MAPK/ERK2 phosphorylates ERG at serine 283 in leukemic cells and promotes stem cell signatures and cell proliferation

© 2016 Macmillan Publishers Limited. Aberrant ERG (v-ets avian erythroblastosis virus E26 oncogene homolog) expression drives leukemic transformation in mice and high expression is associated with poor patient outcomes in acute myeloid leukemia (AML) and T-acute lymphoblastic leukemia (T-ALL). Protein phosphorylation regulates the activity of many ETS factors but little is known about ERG in leukemic cells. To characterize ERG phosphorylation in leukemic cells, we applied liquid chromatography coupled tandem mass spectrometry and identified five phosphorylated serines on endogenous ERG in T-ALL and AML cells. S283 was distinct as it was abundantly phosphorylated in leukemic cells but not in healthy hematopoietic stem and progenitor cells (HSPCs). Overexpression of a phosphoactive mutant (S283D) increased expansion and clonogenicity of primary HSPCs over and above wild-type ERG. Using a custom antibody, we screened a panel of primary leukemic xenografts and showed that ERG S283 phosphorylation was mediated by mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK) signaling and in turn regulated expression of components of this pathway. S283 phosphorylation facilitates ERG enrichment and transactivation at the ERG +85 HSPC enhancer that is active in AML and T-ALL with poor prognosis. Taken together, we have identified a specific post-translational modification in leukemic cells that promotes progenitor proliferation and is a potential target to modulate ERG-driven transcriptional programs in leukemia

Runx1 contributes to neurofibromatosis type 1 neurofibroma formation

Neurofibromatosis type 1 (NF1) patients are predisposed to neurofibromas but the driver(s) that contribute to neurofibroma formation are not fully understood. By cross comparison of microarray gene lists on human neurofibroma-initiating cells and developed neurofibroma Schwann cells (SCs) we identified RUNX1 overexpression in human neurofibroma initiation cells, suggesting RUNX1 might relate to neurofibroma formation. Immunostaining confirmed RUNX1 protein overexpression in human plexiform neurofibromas. Runx1 overexpression was confirmed in mouse Schwann cell progenitors (SCPs) and mouse neurofibromas at the messenger RNA and protein levels. Genetic inhibition of Runx1 expression by small hairpin RNA or pharmacological inhibition of Runx1 function by a Runx1/Cbfβ interaction inhibitor, Ro5-3335, decreased mouse neurofibroma sphere number in vitro. Targeted genetic deletion of Runx1 in SCs and SCPs delayed mouse neurofibroma formation in vivo. Mechanistically, loss of Nf1 increased embryonic day 12.5 Runx1(+)/Blbp(+) progenitors that enable tumor formation. These results suggest that Runx1 has an important role in Nf1 neurofibroma initiation, and inhibition of RUNX1 function might provide a novel potential therapeutic treatment strategy for neurofibroma patients