Defective response to thrombopoietin and impaired expression of c-mpl mRNA of bone marrow cells in congenital amegakaryocytic thrombocytopenia

Pediatric Bone Marrow Failure: A Broad Landscape in Need of Personalized Management

Irreversible severe bone marrow failure (BMF) is a life-threatening condition in pediatric patients. Most important causes are inherited bone marrow failure syndromes (IBMFSs) and (pre)malignant diseases, such as myelodysplastic syndrome (MDS) and (idiopathic) aplastic anemia (AA). Timely treatment is essential to prevent infections and bleeding complications and increase overall survival (OS). Allogeneic hematopoietic stem cell transplantation (HSCT) provides a cure for most types of BMF but cannot restore non-hematological defects. When using a matched sibling donor (MSD) or a matched unrelated donor (MUD), the OS after HSCT ranges between 60 and 90%. Due to the introduction of post-transplantation cyclophosphamide (PT-Cy) to prevent graft versus host disease (GVHD), alternative donor HSCT can reach similar survival rates. Although HSCT can restore ineffective hematopoiesis, it is not always used as a first-line therapy due to the severe risks associated with HSCT. Therefore, depending on the underlying cause, other treatment options might be preferred. Finally, for IBMFSs with an identified genetic etiology, gene therapy might provide a novel treatment strategy as it could bypass certain limitations of HSCT. However, gene therapy for most IBMFSs is still in its infancy. This review summarizes current clinical practices for pediatric BMF, including HSCT as well as other disease-specific treatment options.

Platelet genetic testing by next-generation sequencing: A practical update

Inherited platelet disorders (IPDs) are a heterogeneous group of disorders characterized by normal or reduced platelet counts, bleeding diatheses of varying severities, and the presence (syndromic) or absence (non-syndromic) of involvement of other organs. Due to the lack of highly specific platelet function tests and overlapping clinical and laboratory features, diagnosing the underlying cause of IPDs remains challenging. In recent years, genetic testing via next-generation sequencing (NGS) technologies to rapidly analyze multiple genes has gradually emerged as an important part of the laboratory investigation of patients with IPDs. A systemic clinical and laboratory testing approach and thorough phenotype and genotype correlation studies of both patients and their family members are crucial for accurate diagnoses of IPDs.

Treatment of drug-induced immune thrombocytopenias

Several therapeutic agents can cause thrombocytopenia by either immune-mediated or non-immune-mediated mechanisms. Non-immune-mediated thrombocytopenia is due to direct toxicity of drug molecules to platelets or megakaryocytes. Immune-mediated thrombocytopenia, on the other hand, involves the formation of antibodies that react to platelet-specific glycoprotein complexes, as in classic drug-induced immune thrombocytopenia (DITP), or to platelet factor 4, as in heparin-induced thrombocytopenia (HIT) and vaccine-induced immune thrombotic thrombocytopenia (VITT). Clinical signs include a rapid drop in platelet count, bleeding or thrombosis. Since the patient's condition can deteriorate rapidly, prompt diagnosis and management are critical. However, the necessary diagnostic tests are only available in specialized laboratories. Therefore, the most demanding step in treatment is to identify the agent responsible for thrombocytopenia, which often proves difficult because many patients are taking multiple medications and have comorbidities that can themselves also cause thrombocytopenia. While DITP is commonly associated with an increased risk of bleeding, HIT and VITT have a high mortality rate due to the high incidence of thromboembolic complications. A structured approach to drug-associated thrombocytopenia/thrombosis can lead to successful treatment and a lower mortality rate. In addition to describing the treatment of DITP, HIT, VITT, and vaccine-associated immune thrombocytopenia, this review also provides the pathophysiological and clinical information necessary for correct patient management.

Treatment of chemotherapy-induced thrombocytopenia in patients with non-hematologic malignancies

Chemotherapy-induced thrombocytopenia (CIT) is a common complication of the treatment of non-hematologic malignancies. Many patient-related variables (e.g., age, tumor type, number of prior chemotherapy cycles, amount of bone marrow tumor involvement) determine the extent of CIT. CIT is related to the type and dose of chemotherapy, with regimens containing gemcitabine, platinum, or temozolomide producing it most commonly. Bleeding and the need for platelet transfusions in CIT are rather uncommon except in patients with platelet counts below 25x10⁹/L in whom bleeding rates increase significantly and platelet transfusions are the only treatment. Nonetheless, platelet counts below 70x10⁹/L present a challenge. In patients with such counts, it is important to exclude other causes of thrombocytopenia (medications, infection, thrombotic microangiopathy, post-transfusion purpura, coagulopathy and immune thrombocytopenia). If these are not present, the common approach is to reduce chemotherapy dose intensity or switch to other agents. Unfortunately decreasing relative dose intensity is associated with reduced tumor response and remission rates. Thrombopoietic growth factors (recombinant human thrombopoietin, pegylated human megakaryocyte growth and development factor, romiplostim, eltrombopag, avatrombopag and hetrombopag) improve pretreatment and nadir platelet counts, reduce the need for platelet transfusions, and enable chemotherapy dose intensity to be maintained. National Comprehensive Cancer Network guidelines permit their use but their widespread adoption awaits adequate phase III randomized, placebo-controlled studies demonstrating maintenance of relative dose intensity, reduction of platelet transfusions and bleeding, and possibly improved survival. Their potential appropriate use also depends on consensus by the oncology community as to what constitutes an appropriate pretreatment platelet count as well as identification of patient-related and treatment variables that might predict bleeding.

Screening and diagnosis of inherited platelet disorders

Inherited platelet disorders are important conditions that often manifest with bleeding. These disorders have heterogeneous underlying pathologies. Some are syndromic disorders with non-blood phenotypic features, and others are associated with an increased predisposition to developing myelodysplasia and leukemia. Platelet disorders can present with thrombocytopenia, defects in platelet function, or both. As the underlying pathogenesis of inherited thrombocytopenias and platelet function disorders are quite diverse, their evaluation requires a thorough clinical assessment and specialized diagnostic tests, that often challenge diagnostic laboratories. At present, many of the commonly encountered, non-syndromic platelet disorders do not have a defined molecular cause. Nonetheless, significant progress has been made over the past few decades to improve the diagnostic evaluation of inherited platelet disorders, from the assessment of the bleeding history to improved standardization of light transmission aggregometry, which remains a "gold standard" test of platelet function. Some platelet disorder test findings are highly predictive of a bleeding disorder and some show association to symptoms of prolonged bleeding, surgical bleeding, and wound healing problems. Multiple assays can be required to diagnose common and rare platelet disorders, each requiring control of preanalytical, analytical, and post-analytical variables. The laboratory investigations of platelet disorders include evaluations of platelet counts, size, and morphology by light microscopy; assessments for aggregation defects; tests for dense granule deficiency; analyses of granule constituents and their release; platelet protein analysis by immunofluorescent staining or flow cytometry; tests of platelet procoagulant function; evaluations of platelet ultrastructure; high-throughput sequencing and other molecular diagnostic tests. The focus of this article is to review current methods for the diagnostic assessment of platelet function, with a focus on contemporary, best diagnostic laboratory practices, and relationships between clinical and laboratory findings.

Non-myeloablative conditioning is sufficient to achieve complete donor myeloid chimerism following matched sibling donor bone marrow transplant for myeloproliferative leukemia virus oncogene (MPL) mutation-driven congenital amegakaryocytic thrombocytopenia: Case report

BACKGROUND

Congenital amegakaryocytic thrombocytopenia (CAMT) is a rare platelet production disorder caused mainly by loss of function biallelic mutations in myeloproliferative leukemia virus oncogene (MPL), the gene encoding the thrombopoietin receptor (TPOR). Patients with MPL-mutant CAMT are not only at risk for life-threatening bleeding events, but many affected individuals will also ultimately develop bone marrow aplasia owing to the absence of thrombopoietin/TPOR signaling required for maintenance of hematopoietic stem cells. Curative allogeneic stem cell transplant for patients with CAMT has historically used myeloablative conditioning; however, given the inherent stem cell defect in MPL-mutant CAMT, a less intensive regimen may prove equally effective with reduced morbidity, particularly in patients with evolving aplasia.

METHODS

We report the case of a 2-year-old boy with MPL-mutant CAMT and bone marrow hypocellularity who underwent matched sibling donor bone marrow transplant (MSD-BMT) using a non-myeloablative regimen consisting of fludarabine, cyclophosphamide, and antithymocyte globulin (ATG).

RESULTS

The patient achieved rapid trilinear engraftment and resolution of thrombocytopenia. While initial myeloid donor chimerism was mixed (88% donor), due to the competitive advantage of donor hematopoietic cells, myeloid chimerism increased to 100% by 4 months post-transplant. Donor chimerism and blood counts remained stable through 1-year post-transplant.

CONCLUSION

This experience suggests that non-myeloablative conditioning is a suitable approach for patients with MPL-mutant CAMT undergoing MSD-BMT and is associated with reduced risks of conditioning-related toxicity compared to traditional myeloablative regimens.

Congenital amegakaryocytic thrombocytopenia - Not a single disease

Congenital amegakaryocytic thrombocytopenia (CAMT) is a rare inherited bone marrow failure syndrome (IBMFS) that is characterized by severe thrombocytopenia at birth due to ineffective megakaryopoiesis and development towards aplastic anemia during the first years of life. CAMT is not a single monogenetic disorder; rather, many descriptions of CAMT include different entities with different etiologies. CAMT in a narrow sense, which is primarily restricted to the hematopoietic system, is caused mainly by mutations in the gene for the thrombopoietin receptor (MPL), sometimes in the gene for its ligand (THPO). CAMT in association with radio-ulnar synostosis, which is not always clinically apparent, is mostly caused by mutations in MECOM, rarely in HOXA11. Patients affected by other IBMFS - especially Fanconi anemia or dyskeratosis congenita - may be misdiagnosed as having CAMT when they lack typical disease features of these syndromes or have only mild symptoms. This article reviews scientific and clinical aspects of the various disorders associated with the term "CAMT" with a main focus on the disease caused by mutations in the MPL gene.

New Small Molecule Drugs for Thrombocytopenia: Chemical, Pharmacological, and Therapeutic Use Considerations

This review provides details about three small molecules that were recently approved by the FDA for the treatment of thrombocytopenia. The new treatments include lusutrombopag, avatrombopag, and fostamatinib. The first two drugs are orally active thrombopoietin receptor (TPO-R) agonists which are FDA-approved for the treatment of thrombocytopenia in adult patients with chronic liver disease who are scheduled to undergo a procedure. Fostamatinib is orally active prodrug that, after activation, becomes spleen tyrosine kinase (SYK) inhibitor. Fostamatinib is currently used to treat chronic and refractory immune thrombocytopenia in patients who have had insufficient response to previous treatment. Chemical structures, available dosage forms, recommended dosing, pharmacokinetics, results of toxicity studies in animals, most frequent adverse effects, significant outcomes of the corresponding clinical trials, and their use in specific patient populations are thoroughly described. Described also is a comparative summary of the different aspects of five currently available therapies targeting TPO-R or SYK for the treatment of thrombocytopenia.

The Thrombopoietin Receptor: Structural Basis of Traffic and Activation by Ligand, Mutations, Agonists, and Mutated Calreticulin

A well-functioning hematopoietic system requires a certain robustness and flexibility to maintain appropriate quantities of functional mature blood cells, such as red blood cells and platelets. This review focuses on the cytokine receptor that plays a significant role in thrombopoiesis: the receptor for thrombopoietin (TPO-R; also known as MPL). Here, we survey the work to date to understand how this receptor functions at a molecular level throughout its lifecycle, from traffic to the cell surface, dimerization and binding cognate cytokine via its extracellular domain, through to its subsequent activation of associated Janus kinases and initiation of downstream signaling pathways, as well as the regulation of these processes. Atomic level resolution structures of TPO-R have remained elusive. The identification of disease-causing mutations in the receptor has, however, offered some insight into structure and function relationships, as has artificial means of receptor activation, through TPO mimetics, transmembrane-targeting receptor agonists, and engineering in dimerization domains. More recently, a novel activation mechanism was identified whereby mutated forms of calreticulin form complexes with TPO-R via its extracellular N-glycosylated domain. Such complexes traffic pathologically in the cell and persistently activate JAK2, downstream signal transducers and activators of transcription (STATs), and other pathways. This pathologic TPO-R activation is associated with a large fraction of human myeloproliferative neoplasms.

Durable engraftment and correction of hematological abnormalities in children with congenital amegakaryocytic thrombocytopenia following myeloablative umbilical cord blood transplantation

The use of HSCT is the only potentially curative treatment for CAMT, but access is limited by the availability of suitable donors. We report five consecutive patients with CAMT who received MAC and partially HLA-mismatched, UCBT (unrelated, n = 4). Median times to neutrophil (>500/μL) and platelet (≥20 000 and ≥50 000/μL) engraftment were 19, 57, and 70 days, respectively. Acute GvHD, grade II, developed in one patient, who subsequently developed limited chronic GvHD. At median follow-up of 14 yr, all patients are alive with sustained donor cell engraftment. To our knowledge, this is the largest single-center series of UCBT for patients with this disease and suggests that UCBT is a successful curative option for patients with CAMT.

Mouse prenatal platelet-forming lineages share a core transcriptional program but divergent dependence on MPL

Prenatal platelet-forming lineages are subject to common transcription factor controls despite distinct spatial and ancestral origins. Platelet-forming lineage production is MPL-independent on emergence, but MPL is required in the late fetus for efficient thrombopoiesis.

Myelodysplastic Syndrome Presenting as Amegakaryocytic Thrombocytopenia in a Collodion Baby

We report a rare case of myelodysplastic syndrome that presented early as amegakaryocytic thrombocytopenia in a collodion baby, which is a rare congenital disorder characterized by thick, taut membrane resembling oiled parchment or collodion, which is subsequently shed. To our knowledge, this is the first reported case of a collodion baby who presented with amegakaryocytic thrombocytopenia and who has a significant family history of the same condition. We document the rarity of this possible association and also the need for further study to establish whether a causal relationship exists.

Thrombopoietin from beginning to end

In the two decades since its cloning, thrombopoietin (TPO) has emerged not only as a critical haematopoietic cytokine, but also serves as a great example of bench-to-bedside research. Thrombopoietin, produced by the liver, is the primary regulator of megakaryocyte progenitor expansion and differentiation. Additionally, as TPO is vital for the maintenance of haematopoietic stem cells, it can truly be described as a pan-haematopoietic cytokine. Since recombinant TPO became available, the molecular mechanisms of TPO function have been the subject of extensive research. Via its receptor, c-Mpl (also termed MPL), TPO activates a wide array of downstream signalling pathways, promoting cellular survival and proliferation. Due to its central, non-redundant role in haematopoiesis, alterations of both the hormone and its receptor contribute to human disease; congenital and acquired states of thrombocytosis and thrombocytopenia and aplastic anaemia as a result from dysregulated TPO expression or functional alterations of c-Mpl. With TPO mimetics now in clinical use, the story of this haematopoietic cytokine represents a great success for biomedical research.

Functional characterization of c-Mpl ectodomain mutations that underlie congenital amegakaryocytic thrombocytopenia

Activation of the cell surface receptor, c-Mpl, by the cytokine, thrombopoietin (TPO), underpins megakaryocyte and platelet production in mammals. In humans, mutations in c-Mpl have been identified as the molecular basis of Congenital Amegakaryocytic Thrombocytopenia (CAMT). Here, we show that CAMT-associated mutations in c-Mpl principally lead to defective receptor presentation on the cell surface. In contrast, one CAMT mutant c-Mpl, F104S, was expressed on the cell surface, but showed defective TPO binding and receptor activation. Using mutational analyses, we examined which residues adjacent to F104 within the membrane-distal cytokine receptor homology module (CRM) of c-Mpl comprise the TPO-binding epitope, revealing residues within the predicted Domain 1 E-F and A-B loops and Domain 2 F'-G' loop as key TPO-binding determinants. These studies underscore the importance of the c-Mpl membrane-distal CRM to TPO-binding and suggest that mutations within this CRM that perturb TPO binding could give rise to CAMT.

Congenital amegakaryocytic thrombocytopenia iPS cells exhibit defective MPL-mediated signaling

Congenital amegakaryocytic thrombocytopenia (CAMT) is caused by the loss of thrombopoietin receptor-mediated (MPL-mediated) signaling, which causes severe pancytopenia leading to bone marrow failure with onset of thrombocytopenia and anemia prior to leukopenia. Because Mpl(-/-) mice do not exhibit the human disease phenotype, we used an in vitro disease tracing system with induced pluripotent stem cells (iPSCs) derived from a CAMT patient (CAMT iPSCs) and normal iPSCs to investigate the role of MPL signaling in hematopoiesis. We found that MPL signaling is essential for maintenance of the CD34+ multipotent hematopoietic progenitor (MPP) population and development of the CD41+GPA+ megakaryocyte-erythrocyte progenitor (MEP) population, and its role in the fate decision leading differentiation toward megakaryopoiesis or erythropoiesis differs considerably between normal and CAMT cells. Surprisingly, complimentary transduction of MPL into normal or CAMT iPSCs using a retroviral vector showed that MPL overexpression promoted erythropoiesis in normal CD34+ hematopoietic progenitor cells (HPCs), but impaired erythropoiesis and increased aberrant megakaryocyte production in CAMT iPSC-derived CD34+ HPCs, reflecting a difference in the expression of the transcription factor FLI1. These results demonstrate that impaired transcriptional regulation of the MPL signaling that normally governs megakaryopoiesis and erythropoiesis underlies CAMT.

A novel nonsense mutation in the MPL gene in congenital amegakaryocytic thrombocytopenia

Congenital amegakaryocytic thrombocytopenia (CAMT) is a rare autosomal recessive disorder characterized by thrombocytopenia from failure of megakaryopoiesis. CAMT is one of the bone marrow failure syndromes, and the disease progression may involve other lineages leading to pancytopenia. The genetic background of CAMT is mutations in the MPL gene encoding the thrombopoietin receptor. Here, we describe a Korean male with CAMT. Molecular genetic analyses by direct sequencing revealed that he was compound heterozygous for two nonsense mutations in MPL, Tyr63X (c.189C>A), and Arg357X (c.1069C>T), the latter being a novel mutation.

Congenital amegakaryocytic thrombocytopenia: a brief review of the literature

Congenital amegakaryocytic thrombocytopenia (CAMT) is a rare inherited autosomal recessive disorder that presents with thrombocytopenia and absence of megakaryocytes. It presents with bleeding recognized on day 1 of life or at least within the first month. The cause for this disorder appears to be a mutation in the gene for the thrombopoeitin (TPO) receptor, c-Mpl, despite high levels of serum TPO. Patients with severe Type I-CAMT carry nonsense Mpl mutations which causes a complete loss of the TPO receptor whereas those with Type II CAMT carry missense mutations in the Mpl gene affecting the extracellular domain of the TPO receptor. Differential diagnosis for severe CAMT includes thrombocytopenia with absent radii (TAR) and Wiskott-Aldrich syndrome (WAS). The primary treatment for CAMT is bone marrow transplantation. Bone Marrow/Stem Cell Transplant (HSCT) is the only thing that ultimately cures this genetic disease. Newer modalities are on the way, such as TPO-mimetics for binding towards partially functioning c-Mpl receptors and gene therapy. Prognosis of CAMT patients is poor, because all develop in childhood a tri-linear marrow aplasia that is always fatal when untreated. Thirty percent of patients with CAMT die due to bleeding complications and 20% -due to HSCT if it has been done.

Advances in the understanding of congenital amegakaryocytic thrombocytopenia

Congenital amegakaryocytic thrombocytopenia (MIM #604498) is an extremely rare inherited bone marrow failure syndrome, usually presenting as a severe thrombocytopenia at birth due to ineffective megakaryocytopoiesis and no characteristic physical anomalies. Usually the isolated thrombocytopenia progresses to pancytopenia during the first years of life. The only curative therapy to date is haematopoietic stem cell transplantation. Most of the cases of congenital amegakaryocytic thrombocytopenia are caused by defective expression or function of the thrombopoietin receptor due to homozygous or compound heterozygous mutations in the gene MPL. The essential roles of thrombopoietin as a lineage specific regulator of platelet production and as a regulator of haematopoietic stem cell function are reflected in the haematological defects seen in affected individuals.

Congenital amegakaryocytic thrombocytopenia: the diagnostic importance of combining pathology with molecular genetics

Congenital Amegakaryocytic Thrombocytopenia (CAMT) is a rare bone marrow failure syndrome that presents with isolated thrombocytopenia within the first year of life. Classic diagnostic bone marrow findings reveal absent or significantly decreased megakaryocytes with otherwise normal marrow cellularity. We present a newborn with thrombocytopenia whose initial bone marrow aspirate showed an appropriate number of megakaryocytes. CAMT was subsequently diagnosed after molecular testing demonstrated a mutation in the thrombopoietin receptor. The presence of a normal number of megakaryocytes on an initial bone marrow aspirate should not exclude CAMT from the differential diagnosis of thrombocytopenia within the first year of life.

Thrombocytopenia in the neonate

Thrombocytopenia is one of the commonest haematological problems in neonates, affecting at least 25% of all admissions to neonatal intensive care units (NICUs) [Murray NA, Howarth LJ, McCloy MP et al. Platelet transfusion in the management of severe thrombocytopenia in neonatal intensive care unit patients. Transfus Med 2002;12:35-41; Garcia MG, Duenas E, Sola MC et al. Epidemiologic and outcome studies of patients who received platelet transfusions in the neonatal intensive care unit. J Perinatol 2001;21:415-20; Del Vecchio A, Sola MC, Theriaque DW et al. Platelet transfusions in the neonatal intensive care unit: factors predicting which patients will require multiple transfusions. Transfusion 2001;41:803-8]. Although a long list of disorders associated with neonatal thrombocytopenia can be found in many textbooks, newer classifications based on the timing of onset of thrombocytopenia (early vs. late) are more useful for planning diagnostic investigations and day-to-day management. The mainstay of treatment of neonatal thrombocytopenia remains platelet transfusion although it is important to note that no studies have yet shown clinical benefit of platelet transfusion in this setting. Indeed some reports even suggest that there may be significant adverse effects of platelet transfusion in neonates, including increased mortality, and that the effects of transfusion may differ in different groups of neonates with similar degrees of thrombocytopenia [Bonifacio L, Petrova A, Nanjundaswamy S, Mehta R. Thrombocytopenia related neonatal outcome in preterms. Indian J Pediatr 2007;74:269-74; Kenton AB, Hegemier S, Smith EO et al. Platelet transfusions in infants with necrotizing enterocolitis do not lower mortality but may increase morbidity. J Perinatol 2005;25:173-7]. There is also considerable variation in transfusion practice between different countries and between different neonatal units. Here we review recent progress in understanding the prevalence, causes and pathogenesis of thrombocytopenia in the newborn, the clinical consequences of thrombocytopenia and developments in neonatal platelet transfusion.

Congenital amegakaryocytic thrombocytopenia-3 novel c-MPL mutations and their phenotypic correlations

Congenital amegakaryocytic thrombocytopenia (CAMT) is a rare bone marrow failure syndrome associated with thrombocytopenia and a tendency to progress to aplastic anemia. Mutations in the c-MPL gene encoding for thrombopoietin receptor have been identified in the majority of the patients. Previous studies suggest a genotype-phenotype correlation wherein the severity of the disease depends on the type of mutation present and residual thrombopoietin receptor activity. The present study describes the clinical and genetic findings on a series of 7 patients with CAMT, 3 of them siblings. The patients were homozygous for 5 mutations in the c-MPL gene, including 3 unique ones: c.212+5G>A, C76T, and G1162C. The clinical picture was variable; 1 patient who was homozygous for a nonsense mutation in exon 1 (C76T) developed infantile acute lymphoblastic leukemia, whereas patients who were homozygous for a splice-site mutation (c.212+5G>A) expressing both normal and mutated transcripts had a milder clinical course. As previously suggested, c-MPL mutation analysis in CAMT patients helps to predict the clinical course and to provide optimal therapy.

Inherited thrombocytopenia: when a low platelet count does not mean ITP

Congenital thrombocytopenias, once considered rare and obscure conditions, are today recognized with increasing frequency, especially due to the measurement of platelet number as part of routine blood testing. The clinical spectrum of congenital thrombocytopenia ranges from severe bleeding diatheses, recognized within the first few weeks of life, to mild conditions that may remain undetected even in adulthood. For the latter group of diseases, distinguishing between inherited (primary) and acquired (secondary) thrombocytopenia, especially immune thrombocytopenia purpura (ITP), is essential to avoid unnecessary and potentially harmful treatments. In this review, the congenital thrombocytopenia syndromes are discussed with specific attention focused on diagnostic criteria, clinical presentations, genetic etiology, and current medical management. The mutated genes responsible for each syndrome are reviewed as well as the potential implications for using gene therapy or gene repair in the future.

Three parameters, plasma thrombopoietin levels, plasma glycocalicin levels and megakaryocyte culture, distinguish between different causes of congenital thrombocytopenia

Fourteen children with congenital thrombocytopenia were analysed in order to unravel the mechanisms underlying their thrombocytopenia and to evaluate the value of new laboratory tests, namely measurement of plasma thrombopoietin (Tpo) and glycocalicin (GC) levels and analysis of megakaryocytopoiesis in vitro. Three groups of patients were included. The first group (n = 6) was diagnosed with congenital amegakaryocytic thrombocytopenia. They had no megakaryocytes in the bone marrow, three out of four patients showed no megakaryocyte formation in vitro, and all had high Tpo and low GC levels. Mutations in the thrombopoietin receptor gene, c-mpl, were the cause. The second group of patients (n = 3) had normal Tpo and severely decreased GC levels. In bone marrow, normal to increased numbers of atypical, dysmature megakaryocytes were present. In vitro megakaryocyte formation was quantitatively normal. A defect in final megakaryocyte maturation and subsequent (pro-)platelets may be the cause of the thrombocytopenia. The patients in the third group (n = 5) had Wiskott-Aldrich syndrome (WAS). They had normal Tpo and GC levels and normal megakaryocyte formation both in vivo and in vitro. This corresponded with the generally accepted hypothesis that thrombocytopenia in WAS is due to increased platelet turnover. In conclusion, different causes of congenital thrombocytopenia can be distinguished using three parameters: Tpo and GC plasma levels and in vitro analysis of megakaryocytopoiesis. Therefore, these parameters may be helpful in early diagnosis of different forms of congenital thrombocytopenia.

In multiple myeloma increased thrombopoietin (Tpo) production may be involved in the maintenance of platelet production

In multiple myeloma (MM), suppression of haematopoiesis occurs as a result of expansion of malignant cells in the bone marrow. Thrombopoietin (Tpo) levels in patients with impaired platelet production are generally found to be highly elevated. To examine the circulating Tpo levels in patients with MM, Tpo levels were measured in 50 serum samples from 34 patients. Tpo levels were subsequently related to disease stage, and cell numbers and markers, i.e. platelet count, leukocyte count and haemoglobin (Hb) concentration. Elevated Tpo levels were found in association with decreased platelet counts (n=8), but also in patients with normal platelet counts (n=14). The latter group included patients without and with signs of impaired haematopoiesis, i.e. with decreased Hb concentration and decreased leukocyte count. These results show that neither platelet counts nor Tpo levels are reliable parameters to judge bone-marrow failure in patients with MM. Furthermore, in patients with MM, increased Tpo levels may play a role in the maintenance of thrombocytopoiesis. The origin of the increased Tpo levels remains to be determined.

c-mpl mutations are the cause of congenital amegakaryocytic thrombocytopenia

Congenital amegakaryocytic thrombocytopenia (CAMT) is a rare disease presenting with isolated thrombocytopenia in infancy and developing into a pancytopenia in later childhood. Thrombopoietin (TPO) is the main regulator of thrombocytopoiesis and has also been demonstrated to be an important factor in early hematopoiesis. We analyzed 9 patients with CAMT for defects in TPO production and reactivity. We found high levels of TPO in the sera of all patients. However, platelets and hematopoietic progenitor cells of patients with CAMT did not show any reactivity to TPO, as measured by testing TPO-synergism to adenosine diphosphate in platelet activation or by megakaryocyte colony assays. Flow cytometric analysis revealed absent surface expression of the TPO receptor c-Mpl in 3 of 3 patients. Sequence analysis of the c-mpl gene revealed point mutations in 8 of 8 patients: We found frameshift or nonsense mutations that are predicted to result in a complete loss of c-Mpl function in 5 patients. Heterozygous or homozygous missense mutations predicted to lead to amino acid exchanges in the extracellular domain of the receptor were found in 3 other patients. The type of mutations correlated with the clinical course of the disease. We propose a defective c-Mpl expression due to c-mpl mutations as the cause for thrombocytopenia and progression into pancytopenia seen in patients with CAMT.

Peripheral stem cell transplantation in a child with amegakaryocytic thrombocytopenia

Congenital amegakaryocytic thrombocytopenia (CAMT) is an unusual cause of thrombocytopenia without radial or other congenital anomalies in the newborn. Generalized bone marrow dysfunction developing later in life has been reported. We present a 13-month-old girl who was diagnosed as having congenital amegakaryocytic thrombocytopenia and was successfully treated with allogeneic peripheral stem cell transplantation (PSCT) from her fully matched sibling donor. The neutrophil engraftment was on post transplant day 12 and platelet engraftment was on day 14. Her last hemogram revealed platelets of 168 x 10(9)/l 20 months post transplant.

Evaluation and treatment of thrombocytopenia in the neonatal intensive care unit

Thrombocytopenia is a very frequent problem among sick neonates, affecting up to 35% of all infants admitted to the NICU. Although multiple clinical conditions have been causally associated with neonatal thrombocytopenia, the cause of the thrombocytopenia is unclear in up to 60% of affected neonates. This article provides neonatologists with a practical approach to the thrombocytopenic neonate, with an emphasis on conditions that could be life-threatening or could have significant implications for further pregnancies. An overview of the current therapeutic modalities is also presented, including a discussion of the possible use of recombinant thrombopoietic cytokines to treat certain groups of thrombocytopenic neonates.

Mutations in the thrombopoietin receptor, Mpl, in children with congenital amegakaryocytic thrombocytopenia

Congenital amegakaryocytic thrombocytopenia (CAMT) is a rare disorder of undefined aetiology. The disease presents with severe thrombocytopenia and absence of megakaryocytes in the bone marrow. Furthermore, CAMT patients may develop bone marrow aplasia. To obtain more insight into the mechanism underlying CAMT, five children were analysed. All patients had increased plasma thrombopoietin (Tpo) levels, indicating a platelet production defect. Bone marrow-derived CD34+ stem cells from three patients were cultured in an in vitro liquid culture system to study megakaryocytopoiesis. CD34+ cells from two of the three patients failed to differentiate into megakaryocytes. The lack of megakaryocyte formation could imply that a defect in the c-mpl gene, encoding the Tpo receptor, exists. Sequencing of c-mpl revealed mutations in four of five patients. Three patients had point mutations and/or a deletion in the coding regions of c-mpl. All point mutations led to an amino acid substitution or to a premature stop codon. In one patient, a homozygous mutation in the last base of intron 10 was found that resulted in loss of a splice site. This study showed that mutations in c-mpl could be the cause of thrombocytopenia in CAMT in the majority of patients. Furthermore, Tpo has been shown to have an anti-apoptotic effect on stem cells. Therefore, mutations in c-mpl might not only affect megakaryocyte formation but may also impair stem cell survival, which could explain the occurrence of bone marrow failure as final outcome in patients with CAMT.

Autosomal dominant thrombocytopenia: incomplete megakaryocyte differentiation and linkage to human chromosome 10

We studied a large kindred with nonsyndromic autosomal dominant thrombocytopenia to define the phenotype and used genomic linkage analysis to determine the locus of the abnormal gene. Affected family members are characterized by lifelong moderate thrombocytopenia (mean = 42.7 × 109/L) with moderate propensity toward easy bruising and minor bleeding. Megakaryocytes are present in bone marrow with reduced frequency, and there are no apparent abnormalities of myeloid or erythroid cells. This type of inherited thrombocytopenia has no evident association with hematopoietic malignancy or progression to aplastic anemia. In the past, members of this family have failed therapeutic trials of immunosuppression and splenectomy. In our investigation, we found that affected individuals had normal platelet size compared with unaffected family members and modestly increased thrombopoietin levels. Hematopoietic colony assays from bone marrow and peripheral blood demonstrated that megakaryocyte precursors (CFU-Mk) were dramatically increased in both number and size in affected individuals. Bone marrow cells grown in liquid culture with thrombopoietin failed to develop polyploid cells greater than 8N. Also, electron microscopy demonstrated that megakaryocytes from an affected individual had markedly delayed nuclear and cytoplasmic differentiation. Genome-wide linkage analysis established a single locus for the disease gene on the short arm of chromosome 10 with a maximum 2-point lod score of 5.68 (at θ = 0). By recruiting additional family members, the genomic region was narrowed to 17 centimorgans. We conclude that a gene in this locus plays an important role in megakaryocyte endomitosis and terminal maturation.

Autosomal dominant thrombocytopenia: incomplete megakaryocyte differentiation and linkage to human chromosome 10

We studied a large kindred with nonsyndromic autosomal dominant thrombocytopenia to define the phenotype and used genomic linkage analysis to determine the locus of the abnormal gene. Affected family members are characterized by lifelong moderate thrombocytopenia (mean = 42.7 × 109/L) with moderate propensity toward easy bruising and minor bleeding. Megakaryocytes are present in bone marrow with reduced frequency, and there are no apparent abnormalities of myeloid or erythroid cells. This type of inherited thrombocytopenia has no evident association with hematopoietic malignancy or progression to aplastic anemia. In the past, members of this family have failed therapeutic trials of immunosuppression and splenectomy. In our investigation, we found that affected individuals had normal platelet size compared with unaffected family members and modestly increased thrombopoietin levels. Hematopoietic colony assays from bone marrow and peripheral blood demonstrated that megakaryocyte precursors (CFU-Mk) were dramatically increased in both number and size in affected individuals. Bone marrow cells grown in liquid culture with thrombopoietin failed to develop polyploid cells greater than 8N. Also, electron microscopy demonstrated that megakaryocytes from an affected individual had markedly delayed nuclear and cytoplasmic differentiation. Genome-wide linkage analysis established a single locus for the disease gene on the short arm of chromosome 10 with a maximum 2-point lod score of 5.68 (at θ = 0). By recruiting additional family members, the genomic region was narrowed to 17 centimorgans. We conclude that a gene in this locus plays an important role in megakaryocyte endomitosis and terminal maturation.

Haematopoietic stem cell transplantation for amegakaryocytic thrombocytopenia

Congenital amegakaryocytic thrombocytopenia (CAT), a rare syndrome with failure of megakaryopoiesis, cannot be cured by immunoglobulins, steroids or cyclosporin, but only by allogeneic bone marrow transplantation (BMT). We report on eight patients with CAT, all of whom were dependent at the time of BMT on platelet transfusion. Sources of haematopoietic progenitor cells were bone marrow (n = 5), peripheral stem cells (n = 2) and cord blood (n = 1). Seven patients engrafted. Both patients with matched unrelated donor transplants died, six patients are well with stable platelet counts 3-27 months after transplantation. BMT represents a curative option for CAT. The benefit of using alternative marrow donors should be carefully evaluated.

Role of thrombopoietin in hematopoietic stem cell and progenitor regulation

Thrombopoietin performs an essential role during hematopoiesis by regulating the expansion and maturation of megakaryocytes. In keeping with this function, megakaryocytes, platelets, and their precursors all express the thrombopoietin receptor, Mpl, on their cell surface. However, Mpl is also expressed on primitive, pluripotent hematopoietic progenitors and plays an important role in the regulation of lineages other than megakaryocytes as well as primitive progenitors. Recently, the ability of thrombopoietin to maintain and expand repopulating stem cells has been demonstrated. Thus, thrombopoietin is unique among the hematopoietic cytokines because it is necessary both for terminal maturation and regulation of lineage-specific megakaryocytes and also for maintenance of the most primitive hematopoietic stem cells. Many new strategies are evolving to exploit the activity of thrombopoietin on primitive progenitors. This may lead to faster hematopoietic recovery from marrow-suppressive therapy, effective methods of ex vivo expansion of hematopoietic stem cells, and retroviral transduction of stem cells to facilitate gene therapy.

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

Polycythaemia vera (PV) is a myeloproliferative disorder (MPD) characterized by an increased production of mature blood cells. The underlying pathogenic mechanisms behind PV are largely unknown. Thrombopoietin (TPO) is the most important cytokine for stimulation of megakaryocyte growth and formation of functional platelets. Recently, it has been shown that the receptor for TPO, c-mpl, is expressed on haematopoietic stem cells, and that TPO promotes the growth of these stem cells via binding to c-mpl. Quantitative or qualitative abnormalities of c-mpl function could thus theoretically play a role in the pathogenesis of different MPDs. Previous studies of the integrity of the c-mpl system in PV have produced conflicting results. We therefore studied c-mpl protein expression using immunoblot analysis in 15 PV patients and 10 healthy controls. Seven out of 15 PV patients (47%) exhibited similar c-mpl protein levels to the controls, whereas eight out of 15 patients (53%) showed either markedly reduced or absent levels of c-mpl. Five of the seven c-mpl-positive patients had only been treated by phlebotomy, whereas six out of eight c-mpl-negative patients were receiving treatment with hydroxyurea, anagrelide or alpha-interferon. Disease duration tended to be slightly longer in c-mpl-negative patients compared with c-mpl-positive patients (mean = 55 vs. 43 months). Tyrosine phosphorylation of JAK-2 in immunoprecipitates of platelets obtained after stimulation with TPO (100 and 1000 ng/ml) was normal in c-mpl-positive patients, whereas it could not be detected in c-mpl-negative patients. We therefore conclude that there exists a marked heterogeneity in c-mpl protein levels and functional integrity in PV. However, it seems less likely that c-mpl abnormalities per se are directly involved in the pathogenesis leading to the occurrence of PV, as c-mpl levels were similar to those seen in healthy individuals in about half of the patients under study.

WASP is involved in proliferation and differentiation of human haemopoietic progenitors in vitro

The Wiskott-Aldrich syndrome (WAS) is an X-linked recessive disorder characterized by thrombocytopenia, immunodeficiency and eczema. X-linked thrombocytopenia (XLT) is a mild form of WAS with isolated thrombocytopenia. Both phenotypes are caused by mutation of the Wiskott-Aldrich syndrome protein (WASP) gene. In this study we investigated the role of WASP in the differentiation of CD34-positive (CD34+) cells isolated from the bone marrow of patients with WAS (n = 5) or with XLT (n = 4). Megakaryocyte colony formation was significantly decreased in patients with WAS when compared with normal controls. The formation of granulocyte-macrophage colonies and erythroid bursts were also decreased in WAS patinets. In contrast, in XLT patients, formation of all these colonies was normal. However, in vitro proplatelet formation of megakaryocytes induced by thrombopoietin was markedly decreased in both XLT and WAS. Electron microscopic examination revealed that megakaryocytes obtained from WAS or XLT patients grown in vitro had abnormal morphologic features, which seemed to be caused by defective actin cytoskeletal organization, including labyrinth-like structures of the demarcation membrane system and deviated distribution of the alpha-granules and demarcation membrane system. These observations indicate that WASP is involved in the proliferation and differentiation of CD34+ haemopoietic progenitor cells probably by its participation in signal transduction and in the regulation of the cytoskeleton.

Platelet c-mpl expression is dysregulated in patients with essential thrombocythaemia but this is not of diagnostic value

Essential thrombocythaemia (ET) can be difficult to discriminate from an occult case of reactive thrombocytosis (RT). Since thrombopoietin (TPO) is the primary regulator of thrombopoiesis, we have investigated whether levels of TPO and/or its receptor, c-mpl, are of value in the differential diagnosis of ET. Plasma TPO levels in patients with ET, RT and other myeloproliferative disorders (MPDs) did not differ significantly from normal controls. However, surface c-mpl expression was significantly reduced in platelets from 18 ET patients, 0-65.5% of controls (P < 0.001). Immunoblots on five of these and five additional patients were consistent with absent or reduced c-mpl protein levels. The surface c-mpl expression results were significantly different from those in eight RT patients (21. 3-95.5%, P = 0.0015), but there was considerable overlap between the two groups and a reduced level was not restricted to ET. Furthermore, c-mpl expression in ET patients was not different from eight patients with other MPDs (0-87.6%, P = 0.06), nor could it differentiate between ET patients with monoclonal and polyclonal haemopoiesis. Although a low or absent c-mpl level is suggestive of a primary rather than a secondary thrombocytosis, it is insufficiently discriminatory to be used as a diagnostic marker for ET.

Identification of mutations in the c-mpl gene in congenital amegakaryocytic thrombocytopenia

Congenital amegakaryocytic thrombocytopenia (CAMT) is a rare disorder expressed in infancy and characterized by isolated thrombocytopenia and megakaryocytopenia with no physical anomalies. Our previous hematological analysis indicated similarities between human CAMT and murine c-mpl (thrombopoietin receptor) deficiency. Because the c-mpl gene was considered as one of the candidate genes for this disorder, we analyzed the genomic sequence of the c-mpl gene of a 10-year-old Japanese girl with CAMT. We detected two heterozygous point mutations: a C-to-T transition at the cDNA nucleotide position 556 (Q186X) in exon 4 and a single nucleotide deletion of thymine at position 1,499 (1,499 delT) in exon 10. Both mutations were predicted to result in a prematurely terminated c-Mpl protein, which, if translated, lacks all intracellular domains essential for signal transduction. Each of the mutations was segregated from the patient's parents. Accordingly, the patient was a compound heterozygote for two mutations of the c-mpl gene, each derived from one of the parents. The present study suggests that at least a certain type of CAMT is caused by the c-mpl mutation, which disrupts the function of thrombopoietin receptor.

Effects of thrombopoietin on the proliferation and differentiation of primitive and mature haemopoietic progenitor cells in cord blood

Thrombopoietin (TPO) is considered to be the primary growth factor for regulating megakaryopoiesis and thrombopoiesis. In this study we investigated the in vitro effect of TPO on relatively immature and mature CD34+ progenitor cells in cord blood. Cells were cultured in both liquid and semi-solid cultures containing 50 ng/ml TPO. The CD34+/CD45RA- and CD34+/CD38- subfractions in cord blood were both enriched for megakaryocyte progenitors as determined in a semisolid CFU-meg assay. Progenitor cells derived from the CD34+/CD45RA- and CD34+/CD38- subfractions showed high proliferative capacity in liquid cultures. We observed a mean 19-fold expansion of the total CD34+ cell fraction, whereas in the CD34+/CD45RA- and CD34+/CD38- subfractions the mean expansion was 23- and 50-fold respectively. The expansion of the immature progenitor cell subfractions resulted in a highly purified megakaryocyte suspension containing > 80% megakaryocytes after 14 d in culture. However, these expanded megakaryocytes remained in a diploid (2N) and tetraploid (4N) state. Maturation could not be further induced by low concentration of TPO (0.1 ng/ml). The majority of the cells were 2N (80%) and 4N (15%) and only 5% of the cells had a ploidy of more than 4N. These results indicate that megakaryocyte progenitor cells in cord blood residing in the immature stem cell fraction exhibit a high proliferative capacity when cultured in the presence of TPO as the single growth factor, without maturation to hyperploid megakaryocytes.