Enamel Defects in Enamelin Knockout/LacZ Knockin Mice

Enamelin is a 186-kDa glycoprotein that is secreted by ameloblasts and is critical for proper dental enamel formation. Mutations in the human enamelin gene (ENAM) cause autosomal dominant hypoplastic amelogenesis imperfecta. Objectives: To better understand the role played by enamelin in enamel biomineralization, we have used gene targeting to knock out normal enamelin expression while replacing the 5' enamelin coding sequence with a ß-galactosidase reporter in mice. Methods: Correct targeting of the transgene was confirmed by Southern blotting and PCR analyses and no enamelin protein could be detected by Western blotting in the Enam-null mice. The enamel of wild type, Enam^+/-, and Enam^-/- mice was characterized by radiography, micro-computed tomography, light microscopy, histochemistry, and scanning electron microscopy. Results: Histochemical X-gal staining demonstrated that enamelin expression was restricted to ameloblasts; all other tissues and cell types were negative. The major finding, confirmed by radiography, micro-computed tomography, light and scanning electron microscopy, was that the Enam^-/- mice produced no true enamel, but bone, dentin and all other tissues appeared undisturbed. Although a thick layer of enamel proteins covered normal-appearing dentin of unerupted teeth, von Kossa staining revealed almost a complete absence of mineral formation within this protein layer. However, a thin, highly irregular, mineralized crust covered the dentin on erupted teeth, apparently arising from the formation and fusion of small mineralization foci (calcospherites) in the deeper part of the accumulated enamel protein layer. Conclusions: Enamelin is specifically expressed by ameloblasts and is essential for the formation of a mineralization front immediately adjacent to the ameloblasts where enamel crystal elongation occurs; no true enamel forms in the absence of enamelin. This study was supported by NIDCR grant DE011301.