Genetic and nongenetic sources of variation in phospholipid transfer protein activity.

Phospholipid transfer protein (PLTP) belongs to the lipid transfer/lipopolysacharide binding protein gene family. It is expressed in the liver and in macrophages (1). PLTP is responsible for most of the transfer of phospholipids from plasma VLDL to HDL (2–4). Mouse studies support the hypothesis that increased PLTP level is positively associated with atherosclerotic risk. Increased PLTP expression in mice has been associated with atherosclerosis (5–7), impaired reverse cholesterol transport (7), and decreased HDL, LDL, and VLDL levels (5). Elevated PLTP may lower HDL by increased hapatic uptake in mice (8). Mice with PLTP deficiency also absorbed less dietary cholesterol (9). Macrophage cholesterol efflux is impaired in PLTP transgenic mice, which may promote atherosclerosis (7). Acute elevation of PLTP was found to increase both atherosclerotic lesions size and macrophage content in transgenic mice (6). Also, transplantation of bone marrow from PTLP-deficient mice into irradiated LDL receptor-deficient mice reduced atherosclerosis compared with mice possessing functional macrophage PLTP (10). In contrast, macrophage PLTP expression introduced through bone marrow transplantation in double LDL receptor and PLTP-deficient mice decreased atherosclerotic lesions and total cholesterol, while increasing HDL (11). Similarly, bone marrow knockouts of PLTP have also been reported to have lower atherosclerosis on an LDL receptor background (11). In humans, PLTP has been reported to influence the size, density, and levels of VLDL and HDL (12–15) as well as LDL (15, 16). We recently described the relationship between PLTP mass, activity, and lipid levels. PLTP mass and activity were poorly correlated, but PLTP mass was correlated with HDL size and concentration, while PLTP activity was correlated with VLDL size, total cholesterol, and triglyceride levels. These correlations differ by gender (17). The relationship of PLTP activity and expression to vascular disease in humans requires further investigation. While studies have associated elevated PLTP with coronary artery disease (18), left ventricular dysfunction (19), and coronary artery events in subjects on statins (20), the opposite result was reported for peripheral artery disease. Peripheral artery vascular disease has been associated with lower PLTP (21), while carotida intima-media thickness (22) has been associated with raised PLTP in humans. PLTP expression in macrophages in the vessel wall has been proposed to be induced in the conversion of macrophages to foam cells, thus impacting atherosclerotic plaque progression (1, 23, 24). Prior investigation identified genetic variation associated with PLTP level and with lipids. PLTP single nucleotide polymorphism (SNP) rs7679 was found to predict serum triglyceride level as well as RNA expression of PLTP in liver (25). In that report, the rs7679 T allele was associated with increased PLTP expression, increased HDL level, and decreased triglyceride level. SNP rs2294213 has been reported to be associated with raised HDL, lowered triglycerides, and higher PLTP activity (26). The lipid profiles would suggest that increased expression might be cardioprotective. This is inconsistent with elevated PLTP predicting atherosclerosis in humans. Our goal was to evaluate the relationship of common genetic variation in PLTP with both carotid artery disease (CAAD) and with PLTP activity using a tagging SNP (tagSNP) approach. A cohort of subjects with and without severe carotid artery disease was considered, and a subset was selected for the labor intensive PLTP activity measure. An additional replication cohort was available for evaluation of PLTP activity.