Inherited thrombocytopenias: toward a molecular understanding of disorders of platelet production

Genomic Variation Affecting MPV and PLT Count in Association with Development of Ischemic Stroke and Its Subtypes

Platelets play a significant role in the pathophysiology of ischemic stroke since they are involved in the formation of intravascular thrombus after erosion or rupture of the atherosclerotic plaques. Platelet (PLT) count and mean platelet volume (MPV) are the two significant parameters that affect the functions of platelets. In the current study, MPV and PLT count was evaluated using flow cytometry and a cell counter. SonoClot analysis was carried out to evaluate activated clot timing (ACT), clot rate (CR), and platelet function (PF). Genotyping was carried out using GSA and Sanger sequencing, and expression analysis was performed using RT-PCR. In silico analysis was carried out using the GROMACS tool and UNAFold. The interaction of significant proteins with other proteins was predicted using the STRING database. Ninety-six genes were analyzed, and a significant association of THPO (rs6141) and ARHGEF3 (rs1354034) was observed with the disease and its subtypes. Altered genotypes were associated significantly with increased MPV, decreased PLT count, and CR. Expression analysis revealed a higher expression in patients bearing the variant genotypes of both genes. In silico analysis revealed that mutation in the THPO gene leads to the reduced compactness of protein structure. mRNA encoded by mutated ARHGEF3 gene increases the half-life of mRNA. The two significant proteins interact with many other proteins, especially the ones involved in platelet activation, aggregation, erythropoiesis, megakaryocyte maturation, and cytoskeleton rearrangements, suggesting that they could be important players in the determination of MPV values. In conclusion, the current study demonstrated the role of higher MPV affected by genetic variation in the development of IS and its subtypes. The results of the current study also indicate that higher MPV can be used as a biomarker for the disease and altered genotypes, and higher MPV can be targeted for better therapeutic outcomes.

Role of Nitric Oxide in Megakaryocyte Function

Megakaryocytes are the main members of the hematopoietic system responsible for regulating vascular homeostasis through their progeny platelets, which are generally known for maintaining hemostasis. Megakaryocytes are characterized as large polyploid cells that reside in the bone marrow but may also circulate in the vasculature. They are generated directly or through a multi-lineage commitment step from the most primitive progenitor or Hematopoietic Stem Cells (HSCs) in a process called "megakaryopoiesis". Immature megakaryocytes enter a complicated development process defined as "thrombopoiesis" that ultimately results in the release of extended protrusions called proplatelets into bone marrow sinusoidal or lung microvessels. One of the main mediators that play an important modulatory role in hematopoiesis and hemostasis is nitric oxide (NO), a free radical gas produced by three isoforms of nitric oxide synthase within the mammalian cells. In this review, we summarize the effect of NO and its signaling on megakaryopoiesis and thrombopoiesis under both physiological and pathophysiological conditions.

Application of High-Throughput Sequencing in the Diagnosis of Inherited Thrombocytopenia

Inherited thrombocytopenia is a group of hereditary diseases with a reduction in platelet count as the main clinical manifestation. Clinically, there is an urgent need for a convenient and rapid diagnosis method. We introduced a high-throughput, next-generation sequencing (NGS) platform into the routine diagnosis of patients with unexplained thrombocytopenia and analyzed the gene sequencing results to evaluate the value of NGS technology in the screening and diagnosis of inherited thrombocytopenia. From a cohort of 112 patients with thrombocytopenia, we screened 43 patients with hereditary features. For the blood samples of these 43 patients, a gene sequencing platform for hemorrhagic and thrombotic diseases comprising 89 genes was used to perform gene detection using NGS technology. When we combined the screening results with clinical features and other findings, 15 (34.9%) of 43patients were diagnosed with inherited thrombocytopenia. In addition, 19 pathogenic variants, including 8 previously unreported variants, were identified in these patients. Through the use of this detection platform, we expect to establish a more effective diagnostic approach to such disorders.

Platelet Abnormalities in the Oral Maxillofacial Surgery Patient

Platelet abnormalities result from a wide range of congenital and acquired conditions, which may be known or unknown to patients presenting for oral maxillofacial surgery. It is critical to obtain a thorough history, including discussion of any episodes of bleeding or easy bruising, to potentially discern patients with an underlying platelet disorder. If patients indicate a positive history, preoperative laboratory studies are indicated, with potential referral or consultation with a hematologist. Appropriate preoperative planning may reduce the risk of bleeding associated with platelet dysfunction, potentially avoiding serious perioperative and postoperative complications.

Bengal macrothrombocytopenia is not totally an innocuous condition

Inherited macrothrombocytopenia is a subgroup of thrombocytopenias, and is characterised by the presence of giant platelets and decreased platelet count with variable bleeding manifestations. Bengal macrothrombocytopenia is a newly described entity, previously called asymptomatic constitutional macrothrombocytopenia (ACMT), presented with variable bleeding tendencies; with mild to severe thrombocytopenia and macro-platelets in their peripheral blood smear and it is not totally an innocuous condition as described previously.

Laboratory diagnostics of inherited platelet disorders

Abstract Inherited platelet disorders (IPDs) are the general and common denomination of a broad number of different rare and congenital pathologies affecting platelets. Even if these disorders are characterized by widely heterogeneous clinical presentations, all of them are commonly present as defects in hemostasis. Platelet number and/or function are affected by a wide spectrum of severity. IPDs might be associated with defects in bone marrow megakaryocytopoiesis and, rarely, with somatic defects. Although in the last few years new insights in the genetic bases and pathophysiology of IPDs have greatly improved our knowledge of these disorders, much effort still needs to be made in the field of laboratory diagnosis. This review discusses the laboratory approach for the differential diagnosis of the most common IPDs, suggesting a common multistep flowchart model which starts from the simpler test (platelet count) ending with the more selective and sophisticated analyses.

Small-platelet thrombocytopenia in a family with autosomal recessive inheritance pattern

We describe the clinical and laboratory features of a family of Arab ancestry and consanguinity. Five affected individuals were diagnosed in two sibships. All affected members have small platelets, severe to moderate thrombocytopenia of neonatal onset, increased bleeding tendency and bleeding complications such as: life-threatening massive hemoperitoneum due to corpus luteum rupture during ovulation and severe mucosal bleeding. The familial involvement and early onset of the disease support the presence of a congenital genetic disorder with an autosomal recessive inheritance pattern. This does not fit the clinical spectrum of any of the currently known thrombocytopenia disorders.

The incredible journey: From megakaryocyte development to platelet formation

Circulating blood platelets are specialized cells that prevent bleeding and minimize blood vessel injury. Large progenitor cells in the bone marrow called megakaryocytes (MKs) are the source of platelets. MKs release platelets through a series of fascinating cell biological events. During maturation, they become polyploid and accumulate massive amounts of protein and membrane. Then, in a cytoskeletal-driven process, they extend long branching processes, designated proplatelets, into sinusoidal blood vessels where they undergo fission to release platelets. Given the need for platelets in many pathological situations, understanding how this process occurs is an active area of research with important clinical applications.

Pathogenesis and management of inherited thrombocytopenias: rationale for the use of thrombopoietin-receptor agonists

Knowledge in the field of inherited thrombocytopenias (ITs) has considerably improved over the recent years. In the last 5 years, nine new genes whose mutations are responsible for thrombocytopenia have been identified, and this also led to the recognition of several novel nosographic entities, such as thrombocytopenias deriving from mutations in CYCS, TUBB1, FLNA, ITGA2B/ITGB3, ANKRD26 and ACTN1. The identification of novel molecular alterations causing thrombocytopenia together with improvement of methodologies to study megakaryopoiesis led to considerable advances in understanding pathophysiology of ITs, thus providing the background for proposing new treatments. Thrombopoietin-receptor agonists (TPO-RAs) represent an appealing therapeutic hypothesis for ITs and have been tested in a limited number of patients. In this review, we provide an updated description of pathogenetic mechanisms of thrombocytopenia in the different forms of ITs and recapitulate the current management of these disorders. Moreover, we report the available clinical and preclinical data about the role of TPO-RAs in ITs and discuss the rationale for the use of these molecules in view of pathogenesis of the different forms of thrombocytopenia of genetic origin.

Platelet morphology analysis

Platelets are very small blood cells (1.5-3 μm), which play a major role in primary haemostasis and in coagulation mechanisms. Platelet characterization requires their counting (see Chapter 15 ) associated with accurate morphology analysis. We describe the major steps in order to correctly obtain stained blood films, which can be analyzed by optical microscope. Platelet morphology abnormalities are found in acquired malignant hematological diseases such myeloproliferative or myelodysplastic syndromes and acute megakaryoblastic leukemia. A careful analysis of the platelet size and morphology, by detecting either normal platelets with or without excessive anisocytosis, microplatelets, or large/giant platelets, will contribute to inherited thrombocytopenia diagnosis and gather substantial data when looking for an acquired platelet disorders.

Protein kinase C alpha and beta are positive regulators of thrombus formation in vivo in a zebrafish (Danio rerio) model of thrombosis

BACKGROUND

The zebrafish (Danio rerio) is becoming an attractive model organism for the assessment of gene function in thrombosis in vivo. Zebrafish, as a thrombosis model, have several advantages, with the capacity to follow thrombus formation at high resolution in real time using intravital microscopy, without the need for complex surgical techniques, and the capability to rapidly knockdown gene expression using morpholino antisense approaches.

OBJECTIVES

We have recently shown, in mouse models, that protein kinase C alpha (PKCα) plays a critical role in regulating thrombus formation in vivo. PKC beta (β) plays a non-redundant role also in platelet function in vitro, but the function of this gene had not yet been assessed in vivo.

METHODS

In the present study, we analyzed the function of both PKCα and PKCβ in the zebrafish model in vivo, by live imaging using a laser-induced injury of the main caudal artery in 3-day-old larvae.

RESULTS

We showed that D. rerio express orthologs of both the PKCα and PKCβ genes, with high sequence identity. Translation blocking and splice-blocking morpholinos effectively and specifically knockdown expression of these genes and knockdown with either morpholino leads to attenuated thrombus formation, as assessed by several quantitative parameters including time to initial adhesion and peak thrombus surface area.

CONCLUSIONS

Our data indicate that these two highly related genes play non-redundant roles in regulating thrombosis, an observation that supports our previous in vitro murine data, and suggests unique roles, and possibly unique regulation, for PKCα and PKCβ in controlling platelet function in vivo.

The incidence of thrombocytopenia in children with Cornelia de Lange syndrome

Thrombocytopenia was first reported in Cornelia de Lange syndrome (CdLS) by Froster in 1993. Despite early reports, thrombocytopenia has been rarely reported in this disorder. We performed a retrospective analysis of a large cohort of patients with CdLS. We calculated prevalence of thrombocytopenia in three subsets of this cohort: the entire cohort (n = 1,740), a subset of subjects with substantial clinical records (n = 695) and a subset of subjects with clinical information regarding platelet counts (n = 85). This analysis revealed that 15 have had thrombocytopenia (18% of those with available blood counts); seven had immune thrombocytopenia (ITP). The reported prevalence of pediatric ITP is between 5 and 13 per 100,000 persons. The prevalence of ITP in this cohort is between 7/1,740 and 7/85, giving a relative risk of ITP of between 30 (CI 12-77) and 633 (CI 259-1,549). Contrary to the reported cases in the literature, none of our patients have had progression of the thrombocytopenia nor have they developed other cytopenias. All 15 patients with thromobocytopenia had CdLS based on clinical criteria. Of the 10 patients tested for mutations in NIBPL, 8 had mutations identified. These data support an increased incidence of thrombocytopenia and ITP in CdLS. Subsequently, patients are at risk for spontaneous hemorrhage, and likely increased risk secondary to the high frequency of self-injurious behavior. Although further studies are needed to better define the scope of the problem and to define the mechanisms of thrombocytopenia in CdLS, we would recommend screening for thrombocytopenia upon diagnosis and at 5-year intervals thereafter.

In vivo inactivation of MASTL kinase results in thrombocytopenia

OBJECTIVE

A missense mutation in the microtubule-associated serine/threonine-like kinase gene (MASTL, FLJ14813) on human chromosome 10 was previously linked to a novel form of autosomal dominant inherited thrombocytopenia in a single pedigree. The mutation results in an amino acid change from glutamic acid at position 167 to aspartic acid and segregates perfectly with thrombocytopenic individuals within this extended family. The phenotype is characterized by mild thrombocytopenia with an average platelet count of 60,000 platelets per microliter of blood. We wanted to determine the expression and localization of MASTL, as well as its role in developing thrombocytes using an in vivo model system.

MATERIALS AND METHODS

Northern blot analysis allowed us to examine expression patterns. Morpholino knockdown assays in zebrafish (Danio rerio) were employed to determine in vivo contribution to thrombocyte development. Transient expression in baby hamster kidney cells resulted in localization of both the wild-type and E167D mutant forms of MASTL kinase to the nucleus.

RESULTS

Northern blot analysis indicates that MASTL messenger RNA is restricted in its expression to hematopoietic and cancer cell lines. A transient knockdown of MASTL in zebrafish results in deficiency of circulating thrombocytes. Transient expression of recombinant MASTL kinase in vitro demonstrates localization to the nucleus.

CONCLUSIONS

Functional studies presented here demonstrate a direct relationship between transient knockdown of MASTL kinase gene expression and reduction of circulating thrombocytes in zebrafish. This transient knockdown of MASTL in zebrafish correlates with a decrease in the expression of the thrombopoietin receptor, c-mpl, and the CD41 platelet adhesion protein, GpIIb, but has no effect on essential housekeeping zebrafish gene, EF1alpha.

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.

Historical review: megakaryopoiesis and thrombopoiesis

The study of thrombopoiesis has evolved greatly since an era when platelets were termed "the dust of the blood," only about 100 years ago. During this time megakaryocytes were identified as the origin of blood platelets; marrow-derived megakaryocytic progenitor cells were functionally defined and then purified; and the primary regulator of the process, thrombopoietin, was cloned and characterized and therapeutic thrombopoietic agents developed. During this journey we continue to learn that the physiologic mechanisms that drive proplatelet formation can be recapitulated in cell-free systems and their biochemistry evaluated; the molecular underpinnings of endomitosis are being increasingly understood; the intracellular signals sent by engagement of a large number of megakaryocyte surface receptors have been defined; and many of the transcription factors that drive megakaryocytic fate determination have been identified and experimentally manipulated. While some of these biologic processes mimic those seen in other cell types, megakaryocytes and platelets possess enough unique developmental features that we are virtually assured that continued study of thrombopoiesis will yield innumerable clinical and scientific insights for many decades to come.

Diagnostic approach to platelet function disorders

Platelet disorders are common bleeding disorders, with a variety of congenital and acquired causes. The diagnostic evaluation of platelet disorders challenges both clinicians and clinical laboratories, as testing for these conditions is complex, not well standardized and time consuming. An understanding of normal platelet function has provided insights on the pathogenesis of many platelet function disorders. Knowledge of the key features of platelet disorders aids their diagnostic assessment. Tests for aggregation, secretion and dense granule defects continue to be the most helpful for the evaluation of suspected platelet function disorders.

Inherited platelet disorders

Inherited platelet disorders are a rare, but probably underdiagnosed, cause of symptomatic bleeding. They are characterized by abnormalities of platelet number (inherited thrombocytopenias), function (inherited disorders of platelet function) or both. This review briefly discusses the inherited platelet disorders with respect to molecular defects, diagnostic evaluation and treatment strategies.

Inherited platelet function disorders versus other inherited bleeding disorders: an Indian overview

Inherited deficiencies of plasma proteins involved in blood coagulation generally lead to lifelong bleeding disorders, whose severity is directly proportional to the degree of factor deficiency. Platelet and other coagulation factors play an important role in the haemostasis mechanism. We attempted to study the prevalence of inherited platelet function disorders (PFDs) and correlate with other coagulation factor disorders in the Indian population. Patients with PFDs and other coagulation factor disorders who presented at our hospital during the 5 year period (from January, 2001 to December, 2005) were the subjects of the study. A total of 1576 patients were diagnosed to have congenital bleeding disorders including PFDs, von Willebrand disease, haemophilia A and B and rare coagulation disorder cases. Haemophilia A (HA) was the most common and was seen in 52.31% of the patients followed by total PFDs seen in 27.77% of the patients. Based on severity of the disease, the results of PFDs were highly significant when compared to haemophilia and von Willebrand disease (VWD) (p=0.000). Severity was found higher in HA (77.8%) followed by HB (69.6%) and was found lower for PF3 availability defect (9.0%). It has been concluded that the prevalence of PFDs is relatively low as compared to coagulation factors related disorder and also it has been established that type-1 VWD is relatively less frequent in India as compared to the West.

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.

Inherited Thrombocytopenia: Congenital Amegakaryocytic Thrombocytopenia and Thrombocytopenia With Absent Radii

Thrombocytopenia in the newborn period can signify an inherited platelet disorder. Congenital amegakaryocytic thrombocytopenia (CAMT) and thrombocytopenia with absent radii (TAR) share features of isolated thrombocytopenia, reduced or absent marrow megakaryocytes, impaired responsiveness to thrombopoietin (TPO), and high plasma TPO levels. These disorders are most readily distinguished from each other by the finding of radial aplasia in TAR and the presence of c-MPL mutations in CAMT. In addition, their long-term outcomes are strikingly different: the development of trilineage marrow failure in CAMT in contrast to the general improvement of thrombocytopenia in TAR. The differential diagnosis for CAMT and TAR also includes other congenital disorders in which thrombocytopenia and radial abnormalities can be seen. In this article we will review our molecular and clinical understanding of these two inherited disorders of amegakaryocytosis.

Congenital platelet disorders: overview of their mechanisms, diagnostic evaluation and treatment

The bleeding problems associated with common and rare inherited platelet disorders illustrate the importance of platelets to normal haemostasis. At sites of injury, platelets normally adhere, undergo activation, secretion and aggregate formation, and they provide the membrane surface for the assembly of coagulation to generate thrombin. The causes of inherited disorders that alter platelet haemostatic functions are quite diverse, ranging from defects in receptors critical to platelet adhesion and aggregation, to defects in signalling molecules or in transcription factors important for production of functional platelets. The mechanisms of impaired platelet function are largely unknown for the more common disorders that alter platelet activation, secretion and the secondary wave of platelet aggregation. The diagnostic evaluation of congenital platelet disorders has been challenging as some 'platelet-type' bleeding symptoms, such as bruising, are quite common in the general population. Moreover, the diagnostic tests used by clinical laboratories to evaluate disorders of platelet function have not been standardized. In individuals recognized to have an inherited defect in platelet function, therapy is important for controlling and preventing bleeding episodes. Presently, there are a number of choices to consider for the management of bleeding symptoms, including menorrhagia. This paper reviews the causes, diagnostic evaluation and therapies for common and rare congenital platelet disorders.

Inherited disorders of platelets: an update

PURPOSE OF REVIEW

To overview inherited syndromes that affect platelets and to discuss current data on the molecular origin and management of these rare diseases.

RECENT FINDINGS

An increasing number of genes responsible for inherited thrombocytopenias have been identified and these now extend to glycosylation defects. Although Glanzmann thrombasthenia remains the predominant disorder of platelet function, knowledge is increasing of pathologies concerning primary receptors for adhesion and signalling, the activation and secretory pathways, and even the development of procoagulant activity.

SUMMARY

These syndromes affect cell adhesion, cell activation, and cell-to-cell contact interactions fundamental in cell biology. Studies on the pathophysiology of alphaIIbbeta3 in platelets have helped unravel the molecular mechanisms of integrin function, and the information gained has resulted in improved antithrombotic therapy. The establishment of national registries and the use of state-of-the-art genomic and proteomic technologies will accelerate progress and help to define how mutations affecting a much larger range of proteins contribute alone or in combination to defining specific platelet phenotypes.

Congenital macrothrombocytopenias

Congenital macrothrombocytopenias comprise a heterogeneous group of rare disorders, characterized by abnormal giant platelets, thrombocytopenia and bleeding tendency with variable severity. Many of these disorders share common clinical and laboratory features, making accurate diagnosis difficult and patients are often misdiagnosed with and treated for idiopathic thrombocytopenic purpura. Recent progress in the elucidation of underlying defects and further developments of specific diagnostic techniques for several congenital macrothrombocytopenias have renewed our approach to the classification and the diagnosis of the disease. This review summarizes the current knowledge on the clinical and laboratory features of common congenital macrothrombocytopenias and discusses how that knowledge aids in making a proper diagnosis.

The rule of four: a systematic approach to diagnosis of common pediatric hematologic and oncologic disorders

The "Rule of Four" facilitates a rapid and focused approach to the diagnosis of the common hematologic and oncologic disorders encountered in general pediatric practice. This system relies on four recurrent but different clinical entities or laboratory tests relevant to the diagnosis of children with anemia, excessive bleeding or clotting, and common malignancies. For each disorder, there is a discussion of a variety of four lab tests or factors pertinent to a differential diagnosis.

Pathogenetic mechanisms of hematological abnormalities of patients with MYH9 mutations

Mutations of MYH9, the gene for non-muscle myosin heavy chain IIA (NMMHC-IIA), cause a complex clinical phenotype characterized by macrothrombocytopenia and granulocyte inclusion bodies, often associated with deafness, cataracts and/or glomerulonephritis. The pathogenetic mechanisms of these defects are either completely unknown or controversial. In particular, it is a matter of debate whether haploinsufficiency or a dominant-negative effect of mutant allele is responsible for hematological abnormalities. We investigated 11 patients from six pedigrees with different MYH9 mutations. We evaluated NMMHC-IIA levels in platelets and granulocytes isolated from peripheral blood and in megakaryocytes (Mks) cultured from circulating progenitors. NMMHC-IIA distribution in Mks and granulocytes was also assessed. We demonstrated that all the investigated patients had a 50% reduction of NMMHC-IIA expression in platelets and that a similar defect was present also in Mks. In subjects with R1933X and E1945X mutations, the whole NMMHC-IIA of platelets and Mks was wild-type. No NMMHC-IIA inclusions were observed at any time of Mk maturation. In granulocytes, the extent of NMMHC-IIA reduction in patients with respect to control cells was significantly greater than that measured in platelets and Mks, and we found that wild-type protein was sequestered within most of the NMMHC-IIA inclusions. Altogether these results indicate that haploinsufficiency of NMMHC-IIA in megakaryocytic lineage is the mechanism of macrothrombocytopenia consequent to MYH9 mutations, whereas in granulocytes a dominant-negative effect of mutant allele is involved in the formation of inclusion bodies. The finding that the same mutations act through different mechanisms in different cells is surprising and requires further investigation.

Qualitative disorders of platelets and megakaryocytes

Qualitative disorders of platelet function and production form a large group of rare diseases which cover a multitude of genetic defects that by and large have as a common symptom, excessive mucocutaneous bleeding. Glanzmann thrombasthenia, is enabling us to learn much about the pathophysiology of integrins and of how alphaIIb beta3 functions. Bernard-Soulier syndrome, an example of macrothrombocytopenia, combines the production of large platelets with a deficit or non-functioning of the major adhesion receptor of platelets, the GPIb-IX-V complex. Amino acid substitutions in GPIb alpha, may lead to up-regulation and spontaneous binding of von Willebrand factor as in Platelet-type von Willebrand disease. In disorders with defects in the MYH9 gene, macrothrombocytopenias are linked to modifications in kidney, eye or ear, whereas other inherited thrombocytopenias variously link a low platelet count with a propensity to leukemia, skeletal defects, learning impairment, and abnormal red cells. Defects of secretion from platelets include an abnormal alpha-granule formation as in the gray platelet syndrome (with marrow myelofibrosis), and of organelle biogenesis in the Hermansky-Pudlak and Chediak-Higashi syndromes where platelet dense body defects are linked to abnormalities of other lysosomal-like organelles including melanosomes. Finally, defects involving surface receptors (P2Y(12), TPalpha) for activating stimuli, of proteins essential for signaling pathways (including Wiskott-Aldrich syndrome), and of platelet-derived procoagulant activity (Scott syndrome) show how studies on platelet disorders are helping unravel the pathways of primary hemostasis.

Altered cytoskeleton organization in platelets from patients with MYH9-related disease

MYH9-related disease (MYH9-RD) is an autosomal dominant disorder deriving from mutations in the MYH9 gene encoding for the heavy chain of non-muscle myosin IIA, and characterized by thrombocytopenia and giant platelets. Isoform IIA of myosin is the only one expressed in platelets, but the possibility that MYH9 mutations affect the organization of contractile structures in these blood elements has never been investigated. In this work we have analyzed the composition and the agonist-induced reorganization of the platelet cytoskeleton from seven MYH9-RD patients belonging to four different families. We found that an increased amount of myosin was constitutively associated with actin in the cytoskeleton of resting MYH9-RD platelets. Upon platelet stimulation, an impaired increase in the total cytoskeletal proteins was observed. Moreover, selected membrane glycoproteins, tyrosine kinases, and small GTPases failed to interact with the cytoskeleton in agonist-stimulated MYH9-RD platelets. These results demonstrate for the first time that mutations of MYH9 result in an alteration of the composition and agonist-induced reorganization of the platelet cytoskeleton. We suggest that these abnormalities may represent the biochemical basis for the previously reported functional alterations of MYH9-RD platelets, and for the abnormal platelet formation from megakaryocytes, resulting in thrombocytopenia and giant platelets.

Neonatal thrombocytopenia

Thrombocytopenia occurs in up to a third of preterm neonates admitted to intensive care units. In these babies, thrombocytopenia typically presents in one of two patterns: early-onset thrombocytopenia occurring within 72 h of birth and late-onset thrombocytopenia which develops after 72 h. Early-onset thrombocytopenia is most commonly caused by disorders associated with placental insufficiency (e.g. maternal hypertension), is mild-moderate, self-limiting and requires no treatment; it is caused by reduced platelet production. Late-onset thrombocytopenia is usually due to bacterial sepsis or necrotising enterocolitis; it is often severe (platelets <50 x 10(9)/l), prolonged and requires treatment with platelet transfusions. In term babies, neonatal thrombocytopenia is usually severe and most commonly caused by bacterial sepsis, perinatal asphyxia or neonatal alloimmune thrombocytopenia. There is a lack of evidence-based guidelines for treatment of neonatal thrombocytopenia. The most important future developments will depend upon studies aimed at determining optimal platelet transfusion schedules for term and preterm neonates.