Analysis of Over Erupted Incisors in Twist-1 Deficient Mice

Twist-1 is a nuclear protein that contains a basic-helix-loop-helix domain. In mice, Twist-1 influences the early migration of cranial neural crest mesenchyme and the onset of their differentiation. While Twist-1 (-/-) mice die around E11.5 due to a failure of cranial neural folds to close, Twist-1 (+/-) mice survive and display a phenotype of craniosynstoses and duplication of digit 1. We recently identified the presence of pulpal calcifications in teeth of Twist-1 (+/-) mice. These features resemble Saethre-Chotzen syndrome that is associated with loss-of-function mutations in the human TWIST-1. Objectives: To extend our phenotypic studies of Twist-1–deficient dentition at postnatal stages and to perform molecular analyses to provide new insights into the pathogenesis of these defects. Methods: Heads from Twist-1 (+/-) mice and their normal littermates at several postnatal stages ranging from neonates to Day 120 were studied histologically and by Faxitron radiography and image analysis. In situ hybridization and Real-time PCR analysis was performed on microdissected incisor organs to study changes in gene expression. To study whether defects in dentition are related to the genetic interaction between Twist-1 and its antagonistic partner Runx2, genetic backcrosses were performed. Results: 60% of Twist-1 (+/-) mice studied revealed deviation of the nasal septum and interlocking of the incisors that are markedly over erupted. Faxitron radiographs and image analysis showed a higher degree of mineralization in Twist-1 (+/-) incisors and molars. No marked changes in Runx2, periostin, Shh, Fgf3 and Fgf10 expression levels was noted. The phenotype of over erupted incisors was rescued in all Twist-1 (+/-) X Runx2 (+/-) mice studied. Conclusions: The phenotype of over erupted incisors observed in Twist-1 (+/-) mice may be related to excess bone deposition caused by Runx2 rather than aberrant signaling in the cervical loop area. Supported by NIH grant RO1-DE013368 to RDS.