Abstract 14621: The Endothelial Lipase Loss of Function Mutation R450G Causes Hyperalphalipoproteinemia That Does Not Protect Against ASCVD

Abstract

Background: High-density lipoprotein cholesterol (HDL-C) levels and cardiovascular risk are inversely related in epidemiological studies. However, some rare monogenetic mutations that raise HDL do not appear to confer atheroprotection. Methods: The LIPG gene, encoding Endothelial Lipase (EL), was sequenced in 337 patients with hyperalphalipoproteinemia (HDL-C>80mg/dL) for whom a causative variant in other genes affecting plasma HDL levels ( CETP, APOA1, ABCA1, LCAT, LPL, GALNT2 ) had been excluded. HDL particle size distribution was determined using a Lipoprint HDL subtraction test and further assessed by non-denaturing gradient gel electrophoresis followed by densitometric scanning. EL variants were cloned by site directed mutagenesis, expressed in HEK293 cells and compared to wild-type EL. Their expression in cell extracts and secretion in culture medium were measured by western-blot and ELISA. EL phospholipase enzymatic activity was assessed by measuring the production of non-esterified free fatty acids (FFA) using native and reconstituted HDL (rHDL) as substrates. Results: We identified the novel EL R450G missense mutation in three individuals (2 heterozygotes and one homozygote). Their HDL particles were larger than those of normolipemic controls (66±14% vs. 36±6% of large HDL1 particles; P=0.02). Six week treatment with Probucol 1g/day reduced the amount of HDL1 in those patients. Despite elevated circulating HDL levels, these patients presented with atherosclerotic cardiovascular disease. In vitro, the R450G variant was expressed and secreted normally from cells but displayed markedly reduced enzymatic activity compared with wild-type EL (Vmax = 6±4, vs. 32±5 nmol/Hr and Km = 0.01±0.07 vs. 0.30±0.15 mM, respectively; P<0.01, all). Conclusion: EL-R450G is a loss-of-function variant associated with hyperalphalipoproteinemia. Carriers of this mutation do not appear to be protected against atherosclerosis.