Clonality in chronic myeloproliferative disorders defined by X-chromosome linked probes: demonstration of heterogeneity in lineage involvement

Pathogenesis of Myeloproliferative Disorders

Myeloproliferative neoplasms (MPNs) are a set of chronic hematopoietic neoplasms with overlapping clinical and molecular features. Recent years have witnessed considerable advances in our understanding of their pathogenetic basis. Due to their protracted clinical course, the evolution to advanced hematological malignancies, and the accessibility of neoplastic tissue, the study of MPNs has provided a window into the earliest stages of tumorigenesis. With the discovery of mutations in CALR, the majority of MPN patients now bear an identifiable marker of clonal disease; however, the mechanism by which mutated CALR perturbs megakaryopoiesis is currently unresolved. We are beginning to understand better the role of JAK2(V617F) homozygosity, the function of comutations in epigenetic regulators and spliceosome components, and how these mutations cooperate with JAK2(V617F) to modulate MPN phenotype.

The evolving genomic landscape of myeloproliferative neoplasms

Our understanding of the genetic basis of the Philadelphia chromosome-negative myeloproliferative neoplasms (MPNs) has moved forward at a staggering pace over the last decade. With the discoveries of underlying mutations in JAK2, MPL, and, most recently, calreticulin (CALR), that together account for ∼90% of patients with MPNs, these conditions are now among the best characterized of hematological malignancies. While JAK-STAT pathway activation has been shown to be central to the pathogenesis of the MPN phenotype, the mechanism by which mutant CALR alters cellular function to result in myeloid proliferation remains unclear. Other mutations in several epigenetic modifiers, such as ASXL1, DNMT3a, TET2, EZH2, IDH1, and IDH2, as well as in genes involved in mRNA splicing, such as SF3B1 and U2AF2, have also been described in recent years in patients with MPNs, and evidence is emerging as to how these may be contributing to disease biology. From a therapeutic perspective, the discovery of aberrations in JAK2 has rapidly translated into the successful clinical use of JAK inhibitors in MPNs. Mutant calreticulin has the potential to be a tumor-specific therapeutic target because the mutations generate a novel protein C-terminus. In this chapter, we detail the genomic alterations that underlie MPNs, with a focus on the recent discovery of mutations in CALR, and explore the clinical and biological relevance of the altered genomic landscape in MPNs.

Spleens of myelofibrosis patients contain malignant hematopoietic stem cells

Cancer stem cell behavior is thought to be largely determined by intrinsic properties and by regulatory signals provided by the microenvironment. Myelofibrosis (MF) is characterized by hematopoiesis occurring not only in the marrow but also in extramedullary sites such as the spleen. In order to study the effects of these different microenvironments on primitive malignant hematopoietic cells, we phenotypically and functionally characterized splenic and peripheral blood (PB) MF CD34+ cells from patients with MF. MF spleens contained greater numbers of malignant primitive HPCs than PB. Transplantation of PB MF CD34+ cells into immunodeficient (NOD/SCID/IL2Rγ(null)) mice resulted in a limited degree of donor cell chimerism and a differentiation program skewed toward myeloid lineages. By contrast, transplanted splenic MF CD34+ cells achieved a higher level of chimerism and generated both myeloid and lymphoid cells that contained molecular or cytogenetic abnormalities indicating their malignant nature. Only splenic MF CD34+ cells were able to sustain hematopoiesis for prolonged periods (9 months) and were able to engraft secondary recipients. These data document the existence of MF stem cells (MF-SCs) that reside in the spleens of MF patients and demonstrate that these MF-SCs retain a differentiation program identical to that of normal hematopoietic stem cells.

New JAK2 inhibitors for myeloproliferative neoplasms

INTRODUCTION

The discovery of the JAK(V617F) kinase established a common pathogenetic link to the most important types of Philadelphia-chromosome-negative myeloproliferative neoplasms (MPNs): polycythemia vera (PV), essential thrombocythemia (ET), and primary myelofibrosis (PMF). More importantly, the demonstration of constitutive kinase activity emanating from the JAK2 protein provided the rationale for the development of small-molecule JAK2 kinase inhibitors.

AREAS COVERED

Several JAK2 kinase inhibitors are being tested in clinical trials for patients with MPNs. In PMF trials, JAK2 inhibitors have been shown to produce rapid reductions in spleen size and marked improvements in constitutional symptoms and quality of life. In ET and/or PV, JAK2 inhibitors normalize hematocrit, platelets and WBC, and spleen size in a large number of patients that are resistant or intolerant to hydroxyurea. JAK2 inhibitors are not specific for the JAK2V617F mutant protein. Rather, they inhibit the JAK2- signal transducer and activator of transcription (STAT) pathway and therefore any patient with MPN may benefit from therapy regardless of JAK2 mutational status.

EXPERT OPINION

JAK2 inhibitors induce clinically relevant responses in a large proportion of patients with MPNs. Because JAK kinase activation underlies the pathogenesis of other disorders, such as autoimmune and rheumatological disorders, the paradigm of JAK inhibition may translate into novel therapies for a variety of human diseases.

JAK2 V617F and beyond: role of genetics and aberrant signaling in the pathogenesis of myeloproliferative neoplasms

Dysregulated signaling is a hallmark of chronic myeloproliferative neoplasms (MPNs), as evidenced by the identification of the activating JAK2 V617F somatic mutation in almost all patients with polycythemia vera (PV) and 50-60% of essential thrombocythemia and primary myelofibrosis patients. These disorders are clinically distinct, raising the question of how a single mutation can result in such phenotypic diversity. Mouse models have demonstrated that the level of JAK2 V617F expression can modulate the phenotype, and clinical studies of JAK2 V617F allele burden have reported similar findings. It has also been hypothesized that one or more pre-JAK2 V617F events may modify the MPN phenotype. However, the molecular basis of JAK2 V617F-negative essential thrombocythemia and primary myelofibrosis remains largely unexplained. Mutations in the TET2 gene have been identified in both JAK2 V617F-positive and -negative MPNs and other myeloid neoplasms, but their functional and clinical significance have yet to be clarified. In addition, recent reports have identified a specific germline haplotype that increases the predisposition to MPNs. The role of inhibitory pathways (e.g., SOCS and LNK) in regulating JAK-STAT signaling in MPNs is being increasingly recognized. The implications of these findings and their clinical relevance are the focus of this article.

Janus kinase inhibitors for the treatment of myeloproliferative neoplasias and beyond

Recent advances in our understanding of the pathogenesis of the Philadelphia chromosome-negative myeloproliferative neoplasms, polycythaemia vera, essential thrombocythaemia and myelofibrosis have led to the identification of the mutation V617F in Janus kinase (JAK) as a potential therapeutic target. This information has prompted the development of ATP-competitive JAK2 inhibitors. Therapy with JAK2 inhibitors may induce rapid and marked reductions in spleen size and can lead to remarkable improvements in constitutional symptoms and overall quality of life. Because JAKs are involved in the pathogenesis of inflammatory and immune-mediated disorders, JAK inhibitors are also being tested in clinical trials in patients with rheumatoid arthritis and psoriasis, as well as for the treatment of other autoimmune diseases and for the prevention of allograft rejection. Preliminary results indicate that these agents hold great promise for the treatment of JAK-driven disorders.

Involvement of mast cells by the malignant process in patients with Philadelphia chromosome negative myeloproliferative neoplasms

The Philadelphia chromosome negative myeloproliferative neoplasms (MPNs) are clonal hematologic malignancies frequently characterized by a mutation in JAK2 (JAK2V617F). Peripheral blood (PB) CD34(+) cells from patients with polycythemia vera (PV) and primary myelofibrosis (PMF) generated in vitro significantly fewer mast cells (MCs) than normal PB CD34(+) cells. The numbers of MC progenitors assayed from MPN CD34(+) cells were, however, similar to that assayed from normal CD34(+) cells. A higher percentage of the cultured MPN MCs expressed FcvarepsilonRIalpha, CD63 and CD69 than normal MCs, suggesting that cultured MPN MCs are associated with an increased state of MC activation. Further analysis showed that a higher proportion of cultured PV and PMF MCs underwent apoptosis in vitro. By using JAK2V617F, MplW515L and chromosomal abnormalities as clonality markers, we showed that the malignant process involved MPN MCs. JAK2V617F-positive MC colonies were assayable from the PB CD34(+) cells of each of the 17 JAK2V617F positive MPN patients studied. Furthermore, erlotinib, a JAK2 inhibitor, was able to inhibit JAK2V617F-positive PV MC progenitor cells, indicating that malignant MC progenitor cells are a potential cellular target for such JAK2 inhibitor-directed therapy.

A germline JAK2 SNP is associated with predisposition to the development of JAK2(V617F)-positive myeloproliferative neoplasms

Polycythemia vera, essential thrombocythemia and primary myelofibrosis are myeloproliferative neoplasms (MPN) characterized by multilineage clonal hematopoiesis. Given that the identical somatic activating mutation in the JAK2 tyrosine kinase gene (JAK2(V617F)) is observed in most individuals with polycythemia vera, essential thrombocythemia and primary myelofibrosis, there likely are additional genetic events that contribute to the pathogenesis of these phenotypically distinct disorders. Moreover, family members of individuals with MPN are at higher risk for the development of MPN, consistent with the existence of MPN predisposition loci. We hypothesized that germline variation contributes to MPN predisposition and phenotypic pleiotropy. Genome-wide analysis identified an allele in the JAK2 locus (rs10974944) that predisposes to the development of JAK2(V617F)-positive MPN, as well as three previously unknown MPN modifier loci. We found that JAK2(V617F) is preferentially acquired in cis with the predisposition allele. These data suggest that germline variation is an important contributor to MPN phenotype and predisposition.

Myeloproliferative disorders

In 1951 William Dameshek classified polycythemia vera (PV), essential thombocytosis (ET), and primary myelofibrosis (PMF) as pathogenetically related myeloproliferative disorders (MPD). Subsequent studies demonstrated that PV, ET, and PMF are clonal disorders of multipotent hematopoietic progenitors. In 2005, a somatic activating mutation in the JAK2 nonreceptor tyrosine kinase (JAK2V617F) was identified in most patients with PV and in a significant proportion of patients with ET and PMF. Subsequent studies identified additional mutations in the JAK-STAT pathway in some patients with JAK2V617F(-) MPD, suggesting that constitutive activation of this signaling pathway is a unifying feature of these disorders. Although the discovery of mutations in the JAK-STAT pathway is important from a pathogenetic and diagnostic perspective, important questions remain regarding the role of this single disease allele in 3 related but clinically distinct disorders, and the role of additional genetic events in MPD disease pathogenesis. In addition, these observations provide a foundation for development of small molecule inhibitors of JAK2 that are currently being tested in clinical trials. This review will discuss our understanding of the pathogenesis of PV, ET, and PMF, the potential role of JAK2-targeted therapy, and the important unanswered questions that need to be addressed to improve clinical outcome.

JAK2 and MPL mutations in myeloproliferative neoplasms: discovery and science

Although it has long been known that the myeloproliferative neoplasms (MPN) polycythemia vera (PV), essential thrombocythemia (ET) and primary myelofibrosis (PMF) are clonal hematopoietic stem-cell disorders, for many years the genetic basis for these disorders was elusive. A new era in MPN biology began in 2005 with the discovery of a somatic point mutation in JAK2 tyrosine kinase (JAK2V617F), which was identified in a significant proportion of patients with PV, ET and PMF. Based on the hypothesis that JAK-STAT signaling is central to the pathogenesis of JAK2V617F-negative MPN, genomic studies have identified JAK2 exon 12 mutations in JAK2V617F-negative PV and activating mutations in MPL in patients with JAK2V617F-negative ET and PMF. In this review, we will discuss the role of these mutant alleles in the pathogenesis of PV, ET and PMF, the potential therapeutic implications of these discoveries, and the implications of these discoveries for genomic studies of hematopoietic malignancies.

T cells from patients with polycythemia vera elaborate growth factors which contribute to endogenous erythroid and megakaryocyte colony formation

In the present study, we report that media conditioned by polycythemia vera (PV) CD3+ cells promote BFU-E and CFU-Mk colony formation by both cord blood and PV peripheral blood CD34+ cells in the absence of exogenous cytokines and promoting megakaryocyte proplatelet formation. CD3+ cells constitutively produce elevated levels of IL-11, while stimulation with the addition of phytohemagglutinin (PHA) increased GM-CSF levels in most of the patients with PV. Anti-IL-11-neutralizing antibody partially inhibited the formation of BFU-E and CFU-Mk colonies promoted by PV CD3+ cell-conditioned media. Although IL-11 is not produced by normal T cells, real-time PCR and flow cytometric analysis showed that IL-11 was upregulated in the CD3+ cells of most PV patients as compared to normal CD3+ cells. In addition, a greater percentage of BFU-E colonies formed by PV CD34+ cells in the presence of PV CD3+ cell-conditioned media alone were JAK2V617F-positive as compared with that induced by EPO. We conclude that dysregulated production of soluble growth factor(s), including IL-11 and GM-CSF by PV T cells, contributes to the in vitro formation of erythroid colonies in the absence of exogenous cytokines by PV CD34+ cells and likely plays a role in sustaining hematopoiesis in PV.

The JAK2 V617F mutation involves B- and T-lymphocyte lineages in a subgroup of patients with Philadelphia-chromosome negative chronic myeloproliferative disorders

The JAK2 V617F mutation is a frequent genetic event in the three classical Philadelphia-chromosome negative chronic myeloproliferative disorders (Ph(neg.)-CMPD), polycythemia vera (PV), essential thrombocythemia (ET) and idiopathic myelofibrosis (IMF). Its occurrence varies in frequency in regards to phenotype. The mutation is found in the majority of patients with PV and about half of the patients with ET and IMF. These diseases are clonal stem cell disorders arising in an early stem cell progenitor. The level in the stem cell hierarchy on which the initiating genetic events and the JAK2 V617F mutation occurs is not known. The mutation has so far been detected in all cells of the myeloid lineage, whereas the potential clonal involvement of the lymphoid lineage is controversial. In this study, we detected the JAK2 V617F mutation by real-time quantitative PCR (qPCR) in both B-lymphocytes and T-lymphocytes in a subgroup of patients with Ph(neg.)-CMPDs. These results demonstrate the origin of the JAK2 V617F positive disorders in an early stem cell with both lymphoid and myeloid differentiation potential.

Myelofibrosis with myeloid metaplasia: disease overview and non-transplant treatment options

Myelofibrosis with myeloid metaplasia (MMM) is currently classified as a classic (i.e. not yet molecularly defined) myeloproliferative disorder (MPD), along with essential thrombocythemia (ET) and polycythemia vera (PV). All three MPDs represent stem-cell-derived clonal myeloproliferation that, in the case of MMM, is accompanied by an intense bone marrow stromal reaction that includes collagen fibrosis, osteosclerosis, and angiogenesis. To date, both the molecular basis of the primary clonal process and the pathogenetic mechanisms that underlie the secondary histological changes remain elusive. Clinically, MMM is characterized by anemia, multi-organ extramedullary hematopoiesis that often involves the spleen and liver, constitutional symptoms, and premature death from either leukemic transformation or other disease complications. Current diagnosis is based on characteristic but not diagnostic bone marrow histological features. Modern therapy remains palliative but allogeneic stem cell transplantation might be curative to a selected group of patients. This chapter reviews both the old and the new therapy with regard to non-transplant treatment options for MMM.

Classification of chronic myeloid disorders: From Dameshek towards a semi-molecular system

Hematological malignancies are phenotypically organized into lymphoid and myeloid disorders, although such a distinction might not be precise from the standpoint of lineage clonality. In turn, myeloid malignancies are broadly categorized into either acute myeloid leukemia (AML) or chronic myeloid disorder (CMD), depending on the presence or absence, respectively, of AML-defining cytomorphologic and cytogenetic features. The CMD are traditionally classified by their morphologic appearances into discrete clinicopathologic entities based primarily on subjective technologies. It has now become evident that most CMD represent clonal stem cell processes where the primary oncogenic event has been characterized in certain instances; Bcr/Abl in chronic myeloid leukemia, FIP1L1-PDGFRA or c-kit(D816V) in systemic mastocytosis, rearrangements of PDGFRB in chronic eosinophilic leukemia, and rearrangements of FGFR1 in stem cell leukemia/lymphoma syndrome. In addition, Bcr/Abl-negative classic myeloproliferative disorders are characterized by recurrent JAK2(V617F) mutations, whereas other mutations affecting the RAS signaling pathway molecules have been associated with juvenile myelomonocytic leukemia. Such progress is paving the way for a transition from a histologic to a semi-molecular classification system that preserves conventional terminology, while incorporating new information on molecular pathogenesis.

Involvement of various hematopoietic-cell lineages by the JAK2V617F mutation in polycythemia vera

The JAK2(V617F) mutation has been shown to occur in the overwhelming majority of patients with polycythemia vera (PV). To study the role of the mutation in the excessive production of differentiated hematopoietic cells in PV, CD19+, CD3+, CD34+, CD33+, and glycophorin A+ cells and granulocytes were isolated from the peripheral blood (PB) of 8 patients with PV and 3 healthy donors mobilized with G-CSF, and the percentage of JAK2(V617F) mutant allele was determined by quantitative real-time polymerase chain reaction (PCR). The JAK2(V617F) mutation was present in cells belonging to each of the myeloid lineages and was also present in B and T lymphocytes in a subpopulation of patients with PV. The proportion of hematopoietic cells expressing the JAK2(V617F) mutation decreased after differentiation of CD34+ cells in vitro in the presence of optimal concentrations of SCF, IL-3, IL-6, and Epo. These data suggest that the JAK2(V617F) mutation may not provide a proliferative and/or survival advantage for the abnormal PV clone. Although the JAK2(V617F) mutation plays an important role in the biologic origins of PV, it is likely not the sole event leading to PV.

Idiopathic myelofibrosis: pathogenesis to treatment

Idiopathic myelofibrosis (IMF) is the least common of the chronic myeloproliferative disorders and carries the worst prognosis with a median survival of 4 years. It is a clonal haematopoietic stem-cell disorder and, although the pathogenesis remains unclear, approximately 50% of cases are known to possess an activating JAK2 V617F mutation. In contrast, the characteristic stromal proliferation is a reactive, or secondary, event that results from the aberrant release of a variety of growth factors from megakaryocytes and monocytes. Treatment for most cases is supportive, although androgens, recombinant erythropoietin, steroids and thalidomide are effective modalities for the amelioration of anaemia. Myelosuppression, splenectomy and irradiation are valuable therapeutic modalities for specific clinical situations. Prognostic scores are available to aid the identification of cases for whom bone marrow transplantation should be considered. Recently, the use of reduced intensity conditioning has resulted in prolonged survival and lower transplant-related mortality. This review summarises the recent advances in the disease's pathogenesis and discusses the role of the various therapeutic options.

Prognostic relevance of cytogenetics determined by fluorescent in situ hybridization in patients having myelofibrosis with myeloid metaplasia

BACKGROUND

In chronic myelofibrosis (MF), distinct recurrent cytogenetic aberrations have been identified but their true prognostic relevance remains uncertain. In this disease, cytogenetic studies as assessed by conventional metaphase karyotyping are limited due to the inherent difficulties in obtaining adequate bone marrow aspirates and the low proliferative capacity of the clonal cells. Interphase fluorescent in situ hybridization (FISH) can partly overcome these limitations and increase the sensitivity of cytogenetic assessment in MF.

METHODS

We retrospectively analyzed formalin-fixed, paraffin embedded bone marrow sections of 107 MF patients by FISH and correlated cytogenetic findings with clinical presentation and survival.

RESULTS

Chromosomal aberrations were detected in 56% of patients, with 20q- (24.3%) and 13q- (16.8%) being the most frequent ones. Importantly, cytogenetic abnormalities were found in 8/17 patients displaying a normal karyotype as assessed by conventional cytogenetics.

CONCLUSIONS

Cytogenetic abnormalities in patients with MF can be detected reliably using FISH. Rare abnormalities confer an adverse outcome, but the main recurrent chromosomal aberrations do not correlate with clinical features and prognosis.

Pathogenesis of myelofibrosis with myeloid metaplasia

The primary disease process in myelofibrosis with myeloid metaplasia (MMM) is clonal myeloproliferation with varying degrees of phenotypic differentiation. This is characteristically accompanied by secondary intramedullary collagen fibrosis, osteosclerosis, angiogenesis, and extramedullary hematopoiesis. Modern clonality studies have confirmed the multipotent stem-cell origin of the neoplastic process in MMM. The nature of the specific oncogenic mutation(s) is currently being unraveled with the recent discovery of an association between a somatic point mutation of JAK2 tyrosine kinase (V617F) and bcr/abl-negative myeloproliferative disorders, including MMM. The pathogenetic mechanisms that underlie the secondary bone marrow stromal changes in MMM are also incompletely understood. Mouse models of this latter disease aspect have been constructed by either in vivo overexpression of thrombopoietin (TPOhigh mice) or megakaryocyte lineage restricted underexpression of the transcription factor GATA-1 (GATA-1low mice). Gene knockout experiments using such animal models have suggested the essential role of hematopoietic cell-derived transforming growth factor beta1 in inducing bone marrow fibrosis and stromal cell-derived osteoprotegerin in promoting osteosclerosis. However, experimental myelofibrosis in mice does not recapitulate clonal myeloproliferation that is fundamental to human MMM. Other cytokines that are implicated in mediating myelofibrosis and angiogenesis in MMM include basic fibroblast, platelet-derived, and vascular endothelial growth factors. It is currently assumed that such cytokines are abnormally released from clonal megakaryocytes as a result of a pathologic interaction with neutrophils (eg, emperipolesis). This latter phenomenon, through neutrophil-derived elastase, could also underlie the abnormal peripheral-blood egress of myeloid progenitors in MMM.

The kinetics of clonal dominance in myeloproliferative disorders

To study clonal evolution in myeloproliferative disorders, we used stochastic models of hematopoiesis for mouse and cat, species for which the in vivo kinetics of hematopoietic stem cells (HSCs) have been experimentally defined. We determined the consequence if 1 HSC became able to survive without the support of a microenvironmental niche while the rest of its behavior did not change. Neoplastic cells persisted and dominated hematopoiesis in 14% of mice and 17% of cats, requiring mean times of 2.5 +/- 0.5 and 7.0 +/- 1.2 years, respectively (n=1000 simulations/species). In both species, when the number of neoplastic HSCs exceeded 0.5% of all HSCs, clonal dominance was inevitable. Our results can explain the absence of clonal myeloproliferative disorders in mice (lifetime, 2 years), are consistent with clinical observations in cats, and provide insight into the progression of chronic myelogenous leukemia (CML) in humans. They also demonstrate that competition for microenvironmental support can lead to the suppression of normal hematopoiesis as neoplasia evolves. Toxic or immunologic suppression of normal HSCs is not required.

The JAK2V617F tyrosine kinase mutation in myeloproliferative disorders: status report and immediate implications for disease classification and diagnosis

Janus kinase 2 (JAK2) is a cytoplasmic protein-tyrosine kinase that catalyzes the transfer of the gamma-phosphate group of adenosine triphosphate to the hydroxyl groups of specific tyrosine residues in signal transduction molecules. JAK2 mediates signaling downstream of cytokine receptors after ligand-induced autophosphorylation of both receptor and enzyme. The main downstream effectors of JAK2 are a family of transcription factors known as signal transducers and activators of transcription (STAT) proteins. The myeloproliferative disorders (MPD), a subgroup of myeloid malignancies, are clonal stem cell diseases characterized by an expansion of morphologically mature granulocyte, erythroid, megakaryocyte, or monocyte lineage cells. Among the traditionally classified MPD, the disease-causing mutation has been delineated, thus far, for only chronic myeloid leukemia (ie, bcr/abl). In the past 3 months, 7 different studies have Independently described a close association between an activating JAK2 mutation (JAK2V617F) and the classic bcr/abi-negative MPD (ie, polycythemia vera, essential thrombocythemia, myelofibrosis with myeloid metaplasia) as well as the less frequent occurrence of the same mutation in both atypical MPD and the myelodysplastic syndrome. The particular finding is consistent with previous observations that have implicated the JAK/STAT signal transduction pathway in the pathogenesis of bcr/abl-negative MPD, Including the phenotype of growth factor independence and/or hypersensitivity. The current article summarizes this new information and discusses its implications for both classification and diagnosis of MPD.

Idiopathic myelofibrosis with generalized periostitis in a 4-year-old girl

Idiopathic myelofibrosis, a chronic myeloproliferative disorder of unknown origin, is characterized by splenomegaly, extramedullary hematopoiesis, leukoerythroblastosis, teardrop erythrocytes, and myelofibrosis. It is a rare disorder in childhood. The authors describe a 4-year-old girl with features consistent with idiopathic myelofibrosis, who also had generalized solid laminated periosteal reaction involving all long bones. The presence of thrombocytopenia at the onset and lack of leukocytosis were in contrast to the reported features seen in children. Recent case reports describe a relatively indolent course in children. Spontaneous remissions have also been described in pediatric cases. The fulminant course of this patient without any features of malignant transformation was noteworthy in this regard.

Gains on 9p are common genomic aberrations in idiopathic myelofibrosis: a comparative genomic hybridization study

Ideopathic myelofibrosis (IMF) is a chronic myeloproliferative disorder resulting in bone marrow fibrosis as a consequence of growth factor release from clonal haematopoiesis. Conventional cytogenetic analysis identifies abnormalities in approximately a third of cases at diagnosis, although rarely uncovers unique, primary genetic events. We have used comparative genomic hybridization (CGH) to study 25 IMF cases and have compared the results with conventional cytogenetics. Metaphase cells were available for analysis in 13 cases, of which seven showed an abnormal karyotype. CGH chromosomal profiles showed imbalances in 21 of 25 cases. The most frequent aberrations were gains of 9p (12 cases), 2q (seven cases), 3p (seven cases), chromosome 4 (seven cases), 12q (seven cases), 13q (eight cases). The main losses were at 17q and occurred in six cases. The results for CGH and cytogenetics were matched for one case only. Investigation of IMF by CGH suggests that genomic aberrations are much more common than has been previously indicated by conventional cytogenetic analysis and occur in the majority of cases. Gains of 9p were the most frequent finding, occurring in 50% of patients and suggests that genes on 9p may play a crucial role in the pathogenesis of IMF.

Myelofibrosis with myeloid metaplasia: new developments in pathogenesis and treatment

Myeloid metaplasia with myelofibrosis (MMM) is a chronic myeloproliferative disorder (CMPD) characterized by progressive anemia, massive splenomegaly, both hepatosplenic and non-hepatosplenic extramedullary hematopoiesis (EMH), a leukoerythroblastic blood smear, circulating progenitor cells, and marked bone marrow stromal reaction including collagen fibrosis, osteosclerosis and angiogenesis. The overall median survival is 5 years although it might range from 2 to 15 years depending on the presence or absence of clinically defined prognostic factors. Death is often due to leukemic transformation, portal hypertension or infection. In addition to shortened survival, quality of life is often affected by frequent red blood cell transfusions, profound constitutional symptoms, and cachexia. Drug therapy and autologous hematopoietic stem cell transplantation (HSCT) are of only palliative value and have not been shown to improve survival. The role of allogeneic HSCT, both myeloablative and non-myeloablative, is actively being investigated. Both splenectomy and radiation therapy have defined therapeutic roles to control EMH-associated symptoms. Analysis of the molecular biology of the disease is underway with the aid of animal models leading to the identification of novel therapeutic targets. Among the novel agents tested, thalidomide seems the most promising although newer agents are on the horizon.

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.

Spontaneous STAT5 activation induces growth factor independence in idiopathic myelofibrosis: possible relationship with FKBP51 overexpression

Spontaneous growth of megakaryocyte progenitors is one of the biologic hallmarks of idiopathic myelofibrosis (IMF). The molecular mechanisms underlying this hypersensitivity to cytokines are poorly understood. Using a differential display approach, we previously observed FK506 binding protein 51 (FKBP51) overexpression in pathologic megakaryocytes from IMF. Using an FKBP51-overexpressing cell line, we found sustained STAT5 activation associated with JAK2 phosphorylation. We subsequently tested whether this transcription factor was activated in patient samples. We detected a STAT5 nuclear translocation and activation in spontaneously grown megakaryocytes and in circulating CD34(+) cells from the majority of patients studied. The biologic role of this JAK/STAT pathway activation was demonstrated by inhibiting both the anti-apoptotic phenotype mediated by FKBP51 overexpression in UT7 cells and the spontaneous megakaryocytic growth by addition in culture of the JAK2 inhibitor AG490 or overexpression of a STAT5b dominant negative or SOCS-1. These results demonstrate that a constitutive STAT5 activation in IMF is indispensable for spontaneous growth of megakaryocytes. They also suggest that FKBP51 overexpression could be involved in STAT5 activation in IMF cells and in subsequent abnormal growth.

Discrimination of polycythemias and thrombocytoses by novel, simple, accurate clonality assays and comparison with PRV-1 expression and BFU-E response to erythropoietin

Essential thrombocythemia (ET) and polycythemia vera (PV) are clonal myeloproliferative disorders that are often difficult to distinguish from other causes of elevated blood cell counts. Assays that could reliably detect clonal hematopoiesis would therefore be extremely valuable for diagnosis. We previously reported 3 X-chromosome transcription-based clonality assays (TCAs) involving the G6PD, IDS, and MPP1 genes, which together were informative in about 65% of female subjects. To increase our ability to detect clonality, we developed simple TCA for detecting the transcripts of 2 additional X-chromosome genes: Bruton tyrosine kinase (BTK) and 4-and-a-half LIM domain 1 (FHL1). The combination of TCA established the presence or absence of clonal hematopoiesis in about 90% of female subjects. We show that both genes are subject to X-chromosome inactivation and are polymorphic in all major US ethnic groups. The 5 TCAs were used to examine clonality in 46 female patients along with assays for erythropoietin-independent erythroid colonies (EECs) and granulocyte PRV-1 mRNA levels to discriminate polycythemias and thrombocytoses. Of these, all 19 patients with familial polycythemia or thrombocytosis had polyclonal hematopoiesis, whereas 22 of 26 patients with clinical evidence of myeloproliferative disorder and 1 patient with clinically obscure polycythemia were clonal. Interestingly, interferon alpha therapy in 2 patients with PV was associated with reversion of clonal to polyclonal hematopoiesis. EECs were observed in 14 of 14 patients with PV and 4 of 12 with ET, and increased granulocyte PRV-1 mRNA levels were found in 9 of 13 patients with PV and 2 of 12 with ET. Thus, these novel clonality assays are useful in the diagnosis and follow-up of polycythemic conditions and disorders with increased platelet levels.

Both B and T lymphocytes may be clonally involved in myelofibrosis with myeloid metaplasia

A combination of magnetic cell sorting (MACS) and fluorescent in situ hybridization (FISH) techniques was used to detect clonal cytogenetic markers in different myeloid and lymphoid cell types of the peripheral blood from 4 patients with myelofibrosis with myeloid metaplasia (MMM) that was associated with either a 13q- or a 20q- karyotypic abnormality. Interphase cytogenetics studies demonstrated abnormal clonal FISH signal patterns in neutrophil, myeloid, erythroid, megakaryocyte, and B- and T-cell preparations in 3 of the 4 patients. In one patient, FISH results were within normal limits in T cells and slightly abnormal in B cells. In general, the percentage of abnormal nuclei was variable in both lymphocyte populations but always higher in B lymphocytes compared with T lymphocytes. The current study provides direct evidence for the clonal involvement of both B and T lymphocytes in MMM. A larger study is needed to clarify the relevance of the observed interpatient heterogeneity in clonal constitution.

The therapy of myelofibrosis: targeting pathogenesis

Myelofibrosis with myeloid metaplasia (MMM) encompasses the diagnoses of agnogenic myeloid metaplasia (idiopathic myelofibrosis), as well as the advanced phases of polycythemia vera and essential thrombocythemia (post polycythemic and post thrombocythemia myeloid metaplasia, respectively). MMM is a clonal, hematopoietic stem cell disorder in which neither the pathogenesis, nor a broadly applicable effective therapy have been described. Clinically, these patients experience progressive marrow replacement by fibrotic tissue, ineffective hematopoiesis, problematic cytopenia's, significant hepato-splenomegaly, extramedullary hematopoiesis, profound constitutional symptoms, and a risk of blastic transformation. Historically, therapies have been targeted at palliating symptoms (i.e. splenectomy, transfusions, hydroxyurea, erythropoietin, androgens, localized radiotherapy). Stem cell transplantation appears promising, but is often toxic and not broadly applicable due to co-morbidities and age of MMM patients. Non-myeloablative approaches to conditioning may broaden the applicability of stem cell transplantation in MMM, yet results to date are preliminary. Although a definitive molecular abnormality responsible for the pathogenesis of MMM has not been described, much has been learned about the aberrant expression of pro-fibrotic cytokines and the presence of increased angiogenesis in MMM. These pathogenetic insights have led to a series of pilot clinical trials with therapeutic agents targeting aberrantly expressed cytokines (and possibly angiogenesis) including Thalidomide (alone or in combination), Etanercept, and STI-571. Amongst these later agents Thalidomide has demonstrated the most promise (palliating disease associated cytopenia's), whereas the TNF-alpha inhibitor Etanercept has aided with MMM associated constitutional symptoms. Although these later trials have been helpful in a subset of patients, no agent to date has led to solid complete responses in MMM across the spectrum of disease manifestations. Further insights into the pathogenetic mechanisms responsible for myeloproliferation (aberrant cell signaling pathways, apoptotic resistance, other) are necessary to guide selection and testing of the expanding number of novel anti-neoplastic agents in chronic myeloid disorders and MMM.

Thalidomide in agnogenic and secondary myelofibrosis

Myelofibrosis with myeloid metaplasia (MMM) is a clonal disorder involving disregulation of angiogenesis and immunomodulatory mechanisms. Thalidomide (Thal) retains antiangiogenic, immunomodulatory and cytokine regulatory properties and recently it has been used successfully in multiple myeloma. Here, we report our experience in 10 MMM patients treated with Thal. Patients with agnogenic MMM treated in an early phase of the disease obtained significant benefits from the therapy and remain transfusion-free. In contrast, all secondary MMM failed to respond. These preliminary findings confirm that Thal plays a role in MMM therapy, although the efficacy in the different phases of the disease must be further evaluated.

Myeloablation and autologous peripheral blood stem cell rescue results in hematologic and clinical responses in patients with myeloid metaplasia with myelofibrosis

Current therapeutic options for myeloid metaplasia with myelofibrosis (MMM) are limited. A pilot study was conducted of autologous peripheral blood stem cell (PBSC) collection in 27, followed by transplantation in 21 patients with MMM. The median age was 59 (range 45-75) years. PBSCs were mobilized at steady state (n = 2), after granulocyte colony-stimulating factor (G-CSF) alone (n = 17), or after anthracycline-cytarabine induction plus G-CSF (n = 8). A median of 11.6 x 10(6) (range 0 to 410 x 10(6)) CD34(+) cells per kilogram were collected. Twenty-one patients then underwent myeloablation with oral busulfan (16 mg/kg) and PBSC transplantation. The median times to neutrophil and platelet recovery after transplantation were 21 (range 10-96) and 21 (range, 13 to > or = 246) days, respectively. Five patients received back-up PBSC infusion because of delayed neutrophil or platelet recovery. The median follow-up is 390 (range 70-1623) days after transplantation, and the 2-year actuarial survival is 61%. After transplantion, 6 patients died: 3 of nonrelapse causes (1 within 100 days of PBSC infusion) and 3 of disease progression. Erythroid response (hemoglobin > or = 100 g/L [10 gm/dL] without transfusion for > or = 8 weeks) occurred in 10 of 17 anemic patients. Four of 8 patients with a platelet count less than 100 x 10(9)/L (100 000/microL) responded with a durable platelet count more than 100 x 10(9)/L (100 000/microL). Symptomatic splenomegaly improved in 7 of 10 patients. It is concluded that (1) PBSC collection was feasible and stable engraftment occurred after transplantation in most patients with MMM, (2) myeloablation with busulfan was associated with acceptable toxicity, (3) a significant proportion of patients derived clinical benefit after treatment, and (4) further investigation of this novel approach is warranted. (Blood. 2001;98:586-593)

STAT3 is constitutively active in some patients with Polycythemia rubra vera

OBJECTIVE

Polycythemia vera is a clonal stem cell disorder characterized by hyperproliferation of the erythroid, myeloid, and megakaryocytic lineages. While it has been shown that progenitor cells of P. vera patients are hypersensitive to several growth factors including erythropoietin, insulin-like growth factor-1, thrombopoietin, interleukin-3, and granulocyte/monocyte colony-stimulating factor, the molecular pathogenesis of this disease remains unknown. Growth factor hypersensitivity could be mediated by changes in signal transduction pathways. We therefore investigated a common downstream effector of cytokines, the signal transducers and activators of transcription (STATs). A constitutive activation of STAT factors could explain the increased proliferation of P. vera cells even in the absence of growth factor stimulation.

METHODS

Peripheral granulocytes from patients with P. vera and from healthy volunteers were assayed for STAT1, 3, and 5 DNA binding by electrophoretic mobility shift assay.

RESULTS

Four of 14 P. vera patients analyzed showed constitutive STAT3 DNA binding in unstimulated peripheral granulocytes, while none of the 17 healthy volunteers tested did. None of the subjects showed constitutive STAT1 or STAT5 activity. Western blotting demonstrated that, in the three patients, STAT3 is constitutively phosphorylated on Tyr 705, whereas it is unphosphorylated in the other patients and in controls. Interestingly, constitutive STAT3 activity did not correlate with the duration of disease or the treatment regimen. It was observed in a recently diagnosed patient and in two patients treated only with phlebotomy.

CONCLUSION

Our data suggest that constitutive phosphorylation and activation of STAT3 is not a secondary event induced by mutagenizing agents or by prolonged hyperproliferation of hematopoietic cells, but rather represents a primary molecular aberration. Constitutively active STAT3 may contribute to the growth factor hypersensitivity of P. vera cells.

Clonal hemopoiesis and risk of thrombosis in young female patients with essential thrombocythemia

OBJECTIVE

Several studies demonstrated a high prevalence of nonrandom X-chromosome inactivation pattern (X-CIP) in essential thrombocythemia (ET). This study explored the incidence of clonal hemopoiesis in myeloid precursors and endogenous erythroid colonies (EECs) in ET patients and its correlation with thrombotic manifestations.

MATERIALS AND METHODS

Clonal analysis of hemopoiesis using X-CIP was performed in 40 female patients with ET. Median age was 40.5 years (range 20-64), and median platelet count at testing time was 700 x 10(9)/L (range 220-1300 x 10(9)/L). Patients older than 65 years were excluded to reduce age-related skewing. Clonality was assessed on neutrophils, platelets, EECs, and bone marrow CD34(+) cells.

RESULTS

Eight (20%) of 40 patients developed thrombosis mainly at diagnosis. Clonal hemopoiesis was found in 17 (42.5%) patients, 15 (37.5%) had polyclonal hemopoiesis, and 8 (20%) were considered uninterpretable due to constitutive skewing. Clonality was confirmed on purified CD34(+) subpopulations from bone marrow, documenting that clonality does not appear lineage-restricted. There were no statistical differences in age at diagnosis, median platelet count at testing time, and length of follow-up. Thrombotic episodes were significantly more frequent in the monoclonal group (p = 0.04, Fisher exact test).

CONCLUSIONS

Young female patients with ET exhibiting a clonal pattern of hemopoiesis by X-CIP analysis are at higher risk for thrombosis. X-CIP analysis may contribute to defining the individual risk leading to appropriate treatment. X-CIP will allow a correct diagnosis in patients with latent myeloproliferative disorders and thrombosis in unusual sites. Clonal hemopoiesis is easily recognized by X-CIP, but its applicability is limited to the female sex and is hampered by the presence of age-related or constitutive skewing.

Clonality analysis using X-chromosome inactivation patterns by HUMARA-PCR assay in female controls and patients with idiopathic thrombocytosis in Taiwan

OBJECTIVE

Analysis of X-chromosome inactivation patterns (XCIPs) is a useful tool in the diagnosis of clonal disorders. The human androgen receptor (HUMARA) locus is especially useful for clonality study. The present study was conducted 1) to determine the heterozygosity rate for HUMARA locus in Taiwanese women, 2) to determine the frequency of excessive skewing in different cell types, and 3) to determine the utility of XCIPs in the differential diagnosis of thrombocytosis.

PATIENTS AND METHODS

XCIPs by HUMARA-PCR assay were performed on purified granulocytes and T cells from 73 female patients presenting with idiopathic persistent thrombocytosis (IT), 10 patients with reactive thrombocytosis (RT), and 46 bone marrow samples from female controls. XCIPs of buccal mucosa cells were also compared with those of T cells in 57 patients with IT. The percentage of clonal granulocytes was calculated after correcting for the degree of Lyonization in T cells.

RESULTS

The heterozygosity rate for the HUMARA gene was 89.1% in Taiwanese females. The median age of informative IT patients and controls was 59 (18-92) and 58 (19-89), respectively. Excessive skewing (allele ratio <0.33) was more frequent in granulocytes than in T cells in both controls (12/43 vs 9/43, p = 0.080) and IT patients (56/64 vs 25/64, p < 0.001). XCIPs were the same for both buccal mucosa and T cells in 43 patients but were different in 14 patients. Of the 43 informative controls, 31 had a polyclonal pattern; an ambiguous pattern was found in nine; and the remaining three, aged 71, 73, and 80, respectively, had a clonal pattern. A clonal pattern was found in 42 IT patients, a polyclonal pattern in 12, and an ambiguous pattern in 10 of the 64 IT patients. The frequency of clonal, polyclonal, and ambiguous patterns in the 40 IT patients with age < or = 65 was 55.0%, 30.0%, and 15.0%, respectively. None of the IT patients aged >65 had a polyclonal disease. IT patients aged >65 had a significantly higher frequency of clonal pattern (p = 0.030) and a significantly lower frequency of polyclonal pattern (p = 0.002) than those with age <65. Of the eight heterozygous patients with RT, one aged 80 exhibited a clonal pattern, and the remaining seven had a polyclonal pattern.

CONCLUSIONS

The present study on Taiwanese females showed a heterozygosity rate of 89.1% for the HUMARA gene. Our results confirmed that IT is a heterogeneous disorder in terms of clonality. Twenty-three percent of IT patients exhibited a greater than 20% difference in allele expression for buccal mucosa and T cells. Presence of a clonal XCIP in young patients with IT can serve as a positive marker for the diagnosis of clonal thrombocytosis, and elderly patients with polyclonal XCIPs are unlikely to have essential thrombocythemia.

The Pathogenesis of Chronic Myeloproliferative Diseases

Chronic myeloproliferative disorders are operationally classified to include essential thrombocythemia, polycythemia vera, and agnogenic myeloid metaplasia. In most cases, clonal hematopoiesis, involving all 3 myeloid lineages, can be demonstrated. However, the underlying molecular lesions that are responsible for disease initiation and progression remain elusive. There are ongoing efforts to clarify the pathogenetic role of cytokines, bone marrow stromal cells and molecules, and intracellular aberrations in either signal transduction or apoptosis. This review discusses some of the current and past observations regarding the pathogenesis of chronic myeloproliferative disorders.

Towards a molecular understanding of polycythemia rubra vera

Polycythemia rubra vera (PV) is one of four diseases collectively called the myeloproliferative disorders (MPDs). Each disorder leads to an increased production of one or several hematopoietic cell lineages. MPDs arise from acquired mutations in a pluripotent hematopoietic stem cell. However, the molecular mechanisms leading to the development of these diseases are poorly understood. This review will summarize and evaluate recent advances in our understanding of one particular MPD, PV.

Cloning of PRV-1, a novel member of the uPAR receptor superfamily, which is overexpressed in polycythemia rubra vera

Polycythemia vera (PV) is a clonal stem cell disorder characterized by hyperproliferation of the erythroid, myeloid, and megakaryocytic lineages. Although it has been shown that progenitor cells of patients with PV are hypersensitive to several growth factors, the molecular pathogenesis of this disease remains unknown. To investigate the molecular defects underlying PV, we used subtractive hybridization to isolate complementary DNAs (cDNAs) differentially expressed in patients with PV versus normal controls. We isolated a novel gene, subsequently named PRV-1, which is highly expressed in granulocytes from patients with PV (n = 19), but not detectable in normal control granulocytes (n = 21). Moreover, PRV-1 is not expressed in mononuclear cells from patients with chronic myelogenous leukemia (n = 4) or acute myelogenous leukemia (n = 5) or in granulocytes from patients with essential thrombocythemia (n = 4) or secondary erythrocytosis (n = 4). Northern blot analysis showed that PRV-1 is highly expressed in normal human bone marrow and to a much lesser degree in fetal liver. It is not expressed in a variety of other tissues tested. Although PRV-1 is not expressed in resting granulocytes from normal controls, stimulation of these cells with granulocyte colony-stimulating factor induces PRV-1 expression. The PRV-1 cDNA encodes an open reading frame of 437 amino acids, which contains a signal peptide at the N-terminus and a hydrophobic segment at the C-terminus. In addition, PRV-1 contains 2 cysteine-rich domains homologous to those found in the uPAR/Ly6/CD59/snake toxin-receptor superfamily. We therefore propose that PRV-1 represents a novel hematopoietic receptor.

Cloning of PRV-1, a novel member of the uPAR receptor superfamily, which is overexpressed in polycythemia rubra vera

Polycythemia vera (PV) is a clonal stem cell disorder characterized by hyperproliferation of the erythroid, myeloid, and megakaryocytic lineages. Although it has been shown that progenitor cells of patients with PV are hypersensitive to several growth factors, the molecular pathogenesis of this disease remains unknown. To investigate the molecular defects underlying PV, we used subtractive hybridization to isolate complementary DNAs (cDNAs) differentially expressed in patients with PV versus normal controls. We isolated a novel gene, subsequently named PRV-1, which is highly expressed in granulocytes from patients with PV (n = 19), but not detectable in normal control granulocytes (n = 21). Moreover, PRV-1 is not expressed in mononuclear cells from patients with chronic myelogenous leukemia (n = 4) or acute myelogenous leukemia (n = 5) or in granulocytes from patients with essential thrombocythemia (n = 4) or secondary erythrocytosis (n = 4). Northern blot analysis showed that PRV-1 is highly expressed in normal human bone marrow and to a much lesser degree in fetal liver. It is not expressed in a variety of other tissues tested. Although PRV-1 is not expressed in resting granulocytes from normal controls, stimulation of these cells with granulocyte colony-stimulating factor induces PRV-1 expression. The PRV-1 cDNA encodes an open reading frame of 437 amino acids, which contains a signal peptide at the N-terminus and a hydrophobic segment at the C-terminus. In addition, PRV-1 contains 2 cysteine-rich domains homologous to those found in the uPAR/Ly6/CD59/snake toxin-receptor superfamily. We therefore propose that PRV-1 represents a novel hematopoietic receptor.

Myelofibrosis with myeloid metaplasia: diagnostic definition and prognostic classification for clinical studies and treatment guidelines

PURPOSE

Myelofibrosis with myeloid metaplasia (MMM) is a chronic myeloproliferative disorder characterized by bone marrow fibrosis and extramedullary hematopoiesis. Recent studies provide definite diagnostic criteria and prognostic classifications of the disease, and allogeneic stem-cell transplantation (SCT) now offers a chance of curing the disease. In order to put diagnostic criteria and prognostic classifications of the disease into the perspective of developing guidelines for treatment strategies, all studies published in the English literature over the last 30 years were reviewed.

MATERIALS AND METHODS

Studies were identified through a MEDLINE search (1966 to present) and from the bibliographies of relevant articles.

RESULTS

The Italian Consensus Conference on diagnostic criteria is a structured enterprise aimed at formulating a definition of MMM that will be used for enrolling patients onto clinical studies. It relies on the obligatory presence of myelofibrosis and on the exclusion of the BCR-ABL rearrangement or Philadelphia chromosome, in association with combinations of traditional features. Prognostic scores allow us to identify classes of patients on the basis of hemoglobin, age, WBC count, and chromosomal abnormalities. Several nonrandomized studies have indicated that allogeneic SCT for patients under the age of 55 is effective in prolonging survival in more than 50% of cases and in possibly curing the disease. Patients with the most severe prognosis are candidates.

CONCLUSION

"Consensus" methodology offers a definition of MMM useful for conducting and reporting clinical studies. A detailed knowledge of prognostic factors can help to delineate guidelines for addressing patients with allogeneic SCT.

A Large Proportion of Patients With a Diagnosis of Essential Thrombocythemia Do Not Have a Clonal Disorder and May Be at Lower Risk of Thrombotic Complications

Essential thrombocythemia (ET) is traditionally considered to be a clonal disorder. No specific karyotypic abnormalities have been described, but the demonstration of clonality using X-chromosome inactivation patterns (XCIPs) has been used to differentiate ET from a non-clonal reactive thrombocytosis. However, these assays may be difficult to interpret, and contradictory results have been reported. We have studied 46 females with a diagnosis of ET according to the Polycythemia Vera Study Group (PVSG) criteria. XCIP results in 23 patients (50%) were uninterpretable due to either constitutive or possible acquired age-related skewing. Monoclonal myelopoiesis could be definitively shown in only 10 patients. Thirteen patients had polyclonal myelopoiesis, and in 8, it was possible to exclude clonal restriction to the megakaryocytic lineage. Furthermore, there was no evidence of clonal progenitors in purified CD34+CD33− and CD34+CD33+ subpopulations from bone marrow of 2 of these 13 patients. There was no difference between patients with monoclonal and polyclonal myelopoiesis with respect to age or platelet count at diagnosis, duration of follow-up, incidence of hepatosplenomegaly, or hemorrhagic complications. However, polyclonal patients were less likely to have experienced thrombotic events (P = .039). These results suggest that ET is a heterogeneous disorder, and the clinical significance of clonality status warrants investigation in a larger study.

A Large Proportion of Patients With a Diagnosis of Essential Thrombocythemia Do Not Have a Clonal Disorder and May Be at Lower Risk of Thrombotic Complications

Essential thrombocythemia (ET) is traditionally considered to be a clonal disorder. No specific karyotypic abnormalities have been described, but the demonstration of clonality using X-chromosome inactivation patterns (XCIPs) has been used to differentiate ET from a non-clonal reactive thrombocytosis. However, these assays may be difficult to interpret, and contradictory results have been reported. We have studied 46 females with a diagnosis of ET according to the Polycythemia Vera Study Group (PVSG) criteria. XCIP results in 23 patients (50%) were uninterpretable due to either constitutive or possible acquired age-related skewing. Monoclonal myelopoiesis could be definitively shown in only 10 patients. Thirteen patients had polyclonal myelopoiesis, and in 8, it was possible to exclude clonal restriction to the megakaryocytic lineage. Furthermore, there was no evidence of clonal progenitors in purified CD34+CD33− and CD34+CD33+ subpopulations from bone marrow of 2 of these 13 patients. There was no difference between patients with monoclonal and polyclonal myelopoiesis with respect to age or platelet count at diagnosis, duration of follow-up, incidence of hepatosplenomegaly, or hemorrhagic complications. However, polyclonal patients were less likely to have experienced thrombotic events (P = .039). These results suggest that ET is a heterogeneous disorder, and the clinical significance of clonality status warrants investigation in a larger study.

Treatment of polycythaemia vera and essential thrombocythaemia

The clinical course in both polycythaemia vera (PV) and essential thrombocythaemia (ET) is characterized by significant thrombohaemorrhagic complications and variable risk of disease transformation into myeloid metaplasia with myelofibrosis or acute myeloid leukaemia. Randomized studies have shown that the risk of thrombosis was significantly reduced in ET with the use of hydroxyurea (HU) and in PV with the use of chlorambucil or 32P. However, the use of chlorambucil or 32P has been associated with an increased risk of leukaemic transformation. Subsequently, other studies have suggested that both HU and pipobroman may be less leukaemogenic and as effective as chlorambucil and 32P for preventing thrombosis in PV. However, the results from these prospective studies have raised concern that even HU and pipobroman may be associated with excess leukaemic events in both ET and PV. The recent introduction of anagrelide as a specific platelet-lowering agent, the demonstration of treatment efficacy with interferon-alpha, and the revived interest in using low-dose acetylsalicylic acid provide the opportunity to initiate prospective randomized studies incorporating these treatments.

Molecular genetics and cytogenetics of myeloproliferative disorders

The myeloproliferative disorders are believed to represent clonal malignancies resulting from transformation of a pluripotent stem cell. X-inactivation patterns of peripheral blood cells have been proposed as a useful diagnostic tool but this method is limited by the finding of a clonal X-inactivation pattern in a significant proportion of normal elderly women. There is no pathognomonic chromosomal abnormality associated with the myeloproliferative disorders. However, consistent acquired cytogenetic changes include del(20q), del(13q), trisomy 8 and 9 and duplication of segments of 1q, all of which have been observed at diagnosis or before cytoreductive therapy and therefore represent early lesions which contribute to the pathogenesis of these disorders. Although, the acquired molecular defects underlying most myeloproliferative disorders have not yet been elucidated, translocations associated with the rare 8p11 syndrome have permitted identification of a novel fusion protein. The role of a number of candidate genes in the other myeloproliferative disorders has also been studied, but no mutations have been identified so far. It is likely that a number of genes will be involved, given the varied phenotypes of the diseases. Identification of causal genes will be of considerable interest to both clinicians, who currently lack a specific and sensitive diagnostic test, and scientists interested in fundamental issues of stem cell behaviour.

Clonality markers in polycythaemia and primary thrombocythaemia

Our current understanding of the pathogenesis of the myeloproliferative disorders is based on the assumption that they represent a clonal disorder resulting from transformation of a haematopoietic stem cell. Clonality assays exploit the fact that female cells have only one active X-chromosome. Methods for determining X chromosome inactivation have been devised at the protein (G6PD isoenzymes); DNA (HUMARA; phosphoglycerate kinase (PGK); hypoxanthine-phosphoribosyl transferase (HPRT) and RNA (G6PD; P55; IDS) levels. In this type of RNA assay the product of the active X chromosome is quantified by studying polymorphisms present in mRNA. The presence of skewed lyonization in normal females is a potential limitation to the method, although the use of T cells as a control makes it possible to distinguish clonal haematopoiesis from skewed lyonization. However, the phenomenon of acquired skewing in normal elderly women means that X-inactivation patterns in elderly patients must be interpreted with caution. Clonality studies have been conducted in polycythaemia vera (PV) and essential thrombocythaemia (ET) patients. They usually demonstrate a clonal X-inactivation pattern in granulocytes and/or platelets but a polyclonal pattern in T cells. However, in both ET and PV a significant minority of patients exhibit polyclonal haematopoiesis with polyclonal patterns in granulocytes/platelets. Female patients with polyclonal haematopoiesis differ from those with clonal haematopoiesis in terms of age and platelet count and possibly in their requirements for treatment. This new technology for the investigation of the MPD seems promising for understanding certain clinical aspects of these diseases and may be introduced for evaluation of new modalities of treatment.

Clonality analysis of hematopoiesis and thrombopoietin levels in patients with essential thrombocythemia

Essential thrombocythemia (ET) is a myeloproliferative disorder, characterized by sustained thrombocytosis. Diagnosis requires the elimination of all known causes of thrombocytosis. ET is believed to be a clonal disorder, and we investigated the frequency of a clonal hematopoiesis in this disease with the aim of using this as a positive diagnostic criterion. However, a non-random inactivation pattern can be encountered in normal females which mimics clonal hematopoiesis. In addition, the percentage of normal females with skewed lyonization seems higher using techniques based on the difference in DNA methylation, compared to G6PD enzyme polymorphism. Recently, new techniques based on transcript analysis have been developed. We report here the results of clonality studies of hematopoiesis in 53 ET patients using two different techniques based on DNA and RNA polymorphisms, and T-lymphocytes as a control tissue of lyonization. The majority of ET patients showed monoclonal hematopoiesis in the presence of polyclonality of T-lymphocytes. Because all ET patients did not show the same clonal pattern of hematopoiesis, we searched for inappropriate secretion of thrombopoietin (TPO) in patients with polyclonal disease. This assay was performed in 48 patients, of whom 9 showed polyclonal hematopoiesis and 27 monoclonal hematopoiesis. We found no difference in TPO levels between ET patients and normal controls, nor between patients with polyclonal hematopoiesis and those with monoclonal hematopoiesis. Our results confirm the high frequency of monoclonal hematopoiesis in ET, the usefulness of RNA markers, and the possibility of using T-lymphocytes as a control tissue for X-chromosome inactivation patterns. On the other hand, TPO levels are not decreased even in ET patients with high platelet counts, suggesting an increased production or decreased clearance of TPO in this disease.

A histomorphometric study of haematological disorders with respect to marrow fibrosis and osteosclerosis

A retrospective investigation of 75 EDTA-decalcified Jamshidi biopsies collected over a 2-year period at Aarhus University Hospital was performed. The biopsies originated from 75 patients suffering from idiopathic myelofibrosis, other chronic myeloproliferative disorders, or other conditions with known associations with bone marrow fibrosis. The relative volumes of trabecular and woven bone, as well as haematopoietic and non-haematopoietic tissue, were estimated histomorphometrically. The degree of fibrosis was estimated semiquantitatively. Finally, the thickness of trabecular osteons was estimated from the number of lamellae by counting. Patients with idiopathic myelofibrosis had statistically significantly more bone tissue than the other groups of patients. The osteosclerosis was primarily due to woven bone. Larger cancellous osteons also suggested a positive balance in bone remodelling. The amount of bone tissue showed furthermore a statistically significant increase through the groups of polycythaemia vera, essential thrombocythaemia, chronic myelogenous leukaemia and idiopathic myelofibrosis. Parallel to the increase in the amount of bone, an increase in the degree of marrow fibrosis was detected. The positive correlation between the amount of bone and the degree of marrow fibrosis was statistically highly significant (p=0.0008).

X-Chromosome inactivation in healthy females: incidence of excessive lyonization with age and comparison of assays involving DNA methylation and transcript polymorphisms

Skewed lyonization in healthy females represents the major disadvantage of X-chromosome-based clonality assays. Because most techniques are based on the difference in DNA methylation between active and inactive X-chromosomes, incomplete DNA digestion may occur, giving an unreliable clonality result. Here, we compare two different techniques carried out in healthy females belonging to three age groups and in a group of patients with essential thrombocythemia. The first technique involved the human androgen receptor gene, the second the transcript analysis of the iduronate-2-sulfatase, P55, and glucose-6-phospate dehydrogenase genes. Results between both techniques were concordant in most cases except in neonates, and the same pattern was observed in all fractions in healthy females. We conclude that: (a) clonality assays involving DNA and RNA polymorphisms are usually concordant except in neonates; (b) appropriate control tissue embryologically related to the sample must be chosen to eliminate excessive lyonization; (c) acquired skewing increases with age, whereas nonrandom lyonization is a rare phenomenon.