Mutations in the PHEX gene cause X-linked familial hypophosphatemic rickets (XLH) with severe bone (osteomalacia) and tooth abnormalities being the distinguishing features of this disease. The PHEX mutations lead to an increase in ASARM peptides (acidic serine- and aspartate-rich motif) and osteopontin fragments which inhibit bone extracellular matrix mineralization. MEPE-derived ASARM has been shown to accumulate in tooth dentin of patients with XLH where it may impair dentinogenesis. Here, we investigated the effects of ASARM peptides on odontoblast differentiation and matrix mineralization. Dental pulp stem cells obtained from human exfoliated deciduous teeth (SHEDs) were first characterized for mesenchymal stem cell markers by cell sorting analysis. The cells were then seeded into a 3D collagen-tooth slice scaffold, and induced towards odontoblastic differentiation using appropriate culture conditions (supplements). Cultures were treated with synthetic ASARM peptides (phosphorylated and nonphosphorylated) derived from the human MEPE sequence. Phosphorylated ASARM peptide inhibited SHED differentiation, with no mineralized nodule formation, decreased odontoblast marker expression, and upregulation of MEPE. When implanted in a tooth pulp injury model, this peptide impaired reparative dentin formation and mineralization, and increased MEPE immunohistochemical staining was detected. In conclusion, using original models to study tooth dentin abnormalities observed in XLH, we show that the MEPE-derived ASARM peptide inhibits both odontogenic differentiation and matrix mineralization, while increasing MEPE expression. These results provide a partial mechanistic explanation of XLH pathogenesis; that direct inhibition of mineralization by ASARM peptide leads to the mineralization defects observed in XLH teeth. This process appears to be positively reinforced by the increased MEPE expression induced by ASARM. The MEPE-ASARM system should be considered as a potential therapeutic target for treatment of XLH.
18 May 2013 - 22 May 2013