E2a/Pbx1 Induces the Rapid Proliferation of Stem Cell Factor-Dependent Murine Pro-T Cells That Cause Acute T-Lymphoid or Myeloid Leukemias in Mice

Abstract

Oncoprotein E2a/Pbx1 is produced by the t(1;19) chromosomal translocation of human pre-B acute lymphoblastic leukemia. E2a/Pbx1 blocks differentiation of primary myeloid progenitors but, paradoxically, induces apoptosis in established pre-B-cell lines, and no transforming function of E2a/Pbx1 has been reported in cultured lymphoid progenitors. Here, we demonstrate that E2a/Pbx1 induces immortal proliferation of stem cell factor (SCF)-dependent pro-T thymocytes by a mechanism dependent upon both its transactivation and DNA-binding functions. E2a-Pbx1 cooperated with cytokines or activated signaling oncoproteins to induce cell division, as inactivation of conditional E2a/Pbx1 in either factor-dependent pro-T cells or pro-T cells made factor independent by expression of Bcr/Abl resulted in pro-T-cell quiescence, while reactivation of E2a/Pbx1 restored cell division. Infusion of E2a/Pbx1 pro-T cells in mice caused T lymphoblastic leukemia and, unexpectedly, acute myeloid leukemia. The acute lymphoblastic leukemia did not evidence further maturation, suggesting that E2a/Pbx1 establishes an early block in pro-T-cell development that cannot be overcome by marrow or thymic microenvironments. In an E2a/Pbx1 pro-T thymocyte clone that induced only pro-T acute lymphoblastic leukemia, coexpression of Bcr/Abl expanded its leukemic phenotype to include acute myeloid leukemia, suggesting that unique functions of cooperating signaling oncoproteins can influence the lymphoid versus myeloid character of E2a/Pbx1 leukemia and may cooperate with E2a/Pbx1 to dictate the pre-B-cell phenotype of human leukemia containing t(1;19).

Many thanks go to Ellen Rothenberg and Steven Hedrick for providing a wealth of information regarding T-cell biology. Barbara Kee provided both the J558 (IL-7) and CHO (MGF/SCF) cells, among other reagents, as well as the strategy for culturing pre-B cells. Special thanks go to Dennis Young for help with the flow cytometric analysis, to Cheryl Yost and Carol Micheletti for the peripheral blood analysis, and to Ciara Ryan and Guy Salvesen's laboratory for help with the apoptosis assays. We thank Peter M. C. Wong for the BL3 cells, Schikwann Tsai for the EML.C1 cells, Michael Carelton for the Scid.adh cells, and Rich Rivera and Angela Weiss for providing probes for Northern analysis and RNA from EML.C1 cells, respectively. Thanks also go to Martina Pasillas for helping in the analysis of samples by Western and Northern blotting.

This work was supported by Public Health Service grant NIH CA56876. D.B.S. is supported by Department of Defense grant F49620-99-C-0054.