The abnormal C-terminus caused by DVL1 variants impacts Robinow syndrome phenotypes

Robinow Syndrome is a genetically heterogeneous, rare skeletal disorder characterized by craniofacial and limb defects. All 7 causative genes lie in the Wingless-related Integration site-1 (WNT) pathway. Here we study the pathogenesis of DVL1 (Dishevelled 1), the most commonly affected gene, where variants cause a frameshift that replaces the C terminus with a novel peptide. We compared phenotypes caused by DVL11519ΔT to the effects of wtDVL1 or DVL1 with a stop codon at position 1519. Misexpression of DVL11519ΔT in chicken embryos with an avian retrovirus, leads to increased width of the frontonasal mass similar to the facial phenotype in RS. Ultimately skeletogenesis is inhibited, which was verified in primary cultures of frontonasal mass mesenchyme. In luciferase assays carried out in facial mesenchyme, wtDVL1 activated canonical and JNK-PCP WNT signalling whereas the DVL11519* and the DVL11519ΔT variant had significantly lower signaling activity. These data confirm that the C-terminus plays an important role in WNT signal transduction and skeletogenesis. We also determined that there is mislocalization of the protein expressed from DVL11519ΔT in the nucleus while the other two constructs were expressed in the cytoplasm. Nuclear expression of DVL1 may alter transcription in RS. In complementary Drosophila experiments using a variety of readouts, only the DVL11519ΔT variant and not the 1519* impacted morphogenesis and signaling. This is the first study to show that the novel C-terminus of DVL1 is sufficient to interfere with the function of DVL1 protein expressed from the normal allele in heterozygous, autosomal dominant RS.