A dimerization defect caused by a glycine substitution at position 420 by serine in tissue-nonspecific alkaline phosphatase associated with perinatal hypophosphatasia

Authors

  • Saori Makita,

    1. Division of Comprehensive Prosthodontics, Department of Oral Health Science, Niigata University Graduate School of Medical and Dental Sciences, Japan
    2. Division of Oral Biochemistry, Department of Tissue Regeneration and Reconstruction, Niigata University Graduate School of Medical and Dental Sciences, Japan
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  • Hiba A. Al-Shawafi,

    1. Division of Oral Biochemistry, Department of Tissue Regeneration and Reconstruction, Niigata University Graduate School of Medical and Dental Sciences, Japan
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  • Sara Sultana,

    1. Division of Oral Biochemistry, Department of Tissue Regeneration and Reconstruction, Niigata University Graduate School of Medical and Dental Sciences, Japan
    2. Oral Maxillofacial Surgery, Department of Tissue Regeneration and Reconstruction, Niigata University Graduate School of Medical and Dental Sciences, Japan
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  • Miwa Sohda,

    1. Division of Oral Biochemistry, Department of Tissue Regeneration and Reconstruction, Niigata University Graduate School of Medical and Dental Sciences, Japan
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  • Shuichi Nomura,

    1. Division of Comprehensive Prosthodontics, Department of Oral Health Science, Niigata University Graduate School of Medical and Dental Sciences, Japan
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  • Kimimitsu Oda

    Corresponding author
    1. Division of Oral Biochemistry, Department of Tissue Regeneration and Reconstruction, Niigata University Graduate School of Medical and Dental Sciences, Japan
    • Division of Comprehensive Prosthodontics, Department of Oral Health Science, Niigata University Graduate School of Medical and Dental Sciences, Japan
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Correspondence

K. Oda, Division of Oral Biochemistry, Niigata University Graduate School of Medical and Dental Sciences, 2-5274 Gakkocho-dori, Chuo-ku, Niigata 951-8514, Japan

Fax: +81 25 227 0803

Tel: +81 25 227 2827

E-mail: oda@dent.niigata-u.ac.jp

Abstract

Mutations in the tissue-nonspecific alkaline phosphatase (TNSALP) gene cause hypophosphatasia (HPP), an inborn error of metabolism characterized by defects in bone and teeth mineralization accompanying subnormal levels of serum alkaline phosphatase activity. Missense mutations at position 420 of TNSALP (standard nomenclature), which convert glycine to serine [TNSALP (G420S)] or alanine [TNSALP (G420A)], have been reported in perinatal and childhood HPP, respectively. When expressed in COS-1 cells, both TNSALP mutants were indistinguishable from wild-type TNSALP [TNSALP (W)] as evidenced by immunofluorescence and western blotting. Nevertheless, the two TNSALP mutants did not show substantial alkaline phosphatase activity. In agreement with transiently transfected cells, TNSALP (G420S) expressed in a Tet-On inducible expression system lacked its alkaline phosphatase activity, although this mutant was anchored to the cell surface lipid bilayers by glycosylphosphatidylinositol as an 80 kDa mature form bearing complex-type oligosaccharides like TNSALP (W). Importantly, TNSALP (G420S) was found to largely fail to assemble into the homodimer in contrast to TNSALP (W). Taken together, these results demonstrate that the glycine residue at position 420 is crucial for the subunit interaction of TNSALP and hence its catalytic function without affecting trafficking of monomeric TNSALP. We conclude that the dimerization defect is the molecular basis for perinatal HPP associated with the genotype G420S/G420S.

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