The Wilms’ Tumor Gene WT1 Is Over-Expressed in Immature Leukemia Cells but Not Necessary for Leukemia Development in Mouse Leukemia Models.

The Wilms tumor gene WT1, which is over-expressed in almost all leukemia, is one of the most promising targets for immunotherapy. To clarify which cells express WT1, we generated a knock-in reporter GFP mice (WT1^GFP/+) and assayed for WT1 expression in normal and leukemic hematopoietic cells. In normal hematopoietic cells, WT1 was expressed in none of the long-term hematopoietic stem cells (HSCs) and very few (<1%) of multipotent progenitor cells. WT1 was expressed in 17.4% of megakaryocyte-erythrocyte progenitor cells and 4.1% of common myeloid progenitor cells, while the frequencies of WT1 expressing cells in mature granulocyte or lymphocyte were very low (~1%). In contrast, in murine leukemias induced by TEL/PDGFβR + AML1/ETO or BCR/ABL, WT1 was expressed in 40.5% or 38.9% of c-kit+lin-Sca-1+ (KLS) leukemia cells, which contained a subset, but not all, of transplantable leukemic stem cells (LSC). Furthermore, WT1 was not expressed fetal liver HSC population, mobilized HSCs by cyclophosphamide and granulocyte-colony stimulating factor, or HSCs stimulated with SCF, Flt3L, and IL3 in-vitro. These results suggest that WT1 expression in is not a surrogate marker of cell proliferation. To examine the function of WT1 in leukemogenesis, we used recombinant lentivirus to overexpress WT1. HSCs transduced with either empty or WT1-expressing lentivirus were transplanted into lethally irradiated recipients. The FACS analysis of BM cells at 4 months post transplant showed very low chimerism of WT1-expressing cells compared to control GFP-expressing cells. Moreover, differentiation block of WT1-expressing cells was not observed. These results indicate that WT1 does not induce proliferation or block differentiation in-vivo. Next, we tested whether leukemias could be induced from the WT1-deficient fetal liver hematopoietic cells. All of three mice transplanted with E13.5 WT1-defeicient fetal liver cells transduced with TEL/PDGFβR+AML1/ETO developed leukemia. Similarly, 3 out of 4 mice transplanted with p210BCR/ABL-transduced WT1-deficient fetal liver cells developed leukemia. These results indicate that WT1 is not necessary for leukemia development in two leukemia models examined. Collectively, WT1 is expressed on immature leukemic cells but not normal HSCs, highlighting its potential as a specific target for therapy. However, the exact function of WT1 in leukemogenesis remains unclear from these studies.