Transcriptional regulation of Hox genes during hindbrain development.

Hox genes play a crucial role in patterning the anteroposterior axis. Therefore, it is important to understand the mechanisms regulating their expression. One way of identifying regulatory regions is to compare related genomic sequences in order to find conserved elements. Previous work in transgenic mice has defined the elements required to recapitulate Hoxa2 expression in rhombomeres 3 and 5. However, the mechanisms regulating expression of Hoxa2 in rhombomeres 2 and 4 were unknown. In this study, I demonstrate that a highly conserved region in the intron of Hoxa2 contains the control elements directing rhombomere 4 expression. Further, I show that the rhombomere 2 enhancer is located in the second exon of Hoxa2. Due to genome-wide duplication events, the number of Hox genes increased during vertebrate evolution. In the pufferfish, this led to the presence of two Hoxa2 genes, Hoxa2(a) and Hoxa2(b). The two co-paralogous genes show differential expression. I compared the control regions of these two genes and identified subtle changes in their cw-regulatory regions. Using chick electroporation, I show that several changes in these elements are responsible for the differential expression. Hoxbl has a broad expression pattern in the hindbrain at early stages. This expression becomes restricted to rhombomere 4 during later development. I show that Krox20 binds to a highly conserved repressor region and that removal of this element in transgenic constructs leads to an expansion of reporter expression into rhombomeres 3 and 5. Finally, I show that Hoxbl expression in the second branchial arch is repressed by a mammalian-specific repressor element. This work has shed important insight into the mechanisms and factors that modulte expression of Hoxa2 and Hoxbl during hindbrain development