Most epidemiological studies have shown that high density lipoprotein cholesterol (HDL) levels are inversely correlated to cardiovascular disease (CVD). Nonetheless, a subset of patients with elevated HDL (hyperalphalipoproteinemia, HA) and elevated LDL cholesterol (hyperbetalipoproteinemia, HB) levels are at increased risk for CVD. This phenotype (HA/HB) with increased risk for CVD is similar to that described in the HDL receptor, scavenger receptor, class B, type I (SR-BI) knockout mouse. In humans, polymorphisms of the SR-BI gene have been associated with HDL and LDL cholesterol levels and body mass index, but to date, no functional defects in the receptor have been reported. We have identified the first case of an adult female heterozygote for SR-BI deficiency; this was based on finding markedly lower SR-BI RNA and protein and decreased SR-BI function in the proband's macrophages, and a novel splice variant within the region of the gene that encodes the functional extracellular loop. We also found decreased SR-BI RNA expression in macrophages cultured from the proband's mother, sister, and an unrelated adult female with HA/HB. We hypothesize that mutations/polymorphisms in the SR-BI gene strongly correlate with decreased SR-BI function and are associated with HA/HB and increased risk for atherosclerosis.
The aims of this proposal are 1) to thoroughly characterize the lipoproteins and the fractional clearance of HDL-cholesteryl ester in a cohort of 40 adult subjects with HA/HB; 2) to assess SR-BI expression and function in monocyte-derived macrophages isolated from HA/HB and control subjects. The human macrophage model for SR-BI expression and function is expected to act as a surrogate for hepatic SR-BI, given the limitations in obtaining liver tissue from these donors. SR-BI knockout mice will be used to correlate changes in macrophage SR-BI expression and function with hepatic SR-BI function and to corroborate our findings in human SR-BI deficient macrophages; and 3) to identify polymorphisms and/or mutations of the SR-BI gene in HA/HB and control subjects. Transient transfection experiments with COS-7 cells will also verify the significance of SR-BI variants. This work will provide new insights into the role of SR-BI in human dyslipidemia and atherosclerosis and challenge the concept that desirable HDL cholesterol levels are always cardioprotective.
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