Structure, evolution, and polymorphisms of the human apolipoprotein A4 gene (APOA4).
The genes coding for three proteins of the plasma lipid transport system--apolipoproteins A1 (APOA1), C3 (APOC3), and A4 (APOA4)--are closely linked and tandemly organized on the long arm of human chromosome 11. In this study the human APOA4 gene has been isolated and characterized. In contrast to APOA1 and APOC3 genes, which contain three introns, the APOA4 gene contains only two. An intron interrupting the 5' noncoding region of the APOA1 and APOC3 mRNAs is absent from the corresponding position of the APOA4 mRNA. However, similar to APOA1 and APOC3 genes, the introns of the APOA4 gene separate nucleotide sequences coding for the signal peptide and the amphipathic domains in APOA4. These results suggest that the APOA1, APOC3, and APOA4 genes were derived from a common evolutionary ancestor and indicate that during evolution the APOA4 gene lost one of its ancestral introns. Two restriction endonuclease sites, an Xba I located in the second intron of the APOA4 gene and a different Xba I located 9 kilobases 3' to the APOA4 gene, are polymorphic in Mediterranean and Northern European populations. Haplotype analysis indicated that even though these polymorphic sites are located within 9 kilobases they do not display significant nonrandom association. Finally, restriction mapping analysis of DNA from a patient with combined APOA1-APOC3 deficiency and premature coronary artery disease indicated that this patient has a structurally normal APOA4 gene.