Saethre-Chotzen syndrome is an autosomal dominant disorder that affects between 1 and 2 in every 50,000 people. The syndrome is characterized by a spectrum of malformations, including early closure of the cranial sutures leading to a misshapen head and facial asymmetry, underdeveloped cheek bones, low-set or abnormal ears with a mild hearing deficit, dental defects, and limb abnormalities. In addition, some individuals have mild to moderate mental retardation. Individuals with Saethre-Chotzen syndrome usually require some medical or surgical intervention to manage these problems.
"We suspected that TWIST might be the disease gene when we saw the pattern of its expression in the mouse embryo," explains Ethylin Jabs, M.D., associate professor of pediatrics, medicine, and surgery. The mouse gene TWIST normally is active in the early embryonic mesoderm of the head and limbs, tissues involved in forming the skeleton and muscle of the head, face, hands, and feet. Mice lacking both copies of the gene die before birth with specific defects, including failure of the neural tube to close and disorganized mesenchyme in the head and limbs. "The major clinical features in humans could be due to disorganization of mesodermally derived tissues," explains Jabs.
Since a region on the short arm of chromosome 7 had been implicated in Saethre-Chotzen syndrome, Jabs's speculation was accurate. The region on chromosome 7 has homology to chromosome 12 in mice, known to contain the mouse TWIST gene. Jabs's team went on to map the human TWIST gene to the Saethre-Chotzen locus on chromosome 7. Then they searched for mutations in the TWIST gene in 38 affected family members and in 100 healthy individuals. They found five different types of mutations in TWIST in affected individuals. None of these mutations was found in unaffected individuals, confirming TWIST as the disease gene.
Most of the mutations identified in the TWIST gene seem to interfere with the TWIST protein's binding to DNA. TWIST is a member of the basic helix-loop-helix family of proteins. This is the first time any helix-loop-helix protein has been implicated in human malformations. Normally, two TWIST molecules combine like two chopsticks to hold the DNA helix at specific places, regulating the transcription of the genes there. Without the intact TWIST gene, genes important in embryonic development may not be expressed. One mutation actually creates a much-shortened protein that likely lacks all function. A father and daughter with this mutation had especially severe cases of the syndrome as well as cleft palates, short statures, and learning disabilities.
Saethre-Chotzen syndrome is frequently mistaken for Crouzon syndrome, another craniofacial disorder characterized by craniosynostosis and caused by mutations in the fibroblast growth factor receptor (FGFR) family of genes. Jabs's work provides an explanation for the confusion. In the fruit fly Drosophila, the TWIST protein actually induces the expression of the FGFR genes, and mutations in FGFR and TWIST have similar effects on fruit fly development. A comparable role for TWIST in humans could explain the resemblance of Saethre-Chotzen to Crouzon syndrome. "Our results suggest that, like in Drosophila, FGFRs are downstream targets of TWIST in humans," says Jabs.
Her results paint a neat picture of craniofacial development; TWIST protein expressed in the embryonic mesoderm turns on FGFR genes, which in turn instruct various cells to divide, differentiate, or migrate to form the appropriate head, face, and limb structures. Failure of any of these genes to act leads to improper development and human syndromes including Saethre-Chotzen.
The research was funded by the National Institutes of Health, the Shriners Hospital for Crippled Children, and others. Other study authors include members of Jab's lab at Hopkins, led by post-doctoral student Timothy D. Howard, Ph.D.
The Johns Hopkins Children's Center is the children's hospital of the Johns Hopkins Medical Institutions. Maryland's only comprehensive acute-care hospital for children, the Center, with its 177-bed hospital and more than 40 divisions and services, treats some 8,000 inpatients annually, with more than 90,000 outpatient visits.