Craniofacial studies in chicken embryos confirm the pathogenicity of Frizzled2 variants associated with Robinow syndrome

Abstract

Robinow syndrome (RS) is a rare, genetically heterogeneous condition caused by mutations in seven WNT pathway genes. The clinical presentation includes craniofacial widening and jaw hypoplasia. Here we tested the functional impact of two missenseFZD2variants 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 ofCTNNB1and abnormal BMP signaling. In primary cultures, variants of FZD2, inhibited chondrogenesis, caused increased nuclear shuttling of β-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 theRor2co-receptor. This is the first study to show that the missense variants inFZD2have 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.