David Wade

The Tangier disease gene product ABC1 controls the cellular apolipoprotein-mediated lipid removal pathway

The ABC1 transporter was identified as the defect in Tangier disease by a combined strategy of gene expression microarray analysis, genetic mapping, and biochemical studies. Patients with Tangier disease have a defect in cellular cholesterol removal, which results in near zero plasma levels of HDL and in massive tissue deposition of cholesteryl esters. Blocking the expression or activity of ABC1 reduces apolipoprotein-mediated lipid efflux from cultured cells, and increasing expression of ABC1 enhances it. ABC1 expression is induced by cholesterol loading and cAMP treatment and is reduced upon subsequent cholesterol removal by apolipoproteins. The protein is incorporated into the plasma membrane in proportion to its level of expression. Different mutations were detected in the ABC1 gene of 3 unrelated patients. Thus, ABC1 has the properties of a key protein in the cellular lipid removal pathway, as emphasized by the consequences of its defect in patients with Tangier disease.

C/T Polymorphism in the 5′ Untranslated Region of the Apolipoprotein(a) Gene Introduces an Upstream ATG and Reduces In Vitro Translation

Elevated plasma levels of lipoprotein(a) [Lp(a)] are a significant-independent risk factor for arteriosclerosis. Interindividual levels of Lp(a) vary nearly 1000-fold and are mainly due to inheritance that is linked to the locus of the apolipoprotein(a) [apo(a)] gene. A search was made for sequence variants in the 5 flanking region of the apo(a) gene that affect its expression. A C to T transition at position +93 from the transcription start site was found with a frequency of 14% in the study population. In transient transfection assays in HepG2 cells, luciferase reporter gene constructs with a T at this position were associated with a 58% reduction in luciferase activity compared with the more common allele. This single base variant had no significant effect on the binding of nuclear regulatory proteins; however, it introduced an additional upstream ATG initiation codon with its own in-frame stop codon. Furthermore, equivalent levels of mRNA were produced in HepG2 cells transfected with reporter gene constructs containing either a T or a C at position +93. In vitro translation experiments using transcripts derived from either variant apo(a) promoter revealed a 60% reduction in translation associated with the T allele. Hence, the additional ATG created by the T at position +93 in the 5 flanking region of the apo(a) gene impairs the efficiency of translation from the bona fide ATG initiation codon.

THE RECURRING EVOLUTION OF LIPOPROTEIN(A) : INSIGHTS FROM CLONING OF HEDGEHOG APOLIPOPROTEIN(A)

The lipoprotein Lp(a), a major inherited risk factor for atherosclerosis, consists of a low density lipoprotein-like particle containing apolipoprotein B-100 plus the distinguishing component apolipoprotein(a) (apo(a)). Human apo(a) contains highly repeated domains related to plasminogen kringle four plus single kringle five and protease-like domains. Apo(a) is virtually confined to primates, and the gene may have arisen during primate evolution. One exception is the occurrence of an Lp(a)-like particle in the hedgehog. Cloning of the hedgehog apo(a)-like gene shows that it is distinctive in form and evolutionary history from human apo(a), but that it has acquired several common features. It appears that the primate and hedgehog apo(a) genes evolved independently by duplication and modification of different domains of the plasminogen gene, providing a novel type of “convergent” molecular evolution.