Loosening the cuff: important new advances in modeling antihypertensive treatment effects in genetic studies of hypertension

Genetic influence on the relation between exhaled nitric oxide and pulse wave reflection

Nitric oxide has an important role in the development of the structure and function of the airways and vessel walls. Fractional exhaled nitric oxide (FE(NO)) is inversely related to the markers and risk factors of atherosclerosis. We aimed to estimate the relative contribution of genes and shared and non-shared environmental influences to variations and covariation of FE(NO) levels and the marker of elasticity function of arteries. Adult Caucasian twin pairs (n = 117) were recruited in Hungary, Italy and in the United States (83 monozygotic and 34 dizygotic pairs; age: 48 ± 16 SD years). FE(NO) was measured by an electrochemical sensor-based device. Pulse wave reflection (aortic augmentation index, Aix(ao)) was determined by an oscillometric method (Arteriograph). A bivariate Cholesky decomposition model was applied to investigate whether the heritabilities of FE(NO) and Aix(ao) were linked. Genetic effects accounted for 58% (95% confidence interval (CI): 42%, 71%) of the variation in FE(NO) with the remaining 42% (95%CI: 29%, 58%) due to non-shared environmental influences. A modest negative correlation was observed between FE(NO) and Aix(ao) (r = -0.17; 95%CI:-0.32,-0.02). FE(NO) showed a significant negative genetic correlation with Aix(ao) (r(g) = -0.25; 95%CI:-0.46,-0.02). Thus in humans, variations in FE(NO) are explained both by genetic and non-shared environmental effects. Covariance between FE(NO) and Aix(ao) is explained entirely by shared genetic factors. This is consistent with an overlap among the sets of genes involved in the expression of these phenotypes and provides a basis for further genetic studies on cardiovascular and respiratory diseases.

Heritability and Stability of Resting Blood Pressure

We examined the contribution of genetic and environmental influences to variation in resting systolic (SBP) and diastolic (DBP) blood pressure in participants from 4 twin studies carried out between 1986 and 2003. A total of 1577 subjects (682 males, 895 females) participated. There were 580 monozygotic twins, 664 dizygotic twins and 333 of their siblings. The 4 studies sampled subjects in different age groups (average age 17, 32, 37, 44 years), allowing for comparison of the relative contribution of genetic and environmental factors across the first part of the life span. Blood pressure was assessed under laboratory conditions in 3 studies and by ambulatory monitoring in 1 study. Univariate analyses of SBP and DBP showed significant heritability of blood pressure in all studies (SBP h2 48% to 60%, DBP h2 34% to 67%). Overall, there was little evidence for sex differences in blood pressure heritability, and no evidence for differences in heritability due to measurement strategy (laboratory vs. ambulatory). For 431 subjects there were data from 2 or more occasions that allowed us to assess the tracking of blood pressure over time and to estimate the genetic and environmental contributions to blood pressure tracking. Correlations over time across an average period of 7.1 years (tracking) were between .41 and .70. Multivariate genetic analyses showed that blood pressure tracking was entirely explained by the same genetic factors being expressed across time. It was concluded that whole genome scans for resting blood pressure can safely pool data from males and females, laboratory and ambulatory recordings, and different age cohorts.

Heritability and Stability of Resting Blood Pressure in Australian Twins

In Australian twins participating in three different studies (1979–1996), the contribution of genetic and environmental influences to variation in resting systolic (SBP) and diastolic blood pressure (DBP) was studied. The sample consisted of 368 monozygotic and 335 dizygotic twin pairs with measurements for both individuals. Blood pressure measurements in two studies were available for 115 complete twin pairs, and 49 twin pairs had measurements in three studies. This allowed assessment of blood pressure tracking over an average period of 12 years in the age range of 23 to 45 years. Multivariate analyses showed significant heritability (h2) of blood pressure in all studies (SBP h2= 19%–56%, DBP h2= 37%–52%). In addition, the analyses showed that the blood pressure tracking was explained by the same set of genetic factors. These results replicate an earlier finding in Dutch twins that also showed stability of the contribution of genetic factors to blood pressure tracking.

Suggestive linkage detected for blood pressure related traits on 2q and 22q in the population on the Samoan islands

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.

Genome-wide scan for blood pressure in Australian and Dutch subjects suggests linkage at 5P, 14Q, and 17P

Large-scale studies estimate the heritability of blood pressure at approximately 50%. We carried out a genome-wide linkage analysis to search for chromosomal loci that might explain this heritability using longitudinal, multiple measures of systolic and diastolic blood pressure obtained in sibling pairs and dizygotic twin pairs from 2 countries (a total of 286 pairs from Australia and 636 pairs from the Netherlands). These pairs and a large number of their parents were genotyped with microsatellite markers. Multivariate linkage analysis of the combined data of both countries, using a variance components approach, showed suggestive linkage for diastolic blood pressure on chromosomes 5p13.1 (logarithm of odds score: 2.48), 14q12 (logarithm of odds score: 2.40) and 17q24.3 (logarithm of odds score: 2.36). The highest logarithm of odds score of 1.21 for systolic blood pressure was observed on chromosome 13q34. These results replicate earlier findings and add to a slowly emerging picture of multiple loci contributing to quantitative blood pressure variation.

Heritability of daytime ambulatory blood pressure in an extended twin design

The present study estimated the genetic influences on ambulatory systolic and diastolic blood pressure, and on hypertensive status derived from ambulatory levels, in a family sample of 535 twins and 257 singleton siblings. This "extended twin design" was used to explicitly test the possibility that results obtained in singleton siblings are different from those obtained in twins. To examine the effects of excluding (medicated) hypertensive subjects, the genetic analyses were first performed under strict exclusion (medication and/or blood pressure >135/85 mm Hg), then without the medicated subjects, and, finally, without any exclusion. For the latter analysis, the untreated blood pressure values in subjects using antihypertensive medication were estimated by augmenting the observed blood pressure by the published efficacy of the specific antihypertensive medication used. No evidence was found for differential means, variances, or covariances of ambulatory blood pressure in singletons compared with twins. This indicates that estimates of heritability of ambulatory blood pressure from twin studies can be generalized to the singleton population. Heritability of hypertension, defined as a mean daytime blood pressure >135/85 mm Hg or antihypertensive medication use, was 61%. Genetic contribution to ambulatory blood pressure was highest when all subjects were included (systolic, 44% to 57%; diastolic, 46% to 63%) and lowest under strict exclusion (systolic, 32% to 50%; diastolic, 31% to 55%). We conclude that exclusion of (medicated) hypertensives removes part of the true genetic variance in ambulatory blood pressure.

Genome-wide scan for blood pressure suggests linkage to chromosome 11, and replication of loci on 16, 17, and 22

Hypertension was one of the first complex traits to be studied and is thought to be influenced by polygenic and multiple environmental risk factors. Several genomic studies have found suggestive logarithm of odds (LOD) scores for either blood pressure or essential hypertension, but few loci have been replicated. In this study, we performed a genome-wide linkage analysis for systolic blood pressure (SBP) and diastolic blood pressure (DBP) on 1109 white female dizygotic twin pairs from the TwinsUK registry in London. Multipoint linkage analysis replicated the locations of 3 previously reported linkage peaks: on chromosome 16 at 65 cM (LOD 0.8 for SBP and 1.8 for DBP); on chromosome 17 at 70 cM (LOD 1.8 SBP); and at 35 cM on chromosome 22 (LOD 0.97 SBP and 0.99 DBP). Results from multipoint analysis showed 1 novel suggestive linkage for SBP (multipoint LOD 2.28; 2-point P=0.0007) at 35 cM on chromosome 11. Results were similar when those on blood pressure medication were excluded. These are encouraging results for hypertensive research and demonstrate that despite past disappointments, linkage studies can be used to replicate regions from other studies and potentially discover new genetic risk factors of moderate to large effect size. Considering the differences in selection and ascertainment of the previous linkage studies, these results also suggest that some quantitative trait loci are likely to influence the normal range of blood pressure and clinical hypertension, whereas others will be specific to each trait. Future studies should focus on the fine mapping of these replicated regions, which include potential candidate genes.

Genome-wide linkage analysis of longitudinal phenotypes using sigma2A random effects (SSARs) fitted by Gibbs sampling

The study of change in intermediate phenotypes over time is important in genetics. In this paper we explore a new approach to phenotype definition in the genetic analysis of longitudinal phenotypes. We utilized data from the longitudinal Framingham Heart Study Family Cohort to investigate the familial aggregation and evidence for linkage to change in systolic blood pressure (SBP) over time. We used Gibbs sampling to derive sigma-squared-A-random-effects (SSARs) for the longitudinal phenotype, and then used these as a new phenotype in subsequent genome-wide linkage analyses.

Additive genetic effects (σ²_A.time) were estimated to account for ~9.2% of the variance in the rate of change of SBP with age, while additive genetic effects (σ²_A) were estimated to account for ~43.9% of the variance in SBP at the mean age. The linkage results suggested that one or more major loci regulating change in SBP over time may localize to chromosomes 2, 3, 4, 6, 10, 11, 17, and 19. The results also suggested that one or more major loci regulating level of SBP may localize to chromosomes 3, 8, and 14.

Our results support a genetic component to both SBP and change in SBP with age, and are consistent with a complex, multifactorial susceptibility to the development of hypertension. The use of SSARs derived from quantitative traits as input to a conventional linkage analysis appears to be valuable in the linkage analysis of genetically complex traits. We have now demonstrated in this paper the use of SSARs in the context of longitudinal family data.

Linkage analysis of systolic blood pressure: a score statistic and computer implementation

A genome-wide linkage analysis was conducted on systolic blood pressure using a score statistic. The randomly selected Replicate 34 of the simulated data was used. The score statistic was applied to the sibships derived from the general pedigrees. An add-on R program to GENEHUNTER was developed for this analysis and is freely available.

Bivariate variance-component analysis, with application to systolic blood pressure and total cholesterol levels in the Framingham Heart Study

BACKGROUND

The correlations between systolic blood pressure (SBP) and total cholesterol levels (CHOL) might result from genetic or environmental factors that determine variation in the phenotypes and are shared by family members. Based on 330 nuclear families in the Framingham Heart Study, we used a multivariate normal model, implemented in the software FISHER, to estimate genetic and shared environmental components of variation and genetic and shared environmental correlation between the phenotypes. The natural logarithm of the phenotypes measured at the last visit in both Cohort 1 and 2 was used in the analyses. The antihypertensive treatment effect was corrected before adjustment of the systolic blood pressure for age, sex, and cohort.

RESULTS

The univariate correlation coefficient was statistically significant for sibling pairs and parent-offspring pairs, but not significant for spouse pairs. In the bivariate analysis, the cross-trait correlation coefficients were not statistically significant for all relative pairs. The shared environmental correlation was statistically significant, but the genetic correlation was not significant.

CONCLUSION

There is no significant evidence for a close genetic correlation between systolic blood pressure and total cholesterol levels. However, some shared environmental factors may determine the variation of both phenotypes.

Genome-wide linkage analysis of blood pressure under locus heterogeneity

We describe a method for mapping quantitative trait loci that allows for locus heterogeneity. A genome-wide linkage analysis of blood pressure was performed using sib-pair data from the Framingham Heart Study. Evidence of linkage was found on four markers (GATA89G08, GATA23D06, GATA14E09, and 049xd2) at a significance level of 0.01. Two of them (GATA14E09 and 049xd2) seem to overlap with linkage signals reported previously, while the other two are not linked to any known signals.

Lack of reproducibility of linkage results in serially measured blood pressure data

Background

Using the longitudinal Framingham Heart Study data on blood pressure, we analyzed the reproducibility of linkage measures from serial cross-sectional surveys of a defined population by performing genome-wide model-free linkage analyses to systolic blood pressure (SBP) and history of hypertension (HTN) measured at five separate time points.

Results

The heritability of SBP was relatively stable over time, ranging from 11.6 to 23.5% (coefficient of variation = 25.7%). However, the variability in linkage results was much greater. The average correlation in LOD scores at any pair of time points was 0.46 for HTN (NPL All LOD) and 0.17 for SBP (Variance Components LOD). No evidence of reproducible linkage results was found, with a mean κ of 0.02 for linkage to HTN and -0.03 for SBP linkage. At loci with potential evidence for linkage (LOD > 1.0 at one or more time points), the correlation was even lower. The coefficient of variation at loci with potential evidence of linkage was 126% for HTN and 135% for SBP. None of 15 chromosomal regions for HTN and only one of 28 regions for SBP with potential evidence for linkage had a LOD > 1.0 at more than two of the five time points.

Conclusion

These data suggest that, although heritability estimates at different time points are relatively robust, the reproducibility of linkage results in serial cross-sectional samples of a geographically defined population at successive time points is poor. This may explain in part the difficulty encountered in replicating linkage studies of complex phenotypes.