MPLW515L mutation in acute megakaryoblastic leukaemia

Differentiation therapy for murine myelofibrosis model with MLN8237 loaded low-density lipoproteins

Primary myelofibrosis (PMF) is a severe myeloproliferative neoplasm that is characterized by low-differentiation megakaryoblasts and progressive bone marrow fibrosis. Although an Aurora kinase A (AURKA) targeting small-molecule inhibitor MLN8237 has been approved in clinical trials for differentiation therapy of high-risk PMF patients, its off-target side effects lead to a partial remission and serious complications. Here, we report a dual-targeting therapy agent (rLDL-MLN) with great clinical translation potential for differentiation therapy of PMF disease. In particular, the reconstituted low-density lipoprotein (rLDL) nanocarrier and the loaded MLN8237 can actively target malignant hematopoietic stem/progenitor cells (HSPCs) via LDL receptors and intracellular AURKA, respectively. In contrast to free MLN8237, rLDL-MLN effectively prohibits the proliferation of PMF cell lines and abnormal HSPCs and significantly induces their differentiation, as well as prevents the formation of erythrocyte and megakaryocyte colonies from abnormal HSPCs. Surprisingly, even at a 1500-fold lower dosage (0.01 mg/kg) than that of free MLN8237, rLDL-MLN still exhibits a much more effective therapeutic effect, with the PMF mice almost clear of blast cells. More importantly, rLDL-MLN promotes hematological recovery without any toxic side effects at the effective dosage, holding great promise in the targeted differentiation therapy of PMF patients.

Unmanipulated haploidentical hematopoietic stem cell transplantation for pediatric de novo acute megakaryoblastic leukemia without Down syndrome in China: A single-center study

Background

AMKL without DS is a rare but aggressive hematological malignant disease in children, and it is associated with inferior outcomes. Several researchers have regarded pediatric AMKL without DS as high-risk or at least intermediate-risk AML and proposed that upfront allogenic hematopoietic stem cell transplantation (HSCT) in first complete remission might improve long-term survival.

Patients and method

We conducted a retrospective study with twenty-five pediatric (< 14 years old) AMKL patients without DS who underwent haploidentical HSCT in the Peking University Institute of Hematology, Peking University People's Hospital from July 2016 to July 2021. The diagnostic criteria of AMKL without DS were adapted from the FAB and WHO: ≥ 20% blasts in the bone marrow, and those blasts expressed at least one or more of the platelet glycoproteins: CD41, CD61, or CD42. AMKL with DS and therapy related AML was excluded. Children without a suitable closely HLA-matched related or unrelated donor (donors with more than nine out of 10 matching HLA-A, HLA-B, HLA-C, HLA-DR, and HLA-DQ loci), were eligible to receive haploidentical HSCT. Definition was adapted from international cooperation group. All statistical tests were conducted with SPSS v.24 and R v.3.6.3.

Results

The 2-year OS was 54.5 ± 10.3%, and the EFS was 50.9 ± 10.2% in pediatric AMKL without DS undergoing haplo-HSCT. Statistically significantly better EFS was observed in patients with trisomy 19 than in patients without trisomy 19 (80 ± 12.6% and 33.3 ± 12.2%, respectively, P = 0.045), and OS was better in patients with trisomy 19 but with no statistical significance (P = 0.114). MRD negative pre-HSCT patients showed a better OS and EFS than those who were positive (P < 0.001 and P = 0.003, respectively). Eleven patients relapsed post HSCT. The median time to relapse post HSCT was 2.1 months (range: 1.0-14.4 months). The 2-year cumulative incidence of relapse (CIR) was 46.1 ± 11.6%. One patient developed bronchiolitis obliterans and respiratory failure and died at d + 98 post HSCT.

Conclusion

AMKL without DS is a rare but aggressive hematological malignant disease in children, and it is associated with inferior outcomes. Trisomy 19 and MRD negative pre-HSCT might contribute to a better EFS and OS. Our TRM was low, haplo-HSCT might be an option for high-risk AMKL without DS.

Pediatric non–Down’s syndrome acute megakaryoblastic leukemia patients in China: A single center's real-world analysis

Non-Down’s syndrome acute megakaryocytic leukemia (non-DS-AMKL) is a subtype of childhood acute myeloid leukemia (AML), whose prognosis, prognostic factors and treatment recommendations have not yet to be defined in children. We conducted a retrospective study with 65 newly diagnosed non-DS-AMKL children from August 2003 to June 2020 to investigate the clinical impact of factors and clinical outcome. Among all 65 patients, 47 of them were treated at our center who received three different regimens due to time point of admission (CAMS-another, CAMS-2009 and CAMS-2016 protocol), and the efficacy were compared. Patients with newly diagnosed non-DS-AMKL accounted for 7.4% of pediatric AML cases. The median age of the patients was 18 months at diagnosis, and over 90% of them were under three-years-old. The overall survival (OS) rates were 33.3% ± 1.7%, 66.7% ± 24.4% and 74.2% ± 4.0% for three groups (CAMS-another, CAMS-2009 and CAMS-2016 regimen), respectively. In CAMS-2016 group, the complete remission (CR) rate after induction was 67.7% (21/31), while the total CR rate after all phases of chemotherapy was 80.6% (25/31). The 2-year survival probability did not significantly improve in patients underwent HSCT when compared with non-HSCT group (75.0% ± 4.7% vs. 73.9% ± 4.6%, p=0.680). Those who had a “dry tap” during BM aspiration at admission had significantly worse OS than those without “dry tap” (33.3% ± 8.6% vs. 84.0% ± 3.6%, p=0.006). Moreover, the results also revealed that patients with CD34+ had significantly lower OS (50.0% ± 6.7% vs. 89.5% ± 3.5%, p=0.021), whereas patients with CD36+ had significantly higher OS than those who were negative (85.0% ± 4.0% vs. 54.5% ± 6.6%, p=0.048). In conclusion, intensive chemotherapy resulted in improved prognosis of non-DS-AMKL children and subclassification may base on “dry tap” and immunophenotypic. Although some progress has been made, outcomes of non-DS-AMKL children remain unsatisfactory, especially in HSCT group, when compared with other AML types.

Dissecting stepwise mutational impairment of megakaryopoiesis in a model of Down syndrome-associated leukemia

Individuals with Down syndrome (DS) have more than 100-fold increased risk of acute megakaryoblastic leukemia (AMKL), but its pathogenesis is poorly understood. In this issue of the JCI, Arkoun et al. engineered stepwise DS-AMKL-associated mutations in GATA1, MPL, and SMC3 in human induced pluripotent stem cell (iPSC) clones from individuals with DS to dissect how each mutation affects gene expression control and megakaryocytic differentiation. The authors showed that the mutations cooperatively promote progression from transient myeloproliferative disorder to DS-AMKL. This study highlights the importance of mutation order and context in the perturbations of transcriptional and differentiation pathways involved in the evolution of hematologic malignancies, which will be critical for the development of preventative and therapeutic interventions.

Development of a Real-Time qPCR Assay for Detection of Common MPL Mutations in Myeloproliferative Neoplasms (MPNS)

Myeloproliferative neoplasms (MPNs) are blood cell disorders, characterized by overproduction of abnormal cells in bone marrow due to stem cell mutation. The proliferations of blood cell are controlled by many genes particularly MPL gene which encodes thrombopoietin receptor, a hematopoietic growth factor involved in the production and regulation of the platelets and multipotent hematopoietic progenitor cells. Acquired mutations including (W515L and W515K) in this gene have been observed in patients with primary myelofibrosis or essential thrombocythemia lacking JAK2 (V617F) mutations. MPL mutation detection is important for MPNs diagnosis, but due to low frequency of mutant allele burden (< 15%) may be missed by already available common assays such as Sanger sequencing. Furthermore, these techniques are costly, time-consuming, and less sensitive. In present study, we aimed to develop sensitive, less time-consuming, and cost-effective real-time PCR assay for the detection of MPL mutations that is based on TaqMan fluorescent probes. DNA was extracted from blood sample of 128 MPNs patients collected and further analysis was performed on TaqMan RT-PCR. Reference curve was obtained for amplified product of MPL gene containing mutated sequence. The predicted sensitivity level was at least 5% mutant allele burden by our developed assay that is much higher than sequencing output. Out of 128, 2 (1.56%) patients harbored W515L mutation and 1 (0.78%) harbored W515K mutation. It was concluded that TaqMan qRT-PCR assay is an efficient, sensitive, cost-effective, and less time-consuming method capable of detecting MPL mutation in MPNs patients. We suggested that this assay might be helpful in investigating mutant allele load in MPNs patients.

The MPL mutation

Myeloproliferative neoplasms (MPN) patients share driver mutations in JAK2, MPL or CALR genes leading to the activation of the thrombopoietin receptor (TPOR) and downstream signaling pathways. JAK2 mutation drives all the three major entities of MPN (Polycythemia Vera, Essential Thrombocythemia and Primary Myelofibrosis) through the constitutive activation of TPOR, erythropoietin (EPOR) and colony stimulating factor 3 receptor (CSF3R) signaling. MPL is a proto-oncogene encoding for TPOR, the hematopoietic growth factor receptor of myeloid stem cells. MPL mutants induce the stable dimerization of TPOR that in turn activate JAK2 and the thrombopoietin pathway. The thrombopoietin pathway plays an important role in the development of megakaryocytes and platelets as well as the self-renewal of hematopoietic stem cells. Little wonder therefore that mutations of MPL result in thrombocytosis, leading to an abnormal MPL trafficking or receptor activation. Finally, some extremely rare germline genetic variants in MPL can induce MPN-like hereditary disease. Against this molecular background, TPOR is a key actor in the MPN development and MPL mutations are of major relevance to fully elucidate the molecular mechanisms underlying the clinical manifestations of MPN and to arrange novel therapeutic strategies aiming to disrupt the dysegulated signaling cascade. This chapter will focus on the role MPL in the pathogenesis of MPN and in familial thrombocytosis and will review these different subtypes of somatic and germline genetic variants by dissecting how they impact clinical phenotype.

Myeloproliferative neoplasms: Current molecular biology and genetics

Myeloproliferative neoplasms (MPNs) are clonal disorders characterized by increased production of mature blood cells. Philadelphia chromosome-negative MPNs (Ph-MPNs) consist of polycythemia vera (PV), essential thrombocythemia (ET), and primary myelofibrosis (PMF). A number of stem cell derived mutations have been identified in the past 10 years. These findings showed that JAK2V617F, as a diagnostic marker involving JAK2 exon 14 with a high frequency, is the best molecular characterization of Ph-MPNs. Somatic mutations in an endoplasmic reticulum chaperone, named calreticulin (CALR), is the second most common mutation in patients with ET and PMF after JAK2 V617F mutation. Discovery of CALR mutations led to the increased molecular diagnostic of ET and PMF up to 90%. It has been shown that JAK2V617F is not the unique event in disease pathogenesis. Some other genes' location such as TET oncogene family member 2 (TET2), additional sex combs-like 1 (ASXL1), casitas B-lineage lymphoma proto-oncogene (CBL), isocitrate dehydrogenase 1/2 (IDH1/IDH2), IKAROS family zinc finger 1 (IKZF1), DNA methyltransferase 3A (DNMT3A), suppressor of cytokine signaling (SOCS), enhancer of zeste homolog 2 (EZH2), tumor protein p53 (TP53), runt-related transcription factor 1 (RUNX1) and high mobility group AT-hook 2 (HMGA2) have also identified to be involved in MPNs phenotypes. Here, current molecular biology and genetic mechanisms involved in MNPs with a focus on the aforementioned factors is presented.

Early lineage priming by trisomy of Erg leads to myeloproliferation in a Down syndrome model

Down syndrome (DS), with trisomy of chromosome 21 (HSA21), is the commonest human aneuploidy. Pre-leukemic myeloproliferative changes in DS foetal livers precede the acquisition of GATA1 mutations, transient myeloproliferative disorder (DS-TMD) and acute megakaryocytic leukemia (DS-AMKL). Trisomy of the Erg gene is required for myeloproliferation in the Ts(17¹⁶)65Dn DS mouse model. We demonstrate here that genetic changes specifically attributable to trisomy of Erg lead to lineage priming of primitive and early multipotential progenitor cells in Ts(17¹⁶)65Dn mice, excess megakaryocyte-erythroid progenitors, and malignant myeloproliferation. Gene expression changes dependent on trisomy of Erg in Ts(17¹⁶)65Dn multilineage progenitor cells were correlated with those associated with trisomy of HSA21 in human DS hematopoietic stem and primitive progenitor cells. These data suggest a role for ERG as a regulator of hematopoietic lineage potential, and that trisomy of ERG in the context of DS foetal liver hemopoiesis drives the pre-leukemic changes that predispose to subsequent DS-TMD and DS-AMKL.

An excess number of genes in trisomy on human chromosome 21 leads to the development of specific diseases in human Down syndrome. An excess copy of the gene, ERG, an ETS family transcription factor, has been implicated in abnormal blood system development in Down syndrome. In this study we show how trisomy of Erg in a murine Down syndrome model perturbs hematopoietic progenitor cells in a manner similar to that observed in human Down syndrome by inducing gene expression changes and lineage priming in early multi-potential progenitors. We show that the gene expression signature specifically attributable to trisomy of Erg in the murine model is strongly correlated with gene expression changes in human Down syndrome hematopoietic cells. The data suggest that Erg is an important regulator of megakaryocyte-erythroid lineage specification in multipotential hematopoietic cells and that trisomy of Erg in the context of DS prediposes to a transient myeloproliferative disorder and acute megakaryocyte leukaemia in a multi-step model of leukemogenesis.

Stem and progenitor cell dysfunction in human trisomies

Trisomy 21, the commonest constitutional aneuploidy in humans, causes profound perturbation of stem and progenitor cell growth, which is both cell context dependent and developmental stage specific and mediated by complex genetic mechanisms beyond increased Hsa21 gene dosage. While proliferation of fetal hematopoietic and testicular stem/progenitors is increased and may underlie increased susceptibility to childhood leukemia and testicular cancer, fetal stem/progenitor proliferation in other tissues is markedly impaired leading to the characteristic craniofacial, neurocognitive and cardiac features in individuals with Down syndrome. After birth, trisomy 21-mediated premature aging of stem/progenitor cells may contribute to the progressive multi-system deterioration, including development of Alzheimer's disease.

The landscape of somatic mutations in Down syndrome-related myeloid disorders

Transient abnormal myelopoiesis (TAM) is a myeloid proliferation resembling acute megakaryoblastic leukemia (AMKL), mostly affecting perinatal infants with Down syndrome. Although self-limiting in a majority of cases, TAM may evolve as non-self-limiting AMKL after spontaneous remission (DS-AMKL). Pathogenesis of these Down syndrome-related myeloid disorders is poorly understood, except for GATA1 mutations found in most cases. Here we report genomic profiling of 41 TAM, 49 DS-AMKL and 19 non-DS-AMKL samples, including whole-genome and/or whole-exome sequencing of 15 TAM and 14 DS-AMKL samples. TAM appears to be caused by a single GATA1 mutation and constitutive trisomy 21. Subsequent AMKL evolves from a pre-existing TAM clone through the acquisition of additional mutations, with major mutational targets including multiple cohesin components (53%), CTCF (20%), and EZH2, KANSL1 and other epigenetic regulators (45%), as well as common signaling pathways, such as the JAK family kinases, MPL, SH2B3 (LNK) and multiple RAS pathway genes (47%).

Molecular pathways: induction of polyploidy as a novel differentiation therapy for leukemia

Differentiation therapy has emerged as a powerful way to target specific hematologic malignancies. One of the best examples is the use of all-trans retinoic acid (ATRA) in acute promyelocytic leukemia (APL), which has significantly improved the outcome for patients with this specific form of acute myeloid leukemia (AML). In considering how differentiation therapy could be used in other forms of AML, we predicted that compounds that induce terminal differentiation of megakaryocytes would be effective therapies for the megakaryocytic form of AML, named acute megakaryocytic leukemia (AMKL). We also speculated that such agents would reduce the burden of abnormal hematopoietic cells in primary myelofibrosis and alter the differentiation of megakaryocytes in myelodysplastic syndromes. Using a high-throughput chemical screening approach, we identified small molecules that promoted many features of terminal megakaryocyte differentiation, including the induction of polyploidization, the process by which cells accumulate DNA to 32N or greater. As the induction of polyploidization is an irreversible process, cells that enter this form of the cell cycle do not divide again. Thus, this would be an effective way to reduce the tumor burden. Clinical studies with polyploidy inducers, such as aurora kinase A inhibitors, are under way for a wide variety of malignancies, whereas trials specifically for AMKL and PMF are in development. This novel form of differentiation therapy may be clinically available in the not-too-distant future. Clin Cancer Res; 19(22); 6084-8. ©2013 AACR.

Different mutations of the human c-mpl gene indicate distinct haematopoietic diseases

The human c-mpl gene (MPL) plays an important role in the development of megakaryocytes and platelets as well as the self-renewal of haematopoietic stem cells. However, numerous MPL mutations have been identified in haematopoietic diseases. These mutations alter the normal regulatory mechanisms and lead to autonomous activation or signalling deficiencies. In this review, we summarise 59 different MPL mutations and classify these mutations into four different groups according to the associated diseases and mutation rates. Using this classification, we clearly distinguish four diverse types of MPL mutations and obtain a deep understand of their clinical significance. This will prove to be useful for both disease diagnosis and the design of individual therapy regimens based on the type of MPL mutations.

Thrombopoietin/MPL participates in initiating and maintaining RUNX1-ETO acute myeloid leukemia via PI3K/AKT signaling

Oncogenic mutations in components of cytokine signaling pathways elicit ligand-independent activation of downstream signaling, enhancing proliferation and survival in acute myeloid leukemia (AML). The myeloproliferative leukemia virus oncogene, MPL, a homodimeric receptor activated by thrombopoietin (THPO), is mutated in myeloproliferative disorders but rarely in AML. Here we show that wild-type MPL expression is increased in a fraction of human AML samples expressing RUNX1-ETO, a fusion protein created by chromosome translocation t(8;21), and that up-regulation of Mpl expression in mice induces AML when coexpressed with RUNX1-ETO. The leukemic cells are sensitive to THPO, activating survival and proliferative responses. Mpl expression is not regulated by RUNX1-ETO in mouse hematopoietic progenitors or leukemic cells. Moreover, we find that activation of PI3K/AKT but not ERK/MEK pathway is a critical mediator of the MPL-directed antiapoptotic function in leukemic cells. Hence, this study provides evidence that up-regulation of wild-type MPL levels promotes leukemia development and maintenance through activation of the PI3K/AKT axis, and suggests that inhibitors of this axis could be effective for treatment of MPL-positive AML.

Polycythemia vera and essential thrombocythemia: 2012 update on diagnosis, risk stratification, and management

DISEASE OVERVIEW

Polycythemia vera (PV) and essential thrombocythemia (ET) are myeloproliferative neoplasms primarily characterized by erythrocytosis and thrombocytosis, respectively. Other disease features include leukocytosis, splenomegaly, thrombohemorrhagic complications, vasomotor disturbances, pruritus, and a small risk of disease progression into acute myeloid leukemia or myelofibrosis.

DIAGNOSIS

Almost all patients with PV harbor a JAK2 mutation. When PV is suspected, the presence of a JAK2 mutation confirms the diagnosis and its absence, combined with normal or increased serum erythropoietin level, excludes the diagnosis. Differential diagnosis of ET had to include chronic myelogenous leukemia and prefibrotic myelofibrosis. A JAK2 mutation is found in approximately 60% of patients with ET.

RISK STRATIFICATION

Current risk stratification in PV and ET is designed to estimate the likelihood of thrombotic complications: high-risk is defined by the presence of age >60 years or presence of thrombosis history; low-risk is defined by the absence of both of these two risk factors. Presence of extreme thrombocytosis (platelet count >1,000 × 10(9)/L) might be associated with acquired von Willebrand syndrome (AvWS) and, therefore, risk of bleeding. Risk factors for shortened survival in both PV and ET include advanced age, leukocytosis, and history of thrombosis.

RISK-ADAPTED THERAPY

Survival is near-normal in ET and reasonably long in PV. The 10-year risk of leukemic/fibrotic transformation is <1%/1% in ET and <3%/10% in PV. In contrast, the risk of thrombosis exceeds 20%. The main goal of therapy is therefore to prevent thrombohemorrhagic complications and this is effectively and safely accomplished by the use of low-dose aspirin (PV and ET), phlebotomy (PV) and hydroxyurea (high risk PV and ET). Treatment with busulfan or interferon-α is usually effective in hydroxyurea failures. Screening for clinically significant AvWS is recommended before administrating aspirin in the presence of extreme thrombocytosis.

Increased dosage of the chromosome 21 ortholog Dyrk1a promotes megakaryoblastic leukemia in a murine model of Down syndrome

Individuals with Down syndrome (DS; also known as trisomy 21) have a markedly increased risk of leukemia in childhood but a decreased risk of solid tumors in adulthood. Acquired mutations in the transcription factor-encoding GATA1 gene are observed in nearly all individuals with DS who are born with transient myeloproliferative disorder (TMD), a clonal preleukemia, and/or who develop acute megakaryoblastic leukemia (AMKL). Individuals who do not have DS but bear germline GATA1 mutations analogous to those detected in individuals with TMD and DS-AMKL are not predisposed to leukemia. To better understand the functional contribution of trisomy 21 to leukemogenesis, we used mouse and human cell models of DS to reproduce the multistep pathogenesis of DS-AMKL and to identify chromosome 21 genes that promote megakaryoblastic leukemia in children with DS. Our results revealed that trisomy for only 33 orthologs of human chromosome 21 (Hsa21) genes was sufficient to cooperate with GATA1 mutations to initiate megakaryoblastic leukemia in vivo. Furthermore, through a functional screening of the trisomic genes, we demonstrated that DYRK1A, which encodes dual-specificity tyrosine-(Y)-phosphorylation-regulated kinase 1A, was a potent megakaryoblastic tumor-promoting gene that contributed to leukemogenesis through dysregulation of nuclear factor of activated T cells (NFAT) activation. Given that calcineurin/NFAT pathway inhibition has been implicated in the decreased tumor incidence in adults with DS, our results show that the same pathway can be both proleukemic in children and antitumorigenic in adults.

Myeloid leukemia in Down syndrome

Although adults with Down syndrome (DS) show a decreased incidence of cancer compared to individuals without DS, children with DS are at an increased risk of leukemia. Nearly half of these childhood leukemias are classified as acute megakaryoblastic leukemia (AMKL), a relatively rare subtype of acute myeloid leukemia (AML). Here, we summarize the clinical features of myeloid leukemia in DS, review recent research on the mechanisms of leukemogenesis, including the roles of GATA1 mutations and trisomy 21, and discuss treatment strategies. Given that trisomy 21 is a relatively common event in hematologic malignancies, greater knowledge of how the genes on chromosome 21 contribute to DS-AMKL will increase our understanding of a broader class of patients with leukemia.

The thrombopoietin/MPL/Bcl-xL pathway is essential for survival and self-renewal in human preleukemia induced by AML1-ETO

AML1-ETO (AE) is a fusion product of translocation (8;21) that accounts for 40% of M2 type acute myeloid leukemia (AML). In addition to its role in promoting preleukemic hematopoietic cell self-renewal, AE represses DNA repair genes, which leads to DNA damage and increased mutation frequency. Although this latter function may promote leukemogenesis, concurrent p53 activation also leads to an increased baseline apoptotic rate. It is unclear how AE expression is able to counterbalance this intrinsic apoptotic conditioning by p53 to promote survival and self-renewal. In this report, we show that Bcl-xL is up-regulated in AE cells and plays an essential role in their survival and self-renewal. Further investigation revealed that Bcl-xL expression is regulated by thrombopoietin (THPO)/MPL-signaling induced by AE expression. THPO/MPL-signaling also controls cell cycle reentry and mediates AE-induced self-renewal. Analysis of primary AML patient samples revealed a correlation between MPL and Bcl-xL expression specifically in t(8;21) blasts. Taken together, we propose that survival signaling through Bcl-xL is a critical and intrinsic component of a broader self-renewal signaling pathway downstream of AML1-ETO-induced MPL.

The thrombopoietin/MPL pathway in hematopoiesis and leukemogenesis

Hematopoietic stem cells (HSC) comprise a small percentage of total hematopoietic cells. Their ability to self-renewal is key to the continuous replenishment of the hematopoietic system with newly formed functional blood cell types while maintaining their multipotential capacity. Understanding the extrinsic signals that are essential to HSC maintenance will provide insights into the regulation of hematopoiesis at its most primitive stage, and with the knowledge applied, will potentially lead to improved clinical transplantation outcomes. In this review, we will summarize the current understanding of the role of the thrombopoietin/MPL signaling pathway in HSC maintenance during adult and fetal hematopoiesis. We will also speculate on the downstream key players in the pathway based on published data, and summarize the role of this pathway in leukemia.

Primary myelofibrosis with or without mutant MPL: comparison of survival and clinical features involving 603 patients

MPL and JAK2V617F mutation analysis was performed in 603 patients with primary myelofibrosis (PMF) seen at the Mayo Clinic, USA (n=329) or University of Florence, Italy (n=274). Mutant MPL was detected in 49 (8.1%) patients and JAK2V617F in 350 (58%); 4 patients showed both mutations. MPLW515L/K was the commonest mutation; 2 patients showed novel mutations (L513ins and Q516-P518insAAAA). The US and Italy patient cohorts were separately analyzed for comparison of survival and clinical features between MPL-mutated, JAK2-mutated and JAK2/MPL-unmutated cases. JAK2/MPL-unmutated patients were significantly younger than their JAK2-mutated counterparts, in both patient cohorts (P<0.01). In the Florence only cohort, the presence of mutant MPL was associated with older age (P<0.01) and constitutional symptoms (P=0.04) and JAK2V617F with higher hemoglobin (P<0.01) and leukocyte (P=0.03) count; neither patient cohort showed significant associations with platelet count, hemoglobin <10 g/dl, abnormal/unfavorable karyotype, spleen size or prognostic score distribution. To date, 240 deaths and 79 leukemic transformations have been documented among all 603 study patients. Multivariable analysis disclosed no significant difference in overall or leukemia-free survival between the three molecular subgroups. We conclude that the presence of mutant MPL has narrow and inconsistent phenotypic effect in PMF and does not influence overall or leukemia-free survival.

Novel mutations and their functional and clinical relevance in myeloproliferative neoplasms: JAK2, MPL, TET2, ASXL1, CBL, IDH and IKZF1

Myeloproliferative neoplasms (MPNs) originate from genetically transformed hematopoietic stem cells that retain the capacity for multilineage differentiation and effective myelopoiesis. Beginning in early 2005, a number of novel mutations involving Janus kinase 2 (JAK2), Myeloproliferative Leukemia Virus (MPL), TET oncogene family member 2 (TET2), Additional Sex Combs-Like 1 (ASXL1), Casitas B-lineage lymphoma proto-oncogene (CBL), Isocitrate dehydrogenase (IDH) and IKAROS family zinc finger 1 (IKZF1) have been described in BCR-ABL1-negative MPNs. However, none of these mutations were MPN specific, displayed mutual exclusivity or could be traced back to a common ancestral clone. JAK2 and MPL mutations appear to exert a phenotype-modifying effect and are distinctly associated with polycythemia vera, essential thrombocythemia and primary myelofibrosis; the corresponding mutational frequencies are approximately 99, 55 and 65% for JAK2 and 0, 3 and 10% for MPL mutations. The incidence of TET2, ASXL1, CBL, IDH or IKZF1 mutations in these disorders ranges from 0 to 17%; these latter mutations are more common in chronic (TET2, ASXL1, CBL) or juvenile (CBL) myelomonocytic leukemias, mastocytosis (TET2), myelodysplastic syndromes (TET2, ASXL1) and secondary acute myeloid leukemia, including blast-phase MPN (IDH, ASXL1, IKZF1). The functional consequences of MPN-associated mutations include unregulated JAK-STAT (Janus kinase/signal transducer and activator of transcription) signaling, epigenetic modulation of transcription and abnormal accumulation of oncoproteins. However, it is not clear as to whether and how these abnormalities contribute to disease initiation, clonal evolution or blastic transformation.

Screening for MPL mutations in essential thrombocythemia and primary myelofibrosis: normal Mpl expression and absence of constitutive STAT3 and STAT5 activation in MPLW515L-positive platelets

OBJECTIVES

To evaluate the frequency of MPL W515L, W515K and S505N mutations in essential thrombocythemia (ET) and primary myelofibrosis (PMF) and to determine whether MPLW515L leads to impaired Mpl expression, constitutive STAT3 and STAT5 activation and enhanced response to thrombopoietin (TPO).

METHODS

Mutation detection was performed by allele-specific PCR and sequencing. Platelet Mpl expression was evaluated by flow cytometry, immunoblotting and real-time RT-PCR. Activation of STAT3 and STAT5 before and after stimulation with increasing concentrations of TPO was studied by immunoblotting. Plasma TPO was measured by ELISA.

RESULTS

MPLW515L was detected in 1 of 100 patients with ET and 1 of 11 with PMF. Platelets from the PMF patient showed 100% mutant allele, which was <50% in platelets from the ET patient, who also showed the mutation in granulocytes, monocytes and B cells. Mpl surface and total protein expression were normal, and TPO levels were mildly increased in the MPLW515L-positive ET patient, while MPL transcripts did not differ from controls in both MPLW515L-positive patients. Constitutive STAT3 and STAT5 phosphorylation was absent and dose response to TPO-induced phosphorylation was not enhanced.

CONCLUSIONS

The low frequency of MPL mutations in this cohort is in agreement with previous studies. The finding of normal Mpl levels in MPLW515L-positive platelets indicates this mutation does not lead to dysregulated Mpl expression, as frequently shown for myeloproliferative neoplasms. The lack of spontaneous STAT3 and STAT5 activation and the normal response to TPO is unexpected as MPLW515L leads to constitutive receptor activation and hypersensitivity to TPO in experimental models.

Quantification of transforming capacity and cooperation of defined genetic alterations in myeloid malignancies

OBJECTIVE

Mutations found in myeloid malignancies are qualitatively classified as conferring proliferative and survival advantages or impairing cellular differentiation. However, no suitable experimental model to quantify transforming potential of individual mutations and functional cooperation between defined genetic/epigenetic alterations has been established so far.

MATERIALS AND METHODS

Based on cytokine-independent proliferation as a marker for cellular transformation, we used limiting dilution and clonal expansion of retrovirally transduced cells in the presence or absence of cytokines to quantify the transformation potential of constitutively active receptor mutants and short hairpin RNAs (shRNA) targeting transcription factors by RNA interference. Interleukin-3-dependent 32D cells were transduced with betaGMR-I374N, c-KitV558D, or c-MplS368C, and cloning efficiencies were normalized to viral integration numbers as determined by quantitative polymerase chain reaction.

RESULTS

In this assay, c-KitV558D and c-MplS368C were about 25-fold more effective than betaGMR-I374N. To study cooperation of defined genetic/epigenetic aberrations, receptor mutants were coexpressed with shRNAs targeting PU.1 and p53. In p53-hypomorphic, but not in 32D wild-type cells, RNA interference against PU.1 significantly enhances transformation efficacy by c-KitV558D, but not by c-MplS368C, as compared to control shRNA. These data demonstrate nonredundant, receptor-specific and p53-dependent responses to reduced PU.1 expression in 32D cells.

CONCLUSION

This cell culture model represents a useful tool to quantify hematopoietic cell transformation by defined genetic and epigenetic alterations.

Screening and monitoring of MPL W515L mutation with real-time PCR in patients with myelofibrosis undergoing allogeneic-SCT

Monitoring of minimal residual disease (MRD) after allogeneic (allo)-SCT for myelofibrosis (MF) allows recognizing the depth of remission and thus guides application of appropriate therapeutic interventions. MPL W515L/K mutations, which are detected in 5-10% of JAK2V617F-negative patients, may be useful for this purpose. Using a highly sensitive quantitative PCR method, we tested 90 patients with MF who underwent allo-SCT for the presence of MPL W515L/K mutations. Two patients with primary MF were found to harbor MPLW515L while no patient was positive for MPLW515K mutation. Both patients were JAK2V617F negative and cleared the mutation rapidly after allo-SCT and remained negative for a median follow-up of 19 months. The results of molecular monitoring correlated well with other remission parameters such as normalization of peripheral blood counts and morphology and complete donor chimerism. We conclude that MPLW515L can be cleared after allo-SCT and hence may be used as an MRD marker in a proportion of JAK2V617F-negative MF patients.

Identifying differentially methylated genes using mixed effect and generalized least square models

Background

DNA methylation plays an important role in the process of tumorigenesis. Identifying differentially methylated genes or CpG islands (CGIs) associated with genes between two tumor subtypes is thus an important biological question. The methylation status of all CGIs in the whole genome can be assayed with differential methylation hybridization (DMH) microarrays. However, patient samples or cell lines are heterogeneous, so their methylation pattern may be very different. In addition, neighboring probes at each CGI are correlated. How these factors affect the analysis of DMH data is unknown.

Results

We propose a new method for identifying differentially methylated (DM) genes by identifying the associated DM CGI(s). At each CGI, we implement four different mixed effect and generalized least square models to identify DM genes between two groups. We compare four models with a simple least square regression model to study the impact of incorporating random effects and correlations.

Conclusions

We demonstrate that the inclusion (or exclusion) of random effects and the choice of correlation structures can significantly affect the results of the data analysis. We also assess the false discovery rate of different models using CGIs associated with housekeeping genes.

Recent advances in bone marrow biopsy pathology

The second quarter of 2009 saw steady advances in bone marrow biopsy (BMB) pathology. The following publications are a personal selection of the highlights. Quality issues in diagnostic immunohistochemistry for BMB have largely been ignored in external quality assurance programmes, and this issue is highlighted. In other areas, publications reflecting advances in flow cytometry and aspirate morphology are discussed where translation to the BMB is possible. Classifications undergo constant change, and several publications address the redefinition of the cut off points between malignancy, benign, and normal. Lastly, current scientific research is presented where it is relevant to the understanding of BMB pathobiology.