SMPD1 and lysosomal storage disease: Niemann-Pick disease type A (NPA) is a rare autosomal recessive, neuropathic lysosomal storage disease that is caused by mutations in the SMPD1 gene, leading to a deficiency in acid sphingomyelinase (< 10% enzyme activity) in the cells (Schuchman and Wasserstein, 2015).