At the base of colinear Hox gene expression: cis-features and trans-factors orchestrating the initial phase of Hox cluster activation

Hox genes are crucial players in the generation and pattering of the vertebrate trunk and posterior body during embryogenesis. Their initial expression takes place shortly after the establishment of the primitive streak, in the posterior-most part of the mouse embryo and is a determinant step for setting up the definitive Hox expression boundaries along the antero-posterior body axis. The developmental signals and epigenetic mechanisms underlying this early activation remained unsolved until recently. The development of novel embryo-derived model systems, combined with methods that examine chromatin status and chromosome conformation, led to deeper understanding of the process of Hox activation in the early embryo. Here we summarize how the early Hox cis-regulatory landscape becomes active upon receiving the appropriate developmental signal, and we discuss the importance of the local topological segmentation of the HoxA cluster during early Hox activation.