Absence of mutations in the key megakaryocyte transcriptional regulator FOG-1 in patients with idiopathic myelofibrosis

Proteasome inhibitor bortezomib impairs both myelofibrosis and osteosclerosis induced by high thrombopoietin levels in mice

Primary myelofibrosis (PMF) is the most serious myeloproliferative disorder, characterized by clonal myeloproliferation associated with cytokine-mediated bone marrow stromal reaction including fibrosis and osteosclerosis. Current drug therapy remains mainly palliative. Because the NF-kappaB pathway is implicated in the abnormal release of cytokines in PMF, the proteasome inhibitor bortezomib might be a potential therapy. To test its effect, we used the lethal murine model of myelofibrosis induced by thrombopoietin (TPO) overexpression. In this TPO(high) model, the development of the disease is related to a deregulated MPL signaling, as recently described in PMF patients. We first demonstrated that bortezomib was able to inhibit TPO-induced NF-kappaB activation in vitro in murine megakaryocytes. It also inhibited NF-kappaB activation in vivo in TPO(high) mice leading to decreased IL-1alpha plasma levels. After 4 weeks of treatment, bortezomib decreased TGF-beta1 levels in marrow fluids and impaired marrow and spleen fibrosis development. After 12 weeks of treatment, bortezomib also impaired osteosclerosis development through osteoprotegerin inhibition. Moreover, this drug reduced myeloproliferation induced by high TPO level. Finally, bortezomib dramatically improved TPO(high) mouse survival (89% vs 8% at week 52). We conclude that bortezomib appears as a promising therapy for future treatment of PMF patients.

Pathogenesis of myelofibrosis with myeloid metaplasia

The primary disease process in myelofibrosis with myeloid metaplasia (MMM) is clonal myeloproliferation with varying degrees of phenotypic differentiation. This is characteristically accompanied by secondary intramedullary collagen fibrosis, osteosclerosis, angiogenesis, and extramedullary hematopoiesis. Modern clonality studies have confirmed the multipotent stem-cell origin of the neoplastic process in MMM. The nature of the specific oncogenic mutation(s) is currently being unraveled with the recent discovery of an association between a somatic point mutation of JAK2 tyrosine kinase (V617F) and bcr/abl-negative myeloproliferative disorders, including MMM. The pathogenetic mechanisms that underlie the secondary bone marrow stromal changes in MMM are also incompletely understood. Mouse models of this latter disease aspect have been constructed by either in vivo overexpression of thrombopoietin (TPOhigh mice) or megakaryocyte lineage restricted underexpression of the transcription factor GATA-1 (GATA-1low mice). Gene knockout experiments using such animal models have suggested the essential role of hematopoietic cell-derived transforming growth factor beta1 in inducing bone marrow fibrosis and stromal cell-derived osteoprotegerin in promoting osteosclerosis. However, experimental myelofibrosis in mice does not recapitulate clonal myeloproliferation that is fundamental to human MMM. Other cytokines that are implicated in mediating myelofibrosis and angiogenesis in MMM include basic fibroblast, platelet-derived, and vascular endothelial growth factors. It is currently assumed that such cytokines are abnormally released from clonal megakaryocytes as a result of a pathologic interaction with neutrophils (eg, emperipolesis). This latter phenomenon, through neutrophil-derived elastase, could also underlie the abnormal peripheral-blood egress of myeloid progenitors in MMM.

Idiopathic Myelofibrosis

Idiopathic myelofibrosis (IMF) is characterized by anemia, progressive splenomegaly, bone marrow fibrosis, and extramedullary hematopoiesis. However, patients with a transitional myeloproliferative disorder (MPD), a prefibrotic form of myelofibrosis, or myelofibrosis with a fatty bone marrow share many features of IMF but have clinical characteristics that deviate from the classical description of IMF. A phenomenon that serves as a unique biomarker of IMF is the constitutive mobilization of hematopoietic progenitor cells (HPCs) and/or endothelial progenitor cells (EPCs) from the bone marrow to the peripheral blood and other extramedullary sites. Using such parameters as hemoglobin level, white blood cell count, and number of blasts in the peripheral blood, prognostic scores can be developed by which to base therapeutic decisions. Androgens, recombinant human erythropoietin (rHuEpo), and thalidomide are effective modalities of treatment of the anemia of IMF. Systemic symptoms of excess myeloproliferation are the primary indication for treatment with chemotherapeutic agents. Hydroxyurea is the most commonly used drug. Ablation of the abnormal hematopoietic clone with high-dose chemotherapy and allogeneic stem cell transplantation offers an opportunity to cure patients with IMF. The use of fully myeloablative conditioning regimens, with or without total body irradiation, is associated with a high transplant-related mortality rate (27% to 48%), especially in patients with advanced disease and in the elderly. The use of reduced intensity conditioning (RIC) regimens has resulted in prolonged survival and lower transplant-related mortality.