FIP1L1-PDGFRalpha imposes eosinophil lineage commitment on hematopoietic stem/progenitor cells

Phosflow assessment of PDGFRA phosphorylation state: A guide for tyrosine kinase inhibitor targeted therapy in hypereosinophilia patients

Clonal eosinophilia is a hematologic disorder caused by translocation in growth factor receptor (GFR) genes. Despite the identified molecular mechanisms underlying clonal hypereosinophilia, the distinction between clonal and reactive eosinophilia has remained challenging due to the diversity of partner genes for translocated GFRs. This study aimed to examine the feasibility of phosphoflow cytometry in the diagnosis of clonal hypereosinophilia through evaluating the level of platelet-derived growth factor receptor alpha (PDGFRA) phosphorylation and its correlation with PDGFRA genetic aberration. Blood samples were collected from 45 hypereosinophilia patients and 10 healthy controls. Using phosphoflow cytometry method, the phosphorylation state of PDGFRA was assessed. The specificity of phosflow results was confirmed by western blotting and eventually compared with qRT-PCR expression analysis of 3'-region of PDGFRA. To detect the genetic aberration of PDGFRA, 5'-rapid amplification of cDNA ends (5'-RACE) was performed. Phosflow analysis illustrated that 9 of 45 hypereosinophilic patients had higher level of PDGFRA phosphorylation while sequence analysis of 5'-RACE-PCR fragments confirmed that in seven cases of them, there was a PDGFRA-FIP1L1 fusion. We also verified that two of nine patients with hyperposphorylated PDGFRA hold ETV6-PDGFRA and STRN-PDGFRA rearrangements. Importantly, nine cases also had significantly higher levels of PDGFRA mRNA expression when compared with healthy controls, and cases with no PDGFRA rearrangement. These findings highlight a robust correlation between hyperphosphorylation state of PDGFRA and aberrant PDGFRA gene fusions. This implicates phosflow as an efficient and reliable technique raising an intriguing possibility that it could replace other genomic and cDNA-amplification-based diagnostic approaches with limited effectiveness.

Molecular Pathogenesis and Treatment Perspectives for Hypereosinophilia and Hypereosinophilic Syndromes

Hypereosinophilia (HE) is a heterogeneous condition with a persistent elevated eosinophil count of >350/mm³, which is reported in various (inflammatory, allergic, infectious, or neoplastic) diseases with distinct pathophysiological pathways. HE may be associated with tissue or organ damage and, in this case, the disorder is classified as hypereosinophilic syndrome (HES). Different studies have allowed for the discovery of two major pathogenetic variants known as myeloid or lymphocytic HES. With the advent of molecular genetic analyses, such as T-cell receptor gene rearrangement assays and Next Generation Sequencing, it is possible to better characterize these syndromes and establish which patients will benefit from pharmacological targeted therapy. In this review, we highlight the molecular alterations that are involved in the pathogenesis of eosinophil disorders and revise possible therapeutic approaches, either implemented in clinical practice or currently under investigation in clinical trials.

Involvement of platelet-derived growth factor ligands and receptors in tumorigenesis

Platelet-derived growth factor (PDGF) isoforms and their receptors have important roles during embryogenesis, particularly in the development of various mesenchymal cell types in different organs. In the adult, PDGF stimulates wound healing and regulates tissue homeostasis. However, overactivity of PDGF signalling is associated with malignancies and other diseases characterized by excessive cell proliferation, such as fibrotic conditions and atherosclerosis. In certain tumours, genetic or epigenetic alterations of the genes for PDGF ligands and receptors drive tumour cell proliferation and survival. Examples include the rare skin tumour dermatofibrosarcoma protuberance, which is driven by autocrine PDGF stimulation due to translocation of a PDGF gene, and certain gastrointestinal stromal tumours and leukaemias, which are driven by constitute activation of PDGF receptors due to point mutations and formation of fusion proteins of the receptors, respectively. Moreover, PDGF stimulates cells in tumour stroma and promotes angiogenesis as well as the development of cancer-associated fibroblasts, both of which promote tumour progression. Inhibitors of PDGF signalling may thus be of clinical usefulness in the treatment of certain tumours.

Leukemogenic kinase FIP1L1-PDGFRA and a small ubiquitin-like modifier E3 ligase, PIAS1, form a positive cross-talk through their enzymatic activities

Fusion tyrosine kinases play a crucial role in the development of hematological malignancies. FIP1L1‐PDGFRA is a leukemogenic fusion kinase that causes chronic eosinophilic leukemia. As a constitutively active kinase, FIP1L1‐PDGFRA stimulates downstream signaling molecules, leading to cellular proliferation and the generation of an anti‐apoptotic state. Contribution of the N‐terminal FIP1L1 portion is necessary for FIP1L1‐PDGFRA to exert its full transforming activity, but the underlying mechanisms have not been fully characterized. We identified PIAS1 as a FIP1L1‐PDGFRA association molecule by yeast two‐hybrid screening. Our analyses indicate that the FIP1L1 portion of FIP1L1‐PDGFRA is required for efficient association with PIAS1. As a consequence of the association, FIP1L1‐PDGFRA phosphorylates PIAS1. Moreover, the kinase activity of FIP1L1‐PDGFRA stabilizes PIAS1. Therefore, PIAS1 is one of the downstream targets of FIP1L1‐PDGFRA. Moreover, we found that PIAS1, as a SUMO E3 ligase, sumoylates and stabilizes FIP1L1‐PDGFRA. In addition, suppression of PIAS1 activity by a knockdown experiment resulted in destabilization of FIP1L1‐PDGFRA. Therefore, FIP1L1‐PDGFRA and PIAS1 form a positive cross‐talk through their enzymatic activities. Suppression of sumoylation by ginkgolic acid, a small molecule compound inhibiting a SUMO E1‐activating enzyme, also destabilizes FIP1L1‐PDGFRA, and while the tyrosine kinase inhibitor imatinib suppresses FIP1L1‐PDGFRA‐dependent cell growth, ginkgolic acid or siRNA of PIAS1 has a synergistic effect with imatinib. In conclusion, our results suggest that sumoylation by PIAS1 is a potential target in the treatment of FIP1L1‐PDGFRA‐positive chronic eosinophilic leukemia.

Lyn mediates FIP1L1-PDGFRA signal pathway facilitating IL-5RA intracellular signal through FIP1L1-PDGFRA/JAK2/Lyn/Akt network complex in CEL

The Fip1-like1 (FIP1L1)–platelet-derived growth factor receptor alpha (PDGFRA) (F/P) oncogene can cause chronic eosinophilic leukemia (CEL), but requires IL-5 cytokine participation. In this study, we investigate the mechanism of F/P in collaboration with IL-5 in CEL. The results showed that Lyn, a key effector in the IL-5-motivated eosinophil production, is extensively activated in F/P-positive CEL cells. Lyn can associate and phosphorylate IL-5 receptor α (IL-5RA) in F/P-positive cells. Moreover, the activation of Lyn and IL-5R kinase were strengthened when the cells were stimulated by IL-5. Lyn inhibition in F/P-positive CEL cells attenuated cellular proliferation, induced apoptosis, and blocked cell migration and major basic protein (MBP) release. We identified the FIP1L1-PDGFRA/JAK2/Lyn/Akt complex in the F/P-expressing cells which can be disrupted by dual inhibition of JAK2 and Lyn, repressing cell proliferation in both EOL-1(F/P-positive human eosinophilic cell line) and imatinib-resistance (IR) cells. Altogether, our data demonstrate that Lyn is a vital downstream kinase activated by F/P converged with IL-5 signals in CEL cells. Lyn activate and expand IL-5RA intracellular signaling through FIP1L1-PDGFRA/JAK2/Lyn/Akt network complex, provoking eosinophils proliferation and exaggerated activation manifested as CEL.

Murine models of eosinophilic leukemia: a model of FIP1L1-PDGFRα initiated chronic eosinophilic leukemia/systemic mastocytosis

Chronic eosinophilic leukemia (CEL) was distinguished from hypereosinophilic syndrome (HES) in the 2001 World Health Organization (WHO) criteria. Subsequently, the FIP1L1-PDGFRα (F/P) fusion tyrosine kinase was identified in patients with HES and found to be the most common clonal defect in CEL and the second most frequent mutation in systemic mastocytosis (SM). Introduction of F/P into bone marrow hematopoietic stem cells and progenitors has been used to establish murine models of F/P-myeloproliferative neoplasm and F/P-CEL. IL-5 overexpression and introduction of F/P is required to develop murine CEL. This F/P-CEL model is thought to be an accurate model of the clinical disease. Here we describe the method of F/P-CEL/SM model development and assessment.

Hes1 upregulation contributes to the development of FIP1L1-PDGRA-positive leukemia in blast crisis

We have previously shown that elevated expression of Hairy enhancer of split 1 (Hes1) contributes to blast crisis transition in Bcr-Abl-positive chronic myelogenous leukemia. Here we investigate whether Hes1 is involved in the development of other myeloid neoplasms. Notably, Hes1 expression was elevated in only a few cases of 65 samples with different types of myeloid neoplasms. Interestingly, elevated expression of Hes1 was found in two of five samples of Fip1-like1 platelet-derived growth factor receptor-α (FIP1L1-PDGFA)-positive myeloid neoplasms associated with eosinophilia. Whereas FIP1L1-PDGFRα alone induced acute T-cell leukemia or myeloproliferative neoplasms in mouse bone marrow transplantation models, mice transplanted with bone marrow cells expressing both Hes1 and FIP1L1-PDGFRα developed acute leukemia characterized by an expansion of myeloid blasts and leukemic cells without eosinophilic granules. FIP1L1-PDGFRα conferred cytokine-independent growth to Hes1-transduced common myeloid progenitors, interleukin-3-dependent cells. Imatinib inhibited the growth of common myeloid progenitors expressing Hes1 with FIP1L1-PDGFRα, but not with imatinib-resistant FIP1L1-PDGFRα mutants harboring T674I or D842V. In contrast, ponatinib efficiently eradicated leukemic cells expressing Hes1 and the imatinib-resistant FLP1L1-PDGFRΑ mutant in vitro and in vivo. Thus, we have established mouse models of FIP1L1-PDGFRA-positive leukemia in myeloid blast crisis, which will help elucidate the pathogenesis of the disease and develop a new treatment for it.

Targeting the PDGF signaling pathway in tumor treatment

Platelet-derived growth factor (PDGF) isoforms and PDGF receptors have important functions in the regulation of growth and survival of certain cell types during embryonal development and e.g. tissue repair in the adult. Overactivity of PDGF receptor signaling, by overexpression or mutational events, may drive tumor cell growth. In addition, pericytes of the vasculature and fibroblasts and myofibroblasts of the stroma of solid tumors express PDGF receptors, and PDGF stimulation of such cells promotes tumorigenesis. Inhibition of PDGF receptor signaling has proven to useful for the treatment of patients with certain rare tumors. Whether treatment with PDGF/PDGF receptor antagonists will be beneficial for more common malignancies is the subject for ongoing studies.

The conformational control inhibitor of tyrosine kinases DCC-2036 is effective for imatinib-resistant cells expressing T674I FIP1L1-PDGFRα

The cells expressing the T674I point mutant of FIP1-like-1-platelet-derived growth factor receptor alpha (FIP1L1-PDGFRα) in hypereosinophilics syndrome (HES) are resistant to imatinib and some second-generation tyrosine kinase inhibitors (TKIs). There is a desperate need to develop therapy to combat this acquired drug resistance. DCC-2036 has been synthesized as a third-generation TKI to combat especially the Bcr-Abl T315I mutant in chronic myeloid leukemia. This study evaluated the effect of DCC-2036 on FIP1L1-PDGFRα-positive cells, including the wild type (WT) and the T674I mutant. The in vitro effects of DCC-2036 on the PDGFRα signal pathways, proliferation, cell cycling and apoptosis of FIP1L1-PDGFRα-positive cells were investigated, and a nude mouse xenograft model was employed to assess the in vivo antitumor activity. We found that DCC-2036 decreased the phosphorylated levels of PDGFRα and its downstream targets without apparent effects on total protein levels. DCC-2036 inhibited proliferation, and induced apoptosis with MEK-dependent up-regulation of the pro-apoptotic protein Bim in FIP1L1-PDGFRα-positive cells. DCC-2036 also exhibited in vivo antineoplastic activity against cells with T674I FIP1L1-PDGFRα. In summary, FIP1L1-PDGFRα-positive cells are sensitive to DCC-2036 regardless of their sensitivity to imatinib. DCC-2036 may be a potential compound to treat imatinib-resistant HES.

Review of current classification, molecular alterations, and tyrosine kinase inhibitor therapies in myeloproliferative disorders with hypereosinophilia

Recent advances in our understanding of the molecular mechanisms underlying hypereosinophilia have led to the development of a ‘molecular’ classification of myeloproliferative disorders with eosinophilia. The revised 2008 World Health Organization classification of myeloid neoplasms included a new category called “myeloid and lymphoid neoplasms with eosinophilia and abnormalities of PDGFRA, PDGFRB or FGFR1.” Despite the molecular heterogeneity of PDGFR (platelet-derived growth factor receptor) rearrangements, tyrosine kinase inhibitors at low dose induce rapid and complete hematological remission in the majority of these patients. Other kinase inhibitors are promising. Further discoveries of new molecular alterations will direct the development of new specific inhibitors. In this review, an update of the classifications of myeloproliferative disorders associated with hypereosinophilia is discussed together with open and controversial questions. Molecular mechanisms and promising results of tyrosine kinase inhibitor treatments are reviewed.

Bridges between Cell Cycle Regulation and Self-Renewal Maintenance

Stem cells are a unique population that lies at the summit of any, or at least most, biological systems. They can differentiate in a variety of mature cell types, but they also have the ability to self-renew, that is, the capacity to divide and retain all the features of the mother cell. The regulation of self-renewal has been studied for many years, but several aspects of this regulation are still vague. The combined decision to divide and self-renew or differentiate suggests that the mechanisms that regulate self-renewal and cell cycle activity are intermingled. While inactivation of many cell cycle regulators impacts the physiological and pathological biology of stem cells, the exact mechanisms that link the decision to enter the cell cycle and the choice of the cellular fate are poorly understood. The multiplicity of signals and pathways regulating self-renewal add to the complexity of the phenomenon. Here, I will review the described links between the cell cycle and self-renewal and discuss the role of the niche in the regulation of both mechanisms.

Diagnostic Complexities of Eosinophilia

Context.— The advent of molecular tools capable of subclassifying eosinophilia has changed the diagnostic and clinical approach to what was classically called hypereosinophilic syndrome.

Objectives.— To review the etiologies of eosinophilia and to describe the current diagnostic approach to this abnormality.

Data Sources.— Literature review.

Conclusions.— Eosinophilia is a common, hematologic abnormality with diverse etiologies. The underlying causes can be broadly divided into reactive, clonal, and idiopathic. Classically, many cases of eosinophilia were grouped together into the umbrella category of hypereosinophilic syndrome, a clinical diagnosis of exclusion. In recent years, an improved mechanistic understanding of many eosinophilias has revolutionized the way these disorders are understood, diagnosed, and treated. As a result, specific diagnoses can now be assigned in many cases that were previously defined as hypereosinophilic syndrome. Most notably, chromosomal rearrangements, such as FIP1L1-PDGFRA fusions caused by internal deletions in chromosome 4, are now known to be associated with many chronic eosinophilic leukemias. When present, these specific molecular abnormalities predict response to directed therapies. Although an improved molecular understanding is revolutionizing the treatment of patients with rare causes of eosinophilia, it has also complicated the approach to evaluating and treating eosinophilia. Here, we review causes of eosinophilia and present a framework by which the practicing pathologist may approach this diagnostic dilemma. Finally, we consider recent cases as clinical examples of eosinophilia from a single institution, demonstrating the diversity of etiologies that must be considered.

Identification of JAK2 as a mediator of FIP1L1-PDGFRA-induced eosinophil growth and function in CEL

The Fip1-like1 (FIP1L1)-platelet-derived growth factor receptor alpha fusion gene (F/P) arising in the pluripotent hematopoietic stem cell (HSC),causes 14% to 60% of patients with hypereosinophilia syndrome (HES). These patients, classified as having F/P (+) chronic eosinophilic leukemia (CEL), present with clonal eosinophilia and display a more aggressive disease phenotype than patients with F/P (–) HES patients. The mechanisms underlying predominant eosinophil lineage targeting and the cytotoxicity of eosinophils in this leukemia remain unclear. Given that the Janus tyrosine kinase (JAK)/signal transducers and activators of transcription (Stat) signaling pathway is key to cytokine receptor-mediated eosinophil development and activated Stat3 and Stat5 regulate the expression of genes involved in F/P malignant transformation, we investigated whether and how JAK proteins were involved in the pathogenesis of F/P-induced CEL. F/P activation of JAK2, Stat3 and Stat5, were confirmed in all the 11 F/P (+) CEL patients examined. In vitro inhibition of JAK2 in EOL-1, primary F/P(+) CEL cells (PC) and T674I F/P Imatinib resistant cells(IR) by either JAK2-specific short interfering RNA (siRNA) or the tryphostin derivative AG490(AG490), significantly reduced cellular proliferation and induced cellular apoptosis. The F/P can enhance the IL-5-induced JAK2 activation, and further results indicated that JAK2 inhibition blocked IL-5-induced cellular migration and activation of the EOL-1 and PC cells in vitro. F/P-stimulation of the JAK2 suppressed cells led to a significantly reduction in Stat3 activation, but relatively normal induction of Stat5 activation. Interestingly, JAK2 inhibition also reduced PI3K, Akt and NF-κB activity in a dose-dependent manner, and suppressed expression levels of c-Myc and Survivin. These results strongly suggest that JAK2 is activated by F/P and is required for F/P stimulation of cellular proliferation and infiltration, possibly through induction of c-Myc and Survivin expression via activation of multiple signaling pathways, including NF-κB, Stat3, and PI3K/Akt.

ETV6-PDGFRB and FIP1L1-PDGFRA stimulate human hematopoietic progenitor cell proliferation and differentiation into eosinophils: the role of nuclear factor-κB

BACKGROUND

ETV6-PDGFRB (also called TEL-PDGFRB) and FIP1L1-PDGFRA are receptor-tyrosine kinase fusion genes that cause chronic myeloid malignancies associated with hypereosinophilia. The aim of this work was to gain insight into the mechanisms whereby fusion genes affect human hematopoietic cells and in particular the eosinophil lineage.

DESIGN AND METHODS

We introduced ETV6-PDGFRB and FIP1L1-PDGFRA into human CD34(+) hematopoietic progenitor and stem cells isolated from umbilical cord blood.

RESULTS

Cells transduced with these oncogenes formed hematopoietic colonies even in the absence of cytokines. Both oncogenes also stimulated the proliferation of cells in liquid culture and their differentiation into eosinophils. This model thus recapitulated key features of the myeloid neoplasms induced by ETV6-PDGFRB and FIP1L1-PDGFRA. We next showed that both fusion genes activated the transcription factors STAT1, STAT3, STAT5 and nuclear factor-κB. Phosphatidylinositol-3 kinase inhibition blocked nuclear factor-κB activation in transduced progenitor cells and patients' cells. Nuclear factor-κB was also activated in the human FIP1L1-PDGFRA-positive leukemia cell line EOL1, the proliferation of which was blocked by bortezomib and the IκB kinase inhibitor BMS-345541. A mutant IκB that prevents nuclear translocation of nuclear factor-κB inhibited cell growth and the expression of eosinophil markers, such as the interleukin-5 receptor and eosinophil peroxidase, in progenitors transduced with ETV6-PDGFRB. In addition, several potential regulators of this process, including HES6, MYC and FOXO3 were identified using expression microarrays.

CONCLUSIONS

We show that human CD34(+) cells expressing PDGFR fusion oncogenes proliferate autonomously and differentiate towards the eosinophil lineage in a process that requires nuclear factor-κB. These results suggest new treatment possibilities for imatinib-resistant myeloid neoplasms associated with PDGFR mutations.

Platelet-derived growth factors and their receptors in normal and malignant hematopoiesis

Platelet-derived growth factors (PDGF) bind to two closely related receptor tyrosine kinases, PDGF receptor α and β, which are encoded by the PDGFRA and PDGFRB genes. Aberrant activation of PDGF receptors occurs in myeloid malignancies associated with hypereosinophilia, due to chromosomal alterations that produce fusion genes, such as ETV6-PDGFRB or FIP1L1-PDGFRA. Most patients are males and respond to low dose imatinib, which is particularly effective against PDGF receptor kinase activity. Recently, activating point mutations in PDGFRA were also described in hypereosinophilia. In addition, autocrine loops have been identified in large granular lymphocyte leukemia and HTLV-transformed lymphocytes, suggesting new possible indications for tyrosine kinase inhibitor therapy. Although PDGF was initially purified from platelets more than 30 years ago, its physiological role in the hematopoietic system remains unclear. Hematopoietic defects in PDGF-deficient mice have been reported but appear to be secondary to cardiovascular and placental abnormalities. Nevertheless, PDGF acts directly on several hematopoietic cell types in vitro, such as megakaryocytes, platelets, activated macrophages and, possibly, certain lymphocyte subsets and eosinophils. The relevance of these observations for normal human hematopoiesis remains to be established.

The two faces of myeloproliferative neoplasms: Molecular events underlying lymphoid transformation

Multipotent haematopoietic stem cells pass through stages of differentiation with the progressive loss of developmental options leading to the production of terminally differentiated mature blood cells. This process is regulated by soluble cytokines binding to a ligand specific cell surface receptor on a precursor cell. Key to signal transduction are tyrosine kinase proteins which can be divided into two sub families, the receptor protein tyrosine kinases which are transmembrane receptors and retain an intact catalytic kinase domain and the cytoplasmic tyrosine kinases which bind to cytokine receptors. Abnormalities of tyrosine kinase proteins are well recognised in myeloid malignancies, mutation in the cytoplasmic tyrosine kinase JAK2 (V617F) is key in the pathogenesis of myeloproliferative neoplasms, and translocations involving ABL key in the development of chronic myeloid leukaemia. However tyrosine kinase mutations are increasingly recognised to play a role in the pathogenesis of a wider range of haematological cancers. This review focuses on the role of deregulated tyrosine kinase genes either as part of novel fusion proteins involving FGFR1, PDGFRα, PDGFRβ, JAK2 and ABL, or as a consequence of point mutation in JAK1 or JAK2 in the development of precursor T and B lymphoid malignancies or mixed myeloid/lymphoid disorders. We also set out some of the postulated mechanisms which underlie the association of tyrosine kinase mutations with the development of lymphoid malignancy.

Preclinical xenograft models of human sarcoma show nonrandom loss of aberrations

BACKGROUND

Human tumors transplanted into immunodeficient mice (xenografts) are good preclinical models, and it is important to identify possible systematic changes during establishment and passaging in mice.

METHODS

High-resolution microarray-based comparative genomic hybridization (array CGH) was used to investigate how well a series of sarcoma xenografts, including 9 patient/xenograft pairs and 8 early versus late xenograft passage pairs, represented the patient tumor from which they originated.

RESULTS

In all analyses, the xenografts were more similar to their tumor of origin than other xenografts of the same type. Most changes in aberration patterns were toward a more normal genome complement, and the increased aberrations observed were mostly toward more loss. In general, the changes were scattered over the genome, but some changes were significant in osteosarcomas. These were rather focused and consistent with amplifications frequent in patient samples, involving the genes platelet-derived growth factor receptor A (PDGFRA), cysteine-rich hydrophobic domain 2 (CHIC2), FIP-like 1 (FIP1L1), ligand of numb-protein X1 (LNX1), RAS-like family 11 member B (RASL11B), and sec1 family domain containing 2 (SCFD2), probably a sign of continued tumor progression. Some changes that disappeared may have been involved in host-stroma interactions or chemotherapy resistance, possibly because of the absence of selection in the mouse.

CONCLUSIONS

Direct xenografts reflected well the genomic patterns of their tumors of origin. The few significant aberrations that were lost during passaging in immune-defective mice may have been caused by the lack of selection in the new host, whereas aberrations that were gained appeared to be the result of general tumor progression rather than model-specific artifacts.

World health organization classification, evaluation, and genetics of the myeloproliferative neoplasm variants

There is no single category in the fourth edition (2008) of the World Health Organization (WHO) classification of myeloid neoplasms that encompasses all of the diseases referred to by some authors as the myeloproliferative neoplasm (MPN) "variants." Instead, they are considered as distinct entities and are distributed among various subgroups of myeloid neoplasms in the classification scheme. These relatively uncommon neoplasms do not meet the criteria for any so-called "classical" MPN (chronic myelogenous leukemia, polycythemia vera, primary myelofibrosis, or essential thrombocythemia) and, although some exhibit myelodysplasia, none meets the criteria for any myelodysplastic syndrome (MDS). They are a diverse group of neoplasms ranging from fairly well-characterized disorders such as chronic myelomonocytic leukemia to rare and thus poorly characterized disorders such as chronic neutrophilic leukemia. Recently, however, there has been a surge of information regarding the genetic infrastructure of neoplastic cells in the MPN variants, allowing some to be molecularly defined. Nevertheless, in most cases, correlation of clinical, genetic, and morphologic findings is required for diagnosis and classification. The fourth edition of the WHO classification provides a framework to incorporate those neoplasms in which a genetic abnormality is a major defining criterion of the disease, such as those associated with eosinophilia and abnormalities of PDGFRA, PDGFRB, and FGFR1, as well as for those in which no specific genetic defect has yet been discovered and which remain clinically and pathologically defined. An understanding of the clinical, morphologic, and genetic features of the MPN variants will facilitate their diagnosis.

BCR-ABL but not JAK2 V617F inhibits erythropoiesis through the Ras signal by inducing p21CIP1/WAF1

BCR-ABL is a causative tyrosine kinase (TK) of chronic myelogenous leukemia (CML). In CML patients, although myeloid cells are remarkably proliferating, erythroid cells are rather decreased and anemia is commonly observed. This phenotype is quite different from that observed in polycythemia vera (PV) caused by JAK2 V617F, whereas both oncogenic TKs activate common downstream molecules at the level of hematopoietic stem cells (HSCs). To clarify this mechanism, we investigated the effects of BCR-ABL and JAK2 V617F on erythropoiesis. Enforced expression of BCR-ABL but not of JAK2 V617F in murine LSK (Lineage(-)Sca-1(hi)CD117(hi)) cells inhibited the development of erythroid cells. Among several signaling molecules downstream of BCR-ABL, an active mutant of N-Ras (N-RasE12) but not of STAT5 or phosphatidylinositol 3-kinase (PI3-K) inhibited erythropoiesis, while N-RasE12 enhanced the development of myeloid cells. BCR-ABL activated Ras signal more intensely than JAK2 V617F, and inhibition of Ras by manumycin A, a farnesyltransferase inhibitor, ameliorated erythroid colony formation of CML cells. As for the mechanisms of Ras-induced suppression of erythropoiesis, we found that GATA-1, an erythroid-specific transcription factor, blocked Ras-mediated mitogenic signaling at the level of MEK through the direct interaction. Furthermore, enforced expression of N-RasE12 in LSK cells derived from p53-, p16(INK4a)/p19(ARF)-, and p21(CIP1/WAF1)-null/wild-type mice revealed that suppressed erythroid cell growth by N-RasE12 was restored only by p21(CIP1/WAF1) deficiency, indicating that a cyclin-dependent kinase (CDK) inhibitor, p21(CIP1/WAF1), plays crucial roles in Ras-induced suppression of erythropoiesis. These data would, at least partly, explain why respective oncogenic TKs cause different disease phenotypes.

Fractionation of mature eosinophils in GATA-reporter transgenic mice

Eosinophils contribute to the pathophysiology of allergic and infectious diseases, albeit their molecular functions remain unknown. Mature eosinophils are identified by their unique morphology and staining characteristics. However, it is difficult to fractionate living eosinophils by flow cytometry because these granulocytes express multiple cell surface markers that are shared by other cells of hematopoietic or non-hematopoietic origin. In this study, we describe a flow cytometry-based method to enumerate and fractionate eosinophils by developmental stages. To fractionate these cell types, we used transgenic mouse lines that express fluorescent proteins under control of the Gata1 gene hematopoietic regulatory region (Gata1-HRD), which is exclusively active in Gata1-expressing hematopoietic cells, including eosinophils. As expected, mature eosinophils were highly enriched in the fluorescent reporter-expressing subfraction of bone marrow myeloid cells that were negatively selected by using multiple antibodies against B220, CD4, CD8, Ter119, c-Kit and CD71. Cytochemical analyses of flow-sorted cells identified the cells in this fraction as eosinophils harboring eosinophilic granules. Additionally, expression of eosinophil-specific genes, for instance eosinophil enzymes and the IL-5 receptor alpha gene, were specifically detected in this fraction. The expression of these eosinophil-specific genes increased as the cells differentiated. This method for enrichment of bone marrow eosinophils is applicable to fractionation of eosinophils and bronchoalveolar lavage fluid from transgenic mice with atopic asthma. Thus, both pathological and developmental stages of eosinophils are efficiently fractionated by this flow cytometry-based method using Gata1-HRD transgenic reporter mice. This study, therefore, proposes a useful means to study the experimental allergic and inflammatory systems.

A review of treatment with mepolizumab, an anti-IL-5 mAb, in hypereosinophilic syndromes and asthma

The hypereosinophilic syndromes (HESs) are a heterogeneous group of diseases characterized by peripheral blood eosinophilia with end-organ damage and varying in severity from nonspecific symptoms to life-threatening. Treatment objectives are a safe reduction of blood and tissue eosinophil levels and prevention of eosinophil-mediated tissue damage. Current treatment of patients with HESs, who lack the FIP-1-like 1-platelet-derived growth factor receptor alpha (FIP1L1-PDGFRA) fusion gene, is mainly systemic corticosteroid therapy. Eosinophil development from hematopoietic progenitor cells is regulated by IL-5, which influences maturation, differentiation, mobilization, activation, and survival. Consequently, inhibiting IL-5 is a logical therapeutic objective for patients with HESs or selected patients with asthma. Mepolizumab is a fully humanized anti-IL-5 monoclonal IgG(1) antibody that binds to free IL-5 with high affinity and specificity to prevent IL-5 from associating with the IL-5 receptor complex alpha-chain on the surface of eosinophils. In clinical trials with patients with HESs, mepolizumab reduced blood eosinophil counts and the maintenance corticosteroid dose and had no major safety concerns. Mepolizumab reduced airway and blood eosinophils and prevented asthma exacerbations. Thus, mepolizumab may be effective for long-term treatment of patients with selected eosinophilic disorders.