Insertion of the 3' ABL region into the long arm of chromosome 1 in a Philadelphia chromosome-negative chronic myeloid leukemia case

Chronic myeloid leukemia (CML) is a pluripotent hematopoietic stem cell disorder almost always characterized by the presence of the Philadelphia chromosome (Ph), usually due to t(9;22)(q34;q11). The presence of Ph results in the formation of the BCR/ABL fusion gene, which is a constitutively activated tyrosine kinase. Approximately 1% of CML patients appear to have a Ph-negative karyotype but carry a cryptic BCR/ABL fusion that can be located by fluorescence in situ hybridization (FISH) at chromosome 22q11, 9q34 or a third chromosome. This study investigated a rare Ph-negative CML case with insertion of the 3' ABL region into the long arm of derivative chromosome 1 but lacking the 5' BCR region on der(22).     

Tyrosine kinase fusion genes in chronic myeloproliferative diseases.

With the exception of chronic myeloid leukemia (CML), chronic myeloproliferative disorders (CMPDs) are a heterogeneous spectrum of conditions for which the molecular pathogenesis is not well understood. Most cases have a normal or aneuploid karyotype, but a minority present with a reciprocal translocation that disrupts specific tyrosine kinase genes, most commonly PDGFRB or FGFR1. These translocations result in the production of constitutively active tyrosine kinase fusion proteins that deregulate hemopoiesis in a manner analogous to BCR-ABL. With the advent of targeted signal transduction therapy, an accurate clinical and molecular diagnosis of CMPDs has become increasingly important. Currently, patients with PDGFRB or ABL fusion genes are candidates for treatment with Imatinib (STI571), but it is likely that alternative strategies will be necessary for the treatment of most other patients.     

Acute myeloid leukemia developing during imatinib mesylate therapy for chronic myeloid leukemia in the absence of new cytogenetic abnormalities

The BCR/ABL tyrosine kinase inhibitor imatinib mesylate produces a high rate of cytogenetic responses in patients with Philadelphia (Ph)-positive chronic myeloid leukemia (CML), but secondary clonal chromosome abnormalities may develop in Ph-negative cells, and acute myeloid leukemia (AML) has been reported in patients with secondary chromosome abnormalities. We report a patient who developed AML during imatinib treatment of Ph-positive CML despite a cytogenetic response and absence of secondary chromosome abnormalities. Thus, development of AML as a rare event in CML patients with cytogenetic responses to imatinib therapy does not depend on the development of secondary cytogenetic abnormalities.     

Targeted Signal Transduction Therapies in Myeloid Malignancies

The myeloid malignancies include the myeloproliferative neoplasms (MPN) including chronic myeloid leukemia (CML), and acute myeloid leukemia (AML). A growing body of evidence documents that these diseases are caused by genetic mutations that constitutively activate tyrosine kinases. They include the BCR/ABL in CML, the V617F JAK2 in Philadelphia chromosome–negative MPN, and the Flt3 ITD and TKD mutations in AML. Trials of the ABL kinase inhibitor, imatinib, have revolutionized the treatment of CML, and there are ongoing studies with other kinase inhibitors in MPN and AML. Here we review results of recent studies with first-generation JAK2 inhibitors in the treatment of MPN and second-generation ABL and Flt3 inhibitors in CML and AML, respectively. It is becoming apparent that although these kinase mutations have similar effects in vitro, each of the diseases has unique features that alter the use of kinase inhibitors in the clinic.     

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.     

The t(8;9)(p22;p24) translocation in atypical chronic myeloid leukaemia yields a new PCM1-JAK2 fusion gene

Several tyrosine kinase genes are involved in chromosomal translocations in chronic myeloproliferative disorders, but there are still uncharacterized translocations in some cases. We report two such cases corresponding to atypical chronic myeloid leukaemia with a t(8;9)(p22;p24) translocation. By fluorescence in situ hybridisation (FISH) on the corresponding metaphases with a bacterial artificial chromosome probe encompassing the janus kinase 2 (JAK2) gene at 9p24, we observed a split for both patients, suggesting that this gene was rearranged. The locus at 8p22 contains different candidate genes including the pericentriolar material 1 gene (PCM1), already implicated in reciprocal translocations. The rearrangement of the PCM1 gene was demonstrated by FISH, for both patients. By RT-PCR, we confirmed the fusion of 3' part of JAK2 with the 5' part of PCM1. Sequence analysis of the chimeric PCM1-JAK2 mRNA suggests that the putative protein displays the coiled-coil domains of PCM1 and the tyrosine kinase domain of JAK2. This new translocation identifies JAK2 as a possible therapeutic target for compounds with anti-tyrosine kinase activity.     

Advances in the molecular characterization of Philadelphia-negative chronic myeloproliferative disorders

PURPOSE OF REVIEW: The identification and characterization of somatic disease alleles have greatly improved our understanding of the molecular pathogenesis of myeloproliferative disorders. This review focuses on recent studies investigating the role of activated tyrosine kinase signaling in the Philadelphia chromosome negative myeloproliferative disorders. RECENT FINDINGS: Previously identified tyrosine kinase mutations in chronic myeloid leukemia and other myeloproliferative disorders suggested the possibility that polycythemia vera, essential thrombocythemia and primary myelofibrosis are also caused by activated tyrosine kinases. Recent studies identified an activating mutation in the JAK2 tyrosine kinase (JAK2V617F) in most patients with polycythemia vera and in approximately half of those with essential thrombocythemia and primary myelofibrosis. More recently, activating mutations in the thrombopoietin receptor and in JAK2 exon 12 have been identified in JAK2V617F negative myeloproliferative disorders. SUMMARY: The discovery of activated tyrosine kinases in the majority of patients with polycythemia vera, essential thrombocythemia and primary myelofibrosis has diagnostic and pathogenetic implications. Subsequent studies are needed to elucidate the cause of myeloproliferative disorders without known disease alleles and to determine if inhibition of JAK2 signaling has therapeutic efficacy in the treatment of polycythemia vera, essential thrombocythemia and primary myelofibrosis.     

Atypical chronic myeloid leukemia as defined in the WHO classification is a JAK2 V617F negative neoplasm

Atypical chronic myeloid leukemia (aCML) as defined by the WHO classification is a rare hematopoietic stem cell disorder, which shows both myeloproliferative as well as myelodysplastic features. Because of the presence of neutrophilic leukocytosis, aCML may resemble chronic myelogenous leukemia. However, in contrast with the latter, aCML lacks a Philadelphia chromosome or the BCR/ABL fusion gene. The molecular pathogenesis of aCML and its relationship to other myeloproliferative neoplasms is unknown. To clarify these points, the presence of JAK2 V617F was examined by a retrospective analysis of archival specimens obtained from two large medical institutions.Paraffin-embedded bone marrow (BM) trephines and clot sections were examined by an allele-specific TaqMan PCR suitable for use with decalcified tissue. Fifty-nine cases of Philadelphia (Ph) chromosome negative chronic myeloproliferative neoplasms (CMPN) and normal bone marrows (BM) served as controls. None of the nine amplifiable cases of aCML and none of the normal BM controls showed a JAK2 V617F mutation, in contrast to 45/59 (76%) of the Ph chromosome negative CMPN cases. Atypical CML should therefore be considered as a JAK2 negative chronic myeloid neoplasm that remains properly categorized, alongside chronic myelomonocytic leukemia and juvenile myelomonocytic leukemia, within the WHO group of myelodysplastic/myeloproliferative neoplasms.     

Genetic Mechanisms of Chronic Myeloid Leukemia Blastic Transformation

The BCR-ABL1 oncogenic tyrosine kinase can transform pluripotent hematopoietic stem cells and initiate chronic myeloid leukemia in chronic phase (CML-CP), a myeloproliferative disorder characterized by excessive accumulation of mature myeloid cells. Patients in CML-CP usually respond to treatment with ABL1 tyrosine kinase inhibitors (TKIs) such as imatinib, though some patients who respond initially may become resistant later. CML-CP leukemia stem cells (LSCs) are intrinsically insensitive to TKIs and thus survive in the long term. These LSCs or their progeny may at some stage acquire additional genetic changes that cause the leukemia to transform further, from CML-CP to a more advanced phase, which has been subclassified as either accelerated phase (CML-AP) or blastic phase (CML-BP). CML-BP is characterized by a major clonal expansion of immature progenitors, which have either myeloid or lymphoid features. CML-BP responds poorly to treatment and is usually fatal. This review discusses the role of genomic instability leading to blastic transformation of CML and proposes some novel therapeutic approaches.     

Experimental therapeutics for patients with myeloproliferative neoplasias

Philadelphia chromosome (Ph)-negative myeloproliferative neoplasms (MPNs) are characterized by stem cell-derived, unrestrained clonal myeloproliferation. The World Health Organization classification system, proposed in 2008, identifies 7 distinct categories of Ph-negative MPNs including essential thrombocythemia (ET); polycythemia vera (PV); primary myelofibrosis (PMF); mastocytosis; chronic eosinophilic leukemia; chronic neutrophilic leukemia; and MPN, unclassifiable. For many years, the treatment of ET, PV, and PMF, the most frequently diagnosed Ph-negative MPNs, has been largely supportive. In recent years, that paradigm has been challenged because of the discovery of a recurrent point mutation in the Janus kinase 2 (JAK2) gene (JAK2(V617F)). This mutation can be detected in the vast majority of patients with PV and approximately half of patients with ET or PMF and serves as both a diagnostic marker as well as representing a putative molecular target for drug development. Several putative targeted agents with significant in vitro JAK2 inhibitory activity and various degrees of JAK2 specificity are currently undergoing clinical evaluation. Furthermore, other investigational non-tyrosine kinase inhibitor approaches such as immunomodulatory agents and pegylated interferon- have also shown promising results in MPNs.     

JAK2 V617F in myeloid disorders: what do we know now, and where are we headed?

Activating tyrosine kinase (TK) mutations disrupt cellular proliferation and survival pathways and are increasingly recognized as a fundamental cause of human cancers. Until very recently, the only TK mutations widely observed in myeloid neoplasia were the BCR/ABL1 fusions characteristic of chronic myeloid leukemia and some acute leukemias, and FLT3 activating mutations in a minority of acute myeloid leukemias. Several rare TK mutations are found in various atypical myeloproliferative disorders, but big pieces of the pathobiological puzzle were glaringly missing. In the first half of 2005, one gap was filled in: 7 studies identified the same acquired amino acid substitution (V617F) in the Janus kinase 2 (JAK2) TK in large numbers of patients with diverse clonal myeloid disorders. Most affected patients suffer from the classic BCR/ABL1-negative myeloproliferative disorders (MPD), especially polycythemia vera (74% of n = 506), but a subset of people with essential thrombocythemia (36% of n = 339) or myelofibrosis with myeloid metaplasia (44% of n = 127) bear the identical mutation, as do a few individuals with myelodysplastic syndromes or an atypical myeloid disorder (7% of n = 556). This long-sought common mutation in BCR/ABL1-negative MPD raises many provocative biological and clinical questions, and demands re-evaluation of prevailing diagnostic algorithms for erythrocytosis and thrombocytosis. JAK2 V617F may provide novel molecular targets for drug therapy, and suggests other places to seek cooperating mutations or mutations associated with similar phenotypes. The story of this exciting finding will unfold rapidly in the years ahead, and ongoing developments will be important for all hematologists to understand.     

t(3;21)(q22;q22) leading to truncation of the RYK gene in atypical chronic myeloid leukemia

The analysis of a small number of patients with atypical chronic myeloid leukemia showing balanced chromosomal translocations has revealed diverse tyrosine kinase fusion genes, most commonly involving FGFR1, PDGFRA, PDGFRB, JAK2, and ABL. We present a case of aCML with a 3q22;21q22-translocation that led to truncation of the receptor-like tyrosine kinase (RYK) gene and its juxtaposition with sequences from chromosome 21 including the ATP5O gene coding for a mitochondrial ATP synthase. The resulting fusion was not in frame, however, which is why we speculate that an abrogated RYK gene product rather than a chimeric protein might be the leukemogenic result.     

BCR translocation to derivative chromosome 2: a new case of chronic myeloid leukemia with a complex variant translocation and Philadelphia chromosome

The well-known typical fusion gene BCR/ABL is observed in connection with a complex translocation event in 5-8% of cases of chronic myeloid leukemia (CML). The present study described an exceptional CML case with complex chromosomal aberrations not previously observed. Aberrations included a translocated BCR to the derivative chromosome 2 [der(2)] that also involved a four-chromosome translocation, implying chromosomal regions 1p32 and 2q21, besides 9q34 and 22q11.2, which were characterized by molecular cytogenetics.     

Absence in Ph-negative, M-BCR rearrangement-positive chronic myelogenous leukemia of linkage between 5' ABL and 3' M-BCR sequences in Philadelphia translocation.

The Philadelphia (Ph) chromosome can be detected in the vast majority of patients with chronic myelogenous leukemia (CML). We performed a long-range analysis of chromosomal translocation junction by pulsed-field gel electrophoresis (PFGE) techniques, to examine whether molecular evidence of a reciprocal Ph translocation exists in Ph-positive CML as well as Ph-negative, M-BCR rearrangement-positive CML. The rearrangement within M-BCR and ABL was detected in all patients including nine Ph-positive CML, and three Ph-negative CML. The rearranged 3'-abl fragments showed comigration with rearranged 5'-bcr fragment in rare-cutting restriction enzyme digests in all patients with Ph-positive CML. Thus, the physical linkage of the 3' part of ABL to the 5' side of M-BCR on 22q-chromosome was shown. The same linkage was also demonstrated in all three patients with Ph-negative CML. Meanwhile, the rearranged 3'-bcr fragments showed comigration with rearranged pHabl5' (or T39-1-2) fragments in all patients with Ph-positive CML, indicating the linkage of the 5' end of ABL to the 3' part of M-BCR on 9q+ chromosome. However, this linkage was absent in two Ph-negative CML patients who could be studied. The results suggest that a genomic insertion of 3' ABL into M-BCR in Ph-negative CML occurs by a single cytogenetic event rather than a two-translocation mechanism.