Loss of citron-kinase results in cortical malformation and early-onset epilepsy

Disruptions in the timing and interactions of a single gene or protein can alter the survival and fates of newly generated cells in CNS. In humans these alterations during development can lead to gross malformations of the cerebral cortex and often can be associated with disorders such as epilepsy. In this study, I describe the identification of a novel autosomal recessive mutation in rat, Flathead (fh/fh), which leads to an overall reduction in CNS growth and a cerebral cortex that is one half the size of wildtype. Previous studies in fh/ fh mutants have shown that the fh gene is CNS-specific, and that the reduced brain size correlates with massive apoptosis in proliferative areas during the time of maximal neurogenesis. The group of experiments in this thesis show that homozygous mutants suffer from intractable, spontaneous and high-frequency generalized seizures which begin one week after birth. Throughout the entire cortex of fh/fh mutants, there is a disproportionate loss of inhibitory interneurons which is present before and during the time of seizure generation. Remaining interneurons in fh/fh cortex display dramatic hypertrophy while pyramidal neurons do not show similar changes. The reduction of neocortical interneurons is associated with high levels of cell death in the ganglionic eminence, the primary embryonic source for neocortical interneurons, and the death appears to result from a failed cytokinesis process. Moreover many precursors within the ventricular zone (VZ) are binucleate and binucleate interneurons and pyramidal neurons are observed in postnatal fh/ fh cortex as well. Finally, I show that flathead is caused by a mutation in the first exon of the N-terminal kinase domain in the gene encoding Citron-kinase (Citron-K). Citron-K expression is maximal during neurogenesis and concentrates along the VZ surface where it localizes to cytokinesis furrows and is activated by the small GTPase RhoA. This study shows that Citron-K is required for cytokinesis in neural progenitors and that Citron-K is essential for the survival, fate and differentiation of different cortical neurons. These findings both enhance our understanding of CNS development and provide us with insight into novel pathways which may alter brain excitability and lead to disorders such as epilepsy.