Genome-wide scan for hypertension in Sydney Sibships: the GENIHUSS study

A haplotype at the UCP1 gene locus contributes to genetic risk for type 2 diabetes in Asian Indians (CURES-72)

BACKGROUND

The gene encoding for uncoupling protein-1 (UCP1) is considered to be a candidate gene for type 2 diabetes because of its role in thermogenesis and energy expenditure. The objective of the study was to examine whether genetic variations in the UCP1 gene are associated with type 2 diabetes and its related traits in Asian Indians.

METHODS

The study subjects, 810 type 2 diabetic subjects and 990 normal glucose tolerant (NGT) subjects, were chosen from the Chennai Urban Rural Epidemiological Study (CURES), an ongoing population-based study in southern India. The polymorphisms were genotyped using the polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) method. Linkage disequilibrium (LD) was estimated from the estimates of haplotypic frequencies.

RESULTS

The three polymorphisms, namely -3826A-->G, an A-->C transition in the 5'-untranslated region (UTR) and Met229Leu, were not associated with type 2 diabetes. However, the frequency of the A-C-Met (-3826A-->G-5'UTR A-->C-Met229Leu) haplotype was significantly higher among the type 2 diabetic subjects (2.67%) compared with the NGT subjects (1.45%, P < 0.01). The odds ratio for type 2 diabetes for the individuals carrying the haplotype A-C-Met was 1.82 (95% confidence interval, 1.29-2.78, P = 0.009).

CONCLUSIONS

The haplotype, A-C-Met, in the UCP1 gene is significantly associated with the increased genetic risk for developing type 2 diabetes in Asian Indians.

A novel locus for arterial hypertension on chromosome 1p36 maps to a metabolic syndrome trait cluster in the Sorbs, a Slavic population isolate in Germany

OBJECTIVE

Genome-wide linkage studies and genome-wide association studies have not as yet identified major genes contributing to primary hypertension in the general population. This state-of-affairs suggests considerable heterogeneity with small contributing effects for primary hypertension, or other complex genetic traits, in outbred populations. Isolated populations, as recent data from Iceland and French Canada suggest, could offer a solution to this problem.

METHODS

We studied a Slavic isolate in Germany, the Sorbs, and genotyped 1040 polymorphic microsatellite markers in 87 multigeneration families.

RESULTS

Our genome-wide linkage scan revealed a locus on chromosome 1p36.13 at D1S3669-D1S2826 (40.95 cM Marshfield coordinates; logarithm of the odds = 3.45, nominal P = 0.00003) that reached genome-wide significance (P = 0.004), indicating the increased power in isolated populations. The chromosome 1 locus maps to a region in which traits such as diabetes, hyperlipidemia, obesity and BMI cluster.

CONCLUSION

Our results suggest that this locus contributes to the metabolic syndrome, and that further attention in this and other populations is warranted.

Are RGS2 gene polymorphisms associated with high blood pressure in an ethnicity- and gender-specific manner?

BACKGROUND

Polymorphisms in the Regulator of G-protein Signaling 2 (RGS2) gene have been reported to be associated with hypertension (HT) in Japanese women and black Americans of either gender but not in white Americans or Japanese men. We have tested whether these proposed ethnicity- and gender-specific associations between RGS2 gene polymorphisms and HT can be confirmed in an independent population of male and female blacks, whites, and south Asians.

METHODS

A population-based sample of 1379 black, white Dutch, and south Asian subjects from the Amsterdam area was genotyped for eight polymorphisms in the RGS2 gene. All analyses were done separately per ethnic group. The phenotype high blood pressure was defined as a dichotomous variable comparing HT vs. normotension (NT) and as a linear variable using systolic blood pressure (SBP) in a multiple regression analysis with concomitant antihypertensive medication, age and body mass index as coexplanatory variables.

RESULTS

Ethnic differences in the frequency of polymorphisms and haplotypes (HAPs) derived thereof were in line with previous studies. Our data do not confirm previously reported ethnicity- or gender-specific associations regardless which phenotype definition was used. While the D allele of 1891-1892TC insertion/deletion polymorphism showed association in several groups, they differed from previously reported ones. Haplotype-phenotype analysis was not more sensitive to detect genotype-phenotype associations than individual alleles.

CONCLUSIONS

Previously reported ethnicity- and gender-specific associations of RGS2 genotype and hypertensive phenotype are not robust. Nevertheless, the 1891-1892TC insertion/deletion polymorphism warrants further investigation.

Genetic effect on blood pressure is modulated by age: the Hypertension Genetic Epidemiology Network Study

Genome-wide linkage analysis was performed for systolic and diastolic blood pressures in the Hypertension Genetic Epidemiology Network. We investigated the role of gene-age interactions using a recently developed variance components method that incorporates age variation in genetic effects. Substantially improved linkage evidence, in terms of both the number of linkage peaks and their significance levels, was observed. Twenty-six linkage peaks were identified with maximum logarithm of odds scores ranging between 3.0 and 4.6, 15 of which were cross-validated by the literature. The chromosomal region 1p36 that showed the highest logarithm of odds score in our study was found to be supported by evidence from 3 studies. The new method also led to vastly improved validation across ethnic groups. Ten of the 15 supported linkage peaks were cross-validated between 2 different ethnic groups, and 2 peaks on chromosomal region 1q31 and 16p11 were validated in 3 ethnic groups. In conclusion, this investigation demonstrates that genetic effects on blood pressure vary by age. The improved genetic linkage results presented here should help to identify the specific genetic variants that explain the observed results.

Linkage mapping of CVD risk traits in the isolated Norfolk Island population

To understand the underlying genetic architecture of cardiovascular disease (CVD) risk traits, we undertook a genome-wide linkage scan to identify CVD quantitative trait loci (QTLs) in 377 individuals from the Norfolk Island population. The central aim of this research focused on the utilization of a genetically and geographically isolated population of individuals from Norfolk Island for the purposes of variance component linkage analysis to identify QTLs involved in CVD risk traits. Substantial evidence supports the involvement of traits such as systolic and diastolic blood pressures, high-density lipoprotein-cholesterol, low-density lipoprotein-cholesterol, body mass index and triglycerides as important risk factors for CVD pathogenesis. In addition to the environmental influences of poor diet, reduced physical activity, increasing age, cigarette smoking and alcohol consumption, many studies have illustrated a strong involvement of genetic components in the CVD phenotype through family and twin studies. We undertook a genome scan using 400 markers spaced approximately 10 cM in 600 individuals from Norfolk Island. Genotype data was analyzed using the variance components methods of SOLAR. Our results gave a peak LOD score of 2.01 localizing to chromosome 1p36 for systolic blood pressure and replicated previously implicated loci for other CVD relevant QTLs.

Association study between hypertension and A/G polymorphism at codon 637 of the transporter associated with antigen processing 1 gene

To explore the effect of A/G polymorphisms at codon 637 of the transporter associated with antigen processing 1 (TAP1) gene on the risk of hypertension. A case-control study of epidemiology was conducted. The case group included 277 community-based patients (136 males and 141 females; mean age 58.7+/-12.1 years) diagnosed with hypertension, and the control group consisted of 227 healthy subjects (95 males and 132 females; mean age 51.29+/-12.16 years) from the same community. The A/G polymorphisms at codon 637 of the TAP1 gene was examined by the polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) method with genomic DNA. The effect of A/G polymorphisms at codon 637 of the TAP1 gene on hypertension was analyzed by using multivariate unconditional logistic regression models. The contribution of TAP1 637 A/G allele frequencies of the control group was consistent with that predicted by the Hardy-Weinberg equilibrium test (x2=230, p=0.632). There was a significant difference in the frequency of the A/G polymorphisms at codon 637 of the TAP1 gene between hypertensive patients (74.4/25.6%) and controls (82.4%/17.6%), x2=9.324, p=0.002. Genotype model (AA-AG-GG) analysis showed that there was a significant difference in the frequency of the recessive genotype between cases and controls (AA/AG vs. GG: odds ratio [OR]=3.046, 95% confidence interval [CI]=1.138-8.153) after adjustment for the covariates of age, serum total cholesterol, triglycerides, body mass index (BMI) and smoking. But there were no significant differences in the frequency of the genotype for the dominant model (AA vs.

AG/GG

p=0.293) or additive model (AA vs. AG vs. GG: p=0.081) after adjustment. One-way ANOVA analysis showed that the systolic blood pressure, diastolic blood pressure, and BMI levels of the GG genotype were significantly higher than those of the AA or AG genotypes. In conclusion, our findings suggest that the A/G polymorphisms at codon 637 of the TAP1 gene contributes to the risk of hypertension, possibly via the increases in blood pressure and BMI.

IGG: A tool to integrate GeneChips for genetic studies

To facilitate genetic studies using high-throughput genotyping technologies, we have developed an open source tool to integrate genotype data across the Affymetrix and Illumina platforms. It can efficiently integrate a large amount of data from various GeneChips, add genotypes of the HapMap Project into a specific project, flexibly trim and export the integrated data with different formats of popular genetic analysis tools, and highly control the quality of genotype data. Furthermore, this tool has sufficiently simplified its usage through its user-friendly graphic interface and is independent of third-party databases. IGG has successfully been applied to a genome-wide linkage scan in a Charcot-Marie-Tooth disease pedigree by integrating three types of GeneChips and HapMap project genotypes.

A review of the genetics of essential hypertension

PURPOSE OF REVIEW

Essential hypertension affects more than 20% of the adult population, and has a multifactorial origin arising from an interaction between susceptibility genes and environmental factors. Several strategies have been used to identify hypertension susceptibility genes. This review highlights recent efforts in genetic dissection of essential hypertension.

RECENT FINDINGS

Recently, further chromosomal regions harboring blood pressure loci have emerged in genome-wide linkage studies. Findings from a new systematic two-dimensional genome scan are presented, as well as sex-specific loci linked to hypertension in inbred rodent models. Many case-control association studies have been carried out, but results so far have been equivocal. This review discusses some interesting studies combining linkage and association strategies using gene-gene interactions, and studies the use of haplotypes instead of SNPs. Two novel hypertension susceptibility genes are presented, and a short summary on new insights into genes of the renin-angiotensin and adrenergic systems is given.

SUMMARY

To date, linkage and association studies have not been convincing. Genome-wide association studies may prove to be an effective approach to the problems posed by complex traits. Combined with candidate gene approaches, it is hoped this strategy will yield convincing evidence for genes associated with essential hypertension.

A hierarchical model for estimating significance levels of non-parametric linkage statistics for large pedigrees

The significance level of a non-parametric linkage (NPL) statistic is often found by simulation since the distribution of the test statistic is complex and unknown. Ideally, simulation occurs by randomly assigning founder genotypes and then simulating meiotic events for the descendants in the pedigree, commonly referred to as 'gene dropping'. The missing data pattern in the original pedigree, including lack of phase information (due to unordered genotypes), is then imposed on the simulated pedigree. However, this approach is usually computationally infeasible for larger pedigrees which require Markov chain Monte Carlo (MCMC) techniques to calculate the statistic, as an additional MCMC run is required to estimate the statistic for each gene drop. In this work, we propose a novel method to estimate the significance level of the NPL statistic in large pedigrees. This is accomplished by constructing a hierarchical model, which allows estimation of the NPL statistic variability via separate estimation of the Markov chain and gene dropping variability. The significance level is estimated by fitting a parametric model to the statistic and using the method of moments to obtain parameter estimates. In a simulation study we found our hierarchical model estimates to be very close to the gold standard empirical estimates, and offer substantial improvements over the existing conservative method used by the software SimWalk2. The estimation procedure significantly reduces the computational time relative to the ideal empirical estimate, allowing for an accurate estimate of the significance level in a more manageable amount of time.

The Future of Antihypertensive Treatment

Despite progress in recent years in the prevention, detection, and treatment of high blood pressure (BP), hypertension remains an important public health challenge. Hypertension affects approximately 1 billion individuals worldwide. High BP is associated with an increased risk of mortality and morbidity from stroke, coronary heart disease, congestive heart failure, and end-stage renal disease; it also has a negative impact on the quality of life. Hypertension cannot be eliminated because there are no vaccines to prevent the development of hypertension, but, its incidence can be decreased by reducing the risk factors for its development, which include obesity, high dietary intake of fat and sodium and low intake of potassium, physical inactivity, smoking, and excessive alcohol intake. For established hypertension, efforts are to be directed to control BP by lifestyle modification (LSM). However, if BP cannot be adequately controlled with LSM, then pharmacotherapy can be instituted along with LSM. Normalization of BP reduces cardiovascular risk (for cardiovascular death, myocardial infarction, and cardiac arrest), provides renoprotection (prevention of the onset or slowing of proteinuria and progression of renal dysfunction to end-stage renal disease in patients with hypertension, diabetes mellitus types 1 and 2, and chronic renal disease), and decreases the risk of cerebrovascular events (stroke and cognition impairment), as has been amply demonstrated by a large number of randomized clinical trials. In spite of the availability of more than 75 antihypertensive agents in 9 classes, BP control in the general population is at best inadequate. Therefore, antihypertensive therapy in the future or near future should be directed toward improving BP control in treated hypertensive patients with the available drugs by using the right combinations at optimum doses, individually tailored gene-polymorphism directed therapy, or development of new modalities such as gene therapy and vaccines. Several studies have shown that BP can be reduced by lifestyle/behavior modification. Although, the reductions appear to be trivial, even small reductions in systolic BP (for example, 3-5 mm Hg) produce dramatic reduction in adverse cardiac events and stroke. On the basis of the results of clinical and clinical/observational studies, it has been recommended that more emphasis be placed on lifestyle/behavior modification (obesity, high dietary intake of fat and sodium, physical inactivity, smoking, excessive alcohol intake, low dietary potassium intake) to control BP and also to improve the efficacy of pharmacologic treatment of high BP. New classes of antihypertensive drugs and new compounds in the established drug classes are likely to widen the armamentarium available to combat hypertension. These include the aldosterone receptor blockers, vasodilator beta-blockers, renin inhibitors, endothelin receptor antagonists, and dual endopeptidase inhibitors. The use of fixed-dose combination drug therapy is likely to increase. There is a conceptual possibility that gene therapy may yield long-lasting antihypertensive effects by influencing the genes associated with hypertension. But, the treatment of human essential hypertension requires sustained over-expression of genes. Some of the challenging tasks for successful gene therapy that need to be mastered include identification of target genes, ideal gene transfer vector, precise delivery of genes into the required site (target), efficient transfer of genes into the cells of the target, and prompt assessment of gene expression over time. Targeting the RAS by antisense gene therapy appears to be a viable strategy for the long-term control of hypertension. Several problems that are encountered in the delivery of gene therapy include 1) low efficiency for gene transfer into vascular cells; 2) a lack of selectivity; 3) problem in determining how to prolong and control transgene expression or antisense inhibition; and 4) difficulty in minimizing the adverse effects of viral or nonviral vectors. In spite of the hurdles that face gene therapy administration in humans, studies in animals indicate that gene therapy may be feasible in treating human hypertension, albeit not in the near future. DNA testing for genetic polymorphism and determining the genotype of a patient may predict response to a certain class of antihypertensive agent and thus optimize therapy in individual patients. In this regard, there are some studies that report the effectiveness of antihypertensive therapy based upon the genotype of selected patients. Treatment of human hypertension with vaccines is feasible but is not likely to be available in the near future.

Familial aggregation of insulin resistance and cardiovascular risk factors in hypertension

The authors assessed the familial aggregation of cardiometabolic abnormalities (elevated homeostasis model assessment [HOMA], triglycerides [TG], and low-density lipoprotein [LDL] and reduced high-density lipoprotein [HDL]) among hypertensive siblings (N=287 from 138 families). Evidence for familial aggregation required sibling-pair concordance of outcome variables dichotomized according to predefined values (concordance for highest-quartile HOMA [>3.3], TG [>170 mg/dL], and LDL [>138 mg/dL] and lowest-quartile HOMA for HDL [<32 mg/dL]). Hypertensive individuals with insulin resistance (high-quartile HOMA) had higher TG and lower HDL and LDL levels compared with insulin-sensitive hypertensives. High-quartile HOMA, TG, and LDL aggregated in hypertensive families, and TG plus HOMA coaggregated. HDL did not show aggregation. In a multivariate logistic regression, the only significant predictor of an individual's HOMA status was a sibling's HOMA status in a model including age, sex, race, and body mass index (odds ratio=9.12; 95% confidence interval, 3.64-23.14; P<.001). Cardiometabolic variables demonstrate heritability in hypertensive families. Further exploration of common genetic susceptibility loci in hypertension involving these factors is warranted.

The functional variant of the CLC-Kb channel T481S is not associated with blood pressure or hypertension in Swedes

OBJECTIVE

A common threonine481serine polymorphism (T481S) has been shown in vitro to strongly activate the chloride channel Kb (CLC-Kb) expressed in the kidney, and the 481S allele has been associated with human hypertension. The study aim was to evaluate the association of the T481S polymorphism with blood pressure (BP) levels and the BP progression rate in Swedes.

DESIGN AND METHODS

The cardiovascular cohort of the Malmö Diet and Cancer (MDC) study is a population surveyed in 1991-1996 (n=6103, DNA available on n=6055), 53% of whom had also been examined 11 +/- 4.4 years earlier in the Malmö Preventive Project (MPP). Hypertension was defined as having BP above 140/90 mmHg or being on antihypertensive therapy (AHT). Carriers of one or two copies of the 481S allele were compared with T481T homozygotes (noncarriers).

RESULTS

Among individuals without AHT in the MDC study (n=4988) there was no difference between carriers (n=1164, 23%) and noncarriers (n=3824, 77%) in systolic BP (139.3 +/- 8.3 vs 139.2 +/- 8.3 mmHg, P = 0.82) or diastolic BP (86.0 +/- 9.1 vs 86.0 +/- 9.2 mmHg, P = 0.95). In subjects free from AHT at the MPP and MDC studies (n=2627) there was no difference between carriers (n=607, 23%) and noncarriers (n=2020, 77%) in progression of systolic BP (2.1 +/- 2.6 vs 2.0 +/- 2.8 mmHg/year, P = 0.72) or diastolic BP (0.57 +/- 1.4 vs 0.58 +/- 1.6 mmHg/year, P = 0.85) from MPP to MDC. Multivariate analysis gave no support of interaction between the CLC-Kb T481S polymorphism, gender, age or body mass index regarding their effect on BP.

CONCLUSION

Our data do not support a role of the CLC-Kb T481S polymorphism in BP regulation in Swedes.

The molecular basis of pediatric hypertension

Blood pressure, the product of cardiac output and peripheral vascular resistance, follows a circadian rhythm and is altered by a host of circulating and local substances and by many physiologic events. The number of genes, signaling pathways, and systems involved in blood pressure regulation is enormous, and dissecting those factors that are most important in hypertension has proven challenging. This article discusses molecular mechanisms of hypertension in several conditions in which mutations in a single gene give rise to hypertension and then considers the contribution of these and other genes to essential hypertension.

Reduced expression of regulator of G-protein signaling 2 (RGS2) in hypertensive patients increases calcium mobilization and ERK1/2 phosphorylation induced by angiotensin II

CONTEXT

RGS2 (regulators of G-protein signaling) is a negative regulator of Galphaq protein signaling, which mediates the action of several vasoconstrictors. RGS2-deficient mouse line exhibits a hypertensive phenotype and a prolonged response to vasoconstrictors.

OBJECTIVE

To compare RGS2 expression in peripheral blood mononuclear cells (PBMs) and cultured fibroblasts from normotensive subjects and hypertensive patients.

METHODS

PBMs were isolated from 100 controls and 150 essential hypertensives. Additionally, fibroblasts were isolated from skin biopsy of 11 normotensives and 12 hypertensives and cultured up to the third passage. Quantitative mRNA and protein RGS2 expression were performed by real-time quantitative reverse transcriptase-polymerase chain reaction and by immunoblotting, respectively. Free Ca measurement was performed in monolayers of 24-h serum-deprived cells, using FURA-2 AM. Phosphorylation of the extracellular signal-regulated kinases ERK1/2 was measured by immunoblotting. Polymorphism (C1114G) in the 3' untranslated region of the RGS2 gene was investigated by direct sequencing and real-time polymerase chain reaction (PCR).

RESULTS

RGS2 mRNA expression was significantly lower in PBM and in fibroblasts from hypertensives, in comparison to normotensives. C1114G polymorphism was associated with RGS2 expression, with the lowest values in GG hypertensives. The 1114G allele frequency was increased in hypertensives compared with normotensives. Angiotensin II-stimulated intracellular Ca increase and ERK1/2 phosphorylation were higher in fibroblasts from hypertensive patients compared with control subjects, and in those with the G allele, independently of the blood pressure status. The angiotensin II-stimulated Ca mobilization and ERK1/2 phosphorylation were negatively correlated with RGS2 mRNA expression.

CONCLUSION

Low expression of RGS2 contributes to increased G-protein-coupled signaling in hypertensive patients. The allele G is associated with low RGS2 expression and blood pressure increase in humans.

Genome-Wide Scan for Linkage to Obesity-Associated Hypertension in French Canadians

Essential hypertension is a heterogeneous disorder that is thought to develop because of several overlapping subsets of underlying mechanisms. One such causal pathway may involve pathophysiological alterations induced by obesity. In the present study, we examined whether investigating clinically defined subtypes of hypertension, such as obesity-associated hypertension, facilitates the search for its genes. Fifty-five extended families were selected on the basis of having ≥2 siblings affected by hypertension from a geographically remote French-Canadian population. Fifteen of these families showed a high prevalence (≥70%) of obesity. Genome-wide scan using qualitative multipoint linkage analysis (GeneHunter 2.1; marker density <10 cM) was performed in the entire set of hypertensive families and the subset with high prevalence of obesity. In the scan involving all 55 families, the most significant loci (logarithm of odds [LOD] score=2.5) were identified on chromosomes 1 (D1S1597) and 11 (D11S1999). In the scan including only the subset of families with obesity-hypertension, the most significant locus (LOD score=3.1) was found on chromosome 1 in the same region as the scan involving all families (D1S1597). Genotyping additional markers increased the significance of this locus (LOD score=3.5) and refined its position (D1S2672). Several candidate genes of obesity-hypertension are located in close proximity; these include the tumor necrosis factor receptor 2 and atrial natriuretic peptide genes. These results suggest that investigating clinically defined subtypes of hypertension, such as obesity-associated hypertension, may facilitate the search for genes of this complex disorder.