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Lipids and Lipoproteins |
1
Laboratoire du Centre de Médecine Préventive and
2
Département statistiques, 2, avenue du Doyen Jacques Parisot, 54500 Vandoeuvre-lès-Nancy, France.
3
Université Henri Poincaré, 54000 Nancy,
France.
a Address correspondence to this author at: Laboratoire du Centre de Médecine Préventive, 2, avenue du Doyen Jacques Parisot, 54500 Vandoeuvre-lès-Nancy, France. Fax 33 (0)3 83 44 87 21; e-mail Gerard.Siest{at}cmp.u-nancy.fr.
| Abstract |
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| Introduction |
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2,
3, and
4. Apo E3 is the
most common isoform, with a prevalence of ~77% in Caucasian
populations. Apo E4 is the second most common isoform (~15%), and
apo E2 is the rarest (~8%). Apo
E polymorphism is functional and influences a variety of
physiological and pathological processes. Its influence on serum
concentrations of total cholesterol (TC) and LDL-cholesterol and the
concentration of apo B is well known (1)(2)(3)(4)(5). Apo E
polymorphism has also been implicated in the etiology of several
diseases: cardiovascular disease, neurodegenerative diseases such as
Alzheimer's disease, and many others (6). In addition, this polymorphism strongly influences the apo E concentration (6), and there is growing evidence that variations of the apo E concentration have a direct influence on metabolic processes. Moreover, it has been suggested that both the apo E concentration and apo E genotype play an important role in lipoprotein metabolism (7)(8). Increased concentrations have been reported in patients with familial dysbetalipoproteinemia (9). Published physiological apo E concentrations vary between 30 and 250 mg/L (6). One of the main reasons for these discrepancies could be the variety of methods and calibrators used for apo E measurements. As in the example of apo AI and apo B, the use of a common reference material should improve the consistency of results. Another important reason for differences in apo E concentrations could be the sample population studied. Yet another reason for the discrepancies could be the effect of biological factors.
The assessment of apo E concentration as a marker of cardiovascular
risk requires the knowledge of its biological variations and of clearly
defined reference limits and specific decision limits. The aims of this
study were to identify the most important causes of biological
variation in apo E concentrations, to determine serum apo E reference
limits for the most frequent genotypes on a well-selected population
sample belonging to the STANISLAS cohort study, and to estimate the
reference values for the other apo E genotypes, using a translation
factor and the
3/
3 genotype as a reference.
| Materials and Methods |
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The factors studied that could potentially affect the serum apo E concentration were apo E polymorphism, age, body mass index [BMI; calculated according to the Quetelet's formula, weight (kg)/height (m)], waist-to-hip ratio (WHR), alcohol and tobacco consumption, lipid-lowering drugs, oral contraceptive use, hormone replacement therapy, menopause, and puberty. For sexual maturation, the puberty variable was divided into three classes in males (before, during, and after puberty) and two classes in females (before and after menarche).
blood samples
Blood was collected by venipuncture after overnight fasting,
either in Vacutainer Tubes containing EDTA for DNA preparation or in
Vacutainer Tubes containing a gel for serum separation (Becton
Dickinson). Blood was centrifuged promptly at 1000g for 15
min at room temperature for serum separation and buffy coat
preparation. The sera and buffy coat were frozen in liquid nitrogen
until analysis or the extraction of DNA.
analytical methods
DNA extraction was performed according to the method of Miller et
al. (11). Apo E genotype was determined by PCR amplification
and subsequent digestion with the restriction enzyme HhaI as
described by Hixson and Vernier (12). Aspartate
aminotransferase (EC 2.6.1.1), alanine aminotransferase (EC 2.6.1.2),
-glutamyltransferase (EC 2.3.2.2), TC, and triglycerides (TGs) were
measured in fresh serum using established enzymatic methods
(Merck) on an AU5000 automated analyzer (Olympus Merck). Serum
lipoprotein(a), apo AI, and apo B were determined on a Behring
automated nephelometer (Behring). Serum HDL-cholesterol and
apo E were measured on a Cobas-Mira analyzer (Roche Diagnostics). Serum
apo E concentrations were determined by immunoturbidimetry, using a kit
from Daiichi (Apo E Auto"Daiichi," reference 114861) and according
to the manufacturer's recommendations (13). Calibration
curves were obtained by serial dilution of a serum calibrator (Daiichi
High Level Standard, reference 125799; target value, 105 mg/L). The
detection limit of the method was 6.3 mg/L with an upper limit of 100
mg/L. Sera were analyzed without pretreatment and diluted in
double-distilled water when the apo E concentration exceeded 100 mg/L.
Control sera (lyophilized Daiichi Control and pool sera) were included
in each series of measurements. The within-series imprecision of apo E
measurements was tested on three different serum pools freshly prepared
and stored at 4 °C; it varied from 2.8 to 3.4%. The day-to-day
reproducibility was estimated to be 3.4% on a pooled serum (stored
frozen at -20 °C). For the commercially available Daiichi control
serum (stored at 4 °C), the reproducibility was 4.2% (one month)
and 7.0% (12 months).
statistics
Statistical analyses were performed using
BMDP® statistical software (University of California,
Los Angeles, CA) and using log-transformed values for serum apo E
concentrations [Ln (apo E)]. We excluded 156 subjects with missing
data and/or pathological values according to the following criteria:
abnormal liver metabolism as defined by increased enzyme activities
[alanine aminotransferase >200 U/L (5 subjects); aspartate
aminotransferase >200 U/L (14 subjects);
-glutamyltransferase >300
U/L (1 subject)], abnormal lipid profiles [TGs >10 mmol/L (3
subjects), and TC >11 mmol/L (1 subject)] and pregnancy (one woman).
All statistical analyses were conducted separately on fathers, mothers,
sons, and daughters (14)(15). In the first step,
unidimensional comparisons were performed by a one-way ANOVA to assess
the degree of significance of the main biological factors on the serum
apo E concentration. Significant factors were next introduced in a
multiple regression analysis that was used to quantify the
relationships between the apo E concentration and the biological
factors. The significant factors served to define exclusion and
partition criteria for selection of the reference sample population.
The following variables were included in the regression: apo E
polymorphism, age, BMI, WHR in parents only, puberty, oral
contraceptive use, and the five genotype groups (
2/
2,
2/
3,
2/
4,
3/
4, and
4/
4); the
3/
3 genotype served as
reference. Interaction testing involving genotypes with BMI, WHR, age,
puberty, and oral contraceptive use was performed; however, the
interaction effects increased by <1% the explained Ln (apo E)
variability in each group, and the differences were not statistically
significant (P >0.05). Finally, we estimated the 2.5th,
5th, 50th, 95th, and 97.5th percentiles of the apo E distribution in
each reference group.
reference samples
Taking into account the results of the regression analyses,
we excluded subjects with BMI values >30 for parents and >20 (511
years), >23 (1114 years), >26 (1417 years), and >29 (1726
years) for children (16); we also excluded adults with WHR
values >1 for fathers and >0.9 for mothers. The effect of oral
contraceptive intake on the serum apo E concentration was adjusted by
correcting values in women taking oral contraceptives by the regression
coefficients obtained in the multiple regression analysis. Percentiles
of apo E distribution were directly estimated in the
3/
3,
3/
2, and
3/
4 genotypes. Reference values for Ln (apo E) in
genotypes other than
3/
3 were derived from those of the
3/
3
group by a translation coefficient estimated from the regression
analysis. Reference values for Ln (apo E) were then converted to
reference values of apo E by exponential transformation.
| Results |
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distribution of apo e values
In the overall sample (n = 4284 subjects), the serum
concentration of apo E ranged from 16 to 169 mg/L. The distribution of
apo E in each subgroup is shown in Fig. 1
, with mean and SD values as described in Table 1
. All
distributions were skewed, with skewness coefficients varying from 2.47
in fathers to 0.67 in daughters. A logarithmic transformation was
therefore performed. In parents, the fathers had a mean apo E
concentration greater than the mothers (49.7 ± 15.9 mg/L vs
44.7 ± 11.1 mg/L). In children, the means and SD values were
44.5 ± 11.1 mg/L in sons and 47.5 ± 11.7 mg/L in daughters.
The total interindividual biological variation of serum apo E,
including analytical variation, was ~26% in mothers, 25% in sons
and daughters, and 32% in fathers.
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biological factors influencing apo e concentration
The ANOVA showed that alcohol and tobacco consumption,
lipid-lowering drugs, hormonal replacement therapy, and menopause had
no effect on serum apo E concentrations in this sample population
(P >0.10). In contrast, age, BMI, WHR, puberty, and
oral contraceptive intake had a significant influence on the serum apo
E concentration (P <0.01 to P <0.001). Thus,
these variables were included with apo E genotypes (using
3/
3
genotype as the reference) as explanatory variables in the multiple
regression analysis. Table 2
shows the respective regression coefficients and SDs for each
variable found to be significant (P <0.05). These
biological indices explained 1932% of the Ln (apo E) variability. In
children, Ln (apo E) decreased with age only in males (P
<0.05) and with puberty in both sexes (P <0.01). In adults
26- to 56-years-old, age did not affect the Ln (apo E) value. We
decided therefore to present apo E reference values by age in children
and without age stratification in adults.
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Oral contraceptive use decreased the Ln (apo E) value in mothers and in daughters (P <0.001) with respective regression coefficients of -0.150 ± 0.017 and -0.191 ± 0.022. For the estimation of the apo E reference values, we adjusted Ln (apo E) values for oral contraceptive intake, using the previously determined regression coefficients. Because BMI and WHR significantly (P <0.01 to P <0.001) affected apo E concentrations, we excluded subjects with increased values (see Materials and Methods).
Polymorphism of apo E was the most important factor known to modify
serum apo E concentrations. When the
3/
3 genotype was used for
comparison, multiple regression analysis gave the mean deviation for
each genotype in regard to the Ln (apo E) values observed in the
3/
3 group. As expected in fathers, for example, the most
important positive deviation was observed for the
2/
2 group
(0.878 ± 0.125). The deviations for the
3/
2 group and for
the
2/
4 group were 0.211 ± 0.024 and 0.126 ± 0.059,
respectively. For
3/
4 subjects (-0.034 ± 0.021) and for
4/
4 subjects (-0.038 ± 0.065), negative deviations were
obtained.
We repeated the regression analysis with no more than one child of each sex from each family. No bias was observed in the age structure, the genotype proportions, means, and SDs for all of the variables tested, and the regression coefficients were not statistically different. Considering the small discrepancies between the two models of regression, we decided to present the first model, which included more subjects.
reference values of serum apo e concentration
The reference sample represented 3956 subjects, i.e., 92.3%
of the overall sample. Among them, 2434 individuals carried the
3/
3 genotype and 1522 carried a different genotype. One
hundred and seventy-two subjects were excluded because of increased BMI
and/or WHR values. The characteristics of the reference sample are
presented in Table 3
; these characteristics were different from the overall sample
for lipids, enzymes, and BMI values, with a less important dispersion.
The reference values for serum apo E concentrations in the three most
common genotype groups are summarized in Table 4
. The reference values for serum apo E concentrations in the
three rare genotype groups are shown in Table 5
. These reference values were estimated from those of the
3/
3 genotype, using the regression coefficients listed in Table 2
. Regarding age groups and median values, males exhibited lower apo E
values than females until 1426 years of age. In contrast, in adults
older than 26 years, men had higher apo E values than women regardless
of genotype. In both sexes, the lowest apo E concentration was found
between the ages of 14 and 25. The effect of age, related to puberty in
boys and girls, remained in girls even after the adjustment for oral
contraceptive intake was made. For more information, we also examined
apo E reference values between individuals 511, 1114, 1417, and
1726 years of age in children carrying the
3/
3 genotype. When
these values were compared with the reference values mentioned on Table 4
, we observed a decrease of apo E values estimated at the 50th
percentile in children aged 11 to 25 years: 43.8 mg/L to 39.7 mg/L for
boys and 48.9 mg/L to 45.2 mg/L for girls.
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| Discussion |
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The total interindividual variation of serum apo E concentrations in the overall sample population studied is considerable (2532%) but similar to that reported for other apolipoproteins (21)(22)(23).
Biological factors that significantly modified serum apo E
concentrations were, in decreasing order of importance, apo E genetic
polymorphism (mainly
2/
2 genotype), WHR in adults, oral
contraceptive intake, puberty in both sexes, BMI, and age in sons.
Age and sex significantly influence serum apo E concentrations. We found lower apo E values in boys than in girls. However, in adults over 26 years of age, the apo E concentration was higher in men than in women. This interaction of age and sex on apo E concentrations is similar to that observed for apo B, with lower values in males than in females until 2535 years of age and higher apo B concentrations in older males (21). These data could explain the different results published in the literature, i.e., higher apo E concentrations in women (9)(24), in men (25), or no difference (26). The decrease of apo E values with age in children and the lack of age variation in adults are in agreement with other studies (27)(28).
In addition to the age effect, puberty significantly decreases serum apo E concentrations in both sexes. This independent impact of puberty on apo E concentrations has not been reported previously. This finding could be compared in girls with the diminishing effect of estrogens or the use of oral contraceptives on serum apo E concentrations as described by others (17)(29)(30)(31). In the overall sample studied, the decrease in apo E concentrations in women taking oral contraceptives leads to a shift of the apo E distribution towards lower values in mothers and in daughters.
Alcohol and tobacco consumption did not significantly modify apo E concentrations. However, the consumption of alcohol and tobacco was relatively moderate in this group. It has been reported that alcohol abuse increases apo E concentrations (32)(33), although moderate drinking (34) and smoking do not seem to influence the concentration of apo E (27)(33).
Increased BMI and WHR values were associated with increased apo E
concentrations, in agreement with others
(17)(35). BMIs and/or WHRs were positively
correlated with TGs, LDL-cholesterol, and apo B and negatively
correlated with HDL-cholesterol and apo AI
(17)(35). Relationships between body mass, TGs,
and some hepatic enzymes, especially alanine aminotransferase and
-glutamyltransferase, were described some years ago
(36)(37).
-Glutamyltransferase activity is
often increased in non-insulin-dependent diabetics. This observation
could be related in part to the modified lipid metabolism in patients
with insulin-resistant syndrome (unpublished results).
The apo E genotype is the most important factor affecting its serum
concentrations. Between 6% and 20% of the total variability of the
apo E concentration has been attributed to its polymorphism by several
authors
(2)(5)(27)(38)(39).
Results from a recent study conducted in the same cohort provided
evidence that apo E variability is determined by its genetic
polymorphism and clearly demonstrated the nonadditive effects of
2
and
4 alleles (17). As expected, the highest apo E
concentrations are found in the
2/
2 genotype and the lowest in
the
4/
4 genotype (see Table 2
).
Mean deviations estimated by multiple regression analysis on Ln (apo E)
are in agreement with those described in previous studies [for review
see (6)]. The great impact of polymorphism on apo E
concentrations justifies the selection of a homogeneous sample
regarding its genotype for the establishment of apo E reference values.
Because the
3/
3 genotype occurs most frequently, we chose to
define apo E reference values using
3/
3 subjects as the baseline
to estimate reference values in the three less frequent genotypes by
applying a translation coefficient to Ln (apo E). In addition, this
genotype is not reported to be associated with pathological states.
Apo E polymorphism not only influences apo E concentration but also
cholesterol and apo B concentrations, which are cardiovascular risk
factors. The concentration of apo E has been shown to modulate lipid
metabolism (8), and it was suggested that the apo E
concentration, in addition to polymorphism, might become a risk factor
for cardiovascular disease (6)(27). The use of
reference intervals, the knowledge of the main factors causing
variation, and case-control studies to assess relationships between apo
E concentrations and pathological states will help to determine the
relevance of apo E in clinical chemistry. An apo E concentration in a
nongenotyped individual can first be compared with apo
3/
3
reference limits. If it is outside the 95% reference interval, a
genotype could be determined. An apo E concentration in a genotyped
individual should, of course, be compared with its corresponding
genotype reference interval. However, the question remains open about
the predictive value of an apo E concentration outside the reference
limits in each genotype.
In conclusion, we have shown that the serum apo E concentration is
affected by its polymorphism and by most of the current biological
factors of variation: age, gender, BMI, WHR, puberty, and oral
contraceptive use. The most important sources of variation in the
studied population were the
2 allele and WHR. Age was the least
important. The large number of subjects and the amount of information
collected in the STANISLAS cohort study allowed us to quantify the
different observed variations. For the first time, we produced
genetically derived reference values for serum apo E concentrations. We
therefore recommend, to interpret results of apo E measurements, taking
into account all possible source of variability. Consequently, the use
of reference limits according to the apo E genotype is essential for
the interpretation of its concentration.
| Acknowledgments |
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| Footnotes |
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1 Nonstandard abbreviations: apo, apolipoprotein; TC, total cholesterol; STANISLAS: Suivi Temporaire Annuel Non Invasif de la Santé des Lorrains Assurés Sociaux; BMI, body mass index; WHR, waist-to-hip ratio; TG, triglyceride; and Ln (apo E), log-transformed apo E value. ![]()
| References |
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4/
2 in the STANISLAS cohort studydominance of the E2 allele?. Ann Hum Genet 1996b;60:509-516.[ISI][Medline]
[Order article via Infotrieve]
-Glutamyltransferase activity in plasma: statistical distributions, individual variations and reference intervals. Clin Chem 1977;23:1023-1028.
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