To study isoform-specific effects of apolipoprotein E (apoE) in vivo, we 
generated mice with a human APOE*2 allele in place of the mouse Apoe gene via 
targeted gene replacement in embryonic stem cells. Mice expressing human apoE2 
(2/2) have virtually all the characteristics of type III hyperlipoproteinemia. 
Their plasma cholesterol and triglyceride levels are both twice to three times 
those in (normolipidemic) mice that are expressing human apoE3 (3/3) made in an 
identical manner. The 2/2 mice are markedly defective in clearing beta-migrating 
VLDL particles, and spontaneously develop atherosclerotic plaques, even on a 
regular diet. An atherogenic diet, high in fat and cholesterol, exacerbates 
development of atherosclerosis and xanthomas in the 2/2 mice. Thus, comparisons 
between the 2/2 and 3/3 mice unequivocally demonstrate that a single amino acid 
difference (Arg158 Cys) in the apoE protein is sufficient to cause type III HLP 
and spontaneous atherosclerosis in mice.