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

A frameshift mutation in the SCNN1B gene in a family with Liddle syndrome: A case report and systematic review

Liddle syndrome is an autosomal dominant form of monogenic hypertension that is caused by mutations in SCNN1A, SCNN1B or SCNN1G, which respectively encode the α, β and γ subunits of the epithelial sodium channel. In the present study, DNA was extracted from leukocytes in peripheral blood obtained from all members of a family with Liddle syndrome. Whole‑exome sequencing and Sanger sequencing were performed to assess the candidate variant and a co‑segregation analysis was conducted. A frameshift mutation in SCNN1B (NM_ 000336: c.1806dupG, p.Pro603Alafs*5) in the family was identified, characterized by early‑onset hypertension and hypokalemia. The mutation led to the truncation of the β subunit of the epithelial sodium channel and a lack of the conservative PY motif. Furthermore, a systematic review of follow‑up data from patients with Liddle syndrome with SCNN1B mutations was performed. The follow‑up data of 108 patients with pathogenic SCNN1B mutations from 47 families were summarized. Phenotypic heterogeneity was evident in patients with Liddle syndrome and early‑onset hypertension was the most frequent symptom. Patients responded well to targeted amiloride therapy with significant improvements in blood pressure and serum potassium concentration. The present study demonstrates that confirmatory genetic testing and targeted therapy can prevent premature onset of clinical endpoint events in patients with Liddle syndrome.

Associated factors with hypertension, known poorly controlled hypertension, and newly diagnosed hypertension among people aged 18-70 in Senegal

Introduction

In this study, we aimed to determine the factors associated with hypertension, known poorly controlled hypertension, and newly diagnosed hypertension in 2015 to improve the prevention of this pathology in our country.

Methods

This was a secondary analysis of the national WHO STEPwise survey database. It had a quantitative, descriptive, and analytical aim. The sampling was done in stratification in three stages of sampling. Statistical analysis was performed using Rstudio 4.0.2 software. It was a descriptive, bivariate, and multivariate analysis using binomial logistic regression for explanatory purposes.

Results

The risk factors for being hypertensive among 18-70- year-olds in Senegal in 2015 were: having taken their blood pressure at least once in their life; having taken their cholesterol levels at least once in their life; having received advice to reduce salt consumption; having received advice on how to reduce the amount of sugar; having a lean body mass index; being overweight; being obese III; and being between 35-39 and 65-70 years old. The protective factors are: checking the salt content indicated on the labels; and consuming salt-rich dishes "often" compared to "never". The risk factors for being newly diagnosed with hypertension among 18-70-year-olds in Senegal in 2015 were: having a history of stroke; having a resting heart rate greater than 80 beats per minute; consuming palm oil; and having high fasting blood sugar. The protective factors are: having a high average number of meals eaten per week not prepared at home; checking the salt content indicated on the label; and being of female sex. The risk factors for being known to have poorly controlled hypertension among 18-70- year-olds in Senegal in 2015 were: judging their reduction in salt consumption as "very important"; and having received advice to reduce their salt consumption. The protective factors are: performing physical activity only when traveling; and limiting the consumption of salty meals.

Conclusion

Senegalese family cooking exposes to hypertension, small actions limiting salt consumption strongly protect against hypertension, and simple physical activity during travel strongly protects against hypertension, but the health system is very little proactive in the primordial and primary prevention of hypertension.

A Novel Frame-Shift Mutation in SCNN1B Identified in a Chinese Family Characterized by Early-Onset Hypertension

Background

Liddle syndrome is a form of monogenic hypertension caused by mutations in the three homologous subunits of the epithelial sodium channels (ENaCs), α, β, and γ. It is characterized by early-onset refractory hypertension, hypokalemia, low renin activity, and hypoaldosteronism. In this study, we report a novel frame-shift mutation in SCNN1B responsible for Liddle syndrome in a Chinese family.

Methods

DNA samples were collected from all participants. Whole-exome sequencing was performed in the proband to detect possible causative variants. Sanger sequencing was then conducted in the other family members to verify the candidate variant, and in 100 patients with hypertension and 100 normotensive controls to exclude population genetic polymorphism.

Results

We identified a novel frame-shift mutation (c.1691_1693delinsG) in SCNN1B that was responsible for Liddle syndrome in this family. This mutation leads to the substitution of Arg in place of Gln at codon site 564 and generates a new stop codon at 592, influencing the crucial PY motif and resulting in reduced inactivation of the ENaCs. Aside from the proband, eight family members carried the mutation. Intra-familial phenotypic heterogeneity was observed in the blood pressure and serum potassium levels. Amiloride therapy combined with a low sodium diet is effective to alleviate the symptoms of patients with Liddle syndrome.

Conclusion

c.1691_1693delinsG, a novel frame-shift mutation in the β subunit of ENaC, was identified in a Chinese family with Liddle syndrome by whole-exome sequencing. Phenotypic heterogeneity can make diagnosis of Liddle syndrome difficult on the basis of clinical or biochemical characteristics alone. Genetic analysis is a useful tool allowing timely and accurate diagnosis of Liddle syndrome and playing a guiding role in precise treatment of the disease.

Salt Sensitivity of Blood Pressure in Blacks and Women: A Role of Inflammation, Oxidative Stress, and Epithelial Na+ Channel

Significance: Salt sensitivity of blood pressure (SSBP) is an independent risk factor for mortality and morbidity due to cardiovascular disease, and disproportionately affects blacks and women. Several mechanisms have been proposed, including exaggerated activation of sodium transporters in the kidney leading to salt retention and water. Recent Advances: Recent studies have found that in addition to the renal epithelium, myeloid immune cells can sense sodium via the epithelial Na+ channel (ENaC), which leads to activation of the nicotinamide adenine dinucleotide phosphate oxidase enzyme complex, increased fatty acid oxidation, and production of isolevuglandins (IsoLGs). IsoLGs are immunogenic and contribute to salt-induced hypertension. In addition, aldosterone-mediated activation of ENaC has been attributed to the increased SSBP in women. The goal of this review is to highlight mechanisms contributing to SSBP in blacks and women, including, but not limited to increased activation of ENaC, fatty acid oxidation, and inflammation. Critical Issues: A critical barrier to progress in management of SSBP is that its diagnosis is not feasible in the clinic and is limited to expensive and laborious research protocols, which makes it difficult to investigate. Yet without understanding the underlying mechanisms, this important risk factor remains without treatment. Future Directions: Further studies are needed to understand the mechanisms that contribute to differential blood pressure responses to dietary salt and find feasible diagnostic tools. This is extremely important and may go a long way in mitigating the racial and sex disparities in cardiovascular outcomes. Antioxid. Redox Signal. 35, 1477-1493.

The importance of the epithelial sodium channel in determining salt sensitivity in people of African origin

Hypertension is highly prevalent in Black Africans and has been found to be associated with worse blood pressure (BP) control and more cardiovascular disease. Black Africans are more salt sensitive with low renin and aldosterone levels. This can be explained in part by variants in the epithelial sodium channel (ENaC) causing an increase in channel activity resulting in sodium and water retention. These variants in the ENaC are increased in the Black African populations presumably due to selective pressure for sodium retention in traditionally low-salt diets. Furthermore, increased endothelial sodium channel activity contributes to the risk of vascular stiffness, which may also result in more difficult to control hypertension. Patients with increased activity of the ENaC are more likely to respond to amiloride (a selective sodium channel antagonist), which has implications for the management of severe and resistant hypertension in Black Africans. A large-scale controlled trial on the use of amiloride compared to usual care is warranted in Blacks with severe or resistant hypertension.

Epithelial Sodium Channel and Salt-Sensitive Hypertension

The development of high blood pressure is influenced by genetic and environmental factors, with high salt intake being a known environmental contributor. Humans display a spectrum of sodium-sensitivity, with some individuals displaying a significant blood pressure rise in response to increased sodium intake while others experience almost no change. These differences are, in part, attributable to genetic variation in pathways involved in sodium handling and excretion. ENaC (epithelial sodium channel) is one of the key transporters responsible for the reabsorption of sodium in the distal nephron. This channel has an important role in the regulation of extracellular fluid volume and consequently blood pressure. Herein, we review the role of ENaC in the development of salt-sensitive hypertension, and present mechanistic insights into the regulation of ENaC activity and how it may accelerate sodium-induced damage and dysfunction. We discuss the traditional role of ENaC in renal sodium reabsorption and review work addressing ENaC expression and function in the brain, vasculature, and immune cells, and how this has expanded the implications for its role in the initiation and progression of salt-sensitive hypertension.

Screening Rates for Primary Aldosteronism in Resistant Hypertension: A Cohort Study

Resistant hypertension is associated with higher rates of cardiovascular disease, kidney disease, and death than primary hypertension. Although clinical practice guidelines recommend screening for primary aldosteronism among persons with resistant hypertension, rates of screening are unknown. We identified 145 670 persons with hypertension and excluded persons with congestive heart failure or advanced chronic kidney disease. Among this cohort, we studied 4660 persons ages 18 to <90 from the years 2008 to 2014 with resistant hypertension and available laboratory tests within the following 24 months. The screening rate for primary aldosteronism in persons with resistant hypertension was 2.1%. Screened persons were younger (55.9±13.3 versus 65.5±11.6 years; P<0.0001) and had higher systolic (145.1±24.3 versus 139.6±20.5 mm Hg; P=0.04) and diastolic blood pressure (81.8±13.6 versus 74.4±13.8 mm Hg; P<0.0001), lower rates of coronary artery disease (5.2% versus 14.2%; P=0.01), and lower serum potassium concentrations (3.9±0.6 versus 4.1±0.5 mmol/L; P=0.04) than unscreened persons. Screened persons had significantly higher rates of prescription for calcium channel blockers, mixed α/β-adrenergic receptor antagonists, sympatholytics, and vasodilators, and lower rates of prescription for loop, thiazide, and thiazide-type diuretics. The prescription of mineralocorticoid receptor antagonists or other potassium-sparing diuretics was not significantly different between groups (P=0.20). In conclusion, only 2.1% of eligible persons received a screening test within 2 years of meeting criteria for resistant hypertension. Low rates of screening were not due to the prescription of antihypertensive medications that may potentially interfere with interpretation of the screening test. Efforts to highlight guideline-recommended screening and targeted therapy are warranted.

Analyses of epithelial Na+ channel variants reveal that an extracellular β-ball domain critically regulates ENaC gating

Epithelial Na⁺ channel (ENaC)-mediated Na⁺ transport has a key role in the regulation of extracellular fluid volume, blood pressure, and extracellular [K⁺]. Among the thousands of human ENaC variants, only a few exist whose functional consequences have been experimentally tested. Here, we used the Xenopus oocyte expression system to investigate the functional roles of four nonsynonymous human ENaC variants located within the β7-strand and its adjacent loop of the α-subunit extracellular β-ball domain. αR350Wβγ and αG355Rβγ channels exhibited 2.5- and 1.8-fold greater amiloride-sensitive currents than WT αβγ human ENaCs, respectively, whereas αV351Aβγ channels conducted significantly less current than WT. Currents in αH354Rβγ-expressing oocytes were similar to those expressing WT. Surface expression levels of three mutants (αR350Wβγ, αV351Aβγ, and αG355Rβγ) were similar to that of WT. However, three mutant channels (αR350Wβγ, αH354Rβγ, and αG355Rβγ) exhibited a reduced Na⁺ self-inhibition response. Open probability of αR350Wβγ was significantly greater than that of WT. Moreover, other Arg-350 variants, including αR350G, αR350L, and αR350Q, also had significantly increased channel activity. A direct comparison of αR350W and two previously reported gain-of-function variants revealed that αR350W increases ENaC activity similarly to αW493R, but to a much greater degree than does αC479R. Our results indicate that αR350W along with αR350G, αR350L, and αR350Q, and αG355R are novel gain-of-function variants that function as gating modifiers. The location of these multiple functional variants suggests that the αENaC β-ball domain portion that interfaces with the palm domain of βENaC critically regulates ENaC gating.

Hypertension among US-born and foreign-born non-Hispanic Blacks: National Health and Nutrition Examination Survey 2003-2014 data

OBJECTIVES

Non-Hispanic Blacks in the United States have the highest reported prevalence of hypertension (44%) worldwide. However, this does not consider the heterogeneity of Blacks within the United States, particularly comparing US-born to long-standing or recent (foreign-born) immigrants. The objective of this study is to compare odds of hypertension between US-born and foreign-born Blacks in the United States.

METHODS

We assessed the prevalence of hypertension among US-born (n = 4511) vs. foreign-born (n = 522) non-Hispanic Black adults aged 22-79 years, based on pooled nationally representative data (2003-2014); as well by length of US residency among immigrants. Multivariable-adjusted logistic regression was used to investigate the association between nativity and hypertension odds.

RESULTS

Nearly half (42.8%) of US-born Blacks but only 27.4% of foreign-born Blacks had hypertension. After adjusting for major covariates, foreign-born Blacks were 39.0% less likely (odds ratio 0.61 95% confidence interval 0.49, 0.77) to have hypertension than their US-born counterparts. Among foreign-born Blacks, length of US residency was not significantly associated with odds of hypertension.

CONCLUSION

Foreign-born vs. US-born non-Hispanic Blacks have substantially lower prevalence of hypertension. Considering nativity among US Blacks in clinical research and public health efforts may improve accuracy of characterizing health disparities and facilitate development of targeted interventions to reduce hypertension in this diverse population.

Epithelial sodium channel blockade and new β-ENaC polymorphisms among normotensive and hypertensive adult Nigerians

We sought to determine the effect of amiloride on blood pressure (BP) and the presence of polymorphisms of the β-subunit of the epithelial sodium channel (ENaC) among normotensive (NT) and hypertensive (HT) Nigerians. Healthy volunteers-47 NT and 53 age-matched HT were recruited after giving informed consent. Subjects were salt-loaded with 200 mmol of NaCl daily for 5 days. Following a week washout period, salt-loading was repeated in addition to the administration of 5 mg amiloride daily for five days. Blood pressure, plasma and urine electrolytes were measured at baseline, after salt-loading and after salt-loading plus amiloride. PCR amplicons were sequenced for β-ENaC polymorphisms. Salt-loading led to a significant increase (p < 0.05) in SBP among NT and HT and in DBP (p < 0.001) only among HT. Amiloride reduced SBP and DBP to below baseline levels in NT (p < 0.05) and HT (p < 0.001) subjects. Five of the subjects had the β-T594M polymorphism, HT 3/53; NT 2/47 (p = 0.75). Four previously unreported β-ENaC mutations were recorded: E632V and E636V, respectively, among two HT subjects, D638Y in another HT and L628Q in one NT subject. We showed the presence of β-ENaC polymorphisms among our populace and the possible usefulness of amiloride as a single antihypertensive among Nigerians.

Serum Creatine Kinase-MB Isoenzyme Activity among Subjects with Uncomplicated Essential Hypertension: Any Sex Differences

Hypertension (high blood pressure) is a major health challenge and more women than men are affected by the condition. Complications as a result of this condition often lead to disabilities and premature death. The objective of this study was to evaluate creatine kinase-MB (CK-MB) activity in uncomplicated hypertension and to know whether sex differences exist in the activity of the enzyme. Serum creatine kinase-MB isoenzyme activity, troponin I, and lipid profile were evaluated in 140 male and 100 female Nigerians with hypertension. The control group was comprised of 100 (50 males and 50 females) normotensive subjects. Measured parameters were assayed using Selectra Pros chemistry analyzer. The means were compared between males and females using Students't-test. The mean CK-MB activity of the female hypertensive subjects was significantly higher (p < 0.001) than the males. Similarly, the mean cardiac troponin I (cTnI) of the female hypertensive subjects was significantly higher (p < 0.001) than the males. Conversely, the mean CK-MB activity of the female normotensive subjects was significantly lower (p < 0.001) than the male counterparts. There was no difference in the levels of cTnI between male and female normotensive subjects. Serum CK-MB activity was higher in female than male hypertensive subjects. In the light of these results, cardiac markers should be routinely done in the evaluation of hypertensive subjects and sex-specific consideration may be recognized in the management of these patients.

Gitelman syndrome in a South African family presenting with hypokalaemia and unusual food cravings

BACKGROUND

Gitelman syndrome (GS) is an autosomal recessive renal tubular disorder characterised by renal salt wasting with hypokalaemia, metabolic alkalosis, hypomagnesaemia and hypocalciuria. It is caused by mutations in SLC12A3 encoding the sodium-chloride cotransporter on the apical membrane of the distal convoluted tubule. We report a South African family with five affected individuals presenting with hypokalaemia and unusual food cravings.

METHODS

The affected individuals and two unaffected first degree relatives were enrolled into the study. Phenotypes were evaluated through history, physical examination and biochemical analysis of blood and urine. Mutation screening was performed by sequencing of SLC12A3, and determining the allele frequencies of the sequence variants found in this family in 117 ethnically matched controls.

RESULTS

The index patient, her sister, father and two aunts had a history of severe salt cravings, fatigue and tetanic episodes, leading to consumption of large quantities of salt and vinegar. All affected individuals demonstrated hypokalaemia with renal potassium wasting. Genetic analysis revealed that the pseudo-dominant pattern of inheritance was due to compound heterozygosity with two novel mutations: a S546G substitution in exon 13, and insertion of AGCCCC at c.1930 in exon 16. These variants were present in the five affected individuals, but only one variant each in the unaffected family members. Neither variant was found in any of the controls.

CONCLUSIONS

The diagnosis of GS was established in five members of a South African family through clinical assessment, biochemical analysis and mutation screening of the SLC12A3 gene, which identified two novel putative pathogenic mutations.

Protein trafficking defects in inherited kidney diseases

The nephron, the basic structural and functional unit of the kidney, is lined by different, highly differentiated polarized epithelial cells. Their concerted action modifies the composition of the glomerular ultrafiltrate through reabsorption and secretion of essential solutes to finally produce urine. The highly specialized properties of the different epithelial cell types of the nephron are remarkable and rely on the regulated delivery of specific proteins to their final subcellular localization. Hence, mutations affecting sorting of individual proteins or inactivating the epithelial trafficking machinery have severe functional consequences causing disease. The presence of mutations leading to protein trafficking defect is indeed a mechanism of pathogenesis seen in an increasing number of disorders, including about one-third of monogenic diseases affecting the kidney. In this review, we focus on representative diseases to discuss different molecular mechanisms that primarily lead to defective protein transport, such as endoplasmic reticulum retention, mistargeting, defective endocytosis or degradation, eventually resulting in epithelial cell and kidney dysfunction. For each disease, we discuss the type of reported mutations, their molecular and cellular consequences and possible strategies for therapeutic intervention. Particular emphasis is given to new and prospective therapies aimed at rescuing the trafficking defect at the basis of these conformational diseases.

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.

Deletion of α-subunit exon 11 of the epithelial Na+ channel reveals a regulatory module

Epithelial Na(+) channel (ENaC) subunits (α, β, and γ) found in functional complexes are translated from mature mRNAs that are similarly processed by the inclusion of 13 canonical exons. We examined whether individual exons 3-12, encoding the large extracellular domain, are required for functional channel expression. Human ENaCs with an in-frame deletion of a single α-subunit exon were expressed in Xenopus oocytes, and their functional properties were examined by two-electrode voltage clamp. With the exception of exon 11, deletion of an individual exon eliminated channel activity. Channels lacking α-subunit exon 11 were hyperactive. Oocytes expressing this mutant exhibited fourfold greater amiloride-sensitive whole cell currents than cells expressing wild-type channels. A parallel fivefold increase in channel open probability was observed with channels lacking α-subunit exon 11. These mutant channels also exhibited a lost of Na(+) self-inhibition, whereas we found similar levels of surface expression of mutant and wild-type channels. In contrast, in-frame deletions of exon 11 from either the β- or γ-subunit led to a significant loss of channel activity, in association with a marked decrease in surface expression. Our results suggest that exon 11 within the three human ENaC genes encodes structurally homologous yet functionally diverse domains and that exon 11 in the α-subunit encodes a module that regulates channel gating.

Intensive risk factor control in stroke prevention

Stroke prevention is an urgent priority because of the aging of the population and the steep association of age and risk of stroke. Direct costs of stroke are expected to more than double in the US between 2012 and 2030. By getting everything right, patients can reduce the risk of stroke by 80% or more; however, getting everything right is a tall order. Roughly in order of importance, this requires smoking cessation, maintenance of a healthy weight, a Cretan Mediterranean diet, blood pressure control, lipid-lowering drugs, appropriate use of antiplatelet agents and anticoagulants, and appropriate carotid endarterectomy and stenting. A new approach called "treating arteries instead of targeting risk factors" appears promising but requires validation in randomized trials.

Recent advances in hypertension in sub-Saharan Africa

BACKGROUND

Hypertension was once considered rare in sub-Saharan Africa (SSA), but currently it has become a widespread problem with immense socioeconomic importance. The purpose of this review is to summarise new information on hypertension in SSA that has been published since the last major review in 2008.

METHODS AND RESULTS

A literature search was performed in Pubmed, Embase, WHO Global Cardiovascular Infobase, African Journal On-Line, and African Index Medicus using the following search criteria: hypertension, high blood pressure, and Africa/SSA. Epidemiological surveys that used the WHO STEPS approach or similar methods were also included. The overall prevalence of hypertension in SSA was estimated at 16.2% (95% CI 14.2% to 20.3%) with an estimated number of hypertensive individuals to be 74.7 million. The prevalence of hypertension varies widely from country to country. It is projected that the number of affected individuals will increase by 68% (125.5 million) by 2025. Mass migration of rural Africans to urban areas and rapid changes in lifestyle and risk factors account for the rising prevalence of hypertension.

CONCLUSIONS

Proactive public health interventions at a population level need to be introduced to control the growing hypertension epidemic, and there needs to be a major improvement in access to hypertensive care for the individual. There is an important need for better epidemiological data and hypertension related outcome trials in SSA.

Gain-of-function variant of the human epithelial sodium channel

Epithelial Na(+) channel (ENaC) mutations are associated with several human disorders, underscoring the importance of these channels in human health. Recent human genome sequencing projects have revealed a large number of ENaC gene variations, several of which have been found in individuals with salt-sensitive hypertension, cystic fibrosis, and other disorders. However, the functional consequences of most variants are unknown. In this study, we used the Xenopus oocyte expression system to examine the functional properties of a human ENaC variant. Oocytes expressing αβγL511Q human ENaCs showed 4.6-fold greater amiloride-sensitive currents than cells expressing wild-type channels. The γL511Q variant did not significantly alter channel surface expression. Single channel recordings revealed that the variant had fourfold higher open probability than wild type. In addition, γL511Q largely eliminated the Na(+) self-inhibition response, which reflects a downregulation of ENaC open probability by extracellular Na(+). Moreover, γL511Q diminished chymotrypsin-induced activation of the mutant channel. We conclude that γL511Q is a gain-of-function human ENaC variant. Our results suggest that γL511Q enhances ENaC activity by increasing channel open probability and dampens channel regulation by extracellular Na(+) and proteases.

Between candidate genes and whole genomes: time for alternative approaches in blood pressure genetics

Blood pressure has a significant genetic component, but less than 3% of the observed variance has been attributed to genetic variants identified to date. Candidate gene studies of rare, monogenic hypertensive syndromes have conclusively implicated several genes altering renal sodium balance, and studies of essential hypertension have inconsistently implicated over 50 genes in pathways affecting renal sodium balance and other functions. Genome-wide linkage scans have replicated numerous quantitative trait loci throughout the genome, and over 50 single nucleotide polymorphisms (SNPs) have been replicated in multiple genome-wide association studies. These studies provide considerable evidence that epistasis and other interactions play a role in the genetic architecture of blood pressure regulation, but candidate gene studies have limited scope to test for epistasis, and genome-wide studies have low power for both main effects and interactions. This review summarizes the genetic findings to date for blood pressure, and it proposes focused, pathway-based approaches involving epistasis, gene-environment interactions, and next-generation sequencing to further the genetic dissection of blood pressure and hypertension.

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.

ENaCs and ASICs as therapeutic targets

The epithelial Na(+) channel (ENaC) and acid-sensitive ion channel (ASIC) branches of the ENaC/degenerin superfamily of cation channels have drawn increasing attention as potential therapeutic targets in a variety of diseases and conditions. Originally thought to be solely expressed in fluid absorptive epithelia and in neurons, it has become apparent that members of this family exhibit nearly ubiquitous expression. Therapeutic opportunities range from hypertension, due to the role of ENaC in maintaining whole body salt and water homeostasis, to anxiety disorders and pain associated with ASIC activity. As a physiologist intrigued by the fundamental mechanics of salt and water transport, it was natural that Dale Benos, to whom this series of reviews is dedicated, should have been at the forefront of research into the amiloride-sensitive sodium channel. The cloning of ENaC and subsequently the ASIC channels has revealed a far wider role for this channel family than was previously imagined. In this review, we will discuss the known and potential roles of ENaC and ASIC subunits in the wide variety of pathologies in which these channels have been implicated. Some of these, such as the role of ENaC in Liddle's syndrome are well established, others less so; however, all are related in that the fundamental defect is due to inappropriate channel activity.