Marked heterogeneity in protein levels and functional integrity of the thrombopoietin receptor c-mpl in polycythaemia vera

Advances in molecular diagnostics of myeloproliferative disorders

Incremental advances in the molecular pathogenesis of myeloproliferative disorders (MPDs) have had a substantial impact on clinical practice in terms of both diagnosis and treatment. An array of novel molecular methods are being developed and integrated into the current battery of tests for diagnosis and monitoring of treatment response. Primarily, subjective clinico-histologic approaches to diagnosis are being replaced by more objective semimolecular diagnostic algorithms. Furthermore, identification of disease-specific molecular markers has facilitated the development of small-molecule drugs for targeted therapy. This review provides an overview of MPDs with emphasis on molecular diagnostic tests and their incorporation into contemporary diagnostic and therapeutic algorithms.

The diagnosis of polycythemia vera: New tests and old dictums

Pathogenetically fundamental observations have identified polycythemia vera (PV) as a clonal stem cell disease with bone marrow histological and other biological features that distinctly differentiate it from other causes of 'increased' hematocrit. However, relatively little attention has been given to the effective utilization of pathology and laboratory markers of clonal myeloproliferation as diagnostic tools in PV. In contrast, the diagnostic use of red cell mass (RCM) measurement in PV stemmed from the accidental endorsement, as 'diagnostic criteria', of 'study eligibility criteria' that were formulated for clinical trials. It has since become evident that RCM measurement is a tedious procedure that is fraught with multiple-level imprecision, as well as suboptimal diagnostic accuracy. Therefore, it is reasonable to consider dispensing with RCM measurement as a diagnostic test for PV and instead utilize a diagnostic algorithm that combines clinical information with easily accessible laboratory data, including serum erythropoietin level and bone marrow histology. Recent discoveries of myeloproliferative-disease-specific molecular markers, including the JAK2 V617F tyrosine kinase mutation that is found in the majority of patients with PV, provide further support for such a measure.

Chronic idiopathic myelofibrosis: clinicopathologic features, pathogenesis, and prognosis

CONTEXT

Chronic idiopathic myelofibrosis (CIMF) is a clonal myeloproliferative disease characterized by panmyelosis with intact maturation, progressive bone marrow fibrosis, and multiorgan extramedullary hematopoiesis.

OBJECTIVE

This review article aims to summarize the recent updates regarding the clinicopathologic features, molecular pathogenesis, cytogenetic abnormalities, diagnostic criteria, new diagnostic ancillary tests, and prognostic factors of CIMF.

DATA SOURCES

Important relevant articles indexed in PubMed/MEDLINE (National Library of Medicine) through the end of 2005 and referenced medical texts.

CONCLUSIONS

Because CIMF has a variety of clinical presentations, diagnosis may be challenging; the prefibrotic stage of CIMF has always been a challenging disease for pathologists to diagnose accurately. The recently proposed European Clinical and Pathological criteria can be helpful in the diagnosis of CIMF, especially in its prefibrotic stage. The enumeration of CD34-positive cells in the peripheral blood and the presence of circulating endothelial progenitor cells are the new important ancillary tests for the diagnosis of a small subset of patients with CIMF with atypical presentation. The recent discovery of the new mutation affecting the Janus tyrosine kinase 2 (JAK2V617F), more frequently observed in patients with polycythemia vera, is seen in approximately 35% to 57% of patients with CIMF. This mutation can serve as another diagnostic tool. Important factors affecting prognosis in CIMF are anemia, age of the patient, white blood cell count, degree of fibrosis, and number of blasts in the peripheral blood.

Polycythemia Vera: New Clinicopathologic Perspectives

Context.—Polycythemia vera (PV) is a clonal myeloproliferative disease characterized by an erythroid dominant trilineage proliferation of hematopoietic precursor cells. Classified as a chronic myeloproliferative disease, PV represents a histopathologic spectrum of 2 recognized stages, the polycythemic and postpolycythemic phase. The clinical manifestations of hemorrhage, thrombosis, and increased red cell mass are directly related to primary bone marrow dysfunction. Prognosis is strongly associated with thrombosis risk and disease progression; thus, treatment is directed toward minimizing coagulopathic complications and preventing leukemic transformation. Recently, a specific point mutation in the Janus kinase 2 (JAK2) gene was described in a majority of PV patients. The potential diagnostic and/or prognostic value of JAK2V617F is discussed.

Objective.—To review important developments from the recent and historical literature. Modern diagnostic criteria and emerging molecular findings are emphasized.

Data Sources.—A comprehensive review was performed of the relevant literature indexed in PubMed (National Library of Medicine) and referenced medical texts.

Conclusions.—Modified clinical, histologic, and laboratory criteria have clarified the diagnosis of PV. Also, continuing studies on the recently discovered JAK2V617F gene mutation may significantly improve our understanding of PV pathogenesis and facilitate its medical management.

Chromosomal Abnormalities and Molecular Markers in Myeloproliferative Disorders

The first possibly causative molecular aberration in patients with myeloproliferative disorders has recently been described. A point mutation in the Janus kinase 2 exchanging a valine for a phenylalanine at position 617 (JAK2 V617F) was found in 65% to 97% of polycythemia vera (PV) patients, as well as in approximately 50% of essential thrombocythemia (ET) and idiopathic myelofibrosis (IMF) patients. In addition, a growing set of molecular and genetic markers, some possibly contributing to disease development, some more likely epiphenomena, has been characterized in these patients over the last few years. Compiling and synthesizing the increasing knowledge on the genetic changes observed in myeloproliferative disorder (MPD) patients will allow us to generate testable hypotheses on the molecular etiology of disease development. Therefore, this review will summarize the current knowledge on chromosomal aberrations, molecular markers, and gene expression studies in MPD patients. From these data, a model depicting our current understanding of the interplay between these markers is presented.

Primary and secondary thrombocytosis in childhood

This review summarizes current data on the pathomechanisms and clinical aspects of primary and secondary thrombocytosis in childhood. Primary thrombocytosis is extremely rare in childhood, mostly diagnosed at the beginning of the second decade of life. As in adults, the criteria of the Polycythemia Vera Group are appropriate to diagnose primary thrombocytosis. The pathomechansims of non-familial forms are complex and include spontaneous formation of megakaryopoietic progenitors and increased sensitivity to thrombopoietin (Tpo). Familial forms can be caused by mutations in Tpo or Tpo receptor (c-mpl) genes. These mutations result in overexpression of Tpo, sustained intracellular signalling or disturbed regulation of circulating Tpo. Treatment of primary thrombocytosis is not recommended if platelet counts are <1500/nl and bleeding or thrombosis did not occur in patient's history. In severe cases, decision on treatment should weigh potential risks of treatment options (hydroxyurea, anagrelide) against expected benefits for preventing thrombosis or haemorrhage. Secondary thrombocytosis is frequent in children, in particular in the first decade of life. Hepatic Tpo production is stimulated in acute response reaction to a variety of disorders. Thrombosis prophylaxis is not required, even at platelet counts >1000/nl, except for cases with additional prothrombotic risk factors.

Molecular pathogenesis of Philadelphia chromosome negative myeloproliferative disorders

We summarize the current knowledge on molecular alterations in myeloproliferative disorders (MPD), in particular altered in vitro responses of progenitor cells, cytokine signaling, gene expression patterns and genetic lesions. Newly characterized markers, such as altered expression of polycythemia rubra vera-1 (PRV-1) and the thrombopoietin receptor (c-MPL) as well as deletions on chromosome 20q (del20q) and loss of heterozygosity on chromosome 9p (9pLOH) provide an opportunity to diagnose and identify subpopulations of MPD patients. Furthermore, we review familial syndromes that share phenotypic features with sporadic MPD. In some of these families, mutations in the genes for thrombopoietin (TPO), c-MPL, EPO-receptor and the von Hippel-Lindau (VHL) gene have been shown to cause the disease. However, in the majority of familial cases the molecular causes remain unknown. Some of these families display clonal hematopoiesis and other features previously only found in sporadic MPD. Elucidating the molecular defect(s) in these pedigrees will likely be relevant for understanding sporadic MPD pathogenesis.

Constitutively activated phosphatidylinositol 3-kinase primes platelets from patients with chronic myelogenous leukemia for thrombopoietin-induced aggregation

In this study, we examined the effect of thrombopoietin (TPO) on the aggregation of platelets from 40 patients with myeloproliferative disorders (MPDs), including 17 patients with chronic myelogenous leukemia in the chronic phase (CML-CP), 10 with polycythemia vera, 10 with essential thrombocythemia, and three with myelofibrosis. TPO by itself dose-dependently induced the aggregation of platelets from patients with CML-CP but not from those with other MPDs or with CML-CP in cytogenetical complete remission. The expression of CD63 in CML-CP platelets was induced by TPO treatment. Phosphatidylinositol 3-kinase (PI3-kinase) was constitutively activated in CML-CP platelets. Pretreatment with PI3-kinase inhibitors (wortmannin and LY294002) dose-dependently inhibited TPO-induced aggregation of CML-CP platelets. The Abl kinase inhibitor imatinib mesylate and the Jak inhibitor AG490 suppressed TPO-induced aggregation of CML-CP platelets. Pretreatment with imatinib mesylate, but not with AG490, inhibited the activity of PI3-kinase in CML-CP platelets. In addition, tyrosine phosphorylation of Jak2 was undetected in CML-CP platelets before TPO treatment. These findings indicate that the constitutive activation of PI3-kinase primes CML-CP platelets for the aggregation induced by TPO, and that Bcr-Abl, but not Jak family protein tyrosine kinases, are involved in the constitutive activation of PI3-kinase in CML-CP platelets.

Comparison of molecular markers in a cohort of patients with chronic myeloproliferative disorders

Decreased expression of c-MPL protein in platelets, increased expression of polycythemia rubra vera 1 (PRV-1) and nuclear factor I-B (NFIB) mRNA in granulocytes, and loss of heterozygosity on chromosome 9p (9pLOH) were described as molecular markers for myeloproliferative disorders (MPDs). To assess whether these markers are clustered in subgroups of MPDs or represent independent phenotypic variations, we simultaneously determined their status in a cohort of MPD patients. Growth of erythropoietin-independent colonies (EECs) was measured for comparison. We observed concordance between EECs and PRV-1 in MPD patients across all diagnostic subclasses, but our results indicate that EECs remain the most reliable auxiliary test for polycythemia vera (PV). In contrast, c-MPL, NFIB, and 9pLOH constitute independent variations. Interestingly, decreased c-MPL and elevated PRV-1 also were observed in patients with hereditary thrombocythemia (HT) who carry a mutation in the thrombopoietin (TPO) gene. Thus, altered c-MPL and PRV-1 expression also can arise through a molecular mechanism different from sporadic MPD.

Molecular markers in myeloproliferative disorders: from classification to prognosis?

The recent description of molecular markers in patients with myeloproliferative disorders (MPDs) has raised several questions: does the presence of multiple markers coincide in individual patients or can a patient acquire some markers selectively? Do the markers distinguish molecular categories of MPDs? Do these categories coincide with the clinically defined subgroups of MPDs: PV, ET and IMF? If not, which system of categorization is more useful to the patient and his physician, the molecular one or the clinical one, and why? The present review will summarize the current knowledge of molecular markers in MPDs and discuss today's answers to the above questions. Since our knowledge of the molecular basis of MPDs is rapidly expanding, it is my hope that this review will soon be outdated.

Basic sciences of the myeloproliferative diseases: pathogenic mechanisms of ET and PV

The molecular pathogenesis of ET and PV is unknown, although the relatively indolent clinical course observed in most patients suggests that the defect may be subtle and difficult to establish. Clonality analysis using X-chromosome inactivation patterns in females on purified CD34+ cells have confirmed that a defect is present in the hematopoietic stem cell. However, at least in ET, a significant proportion of patients have polyclonal hemopoiesis, and this presumably reflects the heterogeneous nature of the disorder(s). Attention has focussed on the potential disruption of the physiological regulators EPO and TPO and their respective receptors. In familial disorders, pathological mutations have been identified in some, but by no means all, cases: EPO receptor mutations in PFCP, TPO mutations in FT and, conversely, TPO receptor (c-mpl) mutations in CAMT. Equivalent ligand or receptor mutations have not been detected in ET or PV patients. However, there is evidence to suggest that c-mpl expression may be dysregulated, with low or absent c-mpl mRNA or protein reported in ET and/or PV patients. At present it is not clear whether this is the cause or consequence of the paradoxically normal/increased TPO levels found with both primary and secondary thrombocytosis. In vitro culture analysis has demonstrated both cytokine independence and hyper-sensitivity as a generalised feature of progenitor cells from many patients, but differences exist depending on the assays used and there is little understanding of the mechanism(s) underlying these responses. Two genes have recently been identified with increased mRNA expression in PV granulocytes: PRV-1, a novel cell surface receptor closely related to the uPAR/Ly6/CD59/snake toxin family of proteins, and NFI-B, a member of the nuclear factor I family which may be associated with TGF-beta resistance. Investigation of their regulation and biological effects may assist in determining the pathobiology of these elusive disorders.

Captopril-impaired production of tumor necrosis factor-alpha-induced interleukin-1beta in human monocytes is associated with altered intracellular distribution of nuclear factor-kappaB

We evaluated the effects of the angiotensin-converting enzyme (ACE) inhibitor captopril on pathways for monocyte production of interleukin (IL)-1beta in vitro. Human monocytes were treated with captopril and stimulated with tumor necrosis factor (TNF)-alpha or lipopolysaccharide. Captopril caused a dose-dependent reduction of TNF-alpha induced IL-1beta. LPS-induced IL-1beta generation was not reduced by the ACE inhibitor. Pro-IL-1beta concentrations followed the same pattern as that for mature IL-1beta when monocytes were preincubated with captopril. Also, IL-1beta mRNA concentrations were reduced by captopril pretreatment in parallel with IL-1beta. We sought to determine whether captopril affected the nuclear factor (NF)-kappaB complex in monocytic cells. We found that the translocation of the p-65 component of NF-kappaB to the nucleus was inhibited by captopril. Thus captopril reduced TNF-alpha-induced IL-1beta and IL-1betamRNA synthesis in monocytes, in vitro, probably through interference with NF-kappaB activation of the IL-1beta gene. These results support the hypothesis that captopril has immunomodulating properties.

Current opinion in essential thrombocythemia: pathogenesis, diagnosis, and management

A working diagnosis of essential thrombocythemia (ET) is made in the presence of nonreactive thrombocytosis and after the exclusion of another chronic myeloid disorder that may mimic ET in its presentation. Clinically, ET is characterized by vasomotor symptoms, thrombohemorrhagic complications, recurrent fetal loss, and transformation of the disease into either myelofibrosis with myeloid metaplasia or acute myeloid leukemia. Median survival in the majority of patients is close to that of an age-adjusted normal population, and current therapy has not been shown to either retard or hasten leukemic transformation, which is reported to occur in 1% to 20% of patients. The use of hydroxyurea in high-risk patients with ET has reduced the incidence of thrombosis, and recent studies have suggested the value of keeping the platelet count below 400 x 10(9)/L in such cases. The incidence of thrombosis in low-risk patients may not be high enough to warrant the use of cytoreductive therapy. Although effective in controlling vasomotor symptoms, aspirin therapy has not been shown to influence the risk of either recurrent thrombosis or first-trimester miscarriage in ET. Recent laboratory studies have suggested that hematopoiesis in ET may not always be clonal. Similarly, there is substantial heterogeneity in both megakaryocyte/platelet surface expression of the thrombopoietin receptor (c-Mpl) and bone marrow microvessel density. Clinicopathologic correlates to these biologic parameters are currently being defined.

Recent progress in the pathogenesis and management of essential thrombocythemia

In the last decade, the diagnosis of essential thrombocythemia (ET) has been refined by appreciation of the occurrence of karyotypically occult but molecularly evident chronic myelogenous leukemia and morphologically subtle myelodysplastic syndrome (MDS) and cellular-phase agnogenic myeloid metaplasia (AMM). Although ET continues to be defined by the presence of nonreactive thrombocythemia that is not accounted for by another chronic myeloid disorder, recent studies of clonality and other laboratory parameters have suggested clinically relevant biologic heterogeneity among affected patients. Furthermore, randomized, prospective, and controlled retrospective data have provided additional clinical information that has resulted in the development of risk categories and risk-adjusted treatment recommendations.