WNK4 Intron 10 Polymorphism Is Not Associated With Hypertension

Molecular genetics of essential hypertension

Hypertension is a major public health problem in the developing as well as in developed countries due to its high prevalence and its association with coronary heart disease, renal disease, stroke, peripheral vascular disease, and related disorders. Essential hypertension (EH) is the most common diagnosis in this disease, suggesting that a monocausal etiology has not been identified. However, a number of risk factors associated with EH have also been identified such as age, sex, demographic, environmental, genetic, and vascular factors. Recent advances in molecular biological research had achieved clarifying the molecular basis of Mendelian hypertensive disorders. Molecular genetic studies have now identified mutations in several genes that cause Mendelian forms of hypertension in humans. However, none of the single genetic variants has emerged from linkage or association analyses as consistently related to the blood pressure level in every sample and in all populations. Besides, a number of polymorphisms in candidate genes have been associated with differences in blood pressure. The most prominent candidate has been the polymorphisms in the renin-angiotensin-aldosterone system. In total, EH is likely to be a polygenic disorder that results from inheritance of a number of susceptibility genes and involves multiple environmental determinants. These determinants complicate the study of blood pressure variations in the general population. The complex nature of the hypertension phenotype makes large-scale studies indispensable, when screening of familial and genetic factors was intended. In this review, recent genetic studies exploring the molecular basis of EH, including different molecular pathways, are highlighted.

Comprehensive assessment of the association of WNK4 polymorphisms with hypertension: evidence from a meta-analysis

The relationship between with-no-lysine [K] kinase 4 (WNK4) gene polymorphisms and hypertension has been widely investigated, However, the studies yielded contradictory results. To evaluate these inconclusive findings comprehensively, we therefore performed a meta-analysis. Ten articles encompassing 16 independent case-control studies with 6089 hypertensive cases and 4881 normotensive controls were selected for this meta-analysis. Four WNK4 gene polymorphisms were identified (G1155942T, G1156666A, T1155547C, and C6749T). The results showed statistically significant associations of G1155942T polymorphism (allelic genetic model: odds ration or OR = 1.62, 95% confidence interval or CI: 1.11–2.38, P = 0.01; dominant model: OR = 1.85, 95% CI: 1.07–3.19, P = 0.03) and C6749T polymorphism (allele contrast: OR = 2.04, 95% CI: 1.60–2.59, P<0.01; dominant model: OR = 2.04, 95%CI: 1.59–2.62, P<0.01; and homozygous model: OR = 5.01, 95% CI: 1.29–19.54, P = 0.02) with hypertension risk. However, neither C1155547T nor G1156666A was associated significantly with hypertension susceptibility. In conclusion, this meta-analysis suggested that WNK4 G1155942T and C6749T gene polymorphisms may contribute to the susceptibility and development of hypertension. Further well-designed studies with larger sample size are required to elucidate the association of WNK4 gene multiple polymorphisms with hypertension risk.

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 WNKs: atypical protein kinases with pleiotropic actions

WNKs are serine/threonine kinases that comprise a unique branch of the kinome. They are so-named owing to the unusual placement of an essential catalytic lysine. WNKs have now been identified in diverse organisms. In humans and other mammals, four genes encode WNKs. WNKs are widely expressed at the message level, although data on protein expression is more limited. Soon after the WNKs were identified, mutations in genes encoding WNK1 and -4 were determined to cause the human disease familial hyperkalemic hypertension (also known as pseudohypoaldosteronism II, or Gordon's Syndrome). For this reason, a major focus of investigation has been to dissect the role of WNK kinases in renal regulation of ion transport. More recently, a different mutation in WNK1 was identified as the cause of hereditary sensory and autonomic neuropathy type II, an early-onset autosomal disease of peripheral sensory nerves. Thus the WNKs represent an important family of potential targets for the treatment of human disease, and further elucidation of their physiological actions outside of the kidney and brain is necessary. In this review, we describe the gene structure and mechanisms regulating expression and activity of the WNKs. Subsequently, we outline substrates and targets of WNKs as well as effects of WNKs on cellular physiology, both in the kidney and elsewhere. Next, consequences of these effects on integrated physiological function are outlined. Finally, we discuss the known and putative pathophysiological relevance of the WNKs.

A quantitative trait locus for SBP maps near KCNB1 and PTGIS in a population isolate

BACKGROUND

Population isolates are characterized by simplified genetic background and as such present promising opportunities for studying complex diseases. We performed a genome-wide linkage analysis for systolic (SBP) and diastolic blood pressure (DBP) followed up by the association analysis in the Croatian isolated island of Vis, where a very high prevalence of hypertension was reported (75%).

METHODS

Variance-components linkage analysis was used to map quantitative trait loci (QTL) for SBP and DBP in 125 families with 1,389 members. Follow-up association analysis was performed in a sample of 421 subjects from the island of Vis. The 15 top-ranking single nucleotide polymorphisms (SNPs) were selected and tested for the association by in silico replication in the British 1958 Birth Cohort DNA Collection.

RESULTS

Linkage results showed evidence for a QTL influencing DBP (lod = 1.89) on chromosome 7p14.2 and two QTL influencing SBP (lod = 2.03 on chromosome 1p36 and lod = 1.75 on chromosome 20q13). For the association results, the replication was observed for the rs237484 polymorphism on chromosome 20 that was associated with SBP with the effect size beta = -5.2 (P = 0.001; per A allele) in Vis population and beta = -1.1 (P = 0.04) in the British 1958 Birth Cohort. rs237484 is in proximity to the potassium voltage gate channel gene (KCNB1) and close to the prostaglandin I2 (prostacyclin) synthase gene (PTGIS).

CONCLUSIONS

These results provide evidence of a QTL influencing blood pressure (BP) variability in this region and support the notion that the isolated population of the island of Vis is a suitable population for conducting linkage and association analyses of cardiovascular-related phenotypes.

Xylosyltransferase gene variants and their role in essential hypertension

BACKGROUND

An accumulation of extracellular matrix molecules, such as proteoglycans, is observed in the vascular wall of hypertensive patients. Xylosyltransferases I and II (XT-I and XT-II), the chain-initiating enzymes in the biosynthesis of proteoglycans, catalyze the transfer of D-xylose from UDP-D-xylose to specific serine residues of the core protein. Because associations between XYLT polymorphisms and an altered blood pressure have been observed, genetic variations in the XYLT genes might predispose to essential hypertension. The localization of the XYLT2 gene on chromosome 17q increases its attractiveness as this region has been reported to be a potential candidate locus for essential hypertension.

METHODS

Genotyping of four polymorphisms in the genes XYLT1 and XYLT2 was performed in 150 unrelated essential hypertension patients and 150 age- and sex-matched normotensive controls using restriction fragment length polymorphism analysis.

RESULTS

The allele and genotype frequencies of the XYLT variants investigated did not show any significant differences between patients and controls, among allele-carriers and nonallele-carriers and among recessive and nonrecessive allele-carriers comparing patients and controls. Systolic and diastolic blood pressures did not differ significantly between the genotypes concerning all XYLT variants analyzed. Two XYLT2 variants deviated from Hardy-Weinberg equilibrium (HWE) in the hypertensive group.

CONCLUSIONS

No statistically significant association was found between four XYLT variants and hypertension or blood pressure, suggesting that they do not play a significant role in the development of essential hypertension. The deviation from HWE of two XYLT2 variants might be due to gene-phenotype associations which remain to be explored, as well as the possibility of gene-gene interactions.

Association of Ala589Ser polymorphism of WNK4 gene with essential hypertension in a high-risk Chinese population

Recent studies on the association between particular single nucleotide polymorphisms of serine-threonine kinase with no lysine (K) 4 gene (WNK4) and essential hypertension have yielded controversial results. Here, frequencies of Ala589Ser polymorphism within exon 8 of the WNK4 gene were assessed among 259 unrelated ethnic Chinese patients with essential hypertension and 235 strictly matched normotensive controls. All subjects were derived from a relatively isolated population identified in the Kerqin desert region in Zhangwu county of Liaoning, northeastern China, which features a dry climate and the people having a high dietary salt intake, in addition to a significantly higher prevalence (approximately 35%) of essential hypertension. Genotypes were verified with polymerase chain reaction-restriction fragment length polymorphism and confirmed by direct sequencing. Expression pattern and regulatory mechanisms of the WNK4 gene were also explored using Northern blotting and in vitro hormone stimulation assays. Strong associations between the Ala589Ser polymorphism and both raised systolic and diastolic blood pressures were identified. In addition to the kidneys, WNK4 gene expression was also found in many other organs. Several cis-acting elements had been discovered in the promoter region of the gene. As revealed by preliminary experiment, various hormones can down-regulate the expression of WNK4, among which glucocorticoid hormone seems to act in a dose-dependent manner. The WNK4 gene probably plays an important role in the pathogenesis of essential hypertension. As a missense mutation, the Ala589Ser polymorphism may bring changes to the enzyme's function(s), resulting in increased susceptibility to the disease.

Genetics of arterial hypertension and hypotension

Human hypertension affects affects more than 20% of the adult population in industrialized countries, and it is implicated in millions of deaths worldwide each year from stroke, heart failure and ischemic heart disease. Available evidence suggests a major genetic impact on blood pressure regulation. Studies in monogenic hypertension revealed that renal salt and volume regulation systems are predominantly involved in the genesis of these disorders. Mutations here affect the synthesis of mineralocorticoids, the function of the mineralocorticoid receptor, epithelial sodium channels and their regulation by a new class of kinases, termed WNK kinases. It has been learned from monogenic hypotension that almost all ion transporters involved in the renal uptake of Na(+) have a major impact on blood pressure regulation. For essential hypertension as a complex disease, many candidate genes have been analysed. These include components of the renin-angiotensin-aldosterone system, adducin, beta-adrenoceptors, G protein subunits, regulators of G protein signalling (RGS) proteins, Rho kinases and G protein receptor kinases. At present, the individual impact of common polymorphisms in these genes on the observed blood pressure variation, on risk for stroke and as predictors of antihypertensive responses remains small and clinically irrelevant. Nevertheless, these studies have greatly augmented our knowledge on the regulation of renal functions, cellular signal transduction and the integration of both. Together, this provides the basis for the identification of novel drug targets and, hopefully, innovative antihypertensive drugs.

The search for the genetic basis of hypertension

PURPOSE OF REVIEW

This review surveys the literature on the search for the genetic basis of hypertension during the 10 months since November 2003. The goals set forth by this search are defined and the highlights of the work accomplished are provided.

RECENT FINDINGS

The search for the genetic basis of hypertension is ongoing, generating an abundance of new data. These data consist of a large number of candidate genes, association of previously known and novel candidate genes with various facets of hypertension, detection of new quantitative trait loci and identification of genes that mediate susceptibility to hypertension. The renin-zangiotensin-aldosterone system continues to dominate the interest of investigators. Other gene systems are also emerging but a single-gene system cannot be singled out beyond the renin-angiotensin-aldosterone system and the data are mostly sporadic and do not reflect a guided or coordinated effort to resolve unanswered issues. The notion that hypertension is polygenic is reinforced, yet few data are provided as to the actual number of genes involved, gene-gene interaction or gene-environment interaction. Advanced biotechnological tools involving transcriptomics and proteomics are underused.

SUMMARY

Research on the genetic basis of hypertension has generated over the past year a large number of candidate genes and tied them to various aspects of hypertension. How these genes fit into the complex pathophysiological network that induces hypertension remains unclear. The task of putting together these genes into a cohesive framework still lies ahead, but promises to enlighten us as to the true nature of hypertension, the pathogenic mechanisms involved and improved therapeutic and preventive measures.

Association of WNK1 gene polymorphisms and haplotypes with ambulatory blood pressure in the general population

BACKGROUND

Blood pressure (BP) is a heritable trait of major public health concern. The WNK1 and WNK4 genes, which encode proteins in the WNK family of serine-threonine kinases, are involved in renal electrolyte homeostasis. Mutations in the WNK1 and WNK4 genes cause a rare monogenic hypertensive syndrome, pseudohypoaldosteronism type II. We investigated whether polymorphisms in these WNK genes influence BP in the general population.

METHODS AND RESULTS

Associations between 9 single-nucleotide polymorphisms (SNPs) in WNK1 and 1 in WNK4 with ambulatory BP were studied in a population-based sample of 996 subjects from 250 white European families. The heritability estimates of mean 24-hour systolic BP (SBP) and diastolic BP (DBP) were 63.4% and 67.9%, respectively. We found statistically significant (P<0.05) associations of several common SNPs and haplotypes in WNK1 with mean 24-hour SBP and/or DBP. The minor allele (C) of rs880054, with a frequency of 44%, reduced mean 24-hour SBP and DBP by 1.37 (95% confidence interval, -2.45 to -0.23) and 1.14 (95% confidence interval, -1.93 to -0.38) mm Hg, respectively, per copy of the allele.

CONCLUSIONS

Common variants in WNK1 contribute to BP variation in the general population. This study shows that a gene causing a rare monogenic form of hypertension also plays a significant role in BP regulation in the general population. The findings provide a basis to identify functional variants of WNK1, elucidate any interactions of these variants with dietary intake or with response to antihypertensive drugs, and determine their impact on cardiovascular morbidity and mortality.

No association with hypertension of CLCNKB and TNFRSF1B polymorphisms at a hypertension locus on chromosome 1p36

OBJECTIVE

Chromosome 1p36 has been linked to essential hypertension and systolic blood pressure. This locus contains the chloride channel-Kb gene (CLCNKB) and the tumour necrosis factor receptor 2 gene (TNFRSF1B). Polymorphisms of each of these have shown association with hypertension, and a CLCNKB T481S variant alters receptor function. Here we performed association studies in a well-characterized cohort of hypertensives and normotensives whose blood pressure status matched that of both their parents.

METHODS

The study involved 196 essential hypertensives and 321 normotensives. These were genotyped for TNFRSF1B variants T-1710A upstream, A257G in exon 2, a CA-repeat polymorphism in intron 4, E232K and M196R in exon 6, and T1668G and T1690C in the 3'-untranslated region, and the T481S variant of CLCNKB.

RESULTS

The CLCNKB T481S variant showed no association with hypertension. Thermodynamic modelling of the 3'-untranslated region of TNFRSF1B mRNA predicted that the T1668G variant alters the stem-loop structure and thus the mRNA stability and expression. However, neither this nor the other TNFRSF1B polymorphisms, either alone or after haplotype analysis, were associated with hypertension. Moreover, for each gene the blood pressure, body mass index, plasma sodium and plasma lipid concentrations were generally similar across genotypes.

CONCLUSION

Our data fail to support previous association findings for TNFRSF1B and CLCNKB at the chromosome 1p36 locus implicated in hypertension.

Links between dietary salt intake, renal salt handling, blood pressure, and cardiovascular diseases

Epidemiological, migration, intervention, and genetic studies in humans and animals provide very strong evidence of a causal link between high salt intake and high blood pressure. The mechanisms by which dietary salt increases arterial pressure are not fully understood, but they seem related to the inability of the kidneys to excrete large amounts of salt. From an evolutionary viewpoint, the human species is adapted to ingest and excrete <1 g of salt per day, at least 10 times less than the average values currently observed in industrialized and urbanized countries. Independent of the rise in blood pressure, dietary salt also increases cardiac left ventricular mass, arterial thickness and stiffness, the incidence of strokes, and the severity of cardiac failure. Thus chronic exposure to a high-salt diet appears to be a major factor involved in the frequent occurrence of hypertension and cardiovascular diseases in human populations.

Identification of 108 SNPs in TSC, WNK1, and WNK4 and their association with hypertension in a Japanese general population

The deletion of thiazide-sensitive Na-Cl cotransporter ( TSC, SLC12A3) causes Gitelman's syndrome characterized by low blood pressure, while deletions of the WNK1 ( PRKWNK1) and WNK4 ( PRKWNK4) genes cause familial hypertension known as pseudohypoaldosteronism type II. Recent studies have revealed that cell surface expression of TSC is regulated by WNK1 and WNK4. We hypothesized that molecular variations in TSC, WNK1, and WNK4 could lead to an increased morbidity of hypertension. We identified 52, 35, and 21 polymorphisms in Japanese hypertensives by sequencing the entire coding regions of TSC, WNK1 and WNK4, respectively. Twenty-one representative polymorphisms were genotyped in 1,818 Japanese individuals (771 subjects with hypertension and 1,047 controls) randomly sampled in Suita city. The results indicated that the systolic blood pressure in men with the CT+TT genotype in WNK4 C14717T was 3.1 mmHg higher than those with the CC genotype ( p=0.042) after adjustment with confounding factors such as age, BMI, hyperlipidemia, diabetes mellitus, antihypertensive drug use, smoking, and drinking. Multivariate logistic regression analysis (with adjustment for the same parameters) in men revealed that the odds ratio for the presence of hypertension of the CT+TT genotype in C14717T to the CC genotype was 1.62 ( p=0.010, 95% confidence interval, 1.12-2.33). Association of TSC and WNK1 with hypertension was not observed. In conclusion, our study suggests the possible involvement of WNK4 in essential hypertension in a Japanese general population.