DNA-BINDING BY ONCOPROTEIN E2A-PBX1 IS IMPORTANT FOR BLOCKING DIFFERENTIATION BUT DISPENSABLE FOR FIBROBLAST TRANSFORMATION

The t(1;19) chromosomal translocation of pediatric pre-B cell lymphobl astic leukemia produces the E2A-PBX1 oncogene, which can transform fib roblasts, induce acute myeloid leukemia and T cell lymphomas in mice, and immortalize factor-dependent myeloid progenitors in cultured marro w. The homeodomain of Pbx1 binds ATCAATCAA, and while Pbx1 does not ac tivate transcription through this motif, E2A-Pbx1 induces constitutive transactivation. Here, we investigate whether DNA-binding by Pbx1 or transcriptional activation by E2A are essential for the transforming a bilities off E2A-Pbx1. Elimination of DNA-binding in E2A-Pbx1 by point mutations in the Pbx1 homeodomain or by large deletions that removed the Pbx1 homeodomain and carboxyl terminus did not alter ability of E2 A-Pbx1 to induce focus-formation in fibroblast, even though these muta tions completely eliminated its ability to activate transcription thro ugh the PRS. These same DNA-binding mutations, however, severely impai red or eliminated the ability of E2A-Pbx1 to immortalize factor-depend ent myeloid progenitors in marrow cultures. Elimination of the first t ranscriptional activation domain of E2A abolished both fibroblast and myeloid transforming activities while elimination of the second altere d neither of these activities. We conclude that DNA-binding is importa nt for the ability of E2A-Pbx1 to disrupt differentiation, as evidence d in myeloblast immortalization, but dispensable for its ability to in duce focus-formation, and that the aminoterminal domain of E2A, which strongly activates transcription, is essential for both transforming a ctivities.