Common Leukemic Signaling Pathways Identified by Comparative Analysis of GM-CSF and FLT3 Activated Receptor Mutations.

Abstract

As a strategy to identify key targets in myeloid leukemia we examined directly the overlap in signaling and downstream events induced by several activated receptor mutants associated with AML. We compared the events induced by a leukemic mutant (V449E) of the GM-CSF receptor with events associated with the activating FLT3 internal tandem duplication mutant (FLT3-ITD) and the D835Y kinase domain mutant (FLT3-TKD). Receptor mutants were introduced by retroviral transduction into the FDB-1 bi-potential myeloid cell line model (McCormack MP et al., Blood. 95:120–127, 2000). We used biochemical analysis and pathway inhibitors to demonstrate that the GMR V449E mutant selectively activates the JAK2-STAT5 and p44/42 MAPK pathways which are central to the ability of this mutant to confer continued growth and arrested differentiation in the absence of growth factor. Like the GMR V449E mutant, the FLT3-ITD and -TKD mutants arrest granulocyte-macrophage differentiation and support continued growth in the absence of added growth factor. For both of these mutants we also observed constitutive JAK2 and STAT5 phosphorylation and sensitivity to a selective JAK2 inhibitor (JAK2 Inhibitor II, Merck) suggesting a potential key role for JAK2 in signalling from FLT3 activating mutants. Both FLT3 mutants activated the p44/42 MAPK pathway, resulted in Akt phosphorylation and were sensitive to PI3-kinase and MEK inhibitors. As activation of the p44/42 MAPK pathway is common to all mutants, and has recently been associated with suppressed differentiation in AML (Radomska HS et al, J Exp Med. 203:371–381, 2006), we used the selective MEK inhibitor, U0126 (Merck), to investigate the contribution of this pathway to maintenance of the differentiation block. Treatment of factor-independent FDB-1 cells expressing the GMR V449E mutant, or either FLT3 mutant, with U0126 for 48 hours induced morphological differentiation suggesting that the sustained activation of p44/42 MAPK contributes to maintaining the arrest in myeloid differentiation. To explore further the overlapping nature of signalling by the GMRV449E and FLT3-ITD activated mutants we utilised the Wilcoxan rank sum test to perform gene-set enrichment analysis. Analysis of 119 FLT3-ITD associated genes (126 probes) (Mizuki M et al, Blood 101: 3164–3173, 2003) revealed that this gene-set shows evidence of differential expression in response to GMR V449E signaling (p=2.8x10−7). In particular, we identify the tumor suppressor gene, Gadd45α, as a common gene, down-regulated in response to signalling via the FLT3-ITD, FLT3-TKD and GMR V449E activated receptors. Analysis of AML gene expression data (Valk PJ et al, New Engl. J. Med : 350:1617–1628, 2004) indicates that activation of Gadd45α maybe suppressed by multiple pathways in AML blasts.