Exploring the Causal Relationship Between Body Mass Index and Kidney Function Using Tissue-Partitioned Mendelian Randomization

Chronic kidney disease (CKD) represents a leading non-communicable disease, significantly contributing to global morbidity and mortality. Mendelian randomization studies (MR) have been integral in providing robust evidence that increased body mass index (BMI) has a causal impact on CKD. However, dissecting which specific mechanisms are primarily responsible for disease development remains challenging.

Objective

To explore whether the effects of BMI to kidney function are driven primarily by brain-or adipose-tissue derived gene expression using tissue-partitioned Mendelian randomization (MR).

Methods

We employ two-sample univariable and multivariable MR methodology that segregates genetic variants associated with BMI based on colocalization with gene expression in either brain or subcutaneous adipose tissue. We utilize sets of adipose and brain expression quantitative trait loci (eQTLs) that demonstrated colocalization with BMI (86 and 140 loci respectively). We also use GWAS summary statistics of creatinine and cystatin C based eGFR (eGFRcrea and eGFRcys; N=460,826), blood urea nitrogen (BUN; N = 852,678), eGFR decline (N= 34,874 cases) and CKD (defined as eGFRcrea <60=ml=min ⁻¹ per 1.73=m ² ; N= 41,395) of European ancestry.

Results

Univariable MR showed consistent positive associations between BMI and CKD (OR = 1.24, 95% CI: 1.2–1.3) and inverse associations with eGFRcys (beta = –0.05, 95% CI: –0.06 to –0.046). Both brain-and adipose-instrumented BMI showed similar effect sizes. However, in multivariable MR, neither brain-nor adipose-specific BMI variants showed clear independent effects on CKD (OR _adipose = 1.24, 95% CI _adipose = 0.87 to 1.65, OR _brain= 1.18, 95%CI _brain = 0.9 to 1.54) or other kidney function traits.

Conclusions

While genetically predicted BMI was associated with kidney function, our tissue-partitioned MR analysis found no strong evidence that brain or subcutaneous adipose tissue derived gene expression independently drive this relationship. This suggests overlapping or additive mechanisms through which BMI influences kidney function.