Overexpression of Human Bone Morphogenetic Protein-4 (BMP4) Results in Defects and Reduced Mineralization in the Developing Tooth

Objective: Bmp4 is a crucial signaling molecule for early tooth formation and morphogenesis. However, the effects of overexpression of BMP4 on tooth development are not known. Methods: Previously we have identified tooth control elements in the 1.1 kb mouse Bmp4 promoter fragment using a transgenic approach. In this study, we generated transgenic mice overexpressing the human Bmp4 gene driven by this promoter fragment. The teeth and their development were examined using histology, SEM, energy dispersive spectrometry (EDS), micro-radiography and Piximus bone densitometry; in situ hybridization and immunostaining as well as TUNNEL and PCNA assays. Results: Newborn Bmp4 transgenic mice grossly appear normal by both x-ray and histology. By ~10 days after birth, the Bmp4 transgenic mice begin to display profound tooth abnormalities. These include: 1) normally shaped incisors that become chalky white and begin to abrade; 2) by x-ray analysis, Piximus bone densitometry, and by EDS, there is a dramatic decrease in mineral content in both enamel and dentin; 3) in ameloblats and odontoblasts there is a significant reduction in proliferation with a concomitant increase of apoptotic cells; 4) a thinner enamel and a decrease of the density of organic matrix contents in dentin is observed using SEM and Masson’s trichrome stain; and 5) a decrease of collagen I expression in odontoblasts is observed using in situ hybridization and immunostaining. By 6 months, the incisors and molars were severely abraded in appearance and by 10 months, both mandibular and maxilla molar cusps were completely abraded. Conclusions: Overexpression of Bmp4 in ameloblasts can also affect development and function of other cell types such as odontoblasts. The above findings suggest that Bmp4 can control mineralization in both enamel and dentin indirectly through the cell cycle mechanism. This work is supported by NIH-NIDCR grants DE00455 and DE13480 (JQF), and DE13221 (MM).