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Exp Clin Endocrinol Diabetes 2009; 117(1): 28-33
DOI: 10.1055/s-2008-1073157
Article

© Georg Thieme Verlag KG Stuttgart · New York

Infantile Hypophosphatasia due to a New Compound Heterozygous TNSALP Mutation – Functional Evidence for a Hydrophobic Side-Chain?

B. Utsch 1 , I. Brun-Heath 2 , G. Staatz 3 , C. Gravou-Apostolatou 1 , S. Karle 1 , U. Jacobs 1 , M. Ludwig 4 , M. Zenker 5 , H.-G. Dörr 1 , W. Rascher 1 , E. Mornet 2 , J. Dötsch 1
  • 1Department of Pediatrics, University of Erlangen-Nuremberg, Erlangen, Germany
  • 2Equipe Structure-Fonction et Génétique, CHU Paris Ile de France Ouest, Université de Versailles-Saint Quentin en Yvelines, France
  • 3Department of Radiology, Division of Pediatric Radiology, University of Erlangen-Nuremberg, Erlangen, Germany
  • 4Department of Clinical Biochemistry, University of Bonn, Bonn, Germany
  • 5Department of Human Genetics, University of Erlangen-Nuremberg, Erlangen, Germany
Further Information

Publication History

received 15.11.2007 first decision 20.12.2007

accepted 25.03.2008

Publication Date:
03 June 2008 (online)

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Abstract

Background: Infantile hypophosphatasia (IH) is an inherited disorder characterized by defective bone mineralization and a deficiency of alkaline phosphatase activity.

Objective/Design: The aim of the study was to evaluate a new compound heterozygous TNSALP mutation for its residual enzyme activity and localization of the comprised amino acid residues in a 3D-modeling.

Patient: We report on a 4-week old girl with craniotabes, severe defects of ossification, and failure to thrive. Typical clinical features as low serum alkaline phosphatase, high serum calcium concentration, increased urinary calcium excretion, and nephrocalcinosis were observed. Vitamin D was withdrawn and the patient was started on calcitonin and hydrochlorothiazide. Nonetheless, the girl died at the age of 5 months from respiratory failure.

Results: Sequence analysis of the patient's TNSALP gene revealed two heterozygous mutations [c.653T>C (I201T), c.1171C>T (R374C)]. Transfection studies of the unique I201T variant in COS-7 cells yielded a mutant TNSALP protein with only a residual enzyme activity (3.7%) compared with wild-type, whereas the R374C variant was previously shown to reduce normal activity to 10.3%. 3D-modeling of the mutated enzyme showed that I201T resides in a region that does not belong to any known functional site.

Conclusion: We note that I201, which has been conserved during evolution, is buried in a hydrophobic pocket and, therefore, the I>T-change should affect its functional properties. Residue R374C is located in the interface between monomers and it has been previously suggested that this mutation affects dimerization. These findings explain the patient‘s clinical picture and severe course.

Key words

hypophosphatasia - TNSALP gene - TNSALP protein - 3D-modeling - transfection studies - enzyme activity - alkaline phosphatase

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Correspondence

B. UtschMD 

Department of Pediatrics

University Children's Hospital of Berne

Inselspital

3010 Berne

Switzerland

Phone: +41/31/632 95 12

Fax: +41/31/632 94 20

Email: boris.utsch@insel.ch