Demarcated, Hypomineralized Enamel Opacities Depend on Overexpressed Ameloblastin

Objectives: Demarcated enamel opacities are developmental defects linked to molar-incisor hypomineralization (MIH) with a robust prevalence in children. Etiologic factors include environmental and epigenetic factors, however, the pathophysiology is not clear. Understanding of the pathways is complicated as ameloblasts undergo apoptosis and are lost during tooth eruption. The retention of enamel proteins has been implicated in MIH. The goal of the study was to analyze enamel in the presence of the overexpression of the enamel protein ameloblastin (Ambn) in mice.
Methods: Transgenic Ambn was overexpressed in mice from the amelogenin promoter encoding full-length Ambn. Ambn was overexpressed in separate mouse lines at four increasing concentrations and analyzed by Western Blot, mCT, histology and immunostaining.
Results: Mice overexpressing Ambn displayed opaque enamel at low concentration and demarcated lesions at high concentrations of reduced mineral content. At low Ambn concentration, enamel opacities started close to the dentino-enamel junction (DEJ) in the inner enamel and contained 17-kDa Ambn cleavage products. At high Ambn concentration, opacities were demarcated and Ambn species of 17 kDa and higher were found. Ameloblasts demonstrated prolonged secretory and transition stages, thin basement membranes and shortened maturation stages. When opacities expanded to the enamel surface adjacent ameloblasts were detached and formed cysts within the enamel organ.
Conclusions: The overexpression of Ambn in murine secretory ameloblasts results in enamel hypomineralization with opaque or sharply demarcated boundaries, phenotypically similar to MIH.