Self-Association Contributes to E2a-Pbx1-Mediated Oncogenesis.

Pbx1 is a proto-oncogene that was originally discovered at the site of t(1;19) chromosomal translocations in pediatric acute B cell precursor leukemia. It codes for a TALE (three amino acid loop extension) class homeodomain transcription factor, which is a component of hetero-oligomeric protein complexes that regulate developmental gene expression. As a result of chromosomal translocations, Pbx1 is oncogenically activated by in-frame fusions with the E2a bHLH protein, which confers strong transcriptional activator properties and constitutive nuclear localization, bypassing the need for dimerization with Meinox homeodomain proteins to stabilize and import Pbx1 into the nucleus. E2a-Pbx1 oncoproteins retain an ability to bind DNA as a complex with Hox transcription factors, and co-expressed HoxA9 accelerates leukemogenesis. Using a biochemical approach, we have recently observed that E2a-Pbx1 self-associates through the Pbx moiety of the chimeric protein to form higher-order oligomers. Structure/function studies suggest that self-association is required for oncogenic activity since mutant E2a-Pbx1 proteins unable to self-associate are transformation defective in an in vitro myeloid progenitor serial replating assay. Interestingly, their oncogenic activity in this assay is rescued using synthetic oligomerization domains of FKBP. The drug AP21998, which disrupts FKBP-mediated oligomerization, blocks the proliferation of transformed myeloid progenitors and facilitates their terminal myeloid differentiation. In addition to self-association, the DNA binding homeodomain of Pbx1 is also required for transformation, but mutagenesis studies indicate that Pbx1 domains involved in cooperative DNA binding with Hox partners are dispensable. These studies suggest an alternative mechanism for leukemogenesis in which E2a-Pbx1 deregulates subordinate gene expression as an oligomeric complex that circumvents interactions with heterologous homeodomain proteins that otherwise modulate Pbx1 nuclear localization, DNA binding, and transcriptional activity.