Molecular pathological analysis of inborn error of metabolism with mineralization defects

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Hypophosphatase is a genetic disease characterized by defective mineralization of bone and tooth and reduction in serum alkaline phosphatase activity. Various mutations in the tissue-nonspecific alkaline phosphatase gene (TNSALP) are responsible for a wide variety of clinical symptoms of the disease. So far 178 mutations have been reported, though little is known about the molecular basis of this disease. We have been studying effects of several missense mutations on the biosynthesis and catalytic function of TNSALP molecule in mammalian cells expressing each TNSALP mutant. The aim of this research was conducted to elucidate the effect of two mutations : a frame shift caused by a thymidine deletion at 1559 of cDNA (1559delT) and replacement of alanine with threonine at position 99 of TNSALP polypeptide (A99T).1. 1559delT : In combination with an in vitro translational system, we confirmed that 1559delT is synthesized as a larger polypeptide with 80 amino acid residues at C-terminus. Due to this extra amino acids, 1559delT is no longer a membrane bound enzyme via GPI (glycosylphosphatidyl-inositol), but a soluble enzyme possessing alkaline phosphatase activity. However, since majority of newly synthesized 1559delT forms a disulfide-bonded high-molecular-mass aggregate in the ER, undergo polyubiquitination and is degraded in the proteasome, only a small portion of the 1559delT was secreted into the medium.2. A99T : Like wild-type TNSALP, A99T was found to expressed as a 80 kDa mature form on the cell surface via GPI, though in contrast to the wild type, though A99T exhibited no enzyme activity. This indicates that alanine at position 99 is involved in catalytic function of TNSALP. Importantly, A99T was found to form a heterodimer with the wild-type polypeptide when coexpressed with the wild type, presumably accounting for its dominant mode of transmission.