Original Article

Journal of Human Hypertension (2003) 17, 555–559. doi:10.1038/sj.jhh.1001587

Relation between the angiotensinogen (AGT) M235T gene polymorphism and blood pressure in a large, homogeneous study population

J R Ortlepp1, J Metrikat2, V Mevissen1, F Schmitz1, M Albrecht2, P Maya-Pelzer2, P Hanrath1, K Zerres3 and R Hoffmann1

  1. 1Medical Clinic I, University Hospital of Aachen, Pauwelsstrasse 30, Aachen, Germany
  2. 2German Air Force Institute of Aviation Medicine, Furstenfeldbruck, Germany
  3. 3Institute of Human Genetics, University Hospital of Aachen, Pauwelsstrasse 30, Aachen, Germany

Correspondence: Dr JR Ortlepp, Medical Clinic I, University Hospital of Aachen, Pauwelsstrasse 30, Aachen 52057, Germany. E-mail: jrortlepp@ukaachen.de

Received 6 February 2003; Revised 23 April 2003; Accepted 25 April 2003.

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Abstract

The aim of this study was to assess the association of the angiotensinogen M235T polymorphism with arterial blood pressure (BP) at rest and under physical stress in a homogeneous large-scale study population. In all, 1903 men who passed routine medical examination for military flying duty were recruited. BP and heart rate were measured at rest, during, and after bicycle ergometry. Genotyping for the AGT M235T polymorphism was carried out by PCR and RFLP technique. The AGT T235 allele was associated with a significantly higher diastolic BP (n=1903; MM 81plusminus8, MT 83plusminus7, TT 83plusminus8; P=0.003). Pulse pressure (PP) at rest differed significantly between AGT genotypes (n=1903; MM 51plusminus10 mmHg, MT 49plusminus10 mmHg, TT 49plusminus10 mmHg; P=0.001). During physical activity, BP values showed no significant difference between genotypes. In healthy young men, the AGT T235 allele is significantly associated with elevated diastolic BP but also reduced PP at rest. During physical activity, the AGT polymorphism had no impact on blood pressure, indicating the existence of other counteracting mechanisms, which might balance the influence of this gene.

Keywords:

angiotensinogen, AGT, polymorphism, blood pressure, genetics

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Introduction

The regulation of arterial blood pressure (BP) is complex and under the control of different physiological systems. The renin–angiotensin–aldosterone system (RAAS) is one of the important regulatory systems. Genes encoding products of the RAAS are plausible candidate genes for modifying blood pressure. The angiotensinogen gene (AGT) is one of the few candidates that has been investigated extensively and its genetic variant in exon 2 shows a transition resulting in replacement of methionine by threonine at amino-acid position 235 (M235T). Positive associations between the AGT M235T polymorphism, plasma angiotensinogen levels, and essential arterial hypertension indicate a pathway by which the AGT gene determines arterial BP.1,2,3,4,5 However, studies in African and Asian populations could not equally support these findings,6,7,8,9 indicating the importance of other genetic factors interfering with the AGT gene. Moreover, the allele frequency of the AGT M235T polymorphism varies strongly within the different ethnic groups. One study found a significant increase in T235 allele frequency in patients with hypertension that was restricted to men,10 thus gender might also influence the effect of the AGT gene. The diagnosis of essential arterial hypertension is based on BP above a defined cutoff value. However, a recent study indicated that individuals with BP values that have been considered to be at the upper limit of normality already have an increased risk of cardiovascular disease.11 Thus, arterial hypertension should not be considered as a dichotomous diagnosis of having a BP below or above a certain threshold. Similarly, the same seems to be true for genetic determination of arterial hypertension. Many genes might influence the arterial BP without generating an arterial hypertension. It is the concurrence of multiple factors (gene polymorphisms, gender, age, environmental conditions), which leads to the manifestation of arterial hypertension. Thus, to study the effect of one gene polymorphism, it might be appropriate to analyse quantitative traits in a homogeneous (homogeneous for: gender, race, occupation, social status, physical fitness, and age) population, rather than categorical traits (like healthy vs arterial hypertension) in heterogeneous populations. The aim of this study was to assess the association of the AGT M235T polymorphism with arterial BP in a large-scale study population of completely healthy young men.

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Materials and methods

Study population

The subjects were German male aviators who passed routine medical examination for military flying duties. Physical activity was assessed by standardized questionnaires. Participants were included into the study during routine medical qualification for flying duty. A total of 2000 men were screened and 1903 gave written informed consent (95%) and formed the study population. The study was approved by the local ethical committees.

Body mass index

Body weight and height were measured between 7 a.m. and 8 a.m. before breakfast, after urination, and undressed, to calculate BMI by division of weight by square of height (kg/m2).

Blood pressure and heart rate measurement at rest

BP and heart rate (HR) were measured manually by trained personnel after 5 min of rest in supine position in a quiet room between 7 a.m. and 9 a.m. at neutral, ambient temperature. Systolic (SBP), diastolic (DBP), and mean blood pressure (MBP) as well as pulse pressure (PP=SBP-DBP) were assessed. DBP was taken as Korotkov V.

BP and HR during and after physical exercise, physical fitness (PWC 170)

Ergometry was performed in 1750 men. Physical fitness (determined by physical working capacity at an HR of 170 bpm=PWC 170) was assessed by standard cycle ergometry in 45° sitting position and expressed as the achieved performance (watts (W)) per kilogram of body weight (kg) at an actual HR of 170 bpm. Ergometric power was increased every 3 min by steps of 50 W, starting from 100 W up to the point of physical exhaustion or commonly accepted termination criteria. Ergometric performance was monitored by continuous ECG, supervised by an experienced physician. HR, SBP, DBP, MBP, and PP were assessed 3 min, and 6 min during ergometry and 3 min after ergometry. At 3 min ergometry corresponds to 3 min 100 W, 6 min ergometry corresponds to 3 min 100 W plus 3 min 150 W.

Genotyping

Genomic DNA of patients was extracted from whole blood using the InVitek Blood Kit (InVitek, Berlin, Germany). The AGT M235T polymorphism was tested as described previously.1

Statistical analysis

Continuous data are presented as meanplusminuss.d. and categorical data are presented as frequencies. Differences in BP were calculated by ANOVA. A linear regression analysis was performed to evaluate the impact of AGT on pulse pressure. A multivariable analysis was performed including BMI, age, and AGT genotype. Significant differences were defined as a two-sided P-value<0.05. Statistical figures show mean values with 95% confidence interval. All calculations were performed with SPSS for Windows® 10.0. The null hypothesis was that men with different AGT genotypes do not have different BP values.

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Results

Phenotypes

The characteristics of the study population are given in Table 1. The study population represents a 'hyperhealthy' population with few obese men and high physical activity and fitness.


Genotypes

The frequencies of the AGT M235T genotypes and alleles are shown in Table 2.


Relation of AGT genotype and BP values

As given in Table 3, the AGT genotypes were associated with different DBP and PP at rest. Men homozygous for the T allele had the lowest PP at rest. These findings are further illustrated in Figure 1. During and after physical activity BP values did not differ between the AGT genotypes (Table 3).

Figure 1.
Figure 1 - Unfortunately we are unable to provide accessible alternative text for this. If you require assistance to access this image, please contact help@nature.com or the author

Relation of the AGT M235T genotype with SBP, DBP, MBP, and PP at rest in a study population of 1903 young male active aviators of the German Armed Forces. SBP=systolic blood pressure; DBP=diastolic blood pressure, MBP=mean blood pressure; PP=pulse pressure.

Full figure and legend (49K)


Multivariable analysis

In a multivariable analysis for DBP and PP including age, BMI, and AGT genotype, all variables were significantly associated with DBP (age: F=139.664, P<0.001; BMI: F=9.535, P=0.002; AGT genotype: F=5.390, P=0.005) and PP (age: F=32.001, P<0.001; BMI: F=29.240, P<0.001; AGT genotype: F=8.012, P<0.001).

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Discussion

The impact of the angiotensinogen gene (AGT) M235T polymorphism on plasma angiotensinogen levels and essential arterial hypertension has been reported.1,2,3,4,5 However, the finding of an association could not be repeated in all subsequent studies.6,7,8,9 A recent study found a significant impact of the AGT genotype on arterial blood pressure during exercise in 190 men, participating in the HERITAGE family study.12 However, the impact of the AGT M235T polymorphism on the regulation of arterial blood pressure remains incompletely defined. This study evaluated the relation of the AGT genotype to arterial BP in almost 2000 male active aviators of the German Armed Forces. The major findings of this study are that the AGT M235T gene polymorphism is significantly associated with increased DBP and reduced PP in men at a relatively young age, whereas SBP did not differ significantly between the AGT genotypes. Even when the F-value for the AGT genotype in the multivariable analysis for blood pressure is low, the effect of the AGT genotype on BP regulation is detectable in a period of life far before the general incidence of arterial hypertension. During physical stress, the influence of the AGT gene polymorphism on blood pressure regulation could not be detected. Other additional yet unknown genetic or nongenetic factors could influence this genotype–phenotype relation. We speculate that individuals with the T allele might be more prone to cardiovascular diseases later in life, because of the elevation of DBP very early in life. The effect of reduced PP in this young population remains uncertain. Moreover, a difference in the DBP of 2 mmHg might also have only minor relevance. Owing to the relatively small extent of the effect of the AGT genotype polymorphisms of other genes might be associated with BP regulation. In addition, multiple other polymorphisms within the AGT gene have been described and the combination of these multiple AGT polymorphisms might have a higher influence on BP regulation than just a single polymorphism. Further large studies should therefore investigate the haplotype analysis of several candidate genes of the RAAS.

The strength of this study is the number of included subjects and the homogeneity of the examined population. In this study population, all subjects had the same gender, race and nationality, similar age, physical fitness, occupation, annual income, social status, and their principal meals were provided by the military five days a week. Thus, this population was controlled for more exogenic factors than it would be possible for patient or population selected study populations.

The limitation of this study is that this cohort is not representative of a 'regular' population. For example, we cannot predict the effect of the AGT gene on blood pressure of females, old men, or classical hypertensives. Furthermore, because of the cross-sectional character of this study, we cannot prove that gene carriers with relatively high BP will develop cardiovascular disease years or decades later. However, even if the examined military pilots do not match a regular population, they are in particular suitable to serve as an example for genetic–environmental interactions because of their highly homogeneous population characteristics.

In summary, we conclude that the effect of the AGT M235T gene polymorphism on blood pressure regulation, even to a small extent, is detectable very early in life, far before the onset of arterial hypertension.

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References

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Acknowledgements

This study was supported by the START program of the University Hospital of Aachen. The work resulting in this report was performed under German Federal Ministry of Defense Contract 21L2-S-200103. The opinions expressed in this paper are those of the authors and do not necessarily reflect the opinions or official policy of the German Federal Ministry of Defense. We gratefully acknowledge the support of the medical assistants of Dep. I and the backing of Dep. III of the German Air Force Institute of Aviation Medicine.

JR Ortlepp designed the study, did statistical data analysis, J Metrikat coordinated and realized data collection, M Albrecht and P Maya-Pelzer helped with data collection, V Mevissen, F Schmitz, and K Zerres fulfilled genetic analysis, P Hanrath, K Zerres, and R Hoffmann took part in interpretation of data and critical revision. All authors helped to write the report. This work is part of the doctoral thesis of JM Conflict of Interest Statement: None declared.

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