We describe the successful application of a strategy that potentially provides for an efficient and universal screen for downstream gene targets. We used the promoter of the Gsh-1 homeobox gene to drive expression of the SV40 T-antigen gene in transgenic mice. We have previously shown that the Gsh-1 homeobox gene is expressed in discrete domains of the ganglionic eminences, diencephalon, and hindbrain during brain development. Gsh-1-SV40 T transgenic mice showed cellular hyperplasia in regions of the brain coincident with Gsh-1 expression. The Gsh-1-SV40 T transgene was introduced, by breeding, into Gsh-1 homozygous mutant mice, and Gsh-1 -/- cell lines were made. Clonal cell lines were generated and analyzed by Northern blot hybridizations and Affymetrix Gene Chip probe arrays to determine gene expression profiles. The results indicate that the cell lines remain representative of early developmental stages. Further, immunocytochemistry showed uniformly high levels of nestin expression, typical of central nervous system progenitor cells, and the absence of terminal differentiation markers of neuronal cells. One clonal cell line, No. 14, was then stably transfected with a tet-inducible Gsh-1 expression construct and subcloned. The starting clone 14, together with the uninduced and induced subclones, provided cell populations with varying levels of Gsh-1 expression. Differential display and Affymetrix Gene Chip probe arrays were then used to identify transcript differences that represent candidate Gsh-1 target genes. Of particular interest, the drm and gas1 genes, which repress cell proliferation, were observed to be activated in Gsh-1-expressing cells. These observations support models predicting that homeobox genes function in the regulation of cell proliferation.
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