Straightening out the mechanisms of axial elongation using mouse mutant analysis

Understanding the mechanisms that lead to axial elongation in the mouse has direct relevance to elucidating the etiology of vertebral defects in humans. Through the characterization of a spontaneous mouse mutant, kinked tail, and the analysis of Tbx6 protein modulation in vivo, I uncovered two distinct mechanisms affecting axial elongation in the mouse. The kinked tail mutation is a spontaneous mutation, inherited dominantly, that results in a kinky tail phenotype in heterozygotes and early embryonic lethality in homozygotes. Defective axial elongation in kinked tail heterozygotes is displayed as shortened tails and multiple tail kinks resulting from wedge, hemi- and fused vertebrae, similar to those observed in scoliosis patients. These vertebral defects are likely due to a primary notochord defect that is thickened and branched. Kinked tail homozygotes fail to undergo gastrulation due to defective distal visceral endoderm cell migration, ultimately resulting in lethality by embryonic day 8.5. The defective cell migration is further compounded by basement membrane defects and gross dysmorphology of the mutant embryo. Tbx6, a T-box transcription factor, is essential for posterior somite formation, patterning and viability of the mouse embryo. I sought to understand Tbx6 protein regulation and the phenotypic consequences of modulating Tbx6 protein levels in vivo. In vitro analyses revealed that Tbx6 is a relatively stable protein that appears to be regulated in part by the proteasome in addition to other mechanisms. In vivo, less than heterozygous levels of Tbx6 protein results in rib and vertebral defects, enlarged tailbuds and axial shortening while greater than wildtype levels of Tbx6 protein results in small embryonic tailbuds, axial shortening, and lethality. I further examined the consequences of Tbx6 misexpression using a 3-component transgenic system. The primitive streak and presomitic mesoderm are affected in those embryos that misexpress Tbx6.Altogether, my analysis of the spontaneous mutation, kinked tail, demonstrated a possible role of the notochord in proper axial elongation, and the analysis of Tbx6 protein modulation further clarified the importance of maintaining proper levels of Tbx6 for normal axis elongation and embryonic development