NKX3.1 Is a Direct TAL1 Target Gene That Mediates the TAL1 Dependent Proliferation of Human T-Cell Acute Lymphoblastic Leukemia (T-ALL)

The TAL1/SCL gene encodes a bHLH (basic Helix-Loop-helix) protein that acts as a master gene in hematopoiesis. The TAL1/SCL gene is also the most frequently activated gene in human T-ALL but the oncogenic transcriptional programs, downstream of TAL1 in human T-ALL, are not well characterized. Using RNA interference to knockdown TAL1 expression, we show that TAL1 regulates both cell proliferation and death of human T-ALL cells. To determine the TAL1 target genes in human T-ALL, we combine TAL1 knockdown and gene expression profiling and show that TAL1 activates and repress a common subset of genes in cell lines. This subset includes known TAL1 target genes but also the NKX3.1 gene that is a homeobox gene, specifically expressed in the prostate epithelium during prostate development and in adulthood. NKX3.1 gene inactivation is one of the earliest events that occur in prostate cancer initiation, defining NKX3.1 as a major tumor suppressor gene of this cancer. TAL1 expression is associated with NKX3.1 activation in human T-ALL cell lines and NKX3.1 is expressed in TAL1 expressing human T-ALL blasts. TAL1 and GATA-3 are specifically bound in vivo to the [−870/−570] region of the human NKX3.1 gene promoter, and ex vivo, TAL1 can either directly binds an E-box [position −738] or be recruited by GATA-3 on a GATA binding site [position −697]. Finally, functional analyses of the NKX3.1 promoter indicate that these binding sites mediate the transcriptional activity of this promoter in T-cell lines. Sequences analysis of the human and mouse NKX3.1 promoters show that the regulatory sequences involved in the TAL1 activation of the human NKX3.1 gene are not conserved in the mouse gene, indicating why the NKX3.1 gene is not expressed in mouse models of TAL1 mediated leukemogenesis. NKX3.1 knockdown shows that NKX3.1 is necessary for the proliferation of TAL1 expressing T-ALL cell lines and NKX3.1 overexpression can complement the proliferation defects associated with TAL1 knockdown in T-ALL cell lines. Microarray analyses show that TAL1 and NKX3.1 regulate a common subset of genes in T-ALL that includes numerous genes encoding proteins known to be involved in T-cell proliferation and/or signaling. Finally, using a new culture system that enables proliferation of primary human leukemic cells, we show that the NKX3.1 gene is specifically activated in human TAL1 expressing T-ALL together with the defined potential TAL1 and/or NKX3.1 target genes. These results characterize NKX3.1 as the first gene directly activated by TAL1 and involved in the TAL1 dependent proliferation of human T-cell Acute Lymphoblastic Leukemia.