Teresa Sadras

The GM-CSF receptor utilizes β-catenin and Tcf4 to specify macrophage lineage differentiation

Granulocyte-macrophage colony stimulating factor (GM-CSF) promotes the growth, survival, differentiation and activation of normal myeloid cells and is essential for fully functional macrophage differentiation in vivo. To better understand the mechanisms by which growth factors control the balance between proliferation and self-renewal versus growth-suppression and differentiation we have used the bi-potent FDB1 myeloid cell line, which proliferates in IL-3 and differentiates to granulocytes and macrophages in response to GM-CSF. This provides a manipulable model in which to dissect the switch between growth and differentiation. We show that, in the context of signaling from an activating mutant of the GM-CSF receptor β subunit, a single intracellular tyrosine residue (Y577) mediates the granulocyte fate decision. Loss of granulocyte differentiation in a Y577F second-site mutant is accompanied by enhanced macrophage differentiation and accumulation of β-catenin together with activation of Tcf4 and other Wnt target genes. These include the known macrophage lineage inducer, Egr1. We show that forced expression of Tcf4 or a stabilised β-catenin mutant is sufficient to promote macrophage differentiation in response to GM-CSF and that GM-CSF can regulate β-catenin stability, most likely via GSK3β. Consistent with this pathway being active in primary cells we show that inhibition of GSK3β activity promotes the formation of macrophage colonies at the expense of granulocyte colonies in response to GM-CSF. This study therefore identifies a novel pathway through which growth factor receptor signaling can interact with transcriptional regulators to influence lineage choice during myeloid differentiation.

Interleukin‐3‐mediated regulation of β‐catenin in myeloid transformation and acute myeloid leukemia

Aberrant activation of β‐catenin is a common event in AML and is an independent predictor of poor prognosis. Although increased β‐catenin signaling in AML has been associated with oncogenic translocation products and activating mutations in the FLT3R, the mechanisms that activate β‐catenin in AML more broadly are still unclear. Here, we describe a novel link between IL‐3 signaling and the regulation of β‐catenin in myeloid transformation and AML. In a murine model of HoxB8 and IL‐3 cooperation, we show that β‐catenin protein levels are modulated by IL‐3 and that Cre‐induced deletion of β‐catenin abolishes IL‐3‐dependent growth and colony formation. In IL‐3‐dependent leukemic TF‐1.8 cells, we observed increased β‐catenin protein levels and nuclear localization in response to IL‐3, and this correlated with transcriptional induction of β‐catenin target genes. Furthermore, IL‐3 promoted β‐catenin accumulation in a subset of AML patient samples, and gene‐expression profiling of these cells revealed induction of WNT/β‐catenin and TCF4 gene signatures in an IL‐3‐dependent manner. This study is the first to link β‐catenin activation to IL‐3 and suggests that targeting IL‐3 signaling may be an effective approach for the inhibition of β‐catenin activity in some patients with AML.

High prevalence of relapse in children with Philadelphia-like acute lymphoblastic leukemia despite risk-adapted treatment

Acute lymphoblastic leukemia (ALL) remains a leading cause of cancer-related death in children and young adults. Since the 1960s, improvements in the treatment of children with ALL have led to 10-year survival rates now exceeding 85%.[1][1] Philadelphia-like (Ph-like) ALL is characterized by a gene

miR-155 as a potential target of IL-3 signaling in primary AML cells

miR-155 has emerged as one of the key microRNAs (miRNAs) involved in normal and malignant myelopoiesis, and high expression of this miRNA has been flagged as a strong independent prognostic marker in Acute Myeloid Leukemia (AML). While elevated expression of miR-155 has been associated with FLT3-ITD mutations, other mechanisms which may regulate expression of this miRNA in AML remain largely unknown. Here, we present new evidence that miR-155 may be a prime target of IL-3 signaling in primary AML cells. This finding, together with the increasingly apparent role for miR-155 in oncogenesis, and the upregulation of the IL-3 receptor alpha subunit in AML, lead us to propose this pathway may significantly contribute to the leukemic transformation.

A novel somatic JAK2 kinase-domain mutation in pediatric acute lymphoblastic leukemia with rapid on-treatment development of LOH

We report a novel somatic mutation in the kinase domain of JAK2 (R938Q) in a high-risk pediatric case of B-cell acute lymphoblastic leukemia (ALL). The patient developed on-therapy relapse at 12 months, and interestingly, the JAK2 locus acquired loss of heterozygosity during treatment resulting in 100% mutation load. Furthermore, we show that primary ALL mononuclear cells harboring the JAK2 R938Q mutation display reduced sensitivity to the JAK1/2 ATP-competitive inhibitor ruxolitinib in vitro, compared to ALL cells that carry a more common JAK2 pseudokinase domain mutation. Our findings are in line with previous reports that demonstrate that mutations within the kinase domain of JAK2 are associated with resistance to type I JAK inhibitors. Importantly, given the recent inclusion of ruxolitinib in trial protocols for children with JAK pathway alterations, we predict that inter-patient genetic variability may result in suboptimal responses to JAK inhibitor therapy in a subset of cases. The need for alternate targeted and/or combination therapies for patients who display inherent or developed resistance to JAK inhibitor therapy will be warranted, and we propose that kinase-mutants less sensitive to type I JAK inhibitors may present a currently unexplored platform for investigation of improved therapies.