Examination of the Paradoxical Role of TCF3-PBX1 in Human Hematopoietic Cell Proliferation and Apoptosis

Abstract

B cell formation begins when multi-potent lympho-myeloid progenitor cells differentiate into pre-pro-B cells, but B cell programming is not fully committed until advancement to the pro-B cell stage. To successfully proceed through this critical step, the transcription factor TCF3 must activate necessary target genes and signaling cascades. When a TCF3-PBX1 chromosomal translocation occurs, TCF3 promoter and transcription factors turn on the translocated gene, which expresses a chimera protein product that binds to and activates PBX1gene targets. Consequently, accumulation and aberrant proliferation of pre-pro-B cells leads to development of B-cell acute lymphoblastic leukemia (B-ALL). In separate studies of TCF3 and PBX1, mutation of either gene has led to dysregulation of apoptosis and cell proliferation in various human cell types. This study aimed to investigate the paradoxical role of TCF3-PBX1-induced apoptosis and cell proliferation in myeloid and lymphoid populations. A TCF3-PBX1 cDNA-encoding lentivirus was used to transduce human myeloid and lymphoid cells, which were then cultured either in vitro or in vivo. Upon induced TCF3-PBX1 expression, myeloid and lymphoid cells were collected and enumerated to compare cell number differences between TCF3-PBX1-expressing and control cells. Cell types appearing to be differentially affected were further analyzed for increased apoptosis. In vitro data indicated that myeloid cells were drastically affected by TCF3-PBX1 expression by rapid induction of apoptosis. In contrast, lymphoid cells forced to express TCF3-PBX1 did not show a distinguishable population change, but did exhibit a slight increase in apoptosis. In vivo, cells produced from transduced normal cord blood CD34+ cells displayed no difference between TCF3-PBX1-expressing and control cells among both myeloid and lymphoid progeny. Collectively, this indicates that the TCF3-PBX1 protein alone does not determine the level of apoptosis or proliferation in human lymphoid cells. Additional mutations in other key transcription factors may be needed to duplicate the mechanism seen in patients’ B-ALL cells.