Transforming growth factor-beta and megakaryocytes in the pathogenesis of idiopathic myelofibrosis

Role of IL8 in myeloid malignancies

Aberrant overexpression of Interleukin-8 (IL8) has been reported in Myelodysplastic Syndromes (MDS), Acute Myeloid Leukemia (AML), Myeloproliferative Neoplasms (MPNs) and other myeloid malignancies. IL8 (CXCL8) is a CXC chemokine that is secreted by aberrant hematopoietic stem and progenitors as well as other cells in the tumor microenvironment. IL8 can bind to CXCR1/CXCR2 receptors and activate oncogenic signaling pathways, and also increase the recruitment of myeloid derived suppressor cells to the tumor microenvironment. IL8/CXCR1/2 overexpression has been associated with poorer prognosis in MDS and AML and increased bone marrow fibrosis in Myelofibrosis. Preclinical studies have demonstrated benefit of inhibiting the IL8/CXCR1/2 pathways via restricting the growth of leukemic stem cells as well as normalizing the immunosuppressive microenvironment in tumors. Targeting the IL8-CXCR1/2 pathway is a potential therapeutic strategy in myeloid neoplasms and is being evaluated with small molecule inhibitors as well as monoclonal antibodies in ongoing clinical trials. We review the role of IL8 signaling pathway in myeloid cancers and discuss future directions on therapeutic targeting of IL8 in these diseases.

SRSF2-P95H decreases JAK/STAT signaling in hematopoietic cells and delays myelofibrosis development in mice

Heterozygous mutation targeting proline 95 in Serine/Arginine-rich Splicing Factor 2 (SRSF2) is associated with V617F mutation in Janus Activated Kinase 2 (JAK2) in some myeloproliferative neoplasms (MPNs), most commonly primary myelofibrosis. To explore the interaction of Srsf2^P95H with Jak2^V617F, we generated Cre-inducible knock-in mice expressing these mutants under control of the stem cell leukemia (Scl) gene promoter. In transplantation experiments, Srsf2^P95H unexpectedly delayed myelofibrosis induced by Jak2^V617F and decreased TGFβ1 serum level. Srsf2^P95H reduced the competitiveness of transplanted Jak2^V617F hematopoietic stem cells while preventing their exhaustion. RNA sequencing of sorted megakaryocytes identified an increased number of splicing events when the two mutations were combined. Focusing on JAK/STAT pathway, Jak2 exon 14 skipping was promoted by Srsf2^P95H, an event detected in patients with JAK2^V617F and SRSF2^P95 co-mutation. The skipping event generates a truncated inactive JAK2 protein. Accordingly, Srsf2^P95H delays myelofibrosis induced by the thrombopoietin receptor agonist Romiplostim in Jak2 wild-type animals. These results unveil JAK2 exon 14 skipping promotion as a strategy to reduce JAK/STAT signaling in pathological conditions.

Megakaryocyte TGFβ1 partitions erythropoiesis into immature progenitor/stem cells and maturing precursors

Erythropoietin (EPO) provides the major survival signal to maturing erythroid precursors (EPs) and is essential for terminal erythropoiesis. Nonetheless, progenitor cells can irreversibly commit to an erythroid fate well before EPO acts, risking inefficiency if these progenitors are unneeded to maintain red blood cell (RBC) counts. We identified a new modular organization of erythropoiesis and, for the first time, demonstrate that the pre-EPO module is coupled to late EPO-dependent erythropoiesis by megakaryocyte (Mk) signals. Disrupting megakaryocytic transforming growth factor β1 (Tgfb1) disorganized hematopoiesis by expanding the pre-EPO pool of progenitor cells and consequently triggering significant apoptosis of EPO-dependent EPs. Similarly, pharmacologic blockade of TGFβ signaling in normal mice boosted the pre-EPO module, leading to apoptosis of EPO-sensitive EPs. Subsequent treatment with low-dose EPO triggered robust RBC production in both models. This work reveals modular regulation of erythropoiesis and offers a new strategy for overcoming chronic anemias.

Focus on Osteosclerotic Progression in Primary Myelofibrosis

Primary myelofibrosis (PMF) is a myeloproliferative neoplasm characterized by hematopoietic stem-cell-derived clonal proliferation, leading to bone marrow (BM) fibrosis. Hematopoiesis alterations are closely associated with modifications of the BM microenvironment, characterized by defective interactions between vascular and endosteal niches. As such, neoangiogenesis, megakaryocytes hyperplasia and extensive bone marrow fibrosis, followed by osteosclerosis and bone damage, are the most relevant consequences of PMF. Moreover, bone tissue deposition, together with progressive fibrosis, represents crucial mechanisms of disabilities in patients. Although the underlying mechanisms of bone damage observed in PMF are still unclear, the involvement of cytokines, growth factors and bone marrow microenvironment resident cells have been linked to disease progression. Herein, we focused on the role of megakaryocytes and their alterations, associated with cytokines and chemokines release, in modulating functions of most of the bone marrow cell populations and in creating a complex network where impaired signaling strongly contributes to progression and disabilities.

Focus on Osteosclerotic Progression in Primary Myelofibrosis

Primary myelofibrosis (PMF) is a myeloproliferative neoplasm characterized by hematopoietic stem-cell-derived clonal proliferation, leading to bone marrow (BM) fibrosis. Hematopoiesis alterations are closely associated with modifications of the BM microenvironment, characterized by defective interactions between vascular and endosteal niches. As such, neoangiogenesis, megakaryocytes hyperplasia and extensive bone marrow fibrosis, followed by osteosclerosis and bone damage, are the most relevant consequences of PMF. Moreover, bone tissue deposition, together with progressive fibrosis, represents crucial mechanisms of disabilities in patients. Although the underlying mechanisms of bone damage observed in PMF are still unclear, the involvement of cytokines, growth factors and bone marrow microenvironment resident cells have been linked to disease progression. Herein, we focused on the role of megakaryocytes and their alterations, associated with cytokines and chemokines release, in modulating functions of most of the bone marrow cell populations and in creating a complex network where impaired signaling strongly contributes to progression and disabilities.

Mathematical modelling as a proof of concept for MPNs as a human inflammation model for cancer development

The chronic Philadelphia-negative myeloproliferative neoplasms (MPNs) are acquired stem cell neoplasms which ultimately may transform to acute myelogenous leukemia. Most recently, chronic inflammation has been described as an important factor for the development and progression of MPNs in the biological continuum from early cancer stage to the advanced myelofibrosis stage, the MPNs being described as "A Human Inflammation Model for Cancer Development". This novel concept has been built upon clinical, experimental, genomic, immunological and not least epidemiological studies. Only a few studies have described the development of MPNs by mathematical models, and none have addressed the role of inflammation for clonal evolution and disease progression. Herein, we aim at using mathematical modelling to substantiate the concept of chronic inflammation as an important trigger and driver of MPNs.The basics of the model describe the proliferation from stem cells to mature cells including mutations of healthy stem cells to become malignant stem cells. We include a simple inflammatory coupling coping with cell death and affecting the basic model beneath. First, we describe the system without feedbacks or regulatory interactions. Next, we introduce inflammatory feedback into the system. Finally, we include other feedbacks and regulatory interactions forming the inflammatory-MPN model. Using mathematical modeling, we add further proof to the concept that chronic inflammation may be both a trigger of clonal evolution and an important driving force for MPN disease progression. Our findings support intervention at the earliest stage of cancer development to target the malignant clone and dampen concomitant inflammation.

Hmga2 collaborates with JAK2V617F in the development of myeloproliferative neoplasms

High-mobility group AT-hook 2 (HMGA2) is crucial for the self-renewal of fetal hematopoietic stem cells (HSCs) but is downregulated in adult HSCs via repression by MIRlet-7 and the polycomb-recessive complex 2 (PRC2) including EZH2. The HMGA2 messenger RNA (mRNA) level is often elevated in patients with myelofibrosis that exhibits an advanced myeloproliferative neoplasm (MPN) subtype, and deletion of Ezh2 promotes the progression of severe myelofibrosis in JAK2^V617F mice with upregulation of several oncogenes such as Hmga2. However, the direct role of HMGA2 in the pathogenesis of MPNs remains unknown. To clarify the impact of HMGA2 on MPNs carrying the driver mutation, we generated ΔHmga2/JAK2^V617F mice overexpressing Hmga2 due to deletion of the 3' untranslated region. Compared with JAK2^V617F mice, ΔHmga2/JAK2^V617F mice exhibited more severe leukocytosis, anemia and splenomegaly, and shortened survival, whereas severity of myelofibrosis was comparable. ΔHmga2/JAK2^V617F cells showed a greater repopulating ability that reproduced the severe MPN compared with JAK2^V617F cells in serial bone marrow transplants, indicating that Hmga2 promotes MPN progression at the HSC level. Hmga2 also enhanced apoptosis of JAK2^V617F erythroblasts that may worsen anemia. Relative to JAK2^V617F hematopoietic stem and progenitor cells (HSPCs), over 30% of genes upregulated in ΔHmga2/JAK2^V617F HSPCs overlapped with those derepressed by Ezh2 loss in JAK2^V617F/Ezh2^Δ/Δ HSPCs, suggesting that Hmga2 may facilitate upregulation of Ezh2 targets. Correspondingly, deletion of Hmga2 ameliorated anemia and splenomegaly in JAK2^V617F/Ezh2^Δ/wild-type mice, and MIRlet-7 suppression and PRC2 mutations correlated with the elevated HMGA2 mRNA levels in patients with MPNs, especially myelofibrosis. These findings suggest the crucial role of HMGA2 in MPN progression.

The role of the extracellular matrix in primary myelofibrosis

Primary myelofibrosis (PMF) is a myeloproliferative neoplasm that arises from clonal proliferation of hematopoietic stem cells and leads to progressive bone marrow (BM) fibrosis. While cellular mutations involved in the development of PMF have been heavily investigated, noteworthy is the important role the extracellular matrix (ECM) plays in the progression of BM fibrosis. This review surveys ECM proteins contributors of PMF, and highlights how better understanding of the control of the ECM within the BM niche may lead to combined therapeutic options in PMF.

Bone marrow fibrosis in primary myelofibrosis: pathogenic mechanisms and the role of TGF-β

Primary myelofibrosis (PMF) is a Philadelphia chromosome negative myeloproliferative neoplasm (MPN) with adverse prognosis and is associated with bone marrow fibrosis and extramedullary hematopoiesis. Even though the discovery of the Janus kinase 2 (JAK2), thrombopoietin receptor (MPL) and calreticulin (CALR) mutations have brought new insights into the complex pathogenesis of MPNs, the etiology of fibrosis is not well understood. Furthermore, since JAK2 inhibitors do not lead to reversal of fibrosis further understanding of the biology of fibrotic process is needed for future therapeutic discovery. Transforming growth factor beta (TGF-β) is implicated as an important cytokine in pathogenesis of bone marrow fibrosis. Various mouse models have been developed and have established the role of TGF-β in the pathogenesis of fibrosis. Understanding the molecular alterations that lead to TGF-β mediated effects on bone marrow microenvironment can uncover newer therapeutic targets against myelofibrosis. Inhibition of the TGF-β pathway in conjunction with other therapies might prove useful in the reversal of bone marrow fibrosis in PMF.

Inflammation as a Keystone of Bone Marrow Stroma Alterations in Primary Myelofibrosis

Primary myelofibrosis (PMF) is a clonal myeloproliferative neoplasm where severity as well as treatment complexity is mainly attributed to a long lasting disease and presence of bone marrow stroma alterations as evidenced by myelofibrosis, neoangiogenesis, and osteosclerosis. While recent understanding of mutations role in hematopoietic cells provides an explanation for pathological myeloproliferation, functional involvement of stromal cells in the disease pathogenesis remains poorly understood. The current dogma is that stromal changes are secondary to the cytokine “storm” produced by the hematopoietic clone cells. However, despite therapies targeting the myeloproliferation-sustaining clones, PMF is still regarded as an incurable disease except for patients, who are successful recipients of allogeneic stem cell transplantation. Although the clinical benefits of these inhibitors have been correlated with a marked reduction in serum proinflammatory cytokines produced by the hematopoietic clones, further demonstrating the importance of inflammation in the pathological process, these treatments do not address the role of the altered bone marrow stroma in the pathological process. In this review, we propose hypotheses suggesting that the stroma is inflammatory-imprinted by clonal hematopoietic cells up to a point where it becomes “independent” of hematopoietic cell stimulation, resulting in an inflammatory vicious circle requiring combined stroma targeted therapies.

Cytokine Regulation of Microenvironmental Cells in Myeloproliferative Neoplasms

The term myeloproliferative neoplasms (MPN) refers to a heterogeneous group of diseases including not only polycythemia vera (PV), essential thrombocythemia (ET), and primary myelofibrosis (PMF), but also chronic myeloid leukemia (CML), and systemic mastocytosis (SM). Despite the clinical and biological differences between these diseases, common pathophysiological mechanisms have been identified in MPN. First, aberrant tyrosine kinase signaling due to somatic mutations in certain driver genes is common to these MPN. Second, alterations of the bone marrow microenvironment are found in all MPN types and have been implicated in the pathogenesis of the diseases. Finally, elevated levels of proinflammatory and microenvironment-regulating cytokines are commonly found in all MPN-variants. In this paper, we review the effects of MPN-related oncogenes on cytokine expression and release and describe common as well as distinct pathogenetic mechanisms underlying microenvironmental changes in various MPN. Furthermore, targeting of the microenvironment in MPN is discussed. Such novel therapies may enhance the efficacy and may overcome resistance to established tyrosine kinase inhibitor treatment in these patients. Nevertheless, additional basic studies on the complex interplay of neoplastic and stromal cells are required in order to optimize targeting strategies and to translate these concepts into clinical application.

Fibrogenesis in Primary Myelofibrosis: Diagnostic, Clinical, and Therapeutic Implications

Primary myelofibrosis is a stem cell-derived clonal malignancy characterized by unchecked proliferation of myeloid cells, resulting in bone marrow fibrosis, osteosclerosis, and pathologic angiogenesis. Bone marrow fibrosis (BMF) plays a central role in the pathophysiology of the disease. This review describes current issues regarding BMF in primary myelofibrosis, including the pathophysiology and impact of abnormal deposition of excess collagen and reticulin fibers in bone marrow spaces, the modified Bauermeister and the European Consensus grading systems of BMF, and the prognostic impact of BMF on the overall outcome of patients with primary myelofibrosis. The impact of novel therapeutic strategies, including JAK-STAT inhibitors and allogeneic stem cell transplant, on BMF is discussed.

The role of growth differentiation factor 15 in the pathogenesis of primary myelofibrosis

Growth differentiation factor 15 (GDF15) is a pleiotropic cytokine that belongs to the transforming growth factor-β superfamily. Elevated serum concentrations of this cytokine have been reported in patients with various malignancies. To assess the potential roles of GDF15 in hematologic malignancies, we measured its serum levels in patients with these diseases. We found that serum GDF15 levels were elevated in almost all these patients, particularly in patients with primary myelofibrosis (PMF). Immunohistochemical staining of bone marrow (BM) specimens revealed that GDF15 was strongly expressed by megakaryocytes, which may be sources of increased serum GDF15 in PMF patients. Therefore, we further assessed the contribution of GDF15 to the pathogenesis of PMF. Recombinant human (rh) GDF15 enhanced the growth of human BM mesenchymal stromal cells (BM-MSCs), and it enhanced the potential of these cells to support human hematopoietic progenitor cell growth in a co-culture system. rhGDF15 enhanced the growth of human primary fibroblasts, but it did not affect their expression of profibrotic genes. rhGDF15 induced osteoblastic differentiation of BM-MSCs in vitro, and pretreatment of BM-MSCs with rGDF15 enhanced the induction of bone formation in a xenograft mouse model. These results suggest that serum levels of GDF15 in PMF are elevated, that megakaryocytes are sources of this cytokine in BM, and that GDF15 may modulate the pathogenesis of PMF by enhancing proliferation and promoting osteogenic differentiation of BM-MSCs.

Depletion of STAT5 blocks TEL-SYK-induced APMF-type leukemia with myelofibrosis and myelodysplasia in mice

The spleen tyrosine kinase (SYK) was identified as an oncogenic driver in a broad spectrum of hematologic malignancies. The in vivo comparison of three SYK containing oncogenes, SYK(wt), TEL-SYK and IL-2-inducible T-cell kinase (ITK)-SYK revealed a general myeloexpansion and the establishment of three different hematologic (pre)diseases. SYK(wt) enhanced the myeloid and T-cell compartment, without leukemia/lymphoma development. ITK-SYK caused lethal T-cell lymphomas and the cytoplasmic TEL-SYK fusion induced an acute panmyelosis with myelofibrosis-type acute myeloid leukemia (AML) with up to 50% immature megakaryoblasts infiltrating bone marrow, spleen and liver, additional MPN features (myelofibrosis and granulocyte expansion) and MDS stigmata with megakaryocytic and erythroid dysplasia. LKS cells were reduced and all subsets (LT/ST/MPP) showed reduced proliferation rates. SYK inhibitor treatment (R788) of diseased TEL-SYK mice reduced leukocytosis, spleen and liver infiltration, enhanced the hematocrit and prolonged survival time, but could not significantly reduce myelofibrosis. Stat5 was identified as a major downstream mediator of TEL-SYK in vitro as well as in vivo. Consequently, targeted deletion of Stat5 in vivo completely abrogated TEL-SYK-induced AML and myelofibrosis development, proving Stat5 as a major driver of SYK-induced transformation. Our experiments highlight the important role of SYK in AML and myelofibrosis and prove SYK and STAT5 inhibitors as potent treatment options for those diseases.

Signal transduction in the chronic leukemias: implications for targeted therapies

The chronic leukemias, including chronic myeloid leukemia (CML), the Philadelphia-negative myeloproliferative neoplasms (MPNs), and chronic lymphocytic leukemia (CLL), have been characterized extensively for abnormalities of cellular signaling pathways. This effort has led to the elucidation of the central role of dysregulated tyrosine kinase signaling in the chronic myeloid neoplasms and of constitutive B-cell receptor signaling in CLL. This, in turn, has stimulated the development of small molecule inhibitors of these signaling pathways for therapy of chronic leukemia. Although the field is still in its infancy, the clinical results with these agents have ranged from encouraging (CLL) to spectacular (CML). In this review, we summarize recent studies that have helped to define the signaling pathways critical to the pathogenesis of the chronic leukemias. We also discuss correlative studies emerging from clinical trials of drugs targeting these pathways.

Characterization of the TGF-β1 signaling abnormalities in the Gata1low mouse model of myelofibrosis

Primary myelofibrosis (PMF) is characterized by fibrosis, ineffective hematopoiesis in marrow, and hematopoiesis in extramedullary sites and is associated with abnormal megakaryocyte (MK) development and increased transforming growth factor (TGF)-β1 release. To clarify the role of TGF-β1 in the pathogenesis of this disease, the TGF-β1 signaling pathway of marrow and spleen of the Gata1(low) mouse model of myelofibrosis (MF) was profiled and the consequences of inhibition of TGF-β1 signaling on disease manifestations determined. The expression of 20 genes in marrow and 36 genes in spleen of Gata1(low) mice was altered. David-pathway analyses identified alterations of TGF-β1, Hedgehog, and p53 signaling in marrow and spleen and of mammalian target of rapamycin (mTOR) in spleen only and predicted that these alterations would induce consequences consistent with the Gata1(low) phenotype (increased apoptosis and G1 arrest both in marrow and spleen and increased osteoblast differentiation and reduced ubiquitin-mediated proteolysis in marrow only). Inhibition of TGF-β1 signaling normalized the expression of p53-related genes, restoring hematopoiesis and MK development and reducing fibrosis, neovascularization, and osteogenesis in marrow. It also normalized p53/mTOR/Hedgehog-related genes in spleen, reducing extramedullary hematopoiesis. These data identify altered expression signatures of TGF-β1 signaling that may be responsible for MF in Gata1(low) mice and may represent additional targets for therapeutic intervention in PMF.

Spleens of myelofibrosis patients contain malignant hematopoietic stem cells

Cancer stem cell behavior is thought to be largely determined by intrinsic properties and by regulatory signals provided by the microenvironment. Myelofibrosis (MF) is characterized by hematopoiesis occurring not only in the marrow but also in extramedullary sites such as the spleen. In order to study the effects of these different microenvironments on primitive malignant hematopoietic cells, we phenotypically and functionally characterized splenic and peripheral blood (PB) MF CD34+ cells from patients with MF. MF spleens contained greater numbers of malignant primitive HPCs than PB. Transplantation of PB MF CD34+ cells into immunodeficient (NOD/SCID/IL2Rγ(null)) mice resulted in a limited degree of donor cell chimerism and a differentiation program skewed toward myeloid lineages. By contrast, transplanted splenic MF CD34+ cells achieved a higher level of chimerism and generated both myeloid and lymphoid cells that contained molecular or cytogenetic abnormalities indicating their malignant nature. Only splenic MF CD34+ cells were able to sustain hematopoiesis for prolonged periods (9 months) and were able to engraft secondary recipients. These data document the existence of MF stem cells (MF-SCs) that reside in the spleens of MF patients and demonstrate that these MF-SCs retain a differentiation program identical to that of normal hematopoietic stem cells.

Synergism between fibronectin and transforming growth factor-β1 in the production of substance P in monocytes of patients with myelofibrosis

Substance P (SP), also considered a proinflammatory cytokine, as well as others such as transforming growth factor-β1 (TGF-β1) and interleukin-1 (IL-1), and the extracellular matrix protein fibronectin (FN) have been associated with the pathophysiology of myelofibrosis. SP is encoded by the TAC1 gene. The relationships among SP, TGF-β1, IL-1 and FN are poorly understood. This study determined the mechanisms for concomitant production of IL-1, TGF-β1 and SP and also determined the synergistic role of FN in SP release. Enzyme-linked immunosorbent assay (ELISA) indicated increased levels of SP and TGF-β1 in the blood of patients with myelofibrosis. Monocytes, shown to be activated in patients with bone marrow (BM) fibrosis, expressed the TAC1 gene for SP release, in a nuclear factor-κB (NFκB)-dependent manner. Reporter gene assay with the 5' regulatory region of TAC1 indicated its expression by high levels of FN and TGF-β1. Immunohistochemical studies of paraffin-embedded BM biopsies from patients with myelofibrosis, and age-matched controls without fibrosis, indicated co-localization of SP and its receptor neurokinin-1 (NK1). In summary, myelofibrotic monocytes have autocrine loops that stimulate the release of SP and TGF-β1, and that are potentiated by fibronectin. The FN-mediated induction of SP in turn stimulates monocytes through autostimulation by NK1 receptors. These findings, combined with those of previous studies, demonstrate an adhesion-mediated NFκB/IL-1/TGF-β1 axis that can be initiated by increased FN in patients with myelofibrosis for the production of SP. These findings show how TGF-β1 and SP production are coupled, and suggest new therapeutic targets to reverse immune-mediated fibrosis.

Lethal myelofibrosis induced by Bmi1-deficient hematopoietic cells unveils a tumor suppressor function of the polycomb group genes

Polycomb-group (PcG) proteins form the multiprotein polycomb repressive complexes (PRC) 1 and 2, and function as transcriptional repressors through histone modifications. They maintain the proliferative capacity of hematopoietic stem and progenitor cells by repressing the transcription of tumor suppressor genes, namely Ink4a and Arf, and thus have been characterized as oncogenes. However, the identification of inactivating mutations in the PcG gene, EZH2, unveiled a tumor suppressor function in myeloid malignancies, including primary myelofibrosis (PMF). Here, we show that loss of another PcG gene, Bmi1, causes pathological hematopoiesis similar to PMF. In a mouse model, loss of Bmi1 in Ink4a-Arf(-/-) hematopoietic cells induced abnormal megakaryocytopoiesis accompanied by marked extramedullary hematopoiesis, which eventually resulted in lethal myelofibrosis. Absence of Bmi1 caused derepression of a cohort of genes, including Hmga2, which is an oncogene overexpressed in PMF. Chromatin immunoprecipitation assays revealed that Bmi1 directly represses the transcription of Hmga2. Overexpression of Hmga2 in hematopoietic stem cells induced a myeloproliferative state with enhanced megakaryocytopoiesis in mice, implicating Hmga2 in the development of pathological hematopoiesis in the absence of Bmi1. Our findings provide the first genetic evidence of a tumor suppressor function of Bmi1 and uncover the role of PcG proteins in restricting growth by silencing oncogenes.

The kinetics of mRNA transforming growth factor beta1 expression and its serum concentration in graft-versus-host disease after allogeneic hemopoietic stem cell transplantation for myeloid leukemias

Background

Graft-versus-host disease (GVHD) is still a major complication following allogeneic hematopoietic stem cell transplantation (alloHSCT). Recent data indicates that transforming growth factor beta1 (TGF-β1) may play a role in development of GVH reaction.

Material/Methods

Forty patients with acute myeloid leukemia (AML) and chronic myeloid leukemia (CML) were included. Quantitative real time polymerase chain reaction (RT-qPCR) was performed to assess the expression of mRNA TGF-β1. TGF-β1 serum concentration was assessed using a commercial ELISA.

Results

In all patients, a prompt decrease in TGF-β1 mRNA expression and its serum concentration was demonstrated after conditioning. In patients with acute GVHD, TGF-β1 mRNA expression and its serum concentration remained low until day +30 after transplant as compared to the day of transplant (p<0.03 and p<0.006, respectively). TGF-β1 mRNA expression and its serum concentration significantly increased on day +100 in patients who developed chronic GVHD as compared to the day of transplant (p<0.0009 and p<0.02, respectively).

Conclusions

TGF-β1 seems to be an additional regulator of donor engraftment; its low levels probably being one of the factors contributing to the development of acute GVHD. On the other hand, chronic GVHD symptoms seem to correlate with high TGF-β1 mRNA expression and its serum concentration in patients who underwent bone marrow transplantation for myeloid leukemias. Nevertheless, further studies with greater numbers of patients are needed to establish the role of TGF-β1 in graft-versus-host disease pathophysiology.

Osteomyelosclerosis, anemia and extramedullary hematopoiesis in mice lacking the transcription factor NFATc2

BACKGROUND

Nuclear factors of activated T cells (NFAT) are transcription factors that are central to cytokine production in activated T cells and regulate the development and differentiation of various tissues. NFATc2 is expressed in hematopoietic stem cells and regulated during myeloid commitment in a lineage-specific manner. The biological role of NFATc2 in hematopoiesis is, however, unclear.

DESIGN AND METHODS

In the present study, we analyzed steady-state hematopoiesis in young (<3 months) and old (>12 months) mice lacking NFATc2. Complete blood counts were performed in the peripheral blood, bone marrow and spleen. Using cytological and histological analyses, the blood cell differential was determined. Colony-formation assays were used to determine the differentiation potential of hematopoietic cells. Bone cell cultures were derived from the bone marrow, and bone remodeling markers were determined in the serum.

RESULTS

NFATc2(-/-) mice older than 12 months were anemic and thrombocytopenic. The bone marrows of these mice showed a markedly reduced number of hematopoietic cells, of which megakaryocytic and erythroid lineages were most affected. While the number of hematopoietic progenitor cells in NFATc2-deficent bone marrow was reduced, the myeloid differentiation potential of these cells remained intact. Aged NFATc2(-/-) mice showed ossification of their bone marrow space and developed extramedullary hematopoiesis in the spleen. Ex vivo differentiation assays revealed an intrinsic defect of NFATc2-deficient stromal cells, in which NFATc2(-/-) osteoblasts differentiated more efficiently than wild-type cells, whereas osteoclast differentiation was impaired.

CONCLUSIONS

Our data suggest that NFATc2 may play a role in the maintenance of steady-state hematopoiesis and bone remodeling in adult organisms.

Myeloproliferative neoplasms: new translational therapies

The myeloproliferative neoplasms represent a diverse group of hematologic malignancies that have been the subject of intense investigation over the last decade. Although clinical trials of the much anticipated small molecule inhibitors of Janus kinase 2 have shown that these experimental agents are successful in palliating many of the symptoms associated with the myeloproliferative neoplasms, they have not been reported to affect the disease initiating hematopoietic stem cell population or to alter the natural history of these disorders. Investigators remain optimistic that new information about the genetic and cellular origins gained from the efforts of numerous laboratories will ultimately translate in to the identification of new drug targets and more effective therapies. We hypothesize that ultimately, the use of combinations of drugs including chromatin modifying agents, immunomodulatory agents, anti-apoptotic agents, cellular therapies and monoclonal antibodies will be required to effectively treat patients with myeloproliferative neoplasms.

Direct activation of STAT5 by ETV6-LYN fusion protein promotes induction of myeloproliferative neoplasm with myelofibrosis

Myeloproliferative neoplasms (MPN), a group of haematopoietic stem cell (HSC) disorders, are often accompanied by myelofibrosis. We previously identified the fusion of the ETV6 gene to the LYN gene (ETV6-LYN) in idiopathic myelofibrosis with ins(12;8)(p13;q11q21). The introduction of ETV6-LYN into HSCs resulted in fatal MPN with massive myelofibrosis in mice, implicating the rearranged LYN kinase in the pathogenesis of MPN with myelofibrosis. However, the signalling molecules directly downstream from and activated by ETV6-LYN remain unknown. In this study, we demonstrated that the direct activation of STAT5 by ETV6-LYN is crucial for the development of MPN. ETV6-LYN was constitutively active as a kinase through autophosphorylation. ETV6-LYN, but not its kinase-dead mutant, supported cytokine-free proliferation of haematopoietic cells. STAT5 was activated in a JAK2-independent manner in ETV6-LYN-expressing cells. ETV6-LYN interacted with STAT5 and directly activated STAT5 both in vitro and in vivo. Of note, ETV6-LYN did not support the formation of colonies by Stat5-deficient HSCs under cytokine-free conditions and the capacity of ETV6-LYN to induce MPN with myelofibrosis was profoundly attenuated in a Stat5-null background. These findings define STAT5 as a direct target of ETV6-LYN and unveil the LYN-STAT5 axis as a novel pathway to augment proliferative signals in MPN and leukaemia.

FLT3-mediated p38-MAPK activation participates in the control of megakaryopoiesis in primary myelofibrosis

Primary myelofibrosis (PMF) is characterized by increased number of hematopoietic progenitors and a dysmegakaryopoiesis which supports the stromal reaction defining this disease. We showed that increased ligand (FL) levels in plasma, hematopoietic progenitors, and stromal cells from PMF patients were associated with upregulation of the cognate Flt3 receptor on megakaryocytic (MK) cells. This connection prompted us to study a functional role for the FL/Flt3 couple in PMF dysmegakaryopoiesis, as a route to reveal insights into pathobiology and therapy in this disease. Analysis of PMF CD34(+) and MK cell transcriptomes revealed deregulation of the mitogen-activated protein kinase (MAPK) pathway along with Flt3 expression. In PMF patients, a higher proportion of circulating Flt3(+)CD34(+)CD41(+) cells exhibited an increased MAPK effector phosphorylation independently of Jak2(V617F) mutation. Activation of FL/Flt3 axis in PMF MK cell cultures, in response to FL, induced activation of the p38-MAPK cascade, which is known to be involved in inflammation, also increasing expression of its target genes (NFATC4, p53, AP-1, IL-8). Inhibiting Flt3 or MAPK or especially p38 by chemical, antibody, or silencing strategies restored megakaryopoiesis and reduced phosphorylation of Flt3 and p38 pathway effectors, confirming the involvement of Flt3 in PMF dysmegakaryopoiesis via p38 activation. In addition, in contrast to healthy donors, MK cells derived from PMF CD34(+) cells exhibited an FL-induced migration that could be reversed by p38 inhibition. Taken together, our results implicate the FL/Flt3 ligand-receptor complex in PMF dysmegakaryopoiesis through persistent p38-MAPK activation, with implications for therapeutic prospects to correct altered megakaryopoiesis in an inflammatory context.

Proteolytic matrix metallopeptidases and inhibitors in BCR-ABL1-negative myeloproliferative neoplasms: correlation with JAK2 mutation status

BACKGROUND/AIMS

Philadelphia chromosome-negative myeloproliferative neoplasms (MPNs) share the same acquired lesion JAK2(V617F) and may exhibit substantial overlap. Variability in JAK activation and allele burden, complemented by host, genetic and non-genetic modifiers, determine the phenotype. The aim of this study was to investigate the presence of the JAK2 mutation in association with the ratio of metallopeptidases inhibitors (TIMPs) to tissue metallopeptidases (MMPs) in MPNs, where inhibitory rather than proteolytic activity in marrow microenvironment appears to predominate.

METHODS

94 patients with polycythemia vera, essential thrombocythemia and primary myelofibrosis, and 102 healthy individuals were evaluated. Allele-specific PCR and RFLP were used to detect JAK2 and genomic status. Serum concentrations of MMP and TIMP were measured by ELISA. The parameters were assessed with covariance analysis, and adjusted for gender, age and co-morbidity.

RESULTS

Mutation frequency was 81.91%. Abnormal TIMP/MMP ratios were identified in all three diseases. JAK2 mutation was correlated with significant changes in TIMP concentrations.

CONCLUSIONS

Identification of an abnormal TIMP/MMP ratio in all three diseases, regardless of the JAK2 status, indicates invariable marrow remodeling. In this particular group of patients, presence of a JAK2(V617F) mutation, being associated with even higher ratios, appears to be a concurring participant in bone marrow-reforming processes. Additional research may delineate correlates with the JAK2 allelic burden.

Circulating levels of MCP-1, sIL-2R, IL-15, and IL-8 predict anemia response to pomalidomide therapy in myelofibrosis

Cytokine-phenotype associations have recently been described in primary myelofibrosis and increased levels of IL-8, sIL-2R, IL-12, and IL-15 were found to be independently predictive of inferior survival. Pomalidomide therapy is effective for alleviating anemia in myelofibrosis; we examined the relationship between plasma cytokine/chemokine levels and response to treatment with pomalidomide. The study population included 32 Mayo Clinic patients (median age 66 years) who participated in two consecutive clinical trials of pomalidomide therapy for myelofibrosis-associated anemia. Ten (31%) patients achieved anemia response per International Working Group criteria. Anemia response was seen only in the presence of JAK2V617F (P = 0.04) and, in addition, predicted by lower circulating levels of MCP-1 (P = 0.003), IL-2R (P = 0.008), IL-15 (0.01), and IL-8 (P = 0.02). Marked splenomegaly and increased serum LDH level were associated with poor response (P = 0.02 and 0.03, respectively) and with each other (P = 0.02), but not with JAK2V617F. The aforementioned cytokines were not significantly associated with JAK2V617F but increased levels of sIL-2R (P = 0.01), IL-15 (P = 0.06), and MCP-1 (P = 0.07) clustered with marked splenomegaly. Current data suggest that, in the context of pomalidomide treatment, response is more likely in the presence of JAK2V617F and further predicted by the absence of marked splenomegaly or increased levels of proinflammatory cytokines.

Circulating interleukin (IL)-8, IL-2R, IL-12, and IL-15 levels are independently prognostic in primary myelofibrosis: a comprehensive cytokine profiling study

PURPOSE

Abnormal cytokine expression accompanies myelofibrosis and might be a therapeutic target for Janus-associated kinase (JAK) inhibitor drugs. This study describes the spectrum of plasma cytokine abnormalities in primary myelofibrosis (PMF) and examines their phenotypic correlates and prognostic significance.

PATIENTS AND METHODS

Patients included in this study were required to have archived plasma, bone marrow biopsy, and cytogenetic information available at the time of first referral to the Mayo Clinic. Multiplex biometric sandwich immunoassay was used to measure plasma levels of 30 cytokines.

RESULTS

In total, 127 PMF patients were studied; comparison with normal controls (n = 35) revealed significantly increased interleukin-1β (IL-1β), IL-1RA, IL-2R, IL-6, IL-8, IL-10, IL-12, IL-13, IL-15, tumor necrosis factor α (TNF-α), granulocyte colony-stimulating factor (G-CSF), interferon alfa (IFN-α), macrophage inflammatory protein 1α (MIP-1α), MIP-1β, hepatocyte growth factor (HGF), IFN-γ-inducible protein 10 (IP-10), monokine induced by IFN-γ (MIG), monocyte chemotactic protein 1 (MCP-1), and vascular endothelial growth factor (VEGF) levels and decreased IFN-γ levels. In treatment-naive patients (n = 90), increased levels of IL-8 (P < .001), IL-2R (P < .001), IL-12 (P < .001), IL-15 (P = .001), and IP-10 (P = .003) were independently predictive of inferior survival. A similar multivariable analysis that included all 127 study patients confirmed the prognostic value of these five cytokines, and IL-8, IL-2R, IL-12, and IL-15 remained significant when risk stratification, according to the recently revised Dynamic International Prognostic Scoring System (DIPSS plus), was added to the multivariable model. Leukemia-free survival was predicted by IL-8, which was also the only cytokine associated with ≥ 1% circulating blasts. Other cytokine-phenotype associations included increased IL-8 and constitutional symptoms; IL-2R, IL-12, and transfusion need; IL-2R, IL-8, and leukocytosis; IP-10 and thrombocytopenia; HGF, MIG, IL-1RA, and marked splenomegaly; and IL-1RA, IL-2R, IP-10, MIP-1β, and JAK2V617F. A two-cytokine (IL-8/IL-2R) -based risk categorization delineated prognostically different groups within specific DIPSS plus risk categories.

CONCLUSION

This study signifies the presence of specific cytokine-phenotype associations in PMF and a prognostically relevant plasma cytokine signature that might prove useful as a laboratory tool for predicting and monitoring treatment response.

Phase II study of sunitinib in patients with primary or post-polycythemia vera/essential thrombocythemia myelofibrosis

BACKGROUND

Changes in the bone marrow microenvironment in myelofibrosis are triggered by a cytokine burst and consist of fibrosis, osteosclerosis, and angiogenesis. Sunitinib is a multitargeted small-molecule inhibitor of the receptor tyrosine kinases involved in cell proliferation and angiogenesis, including vascular endothelial growth factor receptors.

PATIENTS AND METHODS

Fourteen patients with myelofibrosis were treated with sunitinib at a daily continuous dose of 37.5 mg orally. The median duration of sunitinib treatment was 5.2 months (range, 1-18 months).

RESULTS

One patient (7%) showed a clinical improvement of anemia (increase in hemoglobin of 4 g/dL), with improvement in anemia-associated symptoms. The time to response was 6 months, and the benefit was sustained for 12 months. However, 8 patients (57%) experienced a total of 13 incidents of significant (grade 3-4) adverse events possibly related to sunitinib (fatigue, gastrointestinal disturbances, anemia, leukopenia, and thrombocytopenia were the most common). In 7 patients (50%), sunitinib was held, and subsequently the dose was reduced to 25 mg daily. Overall, 29% of patients withdrew from the study because of toxicity.

CONCLUSION

Sunitinib therapy, as applied here, was not well-tolerated by patients with myelofibrosis, and the benefits were minimal. Our experience with sunitinib combined with previous experience with other antiangiogenic medications suggest that this class of drugs may have limited usefulness in myelofibrosis when used as a single-agent therapy.

Angiogenesis and vascular endothelial growth factor-/receptor expression in myeloproliferative neoplasms: correlation with clinical parameters and JAK2-V617F mutational status

Data on angiogenesis in the bone marrow of BCR-ABL1-negative myeloproliferative neoplasm (MPN) patients suggest an increase of the microvessel density (MVD) and vascular endothelial growth factor (VEGF) expression, but relations to the JAK2-V617F status remain controversial. We performed immunohistochemical studies of MVD and VEGF-expression in 100 MPN, including 24 essential thrombocythemia- (ET), 46 polycythemia vera- (PV), 26 primary myelofibrosis- (PMF), four myelodysplastic (MDS)/MPN- and 20 control reactive bone marrow cases, and correlated these findings with biological and clinical key data and the JAK2-V617F status. We found significantly increased MVD, particularly that assessed by CD105, and VEGF expression in MPN compared to controls (PMF > PV > MDS/MPN > ET). We observed stronger association between CD105-MVD and VEGF expression, fibrosis, and JAK2-V617F mutant allele burden, compared to CD34-MVD. MVD was strongly increased in MPN with high JAK2-V617F mutant allele burden. Our study highlights the importance of newly formed CD105+ vessels in the bone marrow of MPN patients, and indicates that assessment of CD105-MVD better reflects angiogenic activity in MPN. In addition, it provides evidence that despite the fact that angiogenesis is generally independent of the JAK2-V617F status in MPN, new vessel formation might be linked to Jak2 effects in some cases with high JAK2-V617F mutant allele burden.

Expression profiling of apoptosis-related genes in megakaryocytes: BNIP3 is downregulated in primary myelofibrosis

OBJECTIVE

In order to identify factors involved in the aberrantly regulated apoptosis of megakaryocytes in primary myelofibrosis (PMF), the mRNA expression of human megakaryocytes in situ was quantified by real-time polymerase chain reaction low-density arrays.

MATERIALS AND METHODS

The mRNA from 200 to 300 laser-microdissected megakaryocytes per case from PMF (n=22) and control (n=10) bone marrow was reverse-transcribed into cDNA by random priming and subsequently amplified by primer-specific cDNA amplification. The mRNA of corresponding total bone marrow cells was reverse-transcribed into cDNA without the following amplification. For relative mRNA quantification, custom-made TaqMan low-density arrays with a setup of 48 different genes were applied. In addition, methylation analysis and immunohistochemistry of a selected candidate gene were accomplished.

RESULTS

A trend toward an overall downregulation of apoptosis-associated genes could be observed in megakaryocytes, whereas the total bone marrow cellularity exhibited an overall upregulation of these factors. Among several candidates with statistically significant deregulation BCL2/adenovirus E1B 19 kDa interacting protein 3 (BNIP3) and protein kinase C beta1 were shown to be the most aberrantly expressed genes.

CONCLUSION

Apoptosis-related gene expression profiling of human megakaryocytes reveals a set of candidates, most notably BNIP3, indicating that the increase of megakaryocytes in myeloproliferative neoplasia might not only be the result of increased proliferation but also of disturbed apoptosis.

Does primary myelofibrosis involve a defective stem cell niche? From concept to evidence

Primary myelofibrosis (PMF) is the rarest and the most severe Philadelphia-negative chronic myeloproliferative syndrome. By associating a clonal proliferation and a mobilization of hematopoietic stem cells from bone marrow to spleen with profound alterations of the stroma, PMF is a remarkable model in which deregulation of the stem cell niche is of utmost importance for the disease development. This paper reviews key data suggesting that an imbalance between endosteal and vascular niches participates in the development of clonal stem cell proliferation. Mechanisms by which bone marrow niches are altered with ensuing mobilization and homing of neoplastic hematopoietic stem cells in new or reinitialized niches in the spleen and liver are examined. Differences between signals delivered by both endosteal and vascular niches in the bone marrow and spleen of patients as well as the responsiveness of PMF stem cells to their specific signals are discussed. A proposal for integrating a potential role for the JAK2 mutation in their altered sensitivity is made. A better understanding of the cross talk between stem cells and their niche should imply new therapeutic strategies targeting not only intrinsic defects in stem cell signaling but also regulatory hematopoietic niche-derived signals and, consequently, stem cell proliferation.

The hematopoietic stem cell compartment of JAK2V617F-positive myeloproliferative disorders is a reflection of disease heterogeneity

The JAK2V617F somatic point mutation has been described in patients with myeloproliferative disorders (MPDs). Despite this progress, it remains unknown how a single JAK2 mutation causes 3 different MPD phenotypes, polycythemia vera (PV), essential thrombocythemia, and primitive myelofibrosis (PMF). Using an in vivo xenotransplantation assay in nonobese diabetic-severe combined immunodeficient (NOD/SCID) mice, we tested whether disease heterogeneity was associated with quantitative or qualitative differences in the hematopoietic stem cell (HSC) compartment. We show that the HSC compartment of PV and PMF patients contains JAK2V617F-positive long-term, multipotent, and self-renewing cells. However, the proportion of JAK2V617F and JAK2 wild-type SCID repopulating cells was dramatically different in these diseases, without major modifications of the self-renewal and proliferation capacities for JAK2V617F SCID repopulating cells. These experiments provide new insights into the pathogenesis of JAK2V617F MPD and demonstrate that a JAK2 inhibitor needs to target the HSC compartment for optimal disease control in classical MPD.

Hypermethylation of CXCR4 promoter in CD34+ cells from patients with primary myelofibrosis

Constitutive mobilization of CD34(+) cells in patients with primary myelofibrosis (PMF) has been attributed to proteolytic disruption of the CXCR4/SDF-1 axis and reduced CXCR4 expression. We document here that the number of circulating CD34(+)/CXCR4(+) cells in PMF patients, as well as the cellular CXCR4 expression, was directly related to CXCR4 mRNA level and that reduced CXCR4 mRNA level was not due to SDF-1-induced downregulation. To address whether epigenetic regulation contributes to defective CXCR4 expression, we studied the methylation status of the CXCR4 promoter using methylation-specific polymerase chain reaction and methylation-specific sequencing in the JAK2V617F-positive HEL cell line and in CD34(+) cells. We found that CD34(+) cells from PMF patients, unlike those from normal subjects, presented hypermethylation of CXCR4 promoter CpG island 1. Following incubation with the demethylating agent 5-Aza-2'-deoxycytidine (5-AzaD), the percentage of PMF CD34(+) cells expressing CXCR4 increased 3-10 times, whereas CXCR4 mRNA level increased approximately 4 times. 5-AzaD-treated PMF CD34(+) cells displayed almost complete reversal of CpG1 island 1 hypermethylation and showed enhanced migration in vitro in response to SDF-1. These data point to abnormal methylation of the CXCR4 promoter as a mechanism contributing to constitutive migration of CD34(+) cells in PMF. Disclosure of potential conflicts of interest is found at the end of this article.

Periostin and bone marrow fibrosis

Periostin is a secreted protein that shares structural homology with the insect axon guidance protein fasciclin 1. Periostin is expressed predominantly in collagen-rich fibrous connective tissues that are subjected to constant mechanical stresses. We have shown previously that periostin is a novel component of subepithelial fibrosis in bronchial asthma. Here, we investigated the relationship between periostin and bone marrow (BM) fibrosis. Periostin was expressed in the stroma and stromal cells of BM fibrosis specimens and to a great extent its expression levels correlated closely to the grade of fibrosis, as estimated by silver staining. However, in the present study, we found no relationship between plasma periostin levels and the extent of BM fibrosis. We also demonstrated that periostin is secreted by human BM hTERT stromal cells and that its secretion is enhanced by TGF-beta, a cytokine produced by clonal proliferation of megakaryocytes and/or monocytes. These results indicate that periostin is a component of BM fibrosis and that it may play a role in the disease progression.

Bone morphogenetic proteins are overexpressed in the bone marrow of primary myelofibrosis and are apparently induced by fibrogenic cytokines

Primary myelofibrosis (PMF) is a myeloproliferative neoplasia characterized by progressive deposition of extracellular matrix components in the bone marrow. The involvement of members of the bone morphogenetic protein (BMP) family in aberrant bone marrow matrix homeostasis in PMF has not yet been investigated. Therefore, we analyzed expression of BMP1, an activator of latent transforming growth factor beta-1 (TGFbeta-1) and processor of collagen precursors, and other BMPs in bone marrow from PMF patients and controls (n = 95). Expression of BMP1, BMP6, BMP7, and BMP-receptor 2 was significantly increased in advanced stages of myelofibrosis compared with controls (P < or = 0.01), and enhanced levels of BMP6 expression were already evident in prefibrotic stages of PMF. Immunohistochemistry showed that bone marrow stromal cells and megakaryocytes were the major cellular sources of BMP1 protein. Because TGFbeta-1 and basic fibroblast growth factor have been shown to be important in the development of myelofibrosis, we studied the induction of BMPs by these cytokines in cultured fibroblasts. Fibroblasts treated with TGFbeta-1 showed a pronounced up-regulation of BMP6, suggesting that stromal cells may be susceptible to BMP activation by cytokines with a proven role in the pathogenesis of PMF. We conclude that BMP family members may play an important role in the pathogenesis of myelofibrosis in PMF and are apparently induced by cytokines such as TGFbeta-1.

Bone marrow fibrosis: pathophysiology and clinical significance of increased bone marrow stromal fibres

In bone marrow biopsies, stromal structural fibres are detected by reticulin and trichrome stains, routine stains performed on bone marrow biopsy specimens in diagnostic laboratories. Increased reticulin staining (reticulin fibrosis) is associated with many benign and malignant conditions while increased trichrome staining (collagen fibrosis) is particularly prominent in late stages of severe myeloproliferative diseases or following tumour metastasis to the bone marrow. Recent evidence has shown that the amount of bone marrow reticulin staining often exhibits no correlation to disease severity, while the presence of type 1 collagen, as detected by trichrome staining, is often associated with more severe disease and a poorer prognosis. It was originally thought that increases in bone marrow stromal fibres themselves contributed to the haematopoietic abnormalities seen in certain diseases, but recent studies suggest that these increases are a result of underlying cellular abnormalities rather than a cause. A growing body of evidence suggests that increased deposition of bone marrow stromal fibres is mediated by transforming growth factor-beta and other factors elaborated by megakaryocytes, but it is likely that other cells, cytokines and growth factors are also involved. This suggests new avenues for investigation into the pathogenesis of various disorders associated with increased bone marrow stromal fibres.

Pivotal contributions of megakaryocytes to the biology of idiopathic myelofibrosis

In order to investigate the biologic processes underlying and resulting from the megakaryocytic hyperplasia that characterizes idiopathic myelofibrosis (IMF), peripheral blood CD34+ cells isolated from patients with IMF, polycythemia vera (PV), and G-CSF-mobilized healthy volunteers were cultured in the presence of stem cell factor and thrombopoietin. IMF CD34+ cells generated 24-fold greater numbers of megakaryocytes (MKs) than normal CD34+ cells. IMF MKs were also shown to have a delayed pattern of apoptosis and to overexpress the antiapoptotic protein bcl-xL. MK hyperplasia in IMF is, therefore, likely a consequence of both the increased ability of IMF progenitor cells to generate MKs and a decreased rate of MK apoptosis. Media conditioned (CM) by CD61+ cells generated in vitro from CD34+ cells were then assayed for the levels of growth factors and proteases. Higher levels of transforming growth factor-beta (TGF-beta) and active matrix metalloproteinase-9 (MMP9) were observed in media conditioned with IMF CD61+ cells than normal or PV CD61+ cells. Both normal and IMF CD61+ cells produced similar levels of VEGF. MK-derived TGF-B and MMP-9, therefore, likely contribute to the development of many pathological epiphenomena associated with IMF.

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.

Adenoviral-mediated TGF-β1 inhibition in a mouse model of myelofibrosis inhibit bone marrow fibrosis development

Myelofibrosis is characterized by excessive deposits of extracellular matrix proteins, which occur as a marrow microenvironment reactive response to cytokines released from the clonal malignant myeloproliferation. The observation that mice exposed to high systemic levels of thrombopoietin (TPO) invariably developing myelofibrosis has allowed demonstration of the crucial role of transforming growth factor (TGF)-beta1 released by hematopoietic cells in the onset of myelofibrosis. The purpose of this study was to investigate whether TGF-beta1 inhibition could directly inhibit fibrosis development in a curative approach of this mice model. An adenovirus encoding for TGF-beta1 soluble receptor (TGF-beta-RII-Fc) was injected either shortly after transplantation (preventive) or 30 days post-transplantation (curative). Mice were transplanted with syngenic bone marrow cells transduced with a retrovirus encoding for murine TPO. All mice developed a myeloproliferative syndrome. TGF-beta-RII-Fc was detected in the blood of all treated mice, leading to a dramatic decrease in TGF-beta1 level. Histological analysis show that the two approaches (curative or preventive) were successful enough to inhibit bone marrow and spleen fibrosis development in this model. However, lethality of TPO overexpression was not decreased after treatment, indicating that in this mice model, myeloproliferation rather than fibrosis was probably responsible for the lethality induced by the disorder.

Quantitative analysis of growth factor production in the mechanism of fibrosis in agnogenic myeloid metaplasia

OBJECTIVE

The current study quantified the growth factors in megakaryocytes and monocytes and correlated them to the degree of fibrosis, as there is no quantitative analysis of growth factors from megakaryocytes or monocytes reported in patients with agnogenic myeloid metaplasia (AMM).

MATERIALS AND METHODS

Megakaryocytes were obtained from cultured blood CD34+ cells. CD14+ cells were sorted by magnetic cell sorting. Quantitative analyses of the growth factors were obtained by real-time reverse-transcriptase polymerase chain reaction techniques and enzyme-linked immunobsorbent assay (ELISA).

RESULTS

1) We found that mRNA levels of transforming growth factor (TGF) beta1, platelet-derived growth factor (PDGF), and fibroblast growth factor (FGF) produced by the megakaryocytes were significantly elevated in AMM compared with those in normal controls (p < 0.05). Although these growth factors were elevated severalfold in AMM compared with other myeloproliferative disorders (MPDs) including essential thrombocythemia and polycythemia vera, they were not statistically significant. 2) TGF-beta1 was more abundantly produced than PDGF or FGF. 3) The mRNA levels of these growth factors produced from CD14+ cells were not significantly elevated in AMM compared with other MPDs or controls; the AMM mRNA levels were significantly elevated only in some patients. 4) The correlation of mRNA levels of these growth factors with the degree of myelofibrosis in AMM was significant with megakaryocytes (r = 0.73) but not with monocytes (r = 0.23). 5) ELISA of the growth factors from the cultured megakaryocytes showed that in most of the patients with AMM and other MPDs, and in volunteer controls, the growth factors were undetectable, and only a few patients with AMM (three each of TGF-beta1 and PDGF and one of FGF) and other MPDs (two of TGF-beta1 and one each of PDGF and FGF) had significantly elevated protein levels of these growth factors.

CONCLUSIONS

1) In AMM, the mRNA levels of these fibrosing growth factors are significantly elevated in megakaryocytes, and they were only elevated in a few patients in the monocyte-macrophage lineages. 2) mRNA of TGF-beta1 is more abundantly produced than that of PDGF or FGF from megakaryocytes. 3) A statistically significant correlation between the growth factor mRNA levels with the degree of myelofibrosis in AMM suggests that these fibrosing growth factors produced by the megakaryocytes may be associated with the etiology of bone marrow fibrosis in AMM. 4) Failure to substantiate at the protein level that megakaryocytes are the main source of growth factors production suggests that other factors or cells initiating translation of the growth factors in the megakaryocytes may also be important in the process of bone marrow fibrosis in AMM. Further studies are necessary.

Role of GATA-1 in normal and neoplastic hemopoiesis

GATA-1 exerts a concentration-dependent control on the differentiation of erythroid, megakaryocytic, mast, and eosinophilic cells. The concentration of GATA-1 is, in turn, regulated by specific sequences within the GATA-1 locus. On the basis of its levels of expression, the GATA-1 protein becomes associated with suitable partners forming transcription complexes that, by binding to lineage-specific enhancers, activate the expression of the corresponding target genes. Instrumental to our understanding of the role of GATA-1 in hemopoietic differentiation has been the generation of genetically engineered mutant mice and the discovery of naturally occurring mutations associated with either inherited or acquired human pathologies. We review our current understanding of the role of GATA-1 in normal and neoplastic hematopoiesis as emerging from these genetic approaches.

Pathogenesis of myelofibrosis with myeloid metaplasia: Insight from mouse models

Myelofibrosis with myeloid metaplasia or idiopathic myelofibrosis is a myeloproliferative disease. It is known to be a stem-cell disorder that leads to a secondary and reactive stromal reaction in the bone marrow microenvironment that is responsible for impaired haematopoiesis. Although progress has been made in the elucidation of the pathogenesis of idiopathic myelofibrosis, lack of suitable models has limited our understanding of the pathology. The aim of this chapter is to address recent inferred new insights in mouse models into the pathogenesis of osteomyelofibrosis. These insights outline the role of transforming growth factor-beta1 and osteoprotegerin in the promotion of myelofibrosis and osteosclerosis, respectively, paying special regard to the role of abnormal megakaryocyte proliferation and maturation.

Monocyte/macrophage dysfunctions do not impair the promotion of myelofibrosis by high levels of thrombopoietin

Several lines of evidence indicate that the megakaryocyte/platelet lineage is crucial in myelofibrosis induction. The demonstration that NOD/SCID mice with functionally deficient monocytes do not develop fibrotic changes when exposed to thrombopoietin (TPO) also suggests an important role for monocyte/macrophages. However, in this animal model, the development of myelofibrosis is dependent on the level of TPO. This study was conducted to investigate whether NOD/SCID mice exposed to high TPO levels mediated by a retroviral vector would be refractory to the development of bone marrow fibrosis. We show that TPO and TGF-beta1 in plasma from NOD/SCID and SCID mice engrafted with TPO-overexpressing hemopoietic cells reach levels similar to the ones reached in immunocompetent mice, and all animals develop a myeloproliferative disease associated with a dense myelofibrosis at 8 wk posttransplantation. Monocytes in NOD/SCID mice are functionally deficient to secrete cytokines such as IL-1alpha in response to stimuli, even under TPO expression. Surprisingly, the plasma of these mice displays high levels of IL-alpha, which was demonstrated to originate from platelets. Together, these data suggest that completely functional monocytes are not required to develop myelofibrosis and that platelets are able, under TPO stimulation, to synthesize inflammatory cytokines, which may be involved in the pathogenesis of myelofibrosis and osteosclerosis.

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.

Abnormalities of GATA-1 in megakaryocytes from patients with idiopathic myelofibrosis

The abnormal megakaryocytopoiesis associated with idiopathic myelofibrosis (IM) plays a role in its pathogenesis. Because mice with defective expression of transcription factor GATA-1 (GATA-1(low) mutants) eventually develop myelofibrosis, we investigated the occurrence of GATA-1 abnormalities in IM patients. CD 34(+) cells were purified from 12 IM patients and 8 controls; erythroblasts and megakaryocytes were then obtained from unilineage cultures of CD 34(+) cells. Purified CD 61(+), GPA(+), and CD 34(+) cells from IM patients contained levels of GATA-1, GATA-2, and FOG-1 mRNA, as well as of GATA-2 protein, that were similar to controls. In contrast, CD 61(+) cells from IM patients contained significantly reduced GATA-1 protein. Furthermore, 45% of megakaryocytes in biopsies from IM patients did not stain with anti-GATA-1 antibody, as compared to controls (2%), essential thrombocythemia (4%), or polycythemia vera (11%) patients. Abnormalities in immunoreactivity for FOG-1 were not found, and no mutations in GATA-1 coding sequences were found. The presence of GATA-1(neg) megakaryocytes in bone marrow biopsies was independent of the Val 617 Phe JAK 2 mutation, making it unlikely that a downstream functional relationship exists. We conclude that megakaryocytes from IM patients have reduced GATA-1 content, possibly contributing to disease pathogenesis as in the GATA-1(low) mice and also representing a novel IM-associated marker.