Fig. 2.

Variant modeling in Drosophila establishes MAP3K7 p.(Gly191Arg) as a novel pathogenic LoF variant causing CSCFS. A Posteroanterior view of a chest X-ray showing scoliosis and bilateral cervical ribs. Lateral view of a cervical spine X-ray shows a fusion of C5 and C6. An overview of all clinical features is provided. B The MAP3K7 and Tak1 protein contain an evolutionary conserved kinase domain. Two of the known GoF variants (associated with FMD2) are depicted in orange, two of the known LoF variants (associated with CSCFS) are represented in blue. Our new variant (c.571G>A, p.Gly191Arg), depicted in black, is located in de kinase domain and replaces a highly conserved glycine for an arginine residue. C Overexpression of wild-type Drosophila Tak1 and human MAP3K7 in the Drosophila eye using a GMR driver resulted in early pupal lethality. Overexpression of fly Tak1 and human MAP3K7 with GoF variant p.Glu70Gln fully recapitulated the phenotype of wild-type overexpression. With the p.Gly168arg variant a discrepancy between the human and fly protein was noted. Overexpression of the fly Tak1 variant resulted in a severe eye phenotype whereas overexpression of the human variant did not visibly affect the eye, pointing towards a milder GoF effect compared to the other GoF mutation. Overexpression of known LoF variants p.Val50del and p.Gly110Cys in human and fly proteins lost the wild-type eye phenotype. Overexpression of our novel variant p.Gly191Arg resulted in a normal eye thereby establishing the LoF nature of this variant