Molecular basis of congenital Lp(a) deficiency: a frequent apo(a) 'null' mutation in Caucasians

High plasma concentrations of lipoprotein(a) [Lp(a)], a covalent low-densit y lipoprotein-apolipoprotein(a) [apo(a)] complex, are associated with coron ary heart disease and stroke, Heritability of Lp(a) levels is high and the major locus determining Lp(a) concentrations is the apo(a) gene. We here de monstrate that a G-->A substitution at the +1 donor splice site of the apo( a) kringle (K) IV type 8 intron occurs with a high frequency (similar to 6% ) in Caucasians but not in Africans and is associated with congenital defic iency of Lp(a) in plasma, This mutation alone accounts for a quarter of all 'null' apo(a) alleles in Caucasians. RT-PCR analysis based on apo(a) illeg itimate transcription in lymphoblastoid cells demonstrated that the donor s plice site mutation results in an alternative splicing of the K IV type 8 i ntron and encodes a truncated form of apo(a), Expression of the alternative ly spliced cDNA analogue in HepG2 cells showed that the truncated apo(a) fo rm is secreted but is unable to form the covalent Lp(a) complex, Immunoprec ipitated plasma apo(a) from homozygotes for the mutation was almost complet ely fragmented. Taken together, our data indicate that a failure in complex formation followed by fast degradation in plasma of the truncated free apo (a) is one mechanism which underlies the null Lp(a) type associated with th e donor splice site mutation.