ETS2 and ERG promote megakaryopoiesis and synergize with alterations in GATA-1 to immortalize hematopoietic progenitor cells

ETS2 and ERG are transcription factors, encoded on human chromosome 21 (Hsa21), that have been implicated in human cancer. People with Down syndrome (DS), who are trisomic for Hsa21, are predisposed to acute megakaryoblastic leukemia (AMKL). DS-AMKL blasts harbor a mutation in GATA1, which leads to loss of full-length protein but expression of the GATA-1s isoform. To assess the consequences of ETS protein misexpres- sion on megakaryopoiesis, we expressed ETS2, ERG, and the related protein FLI-1 in wild-type and Gatal mutant murine fetal liver progenitors. These studies revealed that ETS2, ERG, and FLI-1 facilitated the expansion of megakaryocytes from wild-type, Gatal-knockdown, and Gatals knockin progenitors, but none of the genes could overcome the differentiation block characteristic of the Gata1- knockdown megakaryocytes. Although overexpression of ETS proteins increased the proportion of CD41 ⁺ cells generated from Gatals-knockin progenitors, their expression led to a significant reduction in the more mature CD42 fraction. Serial replating assays revealed that overexpression of ERG or FLI-1 immortalized Gatal- knockdown and Gatals knockin, but not wild-type, fetal liver progenitors. Immortalization was accompanied by activation of the JAK/STAT pathway, commonly seen in megakaryocytic malignancies. These findings provide evidence for synergy between alterations in GATA-1 and overexpression of ETS proteins in aberrant megakaryopoiesis.