Downregulation of Wilms' tumor gene (WT1) is not a prerequisite for erythroid or megakaryocytic differentiation of the leukemic cell line K562

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.

Wt1 is not essential for hematopoiesis in the mouse

WT1 has been implicated in human leukemia and hematopoiesis, but its role in stem cell differentiation is not yet fully defined. We show that Wt1-null murine fetal liver cells are capable of reconstituting functional hematopoiesis following transplantation into irradiated recipients. There was also no significant difference between the in vitro colony-forming ability of wild-type and Wt1-null cells. Using a reporter gene assay in a transgenic mouse system, expression from the WT1 promoter was detectable in adult bone marrow, but undetectable in subsets of different hematopoietic cells. We conclude that Wt1 is not essential for murine hematopoiesis and that there may be significant differences in its role between mouse and man.

WT1 in acute leukemia, chronic myelogenous leukemia and myelodysplastic syndrome: therapeutic potential of WT1 targeted therapies

Among clinicians, initial awareness of the Wilms' tumor gene was limited mostly to pediatric oncologists. Almost a decade ago, overexpression of Wilms' tumor 1 (WT1) was observed in adult acute leukemia. Subsequent studies indicated that WT1 overexpression occurs in most cases of acute myelogenous leukemia, acute lymphoblastic leukemia, chronic myelogenous leukemia (CML), and myelodysplastic syndrome (MDS). Limited tissue expression of WT1 in adults suggests that WT1 can be a target for leukemia/MDS therapy. WT1 expression in stem/progenitor cells remains unsettled. However, lack of progenitor cell suppression by WT1 antisense or WT1-specific cytotoxic T cells provide some assurance that WT1 expression in progenitor cells is minimal or absent. Immunotherapy-based WT1 approaches are furthest along in preclinical development. WT1-specific cytotoxic lymphocytes can be generated from normals and leukemic patients. In mice, WT1 vaccines elicit specific immune responses without evidence of tissue damage. In this paper, we review studies validating the immunogenicity of WT1 and propose that leukemia and MDS may be a good clinical model to test the efficacy of a WT1 vaccine.

An isoform of the Wilms' tumor suppressor gene potentiates granulocytic differentiation

WT1 is expressed in hematopoietic progenitor cells and in acute leukemia, but its role in normal and malignant hematopoiesis has not been clearly defined. Alternative splicing of the WT1 mRNA yields several protein isoforms with distinct DNA binding and transcriptional regulatory activities. In this study, we investigated the effect of the WT1 isoform lacking two alternatively spliced sequences (WT1 (-/-)) in 32D cl3 cells, a murine myeloid progenitor cell line. The expression of WT1 (-/-) accelerated the granulocyte-colony stimulating factor (G-CSF)-mediated differentiation of these cells, as judged by morphology and by the expression of differentiation-associated genes and cell surface antigens. WT1 (-/-) inhibited G1/S progression in G-CSF but not in interleukin-3, potentially accounting for its ability to accelerate differentiation. It is likely that dominant-negative mutants previously reported in leukemia patients participate in leukemogenesis by inhibiting this function of the wild-type protein.

Role of the WT1 tumor suppressor in murine hematopoiesis

The WT1 tumor-suppressor gene is expressed by many forms of acute myeloid leukemia. Inhibition of this expression can lead to the differentiation and reduced growth of leukemia cells and cell lines, suggesting that WT1 participates in regulating the proliferation of leukemic cells. However, the role of WT1 in normal hematopoiesis is not well understood. To investigate this question, we have used murine cells in which the WT1 gene has been inactivated by homologous recombination. We have found that cells lacking WT1 show deficits in hematopoietic stem cell function. Embryonic stem cells lacking WT1, although contributing efficiently to other organ systems, make only a minimal contribution to the hematopoietic system in chimeras, indicating that hematopoietic stem cells lacking WT1 compete poorly with healthy stem cells. In addition, fetal liver cells lacking WT1 have an approximately 75% reduction in erythroid blast-forming unit (BFU-E), erythroid colony-forming unit (CFU-E), and colony-forming unit-granulocyte macrophage-erythroid-megakaryocyte (CFU-GEMM). However, transplantation of fetal liver hematopoietic cells lacking WT1 will repopulate the hematopoietic system of an irradiated adult recipient in the absence of competition. We conclude that the absence of WT1 in hematopoietic cells leads to functional defects in growth potential that may be of consequence to leukemic cells that have alterations in the expression of WT1.

Abnormal WT1 expression in the CD34-negative compartment in myelodysplastic bone marrow

In normal bone marrow, WT1 expression is restricted to CD34+ cells. We assessed WT1 mRNA expression levels with quantitative, real-time reverse transcription polymerase chain reaction in normal, myelodysplastic (MDS) and secondary acute myeloid leukaemia (sAML) bone marrow subfractions, based on differentiation status. The highest WT1 expression was observed in the primitive CD34+ rhodamine-123 (rho) dull cells, both in healthy donors and MDS or sAML patients. In contrast to normal CD34-negative bone marrow cells, WT1 was present in CD34-negative bone marrow cells in 12 out of 13 MDS patients and two sAML samples. Further analysis of this aberrant WT1 expression was performed in the CD34-negative subfractions of three MDS patients. In one of these, WT1 expression was found exclusively in the erythroid cells. This patient was completely transfusion dependent and showed morphological dyserythropoiesis. In another MDS patient, WT1 expression was found in a non-erythroid compartment. We conclude that abnormal WT1 expression may contribute to the disturbed differentiation of haematopoietic cells in MDS patients.

Wilms tumor gene (WT1) expression as a panleukemic marker

The Wilms tumor gene (WT1) is expressed in blasts of patients with acute leukemia, irrespective of lineage, and WT1 nuclear protein is detectable in the majority of such blasts. Only very few physiologic hematopoietic progenitors express WT1, but the WT1 expression level of these progenitors and that of leukemic blasts are comparable. Although not specific for acute hematologic malignant diseases, continuous WT1 expression in almost all leukemic blasts strikingly contrasts to its rather transient expression in very few physiologic hematopoietic progenitors. Quantitative and semiquantitative WT1 reverse transcriptase polymerase chain reaction (RT-PCR) protocols have limitations in discriminating physiologic from pathologic overall WT1 expression levels in mononuclear cell preparations. Because of these limitations, reports conflict on the usefulness of long-term monitoring of WT1 expression in patients with acute leukemia. Real-time quantitative WT1 RT-PCR protocols, however, have been developed and tested in small series of patients with acute leukemia. Such protocols hold promise to enable evaluation of the individual treatment response (short-term monitoring) and early diagnosis of imminent relapse through the detection and long-term monitoring of minimal residual disease in patients with acute leukemia. These protocols also should facilitate the notoriously difficult distinction between eosinophilic leukemia and hypereosinophilic syndromes. Data on WT1 expression in leukemic blasts and their physiologic counterparts are discussed in light of clinical relevance.

Forced expression of the Wilms tumor 1 (WT1) gene inhibits proliferation of human hematopoietic CD34(+) progenitor cells

The Wilms tumor gene (WT1) encodes a zinc-finger containing transcription factor present in primitive hematopoietic progenitor cells. WT1 is also highly expressed in most cases of acute myeloid leukemia. Moreover, WT1 can interfere with induced differentiation of leukemic cell lines. These data suggest a function of WT1 in the maintenance of a primitive phenotype and a role in leukemogenesis by interfering with differentiation, prompting us to investigate its function in human hematopoietic progenitor cells. By retroviral transfer, human CD34(+) cord blood progenitor cells were transduced with a vector encoding either of two splicing variants of WT1, with or without the KTS insert in the zinc-finger domain, linked to expression of green fluorescent protein (GFP) via an internal ribosomal entry site. When compared to cells transduced with vector containing GFP only, WT1 expressing cells showed strongly reduced colony formation in methylcellulose and inhibited proliferation in suspension culture, with no apparent reduction in viability. Cell cycle phase distribution was not affected by WT1 expression. No signs of impaired differentiation, as judged by the surface markers CD11b, CD14 and glycophorin were detected. In contrast to the results with human CD34(+) progenitor cells, the proliferation of murine bone marrow cells was not significantly affected by WT1, consistent with previous data. We conclude that forced expression of WT1 in highly enriched human hematopoietic progenitor cells leads to strong anti-proliferative effects but is compatible with induced maturation of these cells.

WT1 proteins: functions in growth and differentiation

The Wilms' tumor 1 gene (WT1) has been identified as a tumor suppressor gene involved in the etiology of Wilms' tumor. Approximately 10% of all Wilms' tumors carry mutations in the WT1 gene. Alterations in the WT1 gene have also been observed in other tumor types, such as leukemia, mesothelioma and desmoplastic small round cell tumor. Dependent on the tumor type, WT1 proteins might either function as tumor suppressor proteins or as survival factors. Mutations in the WT1 gene can also result in congenital abnormalities as observed in Denys-Drash and Frasier syndrome patients. Mouse models have proven the critical importance of WT1 expression for the development of several organs, including the kidneys, the gonads and the spleen. The WT1 proteins seem to perform two main functions. They regulate the transcription of a variety of target genes and may be involved in post-transcriptional processing of RNA. The WT1 gene encodes at least 24 protein forms. These isoforms have partially distinct biological functions and effects, which in many cases are also specific for the model system in which WT1 is studied. This review discusses the molecular mechanisms by which the various WT1 isoforms exert their functions in normal development and how alterations in WT1 may lead to developmental abnormalities and tumor growth.

The possible role and application ofWT1 in Human Leukemia

WT1, a tumor suppressor gene responsible for the development of childhood kidney tumors, is now also thought to be involved in the occurrence of human leukemia. First, evidence has shown that WT1 functions during hematopoiesis and regulates the proliferation and differentiation of blood cells. Second, specific expression patterns of this gene correlate with the malignant phenotype of leukemia compared with the physiological situation. Third, mutations of WT1 can be detected, though not frequently, in human leukemia but not in normal hematopoietic cells. Thus, a possible role of WT1 in human leukemogenesis has been proposed. Because the expression of this gene is relatively high during the so-called myelodysplastic stages and in all subtypes of human leukemia compared with normal blood cells, the notion has been raised that WT1 can be used as a "panleukemic marker" for the diagnosis of leukemia at the molecular level. The expression level of WT1 may have significance in predicting prognosis and monitoring relapse. Moreover, with a deeper understanding of its role in leukemogenesis, WT1 may serve as a target molecule in the strategy of gene therapy for leukemia.

Overexpression of murine WT1 + / + and - / - isoforms has no effect on chemoresistance but delays differentiation in the K562 leukemia cell line

The Wilms' tumor gene (WT1) encodes a zinc-finger transcription factor that is expressed as four distinct isoforms designated as, + / +, + / -, - / + and - / -. It is expressed in leukemic cells, and is proposed to play a role in their proliferation and differentiation. In this study we have shown that cell lines of the erythroleukemia, K562, overexpressing the murine + / + and - / - WT1 isoforms grow normally and do not exhibit altered responses to the induction of apoptosis by the reagents cisplatin and adriamycin, or to serum withdrawal. However, differentiation of K562 cells with 12-O-tetradecanoylphorbol 13-acetate, modeling aspects of megakaryopoiesis, was partially inhibited by the persistent expression of both the murine + / + and - / - WT1 isoforms. This finding suggests that WT1 plays a role in the regulation of hematopoietic differentiation and is consistent with an oncogenic role for WT1 in leukemogenesis.