MEPE Cleavage in Human Hypophosphatemic Dentin

INTRODUCTION: Familial hypophosphatemia rickets is in most cases transmitted as an X linked dominant trait and results from the mutation of the PHEX gene. As suggested by recent publications, PHEX may also protect matrix extracellular phosphoglycoprotein (MEPE) from proteolysis by a non-proteolytic interaction and therefore control the inhibiting effect of the ASARM peptide (cleaved C Terminus of MEPE) on matrix mineralization. Hypophosphatemic dentin has been reported to display important defects that may explain the teeth susceptibility to infection. PURPOSE: To explore the structure, the composition and the distribution of non-collagenous proteins (NCP) of the hypophosphatemic dentin. METHODS: Hypophosphatemic teeth collected from patients and control teeth were prepared for SEM, immunochemistry and western blot analysis. Concerning MEPE, 2 antibodies raised against the middle region of the protein and another one against the ASARM motif/ peptide were used. RESULTS: Compared to age matched controls, the dentin from hypophosphatemic patients exhibited major differences: - presence of large interglobular spaces resulting from the lack of fusion of calcospherites in the circumpulpal dentin; - defective mineralization in the interglobular spaces contrasting with normal Ca-P levels in the calcospherites on X-ray microanalysis; - accumulation in the interglobular spaces of immunolabeling with antibodies against MEPE, dentin sialoprotein (DSP), dentin matrix protein (DMP-1), bone sialoprotein (BSP) and osteocalcin (OC); - abnormal presence of low molecular weight protein complexes recognized on Western blots by antibodies against DSP and OPN, -clearly reduced MEPE labeling with the anti-ASARM antibody on western blots, suggesting a C Terminus cleavage of this protein. CONCLUSIONS: Alterations of the post-translational processing or partial degradation of some NCP are found in hypophosphatemic dentin. Furthermore, as described in the bone, an abnormal cleavage of C Terminus MEPE with liberation of the ASARM peptide is observed. Both anomalies may partially explain the impaired dentin mineralization associated to the disease.