Tbx1 Transcriptional Mechanisms and Tooth Development in Tbx1 mutant mice

TBX1 is associated clinically with DiGeorge syndrome (DGS) and Tbx1 mutant mice present with phenotypic effects observed in patients with DGS. DGS individuals have several developmental anomalies including craniofacial defects. Objectives: because Tbx1 is required for craniofacial development and other tissues, we are working to define the molecular mechanisms required for Tbx1 expression and activity during tooth development. Tbx1 has overlapping expression patterns in the tooth bud and cap stages with several transcription factors and we are working to establish the hierarchy of transcription factors required for normal craniofacial development. Methods: We use transient transfections, real-time PCR, mutant mice, chromatin immunoprecipitation (ChIP) assays, electrophoretic mobility shift assays, immunochemical and biochemical assays. Results: During tooth development Tbx1 expression is localized to the dental epithelium and its expression is first observed at E13.5. Tbx1 is expressed in both incisors and molars throughout the dental epithelium including the cervical loop and enamel knot. Tbx1 homozygous mutant mice present with incisor microdontia, suggesting a defect in epithelial cell proliferation or increased apoptosis. These mice have mandibular and tongue hypoplasia, with a reported cleft palate. Pre-ameloblast differentiation appears to be defective. Mechanistically, Tbx1 physically interacts with PITX2 to repress PITX2 transcriptional activation. Tbx1 may act as a transcriptional repressor and we are analyzing potential genes involved in tooth development as Tbx1 downstream targets. Conclusions: Tbx1 is required for tooth development and is expressed at early stages of tooth development specifically in the epithelium. Tbx1 mutant mice demonstrate a distinct role for Tbx1 in incisor, mandible and tongue development. Tbx1 represses PITX2 activity through a direct interaction, which may be required for normal tooth development. These data demonstrate a new role for Tbx1 in tooth morphogenesis. Support for this research was provided from grant DE13941 from the National Institute of Dental and Craniofacial Research.