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fact
sheet
Apolipoprotein
E Polymorphism and Cardiovascular Disease
June
E. Eichner
University of Oklahoma
HuGE Review
Published
December 11, 2002
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Gene |
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Apo
ε is a member of the apolipoprotein gene family, a group of genes that
serve a variety of functions related to lipoprotein metabolism.
Apo ε is located at chromosome 19q13.2 and is closely linked
to the apo C-l/C-II gene complex.
It consists of four exons and three introns spanning 3597
nucleotides (1). The transcript produces a 299
polypeptide (2).
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Prevalence of
Gene Variants |
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The
structural gene is polymorphic with three common alleles-- ε2, ε3,
and ε4-- that code for three isoforms of the protein, known as E2,
E3, and E4.
Of these variants, apo ε3 shows the highest allelic frequency
(> 60%) in all populations studied (3). The variants
of the protein of apo E, E2, and E4, each differ from the wild type E3 by
one amino acid.
Crude estimates are that E2 is carried by 3%-20% of a population
pool and E4 by 20%-40%, depending on the racial or ethnic heritage of the
population studied (3).
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Disease
Burden |
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The
apo ε polymorphism has functional effects on lipoprotein metabolism
mediated through the hepatic binding, uptake, and catabolism of lipid
particles, i.e., chylomicrons, chylomicron remnants, very low density
lipoprotein, and high density lipoprotein
subspecies.
Apo ε contributes to variability in normal cholesterol levels
in populations (3).
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Interactions |
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The
major effect of genetic variation at this locus is its influence
on cholesterol levels, one of the major risk factors for cardiovascular
disease (CVD), particularly coronary artery disease.
With reference to cholesterol effects from the ε3 allele,
ε4 is associated with higher total and low density lipoprotein
cholesterol levels and ε2 with lower levels.
The cholesterol lowering effect of ε2 tends to be greater
than the cholesterol raising effect of ε4.
In total, the
contribution of this gene to cholesterol variability based
on a variety of populations that have been studied is no more than
10%.
Diet and other genes contribute to each individual’s cholesterol
level, as well as to the population’s average level.
Many studies have looked at interactions with variants of
this gene as possible modifiers of other cardiovascular risks, such
as high- and low-fat diets and active vs. sedentary lifestyles.
Interactions with lipid-lowering medications also have been
investigated in relation to apo ε.
In addition, this gene has been studied as a possible risk
factor for other diseases.
The major finding in this regard is that one allele in particular
(ε4) is a risk factor for Alzheimer disease in some populations.
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Laboratory
Tests
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Genotyping
for apo ε is available both for clinical purposes and for laboratory
research. Several techniques have been used to determine an individual’s
genotype, but most involve amplification of genomic sequences containing
polymorphic sites. The test is offered commercially.
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Population
Testing |
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Homozygozity
for ε2 has paradoxically been associated with type III
hyperlipoproteinemia (1-5 per 5,000 persons) and
has been used as a diagnostic criterion for this disease; however,
this is a relatively rare disease, and not every ε2 homozygote has
this lipid disorder.
When applied to screening the general population for coronary
artery disease, with the exception of type III hyperlipoproteinemia, apo
ε genotype is not a sensitive screening test (3).
This means that it is not useful for identifying individuals with
coronary artery disease. Furthermore, a high cholesterol level does not
automatically mean one carries the ε4 genotype, although a lower
cholesterol level is more frequently associated with carriers of the
ε2 genotype.
Individual or population screening for apo ε
does not give more clinically relevant information with respect to
CVD than does a lipid profile.
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References |
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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 1985;71:144-6.
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Rall
SC, Weisgraber KH, Mahley RW. Human apolipoprotein E: the complete amino
acid sequence. J Biol Chem 1982;257:4171-8.
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Eichner
JE, Dunn ST, Perveen G, et al. Apolipoprotein E polymorphism
and cardiovascular disease: A HuGE Review.
Am J Epidemiol 2002;155:487-95.
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Web sites |
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http://genome.gov
http://www.nhlbi.nih.gov/
http://www.framingham.com/heart/timeline.htm
http://www.lipidsonline.org/slides/slide01.cfm?q=apo+E
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