Detection of the single hotspot mutation in the JH2 pseudokinase domain of Janus kinase 2 in bone marrow trephine biopsies derived from chronic myeloproliferative disorders

[Bone marrow biopsy: processing and use of molecular techniques]

The rapid technological development in diagnostic pathology, especially of immunohistochemical and molecular techniques, also has a significant impact on diagnostic procedures for the evaluation of bone marrow trephine biopsies. The necessity for optimal morphology, combined with preservation of tissue antigens and nucleic acids on one hand and the wish for short turnaround times on the other hand require careful planning of the workflow for fixation, decalcification and embedding of trephines. Although any kind of bone marrow processing has its advantages and disadvantages, formalin fixation followed by EDTA decalcification can be considered a good compromise, which does not restrict the use of molecular techniques. Although the majority of molecular studies in haematological neoplasms are routinely performed on bone marrow aspirates or peripheral blood cells, there are certain indications, in which molecular studies such as clonality determination or detection of specific mutations need to be performed on the trephine biopsy. Especially, the determination of B- or T-cell clonality for the diagnosis of lymphoid malignancies requires stringent quality controls and knowledge of technical pitfalls. In this review, we discuss technical aspects of bone marrow biopsy processing and the application of diagnostic molecular techniques.

Visual screening for JAK2V617F mutation by a disposable dipstick

During the last 5 years, it was discovered that the JAK2V617F somatic mutation is present in virtually all patients with polycythemia vera and a large proportion of patients with essential thrombocythemia, primary myelofibrosis, and refractory anemia with ring sideroblasts and thrombocytosis. As a result, JAK2V617F was incorporated as a new clonal marker in the 2008 revision of the WHO diagnostic criteria. Current methods for JAK2 genotyping include direct sequencing, pyrosequencing, allele-specific PCR with electrophoresis, restriction fragment length polymorphism, real-time PCR, DNA-melting curve analysis, and denaturing HPLC. Some of these methods are labor intensive and time consuming, while the others require specialized costly equipment and reagents. We report a method for direct detection of the JAK2V617F allele by the naked eye using a dipstick test in a dry-reagent format. The method comprises a triprimer PCR combined with visual detection of the products within minutes by the dipstick test. Specialized instrumentation is not involved. The requirements for highly qualified technical personnel are minimized. Because the detection reagents exist in dry form on the dipstick, there is no need for multiple pipetting and incubation steps.

Molecular pathology of myeloproliferative neoplasms

Myeloproliferative neoplasms (MPNs; formerly chronic myeloproliferative disorders) are a class of myeloid hematologic malignancies that represent a stem cell-derived expansion of 1 or more hematopoietic cell lineages. The current 2008 World Health Organization system recognizes 8 types of MPN: chronic myelogenous leukemia, chronic neutrophilic leukemia, polycythemia vera, primary myelofibrosis, essential thrombocythemia, chronic eosinophilic leukemia, mastocytosis, and myeloproliferative neoplasm, unclassifiable. This review summarizes the salient characteristics of the MPNs, with emphasis on recent developments in the molecular pathophysiology and therapeutic monitoring of these disorders.

Prodromal myeloproliferative neoplasms: the 2008 WHO classification

The concept of prodromal chronic myeloproliferative neoplasms has been endorsed by the WHO classification implicating a stepwise evolution of disease. Histology of the bone marrow (BM) and borderline to mildly expressed clinical features play a pivotal role for diagnosing prefibrotic-early primary myelofibrosis. By lowering the platelet count for essential thrombocythemia and regarding BM morphology, early manifestations are tackled. Pre-polycythemic stages of polycythemia vera with a low hemoglobin level at onset are diagnosed by positive JAK2V617F mutation status, a low erythropoietin value, and characteristic BM features. The revised WHO classification incorporates hematological, morphological, and molecular-genetic parameters to generate a consensus-based working diagnosis.

SOCS-mediated downregulation of mutant Jak2 (V617F, T875N and K539L) counteracts cytokine-independent signaling

Recently, mutations in the gene of Janus kinase 2 (Jak2) were discovered in patients suffering from chronic myeloproliferative disorders (MPD) and leukemia. As suppressors of cytokine signaling (SOCS) proteins are potent feedback inhibitors of Jak-mediated signaling, we investigated their role in signal transduction through constitutively active Jak2 mutants. We selected two mutants, Jak2-V617F and Jak2-K539L, found in patients with MPDs and Jak2-T875N identified in acute megakaryoblastic leukemia. We found SOCS family members to be induced through Jak2-V617F in human leukemia cell lines expressing the mutant allele and in stable HEK transfectants inducibly expressing constitutively active Jak2 mutants. SOCS proteins were recruited to the membrane and bound to the constitutively active Jaks. In contrast to wild-type Jak2, the mutant proteins were constitutively ubiquitinated and degraded through the proteasome. Taken together, we show a SOCS-mediated downregulation of the constitutively active, disease-associated mutant Jak2 proteins. Furthermore, a threshold level of mutant Jak expression has to be overcome to allow full cytokine-independent constitutive activation of signaling proteins, which may explain progression to homozygocity in MPDs as well as gene amplification in severe phenotypes and leukemia.

Expression of adhesion factor CD239 in bone marrow cells in chronic myeloproliferative diseases

Patients suffering from Philadelphia chromosome-negative chronic myeloproliferative disease (Ph(-) CMPD), such as polycythaemia vera (PV), are frequently JAK2(V617F)-mutated and have an elevated risk for thromboembolic complications. Recent data indicated that the molecular basis of JAK2(V617F) and thrombosis might be related to increased expression of CD239, the Lutheran blood group/basal cell adhesion molecule, in PV-derived red blood cells. The aim of this study was to clarify whether JAK2(V617F) PV with thromboembolism is characterised by CD239 overexpression. Leukocyte-derived JAK2(V617F) was detectable in bone marrow cells but also in liver and spleen explants and transplants after thrombosis (95% JAK2(V617F)-positive Ph(-) CMPD, n = 66). CD239 was expressed in a constitutive fashion in PV-derived bone marrow cells and thus a pro-thrombotic function does not seem to be mediated by increased CD239 expression alone.

Histological and Molecular Classification of Chronic Myeloproliferative Disorders in the Age of JAK2: Persistence of Old Questions despite New Answers

The discovery of the Janus kinase 2 gain-of-function V617F mutation (JAK2(V617F)) provided a major breakthrough in the understanding of Philadelphia chromosome-negative chronic myeloproliferative disorders (Ph(-) CMPD). Among haematologic neoplasm the mutation appears to be almost specific for Ph(-) CMPD but the different entities comprising polycythaemia vera (PV), essential thrombocythaemia and chronic idiopathic myelofibrosis (CIMF) are not discriminated by the mutation. It is unclear how the diversity with heterogeneous clinical and pathoanatomical presentations comes about. It has been suggested that differences in JAK2(V617F) gene dosage or different degrees to which the haematologic lineages are affected by the mutation could explain the heterogeneity of morphology and prognosis. Indeed the mutation mediates a PV-like phenotype but with regard to myelofibrosis JAK2(V617F) does not appear to be a causative factor. Megakaryocytes are homozygous in the majority of fibrotic CIMF and PV, whereas JAK2(V617F) heterozygosity is predominantly encountered in prefibrotic CIMF and essential thrombocythaemia but transition from hetero- to homozygosity with onset of fibrosis is rare. In conclusion, JAK2(V617F) provides a valuable adjunct to the diagnosis of Ph(-) CMPD, in particular with regard to discrimination from reactive proliferations, but the challenge of correct subtyping and hence prognostication persists for clinicians and bone marrow pathologists.

A simple, rapid, and sensitive method for the detection of the JAK2 V617F mutation

The point mutation 1849 (GT) V617F in the JAK2 gene occurs at high frequency in several chronic myeloproliferative diseases. Although a number of V617F mutation detection methods have been described, few are readily implemented in a diagnostic setting. We developed a simple and sensitive allelespecific competitive blocker polymerase chain reaction (ACB-PCR) assay to detect the V617F mutation. DNA was extracted from peripheral whole blood samples of 26 patients with chronic myeloproliferative disease. The ACB-PCR limit of detection was 1%. All positive samples detected by sequencing were detected by ACB-PCR. In 3 patients with essential thrombocythemia, the V617F mutation was readily detected by ACB-PCR but was near the detection limit of sequencing, confirming that ACB-PCR is more effective at detecting V617F when the mutant cell population is low. Detection of the monomorphic JAK2 V617F mutation using the ACB-PCR assay is easy to perform, rapid, sensitive, and cost-effective, which are key features of an ideal diagnostic method.

The suppressor of cytokine signalling-1 (SOCS-1) gene is overexpressed in Philadelphia chromosome negative chronic myeloproliferative disorders

The suppressor of cytokine signalling-1 (SOCS-1) is a negative regulator of signal transduction mediated by cytoplasmic tyrosine kinases such as the Janus kinases (JAKs). We investigated SOCS-1 expression in bone marrow cells from Philadelphia chromosome negative chronic myeloproliferative disorders (Ph(-) CMPD) and normal haematopoiesis (n=121), and additionally in peripheral blood samples (n=18). Except for chronic idiopathic myelofibrosis harbouring wild-type JAK2, other Ph(-) CMPD expressed significantly higher SOCS-1 levels of up to 14-fold compared to the control group (p<0.001) independent of the JAK2 status. The mononuclear cell fraction but not granulocytes in patients with Polycythaemia vera also significantly overexpressed SOCS-1. We conclude that up-regulation of the SOCS-1 gene might reflect a compensatory feedback mechanism with different emphasis among Ph(-) CMPD subtypes independent of an underlying JAK2 (V617F) mutation.

Different involvement of the megakaryocytic lineage by the JAK2V617F mutation in Polycythemia vera, essential thrombocythemia and chronic idiopathic myelofibrosis

Atypical megakaryocytes provide the histomorphological hallmark of all Philadelphia-chromosome negative chronic myeloproliferative disorder (Ph(-) CMPD) subtypes and have not been studied so far for the JAK2(V617F) mutation. The mutant gene dosage was determined in isolated megakaryocytes from 68 cases of JAK2(+)/Ph(-) CMPD by a pyrosequencing assay. Megakaryocytes from essential thrombocythemia (ET) showed significantly lower levels of mutated JAK2 alleles compared to patients with chronic idiopathic myelofibrosis (cIMF) with manifest fibrosis and polycythemia vera (PV) but not to prefibrotic cIMF. Solely, ET JAK2V617F in megakaryocytes is associated with a PV-like phenotype, and at least in one patient, the JAK2 mutation was exclusively acquired within the megakaryocytic lineage. The overt differences between prefibrotic and fibrotic cIMF suggested a causative role of the gene dosage of mutant JAK2 in fibrotic progression. Megakaryocyte analysis of a follow-up of eight individual cases with sequential biopsies, however, showed that progression to homozygosity of V617F mutated JAK2 and onset of manifest fibrosis appeared to be independent events. We conclude that megakaryocytes might be the predominant or even the exclusive lineage that acquires the JAK2(V617F) mutation in ET and that the JAK2(V617F) evolution to higher gene dosages represents a dynamic and complex process substantially involving megakaryocytes.

Transcription factor Fli-1 expression by bone marrow cells in chronic myeloproliferative disorders is independent of an underlying JAK2 (V617F) mutation

OBJECTIVES

Friend leukemia integration-1 (Fli-1), a member of the Ets gene family of transcription factors, has been demonstrated to be a target of a leukaemia inducing virus in mice, and is known to be part of a fusion gene in Ewings' sarcoma in humans. Wild-type Fli-1 is involved in lineage commitment of megakaryocytes and myeloid progenitors through induction of Janus kinases (JAKs) following ligand binding to cytokine and growth factor receptors. Proliferation of atypical megakaryocytes is a predominant histopathological feature in Philadelphia chromosome negative chronic myeloproliferative disorders (Ph(-) CMPD) and a potential aberrant expression of Fli-1 has not been investigated so far.

METHODS

Fli-1 expression was investigated by real-time RT-PCR and immunohistochemistry in bone marrow cells derived from Ph(-) CMPD (n = 80) and non-neoplastic haematopoiesis (n = 21) following determination of the JAK2 status.

RESULTS

Fli-1 mRNA expression was significantly higher in Essential thrombocythaemia (ET) with JAK2 (V617F) compared with other Ph(-) CMPD and control (P < 0.001). By immunohistochemistry, Fli-1 protein could be detected in nuclei of atypical megakaryocytes in Ph(-) CMPD and, less accentuated, in non-neoplastic megakaryocytes. Fli-1 protein expression by myeloid progenitors was considerably heterogenous in Ph(-) CMPD independent of an underlying JAK2 (V617F) mutation and without notable differences to non-neoplastic haematopoiesis.

CONCLUSION

Fli-1 is rather constitutively expressed by bone marrow cells in Ph(-) CMPD independent of the underlying JAK2 status. The overall stronger labelling for Fli-1 in megakaryocytes in Ph(-) CMPD most likely reflects the degree of polyploidisation but aberrant activation of nuclear target genes can not be excluded.

Detection of the activating JAK2 V617F mutation in paraffin-embedded trephine bone marrow biopsies of patients with chronic myeloproliferative diseases

The discovery of the activating V617F mutation in the JAK2 tyrosine kinase in a high proportion of patients with Ph- chronic myeloproliferative diseases (CMPD) represents a diagnostic breakthrough for these disorders. Trephine bone marrow biopsy is an essential part of the diagnostic workup of CMPD and represents a valuable archival source of DNA. Therefore, we studied 152 paraffin-embedded trephines with CMPD and related disorders for the presence of the V617F mutation, using both allele-specific polymerase chain reaction (PCR) and nested PCR with subsequent digestion with BsaXI. Only 6 of 152 (4%) samples were not evaluable because of poor DNA quality. The V617F mutation was detected in 27 of 28 (96%) cases of polycythemia vera, 17 of 23 (74%) cases of essential thrombocythemia, 28 of 45 (62%) cases of chronic idiopathic myelofibrosis, six of eight (75%) cases of CMPD unclassified, and two of four (50%) cases of myelodysplastic/myeloproliferative syndrome. Ph+ chronic myelogenous leukemia (four cases), reactive (secondary) erythrocytosis (14 cases), and thrombocytosis (one case) as well as normal controls (19 cases) all lacked the V617F mutation. Based on results of BsaXI digestion and sequencing, 24 of 54 (44%) evaluable V617F+ cases were considered homozygously mutated. Thus, detection of the V617F JAK2 mutation is feasible in paraffin-embedded trephine biopsies and represents a major advance in the diagnostic evaluation of CMPD.

Aberrant collagenase expression in chronic idiopathic myelofibrosis is related to the stage of disease but not to the JAK2 mutation status

Bone marrow fibrosis in chronic idiopathic myelofibrosis (cIMF) most likely represents an imbalance between synthesis and turnover of collagen fibers. Because the JAK-STAT signaling pathway is involved in the regulation of genes encoding matrix metalloproteinases (MMPs), we examined the expression of MMPs, their tissue inhibitors (TIMPs), and collagen types in relation to the JAK2 status (V617F mutation versus wild-type) in cIMF (n = 64). Whereas no correlation was found between the JAK2 status and MMP gene products, there was an evident association with the stage of disease. Membrane type 1-MMP (MMP-14) was overexpressed by up to 80-fold in advanced stages that progressed to fibrosis (P < 0.001), and megakaryocytes and endothelial cells were unmasked as the major cellular source. By contrast, a significantly higher expression of neutrophil collagenase (MMP-8) was encountered in the prefibrotic stages of cIMF (P < 0.001). Altogether, the stepwise progress of myelofibrosis in cIMF was associated with expression of a defined subset of target genes as shown in sequential trephine biopsies of cIMF patients. We conclude that the expression of matrix-modeling genes in cIMF is not influenced by the JAK2 mutation status but is predominantly related to the stage of disease.

JAK2(V617F): Prevalence in a large Chinese hospital population

Recently, the JAK2(V617F) mutation was found in patients with myeloproliferative disorders (MPDs), including most with polycythemia vera (PV). The mutant JAK2 has increased kinase activity, and it was shown to be pathogenic in mouse models. Herein, we analyzed blood samples randomly collected from a clinical laboratory. Surprisingly, as many as 37 samples from a total of 3935 were found positive for the JAK2 mutation. However, only one of these samples had blood test results indicative for probable PV, but several had nonhematologic diseases. On average, samples with the mutation had normal red cell counts but significantly higher white blood cell and platelet counts, although most were within the normal range. The data suggest that the JAK2(V617F) mutation is apparently much more common than MPDs. Its occurrence may be a prelude to full blood cell abnormalities and other diseases, but it cannot by itself diagnose MPDs.