Apolipoprotein E polymorphism determined by restriction enzyme analysis of DNA amplified by polymerase chain reaction: convenient alternative to phenotyping by isoelectric focusing.

Three common alleles determine six apolipoprotein E (apo E) phenotypes that are associated with variations in serum cholesterol in the population. This genetic variation results from single nucleotide alterations at two DNA loci encoding the amino acid residues 112 and 158 of apo E. We compared results of apo E phenotyping carried out by isoelectric focusing with those of apo E genotyping accomplished by direct DNA analysis. In the latter, the target DNA was amplified by the polymerase chain reaction (PCR) and subsequently analyzed by digestion with the restriction enzyme Hha I, followed by polyacrylamide gel electrophoresis of the cleavage products. With one exception, these two techniques yielded similar results from all 40 samples tested. In addition, a rare variant form of apo E (phenotype E1) was analyzed separately and incorrectly diagnosed as E2 by the Hha I digestion method; the anticipated mutation in the codon 127 was, however, confirmed by demonstration of a new Taq I restriction site in this variant gene. These data confirm that the common isoforms of apo E usually arise from genetic variation of the codons 112 and 158 and demonstrate the feasibility of the PCR technique in apo E genotyping.