DNA-binding dependent and independent functions of WT1 protein during human hematopoiesis

Understanding WT1 Alterations and Expression Profiles in Hematological Malignancies

WT1 is a true chameleon, both acting as an oncogene and tumor suppressor. As its exact role in leukemogenesis is still ambiguous, research with model systems representing natural conditions surrounding the genetic alterations in WT1 is necessary. In a cohort of 59 leukemia/lymphoma cell lines, we showed aberrant expression for WT1 mRNA, which does not always translate into protein levels. We also analyzed the expression pattern of the four major WT1 protein isoforms in the cell lines and primary AML blasts with/without WT1 mutations and demonstrated that the presence of mutations does not influence these patterns. By introduction of key intronic and exonic sequences of WT1 into a lentiviral expression vector, we developed a unique tool that can stably overexpress the four WT1 isoforms at their naturally occurring tissue-dependent ratio. To develop better cellular model systems for WT1, we sequenced large parts of its gene locus and also other important myeloid risk factor genes and revealed previously unknown alterations. Functionally, inhibition of the nonsense-mediated mRNA decay machinery revealed that under natural conditions, the mutated WT1 alleles go through a robust degradation. These results offer new insights and model systems regarding the characteristics of WT1 in leukemia and lymphoma.

The Role of Wilms' Tumor Gene (WT1) Expression as a Marker of Minimal Residual Disease in Acute Myeloid Leukemia

The Minimal Residual Disease(MRD) monitoring in acute myeloid leukemia (AML) is crucial to guide treatment after morphologic complete remission, to define the need for consolidation with allogeneic stem cell transplantation (Allo-SCT), and to detect impending relapse allowing early intervention. However, more than 50% of patients with AML lack a specific or measurable molecular marker to monitor MRD. We reviewed the key studies on WT1 overexpression as a marker of MRD in AML patients undergoing an intensive chemotherapy program, including Allo-SCT. In addition, we provided some practical considerations on how to properly use WT1 expression as an MRD marker, considering its strengths and weaknesses. In order to achieve the best sensitivity and specificity, it is recommended to refer to the standardized method of European LeukemiaNet and its defined threshold (250 WT1 copies/10⁴ Abelson (ABL) on Bone Marrow-BM and 50 WT1 copies/10⁴ ABL on Peripheral Blood-PB), which has been validated in a large and multicenter cohort of patients and normal controls.

The transcriptional coregulator NAB2 is a target gene for the Wilms' tumor gene 1 protein (WT1) in leukemic cells

The Wilms’ tumor gene 1 (WT1) is recurrently mutated in acute myeloid leukemia. Mutations and high expression of WT1 associate with a poor prognosis. In mice, WT1 cooperates with the RUNX1/RUNX1T1 (AML1/ETO) fusion gene in the induction of acute leukemia, further emphasizing a role for WT1 in leukemia development. Molecular mechanisms for WT1 are, however, incompletely understood. Here, we identify the transcriptional coregulator NAB2 as a target gene of WT1. Analysis of gene expression profiles of leukemic samples revealed a positive correlation between the expression of WT1 and NAB2, as well as a non-zero partial correlation. Overexpression of WT1 in hematopoietic cells resulted in increased NAB2 levels, while suppression of WT1 decreased NAB2 expression. WT1 bound and transactivated the proximal NAB2 promoter, as shown by ChIP and reporter experiments, respectively. ChIP experiments also revealed that WT1 can recruit NAB2 to the IRF8 promoter, thus modulating the transcriptional activity of WT1, as shown by reporter experiments. Our results implicate NAB2 as a previously unreported target gene of WT1 and that NAB2 acts as a transcriptional cofactor of WT1.

Anti-apoptotic quinolinate phosphoribosyltransferase (QPRT) is a target gene of Wilms' tumor gene 1 (WT1) protein in leukemic cells

Wilms' tumor gene 1 (WT1) is a zinc finger transcription factor that has been implicated as an oncogene in leukemia and several other malignancies. When investigating possible gene expression network partners of WT1 in a large acute myeloid leukemia (AML) patient cohort, one of the genes with the highest correlation to WT1 was quinolinate phosphoribosyltransferase (QPRT), a key enzyme in the de novo nicotinamide adenine dinucleotide (NAD+) synthesis pathway. To investigate the possible relationship between WT1 and QPRT, we overexpressed WT1 in hematopoietic progenitor cells and cell lines, resulting in an increase of QPRT expression. WT1 knock-down gave a corresponding decrease in QPRT gene and protein expression. Chromatin-immunoprecipitation revealed WT1 binding to a conserved site in the first intron of the QPRT gene. Upon overexpression in leukemic K562 cells, QPRT conferred partial resistance to the anti-leukemic drug imatinib, indicating possible anti-apoptotic functions, consistent with previous reports on glioma cells. Interestingly, the rescue effect of QPRT overexpression was not correlated to increased NAD + levels, suggesting NAD + independent mechanisms. We conclude that QPRT, encoding a protein with anti-apoptotic properties, is a novel and direct target gene of WT1 in leukemic cells.

Lentivirus-induced dendritic cells for immunization against high-risk WT1(+) acute myeloid leukemia

Wilms' tumor 1 antigen (WT1) is overexpressed in acute myeloid leukemia (AML), a high-risk neoplasm warranting development of novel immunotherapeutic approaches. Unfortunately, clinical immunotherapeutic use of WT1 peptides against AML has been inconclusive. With the rationale of stimulating multiantigenic responses against WT1, we genetically programmed long-lasting dendritic cells capable of producing and processing endogenous WT1 epitopes. A tricistronic lentiviral vector co-expressing a truncated form of WT1 (lacking the DNA-binding domain), granulocyte-macrophage colony-stimulating factor (GM-CSF), and interleukin-4 (IL-4) was used to transduce human monocytes ex vivo. Overnight transduction induced self-differentiation of monocytes into immunophenotypically stable "SmartDC/tWT1" (GM-CSF(+), IL-4(+), tWT1(+), IL-6(+), IL-8(+), TNF-α(+), MCP-1(+), HLA-DR(+), CD86(+), CCR2(+), CCR5(+)) that were viable for 3 weeks in vitro. SmartDC/tWT1 were produced with peripheral blood mononuclear cells (PBMC) obtained from an FLT3-ITD(+) AML patient and surplus material from a donor lymphocyte infusion (DLI) and used to expand CD8(+) T cells in vitro. Expanded cytotoxic T lymphocytes (CTLs) showed antigen-specific reactivity against WT1 and against WT1(+) leukemia cells. SmartDC/tWT1 injected s.c. into Nod.Rag1(-/-).IL2rγc(-/-) mice were viable in vivo for more than three weeks. Migration of human T cells (huCTLs) to the immunization site was demonstrated following adoptive transfer of huCTLs into mice immunized with SmartDC/tWT1. Furthermore, SmartDC/tWT1 immunization plus adoptive transfer of T cells reactive against WT1 into mice resulted in growth arrest of a WT1(+) tumor. Gene array analyses of SmartDC/tWT1 demonstrated upregulation of several genes related to innate immunity. Thus, SmartDC/tWT1 can be produced in a single day of ex vivo gene transfer, are highly viable in vivo, and have great potential for use as immunotherapy against malignant transformation overexpressing WT1.

Role of WT1-ZNF224 interaction in the expression of apoptosis-regulating genes

The transcription factor Wilms' tumor gene 1, WT1, is implicated both in normal developmental processes and in the generation of a variety of solid tumors and hematological malignancies. Physical interactions of other cellular proteins with WT1 are known to modulate its function. We previously identified the Krüppel-like zinc-finger protein, ZNF224, as a novel human WT1-associating protein that enhances the transcriptional activation of the human vitamin D receptor promoter by WT1. Here, we have analyzed the effects of WT1-ZNF224 interaction on the expression of apoptosis-regulating genes in the chronic myelogenous leukemia (CML) K562 cell line. The results demonstrated that ZNF224 acts in fine tuning of WT1-dependent control of gene expression, acting as a co-activator of WT1 in the regulation of proapoptotic genes and suppressing WT1 mediated transactivation of antiapoptotitc genes. Moreover, the DNA damaging drug cytosine arabinoside (ara-C) induces expression of ZNF224 in K562 cells and this induction enhances cell apoptotic response to ara-C. These findings suggest that ZNF224 can be a mediator of DNA damage-induced apoptosis in leukemia cells.

The Wilms' tumor gene 1 (WT1) induces expression of the N-myc downstream regulated gene 2 (NDRG2)

The Wilms' tumor gene 1 (WT1) protein is a transcriptional regulator that is highly expressed in immature hematopoietic progenitor cells and in the majority of patients with acute and chronic myeloid leukemia. However, it is still unclear how WT1 exerts its function(s) in hematopoietic cells. The aim of this work was to investigate the function of WT1 as a transcription factor in human hematopoietic progenitor cells. To this end, an oligonucleotide array approach was used to study the gene expression in CD34(+) cells from human cord blood retrovirally transduced with WT1 or a control vector. We found that the expression of the putative tumor suppressor gene N-myc downstream regulated gene 2 (NDRG2) mRNA was induced by WT1 in CD34(+) cells and also in leukemic U937 cells. Furthermore, a novel transcription start site in the NDRG2 gene was identified in WT1-transduced cells, in addition to two previously reported transcription start sites. These results show that the expression of the NDRG2 gene is directly or indirectly induced by WT1, and provide the first insights into transcriptional regulation of the NDRG2 gene, including demonstration of a novel splice variant.

Deregulation of the Wilms' tumour gene 1 protein (WT1) by BCR/ABL1 mediates resistance to imatinib in human leukaemia cells

The Wilms' tumour gene 1 (WT1) protein is highly expressed in most leukaemias. Co-expression of WT1 and the fusion protein AML1-ETO in mice rapidly induces acute myeloid leukaemia (AML). Mechanisms behind expression of WT1, as well as consequences thereof, are still unclear. Here, we report that the fusion protein BCR/ABL1 increases expression of WT1 mRNA and protein via the phosphatidylinositol-3 kinase (PI3K)-Akt pathway. Inhibition of BCR/ABL1 or PI3K activity strongly suppressed transcription from WT1 promoter/enhancer reporters. Forced expression of BCR/ABL1 in normal human progenitor CD34+ cells increased WT1 mRNA and protein, further supporting the notion of BCR/ABL1-driven expression of WT1 in human haematopoietic cells. Forced expression of WT1 in K562 cells provided protection against cytotoxic effects of the ABL1 tyrosine kinase inhibitor imatinib, as judged by effects on viability measured by trypan blue exclusion, metabolic activity, annexin V and DAPI (4', 6-diamidino-2-phenylindole) staining. None of the isoforms provided any detectable protection against apoptosis induced by arsenic trioxide and only very weak protection against etoposide, indicating that WT1 interferes with specific apoptotic signalling pathways. Our data demonstrate that WT1 expression is induced by oncogenic signalling from BCR/ABL1 and that WT1 contributes to resistance against apoptosis induced by imatinib.

DNA methylation profiling of myelodysplastic syndrome hematopoietic progenitor cells during in vitro lineage-specific differentiation

Deregulated epigenetic mechanisms are likely involved in the pathogenesis of myelodysplastic syndromes (MDSs). Which genes are silenced by aberrant promotor methylation during MDS hematopoiesis has not been equivalently investigated. Using an in vitro differentiation model of human hematopoiesis, we generated defined differentiation stages (day 0, day 4, day 7, day 11) of erythro-, thrombo- and granulopoiesis from 13 MDS patients and seven healthy donors. Promotor methylation analysis of key regulatory genes involved in cell cycle control (p14, p15, p16, CHK2), DNA repair (hMLH1), apoptosis (p73, survivin, DAPK), and differentiation (RARb, WT1) was performed by methylation-specific polymerase chain reaction. Corresponding gene expression was analyzed by microarray (Affymetrix, HG-U133A). We provide evidence that p16, survivin, CHK2, and WT1 are affected by promotor hypermethylation in MDSs displaying a selective International Prognostic Scoring System risk association. A methylation-associated mRNA downregulation for specific hematopoietic lineages and differentiation stages is demonstrated for survivin, CHK2, and WT1. We identified a suppressed survivin mRNA expression in methylated samples during erythropoiesis, whereas WT1 and CHK2 methylation-related reduction of mRNA expression was found during granulopoiesis in all MDS risk types. Our data suggest that lineage-specific methylation-associated gene silencing of survivin, CHK2, and WT1 in MDS hematopoietic precursor cells may contribute to the MDS-specific phenotype

A tumor suppressor and oncogene: the WT1 story

The Wilms' tumor 1 (WT1) gene encodes a transcription factor important for normal cellular development and cell survival. The initial discovery of WT1 as the causative gene in an autosomal-recessive condition identified it as a tumor suppressor gene whose mutations are associated with urogenital disease and the development of kidney tumors. However, this view is not in keeping with the frequent finding of wild-type, full-length WT1 in human leukemia, breast cancer and several other cancers including the majority of Wilms' tumors. Rather, these observations suggest that in those conditions, WT1 has an oncogenic role in tumor formation. In this review, we explore the literature supporting both views of WT1 in human cancer and in particular human leukemias. To understand the mechanism by which WT1 can do this, we will also examine its functional activity as a transcription factor and the influence of protein partners on its dual behavior.

Curcumin inhibits WT1 gene expression in human leukemic K562 cells

AIM

Wilms' tumor1 (WT1) gene is highly expressed in leukemic blast cells of myeloid and lymphoid origin. Thus, WT1 mRNA and protein serve as promising tumor markers for the detection of leukemia and monitoring of disease progression. The purpose of this study was to investigate the modulating effects of curcumin on WT1 gene expression in the human leukemic cell line K562.

METHODS

The cytotoxicity of curcumin on the K562 cell line was evaluated by using 3-(4,5-dimethyl-2 thiazoyl)-2,5-diphenyl-tetrazolium bromide (MTT) assay. The K562 cell line was treated with a non-cytotoxic dose of curcumin (5, 10, or 15 micromol/L) for 13 d. The expression levels of WT1 protein and WT1 mRNA were assessed by Western blot analysis and reverse transcription-polymerase chain reaction (RT-PCR), respectively.

RESULTS

Curcumin had a cytotoxic effect on K562 leukemic cells with an inhibitory concentration at 50% (IC50) of approximately 20 microg/mL (54.3 micromol/L). Non-cytotoxic doses of curcumin, at concentrations of 5, 10, and 15 micromol/L for 2 d, decreased the level of WT1 protein and WT1 mRNA in the K562 cell line in a dose-dependent manner. Similarly, curcumin at a concentration of 10 micromol/L significantly decreased the level of WT1 protein and mRNA in a time-dependent manner.

CONCLUSION

The inhibitory effects of curcumin are associated with a decrease in the levels of both WT1 protein and WT1 mRNA. The current study provides a molecular basis for future clinical trials in leukemic patients. Thus, curcumin could be a promising chemotherapeutic agent for human leukemia.