liddle's syndrome) and hypotension (pseudohypoaldosteronism type 1). The regulation of ENaC involves a variety of hormonal signals (aldosterone, vasopressin, insulin), but the molecular mechanisms behind this regulation are mostly unknown. Two regulatory proteins have gained interest in recent years: the ubiquitin-protein ligase neural precursor cell-expressed, developmentally downregulated gene 4 isoform Nedd4-2, which negatively controls ENaC cell surface expression, and serum glucocorticoid-inducible kinase 1 (Sgk1), which is an aldosterone- and insulin-dependent, positive regulator of ENaC density at the plasma membrane. Here, we summarize present ideas about Sgk1 and Nedd4-2 and the lines of experimental evidence, suggesting that they act sequentially in the regulatory pathways governed by aldosterone and insulin and regulate ENaC number at the plasma membrane.

Phosphorylation of Nedd4-2 by Sgk1 regulates epithelial Na(+) channel cell surface expression

The epithelial Na(+) channel (ENaC) plays an essential role in the regulation of whole body Na(+) balance and blood pressure. The cell surface expression of this channel, a complex of three subunits (alpha, beta and gamma ENaC), has been shown to be regulated by hormones such as aldosterone and vasopressin and by intracellular signaling, including ubiquitylation and/or phosphorylation. However, the molecular mechanisms involving phosphorylation in the regulation of ENaC are unclear. Here we show by expression studies in Xenopus laevis oocytes that the aldosterone-induced Sgk1 kinase interacts with the ubiquitin protein ligase Nedd4-2 in a PY motif-dependent manner and phosphorylates Nedd4-2 on Ser444 and, to a lesser extent, Ser338. Such phosphorylation reduces the interaction between Nedd4-2 and ENaC, leading to elevated ENaC cell surface expression. These data show that phosphorylation of an enzyme involved in the ubiquitylation cascade (Nedd4-2) controls cell surface density of ENaC and propose a paradigm for the control of ion channels. Moreover, they suggest a novel and complete signaling cascade for aldosterone-dependent regulation of ENaC.