Abstract 627: A Deleterious Mutation in SCARB1 Significantly Alters High-Density Lipoprotein Level, Composition and Biological Function in Humans

Abstract

The membrane protein Scavenger Receptor Class B, Type 1 (SR-BI), coded by the gene SCARB1, facilitates the selective uptake of cholesteryl esters from HDL in the liver. Its impact on human lipoprotein metabolism however is not fully understood. Our laboratory has recently identified two homozygotes and 16 heterozygotes for a coding mutation in SCARB1 (c.1127C>T, p.Pro376Leu). We assessed the hypothesis that this SCARB1 mutation significantly increases HDL-C and alters HDL composition and biological function. Plasma from subjects homozygous (n=2) or heterozygous (n=8) for the SR-BI Pro376Leu mutation was compared with that of normolipidemic controls. Five major HDL subfractions were isolated from EDTA-treated plasma using isopycnic density gradient ultracentrifugation and their chemical composition was assayed using commercial enzymatic assay kits. Quantification of >160 molecular species of HDL phospho- and sphingolipids was accomplished via a novel LC-ESI/MS/MS based analysis. Cholesterol efflux capacity, measured from THP-1 macrophages, was determined in total HDL. We identified a >2-fold increase in HDL-C in homozygotes and a >1.5-fold increase in heterozygotes compared to controls, owing mainly to an increase in large, light HDL2 particles. The ApoA-I/ApoA-II ratio was increased in a gene-dose dependent fashion. HDL isolated from heterozygotes mediated 13.5% less efflux when compared to particles from control subjects. Analysis of the HDL phosphosphingolipidome revealed a 23% decrease in phosphatidylinositol content between carriers and controls. A gene-dose dependent trend towards diminishment of phosphatidylserine and increased ceramide was noticeable. In conclusion, the SR-BI Pro376Leu significantly increases HDL levels, but potentially reduces the capacity of HDL to mediate cellular cholesterol efflux. This may be due in part to a decrease in minor, negatively charged phospholipid subclasses including PI and PS, which have previously been shown to play a functional role in stimulating cellular cholesterol efflux.