HoxA9-mediated immortalization of myeloid progenitors requires functional interactions with TALE cofactors Pbx and Meis

Specific Hox genes are implicated in leukemic transformation, and their sel ective genetic collaboration with TALE homeobox genes, Pbx and Meis, accent uates their oncogenic potential. The molecular mechanisms under-lying these coordinate functions, however, have not been characterized. In this study, we demonstrate that HoxA9 requires its Pbx interaction motif as well as it s amino terminus to enhance the clonogenic potential of myeloid progenitors in vitro. We further show that HoxA9 forms functional trimeric DNA binding complexes with Pbx and Meis-like proteins on a modified enhancer. DNA bind ing complexes containing HoxA9 and TALE homeoproteins display cooperative t ranscriptional activity and are present in leukemic cells. Trimeric complex formation on its own, however, is not sufficient for HoxA9-mediated immort alization. Rather, structure-function analyses demonstrate that domains of HoxA9 which are necessary for cellular transformation are coincident with t hose required for trimer-mediated transcriptional activation. Furthermore, the amino terminus of HoxA9 provides essential transcriptional effector pro perties and its requirement for myeloid transformation can be functionally replaced by the VP16 activation domain. These data suggest that biochemical interactions between HoxA9 and TALE homeoproteins mediate cellular transfo rmation in hematopoietic cells, and that their transcriptional activity in higher order DNA binding complexes provides a molecular basis for their col laborative roles in leukemo-genesis.