Relationship between JAK2 V617F Mutation Status, Granulocyte CD177 mRNA Expression and CD177 Soluble Protein Level in Patients with Polycythemia Vera.

A unique gain-of-function mutation of the Janus kinase 2 (JAK2) gene has been recently described in patients with polycythemia vera (PV), essential thrombocythemia and chronic idiopathic myelofibrosis [N Engl J Med. 2005 Apr 28;352(17):1779–90]. Although the currently available data clearly demonstrate that the JAK2 V617F mutation participates in the pathogenesis of myeloproliferative disorders, the mutation’s precise place in the hierarchical order of pathogenetic events remains to be established. We have recently reported that altered gene expression in myeloproliferative disorders correlates with activation of signaling by the V617F mutation of JAK2 (Blood. 2005 Aug 4; Epub ahead of print). Granulocyte CD177 (PRV1) mRNA overexpression has been initially reported as a potential marker of PV but later shown by us to rather be a marker of neutrophil activation [Br J Haematol. 2004 Sep;126(5):650–6]. In this study, we analyzed the relationship between JAK2 V617F mutation status, granulocyte CD177 mRNA expression and CD177 soluble protein level in 72 patients with PV. We also investigated the ontogeny of CD177 expression by hematopoietic cells with the aim of defining the stage of mRNA expression during myeloid, erythroid and megakaryocytic cell differentiation. Finally we studied the effect of soluble CD177 protein on hematopoietic cell proliferation and differentiation. Granulocyte CD177 mRNA expression and percentage of JAK2 V617F alleles were evaluated by quantitative Real Time PCR (qRT-PCR), while serum CD177 protein level was measured by a flow cytometry-based competitive antibody-binding assay. Liquid cultures were performed by culturing peripheral blood mononuclear cells obtained from healthy individuals and PV patients in the presence of high CD177-expressing, low CD177-expressing or CD177-depleted sera. After 12 days of culture, cells were collected, counted and evaluated for colony growth, and for flow cytometry analysis of myeloid, erythroid, megakaryocytic and CD34-positive cell subpopulations. qRT-PCR studies showed a close relationship between CD177 mRNA level and percentage of JAK2 V617F alleles (r=0.412, P<0.001). CD177 mRNA expression was almost undetectable in cell populations other than granulocytes. Studies of CFU-GM growth and differentiation indicated that CD177 mRNA expression is a late event restricted to the neutrophil stage of differentiation. Analysis of serum samples showed variable values for mean fluorescence intensity (MFI), indicating variable levels of the soluble CD177 protein in the patients studied. A very close relationship was found between granulocyte CD177 mRNA expression and soluble CD177 protein level (r=0.56, P=0.02). Incubation of mononuclear cells with serum samples showing high levels of soluble CD177 protein resulted in increased numbers of CD34-positive cells (P<0.02) and of erythroid progenitors (P<0.03). This effect was not detectable when low CD177-expressing or CD177-depleted sera were employed. These observations clearly indicate that the JAK2 V617F mutation is associated with enhanced granulocyte CD177 mRNA expression, and that this latter results in high levels of soluble CD177 protein. These elevated levels might contribute to the increased red cell production that characterizes polycythemia vera.