Currently, there are seven well-described genes that increase the risk for PD with variable rates of penetrance: in addition to SNCA, LRRK2, VPS35, PRKN, PINK1, DJ1 and the glucocerebrosidase gene GBA [104], there are PD risk variants, including PITX3, TMEM106B, SNCA Rep1, APOE ε4, COMT and MAPT H1/H1 with respective relationships to cognition [105]. Here, COMT is linked to Parkinson disease.