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BACKGROUND: GLI is an oncodevelopmental gene in the vertebrate hedgehog/patched signaling pathway that is spatiotemporally regulated during development and is amplified in a subset of human cancers. GLI is the prototype for the Gli-Kruppel family of transcription factors, which includes the Drosophila segment polarity gene ci, the C. elegans sex-determining gene tra-1, and human and mouse GLI3, all of which contain a conserved domain of five C2-H2 zinc fingers. GLI3 mutations have been implicated in the mouse mutant extra toes, as well as in human Greig cephalopolydactaly syndrome and the autosomal dominant form of Pallister-Hall syndrome. As such, GLI and the vertebrate hedgehog/patched signaling pathway appear to play important roles in both normal development and neoplasia. MATERIALS AND METHODS: Since it is not known whether aberrant GLI expression is similarly linked to developmental disorders, we developed gain-of-function transgenic mice which express human GLI ectopically. RESULTS: Affected transgenic mice exhibit a phenotype of failure to thrive, early death, and Hirschsprung-like patches of gastrointestinal dilatation. The colons of affected mice have greatly attenuated smooth muscle layers and abnormal overlying epithelium. The density of myenteric plexuses is reduced in the colonic walls. The severity of the phenotype is related to the level of transgene expression. CONCLUSIONS: The transgenic mouse model supports a role for GLI in gastrointestinal development. As part of the vertebrate hedgehog/patched signaling pathway, GLI is essential to mesoderm and CNS ectoderm development and transgenic GLI expression affects neuronal, muscular, and epithelial cell differentiation in the gut. Expression of human GLI in mice results in impairment of enteric neuronal development and a Hirschsprung-like phenotype.