Common Methylenetetrahydrofolate Reductase Polymorphism MTHFR 677C>T (rs1801133), Plasma Homocysteine, and Non-Valvular Atrial Fibrillation in Overweight/Obese Patients: Causality Indicated by Mediation and One-Sample Mendelian Randomization Analy

<b>Background/Objectives</b>: The causal role of homocysteine (tHcy) in atrial fibrillation (AF) is unclear. To (re)explore the causal effect of tHcy in non-valvular AF (NVAF). <b>Methods</b>: In a case-control study in overweight/obese adults, cases were patients with NVAF and controls were their peers without AF. They were assessed for clinical, laboratory, and echocardiographic particulars and were genotyped for <i>MTHFR 677C>T</i> (rs1801133), <i>PITX2 C>T</i> (rs2200733), and <i>KCNE1 112A>G</i> (rs1805127) polymorphisms. We employed a conventional case-control, mediation analysis, and one-sample Mendelian randomization (MR) analyses to evaluate forward and reverse tHcy-NVAF associations. <b>Results</b>: We enrolled 180 cases and 179 controls. With an extensive confounder control (i) the <i>MTHFR 677C>T</i> variant allele associated with higher tHcy; (ii) <i>PITX2 C>T</i> variant allele associated with NVAF while <i>KCNE1 112A>G</i> did not; (iii) <i>MTHFR</i> variant associated with NVAF indirectly, through tHcy assuming wild type but not variant genotype (exposure-mediator interaction); (iv) considering all subjects, tHcy associated with NVAF through the effect on renal function and NT-proBNP levels (no exposure-mediator interaction); (v) considering <i>MTHFR</i> wild-type subjects (<i>n</i> = 160), tHcy "directly" strongly associated with NVAF, and considering variant carriers (<i>n</i> = 199), it indirectly associated with NVAF and directly tended to associate with a lower probability of NVAF; (vi) in MR analysis (<i>MTHFR</i> SNP instrument), tHcy associated with NVAF; and vii) mediation and MR analyses [<i>PITX2</i> SNP (exposure/instrument)-NVAF, (mediator/exposure)-tHcy outcome] excluded the reverse tHcy-NVAF association. <b>Conclusions</b>: Data strongly support the causal role of tHcy in NVAF in overweight/obese patients and suggest that the effect might be modified by the <i>MTHFR 677C>T</i> variant allele.