Collagen Point-Mutation and Altered Enamel Matrix Change Dentin-Enamel Junction Properties

Objectives: The objective of this study was to test the effect on the mechanical and morphological properties across the DEJ of mouse models with one of the following disruptions: collagen point mutation, absence of the major enamel matrix protein amelogenin, lack of amelogenin cleavage, and incomplete removal of enamel matrix proteins.
Methods: We analyzed the DEJ in mandibular incisors of male and female (N=5 each) 6-week-old mice of Amelx-KO, KLK4-KO, MMP20-KO, and osteogenesis imperfecta (OIM) strains. After micro-CT analyses, coronal sections mesial of the first molar were analyzed sequentially by nanoindentation, SEM, and FIB-SEM 3D analysis. Measures were derived for enamel thickness, mineral density, hardness, elastic modulus, microstructure, interface topography, and porosity compared to controls.
Results: Enamel matrix disruptions caused by absence of Amelx, KLK4, or MMP20 result in changes in morphology, mineral content, and mechanical properties across the DEJ; disruptions to the dentin collagen matrix in OIM produce mineral density and mechanical property increases in enamel across the DEJ. In dentin, mineral density was up to 28% lowered near the DEJ of Amelx-KO, KLK4-KO, and MMP20-KO incisors compared to controls, hardness decreased up to 7% in MMP20-KO. In enamel, mineral density was up to 11% increased near the DEJ in the OIM model. Enamel showed sex-specific differences with mineral density 10% lower in WT males compared to females and, in contrast, up to 17% higher in MMP20-KO and Amelx-KO males compared to females. SEM analysis reveals a periodic scalloped appearance of the DEJ structure in KLK4-KO and OIM incisors like WT samples, while MMP20-KO and Amelx-KO strains show a flatter DEJ interface.
Conclusions: Disruptions to both dentin collagen matrix and enamel Amelx expression and cleavage have effects on microstructure and mechanics reaching across the DEJ. Sex and strain differences affecting DEJ structure and mineral content are revealed by advanced imaging and mechanical analysis.