(Hypertension. 1999;33:266-270.)
© 1999 American Heart Association, Inc.
Scientific Contributions |
From the First Department of Internal Medicine, Shiga University of Medical Science, Tsukinowa, Seta, Otsu-city, Shiga-ken, Japan.
Correspondence to Naoharu Iwai, MD, First Department of Internal Medicine, Shiga University of Medical Science, Tsukinowa Seta, Otsu-city 520-2192, Shiga-ken, Japan. E-mail iwai{at}suncuore.shiga-med.ac.jp
| Abstract |
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Key Words: genetics CYP11B2 aldosterone renin blood pressure monitoring, ambulatory hypertension, low-renin
| Introduction |
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Several frequent polymorphisms have recently been described in the transcriptional regulatory region of the CYP11B2 gene, and the T(-344)C polymorphism has recently been reported to be associated with left ventricular mass in young Finnish adults free of clinical heart diseases.3 Pojoga et al4 reported that the T(-344)C polymorphism in the aldosterone synthase gene was associated with significant differences in plasma aldosterone levels. However, they reported that hypertension was not associated with this T(-344)C polymorphism.
In the present study, we investigated possible associations between genetic variations of CYP11B2 [Lys173Arg and T(-344)C] and hypertension in a Japanese population.
| Methods |
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Sequence Analysis and Genotype Determination
DNA was isolated from peripheral leukocytes, and the
genotype of polymorphisms in the CYP11B2 gene
was determined by a polymerase chain reaction (PCR)based method.
Since CYP11B1 is highly homologous to
CYP11B2,6 we focused on the promoter
regions of the two genes, where the sequence homology is not high, and
each gene could be amplified separately by PCR. The promoter region of
CYP11B2 (from -677 to -42) was amplified by PCR using the
following two primers: 5'-CAGGAGGGATGAGCAGGCAGAGCACAG-3'
(sense; from -677 to -651) and
5'-CTCACCCAGGAACCTGCTCTGGAAACATA-3' (antisense 2; from -71 to
-42). The PCR profile included 34 cycles of denaturing at 94°C for
45 seconds, annealing at 58°C for 45 seconds, and polymerization at
72°C for 45 seconds. The PCR-amplified fragments were electrophoresed
on a 1.5% agarose gel (Nippon Gene Co Ltd) and purified using glass
beads (Qiaex II Gel Extraction Kit, Qiagen). Purified PCR products
were directly sequenced using a cycle sequencing kit (TaKaRa Taq Cycle
Sequencing Kit, Takara Shuzou). For the sequence analysis, we
selected 11 hypertensive subjects5 with low PRA (<0.2 ng
angiotensin I [Ang I] ·
h-1 · mL-1), 3
with normal PRA, and 3 with high PRA (>3 ng Ang I ·
h-1 · mL-1)who
had a positive family history of hypertension. In our direct sequence
analyses, we found the T(-344)C and C(-470)T
polymorphisms in CYP11B2. No other sequence variation
was found in promoter regions of the CYP11B2 gene.
The genotype of CYP11B2 was determined by PCR amplification using the primers described above and digestion by the HaeIII restriction enzyme (Takara Shuzou).
The amplified fragments were digested with the HaeIII restriction enzyme and then subjected to electrophoresis on a 2.0% agarose gel and visualized under UV light. Fragments of 231 bp (T allele) and 140+91 bp (C allele) were detected.
Genetic variations of the CYP11B2 (Lys173Arg) gene were detected as described in the literature.2 Briefly, the 5' and 3' primers were 5'-AGGCAGCTTCTACCAGGGCCCCAGTCACT-3' (sense) and 5'-CCCCTCCCCTGCAAATCTCATCCCTTA-3' (antisense), respectively. Genomic DNA was amplified as previously reported with a program that consisted of denaturation at 94°C for 1 minute, followed by 33 cycles of 94°C for 45 seconds, 61°C for 45 seconds, and 72°C for 150 seconds. The amplified fragments were digested with the Bsu36I restriction enzyme (New England Biolabs) and then subjected to electrophoresis on a 2.0% agarose gel and visualized under UV light. Fragments of 1286 bp (Lys allele) and 1037+249 bp (Arg allele) were detected.
Statistical Analysis
Statistical analyses were performed with StatView 4.
Frequency was compared by a contingency table analysis.
Numerical data were analyzed by an unpaired t test
or one-way ANOVA. Multiple regression analysis was performed to
investigate the possible influence of the genotype of
CYP11B2 on ALD/PRA; gender, age, body mass index (BMI),
hypertension (negative=0, positive=1), and the genotype of the
CYP11B2 gene (TT [wild-type]=0, TC+CC [hetero+homo
mutation]=1) were included as independent variables. Moreover,
multiple regression analysis was performed to investigate the
possible influence of the genotype of CYP11B2 on
M-mode echocardiographic measurements. Gender, age,
BMI, systolic BP, diastolic BP and the
genotype of the CYP11B2 gene (TT=0, TC+CC=1) were
included as independent variables. Backward selection was used and
a P value of 0.10 or greater was required for a variable
to be removed. To examine the independent contribution of
CYP11B2 gene polymorphisms to hypertension, while
adjusting for the effects of other clinical characteristics, we used a
logistic analysis. For hypertension, a baseline logistic model
was developed by applying backward stepwise selection with the
following factors as covariates: gender (female=0, male=1), BMI, and
polymorphisms of the CYP11B2 gene (TT=0, TC+CC=1).
Backward selection was used, and a P value of 0.10 or
greater was required for a variable to be removed from the
model.
M-Mode Echocardiographic Measurements
M-mode echocardiographic measurements were taken
in 136 subjects who were not receiving antihypertensive therapy. Left
ventricular end-diastolic dimension (LVDd) was
calculated from M-mode echocardiographic measurements
of the left ventricle using an SSH160A system with 3.75-MHz transducers
(Toshiba). M-mode measurements (guided in two dimensions) of LVDd, left
ventricular end-systolic dimension (LVDs),
end-diastolic interventricular septal
thickness, and end-diastolic posterior wall thickness were
performed at the left ventricular minor axis at the level
of the chordae tendinae just beyond the mitral leaflet tips, as
recommended by the American Society of
Echocardiography.7 Each measurement
was taken three times, and the average value was used.
Plasma Aldosterone and Renin Activities
Plasma aldosterone (ALD) and plasma renin activity
(PRA) were determined at our outpatient clinic in 97 hypertensive
subjects who were under no drug treatment after subjects had rested 30
minutes. ALD was determined by radioimmunoassay (SPAC-S
Aldosterone Kit). PRA was determined by measuring Ang I by
radioimmunoassay (SRL, Japan).
Twenty-FourHour BP Measurement
Twenty-fourhour ambulatory BP monitoring (ABPM) was performed
in 65 subjects with hypertension in the absence of drug treatment. We
usually recommend that all first-visit patients with possible
hypertension have their BP levels evaluated by ABPM. Of all the
subjects, only 65 approved to undergo ABPM. ABPM was performed
noninvasively every hour with an automatic oscillometric device (model
BP8800NC, Nippon Colin) that was attached to the upper left arm.
Subjects were allowed to perform their usual daily activities between
the measurements, except that they were asked to get up at 6
AM and go to sleep at 9 PM. BP and heart rate
measurements were obtained at 30-minute intervals. A nondipper pattern
was defined as a difference in mean BP of <10% between the daytime (6
AM to 9 PM) and nighttime (9 PM to
6 AM) hours. Mean BP was calculated as
diastolic BP plus one third of the difference between
systolic and diastolic BPs.
| Results |
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Since the Arg173 allele in CYP11B2 completely corresponded to the C(-344) allele in our study population (n=100), only the T(-344)C genotype was analyzed further. In addition, since the CC genotype occurred at a low frequency, the TC and CC genotypes were combined into a single group.
Functional Significance of the Polymorphisms
To explore the possible functional significance of the C(-344)T
polymorphism of CYP11B2, we investigated associations
between the genotype of CYP11B2 and renin profile.
Table 2
shows ALD/PRA according to the
genotype of the CYP11B2 gene. Gender, age, BMI, and
BP did not differ significantly in the two groups. The TT
genotype was associated with a lower ALD/PRA than the TC+CC
genotype (P=0.0017). Multiple regression
analysis indicated that the genotype of the
CYP11B2 gene (TT=0, TC+CC=1) (P=0.0047,
ß-coefficient=0.288) and BMI (P=0.0519,
ß-coefficient=0.024) were predictors of ALD/PRA. The frequency of the
low PRA in the TC+CC genotype was significantly higher than in
the TT genotype (
2=6.904,
P=0.0317)
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Association of Polymorphisms With Hypertension
Table 3
shows the characteristics of
the NT and HTN groups. Age did not differ significantly between the two
groups. The HTN group had a higher percentage of men and a higher BMI.
The frequency of the TC+CC genotype in the NT group was 43.2%
(76+22/227), which was significantly lower (P=0.0487) than
that in the HTN group (52.2%, 102+31/255). Logistic analysis
of 482 subjects revealed that gender (P=0.0021; odds
ratio=1.790; 95% confidence interval [CI]=1.234 to 2.595), BMI
(P=0.0024; odds ratio=1.093; 95% CI=1.032 to 1.158), and
genotype of the CYP11B2 gene (TT=0, TC+CC=1)
(P=0.0484; odds ratio=1.452; 95% CI=1.003 to 2.104) were
associated with hypertension (P<0.0001).
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Correlation Between CYP11B2 Polymorphism
and ABPM
Table 4
shows the correlation
between CYP11B2 polymorphism and the nocturnal decline
in BP. There were significantly more nondippers among subjects with the
TC+CC genotype (45.7%, 11+5/35) than among subjects with the
TT genotype (20.0%, 6/30) (P=0.029).
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Correlation Between CYP11B2 Polymorphism and
M-Mode Measurements
Table 5
shows the correlation
between CYP11B2 polymorphism and
echocardiographic parameters.
Echocardiographic assessment (n=136) revealed that
LVDd/height tended to be greater (P=0.0987) in subjects with
the TC+CC genotype than in subjects with the TT
genotype. Other parameters, such as LVDs/height,
end-diastolic interventricular septal
thickness, and end-diastolic posterior wall thickness, were
not significantly associated with CYP11B2 polymorphism
(P=0.2889, P=0.6128, and P=0.8791,
respectively). Multiple regression analysis indicated that BMI
(P=0.0024, ß-coefficient=0.002), age (P=0.0487,
ß-coefficient=3.992), and the genotype of the
CYP11B2 gene (TT=0, TC+CC=1) (P=0.0982,
ß-coefficient=0.008) were predictors of LVDd/height.
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| Discussion |
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In a study by Fardella et al,2 the Arg173 allele was observed more often in hypertensive subjects (37.5%) than in normotensive subjects (22.5%); however, this change did not appear to have any functional significance. On the basis of our finding that the C(-344) allele was completely linked with the Arg173 allele in a Japanese population, the C(-344) allele may be associated with higher transcriptional activity. Indeed, Clyne et al9 suggested that the promoter region between -413 and -221 might influence the responsiveness to angiotensin II. The T(-344)C variation of the CYP11B2 gene or some other unidentified polymorphism associated with T(-344)C may be associated with transcriptional levels. Because the region sequenced in the present study was just between -677 and -42, and we sequenced this promoter region in just 11 patients, other unidentified sequence variations might exist. Further studies will be necessary to confirm the functional significance of the C(-344)T polymorphism.
Kupari et al3 reported that homozygotes for the C(-344) allele average, by cubic approximation, 28% greater end-diastolic volumes and 21% greater mass than homozygotes. Similarly, in our study, echocardiographic assessment (n=136) revealed that LVDd/height tended to be higher in subjects with the TC+CC genotype (P=0.0987) than in subjects with the TT genotype.
The mechanism of the association of left ventricular size and mass with polymorphism of the CYP11B2 promoter is still unclear. Previous echocardiographic studies in humans have demonstrated that increases or decreases in dietary sodium result in corresponding changes in left ventricular volumes and mass.10 Kupari et al3 also reported that in a regression model for left ventricular mass, a statistically significant interaction was observed between CYP11B2 promoter polymorphism and salt intake: The association of left ventricular mass with salt intake was strong and highly significant (P<0.0001) in the -344CC group, intermediate (P=0.0110) in the -344CT group, and nonexistent (P=0.610) in the -344TT group.3 Thus, differences in the body's sodium balance and intravascular volume related to the CYP11B2 genotype are plausible mechanisms for our observations of a dilated left ventricular dimension.
Another explanation may involve the indirect cardiovascular effects of aldosterone, ie, the induction of myocardial hypertrophy and fibrosis.11 12
We studied 24-hour ABPM in 65 subjects with hypertension who were under no drug therapy. The nondipper type was significantly more frequent among subjects with the TC+CC genotype (45.7%,11+5/35) than among subjects with the TT genotype (20.0%, 6/30) (P=0.029). It has been reported that NaCl loading reduces the nocturnal decline in BP in salt-sensitive individuals but not in salt-resistant individuals.13 14 Oshima et al15 also reported that in patients with essential hypertension, intracellular sodium accumulation and inadequate suppression of the renin-angiotensin system may be independently associated with NaCl sensitivity. The higher frequency of nondippers among subjects with the TC+CC genotype suggests that this genotype may be associated with salt sensitivity.
Recently, Brand et al16 reported significant association of the T(-344) allele of the CYP11B2 gene and hypertension. However, our results indicated that the C(-344) allele of the CYP11B2 gene was associated with higher ALD/PRA and that the frequency of the C(-344) allele was higher in the HTN group in a Japanese population. The reasons for the differences between our data and those of Brand et al16 are not evident. One possibility is that the C(-344) allele of the CYP11B2 gene may not have any functional significance itself but may be linked to an as yet unidentified mutation in the promoter region of the gene. Another possibility is that the predisposition to hypertension may be clarified only during high salt intake. Japanese salt intake is reportedly higher than that in whites.17 Environmental factors such as salt intake might influence a genetic predisposition to hypertension.
In conclusion, the C(-344) allele of the CYP11B2 gene was associated with higher ALD/PRA in Japanese subjects. The present results suggest that the TC+CC genotype may be associated with salt sensitivity. Further investigation in a larger population might be necessary to confirm our results.
Received September 15, 1998; first decision October 15, 1998; accepted November 5, 1998.
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