Genetic defects occupy a definite role in the pathogenesis of Parkinson’s disease [79] Mutations known to cause PD are SNCA in aggregating α-synuclein [80], PRKN in impairing proteosomal protein degradation [81], impairment of the oxidative stress response by DJ1 mutation [82], dysfunction of mitochondria by PINK1 mutation [83], cognitive decline due to catechol-O-methyltransferase (COMT) Val/Val mutation [84], and leucine-rich repeat kinase 2 (LRRK2) mutations at the PARK8 locus [20,85,86,87]. This evidence concerns the gene COMT and Parkinson disease.