The Wilms tumor gene, WT1, encodes a zinc-®nger DNA binding protein which is thought to function as a tissue speci®c transcription factor, regulating cell growth and di erentiation. High expression of WT1 has been detected in a range of acute leukemias. To elucidate a role for WT1 in leukemogenesis, we transfected the monoblastic cell line U937, which lacks detectable levels of endogenous WT1, with two isoforms of WT1. We showed that, in contrast to U937 control cells, cells constitutively expressing either of the isoforms, WT1(7KTS) or WT1(+KTS), did not respond to di erentiation induction by retinoic acid or vitamin D3, as judged by the capacity to reduce nitro blue tetrazolium and morphology. Although U937 cells expressing WT1 were hampered in their ability to di erentiate on incubation with retinoic acid and vitamin D3, the induced G1/G0-accumulation was similar to di erentiating control cells treated with inducers. Furthermore, distinct e ects on the maturation process were indicated by downregulation of the myeloid cell surface makers CD13 and CD15, while the upregulation of CD14 and CD11c on WT1 transfected cells was similar to control cells upon incubation with retinoic acid and vitamin D3. Taken together our results demonstrate that a constitutive expression of WT1 in the leukemic cell line U937 leads to impairment of di erentiation responses, indicating that a high expression of WT1 can contribute to the di erentiation block of acute leukemia.

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