Angela Lamantia

Genetic linkage of β and γ subunits of epithelial sodium channel to systolic blood pressure

Summary Background Mutations in the genes on chromosome 16p12 that encode the β and γ subunits of the epithelial sodium channel ( SCNNIB and SCNNIG , respectively) have been linked with rare sodium-dependent forms of low and high blood pressure. Other DNA variants in or around these genes may contribute to variation in blood pressure and the risk of coronary heart disease and stroke. Methods We studied 286 white families from the general population in Victoria, Australia. Each family comprised both parents and two natural children. All participants were genotyped at chromosome 16p12 by use of four highly polymorphic microsatellite markers. Quantitative phenotype measurements were correlated with genotype in identity-by-descent sibling-pair linkage analyses. Findings We found significant linkage between systolic blood pressure and chromosome 16p12 after parametric analyses (p=0·0003) and non-parametric analyses (p=0·001). The mean difference in systolic blood pressure between siblings identical-by-descent at these loci was half as large (7·1 mm Hg) as the difference between siblings non-identical at these loci (14·0 mm Hg, p=0·001). No linkage between chromosome 16p12 and diastolic blood pressure or body-mass index was observed. Interpretation Chromosome 16p12 and the SCNNIB and SCNNIG genes are implicated in the physiological variation of systolic blood pressure. Our findings are important in explaining individual cardiovascular risk within the general population.

Genes controlling postural changes in blood pressure: comprehensive association analysis of ATP-sensitive potassium channel genes KCNJ8 and ABCC9

Buffering of blood pressure during change of posture such as standing is controlled largely by the baroreflex. In our population-based Victorian Family Heart Study (VFHS), we previously demonstrated that, on average, systolic blood pressure (SBP) changes very little on standing; however, interindividual variation is substantial and shows familial aggregation, with approximately 25% of the variance attributable to genetic factors. Our genomewide linkage analysis suggests a region on chromosome 12p that harbors two strong candidate genes, KCNJ8 and ABCC9, encoding the channel-forming inward rectifier subunit Kir6.1 and the ATP-sensitive binding cassette SUR2B, respectively. These are key components of smooth muscle ATP-sensitive potassium (K(ATP)) channels, important regulators of arterial tone and blood flow and central to autonomic baroreceptor control of changes in total peripheral resistance. We performed a comprehensive association analysis of 47 tag single nucleotide polymorphisms (SNPs) spanning the KCNJ8 and ABCC9 gene regions with postural change in SBP (DeltaSBP). To augment power, we took a selective genotyping approach in which we compared allele and genotype frequencies between 150 unrelated individuals with high (positive) DeltaSBP (> or = 7 mmHg) and 150 unrelated individuals with low (negative) DeltaSBP (< or = -7 mmHg) drawn from the offspring generation (18-30 yr) of the VFHS. Association analyses showed that no SNPs demonstrated statistically significant differences in genotype frequencies between groups, particularly after adjustments for multiple testing. We conclude that sequence variants in KCNJ8 and ABCC9 are unlikely to contribute to variation in DeltaSBP. Other genes in the identified chromosome 12p region warrant investigation.

Familial Analysis of Epistatic and Sex-Dependent Association of Genes of the Renin–Angiotensin–Aldosterone System and Blood PressureCLINICAL PERSPECTIVE

Background— Renin–angiotensin–aldosterone system genes have been inconsistently associated with blood pressure, possibly because of unrecognized influences of sex-dependent genetic effects or gene–gene interactions (epistasis). Methods and Results— We tested association of systolic blood pressure with single-nucleotide polymorphisms (SNPs) at renin (REN), angiotensinogen (AGT), angiotensin-converting enzyme (ACE), angiotensin II type 1 receptor (AGTR1), and aldosterone synthase (CYP11B2), including sex–SNP or SNP–SNP interactions. Eighty-eight tagSNPs were tested in 2872 white individuals in 809 pedigrees from the Victorian Family Heart Study using variance components models. Three SNPs (rs8075924 and rs4277404 at ACE and rs12721297 at AGTR1) were individually associated with lower systolic blood pressure with significant (P<0.00076) effect sizes ≈1.7 to 2.5 mm Hg. Sex-specific associations were seen for 3 SNPs in men (rs2468523 and rs2478544 at AGT and rs11658531 at ACE) and 1 SNP in women (rs12451328 at ACE). SNP–SNP interaction was suggested (P<0.005) for 14 SNP pairs, none of which had shown individual association with systolic blood pressure. Four SNP pairs were at the same gene (2 for REN, 1 for AGT, and 1 for AGTR1). The SNP rs3097 at CYP11B2 was represented in 5 separate pairs. Conclusions— SNPs at key renin–angiotensin–aldosterone system genes associate with systolic blood pressure individually in both sexes, individually in one sex only and only when combined with another SNP. Analyses that incorporate sex-dependent and epistatic effects could reconcile past inconsistencies and account for some of the missing heritability of blood pressure and are generally relevant to SNP association studies for any phenotype.

OS 31-01 BLOOD PRESSURE, INITIAL ORTHOSTATIC HYPOTENSION, GLYCERYL TRINITRATE AND THE GLU504LYS POLYMORPHISM OF ALDEHYDE DEHYROGENASE-2.

Objective: Glyceryl trinitrate (GTN) is used to treat angina and lower blood pressure. The enzyme mitochondrial aldehyde dehydrogenase-2 (ALDH2) can activate GTN. The polymorphism (Gly504Lys) in the gene encoding ALDH2 is associated with significantly reduced enzyme activity and potentially diminished responses to GTN. Design and Method: Systolic blood pressure (SBP) was measured continuously (Finometer Midi) during 2 orthostatic challenges in 493 healthy young adults (mean age 22 y), 390 of whom took 300 &mgr;g GTN sublingually 5 min prior to the second challenge and 103 subjects of whom were untreated. We recorded the average SBP for 30 s after 5 minutes lying, the nadir in SBP within 30 s of standing and the average SBP for 30 s after 2 minutes standing. The ALDH2 polymorphism was measured by the SNP rs671 (allele A = Lys, allele G = Gly). A dominant model (GG v AG/AA) was use to compare SBP with adjustments for age, sex and weight. Results: During the control challenge in the GTN group, the mean lying SBP was 117 mmHg (SD 15.8), 73 mmHg (20.4) on initial standing and 109 mmHg (16.9) after 2 minutes standing. After GTN, SBP was significantly (all P < 0.0001) lower than control lying [114 mmHg (15.9)], on standing [60 mmHg (19.3)] and after 2 minutes standing [103 mmHg (16.9)]. In the 103 untreated subjects there were no significant differences between the SBP between challenges. None of the SBP responses after GTN differed between the 2 ALDH2 genotype groups. Conclusions: In healthy young adults acute GTN causes a major exaggeration of the initial orthostatic hypotension within 30 s of standing. The hypotensive actions of GTN were not associated with the Gly504Lys polymorphism. These findings suggest that factors other than ALDH2 activity explain the blood pressure effects of acute exposure to GTN.

OS 08-04 EPISTATIC AND SEX-DEPENDENT ASSOCIATION ANALYSES OF GENES OF THE RENIN-ANGIOTENSIN-ALDOSTERONE SYSTEM AND BLOOD PRESSURE IN FAMILIES.

Objective: Genes encoding key elements of the renin-angiotensin-aldosterone system (RAAS) cascade have been previously but inconsistently associated with blood pressure. Sex-dependency might be important here and functional genetic polymorphisms might exhibit epistatic effects. Design and Method: We assessed variation in the genes encoding renin (REN), angiotensinogen (AGT), angiotensin converting enzyme (ACE), angiotensin II type 1 receptor (AGTR1) and aldosterone synthase (CYP11B2). We determined whether individual SNPs, interactions between SNPs or sex-SNP interactions were associated with systolic blood pressure (SBP). 88 SNPs provided gene-wide coverage of the 5 RAAS genes for 2929 individuals (comprising over 800 2-generation white adult families) from the Victorian Family Heart Study. Variance components models were used that adjusted for measured covariates and within-family correlation. Results: Initial analyses found that three SNPs (rs8075924 and rs4277404 in ACE and rs12721297 in AGTR1) were inversely associated with SBP with significant (p < 0.005) effect sizes around −1.7 mmHg to −2.5 mmHg. No association was observed for markers for well-known polymorphisms: ACE I/D, AGTR1 A1166C and AGT M235T. Significant sex-specific associations were seen for 3 SNPs in males (rs2468523 and rs2478544 in AGT and rs11658531 in ACE) and 1 SNP in females (rs12451328 in ACE). There were significant interactions between 3 pairs of SNPs (rs7079 in AGT with rs2004776 in AGT; rs4736360 in CYP11B with rs2478523 in AGT and rs4736360 in CYP11B with rs2478545 in AGT). The interaction effect estimates ranged from 2.9 to 5.5 mmHg. Conclusions: Our analyses of SNPs in and around key RAAS genes for SBP reveal evidence of direct association, association dependent on sex and association dependent on epistasis. Variants dependent on epistasis or sex were not obvious in direct tests of association with SBP. Analyses that incorporate sex-dependent and epistatic effects could help account for the missing heritability of blood pressure.