Craniofacial Ciliopathies: Role of the Primary Cilia Transition Zone Complex

Craniofacial abnormalities comprise one-third of all birth defects and often require multiple rounds of invasive surgeries to repair the malformations. Primary cilia are microtubule-based organelles that coordinate multiple signaling pathways known to be critical for proper development of the craniofacial complex including Hedgehog, Wnt, FGF, and PDGF pathways. A growing body of work has genetically linked proteins involved in primary cilia formation and membrane composition to a number of craniofacial anomalies, collectively grouped as craniofacial ciliopathies. Ciliopathies with craniofacial defects include Bardet-Biedl syndrome (BBS), oro-facial-digital syndrome (OFD1), Meckel syndrome (MKS), Joubert syndrome, and Ellis-van Creveld syndrome. Our previous work identified a complex of proteins that localize to the region of the primary cilium between the basal body and ciliary axoneme, termed the transition zone (TZ). 

 Objective: Define mechanistically how mutations in transition zone proteins lead to the craniofacial defects seen in the above ciliopathies.

 Methods: Focusing on three proteins found at distinct regions within the TZ complex (Tectonic, B9D1, and TMEM231), we used geometric morphometric analysis to quantify the phenotypic mean and variation in facial shape in mouse models carrying individual loss-of-function alleles for each of these TZ components.

 Results: We found that loss of these TZ proteins leads to severe narrowing of the facial midline and decreased SHH expression in the frontonasal ectodermal zone (FEZ). TZ mutants also displayed neural tube patterning defects with expansion of dorsal cell types evidenced by expansion of Pax6 expression and corresponding decrease in FoxA2 expression along the dorsal-ventral axis. SEM and TEM microscopy revealed structural defects in the primary cilia localized to the neural tube and neural crest populations.

Conclusion: Our findings reinforce the importance of primary cilia in directing proper craniofacial morphogenesis and begin to elucidate the cellular mechanisms by which the TZ complex regulates facial development.