Tophkhane, S. S.; Fu, K.; Richman, J.

Robinow syndrome (RS) is a rare, genetically heterogeneous condition caused by mutations in 7 WNT pathway genes. The clinical presentation includes craniofacial widening and jaw hypoplasia. Here we tested the functional impact of 2 missense FZD2 variants in the frontonasal mass in the chicken embryo, the area homologous to the affected region in RS. Viruses coding for wild-type or variant forms of FZD2 (code for P142L and G434V) inhibited beak ossification in vivo and certain bones failed to differentiate, possibly mediated by decreased levels of CTNNB1 and abnormal BMP signaling. In primary cultures, variants of FZD2, inhibited chondrogenesis, caused increased nuclear shuttling of {beta}-catenin and increased expression of early mesenchyme marker, TWIST which precedes chondrocyte specification. To determine the impact of the variants on WNT pathways we used luciferase reporters. The G434V and P142L plasmids dominantly interfered with wtFZD2 in SuperTopflash canonical assays. The P142L variant repressed the activity of the ATF2 reporter in the presence of the ROR2 co-receptor. This is the first study to show that the missense variants in FZD2 have functional defects and that the upper face skeletal defects in chickens recapitulate features of the jaw hypoplasia and flat profile of people with Robinow Syndrome.