A new genetic isolate of gray platelet syndrome (GPS): clinical, cellular, and hematologic characteristics

Gray Platelet Syndrome Presenting With Pancytopenia, Splenomegaly, and Bone Marrow Fibrosis

OBJECTIVES

Gray platelet syndrome (GPS) is a rare platelet storage pool disorder associated with a marked decrease or absence of platelet α-granules and their contents. It is characterized clinically by mild to moderate bleeding; moderate macrothrombocytopenia with large, agranular platelets; splenomegaly; and bone marrow fibrosis. Electron microscopy confirms markedly reduced or absent α-granules in platelets and megakaryocytes. The classic description of GPS is caused by homozygous mutations in NBEAL2 (neurobeachinlike 2).

METHODS

A 28-year-old Hispanic man with a history of easy bruising and occasional episodes of epistaxis sought treatment for pancytopenia and splenomegaly. Peripheral blood smear and bone marrow analysis, electron microscopy, and next-generation sequencing were performed.

RESULTS

Large and agranular platelets were present in the peripheral blood. There was bone marrow fibrosis. Electron microscopy of the platelets showed absence of α-granules. Next-generation sequencing revealed a germline apparently homozygous nonsense variant in the NBEAL2 gene: c.5674C>T, p.Gln1892X (p.Q1829X).

CONCLUSIONS

The differential diagnosis of GPS includes a myeloid neoplasm such as myelodysplastic syndrome with bone marrow fibrosis. The availability of diagnostic genetic panels for hereditable platelet disorders can assist in the recognition of GPS and other platelet disorders. We also describe a previously unreported pathogenic germline homozygous nonsense variant in the NBEAL2 gene: c.5674C>T, p.Gln1892X (p.Q1829X) in a patient with GPS.

Abnormal proplatelet formation and emperipolesis in cultured human megakaryocytes from gray platelet syndrome patients

The Gray Platelet Syndrome (GPS) is a rare inherited bleeding disorder characterized by deficiency of platelet α-granules, macrothrombocytopenia and marrow fibrosis. The autosomal recessive form of GPS is linked to loss of function mutations in NBEAL2, which is predicted to regulate granule trafficking in megakaryocytes, the platelet progenitors. We report the first analysis of cultured megakaryocytes from GPS patients with NBEAL2 mutations. Megakaryocytes cultured from peripheral blood or bone marrow hematopoietic progenitor cells from four patients were used to investigate megakaryopoiesis, megakaryocyte morphology and platelet formation. In vitro differentiation of megakaryocytes was normal, whereas we observed deficiency of megakaryocyte α-granule proteins and emperipolesis. Importantly, we first demonstrated that platelet formation by GPS megakaryocytes was severely affected, a defect which might be the major cause of thrombocytopenia in patients. These results demonstrate that cultured megakaryocytes from GPS patients provide a valuable model to understand the pathogenesis of GPS in humans.

Spontaneous 8bp Deletion in Nbeal2 Recapitulates the Gray Platelet Syndrome in Mice

During the analysis of a whole genome ENU mutagenesis screen for thrombosis modifiers, a spontaneous 8 base pair (bp) deletion causing a frameshift in exon 27 of the Nbeal2 gene was identified. Though initially considered as a plausible thrombosis modifier, this Nbeal2 mutation failed to suppress the synthetic lethal thrombosis on which the original ENU screen was based. Mutations in NBEAL2 cause Gray Platelet Syndrome (GPS), an autosomal recessive bleeding disorder characterized by macrothrombocytopenia and gray-appearing platelets due to lack of platelet alpha granules. Mice homozygous for the Nbeal2 8 bp deletion (Nbeal2^gps/gps) exhibit a phenotype similar to human GPS, with significantly reduced platelet counts compared to littermate controls (p = 1.63 x 10⁻⁷). Nbeal2^gps/gps mice also have markedly reduced numbers of platelet alpha granules and an increased level of emperipolesis, consistent with previously characterized mice carrying targeted Nbeal2 null alleles. These findings confirm previous reports, provide an additional mouse model for GPS, and highlight the potentially confounding effect of background spontaneous mutation events in well-characterized mouse strains.

Megakaryocytic emperipolesis and platelet function abnormalities in five patients with gray platelet syndrome

The gray platelet syndrome (GPS) is a rare congenital platelet disorder characterized by mild to moderate bleeding diathesis, macrothrombocytopenia and lack of azurophilic α-granules in platelets. Some platelet and megakaryocyte (MK) abnormalities have been described, but confirmative studies of the defects in larger patient cohorts have not been undertaken. We studied platelet function and bone marrow (BM) features in five GPS patients with NBEAL2 autosomal recessive mutations from four unrelated families. In 3/3 patients, we observed a defect in platelet responses to protease-activated receptor (PAR)1-activating peptide as the most consistent finding, either isolated or combined to defective responses to other agonists. A reduction of PAR1 receptors with normal expression of major glycoproteins on the platelet surface was also found. Thrombin-induced fibrinogen binding to platelets was severely impaired in 2/2 patients. In 4/4 patients, the BM biopsy showed fibrosis (grade 2-3) and extensive emperipolesis, with many (36-65%) MKs containing 2-4 leukocytes engulfed within the cytoplasm. Reduced immunolabeling for platelet factor 4 together with normal immunolabeling for CD63 in MKs of two patients demonstrated that GPS MKs display an alpha granule-specific defect. Increased immunolabeling for P-selectin and decreased immunolabeling for PAR1, PAR4 and c-MPL were also observed in MKs of two patients. Marked emperipolesis, specific defect of MK alpha-granule content and defect of PAR1-mediated platelet responses are present in all GPS patients that we could study in detail. These results help to further characterize the disease.

Gray platelet syndrome: proinflammatory megakaryocytes and α-granule loss cause myelofibrosis and confer metastasis resistance in mice

NBEAL2 encodes a multidomain scaffolding protein with a putative role in granule ontogeny in human platelets. Mutations in NBEAL2 underlie gray platelet syndrome (GPS), a rare inherited bleeding disorder characterized by a lack of α-granules within blood platelets and progressive bone marrow fibrosis. We present here a novel Nbeal2(-/-) murine model of GPS and demonstrate that the lack of α-granules is due to their loss from platelets/mature megakaryocytes (MKs), and not by initial impaired formation. We show that the lack of Nbeal2 confers a proinflammatory phenotype to the bone marrow MKs, which in combination with the loss of proteins from α-granules drives the development of bone marrow fibrosis. In addition, we demonstrate that α-granule deficiency impairs platelet function beyond their purely hemostatic role and that Nbeal2 deficiency has a protective effect against cancer metastasis.

Abnormal megakaryocyte development and platelet function in Nbeal2(-/-) mice

Gray platelet syndrome (GPS) is an inherited bleeding disorder associated with macrothrombocytopenia and α-granule-deficient platelets. GPS has been linked to loss of function mutations in NEABL2 (neurobeachin-like 2), and we describe here a murine GPS model, the Nbeal2(-/-) mouse. As in GPS, Nbeal2(-/-) mice exhibit splenomegaly, macrothrombocytopenia, and a deficiency of platelet α-granules and their cargo, including von Willebrand factor (VWF), thrombospondin-1, and platelet factor 4. The platelet α-granule membrane protein P-selectin is expressed at 48% of wild-type levels and externalized upon platelet activation. The presence of P-selectin and normal levels of VPS33B and VPS16B in Nbeal2(-/-) platelets suggests that NBEAL2 acts independently of VPS33B/VPS16B at a later stage of α-granule biogenesis. Impaired Nbeal2(-/-) platelet function was shown by flow cytometry, platelet aggregometry, bleeding assays, and intravital imaging of laser-induced arterial thrombus formation. Microscopic analysis detected marked abnormalities in Nbeal2(-/-) bone marrow megakaryocytes, which when cultured showed delayed maturation, decreased survival, decreased ploidy, and developmental abnormalities, including abnormal extracellular distribution of VWF. Our results confirm that α-granule secretion plays a significant role in platelet function, and they also indicate that abnormal α-granule formation in Nbeal2(-/-) mice has deleterious effects on megakaryocyte survival, development, and platelet production.

Gray platelet syndrome and defective thrombo-inflammation in Nbeal2-deficient mice

Platelets are anuclear organelle-rich cell fragments derived from bone marrow megakaryocytes (MKs) that safeguard vascular integrity. The major platelet organelles, α-granules, release proteins that participate in thrombus formation and hemostasis. Proteins stored in α-granules are also thought to play a role in inflammation and wound healing, but their functional significance in vivo is unknown. Mutations in NBEAL2 have been linked to gray platelet syndrome (GPS), a rare bleeding disorder characterized by macrothrombocytopenia, with platelets lacking α-granules. Here we show that Nbeal2-knockout mice display the characteristics of human GPS, with defective α-granule biogenesis in MKs and their absence from platelets. Nbeal2 deficiency did not affect MK differentiation and proplatelet formation in vitro or platelet life span in vivo. Nbeal2-deficient platelets displayed impaired adhesion, aggregation, and coagulant activity ex vivo that translated into defective arterial thrombus formation and protection from thrombo-inflammatory brain infarction following focal cerebral ischemia. In a model of excisional skin wound repair, Nbeal2-deficient mice exhibited impaired development of functional granulation tissue due to severely reduced differentiation of myofibroblasts in the absence of α-granule secretion. This study demonstrates that platelet α-granule constituents are critically required not only for hemostasis but also thrombosis, acute thrombo-inflammatory disease states, and tissue reconstitution after injury.

Multicolor flow cytometry for evaluation of platelet surface antigens and activation markers

INTRODUCTION

Flow cytometry allows the analysis of multiple antigens in a single tube at a single cell level. We present a rapid and sensitive two tube flow cytometric protocol for the detection of multiple platelet antigens and activation markers gated on a pure platelet population.

MATERIALS AND METHODS

The presence of platelet specific antigens was analyzed in citrated whole blood of normal platelets and from patients diagnosed with platelet abnormalities. Quiescent platelets as well as stimulated platelets were analyzed using a gating strategy based on ubiquitously expressed platelet membrane markers. A ubiquitously expressed platelet marker was combined with antibodies against the activated alpha2b-beta3 (PAC-1), Lysosomal Activated Membrane Protein (CD63) and P-selectin (CD62P).

RESULTS

We were able to detect the platelet antigens CD36, CD41, CD42a, CD42b and CD61 in one single tube. Our approach allowed the single tube determination of PAC-1, CD63 and CD62P after activation of platelets by thrombin, collagen, ADP and PAR-1, and determination of platelet abnormalities.

CONCLUSIONS

Our two tube multi-parameter screening protocol is suited for the analysis of platelet antigens expressed on quiescent and activated platelets and allows the detection of aberrancies as found in blood of patients with thrombocytopathy such as Glanzmann Thrombasthenia, storage pool disease with diminished granule content and patients treated with clopidogrel and acetylsalicylic acid.

Gray platelet syndrome: natural history of a large patient cohort and locus assignment to chromosome 3p

Gray platelet syndrome (GPS) is an inherited bleeding disorder characterized by macrothrombocytopenia and absence of platelet α-granules resulting in typical gray platelets on peripheral smears. GPS is associated with a bleeding tendency, myelofibrosis, and splenomegaly. Reports on GPS are limited to case presentations. The causative gene and underlying pathophysiology are largely unknown. We present the results of molecular genetic analysis of 116 individuals including 25 GPS patients from 14 independent families as well as novel clinical data on the natural history of the disease. The mode of inheritance was autosomal recessive (AR) in 11 and indeterminate in 3 families. Using genome-wide linkage analysis, we mapped the AR-GPS gene to a 9.4-Mb interval on 3p21.1-3p22.1, containing 197 protein-coding genes. Sequencing of 1423 (69%) of the 2075 exons in the interval did not identify the GPS gene. Long-term follow-up data demonstrated the progressive nature of the thrombocytopenia and myelofibrosis of GPS resulting in fatal hemorrhages in some patients. We identified high serum vitamin B(12) as a consistent, novel finding in GPS. Chromosome 3p21.1-3p22.1 has not been previously linked to a platelet disorder; identification of the GPS gene will likely lead to the discovery of novel components of platelet organelle biogenesis. This study is registered at www.clinicaltrials.gov as NCT00069680 and NCT00369421.

The α-granule proteome: novel proteins in normal and ghost granules in gray platelet syndrome

BACKGROUND

Deficiencies in granule-bound substances in platelets cause congenital bleeding disorders known as storage pool deficiencies. For disorders such as gray platelet syndrome (GPS), in which thrombocytopenia, enlarged platelets and a paucity of α-granules are observed, only the clinical and histologic states have been defined.

OBJECTIVES

In order to understand the molecular defect in GPS, the α-granule fraction protein composition from a normal individual was compared with that of a GPS patient by mass spectrometry (MS).

METHODS

Platelet organelles were separated by sucrose gradient ultracentrifugation. Proteins from sedimented fractions were separated by sodium dodecylsulfate polyacrylamide gel electrophoresis, reduced, alkylated, and digested with trypsin. Peptides were analyzed by liquid chromatography-tandem MS. Mascot was used for peptide/protein identification and to determine peptide false-positive rates. MassSieve was used to generate and compare parsimonious lists of proteins.

RESULTS

As compared with control, the normalized peptide hits (NPHs) from soluble, biosynthetic α-granule proteins were markedly decreased or undetected in GPS platelets, whereas the NPHs from soluble, endocytosed α-granule proteins were only moderately affected. The NPHs from membrane-bound α-granule proteins were similar in normal platelets and GPS platelets, although P-selectin and Glut3 were slightly decreased, consistent with immunoelectron microscopy findings in resting platelets. We also identified proteins not previously known to be decreased in GPS, including latent transforming growth factor-β-binding protein 1(LTBP1), a component of the transforming growth factor-β (TGF-β) complex.

CONCLUSIONS

Our results support the existence of 'ghost granules' in GPS, point to the basic defect in GPS as failure to incorporate endogenously synthesized megakaryocytic proteins into α-granules, and identify specific new proteins as α-granule inhabitants.

Serglycin proteoglycan deletion in mouse platelets: physiological effects and their implications for platelet contributions to thrombosis, inflammation, atherosclerosis, and metastasis

Serglycin is found in all nucleated hematopoietic cells and platelets, blood vessels, various reproductive and developmental tissues, and in chondrocytes. The serglycin knockout mouse has demonstrated that this proteoglycan is required for proper generation and function of secretory granules in several hematopoietic cells. The effects on platelets are profound, and include diminishing platelet aggregation responses and formation of platelet thrombi. This chapter will review cell-specific aspects of serglycin structure, its gene regulation, cell and tissue localization, and the effects of serglycin deletion on hematopoietic cell granule structure and function. The effects of serglycin knockout on platelets are described and discussed in detail. Rationales for further investigations into the contribution of serglycin to the known roles of platelets in thrombosis, inflammation, atherosclerosis, and tumor metastasis are presented.

Neutrophil secretory defect in the gray platelet syndrome: A new case

We report the case of a 60-year-old woman who was newly diagnosed for the gray platelet syndrome (GPS). This patient had long-term thrombocytopenia which had been initially misdiagnosed as idiopathic thrombocytopenic purpura (ITP). Blood smear displayed characteristic gray platelets, allowing the diagnosis to be made, which was confirmed by electron microscopy (EM). Polymorphonuclear neutrophils (PMN) appeared poorly granulated on the May-Grunwald-Giemsa-stained blood smear. Flow cytometry analysis of PMN demonstrated increased expression of CD35, CD11b and CD18 at resting PMN surface, without any changes after fMLP stimulation. Ultrastructural study retrieved a decreased number of myeloperoxidase (MPO)-negative secondary granules in PMN. Immunolabeling confirmed the presence of membrane proteins and the absence of soluble content in platelet and megakaryocyte (MK) alpha-granules, and the decrease of secondary granules and secretory vesicles in PMN. This new observation demonstrates that the impairment of the secretory compartment of PMN is definitely a hallmark of GPS, and that the detection of these subtle abnormalities should be searched with adequate and up-to-date technical approaches.

An inherited macrothrombocytopenic disorder with abnormal large granules

Inherited giant platelet syndromes are a heterogeneous group of rare bleeding disorders. In the current study, a patient was reported with prolonged bleeding time, thrombocytopenia and giant platelets. Both the patient's red and white cell counts were normal without morphological abnormalities. The electron microscopy of platelets showed abnormal large electron-dense granules in the cytoplasm with exocytosis being easily observed. Her fathers platelets had the same abnormalities. The expression of glycoprotein (GP)Ib, GPIIb and GPIIIa on the surface of the patient's platelets was normal, and her platelet aggregation in response to ADP and ristocetin was also normal. Immunogold probes combined to monoclonal antibodies against GPIIIa, P-selectin or CD63 could not identify any relationship between the aberrant structures and alpha granules or lysosomes of platelet. Serotonin level was normal in the platelets of the patient, indicating the electron opaque granules are not dense bodies. The morphological abnormalities of the platelets from the patient are clearly distinguishable from other hereditary giant platelet disorders. We propose that the abnormal large granules from the patient's platelets probably represent a novel inherited thrombocytopenic disorder.

The cell biology of cell-in-cell structures

For decades, authors have described unusual cell structures, referred to as cell-in-cell structures, in which whole cells are found in the cytoplasm of other cells. One well-characterized process that results in the transient appearance of such structures is the engulfment of apoptotic cells by phagocytosis. However, many other types of cell-in-cell structure have been described that involve viable non-apoptotic cells. Some of these structures seem to form by the invasion of one cell into another, rather than by engulfment. The mechanisms of cell-in-cell formation and the possible physiological roles of these processes will be discussed.

Serglycin proteoglycan deletion induces defects in platelet aggregation and thrombus formation in mice

Serglycin (SG), the hematopoietic cell secretory granule proteoglycan, is crucial for storage of specific secretory proteins in mast cells, neutrophils, and cytotoxic T lymphocytes. We addressed the role of SG in platelets using SG-/- mice. Wild-type (WT) but not SG-/- platelets contained chondroitin sulfate proteoglycans. Electron microscopy revealed normal alpha-granule structure in SG-/- platelets. However, SG-/- platelets and megakaryocytes contained unusual scroll-like membranous inclusions, and SG-/- megakaryocytes showed extensive emperipolesis of neutrophils. SG-/- platelets had reduced ability to aggregate in response to low concentrations of collagen or PAR4 thrombin receptor agonist AYPGKF, and reduced fibrinogen binding after AYPGKF, but aggregated normally to ADP. 3H-serotonin and ATP secretion were greatly reduced in SG-/- platelets. The alpha-granule proteins platelet factor 4, beta-thromboglobulin, and platelet-derived growth factor were profoundly reduced in SG-/- platelets. Exposure of P-selectin and alphaIIb after thrombin treatment was similar in WT and SG-/- platelets. SG-/- mice exhibited reduced carotid artery thrombus formation after exposure to FeCl3. This study demonstrates that SG is crucial for platelet function and thrombus formation. We propose that SG-/- platelet function deficiencies are related to inadequate packaging and secretion of selected alpha-granule proteins and reduced secretion of dense granule contents critical for platelet activation.

Proteomic analysis of platelet alpha-granules using mass spectrometry

BACKGROUND

Platelets have three major types of secretory organelles: lysosomes, dense granules, and alpha-granules. alpha-Granules contain several adhesive proteins involved in hemostasis, as well as glycoproteins involved in inflammation, wound healing, and cell-matrix interactions. This article represents the first effort to define the platelet alpha-granule proteome using mass spectrometry (MS).

METHODS

We prepared a subcellular fraction enriched in intact alpha-granules from human platelets using sucrose gradient ultracentrifugation. alpha-Granule proteins were separated and identified using sodium dodecylsulfate polyacrylamide gel electrophoresis and liquid chromatography-tandem MS.

RESULTS

In the sucrose fraction enriched in alpha-granules, we identified 284 non-redundant proteins, 44 of which appear to be new alpha-granule proteins, on the basis of a literature review. Immunoelectron microscopy confirmed the presence of Scamp2, APLP2, ESAM and LAMA5 in platelet alpha-granules for the first time. We identified 65% of the same proteins that were detected in the platelet releasate (J. A. Coppinger et al. [Blood 2004;103: 2096-104]) as well as additional soluble and membrane proteins. Our method provides a suitable tool for analyzing the granule proteome of patients with storage pool deficiencies.

Congenital Platelet Disorders

Congenital platelet disorders represent a rare group of diseases classified by either a qualitative or quantitative platelet defect. This article outlines the historical, clinical, laboratory, and genetic features of various inherited platelet disorders with attention given to updated information on disease classification, diagnosis, and genotypes. A separate discussion regarding management addresses the difficulty in treatment strategies, particularly in patients who develop alloimmunization to platelets.

Defective platelet responsiveness to thrombin and protease-activated receptors agonists in a novel case of gray platelet syndrome: correlation between the platelet defect and the alpha-granule content in the patient and four relatives

BACKGROUND

We report a novel case of gray platelet syndrome (GPS). A 14-year-old boy had bleeding diathesis, mild thrombocytopenia, giant platelets with severe defect of alpha-granule secretory proteins, myelofibrosis and splenomegaly.

METHODS AND RESULTS

Platelet function studies showed a marked reduction of aggregation and Ca(2+) mobilization by thrombin, protease-activated receptor 1 (PAR1)-activating peptide (AP) and PAR4-AP, PAR1 expression at 55% of normal levels, and a more than two hundred fold reduction of in vitro whole-blood thromboxane B(2) (TXB(2)) production. Sequencing of coding regions of the PAR1 gene failed to show abnormalities. This patient was initially classified as a sporadic case of GPS, as electron microscopy failed to identify giant platelets and/or alpha-granule deficiency in his relatives. However, further studies on the father and three other relatives showed a relative lack of platelet alpha-granule proteins by immunofluorescence microscopy, a defective platelet response to PAR4-AP, and severely reduced in vitro whole-blood TXB(2) production. On this basis, we suggest that in this family, GPS was transmitted in a dominant fashion with highly variable penetrance.

CONCLUSIONS

Our study suggests that current diagnostic criteria fail to identify some patients with a mild GPS phenotype and that such patients might be identified by the methods cited above. It also better characterizes the pathogenesis of defective platelet responses to thrombin, and raises interesting questions on the correlation between abnormal PAR function and the lack of alpha-granule content in GPS.

X-linked gray platelet syndrome due to a GATA1 Arg216Gln mutation

We identified a family with gray platelet syndrome (GPS) segregating as a sex-linked trait. Affected males had a mild bleeding disorder, thrombocytopenia, and large agranular platelets characteristic of GPS, while obligate carrier females were asymptomatic but had dimorphic platelets on peripheral smear. Associated findings included mild erythrocyte abnormalities in affected males. Linkage analysis revealed a 63 cM region on the X chromosome between markers G10578 and DXS6797, which segregated with the platelet phenotype and included the GATA1 gene. Sequencing of GATA1 revealed a G-to-A mutation at position 759 corresponding to amino acid change Arg216Gln. This mutation was previously described as a cause of X-linked thrombocytopenia with thalassemia (XLTT) but not of gray platelet syndrome. Our findings suggest that XLTT is within a spectrum of disorders constituting the gray platelet syndrome, and we propose that GATA1 is an upstream regulator of the genes required for platelet alpha-granule biogenesis.

The gray platelet syndrome: Clinical spectrum of the disease

The gray platelet syndrome (GPS) is a rare inherited disorder of the megakaryocyte (MK) lineage. Thrombocytopenia and enlarged platelets are associated with a specific absence of alpha-granules and their contents. GPS patients exhibit much heterogeneity both in bleeding severity and in their response to platelet function testing. A unique feature is that proteins endogenously synthesised by megakaryocytes (MK) or endocytosed by MK or platelets fail to enter into the secretable storage pools that characterise alpha-granules of normal platelets. Although the molecular basis of the disease is unknown, evidence suggests that alpha-granules simply fail to mature during MK differentiation. One result is a continued leakage of growth factors and cytokines into the marrow causing myelofibrosis. While for some patients platelet function may be only moderately affected, for others thrombin and/or collagen-induced platelet aggregation is markedly modified and an acquired lack of the GPVI collagen receptor has been reported. In this review, we document the clinical and molecular heterogeneity in GPS, a unique disease of the biogenesis of platelet alpha-granules and of the storage of growth factors and secretable proteins.

A review of inherited platelet disorders with guidelines for their management on behalf of the UKHCDO

The inherited platelet disorders are an uncommon cause of symptomatic bleeding. They may be difficult to diagnose (and are likely to be under-diagnosed) and pose problems in management. This review discusses the inherited platelet disorders summarising the current state of the art with respect to investigation and diagnosis and suggests how to manage bleeding manifestations with particular attention to surgical interventions and the management of pregnancy.

bcr/abl-negative, classic myeloproliferative disorders: diagnosis and treatment

Essential thrombocythemia, polycythemia vera, and myelofibrosis with myeloid metaplasia constitute the "classic" bcr/abl-negative myeloproliferative disorders (MPDs). Each of these MPDs represents a stem cell-derived clonal myeloproliferation with the respective features of thrombocytosis, erythrocytosis, and bone marrow fibrosis. Unlike with cases of chronic myeloid leukemia, in which the bcr/abl mutation is invariably detected, current diagnosis of essential thrombocythemia, polycythemia vera, and myelofibrosis with myeloid metaplasia is based on a consensus-driven set of clinical and laboratory criteria that have undergone substantial modification in recent times. The recent discovery of a recurrent activating Janus tyrosine kinase (JAK2) mutation (JAK2VG17F) in all 3 classic MPDs offers another opportunity for refining current diagnoses and disease classifications. In this article, we outline contemporary diagnostic algorithms for each of these disorders and provide an evidence-based approach to management.

Severe deficiency of glycoprotein VI in a patient with gray platelet syndrome

We report a novel case of gray platelet syndrome (GPS) where a severe deficiency of the platelet collagen receptor, glycoprotein (GP) VI, accompanies classical symptoms of a low platelet count and platelets lacking alpha-granules. Dense granules were normally present. Platelet aggregation with collagen was severely decreased, as was the response to convulxin (Cvx), a GPVI agonist. Quantitative analysis of GPVI using fluorescein isothiocyanate (FITC)-Cvx in flow cytometry showed its virtual absence on the patient's platelets. The GPVI deficiency was confirmed using monoclonal antibodies in Western blotting and in immunogold labeling on frozen thin sections where internal pools of GPVI were confirmed for normal platelets. The Fc receptor gamma-chain, constitutively associated with GPVI in normal platelets, was present in subnormal amounts, and the phospholipase C gamma 2-dependent activation pathway appeared to function normally. No autoantibodies to GPVI were found in the patient's serum using monoclonal antibody immobilization of platelet antigen (MAIPA). Sequencing of coding regions of the GPVI gene failed to show abnormalities, and mRNA for GPVI was present in the patient's platelets, pointing to a probable acquired defect in GPVI expression. Our results may provide a molecular explanation for the subgroup of patients with severely deficient collagen-induced platelet aggregation as previously described for GPS in the literature.

Use of a cDNA microarray to determine molecular mechanisms involved in grey platelet syndrome

The grey platelet syndrome (GPS) is a bleeding disorder of unknown aetiology with phenotypic and genetic heterogeneity. Affected patients exhibit macrothrombocytopenia, decreased alpha-granule content and, sometimes, myelofibrosis. We used microarray technology to investigate changes in gene expression that might reveal mechanisms involved in GPS. The expression of 4900 unique genes and expressed sequence tags was evaluated in fibroblasts from a GPS patient; normal fibroblasts provided the reference standard. Genes that were differentially regulated in the GPS cells were categorized into gene clusters based upon similarity/differences of expression differences. The results showed that genes with functional similarities clustered together. This analysis revealed significant upregulation of selected biological processes involving the production of cytoskeleton proteins, including fibronectin 1, thrombospondins 1 and 2, and collagen VI alpha. These genes appear to play a role in the pathogenesis of GPS. Indeed, Northern blot analyses confirmed that fibronectin, thrombospondin and matrix metalloprotease-2 were overexpressed in GPS fibroblasts compared with normal fibroblasts. Moreover, immunohistochemistry studies revealed robust fibronectin staining in GPS fibroblasts compared with normal ones. Our findings support the feasibility of using cDNA microarray techniques to detect distinctive and informative differences in gene expression patterns relevant to GPS, and suggest that the molecular basis for myelofibrosis in GPS involves upregulation of cytoskeleton proteins.

Development of platelet secretory granules

Platelet granule exocytosis plays a critical role in thrombosis and wound healing. Platelets have three major types of secretory granules that are defined by their unique molecular contents, kinetics of exocytosis and morphologies. Although the ontogeny of platelet granules is poorly understood, a convergence of new insights into megakaryocyte development, the molecular mechanisms of vesicle trafficking and the genetic basis of platelet granule defects, is beginning to define the cellular and molecular pathways responsible for platelet granule ontogeny.