Efficacy of TG101348, a selective JAK2 inhibitor, in treatment of a murine model of JAK2V617F-induced polycythemia vera

We report that TG101348, a selective small-molecule inhibitor of JAK2 with an in vitro IC50 of approximately 3 nM, shows therapeutic efficacy in a murine model of myeloproliferative disease induced by the JAK2V617F mutation. In treated animals, there was a statistically significant reduction in hematocrit and leukocyte count, a dose-dependent reduction/elimination of extramedullary hematopoiesis, and, at least in some instances, evidence for attenuation of myelofibrosis. There were no apparent toxicities and no effect on T cell number. In vivo responses were correlated with surrogate endpoints, including reduction/elimination of JAK2V617F disease burden assessed by quantitative genomic PCR, suppression of endogenous erythroid colony formation, and in vivo inhibition of JAK-STAT signal transduction as assessed by flow cytometric measurement of phosphorylated Stat5.

Identification of driver and passenger mutations of FLT3 by high-throughput DNA sequence analysis and functional assessment of candidate alleles

Mutations in the juxtamembrane and kinase domains of FLT3 are common in AML, but it is not known whether alterations outside these regions contribute to leukemogenesis. We used a high-throughput platform to interrogate the entire FLT3 coding sequence in AML patients without known FLT3 mutations and experimentally tested the consequences of each candidate leukemogenic allele. This approach identified gain-of-function mutations that activated downstream signaling and conferred sensitivity to FLT3 inhibition and alleles that were not associated with kinase activation, including mutations in the catalytic domain. These findings support the concept that acquired mutations in cancer may not contribute to malignant transformation and underscore the importance of functional studies to distinguish "driver" mutations underlying tumorigenesis from biologically neutral "passenger" alterations.

FLT3 mutations confer enhanced proliferation and survival properties to multipotent progenitors in a murine model of chronic myelomonocytic leukemia

Despite their known transforming properties, the effects of leukemogenic FLT3-ITD mutations on hematopoietic stem and multipotent progenitor cells and on hematopoietic differentiation are not well understood. We report a mouse model harboring an ITD in the murine Flt3 locus that develops myeloproliferative disease resembling CMML and further identified FLT3-ITD mutations in a subset of human CMML. These findings correlated with an increase in number, cell cycling, and survival of multipotent stem and progenitor cells in an ITD dose-dependent manner in animals that exhibited alterations within their myeloid progenitor compartments and a block in normal B cell development. This model provides insights into the consequences of constitutive signaling by an oncogenic tyrosine kinase on hematopoietic progenitor quiescence, function, and cell fate.

Uniform sensitivity of FLT3 activation loop mutants to the tyrosine kinase inhibitor midostaurin

Small molecule inhibitors that target fms-like tyrosine kinase 3 (FLT3)-activating mutations have potential in the treatment of leukemias. However, certain mutations can simultaneously activate the tyrosine kinase, and confer resistance to small molecule inhibitors. We therefore tested the sensitivity of 8 FLT3 activation loop mutants to midostaurin. Each mutant conferred IL-3 factor-independent proliferation to Ba/F3 cells, and each resulted in the constitutive activation of FLT3 and its targets, signal transducer and activator of transcription 5 (STAT5) and extracellular stimuli-responsive kinase (ERK). For each mutant tested, midostaurin inhibited cell growth and phosphorylation of FLT3, STAT5, and ERK. In contrast, midostaruin did not inhibit Ba/F3 cells stably transduced with FLT3-internal tandem duplications containing a G697R mutation that confers resistance to midostaurin, demonstrating that midostaurin inhibition of FLT3 activation loop mutants was not due to off-target effects. We conclude that midostaurin is a potent inhibitor of a spectrum of FLT3 activation loop mutations, and that acute myeloid leukemia patients with such mutations are potential candidates for clinical trials involving midostaurin.

Role of JAK2 in the pathogenesis and therapy of myeloproliferative disorders

The myeloproliferative disorders polycythaemia vera (PV), essential thombocythaemia (ET), and primary myelofibrosis (PMF) are clonal disorders of multipotent haematopoietic progenitors. The genetic cause of these diseases was not known until 2005, when several independent groups demonstrated that most patients with PV, ET and PMF acquire a single point mutation in the cytoplasmic tyrosine kinase JAK2 (JAK2V617F). These discoveries have changed the landscape for diagnosis and classification of PV, ET and PMF, and show the ability of genomic technologies to identify new molecular targets in human malignancies with pathogenetic, diagnostic and therapeutic significance.

FGFR3 activates RSK2 to mediate hematopoietic transformation through tyrosine phosphorylation of RSK2 and activation of the MEK/ERK pathway

To better understand the signaling properties of oncogenic FGFR3, we performed phospho-proteomics studies to identify potential downstream signaling effectors that are tyrosine phosphorylated in hematopoietic cells expressing constitutively activated leukemogenic FGFR3 mutants. We found that FGFR3 directly tyrosine phosphorylates the serine/threonine kinase p90RSK2 at Y529, which consequently regulates RSK2 activation by facilitating inactive ERK binding to RSK2 that is required for ERK-dependent phosphorylation and activation of RSK2. Moreover, inhibition of RSK2 by siRNA or a specific RSK inhibitor fmk effectively induced apoptosis in FGFR3-expressing human t(4;14)-positive myeloma cells. Our findings suggest that FGFR3 mediates hematopoietic transformation by activating RSK2 in a two-step fashion, promoting both the ERK-RSK2 interaction and subsequent phosphorylation of RSK2 by ERK.

Proposals and rationale for revision of the World Health Organization diagnostic criteria for polycythemia vera, essential thrombocythemia, and primary myelofibrosis: recommendations from an ad hoc international expert panel

The Janus kinase 2 mutation, JAK2617V>F, is myeloid neoplasm-specific; its presence excludes secondary polycythemia, thrombocytosis, or bone marrow fibrosis from other causes. Furthermore, JAK2617V>F or a JAK2 exon 12 mutation is present in virtually all patients with polycythemia vera (PV), whereas JAK2617V>F also occurs in approximately half of patients with essential thrombocythemia (ET) or primary myelofibrosis (PMF). Therefore, JAK2 mutation screening holds the promise of a decisive diagnostic test in PV while being complementary to histology for the diagnosis of ET and PMF; the combination of molecular testing and histologic review should also facilitate diagnosis of ET associated with borderline thrombocytosis. Accordingly, revision of the current World Health Organization (WHO) diagnostic criteria for PV, ET, and PMF is warranted; JAK2 mutation analysis should be listed as a major criterion for PV diagnosis, and the platelet count threshold for ET diagnosis can be lowered from 600 to 450 x 10(9)/L. The current document was prepared by an international expert panel of pathologists and clinical investigators in myeloproliferative disorders; it was subsequently presented to members of the Clinical Advisory Committee for the revision of the WHO Classification of Myeloid Neoplasms, who endorsed the document and recommended its adoption by the WHO.

Highly sensitive method for genomewide detection of allelic composition in nonpaired, primary tumor specimens by use of affymetrix single-nucleotide-polymorphism genotyping microarrays

Loss of heterozygosity (LOH), either with or without accompanying copy-number loss, is a cardinal feature of cancer genomes that is tightly linked to cancer development. However, detection of LOH is frequently hampered by the presence of normal cell components within tumor specimens and the limitation in availability of constitutive DNA. Here, we describe a simple but highly sensitive method for genomewide detection of allelic composition, based on the Affymetrix single-nucleotide-polymorphism genotyping microarray platform, without dependence on the availability of constitutive DNA. By sensing subtle distortions in allele-specific signals caused by allelic imbalance with the use of anonymous controls, sensitive detection of LOH is enabled with accurate determination of allele-specific copy numbers, even in the presence of up to 70%-80% normal cell contamination. The performance of the new algorithm, called "AsCNAR" (allele-specific copy-number analysis using anonymous references), was demonstrated by detecting the copy-number neutral LOH, or uniparental disomy (UPD), in a large number of acute leukemia samples. We next applied this technique to detection of UPD involving the 9p arm in myeloproliferative disorders (MPDs), which is tightly associated with a homozygous JAK2 mutation. It revealed an unexpectedly high frequency of 9p UPD that otherwise would have been undetected and also disclosed the existence of multiple subpopulations having distinct 9p UPD within the same MPD specimen. In conclusion, AsCNAR should substantially improve our ability to dissect the complexity of cancer genomes and should contribute to our understanding of the genetic basis of human cancers.

TG101209, a small molecule JAK2-selective kinase inhibitor potently inhibits myeloproliferative disorder-associated JAK2V617F and MPLW515L/K mutations

JAK2V617F and MPLW515L/K represent recently identified mutations in myeloproliferative disorders (MPD) that cause dysregulated JAK-STAT signaling, which is implicated in MPD pathogenesis. We developed TG101209, an orally bioavailable small molecule that potently inhibits JAK2 (IC(50)=6 nM), FLT3 (IC(50)=25 nM) and RET (IC(50)=17 nM) kinases, with significantly less activity against other tyrosine kinases including JAK3 (IC(50)=169 nM). TG101209 inhibited growth of Ba/F3 cells expressing JAK2V617F or MPLW515L mutations with an IC(50) of approximately 200 nM. In a human JAK2V617F-expressing acute myeloid leukemia cell line, TG101209-induced cell cycle arrest and apoptosis, and inhibited phosphorylation of JAK2V617F, STAT5 and STAT3. Therapeutic efficacy of TG101209 was demonstrated in a nude mouse model. Furthermore, TG101209 suppressed growth of hematopoietic colonies from primary progenitor cells harboring JAK2V617F or MPL515 mutations.

Ott1(Rbm15) has pleiotropic roles in hematopoietic development

OTT1(RBM15) was originally described as a 5' translocation partner of the MAL(MKL1) gene in t(1,22)(p13;q13) infant acute mega karyocytic leukemia. OTT1 has no established physiological function, but it shares homology with the spen/Mint/SHARP family of proteins defined by three amino-terminal RNA recognition motifs and a carboxyl-terminal SPOC (Spen paralog and ortholog carboxyl-terminal) domain believed to act as a transcriptional repressor. To define the role of OTT1 in hematopoiesis and help elucidate the mechanism of t(1,22) acute megakaryocytic leukemia pathogenesis, a conditional allele of Ott1 was generated in mice. Deletion of Ott1 in adult mice caused a loss of peripheral B cells due to a block in pro/pre-B differentiation. There is myeloid and megakaryocytic expansion in spleen and bone marrow, an increase in the Lin(-)Sca-1(+)c-Kit(+) compartment that includes hematopoietic stem cells, and a shift in progenitor fate toward granulocyte differentiation. These data show a requirement for Ott1 in B lymphopoiesis, and inhibitory roles in the myeloid, megakaryocytic, and progenitor compartments. The ability of Ott1 to affect hematopoietic cell fate and expansion in multiple lineages is a novel attribute for a spen family member and delineates Ott1 from other known effectors of hematopoietic development. It is plausible that dysregulation of Ott1-dependent hematopoietic developmental pathways, in particular those affecting the megakaryocyte lineage, may contribute to OTT1-MAL-mediated leukemogenesis.

A novel fusion of RBM6 to CSF1R in acute megakaryoblastic leukemia

Activated tyrosine kinases have been frequently implicated in the pathogenesis of cancer, including acute myeloid leukemia (AML), and are validated targets for therapeutic intervention with small-molecule kinase inhibitors. To identify novel activated tyrosine kinases in AML, we used a discovery platform consisting of immunoaffinity profiling coupled to mass spectrometry that identifies large numbers of tyrosine-phosphorylated proteins, including active kinases. This method revealed the presence of an activated colony-stimulating factor 1 receptor (CSF1R) kinase in the acute megakaryoblastic leukemia (AMKL) cell line MKPL-1. Further studies using siRNA and a small-molecule inhibitor showed that CSF1R is essential for the growth and survival of MKPL-1 cells. DNA sequence analysis of cDNA generated by 5'RACE from CSF1R coding sequences identified a novel fusion of the RNA binding motif 6 (RBM6) gene to CSF1R gene generated presumably by a t(3;5)(p21;q33) translocation. Expression of the RBM6-CSF1R fusion protein conferred interleukin-3 (IL-3)-independent growth in BaF3 cells, and induces a myeloid proliferative disease (MPD) with features of megakaryoblastic leukemia in a murine transplant model. These findings identify a novel potential therapeutic target in leukemogenesis, and demonstrate the utility of phosphoproteomic strategies for discovery of tyrosine kinase alleles.

FoxOs are lineage-restricted redundant tumor suppressors and regulate endothelial cell homeostasis

Activated phosphoinositide 3-kinase (PI3K)-AKT signaling appears to be an obligate event in the development of cancer. The highly related members of the mammalian FoxO transcription factor family, FoxO1, FoxO3, and FoxO4, represent one of several effector arms of PI3K-AKT signaling, prompting genetic analysis of the role of FoxOs in the neoplastic phenotypes linked to PI3K-AKT activation. While germline or somatic deletion of up to five FoxO alleles produced remarkably modest neoplastic phenotypes, broad somatic deletion of all FoxOs engendered a progressive cancer-prone condition characterized by thymic lymphomas and hemangiomas, demonstrating that the mammalian FoxOs are indeed bona fide tumor suppressors. Transcriptome and promoter analyses of differentially affected endothelium identified direct FoxO targets and revealed that FoxO regulation of these targets in vivo is highly context-specific, even in the same cell type. Functional studies validated Sprouty2 and PBX1, among others, as FoxO-regulated mediators of endothelial cell morphogenesis and vascular homeostasis.

The homeobox gene CDX2 is aberrantly expressed in most cases of acute myeloid leukemia and promotes leukemogenesis

The homeobox transcription factor CDX2 plays an important role in embryonic development and regulates the proliferation and differentiation of intestinal epithelial cells in the adult. We have found that CDX2 is expressed in leukemic cells of 90% of patients with acute myeloid leukemia (AML) but not in hematopoietic stem and progenitor cells derived from normal individuals. Stable knockdown of CDX2 expression by RNA interference inhibited the proliferation of various human AML cell lines and strongly reduced their clonogenic potential in vitro. Primary murine hematopoietic progenitor cells transduced with Cdx2 acquired serial replating activity, were able to be continuously propagated in liquid culture, generated fully penetrant and transplantable AML in BM transplant recipients, and displayed dysregulated expression of Hox family members in vitro and in vivo. These results demonstrate that aberrant expression of the developmental regulatory gene CDX2 in the adult hematopoietic compartment is a frequent event in the pathogenesis of AML; suggest a role for CDX2 as part of a common effector pathway that promotes the proliferative capacity and self-renewal potential of myeloid progenitor cells; and support the hypothesis that CDX2 is responsible, in part, for the altered HOX gene expression that is observed in most cases of AML.

The Retinoblastoma Binding Protein RBP2 Is an H3K4 Demethylase

Changes in histone methylation status regulate chromatin structure and DNA-dependent processes such as transcription. Recent studies indicate that, analogous to other histone modifications, histone methylation is reversible. Retinoblastoma binding protein 2 (RBP2), a nuclear protein implicated in the regulation of transcription and differentiation by the retinoblastoma tumor suppressor protein, contains a JmjC domain recently defined as a histone demethylase signature motif. Here we report that RBP2 is a demethylase that specifically catalyzes demethylation on H3K4, whose methylation is normally associated with transcriptionally active genes. RBP2-/- mouse cells displayed enhanced transcription of certain cytokine genes, which, in the case of SDF1, was associated with increased H3K4 trimethylation. Furthermore, RBP2 specifically demethylated H3K4 in biochemical and cell-based assays. These studies provide mechanistic insights into transcriptional regulation by RBP2 and provide the first example of a mammalian enzyme capable of erasing trimethylated H3K4.

JAK2 exon 12 mutations in polycythemia vera and idiopathic erythrocytosis

BACKGROUND

The V617F mutation, which causes the substitution of phenylalanine for valine at position 617 of the Janus kinase (JAK) 2 gene (JAK2), is often present in patients with polycythemia vera, essential thrombocythemia, and idiopathic myelofibrosis. However, the molecular basis of these myeloproliferative disorders in patients without the V617F mutation is unclear.

METHODS

We searched for new mutations in members of the JAK and signal transducer and activator of transcription (STAT) gene families in patients with V617F-negative polycythemia vera or idiopathic erythrocytosis. The mutations were characterized biochemically and in a murine model of bone marrow transplantation.

RESULTS

We identified four somatic gain-of-function mutations affecting JAK2 exon 12 in 10 V617F-negative patients. Those with a JAK2 exon 12 mutation presented with an isolated erythrocytosis and distinctive bone marrow morphology, and several also had reduced serum erythropoietin levels. Erythroid colonies could be grown from their blood samples in the absence of exogenous erythropoietin. All such erythroid colonies were heterozygous for the mutation, whereas colonies homozygous for the mutation occur in most patients with V617F-positive polycythemia vera. BaF3 cells expressing the murine erythropoietin receptor and also carrying exon 12 mutations could proliferate without added interleukin-3. They also exhibited increased phosphorylation of JAK2 and extracellular regulated kinase 1 and 2, as compared with cells transduced by wild-type JAK2 or V617F JAK2. Three of the exon 12 mutations included a substitution of leucine for lysine at position 539 of JAK2. This mutation resulted in a myeloproliferative phenotype, including erythrocytosis, in a murine model of retroviral bone marrow transplantation.

CONCLUSIONS

JAK2 exon 12 mutations define a distinctive myeloproliferative syndrome that affects patients who currently receive a diagnosis of polycythemia vera or idiopathic erythrocytosis.

Jak2: normal function and role in hematopoietic disorders

Janus kinase 2 (Jak2) associates with cytokine receptors and is essential for signal transduction by mediating tyrosine phosphorylation. Kinase activity is regulated by a series of interactions beginning with the requirement to bind to specific domains in receptors, suppression of activation by the pseudokinase domain, and the requirement for phosphorylation within the activation loop. Recent studies have implicated de-regulation of Jak2 kinase activity by chromosomal translocations in hematopoietic tumors and mutations within the pseudokinase domain in a spectrum of myeloproliferative diseases.

FoxOs Are Critical Mediators of Hematopoietic Stem Cell Resistance to Physiologic Oxidative Stress

To understand the role of FoxO family members in hematopoiesis, we conditionally deleted FoxO1, FoxO3, and FoxO4 in the adult hematopoietic system. FoxO-deficient mice exhibited myeloid lineage expansion, lymphoid developmental abnormalities, and a marked decrease of the lineage-negative Sca-1+, c-Kit+ (LSK) compartment that contains the short- and long-term hematopoietic stem cell (HSC) populations. FoxO-deficient bone marrow had defective long-term repopulating activity that correlated with increased cell cycling and apoptosis of HSC. Notably, there was a marked context-dependent increase in reactive oxygen species (ROS) in FoxO-deficient HSC compared with wild-type HSC that correlated with changes in expression of genes that regulate ROS. Furthermore, in vivo treatment with the antioxidative agent N-acetyl-L-cysteine resulted in reversion of the FoxO-deficient HSC phenotype. Thus, FoxO proteins play essential roles in the response to physiologic oxidative stress and thereby mediate quiescence and enhanced survival in the HSC compartment, a function that is required for its long-term regenerative potential.

JAK-2 mutations and their relevance to myeloproliferative disease

PURPOSE OF REVIEW

The identification of the JAK2V617F allele greatly improved our understanding of the molecular pathogenesis of myeloproliferative disorders. This review focuses on recent studies offering new genetic, biochemical, and functional insight into the role of JAK2V617F in the pathogenesis of these disorders.

RECENT FINDINGS

JAK2V617F mutations are present in almost all patients with polycythemia vera, and in approximately half of those with essential thrombocytosis and myelofibrosis. JAK2V617F has constitutive tyrosine kinase activity, and is able to transform hematopoietic cells and activate JAK-STAT signaling when co-expressed with homodimeric type I cytokine receptors. Murine bone marrow transplant experiments demonstrate that JAK2V617F is sufficient for the development of polycythemia vera in recipient mice. These results suggested that JAK-STAT pathway activation might contribute to the pathogenesis of JAK2-negative myeloproliferative disorders, and led to the discovery of an activating mutation in the thrombopoietin receptor in JAK2-negative myelofibrosis and essential thrombocytosis.

SUMMARY

The discovery of the JAK2V617F allele represents an important advance in our understanding of the molecular pathogenesis of myeloproliferative disorders, though many questions remain regarding the role of a single allele in three clinically distinct disorders, the mechanism of activation of JAK2V617F, and the pathogenesis of JAK2-negative myeloproliferative disorders.

Concurrent MPL515 and JAK2V617F mutations in myelofibrosis: chronology of clonal emergence and changes in mutant allele burden over time

MPLW515L/K and JAK2V617F can co-exist in myelofibrosis with myeloid metaplasia (MMM). The chronology of clonal emergence was studied in three such cases using serially stored bone marrow. At diagnosis, a major MPL515 mutant clone was accompanied by a minor JAK2V617F clone in all three instances. At 25 time points over a period of 4-8 years, allele burden fluctuated but remained high for MPLW515L/K and low for JAK2V617F. We conclude that MPLW515L/K and JAK2V617F are both early events in MMM and allele burden, rather than the mere presence of these mutations, might be relevant to phenotypic variation in myeloproliferative disorders.

Cdx4 dysregulates Hox gene expression and generates acute myeloid leukemia alone and in cooperation with Meis1a in a murine model

HOX genes have emerged as critical effectors of leukemogenesis, but the mechanisms that regulate their expression in leukemia are not well understood. Recent data suggest that the caudal homeobox transcription factors CDX1, CDX2, and CDX4, developmental regulators of HOX gene expression, may contribute to HOX gene dysregulation in leukemia. We report here that CDX4 is expressed normally in early hematopoietic progenitors and is expressed aberrantly in approximately 25% of acute myeloid leukemia (AML) patient samples. Cdx4 regulates Hox gene expression in the adult murine hematopoietic system and dysregulates Hox genes that are implicated in leukemogenesis. Furthermore, bone marrow progenitors that are retrovirally engineered to express Cdx4 serially replate in methylcellulose cultures, grow in liquid culture, and generate a partially penetrant, long-latency AML in bone marrow transplant recipients. Coexpression of the Hox cofactor Meis1a accelerates the Cdx4 AML phenotype and renders it fully penetrant. Structure-function analysis demonstrates that leukemic transformation requires intact Cdx4 transactivation and DNA-binding domains but not the putative Pbx cofactor interaction motif. Together, these data indicate that Cdx4 regulates Hox gene expression in adult hematopoiesis and may serve as an upstream regulator of Hox gene expression in the induction of acute leukemia. Inasmuch as many human leukemias show dysregulated expression of a spectrum of HOX family members, these collective findings also suggest a central role for CDX4 expression in the genesis of acute leukemia.

Eosinophilic disorders: molecular pathogenesis, new classification, and modern therapy

Before the 1990s, lack of evidence for a reactive cause of hypereosinophilia or chronic eosinophilic leukemia (e.g. presence of a clonal cytogenetic abnormality or increased blood or bone marrow blasts) resulted in diagnosticians characterizing such nebulous cases as 'idiopathic hypereosinophilic syndrome (HES)'. However, over the last decade, significant advances in our understanding of the molecular pathophysiology of eosinophilic disorders have shifted an increasing proportion of cases from this idiopathic HES 'pool' to genetically defined eosinophilic diseases with recurrent molecular abnormalities. The majority of these genetic lesions result in constitutively activated fusion tyrosine kinases, the phenotypic consequence of which is an eosinophilia-associated myeloid disorder. Most notable among these is the recent discovery of the cryptic FIP1L1-PDGFRA gene fusion in karyotypically normal patients with systemic mast cell disease with eosinophilia or idiopathic HES, redefining these diseases as clonal eosinophilias. Rearrangements involving PDGFRA and PDGFRB in eosinophilic chronic myeloproliferative disorders, and of fibroblast growth factor receptor 1 (FGFR1) in the 8p11 stem cell myeloproliferative syndrome constitute additional examples of specific genetic alterations linked to clonal eosinophilia. The identification of populations of aberrant T-lymphocytes secreting eosinophilopoietic cytokines such as interleukin-5 establish a pathophysiologic basis for cases of lymphocyte-mediated hypereosinophilia. This recent revival in understanding the biologic basis of eosinophilic disorders has permitted more genetic specificity in the classification of these diseases, and has translated into successful therapeutic approaches with targeted agents such as imatinib mesylate and recombinant anti-IL-5 antibody.

MPL515 mutations in myeloproliferative and other myeloid disorders: a study of 1182 patients

Recently, a gain-of-function MPL mutation, MPLW515L, was described in patients with JAK2V617F-negative myelofibrosis with myeloid metaplasia (MMM). To gain more information on mutational frequency, disease specificity, and clinical correlates, genomic DNA from 1182 patients with myeloproliferative and other myeloid disorders and 64 healthy controls was screened for MPL515 mutations, regardless of JAK2V617F mutational status: 290 with MMM, 242 with polycythemia vera, 318 with essential thrombocythemia (ET), 88 with myelodysplastic syndrome, 118 with chronic myelomonocytic leukemia, and 126 with acute myeloid leukemia (AML). MPL515 mutations, either MPLW515L (n = 17) or a previously undescribed MPLW515K (n = 5), were detected in 20 patients. The diagnosis of patients with mutant MPL alleles at the time of molecular testing was de novo MMM in 12 patients, ET in 4, post-ET MMM in 1, and MMM in blast crisis in 3. Six patients carried the MPLW515L and JAK2V617F alleles concurrently. We conclude that MPLW515L or MPLW515K mutations are present in patients with MMM or ET at a frequency of approximately 5% and 1%, respectively, but are not observed in patients with polycythemia vera (PV) or other myeloid disorders. Furthermore, MPL mutations may occur concurrently with the JAK2V617F mutation, suggesting that these alleles may have functional complementation in myeloproliferative disease.

Transformation from committed progenitor to leukaemia stem cell initiated by MLL-AF9

Leukaemias and other cancers possess a rare population of cells capable of the limitless self-renewal necessary for cancer initiation and maintenance. Eradication of these cancer stem cells is probably a critical part of any successful anti-cancer therapy, and may explain why conventional cancer therapies are often effective in reducing tumour burden, but are only rarely curative. Given that both normal and cancer stem cells are capable of self-renewal, the extent to which cancer stem cells resemble normal tissue stem cells is a critical issue if targeted therapies are to be developed. However, it remains unclear whether cancer stem cells must be phenotypically similar to normal tissue stem cells or whether they can retain the identity of committed progenitors. Here we show that leukaemia stem cells (LSC) can maintain the global identity of the progenitor from which they arose while activating a limited stem-cell- or self-renewal-associated programme. We isolated LSC from leukaemias initiated in committed granulocyte macrophage progenitors through introduction of the MLL-AF9 fusion protein encoded by the t(9;11)(p22;q23). The LSC were capable of transferring leukaemia to secondary recipient mice when only four cells were transferred, and possessed an immunophenotype and global gene expression profile very similar to that of normal granulocyte macrophage progenitors. However, a subset of genes highly expressed in normal haematopoietic stem cells was re-activated in LSC. LSC can thus be generated from committed progenitors without widespread reprogramming of gene expression, and a leukaemia self-renewal-associated signature is activated in the process. Our findings define progression from normal progenitor to cancer stem cell, and suggest that targeting a self-renewal programme expressed in an abnormal context may be possible.

Activating alleles of JAK3 in acute megakaryoblastic leukemia

Tyrosine kinases are aberrantly activated in numerous malignancies, including acute myeloid leukemia (AML). To identify tyrosine kinases activated in AML, we developed a screening strategy that rapidly identifies tyrosine-phosphorylated proteins using mass spectrometry. This allowed the identification of an activating mutation (A572V) in the JAK3 pseudokinase domain in the acute megakaryoblastic leukemia (AMKL) cell line CMK. Subsequent analysis identified two additional JAK3 alleles, V722I and P132T, in AMKL patients. JAK3(A572V), JAK3(V722I), and JAK3(P132T) each transform Ba/F3 cells to factor-independent growth, and JAK3(A572V) confers features of megakaryoblastic leukemia in a murine model. These findings illustrate the biological importance of gain-of-function JAK3 mutations in leukemogenesis and demonstrate the utility of proteomic approaches to identifying clinically relevant mutations.

MPLW515L is a novel somatic activating mutation in myelofibrosis with myeloid metaplasia

BACKGROUND

The JAK2V617F allele has recently been identified in patients with polycythemia vera (PV), essential thrombocytosis (ET), and myelofibrosis with myeloid metaplasia (MF). Subsequent analysis has shown that constitutive activation of the JAK-STAT signal transduction pathway is an important pathogenetic event in these patients, and that enzymatic inhibition of JAK2V617F may be of therapeutic benefit in this context. However, a significant proportion of patients with ET or MF are JAK2V617F-negative. We hypothesized that activation of the JAK-STAT pathway might also occur as a consequence of activating mutations in certain hematopoietic-specific cytokine receptors, including the erythropoietin receptor (EPOR), the thrombopoietin receptor (MPL), or the granulocyte-colony stimulating factor receptor (GCSFR).

METHODS AND FINDINGS

DNA sequence analysis of the exons encoding the transmembrane and juxtamembrane domains of EPOR, MPL, and GCSFR, and comparison with germline DNA derived from buccal swabs, identified a somatic activating mutation in the transmembrane domain of MPL (W515L) in 9% (4/45) of JAKV617F-negative MF. Expression of MPLW515L in 32D, UT7, or Ba/F3 cells conferred cytokine-independent growth and thrombopoietin hypersensitivity, and resulted in constitutive phosphorylation of JAK2, STAT3, STAT5, AKT, and ERK. Furthermore, a small molecule JAK kinase inhibitor inhibited MPLW515L-mediated proliferation and JAK-STAT signaling in vitro. In a murine bone marrow transplant assay, expression of MPLW515L, but not wild-type MPL, resulted in a fully penetrant myeloproliferative disorder characterized by marked thrombocytosis (Plt count 1.9-4.0 x 10(12)/L), marked splenomegaly due to extramedullary hematopoiesis, and increased reticulin fibrosis.

CONCLUSIONS

Activation of JAK-STAT signaling via MPLW515L is an important pathogenetic event in patients with JAK2V617F-negative MF. The bone marrow transplant model of MPLW515L-mediated myeloproliferative disorders (MPD) exhibits certain features of human MF, including extramedullary hematopoiesis, splenomegaly, and megakaryocytic proliferation. Further analysis of positive and negative regulators of the JAK-STAT pathway is warranted in JAK2V617F-negative MPD.

Cell-autonomous and -nonautonomous contributions of STAT1 in murine models of tumorigenesis

In this issue of Cancer Cell, Kovacic and colleagues have reexamined the role of STAT1 in murine models of leukemogenesis. Their studies shed new light on the complex interplay between cell-autonomous contributions and host responsiveness to cancer and elucidate a previously unknown role of STAT1 in tumor progression.

JAK2T875N is a novel activating mutation that results in myeloproliferative disease with features of megakaryoblastic leukemia in a murine bone marrow transplantation model

Acute megakaryoblastic leukemia (AMKL) is a subtype of acute myeloid leukemia associated with a poor prognosis. However, there are relatively few insights into the genetic etiology of AMKL. We developed a screening assay for mutations that cause AMKL, based on the hypothesis that constitutive activation of STAT5 would be a biochemical indicator of mutation in an upstream effector tyrosine kinase. We screened human AMKL cell lines for constitutive STAT5 activation, and then used an approach combining mass spectrometry identification of tyrosine phosphorylated proteins and growth inhibition in the presence of selective small molecule tyrosine kinase inhibitors that would inform DNA sequence analysis of candidate tyrosine kinases. Using this strategy, we identified a new JAK2T875N mutation in the AMKL cell line CHRF-288-11. JAK2T875N is a constitutively activated tyrosine kinase that activates downstream effectors including STAT5 in hematopoietic cells in vitro. In a murine transplant model, JAK2T875N induced a myeloproliferative disease characterized by features of AMKL, including megakaryocytic hyperplasia in the spleen; impaired megakaryocyte polyploidization; and increased reticulin fibrosis of the bone marrow and spleen. These findings provide new insights into pathways and therapeutic targets that contribute to the pathogenesis of AMKL.

X-inactivation-based clonality analysis and quantitative JAK2V617F assessment reveal a strong association between clonality and JAK2V617F in PV but not ET/MMM, and identifies a subset of JAK2V617F-negative ET and MMM patients with clonal hematopoiesis

The JAK2V617F mutation is present in most patients with polycythemia vera (PV) and in some patients with essential thrombocythemia (ET) and myeloid metaplasia/myelofibrosis (MMM). We sought to investigate the relationship between granulocyte clonality and JAK2V617F allelic ratio. A total of 168 of 190 female patients were informative for a clonality assay at the HUMARA locus; 80% of MMM, 75% of PV, and 67% of ET patients demonstrated clonal granulopoiesis. The JAK2V617F allele was detected by quantitative real-time polymerase chain reaction (PCR) in 99% of PV, 72% of ET, and 39% of MMM. A correlation between clonality and JAK2V617F allelic ratio was demonstrated for PV (P < .001) but not for ET or MMM (both P > .6). These data suggest that acquisition of the JAK2V617F mutation may be sufficient for the development of PV, but additional genetic events are necessary in ET and MMM. In addition, some ET and MMM patients with clonal granulopoiesis have somatic mutations other than JAK2V617F.

Activation of FIP1L1-PDGFRalpha requires disruption of the juxtamembrane domain of PDGFRalpha and is FIP1L1-independent

Genetic abnormalities that result in expression of chimeric tyrosine kinase proteins such as BCR-ABL1 and ETV6-PDGFRbeta are common causes of hematopoietic malignancies. The paradigm for constitutive activation of these fusion tyrosine kinases is enforced homodimerization by self-association domains present in the fusion partner proteins. The unique interstitial deletion on chromosome 4q12 that leads to expression of the FIP1L1-PDGFRalpha fusion tyrosine kinase was recently identified as a cause of chronic eosinophilic leukemia. In this report, we demonstrate that FIP1L1 is completely dispensable for PDGFRalpha activation in vitro and in vivo. Instead, truncation of PDGFRalpha between two conserved tryptophan residues in the juxtamembrane (JM) domain is required for kinase activation and transforming potential of FIP1L1-PDGFRalpha. The presence of a complete JM domain in FIP1L1-PDGFRalpha is inhibitory, but this autoinhibition can be overcome by enforced homodimerization. Similar effects of the JM domain in the context of PDGFRbeta were observed. These results suggest that disruption of the autoinhibitory JM domain is an alternative, dimerization-independent mechanism by which chimeric tyrosine kinases are constitutively activated and induce leukemogenesis.

International Working Group (IWG) consensus criteria for treatment response in myelofibrosis with myeloid metaplasia, for the IWG for Myelofibrosis Research and Treatment (IWG-MRT)

Myelofibrosis with myeloid metaplasia (MMM) is a clinicopathologic entity characterized by stem cell-derived clonal myeloproliferation, ineffective erythropoiesis, extramedullary hematopoiesis, and bone marrow fibrosis and osteosclerosis. Patients with MMM have shortened survival and their quality of life is compromised by progressive anemia, marked hepatosplenomegaly, and severe constitutional symptoms including cachexia. After decades of frustration with ineffective therapy, patients are now being served by promising treatment approaches that include allogeneic hematopoietic stem cell transplantation and immunomodulatory drugs. Recent information regarding disease pathogenesis, including a contribution to the myeloproliferative disorder phenotype by a gain-of-function JAK2 mutation (JAK2(V617F)), has revived the prospect of targeted therapeutics as well as molecular monitoring of treatment response. Such progress calls for standardization of response criteria to accurately assess the value of new treatment modalities, to allow accurate comparison between studies, and to ensure that the definition of response reflects meaningful health outcome. Accordingly, an international panel of experts recently convened and delineated 3 response categories: complete remission (CR), partial remission (PR), and clinical improvement (CI). Bone marrow histologic and hematologic remissions characterize CR and CR/PR, respectively. The panel agreed that the CI response category is applicable only to patients with moderate to severe cytopenia or splenomegaly.

Oncogenic K-ras cooperates with PML-RAR alpha to induce an acute promyelocytic leukemia-like disease

Most patients with acute promyelocytic leukemia (APL) express PML-RAR alpha, the fusion product of t(15;17)(q22;q11.2). Transgenic mice expressing PML-RAR alpha develop APL with long latency, low penetrance, and acquired cytogenetic abnormalities. Based on observations that 4% to 10% of APL patients harbor oncogenic ras mutations, we coexpressed oncogenic K-ras from its endogenous promoter with PML-RAR alpha to generate a short-latency, highly penetrant mouse model of APL. The APL disease was characterized by splenomegaly, leukocytosis, extramedullary hematopoiesis (EMH) in spleen and liver with an increased proportion of immature myeloperoxidase-expressing myeloid forms; transplantability to secondary recipients; and lack of cytogenetic abnormalities. Bone marrow cells showed enhanced self-renewal in vitro. This model establishes a role for oncogenic ras in leukemia pathogenesis and thus validates the oncogenic RAS signaling pathway as a potential target for therapeutic inhibition in leukemia patients. This mouse model should be useful for investigating signaling pathways that promote self-renewal in APL and for testing the in vivo efficacy of RAS signaling pathway inhibitors in conjunction with other targeted therapies such as ATRA (all trans retinoic acid) and arsenic trioxide.

Roles of tyrosine 589 and 591 in STAT5 activation and transformation mediated by FLT3-ITD

Acquired mutations in the FLT3 receptor tyrosine kinase are common in acute myeloid leukemia and result in constitutive activation. The most frequent mechanism of activation is disruption of the juxtamembrane autoregulatory domain by internal tandem duplications (ITDs). FLT3-ITDs confer factor-independent growth to hematopoietic cells and induce a myeloproliferative syndrome in murine bone marrow transplant models. We and others have observed that FLT3-ITD activates STAT5 and its downstream effectors, whereas ligand-stimulated wild-type FLT3 (FLT3WT) does not. In vitro mapping of tyrosine phosphorylation sites in FLT3-ITD identified 2 candidate STAT5 docking sites within the juxtamembrane domain that are disrupted by the ITD. Tyrosine to phenylalanine substitution of residues 589 and 591 in the context of the FLT3-ITD did not affect tyrosine kinase activity, but abrogated STAT5 activation. Furthermore, FLT3-ITD-Y589/591F was incapable of inducing a myeloproliferative phenotype when transduced into primary murine bone marrow cells, whereas FLT3-ITD induced myeloproliferative disease with a median latency of 50 days. Thus, the conformational change in the FLT3 juxtamembrane domain induced by the ITD activates the kinase through dysregulation of autoinhibition and results in qualitative differences in signal transduction through STAT5 that are essential for the transforming potential of FLT3-ITD in vivo.

Absence of the transcription factor CCAAT enhancer binding protein alpha results in loss of myeloid identity in bcr/abl-induced malignancy

The lineage-determining transcription factor CCAAT enhancer binding protein alpha (C/EBPalpha) is required for myeloid differentiation. Decreased function or expression of C/EBPalpha is often found in human acute myeloid leukemia. However, the precise impact of C/EBPalpha deficiency on the maturation arrest in leukemogenesis is not well understood. To address this question, we used a murine transplantation model of a bcr/abl-induced myeloproliferative disease. The expression of bcr/abl in C/EBPalphapos fetal liver cells led to a chronic myeloid leukemia-like disease. Surprisingly, bcr/abl-expressing C/EBPalpha-/- fetal liver cells failed to induce a myeloid disease in transplanted mice, but caused a fatal, transplantable erythroleukemia instead. Accordingly, increased expression of the transcription factors SCL and GATA-1 in hematopoietic precursor cells of C/EBPalpha-/-R01-EY-11298 ) fetal livers was found. The mechanism for the lineage shift from myeloid to erythroid leukemia was studied in a bcr/abl-positive cell line. Consistent with findings of the transplant model, expression of C/EBPalpha and GATA-1 was inversely correlated. Id1, an inhibitor of erythroid differentiation, was identified as a critical direct target of C/EBPalpha. Down-regulation of Id1 by RNA interference impaired C/EBPalpha-induced granulocytic differentiation. Taken together, our study provides evidence that myeloid lineage identity of malignant hematopoietic progenitor cells requires the residual expression of C/EBPalpha.

The clinical phenotype of wild-type, heterozygous, and homozygous JAK2V617F in polycythemia vera

BACKGROUND

Several studies have recently reported on the occurrence of a JAK2(V617F) mutation in myeloid cells from the majority of patients with polycythemia vera (PV). The clinical relevance of this novel observation currently is under study.

METHODS

In a single institutional study, mutation screening for JAK2(V617F) was performed in DNA derived from archived blood granulocytes from 63 consecutive patients with PV in whom current diagnostic criteria were strictly applied and the diagnosis confirmed by bone marrow histology.

RESULTS

The JAK2(V617F) mutant allele was detected in 58 of the 63 patients (92%) with 21% homozygosity. The clinical phenotype of the five patients with the wild-type allele was otherwise typical for the disease. A statistical comparison between JAK2(V617F) heterozygotes (n=45 patients) and homozygotes (n=13 patients) did not reveal any significant associations with regard to age, gender, leukocyte or platelet count at the time of diagnosis, duration of disease, or the incidences of thrombosis or bleeding. However, compared with their heterozygote counterparts, JAK2(V617F) homozygote patients displayed a significantly higher hemoglobin level at the time of diagnosis (P=0.001), an increased incidence of pruritus (69% vs. 38%; P=0.04), a higher rate of fibrotic transformation (23% vs. 2%; P=0.009), and higher PRV-1 transcript levels in their blood granulocytes (P=0.07).

CONCLUSIONS

The results of the current clinical study support previous laboratory observations that link JAK2(V617F) with the PV phenotype by demonstrating a mutant allele dose effect on erythrocytosis and clinical and laboratory features characteristic of PV.

Prospective analysis of TEL/AML1-positive patients treated on Dana-Farber Cancer Institute Consortium Protocol 95-01

In a retrospective analysis, we previously reported that children whose leukemia cells harbored the TEL/AML1 gene rearrangement have excellent outcomes. From 1996 to 2000, we conducted a prospective study to determine the incidence and outcomes of children with TEL/AML1-positive acute lymphoblastic leukemia (ALL). Children with newly diagnosed ALL were treated on DFCI ALL Consortium Protocol 95-01. Patients were risk stratified primarily by current National Cancer Institute (NCI)-Rome risk criteria. With a median follow-up of 5.2 years, the 5-year event-free survival for TEL/AML1-positive patients was 89% compared with 80% for TEL/AML1-negative B-precursor patients (P = .05). The 5-year overall survival rate was 97% among TEL/AML-positive patients compared with 89% among TEL/AML1-negative patients (P = .03). However, in a multivariable analysis, risk group (age and leukocyte count at diagnosis) and asparaginase treatment group, but not TEL/AML1 status, were found to be independent predictors of outcome. We conclude that TEL/AML1-positive patients have excellent outcomes, confirming our previous findings. However, factors such as age at diagnosis and presenting leukocyte count should be taken into consideration when treating this group of patients.

Block of C/EBP alpha function by phosphorylation in acute myeloid leukemia with FLT3 activating mutations

Mutations constitutively activating FLT3 kinase are detected in approximately 30% of acute myelogenous leukemia (AML) patients and affect downstream pathways such as extracellular signal-regulated kinase (ERK)1/2. We found that activation of FLT3 in human AML inhibits CCAAT/enhancer binding protein alpha (C/EBPalpha) function by ERK1/2-mediated phosphorylation, which may explain the differentiation block of leukemic blasts. In MV4;11 cells, pharmacological inhibition of either FLT3 or MEK1 leads to granulocytic differentiation. Differentiation of MV4;11 cells was also observed when C/EBPalpha mutated at serine 21 to alanine (S21A) was stably expressed. In contrast, there was no effect when serine 21 was mutated to aspartate (S21D), which mimics phosphorylation of C/EBPalpha. Thus, our results suggest that therapies targeting the MEK/ERK cascade or development of protein therapies based on transduction of constitutively active C/EBPalpha may prove effective in treatment of FLT3 mutant leukemias resistant to the FLT3 inhibitor therapies.

Expression of Jak2V617F causes a polycythemia vera-like disease with associated myelofibrosis in a murine bone marrow transplant model

An acquired somatic mutation, Jak2V617F, was recently discovered in most patients with polycythemia vera (PV), chronic idiopathic myelofibrosis (CIMF), and essential thrombocythemia (ET). To investigate the role of this mutation in vivo, we transplanted bone marrow (BM) transduced with a retrovirus expressing either Jak2 wild-type (wt) or Jak2V617F into lethally irradiated syngeneic recipient mice. Expression of Jak2V617F, but not Jak2wt, resulted in clinicopathologic features that closely resembled PV in humans. These included striking elevation in hemoglobin level/hematocrit, leukocytosis, megakaryocyte hyperplasia, extramedullary hematopoiesis resulting in splenomegaly, and reticulin fibrosis in the bone marrow. Histopathologic and flow cytometric analyses showed an increase in maturing myeloid lineage progenitors, although megakaryocytes showed decreased polyploidization and staining for acetylcholinesterase. In vitro analysis of primary cells showed constitutive activation of Stat5 and cytokine-independent growth of erythroid colony-forming unit (CFU-E) and erythropoietin hypersensitivity, and Southern blot analysis for retroviral integration indicated that the disease was oligoclonal. Furthermore, we observed strain-specific differences in phenotype, with Balb/c mice demonstrating markedly elevated leukocyte counts, splenomegaly, and reticulin fibrosis compared with C57Bl/6 mice. We conclude that Jak2V617F expression in bone marrow progenitors results in a PV-like syndrome with myelofibrosis and that there are strain-specific modifiers that may in part explain phenotypic pleiotropy of Jak2V617F-associated myeloproliferative disease in humans.

Detection of the FIP1L1-PDGFRA fusion in idiopathic hypereosinophilic syndrome and chronic eosinophilic leukemia

Idiopathic hypereosinophilic syndrome (HES) and chronic eosinophilic leukemia (CEL) are related hematological malignancies characterized by sustained, unexplained hypereosinophilia (>1,500 eosinophils/microL). The term CEL is used when there is evidence that the disease is of clonal origin. We recently identified the FIP1L1-PDGFRA fusion gene in approx 50% of HES/CEL cases. Fusion of FIP1L1 to PDGFRA is the consequence of a deletion on chromosome 4, del(4)(q12q12), with the centromeric breakpoint in FIP1L1 and the telomeric breakpoint in PDGFRA. The breakpoints in FIP1L1 are diverse (introns 7 to 10), but the breakpoints in PDGFRA are always in exon 12 (encoding the juxtamembrane region). Because the chromosomal deletion is only 800 kb in size, it remains undetected with standard cytogenetics. In agreement with this, most patients with HES/CEL present with a normal karyotype. Here we describe three different techniques to detect the presence of the FIP1L1-PDGFRA fusion gene in peripheral blood or bone marrow cells.

Expression of a homodimeric type I cytokine receptor is required for JAK2V617F-mediated transformation

A recurrent somatic activating mutation in the nonreceptor tyrosine kinase JAK2 (JAK2V617F) occurs in the majority of patients with the myeloproliferative disorders polycythemia vera, essential thrombocythemia, myelofibrosis with myeloid metaplasia, and, less commonly, chronic myelomonocytic leukemia. We do not understand the basis for the specificity of the JAK2V617F mutation in clonal disorders of the myeloid, but not lymphoid, lineage, nor has the basis for the pleiotropic phenotype of JAK2V617F-associated myeloproliferative disorders been delineated. However, the presence of the identical mutation in patients with related, but clinicopathologically distinct, myeloid disorders suggests that interactions between the JAK2V617F kinase and other signaling molecules may influence the phenotype of hematopoietic progenitors expressing JAK2V617F. Here, we show that coexpression of the JAK2V617F mutant kinase with a homodimeric Type I cytokine receptor, the erythropoietin receptor (EpoR), the thrombopoietin receptor, or the granulocyte colony-stimulating-factor receptor, is necessary for transformation of hematopoietic cells to growth-factor independence and for hormone-independent activation of JAK-STAT signaling. Furthermore, EpoR mutations that impair erythropoietin-mediated JAK2 or STAT5 activation also impair transformation mediated by the JAK2V617F kinase, indicating that JAK2V617F requires a cytokine receptor scaffold for its transforming and signaling activities. Our results reveal the molecular basis for the prevalence of JAK2V617F in diseases of myeloid lineage cells that express these Type I cytokine receptors but not in lymphoid lineage cells that do not.

JAK2 mutation in essential thrombocythaemia: clinical associations and long-term prognostic relevance

Clinical correlates and long-term prognostic relevance of the JAK2(V617F) mutation was studied in 150 patients with essential thrombocythaemia (ET) from a single institution and followed for a median of 11.4 years. During this period, thrombotic complications were documented in 62 patients (41.3%) and transformation into acute myeloid leukaemia (AML), polycythaemia vera (PV), or myelofibrosis with myeloid metaplasia (MMM) occurred in 4 (2.7%), 8 (5.3%), and 15 (10%) patients, respectively. JAK2(V617F) was detected in either archived bone marrow or blood cells from 73 patients (48.7%) but none were homozygous for the mutant allele. Parameters at diagnosis that were significantly associated with the presence of JAK2(V617F) included advanced age and higher counts of both haemoglobin and leucocytes. During follow-up, patients with the mutation were more likely to transform into PV but the incidences of AML, MMM, or thrombotic events were similar between patients with and without the mutation. Multivariate analysis identified advanced age, higher haemoglobin level, and thrombosis history but not the presence of JAK2(V617F) as independent predictors of inferior survival. Therefore, although the presence of JAK2(V617F) in ET appears to promote a PV phenotype, it might not carry treatment-relevant information.

The JAK2V617F tyrosine kinase mutation in myelofibrosis with myeloid metaplasia: lineage specificity and clinical correlates

An association between an activating JAK2 mutation (JAK2(V617F)) and BCR/ABL-negative myeloproliferative disorders was recently reported in multiple simultaneous publications. In the current study, mutation analysis for JAK2(V617F) was performed in peripheral blood mononuclear cells (PBMC) from 157 patients with myelofibrosis with myeloid metaplasia (MMM) including 117 with agnogenic (AMM), 22 with postpolycythaemic (PPMM), and 18 with post-thrombocythaemic (PTMM) myeloid metaplasia. The detection rate for JAK2(V617F) was significantly higher in PPMM (91%; homozygous in 18%) compared with either AMM (45.3%; homozygous in 2.6%) or PTMM (38.9%; homozygous in 11.1%). Concomitant analysis in granulocytes (n=57) and CD34(+) cells (n=25) disclosed a higher incidence of homozygous JAK2(V617F) mutation but the overall mutation rate was similar to that obtained from PBMC. JAK2(V617F) was not detected in DNA derived from T cells (n=19). In AMM, the presence of JAK2(V617F) was associated with an older age at diagnosis and a history of thrombosis or pruritus. Multivariate analysis identified only age and the Dupriez prognostic score as independent prognostic factors; JAK2(V617F) had no prognostic significance. In conclusion, JAK2(V617F) is a myeloid lineage-specific event, its incidence in MMM is significantly higher with an antecedent history of polycythaemia vera (PV), and its presence in AMM does not affect prognosis but is associated with PV-characteristic clinical features.

Activity of the tyrosine kinase inhibitor PKC412 in a patient with mast cell leukemia with the D816V KIT mutation

The majority of patients with systemic mast cell disease express the imatinib-resistant Asp816Val (D816V) mutation in the KIT receptor tyrosine kinase. Limited treatment options exist for aggressive systemic mastocytosis (ASM) and mast cell leukemia (MCL). We evaluated whether PKC412, a small-molecule inhibitor of KIT with a different chemical structure from imatinib, may have therapeutic use in advanced SM with the D816V KIT mutation. We treated a patient with MCL (with an associated myelodysplastic syndrome (MDS)/myeloproliferative disorder [MPD]) based on in vitro studies demonstrating that PKC412 could inhibit D816V KIT-transformed Ba/F3 cell growth with a 50% inhibitory concentration (IC50) of 30 nM to 40 nM. The patient exhibited a partial response with significant resolution of liver function abnormalities. In addition, PKC412 treatment resulted in a significant decline in the percentage of peripheral blood mast cells and serum histamine level and was associated with a decrease in KIT phosphorylation and D816V KIT mutation frequency. The patient died after 3 months of therapy due to progression of her MDS/MPD to acute myeloid leukemia (AML). This case indicates that KIT tyrosine kinase inhibition is a feasible approach in SM, but single-agent clinical efficacy may be limited by clonal evolution in the advanced leukemic phase of this disease.

Concomitant neutrophil JAK2V617F mutation screening and PRV-1 expression analysis in myeloproliferative disorders and secondary polycythaemia

Polycythaemia vera (PV) is closely associated with both an acquired activating mutation of the JAK2 tyrosine kinase (JAK2(V617F)) in granulocyte-derived DNA and increased granulocyte polycythaemia rubra vera-1 (PRV-1) expression. In order to explore the correlation between these two biological markers and compare their diagnostic utility, mutation analysis for JAK2(V617F) and quantitative measurement of granulocyte PRV-1 expression were performed on the same study sample from 100 participants: 38 with PV, 22 with essential thrombocythaemia (ET), 10 with agnogenic myeloid metaplasia (AMM), 19 with secondary polycythaemia (SP) and 11 healthy volunteers. The respective overall (homozygous) JAK2(V617F) mutational frequencies were 95% (26%), 55% (0%), 30% (0%), 0% and 0%. The corresponding figures for increased PRV-1 expression were 89%, 18%, 20%, 21% and 9%. In patients with either ET or AMM, the likelihood of detecting JAK2(V617F) was significantly higher in the presence of an increased PRV-1 expression (83% vs. 38%; P = 0.05). Similarly, in patients with PV, homozygous as compared with heterozygous JAK2(V617F) correlated with higher levels of PRV-1 expression (P = 0.11). The present study suggests an allele dose-dependent effect of JAK2(V617F) on granulocyte PRV-1 expression. However, compared with the PRV-1 assay, mutation screening for JAK2(V617F) displayed greater accuracy in distinguishing PV from SP.

Genetics of Myeloid Malignancies: Pathogenetic and Clinical Implications

Myeloid malignancies are clonal disorders that are characterized by acquired somatic mutation in hematopoietic progenitors. Recent advances in our understanding of the genetic basis of myeloid malignancies have provided important insights into the pathogenesis of acute myeloid leukemia (AML) and myeloproliferative diseases (MPD) and have led to the development of novel therapeutic approaches. In this review, we describe our current state of understanding of the genetic basis of AML and MPD, with a specific focus on pathogenetic and therapeutic significance. Specific examples discussed include RAS mutations, KIT mutations, FLT3 mutations, and core binding factor rearrangements in AML, and JAK2 mutations in polycythemia vera, essential thrombocytosis, and chronic idiopathic myelofibrosis.

Drug therapy for acute myeloid leukemia

Although improvement in outcomes has occurred in younger adults with acute myeloid leukemia (AML) during the past 4 decades, progress in older adults has been much less conspicuous, if at all. Approximately 50% to 75% of adults with AML achieve complete remission (CR) with cytarabine and an anthracycline such as daunorubicin or idarubicin or the anthracenedione mitoxantrone. However, only approximately 20% to 30% of the patients enjoy long-term disease survival. Various postremission strategies have been explored to eliminate minimal residual disease. The optimal dose, schedule, and number of cycles of postremission chemotherapy for most patients are not known. A variety of prognostic factors can predict outcome and include the karyotype of the leukemic cells and the presence of transmembrane transporter proteins, which extrude certain chemotherapy agents from the cell and confer multidrug resistance and mutations in or over expressions of specific genes such as WT1, CEBPA, BAX and the ratio of BCL2 to BAX, BAALC, EVI1, KIT, and FLT3. Most recently, insights into the molecular pathogenesis of AML have led to the development of more specific targeted agents and have ushered in an exciting new era of antileukemia therapy. Such agents include the immunoconjugate gemtuzumab ozogamicin, multidrug resistance inhibitors, farnesyl transferase inhibitors, histone deacetylase and proteosome inhibitors, antiangiogenesis agents, Fms-like tyrosine kinase 3 (FLT3) inhibitors, and apoptosis inhibitors.

The JAK2V617F activating mutation occurs in chronic myelomonocytic leukemia and acute myeloid leukemia, but not in acute lymphoblastic leukemia or chronic lymphocytic leukemia

Activating mutations in tyrosine kinases have been identified in hematopoietic and nonhematopoietic malignancies. Recently, we and others identified a single recurrent somatic activating mutation (JAK2V617F) in the Janus kinase 2 (JAK2) tyrosine kinase in the myeloproliferative disorders (MPDs) polycythemia vera, essential thrombocythemia, and myeloid metaplasia with myelofibrosis. We used direct sequence analysis to determine if the JAK2V617F mutation was present in acute myeloid leukemia (AML), chronic myelomonocytic leukemia (CMML)/atypical chronic myelogenous leukemia (aCML), myelodysplastic syndrome (MDS), B-lineage acute lymphoblastic leukemia (ALL), T-cell ALL, and chronic lymphocytic leukemia (CLL). Analysis of 222 patients with AML identified JAK2V617F mutations in 4 patients with AML, 3 of whom had a preceding MPD. JAK2V617F mutations were identified in 9 (7.8%) of 116 CMML/a CML samples, and in 2 (4.2%) of 48 MDS samples. We did not identify the JAK2V617F disease allele in B-lineage ALL (n = 83), T-cell ALL (n = 93), or CLL (n = 45). These data indicate that the JAK2V617F allele is present in acute and chronic myeloid malignancies but not in lymphoid malignancies.