The oncoprotein E2A-PBX1 is produced by the t(1;19) chromosomal translocation in pre-B cell acute lymphoblastic leukemia (ALL). We and others have shown that retrovirus-enforced expression of E2A-PBX1 in mouse bone marrow results in a lethal leukemia with short latency. Suprisingly, almost all animals die of myeloid leukemia rather than the B-lymphoid disease observed clinically. The E-protein E47, a product of the wild-type E2A gene, is required in the earliest stages of B-lymphoid commitment. E47 and E2A-PBX1 are identical in their amino-terminal 483 residues and this region includes transcriptional activation domains capable of recruiting CBP/p300 and presumably other co-regulators. Therefore, we hypothesized that E2A-PBX1 may block B-lymphoid commitment by exerting dominant-negative effects on E-proteins including E47. Bone marrow cells transduced with E2A-PBX1 failed to repopulate the B-lymphoid compartment at two weeks post-reconstitution of sub-lethally-irradiated mice, whereas there was exuberant proliferation of transduced myeloid progenitors suggesting selective intereference with B-lymphopoiesis. This phenomenon was examined further using a more experimentally tractable in vitro system. “Lineage negative” hematopoietic progenitors from fetal livers differentiate rapidly to committed B-lymphoid progenitors (B220+, CD19+, Mac-1-, Gr-1-) on co-culture with OP9 stromal cells in the presence of interleukin-7 (IL-7). However, enforced expression of E2A-PBX1 produced a myeloid immunophenotype (B220-, CD19-, Mac-1+, Gr-1+). Furthermore, these cells are dependent on myeloid cytokines, display no IL-7 responsiveness, and have a high forward scatter/side scatter profile. RT-PCR analysis shows severely reduced levels of lymphoid-associated transcripts (E2A, Ebf1, Pax5 and Irf8) and substantially elevated levels of myeloid associated transcripts (Csfr1, Id1 and Pu.1) relative to control lymphoid cells. All of these properties are characteristic of macrophage progenitors, a fate adopted by E2A−/− cells under similar culture conditions. Our findings showing that E2A-PBX1 is capable of altering the lineage fate of early hematopoietic progenitors in a manner similar to gene targeting of E2A provide support, albeit circumstantial, for a mechanism that involves dominant-negative effects on E-proteins and perhaps competition for limiting co-regulators such as CBP/p300. Consistent with the latter possibility, murine bone marrow cells expressing an E2A-PBX1 mutant impaired in CBP/p300 recruitment retain a minor propensity to repopulate the B-lymphoid compartment of irradiated transplant recipients. The results of experiments to evaluate the impact of E2A-PBX1 on transcriptional induction by E47 will be presented.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 3097.