YAP/TAZ Regulate Elevation and Bone Formation of the Mouse Secondary Palate.

Clefting of the secondary palate is one of the most common congenital anomalies, and the multiple corrective surgeries that individuals with isolated cleft palate undergo are associated with major costs and morbidities. Secondary palate development is a complex, multistep process that includes the elevation of the palatal shelves from a vertical to horizontal position, a process that is not well understood. The Hippo signaling cascade is a mechanosensory pathway that regulates morphogenesis, homeostasis, and regeneration by controlling cell proliferation, apoptosis, and differentiation, primarily via negative regulation of the downstream effectors, Yes-associated protein (YAP) and transcriptional coactivator with PDZ-binding motif (TAZ). We deleted Yap/Taz throughout the palatal shelf mesenchyme as well as specifically in the posterior palatal shelf mesenchyme, using the Osr2Cre and Col2Cre drivers, respectively, which resulted in palatal shelf elevation delay and clefting of the secondary palate. In addition, the deletion resulted in undersized bones of the secondary palate. We next determined downstream targets of YAP/TAZ in the posterior palatal shelves, which included Ibsp and Phex, genes involved in mineralization, and Loxl4, which encodes a lysyl oxidase that catalyzes collagen crosslinking. Ibsp, Phex, and Loxl4 were expressed at decreased levels in the ossification region in the posterior palatal shelf mesenchyme upon deletion of Yap/Taz. Furthermore, collagen levels were decreased specifically in the same region prior to elevation. Thus, our data suggest that YAP/TAZ may regulate collagen crosslinking in the palatal shelf mesenchyme, thus controlling palatal shelf elevation, as well as mineralization of the bones of the secondary palate.

A review of craniofacial and dental findings of the RASopathies.

OBJECTIVES: The RASopathies are a group of syndromes that have in common germline mutations in genes that encode components of the Ras/mitogen-activated protein kinase (MAPK) pathway and have been a focus of study to understand the role of this pathway in development and disease. These syndromes include Noonan syndrome (NS), Noonan syndrome with multiple lentigines (NSML or LEOPARD syndrome), neurofibromatosis type 1 (NF1), Costello syndrome (CS), cardio-facio-cutaneous (CFC) syndrome, neurofibromatosis type 1-like syndrome (NFLS or Legius syndrome) and capillary malformation-arteriovenous malformation syndrome (CM-AVM). These disorders affect multiple systems, including the craniofacial complex. Although the craniofacial features have been well described and can aid in clinical diagnosis, the dental phenotypes have not been analysed in detail for each of the RASopathies. In this review, we summarize the clinical features of the RASopathies, highlighting the reported craniofacial and dental findings. METHODS: Review of the literature. RESULTS: Each of the RASopathies reviewed, caused by mutations in genes that encode different proteins in the Ras pathway, have unique and overlapping craniofacial and dental characteristics. CONCLUSIONS: Careful description of craniofacial and dental features of the RASopathies can provide information for dental clinicians treating these individuals and can also give insight into the role of Ras signalling in craniofacial development.

Ras Signaling Regulates Stem Cells and Amelogenesis in the Mouse Incisor.

The role of Ras signaling during tooth development is poorly understood. Ras proteins-which are activated by many upstream pathways, including receptor tyrosine kinase cascades-signal through multiple effectors, such as the mitogen-activated protein kinase (MAPK) and PI3K pathways. Here, we utilized the mouse incisor as a model to study how the MAPK and PI3K pathways regulate dental epithelial stem cells and amelogenesis. The rodent incisor-which grows continuously throughout the life of the animal due to the presence of epithelial and mesenchymal stem cells-provides a model for the study of ectodermal organ renewal and regeneration. Utilizing models of Ras dysregulation as well as inhibitors of the MAPK and PI3K pathways, we found that MAPK and PI3K regulate dental epithelial stem cell activity, transit-amplifying cell proliferation, and enamel formation in the mouse incisor.

Craniofacial and dental development in cardio-facio-cutaneous syndrome: the importance of Ras signaling homeostasis.

Cardio-facio-cutaneous syndrome (CFC) is a RASopathy that is characterized by craniofacial, dermatologic, gastrointestinal, ocular, cardiac, and neurologic anomalies. CFC is caused by activating mutations in the Ras/mitogen-activated protein kinase (MAPK) signaling pathway that is downstream of receptor tyrosine kinase (RTK) signaling. RTK signaling is known to play a central role in craniofacial and dental development, but to date, no studies have systematically examined individuals with CFC to define key craniofacial and dental features. To fill this critical gap in our knowledge, we evaluated the craniofacial and dental phenotype of a large cohort (n = 32) of CFC individuals who attended the 2009 and 2011 CFC International Family Conferences. We quantified common craniofacial features in CFC which include macrocephaly, bitemporal narrowing, convex facial profile, and hypoplastic supraorbital ridges. In addition, there is a characteristic dental phenotype in CFC syndrome that includes malocclusion with open bite, posterior crossbite, and a high-arched palate. This thorough evaluation of the craniofacial and dental phenotype in CFC individuals provides a step forward in our understanding of the role of RTK/MAPK signaling in human craniofacial development and will aid clinicians who treat patients with CFC.