A novel mutation, p.N440del, localized to the TNAP crown domain was identified in the probands in this study. A number of TNAP missense mutations affecting amino acid residues localized in a flexible loop corresponding to the collagen-binding region have been identified in individuals diagnosed with HPP (Table 1). Moreover, Mornet et al. [13], showed that among 10 mutations localized in the crown domain and associated with HPP, at least six were located to this loop, including p.V423A, p.G426C, p.Y436H, p.S445P, p.R450C, and p.R450H. Interestingly, genetic alterations affecting amino acid residues

in the collagen-binding loop correspond to homozygous severe forms of HPP or heterozygous mild phenotypes (Table 1), indicating that this region plays an important, but presently unclear, role in

TNAP function. It is difficult to assign specific dysfunctions to each heterozygous mutation in these probands. Considering the functional and find more structural importance of the crown domain, and based on the pedigree, reduced ALP activity in vivo and in vitro, and predicted alterations in mutant TNAP structures, we hypothesize that the odonto-HPP phenotype is associated with the heterozygous deletion of residue N440, which may indirectly affect the enzyme activity, possibly from failure to reach a correct conformation, or via alterations in interactions with collagen. On the other hand, GSI-IX price based on TNAP protein expression and immunolocalization, analysis of internal contacts, and reports in the literature, we propose that the consequences of heterozygous N440 deletion are intensified by association with the heterozygous missense mutation, p.R152C. However, because the p.R152C mutation

was of maternal heritance and the mother was asymptomatic, we propose that when heterozygous and in the absence of other mutations, this alteration is not in itself sufficient oxyclozanide to significantly and deleteriously affect TNAP function. It remains unclear how particular ALPL mutations cause more severe clinical forms of HPP, in contrast to mild forms, including odonto-HPP. Dental tissues have been reported to be highly sensitive to dysregulation of phosphate and pyrophosphate metabolism [42], [43] and [44], perhaps indicating that less deleterious ALPL mutations may preferentially affect the dentition, but not the broader skeleton. We characterized a novel genetic alteration (c.1318_1320delAAC, p.N440del) in the ALPL gene resulting in odonto-HPP in the probands, monozygotic twins. Based on pedigree information, clinical symptoms, genetic analysis, and residual ALP activity, a genotype–phenotype association was established for p.N440del and odonto-HPP in this case. The heterozygous gene deletion was paternally inherited, and predicted to alter the loop harboring a collagen-binding site in the TNAP protein crown domain. In addition to this gene deletion, the probands feature an p.