The role of Hoxa1 in mammalian hindbrain, inner ear and neural crest development

Homeobox transcription factors belong to a family of proteins involved in an array of developmental processes, the most important being specification of the anterior-posterior axis of the embryo. The first and one of the most anteriorly expressed Hox genes during development is Hoxa1. Mouse knockout studies have revealed that loss of Hoxa1 function leads to mispatterning of the hindbrain, in addition to defects in the inner ear, cranial ganglia, and the breathing inducing cells in rhombomere 3. More recently, humans with homozygous mutations in Hoxa1 have been identified, which sparked new interest in understanding the role this crucial transcription factor plays during development. Interestingly, human patients in addition to the defects described in mice, also display cardiovascular abnormalities.;My dissertation is aimed towards deepening our understanding of the role Hoxa1 plays in the development of the various tissues affected by its loss of function. In the first part, we carried out a genetic lineage-analysis to determine which tissues Hoxa1-expressing cells give rise to. Due to the very early and transient expression of this gene, previous studies have not detected Hoxa1 expression in any of the affected tissues except the neuroectoderm and concluded that all other defects are secondary due to mispatterning of the hindbrain. By generating a new mouse line, we were able to label all the early Hoxa1-expressing cells and determine that they contribute to the inner ear, neural crest derivatives, and the third rhombomere, suggesting that Hoxa1 might play a direct role in the development of these tissues. To test this hypothesis, we carried out conditional analysis to selectively knock out Hoxa1 function in the precursors of each of these tissues. Our experiments revealed that Hoxa1 is only required for a 12--18 hour time-window. Since Cre-mediated recombination in the precursor of a specific organ only takes place after this time window, it was not possible to test the requirement of Hoxa1 in a specific organ. Therefore, the next step was to identify the downstream targets that are activated by this brief spark of Hoxa1 expression. We carried out a large-scale screen for Hoxa1 effector genes, by performing microarray analysis on Hoxa1-/- and wildtype embryos. Thereby we identified and validated several Hox-A1 targets, among them genes involved in early neural crest cell specification and inner ear patterning, indicating that Hoxa1 plays a direct role in the development of these tissues. In conclusion, we identified the tissues, the time-frame and the downstream targets through which Hoxa1 exerts its function during embryogenesis.