Association of Genetic Polymorphisms of Sodium-Calcium Exchanger 1 Gene, NCX1, with Hypertension in a Japanese General Population

Genetic Complexity of Sinoatrial Node Dysfunction

The pacemaker cells of the cardiac sinoatrial node (SAN) are essential for normal cardiac automaticity. Dysfunction in cardiac pacemaking results in human sinoatrial node dysfunction (SND). SND more generally occurs in the elderly population and is associated with impaired pacemaker function causing abnormal heart rhythm. Individuals with SND have a variety of symptoms including sinus bradycardia, sinus arrest, SAN block, bradycardia/tachycardia syndrome, and syncope. Importantly, individuals with SND report chronotropic incompetence in response to stress and/or exercise. SND may be genetic or secondary to systemic or cardiovascular conditions. Current management of patients with SND is limited to the relief of arrhythmia symptoms and pacemaker implantation if indicated. Lack of effective therapeutic measures that target the underlying causes of SND renders management of these patients challenging due to its progressive nature and has highlighted a critical need to improve our understanding of its underlying mechanistic basis of SND. This review focuses on current information on the genetics underlying SND, followed by future implications of this knowledge in the management of individuals with SND.

Interactions between calcium intake and polymorphisms in genes essential for calcium reabsorption and risk of colorectal neoplasia in a two-phase study

The SLC8A1 (solute carrier family 8, member 1) gene, encoding Na⁺ /Ca²⁺ exchanger, is essential in regulating calcium reabsorption and homeostasis. Calcium homeostasis plays a key role in cell proliferation and apoptosis. We hypothesized that polymorphisms in five calcium-regulating genes (SLC8A1, ATP2B1, CALB1, CALB2, and CABP1) interact with calcium intake in relation to the risk of colorectal neoplasia. A two-phase (discovery and replication) study was conducted within the Tennessee Colorectal Polyp Study, including a total of 1275 cases and 2811 controls. In Phase I, we identified six out of 135 SNPs that significantly interacted with calcium intake in relation to adenoma risk. In Phase II, the calcium intake by rs4952490 (SLC8A1) interaction was replicated (P_interaction  = 0.048). We found an inverse association between calcium intake (1000-2000 mg/day) and colorectal adenomas, particularly for multiple/advanced adenomas, among the G-allele carriers but not among homozygous carriers of the common variant (A) in rs4952490. In the joint analysis of SLC8A1, KCNJ1, and SLC12A1 SNPs, carriers of variant alleles in at least two genes and with calcium intake above the DRI (1000 mg/day) were approximately 30-57% less likely to have adenomas than those whose calcium intake was below the DRI. The association was stronger for multiple/advanced adenomas. No association was found among those who did not carry any variant alleles in these genes when calcium intake was below 2500 mg/day. These findings, if confirmed, may provide a new avenue for the personalized prevention of colorectal adenoma and cancer.

Pharmacogenetics of cardiac inotropy

The ability to stimulate cardiac contractility is known as positive inotropy. Endogenous hormones, such as adrenaline and several natural or synthetic compounds possess this biological property, which is invaluable in the modern cardiovascular therapy setting, especially in acute heart failure or in cardiogenic shock. A number of proteins inside the cardiac myocyte participate in the molecular pathways that translate the initial stimulus, that is, the hormone or drug, into the effect of increased contractility (positive inotropy). Genetic variations (polymorphisms) in several genes encoding these proteins have been identified and characterized in humans with potentially significant consequences on cardiac inotropic function. The present review discusses these polymorphisms and their effects on cardiac inotropy, along with the individual pharmacogenomics of the most important positive inotropic agents in clinical use today. Important areas for future investigations in the field are also highlighted.

Attenuated renal vascular responses to acute angiotensin II infusion in smooth muscle-specific Na+/Ca2+ exchanger knockout mice

Recent studies in smooth muscle-specific Na(+)/Ca(2+) exchanger-1 knockout (NCX1(sm-/-)) mice reveal reduced arterial pressure and impaired myogenic responses compared with heterozygous littermates. In this study, we determined renal function in male anesthetized NCX1(sm-/-) mice and NCX1 heterozygous (NCX1(+/-)) littermates before and during acute ANG II infusions. Systolic blood pressure in awake mice was lower in NCX1(sm-/-) mice compared with NCX1(+/-) mice (119 ± 4 vs. 131 ± 3 mmHg, P < 0.05). Acute ANG II infusions (5 ng·min(-1)·g(-1) body wt) increased mean arterial pressure in anesthetized NCX1(+/-) (109 ± 2 to 134 ± 3 mmHg, P < 0.001, n = 8) and NCX1(sm-/-) (101 ± 8 to 129 ± 8 mmHg, P < 0.01, n = 6) mice to a similar extent (Δ25 ± 1 vs. Δ28 ± 4 mmHg, P > 0.05). In response to ANG II infusions, PAH clearance (C(PAH)) decreased from 1.39 ± 0.27 to 0.98 ± 0.22 ml·min(-1)·g(-1) (P < 0.05) and glomerular filtration rate (GFR) was reduced from 0.50 ± 0.09 to 0.32 ± 0.06 ml·min(-1)·g(-1) (P < 0.05) in NCX1(+/-) mice. In contrast, the NCX1(sm-/-) did not exhibit significant reductions in either C(PAH) (1.16 ± 0.30 to 1.22 ± 0.34 ml·min(-1)·g(-1), P > 0.05) or GFR (0.48 ± 0.08 to 0.41 ± 0.05 ml·min(-1)·g(-1), P > 0.05) during acute ANG II infusions. Using flometry to measure renal blood flow continuously, NCX1(sm-/-) mice had significantly attenuated responses to ANG II infusions (-34.2 ± 3.9%, P < 0.05) compared with those in NCX1(+/-) mice (-48 ± 2%) or in wild-type mice (-69 ± 7%). These data indicate that renal vascular responses to ANG II are attenuated in NCX1(sm-/-) mice compared with NCX1(+/-) mice and that NCX1 contributes to the renal vasoconstriction response to acute ANG II infusions.

Association of Asn221Ser mutation in tissue factor pathway inhibitor-beta with plasma total tissue factor pathway inhibitor level

Tissue factor pathway inhibitor (TFPI) is an anticoagulant protease inhibitor that inhibits the tissue factor-initiated blood coagulation cascade reactions. Based on these anticoagulant functions of TFPI, we hypothesized that genetic variations in TFPI may alter the TFPI expression or impair the anticoagulant function and could predispose persons to deep vein thrombosis (DVT). This study was undertaken to examine whether the genetic polymorphisms in TFPI are associated with the plasma TFPI levels and risk for DVT. We sequenced the entire coding regions of TFPI in 175 Japanese DVT patients and identified 12 genetic variants, including one missense mutation, Asn221Ser. The missense mutation occurred at the site presumably attached to the glycosylphosphatidylinositol anchor in the TFPI-beta form. The allele frequency of the mutant Ser-coding allele of the Asn221Ser mutation was 8% in the Japanese general population consisting of 1684 individuals. The Asn221Ser mutation was significantly associated with the total TFPI levels (Asn/Asn, n = 108, total TFPI = 56.57 +/- 0.88 ng/ml (mean +/- SD) vs. Asn/Ser + Ser/Ser, n = 16, total TFPI = 63.44 +/- 2.28 ng/ml, P = 0.0058). The genotype was not associated with the free TFPI level. This Asn221Ser mutation was not associated with DVT. Thus, the Asn221Ser mutation occurring in the TFPI-beta form was associated with the total TFPI level, but not a risk for DVT. The absence of the putative glycosylphosphatidylinositol anchor in TFPI-beta under pathological conditions remains to be studied.

Hypervariable intronic region in NCX1 is enriched in short insertion-deletion polymorphisms and showed association with cardiovascular traits

Background

Conserved non-coding regions (CNR) have been shown to harbor gene expression regulatory elements. Genetic variations in these regions may potentially contribute to complex disease susceptibility.

Methods

We targeted CNRs of cardiovascular disease (CVD) candidate gene, Na(+)-Ca(2+) exchanger (NCX1) with polymorphism screening among CVD patients (n = 46) using DHPLC technology. The flanking region (348 bp) of the 14 bp indel in intron 2 was further genotyped by DGGE assay in two Eastern-European CVD samples: essential hypertension (HYPEST; 470 cases, 652 controls) and coronary artery disease, CAD (CADCZ; 257 cases, controls 413). Genotype-phenotype associations were tested by regression analysis implemented in PLINK. Alignments of primate sequences were performed by ClustalW2.

Results

Nine of the identified NCX1 variants were either singletons or targeted by commercial platforms. The 14 bp intronic indel (rs11274804) was represented with substantial frequency in HYPEST (6.82%) and CADCZ (14.58%). Genotyping in Eastern-Europeans (n = 1792) revealed hypervariable nature of this locus, represented by seven alternative alleles. The alignments of human-chimpanzee-macaque sequences showed that the major human variant (allele frequency 90.45%) was actually a human-specific deletion compared to other primates. In humans, this deletion was surrounded by other short (5-43 bp) deletion variants and a duplication (40 bp) polymorphism possessing overlapping breakpoints. This indicates a potential indel hotspot, triggered by the initial deletion in human lineage. An association was detected between the carrier status of 14 bp indel ancestral allele and CAD (P = 0.0016, OR = 2.02; Bonferroni significance level alpha = 0.0045), but not with hypertension. The risk for the CAD development was even higher among the patients additionally diagnosed with metabolic syndrome (P = 0.0014, OR = 2.34). Consistent with the effect on metabolic processes, suggestive evidence for the association with heart rate, serum triglyceride and LDL levels was detected (P = 0.04).

Conclusions

Compared to SNPs targeted by large number of locus-specific and genome-wide assays, considerably less attention has been paid to short indel variants in the human genome. The data of genome dynamics, mutation rate and population genetics of short indels, as well as their impact on gene expressional profile and human disease susceptibility is limited. The characterization of NCX1 intronic hypervariable non-coding region enriched in human-specific indel variants contributes to this gap of knowledge.

Associations of hypertension and its complications with variations in the xanthine dehydrogenase gene

Hyperuricemia and oxidative stress participate in the pathophysiology of hypertension and its complications. Xanthine dehydrogenase (XDH) produces urate and, in its oxidase isoform, reactive oxygen species. Here we have studied whether or not the genetic variations in XDH could be implicated in hypertension and its complications. By sequencing the promoter region and all exons of XDH in 48 subjects, we identified three missense mutations (G172R, A932T, N1109T) in a heterozygous state in addition to 34 variations, including 15 common single nucleotide polymorphisms (SNPs). The three missense mutations and eight common SNPs (11488C>G, 37387A>G, 44408A>G, 46774G>A, 47686C>T, 49245A>T, 66292C>G, and 69901A>C) were genotyped in 953 hypertensive Japanese subjects and in 1,818 subjects from a general Japanese population. Four hypertensive patients with rare missense mutations (G172R or N1109T) in homozygous form had severe hypertension. Multivariate logistic regression analysis showed a significant association of three SNPs with hypertension in men: 47686C>T (exon 22, odds ratio [OR]: 1.52, p = 0.047) and 69901A>C (intron 31, OR: 3.14, p = 0.039) in the recessive model, and 67873A>C (N1109T) (exon 31, OR: 1.84, p = 0.018) in the dominant model. After full adjustment for all confounding factors, only one polymorphism (69901A>C) was found to be associated with carotid atherosclerosis in the dominant model (p = 0.028). Multiple logistic regression analysis showed that one SNP (66292C>G) was significantly associated with chronic kidney disease (CKD: estimated creatinine clearance < 60 ml/min) in the recessive model (p = 0.0006). Our results suggest that genetic variations in XDH contribute partly to hypertension and its complications, including atherosclerosis and CKD.

Protein tyrosine kinase 2beta as a candidate gene for hypertension

Protein tyrosine kinase 2beta (PTK2B) is a member of the focal adhesion kinase family and is activated by angiotensin II through Ca2+-dependent pathways. An evidence exists that PTK2B is involved in cell growth, vascular contraction, inflammatory responses, and salt and water retention through activation of the angiotensin II type 1 receptor. To examine the contribution of PTK2B, we sequenced the PTK2B gene using 48 patients with hypertension, identified 62 genetic polymorphisms, and genotyped six representative single nucleotide polymorphisms in population-based case-control samples from 3655 Japanese individuals (1520 patients with hypertension and 2135 controls). Multivariate logistic regression analysis after adjustments for age, body mass index, present illness (hyperlipidemia and diabetes mellitus), and lifestyle (smoking and drinking) showed -22A>G to have an association with hypertension in men (AA vs. AG+GG: odds ratio=1.27; 95% confidence interval: 1.02-1.57; P=0.030). Another polymorphism, 53484A>C (K838T), in linkage disequilibrium with -22A>G showed a marginal association with hypertension in men (AA vs. AC+CC: odds ratio=1.25; 95% confidence interval: 0.99-1.57; P=0.059). Diastolic blood pressure was 1.6 mmHg higher in men with the AC+CC genotype of 53484A>C than those with the AA genotype (P=0.003), after adjustments for the same factors. These polymorphisms are in linkage disequilibrium with others in a range of 113 kb in PTK2B. The intracellular distribution of the recombinant PTK2B protein and that of the mutant protein with T838 were indistinguishable even after angiotensin II stimulation, both proteins localizing at a focal point in the peripheral area in the cells. Thus, a haplotype in PTK2B may play a role in essential hypertension in Japanese.

Association of Genetic Polymorphisms of Endothelin-Converting Enzyme-1 Gene with Hypertension in a Japanese Population and Rare Missense Mutation in Preproendothelin-1 in Japanese Hypertensives

Endothelin-1 (EDN1), a 21-amino acid peptide, is a potent vasoconstrictor with various pharmacological responses. EDN1 is synthesized from a 212-amino acid precursor protein, preproEDN1, through multiple proteolytic steps. Endothelin-converting enzyme (ECE) cleaves a Trp73-Val74 peptide bond in big-EDN1 to give rise to mature EDN1. In this study, we examined the possible association of genetic variations in ECE1 with hypertension in a general Japanese population and searched for missense mutations in and around the EDN1 polypeptide. We genotyped 5 single nucleotide polymorphisms (SNPs) in the ECE1 gene in 1,873 individuals from a general Japanese population and identified one SNP associated with hypertension in women (rs212528: TT vs. TC+CC: odds ratio=1.40; 95% confidence intervals: 1.04-1.89; p=0.026), after adjusting for confounding factors. The systolic blood pressure in women with the CC genotype was 6.44 mmHg higher than that in those with the TT genotype (p=0.007), after adjusting for the same factors. Next, to identify the missense mutations that may influence the biological activity of EDN1, we sequenced the genomic region that encodes EDN1 in 942 Japanese hypertensive patients. We identified a novel missense mutation, G36R, in one hypertensive patient, but no mutations were observed in EDN1. A gene polymorphism in EDN1, Lys198Asn, has been reported to be associated with hypertension in obese subjects. Taken together, these findings reveal that the EDN-ECE pathway is an important system involved in essential hypertension in Japanese.

Genetic variations of HSD11B2 in hypertensive patients and in the general population, six rare missense/frameshift mutations

Mutations in the gene encoding 11beta-hydroxysteroid dehydrogenase type 2, HSD11B2, cause a rare monogenic juvenile hypertensive syndrome called apparent mineralocorticoid excess (AME). In AME, defective HSD11B2 enzyme activity results in overstimulation of the mineralocorticoid receptor (MR) by cortisol, causing sodium retention, hypokalemia, and salt-dependent hypertension. Here, we have studied whether genetic variations in HDS11B2 are implicated in essential hypertension in Japanese hypertensives and the general population. By sequencing the entire coding region and the promoter region of HDS11B2 in 953 Japanese hypertensives, we identified five missense mutations in 11 patients (L14F, n = 5; R74H, n = 1; R147H, n = 3; T156I, n = 1; R335H, n = 1) and one novel frameshift mutation (4884Gdel, n = 1) in a heterozygous state, in addition to 19 genetic variations. All genetic variations identified were rare, with minor allele frequencies less than 0.005. Four of 12 patients with the missense/frameshift mutations showed renal failure. Four missense mutations, L14F, R74H, R147H, and R335H, were successfully genotyped in the general population, with a sample size of 3,655 individuals (2,175 normotensives and 1,480 hypertensives). Mutations L14F, R74H, R147H, and R335H were identified in hypertensives (n = 6, 8, 3, and 0, respectively) and normotensives (n = 8, 12, 5, and 0, respectively) with a similar frequency, suggesting that these missense mutations may not strongly affect the etiology of essential hypertension. Since the allele frequency of all of the genetic variations identified in this study was rare, an association study was not conducted. Taken together, our results indicate that missense mutations in HSD11B2 do not substantially contribute to essential hypertension in Japanese.

Association of Sixty-One Non-Synonymous Polymorphisms in Forty-One Hypertension Candidate Genes with Blood Pressure Variation and Hypertension

We previously selected a group of hypertension candidate genes by a key word search using the OMIM database of NCBI and validated 525 coding single nucleotide polymorphisms (SNPs) in 179 hypertension candidate genes by DNA sequencing in a Japanese population. In the present study, we examined the association between 61 non-synonymous SNPs and blood pressure variations and hypertension. We used DNA samples taken from 1,880 subjects in the Suita study, a population-based study using randomly selected subjects. Analyses of covariance adjusting for age, body mass index, hyperlipidemia, diabetes, smoking, drinking, and antihypertensive medication revealed that 17 polymorphisms in 16 genes (APOB, CAST, CLCNKB, CTNS, GHR, GYS1, HF1, IKBKAP, KCNJ11, LIPC, LPL, P2RY2, PON2, SLC4A1, TRH, VWF) were significantly associated with blood pressure variations. Multivariate logistic regression analysis with adjustment for the same factors revealed that 11 polymorphisms in 11 genes (CAST, CTLA4, F5, GC, GHR, LIPC, PLA2G7, SLC4A1, SLCI8A1, TRH, VWF) showed significant associations with hypertension. Five polymorphisms in five genes, CAST(calpastatin), LIPC (hepatic lipase), SLC4A1 (band 3 anion transporter), TRH (thyrotropin-releasing hormone), and VWF (von Willebrand factor), were significantly associated with both blood pressure variation and hypertension. Thus, our study suggests that these five genes were susceptibility genes for essential hypertension in this Japanese population.

Single nucleotide polymorphisms in the interleukin-6 gene associated with blood pressure and atherosclerosis in a Japanese general population

It is known that increased plasma levels of inflammatory markers, such as interleukin-6 (IL-6), are associated with atherosclerosis and myocardial infarction. The aim of this study was to reveal the contribution of the single nucleotide polymorphisms (SNPs) of the IL-6 gene on the blood pressure regulation and progression of atherosclerosis in a general Japanese population. In order to evaluate the potential implications of genetic variability of the IL-6 gene, we explored eight SNPs by direct sequencing for the entire coding region and the promoter region in the IL-6 gene and genotyped two SNPs, -636G > C in the promoter region and 1691C > G in intron 3, for a total of 2,421 Japanese subjects (1,162 men and 1,259 women). As a consequence, -636 G > C was significantly associated with systolic blood pressure (SBP) and carotid intima-media thickness (IMT) in women, and 1691C > G showed a relationship with SBP and carotid IMT in men after adjustment for all confounding factors. Although neither SNP had a significant correlation to the prevalence of hypertension, the haplotype frequency analysis indicated that the number of hypertensive men with a G allele at both -636 and 1691 was significantly greater than the number of nonhypertensive men with this combination. Thus, these two SNPs in the promoter region and intron 3 of the IL-6 gene might play a role in the blood pressure regulation and progression of atherosclerosis in the Japanese.