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Department of Clinical Pathology, Seoul National University College of Medicine, 28 Yongon-dong, Chongno-gu, Seoul 110-799, Korea.
a Author for correspondence. Fax 82-2-745-6653; e-mail jqkim{at}plaza.snu.ac.kr
| Abstract |
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Methods: Plasma NOx was measured in samples from 128 healthy controls and from 110 CAD patients at least 2 months after myocardial infarction. Three genetic polymorphisms that are known or have been suggested to be associated with plasma NOx concentration were also analyzed by PCR-restriction fragment length polymorphism.
Results: Median plasma NOx was significantly higher (P <0.001) in CAD patients (95.9 µmol/L) than in controls (73.8 µmol/L). Furthermore, the median plasma NOx was significantly higher (P <0.001) in hypertensive CAD patients (116.0 µmol/L) than in controls and normotensive CAD patients (86.0 µmol/L). The G-allele frequency of the G10-T polymorphism in intron 23 was significantly higher in CAD patients than in controls. Other polymorphisms showed no differences in allelic frequencies among the control and CAD groups. In controls, individuals with the E298D mutation in exon 7 (136.1 µmol/L) showed significantly higher (P = 0.001) median plasma NOx than those without this mutation (64.5 µmol/L).
Conclusions: Plasma NOx was higher in hypertensive CAD patients than in normotensive CAD patients and controls. The E298D polymorphism of the ecNOS gene was associated with increased plasma NOx. Further study is needed to understand the gene expression and enzyme activity of ecNOS and their association with genotypes.
| Introduction |
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There are at least three isoenzymes of NOS: inducible NOS, constitutive
neuronal NOS, and constitutive endothelial NOS (ecNOS) (7),
which constitute a "gene family" located on different chromosomes
and expressed in different cell lines. The gene encoding ecNOS is
located on chromosome 7q35-36, which comprises 26 exons spanning 21 kb
(8). Moreover, a significant association has been found
between the 27-bp repeat polymorphism in intron 4 of the ecNOS gene
(intron 4 VNTR) and CAD (9). In addition, the
Glu298
Asp polymorphism in exon 7 of the ecNOS
gene (E298D) has been reported to be a strong risk factor for CAD, with
homozygous genotype (T/T) frequencies of 36% in CAD cases vs 10% in
controls (10). No significant association between the G10-T
polymorphism in intron 23 (G10-T) and arterial hypertension has been
found (11).
Studies have rarely been performed on the specific association between the genetic polymorphisms of the ecNOS gene and plasma NOx concentrations in CAD patients. Moreover, because gene pools, life-styles, and gene-environment interactions differ among populations, ethnic differences in the allelic frequencies of ecNOS polymorphisms and in the genetic associations between disease and plasma NOx concentration may exist (12). The purpose of this study was to examine the concentrations of NO2- and NO3- in plasma of CAD patients and to assess the association between these factors and three polymorphisms of ecNOS in the Korean population.
| Materials and Methods |
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lipid and apolipoprotein analysis
Plasma cholesterol and triglycerides were measured by enzymatic
methods (Roche Diagnostics). HDL-cholesterol was measured
directly with the HDL-C diagnostic method (Kyowa Medex) using
either a Hitachi 747 or 7170 automatic chemistry analyzer.
LDL-cholesterol was calculated using the formula of Friedewald et al.
(14), and apolipoproteins A-I and B were measured by
immunonephelometric assay (Bering Nephelometer; Beringwerke).
Lipoprotein(a) was measured using a commercially available ELISA method
(IMMUNO). The within- and between-run CVs were <3% for total
cholesterol, triglycerides, and HDL-cholesterol, and <5% for
apolipoproteins A-I and B, and lipoprotein(a). Body mass index was
calculated by dividing weight by height squared
(kg/m2).
determination of plasma NO2- and NO3-
Plasma NO3- plus
NO2- (NOx) was measured as
NO2- after enzymatic conversion
of NO3- to
NO2- by nitrate reductase, as
described by Schmidt et al. (15) using a commercial
method (R&D Systems). Briefly, plasma samples were diluted twofold with
reaction buffer and ultrafiltered through a
Mr 10 000 cutoff filter to eliminate
protein. After centrifugation at 14 000g for 30 min at room
temperature, 50 µL of each deproteinized plasma sample was incubated
with 25 µL each of 100 U/L nitrate reductase and 0.35 mmol/L NADH.
After a 30-min incubation at 37 °C, Griess reagents I and II (50
µL of each) were added. The mixture was incubated at room temperature
for 10 min, and the absorbance was measured on a microplate reader at a
wavelength of 540 nm. The plasma NOx concentration was determined
relative to a calibration curve prepared with
NO3- calibrators.
dna analysis
Total genomic DNA was prepared from the leukocytes of 10 mL of
blood after lysis of the red blood cells (16). The intron 4
VNTR polymorphism of the ecNOS gene was detected by the method of Wang
et al. (9) with minor modification. Briefly, the DNA
samples were amplified by PCR using primer pairs that flanked the
region of the 27-bp direct repeat in intron 4 of the ecNOS gene. Primer
pairs for PCR were as follows: sense, 5'-AGGCCCTATGGTAGTGCCTTT-3';
antisense, 5'-TCTCTTAGTGCTGTGGTCAC-3'. The amplified DNAs were
separated on 2% agarose gels and visualized by ethidium bromide
staining.
For detection of the E298A polymorphism in exon 7 of the ecNOS gene, a primer pair was used to amplify a part of the ecNOS gene containing exon 7 by PCR. The primer pair for PCR was as follows: sense, 5'-CCCCCTCTGGCCCACT-3'; antisense, 5'-AYACZTCCCTTTGGTGCTCAC-3'. The resulting 152-bp amplification product was incubated at 37 °C for 2 h with 10 U of the restriction enzyme BanII (Roche Diagnostics). The amplified DNAs were digested by BanII into smaller fragments (56 and 96 bp). In the case of a G-to-T substitution at position 894 in exon 7 of the ecNOS gene, a BanII restriction site is lost.
For the G10-T polymorphism in intron 23 of the ecNOS gene, the primer pair for PCR was as follows: sense, 5'-CCCCTGAGTCATCTAAGTATTC-3'; antisense, 5'-AGCTCTGGCACAGTCAAG-3'. The resulting 676-bp amplification products were incubated at 37 °C for 2 h with 10 U of the restriction enzyme HindII (New England Biolabs). The amplified DNAs were then digested by HindII into fragments (577 and 99 bp). In the case of a G-to-T substitution at position 10 of intron 23 of the ecNOS gene, an additional HindII restriction site was produced and the amplified fragments were digested into smaller fragments (374, 203, and 99 bp).
statistical analysis
Statistical analyses were performed with the Statistical Analysis
System package (SAS Institute) and SPSS, Ver. 9.01 (SPSS). Variables in
two or three groups were compared using the MannWhitney
U-test or the KruskalWallis test. To test for independent
relationships between variables, the
2 test
and the Fisher exact test were performed. Statistical significance was
accepted at P <0.05. Genotypic and allelic frequencies were
determined using the gene counting method, and the
2 test was used to ensure that data complied
with the Hardy-Weinberg equilibrium
| Results |
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A comparison of allelic frequencies of the E298D, G10-T, and intron 4
VNTR polymorphisms between CAD patients and controls is shown in
Table 2
. The G-allele frequency of the G10-T polymorphism in intron 23
of the ecNOS gene was higher in CAD patients than in controls. However,
there were no significant differences between the allelic frequencies
of the E298D and intron 4 VNTR polymorphisms in CAD patients and in
controls. Plasma NOx concentrations according to the genotypes of the
polymorphisms are shown in Table 3
. In controls, plasma NOx showed significant differences
according to genotype (P = 0.001); the median plasma
NOx increased in those individuals with an E298D mutation in exon 7
(136.1 µmol/L) compared with those without the mutation (64.5
µmol/L). In hypertensive CAD patients, the median plasma NOx showed
an increasing tendency in the genotype with the 4a allele (127.9
µmol/L) compared with the genotypes without the 4a allele (107.7
µmol/L), although it was not statistically significant
(P = 0.083).
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| Discussion |
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CAD is a multifactorial disease that may differ in each race or ethnic group; for example, the prevalence of CAD differs widely among different populations, and the frequencies of ecNOS gene polymorphisms have been reported to vary among ethnic groups (12). Several polymorphisms of the ecNOS gene associated with CAD have been identified. Wang et al. (9) detected an association between homozygosity for the 4a allele in the intron 4 VNTR polymorphism of the ecNOS gene and an increased risk of CAD only in current and ex-smokers in the Australian population.
In the present study, the association between CAD and three polymorphisms of the ecNOS gene was investigated. Our results showed that the G-allele frequency of the ecNOS gene polymorphism G10-T in intron 23 was significantly higher in the CAD group than in the controls. However, no significant differences were found between patients and controls in terms of the allelic frequencies of the intron 4 VNTR or the E298D polymorphism. This result differs from previous reports on the associations between these polymorphisms and Caucasian CAD patients (9)(10)(11).
The E298D polymorphism in exon 7 of the ecNOS gene has been reported to be a strong risk factor for CAD (10), and Wang et al. (20) detected an association between homozygosity for the 4a allele in the intron 4 VNTR polymorphism of the ecNOS gene and increased plasma NOx in healthy Caucasians. However, studies on the association between genetic polymorphisms of ecNOS and the plasma NOx concentration have been few. The present study shows that only in the control group was plasma NOx significantly dependent on the genotypes of the E298D polymorphism; plasma NOx was increased in those individuals having the E298D mutation. In CAD patients, however, this relationship was not observed. The lack of correlation between controls and patients might be explained as follows. In CAD patients with pathogenic conditions such as atherogenesis, angina, and hypertension, which probably influence plasma NOx, the plasma NOx regulation mechanism could be changed, and therefore the genetic influences seen in healthy subjects might be masked. Otherwise, the significance observed only in healthy subjects might be invalid because of a statistical flaw in the multiple comparison factors. To determine whether the genetic polymorphism of the ecNOS gene, especially the E298D and intron 4 VNTR mutations, may influence plasma NOx, the study of gene expression and protein production is mandatory. We are now studying the relationship between these polymorphisms and mRNA concentrations or the enzyme activity of ecNOS, using human umbilical vein cell culture.
In conclusion, plasma NOx is higher in CAD patients with hypertension than in CAD patients without hypertension and in controls, which might be attributable to the compensatory phenomenon developed in hypertension. A dependency between increased plasma NOx and the polymorphisms of the ecNOS gene is observed in controls with the E298D mutation. To evaluate the exact causal relationships between the ecNOS polymorphisms and plasma NOx, more studies of gene expression and the enzyme activity of ecNOS, and the nature of their dependence on genotypes are required.
| Acknowledgments |
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| Footnotes |
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