A genome-wide association study of hypertension and blood pressure in African Americans

Pre-hypertension in Uganda: a cross-sectional study

BACKGROUND

Persons with a systolic blood pressure (BP) of 120 to < 140 or diastolic BP of 80 to < 90 mm hg are classified as having pre-hypertension. Pre-hypertension is associated with cardiovascular disease (CVD) risk factors, incident CVD and CVD mortality. Understanding determinants of pre-hypertension especially in low income countries is a pre-requisite for improved prevention and control.

METHODS

Data were analyzed for 4142 persons aged 18 years and older with BP measured in a community cross sectional survey in Uganda. The prevalence of pre-hypertension was estimated and a number of risk factors e.g. smoking, use of alcohol, overweight, obesity, physical activity, sex, age, marital status, place of residence, and consumption of vegetables and fruits were compared among different groups (normotension, pre-hypertension, and hypertension) using bivariate and multivariable logistic regression.

RESULTS

The age standardized prevalence of normal blood pressure was 37.6%, pre-hypertension 33.9%, hypertension 28.5% and raised blood pressure 62%. There was no difference between the prevalence of hypertension among women compared to men (28.9% versus 27.9%). However, the prevalence of pre-hypertension was higher among men (41.6%) compared to women (29.4%). Compared to people with normal blood pressure, the risk of pre-hypertension was increased by being 40 years and above, smoking, consumption of alcohol, not being married, being male and being overweight or obese. Compared to pre-hypertension, hypertension was more likely if one was more than 40 years, had infrequent or no physical activity, resided in an urban area, and was obese or overweight.

CONCLUSIONS

More than one in three of adults in this population had pre-hypertension. Preventive and public health interventions that reduce the prevalence of raised blood pressure need to be implemented.

Genetic epidemiology of type 2 diabetes and cardiovascular diseases in Africa

The burdens of type 2 diabetes (T2D) and cardiovascular diseases (CVD) are increasing in Africa. T2D and CVD are the result of the complex interaction between inherited characteristics, lifestyle, and environmental factors. The epidemic of obesity is largely behind the exploding global incidence of T2D. However, not all obese individuals develop diabetes and positive family history is a powerful risk factor for diabetes and CVD. Recent implementations of high throughput genotyping and sequencing approaches have advanced our understanding of the genetic basis of diabetes and CVD by identifying several genomic loci that were not previously linked to the pathobiology of these diseases. However, African populations have not been adequately represented in these global genomic efforts. Here, we summarize the state of knowledge of the genetic epidemiology of T2D and CVD in Africa and highlight new genomic initiatives that promise to inform disease etiology, public health and clinical medicine in Africa.

Association between CDH13 variants and cardiometabolic and vascular phenotypes in a Korean population

PURPOSE

Although some CDH13 single nucleotide polymorphisms (SNPs) have been shown to be determinants of blood adiponectin levels, the clinical implications of CDH13 variants are not yet completely understood. The purpose of this study was to evaluate the effects of SNPs of CDH13 on metabolic and vascular phenotypes.

MATERIALS AND METHODS

We included 238 hypertensive subjects and 260 age- and sex-matched controls. Seven tagging-SNPs were identified in the CDH13 gene by whole gene sequencing. The association between these SNP variants and the risk of hypertension, metabolic traits, and carotid intima-media thickness (IMT) was examined.

RESULTS

Minor allele carriers of rs12444338 had a lower risk of hypertension, but the association turned out just marginal after adjusting confoudners. Blood glucose levels were higher in the minor allele carriers of c.1407C>T (p=0.01), whereas low-density lipoprotein-cholesterol levels were greater in those of rs6565105 (p=0.02). The minor allele of rs1048612 was associated with a higher body mass index (p=0.01). In addition, the mean carotid IMT was significantly associated with rs12444338 (p=0.02) and rs1048612 (p=0.02).

CONCLUSION

These results provide evidence that CDH13 variants are associated with metabolic traits and carotid atherosclerosis in Koreans. This study shows the multifaceted effects of CDH13 variants on cardiometabolic risk.

Functional analysis of seven genes linked to body mass index and adiposity by genome-wide association studies: a review

Genome-wide association studies (GWAS) have identified a total of about 40 single nucleotide polymorphisms (SNPs) that show significant linkage to body mass index, a widely utilised surrogate measure of adiposity. However, only 8 of these associations have been confirmed by follow-up GWAS using more sophisticated measures of adiposity (computed tomography). Among these 8, there is a SNP close to the gene FTO which has been the subject of considerable work to diagnose its function. The remaining 7 SNPs are adjacent to, or within, the genes NEGR1, TMEM18, ETV5, FLJ35779, LINGO2, SH2B1 and GIPR, most of which are less well studied than FTO, particularly in the context of obesity. This article reviews the available data on the functions of these genes, including information gleaned from studies in humans and animal models. At present, we have virtually no information on the putative mechanism associating the genes FLJ35779 and LINGO2 to obesity. All of these genes are expressed in the brain, and for 2 of them (SH2B1 and GIPR), a direct link to the appetite regulation system is known. SH2B1 is an enhancer of intracellular signalling in the JAK-STAT pathway, and GIPR is the receptor for an appetite-linked hormone (GIP) produced by the alimentary tract. NEGR1, ETV5 and SH2B1 all have suggested roles in neurite outgrowth, and hence SNPs adjacent to these genes may affect development of the energy balance circuitry. Although the genes have central patterns of gene expression, implying a central neuronal connection to energy balance, for at least 4 of them (NEGR1, TMEM18, SH2B1 and GIPR), there are also significant peripheral functions related to adipose tissue biology. These functions may contribute to their effects on the obese phenotype.

Association of genetic variants of CELSR1 and 3q28 with hypertension in community-dwelling individuals

Findings of previous studies demonstrated that rs6007897 (C→T, Ala2268Thr) of the cadherin, epidermal growth factor (EGF) laminin A G-type repeats (LAG) seven-pass G-type receptor 1 gene (CELSR1) and rs9846911 (A→G) at chromosome 3q28 were significantly associated with ischemic stroke and chronic kidney disease, respectively. Given that hypertension is a risk factor for both ischemic stroke and chronic kidney disease, it was hypothesized that the association of rs6007897 with ischemic stroke or of rs9846911 with chronic kidney disease might be attributable, at least in part, to their effects on genetic susceptibility to hypertension. The purpose of the present study was to examine a possible association of rs6007897 of CELSR1 or rs9846911 at 3q28 with hypertension in community-dwelling individuals. Study subjects comprised 5,959 community-dwelling individuals (1,670 subjects with hypertension and 4,289 controls) who were recruited to a population-based cohort study. Comparisons of allele frequencies by the Chi-square test revealed that rs6007897 of CELSR1 (P=0.0280) and rs9846911 at 3q28 (P=0.0171) were significantly associated with the prevalence of hypertension. Multivariate logistic regression analysis with adjustment for age, gender, body mass index (BMI), smoking status, the serum concentration of creatinine and the prevalence of dyslipidemia and diabetes mellitus revealed that rs6007897 (P=0.0308; recessive model; odds ratio, 1.56) and rs9846911 (P=0.0353; dominant model; odds ratio, 1.22) were significantly associated with hypertension with the T allele rs6007897 and the G allele rs984691 representing risk factors for this condition. CELSR1 and 3q28 may thus be susceptibility loci for hypertension.

Genome-wide association analysis of blood-pressure traits in African-ancestry individuals reveals common associated genes in African and non-African populations

High blood pressure (BP) is more prevalent and contributes to more severe manifestations of cardiovascular disease (CVD) in African Americans than in any other United States ethnic group. Several small African-ancestry (AA) BP genome-wide association studies (GWASs) have been published, but their findings have failed to replicate to date. We report on a large AA BP GWAS meta-analysis that includes 29,378 individuals from 19 discovery cohorts and subsequent replication in additional samples of AA (n = 10,386), European ancestry (EA) (n = 69,395), and East Asian ancestry (n = 19,601). Five loci (EVX1-HOXA, ULK4, RSPO3, PLEKHG1, and SOX6) reached genome-wide significance (p < 1.0 × 10(-8)) for either systolic or diastolic BP in a transethnic meta-analysis after correction for multiple testing. Three of these BP loci (EVX1-HOXA, RSPO3, and PLEKHG1) lack previous associations with BP. We also identified one independent signal in a known BP locus (SOX6) and provide evidence for fine mapping in four additional validated BP loci. We also demonstrate that validated EA BP GWAS loci, considered jointly, show significant effects in AA samples. Consequently, these findings suggest that BP loci might have universal effects across studied populations, demonstrating that multiethnic samples are an essential component in identifying, fine mapping, and understanding their trait variability.

Construction of gene clusters resembling genetic causal mechanisms for common complex disease with an application to young-onset hypertension

BACKGROUND

Lack of power and reproducibility are caveats of genetic association studies of common complex diseases. Indeed, the heterogeneity of disease etiology demands that causal models consider the simultaneous involvement of multiple genes. Rothman's sufficient-cause model, which is well known in epidemiology, provides a framework for such a concept. In the present work, we developed a three-stage algorithm to construct gene clusters resembling Rothman's causal model for a complex disease, starting from finding influential gene pairs followed by grouping homogeneous pairs.

RESULTS

The algorithm was trained and tested on 2,772 hypertensives and 6,515 normotensives extracted from four large Caucasian and Taiwanese databases. The constructed clusters, each featured by a major gene interacting with many other genes and identified a distinct group of patients, reproduced in both ethnic populations and across three genotyping platforms. We present the 14 largest gene clusters which were capable of identifying 19.3% of hypertensives in all the datasets and 41.8% if one dataset was excluded for lack of phenotype information. Although a few normotensives were also identified by the gene clusters, they usually carried less risky combinatory genotypes (insufficient causes) than the hypertensive counterparts. After establishing a cut-off percentage for risky combinatory genotypes in each gene cluster, the 14 gene clusters achieved a classification accuracy of 82.8% for all datasets and 98.9% if the information-short dataset was excluded. Furthermore, not only 10 of the 14 major genes but also many other contributing genes in the clusters are associated with either hypertension or hypertension-related diseases or functions.

CONCLUSIONS

We have shown with the constructed gene clusters that a multi-causal pie-multi-component approach can indeed improve the reproducibility of genetic markers for complex disease. In addition, our novel findings including a major gene in each cluster and sufficient risky genotypes in a cluster for disease onset (which coincides with Rothman's sufficient cause theory) may not only provide a new research direction for complex diseases but also help to reveal the disease etiology.

A systems biology framework identifies molecular underpinnings of coronary heart disease

OBJECTIVE

Genetic approaches have identified numerous loci associated with coronary heart disease (CHD). The molecular mechanisms underlying CHD gene-disease associations, however, remain unclear. We hypothesized that genetic variants with both strong and subtle effects drive gene subnetworks that in turn affect CHD.

APPROACH AND RESULTS

We surveyed CHD-associated molecular interactions by constructing coexpression networks using whole blood gene expression profiles from 188 CHD cases and 188 age- and sex-matched controls. Twenty-four coexpression modules were identified, including 1 case-specific and 1 control-specific differential module (DM). The DMs were enriched for genes involved in B-cell activation, immune response, and ion transport. By integrating the DMs with gene expression-associated single-nucleotide polymorphisms and with results of genome-wide association studies of CHD and its risk factors, the control-specific DM was implicated as CHD causal based on its significant enrichment for both CHD and lipid expression-associated single-nucleotide polymorphisms. This causal DM was further integrated with tissue-specific Bayesian networks and protein-protein interaction networks to identify regulatory key driver genes. Multitissue key drivers (SPIB and TNFRSF13C) and tissue-specific key drivers (eg, EBF1) were identified.

CONCLUSIONS

Our network-driven integrative analysis not only identified CHD-related genes, but also defined network structure that sheds light on the molecular interactions of genes associated with CHD risk.

The relationship between angiotensinogen gene polymorphisms and essential hypertension in a Northern Han Chinese population

Gene polymorphisms of the renin-angiotensin system are involved in the pathophysiology of hypertension. We genotyped 4 polymorphisms of angiotensinogen (AGT) gene A-20C (rs5050), A-6G (rs5051), C3889T (rs4762), and C4072T (rs699) by polymerase chain reaction-restriction fragment length polymorphism in 652 patients and 780 controls to examine the association of AGT and hypertension in a Northern Han Chinese population. There were significant differences in the distribution of genotypes and allele frequencies at C4072T between the patients and the controls (both P < .01); patients with CC genotype had a higher risk of hypertension (odds ratio = 1.7, 95% confidence interval 1.4-2.1). The distribution of genotypes at A-6G was significantly different between patients and controls (P < .05). No other significant differences in genotypes or frequencies were observed. No association was observed between the haplotypes of AGT and hypertension. The AGT-6A and 4072C alleles are associated with susceptibility to hypertension in this population.

Hypertension and genetic variation in endothelial-specific genes

Genome-wide association (GWA) studies usually detect common genetic variants with low-to-medium effect sizes. Many contributing variants are not revealed, since they fail to reach significance after strong correction for multiple comparisons. The WTCCC study for hypertension, for example, failed to identify genome-wide significant associations. We hypothesized that genetic variation in genes expressed specifically in the endothelium may be important for hypertension development. Results from the WTCCC study were combined with previously published gene expression data from mice to specifically investigate SNPs located within endothelial-specific genes, bypassing the requirement for genome-wide significance. Six SNPs from the WTCCC study were selected for independent replication in 5205 hypertensive patients and 5320 population-based controls, and successively in a cohort of 16537 individuals. A common variant (rs10860812) in the DRAM (damage-regulated autophagy modulator) locus showed association with hypertension (P = 0.008) in the replication study. The minor allele (A) had a protective effect (OR = 0.93; 95% CI 0.88–0.98 per A-allele), which replicates the association in the WTCCC GWA study. However, a second follow-up, in the larger cohort, failed to reveal an association with blood pressure. We further tested the endothelial-specific genes for co-localization with a panel of newly discovered SNPs from large meta-GWAS on hypertension or blood pressure. There was no significant overlap between those genes and hypertension or blood pressure loci. The result does not support the hypothesis that genetic variation in genes expressed in endothelium plays an important role for hypertension development. Moreover, the discordant association of rs10860812 with blood pressure in the case control study versus the larger Malmö Preventive Project–study highlights the importance of rigorous replication in multiple large independent studies.

Powerful tests for association on quantitative trait loci incorporating imprinting effects

Genomic imprinting is an important epigenetic factor in complex traits study, and there has recently been considerable interest in association study for quantitative traits by incorporating imprinting. However, these methods need the assumptions of Hardy-Weinberg equilibrium or only use information from families with one child. In this paper, by taking imprinting into account and making no assumption about the distribution of the quantitative traits, we propose two novel classes of Q-C-TDTI(c) and Q-C-MAX(c) family-based association tests for quantitative traits. The tests flexibly accommodate family data with missing parental genotype and with multiple siblings. Q-C-TDTI(c) is derived from a two-stage analysis, where in the first stage Q-C-PAT(c) is applied to test for imprinting effects and in the second stage we select the most appropriate statistic among three transmission disequilibrium tests for association according to the finding from Q-C-PAT(c). Another proposed Q-C-MAX(c) approach takes the maximum of the three statistics. Compared with the existing alternative methods, the simulation results demonstrate that the two proposed tests are robust to population stratification and have better performance for testing association under various scenarios. Further, the powerful and versatile Q-C-TDTI(c) test is applied to analyze Framingham Heart Study data.

STK39 polymorphism is associated with essential hypertension: a systematic review and meta-analysis

BACKGROUND

A recent genome-wide association study identified STK39as a candidate gene for blood pressure (BP) in Europeans. Subsequently, several studies have attempted to replicate the association across different ethnic populations. However, the results have been inconsistent.

OBJECTIVE AND METHODS

We performed a meta-analysis to elucidate the association between the STK39 rs3754777 polymorphism (or proxy) and hypertension. Published literature from PubMed and Embase databases were retrieved and pooled odds ratio (OR) with 95% confidence interval (CI) was calculated using fixed- or random-effects model.

RESULTS

Using appropriate inclusion/exclusion criteria, we identified 10 studies that included 21, 863 hypertensive cases and 24, 480 controls from different ethnicities. The meta-analysis showed a significant association of STK39 rs3754777 variant with hypertension (OR = 1.10, 95%CI = 1.06-1.15, p = 7.95 × 10(-6)). Further subgroup analysis by ethnicity suggested that the association was significant in Europeans (OR = 1.08, 95% CI = 1.03-1.14, p = 0.002) and in East Asians (OR = 1.16, 95%CI = 1.07-1.25, p = 4.34 × 10(-4)), but not in Africans (OR = 1.01, 95%CI 0.80-1.27, p = 0.932). We further confirmed the positive association by sensitivity analysis. No publication bias was detected (Begg's test, p = 0.721; Egger's test, p = 0.744).

CONCLUSIONS

The present meta-analysis confirms the significant association of STK39 polymorphism with susceptibility to hypertension in Europeans and East Asians. Future studies should include gene-gene and gene-environment interactions to investigate the identified association.

Carotid baroreflex responsiveness in normotensive African Americans is attenuated at rest and during dynamic leg exercise

Evidence suggests differences between African Americans (AAs) and Caucasian Americans (CAs) in cardiovascular responsiveness to physiological stressors. This study tested the hypothesis that carotid baroreflex (CBR) control of heart rate (HR) and blood pressure is reduced in AAs compared to CAs during exercise. Mean arterial pressure (MAP) and HR were continuously recorded at rest and during leg cycling in 23 non-hypertensive male subjects (12 AA; 11 CA; age 19–26 years). CBR control of HR and MAP was assessed with 5-s pulses of neck pressure (NP, simulated hypotension) and neck suction (NS, simulated hypertension) ranging from +45 to −80 Torr. Across all NS stimuli (−20, −40, −60, −80 Torr) at rest, the AA group demonstrated attenuated CBR-mediated reductions in HR (AA, −8.9 ± 1.9 vs. CA, −14.1 ± 2.3 bpm; P < 0.001) and MAP (AA, −6.4 ± 1 vs. CA, −7.8 ± 0.8 mmHg; P < 0.05). Despite similar gain and magnitude of resetting observed in the modeled stimulus response curves, an attenuation among AAs persisted in HR (AA, −8.2 ± 1.6 vs. CA, −11.8 ± 3 bpm; P < 0.05) and MAP (AA, −6.8 ± 0.9 vs. CA, −8.2 ± 1.1 mmHg; P < 0.05) responses to NS during exercise. No differences in CBR-mediated HR and MAP responses to NP were detected between groups at rest or during exercise. These data suggest impairment in the ability to defend against a hypertensive challenge among AAs during steady-state exercise compared to their CA counterparts.

The utility of copy number variation (CNV) in studies of hypertension-related left ventricular hypertrophy (LVH): rationale, potential and challenges

The ultimate goal of human genetics is to understand the role of genome variation in elucidating human traits and diseases. Besides single nucleotide polymorphism (SNP), copy number variation (CNV), defined as gains or losses of a DNA segment larger than 1 kb, has recently emerged as an important tool in understanding heritable source of human genomic differences. It has been shown to contribute to genetic susceptibility of various common and complex diseases. Despite a handful of publications, its role in cardiovascular diseases remains largely unknown. Here, we deliberate on the currently available technologies for CNV detection. The possible utility and the potential roles of CNV in exploring the mechanisms of cardiac remodeling in hypertension will also be addressed. Finally, we discuss the challenges for investigations of CNV in cardiovascular diseases and its possible implications in diagnosis of hypertension-related left ventricular hypertrophy (LVH).

Confounding and heterogeneity in genetic association studies with admixed populations

Association studies among admixed populations pose many challenges including confounding of genetic effects due to population substructure and heterogeneity due to different patterns of linkage disequilibrium (LD). We use simulations to investigate controlling for confounding by indicators of global ancestry and the impact of including a covariate for local ancestry. In addition, we investigate the use of an interaction term between a single-nucleotide polymorphism (SNP) and local ancestry to capture heterogeneity in SNP effects. Although adjustment for global ancestry can control for confounding, additional adjustment for local ancestry may increase power when the induced admixture LD is in the opposite direction as the LD in the ancestral population. However, if the induced LD is in the same direction, there is the potential for reduced power because of overadjustment. Furthermore, the inclusion of a SNP by local ancestry interaction term can increase power when there is substantial differential LD between ancestry populations. We examine these approaches in genome-wide data using the University of Southern California's Children's Health Study investigating asthma risk. The analysis highlights rs10519951 (P = 8.5 × 10(-7)), a SNP lacking any evidence of association from a conventional analysis (P = 0.5).

Comparing methods for performing trans-ethnic meta-analysis of genome-wide association studies

Genome-wide association studies (GWASs) have discovered thousands of variants that are associated with human health and disease. Whilst early GWASs have primarily focused on genetically homogeneous populations of European, East Asian and South Asian ancestries, the next-generation genome-wide surveys are starting to pool studies from ethnically diverse populations within a single meta-analysis. However, classical epidemiological strategies for meta-analyses that assume fixed- or random-effects may not be the most suitable approaches to combine GWAS findings as these either confer low statistical power or identify mostly loci where the variants carry homogeneous effect sizes that are present in most of the studies. In a trans-ethnic meta-analysis, it is likely that some genetic loci will exhibit heterogeneous effect sizes across the populations. This may be due to differences in study designs, differences arising from the interactions with other genetic variants, or genuine biological differences attributed to environmental, dietary or lifestyle factors that modulate the influence of the genes. Here we compare different strategies for meta-analyzing GWAS across genetically diverse populations, where we intentionally vary the effect sizes present across the different populations. We subsequently applied the methods that yielded the highest statistical power to a trans-ethnic meta-analysis of seven GWAS in type 2 diabetes, and showed that these methods identified bona fide associations that would otherwise have been missed by the classical strategies.

Prevalence of genetic renal disease in children

BACKGROUND

Genetic etiology comprises a significant proportion of renal disease in childhood. Completion of the Human Genome Project and increased genetic testing has assisted with the increased recognition of a genetic basis to many renal disorders. Australia and New Zealand have a relatively stable but diverse population, with eight major pediatric nephrology referral centers, which allow ascertainment of disease frequency.

METHODS

To determine prevalence, pediatric nephrologists at the eight centers in Australia and New Zealand were surveyed on their estimated number of patients with renal disease of genetic etiology over a 10-year period. Disease prevalence was calculated using combined national population data.

RESULTS

The overall prevalence of genetic kidney disease in children in Australia and New Zealand is 70.6 children per million age-representative population. Congenital anomalies of the kidney and urinary tract (CAKUT) and steroid-resistant nephrotic syndrome (SRNS) are the most frequent, with a prevalence of 16.3 and 10.7, respectively, per million children.

CONCLUSION

We find a similar prevalence of genetic renal disorders in Australia and New Zealand to those reported in other countries. This is likely to be due to inclusion of children with all forms of renal disease rather than being limited to those with renal impairment.

Variants in CXADR and F2RL1 are associated with blood pressure and obesity in African-Americans in regions identified through admixture mapping

OBJECTIVE

Genetic variants in 296 genes in regions identified through admixture mapping of hypertension, BMI, and lipids were assessed for association with hypertension, blood pressure (BP), BMI, and high-density lipoprotein cholesterol (HDL-C).

METHODS

This study identified coding SNPs identified from HapMap2 data that were located in genes on chromosomes 5, 6, 8, and 21, wherein ancestry association evidence for hypertension, BMI, or HDL-C was identified in previous admixture mapping studies. Genotyping was performed in 1733 unrelated African-Americans from the National Heart, Lung and Blood Institute's Family Blood Pressure Project, and gene-based association analyses were conducted for hypertension, SBP, DBP, BMI, and HDL-C. A gene score based on the number of minor alleles of each SNP in a gene was created and used for gene-based regression analyses, adjusting for age, age, sex, local marker ancestry, and BMI, as applicable. An individual's African ancestry estimated from 2507 ancestry-informative markers was also adjusted for to eliminate any confounding due to population stratification.

RESULTS

CXADR (rs437470) on chromosome 21 was associated with SBP and DBP with or without adjusting for local ancestry (P < 0.0006). F2RL1 (rs631465) on chromosome 5 was associated with BMI (P = 0.0005). Local ancestry in these regions was associated with the respective traits as well.

CONCLUSION

This study suggests that CXADR and F2RL1 likely play important roles in BP and obesity variation, respectively; and these findings are consistent with those of other studies, so replication and functional analyses are necessary.

Effects of rare and common blood pressure gene variants on essential hypertension: results from the Family Blood Pressure Program, CLUE, and Atherosclerosis Risk in Communities studies

RATIONALE

Hypertension affects ≈30% of adults in industrialized countries and is the major risk factor for cardiovascular disease.

OBJECTIVE

We sought to study the genetic effect of coding and conserved noncoding variants in syndromic hypertension genes on systolic blood pressure (BP) and diastolic BP to assess their overall impact on essential hypertension.

METHODS AND RESULTS

We resequenced 11 genes (AGT, CYP11B1, CYP17A1, HSD11B2, NR3C1, NR3C2, SCNN1A, SCNN1B, SCNN1G, WNK1, and WNK4) in 560 European American (EA) and African American ancestry GenNet participants with extreme systolic BP. We investigated genetic associations of 2535 variants with BP in 19997 EAs and in 6069 African Americans in 3 types of analyses. First, we studied the combined effects of all variants in GenNet. Second, we studied 1000 Genomes imputed polymorphic variants in 9747 EA and 3207 African American Atherosclerosis Risk in Communities subjects. Finally, we genotyped 37 missense and common noncoding variants in 6591 EAs and in 6521 individuals (3659 EA/2862 African American) from the CLUE and Family Blood Pressure Program studies, respectively. None of the variants individually reached significant false-discovery rates ≤0.05 for systolic BP and diastolic BP. However, on pooling all coding and noncoding variants, we identified at least 5 loci (AGT, CYP11B1, NR3C2, SCNN1G, and WNK1) with higher association at evolutionary conserved sites.

CONCLUSIONS

Both rare and common variants at these genes affect BP in the general population with modest effects sizes (<0.05 standard deviation units), and much larger sample sizes are required to assess the impact of individual genes. Collectively, conserved noncoding variants affect BP to a greater extent than missense mutations.

Rab38 modulates proteinuria in model of hypertension-associated renal disease

We previously reported that the fawn-hooded hypertensive (FHH) rat is a natural Rab38 knockout, supported by a congenic animal (FHH.BN-Rab38) having less proteinuria than FHH animals. Because these congenic animals contain Brown Norway (BN) alleles for five other named genes; however, a causal role for Rab38 in the FHH phenotype remains uncertain. Here, we used transgenic and knockout models to validate Rab38 and to exclude other genes within the 1.5 Mb congenic region from involvement in causing the FHH phenotype. Transgenic rats homozygous for the wild-type Rab38 BN allele on the FHH background exhibited phenotypic rescue, having 43% lower proteinuria and 75% lower albuminuria than nontransgenic FHH littermates. Conversely, knockout of the Rab38 gene on the FHH.BN-Rab38 congenic line recapitulated a proteinuric phenotype indistinguishable from the FHH strain. In addition, in cultured proximal tubule LLC-PK1 cells, knockdown of Rab38 mRNA significantly decreased endocytosis of colloidal gold-coupled albumin, supporting the hypothesis that Rab38 modulates proteinuria through effects on tubular re-uptake and not by altering glomerular permeability. Taken together, these findings validate Rab38 as a gene having a causal role in determining the phenotype of the FHH rat, which models hypertension-associated renal disease. Furthermore, our data suggest that Rab38 affects urinary protein excretion via effects in the proximal tubule.

Overview of admixture mapping

Admixture mapping is a powerful method of gene mapping for diseases or traits that show differential risk by ancestry. Admixture mapping has been applied most often to African Americans who trace ancestry to Europeans and West Africans. Recent developments in admixture mapping include improvements in methods to take advantage of higher densities of genetic variants, as well as extensions to admixed populations with three or more ancestral populations, such as Latino Americans. In this unit, the author outlines the key concepts of admixture mapping. The author describes several approaches for inferring local ancestry and provides strategies for performing admixture mapping depending on the study design. Finally, the author compares and contrasts linkage analysis, association analysis, and admixture mapping, with an emphasis on integrating admixture mapping and association testing.

An integrated approach of comparative genomics and heritability analysis of pig and human on obesity trait: evidence for candidate genes on human chromosome 2

Background

Traditional candidate gene approach has been widely used for the study of complex diseases including obesity. However, this approach is largely limited by its dependence on existing knowledge of presumed biology of the phenotype under investigation. Our combined strategy of comparative genomics and chromosomal heritability estimate analysis of obesity traits, subscapular skinfold thickness and back-fat thickness in Korean cohorts and pig (Sus scrofa), may overcome the limitations of candidate gene analysis and allow us to better understand genetic predisposition to human obesity.

Results

We found common genes including FTO, the fat mass and obesity associated gene, identified from significant SNPs by association studies of each trait. These common genes were related to blood pressure and arterial stiffness (P = 1.65E-05) and type 2 diabetes (P = 0.00578). Through the estimation of variance of genetic component (heritability) for each chromosome by SNPs, we observed a significant positive correlation (r = 0.479) between genetic contributions of human and pig to obesity traits. Furthermore, we noted that human chromosome 2 (syntenic to pig chromosomes 3 and 15) was most important in explaining the phenotypic variance for obesity.

Conclusions

Obesity genetics still awaits further discovery. Navigating syntenic regions suggests obesity candidate genes on chromosome 2 that are previously known to be associated with obesity-related diseases: MRPL33, PARD3B, ERBB4, STK39, and ZNF385B.

Prediction of blood pressure changes over time and incidence of hypertension by a genetic risk score in Swedes

Recent Genome-Wide Association Studies (GWAS) have pinpointed different single nucleotide polymorphisms consistently associated with blood pressure (BP) and hypertension prevalence. However, little data exist regarding single nucleotide polymorphisms predicting BP variation over time and hypertension incidence. The aim of this study was to confirm the association of a genetic risk score (GRS), based on 29 independent single nucleotide polymorphisms, with cross-sectional BP and hypertension prevalence and to challenge its prediction of BP change over time and hypertension incidence in >17 000 middle-aged Swedes participating in a prospective study, the Malmö Preventive Project, investigated at baseline and over a 23-year average period of follow-up. The GRS was associated with higher systolic and diastolic BP values both at baseline (β ± SEM, 0.968 ± 0.102 mm Hg and 0.585 ± 0.064 mm Hg; P<1E-19 for both) and at reinvestigation (β ± SEM, 1.333 ± 0.161 mm Hg and 0.724 ± 0.086 mm Hg; P<1E-15 for both) and with increased hypertension prevalence (odds ratio [95% CI], 1.192 [1.140-1.245] and 1.144 [1.107-1.183]; P<1E-15 for both). The GRS was positively associated with change (Δ) in BP (β ± SEM, 0.033 ± 0.008 mm Hg/y and 0.023 ± 0.004 mm Hg/y; P<1E-04 for both) and hypertension incidence (odds ratio [95% CI], 1.110 [1.065-1.156]; P=6.7 E-07), independently from traditional risk factors. The relative weight of the GRS was lower in magnitude than obesity or prehypertension, but comparable with diabetes mellitus or a positive family history of hypertension. A C-statistics analysis does not show any improvement in the prediction of incident hypertension on top of traditional risk factors. Our data from a large cohort study show that a GRS is independently associated with BP increase and incidence of hypertension.

Blood pressure and human genetic variation in the general population

PURPOSE OF REVIEW

Hypertension is a complex trait with multiple environmental and genetic contributors. Until recently, linkage studies of rare Mendelian disorders of hypertension and hypotension have produced the most notable progress toward understanding the heritable basis of blood pressure (BP). Association studies to identify common variants have been limited in the past by small sample sizes and most findings have lacked replication.

RECENT FINDINGS

Recently, well powered, targeted candidate gene and genome-wide association studies have reported reproducible associations between rare and common genetic variants and BP and hypertension at the population level.

SUMMARY

Identification of novel genes will lead to an improved understanding of BP regulation and the potential for novel therapies.

Why personalized medicine will fail if we stay the course

Genomic science and associated technologies are providing scientists and clinicians with novel insights that are transforming the delivery of healthcare and the overall well-being of society. However, these insights inform us that historical population sampling approaches for investigating rare and common genetic variations are not representative of the complex ancestral backgrounds of today's patients. In order for personalized medicine to be meaningful and applicable to the global populations, we will need to know how common and rare genetic variants found in different parts of the world influence health and drug response. This article demonstrates the importance of increasing ethnic and racial diversity among participants in genomic research, highlights areas of opportunity for improving our understanding of genomic diversity among populations, and provides examples of successful models that help to resolve these concerns.

Systems-level approaches reveal conservation of trans-regulated genes in the rat and genetic determinants of blood pressure in humans

AIMS

Human genome-wide association studies (GWAS) of hypertension identified only few susceptibility loci with large effect that were replicated across populations. The vast majority of genes detected by GWAS has small effect and the regulatory mechanisms through which these genetic variants cause disease remain mostly unclear. Here, we used comparative genomics between human and an established rat model of hypertension to explore the transcriptional mechanisms mediating the effect of genes identified in 15 hypertension GWAS.

METHODS AND RESULTS

Time series analysis of radiotelemetric blood pressure (BP) was performed to assess 11 parameters of BP variation in recombinant inbred strains derived from the spontaneously hypertensive rat. BP data were integrated with ∼27 000 expression quantative trait loci (eQTLs) mapped across seven tissues, detecting >8000 significant associations between eQTL genes and BP variation in the rat. We then compiled a large catalogue of human genes from GWAS of hypertension and identified a subset of 2292 rat-human orthologous genes. Expression levels for 795 (34%) of these genes correlated with BP variation across rat tissues: 51 genes were cis-regulated, whereas 459 were trans-regulated and enriched for 'calcium signalling pathway' (P = 9.6 × 10(-6)) and 'ion channel' genes (P = 3.5 × 10(-7)), which are important determinants of hypertension. We identified 158 clusters of trans-eQTLs, annotated the underlying 'master regulator' genes and found significant over-representation in the human hypertension gene set (enrichment P = 5 × 10(-4)).

CONCLUSION

We showed extensive conservation of trans-regulated genes and their master regulators between rat and human hypertension. These findings reveal that small-effect genes associated with hypertension by human GWAS are likely to exert their action through coordinate regulation of pathogenic pathways.

Genomic view on the peopling of India

India is known for its vast human diversity, consisting of more than four and a half thousand anthropologically well-defined populations. Each population differs in terms of language, culture, physical features and, most importantly, genetic architecture. The size of populations varies from a few hundred to millions. Based on the social structure, Indians are classified into various caste, tribe and religious groups. These social classifications are very rigid and have remained undisturbed by emerging urbanisation and cultural changes. The variable social customs, strict endogamy marriage practices, long-term isolation and evolutionary forces have added immensely to the diversification of the Indian populations. These factors have also led to these populations acquiring a set of Indian-specific genetic variations responsible for various diseases in India. Interestingly, most of these variations are absent outside the Indian subcontinent. Thus, this review is focused on the peopling of India, the caste system, marriage practice and the resulting health and forensic implications.

Can genetic pleiotropy replicate common clinical constellations of cardiovascular disease and risk?

The relationship between obesity, diabetes, hyperlipidemia, hypertension, kidney disease and cardiovascular disease (CVD) is established when looked at from a clinical, epidemiological or pathophysiological perspective. Yet, when viewed from a genetic perspective, there is comparatively little data synthesis that these conditions have an underlying relationship. We sought to investigate the overlap of genetic variants independently associated with each of these commonly co-existing conditions from the NHGRI genome-wide association study (GWAS) catalog, in an attempt to replicate the established notion of shared pathophysiology and risk. We used pathway-based analyses to detect subsets of pleiotropic genes involved in similar biological processes. We identified 107 eligible GWAS studies related to CVD and its established comorbidities and risk factors and assigned genes that correspond to the associated signals based on their position. We found 44 positional genes shared across at least two CVD-related phenotypes that independently recreated the established relationship between the six phenotypes, but only if studies representing non-European populations were included. Seven genes revealed pleiotropy across three or more phenotypes, mostly related to lipid transport and metabolism. Yet, many genes had no relationship to each other or to genes with established functional connection. Whilst we successfully reproduced established relationships between CVD risk factors using GWAS findings, interpretation of biological pathways involved in the observed pleiotropy was limited. Further studies linking genetic variation to gene expression, as well as describing novel biological pathways will be needed to take full advantage of GWAS results.

Transferability and fine mapping of genome-wide associated loci for lipids in African Americans

Background

A recent, large genome-wide association study (GWAS) of European ancestry individuals has identified multiple genetic variants influencing serum lipids. Studies of the transferability of these associations to African Americans remain few, an important limitation given interethnic differences in serum lipids and the disproportionate burden of lipid-associated metabolic diseases among African Americans.

Methods

We attempted to evaluate the transferability of 95 lipid-associated loci recently identified in European ancestry individuals to 887 non-diabetic, unrelated African Americans from a population-based sample in the Washington, DC area. Additionally, we took advantage of the generally reduced linkage disequilibrium among African ancestry populations in comparison to European ancestry populations to fine-map replicated GWAS signals.

Results

We successfully replicated reported associations for 10 loci (CILP2/SF4, STARD3, LPL, CYP7A1, DOCK7/ANGPTL3, APOE, SORT1, IRS1, CETP, and UBASH3B). Through trans-ethnic fine-mapping, we were able to reduce associated regions around 75% of the loci that replicated.

Conclusions

Between this study and previous work in African Americans, 40 of the 95 loci reported in a large GWAS of European ancestry individuals also influence lipid levels in African Americans. While there is now evidence that the lipid-influencing role of a number of genetic variants is observed in both European and African ancestry populations, the still considerable lack of concordance highlights the importance of continued ancestry-specific studies to elucidate the genetic underpinnings of these traits.

Hypertension in the US Black population: risk factors, complications, and potential impact of central aortic pressure on effective treatment

PURPOSE

The identification of specific factors that contribute to hypertension development and progression among blacks in the US is the focus of much ongoing research. The purpose of this paper is to review these factors and discuss how they present unique opportunities for improving the management of hypertension in this difficult-to-treat population.

METHODS

We searched the published literature for articles discussing the risk factors for hypertension and cardiovascular disease in blacks; the target-organ damage and cardiovascular complications associated with hypertension in this difficult-to-treat population; and the role of central blood pressure in predicting cardiovascular events.

RESULTS

The prevalence of hypertension is higher in blacks than in other race/ethnic groups, with environmental and genetic risk factors likely playing an important role. The cardiovascular and renal consequences of hypertension (eg, left ventricular hypertrophy and renal failure) are also greater in blacks relative to their white counterparts. Preliminary data suggest that central blood pressure may be higher in blacks than in whites and has the potential for greater prognostic capability relative to peripheral blood pressure.

CONCLUSIONS

There is an urgent need to successfully control hypertension in the black population. Although data are limited in blacks, evidence suggests that central blood pressure warrants more continued assessment in future clinical studies.

Multiple loci associated with renal function in African Americans

The incidence of chronic kidney disease varies by ethnic group in the USA, with African Americans displaying a two-fold higher rate than European Americans. One of the two defining variables underlying staging of chronic kidney disease is the glomerular filtration rate. Meta-analysis in individuals of European ancestry has identified 23 genetic loci associated with the estimated glomerular filtration rate (eGFR). We conducted a follow-up study of these 23 genetic loci using a population-based sample of 1,018 unrelated admixed African Americans. We included in our follow-up study two variants in APOL1 associated with end-stage kidney disease discovered by admixture mapping in admixed African Americans. To address confounding due to admixture, we estimated local ancestry at each marker and global ancestry. We performed regression analysis stratified by local ancestry and combined the resulting regression estimates across ancestry strata using an inverse variance-weighted fixed effects model. We found that 11 of the 24 loci were significantly associated with eGFR in our sample. The effect size estimates were not significantly different between the subgroups of individuals with two copies of African ancestry vs. two copies of European ancestry for any of the 11 loci. In contrast, allele frequencies were significantly different at 10 of the 11 loci. Collectively, the 11 loci, including four secondary signals revealed by conditional analyses, explained 14.2% of the phenotypic variance in eGFR, in contrast to the 1.4% explained by the 24 loci in individuals of European ancestry. Our findings provide insight into the genetic basis of variation in renal function among admixed African Americans.

Variation in APOL1 Contributes to Ancestry-Level Differences in HDLc-Kidney Function Association

Low levels of high-density cholesterol (HDLc) accompany chronic kidney disease, but the association between HDLc and the estimated glomerular filtration rate (eGFR) in the general population is unclear. We investigated the HDLc-eGFR association in nondiabetic Han Chinese (HC, n = 1100), West Africans (WA, n = 1497), and African Americans (AA, n = 1539). There were significant differences by ancestry: HDLc was positively associated with eGFR in HC (β = 0.13, P < 0.0001), but negatively associated among African ancestry populations (WA: -0.19, P < 0.0001; AA: -0.09, P = 0.02). These differences were also seen in nationally-representative NHANES data (among European Americans: 0.09, P = 0.005; among African Americans -0.14, P = 0.03). To further explore the findings in African ancestry populations, we investigated the role of an African ancestry-specific nephropathy risk variant, rs73885319, in the gene encoding HDL-associated APOL1. Among AA, an inverse HDLc-eGFR association was observed only with the risk genotype (-0.38 versus 0.001; P = 0.03). This interaction was not seen in WA. In summary, counter to expectation, an inverse HDLc-eGFR association was observed among those of African ancestry. Given the APOL1 × HDLc interaction among AA, genetic factors may contribute to this paradoxical association. Notably, these findings suggest that the unexplained mechanism by which APOL1 affects kidney-disease risk may involve HDLc.

From circulating biomarkers to genomics and imaging in the prediction of cardiovascular events in the general population

Cardiovascular disease (CVD) is the leading cause of death and disability worldwide. In the last decades numerous markers have been considered and investigated for the prediction of CV events, but only a few of them resulted in improved global risk assessment beyond traditional risk factors when incorporated into coronary evaluation scores. Recent genetic studies have pointed out a few but consistent loci or genes which are independently associated with CV risk. The idea is fascinating that these genetic markers could lead to improved individual CV risk assessment and tailored pharmacological interventions. In this brief review we will not make a systematic review of all non-genetic and genetic markers of CV risk but we will try to make a brief overview of the most interesting ones with the aim to underline potential 'pros' and 'cons' of their implementation in clinical practice.

Dopamine, the kidney, and hypertension

There is increasing evidence that the intrarenal dopaminergic system plays an important role in the regulation of blood pressure, and defects in dopamine signaling appear to be involved in the development of hypertension. Recent experimental models have definitively demonstrated that abnormalities in intrarenal dopamine production or receptor signaling can predispose to salt-sensitive hypertension and a dysregulated renin-angiotensin system. In addition, studies in both experimental animal models and in humans with salt-sensitive hypertension implicate abnormalities in dopamine receptor regulation due to receptor desensitization resulting from increased G-protein receptor kinase 4 (GRK4) activity. Functional polymorphisms that predispose to increased basal GRK4 activity both decrease dopamine receptor activity and increase angiotensin II type 1 (AT1) receptor activity and are associated with essential hypertension in a number of different human cohorts.

Genome-wide association study identifies novel loci association with fasting insulin and insulin resistance in African Americans

Insulin resistance (IR) is a key determinant of type 2 diabetes (T2D) and other metabolic disorders. This genome-wide association study (GWAS) was designed to shed light on the genetic basis of fasting insulin (FI) and IR in 927 non-diabetic African Americans. 5 396 838 single-nucleotide polymorphisms (SNPs) were tested for associations with FI or IR with adjustments for age, sex, body mass index, hypertension status and first two principal components. Genotyped SNPs (n = 12) with P < 5 × 10(-6) in African Americans were carried forward for de novo genotyping in 570 non-diabetic West Africans. We replicated SNPs in or near SC4MOL and TCERG1L in West Africans. The meta-analysis of 1497 African Americans and West Africans yielded genome-wide significant associations for SNPs in the SC4MOL gene: rs17046216 (P = 1.7 × 10(-8) and 2.9 × 10(-8) for FI and IR, respectively); and near the TCERG1L gene with rs7077836 as the top scoring (P = 7.5 × 10(-9) and 4.9 × 10(-10) for FI and IR, respectively). In silico replication in the MAGIC study (n = 37 037) showed weak but significant association (adjusted P-value of 0.0097) for rs34602777 in the MYO5A gene. In addition, we replicated previous GWAS findings for IR and FI in Europeans for GCKR, and for variants in four T2D loci (FTO, IRS1, KLF14 and PPARG) which exert their action via IR. In summary, variants in/near SC4MOL, and TCERG1L were associated with FI and IR in this cohort of African Americans and were replicated in West Africans. SC4MOL is under-expressed in an animal model of T2D and plays a key role in lipid biosynthesis, with implications for the regulation of energy metabolism, obesity and dyslipidemia. TCERG1L is associated with plasma adiponectin, a key modulator of obesity, inflammation, IR and diabetes.

Genetic variants in platelet factor 4 modulate inflammatory and platelet activation biomarkers

BACKGROUND

African Americans suffer from higher prevalence and severity of atherosclerosis compared with whites, highlighting racial and ethnic disparities in cardiovascular disease. Previous studies have pointed to the role of vascular inflammation and platelet activation in the formation of atherosclerotic lesions.

METHODS AND RESULTS

We explored the role of genetic variation in 4 chemokine/chemokine receptor genes (CX3CR1, CX3CL1, CXCR3, and PF4) on systemic inflammation and platelet activation serum biomarkers (fractalkine, platelet P-selectin, platelet factor 4 [PF4], and tumor necrosis factor-α). In total, 110 single nucleotide polymorphisms were tested among 1042 African Americans and 763 whites. The strongest association with serum PF4 levels was observed for rs168449, which was significant in both racial groups (P value: African Americans=0.0017, whites=0.014, combined=1.2 × 10(-4)), and remained significant after permutation-based multiple corrections (P(c) value: combined=0.0013). After accounting for the effect of rs168449, we identified another significant single nucleotide polymorphism (rs1435520), suggesting a second independent signal regulating serum PF4 levels (conditional P value: African Americans=0.02, whites=0.02). Together, these single nucleotide polymorphisms explained 0.98% and 1.23% of serum PF4 variance in African Americans and whites, respectively. Additionally, in African Americans, we found an additional PF4 variant (rs8180167), uncorrelated with rs168449 and rs1435520, associated with serum tumor necrosis factor-α levels (P=0.008, P(c)=0.048).

CONCLUSIONS

Our study highlights the importance of PF4 variants in the regulation of platelet activation (PF4) and systemic inflammation (tumor necrosis factor-α) serum biomarkers.

Simple f test reveals gene-gene interactions in case-control studies

Missing heritability is still a challenge for Genome Wide Association Studies (GWAS). Gene-gene interactions may partially explain this residual genetic influence and contribute broadly to complex disease. To analyze the gene-gene interactions in case-control studies of complex disease, we propose a simple, non-parametric method that utilizes the F-statistic. This approach consists of three steps. First, we examine the joint distribution of a pair of SNPs in cases and controls separately. Second, an F-test is used to evaluate the ratio of dependence in cases to that of controls. Finally, results are adjusted for multiple tests. This method was used to evaluate gene-gene interactions that are associated with risk of Type 2 Diabetes among African Americans in the Howard University Family Study. We identified 18 gene-gene interactions (P < 0.0001). Compared with the commonly-used logistical regression method, we demonstrate that the F-ratio test is an efficient approach to measuring gene-gene interactions, especially for studies with limited sample size.

[Advances in genome-wide association studies on essential hypertension]

Since the first genome-wide association study was reported in 2007, hypertension has attracted numerous studies to identify its genetic basis. The first part of the current review summarizes the genetic loci associated with blood pressure/ hypertension identified by genome-wide association studies (GWAS) from January 2007 to September 2011, by race and chromosomal location. In the second part, we stress several important points in GWAS methodology, for example, selecting high-quality phenotypes and using multi-stage study design to increase the power studies to identify loci with minor effect. For statistical analysis, besides multiple testing correction and replication of the GWAS that have been introduced in previous reviews, computer-based genotype imputation has been described for its advantages in compensating GWAS genotyping failures. Although GWAS identifies many unknown genetic variants and improves our understanding for the pathogenesis of hypertension, the loci related to blood pressure / hypertension are common sequence variations with minor effect. The association studies are difficult to be replicated in different populations. Further studies are expected including extensive functional studies and fine mapping using advanced techniques, such as whole genome exon sequencing and pathway analysis, as well as epigenetic study to elucidate the etiology of human essential hypertension.

Heme oxygenase-1 and chronic hypoxia

A myriad of changes are necessary to adapt to chronic hypoxemia. Key among these changes increases in arterial oxygen carrying capacity, ventilation and sympathetic activity. This requires the induction of several gene products many of which are regulated by the activity of HIF-1α, including HO-1. Induction of HO-1 during chronic hypoxia is necessary for the continued breakdown of heme for the enhanced production of hemoglobin and the increased respiratory and sympathetic responses. Several human HO-1 polymorphisms have been identified that can affect the expression or activity of HO-1. Associations between these polymorphisms and the prevalence of hypertension have recently been assessed in specific populations. There are major gaps in our understanding of the mechanisms of how HO-1 mediates changes in the activity of the hypoxia-sensitive chemosensors and whether HO-1 polymorphisms are an important factor in the integrated response to chronic hypoxia. Understanding how HO-1 mediates cardiorespiratory responses could provide important insights into clinical syndromes such as obstructive sleep apnea.

Education, genetic ancestry, and blood pressure in African Americans and Whites

OBJECTIVES

We assessed the relative roles of education and genetic ancestry in predicting blood pressure (BP) within African Americans and explored the association between education and BP across racial groups.

METHODS

We used t tests and linear regressions to examine the associations of genetic ancestry, estimated from a genomewide set of autosomal markers, and education with BP variation among African Americans in the Family Blood Pressure Program. We also performed linear regressions in self-identified African Americans and Whites to explore the association of education with BP across racial groups.

RESULTS

Education, but not genetic ancestry, significantly predicted BP variation in the African American subsample (b=-0.51 mm Hg per year additional education; P=.001). Although education was inversely associated with BP in the total population, within-group analyses showed that education remained a significant predictor of BP only among the African Americans. We found a significant interaction (b=3.20; P=.006) between education and self-identified race in predicting BP.

CONCLUSIONS

Racial disparities in BP may be better explained by differences in education than by genetic ancestry. Future studies of ancestry and disease should include measures of the social environment.

Haplotype variation and genotype imputation in African populations

Sub-Saharan Africa has been identified as the part of the world with the greatest human genetic diversity. This high level of diversity causes difficulties for genome-wide association (GWA) studies in African populations-for example, by reducing the accuracy of genotype imputation in African populations compared to non-African populations. Here, we investigate haplotype variation and imputation in Africa, using 253 unrelated individuals from 15 Sub-Saharan African populations. We identify the populations that provide the greatest potential for serving as reference panels for imputing genotypes in the remaining groups. Considering reference panels comprising samples of recent African descent in Phase 3 of the HapMap Project, we identify mixtures of reference groups that produce the maximal imputation accuracy in each of the sampled populations. We find that optimal HapMap mixtures and maximal imputation accuracies identified in detailed tests of imputation procedures can instead be predicted by using simple summary statistics that measure relationships between the pattern of genetic variation in a target population and the patterns in potential reference panels. Our results provide an empirical basis for facilitating the selection of reference panels in GWA studies of diverse human populations, especially those of African ancestry.

A review of genetics, arterial stiffness, and blood pressure in African Americans

The prevalence of hypertension in African Americans in the USA is among the highest in the world and increasing. The identification of genes and pathways regulating blood pressure in African Americans has been challenging. An early predictor of hypertension is arterial stiffness. The prevalence of arterial stiffness is significantly higher in African Americans compared to Caucasians. Approximately 20 % of the variance in arterial stiffness is estimated to be heritable. Identifying genes and biological pathways regulating arterial stiffness may provide insight into the genetics underlying the increased risk of hypertension in African Americans. This paper reviews the genetic findings to date in the area of arterial stiffness and blood pressure in African Americans with an emphasis on the current limitations and new efforts to move the field forward.