The evolution of megakaryocytes to platelets

Inherited or Immunological Thrombocytopenia: The Complex Nature of Platelet Disorders in 22q11.2 Deletion Syndrome

22q11.2 deletion syndrome (22q11.2DS) is one of the most common congenital malformation syndromes resulting from disrupted embryonic development of pharyngeal pouches. The classical triad of symptoms described by Angelo DiGeorge is frequently accompanied by hematological and immune disorders. While it is well-established that patients with 22q11.2DS have an increased risk of recurrent autoimmune cytopenias, including immune thrombocytopenia, the platelet abnormalities in this population are more complex and multifaceted. Given this issue, we conducted a comprehensive literature review on platelet disorders in 22q11.2DS using accessible databases (PubMed and Scopus). We aimed to outline previous studies limitations and most urgent challenges concerning thrombocytopenia in these patients. One characteristic finding frequently observed in 22q11.2DS is mild macrothrombocytopenia caused presumably by the loss of one GP1BB allele, encoding the element of the GPIb-IX-V complex. This structure plays a central role in thrombocyte adhesion, aggregation, and subsequent activation. Recent studies suggest that defective megakaryopoiesis and impaired vasculogenesis may strongly influence platelet and hemostasis disorders in 22q11.2DS. Furthermore, the phenotypic manifestation may be modulated by epigenetic factors and gene expression modifiers located outside the deletion region. Although the final hemorrhagic phenotype is typically mild, these patients may require more frequent transfusions following major surgical procedures. Despite the risk of thrombocytopenia and thrombocytopathy, there is a lack of large-scale research on hematological anomalies in 22q11.2DS, and the available results are often inconclusive. Given the complexity of hemostatic disorders, it is essential to establish specific recommendations for perioperative management and autoimmune cytopenias treatment within this population.

Glanzmann Thrombasthenia: Perspectives from Clinical Practice on Accurate Diagnosis and Optimal Treatment Strategies

Glanzmann thrombasthenia (GT) is a rare autosomal recessive disorder of fibrinogen-mediated platelet aggregation due to a quantitative or qualitative deficit of the α_IIbβ₃ integrin at the platelet surface membrane resulting from mutation(s) in ITGA2B and/or ITGB3. Patients tend to present in early childhood with easy bruising and mucocutaneous bleeding. The diagnostic process requires consideration of more common disorders of haemostasis and coagulation prior to confirming the disorder with platelet light transmission aggregation, flow cytometry of CD41 and CD61 expression, and/or exon sequencing of ITGA2B and ITGB3. Antifibrinolytic therapy, recombinant activated factor VII, and platelet transfusions are the mainstay of therapy, although the latter may trigger formation of anti-platelet antibodies in GT patients and inadvertent platelet-refractory disease. The management of these patients therefore remains complex, particularly in the context of trauma, labour and delivery, and perioperative care. Bone marrow transplantation remains the sole curative option, although the venue of gene therapy is being increasingly explored as a future alternative for definitive treatment of GT.

MPG and NPRL3 Polymorphisms are Associated with Ischemic Stroke Susceptibility and Post-Stroke Mortality

Ischemic stroke is a complicated disease which is affected by environmental factors and genetic factors. In this field, various studies using whole-exome sequencing (WES) have focused on novel and linkage variants in diverse diseases. Thus, we have investigated the various novel variants, which focused on their linkages to each other, in ischemic stroke. Specifically, we analyzed the N-methylpurine DNA glycosylase (MPG) gene, which plays an initiating role in DNA repair, and the nitrogen permease regulator-like 3 (NPRL3) gene, which is involved in regulating the mammalian target of rapamycin pathway. We took blood samples of 519 ischemic stroke patients and 417 controls. Genetic polymorphisms were detected by polymerase chain reaction (PCR), real-time PCR, and restriction fragment length polymorphism (RFLP) analysis. We found that two NPRL3 polymorphisms (rs2541618 C>T and rs75187722 G>A), as well as the MPG rs2562162 C>T polymorphism, were significantly associated with ischemic stroke. In Cox proportional hazard regression models, the MPG rs2562162 was associated with the survival of small-vessel disease patients in ischemic stroke. Our study showed that NPRL3 and MPG polymorphisms are associated with ischemic stroke prevalence and ischemic stroke survival. Taken together, these findings suggest that NPRL3 and MPG genotypes may be useful clinical biomarkers for ischemic stroke development and prognosis.

[Activators, receptors and signal transduction pathways of blood platelets]

Platelet participation in hemostatic plug formation requires transition into an activated state (or, rather, variety of states) upon action of agonists like ADP, thromboxane A , collagen, thrombin, and others. The mechanisms of action for different agonists, their receptors and signaling pathways associated with them, as well as the mechanisms of platelet response inhibition are the subject of the present review. Collagen exposed upon vessel wall damage induced initial platelet attachment and start of thrombus formation, which involves numerous processes such as aggregation, activation of integrins, granule secretion and increase of intracellular Ca2+. Thrombin, ADP, thromboxane A , and ATP activated platelets that were not initially in contact with the wall and induce additional secretion of activating substances. Vascular endothelium and secretory organs also affect platelet activation, producing both positive (adrenaline) an d negative (prostacyclin, nitric oxide) regulators, thereby determining the relation of activation and inhibition signals, which plays a significant role in the formation of platelet aggregate under normal and pathological conditions. The pathways of platelet signaling are still incompletely understood, and their exploration presents an important objective both for basic cell biology and for the development of new drugs, the methods of diagnostics and of treatment of hemostasis disorders.

A clopidogrel-insensitive inducible pool of P2Y12 receptors contributes to thrombus formation: inhibition by elinogrel, a direct-acting, reversible P2Y12 antagonist

It is known that hepatic metabolism limits the antiaggregatory activity of clopidogrel and, as a consequence, its clinical benefits. In this study, we investigated whether other factors exist that could account for clopidogrel's suboptimal antithrombotic activity. Using an in vivo murine FeCl(3) thrombosis model coupled with intravital microscopy, we found that at equivalent, maximal levels of inhibition of ADP-induced platelet aggregation, clopidogrel (50 mg/kg p.o.) failed to reproduce the phenotype associated with P2Y(12) deficiency. However, elinogrel (60 mg/kg p.o.), a direct-acting reversible P2Y(12) antagonist, achieved maximal levels of inhibition in vivo, and its administration (1 mg/kg i.v.) abolished residual thrombosis associated with clopidogrel dosing. Because elinogrel is constantly present in the plasma, whereas the active metabolite of clopidogrel exists for ∼2 h, we evaluated whether an intracellular pool of P2Y(12) exists that would be inaccessible to clopidogrel and contribute to its limited antithrombotic activity. Using saturation [(3)H]2-(methylthio)ADP ([(3)H]2MeSADP) binding studies, we first demonstrated that platelet stimulation with thrombin and convulxin (mouse) and thrombin receptor activating peptide (TRAP) (human) significantly increased surface expression of P2Y(12) relative to that of resting platelets. We next found that clopidogrel dose-dependently inhibited ADP-induced aggregation, signaling (cAMP), and surface P2Y(12) on resting mouse platelets, achieving complete inhibition at the highest dose (50 mg/kg), but failed to block this inducible pool. Thus, an inducible pool of P2Y(12) exists on platelets that can be exposed upon platelet activation by strong agonists. This inducible pool is not blocked completely by clopidogrel, contributes to thrombosis in vivo, and can be blocked by elinogrel.

The mitogen-activated protein kinase signaling pathways: role in megakaryocyte differentiation

Megakaryopoiesis is a process by which bone marrow progenitor cells develop into mature megakaryocytes (MKs), which in turn produce platelets required for normal hemostasis. The mitogen-activated protein kinases (MAPKs) family comprises four main groups of proteins: extracellular signal-related kinases (ERKs) (ERK1/2 or p44/p42), ERK5, p38MAPKs (alpha, beta, gamma, delta) and c-Jun amino-terminal kinases (JNKs) (JNK 1, 2, 3). These intracellular signaling pathways play a pivotal role in many essential cellular processes including proliferation and differentiation. The purpose of this review is to summarize our current knowledge on the role of MAPKs in MKs, specifically regarding differentiation in immortalized cell lines and primary MKs. A critical role of the MEK (MAPK kinase)-ERK1/2 pathway in MK development has been demonstrated although the details remain controversial. There is at present no functional evidence for a role of p38MAPKs whereas the role of JNKs and ERK5 in MK development is not known. Characterization of these molecular event cascades remains crucial for the understanding of the megakaryopoiesis process.

Differentiation of murine committed megakaryocytic progenitors isolated by a novel strategy reveals the complexity of GATA and Ets factor involvement in megakaryocytopoiesis and an unexpected potential role for GATA-6

OBJECTIVE

The differentiation of megakaryocytes is characterized by polyploidization and cytoplasmic maturation leading to platelet production. Studying these processes is hindered by the paucity of bone marrow megakaryocytes and their precursors. We describe a method for the expansion and purification of committed megakaryocyte progenitors and demonstrate their usefulness by studying changes in the expression of Ets and GATA family transcription factors throughout megakaryocytopoiesis.

METHODS

A two-step serum-free method was developed. Cells isolated using this method were analyzed for surface marker expression by flow cytometry, and for their ability to differentiate using single-cell culture. Purified progenitors were induced to differentiate and analyzed with respect to their ploidy by flow cytometry and expression of specific genes by RT-PCR.

RESULTS

A population of Lin- c-kit+ CD45+ CD41+ CD31+ CD34low CD9low FcgammaRII/IIIlow Sca-1med/low committed megakaryocyte progenitors was purified. These cells could be differentiated efficiently, achieving ploidy of up to 128N. Analysis of RNA demonstrated the expected increases in expression of key megakaryocyte-associated genes. RT-PCR analysis also revealed that a range of Ets and GATA factors are expressed, their individual levels and patterns of expression varying widely. Surprisingly, we find that GATA-6 is specifically expressed in late differentiated megakaryocytes and has the potential to regulate megakaryocyte-expressed genes in cooperation with Ets factors.

CONCLUSION

Purified primary megakaryocytic progenitors are able to differentiate as a cohort into fully mature megakaryocytes. The number of cells obtainable, and the synchrony of the differentiation process, facilitates analysis of the dynamics of molecular processes involved in megakaryocytopoiesis. The expression pattern of Ets and GATA family transcription factors reveals the complexity of the involvement of these key megakaryocytic regulators. The finding of GATA-6 expression and demonstration of its functional activity suggests a novel mechanism for the regulation of certain genes late in megakaryocytopoiesis.

A Phase I Clinical, Pharmacologic, and Biologic Study of Thrombopoietin and Granulocyte Colony-Stimulating Factor in Children Receiving Ifosfamide, Carboplatin, and Etoposide Chemotherapy for Recurrent or Refractory Solid Tumors: A Children's Oncology Group Experience

PURPOSE

Ifosfamide, carboplatin, and etoposide (ICE) are associated with grade III/IV dose-limiting thrombocytopenia. The Children's Oncology Group conducted a phase I dose escalation, pharmacokinetic, and biological study of recombinant human thrombopoietin (rhTPO) after ICE in children with recurrent/refractory solid tumors (CCG-09717) to assess the toxicity and maximum tolerated dose of rhTPO administered at 1.2, 2.4, or 3.6 microg/kg per dose.

EXPERIMENTAL DESIGN

Children received ifosfamide 1,800 mg/m2 on days 0 to 4, carboplatin 400 mg/m2 on days 0 to 1, and etoposide 100 mg/m2 on days 0 to 4. rhTPO was administered i.v. on days +4, +6, +8, +10, and +12 at 1.2, 2.4, or 3.6 microg/kg per dose.

RESULTS

rhTPO was well tolerated and maximum tolerated dose was not reached. Median time to platelet recovery > or =100,000/microL of rhTPO at 1.2, 2.4, and 3.6 microg/kg/d was 24 days (22-24 d), 25 days (23-29 d), and 22 days (16-37 d), respectively. Patients required a median of 2 days of platelet transfusions (0-7 days). Mean (+/- SD) rhTPO maximum serum concentrations were 63.3 +/- 9.7 and 89.3 +/- 15.7 ng/mL and terminal half-lives were 47 +/- 13 and 64 +/- 42 hours after 2.4 and 3.6 microg/kg/d, respectively. There was a significant increase in colony-forming unit megakaryocyte upon WBC count recovery.

CONCLUSIONS

rhTPO was well tolerated. Time to hematologic recovery and median number of platelet transfusions seem to be improved compared with historical controls receiving ICE + granulocyte colony-stimulating factor (CCG-0894).

Immunophenotyping of acute leukaemias

Progress in the management and understanding of acute leukaemia can only be obtained if these diseases are thoroughly investigated, both clinically and with a series of biological tools. This alone has made and still will make possible the identification of prognostic factors and of useful markers for the follow-up of patients in remission. Among the variety of approaches of acute leukaemia definition, immunophenotyping has taken over the past 25 years a predominant and now well-defined place, although room is left for further improvement. In this review, the current state-of-the-art of immunophenotyping of acute leukaemias will be replaced in the context of physiological leukocyte maturation. The recognized classifications and recommended immunophenotyping panels will then be discussed, and the clinical relevance of several key features will be presented. Finally, more recent openings for the use of immunophenotyping will be evoked.

NMDA receptor-mediated regulation of human megakaryocytopoiesis

Identification of the regulatory inputs that direct megakaryocytopoiesis and platelet production is essential for the development of novel therapeutic strategies for the treatment of thrombosis and related hematologic disorders. We have previously shown that primary human megakaryocytes express the N-methyl-d-aspartate acid (NMDA) receptor 1 (NR1) subunit of NMDA-type glutamate receptors, which appear to be pharmacologically similar to those identified at neuronal synapses, responsible for mediating excitatory neurotransmission in the central nervous system. However, the functional role of NMDA receptor signaling in megakaryocytopoiesis remains unclear. Here we provide evidence that demonstrates the fundamental importance of this signaling pathway during human megakaryocyte maturation in vitro. Reverse transcriptase-polymerase chain reaction (RT-PCR) analysis of RNA extracted from CD34+-derived megakaryocytes identified expression of NR2A and NR2D receptor subunits in these cells, as well as the NMDA receptor accessory proteins, Yotiao and postsynaptic density protein 95 (PSD-95). In functional studies, addition of a selective NMDA receptor antagonist, MK-801 inhibited proplatelet formation, without affecting proliferation or apoptosis. Exposure of CD34+ cells to MK-801 cultured for 14 days in the presence of thrombopoietin induced a decrease in expression of the megakaryocyte cell surface markers CD61, CD41a, and CD42a compared with controls. At an ultrastructural level, MK-801-treated cells lacked alpha-granules, demarcated membranes, and multilobed nuclei, which were prominent in untreated mature megakaryocyte controls. Using immunohistochemistry on sections of whole tibiae from c-Mpl knockout mice we demonstrated that megakaryocytic NMDA receptor expression was maintained following c-Mpl ablation. These data support a fundamental role for glutamate signaling in megakaryocytopoiesis and platelet production, which is likely to be independent of thrombopoietin-mediated effects.

BclxL overexpression in megakaryocytes leads to impaired platelet fragmentation

Fragmentation of polyploid megakaryocytes into platelets has great relevance for blood homeostasis. Apoptotic cell death is a highly regulated genetic program, which has been observed in mature megakaryocytes fragmenting into platelets. The antiapoptotic protein BclxL has been reported as up-regulated during megakaryocytic differentiation in vitro, but absent during late megakaryopoiesis. Our study focused on examining BclxL levels in megakaryocytes in vivo and in assessing the effect of its overexpression in transgenic mice (via the platelet factor 4 [PF4] promoter) on megakaryocyte development and platelet fragmentation. Interestingly, in the wild-type and less in PF4-driven transgenic mice, BclxL was not detected in a fraction of the large mature megakaryocytes, suggesting a regulation on the protein level. BclxL overexpression was associated with a moderate increase in megakaryocyte number, with no significant change in ploidy level or platelet counts. When the mice were challenged by induction of immune thrombocytopenia, the rate of platelet recovery was significantly slower in the transgenic mice as compared with controls. Moreover, proplatelet formation in vitro by transgenic megakaryocytes was limited. Transgenic megakaryocytes displayed poorly developed platelet demarcation membranes and cell margin extensions. Our study indicates that regulated expression of BclxL in megakaryocytes is important for the development of cells with a high potential to fragment into platelets.

Molecular basis of Glanzmann's Thrombasthenia and current strategies in treatment

Glanzmann Thrombasthenia, an exceptional inherited platelet disorder is characterized by a complete lack of platelet aggregation due to a defect in the alpha(IIb)beta(3) complex or to a qualitative abnormality of this complex. Advances in molecular biology have permitted to precise the molecular abnormality on alpha(IIb) or beta(3) genes responsible for the disease and have also contributed to a better knowledge of normal platelet physiology. Hemorrhages are the main clinical problem. Current principles of therapeutic management are proposed, with special reference to the risk of platelet alloimmunisation.

Molecular control of megakaryopoiesis and thrombopoiesis

Megakaryopoiesis and subsequent thrombopoiesis occur through complex biologic steps: megakaryocyte precursors that developed from hematopoietic stem cells initially proliferate, then differentiate into mature polyploid megakaryocytes, and finally release platelets. Although a number of growth factors can augment megakaryopoiesis in vitro, thrombopoietin is a physiologic and the most potent regulator of megakaryopoiesis in vitro and in vivo. Thrombopoietin induces the growth of megakaryocyte precursors through activation of multiple signaling cascades, including Ras/mitogen-activated protein kinase (MAPK), signal transducers and activators of transcription 5 (STAT5), phosphatidylinositol 3-kinase (PI3-K)/Akt, and protein kinase C, whereas it induces megakaryocytic maturation primarily through the Ras/MAPK pathway. During the maturation step, megakaryocytes undergo polyploidization characterized by repeated rounds of DNA replication without concomitant cell division. During these rounds of replication, cytokinesis is neglected because of the down-regulated expression of AIM-1, and DNA replication occurs through the increased expression of D-type cyclins. As for transcriptional regulation during megakaryopoiesis, GATA-1 plays a central role in the lineage commitment of hematopoietic stem cells toward erythroid/megakaryocytic lineage and subsequent maturation. p45 NF-E2 is essential for platelet release from terminally differentiated megakaryocytes. At present, mutations of GATA-1, AML1, and HOXA11 genes have been found in hereditary diseases accompanying thrombocytopenia among humans.

Expression of scinderin in megakaryoblastic leukemia cells induces differentiation, maturation, and apoptosis with release of plateletlike particles and inhibits proliferation and tumorigenesis

Rapid proliferation of atypical megakaryoblasts is a characteristic of megakaryoblastic leukemia. Cells from patients with this disorder and cell lines established from this type of leukemia showed the presence of gelsolin but the absence of scinderin expression, 2 filamentous actin-severing proteins present in normal megakaryocytes and platelets. Vector-mediated expression of scinderin in the megakaryoblastic cell line MEG-01 induced a decrease in both F-actin and gelsolin. This was accompanied by increased Rac2 expression and by activation of the PAK/MEKK.SEK/JNK/c-jun, c-fos transduction pathway. The Raf/MEK/ERK pathway was also activated in these cells. Transduction pathway activation was followed by cell differentiation, polyploidization, maturation, and apoptosis with release of platelet-like particles. Particles expressed surface CD41a antigen (glycoprotein IIb/IIIa or fibrinogen receptor), had dense bodies, high-affinity serotonin transport, and circular array of microtubules. Treatment of particles with thrombin induced serotonin release and aggregation that was blocked by CD41a antibodies. PAC-1 antibodies also blocked aggregation. Exposure of cells to PD98059, a blocker of MEK, inhibited antigen CD41a expression, increases in cell volume, and number of protoplasmic extensions. Cell proliferation and cell ability to form tumors in nude mice were also inhibited by the expression of scinderin. MEG-01 cells expressing scinderin had the same fate in vivo as in culture. Thus, when injected into nude mice, they entered apoptosis and released platelet-like particles. The lack of scinderin expression in megakaryoblastic leukemia cells seems to be responsible for their inability to enter into differentiation and maturation pathways characteristic of their normal counterparts.

Megakaryocytic differentiation is accompanied by a reduction in cell migratory potential

Megakaryocytes (MKs) have been found in the peripheral circulation, suggesting that they can migrate out of the bone marrow. In order to evaluate if megakaryocytic differentiation confers a migratory phenotype, we investigated this property in the haematopoietic cell lines MO7e and UT-7/mpl and in CD34+ progenitor cells before and after induction of differentiation by thrombopoietin (TPO). Migration was studied using a bicompartmental culture system in the presence or absence of a bone marrow endothelial cell monolayer. Preincubation with TPO led to a significant reduction in stromal cell-derived factor-1 (SDF-1)-induced migration of MO7e cells (0.7% +/- 0.08% for TPO-treated vs. 2.6% +/- 0.3% for controls P < 0.05). A similar decreased migratory response was seen with UT-7/mpl cells (7.4% +/- 0.4% for TPO-treated vs. 11.1% +/- 0.01% for controls, P<0.05), although these cells did not migrate in response to SDF-1. CD34+ cells partially differentiated with TPO showed decreased migration following further TPO-induced maturation (13.9% +/- 1.8% for TPO-treated vs. 24.1% +/- 1.8% for untreated, P < 0.05). This reduction was more pronounced in the large MK (> or = 4n) fraction. These results demonstrate that megakaryocytic differentiation is accompanied by a partial suppression of the haematopoietic cell migratory phenotype.

Inherited giant platelet disorders. Classification and literature review

Inherited giant platelet disorders are extremely rare. The aim of this article is to review the clinical and laboratory features of this heterogeneous group and to arrive at a working classification. We conducted our literature search using the National Library of Medicine database. A total of 12 clinical entities were described. We classified them into 4 groups depending on the clinical and structural abnormalities. The pathophysiology of these disorders is largely unknown, and more research is needed, particularly in the light of recent advances in laboratory medicine. This review may provide a valuable reference for clinicians and may form a basis for future classification and research.

Expression of a Functional N-Methyl-d-Aspartate–Type Glutamate Receptor by Bone Marrow Megakaryocytes

Better understanding of hemostasis will be possible by the identification of new lineage-specific stimuli that regulate platelet formation. We describe a novel functional megakaryocyte receptor that belongs to a family of ionotropic glutamate receptors of theN-methyl-d-aspartate (NMDA) subtype responsible for synaptic neurotransmission in the central nervous system (CNS). Northern blotting and reverse-transcriptase polymerase chain reaction (RT-PCR) studies identified expression of NMDAR1 and NMDAR2D type subunit mRNA in rat marrow, human megakaryocytes, and MEG-01 clonal megakaryoblastic cells. Immunohistochemistry and in vivo autoradiographic binding of the NMDA receptor-specific antagonist MK-801 confirmed that megakaryocytes expressed open channel-forming NMDA receptors in vivo. Western blots indicated that megakaryocyte NMDAR1 was either unglycosylated or only glycosylated to low levels, and of identical size to CNS-type NMDAR1 after deglycosylation with endoglycosidase F/peptide-N-glycosidase F. In functional studies, we demonstrated that NMDA receptor activity was necessary for phorbol myristate acetate (PMA)-induced differentiation of megakaryoblastic cells; NMDA receptor blockade by specific antagonists significantly inhibited PMA-mediated increases in cell size, CD41 expression, and adhesion of MEG-01 cells. These results provide evidence for a novel pathway by which megakaryocytopoiesis and platelet production may be regulated.

Expression of a Functional N-Methyl-d-Aspartate–Type Glutamate Receptor by Bone Marrow Megakaryocytes

Better understanding of hemostasis will be possible by the identification of new lineage-specific stimuli that regulate platelet formation. We describe a novel functional megakaryocyte receptor that belongs to a family of ionotropic glutamate receptors of theN-methyl-d-aspartate (NMDA) subtype responsible for synaptic neurotransmission in the central nervous system (CNS). Northern blotting and reverse-transcriptase polymerase chain reaction (RT-PCR) studies identified expression of NMDAR1 and NMDAR2D type subunit mRNA in rat marrow, human megakaryocytes, and MEG-01 clonal megakaryoblastic cells. Immunohistochemistry and in vivo autoradiographic binding of the NMDA receptor-specific antagonist MK-801 confirmed that megakaryocytes expressed open channel-forming NMDA receptors in vivo. Western blots indicated that megakaryocyte NMDAR1 was either unglycosylated or only glycosylated to low levels, and of identical size to CNS-type NMDAR1 after deglycosylation with endoglycosidase F/peptide-N-glycosidase F. In functional studies, we demonstrated that NMDA receptor activity was necessary for phorbol myristate acetate (PMA)-induced differentiation of megakaryoblastic cells; NMDA receptor blockade by specific antagonists significantly inhibited PMA-mediated increases in cell size, CD41 expression, and adhesion of MEG-01 cells. These results provide evidence for a novel pathway by which megakaryocytopoiesis and platelet production may be regulated.

Velocardiofacial syndrome patients with a heterozygous chromosome 22q11 deletion have giant platelets

Patients with a microdeletion on chromosome 22q11 demonstrate the clinical picture of the velocardiofacial syndrome. We report on three members of the same family with this microdeletion and velocardiofacial syndrome, all having an increase in platelet size and a mild decrease in platelet number. Their platelet function, however, tested by aggregation and by adherence to collagen in a whole blood perfusion system, was normal. We retrospectively studied the files of 35 other patients with 22q11 deletion and also found that their platelets had an increased size compared with cardiac controls. Moreover, their platelet size correlated negatively with platelet number. Knowing that patients with 22q11 deletion are obligate carriers for a heterozygous glycoprotein Ib beta deletion, these patients can be considered to be heterozygous Bernard-Soulier patients. In addition, a significant increase in platelet size may be a positive predictor for the clinical diagnosis of the velocardiofacial syndrome.

Autoimmune thrombocytopenic purpura (AITP) and acquired thrombasthenia due to autoantibodies to GP IIb-IIIa in a patient with an unusual platelet membrane glycoprotein composition

The subject (E.B.) is a 63-year-old woman with autoimmune thrombocytopenic purpura (AITP) who was first examined some 6 years ago with symptoms of epistaxis and gum bleeding, severe thrombocytopenia, and large platelets. Her serum tested positively with control platelets in the MAIPA assay performed using monoclonal antibodies (MoAb) to glycoprotein (GP) IIIa (XIIF9, Y2/51), yet was negative in the presence of MoAbs to GP IIb (SZ 22) or to the GP IIb-IIIa complex (AP2, P2). The patient's platelets failed to aggregate with all agonists tested except for ristocetin. IgG isolated from the patient's serum inhibited ADP-induced aggregation of control platelets. Unexpectedly, flow cytometry showed an altered expression of membrane glycoproteins on the patient's platelets. Levels of GP Ib-IX were much higher than previously located by us in platelets. In contrast, the expression of GP IIb-IIIa was about half that seen with control subjects. When Western blotting was performed, a striking finding was a strong band of 250 kDa recognized by a series of MoAbs to GP Ib alpha in addition to the band in the normal position of GP Ib alpha. Finally, ADP-stimulated (E.B.) platelets failed to express activation-dependent epitopes on GP IIb-IIIa as recognized by PAC-1, AP6, or F26 and additionally gave a reduced P-selectin expression after thrombin addition. In conclusion, we present a novel patient with a severely perturbed platelet function where an altered membrane GP profile is associated with the presence of an autoantibody recognizing a complex-dependent determinant on GP IIb-IIIa and inhibitory of platelet aggregation.