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Title: Developmental and evolutionary consequences of mutation to transcription factor Gli3
Author: Butt, Jonathan Thomas Delafield
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 2003
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Gli3 is a zinc-finger transcription factor homologous to the Drosophila segment polarity gene Cubitus interruptus. It is expressed early in mouse development in mesoderm and ectoderm, and continues to be expressed during organogenesis of brain, limb, and other structures. In mouse, mutation in Gli3 produces the extra-toe (Xt) phenotype. In man, mutation in GLI3 produces isolate pre- and post-axial polydactylys, and polydactyl complexes associated with craniofacial and neurological abnormalities (Greig's Cephalopolysyndactyly and Pallister-Hall syndrome). GU3 function has been well studied in the limb and spinal cord, but little is known about its mode of action in brain. In this thesis, I examined the function of Gli3 in murine forebrain development. I employed a primary culture technique to test the survival and gene expression characteristics of neocortical cells from Xt/Xt, Xt/+, and +/+ mice during midneurogenesis (embryonic days (E)14.5, 15.5, and 16.5). I found that the survival of neocortical cells is age and density dependent, and that Xt/Xt cells exhibit greater viability than their +/+ counterparts. Furthermore, I found that E16.5 Xt/+ cells exhibit greater viability than +/+ cells, indicating that a murine Xt/+ neurological phenotype might exist. I tested Xt/Xt and Xt/+ telencephalon for disruption to normal cell-type composition using markers of pre- and post-mitotic cell types. Acutely dissociated neocortical and ganglionic eminence cells from E14.5 and E15.5 embryos were immunoreacted with neuronal markers (3-tubulin III and microtubule associated protein 2. Proliferating pre-mitotic cell types were labelled with RC2 and nestin. A further BrDU incorporation technique was employed to test for cells in active cell cycle. The results from these experiments suggest that the cell-type composition of Xt/Xt neocortex does not differ significantly from wild-type proportions, though the proportion of cells undergoing proliferation may be increased. In a separate line of investigation, I examined the neonatal skeleton of Xt/Xt, Xt/+, and +/+ in a mouse colony derived from the Jackson Laboratory. I found that the skeletal phenotypes were comparable to those reported on a similar Xt mouse strain from the M.R.C. Harwell Unit. Furthermore, I crossed the Xt mouse onto two other backgrounds and showed that the heterozygote limb phenotype is, in part, dependent on the background. I also report on the transmission of the Gli3(-) allele from one generation to the next. In data collected over two years, I show that the mutant allele transmits at a greater rate than the wild-type allele. Moreover, I show that this behaviour is restricted to a subset of the mice, and in this subset the transmission rate of Gli3(-) is two-fold higher than would be expected by Mendelian genetics. I term this bizarre finding Gli3(-) positive transmission distortion, and refer to data on the transmission of human GLI3 to suggest this phenomenon is occurring in other species. Finally, I present an essay to suggest evolutionary consequences of mutation to Gli3. I present an overview of the Gli3 gene and the effects of Gli3-mutation on its function as a regulator of tissue patterning in multiple systems. In conclusion, I attempt to place the role of GU3, and mutations to GH3 in a framework that includes modern theories on evolution, and more mature theories on the role of genes and development in evolution.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available