Tohoku J. Exp. Med., 2020 January, 250(1)

An Early-Onset Neuronopathic Form of Acid Sphingomyelinase Deficiency: A SMPD1 p.C133Y Mutation in the Saposin Domain of Acid Sphingomyelinase

SHOZO OTA,¹ ATSUKO NOGUCHI,² DAIKI KONDO,² YOKO NAKAJIMA,³ TETSUYA ITO,³ HIROKAZU ARAI¹ and TSUTOMU TAKAHASHI²

¹Department of Neonatology, Akita Red Cross Hospital, Akita, Akita, Japan
²Department of Pediatrics, Akita University Graduate School of Medicine, Akita, Akita, Japan
³Department of Pediatrics, Fujita Medical University, Nagoya, Aichi, Japan

Acid sphingomyelinase (ASM) is a lysosomal hydrolase that degrades sphingomyelin into ceramide and phosphocholine. Recent crystallographic studies revealed the functional role of the N-terminal ASM saposin domain. ASM deficiency due to mutations in the ASM-encoding sphingomyelin phosphodiesterase 1 (SMPD1) gene causes an autosomal recessive sphingolipid-storage disorder, known as Niemann-Pick disease Type A (NPA) or Type B (NPB). NPA is an early-onset neuronopathic disorder, while NPB is a late-onset non-neuronopathic disorder. A homozygous one-base substitution (c.398G>A) of the SMPD1 gene was identified in an infant with NPA, diagnosed with complete loss of ASM activity in the patient's fibroblasts. This mutation is predicted to substitute tyrosine for cysteine at amino acid residue 133, abbreviated as p.C133Y. The patient showed developmental delay, hepatosplenomegaly and rapid neurological deterioration leading to death at the age of 3 years. To characterize p.C133Y, which may disrupt one of the three disulfide bonds of the N-terminal ASM saposin domain, we performed immunoblotting analysis to explore the expression of a mutant ASM protein in the patient's fibroblasts, showing that the protein was detected as a 70-kDa protein, similar to the wild-type ASM protein. Furthermore, transient expression of p.C133Y ASM protein in COS-7 cells indicated complete loss of ASM enzyme activity, despite that the p.C133Y ASM protein was properly localized to the lysosomes. These results suggest that the proper three-dimensional structure of saposin domain may be essential for ASM catalytic activity. Thus, p.C133Y is associated with complete loss of ASM activity even with stable protein expression and proper subcellular localization.

Keywords —— acid sphingomyelinase deficiency; disulfide bond; Niemann-Pick disease type A; saposin domain; subcellular localization

© 2020 Tohoku University Medical Press

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Tohoku J. Exp. Med., 2020, 250, 5-11

Correspondence: Tsutomu Takahashi, M.D., Ph.D., Department of Pediatrics, Akita University Graduate School of Medicine, 1-1-1 Hondo, Akita, Akita 010-8543, Japan.

e-mail: tomy@med.akita-u.ac.jp