GBA1 and Parkinson disease: PD patients with GBA1 mutations generally have an earlier age of onset (Beavan et al., 2015; Brockmann et al., 2011; Neumann et al., 2009); glucocerebrosidase (GCase) activity is reduced in the substantia nigra of PD brain, particularly in those with GBA1 mutations (Gegg et al., 2012).