HOX13-dependent chromatin accessibility modulates the target repertoires of the HOX factors

Hox genes encode essential transcription factors that control patterning during embryonic development. Distinct combinations of nested Hox expression domains establish cell and tissue identities1-3. Consequently, spatial or temporal de-regulation of Hox genes can cause severe alterations of the body plan3. While HOX factors have very similar DNA binding motifs, their binding specificity is, in part, mediated by co-factors4-6. Yet, the interplay between HOX binding specificities and the cellular context remains largely elusive. To gain insight into this question, we took advantage of developing limbs for which the differential expression of Hox genes is well-characterized7. We show that the transcription factors HOXA13 and HOXD13 (hereafter referred as HOX13) allow another HOX factor, HOXA11, to bind loci initially assumed to be HOX13-specific. Importantly, HOXA11 is unable to bind these loci in distal limbs lacking HOX13 function indicating that HOX13 modulates HOXA11 target repertoire. In addition, we find that the HOX13 factors implement the distal limb developmental program by triggering chromatin opening, a defining property of pioneer factors8,9. Finally, single cell analysis of chromatin accessibility reveals that HOX13 factors pioneer chromatin opening in a lineage specific manner. Together, our data uncover a new mechanism underlying HOX binding specificity, whereby tissue-specific variations in the target repertoire of HOX factors rely, at least in part, on HOX13-dependent chromatin accessibility.