Oncogenic TLS-ERG Fusion Protein Promotes Differentiation of Myeloid Progenitors into Myeloblasts but Blocks Their Terminal Differentiation.

In a subset of acute myelogenous leukemia (AML) and chronic myelogenous leukemia (CML) in blast crisis, the TLS (translocation liposarcoma) gene is fused to the ERG (ets-related gene) gene through a recurrent t(16;21) translocation. This chromosomal translocation results in the generation of the TLS-ERG fusion protein. More recently, the same TLS-ERG fusion protein has also been identified in Ewing’s sarcoma in children. To investigate the oncogenic mechanism of TLS-ERG, we stably expressed TLS-ERG or its mutant TLS-ERGΔets in mouse L-G myeloid progenitor cells through retroviral transduction. L-G cells normally proliferate in the presence of IL-3 and undergo terminal differentiation into hypersegmented neutrophils when treated with G-CSF. In contrast, expression of TLS-ERG in L-G cells led to spontaneous differentiation into immature myeloblasts but G-CSF treatment of these cells failed to induce terminal differentiation. L-G myeloblasts harboring TLS-ERG were characterized by high levels of myeloperoxidase (MPO), a phenomenon also observed in immature myeloblasts isolated from human leukemia patients with the t(16;21) translocation. As deletion of the ets DNA-binding domain from TLS-ERG abolished its ability to promote spontaneous myeloblast differentiation or the ability to up-regulate the MPO gene, we further examined how TLS-ERG fusion protein might affect the functions of other regulators such as PU.1, an ets transcription factor known to play a critical role in the differentiation of myeloid progenitors. Though both proteins have similar DNA-binding property, TLS-ERG differs from PU.1 through binding directly to RNA polymerase II as well as through escaping repression by histone modification enzymes. Together, these findings suggest that the TLS-ERG fusion protein can cause blockage of myeloid differentiation by interfering with the gene expression program normally controlled by key regulators.