Identification of a gene element essential for leukemia-specific expression of transgenes

GATA-1 and GATA-2 binding to 3' enhancer of WT1 gene is essential for its transcription in acute leukemia and solid tumor cell lines

Although oncogenic functions and the clinical significance of Wilms tumor 1 (WT1) have been extensively studied in acute leukemia, the regulatory mechanism of its transcription still remains to be determined. We found a significant correlation among the amounts of WT1, GATA-1 and GATA-2 mRNAs from leukemia and solid tumor cell lines. Overexpression and small interfering RNA (siRNA) transfection experiments of GATA-1 and GATA-2 showed that these GATA transcription factors could induce WT1 expression. Promoter analysis showed that the 5' promoter did not explain the different WT1 mRNA levels between cell lines. The 3' enhancer, especially the distal sites out of six putative GATA binding sites located within the region, but not the intron 3 enhancer, were essential for the WT1 mRNA level. Electrophoretic mobility shift assay (EMSA) showed both GATA-1 and GATA-2 bound to these GATA sites. Besides acute leukemia cell lines, solid tumor cell lines including, TYK-nu-cPr also showed a high level of WT1 mRNA. We showed that GATA-2 expression is a determinant of WT1 mRNA expression in both TYK-nu-cPr cells and HL60 cells without GATA-1 expression. Taken together, these results suggest that GATA-1 and/or GATA-2 binding to a GATA site of the 3' enhancer of WT1 played an important role in WT1 gene expression.

TSA downregulates Wilms tumor gene 1 (Wt1) expression at multiple levels

The Wilms tumor gene WT1 encodes a zinc-finger transcription factor that is inactivated in a subset of pediatric kidney cancers. During embryogenesis, WT1 is expressed in a time- and tissue-specific manner in various organs including gonads and kidney but also in the hematopoietic system. Although widely regarded as a tumor suppressor gene, wild-type WT1 is overexpressed in a variety of hematologic malignancies, most notably in acute lymphoblastic leukemia as well as myelodysplastic syndromes. Reduction of WT1 expression levels leads to decrease of proliferation and apoptosis of leukemic cells, suggesting that in certain contexts WT1 might act as an oncogene. We show here that histone deacetylase inhibitors like Trichostatin A (TSA) can promptly and dramatically downregulate Wt1 expression levels in different cell lines. This effect was mostly due to the cessation of transcription and was mediated by sequences located in intron 3 of Wt1. In addition, TSA also caused enhanced degradation of the Wt1 protein by the proteasome. This was at least in part due to induction of the ubiquitin-conjugating enzyme UBCH8. Thus, downregulation of Wt1 expression might contribute to the beneficial effects of histone deacetylase inhibitors that are currently used in clinical trials as cancer therapeutics.

The transcriptional activity of WT1 gene promoter and enhancer in cell lines with diverse tissue origin

In this study, tissue-specific transcriptional activity of WT1 promoter and enhancer was investigated in a hope to design gene therapy strategy based on the transcriptional regulatory elements of WT1 gene. WT1 promoter and/or enhancer were ligated into pEGFP-1 to construct recombinant vectors with EGFP gene as a reporter and analyzed using Flow cytometry. The results showed that pEWP containingWT1 promoter induced the highest EGFP expression in ECV304 at 16.54 +/- 2.45 times, mildly higher in MCF-7 and SHG44 at 9.46 +/- 1.10 and 7.29 +/- 0.73 times, and in K562 cell line at 2.93 +/- 0.27 times, compared to that of pEGFP-1. However, pEWPA, with WT1 enhancer inserted at Afl II site, increased basal transcription levels of the WT1 promoter in HT-29, SHI-1 and K562 cells by 4.81, 3.06 and 1.01-fold, respectively. pEWPD inserted at NotI and pEWPE inserted at BamI had no ability to increase the transcriptional activity of WT1 promoter. Moreover, reporter gene silenced was observed in transfected host cells by flow cytometryt and real-time PCR. These results suggested that transcriptional activities of WT1 promoter in the recombinant vector seemed not correlated to the constitutional expression level of endogenous WT1 gene. The WT1 enhancer could promote the transcriptional activities of WT1 promoter in some of the cell lines regardless of the hematopoietic tissue origin. The inserted site of enhancer in vector influenced the transcriptional activity of promoter and the extent of reporter gene silencing exerted the influence on the analysis of transcriptional activity.