We investigated the presence of mediated pleiotropy via two-sample MR as this allows us to test for causative mechanisms linking MD, AD, AC-frequency, and AC-quantity. This is a strategy to investigate causal relationships in which evidence on the associations of genetic variants (i.e. instrumental variable) with the risk factor (i.e. exposure) and with the outcome are derived from two samples (Davey Smith and Hemani, 2014; Burgess et al., 2015). The instrumental variables were built considering GWS loci (P < 5× 10−8) and suggestive loci (P < 5× 10−5). Since different MR methods have sensitivities to different potential issues, we considered 28 MR/variant-filtering approaches (online Supplemental Table S1). A Bonferroni correction (P < 1.05 × 10−4) was applied to correct for the number of MR tests performed (n = 17; online Supplemental Table S2) and the number of methods/variant-filtering considered for each test (n = 28; online Supplemental Table S1). Of the 17 MR tests conducted, we observed that 14 survived multiple testing correction (Table 1). This outcome was expected due to the strong genetic correlations observed among the traits investigated.
