Functional Analysis of the Chimpanzee and Human Apo(a) Promoter Sequences: Identification of Sequence Variations Responsible for Elevated Transcriptional Activity in Chimpanzee

Lp(a) concentrations vary considerably among individuals and are primarily determined by the apo(a) gene locus. We have previously shown that mean plasma Lp(a) levels in the chimpanzee are significantly higher than those observed in humans (Doucet, C., Huby, T., Chapman, J., and Thillet, J. (1994) J. Lipid Res 35, 263-270). To evaluate the possibility that this difference may result from a high level of expression of chimpanzee apo(a), we cloned and sequenced 1.4 kilobase (kb) of the 5'-flanking region of the gene and compared promoter activity to that of its human counterpart. Sequence analysis revealed 98% homology between chimpanzee and human apo(a) 5' sequences; among the differences observed, two involved polymorphic sites associated with Lp(a) levels in humans. The TTTTA repeat located 1.3 kb 5' of the apo(a) gene, present in a variable number of copies (n = 5-12) in humans, is uniquely present as four copies in the chimpanzee sequence. The second position concerns the +93 C>T polymorphism that creates an additional ATG start codon in the human apo(a) gene, thereby impairing translation efficiency. In chimpanzee, this position did not appear polymorphic, and a base difference at position +94 precluded the presence of an additional ATG. In transient transfection assays, the chimpanzee apo(a) promoter exhibited a 5-fold elevation in transcriptional activity as compared with its human counterpart. This marked difference in activity was maintained with either 1.4 kb of 5' sequence or the minimal promoter region -98 to +141 of the human and chimpanzee apo(a) genes. Using point mutational analyses, nucleotides present at positions -3, -2, and +8 (relative to the start site of transcription) were found to be essential for the high transcription efficiency of the chimpanzee apo(a) promoter. High transcriptional activity of the chimpanzee apo(a) gene may therefore represent a key factor in the elevated plasma Lp(a) levels characteristic of this non-human primate.