MTHFR and Parkinson disease: From total publications, 40 variants within 18 genes (HOMER1, ADORA2A, ANKK1, MTHFR, DRD2, SLC6A3, COMT, UGT1A, ACE2, BDNF, ABCC8, RYR1, DRD3, GRIN2A, SLC6A4, HTR2A, CYP2D6, CCK) were found to have significant association (p ≤ 0.05) with any type of levodopa induced ADR in PD (Additional file 2: Table S1(8a)).