Evaluating the pre-B-cell receptor as a potential mediator of leukemogenesis downstream of the TCF3-PBX1 oncogenic transcription factor
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Approximately 5% of B-progenitor acute lymphoblastic leukemia (B-ALL) harbor a somatic 1;19 chromosomal translocation which generates a fusion gene coding for TCF3-PBX1. Recent studies have shown that leukemic progenitors with t(1;19) occur in a distinct subset of B-ALL cases that often express a functional pre-B-cell receptor (pre-BCR). It is hypothesized that TCF3-PBX1 drives the leukemogenesis of these cells by promoting aberrant pre-BCR signaling through transcriptional activation of genes encoding pre-BCR components (IGHM, CD79A, CD79B, IGLL1, VPREB1 and VPREB3). To test this hypothesis, we have established short hairpin RNA (shRNA) knockdown models in two different t(1;19) B-ALL cell lines (RCH-ACV and 697). Expression of the knockdown shRNA against the TCF3-PBX1 transcript considerably reduced cell culture-initiation and proliferation. We then performed RNA-Seq in both of our knockdown models and determined that TCF3-PBX1 knockdown surprisingly activates, rather than represses, the expression of IGHM, CD79A, CD79B, IGLL1, VPREB1 and VPREB3. Despite this, we observed a marked reduction in phosphorylation and activation of AKT and ERK in ¬TCF3-PBX1-silenced cells. Additionally, TCF3-PBX1 knockdown had no effect on SYK phosphorylation. Altogether, our results suggest that TCF3-PBX1 promotes t(1;19) B-ALL proliferation by activating PI3K/AKT and MAPK/ERK signaling in a pre-BCR independent manner. In considering alternative mechanisms by which TCF3-PBX1 promotes cell proliferation, TCF3-PBX1 activates the expression of genes encoding key downstream pre-BCR signaling molecules such as the Src-family kinase BLK and the atypical protein kinase C PKCζ either of which could promote pro-survival signaling pathways. Alternatively, we propose that TCF3-PBX1 promotes Notch or Rho signaling and that these pathways might crosstalk with the pre-BCR.