Identification of TAL1/SCL Target Genes through siRNA and Microarray Expression Analysis.

TAL1 is a basic helix-loop-helix (bHLH) protein that is essential for hematopoiesis. Activation of the TAL1/SCL gene occurs in as many as 60% of patients with pediatric and adult T cell acute lymphoblastic leukemia (T-ALL). Mouse models have demonstrated that TAL1 transforms by inhibiting the function of E proteins, however the transcriptional targets of TAL1 that contribute to leukemia are not known. In this study we sought to identify targets of TAL1 that contribute to the transformation of thymocytes through siRNA-mediated knockdown of TAL1 expression in the Jurkat T-ALL cell line that expresses high levels of TAL1. We have generated several clonal cell lines that stably express a siRNA directed against TAL1 with up to 90% knockdown of TAL1 at both the RNA and protein level. Phenotypic characterization of these cells revealed that Jurkat cells with decreased TAL1 expression display a decreased proliferative capacity. This phenotype is augmented in lower serum concentrations in which wild-type and siRNA control Jurkat cells proliferate rapidly. Microarray expression analysis using Affymetrix U133 Plus 2.0 arrays has revealed that TAL1 is a global regulator of gene expression and that many pathways are affected by TAL1 knockdown, including those implicated in cell growth and proliferation. This data will enable us to dissect the downstream pathways of TAL1 by knocking-down or overexpressing these TAL1 targets (depending on whether they are up- or down-regulated in the absence of TAL1), and assessing whether they affect the proliferative rate or survival T-ALL cells. We anticipate that we will identify and validate potential therapeutic targets for TAL1-induced leukemia using this combination of approaches.