Lifelong reduction of LDL-cholesterol related to a common variant in the LDL-receptor gene decreases the risk of coronary artery disease--a Mendelian Randomisation study

Abstract

Background: Rare mutations of the low-density lipoprotein receptor gene (LDLR) cause familial hypercholesterolemia, which increases the risk for coronary artery disease (CAD). Less is known about the implications of common genetic variation in the LDLR gene regarding the variability of cholesterol levels and risk of CAD.

Methods: Imputed genotype data at the LDLR locus on 1 644 individuals of a population-based sample were explored for association with LDL-C level. Replication of association with LDL-C level was sought for the most significant single nucleotide polymorphism (SNP) within the LDLR gene in three European samples comprising 6 642 adults and 533 children. Association of this SNP with CAD was examined in six case-control studies involving more than 15 000 individuals.

Findings: Each copy of the minor T allele of SNP rs2228671 within LDLR (frequency 11%) was related to a decrease of LDL-C levels by 0.19 mmol/L (95% confidence interval (CI) [0.13-0.24] mmol/L, p = 1.5x10(-10)). This association with LDL-C was uniformly found in children, men, and women of all samples studied. In parallel, the T allele of rs2228671 was associated with a significantly lower risk of CAD (Odds Ratio per copy of the T allele: 0.82, 95% CI [0.76-0.89], p = 2.1x10(-7)). Adjustment for LDL-C levels by logistic regression or Mendelian Randomisation models abolished the significant association between rs2228671 with CAD completely, indicating a functional link between the genetic variant at the LDLR gene locus, change in LDL-C and risk of CAD.

Conclusion: A common variant at the LDLR gene locus affects LDL-C levels and, thereby, the risk for CAD.

Figure 1

Figure 1a: Strategy for exploring the association of LDLR polymorphism rs2228671 with LDL-C levels and CAD across multiple populations. Studies based on imputed genotypes for rs2228671 are marked with a *. Figure 1b: Schematic overview of study populations

Figure 2

Figure 2a: Association of the chromosomal locus 19p13.2 with LDL-C levels in population-based sample. Presented are p-values from linear regression of LDL-C on SNPs adjusted for age and gender. p-values of the SNPs genotyped on the Affymetrix GeneChip® 500K Mapping Array Set are displayed as closed squares, p-values predicted by HapMap-based imputation are displayed as blank circles. SNP rs2228671 is highlighted as a red circle. Figure 2b: Linkage disequilibrium structure of the genomic region surrounding SNP rs2228671. Displayed are the HapMap data (r²;[19]) of all SNPs surrounding rs2228671 (11030947–11079189) and the genes within this region.

Figure 3

Figure 3a: Association of SNP rs2228671 with LDL-C levels across populations. Displayed are means of LDL-C and standard error of means stratified by rs2228671 genotype. Unadjusted levels of significance are displayed for each individual study. Effects for the pooled study are from a random effect linear regression model. Figure 3b: Pooled analysis of the association of rs2228671 with LDL-C across populations stratified by age groups and gender. Displayed are means of LDL-C and standard error of means stratified by rs2228671 genotype, separately for age groups (<20, 21–35, 36–50, >50), men and women.

Figure 4. Association of rs2228671 with risk of CAD in six case-control studies.

Displayed are heterozygous and homozygous ORs for CAD and 95% CI per study from two-sided asymptotic Cochrane-Armitage trend test. Pooled ORs, 95% CIs and p-values are from random effect logistic regression models. The number of individuals in each study is represented by size of the closed square. SNP rs2228671 was imputed from the Affymetrix GeneChip® 500K Mapping Array Set in the WTCCC and German MI Family Study I and genotyped directly in the remaining studies.

Figure 5. Results of Mendelian Randomisation study.

Exploration of the causal relationship between LDL-C associated with rs2228671 and CAD (Mendelian Randomisation). In the structural equation model, carriage of the T allele at rs2228671 leads to lower LDL-C levels, and higher LDL-C levels lead to an increased risk of CAD. Given this, there is no additional direct path from rs2228671 to CAD risk, indicating that the functional pathway between the genetic variant at the LDLR gene locus and risk of CAD is through changes in LDL-C.