Analysis of localization of mutated tissue nonspecific alkaline phosphatase proteins associated with neonatal hypophosphatasia using green fluorescent protein chimeras

Hypophosphatasia is associated with a defect of the tissue-nonspecific alkaline phosphatase (TNSALP) gene. The onset and clinical severity are usually correlated in hypophosphatasia; patients with perinatal hypophosphatasia die approximately at the time of birth. In contrast, we describe a male neonatal patient with hypophosphatasia who had no respiratory problems and survived. He was compound heterozygous for the conversion of Phe to Leu at codon 310 (F310L) and the deletion of a nucleotide T at 1735 (delT1735), causing the frame shift with the result of the addition of 80 amino acids at the C-terminal of the protein. Because the C-terminal portion of TNSALP is known to be important for TNSALP to bind to the plasma membrane, the localization of wild-type and mutated TNSALP proteins was analyzed using green fluorescent protein chimeras. The expression vectors containing the complementary DNA of fusion proteins consisting of signal peptide, green fluorescent protein, and wild-type or mutated TNSALP, caused by delT1735 or F310L mutation, were introduced transiently or stably in Saos-2 cells. The delT1735 mutant failed to localize at the cell surface membrane, whereas the wild-type and the F310L mutants were located in the plasma membrane and cytoplasm. The assay for enzymatic activity of TNSALP revealed that the delT1735 mutant lost the activity and that the F310L mutant exhibited an enzymatic activity level that was 72% of the normal level. The F310L mutation was also detected in another neonatal patient with relatively mild (nonlethal) hypophosphatasia (reported in J Clin Endocrinol Metab, 81:4458-4461, 1996), suggesting that residual ALP activity of the F310L mutant contributes to the less severe phenotype. The patient is unique, with respect to a discrepancy between onset and clinical severity in hypophosphatasia.