High Lp(a) levels associate with mortality, through low number of Kringle-IV type 2 repeats
High lipoprotein(a) and high risk of mortality
Introduction and methods
Observational and genetic data have shown that high lipoprotein(a) [Lp(a)] levels are associated with high risk of CV disease, including myocardial infarction and aortic valve stenosis. Thus, the search for therapeutic options to lower Lp(a) are ongoing. No randomized, double-blind evidence is, however, available demonstrating that lowering Lp(a) will reduce the risk of CV disease [1-7].
If lowering Lp(a) lowers CV disease, it is anticipated that it will also reduce CV and all-cause mortality. High Lp(a) levels may, however, also protect against bleeding events [8,9], which implies that lowering the levels could increase mortality risk. The relationship between Lp(a) and mortality is presently unclear.
This study tested the hypothesis that Lp(a) levels are associated with risk of mortality, both observationally and causally, based on human genetic data. Data of 126,936 individuals from the Danish general population [Copenhagen City Heart Study, CCHS) and Copenhagen General Population Study (CGPS)] were used, without loss to follow-up. It was also examined whether such associations are driven by low number of LPA kringle-IV type 2 (KIV-2) repeats and thus small apolipoprotein(a) isoform size. Participants filled out a questionnaire including lifestyle and medical history and underwent a physical examination (with blood sampling for lipid and lipoprotein level measurements).
Main results
- Graphs with the multivariable adjusted HRs according to Lp(a) levels show that high Lp(a) levels were associated with high risk of CV mortality and all-cause mortality, but not of non-CV mortality.
- Compared to those with Lp(a) <10 mg/dL (1st-50th percentiles), the multivariable adj HR of CV mortality was 1.50 (95%CI: 1.28-1.76) for those with Lp(a) >93 mg/dL (96th-100th percentiles), 1.32 (95%CI: 1.12-1.56) for 69-93 mg/dL (91st-95th), 1.02 (95%CI: 0.89-1.16) for 43-68 mg/dL (81st-90th) and 0.97 (95%CI: 0.89-1.07) for 10-42 mg/dL (51st -80th).
- Low KIV-2 number of repeats was associated only with higher CV mortality. In those with <22 repeats (1st-5th percentiles), compared to those with >35 (51st-100th), the adj HR were 1.30 (95%CI: 1.11-1.51) for CV mortality and 1.08 (95%CI: 0.99-1.16) for all-cause mortality. Adjustment for age and sex yielded similar results, but after adjustment for Lp(a) levels, the relations were no longer significant.
- Observational data showed that in those with 50 mg/dL higher Lp(a) levels, the adj HR were 1.16 (95%CI: 1.09-1.23) for CV mortality, 0.97 (95%CI: 0.93-1.01) for non-CV mortality and 1.05 (95%CI: 1.01-1.09) for all-cause mortality.
- In instrumental variable analysis for a genetically 50 mg/dL higher Lp(a) level, the adjusted genetic causal risk ratio for CV mortality was 1.23 (95%CI: 1.08-1.41) based on LPA KIV-2 number of repeats and 0.98 (95%CI: 0.88-1.09) based on the LPA rs10455872 genotype.
Conclusion
In individuals of the Danish general population, high levels of Lp(a) as a consequence of low LPA KIV-2 number of repeats, were associated with high risk of mortality. Interestingly, the LPA rs10455872 genotype and KIV-2 number of repeats showed inconsistent associations with all-cause and CV mortality, which questions the view that the rs10455872 genotype is an indirect reporter of KIV-2 variation. The data suggest that the rs10455872 genotype is primarily associated with plasma Lp(a) levels, while KIV-2 number of repeats is more related with apolipoprotein(a) isoform size, as well as plasma levels.
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