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 Analysis

Background/Objectives: 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). Methods: 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 MTHFR 677C>T (rs1801133), PITX2 C>T (rs2200733), and KCNE1 112A>G (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. Results: We enrolled 180 cases and 179 controls. With an extensive confounder control (i) the MTHFR 677C>T variant allele associated with higher tHcy; (ii) PITX2 C>T variant allele associated with NVAF while KCNE1 112A>G did not; (iii) MTHFR 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 MTHFR wild-type subjects (n = 160), tHcy “directly” strongly associated with NVAF, and considering variant carriers (n = 199), it indirectly associated with NVAF and directly tended to associate with a lower probability of NVAF; (vi) in MR analysis (MTHFR SNP instrument), tHcy associated with NVAF; and vii) mediation and MR analyses [PITX2 SNP (exposure/instrument)—NVAF, (mediator/exposure)—tHcy outcome] excluded the reverse tHcy-NVAF association. Conclusions: Data strongly support the causal role of tHcy in NVAF in overweight/obese patients and suggest that the effect might be modified by the MTHFR 677C>T variant allele.