Haematopoietic progenitors and signal transduction in polycythaemia vera and primary thrombocythaemia

Peripheral blood haematopoietic progenitor cells in patients with beta thalassaemia major receiving desferrioxamine or deferiprone as chelation therapy

OBJECTIVES

The main adverse effect of deferiprone is the development of neutropenia, which occurs via an unknown mechanism. We aimed to gain insight into the pathogenesis of deferiprone-induced neutropenia by assessing the peripheral blood haematopoietic progenitor cells.

METHODS

Sixteen patients with beta thalassaemia were studied; nine (Group A) were receiving desferrioxamine and seven (Group B) deferiprone. Ten healthy individuals comprised the control group (Group C).

RESULTS

Granulocyte-erythrocyte-monocyte-megakaryocyte colony forming units were significantly more in Groups A and B compared with Group C. Granulocyte-macrophage colony forming units (CFU-GM) were significantly more in Group B compared with Group C. Macrophage colony forming units were significantly less in Group B compared with Group C. Granulocyte colony forming units (CFU-G) were significantly more in Group A compared with Group C. We found a trend in the difference in the number of CFU-G between patients' groups (P = 0.123). Adding serum from patients receiving deferiprone to cultures of controls resulted in a maturation arrest of the granulocytic lineage.

CONCLUSION

Our findings point to a maturation arrest at the level of CFU-GM as a potential mechanism of deferiprone-induced neutropenia.

Chromosomal Abnormalities and Molecular Markers in Myeloproliferative Disorders

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

Molecular pathogenesis of Philadelphia chromosome negative myeloproliferative disorders

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

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

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

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.

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.

Chromosome 20 deletions in myeloid malignancies: reduction of the common deleted region, generation of a PAC/BAC contig and identification of candidate genes. UK Cancer Cytogenetics Group (UKCCG)

Deletion of the long arm of chromosome 20 represents the most common chromosomal abnormality associated with the myeloproliferative disorders (MPDs) and is also found in other myeloid malignancies including myelodysplastic syndromes (MDS) and acute myeloid leukaemia (AML). Previous studies have identified a common deleted region (CDR) spanning approximately 8 Mb. We have now used G-banding, FISH or microsatellite PCR to analyse 113 patients with a 20q deletion associated with a myeloid malignancy. Our results define a new MPD CDR of 2.7 Mb, an MDS/AML CDR of 2.6 Mb and a combined 'myeloid' CDR of 1.7 Mb. We have also constructed the most detailed physical map of this region to date--a bacterial clone map spanning 5 Mb of the chromosome which contains 456 bacterial clones and 202 DNA markers. Fifty-one expressed sequences were localized within this contig of which 37 lie within the MPD CDR and 20 within the MDS/AML CDR. Of the 16 expressed sequences (six genes and 10 unique ESTs) within the 'myeloid' CDR, five were expressed in both normal bone marrow and purified CD34 positive cells. These data identify a set of genes which are both positional and expression candidates for the target gene(s) on 20q.

Congenital and inherited polycythemia

Absolute polycythemia is a condition with increased red blood cell mass. There are a number of primary and secondary polycythemic disorders leading to absolute polycythemia. Primary polycythemias are caused by a defect intrinsic to the erythroid progenitor cells. The best characterized primary polycythemia is the autosomal dominant primary familial and congenital polycythemia (PFCP). Familial or childhood occurrence of the myeloproliferative disorder polycythemia vera are also discussed, emphasizing the importance of distinction between polycythemia vera and PFCP. Congenital or familial secondary polycythemic conditions are characterized by increased red cell mass, which is caused by circulating serum factors, typically erythropoietin.

A Polycythemia Vera Update: Diagnosis, Pathobiology, and Treatment

This review focuses on polycythemia vera (PV)—its diagnosis, cellular and genetic pathology, and management. In Section I, Dr. Pearson, with Drs. Messinezy and Westwood, reviews the diagnostic challenge of the investigation of patients with a raised hematocrit. The suggested approach divides patients on their red cell mass (RCM) results into those with absolute (raised RCM) and apparent (normal RCM) erythrocytosis. A standardized series of investigations is proposed for those with an absolute erythrocytosis to confirm the presence of a primary (PV) or secondary erythrocytosis, with abnormal and normal erythropoietic compartments respectively, leaving a heterogenous group, idiopathic erythrocytosis, where the cause cannot be established. Since there is no single diagnostic test for PV, its presence is confirmed following the use of updated diagnostic criteria and confirmatory marrow histology.

In Section II, Dr. Green with Drs. Bench, Huntly, and Nacheva reviews the evidence from studies of X chromosome inactivation patterns that support the concept that PV results from clonal expansion of a transformed hemopoietic stem cell. Analyses of the pattern of erythroid and myeloid colony growth have demonstrated abnormal responses to several cytokines, raising the possibility of a defect in a signal transduction pathway shared by several growth factors. A number of cytogenetic and molecular approaches are now focused on defining the molecular lesion(s).

In the last section, Dr. Barbui with Dr. Finazzi addresses the complications of PV, notably thrombosis, myelofibrosis and acute leukemia. Following an evaluation of published data, a management approach is proposed. All patients should undergo phlebotomy to keep the hematocrit (Hct) below 0.45, which may be all that is required in those at low thrombotic risk and with stable disease. In those at high thrombotic risk or with progressive thrombocytosis or splenomegaly, a myelosuppressive agent should be used. Hydroxyurea has a role at all ages, but 32P or busulfan may be used in the elderly. In younger patients, interferon-α or anagrelide should be considered. Low-dose aspirin should be used in those with thrombotic or ischemic complications.

A Polycythemia Vera Update: Diagnosis, Pathobiology, and Treatment

This review focuses on polycythemia vera (PV)—its diagnosis, cellular and genetic pathology, and management. In Section I, Dr. Pearson, with Drs. Messinezy and Westwood, reviews the diagnostic challenge of the investigation of patients with a raised hematocrit. The suggested approach divides patients on their red cell mass (RCM) results into those with absolute (raised RCM) and apparent (normal RCM) erythrocytosis. A standardized series of investigations is proposed for those with an absolute erythrocytosis to confirm the presence of a primary (PV) or secondary erythrocytosis, with abnormal and normal erythropoietic compartments respectively, leaving a heterogenous group, idiopathic erythrocytosis, where the cause cannot be established. Since there is no single diagnostic test for PV, its presence is confirmed following the use of updated diagnostic criteria and confirmatory marrow histology.

In Section II, Dr. Green with Drs. Bench, Huntly, and Nacheva reviews the evidence from studies of X chromosome inactivation patterns that support the concept that PV results from clonal expansion of a transformed hemopoietic stem cell. Analyses of the pattern of erythroid and myeloid colony growth have demonstrated abnormal responses to several cytokines, raising the possibility of a defect in a signal transduction pathway shared by several growth factors. A number of cytogenetic and molecular approaches are now focused on defining the molecular lesion(s).

In the last section, Dr. Barbui with Dr. Finazzi addresses the complications of PV, notably thrombosis, myelofibrosis and acute leukemia. Following an evaluation of published data, a management approach is proposed. All patients should undergo phlebotomy to keep the hematocrit (Hct) below 0.45, which may be all that is required in those at low thrombotic risk and with stable disease. In those at high thrombotic risk or with progressive thrombocytosis or splenomegaly, a myelosuppressive agent should be used. Hydroxyurea has a role at all ages, but 32P or busulfan may be used in the elderly. In younger patients, interferon-α or anagrelide should be considered. Low-dose aspirin should be used in those with thrombotic or ischemic complications.