Abstract 126: Identification of an Individual With Extremely High HDL-C Who is Homozygous for a Loss-of-function Mutation in Scavenger Receptor BI (SR-BI) and Functional Characterization of the Mutation
Background The role of SR-BI in human HDL metabolism and physiology has yet to be fully clarified and no humans with homozygous SR-BI deficiency have been reported to date. We report the identification of an individual with HDL-C > 160 mg/dL who is homozygous for a Pro376Leu mutation in the SCARB1 gene.
Methods In silico tests were performed to evaluate the effect of the variant on protein function (Polyphen 2 and Condel) and on protein secondary structure (Raptor X and NetSurfP). HDL binding, cholesteryl-ether (CE) uptake and the effect on cholesterol efflux were assessed in vitro in COS7 cells. Serum efflux was measured using J774 macrophages. Subcellular distribution of the mutant protein was assessed through biotinylation of cell surface proteins and trypsin sensitivity.
Results HDL-C was 166mg/dl and apoA-I was and 275 mg/dl in the homozygous subject. HDL particles were much larger and enriched in cholesterol and triglycerides as compared with a control subject matched for HDL-C levels. In silico analysis predicted that the variant would cause secondary structure modifications and be deleterious for function. In vitro studies in COS7 cells transfected with the P376L mutation showed a strongly diminished HDL binding (-69%) and CE uptake (-79%) as compared to cells transfected with the WT form of SR-BI. Efflux was also impaired (-59%). The biotinylation and trypsin sensitivity assays showed a defect in the delivery of the mutant protein to the cell surface.
Conclusions We report the first identification of a homozygote for a loss-of-function mutation in SR-BI; the phenotype includes extremely elevated HDL-C levels. The P376L mutation in SCARB1 gene results in marked loss of SR-BI function at least in part by impairing the transport of the protein to the cell surface.
- © 2013 by American Heart Association, Inc.