Heritability of Arterial Stiffness in Black- and White American Youth and Young Adults

Abstract

Background

To examine the heritability of arterial stiffness measured as pulse wave velocity (PWV) and its dependence on ethnicity, gender and blood pressure (BP).

Methods

As part of the Georgia Cardiovascular Twin Study, we measured aortoradial (radial) and aorto-dorsalis-pedis (foot) PWV in 702 twins (41% black, 49% male) aged 12 – 30 years (mean: 17.7 ± 3.3) including monozygotic and dizygotic pairs of same- as well as opposite-gender. Ethnicity and gender effects on genetic and environmental contributions to PWV were estimated by genetic model fitting.

Results

Diastolic BP was the most important hemodynamic predictor. Best fitting models showed no ethnicity or gender differences in estimates of genetic and environmental influence and indicated substantial heritabilities of 0.43 (95% CI: 0.30–0.54) and 0.53 (95% CI: 0.42–0.62) for radial and foot PWV, respectively. Over a quarter of these heritabilities (0.19 for radial PWV, 0.14 for foot PWV) were due to genes in common with DBP as based on multivariate models.

Conclusions

Individual differences in arterial stiffness of youth and young adults are substantially heritable and more than 25% of this heritability is explained by genes that also influence DBP. Heritability estimates do not show any differences between blacks and whites or males and females.

INTRODUCTION

Arterial stiffness has been shown to be an independent predictor of cardiovascular morbidity and mortality in patients with hypertension ¹, diabetes ², and end-stage renal disease ³, and most likely increases cardiovascular risk in the general population ^4,5.

Demographic factors like gender and ethnicity are one source explaining individual differences in arterial stiffness. Females have intrinsically stiffer conduit arteries than males⁶. Particularly prepubertal females have higher arterial stiffness compared with age-matched males. This difference has been shown to minimize in post puberty and widen again in the postmenopausal years, suggesting effects of female sex steroids ⁶. Ethnic difference in arterial stiffness have also been consistently reported with blacks (Afro-Carribeans) showing stiffer arteries than whites (Caucasians)^7,8.

Genetic factors are another source of individual differences in arterial stiffness. ⁹ Although a rapidly increasing number of studies have investigated potential candidate genes for arterial stiffness (for review see ⁹), the logical prerequisite in the search for candidate genes, namely the determination of heritability of arterial stiffness, especially as measured by PWV, has received very little attention. Starting with the pioneering work of The Bogalusa Heart Study ¹⁰ a number of family studies ^11,12 and one previous twin study of our own ¹³ suggested a genetic background of arterial stiffness for a number of proxy measures such as carotid stiffness and augmentation index. Only two studies assessed the heritability of PWV. The Framingham Heart Study showed a moderate heritability of 0.40 for carotid-femoral PWV, but a low and non-significant heritability of 0.09 for carotid-brachial PWV in 817 pedigrees. ¹⁴ The Erasmus Rucphen Family study measured carotid-femoral PWV in 930 individuals from a single pedigree and reported a heritability of 0.36 which reduced to 0.26 after adjustment of covariates.¹⁵

The picture emerging from the available literature remains incomplete for several reasons. First, all of these studies were conducted within ethnically homogeneous populations and did not allow comparisons of heritability estimates between different ethnic groups. Second, although it is known that characteristics and prognostic significance of central and peripheral arterial stiffness differ ^16,17, very little data is available concerning their potentially distinct genetic background. Finally, previous studies almost exclusively evaluated arterial stiffness in middle-aged and older adults. With a few exceptions ^18,19, very little data is available on the determinants and heritability of arterial stiffness in youth and young adults.

Aims of this first twin study of PWV were twofold. First, we investigated the demographic, anthropometric and hemodynamic determinants of aortoradial (radial) and aorto-dorsalis-pedis (foot) PWV in our large sample of black and white adolescents and young adults. Second, we estimated heritabilities of radial and foot PWV and examined the extent to which these are dependent on race, sex, and/or genes in common with BP. We further examined to what extent similarity between radial and foot PWV could be explained by genetic and/or environmental factors.

METHODS

Study population

PWV data for this study was available from 702 twins (41% black) from the Georgia Cardiovascular Twin Study, including monozygotic (MZ) and dizygotic (DZ) pairs of same- as well as opposite-gender (mean age: 17.7 ± 3.3; range: 11.9 – 30.0). Zygosity determination, the criteria to classify subjects as black or white Americans, and recruitment into the Georgia Cardiovascular Twin Study have been described previously ²⁰. All participants were apparently healthy based upon (parental) report of the child’s medical history. None of the participants were taking any medication. Written informed consent was provided by all participants and by parents if participants were <18 years. The Institutional Review Board at the Medical College of Georgia had given approval for the study.

Measures

Aortoradial (radial) PWV and aorto-dorsalis-pedis (foot) PWV were measured noninvasively with applanation tonometry (Millar Instruments) ⁴ and analysis software (SphygmoCor, AtCor Medical, Sydney, Australia). Pressure waves were recorded at the common carotid and radial arteries for the radial PWV and at the common carotid and dorsalis-pedis arteries for the foot PWV. The dorsalis-pedis was used as a pragmatic alternative to the femoral measurement site as it was considered less sensitive and more readily accepted in our population of youth and young adults. PWV was then automatically calculated from measurements of pulse transit time and the distance traveled by the pulse between the two recording sites: PWV=Distance (meters)/Transit Time (seconds). Systolic BP (SBP) and diastolic BP (DBP) measurements (Dinamap 1864 SX; Criticon Incorporated, Tampa, FL) were taken at 11, 13 and 15 minutes, during a 15-minute laying relaxation period. The average was used to represent resting SBP and DBP.

Analytical approach

First, we investigated the determinants of PWV by: 1) testing ethnicity and gender differences, 2) calculating correlations with age, body mass index (BMI), waist circumference, SBP, DBP and pulse pressure (PP), and 3) performing hierarchic multiple regression analyses to determine which predictors showed independent effects. Second, we used univariate model fitting analyses to estimate the relative influence of genetic and environmental factors on individual differences in radial and foot PWV and test for ethnicity and gender differences as described in detail elsewhere ²¹. Finally, we used multivariate modeling to estimate: 1) the extent to which heritability of PWV could be explained by genetic factors in common with DBP, the most important hemodynamic determinant uncovered and 2) to what extent the correlation between radial and foot PWV could be explained by common genes and/or common environment after adjustment for covariates.

We detailed these analytical approaches, including the hierarchic multiple regression analyses and quantitative genetic model fitting, in Supplementary Material I: Methods.

Statistical analysis and software

Radial and foot PWV, SBP, DBP, PP, BMI and waist circumference were all logarithmically transformed before analysis to obtain normal distributions. Multiple regression models were performed using Generalized Estimating Equations (GEE) to account for the dependency between twins ²². Data handling, preliminary analyses and GEE were done with STATA (StataCorp, College Station, Texas). Quantitative genetic modeling was performed with Mx software ²³.

RESULTS

General characteristics

Table 1 shows mean values of general characteristics and PWV for all four ethnicity-by-gender groups. Effects of ethnicity, gender and their interaction were tested using GEE models with age included as a covariate. The mean age of the sample was 17.7 years (range: 11.9–30.0 years), with blacks slightly younger than whites. Height, waist circumference and PP were similar for blacks and whites, while SBP, DBP and both radial and foot PWV levels were significantly higher in blacks than in whites. BMI and weight were higher in black compared to white females. Males were taller and heavier, had larger waist circumference (only in whites), higher SBP and PP but lower DBP levels than females, while the difference of radial or foot PWV levels between the genders was not significant.

Table 1.

General Characteristics and PWV Data of White- and Black American Males and Females

	Whites		Blacks		Ethnicity & Gender Effects
	Males	Females	Males	Females	Ethnicity, P	Gender, P
N, subjects *	214	199	129	160
Age, y	18.2±3.5	17.9±3.2	17.5±3.1	17.0±3.4	<0.05	NS
Height, m	1.74±0.09	1.63±0.07	1.73±0.10	1.62±0.07	NS	<0.001
Weight, kg	72.5±19.9	60.1±15.2	71.7±19.4	68.9±19.5	0.01†	<0.001
BMI, kg/m2	23.8±5.6	22.6±5.0	23.8±5.4	25.9±6.7	<0.001 †	NS
Waist, cm	32.7±5.6	29.9±4.6	31.0±5.6	31.6±5.3	NS	<0.001‡
SBP, mmHg	114.4±10.3	107.7±8.8	117.1±11.1	110.7±10.2	<0.001	<0.001
DBP, mmHg	57.0±6.4	59.3±6.3	59.0±7.0	61.4±6.9	<0.001	<0.001
PP, mmHg	57.4±9.6	48.3±7.9	58.2±10.1	49.3±8.4	NS	<0.001
Radial PWV, m/s	6.38±1.08	6.28±0.99	6.65±1.03	6.68±1.02	<0.001	NS
Foot PWV, m/s	7.15±0.91	7.03±0.84	7.10±0.83	7.20±0.81	<0.05	NS

Data are mean ± SD unless stated otherwise. BMI indicates body mass index; SBP, systolic blood pressure; DBP, diastolic blood pressure; PP, pulse pressure; radial PWV, aortoradial pulse wave velocity; and foot PWV, aorto-dorsalis-pedis pulse wave velocity.

^*Maximum number of subjects is shown for each ethnicity by gender group, but total numbers were slightly lower for Radial PWV (n=699) and Foot PWV (n=677).

^†Females only;

^‡Whites only

Determinants of PWV

Table 2 shows correlations of radial and foot PWV with age, anthropometric and hemodynamic variables in different ethnicity-by-gender groups. In both ethnicities a moderate correlation of 0.42 was found between radial and foot PWV indicating relatively modest overlap between these 2 measures of arterial stiffness. This was confirmed by different patterns of correlations with covariates for both blacks and whites, with foot PWV showing much stronger correlations than radial PWV with age, height, BMI, waist circumference and SBP. DBP showed strong associations with both radial and foot PWV (r = 0.32 – 0.43) whereas correlations with PP were absent or even negative. Heart rate did not show a significant correlation between either radial or foot PWV (data not shown). Also, environmental factors such as smoking, alcohol use and overnight sodium excretion did not show a significant influence on radial or foot PWV (data not shown). In a subgroup of twins (n=207, 49% black) total cholesterol, HDL, triglycerides and fasting glucose were available, but no significant associations with either radial or foot PWV were found (data not shown).

Table 2.

Correlations [overall (male/female)] of Radial and Foot PWV with Age, Anthropometric and Hemodynamic Variables for White- and Black American Males and Females

Measures	Whites		Blacks
	Radial PWV	Foot PWV	Radial PWV	Foot PWV
Foot PWV	0.42(0.51/0.31)	…	0.42(0.42/0.41)	…
Age	0.27(0.33/0.19)	0.51(0.62/0.38)	0.14(0.10/0.17)	0.40(0.34/0.47)
Height	0.10(0.15/0.01)	0.23(0.32/0.11)	0.00(0.01/0.01)	0.06(0.13/0.09)
BMI	0.01(0.08/−0.08)	0.21(0.24/0.17)	−0.10(−0.11/−0.11)	0.14(0.11/0.13)
Waist	0.06(0.15/−0.08)	0.25(0.29/0.18)	−0.08(−0.07/−0.10)	0.17(0.18/0.15)
SBP	0.18(0.27/0.05)	0.28(0.37/0.16)	0.04(0.10/0.00)	0.22(0.29/0.21)
DBP	0.38(0.42/0.36)	0.43(0.48/0.42)	0.32(0.35/0.31)	0.37(0.41/0.33)
PP	−0.08 (0.00/−0.24)	0.00(0.07/−0.16)	−0.18(−0.14/−0.23)	−0.02(0.02/0.00)

Significant correlations (p<0.05) are in bold. See Table 1 for abbreviations.

Results of hierarchical multiple regression analyses for radial and foot PWV are shown in Table 3. In the full models, SBP and DBP levels as well as the interaction between SBP and ethnicity showed independent significant effects on both radial PWV and foot PWV, whereas BMI (or waist circumference) was only significant for foot PWV. The influence of ethnicity remained significant in the full model with, on average, lower PWV levels in whites than in blacks. However, whites showed steeper increases in both radial and foot PWV with increases in SBP as indicated by the significant SBP by ethnicity interaction. DBP was clearly the most important hemodynamic predictor. Explained variance by the DBP model alone approached that of the full model for both radial and foot PWV. Significant correlations of PWV with height as an index of growth (Table 2) could probably be attributed to its correlation with age (r=0.32) in this sample of young twins, because no significant independent contribution of height to radial or foot PWV was found (data not shown). PP showed no significant contribution to either radial or foot PWV (data not shown). Overall, full models explained 18.8 % and 34.4% of the variance in radial and foot PWV, respectively.

Table 3.

Effects of Age, Gender, Ethnicity, BMI, Waist circumference, SBP and DBP on Radial and Foot PWV

Models	Variables	Beta(SE)	P value	Explained Variance
Radial PWV
1. Base model	Age	0.07(0.01)	<0.001
	Gender	−0.01(0.08)	NS
	Ethnicity	0.40(0.09)	<0.001	7.3%
2. Model 1 + BMI	BMI	−0.02(0.007)	<0.05	8.2%
3. Model 1 + Waist	Waist	−0.01(0.008)	NS
4. Model 1 + SBP	SBP	0.02(0.005)	<0.05
	SBP*Ethnicity	−0.02(0.007)	<0.05	8.7%
5. Model 1 + DBP	DBP	0.05(0.006)	<0.001	17.5%
6. Full model	Age, Gender, Ethnicity, DBP, SBP, SBP*Ethn.			18.8%
Foot PWV
1. Base model	Age	0.12(0.01)	<0.001
	Gender	0.02(0.06)	NS
	Ethnicity	0.17(0.07)	<0.05	22.2%
2. Model 1 + BMI	BMI	0.01(0.006)	<0.05	22.7%
3. Model 1 + Waist	Waist	0.02(0.006)	<0.05	23.3%
4. Model 1 + SBP	SBP	0.02(0.004)	<0.001
	SBP*Ethnicity	−0.01(0.006)	<0.05	26.4%
5. Model 1 + DBP	DBP	0.04(0.004)	<0.001	32.3%
6. Full model	Age, Gender, Ethnicity, BMI, DBP, SBP, SBP*Ethnicity			34.4%

See Table 1 for abbreviations.

Twin correlations and genetic model fitting

Table 4 shows the twin correlations of radial PWV, foot PWV and DBP for each gender-by-zygosity group in whites and blacks. MZ correlations were generally higher than DZ correlations, indicating important genetic effects. One exception is the high DZ correlation for radial PWV and DBP in black males, possibly chance observations due to the small sample size of this group (13 pairs). We included DBP data here because of the consistent correlations and contributions to both radial and foot PWV levels, as shown in Tables 2 and 3.

Table 4.

Twin Correlations for Each Gender by Zygosity group, and Variance Components Estimates of Best-Fitting Univariate Models for Radial PWV, Foot PWV and DBP

Measure	MZM	DZM	MZF	DZF	DOS
Correlation, Whites
N, pairs	48	28	41	24	53
Radial PWV	0.51	0.41	0.41	0.39	0.22
Foot PWV,	0.51	0.23	0.52	0.37	0.23
DBP	0.48	0.13	0.57	0.18	0.26
Correlation, Blacks
N, pairs	27	13	30	23	35
Radial PWV	0.29	0.49	0.33	0.17	0.22
Foot PWV	0.60	0.45	0.57	0.49	0.45
DBP	0.46	0.57	0.70	0.46	0.37
Estimates, overall	h2 (95%CI)			e2 (95%CI)
Radial PWV	0.43 (0.31–0.54)			0.57 (0.46–0.69)
Foot PWV	0.54 (0.43–0.63)			0.46 (0.37–0.57)
DBP	0.53 (0.42–0.62)			0.47 (0.38–0.58)

MZM indicates monozygotic males; DZM, dizygotic males; MZF, monozygotic females; DZF, dizygotic females; and DOS, dizygotic opposite gender. h² indicates heritability; e², unique environmental variance component.

Univariate genetic model fitting including all 10 gender-by-ethnicity-by-zygosity groups from Table 4 showed no significant ethnicity or gender differences in genetic and environmental influences on radial PWV, foot PWV or DBP. For each of the variables, the model composed of additive genetic influences and unique environmental influences (AE model) was the best fitting model, revealing significant heritabilities for radial PWV (43%), foot PWV (54%) and DBP (53%). Genetic and environmental parameter estimates and 95% confidence intervals (CIs) of these best fitting models are shown in Table 4.

We performed bivariate genetic modeling of PWV and DBP levels to assess to what extent genetic and environmental influences were shared between PWV and DBP. Genetic correlations were 0.66 (95% CI: 0.52–0.81) between radial PWV and DBP and 0.51 (95% CI: 0.35–0.65) between foot PWV and DBP indicating significant overlap in genes influencing PWV and DBP. Environmental correlations were much lower, i.e., non-significant between radial PWV and DBP and 0.20 (0.05–0.33) between foot PWV and DBP. As shown in Figure 1, 43% of the radial PWV heritability (0.19/0.44) and 26% of the foot PWV heritability (0.14/0.54) were due to genes in common with DBP. After the removal of the shared genetic and environmental factors with DBP, heritabilities for radial and foot PWV were estimated as 0.31 (95% CI: 0.16–0.43) and 0.48 (95% CI: 0.36–0.58), respectively.

Figure 1.

Sources of variance in radial and foot PWV based on best-fitting bivariate model with DBP

Subsequently, we performed bivariate model fitting of radial and foot PWV to investigate to what extent the correlation between these 2 different indices of arterial stiffness could be explained by common genes and/or common environment after adjustment for significant covariates, i.e., the full models as determined by the multiple regression analyses (Table 3). Figure 2 shows the results of this analysis. The genetic correlation between radial and foot PWV was significant (0.50, 95% CI: 0.25–0.76). The unique environmental correlation was much lower but statistically significant (0.16, 95% CI: 0.03–0.29). According to the best fitting model shown in Figure 2, the phenotypic correlation between these covariate-adjusted measures of radial and foot PWV was 0.28 of which 65% was due to genetic overlap between the traits.

Figure 2.

Genetic and environmental correlations and factor loadings of the best fitting bivariate model for radial and foot PWV adjusted for significant covariates (see Table 3). For clarity only one twin is depicted. Factor loadings (or path coefficients) are expressed as square roots to make clear that squaring those factor loadings yields estimates of genetic and environmental variance components as shown in text. A: additive genetic factor; E: unique environmental factor.

DISCUSSION

In this first classic twin study of arterial stiffness we investigated demographic, anthropometric and hemodynamic determinants of radial and foot PWV and estimated their heritabilities. We showed that arterial stiffness, especially foot PWV, increased with age even in this population of youth and young adults without any major disease. Ethnic differences in arterial stiffness seem to be established by adolescence. DBP as opposed to PP was the strongest hemodynamic predictor of PWV in young individuals. Individual differences in both radial and foot PWV of youth and young adults were substantially heritable and partly explained by genes for DBP. Heritability estimates did not show any differences between blacks and whites or males and females and the genetic background for radial and foot PWV overlapped but still differed substantially.

Few prior studies have evaluated heritability for arterial stiffness, especially measured by PWV. Heritabilities of 40% and 36% reported for carotid-femoral PWV in two recent family studies ^14,15 were slightly lower than our estimate of 54% for foot PWV, whereas the low and non-significant heritability of 9% for carotid-brachial PWV in the Framingham sample ¹⁴ is at odds with our estimate of 43% for radial PWV. To the best of our knowledge, our study is the only one that has been performed in both white- and black- American males and females, allowing for the evaluation for gender and ethnic effects. Although black Americans had stiffer arteries than their white American counterparts in this young population, the heritability estimates did not differ significantly between the two ethnic groups or between males and females.

We detected a genetic correlation of 0.50 between radial and foot PWV. This moderate correlation indicated that the genetic basis of these two arterial stiffness indices overlap but still differ largely. The foot PWV represents a mixture of proximal elastic arteries, mainly the aorta, and distal muscular arteries, such as the femoral, sural, and dorsalis-pedis arteries. The radial PWV only reflects distal muscular arteries. It is generally recognized that elastic arteries stiffen progressively with age, whereas the stiffness of muscular arteries changes little with age ¹⁶. Even in this young population our findings support this differential pattern of aging with foot PWV showing much higher correlations with age than radial PWV. Recent findings suggest that aortic PWV is an independent determinant of cardiovascular death in high risk patients, while PWV in peripheral arterial segments did not predict cardiovascular outcome ¹⁷. Other studies showed that both central and peripheral arterial stiffness were associated with cardiovascular risk factors including impaired glucose metabolism ²⁴ and C-reactive protein levels ²⁵. Our findings raise the possibility that the distinct predictive characteristics of arterial stiffness of elastic and muscular arteries partly have their origin in their different genetic basis.

The genetic mechanisms underlying the heritability of arterial stiffness are largely unknown. However, a number of studies suggested that at least three categories of genes could be potentially related to the development of arterial stiffness: those related to the pathophysiology of high BP (e.g., genes for the angiotensin-converting enzyme, the angiotensin II type 1 receptor and aldosterone synthase), those related to structure of the arterial wall and extra-cellular matrix (e.g., genes for elastin, collagens and matrix metalloproteinases), and those related to cell signaling ^9,26,27.

One important finding of our study is that DBP (as opposed to PP) was the strongest hemodynamic predictor of PWV in our study of youth and young adults. Our results are remarkably similar to a recent study in 77 young healthy males (23 – 35 years old) in which only age and DBP remained significant independent predictors of carotid-femoral PWV in the multiple regression model¹⁸. We observed insignificant or even negative correlations between PP and PWV in this young population, although PP is generally considered a proxy of arterial stiffness and an independent predictor of coronary heart disease ²⁸, particularly in older and/or hypertensive subjects. However, our findings are in accordance with the NHANES study ²⁹, which showed a progressive decrease in pulse pressure between adolescence and middle age, despite an acknowledged increase in arterial stiffness over this age range ³⁰, as also observed in our data. One possible explanation for this phenomenon is the reduction with age of amplification of the pulse wave between the central aorta and brachial artery ³⁰. Therefore, the role of PP as a cardiovascular risk factor in persons under age 40 has been called into question ³⁰. Determinants of PP include left ventricular ejection, compliance and distensibility of arteries, and timing and intensity of wave reflections ⁴. PP as an indicator of arterial stiffness may be most informative only in older subjects for whom ventricular ejection is reduced, leaving arterial stiffness and timing of wave reflections as the main determinants of increased PP ²⁶.

Although use of the dorsalis-pedis as an alternative to the femoral measurement site was considered less sensitive and more readily accepted in our population of youth and young adults, it constitutes a limitation of our study. As stated, foot PWV represents a mixture of both proximal elastic arteries and distal more muscular arteries. Some differences with other reports that typically measured carotid-femoral PWV are thus expected. However, based on its stronger correlation with age and moderate correlation with radial PWV, we believe that foot PWV mostly represents stiffness of the elastic component and can be considered a reasonable proxy of aortic stiffness.

In conclusion, individual differences in both radial and foot PWV of youth and young adults were substantially heritable and more than 25% of this heritability is explained by genes that also influence DBP. Heritability estimates did not show any differences between blacks and whites or males and females. The association between radial and foot PWV could largely be explained by common genetic factors, but overlap in genetic background was only 50%. Our findings warrant more studies aimed at pinpointing individual genes underlying arterial stiffness, some of which will represent pathways common to stiffness of both elastic and muscular arteries, while many will be specific to either one. Identification of genes for arterial stiffness may indicate promising pathways and targets for drug development. At the same time such genes may help tailor treatments and preventive measures to those individuals that show the highest risk.