Molecular basis of apolipoprotein (a) isoform size heterogeneity as revealed by pulsed-field gel electrophoresis.

Lipoprotein(a) [Lp(a)] is a cholesterol-rich lipoprotein that is distinguished by its content of a glycoprotein called apolipoprotein(a) [apo(a)]. Apo(a) varies in size among individuals owing to different numbers of cysteine-rich sequences that are homologous to kringle 4 of plasminogen. The genetic basis for this variation is not understood at the genomic level. In this study we used pulsed-field gel electrophoresis and genomic blotting to identify a highly polymorphic restriction fragment from the apo(a) gene. The fragment contains multiple tandem repeats of a kringle 4-encoding sequence and varies in length from 48 to 190 kb depending on the number of kringle 4-encoding sequences. A total of 19 different alleles were identified among 102 unrelated Caucasian Americans. 94% of individuals studied had two different alleles which could be distinguished by size on pulsed-field gel electrophoresis. The degree of size heterogeneity was much greater than had been previously appreciated based on the analysis of the apparent molecular mass of the protein. The size of the apo(a) gene correlated directly with the size of the apo(a) protein, and inversely with the concentration of Lp(a) in plasma. Segregation analysis of the apo(a) gene was performed in families; siblings with identical apo(a) genotypes had similar plasma levels of Lp(a). These results suggest that in the normal population, the level of plasma Lp(a) is largely determined by alleles at the apo(a) locus.