Hematopoietic Overexpression of the ETS Family Transcription Factor Erg, or the Oncogenic Fusion Protein TLS-ERG, Induces Erythro-Megakaryocytic Leukemia

Abstract 1283

ETS-related gene, ERG, is a key regulator of hematopoietic stem cell (HSC) function, and a potent oncogene. It is involved in chromosomal translocations with the EWS gene in Ewing's sarcoma, the TLS gene in Acute Myeloid Leukemia (AML) and the TMPRSS2 gene in more than half of all prostate cancers. In addition, increased ERG levels are associated with poor prognosis in cytogenetically normal AML and T-cell Acute Lymphoblastic Leukemia; and our recent data suggests that trisomy of ERG is important for development of myeloproliferative disease and Acute Megakaryocytic Leukemia (AMKL) in Down syndrome individuals.

The role ERG, and the oncogenic fusion protein TLS-ERG, play during hematopoietic transformation remains unclear. Hematopoietic overexpression of ERG has been shown to induce T-cell leukemia in mice. Development of a non-lymphoid disease has also been described, however this disease was reported to be an AMKL by one group, and a non-malignant erythroid hyperplasia by another. Hematopoietic overexpression of TLS-ERG in mice has not been described. This fusion has been shown to perturb differentiation and increase self-renewal of human myeloid progenitor cells in vitro, and enable the IL-3 dependent L-G murine myeloid progenitor cell line to induce a leukemia-like disease in vivo.

In order to clarify and compare the role of wild-type and rearranged forms of ERG in leukemia development, we injected lethally irradiated mice with fetal liver cells (FLCs) transduced with retrovirus carrying either Erg or TLS-ERG. These mice succumbed to disease with a median latency of 80 days after receiving Erg-transduced FLCs, or 44 days after receiving TLS-ERG-transduced FLCs. Consistent with published data, 30% of Erg mice developed T-cell leukemias. Interestingly, no TLS-ERG mice developed this disease.

Strikingly, 100% of Erg and TLS-ERG mice developed an identical non-lymphoid disease characterised by hepatosplenomegaly, anemia and leukocytosis. Histopathological and flow cytometric analysis revealed infiltration of the bone marrow, spleen, lung and liver by nucleated erythroblasts, expressing a high level of CD71 and varying levels of Ter119. Interestingly, in some mice a subset of these cells also expressed the megakaryocytic marker CD41. Primary spleen cells were capable of transplanting disease in non-irradiated mice, demonstrating that this disease was malignant.

Spleen cells from Erg and TLS-ERG leukemic mice, but not controls, were capable of generating large numbers of small colonies when cultured in methylcellulose stimulated with IL-3/SCF/EPO. These colonies primarily contained erythroblasts (CD71⁺Ter119^+/−), however some cells also expressed CD41 and were acetylcholinesterase positive. Most acetylcholinesterase positive cells were of small size, indicating either incomplete or early megakaryocyte maturation.

Combined, these data suggest that hematopoietic overexpression of Erg or TLS-ERG in mice leads to transformation of a bi-potential erythroid-megakaryocyte progenitor. Following transformation, this cell appears to retain some bi-potentiality in vitro, however in vivo it primarily develops along the erythroid lineage. Thus, Erg- and TLS-ERG- induced non-lymphoid disease may be best described as an erythro-megakaryocytic leukemia. Finally, the data also indicate that truncation and fusion to TLS abrogates ERG's ability to transform lymphoid progenitors, however TLS-ERG retains the ability to transform myeloid progenitors in a manner that strongly resembles that of wild-type Erg.