| The t(1;19) chromosomal translocation encodes the chimeric transcription factor E2a-Pbx1, and is associated with up to 6% of pediatric acute lymphoblastic leukemias. The genetic rearrangement fuses the transactivation domains of the basic helix-loop-helix (bHLH) protein E2a with the DNA binding homeodomain (HD) of the Pbx1 protein. Although the molecular properties of the E2a-Pbx1 transcription factor have been well characterized, and the oncogenicity of E2a-Pbx1 thought to be due to misregulation of Pbx1 target genes, the transcriptional cascade that is initiated by E2a-Pbx1 in its native pre-B cell context is poorly understood. The studies presented in this dissertation focus on the identification and characterization of transcriptional target genes of E2a-Pbx1 in lymphoid cells. To this end, gene expression profiling using cDNA microarrays was employed to analyze the transcriptional response to E2a-Pbx1b induction in pre-B cells. The results show that E2a-Pbx1b activates a variety of genes that influence cellular proliferation, adhesion, signaling, and homeostasis. Specifically, the growth stimulus response genes SGK and MRG1 were identified to be subordinate transcriptional targets of E2a-Pbx1b. We hypothesize that the ectopic activation of these mitogenic responsive genes contributes to t(1;19) associated leukemogenesis.; Furthermore, we demonstrate that there exists a pathway in which E2a-Pbx1b induces the expression of the proto-oncogene BMI-1, which in turn represses the cell cycle regulatory INK4a locus (p16 INK4a/p14ARF). Elucidation of the regulatory pathway provides an explanation for the unusual absence of mutations in the INK4a locus specifically in t(1;19)-associated leukemias, and identifies the G1/S cell cycle checkpoint as a target for the oncogenic activity of E2a-Pbx1.; Finally, we have investigated potential roles of these and other putative E2a-Pbx1/Pbx1 target genes in the context of the regulatory role of the Pbx1 proto-oncoprotein. Our results suggest that Pbx1 can modulate transcription in a context dependent manner, both as a repressor and activator. We postulate that E2a-Pbx1 abrogates these regulatory functions, and ectopically activates a subset of Pbx1 subordinate genes, particularly those associated with cellular proliferation. We conclude that aberrant activation of these target genes contributes to the initiation of oncogenesis in t(1;19) associated leukemia. |
