[Hyperlipoprotein (a)-emia determined by genetic polymorphisms in apolipoprotein (a) gene]

Prothrombotic and proatherogenic risk factors, including elevated lipoprotein (a), predispose an individual to initial and recurrent ischemic stroke. An increased plasma level of lipoprotein (a), over 25-30 mg/dL, is called hyperlipoprotein (a)-emia, and is largely determined by genetic polymorphisms in apolipoprotein (a). Apolipoprotein (a) is a major protein component of lipoprotein (a), and is connected with apolipoprotein B-100 of low density lipoprotein. The size of apolipoprotein (a) and concentration of lipoprotein (a) vary widely among individuals as well as between ethnic groups. The size of apolipoprotein (a) and plasma concentrations of lipoprotein (a) are inversely related. Accordingly, the number of tandemly repeated structures, named kringle-4 domains, determines the size of apolipoprotein (a), and consequently the plasma lipoprotein (a) level. In addition, the efficiency of apolipoprotein (a) gene expression is a determinant of lipoprotein (a) concentrations, since lipoprotein (a) levels vary more than 200-fold even among the individuals having the same apolipoprotein (a) size. First, we identified haplotypes in the 5'-promoter region of the apolipoprotein (a) gene as a regulating factor of plasma lipoprotein (a) levels. Second, a pentanucleotide repeat polymorphism upstream of the promoter region was also reported to affect plasma lipoprotein (a) levels. Third, two functional single nucleotide polymorphisms were identified in a distal enhancer region situated approximately 20 kb from the apolipoprotein (a) gene. Finally, several polymorphisms were identified in the kringle-4 domains, and found to influence plasma lipoprotein (a) levels as well as the lysine/fibrin-binding function. Since lower molecular weight forms of apolipoprotein (a) are closely associated with the incidence of ischemic stroke, both high plasma concentrations of lipoprotein (a) and small sizes (i. e., number of kringle-4 repeats in the gene) of apolipoprotein (a) are risk factors for the development of atherothrombosis.