Glycoprotein Acetyls and Depression: testing for directionality and potential causality using longitudinal data and Mendelian randomization analyses

Background: Inflammation is implicated in depression, but the issue of causality remains unclear. Objectives: To investigate potential causality and direction of effect between inflammation and depression. Methods: Using data from the ALSPAC birth cohort (n=4021), we used multivariable regression to investigate bidirectional longitudinal associations of GlycA and depression symptoms score and diagnosis, assessed at ages 18y and 24y. We used two-sample Mendelian randomization (MR) to investigate potential causality and directionality. Genetic variants for GlycA were obtained from UK Biobank (UKBB) (N=115,078); for depression from the Psychiatric Genomics Consortium and UKBB (N=500,199); and for depressive symptoms (N=161,460) from the Social Science Genetic Association Consortium. In addition to the Inverse Variance Weighted (IVW) method, we used sensitivity analyses to strengthen causal inference. We conducted multivariable MR adjusting for body mass index (BMI) due to known genetic correlation between inflammation, depression and BMI. Results: After adjusting for potential confounders we found no association between GlycA and depression symptoms score or vice versa. We observed an association between GlycA and depression diagnosis (OR=1.18, 95% CI: 1.03-1.36). MR suggested no causal effect of GlycA on depression, but there was evidence of a causal effect of depression on GlycA (mean difference in GlycA = 0.09; 95% CI: 0.03-0.16), which was maintained in some, but not all, sensitivity analyses. Conclusion: We found no consistent evidence for an effect of the inflammatory marker GlycA on depression. There was some evidence that depression may increase GlycA, but this may be confounded/mediated by BMI.