C2CD3 links tooth patterning to evolutionary tooth loss in birds and dental anomalies across vertebrates

Primary cilia play an essential role in molecular signaling transduction during development, coordinating cell fate decisions and morphogenesis. Defects in components of the primary cilium result in an array of developmental syndromes called ciliopathies. Here we report 24 patients with nine rare and seven novel variants in the primary cilia gene C2CD3, which has previously been implicated in the ciliopathy Orofaciodigital syndrome XIV. Patients displayed an array of dental defects impacting the number, shape, and timing of tooth development. To understand the etiology underlying these defects, murine and avian models were analyzed during embryonic development. Ap2Cre;C2cd3^{ex4-5 fl/fl} mouse mutants showed a range of dental defects prenatally, mimicking several of the phenotypes observed in patients. Shh was upregulated in the developing upper jaw, leading to a widening of the midline, while Shh expression in the lower jaw was reduced, highlighting the context-dependent impact of cilia on hedgehog signaling. In avian mutants carrying a C2CD3 variant, talpid², tooth buds were evident with SHH expression upregulated during early development in both upper and lower jaws. Cartilage defects were additionally observed in both conditional mouse knockouts and talpid² embryos. These changes in Shh signaling provide a mechanism to explain the phenotypes in patients and highlight the conserved role of C2CD3 in coordinating the morphogenetic potential of the jaw and patterning of the forming dentition. Given the conserved phenotypes, alterations in C2CD3-dependent ciliogenesis may have played a role in modulating Shh signaling to alter the timing and position of tooth germs across vertebrate evolution.