Saethre-Chotzen syndrome (acrocephalo-syndactyly type III, ACS III) is an autosomal dominant craniosynostosis with brachydactyly, soft tissue syndactyly and facial dysmorphism including ptosis, facial asymmetry and prominent ear crura. ACS III has been mapped to chromosome 7p21-22. Of interest, TWIST, the human counterpart of the murine Twist gene, has been localized on chromosome 7p21 as well. The Twist gene product is a transcription factor containing a basic helix-loop-helix (b-HLH) domain, required in head mesenchyme for cranial neural tube morphogenesis in mice. The co-localisation of ACS III and TWIST prompted us to screen ACS III patients for TWIST gene mutations especially as mice heterozygous for Twist null mutations displayed skull defects and duplication of hind leg digits. Here, we report 21-bp insertions and nonsense mutations of the TWIST gene (S127X, E130X) in seven ACS III probands and describe impairment of head mesenchyme induction by TWIST as a novel pathophysiological mechanism in human craniosynostoses.

The twist gene is involved in the establishment of germ layers in Drosophila embryos: twist homozygous mutant embryos fail to form the ventral furrow at gastrulation and lack mesoderm and all internal organs. We have determined the sequence of the twist gene, that contains ‘CAX’ repeats in its 5′ moiety, and codes for a protein of 490 amino acids. We have raised anti‐twist antibodies that were used to study the distribution of the twist protein in whole mounts and tissue sections of wild‐type embryos. Twist protein appears to be a nuclear protein at all developmental stages. It is present over both poles and in the midventral region (endoderm and mesoderm anlagen) at cellular blastoderm stage; later in development, it is detected within the mesodermal layer until its differentiation into somatopleura and splanchnopleura in which some cells are still labelled by anti‐twist antibodies.

The twist zygotic gene appears to be involved in the establishment of the dorso-ventral pattern in Drosophila embryos. Homozygous twist embryos are partially dorsalized, their gastrulation is abnormal, and they fail to differentiate mesoderm. We determined the temperature-sensitive period of twist around the gastrulation time, and we isolated the gene. A 300 kb chromosomic walk allowed the detection of the 70 kb deletion that delimits the twist region in Df(2R)twiS60. Southern blot analyses of 21 EMS induced twist allele DNAs and systematic Northern blot analyses all over this 70 kb region lead to the localization of the twist gene: within about 10 kb at the left border of the deletion, 2 twist alleles show each a small deletion that uncover a transcription unit whose expression occurs about at the time of gastrulation.