Type III hyperlipoproteinemia and spontaneous atherosclerosis in mice resulting from gene replacement of mouse Apoe with human Apoe*2.

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

On the lipoprotein abnormality in type III hyperlipoproteinemia

Two lipoprotein species were isolated by starch block electrophoresis from the very low density lipoproteins (VLDL) (S(f) 20-400) of patients with type III hyperlipoproteinemia. One of these, α(2)-VLDL, had a content of lipid and protein and physical characteristics similar to VLDL from normal subjects or patients with other forms of hyperglyceridemia. The other species, β-VLDL, contained more cholesterol and less triglyceride in relation to the protein, than normal VLDL. Only the apoprotein of low density lipoprotein was immunochemically detectable in the β-VLDL; the proteins in the α(2)-VLDL reacted with antisera specific for low density lipoprotein and high density lipoprotein. The electrophoretic mobility of β-VLDL was similar to that of low density lipoprotein and significantly less than that of α(2)-VLDL. Isolated β-VLDL had a lesser mean flotation rate than α(2)-VLDL, but both α(2)- and β-VLDL were found throughout the S(f) 20-400 flotation range. The relative quantities of α(2)- and β-VLDL could be varied by changing the diet or by heparin administration. Most of the VLDL from type III patients on a high carbohydrate diet was in the α(2)-VLDL form. During fasting, α(2)-VLDL fell and β-VLDL increased becoming the predominant species of VLDL. Heparin-induced acceleration of triglyceride clearance also increased β-VLDL and decreased α(2)-VLDL. These findings suggest a precursor-product relationship between the α(2)- and β-forms of VLDL

Type III hyperlipoproteinemia and spontaneous atherosclerosis in mice resulting from gene replacement of mouse Apoe with human Apoe*2.

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