Bilirubin as a potential causal factor in type 2 diabetes risk: a Mendelian randomization study

Abstract

Circulating bilirubin, a natural antioxidant, is associated with decreased risk of type 2 diabetes (T2D), but the nature of the relationship remains unknown. We performed Mendelian randomization in a prospective cohort of 3,381 participants free of diabetes at baseline (age 28-75 years; women 52.6%). We used rs6742078 located in the uridine diphosphate-glucuronosyltransferase locus as an instrumental variable (IV) to study a potential causal effect of serum total bilirubin level on T2D risk. T2D developed in a total of 210 participants (6.2%) during a median follow-up period of 7.8 years. In adjusted analyses, rs6742078, which explained 19.5% of bilirubin variation, was strongly associated with total bilirubin (a 0.68-SD increase in bilirubin levels per T allele; P < 1 × 10(-122)) and was also associated with T2D risk (odds ratio [OR] 0.69 [95% CI 0.54-0.90]; P = 0.006). Per 1-SD increase in log-transformed bilirubin levels, we observed a 25% (OR 0.75 [95% CI 0.62-0.92]; P = 0.004) lower risk of T2D. In Mendelian randomization analysis, the causal risk reduction for T2D was estimated to be 42% (causal OR for IV estimation per 1-SD increase in log-transformed bilirubin 0.58 [95% CI 0.39-0.84]; P = 0.005), which was comparable to the observational estimate (Durbin-Wu-Hausman χ(2) test, P for difference = 0.19). These novel results provide evidence that an elevated bilirubin level is causally associated with the risk of T2D and support its role as a protective determinant.

Figure 1. Study population sample from the PREVEND study cohort

For quality control (QC), samples were excluded (n=367) due to call rates below 0.95, sex mismatch, duplicate discordance, contamination and genetic similarity (identity by state > 0.2). Population stratification was assessed by principal component analysis over the sample correlation matrix, based on 16,842 independent (LD-pruned) SNPs. We excluded samples when they diverged from the mean with at least 3 standard deviations (Z-score > 3) for the first five principal components.

Figure 2. Histogram of serum total bilirubin levels with odds ratios for new onset (i.e., incident) type 2 diabetes over the range of bilirubin

The number of participants represents by the white bars which correspond to the right axis. The solid line denotes the odds ratios which were centered on the median value of bilirubin.

Figure 3. Association of the rs6742078 genotypes (TT, TG and GG) with levels of total bilirubin and incidence of type 2 diabetes in the study population

The mean (s.e.) level of bilirubin for each genotype is represented by the gray bars in the left panel of the figure. The odds ratios (95% confidence intervals) for the association of the genotypes with new onset type 2 diabetes are shown in the right panel of the figure.

Figure 4. Mendelian randomization analysis for the association of levels of serum total bilirubin and the incidence of type 2 diabetes

The observed effect of genotype on bilirubin levels (β_{genotype-bilirubin}) and risk of type 2 diabetes (β_genotype-T2D) was estimated per each copy of the T allele of rs6742078. The observed effect of bilirubin on type 2 diabetes was estimated per 1-SD increase in log-transformed bilirubin levels. The estimated causal risk of type 2 diabetes was calculated by the Wald-type method which is a ratio of β_genotype-T2D to β_{genotype-bilirubin}, where risk= (1-exp(β))×100.