Role of the Ciliary Protein C2CD3 in Mandibular Skelotogenesis

Project Summary/Abstract Primary cilia are non-motile, microtubule-based organelles that protrude from the cellular membrane to sense the cell?s external environment and to coordinate the transduction of multiple signaling pathways. Disruptions in the structure or function of primary cilia result in a class of disorders known as ciliopathies.

The skeleton is often affected in ciliopathies as ciliopathy patients often present with osteochondrodysplasias (OCDPs) - anomalies of bone and cartilage - such as micrognathia and mandibular dysmorphology. Oral-facial-digital syndromes are a subset of ciliopathies that present with several OCDPs.

There is considerable phenotypic overlap between subtypes of oral-facial-digital syndromes and among other skeletal ciliopathies, making them difficult to properly diagnose and differentiate from each other.

The substantial degree of phenotypic overlap of ciliopathies that affect the skeleton combined with the lack of a cure for ciliopathic OCDPs make this an issue of significant biomedical concern.

This proposed work focuses on understanding the requirement of the ciliary protein C2cd3 during the development of the mandibular skeleton by utilizing the avian C2CD3 mutant talpid2 (ta2) and the conditional C2cd3fl/fl mouse. C2cd3 localizes to the distal centrioles of primary cilia and is required for ciliogenesis.

Mutations in C2cd3 result in the human ciliopathy Oral-facial-digital syndrome subtype 14.

My preliminary data indicates that talpid2 embryos present with numerous mandibular OCDPs, such as bilateral Meckel?s chondrodysplasias and hypoplastic skeletal elements. There has yet to be an in-depth study as to how these phenotypes arise.

In Aim 1, I will determine if the bilateral Meckel?s chondrodysplasias are exostoses - cartilage-capped bony tumors defined by lack of cellular polarity and ciliary extension, reduced Ext1/2 expression, and reduced heparan sulfate synthesis. ta2 embryos also possess increased FGF8 expression in the developing mandible, but it is not known if this leads to mandibular bone hypoplasia.

For Aim 2, I will determine if expanded mandibular FGF8 signaling is sufficient to induce ta2 mandibular skeletal hypoplasia using bead implantations in the developing avian mandible.

Lastly, C2cd3 is expressed ubiquitously throughout the developing embryo and it is not known if loss of C2cd3 in the craniofacial epithelium or mesenchyme leads to mandibular OCDP.

For Aim 3, I will breed C2cd3fl/fl mice with the dHAND-Cre and Crect mice to conditionally delete C2cd3 in the mandibular mesenchyme and epithelium to determine if C2cd3 loss in either tissue type results in mandibular OCDP.

This study is important for advancing our understanding of the cellular and molecular etiology of mandibular OCDPs in ciliopathic patients.

A greater understanding of the developmental mechanisms of mandibular OCDPs in ciliopathies is necessary for further refined diagnosis and the development of less invasive therapeutic options.