Ablepharon and craniosynostosis in a patient with a localized TWIST1 basic domain substitution

The TWIST family is a group of highly conserved basic helix–loop–helix transcription factors. In humans, TWIST1 haploinsufficiency causes Saethre–Chotzen syndrome, which is characterized by craniosynostosis. Heterozygous localized TWIST1 and TWIST2 basic domain substitutions exert antimorphic effects to cause Sweeney–Cox syndrome, Barber–Say syndrome, and ablepharon-macrostomia syndrome, respectively. Sweeney–Cox syndrome, Barber–Say syndrome, and ablepharon-macrostomia syndrome share the facial features of ablepharon, hypertelorism, underdevelopment of the eyelids, and cheek pads adjacent to the corners of the mouth. Existence of phenotypic overlap between Saethre–Chotzen syndrome and Sweeney–Cox syndrome remains unknown. Herein, we document a male infant with the distinctive facial features of ablepharon, hypertelorism, cheek pads adjacent to the corners of the mouth, and bilateral coronal suture craniosynostosis who had a de novo heterozygous mutation in the basic domain of TWIST1, that is, c.351C>G p.Glu117Asp. The pathogenicity of this variant was supported by in silico and in vivo evidence. Our review showed that Sweeney–Cox syndrome appears to share many characteristics with Barber–Say syndrome and ablepharon-macrostomia syndrome except for craniosynostosis, which is a cardinal feature of Saethre–Chotzen syndrome. An amino acid substitution from Glu117 to Asp, both of which are the sole members of negatively charged amino acids, resulted in a prototypic Sweeney–Cox syndrome phenotype. This suggests that any amino acid substitutions at Glu117 would likely lead to the Sweeney–Cox syndrome phenotype or lethality. The present observation suggests that a localized TWIST1 basic domain substitution, that is, p.Glu117Asp, in TWIST1 may exert a mild antimorphic effect similar to that of haploinsufficiency, leading to craniosynostosis and ablepharon.