G-Protein β3 Subunit C825T Variant and Ambulatory Blood Pressure in Essential Hypertension
Abstract—Recent studies have identified a novel polymorphism (C825T) of the gene encoding the β3 subunit of heterotrimeric G proteins (Gβ3) associated with enhanced activation of G proteins, which appears to be more common in hypertensive patients. In the present study we examine the relationship between this genetic variant and hypertension in 479 white patients with established essential hypertension recruited from the hypertension clinic of the Universitätsklinikum Benjamin Franklin in Berlin, Germany, and 1000 normotensive gender- and age-matched controls. All patients were screened for the presence of secondary hypertension and were further characterized by ambulatory blood pressure measurements performed in 295 treated and 184 untreated patients. Genotype distribution for the Gβ3-C825T genotype in patients (CC=204, CT=224, TT=51) was significantly different from that in controls (CC=514, CT=412, TT=74; χ2=11.5, P<0.01), and the T allele was associated with an odds ratio of 1.5 (95% CI, 1.1 to 2.2) versus non-T carriers for the presence of hypertension. However, in both the whole group and the untreated subgroup, blood pressure levels between the genotypic groups were virtually identical. Furthermore, age of onset of hypertension and number of antihypertensive medications (in treated patients) were similar between the genotypic groups. Thus, while our data confirm the association between the Gβ3-C825T variant and essential hypertension, they do not support the hypothesis that this marker is associated with more severe blood pressure in patients with already established hypertension.
- angiotensin-converting enzyme
- hypertension, essential
- blood pressure monitoring, ambulatory
- hypertrophy, left ventricular
Arterial blood pressure is a complex quantitative trait influenced by both genetic and environmental factors.1 On the basis of the finding that high blood pressure and target-organ damage tend to cluster in families, it is anticipated that genetic markers can be identified that would allow the early detection of individuals prone to the development of hypertension and its sequelae.2
A recent study has detected a novel polymorphism (C825T) in exon 10 of the gene encoding the β3 subunit of heterotrimeric G proteins (Gβ3),3 which was significantly associated with hypertension in patients recruited from Essen, Berlin, and Heidelberg, Germany. The T variant was also associated with the occurrence of a splice variant GNB-s, in which the nucleotides 498 to 620 of exon 9 are deleted, resulting in the loss of 41 amino acids and 1 tryptophan–aspartic acid (WD) repeat domain of the Gβ subunit. This splice variant was found to be predominantly expressed in cells from individuals carrying the T allele, and it appears to result in significantly enhanced activation of G proteins. It was postulated that expression of Gβ3-s may contribute to the development of hypertension by increasing proliferation of smooth muscle cells and possibly cardiomyocytes, thus resulting in the development of vascular and/or myocardial hypertrophy. It was also postulated that the Gβ3-C825T may be associated with late-onset rather than early-onset hypertension.
In the present study we examined the relationship between the Gβ3-C825T genotype, age of onset, and severity of blood pressure in patients with established essential hypertension referred to a tertiary hypertension clinic at a university medical center.
The protocol of the study was approved by the ethical committee of our hospital, and informed consent for genetic studies was obtained from all participants
Patient Selection and Clinical Investigation
Treated and untreated white patients (n=479) from our outpatient hypertension clinic were enrolled in the study. Resting blood pressure was measured at each presentation by a trained nurse. Hypertension was defined as a systolic blood pressure >140 mm Hg and/or a diastolic blood pressure >95 mm Hg on at least 2 separate occasions. Additionally, ambulatory blood pressure measurements (90207, SpaceLabs Medical Inc) were performed in all patients. Patients with white-coat hypertension, as evidenced by elevated clinic blood pressure but 24-hour average ambulatory blood pressure <135/85 mm Hg,4 were not enrolled in the study. All patients were thoroughly examined for the presence of secondary hypertension and target-organ damage by standard clinical, laboratory, and imaging procedures. Age of onset was determined by history and direct contact with the family physician at the first time the patient was ever told by a physician that he or she was hypertensive. In treated patients, the number of antihypertensive medications was considered a surrogate marker for the severity of hypertension.5 Patients who had never been treated previously or had stopped antihypertensive medication for any reason at least 4 weeks before presenting at our clinic were defined as untreated. Once the presence of hypertension was established, venous blood samples were drawn for DNA extraction and genotyping. We also recruited 1000 healthy normotensive age- and gender-matched volunteers among blood donors and hospital staff.
Genomic DNA was prepared from peripheral white blood cells by a standard column-extraction technique (Qiagen). Subsequently, the Gβ3-C825T genotype was determined by polymerase chain reaction amplification of the relevant region followed by restriction with BseD1 (New England Biolabs), as described previously.6 Complete restriction of the polymerase chain reaction product (268 bp=TT genotype) generated bands of 116 and 152 bp (CC genotype).
All data are presented as mean±SD or as proportions. Continuous variables were compared by 2-sided Student’s t test or the nonparametric Kruskal-Wallis test for independent samples as appropriate, and categorical data were assessed by χ2 statistics. The relationships between both genotypes and blood pressure were examined by linear and logistic multiple regression analysis, with gender, age, duration of hypertension, and body mass index as covariates (SPSS PC+, 1995). In the logistic analysis, the sample population was divided into 2 groups on the basis of blood pressure distribution, whereby patients in the 4th quartile of the resting and ambulatory blood pressure distribution were considered cases. P<0.05 was considered statistically significant.
Fourteen patients were excluded from analysis because of secondary hypertension. Genotype distribution for the Gβ3-C825T genotype in the remaining patients (CC=204, CT=224, TT=51) was significantly different from that in controls (CC=514, CT=412, TT=74; χ2=11.5, P<0.01), and the TT genotype was associated with an odds ratio of 1.5 (95% CI, 1.1 to 2.2) versus non-T for the presence of hypertension. However, in both the treated and untreated subgroups, blood pressure levels between the genotypic groups were virtually identical (Table⇓). Furthermore, age of onset of hypertension and number of antihypertensive medications (in treated patients) were similar between the genotypic groups. Stepwise multiple regression analysis of the relationship between the blood pressure variables (resting blood pressure and ambulatory 24-hour blood pressure) and genotypes in patients consistently revealed a significant result only for the covariates gender and age. After adjustment for gender and treatment, the Gβ3 genotype did not enter the equations in either linear or logistic regression analysis. In a univariate post hoc explorative analysis, gender-specific differences in blood pressure were noted in that the T allele in the male group was associated with higher resting blood pressure (CC: 167±17/103±16; CT: 169±24/105±14; TT: 176±31/105±19 mm Hg). However, these trends were not statistically significant (P=0.25). Furthermore, when a dominant effect of the T allele was assumed, an analysis of CC patients versus the combined CT/TT group likewise failed to reveal a significant influence of genotype on resting blood pressure (CC: 167±17/103±16 versus CT/TT: 170±25/105±15 mm Hg; P>0.1).
The recent study by Siffert et al3 reported a positive association between the T variant of the Gβ3-C825T genotype with hypertension in white subjects. Our data confirm this association, but within the patient group, the Gβ3 genotype was not related to age of onset, resting and 24-hour ambulatory blood pressure, or number of medications in treated patients. Thus, while our findings provide evidence that the Gβ3-C825T allele confers susceptibility to the development of hypertension, they do not support the hypothesis that this genotype is an important determinant of the severity of essential hypertension or of the age of onset of the disease.
Nevertheless, in men there was a trend toward higher resting blood pressure in the TT group, which, however, failed to reach statistical significance. On the basis of the frequency of the T allele and the observed variances in resting blood pressure levels, we calculated that it would take >2000 untreated hypertensives to statistically rule out an influence of the T allele on these variables. Ambulatory blood pressure levels, a more reproducible measure of blood pressure load7 and stronger predictor of end-organ damage,8 were virtually identical between the TT and CC groups.
It is, however, important to realize that genes conferring susceptibility to hypertension must not necessarily determine the severity of hypertension, since several other genetic or nongenetic factors may influence the expression of the phenotype in genetically susceptible individuals. Furthermore, the lack of relationship between genotype and severity of hypertension in this population does not imply that the Gβ3 gene is only of marginal effect. When one considers that “essential” hypertension is a multifactorial entity, a given candidate gene may well have a strong effect on blood pressure in a subset of patients, while it may lack importance in others. This line of reasoning is evident from the fact that the T allele is also present in a large proportion of the normotensive population.
As in the first report by Siffert et al,3 the presence of the G3β-825T allele in our study was associated with an odds ratio of approximately 1.5 to 1.7 for the presence of hypertension. Why the G3β-825T variant apparently confers an increased susceptibility to the development of hypertension is presently not clear. It is probably important to note that this genetic variant, which leads to the increased expression of a splice variant of the Gβ3 subunit, was identified in cell lines derived from hypertensive individuals selected on the basis of an increased activity of the Na-H exchanger in immortalized lymphoblasts and cultured fibroblasts.9 10 Increased activity of the Na-H antiporter has been found in ≈50% of patients with essential hypertension,11 but whether this abnormality of ion transport is directly related to the pathogenesis of hypertension in these individuals remains unclear.
In summary, our study indicates that individuals bearing the Gβ3-825T allele may have an increased risk of developing hypertension, but this is not an indicator of severity in patients with established essential hypertension presenting at a tertiary hypertension center. Whether this genetic marker may prove to be of clinical importance in an unselected group of patients with essential hypertension remains to be determined.
Genotyping was supported in part by the Deutsche Forschungsgemeinschaft (DFG Sh35/2–2). We would like to acknowledge Christel Harsch, Klaus Schlotter, and Brigitte Egbers for their expert technical help.
- Received November 11, 1998.
- Revision received December 2, 1998.
- Accepted December 10, 1998.
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