Blood pressure genetics: time to focus

The AGT epistasis pattern proposed a novel role for ZBED9 in regulating blood pressure: Tehran Cardiometabolic genetic study (TCGS)

INTRODUCTION

High blood pressure is widely regarded as the most important risk factor for cardiovascular diseases. Epistasis analysis may provide additional insight into the genetic basis of hypertension.

METHODS

A nested case-control design was used on 4214 unrelated Tehran Cardiometabolic Genetic Study (TCGS) adults to evaluate 65 SNPs of previously associated genes, including ZBED9, AGT, and TNXB. The integrated effect of each gene was determined using the Sequence-based Kernel Association Test (SKAT). We used model-based multifactor dimension reduction (Mb-MDR) and entropy-based gene-gene interaction (IGENT) methods to determine interaction and epistasis patterns.

RESULTS

The integrated effect of each gene has a statistically significant association with blood pressure traits (P-value < 0.05). The single-locus analysis identified two missense variants in ZBED9 (rs450630) and AGT (rs4762) associated with hypertension. In the ZBED9 gene, significant local interactions were discovered. The G allele in rs450630 showed an antagonistic effect on hypertension, but interestingly, IGENT analysis revealed significant epistasis effects for different combinations of ZBED9, AGT, and TNXB loci.

CONCLUSION

We discovered a novel interaction effect between a significant variant in an essential gene for hypertension (AGT) and a missense variant in ZBED9, which has shifted our focus to ZBED9's role in blood pressure regulation.

Dietary sodium and health: more than just blood pressure

Sodium is essential for cellular homeostasis and physiological function. Excess dietary sodium has been linked to elevations in blood pressure (BP). Salt sensitivity of BP varies widely, but certain subgroups tend to be more salt sensitive. The mechanisms underlying sodium-induced increases in BP are not completely understood but may involve alterations in renal function, fluid volume, fluid-regulatory hormones, the vasculature, cardiac function, and the autonomic nervous system. Recent pre-clinical and clinical data support that even in the absence of an increase in BP, excess dietary sodium can adversely affect target organs, including the blood vessels, heart, kidneys, and brain. In this review, the investigators review these issues and the epidemiological research relating dietary sodium to BP and cardiovascular health outcomes, addressing recent controversies. They also provide information and strategies for reducing dietary sodium.

Genetic Variation in the Human SORBS1 Gene is Associated With Blood Pressure Regulation and Age at Onset of Hypertension: A SAPPHIRe Cohort Study

Essential hypertension is a complex disease involving multiple genetic and environmental factors. A human gene containing a sorbin homology domain and 3 SH3 domains in the C-terminal region, termed SORBS1, plays a significant role in insulin signaling. We previously found a significant association between the T228A polymorphism and insulin resistance, obesity, and type 2 diabetes. It has been hypothesized that a set of genes responsible for insulin resistance may be closely linked with genes susceptible to the development of hypertension. Identification of insulin resistance-related genetic factors may, therefore, enhance our understanding of essential hypertension. This study aimed to examine whether common SORBS1 genetic variations are associated with blood pressure and age at onset of hypertension in an ethnic Chinese cohort.We genotyped 9 common tagged single nucleotide polymorphisms of the SORBS1 gene in 1136 subjects of Chinese origin from the Stanford Asia-Pacific Program for Hypertension and Insulin Resistance family study. Blood pressure was measured upon enrolment. The associations of the SORBS1 single nucleotide polymorphisms with blood pressure and the presence of hypertension were analyzed with a generalized estimating equation model. We used the false-discovery rate measure Q value with a cutoff <0.1 to adjust for multiple comparisons. In the Cox regression analysis for hypertension-free survival, a robust sandwich variance estimator was used to deal with the within-family correlations with age at onset of hypertension. Gender, body mass index, and antihypertension medication were adjustment covariates in the Cox regression analysis.In this study, genetic variants of rs2281939 and rs2274490 were significantly associated with both systolic and diastolic blood pressure. A genetic variant of rs2274490 was also significantly associated with the presence of hypertension. Furthermore, genetic variants of rs2281939 and rs2274490 were associated with age at onset of hypertension after adjustment for gender, body mass index, and antihypertension medication.In conclusion, we provide evidence for an association between common SORBS1 genetic variations and blood pressure, presence of hypertension, and age at onset of hypertension. The biological mechanism of genetic variation associated with blood pressure regulation needs further investigation.

Human GRK4γ142V Variant Promotes Angiotensin II Type I Receptor-Mediated Hypertension via Renal Histone Deacetylase Type 1 Inhibition

The influence of a single gene on the pathogenesis of essential hypertension may be difficult to ascertain, unless the gene interacts with other genes that are germane to blood pressure regulation. G-protein-coupled receptor kinase type 4 (GRK4) is one such gene. We have reported that the expression of its variant hGRK4γ(142V) in mice results in hypertension because of impaired dopamine D1 receptor. Signaling through dopamine D1 receptor and angiotensin II type I receptor (AT1R) reciprocally modulates renal sodium excretion and blood pressure. Here, we demonstrate the ability of the hGRK4γ(142V) to increase the expression and activity of the AT1R. We show that hGRK4γ(142V) phosphorylates histone deacetylase type 1 and promotes its nuclear export to the cytoplasm, resulting in increased AT1R expression and greater pressor response to angiotensin II. AT1R blockade and the deletion of the Agtr1a gene normalize the hypertension in hGRK4γ(142V) mice. These findings illustrate the unique role of GRK4 by targeting receptors with opposite physiological activity for the same goal of maintaining blood pressure homeostasis, and thus making the GRK4 a relevant therapeutic target to control blood pressure.

Renal dopaminergic system: Pathophysiological implications and clinical perspectives

Fluid homeostasis, blood pressure and redox balance in the kidney are regulated by an intricate interaction between local and systemic anti-natriuretic and natriuretic systems. Intrarenal dopamine plays a central role on this interactive network. By activating specific receptors, dopamine promotes sodium excretion and stimulates anti-oxidant and anti-inflammatory pathways. Different pathological scenarios where renal sodium excretion is dysregulated, as in nephrotic syndrome, hypertension and renal inflammation, can be associated with impaired action of renal dopamine including alteration in biosynthesis, dopamine receptor expression and signal transduction. Given its properties on the regulation of renal blood flow and sodium excretion, exogenous dopamine has been postulated as a potential therapeutic strategy to prevent renal failure in critically ill patients. The aim of this review is to update and discuss on the most recent findings about renal dopaminergic system and its role in several diseases involving the kidneys and the potential use of dopamine as a nephroprotective agent.

Common variants of the G protein-coupled receptor type 4 are associated with human essential hypertension and predict the blood pressure response to angiotensin receptor blockade

Non-synonymous GRK4 variants, R65L, A142V and A486V, are associated with essential hypertension in diverse populations. This study replicated the association of GRK4 variants, including GRK4(142V), with human essential hypertension in a Japanese population (n=588; hypertensive, n=486 normotensive controls) and determined whether the presence of GRK4 variants predicted the blood pressure (BP) response to angiotensin receptor blockers (ARBs) in patients with essential hypertension. We analyzed 829 patients and compared the response to ARBs between individuals with no GRK4 variants (n=136) and those with variants at one or any of the three loci (n=693). Carriers of hGRK4(142V) had a greater decrease in systolic BP in response to ARBs than non-carrier hypertensive patients. By contrast, those with variants only at GRK4(486V) were less likely to achieve the BP goal in response to an ARB than those with no variants. These studies showed for the first time the association between GRK4(142V) and a larger decrease in BP with ARBs in hypertensive patients.

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.

Epistatic study reveals two genetic interactions in blood pressure regulation

Background

Although numerous candidate gene and genome-wide association studies have been performed on blood pressure, a small number of regulating genetic variants having a limited effect have been identified. This phenomenon can partially be explained by possible gene-gene/epistasis interactions that were little investigated so far.

Methods

We performed a pre-planned two-phase investigation: in phase 1, one hundred single nucleotide polymorphisms (SNPs) in 65 candidate genes were genotyped in 1,912 French unrelated adults in order to study their two-locus combined effects on blood pressure (BP) levels. In phase 2, the significant epistatic interactions observed in phase 1 were tested in an independent population gathering 1,755 unrelated European adults.

Results

Among the 9 genetic variants significantly associated with systolic and diastolic BP in phase 1, some may act through altering the corresponding protein levels: SNPs rs5742910 (P_adjusted≤0.03) and rs6046 (P_adjusted =0.044) in F7 and rs1800469 (P_adjusted ≤0.036) in TGFB1; whereas some may be functional through altering the corresponding protein structure: rs1800590 (P_adjusted =0.028, SE=0.088) in LPL and rs2228570 (P_adjusted ≤9.48×10^-4) in VDR. The two epistatic interactions found for systolic and diastolic BP in the discovery phase: VCAM1 (rs1041163) * APOB (rs1367117), and SCGB1A1 (rs3741240) * LPL (rs1800590), were tested in the replication population and we observed significant interactions on DBP. In silico analyses yielded putative functional properties of the SNPs involved in these epistatic interactions trough the alteration of corresponding protein structures.

Conclusions

These findings support the hypothesis that different pathways and then different genes may act synergistically in order to modify BP. This could highlight novel pathophysiologic mechanisms underlying hypertension.

Functional epistatic interaction between rs6046G>A in F7 and rs5355C>T in SELE modifies systolic blood pressure levels

Background

Although numerous genetic studies have been performed, only 0.9% of blood pressure phenotypic variance has been elucidated. This phenomenon could be partially due to epistatic interactions. Our aim was to identify epistatic interaction(s) associated with blood pressure levels in a pre-planned two-phase approach.

Methods and Results

In a discovery cohort composed of 3,600 French individuals, we found rs6046A allele in F7 associated with decreased blood pressure levels (P≤3.7×10⁻³) and rs5355T allele in SELE associated with decreased diastolic blood pressure levels (P = 5×10⁻³). Both variants interacted in order to influence blood pressure levels (P≤0.048). This interaction was replicated with systolic blood pressure in 4,620 additional European individuals (P = 0.03). Similarly, in this replication cohort, rs6046A was associated with decreased blood pressure levels (P≤8.5×10⁻⁴). Furthermore, in peripheral blood mononuclear cells of a subsample of 90 supposed healthy individuals, we found rs6046A positively associated with NAMPT mRNA levels (P≤9.1×10⁻⁵), suggesting an eventual involvement of NAMPT expression in blood pressure regulation. Confirming this hypothesis, further transcriptomic analyses showed that increased NAMPT mRNA levels were positively correlated with ICAM1, SELL, FPR1, DEFA1-3, and LL-37 genes expression (P≤5×10⁻³). The last two mRNA levels were positively associated with systolic blood pressure levels (P≤0.01) and explained 4% of its phenotypic variation.

Conclusion

These findings reveal the importance of epistatic interactions in blood pressure genetics and give new insights for the role of inflammation in its complex regulation.

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.

The epithelial sodium channel γ-subunit gene and blood pressure: family based association, renal gene expression, and physiological analyses

Variants in the gene encoding the γ-subunit of the epithelial sodium channel (SCNN1G) are associated with both Mendelian and quantitative effects on blood pressure. Here, in 4 cohorts of 1611 white European families composed of a total of 8199 individuals, we undertook staged testing of candidate single-nucleotide polymorphisms for SCNN1G (supplemented with imputation based on data from the 1000 Genomes Project) followed by a meta-analysis in all of the families of the strongest candidate. We also examined relationships between the genotypes and relevant intermediate renal phenotypes, as well as expression of SCNN1G in human kidneys. We found that an intronic single-nucleotide polymorphism of SCNN1G (rs13331086) was significantly associated with age-, sex-, and body mass index-adjusted blood pressure in each of the 4 populations (P<0.05). In an inverse variance-weighted meta-analysis of this single-nucleotide polymorphism in all 4 of the populations, each additional minor allele copy was associated with a 1-mm Hg increase in systolic blood pressure and 0.52-mm Hg increase in diastolic blood pressure (SE=0.33, P=0.002 for systolic blood pressure; SE=0.21, P=0.011 for diastolic blood pressure). The same allele was also associated with higher 12-hour overnight urinary potassium excretion (P=0.04), consistent with increased epithelial sodium channel activity. Renal samples from hypertensive subjects showed a nonsignificant (P=0.07) 1.7-fold higher expression of SCNN1G compared with normotensive controls. These data provide genetic and phenotypic evidence in support of a role for a common genetic variant of SCNN1G in blood pressure determination.

Dopamine and renal function and blood pressure regulation

Dopamine is an important regulator of systemic blood pressure via multiple mechanisms. It affects fluid and electrolyte balance by its actions on renal hemodynamics and epithelial ion and water transport and by regulation of hormones and humoral agents. The kidney synthesizes dopamine from circulating or filtered L-DOPA independently from innervation. The major determinants of the renal tubular synthesis/release of dopamine are probably sodium intake and intracellular sodium. Dopamine exerts its actions via two families of cell surface receptors, D1-like receptors comprising D1R and D5R, and D2-like receptors comprising D2R, D3R, and D4R, and by interactions with other G protein-coupled receptors. D1-like receptors are linked to vasodilation, while the effect of D2-like receptors on the vasculature is variable and probably dependent upon the state of nerve activity. Dopamine secreted into the tubular lumen acts mainly via D1-like receptors in an autocrine/paracrine manner to regulate ion transport in the proximal and distal nephron. These effects are mediated mainly by tubular mechanisms and augmented by hemodynamic mechanisms. The natriuretic effect of D1-like receptors is caused by inhibition of ion transport in the apical and basolateral membranes. D2-like receptors participate in the inhibition of ion transport during conditions of euvolemia and moderate volume expansion. Dopamine also controls ion transport and blood pressure by regulating the production of reactive oxygen species and the inflammatory response. Essential hypertension is associated with abnormalities in dopamine production, receptor number, and/or posttranslational modification.

Meta-analysis of genome-wide gene expression differences in onset and maintenance phases of genetic hypertension

Gene expression differences accompany both the onset and established phases of hypertension. By an integrated genome-transcriptome approach we performed a meta-analysis of data from 74 microarray experiments available on public databases to identify genes with altered expression in the kidney, adrenal, heart, and artery of spontaneously hypertensive and Lyon hypertensive rats. To identify genes responsible for the onset of hypertension we used a statistical approach that sought to eliminate expression differences that occur during maturation unrelated to hypertension. Based on this adjusted fold-difference statistic, we found 36 genes for which the expression differed between the prehypertensive phase and established hypertension. Genes having possible relevance to hypertension onset included Actn2, Ankrd1, ApoE, Cd36, Csrp3, Me1, Myl3, Nppa, Nppb, Pln, Postn, Spp1, Slc21a4, Slc22a2, Thbs4, and Tnni3. In established hypertension 102 genes exhibited altered expression after Bonferroni correction (P<0.05). These included Atp5o, Ech1, Fabp3, Gnb3, Ldhb, Myh6, Lpl, Pkkaca, Vegfb, Vcam1, and reduced nicotinamide-adenine dinucleotide dehydrogenases. Among the genes identified, there was an overrepresentation of gene ontology terms involved in energy production, fatty acid and lipid metabolism, oxidation, and transport. These could contribute to increases in reactive oxygen species. Our meta-analysis has revealed many new genes for which the expression is altered in hypertension, so pointing to novel potential causative, maintenance, and responsive mechanisms and pathways.

Hypothesis: it is time to reconsider phenotypes in hypertension

The study of genes and mechanisms associated with hypertension is hampered by the heterogeneity of hypertensive patients. Refining the definition of hypertension is a potential means of improving the clarity of mechanistic studies, but the lack of intermediate phenotypes hinders the assessment of causal relationships. Looking at younger individuals and hemodynamic subsets of hypertension is one such refinement. The authors argue that the separate analysis of patients with isolated diastolic hypertension, predominantly diastolic hypertension, and isolated systolic hypertension in the young in combination with common biomarkers may be an initial step to decrease heterogeneity within patient subsets, thus providing new avenues for genetic and pathophysiological studies.

Dopamine and G protein-coupled receptor kinase 4 in the kidney: role in blood pressure regulation

Complex interactions between genes and environment result in a sodium-induced elevation in blood pressure (salt sensitivity) and/or hypertension that lead to significant morbidity and mortality affecting up to 25% of the middle-aged adult population worldwide. Determining the etiology of genetic and/or environmentally-induced high blood pressure has been difficult because of the many interacting systems involved. Two main pathways have been implicated as principal determinants of blood pressure since they are located in the kidney (the key organ responsible for blood pressure regulation), and have profound effects on sodium balance: the dopaminergic and renin-angiotensin systems. These systems counteract or modulate each other, in concert with a host of intracellular second messenger pathways to regulate sodium and water balance. In particular, the G protein-coupled receptor kinase type 4 (GRK4) appears to play a key role in regulating dopaminergic-mediated natriuresis. Constitutively activated GRK4 gene variants (R65L, A142V, and A486V), by themselves or by their interaction with other genes involved in blood pressure regulation, are associated with essential hypertension and/or salt-sensitive hypertension in several ethnic groups. GRK4γ 142Vtransgenic mice are hypertensive on normal salt intake while GRK4γ 486V transgenic mice develop hypertension only with an increase in salt intake. GRK4 gene variants have been shown to hyperphosphorylate, desensitize, and internalize two members of the dopamine receptor family, the D(1) (D(1)R) and D(3) (D(3)R) dopamine receptors, but also increase the expression of a key receptor of the renin-angiotensin system, the angiotensin type 1 receptor (AT(1)R). Knowledge of the numerous blood pressure regulatory pathways involving angiotensin and dopamine may provide new therapeutic approaches to the pharmacological regulation of sodium excretion and ultimately blood pressure control.