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We provide an overview of ongoing discovery efforts in the genetics of blood pressure (BP) and hypertension (HTN) traits. Two large genome-wide association meta-analyses of individuals of European descent were recently published, revealing ~13 new loci for BP traits. Only two of these loci harbor genes in a pathway known to affect BP (CYP17A1 and NPPA/NPPB). Functional variants in these loci are still unknown. Few genome-wide association studies (GWAS) of complex diseases have been published from non-European populations. The study of populations with different evolutionary history and linkage disequilibrium (LD) structure, such as individuals of African ancestry, may provide an opportunity to further narrow these regions to identify the causal gene(s). Several collaborative efforts toward discovery of low-frequency variants and copy number variation for BP traits are currently underway. As evidence for new loci for complex diseases accumulates the assessment of the epidemiologic architecture of these variants in populations assumes higher priority. The impact of public health–relevant contexts such as diet, physical activity, psychosocial factors, and aging has not been examined for most common variants associated with BP.

Background

Elevated resting heart rate has been shown in multiple studies to be a strong predictor of cardiovascular disease. Previous family studies have shown a significant heritable component to heart rate with several groups conducting genomic linkage scans to identify quantitative trait loci.

Methods

We performed a genome-wide linkage scan to identify quantitative trait loci influencing resting heart rate among 3,282 Caucasians and 3,989 African-Americans in three independent networks comprising the Family Blood Pressure Program (FBPP) using 368 microsatellite markers. Mean heart rate measurements were used in a regression model including covariates for age, body mass index, pack-years, currently drinking alcohol (yes/no), hypertension status and medication usage to create a standardized residual for each gender/ethnic group within each study network. This residual was used in a nonparametric variance component model to generate a LOD score and a corresponding P value for each ethnic group within each study network. P values from each ethnic group and study network were merged using an adjusted Fisher's combining P values method and the resulting P values were converted to LOD scores. The entire analysis was redone after individuals currently taking beta-blocker medication were removed.

Results

We identified significant evidence of linkage (LOD = 4.62) to chromosome 10 near 142.78 cM in the Caucasian group of HyperGEN. Between race and network groups we identified a LOD score of 1.86 on chromosome 5 (between 39.99 and 45.34 cM) in African-Americans in the GENOA network and the same region produced a LOD score of 1.12 among Caucasians within a different network (HyperGEN). Combining all network and race groups we identified a LOD score of 1.92 (P = 0.0013) on chromosome 5p13-14. We assessed heterogeneity for this locus between networks and ethnic groups and found significant evidence for low heterogeneity (P ≤ 0.05).

Conclusion

We found replication (LOD > 1) between ethnic groups and between study networks with low heterogeneity on chromosome 5p13-14 suggesting that a gene in this region influences resting heart rate.

Background

High blood pressure or hypertension is a major risk factor involved in the development of cardiovascular diseases. We conducted genome-wide variance component linkage analyses to search for loci influencing five blood pressure related traits including the quantitative traits systolic blood pressure (SBP), diastolic blood pressure (DBP) and pulse pressure (PP), the dichotomous trait hypertension (HT) and the bivariate quantitative trait SBP-DBP in families residing in American Samoa and Samoa, as well as in the combined sample from the two polities. We adjusted the traits for a number of environmental covariates such as smoking, alcohol consumption, physical activity and material life style.

Results

We found suggestive univariate linkage for SBP on chromosome 2q35-q37 (LOD 2.4) and for PP on chromosome 22q13 (LOD 2.2), two chromosomal regions that recently have been associated with SBP and PP, respectively.

Conclusion

We have detected additional evidence for a recently reported locus associated with SBP on chromosome 2q and a susceptibility locus for PP on chromosome 22q. However, differences observed between the results from our three partly overlapping genetically homogenous study samples from the Samoan islands suggest that additional studies should be performed in order to verify these results.

Essential hypertension affects 75% of post-menopausal women in the United States causing greater cardiovascular complications compared with age-matched men and pre-menopausal women. Hormone replacement and current anti-hypertensive therapies do not correct this post-menopausal increased risk suggesting a distinct pathogenic framework. We investigated the hypothesis that distinct genetic determinants might underlie susceptibility to salt sensitive hypertension in pre-menopausal and post-menopausal states. To determine whether distinct genetic loci contribute to post-menopausal salt-sensitive hypertension, we performed a genome-wide scan for quantitative trait loci (QTLs) affecting blood pressure (BP) in 16-month old post-menopausal F2 (Dahl S×R)-intercross female rats characterized for blood pressure by radiotelemetry. Given identical environments and high salt challenge, post-menopausal BP levels were significantly higher than observed in pre-menopausal (post-menopausal versus pre-menopausal SBP, P<0.0001) and ovariectomized (post-menopausal versus ovariectomized SBP, P<0.001) F2-intercross female rats. We detected four significant to highly significant BP-QTLs (BP-pm1 on chromosome 13, LOD 3.78; BP-pm2 on chromosome 11, LOD 2.76; BP-pm3 on chromosome 2, LOD 2.61; BP-pm4 on chromosome 4, LOD 2.50) and two suggestive BP-QTLs (BP-pm5 on chromosome 15, LOD 2.37; BP-f1 on chromosome 5, LOD 1.65), four of which (BP-pm2, BP-pm3, BP-pm4, BP-pm5) were unique to this post-menopausal cohort. These data demonstrate distinct polygenic susceptibility underlying post-menopausal salt-sensitive hypertension providing a pathway towards the identification of mechanism-based therapy for post-menopausal hypertension and ensuing target-organ complications.

Essential hypertension affects 20 to 30% of the population worldwide and contributes significantly to cardiovascular mortality and morbidity. Heridability of blood pressure is around 15 to 40% but there are also substantial environmental factors affecting blood pressure variability. It is assumed that blood pressure is under the control of a large number of genes each of which has only relatively mild effects. It has therefore been difficult to discover the genes that contribute to blood pressure variation using traditional approaches including candidate gene studies and linkage studies. Animal models of hypertension, particularly in the rat, have led to the discovery of quantitative trait loci harbouring one or several hypertension related genes, but translation of these findings into human essential hypertension remains challenging. Recent development of genotyping technology made large scale genome-wide association studies possible. This approach and the study of monogenic forms of hypertension has led to the discovery of novel and robust candidate genes for human essential hypertension, many of which require functional analysis in experimental models.

Aim

To determine the independent and combined effects of three quantitative trait loci (QTL) for blood pressure in the Genetically Hypertensive (GH/Omr) rat by generating and characterizing single and combined congenic strains that have QTL on rat chromosomes (RNO) 2, 6, and 18 from the GH rat introduced into a hypertension resistant Brown Norway (BN) background.

Methods

Linkage analysis and QTL identification (genome wide QTL scan) were performed with MapMaker/EXP to build the genetic maps and MapMaker/QTL for linking the phenotypes to the genetic map. The congenic strains were derived using marker-assisted selection strategy from a single male F1 offspring of an intercross between the male GH/Omr and female BN/Elh, followed by 10 generations of selective backcrossing to the female BN progenitor strain. Single congenic strains generated were BN.GH-(D2Rat22-D2Mgh11)/Mcwi (BN.GH2); BN.GH-(D6Mit12-D6Rat15)/Mcwi (BN.GH6); and BN.GH-(D18Rat41-D18Mgh4)/Mcwi (BN.GH18). Blood pressure measurements were obtained either via a catheter placed in the femoral artery or by radiotelemetry in the single and combined congenics. Responses to angiotensin II (ANGII), norepinephrine (NE), and baroreceptor sensitivity were measured in the single congenics.

Results

Transferring one or more QTL from the hypertensive GH into normotensive BN strain was not sufficient to cause hypertension in any of the developed congenic strains. There were no differences between the parental and congenic strains in their response to NE. However, BN.GH18 rats demonstrated significantly lower baroreceptor sensitivity (β = -1.25 ± 0.17), whereas BN.GH2 (β = 0.66 ± 0.09) and BN.GH18 (β = 0.71 ± 0.07) had significantly decreased responses to ANGII from those observed in the BN (β = 0.88 ± 0.08).

Conclusion

The failure to alter blood pressure levels by introducing the hypertensive QTL from the GH into the hypertension resistant BN background suggests that the QTL effects are genome background-dependent in the GH rat. BN.GH2 and BN.GH18 rats reveal significant differences in response to ANGII and impaired baroreflex sensitivity, suggesting that we may have captured a locus responsible for the genetic control of baroreceptor sensitivity, which would be considered an intermediate phenotype of blood pressure.

Purpose of review

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

Recent Findings

Recently, well-powered, targeted candidate gene and genome-wide association studies have reported reproducible associations between common genetic variants and blood pressure.

Summary

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

Contemporary genomic tools now allow the fast and reliable genotyping of hundreds of thousands of variants and permit an unbiased interrogation of the common variability across the human genome. These technical advances have been the basis of numerous recent investigations of genes underlying complex genetic traits, and the results for blood pressure and hypertension have been of particular interest. The pathophysiology of the complex genetic trait blood pressure and hypertension is unclear. The heritability of essential hypertension is high and insights can be gained by finding associated genes. Current genome-wide association studies (GWAS) have identified 10 to 20 loci in or near genes that generally were not expected to be associated with blood pressure or essential hypertension; more significant variants will be discovered when even larger and more refined studies become available. This article gives a short introduction to GWAS and summarizes the current findings for blood pressure and hypertension.

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 (SBP and DBP), high-density lipoprotein-cholesterol (HDL-C), low-density lipoprotein-cholesterol (LDL-C), body mass index (BMI) and triglycerides (TG) 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 10cM 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.

Background

Because high blood pressure, altered lipid levels, obesity, and diabetes so frequently occur together, they are sometimes collectively referred to as the metabolic syndrome. While there have been many studies of each metabolic syndrome trait separately, few studies have attempted to analyze them combined, i.e., as one composite variable, in quantitative trait linkage or association analysis. We used genotype and phenotype data from the Framingham Heart Study to perform a full-genome scan for quantitative trait loci underlying the metabolic syndrome.

Results

Heritability estimates for all of the covariate-adjusted and age- and gender-standardized individual traits, and the composite metabolic syndrome trait, were all fairly high (0.39–0.62), and the composite trait was among the highest at 0.61. The composite trait yielded no regions with suggestive linkage by Lander and Kruglyak's criteria, although there were several noteworthy regions for individual traits, some of which were also observed for the composite variable.

Conclusion

Despite its high heritability, the composite metabolic syndrome trait variable did not increase the power to detect or localize linkage peaks in this sample. However, this strategy and related methods of combining correlated individual traits deserve further investigation, particularly in settings with complex causal pathways.

Background

The design of appropriate strategies to analyze and interpret linkage results for complex human diseases constitutes a challenge. Parameters such as power, definition of phenotype, and replicability have to be taken into account in order to reach meaningful conclusions. Incorporating data on repeated phenotypic measures may increase the power to detect linkage but requires sophisticated analysis methods. Using the simulated Genetic Analysis Workshop 13 data set, we have estimated a variety of systolic blood pressure (SBP) phenotypic measures and examined their performance with respect to consistency among replicates and to true and false positive linkage signals.

Results

The whole-genome scan conducted on a dichotomous hypertension phenotype indicated the involvement of few true loci with nominal significance and gave rise to a high rate of false positives. Analysis of a cross-sectional quantitative SBP measure performed better, although genome-wide significance was again not reached. Additional phenotypic measures were derived from the longitudinal data using random effects modelling for censored data with varying levels of covariate adjustment. These models provided evidence for significant linkage to most genes influencing SBP and produced few false positive results. Overall, replicability of results was poor for loci, representing weak effects.

Conclusion

Longitudinally derived phenotypes performed better than cross-sectional measures in linkage analyses. Bearing in mind the sample design and size of these data, linkage results that fail to replicate should not be dismissed; instead, different lines of evidence derived from complementary analysis methods should be combined to prioritize follow up.

Genome-wide association scans are beginning to identify risk alleles for a number of complex diseases and traits. Essential hypertension looked as though it would be an exception to this trend after the Wellcome Trust Case Control Consortium data were published in 2007. However, more recent scans and meta-analyses have reversed the fortunes of essential hypertension. A number of loci have been identified, including a new antihypertensive drug target in the guise of the serine/threonine kinase SPAK. This kinase forms part of a novel kinase cascade that regulates the NCCT (Na+/Cl- co-transporter; SLC12A3) in the kidney and is defective in a rare Mendelian hypertension syndrome (Gordon's syndrome). Genome-wide scans are also being used to look for alleles to predict individual response to antihypertensive drugs and their risk of causing side-effects. The results of these are expected in the near future and may finally deliver the long-awaited goal of personalized drug therapy for hypertensive patients.

The QTc interval is a complex quantitative trait and a strong prognostic indicator of cardiovascular mortality in general, healthy people. The aim of this study was to identify non-genetic factors and quantitative trait loci that govern the QTc interval in an isolated Mongolian population. We used multiple regression analysis to determine the relationship between the QTc interval and non-genetic factors including height, blood pressure, and the plasma lipid level. Whole genome linkage analyses were performed to reveal quantitative trait loci for the QTc interval with 349 microsatellite markers from 1,080 Mongolian subjects. Among many factors previously known for association with the QTc interval, age, sex, heart rate, QRS duration of electrocardiogram and systolic blood pressure were also found to have influence on the QTc interval. A genetic effect for the QTc interval was identified based on familial correlation with a heritability value of 0.31. In a whole genome linkage analysis, we identified the four potential linkage regions 7q31-34, 5q21, 4q28, and 2q36.

Essential hypertension is a major cardiovascular risk factor and a large proportion of this risk is genetic. Identification of genomic regions consistently associated with hypertension has been difficult in association studies to date since this requires large sample sizes.

We previously published a large genome-wide linkage scan in Americans of African (AA) and European (EA) descent in the GenNet Network of the Family Blood Pressure Program (FBPP). A highly significant linkage peak was identified on chr1q spanning a region of 100cM. In the current study, we genotyped 1,569 SNPs under this linkage peak in 2,379 individuals in order to identify whether common genetic variants were associated with blood pressure (BP) at this locus.

Our analysis, using two different family-based association tests, provides suggestive evidence (P≤2×10-5) for a collection of single nucleotide polymorphisms (SNPs) associated with BP. In EAs, using diastolic BP as a quantitative phenotype, three variants located in or near the GPA33, CD247, and F5 genes, emerge as our top hits; for systolic BP, variants in GPA33, CD247, and REN are our best findings. No variant in AAs came close to suggestive evidence (P≥8×10-5) after multiple-test corrections.

In summary, we show that systematic follow-up of a linkage signal can help discover candidate variants for essential hypertension that require follow-up in yet larger samples. The failure to identify common variants is either due to low statistical power or the existence of rare coding variants in specific families or both, that require additional studies to clarify.

Essential hypertension is a major cardiovascular risk factor and a large proportion of this risk is genetic. Identification of genomic regions consistently associated with hypertension has been difficult in association studies to date as this requires large sample sizes.

We previously published a large genome-wide linkage scan in Americans of African (AA) and European (EA) descent in the GenNet Network of the Family Blood Pressure Program (FBPP). A highly significant linkage peak was identified on chr1q spanning a region of 100 cM. In this study, we genotyped 1569 SNPs under this linkage peak in 2379 individuals to identify whether common genetic variants were associated with blood pressure (BP) at this locus.

Our analysis, using two different family-based association tests, provides suggestive evidence (P≤2 × 10−5) for a collection of single nucleotide polymorphisms (SNPs) associated with BP. In EAs, using diastolic BP as a quantitative phenotype, three variants located in or near the GPA33, CD247, and F5 genes, emerge as our top hits; for systolic BP, variants in GPA33, CD247, and REN are our best findings. No variant in AAs came close to suggestive evidence after multiple-test corrections (P≥8 × 10−5).

In summary, we show that systematic follow-up of a linkage signal can help discover candidate variants for essential hypertension that require a follow-up in yet larger samples. The failure to identify common variants is either because of low statistical power or the existence of rare coding variants in specific families or both, which require additional studies to clarify.

Blood pressure (BP) is a heritable trait1 influenced by multiple biological pathways and is responsive to environmental stimuli. Over one billion people worldwide have hypertension (BP ≥140 mm Hg systolic [SBP] or ≥90 mm Hg diastolic [DBP])2. Even small increments in BP are associated with increased risk of cardiovascular events3. This genome-wide association study of SBP and DBP, which used a multi-stage design in 200,000 individuals of European descent, identified 16 novel loci: six of these loci contain genes previously known or suspected to regulate BP (GUCY1A3-GUCY1B3; NPR3-C5orf23; ADM; FURIN-FES; GOSR2; GNAS-EDN3); the other 10 provide new clues to BP physiology. A genetic risk score based on 29 genome-wide significant variants was associated with hypertension, left ventricular wall thickness, stroke, and coronary artery disease, but not kidney disease or kidney function. We also observed associations with BP in East Asian, South Asian, and African ancestry individuals. Our findings provide new insights into the genetics and biology of BP, and suggest novel potential therapeutic pathways for cardiovascular disease prevention.

Blood pressure (BP) is a major cardiovascular disease risk factor. To date, few variants associated with inter-individual BP variation have been identified. A genome-wide association study of systolic (SBP), diastolic BP (DBP), and hypertension in the CHARGE Consortium (n=29,136) identified 13 SNPs for SBP, 20 for DBP, and 10 for hypertension at p <4×10-7. The top 10 loci for SBP and DBP were incorporated into a risk score; mean BP and prevalence of hypertension increased in relation to number of risk alleles carried. When 10 CHARGE SNPs for each trait were meta-analyzed jointly with the Global BPgen Consortium (n=34,433), four CHARGE loci attained genome-wide significance (p<5×10-8) for SBP (ATP2B1, CYP17A1, PLEKHA7, SH2B3), six for DBP (ATP2B1, CACNB2, CSK/ULK3, SH2B3, TBX3/TBX5, ULK4), and one for hypertension (ATP2B1). Identifying novel BP genes advances our understanding of BP regulation and highlights potential drug targets for the prevention or treatment of hypertension.

Background

A multiplicity of study designs such as gene candidate analysis, genome wide search (GWS) and, recently, whole genome association studies have been employed for the identification of the genetic components of essential hypertension (EH). Several genome-wide linkage studies of EH and blood pressure-related phenotypes demonstrate that there is no single locus with a major effect while several genomic regions likely to contain EH-susceptibility loci were validated by multiple studies.

Methods

We carried out the clinical assessment of the entire adult population in a Sardinian village (Talana) and we analyzed 16 selected families with 62 hypertensive subjects out of 267 individuals. We carried out a double GWS using a set of 902 uniformly spaced microsatellites and a high-density SNPs map on the same group of families.

Results

Three loci were identified by both microsatellites and SNP scans and the obtained linkage results showed a remarkable degree of similarity. These loci were identified on chromosome 2q24, 11q23.1–25 and 13q14.11–21.33. Further support to these findings is their broad description present in literature associated to EH or related phenotypes. Bioinformatic investigation of these loci shows several potential EH candidate genes, several of whom already associated to blood pressure regulation pathways.

Conclusion

Our search for major susceptibility EH genetic factors evidences that EH in the genetic isolate of Talana is due to the contribution of several genes contained in loci identified and replicated by earlier findings in different human populations.

Background

Tachycardia is commonly observed in hypertensive patients, predominantly mediated by regulatory mechanisms integrated within the autonomic nervous system. The genetic loci and genes associated with increased heart rate in hypertension, however, have not yet been identified.

Methods

An F2 intercross of Spontaneously Hypertensive Rats (SHR) × Brown Norway (BN) linkage analysis of quantitative trait loci mapping was utilized to identify candidate genes associated with an increased heart rate in arterial hypertension.

Results

Basal heart rate in SHR was higher compared to that of normotensive BN rats (365 ± 3 vs. 314 ± 6 bpm, p < 0.05 for SHR and BN, respectively). A total genome scan identified one quantitative trait locus in a 6.78 cM interval on rat chromosome 8 (8q22–q24) that was responsible for elevated heart rate. This interval contained 241 genes, of which 65 are known genes.

Conclusion

Our data suggest that an influential genetic region located on the rat chromosome 8 contributes to the regulation of heart rate. Candidate genes that have previously been associated with tachycardia and/or hypertension were found within this QTL, strengthening our hypothesis that these genes are, potentially, associated with the increase in heart rate in a hypertension rat model.

Background

Age-related macular degeneration (AMD) is a complex disorder that is responsible for the majority of central vision loss in older adults living in developed countries. Phenotypic and genetic heterogeneity complicate the analysis of genome-wide scans for AMD susceptibility loci. The ordered subset analysis (OSA) method is an approach for reducing heterogeneity, increasing statistical power for detecting linkage, and helping to define the most informative data set for follow-up analysis. OSA assesses the linkage evidence in subsets of potentially more homogeneous families by rank-ordering family-specific lod scores with respect to trait-associated covariates or phenotypic features. Here, we present results of incorporating five continuous covariates into our genome-wide linkage analysis of 389 microsatellite markers in 62 multiplex families: Body mass index (BMI), systolic (SBP) and diastolic (DBP) blood pressure, intraocular pressure (IOP), and pack-years of cigarette smoking. Chromosome-wide significance of increases in nonparametric multipoint lod scores in covariate-defined subsets relative to the overall sample was assessed by permutation.

Results

Using a correction for testing multiple covariates, statistically significant lod score increases were observed for two chromosomal regions: 14q13 with a lod score of 3.2 in 28 families with average IOP ≤ 15.5 (p = 0.002), and 6q14 with a lod score of 1.6 in eight families with average BMI ≥ 30.1 (p = 0.0004). On chromosome 16p12, nominally significant lod score increases (p ≤ 0.05), up to a lod score of 2.9 in 32 families, were observed with several covariate orderings. While less significant, this was the only region where linkage evidence was associated with multiple clinically meaningful covariates and the only nominally significant finding when analysis was restricted to advanced forms of AMD. Families with linkage to 16p12 had higher averages of SBP, IOP and BMI and were primarily affected with neovascular AMD. For all three regions, linkage signals at or very near the peak marker have previously been reported.

Conclusion

Our results suggest that a susceptibility gene on chromosome 16p12 may predispose to AMD, particularly to the neovascular form, and that further research into the previously suggested association of neovascular AMD and systemic hypertension is warranted.

Background

We analyzed the Genetic Analysis Workshop 13 (GAW13) simulated data to contrast and compare different methods for the genetic linkage analysis of hypertension and change in blood pressure over time. We also examined methods for incorporating covariates into the linkage analysis. We used methods for quantitative trait loci (QTL) linkage analysis with and without covariates and affected sib-pair (ASP) analysis of hypertension followed by ordered subset analysis (OSA), using variables associated with change in blood pressure over time.

Results

Four of the five baseline genes and one of the three slope genes were not detected by any method using conventional criteria. OSA detected baseline gene b35 on chromosome 13 when using the slope in blood pressure to adjust for change over time. Slope gene s10 was detected by the ASP analysis and slope gene s11 was detected by QTL linkage analysis as well as by OSA analysis. Analysis of null chromosomes, i.e., chromosomes without genes, did not reveal significant increases in type I error. However, there were a number of genes indirectly related to blood pressure detected by a variety of methods.

Conclusions

We noted that there is no obvious first choice of analysis software for analyzing a complicated model, such as the one underlying the GAW13 simulated data. Inclusion of covariates and longitudinal data can improve localization of genes for complex traits but it is not always clear how best to do this. It remains a worthwhile task to apply several different approaches since one method is not always the best.

The spontaneously hypertensive rat (SHR) is the most widely used animal model of essential hypertension and associated metabolic disturbances. Multiple quantitative trait loci associated with hemodynamic and metabolic parameters have been mapped in the SHR. Recently, it has become possible to identify some of the specific quantitative trait gene (QTG) variants that underlie quantitative trait loci linked to complex cardiovascular and metabolic traits in SHR related strains. Recombinant inbred strains derived from SHR and Brown Norway progenitors, together with SHR congenic and transgenic strains, have proven useful for establishing the identity of several QTGs in SHR models. It is anticipated that the combined use of linkage analyses and gene expression profiles, together with the recently available genome sequences of both the SHR and Brown Norway strains and new methods for manipulating the rat genome, will soon accelerate progress in identifying QTGs for complex traits in SHR-related strains.

Background

Arterial stiffness is reported in numerous family studies to be heritable; linkage analysis has identified genomic regions that likely harbor genes contributing to its phenotypic expression. We sought to identify loci contributing to arterial stiffness in a large group of African American hypertensive families.

Methods

We performed a genome scan on 1251 African Americans in families participating in the HyperGEN (Hypertension Genetic Epidemiology Network) study. Children of the hypertensive proband generation were also included in the analysis. Arterial stiffness was estimated as pulse pressure (PP: systolic –diastolic blood pressure) divided by echocardiographically determined stroke volume (SV). PP/SV ratio was adjusted for several non-genetic sources of variation, including demographic and lifestyle factors; the residual phenotype was analyzed using multipoint variance components linkage implemented in SOLAR 2.0.3.

Results

Arterial stiffness was 20% heritable in African Americans. Two regions were highly suggestive of linkage, one between markers D1S1665 and D1S1728 in the 215 cM region of chromosome 1 (LOD = 3.08), and another between D14S588 and D14S606 in the 85 cM region of chromosome 14 (LOD = 2.42). Two candidate genes (GPR-25, SMOC-1) are located in the linked regions. SMOC-1 is of physiological interest because it codes a secreted glycoprotein with five domains, each containing regions homologous to those on other proteins that mediate cell-matrix interactions. GPR-25 is homologous to receptors involved in blood pressure regulation.

Conclusions

At least two chromosomal regions in humans are likely to harbor genes contributing to interindividual variation in PP/SV ratio, an index of arterial stiffness, in African Americans.

Recent advances in genome technology have enabled genome-wide searching for disease predisposition loci, using dense SNP and haplotype maps. Over the past year, such approaches have yielded positive results in human hypertension. Here we outline factors underlying the rationale for the approach, and consider reasons for false positive and negative results. While the approach has positive results, typically the trait-associated loci explain only a small fraction of the heritable fraction of trait variance. Finally, we consider alternative approaches and emerging strategies to probe the role of heredity in control of blood pressure.

Introduction

A major predictor of age-related osteoporotic fracture is peak areal bone mineral density (aBMD) which is a highly heritable trait. However, few linkage and association studies have been performed in men to identify the genes contributing to normal variation in aBMD. The aim of this study was to perform a genome wide scan in healthy men to identify quantitative trait loci (QTL) that were significantly linked to aBMD and to test whether any of these might be sex-specific.

Methods

aBMD at the spine and hip were measured in 515 pairs of brothers, aged 18-61 (405 white pairs, 110 black pairs). Linkage analysis in the brother sample was compared with results in a previously published sample of 774 sister pairs to identify sex-specific quantitative trait loci (QTL).

Results

A genome wide scan identified significant QTL (LOD>3.6) for aBMD on chromosomes 4q21 (hip), 7q34 (spine), 14q32 (hip), 19p13 (hip), 21q21 (hip), and 22q13 (hip). Analysis suggested that the QTL on chromosome 7q34, 14q32, and 21q21 were male-specific whereas the others were not sex-specific.

Conclusions

This study demonstrates that six QTL were significantly linked with aBMD in men. One was linked to spine and five were linked to hip. When compared to published data in women from the same geographical region, the QTL on chromosomes 7, 14 and 21 were male-specific. The occurrence of sex-specific genes in humans for aBMD has important implications for the pathogenesis and treatment of osteoporosis.