SIGNIFICANT IMPACT OF THE -C(93)T-POLYMORPHISM IN THE APOLIPOPROTEIN(A) GENE ON LP(A) CONCENTRATIONS IN AFRICANS BUT NOT IN CAUCASIANS - CONFOUNDING EFFECT OF LINKAGE DISEQUILIBRIUM/

Lipoprotein(a) [Lp(a)] is a quantitative genetic trait in human plasma associated with atherothrombotic disease, The major determinant of Lp (a) concentration is the apolipoprotein(a) [apo(a)] gene locus, Variat ion in the number of kringle IV repeats (K-IV VNTR) in apo(a) has a di rect effect on Lp(a) concentrations but explains only a fraction of th e large intra-and interpopulation variance in Lp(a) levels, Effects on Lp(a) of other intragenic polymorphisms including a pentanucleotide r epeat (PNRP) in the promoter likely reflect allelic associations with as yet unidentified sequence variation in the apo(a) gene, We have stu died a candidate C-->T transition in two European and two African popu lations, This polymorphism in the 5' region of the apo(a) gene creates an ATG start codon thereby reducing apo(a) translation in vitro by 60 %. All samples were also analyzed for the K-IV VNTR and the PNRP to st ratify for their effects and to consider allelic associations, Consist ent with the in vitro effect the C-->T transition was associated with a significant reduction in Lp(a) levels in both African populations (P < 0.0056), In Caucasians, however, the effect was not significant, Thi s was explained by linkage disequilibrium of the +93 T with apo(a) all eles of intermediate length (K-24-K-34) and with nine PNRs, In Europea ns these alleles are associated with low Lp(a) which makes any potenti al effect of the +93 T undetectable in the total sample. From our resu lts we conclude (i) that the +93 C/T polymorphism is the second known intragenic apo(a) polymorphism which affects Lp(a) levels directly in vivo; (ii) that allelic associations may mask the effect of a mutation ; and (iii) that heterogeneity of an effect of a mutation across popul ations does not disprove causality.