Objective-To better understand the role of lecithin:cholesterol acyltransferase (LCAT) in lipoprotein metabolism through the genetic and biochemical characterization of families carrying mutations in the LCAT gene. Methods and Results-Thirteen families carrying 17 different mutations in the LCAT gene were identified by Lipid Clinics and Departments of Nephrology throughout Italy. DNA analysis of 82 family members identified 15 carriers of 2 mutant LCAT alleles, 11 with familial LCAT deficiency (FLD) and 4 with fish-eye disease (FED). Forty-four individuals carried 1 mutant LCAT allele, and 23 had a normal genotype. Plasma unesterified cholesterol, unesterified/total cholesterol ratio, triglycerides, very-low-density lipoprotein cholesterol, and pre-␤ high-density lipoprotein (LDL) were elevated, and high-density lipoprotein (HDL) cholesterol, apolipoprotein A-I, apolipoprotein A-II, apolipoprotein B, LpA-I, LpA-I:A-II, cholesterol esterification rate, LCAT activity and concentration, and LDL and HDL 3 particle size were reduced in a gene-dose-dependent manner in carriers of mutant LCAT alleles. No differences were found in the lipid/lipoprotein profile of FLD and FED cases, except for higher plasma unesterified cholesterol and unesterified/total cholesterol ratio in the former. Conclusion-In a large series of subjects carrying mutations in the LCAT gene, the inheritance of a mutated LCAT genotype causes a gene-dose-dependent alteration in the plasma lipid/lipoprotein profile, which is remarkably similar between subjects classified as FLD or FED. Key Words: familial lecithin:cholesterol acyltransferase deficiency Ⅲ fish eye disease Ⅲ high-density lipoproteins Ⅲ lecithin:cholesterol acyltransferase Ⅲ mutation T he lecithin:cholesterol acyltransferase (LCAT) (phosphatidylcholine:sterol-O-acyltransferase; EC 2.3.1.43) enzyme is responsible for the synthesis of cholesteryl esters (CE) in plasma. 1 Through this action, LCAT plays a central role in the formation and maturation of high-density lipoproteins (HDL), and in the intravascular stage of reverse cholesterol transport, the major mechanism by which HDL modulate the development and progression of atherosclerosis. A defect in LCAT function would be expected to enhance atherosclerosis by interfering with this process.The human LCAT gene encompasses 4.2 kilobases and is localized in the q21-22 region of chromosome 16. Methods SubjectsProbands with primary hypoalphalipoproteinemia (HALP), defined by a plasma HDL-C level below the fifth percentile for the age-and sex-matched general population, were identified by Lipid Clinics and Departments of Nephrology throughout Italy. Plasma samples were analyzed for total and unesterified cholesterol; in 18 unrelated index cases, the results were suggestive of a defect in the LCAT gene. Genetic analysis revealed that 13 of 18 index cases carried at least 1 mutant LCAT allele. Relatives of the 13 probands were invited to participate in the study. All subjects gave an informed consent. Blood samples were collected after an overni...