Platelet Disorders and Von Willebrand Disease

Platelet Structure

Blood platelets are highly complex, anucleate cells that derive from bone marrow megakaryocytes. Peripheral blood film offers the opportunity for evaluation of platelet numbers, size, distribution, and structure under the light microscope.

Electron microscopy allows more precise characterization of the structural abnormalities ( ), with the opportunity to examine in greater detail specific platelet organelles ( ). An array of 3-dimensional high-resolution structural approaches have emerged and provide novel insights into platelets ( ).

Platelet features that may be visualized ultrastructurally are shown in Figure 41.1 . The outer surface of the platelet, the glycocalyx , is rich in glycoproteins. A submembranous band of microtubules , composed of the protein tubulin, provides structural support for the discoid platelet shape. Contractile microfilaments composed of actin and myosin may also be seen. An extensive open canalicular system within the platelet is in direct communication with the extracellular environment. Often seen in proximity to the open canalicular system is the dense tubular system . The dense tubular system also functions as a calcium-sequestering pump, maintaining low levels of cytoplasmic calcium in the resting platelet.

A variety of inclusions are recognized within the platelet cytoplasm, including mitochondria , glycogen , and granules . Lighter staining α-granules, less frequent dense core (or “bull’s-eye”) granules, lysosomes, and peroxisomes are also seen. Three-dimensional electron microscopic studies have revealed morphologically distinct α-granule subtypes and demonstrated that high spatial segregation of different stored proteins exists within individual α-granules ( ). The α-granule proteins derive both from endocytic uptake and de novo synthesis by megakaryocytes, and include fibrinogen, platelet-derived growth factor (PDGF), von Willebrand factor (vWF), the factor V binding protein multimerin, β-thromboglobulin (βTG), and the heparin-neutralizing platelet factor (PF) 4. P-selectin, a constituent of the α-granule membrane, translocates to the plasma membrane of the platelet when α-granule contents are released. The dense granules contain a nonmetabolic pool of adenosine diphosphate (ADP) and adenosine triphosphate (ATP), 5-hydroxytryptamine (5-HT, serotonin), calcium, and inorganic polyphosphate (polyP).

Platelet Membrane Glycoproteins and Phospholipids

Historically, analyses of platelet surface glycoproteins in patients with inherited platelet disorders have been key to our current understanding of fundamental aspects of platelet function. Table 41.1 shows the major platelet membrane glycoproteins that function as receptors for adhesive ligands. The GPIIb-IIIa receptor complex (also referred to as integrin αIIbβ3 ), which in its activated form serves as a receptor for fibrinogen, is the most abundant platelet receptor, with ∼80,000 to 100,000 copies per platelet. This is followed by the major platelet receptor for vWF, GPIbα, at ∼30,000 copies. GPIbα is disulfide linked to GPIbβ and noncovalently linked to GPIX and GPV; together, these glycoproteins make up the GPIb-IX-V complex. The copy number for other platelet surface glycoproteins is lower. GPVI and GPIa-IIa mediate platelet interactions with collagen.

Platelet membranes contain several phospholipids that play a major role in platelet function, including phosphatidylinositol, phosphatidylcholine, phosphatidylserine (PS), and phosphatidylethanolamine. Phosphatidylinositides are the source of signaling molecules inositoltrisphosphate and diacylglycerol, which are produced following platelet activation. Negatively charged PS is expressed in the inner membrane leaflet of the resting platelet; on platelet activation, PS translocates to the outer leaflet, an essential step in the formation of a procoagulant platelet surface that is critical in several blood coagulation mechanisms.

Role of Platelets in Hemostasis and Platelet-Activation Mechanisms

Following injury to the blood vessel, platelets adhere to exposed subendothelium by a process ( adhesion ) that involves the interaction of a plasma protein, vWF, and a specific glycoprotein complex on the platelet surface, GPIb-IX-V ( Fig. 41.2 ), which binds vWF particularly under conditions of high shear stress. Adhesion is followed by recruitment of additional platelets, which interact with each other to form clumps, a process called aggregation . This involves binding of fibrinogen to specific platelet surface receptors—a complex composed of GPIIb-IIIa, which is platelet- and megakaryocyte-specific and binds vWF as well. Resting platelets do not bind fibrinogen. Stimulation of platelets leads to a series of intracellular signaling responses, including binding of the adapter proteins talin and kindlin to the cytoplasmic tails of the GPIIb-IIIa receptor complexes ( ). These events produce conformational changes in the GPIIb-IIIa complex that expose binding sites for fibrinogen, a prerequisite for platelet aggregation. Activated platelets release contents of their granules ( secretion or release reaction ), such as ADP and serotonin from the dense granules, which cause recruitment of additional platelets. In addition, platelets play a major role in coagulation mechanisms; several key enzymatic reactions occur on the platelet membrane lipoprotein surface ( platelet coagulant activity ).

Initial Laboratory Evaluation

The diagnostic process begins with a detailed clinical evaluation followed by laboratory studies ( ; ). Platelet disorders may occur in association with other inherited or acquired disorders or syndromes; thus, a detailed clinical evaluation is essential. To better assess the intensity of bleeding symptoms, a number of validated standardized bleeding assessment tools are available ( ). This evaluation should also include an assessment of ingestion of medications, herbal supplements, and over-the-counter substances. Many such compounds affect platelet number or function.

The initial laboratory evaluation of platelet disorders includes a complete blood count, including the platelet count. The normal platelet count is 150 to 400 × 10 ⁹ /L. In severe thrombocytopenia, below the established linearity of a given automated instrument or in the presence of cellular fragments spuriously affecting the automated count, a manual phase contrast count with a hemocytometer chamber may be required. Many instruments also provide the mean platelet volume (MPV) and a histogram of MPV. Some automated instruments also assess platelet maturity, as reflected by the RNA content of “reticulated platelets”—analogous to reticulocyte measurements in the erythroid series ( ; ). Such measurements offer the opportunity to detect a thrombopoietic response from the bone marrow. Evaluation of the peripheral blood film provides valuable information regarding platelet count and size distribution, and on associated changes in other blood cells. A simple diagnostic approach that utilizes immunofluorescence labeling of peripheral blood smears using antibodies specific for proteins known to be affected has been proposed as a diagnostic tool for several inherited platelet disorders ( ).

Studies of Platelet Function

No currently available laboratory test faithfully reflects the enormously complex platelet role in hemostasis. Several instruments and approaches have been developed to study platelet function ( ; ; ; ). A large number of biochemical and functional approaches, many in research laboratories, have been applied as a part of phenotype-driven and candidate gene approaches in unraveling the underlying mechanisms in patients identified ( ). These approaches have taken a major change in direction with the recent application of high-throughput deoxyribonucleic acid (DNA) sequencing methods to define the genetic basis in patients ( ; ; ; ).

Platelet Aggregation and Secretion Studies

Platelet function can be evaluated in vitro through study of platelet aggregation and secretion in response to a battery of platelet-stimulating agents ( ; ; ; ). These studies are generally performed using platelet-rich plasma (PRP) harvested from blood collected using sodium citrate as the anticoagulant. Ethylenediaminetetraacetic acid (EDTA), routinely used to collect samples for blood counts, dissociates the platelet GPIIb-IIIa complexes; thus, aggregation studies cannot be performed using such a sample. When citrated PRP is continuously stirred in a platelet aggregometer and a light beam is passed through the suspension; platelet aggregation in response to an added agonist can be monitored by changes in light transmittance. Change in platelet shape from discoid to spheroid is seen as an initial decrease in transmittance, whereas the subsequent formation of platelet clumps allows more light to pass through the suspension to the photodetector and is recorded as an increase in light transmittance ( ). Recommendations have been published from the International Society on Thrombosis and Haemostasis (ISTH) ( ) and others ( ) for the standardization of light-transmission aggregometry. In instruments equipped with a second channel for monitoring secretion, the release of ATP from platelet-dense granules is simultaneously measured ( Fig. 41.3 ). This is accomplished by adding the firefly luminescence substrate and enzyme, luciferin and luciferase, to the PRP. Released ATP then functions as a cofactor in the light-producing luciferin-luciferase reaction, and light emission is recorded with a second photodetector. Thus, aggregation and secretion can be monitored simultaneously and independently ( ). In most cases, the release of ATP may be assumed to reflect the release of the other dense granule constituents, which are less easily measured (i.e., ADP, serotonin, calcium, polyphosphates). Direct biochemical measurements of serotonin, ATP, and ADP can also be performed ( ). Platelet-dense granule secretion may be studied in parallel with aggregation by labeling platelets with ¹⁴ C-serotonin and then measuring the release of radioactivity following platelet activation ( ). On platelet activation, contents of the α-granules, dense granules, and the lysosomal granules are released. These can monitored by different methods ( ).

Platelets have surface receptors for several agonists (see Fig. 41.2 ). Frequently employed agonists in the laboratory include collagen, epinephrine, ADP, U46619 (a TXA2 analog), AA, ristocetin, PAR-1 agonist (SFLLRN), and the calcium ionophore A23187. Platelet responses to these stimuli, with the exception of ristocetin, are dependent on active metabolic processes. In contrast, ristocetin-induced clumping of platelets occurs to a large degree even in the presence of metabolic inhibitors and in formalin-fixed platelets; therefore, it is often referred as agglutination .

Although of paramount importance in vivo, thrombin is difficult to employ as an agonist with PRP because of interference from fibrin formation. The partially trypsinized γ-thrombin, however, retains platelet-stimulating activity but largely lacks clotting activity and can be useful. Thrombin cleaves the G-protein–coupled transmembrane platelet PAR1 receptor. Thus, thrombin receptor–activating peptide (TRAP), consisting of amino acid sequence SFLLRN, derived from the extracellular “tethered ligand” region of the thrombin receptor, activates platelets and is useful in platelet function testing. Strong agonists such as collagen and thrombin are capable of directly inducing secretion and TXA2 synthesis in addition to inducing aggregation. In contrast, secretion and TXA2 synthesis following platelet stimulation with weaker agonists such as ADP or epinephrine may be considered secondary responses following attainment of a threshold level of aggregation ( ). The potential diagnostic yields and limitations of aggregation and secretion studies in patients have been reviewed ( ; ).

Other methods have also been developed to test platelet function ( ). Electrical impedance measurements, as opposed to monitoring light transmission, have been used to study platelet aggregation not only in PRP but also in whole blood ( ). The resulting curves of electrical impedance over time share many similarities to those of light transmittance recordings, although there are characteristic differences between these two approaches ( ; ). When impedance aggregometry is combined with ATP secretion measurement on whole-blood samples ( ), relatively rapid evaluation of platelet function, requiring only a small volume of blood, may be performed ( ). Additionally, platelet aggregation in whole blood may be monitored optically by means of the coaggregation of fibrinogen-coated beads ( Verify Now ) impregnated with a dye that absorbs light in the infrared region of the electromagnetic spectrum ( ). This approach has been applied for assessing the effect of treatment with platelet inhibitors (GPIIb-IIIa inhibitors, aspirin, and clopidogrel).

The contractile abilities of activated platelets also result in contraction (or “retraction”) of formed clots. In the test tube, clot retraction may be quantitatively assessed. In thrombocytopenia or Glanzmann thrombasthenia, clot retraction is delayed or incomplete. GPIIb-IIIa ( ) as well as a number of additional platelet cytoskeletal and contractile proteins ( ) are required for effective clot retraction.

Evaluation of Platelet Function Using Flow Cytometry

Flow cytometry has proved useful for assessing the expression of specific platelet surface glycoprotein receptors and for assessing platelet activation, including ability of GPIIb-IIIa to undergo conformational change, and expression of markers of platelet secretion (e.g., P-selectin from α-granules; Fig. 41.4 ) ( ). Flow cytometry has been used to assess platelet expression of annexin V, dense granules, and release of microparticles. Due to the small volumes of blood required, flow cytometry can be particularly valuable in diagnostic evaluations of children and neonates. Recently, mass cytometry using multiple metal conjugated antibodies has been applied to document heterogeneity of normal platelets in circulation and novel alterations in platelet glycoproteins in Glanzmann thrombasthenia ( ).

High-Throughput DNA Sequencing for the Diagnosis of Inherited Platelet Disorders

The advent of high-throughput DNA sequencing (HTS) is a major advance in the diagnostic approach to patients with inherited platelet disorders ( ; ; ; ). This has transformed the diagnostic landscape from a phenotype-based, laboratory-based, and candidate-gene approach to one that is genotype based. It has provided new insights into genotype-phenotype correlations, discovery of novel genes causing inherited platelet disorders, and the unraveling of unknown molecular pathways regulating megakaryocyte-platelet biology. The application of HTS to patients by different groups has been based on targeted gene panels and whole-exome sequencing (WES) ( ; ; ; ; , ), with whole-genome sequencing being tested. Overall, these approaches provide a genetic diagnosis in about 40% to 50% of patients with inherited platelet disorders, combining those with defects in platelet number and function ( ; ). However, in a large fraction of such patients, the genetic diagnosis remains elusive despite state-of-the art studies. They may have abnormalities in unknown genes or in noncoding regions of the genome. Moreover, a substantial number of variants detected in known genes remain as variants of unknown significance. The advantages of a genetic diagnosis are clear and include a predictive value beyond bleeding (such as predisposition to leukemia or renal failure). As of now, access to these HTS approaches and the required expertise is limiting.

Thrombocytopenia

Decreased platelet production and increased platelet destruction ( Table 41.2 ) constitute two of the major causes of thrombocytopenia. Additionally, thrombocytopenia may be seen with splenomegaly of any cause because of increased splenic sequestration of platelets.

Inherited Thrombocytopenias

Inherited thrombocytopenias are being recognized with increasing frequency; several distinct genetic abnormalities have been documented in such patients ( ; , ; ; ; ) ( Fig. 41.5 ). These patients may present with isolated thrombocytopenia or in conjunction with characteristic syndromes. In some patients, the platelets have defects in function as well. These entities are discussed further in the section on Inherited Disorders of Platelet Function.

From a mechanistic perspective, inherited thrombocytopenias are associated with mutations in genes with diverse function. However, for some, the specific function in megakaryocytes/platelets is unclear. In the last 2 decades, there has been an explosive increase in our knowledge of gene mutations linked to inherited thrombocytopenias; over 20 genes are implicated ( ; ,; ; ) (see Fig. 41.5 ). These genes include transcription factors ( RUNX1 , FLI1 , GATA1, GFI1b , HOXA11 , ETV6 , MECOM/EVI1 , IKZF5 ), thrombopoietin receptor c- MpL , cytoskeletal proteins and related signaling ( MYH9 , TUBB1 , ACTN1 , FLNA , WASP , DIAPH1 , PTPRJ , SRC ), surface membrane glycoproteins and related signaling (GPIBA , GPIBB , GPIX , ITGA2B , ITGB3 ), proteins involved in vesicle transport ( NBEAL2 ), exon-junction complex involved in RNA processing ( RBM8A ), and cAMP-dependent protein kinase ( PRKACG ). Mutations in the ANKRD26 5′ untranslated region result in loss of RUNX1 and FLI1 binding, and ANKRD26 silencing ( ; ; ; ). The application of high-throughput sequencing permits identification of the gene abnormality in ∼50% of patients with inherited thrombocytopenias.

A better understanding of inherited thrombocytopenias is important beyond the predisposition to bleeding, which is variable and often mild. First, many patients are initially recognized in their adulthood ( ). In the absence of a family history, there is the risk for misdiagnosis as immune thrombocytopenia and the potential for unnecessary therapy, such as with steroids or splenectomy. Second, some gene mutations have prognostic implications, such as the predisposition to myeloid or lymphoid malignancies with RUNX1 , ANKRD26 , and ETV6 mutations ( ; ); or worsening renal function or hearing loss, as with MYH9 mutations . Further, there may be therapeutic implications, as in the potential role of eltrombopag in patients with MYH9 mutations ( ). Moreover, some reports have documented recurrence of leukemia following hematopoietic stem cell transplantation of the patient from an undiagnosed sibling donor with an unrecognized RUNX1 mutation ( ; ). Lastly, several entities are associated with platelet function abnormalities as well, as in patients with RUNX1 and other transcription factor mutations ( ) (see later discussion).

From a clinical perspective, patients with inherited thrombocytopenias may be characterized based on inheritance patterns or platelet size ( Table 41.3 ) or diameter ( , ; ; , ).

TABLE 41.3

Classification of Inherited Thrombocytopenias by Platelet Size

Modified with permission from Kumar R, Kahr WHA: Inherited thrombocytopenia: Clinical manifestations, laboratory abnormalities, and molecular defects of a heterogeneous group of conditions. In Rao AK, editor: Hematology/oncology Clinics of North America: disorders of the platelets (vol 27), Philadelphia, 2013, Elsevier, pp 465–494.

Small Platelets (MPV <7 fL)	Normal-Sized Platelets (MPV 7–11 fL)	Large Platelets (MPV>11 fL)
Wiskott-Aldrich syndrome	Inherited amegakaryocytic thrombocytopenia	MYH9- related disorders
X-linked thrombocytopenia	Thrombocytopenia absent radius syndrome	Bernard-Soulier syndrome
FYB -related thrombocytopenia	Radioulnar synostosis with amegakaryocytic thrombocytopenia	ITGA2B/ITGB3 -related thrombocytopenia
PTPRJ -related thrombocytopenia	RUNX1 mutations (FPD/AML)	Gray platelet syndrome
	Jacobsen/Paris-Trousseau syndrome	Velocardiofacial syndrome
	ANKRD26- related thrombocytopenia	GATA1 –related thrombocytopenia
	ETV6 -related thrombocytopenia IKZF5 -related thrombocytopenia	Type 2B von Willebrand disease/Platelet-type von Willebrand disease
	CYCS -related thrombocytopenia	ACTN1 -related thrombocytopenia
		TUBB1 -related macrothrombocytopenia
		FLNA -related macrothrombocytopenia
		DIAPH1 -related macrothrombocytopenia
		PRKACG -related macrothrombocytopenia
		GNE -related macrothrombocytopenia
		SLFN14 -related macrothrombocytopenia
		TPM4 -related macrothrombocytopenia
		GALE -related macrothrombocytopenia
		Thrombocytopenia associated with sitosterolemia

FPD/AML, Familial platelet disorder with predisposition to acute myeloid leukemia.

Autosomal-Dominant Thrombocytopenias

The best recognized and most common of the inherited thrombocytopenias are the diverse group of autosomal-dominant syndromes, referred to as MYH9-related disorder ( MYH9-RD ) ( ; ). These include the May-Hegglin anomaly, Fechtner syndrome, Epstein syndrome, and Sebastian syndrome and share features of increased platelet size, cytoplasmic inclusions in leukocytes (Döhle bodies), and premature release of platelets. They arise from mutations in MYH9 encoding the nonmuscle myosin heavy chain IIA, a contractile cytoskeletal protein. All MYH9 syndromes have macrothrombocytopenia; other features such as nephritis, sensorineural deafness, and cataracts serve to distinguish them ( ; ). In patients having familial platelet disorder with predisposition to acute myeloid leukemia (FPD/AML), autosomal-dominant thrombocytopenia is secondary to mutations in the RUNX1 gene ( ; ). In these patients, platelet size is normal and their function is abnormal ( , ; ) (see later discussion). Patients with platelet-type von Willebrand disease (VWD) are characterized by thrombocytopenia, gain-of-function mutations in GPIBA , and enhanced responsiveness to ristocetin on platelet aggregation ( ). Other thrombocytopenias inherited in an autosomal-dominant manner include velocardiofacial syndrome and DiGeorge syndrome, which arise due to deletions within chromosome 22q11 and are associated with cardiac abnormalities, parathyroid and thymus insufficiencies, cognitive impairment, and facial dysmorphology (velocardiofacial only); and the Paris-Trousseau/Jacobsen syndrome, which is characterized by psychomotor retardation and facial and cardiac abnormalities, and arises due to deletion of a portion of chromosome 11,11q23-24, which encompasses the transcription factor FLI1 gene. Platelets in the latter disorder are increased in size and have giant α-granules. A dominant form of macrothrombocytopenia (Mediterranean macrothrombocytopenia, Bolzano variant) has been described in southern Europe; this has been associated with mutations in GPIbα and considered a heterozygous form of Bernard-Soulier syndrome (BSS) ( ). Autosomal-dominant BSS resulting from a point mutation in a leucine tandem repeat of GPIbα has been described in the American population ( ). Autosomal-dominant thrombocytopenias have been reported in association with mutations in the genes encoding cytoskeletal protein β-1 tubulin TUBB1 ( TUBB1 ) and α actinin-1 ( ACTN1 ), cytochrome c ( CYCS ), ANKRD26 , and PRKACG ( ; , ; ; ). ANKRD26 mutations are associated with increased risk of hematologic myeloid malignancies ( ). Autosomal-dominant inherited macrothrombocytopenia has been linked to DIAPH1 , which, like MYH9-RD, is associated with hearing loss ( ), and to mutations in genes encoding GPIIb (ITGA2B) or GPIIIa (ITGB3) ; these lead to constitutive activation of the GPIIb-IIIa integrin complex ( , ). Other gene mutations that have been linked to autosomal-dominant inherited thrombocytopenia include SFLN4 , TPM4 , and SRC ( ). SRC mutations have also been associated with α-granule deficiency and myelofibrosis (MF) ( ). Most recently, mutations in another TF, IKZF5, have also been linked to autosomal-dominant thrombocytopenia ( ).

Autosomal-Recessive Thrombocytopenias

In classical BSS, macrothrombocytopenia results from biallelic mutations involving the GPIb-IX-V complex (see later discussion). Congenital amegakaryocytic thrombocytopenia (CAMT) is an autosomal-recessive disorder associated with mutations in the thrombopoietin receptor MPL . It is characterized by severe thrombocytopenia and absence of megakaryocytes in the bone marrow. Other causes of recessive thrombocytopenia include the gray platelet syndrome (see later discussion) linked to mutations in the NBEAL2 gene, which encodes a BEACH protein involved in vesicular trafficking ( ; ; ). Thrombocytopenia with absent radii (TAR) syndrome is associated with skeletal abnormalities and inherited in an autosomal-recessive manner. Other genes that cause an autosomal-recessive thrombocytopenia involve mutations in FYB , GNE , and GALE , each with distinct features or mechanisms. Small platelets are present in patients with FYB mutation, GNE mutations increase platelet clearance, and mutations in GALE have effects on galactose metabolism and glycosylation ( ). Recently, biallelic loss-of-function variants in PTPRJ , which encodes for a receptor-like protein tyrosine phosphatase, have also been associated with microthrombocytopenia, with spontaneous bleeding and impaired platelet responses to collagen ( ).

Sex-Linked Inherited Thrombocytopenias

Patients with Wiskott-Aldrich syndrome (WAS) and the related X-linked thrombocytopenia have mutations in the WAS gene ( ; ). These patients characteristically have small platelets and the thrombocytopenia may occur in the absence of other immunologic features of the syndrome. Mutations in transcription factor GATA1 are associated with thrombocytopenia, anemia, and alteration in red cell morphology ( ). Mutations in the FLNA gene, which encodes filamin A, are associated with the X-linked dominant syndromes encompassing periventricular nodular heterotopia and otopalatodigital spectrum of disorders ( ); they have macrothrombocytopenia. The FLNA mutation has been also identified as a cause of nonsyndromic isolated thrombocytopenia.

Thrombocytosis

In most patients, thrombocytosis is acquired. In the rare patients with inherited thrombocytosis, this results from gain-of-function mutations in either thrombopoietin or its receptor (MPL) ( ; ). Acquired thrombocytosis may be primary, related to autonomous clonal bone marrow disorders, such as the myeloproliferative neoplasms (MPNs) and myelodysplastic syndromes, or reactive and secondary to conditions such as infections, chronic inflammation, malignancy, hemolysis, iron-deficiency anemia, or splenectomy ( Box 41.1 ) ( ; ; ; ) (see later discussion). The blood film should be reviewed to confirm the increase in platelets and exclude spurious thrombocytosis due to red cell abnormalities (microspherocytes and fragments). A number of clinical and laboratory findings may provide clues as to whether one is dealing with an autonomous clonal process or a reactive process ( Table 41.5 ). This distinction is important because of the implications regarding the potential for thrombohemorrhagic complications and disease transformation in MPN, and regarding treatment strategies (cytoreduction). The platelet counts do not provide a reliable clue in this distinction. Extreme thrombocytosis (platelet counts greater than 1000 × 10 ⁹ /L), may occur in patients with either process; surgical complications and hematologic malignancies were the two dominant causes in one series with a substantial difference in etiology between inpatients and outpatients ( ). Patients with MPN may have abnormalities in platelet and megakaryocyte structure and platelet function (see later discussion). The presence of cytogenetic abnormalities, such as in JAK2V617F and calreticulin, are strongly associated with primary thrombocytosis ( ; ; ; ). Lastly, studies utilizing platelet transcript profiling of patients with thrombocytosis have shown that specific biomarker gene subsets may predict their thrombocytosis class ( ; ).

Patients with MPN may develop bleeding or thrombotic events. Unless additional associated risk factors, such as malignancy, are present, thrombotic events are unlikely in reactive thrombocytosis ( ). However, in one study ( ), reactive thrombocytosis occurred in about 10% of patients in the recovery phase after an admission to the intensive care unit and was associated with an increase in venous thromboembolism.

Overview

Box 41.2 provides a list of inherited platelet disorders associated with impaired platelet function ( Fig. 41.6 ). Figure 41.5 shows the genes underlying inherited platelet disorders, including those of function and number. Although rare, these disorders provide enormous insights into platelet physiology. In most patients with inherited abnormalities of platelet aggregation and secretion, the underlying molecular-genetic mechanisms remain unknown. Not all disorders resulting in an impaired platelet role in hemostasis are due to a defect in the platelets per se. Examples are patients with VWD and afibrinogenemia, with deficiencies of specific plasma proteins essential for normal platelet function. In VWD, platelet adhesion to subendothelium is abnormal, which is also the case in patients with BSS, in whom platelets are deficient in the GPIb-IX-V complex, the receptor for VWF on platelets (see Fig. 41.6 ). Binding of fibrinogen to the GPIIb-IIIa complex is a prerequisite for platelet aggregation. Thus, platelet aggregation on activation is impaired in both inherited afibrinogenemia and in Glanzmann thrombasthenia, in which platelets are deficient in platelet membrane GPIIb-IIIa complex. Platelet aggregation and secretion are the end result of numerous processes that follow the initial interaction of an agonist with the platelet (see Figs. 41.2 and 41.6 ). Thus, abnormalities in many platelet events may lead to a decrease in these responses. Patients with defects in “platelet secretion and signal transduction” are a heterogeneous group considered together for convenience rather than based on an understanding of the molecular abnormality. The common characteristics in these patients are abnormal aggregation responses and an inability to release dense granule contents upon activation of platelet-rich plasma with agonists such as ADP, epinephrine, and collagen ( ). In aggregation studies, the second wave of aggregation is blunted or absent. The platelet dysfunction in such patients arises from diverse mechanisms. A small proportion of these patients have a deficiency of dense granule stores (storage pool deficiency). In some of the others, impaired platelet function arises due to aberrations in signal transduction events that lead to secretion and aggregation. Another group consists of patients who have an abnormality in platelet interactions with coagulation proteins; the best described is Scott syndrome, characterized by impaired expression of surface PS on activation ( , ; ). Defects related to platelet cytoskeleton or structural proteins may also be associated with platelet dysfunction, as in WAS or in patients with abnormalities related to kindlin-3. Several reports document impaired platelet function associated with mutations in transcription factors (RUNX1, GATA1, FLI1, GFI1b, and others) that regulate gene expression in megakaryocytes and platelets. As a group, they may be more common than appreciated. In addition to the patients described earlier, there are patients who have abnormal platelet function associated with systemic disorders or are syndromic, such as Hermansky-Pudlak syndrome or Chédiak-Higashi syndrome ( ; ). These and other entities are described in the following sections. The gene symbols, where relevant, are shown in italics.

Abnormalities of Glycoprotein Adhesion Receptors