SMPD1 and Niemann-Pick disease: Since SPC accumulation has been speculated to result from a defect of aSMase (because SPC is a substrate for aSMase) [48], it would be intriguing to determine whether there is a similar upregulated expression of SM deacylase in Niemann–Pick disease that could provide a mechanism for the production of SPC in that disease as it does for AD [50,55].