1.Study design
Figure 1 illustrates the design of this study. Our aim is to assess the causal effect of SGLT-2 inhibitors on thyroid dysfunction using a two-sample drug-target MR method. The specific methodology includes: 1) Selecting genetic variants related to SGLT-2 inhibitors; 2) Obtain research outcomes through UK Biobank and FinnGen databases; 3) Utilizing drug-target MR analysis to estimate the causal relationship between SGLT-2 inhibitors and thyroid dysfunction.
2.Selection and validation of SGLT-2 inhibitors genetic instrumental variables
The selection and validation of genetic instrumental variables for SGLT-2 inhibitors were conducted through four sequential steps, as previously described(14). Firstly, we identified genetic variants associated with mRNA expression levels of the SLC5A2 gene (the target gene for SGLT-2 inhibitors) in the Genotype-Tissue Expression (GTEx) and eQTLGen consortium databases, considering only variants with a significance level of P < 0.001. Additionally, functional variants with the potential for SGLT-2 inhibitors were also incorporated. Secondly, we assessed the association between each SLC5A2 variant and HbA1c levels (a marker for the glucose-lowering effect of SGLT-2 inhibitors) in a subpopulation of unrelated individuals of European ancestry without diabetes from the UK Biobank. We selected variants that showed regional association with HbA1c at a significance level of P < 1 × 10-4. Thirdly, we employed a genetic colocalization approach to determine if SLC5A2 and HbA1c shared the same causal variants, with evidence of colocalization defined as a probability of colocalization between SLC5A2 expression and HbA1c exceeding 70%. Finally, we performed standard clustering procedures, removing variants with a pairwise correlation exceeding 0.8 to alleviate issues related to high correlation. After this rigorous screening and validation process, a total of 6 SNPs were identified as genetic instrumental variables strongly associated with both HbA1c and SGLT-2 inhibitors for subsequent MR analysis.
3.Study outcomes
The outcomes of this study are based on data derived from the two largest currently available GWAS databases, namely the UK Biobank database and the FinnGen study. In particular, the data for hyperthyroidism were obtained from the UK Biobank database, consisting of 3,545 cases. The data for thyroid disease (56,574 cases), hypothyroidism (40,926 cases), and immune-related diseases (96,150 cases) were obtained from the ninth release of the FinnGen study (Table S1).
4.Statistical analysis
We employed five main MR analysis methods to determine the results, namely inverse variance-weighted (IVW), MR-Egger, weighted median, simple mode, and weighted mode methods. Among them, the IVW method was used as the primary approach. In order to ensure the accuracy of the experimental results and avoid violating the three assumptions of MR, we conducted sensitivity analysis using tests for pleiotropy test, heterogeneity test, and leave-one-out sensitivity analysis.
In this experiment, we utilized the MR-Egger and the Mendelian randomization pleiotropy residual sum and outlier (MR-PRESSO) global tests to confirm the presence of horizontal pleiotropy. A p-value greater than 0.05 indicates the reliability of the obtained results. Simultaneously, the MR-PRESSO outlier test was performed to identify and eliminate outlier SNPs in the final result analysis.
For heterogeneity analysis, the main approaches used were the IVW and MR-Egger analysis methods, based on the evaluation of Cochran's Q statistic. An observed p-value greater than 0.05 indicates the absence of heterogeneity among the utilized tools, affirming the robustness and reliability of the findings. Additionally, we visualized the heterogeneity of causal estimates through forest plots and funnel plots.
To assess the robustness of the results, we utilized the leave-one-out method for conducting sensitivity analysis. The method entails the sequential exclusion of individual SNPs. If all the curves on the leave-one-out plot align consistently on one side of 0, it suggests that omitting any SNP will have no fundamental impact on the research results, thus demonstrating the robustness of the findings.
All analyses were performed using the TwoSampleMR and MendelianRandomization packages in R (version 4.3.0).