Apolipoprotein E gene mapping and expression: localization of the structural gene to human chromosome 19 and expression of ApoE mRNA in lipoprotein- and non-lipoprotein-producing tissues

Abstract

Apolipoprotein E (apoE) binds to specific cell-surface receptors and appears to be an important determinant in lipoprotein metabolism in man. Cloned human apoE c[complementary]DNA (pAE155) was used as a probe in chromosome mapping studies to detect the structural gene sequences in human-Chinese hamster cell hybrids. Southern blot analysis of HincII-digested DNA from 13 hybrids localized the gene to human chromosome 19. ApoE is syntenic to at least 2 other genes related to lipid metabolism, those for the low-density lipoprotein (LDL) receptor (the LDLR) and apoC-II. The cloned apoE cDNA was further used to detect the presence of apoE mRNA in RNA extracts of various human and baboon tissues. Northern gel analysis using the 32P-labeled pAE155 as a probe demonstrated the presence of hybridizable apoE mRNA in human liver and in baboon liver, intestine, spleen, kidney, adrenal gland and brain but not in baboon skeletal muscle. The apoE mRNA appear to be intact and migrate on an agarose gel under denaturing conditions at .apprx. 18 S. To assay for the biological activity of the apoE mRNA in these tissues, they were translated in a reticulocyte lysate system in vitro. Immunoprecipitation with an apoE-specific antiserum followed by sodium dodecyl sulfate gel electrophoresis and fluorography demonstrated that immunoreactive apoE with the expected apparent size was a product of translation of mRNA from baboon liver, intestine, kidney, spleen and brain but not that from baboon skeletal muscle. By quantitative slot-blot hybridization, taking baboon liver RNA as 100%, the various organs contained hybridizable apoE mRNA sequences at the following relative concentrations: adrenal gland, 59.6%; spleen, 11.8%; brain, 5.6%; kidney, 3.2%; small intestine, 0.8%; skeletal muscle, 0.5% or less. Evidently apoE is synthesized in lipoprotein- and non-lipoprotein-producing tissues. The peripheral synthesis of apoE may play an important role in lipid transport and metabolism and/or some other as yet unidentified functions (especially in brain).