Genetic variation in the apolipoprotein D gene among African blacks and its significance in lipid metabolism

Abstract

Apolipoprotein D (APOD, gene; apoD, protein) is a plasma high-density lipoprotein (HDL)—associated glycoprotein, with a putative role in the cholesterol (CHOL) transport pathway. An apoD protein polymorphism has been previously reported by us. The cathodically shifted pattern seen on isoelectric focusing gels, controlled by the APOD* 2 allele, was found to be unique to populations of African ancestry. To characterize the molecular basis of the protein polymorphism and to identify new mutations, we used a combination of SSCP, DHPLC and DNA sequencing techniques to screen the entire coding region of the APOD gene. We identified three distinct missense mutations, including Phe36Val, Tyr108Cys, and Thr158Lys with frequencies ranging from 2.1 to 2.8% in 722 African blacks from Nigeria. In addition, a common 8 bp deletion polymorphism was observed in intron 1 with a carrier frequency of 30.1%. The missense mutation, Thr158Lys correlated with the APOD* 2 allele of the protein polymorphism. None of the 454 Caucasians screened for these polymorphisms showed any variation. We also determined the effect of these polymorphisms on plasma lipid levels in the African black population by generalized linear model (GLM). The Val36 allele was associated with significantly decreased HDL3-C ( P=0.027) and apoA-I ( P=0.030) levels among females. The Lys158 allele was associated with significantly increased Lp(a) ( P=0.018) and triglyceride ( P=0.017) levels, among females and males, respectively. In addition, males heterozygous for both intron 1 and codon 108 polymorphisms showed significantly increased HDL-C ( P=0.011), HDL3-C ( P=0.041), HDL2-C ( P=0.009), apoA-I ( P=0.005) and decreased LDL-C ( P=0.025) levels. The results of our study show that the APOD gene harbors several polymorphisms, which are unique to African populations. Further study of these polymorphisms may help to characterize the role of apoD in lipid metabolism, and in cardiovascular disease among African populations.