Genetics of salt-sensitive hypertension

Genetic Factors Contributing to the Pathogenesis of Essential Hypertension in Two African Populations

The African continent has the highest prevalence of hypertension globally, with South Africa reporting the highest prevalence in Southern Africa. While the influence of genetic variability in the pathogenesis of hypertension is well described internationally, limited reports are available for African populations. This study aimed to assess the association of genetic variants and essential hypertension in a cohort of two ethnic South African population groups. Two hundred and seventy-seven hypertensive and one hundred and seventy-six normotensive individuals were genotyped for 78 variants. Genotyping was performed using the Illumina GoldenGate Assay and allele-specific polymerase chain reaction. The association of variants was assessed using the Fisher Exact test under the additive and allelic genetic models, while multivariate logistic regression was used to predict the development of hypertension. Five variants (CYP11B2 rs179998, AGT rs5051 and rs699, AGTR1 rs5186, and ACE rs4646994) were significantly associated with essential hypertension in the cohort under study. Furthermore, AGTR1 rs5186 and AGT rs699 were identified as risk factors for the development of hypertension in both ethnic groups. In two ethnic South African populations, an association was observed between renin-angiotensin-aldosterone system (RAAS)-related genes and the development of hypertension.

Pathophysiology and genetics of salt-sensitive hypertension

Most hypertensive cases are primary and heavily associated with modifiable risk factors like salt intake. Evidence suggests that even small reductions in salt consumption reduce blood pressure in all age groups. In that regard, the ACC/AHA described a distinct set of individuals who exhibit salt-sensitivity, regardless of their hypertensive status. Data has shown that salt-sensitivity is an independent risk factor for cardiovascular events and mortality. However, despite extensive research, the pathogenesis of salt-sensitive hypertension is still unclear and tremendously challenged by its multifactorial etiology, complicated genetic influences, and the unavailability of a diagnostic tool. So far, the important roles of the renin-angiotensin-aldosterone system, sympathetic nervous system, and immune system in the pathogenesis of salt-sensitive hypertension have been studied. In the first part of this review, we focus on how the systems mentioned above are aberrantly regulated in salt-sensitive hypertension. We follow this with an emphasis on genetic variants in those systems that are associated with and/or increase predisposition to salt-sensitivity in humans.

Advances in Genomics Research of Blood Pressure Responses to Dietary Sodium and Potassium Intakes

More than half of US adults have hypertension by 40 years of age and a subsequent increase in atherosclerotic cardiovascular disease risk. Dietary sodium and potassium are intricately linked to the pathophysiology of hypertension. However, blood pressure responses to dietary sodium and potassium, phenomena known as salt and potassium sensitivity of blood pressure, respectively, are heterogenous and normally distributed in the general population. Like blood pressure, salt and potassium sensitivity are complex phenotypes, and previous research has shown that up to 75% of individuals experience a blood pressure change in response to such dietary minerals. Previous research has also implicated both high salt sensitivity and low salt sensitivity (or salt resistance) of blood pressure to an increased risk of hypertension and potentially atherosclerotic cardiovascular disease risk. Given the clinical challenges required to accurately measure the sodium and potassium response phenotypes, genomic characterization of these traits has become of interest for hypertension prevention initiatives on both the individual and population levels. Here, we review advances in human genomics research of blood pressure responses to dietary sodium and potassium by focusing on 3 main areas, including the phenotypic characterization of salt sensitivity and resistance, clinical challenges in diagnosing such phenotypes, and the genomic mechanisms that may help to explain salt and potassium sensitivity and resistance. Through this process, we hope to further underline the value of leveraging genomics and broader multiomics for characterizing the blood pressure response to sodium and potassium to improve precision in lifestyle approaches for primordial and primary atherosclerotic cardiovascular disease prevention.

A cross-sectional study on factors associated with hypertension and genetic polymorphisms of renin-angiotensin-aldosterone system in Chinese hui pilgrims to hajj

Background

Hypertension is the leading risk factor for cardiovascular disease (CVD), however, the studies on lifestyle and genetic risks in Chinese pilgrims to Hajj was limited. The aim of this study is to examine the prevalence and associated lifestyle and genetic risks for hypertension among Hui Hajj pilgrims in China.

Methods

We performed a cross-sectional analysis of data in 1,465 participants aged 30–70 years who participated in a medical examination for Hui Hajj pilgrims from Gansu province, China in 2017. Multiple logistic regression was used to evaluate the association of potential risk factors with hypertension. Deoxyribonucleic acid (DNA) polymorphism was examined at sites in the renin-angiotensin-aldosterone system (RAAS).

Results

The prevalence of hypertension was 47% among this population. Lifestyle factors such as fried food preference (like vs. dislike: odds ratio [OR]: =1.53, 95% confidence interval [CI]: 1.13–2.09) and barbecued food preference (like vs. dislike: OR = 1.45, 95% CI: 1.06–1.97) were associated with elevated risk of hypertension among Hui pilgrims. Comparing with Angiotensin converting enzyme (ACE) rs4425 AA genotype, TT genotype was associated with hypertension risk (OR = 2.16, 95% CI: 1.17–4.00). Similar results were also observed for ACE rs4437 CC genotype (OR = 1.95, 95% CI: 1.07–3.55), Angiotensin II receptor (ATR) rs129876 AA genotype (OR = 4.10, 95% CI: 2.30–7.32) and Aldosterone synthase (CYP11B2) rs1912 TT genotype (OR = 2.82, 95% CI: 1.57–5.06) genotypes.

Conclusions

Unhealthy lifestyle and genetic factors were associated with the prevalence of hypertension in Chinese Hui pilgrims and their interactions were also observed.

Electronic supplementary material

The online version of this article (10.1186/s12889-019-7357-1) contains supplementary material, which is available to authorized users.

Genetic susceptibility to salt-sensitive hypertension in a Han Chinese population: a validation study of candidate genes

Salt-sensitive hypertension is a complex disease associated with genetic factors. This study aimed to identify the association between 29 candidate single-nucleotide polymorphisms and salt-sensitive hypertension in a Han Chinese population. Sixty-three participants with salt-sensitive hypertension and 279 controls with salt-resistant hypertension were recruited. A modified Sullivan's acute oral saline load and diuresis shrinkage test was used to detect blood pressure salt sensitivity. Lifestyle risk factors were obtained via a questionnaire. We used the Sequenom Mass ARRAY Platform to genotype the 29 candidate single-nucleotide polymorphisms, and the cumulative genetic risk score was used to evaluate the joint genetic effect. The frequencies of eight genotypes and five alleles in CYP11B2, PRKG1, ADRB2, FGF5, SLC8A1 and BCAT1 genes differed significantly between the salt-sensitive and salt-resistant hypertension groups. Multiple logistic regression adjusted for age and sex showed that subjects carrying rs7897633-A (PRKG1), rs434082-A (SLC8A1) and rs1042714-G (ADRB2) risk alleles had 1.83-, 2.84- and 2.40-fold increased risk for salt-sensitive hypertension, respectively. Combined risk allele analysis using the cumulative genetic risk score showed that subjects carrying one risk had 2.30-fold increased risk, and those carrying 2-4 risks had 3.32-fold increased risk for salt-sensitive hypertension. Among 29 candidate single-nucleotide polymorphisms, rs7897633-A in PRKG1, rs434082-A in SLC8A1 and rs1042714-G in ADRB2 were significantly associated with salt-sensitive hypertension. A joint effect of single-nucleotide polymorphisms from different pathways contributed to a high risk of salt-sensitive hypertension.

The blood pressure-salt sensitivity paradigm: pathophysiologically sound yet of no practical value

Sodium plays an important pathophysiological role in blood pressure (BP) values and in the development of hypertension, and epidemiological studies such as the Intersalt Study have shown that the increase in BP occurring with age is determined by salt intake. Recently, a meta-analysis of 13 prospective studies has also shown the close relationship between excess sodium intake and higher risk of stroke and total cardiovascular events. However, the BP response to changing salt intake displayed a marked variability, as first suggested by Kawasaki et al. (The effect of high-sodium and low-sodium intakes on blood pressure and other related variables in human subjects with idiopathic hypertension. Am J Med 1978; 64: 193-198) and later by Weinberger et al. (Definitions and characteristics of sodium sensitivity and blood pressure resistance. Hypertension 1986; 8: II127-II134), who recognized the heterogeneity of the BP response to salt and developed the concept of salt sensitivity. We have a large body of evidence in favour of a major role of metabolic and neuro-hormonal factors in determining BP salt sensitivity in association with the effect of genetic variation. There is evidence that salt sensitivity influences the development of organ damage, even independently-at least in part-of BP levels and the occurrence of hypertension. In addition, several observational studies indicate that salt sensitivity is clearly associated with a higher rate of cardiovascular events and mortality, independently of BP levels and hypertension. A cluster of factors with well-known atherogenic potential such as hyperinsulinaemia, dyslipidaemia and microalbuminuria-all known to be prevalent in salt-sensitive hypertension-might at least partially explain the increased cardiovascular risk observed in salt sensitive individuals. The gold standard for the evaluation of BP salt sensitivity is the BP response to a moderate reduction of salt intake for several weeks; nevertheless, these protocols often suffer of poor patient compliance to dietary instructions. To overcome this problem, short-term tests have been proposed that evaluate either large differences in salt intake for a few days or the response to intravenous administration of saline solution and short-acting diuretics. Recently, the use of ambulatory BP measurement has been proposed for the clinical assessment of BP salt sensitivity. Noteworthy, BP salt sensitivity, in whomever or however assessed, behaves as a continuous variable but salt sensitivity is used as a categorical parameter, with salt-sensitive individuals being defined as those with a difference in BP between low- and high-sodium intake >10%, and salt-resistant subjects those in whom BP does not increase or shows an increase <5% under sodium loading. The general conclusion that can and should be drawn from the above considerations is that the paradigm of salt sensitivity, despite its important pathophysiological meaning, is not helpful, so far, to the practising physician in clinical practice nor is it relevant or useful to the design and implementation of a population-based strategy of salt intake reduction; however, further studies are warranted for an accurate assessment of the salt-sensitivity phenotype in clinical practice. In the absence of a population strategy for salt intake reduction, the aim should be the generation of a 'low sodium environment' allowing for a dietary salt intake tailored on true human requirements and not on deleterious lifestyle habits.

Diagnostic tools for hypertension and salt sensitivity testing

PURPOSE OF REVIEW

One-third of the world's population has hypertension and it is responsible for almost 50% of deaths from stroke or coronary heart disease. These statistics do not distinguish salt-sensitive from salt-resistant hypertension or include normotensives who are salt-sensitive even though salt sensitivity, independent of blood pressure, is a risk factor for cardiovascular and other diseases, including cancer. This review describes new personalized diagnostic tools for salt sensitivity.

RECENT FINDINGS

The relationship between salt intake and cardiovascular risk is not linear, but rather fits a J-shaped curve relationship. Thus, a low-salt diet may not be beneficial to everyone and may paradoxically increase blood pressure in some individuals. Current surrogate markers of salt sensitivity are not adequately sensitive or specific. Tests in the urine that could be surrogate markers of salt sensitivity with a quick turn-around time include renal proximal tubule cells, exosomes, and microRNA shed in the urine.

SUMMARY

Accurate testing of salt sensitivity is not only laborious but also expensive, and with low patient compliance. Patients who have normal blood pressure but are salt-sensitive cannot be diagnosed in an office setting and there are no laboratory tests for salt sensitivity. Urinary surrogate markers for salt sensitivity are being developed.

Salt: important element, invisible menace

Public health authorities have mounted campaigns aimed at educating Americans about the obesity epidemic and urging them to consume less sugar. Another food additive-salt-is also a culprit, and many experts believe it should be the target of our next major public health campaign. In addition to obesity, salt is associated with increased rates of cardiovascular disease (especially hypertension), gastric cancer, and osteoporosis. Most Americans consume much more salt than they need or is healthy, with up to 75% of it coming from prepared foods. To be successful, these campaigns must educate young consumers. These campaigns must also incorporate food manufacturers and change our dining environments so that low-sodium foods are accessible and affordable.

Population based strategy for dietary salt intake reduction: Italian initiatives in the European framework

Excess dietary sodium chloride (salt) intake is etiologically related to hypertension and cardiovascular disease (CVD). Moderate reduction of salt intake reduces blood pressure (BP) and is expected to contribute to reduce the risk of CVD. Previous community-based trials to reduce BP by means of salt reduction were very successful. The initial positive results of national strategies of dietary salt intake reduction in several European countries, driven by the initiative of the World Health Organisation (WHO) and non-governmental organisations such as the World Action of Salt and Health (WASH), have paved the way for action in other European Union (EU) member states. In Italy, several initiatives aiming at reduction of salt intake at the population level have been recently undertaken. These initiatives include i) the evaluation of current dietary habits promoted by the Working Group for Dietary Salt Reduction in Italy (GIRCSI); ii) the chemical analysis of the bread salt content, a major source of sodium intake in Italy, and the agreement between the bakers' associations and the Ministry of Health for a gradual reduction of the bread salt content; iii) the implementation of educational campaigns to increase population awareness, iv) the involvement of the food catering system. In the immediate future, food reformulation must be extended to other food categories in collaboration with industry, foods' salt targets ought to be defined, the food labelling system must be improved and population salt awareness must be further increased through educational campaigns. The GIRCSI Working Group is committed to pursue these objectives.

Genetics of salt-sensitive hypertension

The assessment of salt sensitivity of blood pressure is difficult because of the lack of universal consensus on definition. Regardless of the variability in the definition of salt sensitivity, increased salt intake, independent of the actual level of blood pressure, is also a risk factor for cardiovascular morbidity and mortality and kidney disease. A modest reduction in salt intake results in an immediate decrease in blood pressure, with long-term beneficial consequences. However, some have suggested that dietary sodium restriction may not be beneficial to everyone. Thus, there is a need to distinguish salt-sensitive from salt-resistant individuals, but it has been difficult to do so with phenotypic studies. Therefore, there is a need to determine the genes that are involved in salt sensitivity. This review focuses on genes associated with salt sensitivity, with emphasis on the variants associated with salt sensitivity in humans that are not due to monogenic causes. Special emphasis is given to gene variants associated with salt sensitivity whose protein products interfere with cell function and increase blood pressure in transgenic mice.

Computational analysis of candidate disease genes and variants for salt-sensitive hypertension in indigenous Southern Africans

Multiple factors underlie susceptibility to essential hypertension, including a significant genetic and ethnic component, and environmental effects. Blood pressure response of hypertensive individuals to salt is heterogeneous, but salt sensitivity appears more prevalent in people of indigenous African origin. The underlying genetics of salt-sensitive hypertension, however, are poorly understood. In this study, computational methods including text- and data-mining have been used to select and prioritize candidate aetiological genes for salt-sensitive hypertension. Additionally, we have compared allele frequencies and copy number variation for single nucleotide polymorphisms in candidate genes between indigenous Southern African and Caucasian populations, with the aim of identifying candidate genes with significant variability between the population groups: identifying genetic variability between population groups can exploit ethnic differences in disease prevalence to aid with prioritisation of good candidate genes. Our top-ranking candidate genes include parathyroid hormone precursor (PTH) and type-1angiotensin II receptor (AGTR1). We propose that the candidate genes identified in this study warrant further investigation as potential aetiological genes for salt-sensitive hypertension.

Population-Based Genomewide Genetic Analysis of Common Clinical Chemistry Analytes

Background: Recent technologies enable genetic association studies of common clinical analytes on a genomewide basis in populations numbering thousands of individuals. The first publications using these technologies are already revealing novel biological functions for both genic and nongenic loci, and are promising to transform knowledge about the biological networks underlying disease pathophysiology. These early studies have also led to development of a set of principles for conducting a successful genomewide association study (GWAS).

Content: This review focuses on these principles with emphasis on the use of GWAS for plasma-based analytes to better understand human disease, with examples from cardiovascular biology.

Conclusions: The correlation of common genetic variation on a genomewide basis with clinical analytes, or any other outcome of interest, promises to reveal how parts of the genome work together in human physiology. Nonetheless, performing a genomewide association study demands an awareness of very specific epidemiologic and analytic principles.

Interindividual variation in serum sodium and longitudinal blood pressure tracking in the Framingham Heart Study

BACKGROUND

Recent cross-sectional studies have suggested that higher serum sodium levels may be a marker of elevated blood pressure. It is unclear whether serum sodium levels are related to the risk of developing hypertension in the community.

METHODS

We investigated the association of serum sodium with longitudinal blood pressure tracking and incidence of hypertension in 2172 nonhypertensive Framingham Offspring Study participants (mean age 42 years, 54% women). We defined an increase in blood pressure as an increment of at least one category (as defined by the Joint National Committee on Prevention, Detection, Evaluation, and Treatment of High Blood Pressure), and incident hypertension as a systolic blood pressure of at least 140 or a diastolic blood pressure of at least 90 mmHg, or use of antihypertensive medications. Serum sodium was analyzed as a continuous variable, and as categories.

RESULTS

Cross-sectionally, serum sodium was not associated with systolic or diastolic blood pressure (P exceeded 0.10). On follow-up (mean 4.4 years), 805 participants (37%, 418 women) progressed by at least one blood pressure category, and 318 (15%, 155 women) developed new-onset hypertension. In multivariable logistic regression analyses (adjusting for age, sex, baseline blood pressure, diabetes, BMI, weight gain and smoking), serum sodium was not associated with blood pressure progression (odds ratio per SD increment 0.93, 95% confidence interval 0.85-1.03), or with hypertension incidence (odds ratio per SD increment 0.94, 95% confidence interval 0.82-1.08).

CONCLUSION

In our large community-based sample, serum sodium was not associated with blood pressure cross-sectionally, or with blood pressure tracking or hypertension incidence longitudinally.

Polymorphisms of EDNRB, ATG, and ACE genes in salt-sensitive hypertension

Almost 50% of hypertensive individuals manifest blood pressure changes in response to salt depletion or repletion and are termed "salt sensitive" (SS). Blunted activity of the endothelin (ET) system and the renin-angiotensin-aldosterone system (RAAS) have been reported as possible mechanisms contributing to salt sensitivity. Data are available that endothelin receptor subtype B (ETBR)-deficient rats develop salt-sensitive hypertension when fed a high-salt diet. Whether the ETBR gene (EDNRB) is involved in genetic predisposition to human salt-sensitive hypertension has not been studied so far. We screened EDNRB in 104 hypertensive patients (49 salt sensitive and 55 salt resistant) and 110 normotensive controls. No new sequence variation was found, but genotype distribution of the common polymorphism G1065A revealed that the AA + GA genotypes were significantly more frequent in salt-resistant than in salt-sensitive individuals (p = 0.007), suggesting a protective role for the A allele. We also screened angiotensinogen gene AGT M235T and angiotensin-converting enzyme insertion/deletion polymorphism ACE I/D and found an association between TT genotype and hypertension. A possible synergistic effect to salt-sensitive hypertension was found by combining EDNRB GG with ACE DD/ID genotypes. In conclusion, our data confirm the role of ET system and RAAS in salt-sensitive hypertension.

Salt craving: the psychobiology of pathogenic sodium intake

Ionic sodium, obtained from dietary sources usually in the form of sodium chloride (NaCl, common table salt) is essential to physiological function, and in humans salt is generally regarded as highly palatable. This marriage of pleasant taste and physiological utility might appear fortunate--an appealing taste helps to ensure that such a vital substance is ingested. However, the powerful mechanisms governing sodium retention and sodium balance are unfortunately best adapted for an environment in which few humans still exist. Our physiological and behavioral means for maintaining body sodium and fluid homeostasis evolved in hot climates where sources of dietary sodium were scarce. For many reasons, contemporary diets are high in salt and daily sodium intakes are excessive. High sodium consumption can have pathological consequences. Although there are a number of obstacles to limiting salt ingestion, high sodium intake, like smoking, is a modifiable behavioral risk factor for many cardiovascular diseases. This review discusses the psychobiological mechanisms that promote and maintain excessive dietary sodium intake. Of particular importance are experience-dependent processes including the sensitization of the neural systems underlying sodium appetite and the effects of sodium balance on hedonic state and mood. Accumulating evidence suggests that plasticity within the central nervous system as a result of experience with high salt intake, sodium depletion, or a chronic unresolved sodium appetite fosters enduring changes in sodium related appetitive and consummatory behaviors.