| In humans, hereditary cerebellar ataxias and epilepsy are quite prevalent and often debilitating. Despite their clinical prevalence, the genetic and developmental bases of these diseases are poorly understood. Neurological mutant mice have provided important models for these diseases. While many classical spontaneous neurological mouse mutants have been characterized, both genetically and phenotypically, there still exist several mutants whose genetic lesion and detailed cellular and molecular phenotype remain unknown, representing rich resources that are available yet unexploited. Included among these is the mouse mutant tippy, which arose as a spontaneous mutation in a linkage cross at the Jackson Laboratory in 1977. Homozygous tippy mice are small, severely ataxic, hyperactive, prone to seizures and typically die by post-natal day 21 (P21) to P25. Although the tippy mutant cerebellum possesses grossly normal morphology, I have discovered that mutant Purkinje cells (PCs) display developmental defects in branching pattern, with loss of shaft dendrites and sprouting of distal spiny branchlets in proximal locations. In conjunction, dendritic spines exhibit immature morphology and aberrant locations on mutant PC somas and proximal dendrites. As a result of the morphological changes in the dendritic tree, there are alterations in the pattern of afferent innervation of PCs in tippy mutant mice that change physiological input to the PCs and consequently give rise to behavioral ataxia. I have also determined that tippy mutant mice have striking morphological defects in the dentate gyrus (DG) of the hippocampus that are likely involved in the spontaneous seizure activity exhibited by these mice. The defect is epitomized by a distinct, organized, ectopic third blade of the DG. Defects in the radial glia scaffolding causing mismigration of granule neurons in conjunction with defects in the putative stem cells causing aberrant proliferation and neurogenesis, likely underlie the emergence and formation of the ectopic blade. Immunohistochemical evidence suggests that the ectopic blade is integrated into the hippocampal network, possibly creating an epileptogenic circuit and promoting seizure activity. Through analysis of 226 affected F2 progeny from an intersubspecific intercross with Mus mus. molissinus mice, I have localized the tippy gene to a 1.0 cM/2.1 Mb region on distal chromosome 9. Sequencing of all exons within the interval has identified no changes, suggesting a regulatory mutation. Expression microarray analysis revealed non-contiguous candidate genes that are significantly downregulated in tippy mutant mice---Bfsp2 and Trf. However, complementation tests have demonstrated non-allelism of both genes with tippy. Although not causative of the tippy phenotype, morphological and expression analyses strongly suggest that Trf may play a contributory role in the Purkinje cell and dentate gyrus structural defects seen in tippy mutant mice. Therefore, the tippy mutation likely resides in a regulatory region, consequently altering transcript level and distribution of both Trf and Bfsp2 or of a novel gene yet to be identified, ultimately perturbing Purkinje cell and dentate gyrus morphogenesis to give rise to ataxia and seizure activity. |
