Proto-Oncogenes β- and γ-Catenin in Leukemogenesis in Severe Congenital Neutropenia (CN).

Severe congenital neutropenia (CN) is characterized by a “maturation arrest” of myeloid progenitors at the promyelocytic stage with few or no mature neutrophils in the bone marrow and blood. Administration of granulocyte colony-stimulating factor (G-CSF) increases neutrophil numbers in most CN patients. Approximately 10–15 % of CN patients develop AML or MDS by mechanisms that are as yet unknown. Since AML/MDS are not observed in cyclic (CyN) or idiopathic neutropenia patients treated with G-CSF, an underlying defect of hematopoiesis rather than G-CSF therapy per se predisposes to malignant transformation in CN patients. Recently, activation of Wnt/β-catenin-/γ-catenin-signaling cascade has been considered as important mechanism in the pathogenesis of AML and CML by enhancement of self-renewal activity and by increase of leukemic potential of myeloid progenitors. Moreover, stabilization of β-catenin led to an increased formation of nuclear β-catenin-T-cell factor complexes and altered expression of Wnt-inducible target genes in a variety of human malignancies.

In the present study we investigated the role of β-catenin/γ-catenin in leukemogenesis in CN patients. CD33⁺ progenitors from CN patients expressed 2.5 times higher levels of b-catenin and 4 times higher levels of γ-catenin mRNA and protein, as assessed by quantitative real-time PCR and Western Blot analysis. Most important, in CN patients this increase was paralleled by dramatically elevated levels of activated nuclear β-catenin and intracellular γ-catenin proteins in CD33⁺ cells, as compared to G-CSF-treated healthy controls and CyN patients. Moreover, mRNA and protein levels of β- and γ-catenins were further increased in CD33⁺ cells and leukemic blasts from 4 CN patients, who developed AML. In line with high β-/γ-catenins levels, expression of target genes c-jun, fra-1 and PPARD was also up-regulated. There was no correlation between activated Wnt/β-/γ-catenin signaling system and mutations in G-CSF receptor, or ELA2 gene.

To investigate the mechanisms of stabilization and increased nuclear translocation of b-catenin, we analyzed the components of b-catenin-degradation multiprotein complex, which contains of Axin, GSK3β, and APC. No differences in expression of Axin, GSK3β and APC as well as in phosphorylation status of GSK3β in CD33⁺ cells from CN patients and controls were observed. Sequence analysis revealed no mutations in β-catenin gene. Furthermore we analysed the expression of E-cadherin, which forms the transmembrane core of adherent junctions by bridging to β-catenin and therefore modulates its subcellular localization and nuclear translocation. E-cadherin mRNA and protein expression was dramatically downregulated in CD33⁺ myeloid progenitors from CN patients, in comparison to G-CSF treated healthy controls. Moreover, confocal microscopy revealed very low levels of co-localized E-cadherin and β-catenin in CD33⁺ cells from CN patients. Therefore, we hypothesize that loss of E-cadherin expression results in nuclear accumulation of β-catenin and activation of its downstream signaling in CN. Taken together, high expression of the proto-oncogenes β- and γ-catenins and nuclear accumulation of β-catenin could contribute to the malignant transformation of myelopoiesis in CN.