Remnants of
triglyceride-rich
lipoproteins accumulate in plasma of subjects with
type III hyperlipoproteinemia (HLP) due to defective clearance by hepatic receptors. Although most subjects with type III HLP are homozygous for
apolipoprotein (
apo) E2 (arg158-->cys, R158C), a variant that binds defectively to
cell surface receptors, some individuals with type III HLP have rare mutations of
apo E. We identified six subjects from three families with type III HLP who had either an
apo E3/1 or E4/1 phenotype by isoelectric focusing. Using
DNA restriction isotyping with HhaI, all six subjects were determined to have only one
apo E allele encoding cys158 and the other encoding arg158. Subsequently, digestion of polymerase chain reaction-amplified portions of exon 4 of the
apo E gene with
endonucleases HaeIII, TaqI, and Sau3AI demonstrated a second
DNA variant that encoded a single amino acid substitution (gly127-->asp, G127D) due to a
guanosine-to-
adenosine nucleotide change resulting in the
apo E1 isoform (G127D, R158C), which had arisen from a parent
apo E2 allele. This mutation was confirmed with direct
DNA sequencing. Incubation of
very low density lipoprotein (VLDL) isolated from hyperlipidemic
apo E1 subjects with J774 macrophages resulted in
a 7- to 12-fold increase in cellular
cholesterol ester compared with VLDL from
apo E2/2 subjects. Although heterozygosity for
apo E1 alone did not impair the interaction of VLDL with cellular receptors in vitro, its presence in subjects with type III HLP suggests that
apo E1, perhaps in combination with secondary factors, may be causative for the
dyslipidemia.