Cholesterol and apolipoprotein B metabolism in Tangier disease

Abstract

Tangier disease (TD), caused by mutations in the gene encoding ATP-binding cassette 1 (ABCA1), is a rare genetic disorder in which homozygotes have a marked deficiency of high density lipoproteins (HDL), as well as concentrations of low density lipoproteins (LDL) that are typically 40% of normal. Although it is well known that the reduced levels of HDL in TD are due to hypercatabolism, the mechanism responsible for the low LDL levels has not been defined. Recently, it has been reported that intestinal cholesterol absorption is altered in ABCA1 deficient mice, suggesting that aberrant cholesterol metabolism may contribute to the LDL reductions in TD. In order to explore this possibility, as well as to define the role that ABCA1 plays in the metabolism of apolipoprotein (apoB)-containing lipoproteins, we determined the kinetics of apoB-100 within lipoproteins, and cholesterol absorption, biosynthesis, and turnover, in a compound heterozygote for TD. The levels of HDL cholesterol, LDL cholesterol and LDL apoB-100 in this subject were 7, 27 and 69% of normal, respectively, the latter of which was due to a two-fold increase in LDL catabolism (0.54 vs. 0.26±0.07 pools day −1) relative to controls ( n=11). NMR analysis of plasma lipoproteins revealed that 91% of the LDL cholesterol in the TD subject was contained within small, dense LDL, as compared with only 20% for controls ( n=70). Cholesterol absorption was 97% of the value for controls ( n=15) in the TD subject, at 45%, with cholesterol synthesis and turnover increased modestly by 17 and 25%, respectively. Our data are consistent with the concept that the reductions of LDL observed in TD are due to enhanced catabolism, secondary to changes in LDL composition and size, with neither cholesterol absorption nor metabolism significantly influenced by mutations in ABCA1.