Genetic variation in the epithelial sodium channel: a risk factor for hypertension in people of african origin

Altered placental ion channel gene expression in preeclamptic high-altitude pregnancies

High-altitude (>2,500 m) residence increases the risk of pregnancy vascular disorders such as fetal growth restriction and preeclampsia, each characterized by impaired placental function. Genetic attributes of highland ancestry confer relative protection against vascular disorders of pregnancy at high altitudes. Although ion channels have been implicated in placental function regulation, neither their expression in high-altitude placentas nor their relationship to high-altitude preeclampsia has been determined. Here, we measured the expression of 26 ion-channel genes in placentas from preeclampsia cases and normotensive controls in La Paz, Bolivia (3,850 m). In addition, we correlated gene transcription to maternal and infant ancestry proportions. Gene expression was assessed by PCR, genetic ancestry evaluated by ADMIXTURE, and ion channel proteins localized by immunofluorescence. In preeclamptic placentas, 11 genes were downregulated (ABCC9, ATP2A2, CACNA1C, KCNE1, KCNJ8, KCNK3, KCNMA1, KCNQ1, KCNQ4, PKD2, and TRPV6) and two were upregulated (KCNQ3 and SCNN1G). KCNE1 expression was positively correlated with high-altitude Amerindian ancestry and negatively correlated with non-high altitude. SCNN1G was negatively correlated with African ancestry, despite minimal African admixture. Most ion channels were localized in syncytiotrophoblasts (Cav1.2, TRPP2, TRPV6, and Kv7.1), whereas expression of Kv7.4 was primarily in microvillous membranes, Kir6.1 in chorionic plate and fetal vessels, and MinK in stromal cells. Our findings suggest a role for differential placental ion channel expression in the development of preeclampsia. Functional studies are needed to determine processes affected by these ion channels in the placenta and whether therapies directed at modulating their activity could influence the onset or severity of preeclampsia.

Prevalence of Chronic Kidney Disease as a Marker of Hypertension Target Organ Damage in Africa: A Systematic Review and Meta-Analysis

Introduction

Hypertension is a major global cause of cardiovascular disease and death with rising worldwide prevalence, particularly in low-income countries. With low awareness, poor treatment, and low control of hypertension in Africans, there is an increased number of patients with target organ damage (TOD), especially chronic kidney disease (CKD), as a consequence of hypertension. The aim of our study is to assess the prevalence of CKD from studies in Africa reporting TOD related to hypertension.

Methods

We performed a search of PubMed/MEDLINE, Web of Science, EBSCOhost, and African Journals Online (AJOL) for studies reporting on CKD as TOD in patients with hypertension. The pooled estimate of CKD was then presented by subregions, age group, eGFR equations, and urban or rural location.

Results

We identified 1,334 articles from which 12 studies were included for quantitative analysis. The studies included 5297 participants from 6 countries (Ghana, Nigeria, Uganda, Tanzania, Democratic Republic of Congo, and South Africa). The pooled prevalence of CKD was 17.8% (95% CI 13.0-23.3%), and CKD was significantly more prevalent in West Africa (21.3% (95% CI: 16.1-27.0); p < 0.0001) and in studies conducted in urban settings (p < 0.001). CKD prevalence was not significantly different by type of GFR equation or age.

Conclusion

This study reports a high prevalence of CKD related to hypertension with a higher prevalence in urban than rural areas. This emphasizes the role of hypertension in causing kidney damage, and the need for strategies to improve awareness, treatment, and control of hypertension in Africans. This study is registered with PROSPERO registration number CRD42018089263.

Diminished Returns of Educational Attainment on Heart Disease among Black Americans

Background

Socioeconomic Status (SES) indicators, such as educational attainment, are social determinants of heart disease. Marginalization related Diminished Returns (MDRs) refer to smaller health benefits of high SES for racial and ethnic minorities compared to the majority group. It is still unknown, however, if MDRs also apply to the effects of education on heart disease.

Purpose

Using a nationally representative sample, we explored racial/ethnic variation in the link between educational attainment and heart disease among American adults.

Methods

We analyzed data (n=25,659) from a nationally representative survey of American adults in 2013. The first wave of the Population Assessment of Tobacco and Health - Adult (PATH-Adult) study was used. The independent variable was education (college graduate, high school graduate, less than a high school diploma). The dependent variable was any heart disease. Age and gender were the covariates. Race, as well as ethnicity, were the moderators. Logistic regressions were used to analyze the data.

Results

Individuals with higher educational attainment had lower odds of heart disease. Race and ethnicity showed statistically significant interactions with education, suggesting that the protective effect of higher education on reducing odds of heart disease was smaller for Hispanic and Black people than for non-Hispanic and White individuals.

Conclusion

Education reduces the risk of heart disease better among non-Hispanic Whites than for Hispanics and Blacks. Therefore, we may expect a disproportionately higher than expected risk of heart disease in Hispanics and Blacks with high educational attainment. Future research should test if the presence of high levels of environmental and behavioral risk factors contribute to the high risk of heart disease in highly educated Black and Hispanic Americans. Policymakers should not reduce health inequalities to just gaps in SES because disparities are present across SES levels, with high SES Blacks and Hispanics remaining at risk of health problems.

Genetics and ESKD Disparities in African Americans

African Americans have a 2- to 4-fold greater incidence of end-stage kidney disease (ESKD) than whites, which has long raised the possibility of a genetic cause for this disparity. Recent advances in genetic studies have shown a causal association of polymorphisms at the apolipoprotein L1 gene (APOL1) with the markedly increased risk for the nondiabetic component of the overall disparity in ESKD in African Americans. Although APOL1-associated kidney disease is thought to account for a substantial proportion of ESKD in African Americans, not all the increased risk for ESKD is accounted for, and a complete cataloging of disparities in genetic causes of ESKD eludes our current understanding of genetic-associated kidney disease. Genetic testing aids the screening, diagnosis, prognosis, and treatment of diseases with a genetic basis. Widespread use of genetic testing in clinical practice is limited by the small number of actionable genetic variants, limited health literacy of providers and patients, and underlying complex ethical, legal, and social issues. This perspective reviews racial and ethnic differences associated with genetic diseases and the development of ESKD in African Americans and discusses potential uncertainties associated with our current understanding of penetrance of genetically linked kidney disease and population-attributable risk percent.

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.

A comparative ultrasonographic evaluation of intrarenal artery resistive index among hypertensive and normotensive adults in a black African population compared to a European population

Background

Hypertensive nephropathy is an important medical problem among the black African population. Early detection of renovascular changes using ultrasonography can provide opportunity for immediate intervention towards preventing or at least delaying the irreversible hypertensive nephropathy.

Purpose

To compare intrarenal resistive index (RI) in healthy normotensive and hypertensive adults in Kano, Nigeria.

Material and Methods

A prospective comparative study of intrarenal RI using ultrasound in 150 hypertensives and 150 normotensive controls. The mean renal RI of the interlobar arteries of both kidneys were measured and recorded. The data were analyzed with the aid of computer-based SPSS 16.0 software for Windows.

Results

The age range of the study participants was 35–70 years. The mean interlobar artery RI values were 0.59 ± 0.04 and 0.59 ± 0.03 on the right and left sides, respectively, in normotensive control individuals while those of hypertensive individuals were 0.73 ± 0.03 and 0.73 ± 0.03 for the mean interlobar artery RI values on the right and left sides, respectively.

Conclusion

The intrarenal RIs were lower in normotensives when compared with the hypertensive participants, which were statistically significant. These showed that hypertension has significant effects on the kidneys, and with early detection and intervention, irreversible renal damage may be prevented.

Racial differences in albuminuria, kidney function, and risk of stroke

BACKGROUND

The objective of this study was to examine the joint associations of estimated glomerular filtration rate (eGFR) and urinary albumin excretion with incident stroke in a large national cohort study.

METHODS

Associations of urinary albumin to creatinine ratio (ACR) and eGFR with incident stroke were examined in 25,310 participants of the Reasons for Geographic and Racial Differences in Stroke (REGARDS) study, a prospective study of black and white US adults ≥45 years of age.

RESULTS

A total of 548 incident strokes were observed over a median of 4.7 years of follow-up. Higher ACR values were associated with lower stroke-free survival in both black and white participants. Among black participants, as compared to an ACR <10 mg/g, the hazard ratios of stroke associated with an ACR of 10-29.99, 30-300, and >300 mg/g were 1.41 (95% confidence interval [CI] 1.01-1.98), 2.10 (95% CI 1.48-2.99), and 2.70 (95% CI 1.58-4.61), respectively, in analyses adjusted for traditional stroke risk factors and eGFR. In contrast, the hazard ratios among white subjects were only modestly elevated and not statistically significant after adjustment for established stroke risk factors. eGFR <60 mL/min/1.73 m(2) was not associated with incident stroke in black or white participants after adjustment for established stroke risk factors.

CONCLUSIONS

Higher ACR was independently associated with higher risk of stroke in black but not white participants from a national cohort. Elucidating the reasons for these findings may uncover novel mechanisms for persistent racial disparities in stroke.

Physiologic tailoring of treatment in resistant hypertension

Resistant hypertension is a major opportunity for prevention of cardiovascular disease. Despite widespread dissemination of consensus guidelines, most patients are uncontrolled with approaches that assume that all patients are the same.

Causes of resistant hypertension include 1) non-compliance 2) consumption of substances that aggravate hypertension (such as salt, alcohol, nonsteroidal anti-inflammatory drugs, licorice, decongestants) and 3) secondary hypertension.

Selecting the appropriate therapy for a patient depends on finding the cause of the hypertension. Once rare causes have been eliminated (such as pheochromocytoma, licorice, adult coarctation of the aorta), the cause will usually be found by intelligent interpretation (in the light of medications then being taken) of plasma renin and aldosterone.

If stimulated renin is low and the aldosterone is high, the problem is primary aldosteronism, and the best treatment is usually aldosterone antagonists (spironolactone or eplerenone; high-dose amiloride for men where eplerenone is not available). If the renin is high, with secondary hyperaldosteronism, the best treatment is angiotensin receptor blockers or aliskiren. If the renin and aldosterone are both low the problem is over-activity of renal sodium channels and the treatment is amiloride. This approach is particularly important in patients of African origin, who are more likely to have low-renin hypertension.

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.

Role of epithelial sodium channels and their regulators in hypertension

The kidney has a central role in the regulation of blood pressure, in large part through its role in the regulated reabsorption of filtered Na(+). Epithelial Na(+) channels (ENaCs) are expressed in the most distal segments of the nephron and are a target of volume regulatory hormones. A variety of factors regulate ENaC activity, including several aldosterone-induced proteins that are present within an ENaC regulatory complex. Proteases also regulate ENaC by cleaving the channel and releasing intrinsic inhibitory tracts. Polymorphisms or mutations within channel subunits or regulatory pathways that enhance channel activity may contribute to an increase in blood pressure in individuals with essential hypertension.

Implications of ethnicity for the treatment of hypertensive kidney disease, with an emphasis on African Americans

The recognition of chronic kidney disease (CKD) as an important public health issue has fostered an increasing number of strategies to increase CKD awareness and to reduce both the prevalence and the complications of CKD. Despite these advances, end-stage renal disease (ESRD) and cardiovascular events remain the major complications of CKD. Although the ESRD epidemic is attributed in greater part to the increasing rate of diabetes, hypertension remains the second most common reported cause of ESRD and is present in approximately 90% of cases of diabetes-related ESRD. The disproportionately high prevalence of hypertension in ethnic minorities, as well as the difficulty of achieving adequate blood-pressure control in these populations, contributes substantially to the high rate of CKD progression and complications in these groups. Although the role of hypertension as a primary cause of CKD is debated, hypertension is commonly recognized as the most important CKD progression factor. Important differences have been reported in the degree and likelihood of blood-pressure response to antihypertensive medications between ethnic groups, but ethnicity seems to be less important as a determinant of clinical outcomes. In this Review we examine key ethnic variations in hypertensive CKD in terms of pathophysiology, response to antihypertensive therapy, clinical outcomes, and evidence-based recommendations for blood-pressure control, with an emphasis on African Americans.

Polypill: For Pollyanna*

A single pill containing a statin, thiazide diuretic, β-blocker, angiotensin receptor antagonist, folate and ASA has been proposed for all people over age 55, with the aim of reducing cardiovascular disease by 80%. Unfortunately, there are insurmountable problems with choosing appropriate constituents of any such single remedy. Adverse effects, drug interactions, inter-individual variation in drug metabolism, and underlying causes of hypertension that differ between patients require individualized therapy. A single pill that will succeed in all patients is not only practically, but conceptually, an inappropriate approach for the prevention of cardiovascular disease.

Small molecule activator of the human epithelial sodium channel

The epithelial sodium channel (ENaC), a heterotrimeric complex composed of alpha, beta, and gamma subunits, belongs to the ENaC/degenerin family of ion channels and forms the principal route for apical Na(+) entry in many reabsorbing epithelia. Although high affinity ENaC blockers, including amiloride and derivatives, have been described, potent and specific small molecule ENaC activators have not been reported. Here we describe compound S3969 that fully and reversibly activates human ENaC (hENaC) in an amiloride-sensitive and dose-dependent manner in heterologous cells. Mechanistically, S3969 increases hENaC open probability through interactions requiring the extracellular domain of the beta subunit. hENaC activation by S3969 did not require cleavage by the furin protease, indicating that nonproteolyzed channels can be opened. Function of alphabetaG37Sgamma hENaC, a channel defective in gating that leads to the salt-wasting disease pseudohypoaldosteronism type I, was rescued by S3969. Small molecule activation of hENaC may find application in alleviating human disease, including pseudohypoaldosteronism type I, hypotension, and neonatal respiratory distress syndrome, when improved Na(+) flux across epithelial membranes is clinically desirable.

Stroke prevention in the high-risk patient

Strokes are increasing in number due to an ageing population and are largely preventable. In the highest risk patients, a 90% relative risk reduction for stroke is attainable by appropriately using all the measures proven to reduce stroke: smoking cessation, a Mediterranean diet, control of hypertension, anticoagulants or antiplatelet agents, lipid lowering drugs and appropriate carotid endarterectomy. Vitamin therapy to lower homocysteine and carotid stenting are additional measures that may yet prove beneficial. Diet, smoking cessation and appropriate carotid endarterectomy reduce stroke by more than do pharmacotherapies. Blood pressure control depends more on selecting appropriate therapy individualised for the patient, than on using any particular drug class. This review, therefore, places pharmacotherapy in perspective as part of, but not all of, stroke prevention.

Advances in genetic hypertension

PURPOSE OF REVIEW

Mendelian forms of hypertension are rare genetic disorders that cause severe hypertension. This review will explore the recently identified molecular mechanisms and pathogenesis of genetic disorders that cause hypertension in children.

RECENT FINDINGS

Hypertension is now believed to be a polygenic disorder resulting from the interaction of multiple genes and the environment. A few forms of severe hypertension have been linked to single genes. The genes responsible for these disorders have all been cloned and all participate in pathways involved in heightened renal sodium reabsorption. The increased sodium reabsorption arises in the distal nephron and leads to volume expansion and hypertension.

SUMMARY

Investigating forms of monogenic hypertension has advanced the understanding of sodium transport and volume control by the kidney. Future studies will identify novel genes, pathways and treatment targets important in the fight against primary hypertension.

Intensive management of risk factors for accelerated atherosclerosis: the role of multiple interventions

Patients at high risk of vascular events can reduce their risk by 75% to 80% through a combination of lifestyle changes and medical therapy. These include smoking cessation, a Mediterranean diet, daily exercise, maintaining a fit weight, moderate consumption of alcohol, effective control of blood pressure and diabetes, intensive treatment with lipid-lowering drugs and antiplatelet agents, and perhaps treatment with vitamins to lower homocysteine. Much of this is achieved primarily by the patient; physicians need to become better at assisting their patients in making lifestyle changes. Effective control of treatment-resistant hypertension can be improved by individualizing medical therapy to the underlying cause, based on measurement of plasma renin and aldosterone. Measurement of carotid plaque may be useful by providing feedback on the success of therapy.

Can ethical reasoning contribute to better epidemiology? A case study in research on racial health disparities

A common belief among epidemiologists and other scientists is that they ought not engage in ethical evaluation or endorse any particular ethical, political, or social values while involved in scientific reasoning. Such values are irrelevant to collecting and interpreting data and can only lead to bias. This view is also reflected in scientists' education. The authors argue that ethical values are crucial to conducting much epidemiological research. Focusing on epidemiological research on racial health disparities, they show that value judgments are inescapable when performing such research. Values are implicit in the framing of research questions, the identification of the problem, and the choices of design and methodology. By making value judgments explicit, scientists will be more likely to pay attention to them and thus assess them in critical ways. Finally, the implications that this has for scientific training are discussed. Scientific training should prepare scientists to engage in ethical reasoning not only because it will make them more responsible human beings, but also because it will make them better scientists.

Epithelial sodium channel allele T594M is not associated with blood pressure or blood pressure response to amiloride

The T594M allele of the epithelial sodium channel beta-subunit has been proposed as a gain-of-function mutation leading to salt-sensitive hypertension in blacks that is particularly responsive to the specific sodium channel antagonist amiloride. However, the positive associations derive from small convenience samples, and the amiloride challenge study lacked a control group. We determined whether the T594M allele was associated with hypertension and blood pressure (BP) response to amiloride in 2 well-characterized random population samples including 3137 Dallas County subjects and 1666 Jamaican blacks. In multivariate models, the T594M allele was not predictive of systolic BP (adjusted odds ratio for hypertension 1.1; 95% confidence interval, 0.7 to 1.8). Amiloride treatment did not lower the BP of 6 T594M heterozygotes significantly more than in 22 control subjects (P=0.8). We conclude that the T594M allele does not contribute significantly to BP in blacks and does not predict a significantly superior response to amiloride therapy.

Functional polymorphisms in the alpha-subunit of the human epithelial Na+ channel increase activity

Activity of the epithelial Na(+) channel (ENaC) is limiting for Na(+) reabsorption at the distal nephron. Gain-of-function mutations in ENaC cause Liddle's syndrome: a severe form of inheritable hypertension. Several polymorphisms in alpha-hENaC possibly associated with abnormal Na(+) handling by the kidney and the salt-sensitive hypertension prevalent in black populations have been reported. The functional effects of alpha-hENaC polymorphisms on channel activity, however, remain controversial and have not been directly tested in a mammalian background. We ask here whether polymorphisms at positions 334, 618, and 663 in alpha-hENaC influence channel activity. Activity of wild-type (A334, C618, A663) and polymorphic ENaC expressed in Chinese hamster ovary cells was assessed with patch-clamp electrophysiology. While the A334T polymorphism had little effect on macroscopic ENaC currents, the C618F and A663T polymorphisms significantly increased ENaC activity >3.3- and 1.6-fold, respectively. Similarly, polymorphic ENaC had greater activity compared with wild-type channels in excised patches with activity of C618F and A663T channels increased 3.8- and 2.6-fold, respectively. Unitary channel conductances and reversal potentials were not different for polymorphic and wild-type ENaC. Increases in activity resulted primarily from increases in the apparent number of active (polymorphic) channels in the plasma membrane. Moreover, addition of a reducing agent to the cytosol significantly increased activity of wild-type ENaC equal to that of C618F polymorphic channels but had no effect on these latter channels. These results are consistent with the C618F and A663T polymorphisms leading to elevated ENaC activity with the possibility that they facilitate altered Na(+) handling by the kidney.

Update on disparities in the pathophysiology and management of hypertension: focus on African Americans

Hypertension treatment and control is of paramount importance in the prevention of premature cardiovascular disease. African Americans present a special challenge to the clinician due, in part, to their earlier age of onset,greater prevalence, and increased rates of untoward events. A review of the recent studies of genetic epidemiology has not revealed unique genotypes that explain human hypertension or the disparate impact suffered by African Americans. Moreover, a clear message has emerged that environmental factors predominate in their effect on cardiovascular risk and are mutable. These findings suggest that to have an immediate and substantial impact on the ethnic disparity of hypertension, resources and research should be directed toward social and behavioral factors. Prompt and aggressive control of blood pressure is an effective global strategy for cardiovascular risk reduction. In most cases, this approach requires multiple interventions including lifestyle modification and an antihypertensive regimen that is tailored to the individual under the current guidelines and not stipulated by race.