1. ¹Institute for Stem Cell Biology and Regenerative Medicine, Stanford University School of Medicine, Stanford, CA, USA
2. ²Department of Pathology, Stanford University School of Medicine, Stanford, CA, USA
3. ³Department of Development Biology, Stanford University School of Medicine, Stanford, CA, USA
4. ⁴Department of Cancer Immunotherapy, Osaka University School of Medicine, Osaka, Japan
5. ⁵Department of Functional Diagnostic Science, Osaka University School of Medicine, Osaka, Japan
6. ⁶Department of Respiratory Medicine, Allergy and Rheumatic Disease, Osaka University School of Medicine, Osaka, Japan
7. ⁷Genome Information Research Center, Osaka University, Osaka, Japan

Correspondence: Dr N Hosen, Department of Cancer Stem Cell Biology, Osaka University Graduate School of Medicine, 1-7 Yamada-Oka, Suita, Osaka 565-0871, Japan. E-mails: hnaoki@stanford.edu or hnaoki@imed3.med.osaka-u.ac.jp

⁸Present address: Department of Cancer Stem Cell Biology, Osaka University Graduate School of Medicine, Osaka 565-0871, Japan.

Received 12 March 2007; Accepted 16 April 2007; Published online 24 May 2007.

Abstract

The Wilms' tumor gene WT1 is overexpressed in most of human leukemias regardless of disease subtypes. To characterize the expression pattern of WT1 during normal and neoplastic hematopoiesis, we generated a knock-in reporter green fluorescent protein (GFP) mouse (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 (LT) hematopoietic stem cells (HSC) and very few (<1%) of the multipotent progenitor cells. In contrast, in murine leukemias induced by acute myeloid leukemia 1 (AML1)/ETO+TEL/PDGFR or BCR/ABL, WT1 was expressed in 40.5 or 38.9% of immature c-kit⁺lin^-Sca-1⁺ (KLS) cells, which contained a subset, but not all, of transplantable leukemic stem cells (LSCs). WT1 expression was minimal in normal fetal liver HSCs and mobilized HSCs, both of which are stimulated for proliferation. In addition, overexpression of WT1 in HSCs did not result in proliferation or expansion of HSCs and their progeny in vivo. Thus, the mechanism by which expansion of WT1-expressing cells occurs in leukemia remains unclear. Nevertheless, our results demonstrate that the WT1^GFP/+ mouse is a powerful tool for analyzing WT1-expressing cells, and they highlight the potential of WT1, as a specific therapeutic target that is expressed in LSCs but not in normal HSCs.