Tissue-specific effects of DMP1-overexpression in the Hyp tooth

Objectives: To investigate the effect of dentin matrix protein 1 (DMP1) overexpression on the ECM morphology and matrix mineralization in the Hyp mouse model of X-linked hypophosphatemia (XLH).
Methods: Dspp-Dmp1^Tg/Tg (DD) mice were crossed with C57BL/6-Phex^Hyp (Hyp) mice to produce the Dspp-Dmp1^Tg/+/Hyp^Hyp/0 (DDHyp) genotype. Dissected mandibles (1, 2, and 3-months of age) were fixed in 10% formalin and decalcified in 14% EDTA (pH 7.4) for five weeks. Five micron-thick paraffin-embedded sections were stained with Picrosirius Red. Collagen fiber length, width, straightness, and angles were determined using polarized light images and the CT-FIRE software (Bredfeldt et al. 2014). Maxillary first molars were freeze-fractured in liquid nitrogen, dehydrated in hexamethyldisilazane, coated with gold/palladium (10 nm), and imaged using field-emission scanning electron microscopy. MicroCT analysis of dentin, alveolar bone, and PDL space were performed on a cone-beam microCT40(Scanco). Serum calcium and phosphate levels were determined for each age group using colorimetric assays.
Results: Picrosirius red staining resulted in weak staining of Hyp and DDHyp root dentin, when compared to WT and DD controls. Histological analyses indicated thinner, loosely packed and disoriented collagen fibrils in the Hyp PDL space. The presence of red, yellowish-orange fibers indicated thicker collagen bundles in the PDL. CT-FIRE analysis confirmed the presence of shorter and less angular collagen fibers in DDHyp dentin. FESEM analysis, further confirmed a disorganized, heterogenous population of collagen fibers in Hyp dentin, whereas DD dentin demonstrated increased mineral deposition associated with the matrix. Changes were also observed with microCT. No significant differences were observed in the serum calcium levels, but significant differences were observed in serum phosphate levels between Hyp and the WT and DD mice at one and two months of age.
Conclusions: Results show a profound effect on ECM components and mineral in DD dentin and the effect of DMP1-overexpression on Hyp ECM.