Oligonucleotide Ligation Assay for Detection of Apolipoprotein E Polymorphisms

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Abstract
Apolipoprotein (apo) E is a protein component of lipoproteins, 50% of which resides in HDL, 10% in LDL, 20% in IDL, and 20% in VLDL cholesterol fractions (1). Apo E binds to the LDL receptor, also termed the B,E receptor, because the receptor accepts both apo B and apo E. Apo E is also thought to bind to a specific chylomicron remnant receptor by virtue of its structural determinants. The heterogeneity in receptor binding of different varieties of apo E is explained by the affinity of different apo E alleles to various receptors. Apo E polymorphisms may be explained by three major alleles: apo Eε2, apo Eε3, and apo Eε4, which are found in 10%, 76%, and 13%, respectively, of the Caucasian population (2). The polymorphisms are due to substitution of a cysteine for an arginine at residue 112 or 158, or at both residues. The apo Eε2 variant has the lowest affinity for the LDL receptor. There is an LDL concentration gradient in both the healthy population and in those with coronary heart disease. Individuals homozygous for apo Eε2 have the lowest concentrations of LDL, and apo Eε4 homozygotes have the highest LDL concentrations (3). The apo Eε4 allele has also been associated with Alzheimer disease. However, the mechanisms of this association are not yet clear (4). Thus, the interest in apo E polymorphisms is high, both on the basis of epidemiological research and for the purpose of clarifying individual lipid disturbances or dementias. We have successfully applied the oligonucleotide ligation assay (OLA) technique to screen for mutations causing familial hypercholesterolemia (5). We have now adapted this technique for the detection of apo E polymorphisms in large numbers of samples.