Sex-Specific Genetically Predicted Iron Status in relation to 12 Vascular Diseases: A Mendelian Randomization Study in the UK Biobank

Associations Between Genetically Predicted Iron Status and Cardiovascular Disease Risk: A Mendelian Randomization Study

BACKGROUND

Mendelian randomization (MR) studies suggest a causal effect of iron status on cardiovascular disease (CVD) risk, but it is unknown if these associations are confounded by pleiotropic effects of the instrumental variables on CVD risk factors. We aimed to investigate the effect of iron status on CVD risk controlling for CVD risk factors.

METHODS AND RESULTS

Iron biomarker instrumental variables (total iron-binding capacity [n=208 422], transferrin saturation [n=198 516], serum iron [n=236 612], ferritin [n=257 953]) were selected from a European genome-wide association study meta-analysis. We performed 2-sample univariate MR of each iron trait on CVD outcomes (all-cause ischemic stroke, cardioembolic ischemic stroke, large-artery ischemic stroke, small-vessel ischemic stroke, and coronary heart disease) from MEGASTROKE (n=440 328) and CARDIoGRAMplusC4D (Coronary Artery Disease Genome Wide Replication and Meta-Analysis Plus the Coronary Artery Disease Genetics) (n=183 305). We then implemented multivariate MR conditioning on 7 CVD risk factors from independent European samples to evaluate their potential confounding or mediating effects on the observed iron-CVD associations. With univariate MR analyses, we found higher genetically predicted iron status to be associated with a greater risk of cardioembolic ischemic stroke (transferrin saturation: odds ratio, 1.17 [95% CI, 1.03-1.33]; serum iron: odds ratio, 1.21 [95% CI, 1.02-1.44]; total iron-binding capacity: odds ratio, 0.81 [95% CI, 0.69-0.94]). The detrimental effects of iron status on cardioembolic ischemic stroke risk remained unaffected when adjusting for CVD risk factors (all P<0.05). Additionally, we found diastolic blood pressure to mediate between 7.1 and 8.8% of the total effect of iron status on cardioembolic ischemic stroke incidence. Univariate MR initially suggested a protective effect of iron status on large-artery stroke and coronary heart disease, but controlling for CVD factors using multivariate MR substantially diminished these associations (all P>0.05).

CONCLUSIONS

Higher iron status was associated with a greater risk of cardioembolic ischemic stroke independent of CVD risk factors, and this effect was partly mediated by diastolic blood pressure. These findings support a role of iron status as a modifiable risk factor for cardioembolic ischemic stroke.

Iron Status and Risk of Heart Disease, Stroke, and Diabetes: A Mendelian Randomization Study in European Adults

BACKGROUND

The relevance of iron status biomarkers for coronary artery disease (CAD), heart failure (HF), ischemic stroke (IS), and type 2 diabetes (T2D) is uncertain. We compared the observational and Mendelian randomization (MR) analyses of iron status biomarkers and hemoglobin with these diseases.

METHODS AND RESULTS

Observational analyses of hemoglobin were compared with genetically predicted hemoglobin with cardiovascular diseases and diabetes in the UK Biobank. Iron biomarkers included transferrin saturation, serum iron, ferritin, and total iron binding capacity. MR analyses assessed associations with CAD (CARDIOGRAMplusC4D [Coronary Artery Disease Genome Wide Replication and Meta-Analysis Plus The Coronary Artery Disease Genetics], n=181 522 cases), HF (HERMES [Heart Failure Molecular Epidemiology for Therapeutic Targets), n=115 150 cases), IS (GIGASTROKE, n=62 100 cases), and T2D (DIAMANTE [Diabetes Meta-Analysis of Trans-Ethnic Association Studies], n=80 154 cases) genome-wide consortia. Observational analyses demonstrated J-shaped associations of hemoglobin with CAD, HF, IS, and T2D. In contrast, MR analyses demonstrated linear positive associations of higher genetically predicted hemoglobin levels with 8% higher risk per 1 SD higher hemoglobin for CAD, 10% to 13% for diabetes, but not with IS or HF in UK Biobank. Bidirectional MR analyses confirmed the causal relevance of iron biomarkers for hemoglobin. Further MR analyses in global consortia demonstrated modest protective effects of iron biomarkers for CAD (7%-14% lower risk for 1 SD higher levels of iron biomarkers), adverse effects for T2D, but no associations with IS or HF.

CONCLUSIONS

Higher levels of iron biomarkers were protective for CAD, had adverse effects on T2D, but had no effects on IS or HF. Randomized trials are now required to assess effects of iron supplements on risk of CAD in high-risk older people.

Iron status and non-alcoholic fatty liver disease: A Mendelian randomization study

OBJECTIVES

The aim of this study was to assess the association of genetically determined iron status with the risk for non-alcoholic fatty liver disease (NAFLD) using two-sample Mendelian randomization (MR) analysis.

METHODS

We applied single nucleotide polymorphisms (SNPs) associated at genome-wide significance with iron status proxied by serum iron, ferritin, transferrin, and transferrin saturation from the Genetics of Iron status Consortium (N = 48 793), in a genome-wide association study of 1664 NAFLD cases and 400 055 controls from the United Kingdom Biobank. A SNP associated with multiple markers of iron status was only applied to one marker with the strongest association in the main analysis. Their effects on NAFLD were calculated using inverse variance weighting after excluding SNPs associated with alkaline phosphatase and lipid metabolism.

RESULTS

The risk for NAFLD is negatively associated with genetically predicted serum transferrin level with a 20% reduction in NAFLD risk per SD (0.65g/L) increase in transferrin (95% confidence interval [CI], 0.66-0.97), and trending positive association with transferrin saturation (odds ratio [OR], 1.50; 95% CI, 0.96-2.35) but it was not associated with serum iron (OR, 0.90; 95% CI, 0.63-1.29) and ferritin (OR, 1.33; 95% CI, 0.54-3.30).

CONCLUSIONS

MR analysis provided evidence that genetically predicted higher serum transferrin, indicating lower iron status, may be protective against NAFLD, whereas higher transferrin saturation, indicating higher iron status, might increase the risk for NAFLD and its pathogenesis.

Using Mendelian Randomization to Study the Role of Iron in Health and Disease

Iron has been shown to play a dual role in health and disease, with either a protective or harmful effect. Some of the contradictory findings from observational studies may be due to reverse causation, residual confounding, or small sample size. One approach that may overcome these limitations without the high cost of randomized control trials is the use of Mendelian randomization to examine the long-term role of iron in a variety of health outcomes. As there is emerging evidence employing Mendelian randomization as a method of assessing the role of micronutrients in health and disease, this narrative review will highlight recent Mendelian randomization findings examining the role of iron in cardiometabolic disorders, inflammation, neurological disorders, different cancers, and a number of other health-related outcomes.

Increased selenium and decreased iron levels in relation to risk of coronary artery disease in patients with diabetes

Background

Observational studies have reported inconsistent associations between micronutrient levels and the risk of coronary artery disease (CAD) in diabetic patients. We aim to explore the causal association between genetically predicted concentrations of micronutrients (phosphorus, magnesium, selenium, iron, zinc, and copper) and CAD in patients with diabetes.

Methods

Single nucleotide polymorphisms (SNPs) connected to serum micronutrient levels were extracted from the corresponding published genome-wide association studies (GWASs). Summary-level statistics for CAD in diabetic patients were obtained from a GWAS of 15,666 patients with diabetes. The primary analysis was carried out with the inverse variance weighted approach, and sensitivity analyses using other statistical methods were further employed to assess the robustness of the results.

Results

Genetically predicted selenium level was causally associated with a higher risk of CAD in diabetic patients (odds ratio [OR]: 1.25; 95% confidence interval [CI]: 1.10-1.42; p = 5.01 × 10^-4). While, genetically predicted iron concentrations in patients with diabetes were inversely associated with the risk of CAD (OR: 0.82; 95% CI: 0.75-0.90; p = 2.16 × 10^-5). The association pattern kept robust in most sensitivity analyses. Nominally significant associations were observed for magnesium and copper with the risk of CAD in patients with diabetes. No consistent evidence was found for the causal associations between phosphorus and zinc levels, and the risk of CAD in patients with diabetes.

Conclusion

We provide consistent evidence for the causal effect of increased selenium and decreased iron levels on CAD in patients with diabetes, highlighting the necessity of micronutrient monitoring and application in these patients.

Cardiometabolic, Lifestyle, and Nutritional Factors in Relation to Varicose Veins: A Mendelian Randomization Study

Background We conducted a 2-sample Mendelian randomization study to assess the associations of cardiometabolic, lifestyle, and nutritional factors with varicose veins. Methods and Results Independent single-nucleotide polymorphisms associated with height (positive control), body mass index, type 2 diabetes, diastolic and systolic blood pressure, smoking, alcohol and coffee consumption, 7 circulating vitamins (A, B6, B9, B12, C, 25-hydroxyvitamin D, and E), and 5 circulating minerals (calcium, iron, magnesium, selenium, and zinc) at the genome-wide significance level were used as instrumental variables. Summary-level data for the genetic associations with varicose veins were obtained from the UK Biobank (8763 cases and 352 431 noncases) and the FinnGen consortium (13 928 cases and 153 951 noncases). Genetically predicted higher height, body mass index, smoking, and circulating iron levels were associated with an increased risk of varicose veins. The odds ratios (ORs) per 1-SD increase in the exposure were 1.34 (95% CI, 1.25-1.43) for height, 1.39 (95% CI, 1.27-1.52) for body mass index, 1.12 (95% CI, 1.04-1.22) for the prevalence of smoking initiation, and 1.24 (95% CI, 1.16-1.33) for iron. Higher genetically predicted systolic blood pressure and circulating calcium and zinc levels were associated with a reduced risk of varicose veins, whereas the association for systolic blood pressure did not persist after adjustment for genetically predicted height. The OR was 0.75 (95% CI, 0.62-0.92) per 1-SD increase in calcium levels and 0.97 (95% CI, 0.95-0.98) for zinc. Conclusions This study identified several modifiable risk factors for varicose veins.