Aberrant expression of transcription factor oncogenes is a common feature in the pathogenesis of T-cell acute lymphoblastic leukemia (T-ALL), however, oncogenic transcriptional programs downstream of T-ALL oncogenes are mostly unknown. We used chromatin immunoprecipitation combined with promoter microarrays (ChIP on chip) to identify direct transcriptional targets of TAL1/SCL, a bHLH transcription factor aberrantly expressed in 60% of patients with T-ALL. Using the Hu12K arrays that contain over 12,000 human promoter regions, we have identified 71 direct targets of TAL1 in the Jurkat T-ALL cell line by ChIP on chip, and have been able to validate 80% of them by conventional chromatin immunoprecipitation followed by quantitative real time PCR. It has previously been demonstrated that TAL1 needs to bind to E proteins (E2A, HEB or E2-2) to achieve an efficient interaction with DNA. We verified that promoters occupied by TAL1 are bound by both the E-proteins E2A and HEB in at least 50% of TAL1-bound promoter regions, suggesting that TAL1/E2A as well as TAL1/HEB heterodimers play a role in the transformation of T-cell precursors. Using RNA interference to knock down TAL1, we have demonstrated functional regulation through TAL1-binding of a significant percentage of the targets identified by ChIP on chip. Results demonstrate that some TAL1 direct targets are repressed by the binding of this transcription factor, while others seem to be activated or are not altered, indicating a high level of complexity in the organization of TAL1-mediated transcriptional regulatory networks. Also, specific association of the expression levels of TAL1 targets in TAL1 positive primary leukemia samples has been shown by comparing gene expression arrays from the different T-ALL subsets. Our results indicate that TAL1 acts as a master transcriptional regulator at the top of a complex regulatory network that contributes to leukemogenesis by regulating T-cell differentiation, proliferation and survival.

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