Replication and Characterization of Association between ABO SNPs and Red Blood Cell Traits by Meta-Analysis in Europeans

Abstract

Red blood cell (RBC) traits are routinely measured in clinical practice as important markers of health. Deviations from the physiological ranges are usually a sign of disease, although variation between healthy individuals also occurs, at least partly due to genetic factors. Recent large scale genetic studies identified loci associated with one or more of these traits; further characterization of known loci and identification of new loci is necessary to better understand their role in health and disease and to identify potential molecular mechanisms. We performed meta-analysis of Metabochip association results for six RBC traits-hemoglobin concentration (Hb), hematocrit (Hct), mean corpuscular hemoglobin (MCH), mean corpuscular hemoglobin concentration (MCHC), mean corpuscular volume (MCV) and red blood cell count (RCC)-in 11 093 Europeans from seven studies of the UCL-LSHTM-Edinburgh-Bristol (UCLEB) Consortium. We identified 394 non-overlapping SNPs in five loci at genome-wide significance: 6p22.1-6p21.33 (with HFE among others), 6q23.2 (with HBS1L among others), 6q23.3 (contains no genes), 9q34.3 (only ABO gene) and 22q13.1 (with TMPRSS6 among others), replicating previous findings of association with RBC traits at these loci and extending them by imputation to 1000 Genomes. We further characterized associations between ABO SNPs and three traits: hemoglobin, hematocrit and red blood cell count, replicating them in an independent cohort. Conditional analyses indicated the independent association of each of these traits with ABO SNPs and a role for blood group O in mediating the association. The 15 most significant RBC-associated ABO SNPs were also associated with five cardiometabolic traits, with discordance in the direction of effect between groups of traits, suggesting that ABO may act through more than one mechanism to influence cardiometabolic risk.

Fig 1. Meta-analysis association results for hemoglobin in unconditional and conditional analyses in the ABO locus.

Regional plots show (A) unconditional analysis and analysis conditional on (B) O, (C) AO, (D) AA, (E) B and (F) AB blood group haplotype. The most significant SNP in the unconditional analysis, rs507666, is highlighted throughout to facilitate comparison of results. The color coding of the LD between the SNPs ranges from dark blue for r² = 0–0.2 to red for r² = 0.8–1, and is grey where LD information was not available. Blue line represents suggestive and red line significant threshold.

Fig 2. Manhattan plots of meta-analysis association results in unconditional and conditional analyses for hemoglobin (Hb), hematocrit (Hct) and red blood cell count (RCC).

Results show unconditional analysis for all three traits (top row) and analysis of Hb conditional on (A) Hct and (B) RCC, of Hct conditional on (C) Hb and (D) RCC and of RCC conditional on (E) Hb and (F) Hct. Line at–log₁₀(P value) = 5.3 represents suggestive threshold and line at–log₁₀(P value) = 7.3 significant threshold.

Fig 3. Comparison of the effect sizes on 15 ABO SNPs between eight different traits.

Traits include von Willebrand factor (log transformed, logVWF) and factor VIII (FVIII) for coagulation factors, total cholesterol (TC) and low-density lipoprotein (LDL) for lipids, hemoglobin (Hb), red blood cell count (RCC) and hematocrit (Hct) for the red blood cell (RBC) traits and alkaline phosphatase (log transformed, logALP) for liver marker group. The colored bar for each SNP represents the 95% confidence interval of the effect size.