Characterization of 11 novel mutations in the tissue non-specific alkaline phosphatase gene responsible for hypophosphatasia and genotype-phenotype correlations

Hypophosphatasia is an inherited disorder caused by mutations in the bone alkaline phosphatase gene. We report here 11 new mutations responsible for hypophosphatasia. Four of them were deletions or insertions resulting in frameshift, two affected a donor splice site and five were missense mutations. Site-directed mutagenesis and transfection experiments of missense mutations showed that the mutations resulted in loss of most enzymatic activity, confirming the disease-causing role of these mutations. Analysis of the 3D model of tissue non-specific alkaline phosphatase showed that among the five missense mutations, one affected a residue in the crown domain and four affected residues located in the calcium-binding region. Alignment of the protein sequences of the calcium-binding region from 11 species showed that the four residues coordinating the calcium ion and the residues affected by the missense mutations described here are conserved in vertebrates. Together, our results confirm the functional role of the calcium site and suggest that its function is likely to be specific to vertebrates.