Genetic analysis of chromosome 13 deletions in BCR/ABL negative chronic myeloproliferative disorders

Pathological interactions between hematopoietic stem cells and their niche revealed by mouse models of primary myelofibrosis

Primary myelofibrosis (PMF) belongs to the Philadelphia-negative myeloproliferative neoplasms and is a hematological disorder caused by abnormal function of the hematopoietic stem cells. The disease manifests itself with a plethora of alterations, including anemia, splenomegaly and extramedullary hematopoiesis. Its hallmarks are progressive marrow fibrosis and atypical megakaryocytic hyperplasia, two distinctive features used to clinically monitor disease progression. In an attempt to investigate the role of abnormal megakaryocytopoiesis in the pathogenesis of PMF, several transgenic mouse models have been generated. These models are based either on mutations that interfere with the extrinsic (thrombopoietin and its receptor, MPL) and intrinsic (the GATA1 transcription factor) control of normal megakaryocytopoiesis, or on known genetic lesions associated with the human disease. Here we provide an up-to-date review on the insights into the pathobiology of human PMF achieved by studying these animal models, with particular emphasis on results obtained with Gata1(low) mice.

A myeloproliferative disorder may hide another one

Chronic myeloproliferative disorders (MPDs) are divided into Philadelphia-positive chronic myeloid leukemia (CML) and Philadelphia-negative disorders including polycythemia vera, essential thrombocythemia and idiopathic myelofibrosis (IMF). Concomitance of a CML and another MPD is a rare event. We report here the case of a patient presenting initially with IMF who developed a Philadelphia-positive CML 7 years later. At the time of CML diagnosis, two distinct clones were present, one with a 13q deletion and one with a t(9;22). We raise the problem of a CML developing on an initial IMF, or two MPDs occurring from a common or two different stem cells.

The role of cytogenetic abnormalities as a prognostic marker in primary myelofibrosis: applicability at the time of diagnosis and later during disease course

Although cytogenetic abnormalities are important prognostic factors in myeloid malignancies, they are not included in current prognostic scores for primary myelofibrosis (PMF). To determine their relevance in PMF, we retrospectively examined the impact of cytogenetic abnormalities and karyotypic evolution on the outcome of 256 patients. Baseline cytogenetic status impacted significantly on survival: patients with favorable abnormalities (sole deletions in 13q or 20q, or trisomy 9 +/- one other abnormality) had survivals similar to those with normal diploid karyotypes (median, 63 and 46 months, respectively), whereas patients with unfavorable abnormalities (rearrangement of chromosome 5 or 7, or > or = 3 abnormalities) had a poor median survival of 15 months. Patients with abnormalities of chromosome 17 had a median survival of only 5 months. A model containing karyotypic abnormalities, hemoglobin, platelet count, and performance status effectively risk-stratified patients at initial evaluation. Among 73 patients assessable for clonal evolution during stable chronic phase, those who developed unfavorable or chromosome 17 abnormalities had median survivals of 18 and 9 months, respectively, suggesting the potential role of cytogenetics as a risk factor applicable at any time in the disease course. Dynamic prognostic significance of cytogenetic abnormalities in PMF should be further prospectively evaluated.

Cytogenetic and molecular aspects of Philadelphia negative chronic myeloproliferative disorders: clinical implications

Chronic myeloproliferative disorders (CMPD) are clonal disorders of the hematopoietic stem cell. The myeloid lineage shows increased proliferation with effective maturation, while peripheral leukocytosis, thrombocytosis or elevated red blood cell mass are found. In Philadelphia negative CMPD recurrent cytogenetic abnormalities occur, but no specific abnormality has been defined to date. The spectrum of cytogenetic aberrations is heterogeneous ranging from numerical gains and losses to structural changes including unbalanced translocations. The most common chromosomal abnormalities are 20q-, 13q-, 12p-, +8, +9, partial duplication of 1q, balanced translocations involving 8p11 and gains in 9p. Cytogenetic analysis of CMPD by conventional or molecular techniques has an important role in establishing the diagnosis of a malignant disease, adding also more information for disease outcome. Molecular studies may detect the possible role of candidate genes implicated in the neoplastic process, addressing new molecular target therapies. FIP1L1/PDGFRalpha rearrangements, as well as alterations of PDGFRbeta or FGFR1 gene have been found to be associated with specific types of CMPD. Recently, a novel somatic mutation, JAK2V617F, has been reported in most of the polycthemia vera (PV) patients, as well as in a lower percentage in essential thrombocythemia (ET) or idiopathic myelofibrosis (IMF) patients. This finding represents the most important advance in understanding of the molecular mechanisms underlined the pathogenesis of CMPD, contributing to the classification and management of patients.

Cytogénétique et génétique moléculaire dans la myélofibrose avec métaplasie myéloïde et dans la polyglobulie de Vaquez

Myelofibrosis with myeloid metaplasia (MMM) is a rare myeloproliferative disorder (MPD) characterized by clonal proliferation of hematopoietic progenitors. 40-50% of karyotypes on blood (or more rarely on bone marrow) revealed at least one abnormality: 30% at diagnosis and 90% in blastic transformation phase. A minority of patients with newly diagnosed polycythemia vera (PV) presented chromosomal abnormalities in their myeloid cells. The most frequent visible alteration in MMM and PV is a 20q deletion, also characterized in other MPDs and myeloid malignancies. Among other chromosomal changes, deletion 13q is more common in MMM than in other MPDs, trisomy 9 and 9p alterations appear more frequent in PV. Cytogenetic studies have disclosed cryptic anomalies and pointed out the high frequency of 9p alterations. JAK2 (V617F) mutation was found in almost all PV patients and near half of MMM patients. This molecular abnormality takes an increased importance in the knowledge of the physiopathology of MPDs, particularly in PV and also in prognosis of MMM patients.

Role of GATA-1 in normal and neoplastic hemopoiesis

GATA-1 exerts a concentration-dependent control on the differentiation of erythroid, megakaryocytic, mast, and eosinophilic cells. The concentration of GATA-1 is, in turn, regulated by specific sequences within the GATA-1 locus. On the basis of its levels of expression, the GATA-1 protein becomes associated with suitable partners forming transcription complexes that, by binding to lineage-specific enhancers, activate the expression of the corresponding target genes. Instrumental to our understanding of the role of GATA-1 in hemopoietic differentiation has been the generation of genetically engineered mutant mice and the discovery of naturally occurring mutations associated with either inherited or acquired human pathologies. We review our current understanding of the role of GATA-1 in normal and neoplastic hematopoiesis as emerging from these genetic approaches.

Prognostic diversity among cytogenetic abnormalities in myelofibrosis with myeloid metaplasia

BACKGROUND

Approximately 30-50% of patients with myelofibrosis with myeloid metaplasia (MMM) demonstrate detectable cytogenetic abnormalities, the prognostic value of which has not been completely defined by previous retrospective studies. The current prospective study addresses this issue in the context of currently accepted independent prognostic variables.

METHODS

The current study is a single institution study in which patients with MMM were accrued between January 2000 and August 2001 and followed in a prospective fashion. All study patients underwent bone marrow examination with cytogenetic studies as well as comprehensive clinical and laboratory evaluation at the time of karyotype analysis.

RESULTS

Among the study cohort of 81 patients (with a median age of 61 years; 54 males), the cytogenetic findings were normal in 44 patients (54%; Group 1). The remaining 37 patients (46%) demonstrated either interstitial deletions involving the long arm of chromosome 13 or 20 (9 patients; Group 2) or other abnormalities (28 patients; Group 3). All study patients were followed prospectively for a minimum of 40 months (range, 40-55 months). Survival from the time of karyotypic analysis was found to be similar between Groups 1 and 2 but was significantly worse in Group 3. Furthermore, none of the patients in Group 2 experienced leukemic transformation, whereas five patients each from the other two groups did. Multivariate analysis identified an unfavorable cytogenetic profile (Group 3), > or = 1% circulating blasts, a hemoglobin level of <10 g/dL, and constitutional symptoms as adverse prognostic features for overall survival.

CONCLUSIONS

Specific cytogenetic lesions in patients with MMM might carry an independent prognostic effect for both survival and the risk of leukemic transformation. Such information should assist in decision making when considering aggressive treatment approaches.

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.

Variegation of the phenotype induced by the Gata1low mutation in mice of different genetic backgrounds

All mice harboring the X-linked Gata1low mutation in a predominantly CD1 background are born anemic and thrombocytopenic. They recover from anemia at 1 month of age but remain thrombocytopenic all their life and develop myelofibrosis, a syndrome similar to human idiopathic myelofibrosis, at 12 months. The effects of the genetic background on the myelofibrosis developed by Gata1low mice was assessed by introducing the mutation, by standard genetic approaches, in the C57BL/6 and DBA/2 backgrounds and by analyzing the phenotype of the different mutants at 12 to 13 (by histology) and 16 to 20 (by cytofluorimetry) months of age. Although all the Gata1low mice developed fibrosis at 12 to 13 months, variegations were observed in the severity of the phenotype expressed by mutants of different backgrounds. In C57BL/6 mice, the mutation was no longer inherited in a Mendelian fashion, and fibrosis was associated with massive osteosclerosis. Instead, DBA/2 mutants, although severely anemic, expressed limited fibrosis and osteosclerosis and did not present tear-drop poikilocytes in blood or extramedullary hemopoiesis in liver up to 20 months of age. We propose that the variegation in myelofibrosis expressed by Gata1low mutants of different strains might represent a model to study the variability of the clinical picture of the human disease.

Frequency of structural abnormalities of the long arm of chromosome 12 in myelofibrosis with myeloid metaplasia

Among cytogenetic studies of 205 patients diagnosed as myelofibrosis with myeloid metaplasia, we found seven cases with structural abnormalities of the long arm of chromosome 12. The karyotype showed six balanced translocations, that is, t(4;12)(q33;q21), t(5;12)(p14;q21), t(1;12)(q22;q24), t(12;17)(q24;q11), t(7;12) (p11;q24), and t(1;12)(p12;q24), as well as other cytogenetic abnormalities such as del(12)(q21;q24) and inv(12) (p12q24). Some isolated cases involving the 12q21 region have also been described in the literature. Importance of rearrangement of chromosome 12 in 12q21 or 12q24 is underlined by the authors suggesting a proto-oncogene accountable mechanism of leukemogenesis.

Myeloproliferative disorders

The myeloproliferative disorders (MPDs) are a group of pre-leukaemic disorders characterized by proliferation of one or more lineages of the myelo-erythroid series. Unlike the Philadelphia chromosome in chronic myeloid leukaemia, there is no pathognomonic chromosomal abnormality associated with the MPDs. Chromosomal abnormalities are seen in 30-40% of patients with polycythaemia vera (PV) and idiopathic myelofibrosis (IMF) and seem to indicate a poor prognosis. On the other hand, chromosomal abnormalities are rare in essential thrombocythaemia. Consistent acquired changes seen at diagnosis include deletion of the long arm of chromosome 20, del(13q), trisomy 8 and 9 and duplication of parts of 1q. Furthermore del(20q), trisomy 8 and dupl(lq) all arise in multipotent progenitor cells. Molecular mapping of 20q deletions and, to some extent, 13q deletions has identified a number of candidate target genes, although no mutations have yet been found. Finally, translocations associated with the rare 8p11 myeloproliferative syndrome and other atypical myeloproliferative disorders have permitted the identification of a number of novel fusion proteins involving fibroblast growth factor receptor-1.

Cytogenetic findings and their clinical relevance in myelofibrosis with myeloid metaplasia

The prognostic significance of bone marrow cytogenetic lesions in myelofibrosis with myeloid metaplasia (MMM) was investigated in a retrospective series of 165 patients. An abnormal karyotype was demonstrated in 57% of patients. At diagnosis (n = 92), 48% of the patients had detectable cytogenetic abnormalities, and clonal evolution was frequently demonstrated in sequential studies. More than 90% of the anomalies were represented by 20q-, 13q-, +8, +9, 12p-, and abnormalities of chromosomes 1 and 7. Of these, 20q-, 13q- and +8 were the most frequent sole abnormalities, each occurring in 15-25% of the abnormal cases. Trisomy 9 and abnormalities of chromosomes 1 and 7 were equally prevalent but were usually associated with additional cytogenetic lesions. Chromosome 5 abnormalities were infrequent but were over-represented in the group of patients exposed to genotoxic therapy. In a multivariate analysis that incorporated other clinical and laboratory variables, the presence of an abnormal karyotype did not carry an adverse prognosis. Instead, +8, 12p-, advanced age and anaemia were independent prognostic determinants of inferior survival. In particular, survival was not adversely affected by the presence of either 20q- or 13q-.

Late-appearing PML/RARalpha fusion transcript with coincidental t(12;13)(p13.2;q14) in acute promyelocytic leukemia lacking the t(15;17) cytogenetic anomaly

The late appearance of a cytogenetic/molecular hallmark in human leukemias is a rare event. We report on a case of acute myeloid leukemia with morphology, immunophenotype and clinical features typical of promyelocytic subtype (APL), in which the specific PML/RARalpha gene rearrangement was molecularly detected only at second relapse of disease, without cytogenetic evidence of the t(15;17). The emergence of the PML/RARalpha gene may be therapy-related or may represent the exceptional result of a clonal evolution during progression of neoplasia. At second relapse, a novel cell clone bearing a t(12;13)(p13.2;q14) was also observed and a molecular deletion and rearrangement of a locus at 13q14, distinct from retinoblastoma (Rb1) locus, was found. In this unusual case, the PML/RARalpha product seems to be not essential for the expression of the promyelocytic phenotype at diagnosis and, when detectable, it is not the sole genetic defect.

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.

Pathogenesis and management of idiopathic myelofibrosis

Idiopathic myelofibrosis is the least common and carries the worst prognosis of the chronic myeloproliferative disorders. The primary disease process is a clonal haematopoietic stem cell disorder which results in a chronic myeloproliferation and an atypical megakaryocyte hyperplasia. In contrast, the characteristic stromal proliferation is a reactive phenomenon, resulting from the inappropriate release of megakaryocyte/platelet-derived growth factors, including PDGF, TGF-beta bFGF and calmodulin. The median survival is approximately 4 years, although individual survival varies greatly. A variety of prognostic schema have been developed which enable the identification of high-risk patients, for whom bone marrow transplantation should be considered. Management for the majority of patients, however, is directed towards the alleviation of symptoms and improvement in quality of life. This review summarizes the recent advances in our understanding of the disease's pathogenesis and discusses the limited therapeutic options available to clinicians.

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.

Idiopathic myelofibrosis: pathogenesis, natural history and management

Idiopathic myelofibrosis is a chronic myeloproliferative disorder characterized by bone marrow fibrosis, extramedullary haematopoiesis and a leucoerythroblastic blood picture. The marrow fibrosis results from an increased deposition of various interstitial and basement membrane glycoproteins, including collagen types I, III, IV, V and VI, fibronectin, vitronectin, laminin and tenascin. In addition, a marked neovascularization is present, even in the early proliferative phase of the disease. In contrast to the clonal haematopoiesis, the increased bone marrow stromal tissue is thought to be a reactive phenomenon, resulting from the inappropriate release of megakaryocyte/platelet-derived growth factors, including PDGF, TGF-beta, EGF, bFGF and calmodulin. Recent cytogenetic studies have highlighted three defects, namely del(13q), del(20q) and partial trisomy 1q, that account for nearly 70% of all abnormalities at diagnosis, and suggests that in many patients gene loss and/or inactivation may be an important pathogenetic mechanism. The median survival is approximately 4 years, although individual survival varies greatly. Prognostic schema enable the identification of patients with a limited life expectancy, for whom bone marrow transplantation should be considered. However, for the majority of patients therapy is supportive and consists of blood transfusions, androgens to sustain erythropoiesis, cytoreductive agents to prevent thrombocythaemia and, in carefully selected cases, splenectomy. The role for a number of experimental therapies, such as vitamin D3 analogues, alpha and gamma interferons and erythropoietin has yet to be defined.