To evaluate the biochemical and molecular mechanisms leading to glomerulosclerosis and the variable development of atherosclerosis in patients with familial lecithin cholesterol acyl transferase (LCAT) deficiency, we generated LCAT knockout (KO) mice and cross-bred them with apolipoprotein (apo) E KO, low density lipoprotein receptor (LDLr) KO, and cholesteryl ester transfer protein transgenic mice. LCAT-KO mice had normochromic normocytic anemia with increased reticulocyte and target cell counts as well as decreased red blood cell osmotic fragility. A subset of LCAT-KO mice accumulated lipoprotein X and developed proteinuria and glomerulosclerosis characterized by mesangial cell proliferation, sclerosis, lipid accumulation, and deposition of electron dense material throughout the glomeruli. LCAT deficiency reduced the plasma high density lipoprotein (HDL) cholesterol (؊70 to ؊94%) and non-HDL cholesterol (؊48 to ؊85%) levels in control, apoE-KO, LDLr-KO, and cholesteryl ester transfer protein-Tg mice. Transcriptome and Western blot analysis demonstrated up-regulation of hepatic LDLr and apoE expression in LCAT-KO mice. Despite decreased HDL, aortic atherosclerosis was significantly reduced (؊35% to ؊99%) in all mouse models with LCAT deficiency. Our studies indicate (i) that the plasma levels of apoB containing lipoproteins rather than HDL may determine the atherogenic risk of patients with hypoalphalipoproteinemia due to LCAT deficiency and (ii) a potential etiological role for lipoproteins X in the development of glomerulosclerosis in LCAT deficiency. The availability of LCAT-KO mice characterized by lipid, hematologic, and renal abnormalities similar to familial LCAT deficiency patients will permit future evaluation of LCAT gene transfer as a possible treatment for glomerulosclerosis in LCAT-deficient states.As the key enzyme responsible for the esterification of free cholesterol present in circulating lipoproteins, LCAT 1 plays a major role in HDL metabolism (1). Several lines of evidence that include epidemiological data, transgenic animal studies, and, more recently, prospective human clinical trials (2-6) indicate that increased plasma HDL levels protect against the development of atherosclerosis. One of several proposed functions of HDL as an anti-atherogenic lipoprotein is to facilitate reverse cholesterol transport, a process by which cholesterol is transported from peripheral cells to the liver for removal from the body (7,8). LCAT may play a major role in this process by maintaining a free cholesterol gradient between peripheral cells and the HDL particle surface, thus promoting free cholesterol efflux (1). The newly generated cholesteryl esters (CE) accumulating in the HDL core may be transferred directly to the liver via whole particle and/or selective uptake (9, 10). Alternatively HDL-CE may be transferred to apoB-containing lipoproteins as a result of the activity of cholesterol ester transfer protein (CETP) (8).The important role that LCAT plays in HDL metabolism has been established by the ide...