LP(a) is an atherogenic lipoprotein that is distinguished by its content of apolipoprotein [apo(a)]. Although the apo(a) cDNA was cloned in 1987, the apo(a) gene has not been characterized, due to technical problems in dealing with a gene containing multiple repeated sequences that are shared with plasminogen and several pseudogenes. We have overcome many of these problems by using pulsed-field gel electrophoresis to define a fragment of genomic DNA that contains all of the exons encoding kringle 4, the major repeated sequence in apo(a). The size of this genomic fragment, which ranges from 50 kb to 190 kb, correlates with the variable size of the apo(a) protein in different individuals, owing to the fact that different alleles possess different numbers of kringle 4- encoding repeats. This discovery provides a molecular handle with which to analyze the apo(a) gene directly, and to examine its role in determining the level of Lp(a) in humans. The specific questions that will be addressed are: 1) Why do plasma levels of Lp(a) vary between individuals and different ethnic groups? 2) How much of this variation is due to differences at the apo(a) locus? 3) What are the structural determinants at the apo(a) locus that affect the plasma level of Lp(a)? 4) And finally, do genes that influence LDL metabolism modulate the level of plasma Lp(a)? To answer these questions we will perform: 1) Population studies to determine the relationship between the size of the apo(a) gene and the plasma level of Lp(a), and to determine the genetic basis of the differences in Lp(a) levels that occur in different populations. 2) Family studies to determine if the plasma level of Lp(a) segregates with the apo(a) gene. 3) Molecular characterization of selected apo(a) alleles which are associated with either high, or low levels of plasma Lp(a). 4) And finally, pedigree analysis to determine the effect of mutations in the LDL receptor and apo(B) genes on the plasma level of Lp(a). By learning more about the apo(a) gene structure and its regulation to the plasma level of Lp(a), we hope to gain insights into the mechanisms underlying high Lp(a) levels in many individuals, and hopefully, these studies will help in the eventual unraveling of the mechanism by which Lp(a) accelerates atherosclerosis.
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