Abetalipoproteinemia, an inherited human disease characterized by a near-complete absence of the apolipoprotein (apo) B-containing lipoproteins in the plasma, is caused by mutations in the gene for microsomal triglyceride transfer protein (MTP). We used gene targeting to knock out the mouse MTP gene (Mttp). In heterozygous knockout mice (Mttp؉͞؊ ), the MTP mRNA, protein, and activity levels were reduced by 50%, in both liver and intestine. Compared with control mice (Mttp؉͞؉), chow-fed Mttp؉͞؊ mice had reduced plasma levels of low-density lipoprotein cholesterol and had a 28% reduction in plasma apoB100 levels. On a high-fat diet, the Mttp؉͞؊ mice exhibited a marked reduction in total plasma cholesterol levels, compared with those in Mttp؉͞؉ mice. Both the livers of adult Mttp؉͞؊ mice and the visceral endoderm of the yolk sacs from Mttp؉͞؊ embryos manifested an accumulation of cytosolic fat. All homozygous embryos (Mttp؊͞؊) died during embryonic development. In the visceral endoderm of Mttp؊͞؊ yolk sacs, lipoprotein synthesis was virtually absent, and there was a marked accumulation of cytosolic fat droplets. In summary, half-normal MTP levels do not support normal levels of lipoprotein synthesis and secretion, and a complete deficiency of MTP causes lethal developmental abnormalities, perhaps because of an impaired capacity of the yolk sac to export lipids to the developing embryo.Abetalipoproteinemia is an inherited human disease characterized by extremely low plasma levels of cholesterol and triglycerides and a virtual absence of the apolipoprotein (apo) B-containing lipoproteins [chylomicrons, very-low-density lipoproteins (VLDL), and low-density lipoproteins (LDL)] in the plasma (1, 2). Affected humans manifest intestinal fat malabsorption and frequently develop severe neurological problems as a result of deficient intestinal absorption of vitamin E, a fat-soluble vitamin (3). Abetalipoproteinemia is caused by mutations in the gene for the 97-kDa catalytic subunit of microsomal triglyceride transfer protein (MTP) (4-6). MTP is thought to transfer lipids to the apoB polypeptide chain as it is translated on the ribosome, allowing apoB to translocate into the lumen of the endoplasmic reticulum and assume the proper conformation for lipoprotein assembly (7,8). In abetalipoproteinemia, apoB is synthesized but cannot form lipoproteins and is degraded (9).Abetalipoproteinemia is considered to be an autosomal recessive syndrome, requiring two defective MTP alleles for disease expression. Parents of affected patients are said to have plasma lipid levels within the normal range (1). The normal plasma lipid levels in obligate heterozygotes have given rise to the concept that MTP normally is present in great excess within lipoprotein-secreting cells. That is, if MTP normally were present within microsomes in great excess, then halfnormal MTP levels in obligate heterozygotes would not be expected to affect lipoprotein secretion rates or plasma lipid levels. Against this concept, however, are recent in vitro data...

Apolipoprotein B is synthesized by the intestine and the liver in mammals, where it serves as the main structural component in the formation of chylomicrons and very low density lipoproteins, respectively. Apolipoprotein B is also expressed in mammalian fetal membranes. To examine the consequences of apolipoprotein B deficiency in mice, we used gene targeting in mouse embryonic stem cells to generate mice containing an insertional disruption of the 5' region of the apolipoprotein B gene. Mice that were heterozygous for the disrupted apolipoprotein B allele had an "20%o reduction in plasma cholesterol levels, markedly reduced plasma concentrations of the pre-f3 and 8-migrating lipoproteins, and an "70%o reduction in plasma apolipoprotein B levels. When fed a diet rich in fat and cholesterol, heterozygous mice were protected from diet-induced hypercholesterolemia; these mice, which constitute an animal model for hypobetalipoproteinemia, should be useful for studying the effects of decreased apolipoprotein B expression on atherogenesis. The breeding of heterozygous mice yielded no viable homozygous apolipoprotein B knockout mice. Most homozygous embryos were resorbed by midgestation (before gestational day 11.5); several embryos that survived until later in gestation exhibited exencephalus. The embryonic lethal phenotype was rescued by complementation with a human apolipoprotein B transgenei.e., human apolipoprotein B transgenic mice that were homozygous for the murine apolipoprotein B knockout mutation were viable. Our findings indicate that apolipoprotein B plays an essential role in mouse embryonic development.

We previously generated transgenic mice expressing human apolipoprotein (apo-) B and demonstrated that the plasma of chow-fed transgenic animals contained markedly increased amounts of LDL (Linton, M. F., R. V. Farese, Jr., G. Chiesa, D. S. Grass, P. Chin, R. E. Hammer, H. H. Hobbs, and S. G. Young 1992. J. Clin. . In this study, we fed groups of transgenic and nontransgenic mice either a chow diet or a diet high in fat (16%) and cholesterol (1.25%). Lipid and lipoprotein levels were assessed, and after 18 wk of diet, the extent of aortic atherosclerotic lesions in each group of animals was quantified. Compared with the female transgenic mice on the chow diet, female transgenic mice on the high-fat diet had higher plasma levels of cholesterol (312±17 vs 144±7 mg/dl; P < 0.0001) and human apo-B (120±8 vs 84±3 mg/dl; P < 0.0001). The higher human apo-B levels were due to increased plasma levels of human apo-B48; the human apo-B100 levels did not differ in animals on the two diets. In mice on the high-fat diet, most of the human apo-B48 and apo-B100 was found in LDL-sized particles. Compared with nontransgenic mice on the high-fat diet, the transgenic animals on the high-fat diet had significantly increased levels of total cholesterol (312±17 vs 230±19 mg/dl; P < 0.0001) and non-HDL cholesterol (283±17 vs 193±+19 mg/dl; P < 0.0001). The extent of atherosclerotic lesion development within the ascending aorta was quantified by measuring total lesion area in 60 progressive sections, using computerassisted image analysis. Neither the chow-fed transgenic mice nor the chow-fed nontransgenic mice had significant atherosclerotic lesions. Nontransgenic animals on the highfat diet had relatively small atherosclerotic lesions ( < 15,000 imm2/section), almost all of which were confined to the proximal 400 ,im of the aorta near the aortic valve. In contrast, transgenic animals on the high-fat diet had extensive athero- Thus, human apo-B expression, in the setting of a diet rich in fats, causes severe atherosclerosis in mice. (J. Clin. Invest. 1995Invest. . 95:2246Invest. -2257

The formation of chylomicrons by the intestine is important for the absorption of dietary fats and fat-soluble vitamins (e.g., retinol, a-tocopherol). Apo B plays an essential structural role in the formation of chylomicrons in the intestine as well as the VLDL in the liver. We have developed genetically modified mice that express apo B in the liver but not in the intestine. By electron microscopy, the enterocytes of these mice lacked nascent chylomicrons in the endoplasmic reticulum and Golgi apparatus. Because these mice could not form chylomicrons, the intestinal villus enterocytes were massively engorged with fat, which was contained in cytosolic lipid droplets. These mice absorbed D-xylose normally, but there was virtually no absorption of retinol palmitate or cholesterol. The levels of a-tocopherol in the plasma were extremely low. Of note, the absence of chylomicron synthesis in the intestine did not appear to have a significant effect on the plasma levels of the apo B-containing lipoproteins produced by the liver. The mice lacking intestinal apo B expression represent the first genetic model of defective absorption of fats and fat-soluble vitamins and provide a useful animal model for studying nutrition and lipoprotein metabolism. (J. Clin. Invest. 1995. 96:2932-2946