B3GALT4 and Parkinson disease: Alterations in ganglioside metabolism may predate neuronal loss in PD, as suggested by the fact that the mRNA expression of the ganglioside biosynthetic enzymes B3GALT4 (GM1 synthase) and ST3GAL2 (GD1a/GT1b synthase) and the levels of GM1 are lower than normal in residual dopaminergic neurons in the substantia nigra of PD patients (Wu et al., 2012; Schneider, 2018).