Polymorphism in the β1-Adrenergic Receptor Gene and Hypertension
Background— The Arg389 variant of the β1-adrenergic receptor gene mediates a higher isoproterenol-stimulated adenylate cyclase activity than the Gly389 variant in vitro. We investigated whether the Arg389Gly or the Ser49Gly polymorphism is associated with hypertension in Scandinavians.
Methods and Results— A total of 292 unrelated, nondiabetic, hypertensive patients and 265 unrelated healthy control subjects were included in a case-control association study. From 118 families, 102 nondiabetic sibling pairs without antihypertensive medication who were discordant for the Arg389Gly polymorphism were selected for a sibling study. Allele and genotype frequencies of the Arg389Gly and Ser49Gly polymorphisms were compared between hypertensive patients and normotensive control subjects. Blood pressure and heart rate were compared between carriers of the different genotypes. In the case-control study, the age- and body mass index-adjusted odds ratio for hypertension in subjects homozygous for the Arg389 allele was 1.9 (95% confidence interval, 1.3 to 2.7; P=0.0005) when compared with carriers of 1 or 2 copies of the Gly389 allele. The genotype-discordant sibling pair analysis revealed that siblings homozygous for the Arg389 allele had significantly higher diastolic blood pressures (79.4±9.9 versus 76.0±10.1 mm Hg; P=0.003) and higher heart rates (68.3±11.0 versus 65.1±9.4 bpm; P=0.02) than siblings carrying 1 or 2 copies of the Gly389 allele. The Ser49Gly polymorphism was not associated with hypertension.
Conclusion— Our data suggest that individuals homozygous for the Arg389 allele of the β1-adrenergic receptor gene are at increased risk to develop hypertension.
Received March 16, 2001; revision received April 23, 2001; accepted April 26, 2001.
Hypertension is a multifactorial disease with a substantial genetic component.1 Between 30% and 50% of blood pressure variation in the population is determined by genetic factors.2 The blood pressure level is determined by 2 factors: cardiac output and peripheral resistance, both of which are tightly regulated by the sympathetic nervous system.3 The β1-adrenergic receptor, a 7-transmembrane Gs-protein–coupled receptor, is expressed in cardiac myocytes.4 On agonist stimulation, it elicits excitatory reactions in the heart, leading to higher cardiac output through increased cardiac inotropy and chronotropy. In addition, β-blocking agents are widely used in the treatment of hypertension, and their antihypertensive effect is mediated by blocking the β1-adrenergic receptor.3
In 1987, the β1-adrenergic receptor gene was cloned,5 and it is localized to chromosome 10.6 Two common polymorphisms, Ser49Gly and Arg389Gly, were identified in 1999.7 Arg389Gly is located in the intracellular cytoplasmic tail near the seventh transmembrane region of the receptor, which is a putative Gs-protein binding domain. The Arg389 variant mediates a higher isoproterenol-stimulated adenylate cyclase activity than the Gly389 variant in vitro.8 The Ser49Gly polymorphism is located in the extracellular amino-terminal region of the receptor,9 but no studies have been published on the potential functional consequences of this polymorphism.
Because the β1-adrenergic receptor is crucial in regulating cardiac output and agents blocking it lower blood pressure, we hypothesized that genetic variants of the β1-adrenergic receptor gene could be of importance in the development of hypertension.
The aim of the present study was to investigate whether the functionally important Arg389Gly polymorphism or the Ser49Gly polymorphism of the β1-adrenergic receptor gene was associated with hypertension in a case-control study. For confirmation of results, an additional independent sibling-pair study was performed.
In the case-control association study, 292 unrelated patients with hypertension and 265 unrelated healthy control subjects were enrolled (Table 1). All study subjects were from southern Sweden and participated in either the Skaraborg hypertension project (n=353)10,11 or in a family study (n=204; unpublished data). The family study consisted of 250 families; each had at least 2 members with hypertension and 1 healthy spouse, and data were collected from 4 primary health care centers in the Scania region. The diagnosis of hypertension was based on at least 3 consecutive blood pressure measurements ≥160 mm Hg for systolic blood pressure (SBP) and/or ≥90 mm Hg for diastolic blood pressure (DBP).
The inclusion criteria for the patients were as follows: (1) age at diagnosis of hypertension≤60 years, (2) presence of chronic antihypertensive treatment, and (3) absence of diabetes mellitus. Subjects included as controls had the following characteristics: (1) age at the time of the study ≥40 years, (2) SBP≤150 mm Hg and DBP≤80 mm Hg, (3) no personal history of elevated blood pressure or diabetes mellitus, (4) no antihypertensive medication use, and (5) no family history of hypertension in first-degree relatives. Blood pressure was measured in the supine position with a sphygmomanometer after 5 minutes of rest, and heart rate was recorded by palpation of the radial artery and recorded for 30 seconds.
For the genotype-discordant sibling-pair study, 491 siblings without antihypertensive medication were ascertained from 118 families comprising 189 sibships from the Botnia study in Finland.12 Altogether, 455 sibling-pair combinations (note: one sibling can appear in more than one sibling-pair) were identified. Of these, 102 nondiabetic sibling-pairs were genotype-discordant (135 unique individuals); that is, one sibling was homozygous for the Arg389 allele and the other sibling carried 1 or 2 copies of the Gly389 allele (Table 2). Three blood pressure recordings were obtained from the right arm in the sitting position after 30 minutes of rest, and the mean value was calculated. The coefficient of variation of the 3 systolic and diastolic blood pressure measurements was 4.1 and 5.9, respectively.
In all subjects, height was measured to the nearest centimeter and weight to the nearest 0.1 kg. Body mass index (BMI) was calculated as the ratio of the weight in kilograms to the square of the height in meters (kg/m2).
The local ethics committee approved the study, and written informed consent was obtained from all the participants.
Total genomic DNA was extracted from whole blood by standard methods.13 The Ser49Gly and Arg389Gly polymorphisms in the β1-adrenergic receptor gene were genotyped by polymerase chain reaction and restriction fragment length polymorphism methods, as described elsewhere.7
Continuous variables are presented as means±SD. Differences in proportions were tested by the χ2 test, and differences between means were tested by t test, ANOVA, or Kruskal-Wallis test, where appropriate. Multiple logistic regression was performed with hypertension as the dependent variable and age, BMI, sex, and codon 389 genotype (Gly389Gly and Arg389Gly versus Arg389Arg) as independent variables. Data from the genotype-discordant sibling study were analyzed using a modified permutation test for paired replicates.14,15 The 2-tailed probability values were estimated using a large (107) random sample from all possible permutations. If the observed sum of differences entered into the 5% region of rejection, the difference between pairs was considered statistically significant. For sibling-pairs discordant for the Arg389Gly polymorphism, the differences in phenotypic variables were computed as the values in the sibling with the Arg389Arg genotype minus the value in the sibling with the Gly389Gly or Arg389Gly genotype. Analyses, except the permutation test, were performed using NCSS 6.0.21 (Statistical Solutions, Ltd). All statistical tests were 2-sided, and P<0.05 was considered statistically significant.
The observed genotype frequencies in the case-control study were in Hardy-Weinberg equilibrium and were in accordance with genotype frequencies found in other European populations.7,8
The Arg389 allele and the Arg389Arg genotype of the β1-adrenergic receptor gene were more common in patients with hypertension than in controls (Table 3). The age-, sex-, and BMI-adjusted odds ratio for treated hypertension in subjects homozygous for the Arg389-allele was 1.9 (95% confidence interval, 1.3 to 2.7; P=0.0005) when compared with carriers of 1 or 2 Gly389 alleles. SBP, DBP, and heart rate did not differ between carriers of the different Arg389Gly genotypes within the treated hypertension group or within the control group (Table 4). The allele and genotype frequencies of the Ser49Gly polymorphism were similar in patients with treated hypertension and in control subjects (Table 3).
In the 102 sibling-pairs discordant for the Arg389Gly polymorphism, the siblings homozygous for the Arg389 allele had a significantly higher DBP (P=0.003) and heart rate (P=0.02) than carriers of 1 or 2 Gly389 alleles, but there was no difference in SBP. Age and BMI were similar between siblings (Table 2).
In the present study, we describe an association between the Arg389Arg genotype and hypertension in a Swedish case-control study. In addition, a genotype-discordant sibling analysis in Finns showed that subjects with the Arg389Arg genotype had a higher DBP and heart rate than carriers of 1 or 2 Gly389 alleles. Although the Arg389Arg genotype was frequently found in control subjects (50.6%), the frequency was 15.2% higher in patients with hypertension (P=0.0003; Table 3). This suggests that subjects homozygous for the Arg389 allele could be at increased risk of developing hypertension.
A weakness of case-control association studies is the risk of finding an association that is due to population stratification, rather than to a disease-contributing allele. In our case-control association study, this is unlikely for the following reasons: first, the genotype frequency distributions were in Hardy-Weinberg equilibrium among both hypertensive patients and control subjects. Second, the nearby Ser49Gly polymorphism did not show any association with hypertension. Finally and most importantly, in a different population, the Arg389Arg genotype was associated with elevated DBP and heart rate in a genotype-discordant sibling analysis. This approach cannot be affected by population stratification because it is based on intrafamilial comparisons.
In vitro studies have shown that the Arg389 variant of the β1-adrenergic receptor gene mediates an increased response to agonist stimulation compared with the Gly389 variant, suggesting that the Arg389Gly polymorphism is of functional importance.8 The increased activity of the Arg389 variant of the β1-adrenergic receptor in vivo could be expected to lead to a higher cardiac output and could therefore explain our association between the Arg389 allele and hypertension.
A second weakness of the case-control design is the selection of the control subjects. To increase the power of the case-control association study, we selected hypertensive patients with a relatively early age at onset and control subjects without a family history of hypertension. However, this selection may decrease the power of finding a difference in blood pressure level between the different genotype carriers. This may explain why no association was found between the Arg389Gly polymorphism and SBP or DBP levels among the normotensive control subjects or among the hypertensive patients. In addition, in the latter group, all patients were on antihypertensive medication. In contrast, the genotype-discordant sibling analysis is very powerful in detecting intra sibling-pair phenotypic differences that are due to the particular genotype discordance, because siblings share, on average, 50% of their genomes and, therefore, to a great extent, are matched for genetic background. Moreover, siblings often share lifestyle factors influencing blood pressure and the risk for hypertension, such as diet, exercise habits, and socioeconomic status, which further strengthen the use of the discordant sibling-pair analysis.
In conclusion, our data suggest that the Arg389Arg genotype of the β1-adrenergic receptor gene confers an increased risk of developing hypertension.
This research was funded by the Swedish Heart Lung Foundation; the Swedish Medical Research Council; the National Public Health Institute; the Skaraborg Institute; the Skaraborg County Council; the West Region County; the Region Skane; the Faculty of Medicine at Lund University; the Påhlsson Foundation, Malmö University Hospital; the Ernhold Lundström Research Foundation; the Crafoord Foundations; and the NEPI Foundation (The Swedish Network of Pharmacoepidemiology). The Sigrid Juselius Foundation, Academy of Finland, and EC-grant BMH4-CT95-0662 funded the Botnia study. We are most grateful to Ann Carlsson, Agneta Edman, and Lena Rosberg for excellent technical assistance.
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