A Molecular Signature for a Cytokine Receptor Survival Pathway in AML Identifies Unique Prognostic Indicators and Therapeutic Targets

The IL-3 receptor regulates myeloid cell survival and proliferation and is overexpressed on AML blasts and CD34+CD38- AML “stem-like” cells suggesting it plays a role in leukemic cell survival that could be exploited by targetted therapy. We have previously shown that Serine 585 in the cytoplasmic domain of the beta subunit of the IL-3 receptor is phosphorylated in response to sub-picomolar concentrations of cytokine, binds 14-3-3 zeta adaptor and is required for cytokine mediated survival (Guthridge et al, 2006, EMBO). We wished to examine the phosphorylation status of this receptor in AML blasts from patients and the expression levels of downstream Ser585 target genes.

Results: We show that Ser585 of the beta common subunit is constitutively phosphorylated in the majority of leukaemic patients regardless of Flt3 status or cytogenetics (18/21 patients). In contrast, tyrosine phosphorylation of the receptor is not deregulated and remains cytokine-dependent (20/21). We have identified a panel of 139 Ser585-regulated genes by microarray (WT receptor vs Ser to Gly mutant) and show that a specific subset of these genes is deregulated in AML blasts confirmed by RT-PCR compared to normal mononuclear cells consistent with constitutive activation of this pathway in AML. We have identified a 4-gene signature that is a molecular correlate of the Ser585-survival pathway in AML. We further identify Ser585-regulated genes that are prognostic indicators of clinical outcome in a cohort of adult patients AML (n=33) independent of WCC or cytogenetic abnormalities. Targetting Ser585 survival genes using siRNA or blocking mAbs induces apoptosis in AML blasts validating Ser585-regulated genes as potential targets for therapy.

Conclusions: Given the emerging problem of resistance to tyrosine kinase inhibitors (TKI) and limited success of Flt3 inhibitors in AML, new strategies for therapy are needed. By interrogating a cytokine survival pathway that functions independently of tyrosine phosphorylation we have uncovered novel Ser585 gene targets with promising clinical utility in leukaemia.