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(Hypertension. 2001;37:739.)
© 2001 American Heart Association, Inc.
Scientific Contributions |
From the Division of Hypertension, Department of Internal Medicine, Mayo Clinic and Foundation (S.T.T., G.L.S.), Rochester, Minn; Renal Division, Emory University (A.B.C.), Atlanta, Ga; and Human Genetics Center and Institute of Molecular Medicine, University of Texas-Houston Health Science Center (E.B.), Houston, Tex.
Correspondence and reprint requests to Stephen T. Turner, MD, Division of Hypertension, Department of Internal Medicine, Mayo Clinic and Foundation, 200 First St SW, Rochester, MN 55902. E-mail turner.stephen{at}mayo.edu
| Abstract |
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Key Words: hypertension, essential blood pressure diuretics proteins genetics
| Introduction |
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It has long been suspected that interindividual variation in drug responses may be influenced by genetic factors.5 Recently, a polymorphism (C825T) was described in exon 10 of the gene encoding the ß3-subunit of G proteins (GNB3),6 and subsequently it was found to be associated with a shortened splice variant of the Gß3-protein that gives rise to enhanced signal transduction via pertussis toxinsensitive G proteins.7 The C825T polymorphism was originally identified through studies of lymphoblasts, derived from whites with essential hypertension, in which sodium-proton antiport activity was increased as a consequence of enhanced G proteindependent signal transduction in response to a variety of vasoactive and growth-promoting stimuli.8
The present study was prompted by recently reported associations of the 825T allele with obesity9 and with low plasma renin10 that suggest that the 825T allele might also be associated with a volume-expanded, sodium-sensitive, and therefore diuretic-responsive form of hypertension.11 12 Hence, our objective was to determine whether the C825T polymorphism predicts interindividual variation in blood pressure response to diuretic therapy among subjects with essential hypertension. We tested this hypothesis in a community and clinic-based biracial sample of hypertensive women and men undergoing monotherapy with a thiazide diuretic.
| Methods |
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Participants had their antihypertensive medications
withdrawn, and other contraindicated drugs were discontinued. If the
participant had been taking a diuretic and their serum
potassium level was <3.6 mEq/L at the screening visit, a potassium
supplement (potassium chloride 20 mEq/d) was prescribed. A dietitian
instructed each participant in a diet designed to provide a standard
sodium intake of 2 mmol per kilogram of body weight per day.
Compliance was monitored by 24-hour urine collections obtained every
other week on the diet. Study subjects were seen every 2 weeks by the
study nurse for blood pressure monitoring. At a minimum of 4 weeks
after antihypertensive drug therapy was discontinued, subjects began
taking 25 mg of hydrochlorothiazide orally each day for
the next 4 weeks. Compliance was assessed by pill counts. Subjects
weight, blood pressure, and serum potassium concentrations were
remeasured after 2 and 4 weeks of diuretic therapy. Low-dose
oral potassium supplements (20 to 40 mmol/d) were prescribed after
2 weeks of diuretic therapy if serum potassium was <3.4 mEq/L.
At the end of the drug-free period and at the end of the
diuretic-therapy period, subjects slept overnight in the GCRC.
At
6 the next morning, blood for measurement of plasma
aldosterone concentration, renin activity, and serum
potassium concentration was drawn from subjects in the seated position
after 30 minutes of ambulation.
Laboratory Procedures
Aldosterone and renin activity were
determined by radioimmunoassays. Each sample was assayed in triplicate
and the average used in the analyses. The C825T
polymorphism was genotyped by polymerase chain reaction
amplification of the relevant region of genomic DNA, followed by
restriction enzyme digestion, gel electrophoresis, and ethidium bromide
staining.6
Statistical Methods
For quantitative traits, data were summarized by
calculating means and variances for each
GNB3 genotype within
each racial group. A 1-way ANOVA was used to assess differences in
means among genotypes within each racial group. Relative
frequencies of genotypes and alleles were calculated for
each racial group.
2 contingency tests
were used to assess differences in relative frequencies between racial
groups.
We used linear regression to assess whether variation in genotype made a statistically significant contribution to the prediction of systolic and diastolic blood pressure responses to hydrochlorothiazide. We first considered models that included only genotype and subsequently models that included genotype as well as covariate traits (including race, gender, pretreatment blood pressure levels, age, waist-to-hip ratio, and measures of the endocrine renin-angiotensin-aldosterone system). In preliminary analyses, we found no evidence of significant interaction between the effects of genotype and those of the other predictor traits.
| Results |
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Covariate Traits
Means for pretreatment systolic and
diastolic blood pressure levels, waist-to-hip ratio, plasma
renin activity, and plasma aldosterone concentration
differed significantly between blacks and whites
(Table 2). However, only the lower means for waist-to-hip
ratio and plasma renin activity in blacks were consistent
across all genotype groups. Because mean waist-to-hip ratio was
significantly lower in women than in men in both races
(P<0.001, analyses not
shown), lower mean waist-to-hip ratio in blacks could be attributed to
the higher percentage of women among blacks (68%) than among whites
(40%; see
Table 1). In contrast, because mean plasma renin activity
did not differ significantly between black women and men (1.0±1.1
versus 1.1±1.1, P>0.5) and
mean plasma renin activity was significantly lower in white men than
white women (1.6±1.3 versus 2.1±1.4,
P<0.04), gender differences
could not account for lower mean PRA in blacks than whites.
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In each race, means for the covariate traits did not differ significantly among genotype groups (Table 2). However, because of racial differences in the genotype frequencies (see Table 1), means for pretreatment systolic and diastolic blood pressure levels, waist-to-hip ratio, plasma renin activity, and plasma aldosterone concentration appeared to differ significantly among genotype groups when blacks and whites were pooled (Table 2).
Association of Blood Pressure Response With
Genotype
Variation in genotype was a statistically
significant predictor of systolic and diastolic
blood pressure responses to hydrochlorothiazide
treatment
(Table 3 and
Figure 1). In the pooled sample, systolic and
diastolic blood pressure responses increased progressively
with the number of T alleles: mean declines in systolic and
diastolic blood pressure were, respectively, 10.2 and
5.9 mm Hg in CC homozygotes, 13.6 and 7.8 mm Hg in TC
heterozygotes, and 16.3 and 10.5 mm Hg in TT homozygotes. In
pairwise contrasts between genotypes, the mean declines in
systolic blood pressure were significantly greater in TC
heterozygotes and TT homozygotes than in CC homozygotes
(P<0.05 and
P<0.001, respectively), and
the mean decline in diastolic blood pressure was
significantly greater in TT than in CC homozygotes
(P<0.001). This same pattern
of progressively greater systolic and diastolic
blood pressure responses in association with the number of T
alleles was observed in each race-gender group except black women
(Figures 2 and 3). The differences in responses between TT and
CC genotypes were greatest in white women (for systolic
blood pressure response, 30.3 versus 10.0 mm Hg, respectively;
for diastolic blood pressure response, 12.3 versus 5.8
mm Hg, respectively).
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By itself, variation in genotype accounted for 3.1% and 4.5%, respectively, of interindividual variation in the systolic and diastolic blood pressure responses to hydrochlorothiazide (Table 3). Other significant univariate predictors of greater systolic and diastolic blood pressure responses to hydrochlorothiazide included higher pretreatment blood pressure levels, black race, female gender, older age, and lower plasma renin activity and urinary aldosterone excretion (Table 3). Lower waist-to-hip ratio and lower plasma aldosterone concentration were additional significant predictors of systolic and diastolic blood pressure responses, respectively.
In multiple regression models that included genotype in addition to the other predictor traits (which were considered one at a time in Table 3), genotype remained a statistically significant predictor of systolic (P<0.04) and diastolic (P<0.01) blood pressure responses to hydrochlorothiazide (models not shown). In addition to C825T genotype, pretreatment blood pressure levels, plasma renin activity, and urinary aldosterone excretion also remained significant predictors of systolic and diastolic blood pressure responses, whereas gender and age remained significant predictors of the diastolic but not the systolic blood pressure response, and race and plasma aldosterone concentration made no additional contribution to the prediction of either systolic or diastolic blood pressure response. The models that included effects of all of the predictor traits (including genotype) accounted for 32% and 18%, respectively, of interindividual variation in systolic and diastolic blood pressure responses (P<0.001 for both models).
In separate multiple regression models that considered the effects of race, gender, and age, variation in genotype was not a statistically significant predictor of waist-to-hip ratio. In models that also included waist-to-hip ratio, variation in genotype was not a statistically significant predictor of pretreatment blood pressure levels, plasma renin activity, plasma aldosterone concentration, or urinary aldosterone excretion (models not shown).
| Discussion |
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One potential confounder is race. Consistent with previous reports, we found that the 825T allele was more frequent in blacks than in whites14 and that both black race and older age were also predictive of greater blood pressure response to hydrochlorothiazide.15 However, the relationships we observed between the C825T polymorphism and interindividual differences in blood pressure responses to hydrochlorothiazide were present in all race-gender groups except black women. Moreover, in multiple linear regression models, the C825T polymorphism remained a significant predictor of interindividual differences in blood pressure responses after the contributions of race as well as the other identified predictors of response were considered.
More relevant to potential mechanistic pathways in patients with essential hypertension, the 825T allele has also been associated with obesity,9 low plasma renin, and elevated aldosterone-to-renin ratio,10 all of which are correlates of volume expansion and sodium sensitivity.11 12 However, we did not detect significant associations of the C825T polymorphism with measures of body fat or activity of the renin-angiotensin-aldosterone system. Additional potentially relevant associations of the 825T allele have been reported with enhanced in vitro sodium-hydrogen antiport and proliferation of immortalized lymphoblasts from subjects with essential hypertension.16 17 Unfortunately, we did not make direct measures of sodium ion transport or cellular proliferation in the present study. Because G proteins relay signals from a wide variety of hormones and neurotransmitters and transmit them to numerous effector systems,18 it is plausible that some unidentified "gain of function" related to the 825T allele may influence the balance of intravascular fluid volume and vasoconstriction that in turn influences the blood pressure response to a thiazide diuretic.
The contribution of the C825T polymorphism to the explanation of interindividual variation in blood pressure response to the thiazide diuretic was small. Nevertheless, the magnitude of its effect was similar to that of other previously identified predictors of diuretic responsivity, including race, age, and measures of the renin-angiotensin-aldosterone system.15 Only the predictive effects of pretreatment systolic blood pressure level substantially exceeded those of the C825T polymorphism. Insofar as these results are indicative of the expected magnitude of effects of variation at other single genetic loci, clinically useful prediction of complex drug-response phenotypes, such as the antihypertensive responses to a thiazide diuretic, would appear to require measurement of the variation in multiple genetic and environmental factors. Nevertheless, our present findings provide additional evidence that direct measures of genetic variation can improve the ability to predict responses to antihypertensive drug therapy.19
In summary, the present study suggests that measured variation in the gene encoding the ß3-subunit of G proteins predicts interindividual differences in blood pressure response to a thiazide diuretic and that the predictive effects of the C825T polymorphism are additive and similar in magnitude to those of other predictors, including race, age, and measures of the renin-angiotensin-aldosterone system. Knowledge of genetic polymorphisms that are predictive of blood pressure response to antihypertensive drugs with known mechanisms of action could provide insight into the molecular mechanisms that contribute to hypertension and aid in the tailoring of effective antihypertensive drug therapy in individual patients.
| Acknowledgments |
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Received October 26, 2000; first decision December 11, 2000; accepted December 29, 2000.
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