Impaired intranuclear trafficking of AML1 causes transformation-coupled arrest of myeloid maturation

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The t(8;21) chromosomal translocation in acute myeloid leukemias disrupts the Runx1 (AML1) gene that is essential for definitive hematopoiesis. The resulting AML-ETO fusion protein, which contains the amino terminal DNA binding domain and nuclear import signal of AML1 but lacks the subnuclear targeting signal (NMTS), functions as a dominant repressor of AML1 function in vivo. Loss of the obligatory AML1 subnuclear targeting signal results in misdirection of this leukemia fusion protein to nuclear microenvironments distinct from AML1 sites. In this study, we directly addressed the contribution of AML1 subnuclear targeting to myeloid differentiation. A single amino acid substitution in the AML1 NMTS that abrogates intranuclear trafficking was introduced. The mutant protein retains the ability to enter the nucleus, binds to DNA and forms nuclear complexes with co-regulatory and signaling proteins. However, the mutant is incapable of association with architecturally organized subnuclear domains. Expression of the mutant AML1 protein blocks differentiation of myeloid progenitor cells to granulocytes. These cells continue to proliferate, maintain an immature blast-like morphology, and exhibit transformed properties that are hallmarks of leukemogenesis. Changes in cell growth and phenotype were reflected by altered gene expression profiles. These findings functionally link fidelity of AML1 subnuclear organization with competency for myeloid differentiation and expression of the transformed/leukemia phenotype.