Studies by various investigators have indicated that elevated levels of plasma homocyst(e)ine are strongly associated with the occurrence of occlusive vascular diseases. With the eventual aim of determining whether or not elevated plasma homocyst(e)ine concentrations are directly causative of cardiovascular diseases, we have generated mice that are moderately and severely homocyst(e)inemic. Homologous recombination in mouse embryonic stem cells was used to inactivate the cystathionine 13-synthase [L-serine hydrolyase (adding homocysteine), EC 4.2.1.22] gene. Homozygous mutants completely lacking cystathionine 13-synthase were born at the expected frequency from matings ofheterozygotes, but they suffered from severe growth retardation and a majority of them died within 5 weeks after birth. Histological examination showed that the hepatocytes of homozygotes were enlarged, multinucleated, and filled with microvesicular lipid droplets. Plasma homocyst(e)ine levels of the homozygotes were "40 times normal. These mice, therefore, represent a model for severe homocyst(e)inemia resulting from the complete lack of cystathionine 18-synthase. Heterozygous mutants have 509o reduction in cystathionine 8-synthase mRNA and enzyme activity in the liver and have twice normal plasma homocyst(e)ine levels. Thus, the heterozygous mutants are promising for studying the in vivo role of elevated levels of homocyst(e)ine in the etiology of cardiovascular diseases.Homocysteine is an intermediate amino acid in methionine metabolism and is either converted to cysteine by transsulfuration or methylated to form methionine. A decreased rate of metabolism through either of these pathways can lead to homocyst(e)inemia and homocystinuria.The of elevated homocyst(e)ine in the formation of thrombi or in the acceleration of atherogenesis is not well understood, various experiments suggest that homocysteine is a mediator of disease. For example, homocysteine damages cultured human venous and arterial endothelial cells (9, 10). Furthermore, DeGroot et al. (11) showed that cells grown from obligate heterozygotes for CBS deficiency were much more sensitive to methionine-mediated injury than were control cells. Other in vitro studies have demonstrated that homocysteine enhances autooxidation of low density lipoproteins (12), enhances biosynthesis of thromboxane (13), inhibits cell-surface thrombomodulin expression (14), promotes vascular smooth muscle cell growth (15), and enhances binding of lipoprotein(a) to fibrin (16). In vivo, long-term infusion of homocystine in baboons leads to endothelial desquamation, to an increase in platelet consumption, and to arterial lesions (17). However, there are also conflicting data indicating that an increased incidence of vascular diseases was not found in individuals heterozygous for homocystinuria (18,19). Determining whether elevated plasma homocyst(e)ine levels are causative of cardiovascular disease or are a consequence of the disease has been difficult because of the lack of an experimental animal mo...

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Apolipoprotein (apo) E, a constituent of several lipoproteins, is a ligand for the low density lipoprotein receptor, and this interaction is important for maintaining cholesterol and triglyceride homeostasis. We have used a gene replacement strategy to generate mice that express the human apoE3 isoform in place of the mouse protein. The levels of apoE mRNA in various tissues are virtually the same in the human apoE3 homozygous (3/3) mice and their littermates having the wild type mouse allele (؉/؉). Total cholesterol and triglyceride levels in fasted plasma from the 3/3 mice were not different from those in the ؉/؉ mice, when maintained on a normal (low fat) chow diet. We found, however, notable differences in the distribution of plasma lipoproteins and apolipoprotein E between the two groups: ␤-migrating lipoproteins and plasma apoB100 levels are decreased in the 3/3 mice, and the apoE distribution is shifted from high density lipoproteins to larger lipoprotein particles. In addition, the fractional catabolic rate of exogenously administered remnant particles without apoE was 6-fold slower in the 3/3 mice compared with the ؉/؉ mice. When the 3/3 and ؉/؉ animals were fed a high fat/high cholesterol diet, the 3/3 animals responded with a dramatic increase (5-fold) in total cholesterol compared with the ؉/؉ mice (1.5-fold), and after 12 weeks on this same diet the 3/3 animals developed significantly (at least 13-fold) larger atherosclerotic plaques in the aortic sinus area than the ؉/؉ animals. Thus the structural differences between human APOE3 and mouse ApoE proteins are sufficient to cause an increased susceptibility to dietary-induced hypercholesterolemia and atherosclerosis in the 3/3 mice.

Antineutrophil cytoplasmic autoantibodies (ANCAs) are identified in the circulation of approximately 80% of patients with pauci-immune necrotizing and crescentic glomerulonephritis and systemic small vessel vasculitis, such as microscopic polyangiitis and Wegener granulomatosis. The most common antigen target for ANCAs is myeloperoxidase (MPO), which is found in neutrophils and monocytes. We report definitive experimental animal evidence that ANCAs are pathogenic. MPO knockout (Mpo–/–) mice were immunized with mouse MPO. Splenocytes from these mice or from control mice were injected intravenously into recombinase-activating gene-2–deficient (Rag2–/–) mice, which lack functioning B lymphocytes and T lymphocytes. All mice that received splenocytes developed mild to moderate glomerular immune deposits, but only mice that received 1 × 108 or 5 × 107 anti-MPO splenocytes developed severe necrotizing and crescentic glomerulonephritis, granulomatous inflammation, and systemic necrotizing vasculitis, including necrotizing arteritis and hemorrhagic pulmonary capillaritis. To test the pathogenic potential of antibodies alone, purified anti-MPO IgG or control IgG was injected intravenously into Rag2–/– mice and wild-type mice. Mice that received anti-MPO IgG but not mice that received control IgG developed focal necrotizing and crescentic glomerulonephritis with a paucity of glomerular Ig deposition. Thus, anti-MPO IgG alone was able to cause pauci-immune glomerular necrosis and crescent formation in the absence of functional T or B lymphocytes in Rag2–/– mice and in the presence of an intact immune system in wild-type C57BL/6J mice. This animal model offers strong support for a direct pathogenic role for ANCA IgG in human glomerulonephritis and vasculitis