G Protein β3 Subunit 825T Allele and Hypertension
To the Editor:
We recently described a C825T polymorphism in the gene GNB3 encoding the β3 subunit of heterotrimeric G proteins. One mutant allele generates a novel splice variant of Gβ3 and is associated with enhanced G protein signaling. This allele was more frequent in patients with essential hypertension. Although the case for noncaucasian populations is less clear, three independently conducted studies have confirmed a statistically significant association of the 825T allele with hypertension in caucasians.
In a recent issue of Hypertension, Brand et al report their results on the association of the 825T allele with hypertension in two cohorts, ie, the study subjects of the previously reported PEGASE and ECTIM studies. Brand et al found no association of the 825T allele with hypertension. This finding is puzzling but might well be explained by some peculiarities of the design of the studies. Normotension was defined as diastolic blood pressure <95 mmHg in a population with ages as low as 25 years. This definition invites misclassification. Whether young normotensive individuals should be included in such a case-control study appears questionable. Allele frequency (31.3%) of the normotensive control group (Table 2) is significantly higher than that reported in previous studies.
Interestingly, comparison of combined genotypes of the hypertensive individuals from Table 2 (87 TT, 302 TC, and 292 CC) with combined controls from the ECTIM study presented in Table 1 (58 TT, 269 TC, and 306 CC) yields a significant odds ratio for hypertension (TT versus CC) of 1.57 (95% CI: 1.1 to 2.3; P=0.016; Fisher’s exact test), and genotype distribution is significantly different (P<0.05 by χ2 test). Taking all of these arguments into consideration, the conclusion that “the 825C/T polymorphism of the G protein subunit β3 is not related to hypertension” is not supported by absolutely solid data and the jury is certainly not yet in.
Hegele RA, Harris SB, Hanley AJ, Cao H, Zinman B. G Protein β3 subunit gene variant and blood pressure variation in Canadian Oji-Cree. Hypertension. 1998;32:688–692.
Kato N, Sugiyama T, Morita H, Kurihara H, Yamori Y, Yazaki Y. G protein β3 subunit variant and essential hypertension in Japanese. Hypertension. 1998;32:935–938.
Beige J, Hohenbleicher H, Distler A, Sharma AM. G-protein β3 subunit C825T variant and ambulatory blood pressure in essential hypertension. Hypertension. 1999;33:1049–1051.
Benjafield AV, Jeyasingam CL, Nyholt DR, Griffiths LR, Morris BJ. G-protein β3 subunit gene (GNB3) variant in causation of essential hypertension. Hypertension. 1998;32:1094–1097.
Schunkert H, Hense HW, Döring A, Riegger GAJ, Siffert W. Association between a polymorphism in the G protein β3 subunit gene and lower renin and elevated diastolic blood pressure. Hypertension. 1998;32:510–513.
Brand E, Herrmann SM, Nicaud V, Ruidavets JB, Evans A, Arveiler D, Luc G, Plouin PF, Tiret L, Cambien F. The 825C/T polymorphism of the G-protein subunit β3 is not related to hypertension. Hypertension. 1999;33:1175–1178.
In response to the comments by Siffert et al, we would like to provide further information about our results that were recently published in Hypertension.R1 Our purpose was to investigate the association of the GNB3 C825T polymorphism with hypertension and blood pressure levels in two different studies: ECTIM and PEGASE. The ECTIM Study is a case-control study on myocardial infarction, in which the control samples are population-based, that provides an opportunity for studying the association of genetic polymorphisms with intermediate phenotypes in the population at-large. In the ECTIM Study, the cut-off value of diastolic blood pressure (DBP) used to define hypertension versus normotension was 95 mm Hg. Siffert et al argued that in a population with ages as low as 25 years, this definition invites misclassification, and they questioned the relevance of including young normotensive individuals. Although the actual age range of control subjects was 25 to 64 years in order to match that of cases, it should be stressed that 90% of subjects were older than 40 years (mean age 51.3±8.9 years). However, we checked whether the conclusions were modified by using a more stringent cut-off point for defining normotension (DBP ≤90 mm Hg). The 825T allele frequency was 0.311 in this new group, as compared to 0.305 and 0.301 in the normotensive and hypertensive groups previously defined in our paper.
The PEGASE Study is a case-control study on moderate and severe hypertension, in which hypertensive subjects were recruited through a network of general practitioners, while normotensive subjects were selected from the ECTIM French control samples on the basis of a DBP ≤90 mm Hg.R2 Unfortunately, this important point had been omitted in our paper when describing the PEGASE population, and we recognize that this omission was a source of confusion. The mean blood pressure levels in this nomotensive sample was 122.4±14.3/76.1±8.9 mm Hg.
Contrary to the assertion of Siffert et al, the 825T allele frequency in controls of our study (0.304±0.013) was not significantly different from those reported in previous studies (0.280±0.010 from Beige et alR3 and 0.320±0.015 from Schunkert et alR4 ). The lower allele frequencies reported by Benjafield et alR5 (0.249±0.022) and Siffert et al (0.251±0.015) corresponded to samples highly selected on the basis of family history of hypertensionR5 or blood pressure levelsR6 (mean DBP: 72.1±6.2 mm Hg).
Finally, in regard to the proposition by Siffert et al of combining ECTIM controls from France and Northern Ireland for comparison to French hypertensive patients, although this obviously would increase the sample size, it does not appear valid from a genetic perspective and would certainly have led to legitimate criticisms from geneticists.
In conclusion, the lack of a significant association with hypertension and blood pressure levels in two different studies is not in favor of a strong effect of the GNB3 C825T polymorphism on the predisposition of essential hypertension in European subjects.
Brand E, Herrmann S-M, Nicaud V, Ruidavets J-B, Evans A, Arveiler D, Luc L, Plouin P-F, Tiret L, Cambien F. The 825 C/T polymorphism of the G-protein subunit β3 (GNB3) is not related to hypertension. Hypertension.. 1999;33:1175–1178.
Tiret L, Blanc H, Ruidavets JB, Arveiler D, Luc G, Jeunemaitre X, Tichet J, Mallet C, Poirier O, Plouin PF, Cambien F. Gene polymorphisms of the renin-angiotensin system in relation to hypertension and parental history of myocardial infarction and stroke: the PEGASE study, Projet d’Etude des Genes de l’Hypertension Arterielle Severe a moderee Essentielle. J Hyptertens.. 1998;16:37–44.
Beige J, Hohenbleicher H, Distler A, Sharma AM. G-Protein β3 subunit C825T variant and ambulatory blood pressure in essential hypertension. Hypertension.. 1999;33:1049–1051.
Schunkert H, Hense HW, Doring A, Riegger GA, Siffert W. Association between a polymorphism in the G protein β3 subunit gene and lower renin and elevated diastolic blood pressure levels. Hypertension.. 1998;32:510–513.
Benjafield AV, Jeyasingam CL, Nyholt DR, Griffiths LR, Morris BJ. G-protein β3 subunit gene (GNB3) variant in causation of essential hypertension. Hypertension.. 1998;32:1094–1097.
Siffert W, Rosskopf D, Siffert G, Busch S, Moritz A, Erbel R, Sharma AM, Ritz E, Wichmann HE, Jakobs KH, Horsthemke B. Association of a human G-protein beta3 subunit variant with hypertension. Nat Genet.. 1998;18:45–48.