This
paper was published with modifications in Am J
Epidemiol 2002;155(6):487-95 |
Apolipoprotein
E Polymorphism and
Cardiovascular Disease: A HuGE Review
by June E. Eichner,1 S. Terence Dunn2 Ghazala Perveen1 David M. Thompson1
Kenneth E. Stewart1 and Berrit
C. Stoehla1
1Department
of Biostatistics and Epidemiology, College of Public
Health, University of Oklahoma, Oklahoma City, OK.
2Pathology Department, College
of Medicine, University of Oklahoma, Oklahoma City,
OK.
March 15, 2002
(Updated March 19, 2002)
AT-A-GLANCE
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This review examines the association between
the apolipoprotein (apo) gene polymorphism
(or its protein product (apo E)), metabolic
regulation of cholesterol, and cardiovascular
disease. The apo gene is located at chromosome
19q13.2. Among the variants of this gene,
alleles * 2, * 3, and * 4 constitute the common
polymorphism found in most populations. Of
these variants, apo * 3 is the most frequent
(>60%) in all populations studied. The
polymorphism has functional effects on lipoprotein
metabolism mediated through the hepatic binding,
uptake, and catabolism of chylomicrons, chylomicron
remnants, very low density lipoprotein (VLDL),
and high density lipoprotein subspecies. Apo
E is the primary ligand for two receptors,
the low density lipoprotein (LDL) receptor
(also known as the B/E receptor) found on
the liver and other tissues and an apo E-specific
receptor found on the liver. The coordinate
interaction of these lipoprotein complexes
with their receptors forms the basis for the
metabolic regulation of cholesterol. Allelic
variation in apo is consistently associated
with plasma concentrations of total cholesterol,
LDL cholesterol, and apo B (the major protein
of LDL, VLDL, and chylomicrons). Apo has been
studied in disorders associated with elevated
cholesterol levels or lipid derangements (i.e.,
hyperlipoproteinemia type III, coronary heart
disease, strokes, peripheral artery disease,
and diabetes mellitus). The apo genotype yields
poor predictive values when screening for
clinically defined atherosclerosis despite
positive, but modest associations with plaque
and coronary heart disease outcomes. In addition
to genotype-phenotype associations with vascular
disease, the alleles and isoforms of apo have
been related to dementias, most commonly Alzheimer's
disease.
Indexing terms: apolipoproteins E; cardiovascular
diseases; epidemiology; genetics
Abbreviations: apo, apolipoprotein; CHD,
coronary heart disease; CI, confidence interval;
LDL, low density lipoprotein; VLDL, very low
density lipoprotein.
Am J Epidemiol 2002;155:487–95.
Received for publication April 4, 2000, and
accepted for publication September 13, 2001.
Reprint requests to Dr. June E. Eichner, Department
of Biostatistics and Epidemiology, University
of Oklahoma, P.O. Box 26901, Oklahoma City,
OK 73190, june-eichner@ouhsc.edu.
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GENE |
Apolipoprotein (apo) is a member of the
apolipoprotein gene family. Other members
of this multigene family include apo A-I,
apo A-II, apo A-IV, apo C-I, apo C-II, and
apo C-III. The coding regions of these genes
are composed of tandem repeats of 11 codons,
which suggests that they have evolved through
duplications of a primordial gene (1).
The apo gene is located at chromosome 19q13.2
and is closely linked to the apo C-I/C-II
gene complex (2). It consists of four exons
and three introns spanning 3,597 nucleotides
and produces a 299 amino acid polypeptide
(2,3). It is synthesized primarily in the
liver, but other organs and tissues also synthesize
apo E, including brain, spleen, kidneys, gonads,
adrenals, and macrophages (4).
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GENE VARIANTS |
The structural gene is polymorphic
with three common alleles, * 2, * 3, and *
4, producing three isoforms of the protein,
E2, E3, and E4. These isoforms differ in amino
acid sequence at positions 112 and 158. Apo
E3 contains cysteine at 112 and arginine at
158. Apo E2 has cysteine at both positions,
and E4 has arginine at both sites (5). The
apo gene polymorphism also has a strong effect
on the level of its gene product; * 2 is associated
with higher concentrations of apo E and *
4 with lower concentrations (6,7).
While there are rare variants, it is the
polymorphism with its three alleles, * 2,
* 3, and * 4, that has been studied in relation
to cardiovascular disease. From these alleles
arise six phenotypes; their ranking from most
to least common is generally 3/3, 4/3, 3/2,
4/4, 4/2, and 2/2 (8). Table 1 provides gene
frequencies for 11 populations, including
a number of European Caucasian populations
that demonstrate a geographic cline (6,9–18).
Northern Europeans (Finns, Germans) tend to
have higher frequencies (E14–19 percent)
of the * 4 allele than southern Europeans
(French, Italians) (E7–12 percent).
Nigerians, Japanese, and Finns have relatively
low frequencies (E3–4 percent) of *
2. Mexican Americans and American Indians
also have low frequencies (E2–4 percent)
of the * 2 allele. In one group consisting
of nine tribes of South-American Indians (n
=95), no instance of * 2 was reported (19).
Kataoka et al. speculate that the presence
of * 2 in American Indians may be the result
of admixture (11). Table 2 provides genotype
frequencies for these same 11 populations
(6,9–18). References for Tables 1 and
2 were selected from non-diseased population
cohorts to provide gene and phenotype frequencies
based on large and diverse populations for
international and ethnic comparisons.
Function
Plasma lipoproteins are spherical bodies composed
of a nonpolar lipid core, primarily triglycerides
and cholesterylesters, with an external layer
of phospholipids and apolipoproteins (20).
Apolipoproteins, the only protein component
of lipoproteins, combine with free cholesterol,
phospholipids, cholesterol esters, and some
triacylglycerols to form lipoproteins. Human
plasma contains about a dozen different apolipoproteins
represented by five main types (A, B, C, D,
and E), some of which are further categorized
into subtypes (e.g., A-I, -II, and -IV; and
C-I, -II, and -III) (7). Apo E, similar to
other apolipoproteins, helps to stabilize
and solubolize lipoproteins as they circulate
in the blood. In general, the role of apolipoproteins
in lipid metabolism includes maintaining the
structural integrity of lipoproteins, serving
as cofactors in enzymatic reactions, and acting
as ligands for lipoprotein receptors. Apo
E is critical in the formation of very low
density lipoprotein (VLDL) and chylomicrons.
The various apo E isoforms interact differently
with specific lipoprotein receptors, ultimately
altering circulating levels of cholesterol.
Apo E from VLDL, chylomicrons, and chylomicron
remnants binds to specific receptor cells
in the liver. Carriers of the * 2 allele are
less efficient at making and transferring
VLDLs and chylomicrons from the blood plasma
to the liver because of its binding properties.
By contrast, carriers of the * 3 and * 4 alleles
are much more efficient in these processes.
While apo E4 and E3 bind with approximately
equal affinity to lipoprotein receptors, apo
E2 binds with less than 2 percent of this
strength (7). Thus, compared with carriers
of the * 3 or * 4 allele, carriers of the
* 2 allele are slower to clear dietary fat
from their blood (21). The difference in uptake
of postprandial lipoprotein particles results
in differences in regulating hepatic low density
lipoprotein (LDL) receptors, which in turn
contributes to genotypic differences in total
and LDL cholesterol levels (6,8,11,12,22,23).
High levels of LDL cholesterol have been
associated with increased risk of coronary
heart disease (CHD). Sing and Davignon demonstrated
that 8.3 percent of the total variance for
LDL cholesterol is accounted for by the apo
E gene locus (24). However, subsequent studies
estimated variances of as low as 1.0 percent
(12). Apo contributes more to normal cholesterol
variability than any other gene identified
thus far in cholesterol metabolism (24).
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Table 1
Relative frequencies of the most common alleles for the gene locus coding for apolipoprotein. |
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Table 2
Frequencies of apolipoprotein E phenotypes in the 11 populations referred to in Table 1. |
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DISEASES |
Cardiovascular disease claims the lives
of about 1 million people annually in the
United States, accounting for approximately
1 in every 2.4 deaths (25). Roughly 22 percent
of the country's residents have some form
of cardiovascular disease, which includes
CHD, stroke, arrhythmias, diseases of the
arteries, including peripheral artery disease,
bacterial endocarditis, cardiomyopathy, congenital
heart defects, congestive heart failure, rheumatic
heart disease, and valvular heart disease.
Women have lower odds for developing cardiovascular
disease (1 in 10) before age 60 years than
men do, but their risk increases significantly
after the protective effect of estrogen is
lost as they pass through the climateric.
It has been estimated that about half of all
deaths in developed countries are caused by
cardiovascular disease (26).
CHD, which accounts for 1 of every 4.7 deaths
in the United States (25), has been associated
with behavioral, genetic, and environmental
risk factors in epidemiologic investigations.
In 1981, Hopkins and Williams published a
list of 246 factors associated with CHD (27).
The primary risk factors linked with CHD,
according to the American Heart Association,
are cigarette smoking, elevated total and
LDL cholesterol levels, low high density lipoprotein
cholesterol level, hypertension, sedentary
lifestyle, obesity, and diabetes mellitus
(28). Other arterial diseases, such as thrombotic
stroke and peripheral artery disease, are
associated with these risk factors, although
the degree of impact varies by disease. The
cardinal risk factor for stroke is hypertension,
although others have also been associated
positively (25). Two important risk factors
for peripheral artery disease are smoking
and diabetes (29). Apo is not considered a
major risk factor for any of these vascular
disorders.
Epidemiologic studies have investigated the
direct impact of apo on CHD, as well as its
impact on cholesterol levels. These studies
are distinguished by their focus: 1) the apo
polymorphism as an independent risk factor
for disease and 2) its contribution to cholesterol
and lipoprotein levels. One study, addressing
the contribution of apo to CHD, reported that
E6 percent of the variation in risk for CHD
in North America can be attributed to this
locus (30). Another study of middle-aged men
from nine populations estimated a E40 percent
increased risk for CHD mortality for * 4 carriers
compared with * 3* 3 genotype or * 2 carriers
(31). Some studies have also suggested that
* 4 carriers are particularly prone to developing
disseminated coronary lesions or to have an
increased risk of death from CHD (32–35).
It has been proposed that the biochemical
mechanism is related to dysfunction of the
E4 isoform in lipoprotein metabolism and an
increased concentration of serum cholesterol
and triglycerides (8,36,37). Studies from
Finland, Scotland, and northern Ireland have
shown that populations with higher cholesterol
levels and higher CHD mortality rates also
have a higher frequency of the * 4 allele
(31,38). Other studies have also associated
the * 2 allele with increased CHD risk (32).
An association between apo * 2/2 and type
III hyperlipoproteinemia has been known for
decades (39). This disorder is characterized
by increased cholesterol and triglyceride
levels, the presence of b-VLDL (cholesterol-enriched
remnants of intestinal chylomicrons and hepatic
VLDL), xanthomas, and premature vascular disease,
both CHD and peripheral artery disease (40).
Overt hyperlipoproteinemia III occurs with
a frequency of 1–5 per 5,000, whereas
homozygosity for * 2/2 occurs with a frequency
of 0.5–1.0 per 100 in Caucasian populations
(8, 40). Thus, this genotype contributes to
the hyperlipoproteinemia III phenotype without
being its sole cause.
Strains of apo -deficient and apo -overexpressing
transgenic mice have been developed to increase
our understanding of apo in disease processes.
Apo -deficient mice accumulate VLDL and remnant
particles in plasma and develop atherosclerosis,
even on lowfat diets (41). Increased expression
of human apo * 3 in transgenic mice results
in hypertriglyceridemia (42).
In addition to being studied in association
with cardiovascular disease outcomes and intermediate
phenotypes, the apo polymorphism has been
investigated as a risk factor for other chronic
diseases, such as diabetes mellitus, b thalassemia,
rheumatoid arthritis, Alzheimer's disease,
Parkinson's disease, schizophrenia, and psychosis
(43–51).
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ASSOCIATIONS |
There is a wealth of literature on the
apo E polymorphism and attempts to associate
this locus with numerous phenotypes; most
of it is related to cardiovascular disease
or cardiovascular disease risk factors. The
citations that follow were selected to give
a balanced, although not exhaustive view of
genotypephenotype studies.
Apo has been one of the most thoroughly studied
genetic polymorphisms, particularly for its
effects on lipid profiles and CHD risk. In
comparisons made to determine risk, the homozygous
* 3/3 genotype is used as the referent. In
general, * 2 lowers total cholesterol levels
and * 4 raises them. The * 2 cholesterol-lowering
effect is 2–3 times that of the * 4
cholesterol-raising effect. On average, *
2 lowers cholesterol levels by E14 mg/dl and
* 4 raises them by E8 mg/dl (22). This effect
has also been reported in children (52) and
is evident in most populations, despite highly
variable mean concentrations of cholesterol
(22). The gene products of apo seem to function
in a relatively uniform physiologic way in
all populations, despite differences in genetic
backgrounds, diet, and exercise patterns (22).
Various studies of vessel pathology have
been conducted by using postmortem specimens,
angiographic findings, and ultrasound measurements
of intimamedia thickness. In one autopsy study
of young (aged 15–34 years) Caucasian
and African-American males, the apo genotype
accounted for 5.7 percent of the observed
variation in lesions of the thoracic aorta
in Caucasians and 5.9 percent in African Americans
and for 5.9 percent of the variation in lesions
of the abdominal aorta in Caucasians and 7.0
percent in African Americans (53). Adjustment
for cholesterol levels did not appreciably
change these apo genotypic effects. In a study
of the right and left anterior descending
coronary arteries and aortae from 700 male
autopsy cases (Helsinki Sudden Death Study)
ranging in age from 33 to 70 years, Ilveskoski
et al. concluded that apo * 4 is a significant
genetic risk factor for coronary atherosclerosis
in early middle age but that it loses its
importance with age (54). A small, positive
association between carotid intimamedia thickness,
measured by ultrasound, and the * 4 versus
* 3 allele has been documented for asymptomatic,
nondiabetic patients (16). In contrast, the
apo * 3/2 genotype was associated with carotid
artery atherosclerotic disease, after the
contribution of established risk variables
was considered in the Atherosclerosis Risk
in Communities (ARIC) study (55). This association
possibly was attributed to the delayed clearance
of triglyceride-rich lipoproteins for * 2
allele carriers.
Overall, clinical studies of angiography
patients have failed to demonstrate conclusively
a pattern of increased CHD risk for * 4 carriers.
One metaanalysis reported relative odds for
men and women with clinical CHD and angiographic
CHD. The overall odds ratio for CHD risk for
men with the * 4 compared with the * 3 allele
was 1.38 (95 percent confidence interval (CI):
1.22, 1.57); for women, it was 1.82 (95 percent
CI: 1.30, 2.54). Relative odds for angiographic
CHD were less convincing (* 2: odds ratio
= 0.76, 95 percent CI: 0.55, 1.05; * 4: odds
ratio = 1.11, 95 percent CI: 0.88, 1.40) (31).
There is also some suggestion that the apo
* 2 allele may have a protective effect (8,
31); however, despite their lower cholesterol
levels, * 2 carriers are not immune from atherosclerosis.
Figure 1 shows, for Caucasian males, the
apo gene frequencies found in three studies
conducted in the United States. The Framingham
Offspring Study is a community-based study
of the offspring of the original Framingham
cohort (aged 23–77 years) (12). The
Multiple Risk Factor Intervention Trial (MRFIT)
was a multicenter primary prevention trial
of men aged 35–57 years at risk for
CHD (32). The third study examined consecutive
male coronary angiography patients (aged 32–83
years) in Oklahoma (56, 57). Only those patients
with at least one vessel disease were included
in the calculation of gene frequencies in
the last study. Dramatic differences are not
apparent across the spectrum of CHD risk.
Comparison of apolipoprotein (apo) E gene
frequencies determined by protein (apo E)
electrophoresis in three US studies: 1) Framingham
Offspring Study, Framingham, Massachusetts—offspring
of the Framingham Heart Study cohort (n =
1,123 Caucasian men; community based) (12);
2) Multiple Risk Factor Intervention Trial
(MRFIT)—participants selected from the
original cohort involving 22 clinical centers
in the United States (n = 619 Caucasian men;
primary prevention trial of at-risk middle-aged
men) (32); and 3) Oklahoma Angiography Cohort,
Oklahoma City, Oklahoma—coronary heart
disease patients with at least one vessel
disease (n = 505 Caucasian men; hospital based,
1992–1994) (author's (J. E.) data (56,
57)).
A number of studies have examined the frequency
of the apo genotype in fatal CHD cases. The
MONICA (Monitoring of Trends and Determinants
in Cardiovascular Disease) Project, a multinational
study sponsored by the World Health Organization,
monitors trends in cardiovascular mortality
and morbidity and assesses the relation of
these trends to changes in risk factor levels
and/or medical care. The MONICA Project suggests
that an increase of 0.01 in the relative frequency
of the * 4 allele increases the CHD death
rate by 24.5 per 100,000 (31). The authors
of this study also suggest that the geographic
distribution of apo alleles can be used to
predict interpopulation variation in CHD mortality
rates. Gerdes et al. examined the relation
between apo genotype and a major coronary
event or death in 966 Danish and Finnish survivors
of myocardial infarction enrolled in the Scandinavian
Simvastatin Survival Study. After evaluating
5.5 years of follow-up data on these patients,
they concluded that myocardial infarction
survivors carrying the * 4 allele have an
80 percent increased risk of dying compared
with other patients. They also indicated that
the apo genotype did not predict risk of a
major nonfatal coronary event (58).
Studies of centenarians show some survival
advantage associated with the * 2 allele.
Altered frequencies of the apo polymorphism
have been found in the very old compared with
younger persons from the same population (59,60). This finding may be related to both a
slightly reduced risk of cardiovascular disease
and a reduced risk of Alzheimer's disease.
Studies that have investigated stroke risk
and the apo polymorphism have provided mixed
results. Case control studies have reported
increased frequencies of the * 4 and * 2 alleles
among patients with ischemic cerebrovascular
disease compared with controls, while other
studies have shown no difference (61–63).
Apo * 2 and * 4 may impose additional lipid
aberrations on diabetics who have elevated
lipid levels and are at increased risk of
CHD (43–45). One study of diabetic nephropathy
has shown that the * 2 allele is more common
in patients with this complication (45). In
general, there have been fewer studies of
the apo polymorphism among stroke and diabetes
patients, and the results have been less consistent
than those for cholesterol variability.
Risk estimates for apo and Alzheimer's disease
are less equivocal. A metaanalysis obtained
from clinic/autopsy-based studies provides
the following summary odds for Caucasian carriers
of the * 4 allele compared with homozygous
* 3/* 3 carriers: odds ratio for apo * 4/2
=2.6 (95 percent CI: 1.6, 4.0); odds ratio
for apo * 4/3 =3.2 (95 percent CI: 2.8, 3.8);
and odds ratio for apo * 4/4 =14.9 (95 percent
CI: 10.8, 20.6). Summary odds for carriers
of the * 2 allele were as follows: odds ratio
for apo * 2/2 = 0.6 (95 percent CI: 0.2, 2.0)
and odds ratio for apo * 3/2 = 0.6 (95 percent
CI: 0.5, 0.8) (64). The effect of * 4 on risk
was somewhat attenuated among African Americans
and Hispanics, although still present, and
was accentuated among Japanese (64). Lifetime
risk of developing Alzheimer's disease is
15 percent for persons with no family history
of the disease. On the basis of epidemiologic
data and Bayesian statistics, the risk increases
to 29 percent for carriers of one * 4 allele
and is 9 percent for those with no * 4 allele
(65).
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INTERACTIONS |
There are numerous studies of interactions
between apo and possible effect modifiers, such
as diet, age, gender, and habits. Most important
among these studies are those assessing the
interaction between nutrient intake and genotype.
Tikkanen et al. reported that * 4 carriers may
respond more than * 3 and * 2 carriers to a
diet low in total fat (66), and Sarkkinen et
al. showed a greater cholesterol response to
changes in intake of fat and cholesterol among
carriers of the * 4 allele (67). Cobbaert et
al. concluded from their study that the regional
cholesterol differences in subjects from north
and south Belgium, who shared a similar genetic
background, could not be explained by differences
in apo genotype distribution and serum lipoprotein(a)
levels. They indicated that the less favorable
* 2 and * 4 lipid profiles in southerners compared
with northerners might reflect modulation of
the apo gene by particular environments. They
pointed out a well-documented higher intake
of saturated fat and dietary cholesterol in
south compared with north Belgium. On the basis
of this observation, they suggested that the
less favorable fat intake in southerners might
explain the differences in * 4 effects (68).
A metaanalysis conducted by Ordovas et al. also
proposed that the effects of apo genotype might
be modulated via alterations of the amount and
type of dietary fat (69). Other studies have
shown no differential response to changes in
dietary cholesterol when total fat is held constant
or to total and saturated fat when cholesterol
is held constant (70,71). Boerwinkle et al.
showed that, in contrast to dietary saturated
fat, the apo E gene locus did not have a major
effect on the response of lipid levels to increased
dietary cholesterol (70). It has also been suggested
that carriers of the * 2 allele are simply less
sensitive to high levels of dietary cholesterol
(72). Despite conflicting evidence, there appears
to be some modulation of the relation between
apo and plasma cholesterol by fat and cholesterol
intake. In addition to evaluating diet,
studies have been designed to assess interactions
between apo and other genes, apo and behaviors,
and apo and medications. Respective examples
include apo and the angiotensin-converting
enzyme insertion/ deletion polymorphism and
restenosis after coronary angioplasty (73),
apo and variation in physical activity expenditure
(74), and apo and cholesterol response to
lipid-lowering drugs (75).
Gerdes et al. examined whether the beneficial
effects of simvastatin treatment differed
by apo genotype. After providing dietary advice,
they randomized men and women aged 35–70
years with a history of myocardial infarction
or angina, serum total cholesterol concentrations
in the range of 5.5–8.0 mmol/liter,
and serum triglyceride levels of less than
2.5 mmol/liter to placebo or simvastatin groups.
Simvastatin treatment reduced the mortality
risk more in * 4 carriers than in other patients,
although the difference was not statistically
significant for the treatment by genotype
interaction (58). At least two other studies
have examined the influence of the apo E polymorphism
on response to lipid-lowering drug treatments
in patients with combined hyperlipoproteinemia
and familial hypercholesterolemia (76, \77).
Nestel et al. conducted a crossover, randomized
trial to examine the efficacy of simvastatin
and gemfibrozil in patients with combined
hyperlipoproteinemia. Efficacy was noted after
6 and 12 weeks on each treatment for the 66
subjects enrolled. The lipid-lowering responsiveness
was greatest in those with the apo E2 isoform
with both medications (76). Knijff et al.
examined the influence of the apo E polymorphism
on pretreatment plasma lipid levels and on
the response to simvastatin treatment in a
sample of 120 Dutch patients with heterozygous
familial hypercholesterolemia. They found
that the differences in pretreatment lipid
levels were not related to the apo E polymorphism
in these patients. With respect to the effect
of 12 weeks of simvastatin treatment, a reduction
of 33 percent, 38 percent, and 19 percent
(on average) was found in the plasma levels
of total cholesterol, LDL cholesterol, and
triglycerides, respectively. Interindividual
variation in response to simvastatin treatment
was not related to the apo E polymorphism
(77).
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LABORATORY
TESTS |
Clinical and research laboratory
tests for apo generally are concerned with
typing the polymorphism or, less frequently,
determining apo E protein concentrations in
plasma or other biologic fluids. Apo E concentrations
are generally higher in hypertriglyceridemia
than in hypercholesterolemia and are highly
variable in CHD patients and in other pathologies
(7). Concentrations of apo E can be measured
by radioimmunoassay, enzyme-linked immunosorbent
assay, electro- or radial-immunoassay, nephelometry,
or turbidimetry; however, interassay and interlaboratory
comparisons are difficult because of extremely
wide variation in mean values between assay
formats and the lack of standardization of
many of these protocols.
The apo E polymorphism was commonly screened
by using phenotyping methods that detect changes
in electrical charge among the protein isoforms
because of sequence differences in amino acids.
Apo E phenotyping is generally achieved by
isoelectric focusing or two-dimensional electrophoresis.
However, phenotyping is susceptible to occasional
error. Post-translational changes affecting
the charge of the protein are found in some
pathologic conditions, for example, diabetes
(78). Concentrations of apo E are usually
lower in * 4 carriers, giving faint banding
patterns, and, occasionally, a rare variant
has the same charge as a dominant isoform
(7).
By contrast, screening for nucleotide alterations
has become less prone to error. Genotyping
has become relatively simple and inexpensive,
making it the preferable method for analyzing
large populations. Several approaches have
been taken, but all involve amplification
of genomic sequences containing polymorphic
sites. Amplification may entail use of allele-specific
primers or a set of flanking primers, followed
by endonuclease digestion, blot hybridization,
single-stranded conformational polymorphism,
heteroduplex analysis, or sequencing. With
the advent of DNA amplification technologies,
genotyping has replaced phenotyping as the
standard method of determining apo status.
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POPULATION
TESTING |
The sensitivity of * 2 homozygosity in
predicting type III hyperlipoproteinemia exceeds
0.90, and the presence of this genotype is
a diagnostic criterion for type III disease
(79). Genotype specificity is much lower,
however; approximately 5 percent of homozygotes
develop disease, and the positive predictive
value of * 2 homozygosity is quite low (79).
Other factors, such as hypothyroidism, familial
combined hyperlipoproteinemia, or diabetes,
seem to be involved in the full expression
of the disease. Other tests, such as the ratio
between serum apo E and apo B, have been studied.
This ratio is lower in type III patients than
in those without disease. Marz et al. reported
95 percent sensitivity and 88 percent specificity
for detecting type III hyperlipoproteinemia
when a threshold value of 0.09 is used for
the E/B ratio. Because of the low frequency
of this disorder (1–5 per 5,000 persons)
and a low positive predictive value of genotype
testing, population screening is not warranted
(79).
When applied to screening for CHD, the apo
genotype is neither sensitive nor specific.
The diagnostic accuracy of the four most common
apo genotypes (3/3, 4/3, 3/2, and 4/4) from
Eichner's data in patients with clinically
defined CHD was assessed by using receiver
operating characteristic analysis (Figure
2). This study and a similar one (80), although
separated geographically and temporally, yielded
remarkably similar findings. The area under
the receiver operating characteristic curve
(a measure of how frequently the apo genotype
distinguishes between two people, one of whom
has angiographically confirmed CHD and another
who does not) is 0.4914 for Eichner's data
and 0.5090 for Menzel's data (80). These values
indicate that the polymorphism does not distinguish
clinically defined disease.
However, clinically defined disease (E50
percent stenosis) is an imperfect measure
of CHD since the presence of any arterial
stenosis (<50 percent) is considered minimal
or nonobstructive disease and is not normal,
and the arterial lesion may be subject to
hemorrhage or rupture. Lenzen et al. also
investigated the association of the apo polymorphism
with CHD (23). Their data compared survivors
of myocardial infarction and healthy controls.
The area under the receiver operating characteristic
curve computed for the four most common genotypes
from these data is 0.5097, again suggesting
that the apo genotype is not a useful screening
test for CHD (23).
In summary, apo was one of the first polymorphisms
associated with cardiovascular disease to
be studied thoroughly in both health and disease.
It influences lipoprotein metabolism and the
plasma concentration of total cholesterol,
LDL cholesterol, apo B, and apo E and confers
a risk for CHD. The American Heart Association
does not mention the apo E locus as a major
risk factor for CHD but does include family
history as such. While apo E does help to
explain interpopulation rates of CHD mortality,
it does not warrant population screening as
a risk factor. One exception depends on finding
conclusive evidence of a genotypedrug interaction
that might influence the course of disease.
In such a case, screening would involve only
those persons considering using the prescribed
medication.
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ACKNOWLEDGEMENTS |
Oklahoma data were provided by Dr. June
Eichner from a grant (HS2-025) funded by the
Oklahoma Center for the Advancement of Science
and Technology.
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REFERENCES |
- Luo
CC, Li WH, Moore MN, et al. Structure and
evolution of the apolipoprotein multigene
family. J Mol Biol 1986;187:325–40.
- Scott J, Knott TJ, Shaw
DJ, et al. Localization of genes encoding
apolipoprotein CI, CII, and E to the p13->cen
region of human chromosome 19. Hum Genet
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