Phenotypic Changes in Dentition of Runx2 Homozygote-null Mutant Mice

Genetic and molecular studies in humans and mice indicate that Runx2 (Cbfa1) is a critical transcriptional regulator of bone and tooth formation. Heterozygous mutations in Runx2 cause cleidocranial dysplasia (CCD), an inherited disorder in humans and mice characterized by skeletal defects, supernumerary teeth, and delayed eruption. Mice lacking the Runx2 gene die at birth and lack bone and tooth development. Our extended phenotypic studies of Runx2 mutants showed that developing teeth fail to advance beyond the bud stage and that mandibular molar organs were more severely affected than maxillary molar organs. Runx2 (−/−) tooth organs, when transplanted beneath the kidney capsules of nude mice, failed to progress in development. Tooth epithelial-mesenchymal recombinations using Runx2 (+/+) and (−/−) tissues indicate that the defect in mesenchyme cannot be rescued by normal dental epithelium. Finally, our molecular analyses showed differential effects of the absence of Runx2 on tooth extracellular matrix (ECM) gene expression. These data support the hypothesis that Runx2 is one of the key mesenchymal factors that influences tooth morphogenesis and the subsequent differentiation of ameloblasts and odontoblasts.

Identification of tooth-specific downstream targets of Runx2

Odontogenesis or tooth development is a highly regulated process that involves complex epithelial-mesenchymal signaling interactions that lead to cuspal morphogenesis, cell differentiation and the subsequent formation of the specialized matrices of enamel, dentin, cementum and bone. Although studies on tooth epithelial-mesenchymal signaling interactions have greatly increased our understanding of molecules that regulate tooth initiation and early morphogenesis (review: Jernvall and Thesleff, Mech. Dev. 92 (2000) 19), the precise nature of the molecular events controlling late morphogenesis and terminal cytodifferentiation is not known. We have recently reported a unique phenotype involving dentition in mice lacking a functional Runx2 gene (D'Souza et al., Development 126 (1999) 2911). The markedly hypoplastic tooth organs as well as defects in the maturation of ameloblasts and odontoblasts point to an important and non-redundant role for Runx2 in tooth morphogenesis and cytodifferentiation. In order to identify genes that are affected by the absence of Runx2, a cDNA library was generated from Runx2(-/-) and Runx2(+/+) first molar organs. Thus far, our analysis has revealed several tooth-specific downstream target genes of Runx2 that include extracellular matrix proteins, kinases, receptors, growth factors, mitochondrial proteins and transcription molecules. Sequence analysis of 61 differentially expressed genes revealed that 96.03% of the clones matched previously described genes in the GenBank/EBML database and 3.96% did not match any entries in the database. Our preliminary expression analysis of one of the differentially expressed clones which encodes for a zinc finger transcription factor termed Zfp reveals that the gene is temporally regulated during tooth development. In conclusion, we have successfully generated a library enriched in genes expressed in Runx2(+/+) molar tooth organs and performed preliminary studies to assess the role of Zfp in tooth development.

Molecular insights into the lineage-specific determination of odontoblasts: the role of Cbfa1

The role of stable transcription complexes in initiating and consolidating programs of gene expression during lineage specification has been extensively studied. Despite the progress made in the identification of key molecules of tooth initiation and patterning, the mechanisms leading to cell differentiation during odontogenesis are unknown. Odontoblasts are exclusive dentin-producing cells that are phenotypically and functionally distinct from osteoblasts. However, not much is known about the precise determinants of odontoblast terminal differentiation--in particular, how the fate of these cells becomes delineated from that of osteogenic mesenchyme. Cbfa1(-/-) mice completely lack osteoblasts and bone, while tooth development arrests at the time of odontoblast differentiation. The purpose of this paper is to overview our studies on the role of Cbfa1 in odontoblast determination and differentiation using the Cbfa1(-/-) mouse model and various experimental approaches. Our expression analyses confirm the down-regulation of Cbfa1 expression in newly differentiated and functional odontoblasts. Second, we demonstrate that Cbfa1(-/-) incisor organs arrest at a later stage than molars, and that alpha 1 (I) collagen, a marker of odontoblast differentiation shared in common with osteoblasts, is not significantly affected by the absence of the transcription factor. Interestingly, Dspp expression in Cbfa1(-/-) appeared markedly down-regulated in putative odontoblasts. The overexpression of Cbfa1 in an odontoblast cell line (MDPC-23) results in the selective down-regulation of Dspp and not type I collagen. It is likely that, in addition to its influence on tooth epithelial morphogenesis, Cbfa1 plays a non-redundant and stage-specific role in the lineage determination and terminal differentiation of odontoblasts from dental papilla mesenchyme.

Cloning of a T-cell receptor beta-chain enhancer binding protein

Several eukaryotic DNA binding proteins have been isolated by screening lambda expression libraries with DNA probes containing their binding site. This strategy has been employed to isolate clones of the factor that interacts with the T-cell receptor beta-chain enhancer motif. A cDNA clone encoding a protein similar to YB-1 has been isolated with this. It seems probable that this protein YB, might interact with other proteins and regulate the transcription of the T-cell receptor beta-chain gene.