SMPD1 and Parkinson disease: While GBA mutations are the most common genetic alterations in PD, other lysosomal enzymes have also been associated with PD, including the gene causing Niemann-Pick A and B (SMPD1) [2], [3], [4] and SCARB2 [5], which encodes the glucocerebrosidase chaperone LIMP-2.