E2A-PBX1 INDUCES ABERRANT EXPRESSION OF TISSUE-SPECIFIC AND DEVELOPMENTALLY-REGULATED GENES WHEN EXPRESSED IN NIH 3T3 FIBROBLASTS

The E2a-Pbx1 oncoprotein contains the transactivation domain of E2a jo ined to the DNA-binding homeodomain (HD) of Pbx1. In mice, E2a-Pbx1 tr ansforms T lymphoblasts and fibroblasts and blocks myeloblast differen tiation. Pbx1 and E2a-Pbx1 bind DNA as heterodimers with other HD prot eins whose expression is tissue specific. While the transactivation do main of E2a is required for all forms of transformation, DNA binding b y the Pbx1 HD is essential for blocking myeloblast differentiation but dispensable for fibroblast or T-lymphoblast transformation. These pro perties suggest (i) that E2a-Pbx1 causes cellular transformation by ac tivating gene transcription, (ii) that transcription of E2a-Pbx1 targe t genes is normally regulated by ubiquitous Pbx proteins and tissue-sp ecific partners, and (iii) that DNA-binding mutants of E2a-Pbx1 activa te a subset of all gene targets. To test these predictions, genes indu ced in NIH 3T3 fibroblasts by E2a-Pbx1 were identified and examined fo r tissue- and stage-specific expression and their differential abiliti es to be upregulated by E2a-Pbx1 in NIH 3T3 fibroblasts and myeloblast s and by a DNA-binding mutant of E2a-Pbx1 in NIH 3T3 cells. Of 12 RNAs induced by E2a-Pbx1, 4 encoded known proteins (a J-C region of the im munoglobulin kappa light chain, natriuretic peptide receptor C, mitoch ondrial fumarase, and the 3',5'-cyclic nucleotide phosphodiesterase, P DE1A) and 5 encoded new proteins related to angiogenin, ion channels, villin, epidermal growth factor repeat proteins, and the human 2.19 ge ne product. Expression of many of these genes was tissue specific or d evelopmentally regulated, and most were not expressed in fibroblasts, indicating that E2a-Pbx1 can induce ectopic expression of genes associ ated with lineage-specific differentiation.