The Association Between STAT5a Activation, LEF-1 Down Regulation and Leukemic Transformation in Patients with Severe Congenital Neutropenia

Congenital hematological disorders are excellent models for investigating the regulation of hematopoiesis in humans. For instance, Severe Congenital Neutropenia (CN) is a heterogeneous syndrome characterized by a maturation arrest of granulopoiesis at the level of promyelocytes with no mature neutrophils in the peripheral blood. Even though G-CSF treatment results in increased neutrophil numbers in more than 90 % of CN patients, G-CSF-dependent granulocytic differentiation is severely affected in these patients. CN patients are found to be at increased risk of developing acute myeloid leukemia or myelodysplastic syndrome (AML/MDS) (cumulative incidence ~ 20 %). Since AML/MDS are not observed in cyclic or idiopathic neutropenia patients treated with G-CSF, an underlying defect down stream of G-CSF signaling rather than G-CSF therapy per se predisposes to malignant transformation in CN. STAT5 is activated by G-CSF signaling pathway and has also been found to be activated in AML. Recently we found that downregulation of LEF-1 transcription factor and its target gene C/EBPa are a common pathologic mechanism for CN. Therefore, we investigated the effects of G-CSF on the phosphorylation status of STAT5a in CN and whether it leads to the down modulation of LEF-1 expression and functions. Indeed, we detected elevated phosphoSTAT5 expression in CD34⁺ cells from CN patients before G-CSF stimulation in vitro, as compared to these cells from healthy individuals. Moreover, treatment with G-CSF resulted in a significantly higher phosphorylation of STAT5a in CN. Intriguingly, levels of phosphoSTAT5 in myeloid blast cells from one CN patient who developed AML was even higher and was in line with undetectable LEF-1 protein expression. Transduction of constitutive active STAT5a (STAT5a 1*6) in CD34⁺ cells from healthy individuals resulted in significant downregulation of LEF1 levels in a dose dependent manner. A screen of 10 kb upstream region of LEF1 gene revealed two putative STAT5 binding sites (−3891bp to −3909bp and −3714bp to −3732bp) and the specificity of this binding was confirmed in the nuclear extracts of CD34⁺ cells by chromatin immunoprecipitation assay. We found enhanced and prolonged STAT5a binding to the LEF-1 promoter in G-CSF treated CD34⁺ cells from CN patients, as compared to healthy individuals. Additionally, transfection of CD34⁺ cells with LEF-1 cDNA resulted in elevation of LEF-1 promoter activity, which suggests a strong LEF-1 autoregulation. Co-transfection with STAT5a 1*6 significantly disrupted LEF-1- dependent activation of LEF-1 promoter. Moreover STAT5a 1*6 severely abrogated the LEF-1 dependent regulation of C/EBPα gene promoter. Taken together phosphorylation of STAT5 is upregulated in hematopoietic progenitors from CN patients which lead to subsequent down regulation of LEF-1. These downstream effects of activated STAT5a may contribute to the malignant transformation of myelopoiesis in CN.