Elevated plasma concentration of the lipoprotein Lp(a) is a major independent risk factor for atherosclerosis and restenosis. This association, as well as explanations for the enormous inter-individual differences in Lp(a) levels, have largely been based on data from population surveys and family studies. In addition, the pathologic activity of Lp(a) remains unknown, although in vitro studies have suggested possible functions which remain unconfirmed in vivo. Hence, the basic aims of this proposed research are to use transgenic mice to examine in vivo the genetic determinants of apo(a) gene expression and its effects on blood vessels. In order to directly investigate the role of two major hypothesized determinants of Lp(a) plasma concentration, transcription of the apo(a) gene and the variable size of apo(a) isoforms, we propose to study the effects of promoter/enhancer sequences and kringle domain repeat number on Lp(a) plasma levels in transgenic mice and mice transfected with recombinant DNA via non-replicating adenovirus. Apo(a) flanking genomic DNA from several individuals whose apo(a) alleles are associated with high or low plasma levels, as well as in vitro mutagenized gene sequences, will be linked to apo(a) and reporter cDNA for transfection. The level of resulting hepatic mRNA and plasma apo(a) concentration will be determined. Mice will also be made to express apo(a) mini genes containing varying numbers of kringle repeats. Apo(a) transcription, protein synthetic rates, plasma levels, and plasma half lives will be compared in animals containing constructs of identical promoter elements linked to protein coding regions of different size. These studies will assess the effect of various steps in expression and metabolism of apo(a) that could account for the rough, inverse correlation between apo(a) isoform size and concentration reported in population studies. The association between plasma concentration of Lp(a) and atherosclerosis in humans is well established. But despite a number of attractive hypotheses with varied degrees of in vitro support, the pathophysiology of Lp(a) remains unknown. We propose to use transgenic mice as manipulatable models of the activity of Lp(a). In this model, we will study the effects of apo(a) concentration and its interaction with other proteins on atherosclerosis. Key variants in the sequence of the apo(a) protein will also be tested. In addition, we will examine the time course of arterial changes associated with expression of the transgenes and test the hypothesis that apo(a) promotes the proliferation and migration of vascular smooth muscle cells by interfering with the activation of plasminogen and TGF-beta.
