The constitutive mobilization of bone marrow-repopulating cells into the peripheral blood in idiopathic myelofibrosis

Use of Next Generation Sequencing to Define the Origin of Primary Myelofibrosis

Primary myelofibrosis (PMF) is a chronic myeloproliferative neoplasm (MPN) characterized by progressive bone marrow sclerosis, extra-medullary hematopoiesis, and possible transformation to acute leukemia. In the last decade, the molecular pathogenesis of the disease has been largely uncovered. Particularly, genetic and genomic studies have provided evidence of deregulated oncogenes in PMF as well as in other MPNs. However, the mechanisms through which transformation to either the myeloid or lymphoid blastic phase remain obscure. Particularly, it is still debated whether the disease has origins in a multi-potent hematopoietic stem cells or instead in a commissioned myeloid progenitor. In this study, we aimed to shed light upon this issue by using next generation sequencing (NGS) to study both myeloid and lymphoid cells as well as matched non-neoplastic DNA of PMF patients. Whole exome sequencing revealed that most somatic mutations were the same between myeloid and lymphoid cells, such findings being confirmed by Sanger sequencing. Particularly, we found 126/146 SNVs to be the e same (including JAK2V617F), indicating that most genetic events likely to contribute to disease pathogenesis occurred in a non-commissioned precursor. In contrast, only 9/27 InDels were similar, suggesting that this type of lesion contributed instead to disease progression, occurring at more differentiated stages, or maybe just represented “passenger” lesions, not contributing at all to disease pathogenesis. In conclusion, we showed for the first time that genetic lesions characteristic of PMF occur at an early stage of hematopoietic stem cell differentiation, this being in line with the possible transformation of the disease in either myeloid or lymphoid acute leukemia.

Circulating CD34+ cells of primary myelofibrosis patients contribute to myeloid-dominant hematopoiesis and bone marrow fibrosis in immunodeficient mice

INTRODUCTION

Primary myelofibrosis (PMF) is a clonal stem cell disorder characterized by myeloid dominant hematopoiesis and dysregulated proliferation of fibroblasts in the bone marrow. However, how these aberrant myeloid cells and fibroblasts are produced remains unclear.

AIM AND METHODS

In this study, we examined in vivo engraftment kinetics of PMF patient-derived CD34+ cells in immunecompromised NOD/SCID/IL2rgKO (NSG) mice. Engrafted human cells were analyzed with flow cytometry, and proliferation of fibroblastic cells and bone marrow fibrosis were assessed with the histo-pathological examination.

RESULTS

Transplantation of PMF patient-derived circulating CD34+ fractions into NSG newborns recapitulates clinical features of human PMF. Engraftment of human CD45+ leukocytes resulted in anemia and myeloid hyperplasia accompanied by bone marrow fibrosis by six months post-transplantation. Fibrotic bone marrow contained CD45-vimentin+ cells of both human and mouse origin, suggesting that circulating malignant CD34+ subsets contribute to myelofibrotic changes in PMF through direct and indirect mechanisms.

CONCLUSION

A patient-derived xenotransplantation (PDX) model of PMF allows in vivo examination of disease onset and propagation originating from immature CD34+ cells and will support the investigation of pathogenesis and development of therapeutic modalities for the disorder.

Suitability of small diagnostic peripheral-blood samples for cell-therapy studies

BACKGROUND AIMS

Primary hematopoietic stem and progenitor cells (HSPCs) are key components of cell-based therapies for blood disorders and are thus the authentic substrate for related research. We propose that ubiquitous small-volume diagnostic samples represent a readily available and as yet untapped resource of primary patient-derived cells for cell- and gene-therapy studies.

METHODS

In the present study we compare isolation and storage methods for HSPCs from normal and thalassemic small-volume blood samples, considering genotype, density-gradient versus lysis-based cell isolation and cryostorage media with different serum contents. Downstream analyses include viability, recovery, differentiation in semi-solid media and performance in liquid cultures and viral transductions.

RESULTS

We demonstrate that HSPCs isolated either by ammonium-chloride potassium (ACK)-based lysis or by gradient isolation are suitable for functional analyses in clonogenic assays, high-level HSPC expansion and efficient lentiviral transduction. For cryostorage of cells, gradient isolation is superior to ACK lysis, and cryostorage in freezing media containing 50% fetal bovine serum demonstrated good results across all tested criteria. For assays on freshly isolated cells, ACK lysis performed similar to, and for thalassemic samples better than, gradient isolation, at a fraction of the cost and hands-on time. All isolation and storage methods show considerable variation within sample groups, but this is particularly acute for density gradient isolation of thalassemic samples.

DISCUSSION

This study demonstrates the suitability of small-volume blood samples for storage and preclinical studies, opening up the research field of HSPC and gene therapy to any blood diagnostic laboratory with corresponding bioethics approval for experimental use of surplus material.

Myeloproliferative neoplasm stem cells

Myeloproliferative neoplasms (MPNs) arise in the hematopoietic stem cell (HSC) compartment as a result of the acquisition of somatic mutations in a single HSC that provides a selective advantage to mutant HSC over normal HSC and promotes myeloid differentiation to engender a myeloproliferative phenotype. This population of somatically mutated HSC, which initiates and sustains MPNs, is termed MPN stem cells. In >95% of cases, mutations that drive the development of an MPN phenotype occur in a mutually exclusive manner in 1 of 3 genes: JAK2, CALR, or MPL The thrombopoietin receptor, MPL, is the key cytokine receptor in MPN development, and these mutations all activate MPL-JAK-STAT signaling in MPN stem cells. Despite common biological features, MPNs display diverse disease phenotypes as a result of both constitutional and acquired factors that influence MPN stem cells, and likely also as a result of heterogeneity in the HSC in which MPN-initiating mutations arise. As the MPN clone expands, it exerts cell-extrinsic effects on components of the bone marrow niche that can favor the survival and expansion of MPN stem cells over normal HSC, further sustaining and driving malignant hematopoiesis. Although developed as targeted therapies for MPNs, current JAK2 inhibitors do not preferentially target MPN stem cells, and as a result, rarely induce molecular remissions in MPN patients. As the understanding of the molecular mechanisms underlying the clonal dominance of MPN stem cells advances, this will help facilitate the development of therapies that preferentially target MPN stem cells over normal HSC.

Proliferative characteristics of Philadelphia-negative myeloproliferative neoplasms - clinical implications

INTRODUCTION

Philadelphia-negative myeloproliferative neoplasms (Ph^- MPN) are characterized by overproduction of one or more blood cell lines.

METHODS

We studied the proliferative characteristics of 91 patients with de novo Ph^- MPN. Colony-forming cells (CFC) and endogenous colonies (EC), from bone marrow (BM) and/or peripheral blood (PB), were analyzed by colony assay based on methylcellulose. The level of circulating CD34⁺ cells was determined by flow cytometry.

RESULTS

The total number of PB CFC in primary myelofibrosis (PMF) was increased compared to the control sample (P < 0.01) and essential thrombocythemia (ET) (P < 0.05). The highest number of BM and PB EC was observed in polycythemia vera (PV) (P < 0.01). Increased levels of CD34⁺ cells characterized early-prefibrotic (57%) and advanced-fibrotic PMF (90%) as compared to PV (34%) and ET (32%) (P < 0.01). In the whole Ph^- MPN group, the total number of PB CFC (P < 0.01), PB EC (P < 0.05), and CD34⁺ cells (P < 0.01) correlated with the degree of BM fibrosis. Higher levels of circulating CD34⁺ cells in PMF correlated with the total number of PB EC (P < 0.05) and degree of BM fibrosis (P < 0.01).

CONCLUSIONS

Exploration of the PB proliferative characteristics of Ph^- MPN on diagnosis may be helpful in revealing early-prefibrotic PMF. Monitoring the levels of circulating CD34⁺ cells may provide a sensitive indicator of fibrotic evolution in PV and PMF.

Myeloproliferative Neoplasms: Molecular Drivers and Therapeutics

Activating mutations in genes that drive neoplastic cell growth are numerous and widespread in cancer, and specific genetic alterations are associated with certain types of cancer. For example, classic myeloproliferative neoplasms (MPNs) are hematopoietic stem cell disorders that affect cells of the myeloid lineage, including erythrocytes, platelets, and granulocytes. An activating mutation in the JAK2 tyrosine kinase is prevalent in these diseases. In MPN patients that lack such a mutation, other genetic changes that lead to activation of the JAK2 signaling pathway are present, indicating deregulation of JAK2 signaling plays an etiological driving role in MPNs, a concept supported by significant evidence from in vivo experimental MPN systems. Thus, small molecules that inhibit JAK2 activity are ideal drugs to impede the progression of disease in MPN patients. However, even though JAK inhibitors provide significant symptomatic relief, they have failed as a remission-inducing therapy. Nonetheless, the progress made understanding the molecular etiology of MPNs since 2005 is significant and has provided insight for the development and testing of novel molecular targeted therapeutic approaches. The current understanding of driver mutations in MPNs and an overview of current and potential therapeutic strategies for MPN patients will be discussed.

Preleukemia: the normal side of cancer

PURPOSE OF REVIEW

In the present review, we will define the preleukemic state. We aim at increasing awareness and research in the field of preleukemia that will nurture targeted therapy for the earlier steps of leukemia evolution.

RECENT FINDINGS

Emerging evidence supports the role of hematopoietic stem/progenitor cells carrying recurrent leukemia-related mutations as the cell of origin of both myeloid and lymphoid malignancies. The preleukemic stem cells can maintain at least to some extent their functionality; however, they have increased fitness endowed by the preleukemic mutations that lead to clonal expansion.

SUMMARY

The latent preleukemic period before overt leukemia presents can take years, and the majority of carriers will never develop leukemia in their lifetime. The preleukemic state is not rare, with greater than 1% of individuals having acquired one or more of the recognized preleukemic lesions. The high frequency of such abnormalities in the population may be the cost of growing old; however, another view could be that in order to survive to old age, the hematopoietic system must adapt to create robust hematopoietic stem/progenitor cells with an increased fitness and clonal expansion. Hence, leukemia does not necessarily start as a disease, but rather as a need, with the normally functioning preleukemic hematopoietic stem cells trying to maintain health for years but in time succumbing to their own acquired virtues.

CD133 marks a stem cell population that drives human primary myelofibrosis

Primary myelofibrosis is a myeloproliferative neoplasm characterized by bone marrow fibrosis, megakaryocyte atypia, extramedullary hematopoiesis, and transformation to acute myeloid leukemia. To date the stem cell that undergoes the spatial and temporal chain of events during the development of this disease has not been identified. Here we describe a CD133(+) stem cell population that drives the pathogenesis of primary myelofibrosis. Patient-derived circulating CD133(+) but not CD34(+)CD133(-) cells, with a variable burden for JAK2 (V617F) mutation, had multipotent cloning capacity in vitro. CD133(+) cells engrafted for up to 10 months in immunocompromised mice and differentiated into JAK2-V617F(+) myeloid but not lymphoid progenitors. We observed the persistence of human, atypical JAK2-V617F(+) megakaryocytes, the initiation of a prefibrotic state, bone marrow/splenic fibrosis and transition to acute myeloid leukemia. Leukemic cells arose from a subset of CD133(+) cells harboring EZH2 (D265H) but lacking a secondary JAK2 (V617F) mutation, consistent with the hypothesis that deregulation of EZH2 activity drives clonal growth and increases the risk of acute myeloid leukemia. This is the first characterization of a patient-derived stem cell population that drives disease resembling both chronic and acute phases of primary myelofibrosis in mice. These results reveal the importance of the CD133 antigen in deciphering the neoplastic clone in primary myelofibrosis and indicate a new therapeutic target for myeloproliferative neoplasms.

How does JAK2V617F contribute to the pathogenesis of myeloproliferative neoplasms?

A decade on from the discovery of the JAK2V617F mutation in the majority of patients with myeloproliferative neoplasms (MPNs), JAK2V617F is now firmly installed in the hematology curriculum of medical students and the diagnostic-testing algorithm of clinicians. Furthermore, the oral JAK1/JAK2 inhibitor ruxolitinib, rationally designed to target activated JAK2 signaling in MPN, has been approved by the Food and Drug Administration (FDA) of the United States for the past 3 years for the treatment of intermediate- and advanced-phase myelofibrosis. Notwithstanding this, JAK2V617F continues to stimulate the MPN research community and novel insights into understanding the mechanisms by which JAK2V617F contributes to the pathogenesis of MPN are continually emerging. In this chapter, we focus on recent advances in 4 main areas: (1) the molecular processes coopted by JAK2V617F to induce MPN, (2) the role that JAK2V617F plays in phenotypic diversity in MPN, (3) the functional impact of JAK2V617F on hematopoietic stem cells, and (4) therapeutic strategies to target JAK2V617F. Although great strides have been made, significant deficits still exist in our understanding of the precise mechanisms by which JAK2V617F-mutant hematopoietic stem cells emerge and persist to engender clonal hematopoiesis in MPN and in developing strategies to preferentially target the JAK2V617F-mutant clone therapeutically. Critically, although myelofibrosis remains arguably the greatest clinical challenge in JAK2V617F-mediated MPN, the current understanding of myelofibrosis-specific disease biology remains quite rudimentary. Therefore, many important biological questions pertaining to JAK2V617F will continue to engage and challenge the MPN research community in the coming decade.

Osteopetrosis mimicking juvenile myelomonocytic leukemia

A 5-month-old boy developed splenomegaly, anemia, thrombocytopenia with elevated white cells, monocytosis and immature granulocytes in the peripheral blood. Bone marrow showed dysplasia without blastosis. Increased colony-forming unit-granulocyte-macrophage was found in the peripheral blood, mimicking granulocyte-macrophage colony-stimulating factor hypersensitivity. These findings fulfilled the diagnosis criteria for juvenile myelomonocytic leukemia (JMML), but no mutations in the CBL, NRAS, KRAS, or PTPN11 genes were detected. In addition to these findings severe hypogammaglobulinemia and elevated alkaline phosphatase were present. Bone X-ray showed dense and radiopaque bones with a bone-in-bone appearance characteristic of infantile malignant osteopetrosis (IMO). Genetic mutation in T-cell, immune regulator 1 (TCIRG1) was identified, confirming the diagnosis of IMO. Careful differential diagnosis including osteopetrosis, is therefore recommended in patients with clinical features and hematologic findings consistent with JMML.

Myelofibrosis-associated complications: pathogenesis, clinical manifestations, and effects on outcomes

Myelofibrosis (MF) is a rare chronic BCR-ABL1 (breakpoint cluster region-Abelson murine leukemia viral oncogene homologue 1)-negative myeloproliferative neoplasm characterized by progressive bone marrow fibrosis, inefficient hematopoiesis, and shortened survival. The clinical manifestations of MF include splenomegaly, consequent to extramedullary hematopoiesis, cytopenias, and an array of potentially debilitating abdominal and constitutional symptoms. Dysregulated Janus kinase (JAK)-signal transducer and activator of transcription signaling underlies secondary disease-associated effects in MF, such as myeloproliferation, bone marrow fibrosis, constitutional symptoms, and cachexia. Common fatal complications of MF include transformation to acute leukemia, thrombohemorrhagic events, organ failure, and infections. Potential complications from hepatosplenomegaly include portal hypertension and variceal bleeding, whereas extramedullary hematopoiesis outside the spleen and liver - depending on the affected organ - may result in intracranial hypertension, spinal cord compression, pulmonary hypertension, pleural effusions, lymphadenopathy, skin lesions, and/or exacerbation of abdominal symptoms. Although allogeneic stem cell transplantation is the only potentially curative therapy, it is suitable for few patients. The JAK1/JAK2 inhibitor ruxolitinib is effective in improving splenomegaly, MF-related symptoms, and quality-of-life measures. Emerging evidence that ruxolitinib may be associated with a survival benefit in intermediate- or high-risk MF suggests the possibility of a disease-modifying effect. Consequently, ruxolitinib could provide a treatment backbone to which other (conventional and novel) therapies may be added for the prevention and effective management of specific MF-associated complications.

Myelofibrosis-associated complications: pathogenesis, clinical manifestations, and effects on outcomes

Myelofibrosis (MF) is a rare chronic BCR-ABL1 (breakpoint cluster region-Abelson murine leukemia viral oncogene homologue 1)-negative myeloproliferative neoplasm characterized by progressive bone marrow fibrosis, inefficient hematopoiesis, and shortened survival. The clinical manifestations of MF include splenomegaly, consequent to extramedullary hematopoiesis, cytopenias, and an array of potentially debilitating abdominal and constitutional symptoms. Dysregulated Janus kinase (JAK)-signal transducer and activator of transcription signaling underlies secondary disease-associated effects in MF, such as myeloproliferation, bone marrow fibrosis, constitutional symptoms, and cachexia. Common fatal complications of MF include transformation to acute leukemia, thrombohemorrhagic events, organ failure, and infections. Potential complications from hepatosplenomegaly include portal hypertension and variceal bleeding, whereas extramedullary hematopoiesis outside the spleen and liver - depending on the affected organ - may result in intracranial hypertension, spinal cord compression, pulmonary hypertension, pleural effusions, lymphadenopathy, skin lesions, and/or exacerbation of abdominal symptoms. Although allogeneic stem cell transplantation is the only potentially curative therapy, it is suitable for few patients. The JAK1/JAK2 inhibitor ruxolitinib is effective in improving splenomegaly, MF-related symptoms, and quality-of-life measures. Emerging evidence that ruxolitinib may be associated with a survival benefit in intermediate- or high-risk MF suggests the possibility of a disease-modifying effect. Consequently, ruxolitinib could provide a treatment backbone to which other (conventional and novel) therapies may be added for the prevention and effective management of specific MF-associated complications.

Myeloproliferative neoplasm animal models

Myeloproliferative neoplasm (MPN) animal models accurately re-capitulate human disease in mice and have been an important tool for the study of MPN biology and therapy. Transplantation of BCR-ABL transduced bone marrow into irradiated syngeneic mice established the field of MPN animal modeling. Genetically engineered MPN animal models have enabled detailed characterization of the effects of specific MPN-associated genetic abnormalities on hematopoietic stem and progenitor cells (HSPCs). Xenograft models have allowed the study of primary human MPN-propagating cells in vivo. JAK2V617F, the most common molecular abnormality in BCR-ABL negative MPN, has been extensively studied using retroviral, transgenic, knock-in and xenograft models.

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.

The effect of CXCL12 processing on CD34+ cell migration in myeloproliferative neoplasms

Primary myelofibrosis (PMF) and polycythemia vera (PV) are chronic myeloproliferative neoplasms. PMF and, to a lesser degree, PV are characterized by constitutive mobilization of hematopoietic stem cells (HSC) and progenitor cells (HPC) into the peripheral blood (PB). The interaction between the chemokine CXCL12 and its receptor CXCR4 plays a pivotal role in determining the trafficking of CD34(+) cells between the bone marrow (BM) and the PB. PMF, but not PV, is associated with downregulation of CXCR4 by CD34(+) cells due to epigenetic events. Both PV and PMF patients have elevated levels of immunoreactive forms of CXCL12 in the BM and PB. Using electrospray mass spectrometry, the PB and BM plasma of PV and PMF patients was shown to contain reduced amounts of intact CXCL12 but significant amounts of several truncated forms of CXCL12, which are lacking in normal PB and BM plasma. These truncated forms of CXCL12 are the product of the action of several serine proteases, including dipeptidyl peptidase-IV, neutrophil elastase, matrix metalloproteinase-2 (MMP-2), MMP-9, and cathepsin G. Unlike CXCL12, these truncates either lack the ability to act as a chemoattractant for CD34(+) cells and/or act as an antagonist to the action of CXCL12. These data suggest that proteolytic degradation of CXCL12 is characteristic of both PV and PMF and that the resulting truncated forms of CXCL12, in addition to the reduced expression of CXCR4 by CD34(+) cells, lead to a profound mobilization of HSC/HPC in PMF.

Correction of the abnormal trafficking of primary myelofibrosis CD34+ cells by treatment with chromatin-modifying agents

The abnormal trafficking of CD34+ cells is a unique characteristic of primary myelofibrosis (PMF). We have further studied the behavior of PMF CD34+ cells by examining their homing to the marrow and the spleens of nonobese diabetic/severe combined immunodeficient (NOD/SCID) mice. Following the infusion of PMF and normal granulocyte colony-stimulating factor-mobilized peripheral blood (mPB) CD34+ cells into NOD/SCID mice, reduced numbers of PMF CD34+ cells and granulocyte-macrophage colony-forming unit (CFU-GM) compared with mPB were detected in the marrow of these mice, whereas similar numbers of PMF and mPB CD34+ cells and CFU-GM homed to their spleens. The abnormal homing of PMF CD34+ cells was associated with reduced expression of CXCR4, but was not related to the presence of JAK2V617F. The sequential treatment of PMF CD34+ cells with the chromatin-modifying agents 5-aza-2'-deoxycytidine (5azaD) and trichostatin A (TSA), but not treatment with small molecule inhibitors of JAK2, resulted in the generation of increased numbers of CD34+CXCR4+ cells, which was accompanied by enhanced homing of PMF CD34+ cells to the marrow but not the spleens of NOD/SCID mice. Following 5azaD/TSA treatment, JAK2V617F-negative PMF hematopoietic progenitor cells preferentially homed to the marrow but not the spleens of recipient mice. Our data suggest that PMF CD34+ cells are characterized by a reduced ability to home to the marrow but not the spleens of NOD/SCID mice and that this homing defect can be corrected by sequential treatment with chromatin-modifying agents.

NF-E2 overexpression delays erythroid maturation and increases erythrocyte production

The transcription factor Nuclear Factor-Erythroid 2 (NF-E2) is overexpressed in the vast majority of patients with polycythaemia vera (PV). In murine models, NF-E2 overexpression increases proliferation and promotes cellular viability in the absence of erythropoietin (EPO). EPO-independent growth is a hallmark of PV. We therefore hypothesized that NF-E2 overexpression contributes to erythrocytosis, the pathognomonic feature of PV. Consequently, we investigated the effect of NF-E2 overexpression in healthy CD34+ cells. NF-E2 overexpression led to a delay in erythroid maturation, manifested by a belated appearance of glycophorin A-positive erythroid precursors. Maturation delay was similarly observed in primary PV patient erythroid cultures compared to healthy controls. Protracted maturation led to a significant increase in the accumulated number of erythroid cells both in PV cultures and in CD34+ cells overexpressing NF-E2. Similarly, NF-E2 overexpression altered erythroid colony formation, leading to an increase in erythroid burst-forming unit formation. These data indicate that NF-E2 overexpression delays the early phase of erythroid maturation, resulting in an expansion of erythroid progenitors, thereby increasing the number of erythrocytes derived from one CD34+ cell. These data propose a role for NF-E2 in mediating the erythrocytosis of PV.

Involvement of mast cells by the malignant process in patients with Philadelphia chromosome negative myeloproliferative neoplasms

The Philadelphia chromosome negative myeloproliferative neoplasms (MPNs) are clonal hematologic malignancies frequently characterized by a mutation in JAK2 (JAK2V617F). Peripheral blood (PB) CD34(+) cells from patients with polycythemia vera (PV) and primary myelofibrosis (PMF) generated in vitro significantly fewer mast cells (MCs) than normal PB CD34(+) cells. The numbers of MC progenitors assayed from MPN CD34(+) cells were, however, similar to that assayed from normal CD34(+) cells. A higher percentage of the cultured MPN MCs expressed FcvarepsilonRIalpha, CD63 and CD69 than normal MCs, suggesting that cultured MPN MCs are associated with an increased state of MC activation. Further analysis showed that a higher proportion of cultured PV and PMF MCs underwent apoptosis in vitro. By using JAK2V617F, MplW515L and chromosomal abnormalities as clonality markers, we showed that the malignant process involved MPN MCs. JAK2V617F-positive MC colonies were assayable from the PB CD34(+) cells of each of the 17 JAK2V617F positive MPN patients studied. Furthermore, erlotinib, a JAK2 inhibitor, was able to inhibit JAK2V617F-positive PV MC progenitor cells, indicating that malignant MC progenitor cells are a potential cellular target for such JAK2 inhibitor-directed therapy.

Evaluation of circulating endothelial cells, VEGF and VEGFR2 serum levels in patients with chronic myeloproliferative diseases

Authors evaluated some markers of angiogenetic activity in patients with chronic myeloproliferative diseases (CMDs). In this study by using a cytofluorimetric analysis we evaluated circulating endothelial progenitor cells (EPCs) in patients with chronic myeloproliferative disease. Moreover, in the same group of subjects, we evaluated serum levels of vascular endothelial growth factor (VEGF) and vascular endothelial growth factor receptor-2 (VEGFR2). In our patients, we have found an increase in the number of endothelial progenitor cells in primary myelofibrosis (PMF) and polycythaemia vera (PV) patients, while an increase of circulating endothelial cells (CECs) was found in all patients with CMD. Moreover, we found higher serum levels of VEGF with respect to control subjects in every group of patients with CMD, and a not significant reduction of VEGFR2 levels in essential thrombocythaemia (ET) patients. A correlation was also found in PV patients between VEGF levels and erythrocyte number and in PMF subjects with the count of white cells. Our data suggest that some markers of angiogenesis are activated in CMD patients and angiogenesis may have a role in the pathophysiology of chronic myeloproliferative disorders.

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.

The JAK2V617F mutation in polycythemia vera and other myeloproliferative disorders: one mutation for three diseases?

The discovery of the JAK2V617F mutation has made the diagnosis of polycythemia vera (PV) much easier, but the pathogenesis of PV is still incompletely understood. In particular, it is not yet elucidated how a single mutation can be found in multiple myeloproliferative disorders (MPD) and myelodysplastic syndromes with ring sideroblasts and whether the sole JAK2V617F is sufficient to induce a MPD in humans. Several hypotheses are under investigation such as differences in the targeted hematopoietic stem cells (HSC), host modifier polymorphisms, intensity of JAK2V617F signaling, presence of other somatic mutations, or the presence of a pre-JAK2 event that may vary according to the MPD phenotype. Multiple studies have provided some evidence for and against each hypothesis, but it now seems possible to reconcile these hypotheses into a model that will need to be tested using newly developed tools. Recent investigations have also led to new treatment modalities that could benefit patients with PV.

Behavior of CD34+ cells isolated from patients with polycythemia vera in NOD/SCID mice

OBJECTIVE

We investigated if polycythemia vera (PV) peripheral blood (PB) CD34+ cells contain cells capable of engrafting nonobese diabetic/severe combined immunodeficient (NOD/SCID) mice and if the JAK2V617F mutational burden of these cells alters their behavior in NOD/SCID mice.

MATERIALS AND METHODS

CD34+ cells isolated from patients with PV, idiopathic myelofibrosis (IM), or granulocyte colony-stimulating factor-mobilized normal donors were transplanted into sublethally irradiated NOD/SCID mice. Cells engrafted into the NOD/SCID mice were analyzed flow cytometrically using lineage-specific antibodies. Genomic DNA was extracted from granulocytes, CD34+ cells, and sorted human CD45(+) cells purified from the bone marrow cells of these mice to examine their JAK2V617F mutational burdens.

RESULTS

Multilineage human cell engraftment was observed in mice transplanted with CD34+ cells from mobilized normal volunteers, IM patients and PV patients with high JAK2V617F burden, but not in mice receiving grafts from PV patients with low JAK2V617F burden. The differentiation program of engrafting PV CD34+ cells with high JAK2V617F burden was remarkably different than that of IM CD34+ cells. The JAK2V617F allele frequency in the human CD45+ cells isolated from the mice receiving CD34+ cells was lower than that observed in the CD34+ cell grafts, indicating the persistence of a JAK2V617F negative compartment of stem cells.

CONCLUSION

We conclude that PB CD34+ cells from PV patients with high JAK2V617F burden and patients with IM contain NOD/SCID repopulating cells, and that differentiation program of IM and PV CD34+ cells are dramatically different.

Effects of Chromatin-Modifying Agents on CD34+ Cells from Patients with Idiopathic Myelofibrosis

Idiopathic myelofibrosis (IM) is likely the consequence of both the acquisition of genetic mutations and epigenetic changes that silence critical genes that control cell proliferation, differentiation, and apoptosis. We have explored the effects of the sequential treatment with the DNA methyltransferase inhibitor, decitabine [5-aza-2'-deoxycytidine (5azaD)], followed by the histone deacetylase inhibitor, trichostatin A (TSA), on the behavior of IM CD34(+) cells. Unlike normal CD34(+) cells where 5azaD/TSA treatment leads to the expansion of CD34(+) cells and marrow-repopulating cells, treatment of IM CD34(+) cells results in a reduction of the number of total cells, CD34(+) cells, and assayable hematopoietic progenitor cells (HPC). In IM, HPCs are either heterozygous or homozygous for the JAK2V617F mutation or possess wild-type JAK2 in varying proportions. Exposure of IM CD34(+) cells to 5azaD/TSA resulted in a reduction of the proportion of JAK2V617F-positive HPCs in 83% of the patients studied and the reduction in the proportion of homozygous HPCs in 50% of the patients. 5azaD/TSA treatment led to a dramatic reduction in the number of HPCs that contained chromosomal abnormalities in two JAK2V617F-negative IM patients. IM is characterized by constitutive mobilization of HPCs, which has been partly attributed to decreased expression of the chemokine receptor CXCR4. Treatment of IM CD34(+) cells with 5azaD/TSA resulted in the up-regulation of CXCR4 expression by CD34(+) cells and restoration of their migration in response to SDF-1. These data provide a rationale for sequential therapy with chromatin-modifying agents for patients with IM.

Philadelphia chromosome-negative myeloproliferative disorders: biology and treatment

The Philadelphia chromosome (Ph)-negative myeloproliferative disorders (MPDs) include essential thrombocythemia (ET), idiopathic myelofibrosis (IMF), and polycythemia vera (PV). All of these disorders are clonal hematologic malignancies originating at the level of the pluripotent hematopoietic stem cell. Recently, activating mutations of the intracellular cytokine-signaling molecule JAK2 have been identified in > 90% of patients with PV and in 50% of those with IMF and ET. In addition, a mutation of the thrombopoietin receptor, MPLW515L, has been documented in some patients with IMF. Both mutations activate JAK-STAT signaling pathways and likely play a role in disease progression. Both ET and PV are associated with prolonged clinical courses associated with frequent thrombotic and hemorrhagic events, and progression to myelofibrosis and acute leukemia. IMF has a much poorer prognosis and is associated with cytopenias, splenomegaly, extramedullary hematopoiesis, and bone marrow fibrosis. Stratification of risk for the development of complications from Ph-negative MPDs has guided the identification of appropriate therapies for this population. Intermediate/high-risk IMF or myelofibrosis after ET or PV is associated with a sufficiently poor prognosis to justify the use of allogeneic stem cell transplantation, which is capable of curing such patients. Reduced-intensity conditioning in preparation for allogeneic stem cell transplantation has permitted older patients with IMF to undergo transplantation with increasing success.

Effect of fibroblast growth factor 2 on stromal cell-derived factor 1 production by bone marrow stromal cells and hematopoiesis

BACKGROUND

Reduction of intramedullary hematopoiesis and the development of myelofibrosis and splenic hematopoiesis are frequent complications of clonal myeloid disorders that cause severe morbidity and death and present a therapeutic challenge. However, the pathogenesis of these complications is still unknown. We evaluated the effect of fibroblast growth factor 2 (FGF-2), the level of which is elevated in patients with clonal myeloid disorders, on bone marrow stromal cell expression of stromal cell-derived factor 1 (SDF-1), a chemokine that is essential for normal hematopoiesis.

METHODS

Reverse transcription-polymerase chain reaction analysis, immunoblot analysis, and enzyme-linked immunosorbent assays were used to examine effects of human recombinant FGF-2 exposure on SDF-1 expression in mouse stromal MS-5 and S-17 cells. Cocultures of human CD34-positive peripheral blood stem cells or mouse pre-B DW34 cells with mouse stromal cells were used to characterize the functional relevance of the effects of FGF-2 on SDF-1 expression. The in vivo hematologic effects of FGF-2 were determined by systemic administration to mice (n = 10). All statistical tests were two-sided.

RESULTS

FGF-2 reduced constitutive SDF-1 mRNA expression and secretion in stromal cells (SDF-1 levels in supernatants: MS-5 cells cultured for 3 days in medium only versus in medium with FGF-2, 95.4 ng/mL versus 22.2 ng/mL, difference = 73.2 ng/mL, 95% confidence interval [CI] = 60.52 to 85.87 ng/mL; P = .002, two-sided Student's t test; S-17 cultured in medium only versus in medium with FGF-2, 203.53 ng/mL versus 32.36 ng/mL, difference = 171.17 ng/mL, 95% CI = 161.8 to 180.6 ng/mL; P<.001). These effects of FGF-2 were reversible. FGF-2 compromised stromal cell support of the growth and survival of pre-B DW34 and myeloid lineage cells, and these effects were reversed in part by exogenous recombinant SDF-1alpha (rSDF-1alpha) (DW34 pre-B cells recovery on S-17 stromal cells, expressed as a percentage of DW34 cells recovered from medium only: with FGF-2 versus without FGF-2, 27.6% versus 100%, difference = 72.4%, 95% CI = 45.34% to 99.51%, P = .008; with FGF-2 plus rSDF1 versus with FGF-2 only, 60.3% versus 27.6%, difference = 32.7%, 95% CI = 9.35% to 56.08%, P = .034; fold increase in number of myeloid lineage cells after culture on S-17 stromal cells: with FGF-2 versus without FGF-2, 0.25-fold versus 3.8-fold, difference = 3.55-fold, 95% CI = 2.66- to 4.44-fold, P<.001; recovery of myeloid cells on S-17 stromal cells, expressed as a percentage of myeloid cells recovered from medium only: FGF-2 plus rSDF-1alpha versus FGF-2 only, 76.5% versus 32.4%, difference = 44.1%, 95% CI = 32.58% to 55.68%, P<.001). Administration of FGF-2 to mice reversibly reduced bone marrow levels of SDF-1 and cellularity and induced immature myeloid cell mobilization, extramedullary hematopoiesis, and splenomegaly.

CONCLUSIONS

Systemic administration of FGF-2 in mice disrupts normal bone marrow hematopoiesis in part through reduced expression of SDF-1. Thus, endogenous FGF-2 may represent a potential therapeutic target in clonal myeloid disorders characterized by bone marrow failure.

Conventional and experimental drug therapy in myelofibrosis with myeloid metaplasia

Myelofibrosis with myeloid metaplasia (MMM) is currently classified as a classic (ie, BCR-ABL-negative) myeloproliferative disorder characterized by anemia, multiorgan extramedullary hematopoiesis, constitutional symptoms, and premature death from either leukemic transformation or other disease complications. Stem cell transplantation can be curative, but many patients either are not appropriate candidates or do not choose to accept the significant risks associated with transplantation. Current pharmacologic therapy has been beneficial mainly in terms of palliating disease-associated cytopenias, constitutional symptoms, splenomegaly, and other organ damage from excess myeloproliferation. Novel treatment strategies are under investigation, including targeted inhibition of JAK2(V617F), the activating tyrosine kinase point mutation present in about half of patients with MMM. In this article, we review both the old and new pharmacologic options for MMM.

Biology and Treatment of Primary Myelofibrosis

Primary myelofibrosis (PMF) is a chronic myeloproliferative disorder associated with an average survival of less than 5 years. Therapy for PMF has used chemotherapeutic agents, immunomodulatory drugs, or biological-response modifiers that have not always been directed at the biological processes that underlie the origins of PMF. Such strategies are palliative and have an uncertain effect on survival. At present, allogeneic stem cell transplantation (ASCT) is the only means of altering the natural history of patients with PMF and provides the only hope for cure of this disorder. Enthusiasm for ASCT in PMF has been muted due to an unacceptable transplantation-related morbidity and mortality in patients receiving fully myeloablative conditioning regimens. Recently, a variety of reduced-intensity conditioning regimens have been utilized in older patients with PMF with significant comorbidities with promising results. Greater understanding of the cellular and molecular events that lead to the development of PMF have provided the opportunity for targeted therapies for PMF. Such therapies must be first evaluated in phase 1/2 trials using a variety of endpoints to assess their efficacy and their potential associated toxicities. The performance of randomized clinical trials comparing these agents to the present standard of care would permit for the first time evidence-based therapeutic decisions to be made for patients with PMF.

FGF2 posttranscriptionally down-regulates expression of SDF1 in bone marrow stromal cells through FGFR1 IIIc

The chemokine stromal cell-derived factor-1 (SDF-1) is constitutively expressed by bone marrow stromal cells and plays key roles in hematopoiesis. Fibroblast growth factor 2 (FGF2), a member of the FGF family that plays important roles in developmental morphogenic processes, is abnormally elevated in the bone marrow from patients with clonal myeloid disorders and other disorders where normal hematopoiesis is impaired. Here, we report that FGF2 reduces SDF-1 secretion and protein content in bone marrow stromal cells. By inhibiting SDF-1 production, FGF2 compromises stromal cell support of hematopoietic progenitor cells. Reverse-transcriptase-polymerase chain reaction (RT-PCR) analysis revealed that bone marrow stromal cells express 5 FGF receptors (FGFRs) among the 7 known FGFR subtypes. Blocking experiments identified FGFR1 IIIc as the receptor mediating FGF2 inhibition of SDF-1 expression in bone marrow stromal cells. Analysis of the mechanisms underlying FGF2 inhibition of SDF-1 production in bone marrow stromal cells revealed that FGF2 reduces the SDF-1 mRNA content by posttranscriptionally accelerating SDF-1 mRNA decay. Thus, we identify FGF2 as an inhibitor of SDF-1 production in bone marrow stromal cells and a regulator of stromal cell supportive functions for hematopoietic progenitor cells.

Advances in the therapy of chronic idiopathic myelofibrosis

The molecular basis of chronic idiopathic myelofibrosis (CIMF) has remained elusive, thus hampering the development of effective targeted therapies. However, significant progress regarding the molecular mechanisms involved in the pathogenes is of this disease has been made in recent years that will likely provide ample opportunity for the investigation of novel therapeutic approaches. At the fore front of these advances is the discovery that 35%-55% of patients with CIMF harbor mutations in the Janus kinase 2 tyrosine kinase gene. Until very recently, the management of patients with CIMF involved the use of supportive measures, including growth factors, transfusions, or interferon, and the administration of cyto-reductive agents, such as hydroxyurea and anagrelide. However, several trials have demonstrated the efficacy of antiangiogenic agents alone or in combination with corticosteroids. In addition, the use of reduced-intensity conditioning allogeneic stem cell transplantation has resulted in prolonged survival and lower transplant-related mortality.

Molecular Profiling of CD34+Cells in Idiopathic Myelofibrosis Identifies a Set of Disease-Associated Genes and Reveals the Clinical Significance of Wilms' Tumor Gene 1 (WT1)

This study was aimed at the characterization of a gene expression signature of the pluripotent hematopoietic CD34(+) stem cell in idiopathic myelofibrosis (IM), which would eventually provide novel pathogenetic insights and/or diagnostic/prognostic information. Aberrantly regulated genes were revealed by transcriptome comparative microarray analysis of normal and IM CD34(+) cells; selected genes were also assayed in granulocytes. One-hundred seventy four differentially expressed genes were identified and in part validated by quantitative polymerase chain reaction. Altered gene expression was corroborated by the detection of abnormally high CD9 or CD164, and low CXCR4, membrane protein expression in IM CD34(+) cells. According to class prediction analysis, a set of eight genes (CD9, GAS2, DLK1, CDH1, WT1, NFE2, HMGA2, and CXCR4) properly recognized IM from normal CD34(+) cells. These genes were aberrantly regulated also in IM granulocytes that could be reliably differentiated from control polycythemia vera and essential thrombocythemia granulocytes in 100% and 81% of cases, respectively. Abnormal expression of HMGA2 and CXCR4 in IM granulocytes was dependent on the presence and the mutational status of JAK2(V617F) mutation. The expression levels of both CD9 and DLK1 were associated with the platelet count, whereas higher WT1 expression levels identified IM patients with more active disease, as revealed by elevated CD34(+) cell count and higher severity score. In conclusion, molecular profiling of IM CD34(+) cells uncovered a limited number of genes with altered expression that, beyond their putative role in disease pathogenesis, are associated with patients' clinical characteristics and may have potential prognostic application.

Involvement of various hematopoietic-cell lineages by the JAK2V617F mutation in polycythemia vera

The JAK2(V617F) mutation has been shown to occur in the overwhelming majority of patients with polycythemia vera (PV). To study the role of the mutation in the excessive production of differentiated hematopoietic cells in PV, CD19+, CD3+, CD34+, CD33+, and glycophorin A+ cells and granulocytes were isolated from the peripheral blood (PB) of 8 patients with PV and 3 healthy donors mobilized with G-CSF, and the percentage of JAK2(V617F) mutant allele was determined by quantitative real-time polymerase chain reaction (PCR). The JAK2(V617F) mutation was present in cells belonging to each of the myeloid lineages and was also present in B and T lymphocytes in a subpopulation of patients with PV. The proportion of hematopoietic cells expressing the JAK2(V617F) mutation decreased after differentiation of CD34+ cells in vitro in the presence of optimal concentrations of SCF, IL-3, IL-6, and Epo. These data suggest that the JAK2(V617F) mutation may not provide a proliferative and/or survival advantage for the abnormal PV clone. Although the JAK2(V617F) mutation plays an important role in the biologic origins of PV, it is likely not the sole event leading to PV.

Induction of cytokines by radioprotective tocopherol analogs

Tocols are a family of eight isomers consisting of four tocopherols and four tocotrienols that exist in four isomeric forms: alpha (alpha), beta (beta), gamma (gamma), and delta (delta). Recently, tocols were found to have important and unique biological effects on nutrition and health other than antioxidant properties and are, therefore, now receiving increased attention. We have demonstrated the radioprotective efficacy of various tocol analogs and some of their esters. Three forms of tocols - alpha-tocopherol, alpha-tocopherol succinate, and gamma-tocotrienol - significantly protected mice against lethal gamma irradiation when administered subcutaneously 24 h before irradiation. The radioprotective effects of tocols on survival were associated with peripheral blood cell recovery after radiation induced cytopenia. Hematopoietic cytokines are known to promote the proliferation and differentiation of blood cell progenitors. Therefore, we hypothesized that peripheral blood cell recovery is preceded by hematopoietic cytokine induction. To test this hypothesis and compare the various radioprotective and non-radioprotective analogs, we measured serum cytokines using a sandwich ELISA, Luminex, and cytokine array in mice treated with various tocols (alpha-tocopherol succinate, alpha-tocopherol, delta-tocopherol, gamma- tocopherol, gamma-tocotrienol, and tocopherol acetate). Among the serum cytokines measured, ELISA and Luminex studies indicated that alpha-tocopherol, alpha-tocopherol succinate, and gamma-tocotrienol increased G-CSF levels in mice. Alpha-tocopherol succinate was most effective in stimulating G-CSF. IL-6 was detected by Luminex in sera samples from mice treated with the above three analogs. The results of the cytokine array suggest that other cytokines and chemokines in addition to G-CSF and IL-6 are induced. Since G-CSF, IL-6, and certain chemokines are important hematopoietic factors, these results support our hypothesis that the protection of mice from radiation-induced hematopoietic death is mediated by cytokines and chemokines. These studies may indicate that alpha-tocopherol succinate can be used as an adjunct in cancer chemotherapy, where neutropenia is a serious problem with threatening infectious complications.

Idiopathic myelofibrosis: pathogenesis to treatment

Idiopathic myelofibrosis (IMF) is the least common of the chronic myeloproliferative disorders and carries the worst prognosis with a median survival of 4 years. It is a clonal haematopoietic stem-cell disorder and, although the pathogenesis remains unclear, approximately 50% of cases are known to possess an activating JAK2 V617F mutation. In contrast, the characteristic stromal proliferation is a reactive, or secondary, event that results from the aberrant release of a variety of growth factors from megakaryocytes and monocytes. Treatment for most cases is supportive, although androgens, recombinant erythropoietin, steroids and thalidomide are effective modalities for the amelioration of anaemia. Myelosuppression, splenectomy and irradiation are valuable therapeutic modalities for specific clinical situations. Prognostic scores are available to aid the identification of cases for whom bone marrow transplantation should be considered. Recently, the use of reduced intensity conditioning has resulted in prolonged survival and lower transplant-related mortality. This review summarises the recent advances in the disease's pathogenesis and discusses the role of the various therapeutic options.

Relation between JAK2 (V617F) mutation status, granulocyte activation, and constitutive mobilization of CD34+ cells into peripheral blood in myeloproliferative disorders

We studied the relationship between granulocyte JAK2 (V617F) mutation status, circulating CD34(+) cells, and granulocyte activation in myeloproliferative disorders. Quantitative allele-specific polymerase chain reaction (PCR) showed significant differences between various disorders with respect to either the proportion of positive patients (53%-100%) or that of mutant alleles, which overall ranged from 1% to 100%. In polycythemia vera, JAK2 (V617F) was detected in 23 of 25 subjects at diagnosis and in 16 of 16 patients whose disease had evolved into myelofibrosis; median percentages of mutant alleles in these subgroups were significantly different (32% versus 95%, P < .001). Circulating CD34(+) cell counts were variably elevated and associated with disease category and JAK2 (V617F) mutation status. Most patients had granulocyte activation patterns similar to those induced by administration of granulocyte colony-stimulating factor. A JAK2 (V617F) gene dosage effect on both CD34(+) cell counts and granulocyte activation was clearly demonstrated in polycythemia vera, where abnormal patterns were mainly found in patients carrying more than 50% mutant alleles. These observations suggest that JAK2 (V617F) may constitutively activate granulocytes and by this means mobilize CD34(+) cells. This exemplifies a novel paradigm in which a somatic gain-of-function mutation is initially responsible for clonal expansion of hematopoietic cells and later for their abnormal trafficking via an activated cell progeny.

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.

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.

Constitutive mobilization of CD34+ cells into the peripheral blood in idiopathic myelofibrosis may be due to the action of a number of proteases

Idiopathic myelofibrosis (IM) is characterized by increased numbers of CD34+ cells in the peripheral blood (PB). We explored the possible mechanisms underlying this abnormal trafficking of CD34+ cells. Plasma levels of neutrophil elastase (NE), total and active matrix metalloproteinase 9 (MMP-9), and soluble vascular cell adhesion molecule-1 (sVCAM-1) were dramatically increased in IM. The absolute number of CD34+ cells in the PB was correlated with the levels of sVCAM-1. Marked elevations of the levels of NE but not total and active MMP-9 as well as MMP-2 were detected in media conditioned by IM mononuclear cells (MNCs) as compared with that of healthy volunteers. IM MNC-conditioned media, however, was shown by zymographic analysis to contain increased gelatinolytic activity corresponding to the molecular weight of MMP-9. IM MNC-conditioned media also exhibited a greater ability to cleave VCAM-1 and c-kit in vitro, consistent with the biologic actions of NE. In addition, the increased ability of IM PB CD34+ cells to migrate through a reconstituted basement membrane was diminished by several inhibitors of MMP-9 activity, indicating that these cells express increased levels of this MMP. These data indicate that a proteolytic environment exists in IM which might result in the sustained mobilization of CD34+ cells.