Epilepsy affects 1-4 percent of the population, with approximately one third of cases being intractable to current medical therapy. As methods improve for noninvasively imaging the human cortex, an increasing number of epilepsies (15-30 percent) are associated with malformations of the cortex. Understanding the genetic controls of cerebral cortical development is central to understanding the genesis of epileptic malformations. Cell identity in the cortex is determined by a sequential process of patterning, proliferation, progressive cell-type specification, and differentiation. While the first three years of this grant have analyzed cell division patterns that produce the cortex, this project takes a genetic approach to investigate molecular mechanisms that specify cell identity in the cortex. The overall goal of the proposed research is to analyze the role of a few key genes required for normal neuronal development in the cortex. For example, cell lineage appears not to be solely responsible for patterning the cerebral cortex; instead, secreted dorsal and ventral signals appear to specify regions of the forebrain. The hyh gene is required for normal patterning of the dorsal forebrain and cerebral cortex. We will further characterize the hyh mutant mice and will identify the gene mutated in the hyh mouse. Secreted dorsal and ventral patterning signals appear to act by specifying progenitor cells; Pax6 is a paired homeodomain transcription factor that is a candidate downstream target of secreted signals. We have found that Pax6 mutant mice show abnormal proliferation and migration, suggesting that specific progenitors are abnormal. We will further analyze the mutant phenotype using retroviruses to mark fates and to alter the expression of Pax6 gene in wild type host animals. Lhx2 is a LIM-homeodomain gene that is also required for normal cortical development, though is precise role is unclear. Lhx2 gene expression will be inhibited in vivo in cortical progenitor cells using retroviruses that carry the Lhx2 gene in an antisense orientation, and the fates of infected cells will be analyzed. Second, the expression of Lhx2 in cerebral cortical progenitor cells in vitro will be inhibited using antisense oligonucleotides, and patterns of cell fate, cell division, and cell death among cerebral cortical progenitor cells will be determined. The methods developed for these studies may have more general utility for studying the roles of many key genes involved in cortical development.
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