Differential Effects on Cell Cycle Regulators by AML1-ETO and a C-Terminal Truncated AML1-ETO (From Tumor Suppressor to Oncogene).

The fusion protein AML1-ETO created by the 8;21 translocation involved in de novo acute myeloid leukemia requires secondary mutational events to promote leukemia. Here, we report that the loss of the molecular events associated with AML1-ETO C-terminus including an NCoR/SMRT interacting domain transforms AML1-ETO into a potent leukemogenic protein in mice. Furthermore, we present evidence of aberrant protein expression of cell cycle regulators in a hematopoietic cell line expressing AML1-ETO compared to the leukemogenic truncated form of AML1-ETO (AML1-ETOtr). Our studies show that AML1-ETO and AML1-ETOtr are biochemically isolated from the same cellular compartments, that they can oligomerize, and that they can both efficiently immunoprecipitate the transcription factor Gfi-1. However, contrary to AML1-ETO, AML1-ETOtr does not promote growth arrest. Western analyses show that cell cycle promoting factors cyclin D3 and cyclin A are decreased by AML1-ETO, while their RNA levels remain the same. In addition, an increase in the cdk-inhibitor p21WAF1 is observed in the presence of both proteins, while only AML1-ETO induces high expression of the cdk-inhibitor p27KIP1. These changes are associated with a deregulation of the SCF ubiquitin E3 ligase component Skp2 that is involved in controlling the levels of both cyclins and cdk-inhibitors. These observations suggest that AML1-ETO has tumor suppressor activity. Additional mutations to bypass this effect can change it into an oncogenic protein. Therefore, our results lead to a new model of AML1-ETO in leukemogenesis, i.e., the gain/loss of function of cell cycle regulators to promote cell cycle progression or disruption of molecular events associated with AML1-ETO C-terminal NCoR/SMRT interacting domain are required for AML1-ETO involved leukemogenesis. The disrupted molecular events may include 1) the loss of factor(s) physically interacting with the C-terminal domain of AML1-ETO, 2) the alteration by mutagenesis of signaling pathways downstream of AML1-ETO C-terminal domain, and 3) the dysfunction within AML1-ETO C-terminal domain by truncations or mutations.