Apolipoprotein(a) isoform size, lipoprotein(a) concentration, and coronary artery disease: a mendelian randomisation analysis

Abstract

Background: The lipoprotein(a) pathway is a causal factor in coronary heart disease. We used a genetic approach to distinguish the relevance of two distinct components of this pathway, apolipoprotein(a) isoform size and circulating lipoprotein(a) concentration, to coronary heart disease.

Methods: In this mendelian randomisation study, we measured lipoprotein(a) concentration and determined apolipoprotein(a) isoform size with a genetic method (kringle IV type 2 [KIV2] repeats in the LPA gene) and a serum-based electrophoretic assay in patients and controls (frequency matched for age and sex) from the Pakistan Risk of Myocardial Infarction Study (PROMIS). We calculated odds ratios (ORs) for myocardial infarction per 1-SD difference in either LPA KIV2 repeats or lipoprotein(a) concentration. In a genome-wide analysis of up to 17 503 participants in PROMIS, we identified genetic variants associated with either apolipoprotein(a) isoform size or lipoprotein(a) concentration. Using a mendelian randomisation study design and genetic data on 60 801 patients with coronary heart disease and 123 504 controls from the CARDIoGRAMplusC4D consortium, we calculated ORs for myocardial infarction with variants that produced similar differences in either apolipoprotein(a) isoform size in serum or lipoprotein(a) concentration. Finally, we compared phenotypic versus genotypic ORs to estimate whether apolipoprotein(a) isoform size, lipoprotein(a) concentration, or both were causally associated with coronary heart disease.

Findings: The PROMIS cohort included 9015 patients with acute myocardial infarction and 8629 matched controls. In participants for whom KIV2 repeat and lipoprotein(a) data were available, the OR for myocardial infarction was 0·93 (95% CI 0·90-0·97; p<0·0001) per 1-SD increment in LPA KIV2 repeats after adjustment for lipoprotein(a) concentration and conventional lipid concentrations. The OR for myocardial infarction was 1·10 (1·05-1·14; p<0·0001) per 1-SD increment in lipoprotein(a) concentration, after adjustment for LPA KIV2 repeats and conventional lipids. Genome-wide analysis identified rs2457564 as a variant associated with smaller apolipoprotein(a) isoform size, but not lipoprotein(a) concentration, and rs3777392 as a variant associated with lipoprotein(a) concentration, but not apolipoprotein(a) isoform size. In 60 801 patients with coronary heart disease and 123 504 controls, OR for myocardial infarction was 0·96 (0·94-0·98; p<0·0001) per 1-SD increment in apolipoprotein(a) protein isoform size in serum due to rs2457564, which was directionally concordant with the OR observed in PROMIS for a similar change. The OR for myocardial infarction was 1·27 (1·07-1·50; p=0·007) per 1-SD increment in lipoprotein(a) concentration due to rs3777392, which was directionally concordant with the OR observed for a similar change in PROMIS.

Interpretation: Human genetic data suggest that both smaller apolipoprotein(a) isoform size and increased lipoprotein(a) concentration are independent and causal risk factors for coronary heart disease. Lipoprotein(a)-lowering interventions could be preferentially effective in reducing the risk of coronary heart disease in individuals with smaller apolipoprotein(a) isoforms.

Funding: British Heart Foundation, US National Institutes of Health, Fogarty International Center, Wellcome Trust, UK Medical Research Council, UK National Institute for Health Research, and Pfizer.

Figure 1

Overview of analyses Analysis 1: ORs for myocardial infarction with measured values of LPA KIV2 repeats and lipoprotein(a) concentration, adjusted for each other. Analysis 2: Genome-wide association analyses of apolipoprotein(a) protein isoform size and lipoprotein(a) concentration and identification of variants that are exclusively associated with either apolipoprotein(a) protein isoform size or lipoprotein(a) concentration. Analysis 3: Test for causality by comparison of ORs for coronary heart disease associated with either LPA KIV2 repeats or lipoprotein(a) concentration with equivalent differences produced by their trait-specific genetic variants. Analysis 4: Study of soluble OxPL-apoB concentrations in relation to genome-wide genotypes. KIV2=kringle IV type 2. OR=odd ratio. OxPL-apoB=oxidised phospholipid on apolipoprotein B100. *Data only used in genetic analyses. †Data used in mendelian randomisation analyses.

Figure 2

Association of variants specifically associated with apolipoprotein(a) isoform size and lipoprotein(a) concentration with various factors in PROMIS Bars are 95% CIs. LPA=lipoprotein(a) gene. KIV2=kringle IV type 2. eGFR=estimated glomerular filtration rate. LpPLA2=lipoprotein-associated phospholipase A2. CFH=complement factor H. MDA-LDL=malondialdehyde-modified LDL. CXCL12=C-X-C motif chemokine 12. FGF21=fibroblast growth factor 21. ICAM=intercellular adhesion molecule 1. VCAM=vascular cell adhesion molecule. MMP9=matrix metallopeptidase 9. CRP=C-reactive protein. VEGF=vascular endothelial growth factor. RAGE=receptor for advanced glycation end products. NrCAM=neural cell adhesion molecule.

Figure 3

Mutually adjusted association of LPA KIV2 repeats and lipoprotein(a) concentration with coronary heart disease in PROMIS Bars are 95% CIs. (A) Association of LPA KIV2 repeats with coronary heart disease risk. (B) Association of lipoprotein(a) with coronary heart disease risk. KIV2=kringle IV type 2. OR=odds ratio.

Figure 4

Phenotypic and genotypic assessment of apolipoprotein(a) isoform size and lipoprotein(a) concentration in coronary heart disease (A) OR for coronary heart disease per 1-SD increment in LPA KIV2 repeats. (B) OR for coronary heart disease per 1-SD increment in lipoprotein(a) concentration. Phenotypic associations were computed in PROMIS and genetic associations were computed using data from the CARDIoGRAMplusC4D consortium. KIV2=kringle IV type 2. OR=odds ratio.

Figure 5

Association of rs7770628 with OxPL-apoB after adjustment for KIV2 repeats and log-lipoprotein(a) concentration in PROMIS Bars are 95% CIs. KIV2=kringle IV type 2. OxPL-apoB=oxidised phospholipid on apolipoprotein B100.