From Cardiovascular Genetics, University of Utah School of Medicine, Salt
Lake City (S.C.H., R.R.W.); the Division of Preventive Medicine, Brigham and
Women's Hospital and Harvard Medical School, Boston, Mass (N.R.C.,
C.H.H.); the Division of Preventive Medicine, University of Alabama at
Birmingham (A.O.); the Division of Epidemiology and Clinical Applications,
National Heart, Lung, and Blood Institute, Bethesda, Md (J.A.C., P.S.A.); the
Department of Medicine, Johns Hopkins University School of Medicine,
Baltimore, Md (W.G.W.); and the School of Public Health and Tropical Medicine,
Tulane University, New Orleans, La (P.K.W.).
Correspondence to Steven C. Hunt, PhD, Cardiovascular Genetics, University of Utah School of Medicine, 410 Chipeta Way, Room 167, Salt Lake City, UT 84108. E-mail steve{at}ucvg.med.utah.edu
Recently, evidence was published that an A for G nucleotide
substitution in the promoter region of the angiotensinogen
gene 6 nucleotides upstream from the start site of
transcription appears to be a functional
mutation.3 7 The A substitution alters the
binding of a nuclear protein, resulting in increased gene transcription
compatible with increased angiotensinogen levels. The G-6A
alleles are in nearly complete linkage disequilibrium with the
M235T alleles.3 7
Angiotensinogen is expressed in tissues involved in blood
pressure regulation, such as kidney, adrenal, and brain
tissue.8 Angiotensinogen gene
duplication in the mouse has resulted in increased
angiotensinogen levels and increased blood
pressure.9 In humans, it has also been shown that
increased angiotensinogen levels correlate with increased
blood pressure.10 11 Injection of
angiotensinogen increases blood
pressure,12 whereas antibodies against
angiotensinogen decrease blood
pressure.13 Hypertensive persons with the TT
genotype compared with the MM genotype at M235T had
higher systolic and diastolic blood pressure and
plasma angiotensinogen levels, and they were 1.6 times more
likely to use an antihypertensive medication and 2.1 times more likely
to use 2 antihypertensive medications.14
In addition to the genetic evidence from the above studies, it has been
suggested that the angiotensinogen gene influences the salt
sensitivity of blood pressure.7 15 Therefore,
subjects involved in an ongoing clinical trial of blood pressure
reduction were tested for both the M235T and G(-6)A
angiotensinogen loci variants. This allowed hypotheses to
be tested about whether the form of the gene that shows increased
transcription leads to an increased incidence of hypertension over 3
years of follow-up and whether sodium reduction can protect persons
with this genotype from hypertension. This article reports on a
substudy conducted over a 3-year follow-up period during phase II of
the Trials of Hypertension Prevention (TOHP). Participants in this
randomized clinical trial were assigned at the 9 clinical centers to
sodium reduction, weight loss, combined sodium and weight loss, or a
usual care group in a 2x2 factorial design.
The primary hypothesis of TOHP was that diastolic blood
pressure would be significantly decreased by sodium reduction and by
weight loss. The primary hypothesis of our substudy was that the
diastolic and systolic blood pressure reductions
resulting from sodium reduction at the 36-month follow-up visit in the
trial would differ among angiotensinogen genotypes.
We also were able to test whether there were differences in the
incidence of hypertension among angiotensinogen
genotypes. Secondary hypotheses for this substudy included
testing whether weight loss and/or combined weight and sodium reduction
would show similar results to sodium reduction. Differences in results
across different clinic visits were not an original hypothesis and
should be considered as post hoc analyses.
Clinical Variables
Angiotensinogen Genotyping
Denatured PCR products were spotted onto 4 nylon membranes and
hybridized with g-32P end-labeled allele-specific
oligonucleotide probes corresponding to variants of the
AGT gene. The probe sequences were 235T: 5' CCT GAC GGG AGC CAG T 3',
235 M: 5' ACT GGC TCC CAT CAG G 3', -6A: 5' GCC AGG GGA AGA AG 3', and
-6G: 5' GCC GGG GGA AGA AG 3'. The membranes were prehybridized (M and
T filters at 50°C, A and G filters at 45°C) in hybridization
solution (0.5 mol/L Na-PO4, pH 7.2, 7% SDS, 1 mmol/L EDTA) for 10
minutes to 1 hour. The membranes were then hybridized with 1 µL of
labeled probe in 3 mL hybridization solution for 2 hours to overnight
at the same temperatures. The membranes were rinsed briefly in 2xSSC
at room temperature and then 30 minutes in 2xSSC at 52°C (M and T)
or 47°C (A and G). The membranes were drained briefly to remove
excess buffer, then wrapped in plastic wrap. They were placed in an
x-ray cassette with a sheet of Kodak XAR5 x-ray film for 10 minutes to
2 hours, then developed in an x-ray film processor.
Statistical Analysis
In studies finding an association of a marker at amino acid 235 of the
angiotensinogen gene with hypertension, the T allele is
present in persons with higher angiotensinogen levels,
higher blood pressures, and hypertension.1 The
corresponding allele at the -6 position is an A. Therefore, the TT
genotype at position 235 is comparable to an AA
genotype at -6. These two genotypes are in nearly
complete linkage disequilibrium, with only 3.0% of the sample from
white subjects not showing genotypic concordance (Table 1
Statistical analyses were the same as for the main
trial,17 with additional terms in the model for
angiotensinogen gene effects. Changes in study
variables were calculated by subtracting the baseline from the
follow-up value. At each time point, a regression model was fit to the
mean changes with an interaction term included to test genotype
by intervention after adjusting for gender, age, and baseline blood
pressure. A repeated-measures analysis of variance was fit to
the data to test for a genotype by intervention by time
interaction, with blood pressure changes at each clinic visit as the
repeated measure, and age, gender, and baseline blood pressure as
adjustment covariates. Baseline measurements were compared by
analysis of variance. Percentages were compared by
Hypertension Incidence
Similarly, the weight loss intervention decreased hypertension
incidence over the course of the study in the AA genotype group
compared with the usual care group. The heterozygotes also showed a
significant reduction in risk. There was no significant effect of
weight loss intervention on hypertension incidence in the GG group.
There was a trend in reduction of risk across genotypes that
was of borderline significance (P=0.07). Combined sodium
reduction and weight loss did not show a significant reduction in
incidence in the AA genotype group but did show a significant
reduction in incidence in the heterozygote group. The test for trend
was not significant (P=0.23).
Blood Pressure Reduction
Table 5
The difference between the AA and GG homozygote genotypes
in the adjusted net effect of sodium reduction on systolic and
diastolic blood pressure change after 36 months was
2.5/3.3 mm Hg. Mean diastolic blood pressure tended
to increase in the sodium reduction group relative to the usual care
group for the GG genotype, but the difference was not
significant. This increase occurred as a result of the usual care group
showing a greater decrease in blood pressure than the intervention
group rather than an actual increase in mean blood pressure in the
intervention group itself (see Table 4
For weight reduction compared with usual care at the 36-month
visit (Table 6
The effect of the combined intervention did not show a
significant blood pressure trend across genotypes at any of the
follow-up visits (Table 7
Despite higher blood pressures and an increased incidence of
hypertension in the control group participants with the AA
genotype, there was a significant reduction in
diastolic blood pressure and/or hypertension incidence in
those assigned to the sodium reduction or weight loss interventions who
had the AA and AG genotypes. It appears that although persons
with the AA genotype develop hypertension to a greater degree
than those with the other genotypes when there is no
intervention, they respond more favorably to salt reduction or weight
loss intervention. It appears that the GG genotype group may
comprise primarily salt-insensitive individuals.
The net blood pressure decrease with sodium reduction in the AA
genotype was 2.5/3.3 mm Hg greater than the decrease in
the GG genotype after 36 months. This mild effect is not
surprising, given that the genetic effect of the AA genotype
versus the GG genotype raises angiotensinogen
levels by only 20% to 30%. It is only the long-term elevation of
angiotensinogen levels that would be expected to slowly
increase blood pressure as compensatory mechanisms reset or fail, the
rate of that increase depending on other genetic and environmental
factors. All other things being equal and with no intervention,
Table 4
Significant differences in diastolic blood pressure
between the AA and GG genotypes for both the sodium and weight
interventions were apparent only at the 36-month visit. The decrease in
blood pressure at the 6-month visit was nearly identical for all 3
genotypes. Persons were less compliant with sodium reduction or
weight loss at the 18- and 36-month visits, in which the blood pressure
differences by genotype began to appear, although the
differences in compliance were not significant. This might imply that
the stronger intervention at 6 months was powerful enough to mask any
underlying genotypic differences by temporarily overriding a resistance
against blood pressure change in persons with the GG genotype.
As the intervention became less strong, or as long-term control became
equilibrated, only then were the genotypic differences observed.
Because this time-dependent result was not a prior hypothesis, other
studies will need to confirm the time course of blood pressure response
to sodium or weight reduction. A smaller study on an older population
suggested that angiotensinogen genotype differences
in blood pressure change could be seen after only 6 months of sodium
reduction and potassium supplementation (unpublished results). This
suggests that age may play a critical role in how rapidly and how
sustained a genotype-specific sodium effect may be.
Because there were significant genotype differences in
blood pressure change in the sodium and weight reduction groups, it
would be expected that similar or better results would be seen in the
combined intervention group. However, for both the AA and GG
genotype groups, the amounts of urinary sodium reduction in the
combined intervention group were only 22 and 25 mmol/L per 24
hours, respectively. Weight reduction was also the least in the AA
combined intervention group. Because neither systolic nor
diastolic blood pressure in those with the GG
genotype in the sodium reduction group were responsive to
long-term sodium reduction, the findings would not be expected to
change if this genotypic group had been more compliant with the
combined intervention. However, the low compliance in the AA group
would be expected to have larger adverse effects on the blood pressure
change estimate. If similar sodium compliance were reached in the AA
group as in the AG group for the combined intervention, the blood
pressure decrease and hypertension prevention may have at least
paralleled the significant sodium reduction group intervention
results. The argument that sodium reduction does not add any further
beneficial effect to that of weight loss cannot be inferred from either
the main study or this substudy because the study was not designed to
detect such an effect and only limited reduction in sodium excretion
was achieved in the combined intervention
group.19
A meta-analysis of intervention trials suggested that a 21 to
70 mmol/L decrease in sodium intake produces reductions in
systolic and diastolic blood pressures of
3.6/2.2 mm Hg.20 Another
meta-analysis showed that median 77 and 76 mmol decreases
in sodium were associated with systolic and
diastolic blood pressure decreases of 4.8/2.5 and
1.9/1.1 mm Hg in hypertensive and normotensive persons,
respectively.21 Infants who were randomized at
birth to a low sodium diet for the first 6 months after birth had lower
blood pressures 15 years later (3.6/2.2 mm Hg) than infants
assigned to a normal sodium diet.22 In addition,
a study in chimpanzees showed that adding salt within the normal human
dietetic range to a baseline low salt diet increased systolic
and diastolic blood pressure (33/10 mm Hg) over 20
months.23 The sodium-induced blood pressure
changes were completely reversed within 6 months after removing the
additional salt, and although most of the chimpanzees responded, there
were clearly responders and nonresponders. In most human studies of
salt sensitivity, there also has been a wide range of responses, even
within the artificially defined subgroups of salt-sensitive and
salt-resistant persons.24
Our study provides evidence that some of this variation is
explained by genetic factors, specifically the
angiotensinogen gene. Despite maintenance of sodium
reduction, persons with the GG genotype may have become
resistant to the blood pressure–lowering effects of sodium
reduction, whereas persons with the AA genotype maintained
their blood pressure response over 3 years. Constant genetically
determined angiotensinogen elevations in persons with the
AA genotype could explain this finding. However, because
angiotensinogen levels were not available in this study,
caution should be used before inferring that the
angiotensinogen genotype findings prove an actual
physiological role of angiotensinogen
levels in blood pressure response to a sodium or weight loss
intervention.
There are known physiological mechanisms that
could explain the above findings. The kidney normally compensates for
increased blood pressure by increasing fluid and sodium excretion. This
response curve is very steep, so that even small increases in blood
pressure can return fluid and electrolyte balance to
normal.15 Obesity shifts this curve to the right
and may flatten the curve so that higher blood pressure levels are
required to maintain fluid balance.25 26 27 A shift
to the right without flattening, as seen in salt-insensitive subjects,
means that blood pressures will be elevated but that natriuresis in
response to blood pressure change will be nearly normal. A flattening
of the response curve, as seen in more salt-sensitive individuals,
requires a greater blood pressure increase to excrete a similar amount
of sodium and fluid than in less salt-sensitive individuals. One
mechanism proposed for the development of salt sensitivity is the
failure to reduce angiotensin II to appropriate levels as
sodium intake increases.28 The genetically
determined higher angiotensinogen levels of the AA
genotype on a normal (high) salt diet would make it more
difficult for these individuals to lower angiotensinogen
when needed, making them more salt sensitive. The resulting flattening
of the pressure-natriuresis slope for the AA genotype would
result in a greater blood pressure drop after sodium reduction than for
the GG genotype, which would have the steeper response curve,
resulting in a smaller blood pressure response.27
Under this model, one would expect very little change in blood pressure
in the GG group because their angiotensinogen level would
respond appropriately to salt reduction, maintaining the steep curve,
and only temporarily lowering blood pressure. The AA genotype
would also be expected to have a temporary drop until equilibrium is
reached if there were not the constant higher genetically determined
angiotensinogen levels that require a higher blood pressure
to maintain the appropriate fluid volume and sodium concentration. If
the salt change is a chronic change, then it may take time to reset
some physiological set-point before blood pressure
will return to normal levels. Over the 36-month period of intervention,
a lower blood pressure was maintained in persons with the AA
genotype despite a decrease in urinary sodium excretion between
the 6-month and 36-month visits. For the GG group, however, net
diastolic blood pressure was reduced only through 18
months, after which it returned to levels similar to those in the usual
care group.
An important question is whether the effects of weight reduction
operate through similar or different mechanisms as for sodium
reduction. Persons on a normal salt diet who are 235T homozygotes have
blunted renal plasma flow response to a 3 ng/kg per minute
angiotensin II infusion compared with the other 2
genotypes.29 Also, greater body mass
index was associated with significantly lower renal plasma flow in
persons with the TT genotype (r=-0.61) than for the
other 2 genotypes (r=-0.38). Therefore, weight loss
may improve renal plasma flow to a greater extent in the TT
genotype, resulting in reduced blood pressure.
An additional explanation may be that weight reduction results in
loss of adipose tissue. Adipose tissue produces the second greatest
amount of angiotensinogen next to the liver. Loss of fat
mass would directly reduce local angiotensinogen levels,
local vasoconstriction, and possibly resistance changes of a more
central nature. Angiotensinogen levels have been correlated
with blood pressure reduction caused by weight
loss.30 A study with rats showed that fasting for
3 days reduced angiotensinogen amounts released per adipose
cell to 33% of control animal levels.31
Refeeding increased angiotensinogen release to levels that
were 41% to 83% higher than those in the control rats. Increased
angiotensinogen release was limited to the local adipose
tissue, since liver mRNA and central plasma levels of
angiotensinogen were not affected by fasting or
overfeeding.
Therefore, weight loss in the TOHP study participants could reduce both
the amount of adipose tissue and the release of
angiotensinogen per cell. This would be expected to have
the greatest effects in persons with the AA genotype who
already have greater angiotensinogen release. Reduction of
angiotensinogen to near normal levels would remove local
vasoconstriction and insulin resistance and reduce blood pressure to a
greater extent than in persons with the GG genotype who already
have near-normal levels of angiotensinogen. Further
research is required to determine whether the mechanism relating weight
loss to reduced blood pressure, especially in persons with the AA
genotype, is a direct mechanism related to the amount of
adipose tissue or whether it occurs by the same mechanism as for sodium
reduction.
Possible Clinical Implications
It also appears that effective long-term maintenance of
the intervention will be required before adequate control is reached,
at least in younger subjects. Older subjects may have a more rapid
response to sodium reduction or weight loss but may still need
long-term changes in diet and exercise habits to maintain that
response. This study did not have the power to specifically test
whether the slight increase in relative risk for GG genotype
individuals is real or has any clinical implications. Until further
data are obtained, therefore, guidelines from the Sixth Report of the
Joint National Committee on Detection, Evaluation, and Treatment of
High Blood Pressure remain appropriate that all patients with elevated
blood pressures should reduce sodium and
weight.32 Our study only suggests that persons
with the adverse angiotensinogen genotype who are
being considered for lifetime antihypertensive therapy should be given
even greater attention by health professionals.
Received March 2, 1998;
first decision March 31, 1998;
accepted May 12, 1998.
2.
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M, Turner P, Clark AJL. Linkage of the angiotensinogen gene
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© 1998 American Heart Association, Inc.
Scientific Contributions
Angiotensinogen Genotype, Sodium Reduction, Weight Loss, and Prevention of Hypertension
Trials of Hypertension Prevention, Phase II
Abstract
Top
Abstract
Introduction
Methods
Results
Discussion
References
Abstract—The
angiotensinogen gene has been linked to essential
hypertension and increased blood pressure. A functional variant
believed to be responsible for hypertension susceptibility occurs at
position -6 in the promoter region of the gene in which an A for G
base pair substitution is associated with higher
angiotensinogen levels. To test whether an allele
within the angiotensinogen gene is related to subsequent
incidence of hypertension and blood pressure response to sustained
sodium reduction, 1509 white male and female subjects participating in
phase II of the Trials of Hypertension Prevention were
genotyped at the angiotensinogen locus.
Participants had diastolic blood pressures between 83 and
89 mm Hg and were randomized in a 2x2 factorial design to sodium
reduction, weight loss, combined intervention, or usual care groups.
Persons in the usual care group with the AA genotype at
nucleotide position -6 had a higher 3-year incidence rate
of hypertension (44.6%) compared with those with the GG
genotype (31.5%), with a relative risk of 1.4 (95% confidence
interval [0.87, 2.34], test for trend across all 3 genotypes,
P=0.10). In contrast, the incidence of hypertension was
significantly lower after sodium reduction for persons with the AA
genotype (relative risk=0.57 [0.34, 0.98] versus usual care)
but not for persons with the GG genotype (relative risk=1.2
[0.79, 1.81], test for trend P=0.02). Decreases of
diastolic blood pressure at 36 months in the sodium
reduction group versus usual care showed a significant trend across all
3 genotypes (P=0.01), with greater net blood
pressure reduction in those with the AA genotype (-2.2
mm Hg) than those with the GG genotype (+1.1 mm Hg). A
similar trend across the 3 genotypes for net systolic
blood pressure reduction (-2.7 for AA versus -0.2 mm Hg for GG)
was not significant (P=0.17). Trends across
genotypes for the effects of weight loss on hypertension
incidence and decreases in blood pressure were similar to those for
sodium reduction. We conclude that the angiotensinogen
genotype may affect blood pressure response to sodium or weight
reduction and the development of hypertension.
Key Words: blood pressure • clinical trials • genetics • interaction • prospective study • renin
Introduction
Top
Abstract
Introduction
Methods
Results
Discussion
References
Basic research and genetic studies in human populations
have implicated the angiotensinogen gene in the development
of elevated blood pressure and hypertension. Genetic linkage
analysis has shown that the angiotensinogen gene is
linked to hypertension in Utah, French, and English sib
pairs.1 2 Multiple polymorphisms were
examined for association with hypertension in an attempt to identify a
specific mutation that could cause increased expression of
angiotensinogen levels leading to increased blood pressure.
The two polymorphisms receiving the most attention were a
methionine-to-threonine amino acid substitution at amino acid 235
(M235T) in the mature angiotensinogen protein and a
threonine-to-methionine substitution at amino acid 174 (T174
M).1 3 These amino acid substitutions arise from
nucleotide substitutions at positions +704 and +521,
respectively, from the transcription start site. The strongest evidence
for association has been with the M235T polymorphism. However, this
polymorphism is not near the cleavage site where renin acts on
angiotensinogen to form angiotensinogen I, nor
is it thought that this amino acid substitution plays a functional
role. Because of this and the lack of consistency in
association studies of M235T with
hypertension,2 4 5 6 it was suspected that there
may be another site in linkage disequilibrium with M235T that is a
causal mutation.3
Methods
Top
Abstract
Introduction
Methods
Results
Discussion
References
Subjects
TOHP, phase II, was designed to study the efficacy of sodium
reduction and weight loss in reducing diastolic blood
pressure levels.16 This 3-year trial recruited
2382 men and women between the ages of 30 and 54 years who were
moderately overweight (110% to 160% of desirable body weight) and had
mean diastolic blood pressures between 83 and 89
mm Hg averaged over 3 baseline visits. Subjects were randomly assigned
to a usual care group, a sodium reduction group, a weight reduction
group, or a combined sodium and weight reduction group. The
intervention goal was a dietary sodium intake of 
80 mmol/L per
day in the sodium reduction group and a 4.5 kg reduction in weight in
the weight reduction group. Both goals were used for individuals in the
combined sodium and weight loss reduction group. These goals were to be
met by the 6-month clinic visit. Additional details of study subjects
and intervention protocols can be found in the publication of the main
study results.17 Each study center received
Institutional Review Board approval, and written informed consent was
obtained from each participant.
Sitting blood pressures were measured by random zero
sphygmomanometers at baseline and at follow-up visits every 6 months
for 36 months, or, for some subjects, 42 or 48 months. The average of 9
measurements (3 from each of 3 visits) was used to calculate the blood
pressure at baseline, the 18-month visit, and the 36-month visit. Blood
pressures for those on antihypertensive medications were set equal to
the measurement at the last unmedicated study visit. Blood pressures of
persons on medications affecting blood pressure for reasons other than
hypertension and of pregnant women were assumed missing for those
visits. Hypertension was defined as an average systolic blood
pressure 
140 mm Hg, an average diastolic blood
pressure 90 mm Hg, or diagnosis and drug treatment of
hypertension. Blood was drawn only at the 36-month or last examination.
Angiotensinogen levels or plasma renin activity were not
measured. Urinary sodium excretion from a 24-hour sample was obtained
routinely at baseline and at 18 and 36 months. In addition, a 25%
sample had urinary sodium measurements obtained at the 6-month visit.
Body weight in kilograms was also measured at each visit.
White cells were obtained from all study subjects who attended
one of the participating clinics after the start of this substudy and
gave consent to have their blood drawn. White cells were separated from
plasma and sent on ice to the University of Utah, where
genotypes at 2 loci of the angiotensinogen gene
were obtained by assays performed at Myriad Genetics, Inc, Salt Lake
City. Genotypes were determined at amino acid 235 coded for in
exon 2 and at position -6, 6 nucleotides upstream from the
transcription start site in the promoter region of the gene located on
chromosome 1. Multiplex polymerase chain reaction (PCR) for the
sequences around codon 235 in exon 2 and the promoter region of the
human angiotensinogen gene were performed with the
following primers: 5' AGC CAG CAG AGA GGT TTG 3' and 5' AGG TTC ATG CAG
GCT GTG 3' for M235T; 5' GGT CCA AGC GTG AGT GTC 3' and 5' CGG CTT ACC
TTC TGC TGT A 3' for G-6A. The sizes of the two products are 126 bp
and 200 bp, respectively. PCR reactions were performed in a
Perkin-Elmer 9600 with the following cycling parameters:
95°C for 5 minutes; 5 cycles of 95°C for 10 seconds, 62°C for 10
seconds, 72°C for 60 seconds; followed by 35 cycles of 95°C for 10
seconds, 58°C for 10 seconds, and 72°C for 60 seconds.
Genotypes were obtained on all but 7 samples received,
giving a total sample size of 1894 subjects: 1509 white subjects, 323
African Americans, and 62 of other races. Persons of other races were
excluded from analysis because of small numbers. Also, because
only 3% (n=10 of 323) of African Americans had the GG
genotype, the full analysis testing for
genotype interactions with intervention on blood pressure could
only be performed on the 1509 white subjects. 
). The discordance in white subjects was
similar to that in African Americans (2.8%) and agrees with previously
published numbers.7 Because of such strong
linkage disequilibrium, the results were the same when analyzing each
of the two loci, and only the analyses of the -6
genotypes are presented. Gene frequencies of the A (and
T) allele are greatly increased in African Americans (q=0.85)
compared with white subjects (q=0.44), as has been reported in other
studies.5 18
View this table:
[in a new window]
Table 1. Percent (n) Concordance of M235T and G(-6)A
Angiotensinogen Genotypes in White Subjects: TOHP, Phase
II 
2 analysis. Relative risks of
hypertension incidence for each genotype represent
hazard rate ratios and were calculated from a Cox regression model.
Tests for trends were done by using an ordinal variable in the
regression equation representing the 3 genotypes
(1, 2, 3; with 3 as the AA genotype). One-degree-of-freedom
tests were calculated for trends across genotypes. Tests of
trends rather than 2-degrees-of-freedom F tests were used because it
has been shown that angiotensinogen levels of the AG
genotype persons are intermediate between the levels of persons
with the 2 homozygous genotypes. All significance levels and
95% confidence intervals are from 2-sided tests.
Results
Top
Abstract
Introduction
Methods
Results
Discussion
References
Twenty percent of the sample of white subjects was
genotype AA (TT) at position -6 (amino acid 235), whereas 32%
was GG (MM) (Table 1
). Table 2 shows the
baseline sample size, gender, age, blood pressure, urinary sodium
excretion, and weight for the 4 study groups by AGT genotype.
There were no significant differences either for tests between means or
for tests of trends for any variable except for gender in the
sodium intervention group. After combining the 4 intervention groups,
there was a small but significant increase in baseline mean
systolic blood pressure across the GG, AG, and AA
angiotensinogen genotypes (126.8, 127.5, 127.8
mm Hg, respectively; P=0.02). The diastolic
blood pressure trend across genotypes (85.8, 86.0, 86.1
mm Hg, respectively) was only of borderline significance
(P=0.08).
View this table:
[in a new window]
Table 2. Baseline Characteristics of White Study Subjects:
TOHP, Phase II
The Figure and Table 3 show the incidence of hypertension in
each angiotensinogen genotype by intervention
group. Three-year hypertension incidence in the usual care group was
greater (but not statistically significant) in the AA genotype
than the GG genotype, with the heterozygotes having an
incidence rate nearly that of the AA group (test for trend,
P=0.10). The hypertension incidence rate in the sodium
reduction group was significantly reduced below the usual care group at
36 months among those with the AA genotype and the AG
genotype. There was no significant effect of sodium reduction
intervention on hypertension incidence in those with the GG
genotype. There was a significant trend of hypertension
incidence rate reduction across genotypes.
View larger version (44K):
[in a new window]
Figure 1. Three-year hypertension incidence by
angiotensinogen genotype. Hypertension incidence
(%) is shown for each of the 4 study groups by
angiotensinogen genotype at the -6 position having
alleles A and G. The resulting 3 genotypes are AA, AG, and
GG, with the AA genotype showing greatest incidence in the
usual care group but lowest incidence in the sodium reduction and
weight loss intervention groups.
View this table:
[in a new window]
Table 3. Incidence of Hypertension Over 3 Years Among White
Subjects by Intervention Group and Angiotensinogen Genotype: TOHP,
Phase II
As a reference for the genotype-specific results,
the main study found net 2.9/1.6 and 1.2/0.7 mm Hg decreases in
systolic/ diastolic blood pressure for sodium
reduction after 6 and 36 months, respectively, 3.7/2.7 and 1.3/0.9
mm Hg decreases for weight reduction, and 4.0/2.8 and
1.1/0.6 mm Hg reductions in the combined reduction
group.17 Table 4
shows crude blood pressure changes after 36 months for each
genotype and intervention group. These crude blood pressure
changes were used to calculate the net blood pressure change between
intervention and usual care groups.
View this table:
[in a new window]
Table 4. Unadjusted Blood Pressure Changes and Standard
Deviations After 36 Months by Intervention Group and Angiotensinogen
Genotype: TOHP, Phase II shows net adjusted blood pressure
changes from baseline for the sodium reduction group compared with the
usual care group. The trend across genotypes in the net
intervention effect after adjustment for age, gender, and baseline
blood pressure was significant at the 36-month visit for
diastolic blood pressure (P=0.01) but not for
systolic blood pressure (P=0.17). Trends were not
significant for the 6- and 18-month visits for either systolic
or diastolic blood pressure. There was a significant 3-way
interaction effect between time, genotype, and intervention
group (sodium reduction versus usual care) on change in
diastolic blood pressure (P=0.01). The
corresponding interaction was of borderline significance for
systolic blood pressure change (P=0.08). It appeared
that persons with the AA genotype were able to maintain blood
pressure reduction at each time point, whereas persons with the GG
genotype were not, despite maintenance of similar
sodium reduction. In models with additional control for any interaction
of intervention group with baseline blood pressure, results by
genotype were nearly identical. Net sodium excretion changes
did not vary significantly across genotypes (Table 5) at any
time point. Average adjusted net weight change in the sodium
intervention versus usual care groups ranged from -0.4 to +0.6 kg
among genotype groups (data not shown).
View this table:
[in a new window]
Table 5. Adjusted Net Effect of Sodium Reduction on Blood
Pressure Change (±SE) Among White Subjects: TOHP, Phase
II1 ). ), the trend across
genotypes in the adjusted net intervention effect across
genotypes was significant for diastolic blood
pressure (P=0.05) but not for systolic blood
pressure (P=0.23). Change in weight was not significantly
different among the 3 genotype groups, ranging from a net 1.7
to a 2.7 kg weight loss compared with usual care at the 36-month
follow-up visit (Table 6). The average adjusted net sodium change
ranged from -8 to +6 mmol/24 hours among genotype groups
(data not shown). There were no significant trends across
genotype in blood pressure reduction at the 6- and 18-month
visits despite the greater weight loss that was observed at these
visits compared with 36 months. Nonsignificant 3-way interactions of
time, genotype, and intervention group (weight loss versus
usual care) were seen for both systolic (P=0.15) and
diastolic (P=0.10) blood pressure change. At 36
months, there was a net systolic and diastolic
blood pressure difference between AA and GG homozygote
genotypes of 2.4/2.7 mm Hg.
View this table:
[in a new window]
Table 6. Adjusted Net Effect of Weight Reduction on Blood
Pressure Change (±SE) Among White Subjects: TOHP, Phase
II1 ). Although
there were net blood pressure reductions in the AA and AG
genotype groups compared with no net blood pressure reduction
in the GG group at 36 months, these differences were not significant.
At 36 months, the combined intervention group showed apparent
differences in compliance with intervention between genotypes.
The net urinary sodium decrease maintained at the 36-month visit in the
combined reduction group with the AA genotype was only 22
mmol/24 hours, not significantly different from zero
(P=0.39), compared with a net 44 mmol/24 hours decrease
(P<0.01) for the heterozygote genotype. Similarly,
net weight reduction in the AA genotype group at 36 months (1.2
kg) was not maintained as well as in the AG group (3.0 kg), possibly
explaining the lack of a significant trend for blood pressure change
across genotypes. The interaction of time, genotype,
and intervention group (combined versus usual care) was significant for
diastolic blood pressure change (P=0.05),
whereas the interaction was of borderline significance for
systolic blood pressure change (P=0.09).
View this table:
[in a new window]
Table 7. Adjusted Net Effect of Combined Sodium and Weight
Reduction on Blood Pressure Change (±SE) Among White Subjects: TOHP,
Phase II1
Discussion
Top
Abstract
Introduction
Methods
Results
Discussion
References
The design of TOHP, phase II, provided a unique opportunity
to test whether hypertension incidence differed by genotype
over 3 years of follow-up, since persons selected to be in this study
had a high normal level of diastolic blood pressure
averaging between 83 and 89 mm Hg at baseline. This substudy of
TOHP found that after a 3-year period of follow-up in the usual care
group, systolic and diastolic blood pressures and
incidence of hypertension were higher, but with borderline
significance, in persons with the AA genotype of the
angiotensinogen gene than in those with the GG
genotype. The heterozygotes had an intermediate level of blood
pressure and had a higher incidence of hypertension than persons with
the GG genotype. This reinforces the hypothesis that this
nucleotide substitution in the promoter region of the gene
predisposes individuals to hypertension. suggests that over only 3 years the AA genotype group would
differ from the GG genotype by 1.6 mm Hg diastolic blood pressure.
Differences of 2 to 3 mm Hg in blood pressure response between
genotypes can have an important public health impact. Analyses using
data from the Framingham Heart Study and NHANES II showed that a 2
mm Hg reduction in diastolic blood pressure would be associated with a
17% decrease in the prevalence of hypertension, a 6% reduction in
risk of coronary heart disease, and a 15% reduction in risk of stroke
and transient ischemic attacks.18A
Persons with a genetic predisposition to develop hypertension as a
result of the angiotensinogen locus appear to have the
greatest diastolic blood pressure decrease after either
sodium reduction or weight loss. Because persons with the AA
genotype had the greatest 3-year incidence of hypertension
without intervention, they may be an important subgroup to target for
effective diet counseling and weight loss. The data from this study
suggest that weight or sodium intervention was not effective for
persons with the GG genotype. Caution must be used, however,
when interpreting these results. The confidence limits for blood
pressure changes and relative risks do not exclude benefits in such
persons, and the early benefit may have been maintained with better
sustained adherence to the interventions.17
Acknowledgments
Phase II of the Trials of Hypertension Prevention study was
supported by cooperative agreements HL-37852, HL-37924, HL-37907,
HL-37904, HL-37854, HL-37849, HL-37884, HL-37853, HL-37899, and
HL-37906 from the National Heart, Lung, and Blood Institute, National
Institutes of Health, Bethesda, Md. Funding for this substudy was
provided by an investigator-initiated grant from Myriad Genetics, Inc,
Salt Lake City, Utah.
Footnotes
Reprint requests to Trials of Hypertension Prevention Coordinating Center, Brigham and Women's Hospital, 900 Commonwealth Ave E, Boston, MA 02215-1204.
References
Top
Abstract
Introduction
Methods
Results
Discussion
References
1.
Jeunemaitre X, Soubrier F, Kotelevtsev Y, Lifton
RP, Williams CS, Charru A, Hunt SC, Hopkins PN, Williams RR, Lalouel
JM, Corvol P. Molecular basis of human hypertension: role of
angiotensinogen. Cell. 1992;71:169–180.[Medline]
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