Correlations of genotype and phenotype in hypophosphatasia

Hypophosphatasia, a rare inherited disorder characterized by defective bone mineralization, is highly variable in its clinical expression, The disease is due to various mutations in the tissue-non-specific alkaline phosphatas e (TNSALP) gene, We report here the use of clinical data, site-directed mut agenesis and computer-assisted modelling to propose a classification of 32 TNSALP gene mutations found in 23 European patients, 17 affected with letha l hypophosphatasia and six with non-lethal hypophosphatasia, Transfection s tudies of the missense mutations found in non-lethal hypophosphatasia showe d that six of them allowed significant residual in vitro enzymatic activity , suggesting that these mutations corresponded to moderate alleles, Each of the six patients with non-lethal hypophosphatasia carried at least one of these alleles, The three-dimensional model study showed that moderate mutat ions were not found in the active site, and that most of the severe missens e mutations were localized in crucial domains such as the active site, the vicinity of the active site and homodimer interface, Some mutations appeare d to be organized in clusters on the surface of the molecule that may repre sent possible candidates for regions interacting with the C-terminal end in volved in glycosylphosphatidylinositol (GPI) attachment or with other dimer s to form tetramers, Finally, our results show a good correlation between c linical forms of the disease, mutagenesis experiments and the three-dimensi onal structure study, and allowed us to clearly distinguish moderate allele s from severe alleles, They also confirm that the extremely high phenotypic heterogeneity observed in patients with hypophosphatasia was due mainly to variable residual enzymatic activities allowed by missense mutations found in the human TNSALP gene.