Genetic effects of iron levels on liver injury and risk of liver diseases: A two-sample Mendelian randomization analysis

Abstract

Background and aims: Although iron homeostasis has been associated with liver function in many observational studies, the causality in this relationship remains unclear. By using Mendelian Randomization analyses, we aimed to evaluate the genetic effects of increased systemic iron levels on the risk of liver injury and various liver diseases. Moreover, in light of the sex-dependent iron regulation in human beings, we further estimated the sex-specific effect of iron levels in liver diseases.

Methods: Independent single nucleotide polymorphisms associated with systemic iron status (including four indicators) at the genome-wide significance level from the Genetics of Iron Status (GIS) Consortium were selected as instrumental variables. Summary data for six liver function biomarkers and five liver diseases were obtained from the UK Biobank, the Estonian Biobank, the eMERGE network, and FinnGen consortium. Mendelian Randomization assessment of the effect of iron on liver function and liver diseases was conducted.

Results: Genetically predicted iron levels were positively and significantly associated with an increased risk of different dimensions of liver injury. Furthermore, increased iron status posed hazardous effects on non-alcoholic fatty liver disease, alcoholic liver disease, and liver fibrosis/cirrhosis. Sex-stratified analyses indicated that the hepatoxic role of iron might exist in NAFLD and liver fibrosis/cirrhosis development among men. No significantly causal relationship was found between iron status and viral hepatitis.

Conclusion: Our study adds to current knowledge on the genetic role of iron in the risk of liver injury and related liver diseases, which provides clinical and public health implications for liver disease prevention as iron status can be modified.

Keywords: iron; liver fibrosis/ cirrhosis; liver injury; mendelian randomization; non-alcoholic fatty liver disease.

Figure 1

Graphical overview of the two-sample MR study design. Three SNPs, each of which has a genome-wide significant association with increased serum iron, increased ferritin, increased transferrin saturation and decreased transferrin levels, were used as instruments for systemic iron status. By using genetic instruments associated with these four iron status biomarkers, the MR approach can be used to estimate the causal effect of systemic iron status on the risk of liver function (biomarkers including ALP, ALT, AST, GGT, DBIL, TBIL) and liver disease (NAFLD, ALD, fibrosis and cirrhosis, viral hepatitis, malignant neoplasm). Replication and reverse MR analyses were performed in the largest available GWAS studies. MR, Mendelian randomization; SNP, single-nucleotide polymorphism.

Figure 2

Associations of genetically preidicted iron status and liver biomarker. (A) Causal effects of iron status on ALP, ALT, AST, GGT; (B) Causal effects of iron status on DBIL; (C) Causal effects of iron status on TBIL. The beta (95% CI) of standardized liver biomarkers (ALP, ALT, AST, GGT, DBIL, and TBIL) per SD increase of iron status biomarkers were estimated using fixed-effect inverse-variance weighted meta-analysis. Beta, the Mendelian Randomization effect of continuous variable outcome; 95% CI, 95% confidence interval; ALP, alkaline phosphatase; ALT, alanine aminotransferase; AST, aspartate aminotransferase; GGT, gamma glutamyltransferase; DBIL, direct bilirubin; and TBIL, total bilirubin; SD, standard deviation.

Figure 3

Associations of genetically predicted iron status and liver diseases. The OR (95% CI) of liver diseases (NAFLD, ALD, fibrosis and cirrhosis, viral hepatitis, and malignant neoplasm) per SD increase of iron status biomarkers were estimated using fixed-effect inverse-variance weighted meta-analysis. OR, odds ratio; 95% CI, 95% confidence interval; NAFLD, nonalcoholic fatty liver disease; ALD, alcoholic liver disease; SD, standard deviation.

Figure 4

Sex-specific associations of genetically predicted iron status and liver diseases. The OR (95% CI) of liver diseases (NAFLD, fibrosis and cirrhosis and viral hepatitis) by sex per SD increase of iron status biomarkers were estimated using fixed-effect inverse-variance weighted meta-analysis. OR, odds ratio; 95% CI, 95% confidence interval; NAFLD, nonalcoholic fatty liver disease; SD, standard deviation.

Figure 5

Replication of associations of genetically predicted iron status and NAFLD in the largest NAFLD GWAS study. The ORs (95% CI) of NAFLD per SD increase of iron status biomarkers were estimated using a fixed-effect inverse-variance weighted meta-analysis. OR, odds ratio; 95% CI, 95% confidence interval; NAFLD, nonalcoholic fatty liver disease; SD, standard deviation.

Figure 6

Replication of associations of genetically predicted iron status and liver diseases in the updated database. The ORs (95% CI) of liver diseases (ALD, fibrosis and cirrhosis, viral hepatitis) per SD increase of iron status biomarkers were estimated using fixed-effect inverse-variance weighted meta-analysis. OR, odds ratio; 95% CI, 95% confidence interval; ALD, alcoholic liver disease; SD, standard deviation.