Hox genes and the control of limb bud outgrowth

Gain‐ and loss‐of‐function experiments have demonstrated that the establishment of the limb architecture relies upon the function of HoxA and HoxD genes. Moreover, we have shown that the complete inactivation of these two gene clusters results in severe limb truncation. During limb development, there are two phases of Hox expression: At early limb bud stages, Hox genes are sequentially activated in time, with expression of ‘late’ genes (from group 10 to 13) being progressively restricted to the posterior mesenchyme. At later developmental stages, transcription is maintained for only a subset of the HoxA and HoxD genes (group 9 to 13), with a collinear nested pattern along the proximal‐distal axis. While phenotypes associated to individual gene loss‐of‐function correlates with the second wave of transcriptional activation, we found that the function of Hox genes is necessary from early limb bud stages onwards. For instance, by removing both HoxA and HoxD gene clusters, we have uncovered a requirement for these transcription factors in Shh activation and maintenance. We have further analyzed HoxA/HoxD conditional mutants and found that decreasing the Hox dosage within developing limb buds impairs cell survival. Our results suggest that, in addition to patterning functions, Hox genes are needed for the proper outgrowth of developing limbs.