Mouse Hox and Cdx genes: Early regulation and involvement on embryonic axial extension

Abstract

In this thesis we studied Cdx and Hox genes and their function during early embryogenesis. Hox genes are organized in clusters, from Hox1 to Hox13. During axial elongation, the cluster progressively becomes active, starting at the Hox1 side, to finally - at the end of axial elongation - activate Hox13. In this thesis we studied the molecular and epigenetic events that are involved in the earliest onset the HoxA. We have found that thecis-regulatory landscape adjacent to the earliest Hox gene (Hoxa1) primes Hoxa1 to be active.Hoxa1 and it's cis-regulatory elements (‘enhancers’) are in the same genomic segment. Upon exposure to the Wnt signals, the early regulatory landscape becomes active, and the Hoxa1 gene is activated. Once this initial gene is transcribed, the rest of the cluster follows. In this thesis we additionally studied the role of Cdx genes - which are family of Hox - during axial elongation. We show that Cdx genes are required to generate all tissue posterior to the head. Embryos lacking Cdx are not able to generate trunk or tail structures. The mutants are not able to maintain their axial progenitor population, which is essential to form tissue along the antero-posterior axis. We identified the regulatory elements which are bound by the Cdx2 transcription factor - and by a protein with a similar function, T Brachyury. We found that Cdx genes - and T Brachyury - directly regulate signaling pathways (Wnt, Fgf) that are essential for the maintenance of axial progenitors. Moreover, we discovered that Cdx is required for the gradual activation of the Hox cluster. Without Cdx proteins, the DNA sequence of regulatory elements in the Hox cluster remains inaccessible. Importantly, in this thesis we combined classical embryonic techniques with genome-wide profiling of regulatory elements, making use of Epiblast Stem Cell (EpiSCs).