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Molecular basis of congenital lp(a) deficiency: a frequent apo(a) 'null' mutation in caucasians
M Ogorelkova1, A Gruber, G Utermann
1Institute of Medical Biology and Human Genetics, University of Innsbruck, Schoepfstrasse 41, 6020 Innsbruck, Austria.
Insights
A common gene mutation in Caucasians causes a deficiency in lipoprotein(a) [Lp(a)], a particle linked to heart disease and stroke. This splice site mutation leads to a truncated apolipoprotein(a) [apo(a)] that cannot form the Lp(a) complex.
Area of Science:
- Genetics
- Cardiovascular Science
- Molecular Biology
Background:
- High plasma lipoprotein(a) [Lp(a)] concentrations are a risk factor for coronary heart disease and stroke.
- Lipoprotein(a) levels are highly heritable, primarily determined by the apolipoprotein(a) [apo(a)] gene.
- Congenital deficiency of Lp(a) suggests genetic causes influencing apo(a) function.
Purpose of the Study:
- To investigate a specific genetic mutation in the apo(a) gene and its association with Lp(a) deficiency.
- To elucidate the molecular mechanism by which the identified mutation affects Lp(a) complex formation and plasma levels.
Main Methods:
- Genotyping for a G-->A splice site substitution in the apo(a) gene.
- Reverse transcription-polymerase chain reaction (RT-PCR) to analyze apo(a) splicing.
- Expression studies in HepG2 cells to assess apo(a) function and complex formation.
- Immunoprecipitation of plasma apo(a) from affected individuals.
Main Results:
- A G-->A substitution at the +1 donor splice site of the apo(a) Kringle IV type 8 intron is frequent in Caucasians (approx. 6%) but absent in Africans.
- This mutation is strongly associated with congenital deficiency of plasma Lp(a) and accounts for a significant proportion of 'null' apo(a) alleles in Caucasians.
- The mutation leads to alternative splicing, producing a truncated apo(a) that is secreted but fails to form the covalent Lp(a) complex, resulting in rapid degradation.
Conclusions:
- A specific donor splice site mutation in the apo(a) gene is a common cause of Lp(a) deficiency in Caucasians.
- The mutation disrupts Lp(a) complex formation due to an alternatively spliced, truncated apo(a) protein.
- This molecular defect, followed by apo(a) degradation, explains the null Lp(a) phenotype associated with this common genetic variation.
Abstract:
High plasma concentrations of lipoprotein(a) [Lp(a)], a covalent low-density lipoprotein-apolipoprotein(a) [apo(a)] complex, are associated with coronary 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 demonstrate 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 ( approximately 6%) in Caucasians but not in Africans and is associated with congenital deficiency 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) illegitimate transcription in lympho- blastoid cells demonstrated that the donor splice site mutation results in an alternative splicing of the K IV type 8 intron and encodes a truncated form of apo(a). Expression of the alternatively spliced cDNA analogue in HepG2 cells showed that the truncated apo(a) form is secreted but is unable to form the covalent Lp(a) complex. Immunoprecipitated plasma apo(a) from homozygotes for the mutation was almost completely 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 the donor splice site mutation.