ARSA and lysosomal storage disease: Metachromatic leukodystrophy is a lysosomal storage disorder (LSD) caused by decreased levels of Arylsulfatase A (ARSA), an enzyme involved in desulfation of sulfatide, a specific sphingolipid (3-O-sulfogalactosylceramide) that is important for the development and function of OLs and Schwann cells (Marcus et al., 2006; Takahashi and Suzuki, 2012; Xiao et al., 2013; Grassi et al., 2016).