Atherosclerosis and Lipoproteins |
From the Department of Nutrition (S.S.-B., H.C.), Harvard School of Public Health, Boston, Mass, and the Salud Coronaria project, Institute of Health Research (X.S., H.C.), University of Costa Rica, San Pedro.
| Abstract |
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Arg) and 55 (Met
Leu) in
the paraoxonase gene (PON1). We studied
the association of PON1 polymorphisms and myocardial
infarction (MI) in a population-based study consisting of 492 cases and
518 controls matched for age, sex, and area of residence, all living in
Costa Rica. The allele frequency of
PON1192Arg was higher in cases (0.27) than
in controls (0.24, P=0.008), whereas that of
PON155Leu was identical (0.26). Compared
with PON1192Gln-Gln, the
PON1192Arg allele was associated with an
increased risk of MI (odds ratio [OR] 1.36, CI 1.06 to 1.75), and
this association was independent of the
PON155 polymorphism, which was not
associated with MI (OR 1.10, CI 0.82 to 1.48). Adjustment for lipid and
nonlipid risk factors strengthened the association between
PON1192Arg and the risk of MI (OR 1.51, CI
1.13 to 2.03). Interestingly, this association was evident only among
nonsmokers (OR 1.90, CI 1.29 to 2.79): there was no evidence of an
association in smokers (OR 0.95, CI 0.57 to 1.79). The interaction
between PON1192 and smoking status was
statistically significant (P=0.04). Thus, the
PON1192 but not the
PON155 gene polymorphism is associated
with an increased risk of MI. This association is not evident
among smokers.
Key Words: coronary heart disease genetic epidemiology paraoxonase antioxidants lipoproteins
| Introduction |
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Human serum paraoxonase is a 44-kDa
Ca2+-dependent glycoprotein. It
remains exclusively associated with apoA-I on HDL through a hydrophobic
region at its amino terminus.9 10 Paraoxonase may lower
the risk of vascular disease by destroying proinflammatory molecules
formed by the oxidation of LDL.6 11 For example, purified
paraoxonase blocks the proinflammatory effect of oxidized LDL in a
vascular cell culture system, probably by destroying oxidized
arachidonic acid derivatives in the Sn-2 position of
LDL phospholipids.8 There is a 10- to 40-fold variability
in the activity of the enzyme among individuals9 that is
influenced, in part, by differences in susceptibility to
organophosphate poisoning.12 13 This interindividual
variability in activity has been attributed to 2 polymorphisms in
the coding region of the paraoxonase gene
(PON1)14 15 : a Gln
Arg substitution
at position 192 (PON1192Arg) and a
Met
Leu substitution at position 55
(PON155Leu).9
The PON1192 and PON155 polymorphisms are common in white and Asian populations, which show frequencies of between 0.30 and 0.59 for the PON1192Arg allele16 17 18 19 20 21 22 23 24 25 and between 0.27 and 0.91 for the PON155Leu allele.21 24 26 Paraoxonase 192Arg is associated with various levels of activity toward nonphysiological substrates.9 27 28 Paraoxonase 192Arg hydrolyzes paraoxon faster, and diazoxon slower, than 192Gln does, yet the 2 alloenzymes show no difference in activity toward other substrates, such as phenylacetate. Most important, the ability of HDL to protect LDL from lipid peroxidation in vitro is significantly reduced in HDL particles containing paraoxonase 192Arg rather than 192Gln.29 30 Carriers of the PON155 allele show an increased activity toward paraoxon that is independent of the PON1192Arg allele effect.26 31
Several studies have shown a positive association between the PON1192Arg allele and coronary disease,16 20 22 24 32 33 and several other studies have shown no association,17 18 19 23 25 including 1 study of the PON155 polymorphism.21 Only 2 studies with a small sample size21 34 have evaluated the PON1192 and the PON155 polymorphisms simultaneously. In diabetics, the PON1192Arg and PON155Leu alleles have both been consistently associated with coronary disease,26 35 36 although no association has been found with an increased risk of diabetes.34 A lack of randomly selected control groups, small sample sizes, and differences in criteria for case definition may explain these inconsistent conclusions, although it is also possible that other genetic characteristics and the particular environmental conditions of a given population may amplify or attenuate the effect of the PON1 gene on coronary disease.
The present study was designed to test whether the PON1192Arg and PON155Leu alleles are associated with an increased risk of myocardial infarction (MI) in a population-based case-control study of Hispanics living in Costa Rica, Central America.
| Methods |
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All survivors of a first MI who were hospitalized between January 1994 and December 1997, who were aged <75 years old, and who had lived in the catchment area for at least 1 year before the event were recruited as cases (n=531, participation 97%). Ten participants were excluded after recruitment because they died after hospital discharge but before data collection was completed, and 29 were excluded because they did not have a blood sample. All cases met the World Health Organization criteria for MI, which require typical symptoms plus either elevations in cardiac enzyme levels or diagnostic changes in the ECG.40 For consistency, one study cardiologist confirmed the diagnosis of first acute MI for all 3 hospitals before recruitment.
One free-living control subject for each case survivor, matched for age (±5 years), sex, and area of residence, was randomly selected from the general population by using information available at the National Census and Statistics Bureau of Costa Rica. Control subjects were considered ineligible if they had ever had an acute MI or were physically or mentally unable to answer the questionnaire. The participation rate for the controls was 90% (n=531). Thirteen subjects were not included because they did not have a blood sample. All study participants gave informed consent. Whenever possible, house visits were planned so that the interviews for case-control pairs were carried out by the same interviewer within 3 weeks of the pair patients hospital discharge. The present study was approved by the Committee on the Use of Human Subjects in Research at the Harvard School of Public Health and by the Institute of Health Research at the University of Costa Rica.
Data Collection
The general questionnaire included closed-end questions
regarding sociodemographic characteristics, smoking, physical activity,
and medical history (including personal history of diabetes and
hypertension). Self-reported diabetes and hypertension were validated
by using the definitions recommended by the Expert Committee on the
Diagnosis and Classification of Diabetes Mellitus41 and
the Third Joint National Committee on Detection, Evaluation, and
Treatment of High Blood Pressure,42 ie, a fasting
capillary whole blood glucose level
110 mg/dL (measured in the
morning at the subjects home) or the ingestion of glucose control
medications and a systolic blood pressure
140mm Hg, a
diastolic blood pressure
90 mm Hg, or the ingestion
of antihypertensive medications. The sensitivity, specificity,
predictive value positive, and predictive value negative were 80%,
97%, 75%, and 98%, respectively, for self-reported diabetes and
52%, 96%, 93%, and 70%, respectively, for self-reported
hypertension. Thus, the reliability of reports of diabetes and
hypertension by subjects is high in the Costa Rican population.
Physical activity was determined by asking subjects about the average
frequency of several occupational and leisure-time activities during
the past year (before MI for case subjects) and the amount of time
spent on them. These activities were grouped into 6 categories
(sleeping, sitting, and light, moderate, strenuous aerobic, and
strenuous anaerobic activities) according to intensity or
to METS, defined as the energy expenditure for sitting quietly, or
1
kcal · kg body
wt-1 ·
h-1.43 Energy
expenditure was calculated as the product of frequency, time, and
intensity and expressed as kilocalories per kilogram per hour. This
aspect of the questionnaire was validated by checking its ability to
predict fitness level (measured by the Harvard step test) in our
previous studies of cardiovascular risk factors in
residents of Puriscal, Costa Rica.44
Anthropometric measurements were collected in triplicate from subjects wearing light clothing and no shoes. Blood samples were obtained at the subjects home the morning after an overnight fast and were collected into tubes containing 0.1% EDTA. Samples were stored at 4°C in a cooler with ice packs and transported to the field workstation within 4 hours, where blood was centrifuged at 2500 rpm for 20 minutes at 4°C to separate the plasma from white and red blood cells. All samples were separated into aliquots and stored at -80°C, and within 6 months, they were transported on dry ice for analysis at the Harvard School of Public Health.
Laboratory Analysis
DNA was extracted with a Qiagen blood kit at the average genomic
DNA concentration of 250 µg/mL. Isolated DNA was genotyped by
polymerase chain reaction, followed by restriction endonuclease
digestion as described.9 The
PON1192 and
PON155 genotypes were identified by
cleavage with AlwI and NlaIII (New England
Biolabs), respectively, at 37 C° for 4 hours. The products were
then run on a 10% polyacrylamide gel (45 mA current per gel)
and stained with ethidium bromide. Allele frequencies were
estimated by the gene-counting method. Plasma triglyceride,
cholesterol, and HDL cholesterol levels were
assayed with enzymatic reagents (Boehringer-Mannheim). In our
laboratory, cholesterol measurements are standardized
according to the program specified by the Centers for Disease Control
and the National Heart, Lung, and Blood Institute.
Statistical Analysis
The 492 cases and 518 controls for whom there was complete
genotype information (93% and 98%, respectively, of the total
study population) were included in the analysis, which was
performed with software from Statistical Analysis Systems.
After the data had been checked for errors, outliers, and
distributions, crude means and frequencies for health characteristics
and potential confounders were compared by using 2-sided t
tests and the
2 test. Triglyceride
values were normalized by loge transformation,
and data are presented as a geometric mean±approximate SD.
The presence or absence of the
PON1192Arg and
PON155Leu alleles was used to define 2
groups for the gene effect, with the
PON1192Gln-Gln and the
PON155Met-Met genotypes used as
reference categories. Multiple nonconditional logistic regression was
used to calculate odds ratios (ORs) with 95% CIs for case status. The
presence of the PON1192Arg and
PON155Leu alleles was compared
with their absence. The distribution among controls of continuous
variables (income, physical activity, waist-to-hip ratio, and
triglyceride, HDL cholesterol, and total
cholesterol levels) was used to compute quintile categories
that were included in the multiple logistic regression models as
covariates. The presence of diabetes, hypertension, and angina was
compared with the absence of these diseases (reference). Subjects who
smoked
1 cigarette per day were defined as current smokers and were
compared with past smokers and those who had never smoked grouped
together (referent category).
The first model included the polymorphisms of
PON1192 and
PON155 and the covariates age, sex, and
area of residence (urban, periurban, or rural). Two additional models
also included smoking, income, physical activity, waist-to-hip ratio,
diabetes, hypertension, angina, and triglyceride,
cholesterol, and HDL cholesterol levels. Data
for covariates are presented for the highest compared with the
lowest quintile, for the presence compared with the absence of disease,
and for smokers compared with nonsmokers. The cut points for the lowest
versus highest quintiles among covariates were
$192 and
$871 for
monthly income,
1.14 and
2.33 kcal ·
kg-1 · h-1 for
physical activity,
0.88 and
1.00 for waist-to-hip ratio,
128 and
263 mg/dL for triglyceride,
33 and
49 mg/dL for HDL
cholesterol, and
128 and
263 mg/dL for total
cholesterol. All covariates were also tested for their
potential as effect modifiers. Because these analyses revealed
a significant interaction between the
PON1192 polymorphism and smoking
status, the effect of the PON1192
polymorphism was investigated in additional analyses in
smokers and nonsmokers separately. We also examined the
PON1192 polymorphismsmoking
interaction in a model of 4 groups in which the referent category,
nonsmokers with PON1192Gln-Gln, was
compared simultaneously with nonsmokers with
PON1192Arg, smokers with
PON1192Gln-Gln, and smokers with
PON1192Arg. Values of P<0.05 (2-sided)
were the mark of statistically significant differences.
| Results |
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General characteristics in MI cases and randomly selected controls by
PON1 genotype are presented in Table 2
. Waist-to-hip ratio, smoking, and
history of diabetes, hypertension, and angina were significantly higher
in cases than in controls regardless of genotype. In both
PON1 genotypes, compared with controls, cases had
higher triglyceride concentrations, lower HDL
cholesterol concentrations, and similar total
cholesterol concentrations. There was no significant
difference between genotypes for any parameter
within cases or controls.
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Table 3
shows the ORs for the presence
compared with the absence of the PON1192Arg
and PON155Leu alleles. The presence of
PON1192Arg was associated with an increased
risk of MI (OR 1.36, CI 1.06 to 1.75). This association did not change
and remained statistically significant in a
multivariate model that included nonlipid risk factors.
The addition of an adjustment for lipid risk factors strengthened the
association (OR 1.51, CI 1.13 to 2.03). In the same models, the
PON155Leu polymorphism was not
associated with risk of MI (OR 1.12, CI 0.87 to 1.44). Among the
covariates, higher income and HDL cholesterol were
associated with lower risk of MI. Smoking, high plasma
triglyceride levels, and history of diabetes, hypertension,
and angina were associated with an increase in the risk of MI. Because
of the potential for sex differences to confound the data
analysis, we repeated the analysis in males only. The
results for men were similar to those for the whole population, by
univariate or multivariate
analysis. For PON1192Arg, the OR
was 1.63, with the CI 1.17 to 2.27; for
PON155Leu, the OR was 0.98, with the CI
0.70 to 1.37 (adjusting for covariates in model 3).
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An association between PON1192Arg and MI
(Table 4
) was evident only among
nonsmokers (OR 1.64, CI 1.19 to 2.26) compared with smokers (OR 0.89,
CI 0.58 to 1.38), and it was strengthened by an adjustment for lipid
and nonlipid risk factors. The interaction between
PON1192 and smoking status was
statistically significant (P=0.04). No association between
PON155Leu and MI was detected in smokers or
nonsmokers. Because stratification by smoking changed the sex
distribution, we repeated the analysis in men only. The results
for men were similar to those for the entire population: OR 1.90 (CI
1.29 to 2.80) for PON1192Arg and OR 1.09
(CI 0.74 to 1.60) for PON155Leu.
|
The Figure
shows the association
between PON1192Arg and risk of MI when
nonsmokers with the PON1192Gln-Gln
genotype were used as the referent category. Smokers had an
increased risk of MI regardless of their
PON1192 genotype: OR 2.66 (CI 1.75
to 4.05) for PON1192Gln-Gln and OR 2.60 (CI
1.72 to 3.9) for PON1192Arg. The
association between PON1192Arg and risk of
MI was evident only among nonsmokers, although the magnitude of this
effect, OR 1.52 (CI 1.08 to 2.15), was considerably smaller than the
effect of smoking on risk of MI.
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| Discussion |
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Conflicting results are also found for the PON155 polymorphism. One study in French diabetic patients found a significant association between the PON155 polymorphism and coronary disease.26 The PON155 polymorphism was not associated with risk of MI in the present study and in one previous report.21
The mechanism mediating the association between the PON1192Arg allele and an increased risk of MI is not known. In vitro studies show that HDL from carriers of the PON1192Arg allele is less effective in decreasing the accumulation of LDL lipid peroxides.29 30 Perhaps this reduction in LDL protection (or in some other yet-to-be-identified activity conferred by PON1) explains the atherogenic effect of PON1192Arg observed in the present and other studies.16 20 22 24 32 33
HDL may play a significant role in the effect of PON1 on coronary disease. HDL cholesterol levels and paraoxonase protein levels are significantly correlated.45 In one study, the PON1192Arg allele was associated with lower HDL cholesterol levels,46 but this was not confirmed in other studies.35 45 47 We did not find an association between PON1 polymorphisms and HDL cholesterol levels or a significant interaction between HDL cholesterol and PON1 (data not shown).
A significant interaction between smoking status and genotype revealed that the presence of PON1192Arg was associated with a 64% increase in the risk of MI among nonsmokers in the present our study. There was no evidence of this association in smokers. It is possible that these results can be explained by differential survival among smokers. On the other hand, there is a biological basis for these results. Cigarette smoke extract decreases paraoxonase activity against nonphysiological substrates,48 and it may also reduce PON1 activities that are involved in cardioprotection. Thus, the deleterious effects of cigarette smoke may equalize or outweigh the differences in potentially positive enzyme activities conferred by the PON1 genotype. However, because we did not measure PON1 activity in the present study, we cannot determine whether this is the mechanism responsible for our results.
Further work is needed to clarify the mechanisms underlying the gene-environment interaction identified in the present study and to uncover other environmental factors that modify the effect of the PON1 genotype on coronary disease.
| Acknowledgments |
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| Footnotes |
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Received January 10, 2000; accepted May 22, 2000.
| References |
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