platelet function defects

8/8/2019 Platelet Function Defects

http://slidepdf.com/reader/full/platelet-function-defects 1/10

ORIGINAL ARTICLE

Platelet function defectsD. SIMON,* T. KUNICKI and D. NUGENT**From the Children Õs Hospital of Orange County, Orange; and The Scripps Research Institute, La Jolla, CA, USA

Summary. Inherited defects of platelet function area heterogeneous group of disorders that can resultin bleeding symptoms ranging from mild bruisingto severe mucocutaneous haemorrhage. Thesedefects may be classied according to their effecton the various steps of platelet microthrombiformation including initiation, extension and cohe-sion, or based on their particular structural orfunctional deciency. Platelet membrane receptordeciencies result in the rare, but well-character-ized syndromes of defective clot initiation, such asBernard–Soulier Syndrome. Platelet storage pooldefects are the most common disorders affectingthe extension phase of clot formation. Glanzmannthrombasthenia, with absent or dysfunctional

a IIbb3 receptor is the prototypical defect of thecohesion/aggregation phase of microthrombi for-mation. Many of these disorders share commontreatments although some therapies will havegreater efcacy for one patient than another andshould be individualized so as to provide optimalcontrol of symptoms. Currently much effort isbeing put into methods to more rapidly andaccurately diagnose patients with platelet disordersand to initiate appropriate therapy and prevent lifethreatening bleeding.

Keywords : bleeding disorders, clot formation, plate-let function, platelet granules, platelet membranereceptors, platelets

Introduction: normal platelet function

The formation of a stable platelet plug hinges onthe ability of the platelet to interact with thedamaged vascular bed and recruitment of othercells in the process of haemostasis and repair. Anydefect in this process can cause bleeding symptoms,ranging from clinically insignicant to severe.Platelet defects can be classied by their locationin the three phases of clot formation: initiation,extension, and cohesion or aggregation or based ontheir particular structural or functional deciency(Schema 1). Although it would be impractical tosummarize all known platelet qualitative defects,

some of the well-characterized and clinically sig-nicant syndromes will be discussed here with afocus on presentation, diagnosis and treatment.Additional excellent and more detailed articles oninherited platelet disorders, including congenital

thrombocytopenia, which are not covered here,have been published in the last few years and arehighly recommended to the reader [1,2].

Adhesion

The rst step in the initiation phase of thrombusformation involves plasma von Willebrand factor(VWF) binding to exposed collagen on thesubendothelium via its VWF-A1 domain whilesimultaneously binding to the platelet membraneglycoprotein (GP) Ib-IX-V complex (Fig. 1a).Platelets also bind directly to collagen via themembrane GPVI receptor complex and the integrin

a 2b1 collagen receptor [3,4]. This process results in astable layer of platelets to facilitate the formation of the platelet plug. Bernard–Soulier syndrome (BSS) ischaracterized by the absence of the platelet mem-brane GPIb complex and thus is the prototypicalmutation that affects initiation because this defectprevents normal adhesion to the VWF-A1 domain[5]. The normal engagement of these receptors andthe formation of the platelet monolayer enhancesplatelet activation and signals the onset of the nextstage of microthrombus formation.

Correspondence: Diane Nugent, MD, Children Õs Hospital of Orange County, 455 S Main St, Orange, CA 92868, USA.Tel.: +714 532 8744; fax: +714 532 8771;e-mail: [email protected]

Accepted after revision 18 August 2008

Haemophilia (2008), 14 , 1240–1249 DOI: 10.1111/j.1365-2516.2008.01898.x

Ó 2008 The Authors1240 Journal compilation Ó 2008 Blackwell Publishing Ltd



Extension

During this phase, adherent platelets become acti-vated and secrete stored compounds from theira -granules and d-granules (Fig. 1b). This stimulatescirculating platelets to activate and release, whichgreatly enhances the propagation of the platelet plug.A key compound includes adenosine diphosphate

(ADP), which helps to further activate plateletsthrough binding to the respective ADP receptors onneighbouring platelets. During the course of thisprocess, the activated platelet produces and/orreleases additional agonists, including thromboxaneA2 (TXA2). This is also the likely stage at which theactivated platelet surface prothrombinase catalyzesthe conversion of prothrombin providing a platformfor brin clot propagation [6]. Numerous plateletdefects that involve the extension phase of clotformation have been previously described [7]. Thus

far, storage pool defects appear to be the mostcommon of these disorders, but membrane receptorand signal transduction pathway abnormalities arean active area of ongoing research.

Another defect related to the extension phase of clotformation is the rare autosomal recessive disorderScott Syndrome [8]. Patients affected by Scott Syn-drome ultimately have impaired platelet dependant

brin formation leading to a potentially signicantbleeding disorder. Scott syndrome is fundamentally asignal transduction pathway defect. Upon activation,Scott platelets are unable to transport phosphatidyl-serine from the inner to the outer phospholipidmembrane. Consequently, the factor Va–Xa andVIIIa–IXa complexes are unable to bind to themembrane resulting in decreased thrombin generationand subsequently inadequate brin formation. Muta-tions of an ATP-binding cassette transporter A1(ABCA1) are partly responsible for phosphatidylser-

Adhesion(a) (b)

(c)

Blood flow direction

- -

CollagenCollagen VWFVWF

Extension

Initiation of secretionand membraneactivation

GP VIGP VICollagenCollagen

Cohesion/AggregationSecretion, thrombin and prothrombinase activity extends microthrombi formation

Fibrinmonomer

Fibrinogen

α IIbβ3

α IIbβ3

VWF

IIbIIIaIX

Ibβ Ibα

V

α -granule Defects-Gray Platelet Syndrome-Quebec Platelet Syndrome-α -SPD-α,δ -SPD

Dense Granule Defects-Hermansky-Pudlak Syndrome

-Chediak-Higashi Syndrome-Griscelli Syndrome-δ-SPD

Cytoskeletal Defects-Wiskott-Aldrich Syndrome-MYH9 and associatedgiant platelet disorders

GlanzmannThrombasthenia

VIAlteredcollagenbinding

Bernard-Soulier Syndrome

Altered response toagonists: ADP, TXA2,Epinephrine, etc.

Scott Syndrome

α2β1

Most Common Inherited Platelet Defects

Fig. 1. Figs 1a, b and c outline the three phases of platelet plug formation including adhesion (1a), extension (1b) and aggregation/cohesionwith clot formation. Each step requires a coordinated response to ligand and receptor interactions, signaling molecules, membrane

expression of clotting protein components, secretion of granule contents, and cytoskeletal modications. Abnormalities in any of thesecomponents will result in platelet dysfunction, although some maybe much more serious than others, based on the presence or absence of alternative pathways to complete the clot formation.

PLATELET DEFECTS 1241

Ó 2008 The Authors Journal compilation Ó 2008 Blackwell Publishing Ltd Haemophilia (2008), 14 , 1240–1249



ine translocation and have been implicated in thepathogenesis of Scott Syndrome [8,9].

Cohesion/aggregation

The nal step in forming a platelet-rich thrombus is

the cohesion/aggregation phase (Fig. 1c). PlasmaVWF and brinogen bind activated platelets togethervia the platelet integrin a IIbb3 (GPIIb–IIIa) complex.The classic disease state that results in a defectiveconsolidation phase of clot formation is Glanzmannthrombasthenia (GT), which results in decreasedlevels or function of the GPIIb–IIIa complex, leadingto absent or severely reduced platelet aggregation[10].

Thrombocytopenia whether inherited or acquiredwill impact all three phases to varying degrees basedon the severity of the platelet deciency. In a healthyindividual, the platelet count must fall below20 000–30 000 mm ) 3 before signicant mucocuta-neous bleeding will occur. In high turnover states,such as immune mediated thrombocytopenic pur-pura, the platelets are younger overall, slightly largerand more haemostatic, and so the platelet count mayfall to <15 000 mm ) 3 before serious bleeding occurs.Other defects that warrant special consideration inthe hereditary thrombocytopenias are the MYH-9family of giant platelet disorders and Wiskott–Aldrich Syndrome with minute Ôdust-like Õplatelets,both of which are associated with bleeding andstriking morphology on the peripheral smear. Thesesyndromes can be diagnosed with a simple CBC; forthat reason, the importance of reviewing the smear inany evaluation of platelet disorders cannot be overstressed. Inherited thrombocytopenic syndromes arecovered elsewhere in a recent review by Nurden et al.[11].

Incidence, racial/ethnic predilection

Fortunately, the severe forms of congenital plateletdysfunction are extremely rare. While it has beensuggested that some of the disorders may be under

diagnosed, some are known only to occur in ahandful of families worldwide. Therefore, thesedefects are much more common in areas whereconsanguinity is more prevalent or in small, geo-graphically or ethnically isolated communities. In themore common secretory defects, compound hetero-zygote mutations are more frequent. Rarely, thefamily history may suggest an autosomal dominantpattern with mucocutaneous bleeding present inmultiple generations. As with all rare blood diseases,the age of onset and family history is very important

and may aid in the diagnostic work-up and choice of therapy.

General diagnostic evaluation

For the majority of haematologists, the diagnosis of

platelet dysfunction is limited to a general categorybased on the combination of aggregation studies,evaluation of the platelet on smear, and in somecentres, ow cytometry and electron microscopy.Reference centres can provide more specializedstudies in the measurement of ATP/ADP levels,characterization of granules or signalling defects.This remained the state of the art until the mostrecent decade, which brought the promise of moreexact diagnosis based on specic molecular andproteomic screening techniques, thus allowing hae-matologist to provide counselling for patients andfamilies based on their own unique mutations.

Platelet Function Evaluation: In decades past,initial studies used the bleeding time as the main-stay of diagnostic manoeuvres to identify patientswith platelet dysfunction [12]. Milder forms of platelet disorders could be elicited by administrationof aspirin, which resulted in prolongation of thebleeding time, in these patients but not in indi-viduals with normal platelet function. Althoughstill very useful in diagnosis of platelet dysfunction,the bleeding time has fallen out of favour becauseof its unreliability for presurgical screening, partic-ularly in patients with renal disease or with skinchanges associated with medications or collagendefects. As a result, it is no longer widely availableand very few clinical laboratories have technicianswho can perform this assay well, especially onchildren.

Once von Willebrand disease, the most commondiagnosis associated with mucosal bleeding has beenexcluded, the patients referred for evaluation of platelet dysfunction may have a number of studiesperformed either in series or with the initial visit.Importantly, no single assay or technology candiagnose all platelet disorders. Many facilities use

the platelet function analyzer (PFA-100) or plateletthromboelastography in whole blood as an initialscreen for platelet dysfunction prior to surgicalprocedures [13]. Further evaluation is necessary toactually dene the platelet defect and may include;platelet aggregation with various agonists [14],calcium ux or secretion studies, protein biochemis-try, electron microscopy and ow cytometry. Unlikecongenital thrombocytopenia syndromes, such asMYH9 defects or Wiscott Aldrich, there is not yeta molecular screen that could bypass the initial

1242 D. SIMON et al.

Ó 2008 The AuthorsHaemophilia (2008), 14 , 1240–1249 Journal compilation Ó 2008 Blackwell Publishing Ltd



functional or biochemical evaluation in plateletdysfunction.

Not all these assays are required for each diagno-sis, however. Ongoing studies and working groupsare validating, which are most useful for eachdiagnosis, to streamline the diagnostic workup,particularly in children [14,15]. A table summarizingpertinent ndings for the syndromes described in thisclinical review is listed in Table 1. In the future,platelet proteomics or molecular arrays may speedup the process of diagnosis by identifying upfrontwhich protein or DNA sequence is defective orabsent, but functional assays are still required todayand will be for some time.

General approach to treatment

Because the treatment is similar for these syndromes,

a brief review of the common therapeutic regimensused in platelet dysfunction will be discussed hereand additional treatments unique to one particulardisorder will be included under that specic heading.The rst step in treatment and prevention of bleedingis always education of the patient and the family. Allmust be counselled that mucocutaneous bleeding willbe common. Serious haemorrhage can occur in theevent of trauma, surgery, in the gastrointestinal tractand frequently with menses or the postpartumperiod.

In any event, the patients should have a reliableway of contacting their haematologist, or providingtheir surgeons or ER physicians with a treatmentplan in the event prophylaxis is needed or bleedingoccurs. All patients with platelet defects must becounselled on avoiding certain medications thatinterfere with platelet function, such as aspirin andmany non-steroidal anti-inammatory agents orcertain antidepressants [16].

Women with menorrhagia may require hormonalsuppression to prevent menses altogether whileothers have undergone uterine ablation or hysterec-tomy to prevent life threatening bleeding in the mostextreme cases. A comprehensive team approach isnecessary for these patients and should include agynaecologist with a special interest in bleedingdisorders [17,18].

In general, certain non-specic agents have been

used for years to minimize mucosal bleeding or asprophylaxis for minor surgical bleeding. Antibrino-lytic agents such as -aminocaproic acid (Amicar) ortranexamic acid (Cyklokapron) have been used withsome success to decrease mucosal bleeding associatedwith epistaxis, menses or mucosal bleeding afterdental work. In addition, desmopressin acetate(DDAVP) has been shown to be effective in prevent-ing the bleeding in some of the platelet dysfunctionsyndromes [19]. A trial of DDAVP should always beperformed to determine response before its use to

Table 1. Summary of platelet defects.

Diagnosis Phase of clot formation Key diagnostic ndings Thrombocytopenia?

Bernard–Soulier syndrome Initiation Lack of GPIb-IX-VImpaired aggregation to ristocetinGiant platelets

Mild

a -SPD (gray platelet syndrome) Extension Lack of a -granules on EM

Large, gray, agranular platelets

Mild to moderate

d-SPD Extension Lack of d-granules on EMReduced secondary wave of aggregationLow levels of platelet ADP, ATP

None

Chediak–Higashi syndrome Extension Oculocutaneous albinismd-granule defectsProgressive neurological deteriorationCytoplasmic inclusions

None

Hermansky–Pudlak syndrome Extension Oculocutaneous albinismd-granule defectsPulmonary brosis

None

ad -SPD Extension Defects in primary and secondary aggregationLack of a - and d-granules on EM

None to mild

Glanzmann thrombasthenia Consolidation/aggregation Lack of GPIIb–IIIa

Poor aggregation with ADP, collagen, EpiNormal ristocetin induced aggregationAbsent clot retractionMorphologically normal platelets

None

ADP, adenosine diphosphate; ATP, adenosine triphosphate; EM, electron microscopy; GP, glycoprotein; SPD, storage pool disease.





prevent bleeding prophylactically, as DDAVP hasbeen known to trigger brinolysis in certain patients.Many articles have been published recently usingactivated recombinant factor VII (rFVIIa) to slow orarrest bleeding associated with platelet dysfunction[20,21]. Dosages have varied widely, but many

patients have responded to this regimen when othershave failed. Used in combination with anti-brino-lytics, minor bleeding can be controlled in certainpatients. This treatment is often used prior to platelettransfusion to avoid blood product exposure andisoimmunization.

Finally, in life-threatening bleeding, platelet trans-fusions will correct the bleeding defect in most cases,even if only one single-donor apheresis unit is given.Apheresis units (approximately equivalent to sixpooled blood bank units) are strongly recommendedand preferred to minimize multiple donor exposure,which can result in sensitization and a plateletrefractory state. To minimize long-term sensitizationto HLA class I proteins expressed on platelets, theblood products should always be leuko-poor orleuko-depleted. In syndromes with complete absenceof a membrane GP such as GT or BSS, one shouldavoid excessive exposure to normal platelets becauseof the risk of developing an isoantibody to themissing proteins of the receptor complex. Isoimmu-nization appears to be rare in Glanzmann, but therisk of alloimmunization is still a major concern.When it does occur, an isoantibody may be directedagainst a functionally important region of the recep-tor complex, making subsequent transfusion of normal platelets ineffective. These isoantibodies aredistinct from alloantibodies, which are formed tomore subtle differences in the protein sequence, suchas HPA-1 (PlA 1 ), where the membrane protein is stillotherwise fully expressed and functional. Theseantibodies to normal platelets or HLA Class I havethe potential of making the patient completelyrefractory to all future transfusions and, in the caseof platelet directed antibodies, interfering with suc-cessful bone marrow transplantation for thosepatients even with an HLA matched donor.

Bernard–Soulier syndrome

Summary. Bernard–Soulier Syndrome (BSS) is amembrane receptor defect demonstrated to impactthe initiation or adhesion phase of platelet plugformation and was initially described in 1948. Theprominent member of the complex, GPIb, is aheterodimer composed of disulde-bonded GPIb aand GPIb b subunits. GPIb then forms a non-covalentcomplex with two GPIXmolecules. Two of these

trimers then associate non-covalently with one mol-ecule of GPV to form the GPIb-IX-V complex. BSS ischaracterized by the inability of platelets to bind toVWF during the initial steps of platelet adhesionbecause these platelets either lack or have a qualita-tive defect in the platelet membrane glycoprotein

GPIb-IX-V complex [22]. Mutations leading toabsent expression or dysfunction have been uncov-ered in the genes for GPIb and GPIX [22]. BSS isnearly always inherited in an autosomal recessivepattern, with consanguinity a frequent nding. Theplatelets are decreased in number and very large onthe peripheral smear. Of note, some patients withDiGeorge syndrome are missing the GPIb b chain,resulting in large platelets and mild thrombocytope-nia, but there is minimal to no functional defect inthese patients.

Clinical manifestations. Clinically, BSS typically pre-sents in infancy with purpura, epistaxis or gingivalbleeding. The age of onset is related to the severity of the disease. Later symptoms can include menorrha-gia, gastrointestinal or genitourinary bleeding. Trau-ma or surgical procedures can also lead to excessivebleeding. The severity of bleeding symptoms can varygreatly among patients.

Diagnosis. One should start with a careful familyhistory, including consanguinity and bleeding symp-toms. Laboratory ndings include mild thrombo-cytopenia but with much more severe bleeding thanone would expect for the relatively small decrease incount. In type 1 BSS, the patients will also havegiant platelets on peripheral blood smear [23,24].A hallmark of the disease is the failure of platelets toagglutinate in the presence of ristocetin. This willdifferentiate BSS from other rare macrothrombocy-topenic disorders, such as the MYH-9 family of platelet disorders. Lastly, ow cytometry should beperformed with antibodies specic for each compo-nent of the complex (CD 42a-d) to characterize thedecrease in the GPIb-IX-V membrane receptor. Thereis also a variant type BSS, in which the GPIb complex

is dysfunctional with poor or absent binding of VWF,but there is still some complex present on the surfaceof the platelet. Patients with type 2 BSS may havenormal platelet size and number [24]. These patientsare identied by measuring the decreased amount of radiolabelled VWF binding in the presence of rist-ocetin cofactor compared with normal platelets.

Treatment. Signicant bleeding or surgical proce-dures may require platelet transfusions. Given theabsence of the GPIb-IX complex in BSS patients, the





risk of sensitization has to be considered. This canbecome a life-threatening complication becausefuture platelet transfusions may be rendered uselessby antibodies binding to the GPIb-IX-V complex ontransfused platelets. In addition, leucodepleted plate-lets are required to decrease the exposure to HLA

Class I antigens . Desmopressin and rFVIIa can beuseful as well, but platelet transfusion remains themainstay in treatment of severe bleeding. BecauseBSS has a wide clinical spectrum, the prognosis of BSS is related to the severity of an individual Õs diseaseand may change overtime based on hormonal alter-ations and the effects of aging.

Storage pool disease

Summary. Storage pool disease (SPD) is a hetero-geneous group of congenital disorders that have incommon a deciency of granules, or their consti-tuents, that results in a defect in ADP releasefrom activated platelets and abnormal secretion-dependent platelet aggregation [25]. The extensionphase of clot formation and platelet activation ismediated in large part by the release of storedcompounds from platelet granules. The principaltypes of platelet granules are the a -granule and thed-granule (dense body). The a -granule containsnumerous proteins involved in platelet interaction,coagulation factors and proteins important inbrinolysis [26]. The d-granule contains primarilycalcium, adenosine triphosphate, ADP and pyro-phosphate. It is the presence of the high concentra-tion of calcium in the d-granule that gives its denseappearance on electron microscopy and allows it tobe distinguished from the a -granule. Defects thataffect the a -granule are termed a -SPD, those thataffect d-granules are d-SPD and combined defectsare termed ad -SPD [27].

a -Storage pool disease ( a -SPD) or gray platelet syndrome

Summary. First described by Raccuglia in 1971,

Gray platelet syndrome (GPS) is a deciency in thenumber of a -granules and their contents in theplatelet cytoplasm of affected patients. Absence of these granules results in a pale or ÔgreyÕhue on theperipheral smear. GPS is thought to be extremelyrare, with approximately 100 cases worldwide. Theclinical manifestations of bleeding in GPS patientsare because of a lack of these a -granule componentsresulting in a small and fragile platelet plug. GPS isinherited in an autosomal dominant or recessivepattern.

Pathophysiology. Megakaryocytes show defectivea -granule production, with impaired uptake andstorage of endogenously synthesized proteins, such asplatelet factor 4, b-thromboglobulin or VWF, anddefective storage of exogenous proteins, such asbrinogen, albumin or factor V. These are key

components in local platelet interaction and throm-bin generation. It is most widely believed that theprimary molecular defect in GPS occurs during theearliest stages of megakaryocyte maturation andinvolves a defect in the packaging of a -granulecontents. Components of the a -granule contents anda -granule membrane, including P-selectin, have beenfound free in GPS platelet cytoplasm, indicating afailure of packaging [28].

Clinical manifestations. Bleeding symptoms may startfrom infancy, but the disease in GPS patients isusually less severe in general. Easy bruising, pete-chiae, mucosal membrane bleeding and postsurgicalor traumatic bleeding may occur; life threateningspontaneous haemorrhage is rare.

Diagnosis. Peripheral blood Wright–Giemsa smeartypically reveals mild to moderate thrombocytopeniaand large gray agranular platelets. Platelet aggrega-tion studies are variable with no classical responsepattern to ADP, epinephrine, thrombin or collagen.In general, the secretion dependent aggregationstudies are abnormal, but there are some patientswho also show decreased response to thrombin orcollagen [26]. Measurement of platelet adenosinenucleotide (ADP and ATP) content and release areuseful in the diagnosis of storage pool and releasedefects [29]. When available, a primary tool used indiagnosis is electron microscopy, which reveals anear complete absence of a -granule in platelets andmegakaryocytes [30].

d-Storage pool disease ( d-SPD)

Summary. Dense granule storage pool defects wereinitially described in 1972 [31]. The platelets are

morphologically normal on Wright-stained smears,but they are decient in dense bodies by electronmicroscopy. These granules are storage sites forserotonin and the nucleotides ADP and ATP.

Pathophysiology. d-SPD patients lack platelet densegranules resulting in a deciency of ADP, ATP andserotonin, which when released, enhance plateletaggregation. The concentration of ADP correlatesbest with bleeding time and is likely mediated by itseffect on VWF- and brinogen-dependent platelet





aggregation via the GPIIb–IIIa receptor. d-SPD islikely inherited in an autosomal dominant pattern butneither the gene nor the molecular basis is known.

Clinical manifestations. d-SPD patients have a bleed-ing spectrum and severity similar to those with

a -SPD, including easy bruising, epistaxis and post-surgical bleeding.

Diagnosis. The diagnosis of d-SPD is made using acombination of techniques. On peripheral bloodsmear, the platelet number is adequate and plateletsappear normal in structure. A lack of dense granulescan be better documented using transmission elec-tron microscopy or uorescent microscopy usingspecial stains [32,33]. Recently, defects in adhesionunder high shear and generation of the prothrom-binase activity have also been reported in patientswith d-SPD, apparently as a result of decreased ADPsecretion [6]. Platelet aggregation studies typicallyshow a signicantly impaired second wave of aggre-gation when stimulated by ADP, epinephrine orthrombin. The most consistent nding is that adeninenucleotides are reduced with an increased ratio of ATP to ADP and normal levels of lysosomal enzymes[34]. The combination of these ndings with aclinical suspicion leads to the diagnosis of d-SPD[6,32–34].

d-SPD associated disorders

There are several disorders that have platelet densegranule deciency in association with lysosme relatedorganelles as part of their constellation of ndings.Most notable is a melanosomal defect which resultsin a pattern of hypopigmentation. The best knownare Hermansky–Pudlak syndrome (HPS) and relatedGriscelli syndromes, and Chediak–Higashi syndrome(CHS) [35].

HPS is an autosomal recessive disorder character-ized by oculocutaneous albinism, lysosomal granuledefects and platelet dense granule deciency. Thesepatients experience a variety of ocular manifestations

and well-documented pulmonary brosis likely sec-ondary to the lysosomal defects. From a haemato-logical standpoint, these patients have a similarbleeding diathesis as patients with isolated d-SPD.The results of their platelet aggregation studies willbe abnormal in the secondary phase and secretionstudies will be abnormal [6,32,33].

CHS is also characterized by oculocutaneousalbinism and dense granule deciency, but, in addi-tion, features immune deciency and a progressiveneurological deterioration. CHS patients, moreover,

have cytoplasmic inclusions in other cell lines, whichcan be easily seen on the peripheral smear. Thesepatients exhibit the same ndings on platelet aggre-gation studies as in isolated d-SPD, but in theaccelerated phase of the disease, they developthrombocytopenia distinguishing them from milder

disorders. Bleeding symptoms are similar to isolatedd-SPD and can be managed accordingly, but onlyhaematopoietic stem cell transplantation has beenshown to improve the long-term outlook of thisotherwise fatal syndrome [36].

ad -SPD

Briey, combined a and d platelet granule decien-cies are signicantly less common than isolateddefects. In these defects, d-granules are alwaysdecreased, whereas the concentration of a -granulesvaries. These disorders also do not affect the entireplatelet population uniformly because some plateletsmay have signicantly more a and/or d-granules thanother platelets in the same patient. Unlike d-SPD,laboratory testing reveals impaired primary aggrega-tion in addition to secondary aggregation. Clinically,these patients behave much like a - or d-SPD patientsand respond to the same treatments in limitedexperience. Because of the lack of specicity of aggregation studies, the diagnosis also requiresmeasurement of a - and d-granule contents and/orelectron microscopy to conrm the absence of platelet granules [37].

Management. The general treatment guidelines listedabove are useful but vary from patient to patient ineffectiveness. When bleeding occurs, treatment mayinclude topical agents, DDAVP or Stimate nasalspray for responsive patients and antibrinolyticagents. Initial treatment should include regular visitswith healthcare team to be sure that proposedregimens provide hemostasis. In addition, the platelettransfusions may help for planned surgery, particu-larly for the ophthalmologic procedures in CHS, butthis modality has not been studied systematically in

SPD or GPS patients.

Membrane receptor disorders affecting the stages of clot formation

An evolving area of research is the eld of inherited disorders of platelet membrane receptorsaffecting platelet extension and cohesion/aggrega-tion activities. These disorders include the P2Yclass of receptors, which respond to the adeninenucleotides secreted by platelet d-granules, as





well as the TXA2 receptor and the epinephrinereceptor.

The most studied of these receptors is the P2Y12receptor which is responsible for ADP-inducedplatelet aggregation [38]. Defects in P2Y12 areinherited in an autosomal recessive pattern and can

result from a number of different alterations in thegene coding for the receptor. Clinically, thesepatients exhibit a number of the same mild bleedingtendencies as SPD patients and, on laboratoryworkup, are found to have a very weak primaryphase of aggregation when stimulated with ADP andother agonists. Only high concentrations of thrombinproduce normal aggregation studies. These patientsgenerally are treated with DDAVP for prophylaxis orbleeding episodes.

Glanzmann thrombasthenia (GT)

Summary. Recognized rst in 1918, GT has beenextensively studied and denes a defect in theconsolidation phase of thrombus formation. GTpatients have either a qualitative or quantitativedisturbance in the platelet membrane GPIIb–IIIacomplex. GT is a rare disorder and is inherited in anautosomal recessive fashion. Consanguinity has beenidentied in most cases.

Pathophysiology. The GPIIb–IIIa receptor is integralto platelet aggregation because in its activated state,it preferentially binds to brinogen and VWF.Fibrinogen and VWF then cross-link platelets bybinding to the activated GPIIb–IIIa molecule onadjacent platelets. GT platelets are able to adherewith other receptors, such as GPIb-IX-V complex, toexposed subendothelium in damaged capillaries, butare unable to spread effectively without GPIIb–IIIaand cannot form platelet microthrombi because of their impaired aggregation.

Diagnosis. GT patients have normal numbers of platelets that appear morphologically normal onperipheral blood smear. Screening coagulation

studies i.e. Protime and aPTT are unaffected, butbleeding time is prolonged. The typical pattern of platelet aggregation studies shows poor aggregationin response to ADP, collagen, epinephrine andthrombin, but normal aggregation in the presenceof ristocetin. If GT is suspected, ow cytometrycan be performed to conrm the diagnosis bydetermining the quantity of GPIIb–IIIa expressedon the platelet membrane using specic anti-bodies binding to either GPIIIa (CD61) or GPIIb(CD 41).

Molecular basis. GT results from either a qualitativeor quantitative defect of platelet GPIIb–IIIa com-plex. The genes coding for GPIIb and GPIIIa arelocated nearby on the long arm of chromosome 17.More than 100 mutations have been identied thateither inhibit synthesis of the receptor altogether orinterfere with its ability to be processed normally[39,40]. Genetic testing can be performed in specialacademic laboratories to determine the exactlocation of the mutation, but on a research basisonly at this point. (http://sinaicentral.mssm.edu/ intranet/research/glanzmann).

Clinical manifestations. Overall, three types of GThave been recognized based on the level of expressionof GPIIb–IIIa (see Table 2).

Patients with type 1 express <5% of the normalamount, whereas patients with type 2 express 5%to 20%. Heterozygotes express 50% of normal andare typically asymptomatic, whereas patients withtype 2 may have a severe phenotype. On the basis

of these data, it seems that some level between 20%and 50% is needed to prevent bleeding. Unlikemany of the other platelet defects, the bleeding fromGT can be severe and may rarely result in death if not treated appropriately. Bleeding symptomsusually start in early infancy and include epistaxis,oral bleeding and purpura. The onset of menarche isassociated with severe menorrhagia, often requiringtransfusions. Bleeding symptoms in GT patients areusually worse in childhood, particularly epistaxis,but improve with age [40]. Antibodies againstGPIIb–IIIa and HLA Class I have been detected in

multiply transfused patients and cause serious com-plications if the patient becomes refractory toplatelet transfusions.

Management. Localized bleeding can usually betreated with topical thrombin or antibrinolyticagents, but invasive procedures usually requireprophylactic platelet transfusion. Childbirth alsorequires aggressive management with platelettransfusion both during and after delivery [14].HLA-matched platelets should be used if possible.

Table 2. Classication of Glanzmann thrombasthenia.

GlycoproteinIIb–IIIa

Fibrinogenbinding

Clotretraction

Type 1 <5% Absent orseverely decient

Absent

Type 2 10–20% Present Normal or

moderately decientVariant >50% Variable Variable





Recombinant factor VIIa has been used with successin some patients with alloantibodies or in an effort toavoid platelet exposure in the rst place. Allogeneicbone marrow transplantation has been used success-fully in the most severe of cases [41].

Prognosis. Generally with aggressive supportive care,GT patients have a good prognosis. Like many otherplatelet disorders, poor outcomes have most oftenbeen attributed to posttraumatic or unanticipatedpostsurgical bleeding.

Summary

The most severe forms of platelet dysfunction areassociated with deciencies in the platelet membranereceptors, but defects in the number and distributionof secretory granules, signalling or prothrombinasefunction can also cause signicant bleeding. Whereasproper diagnosis of a particular platelet dysfunctionis important to the patient and family for geneticcounselling and prognosis, the current treatmentoptions are fairly limited and generic, aimed attreating the symptoms rather than eliminating theprimary defect. Registries and international muta-tional analysis projects will aid in further character-ization of these syndromes and identify the specicgenes involved. With the further expansion of genetherapy and novel therapeutic agents, newly diag-nosed patients in the future may look forward to anormal life span free of major hemorrhagic events.

Disclosures

The authors stated that they had no interests whichmight be perceived as posing a conict or bias.

References1 Nurden AT. Qualitative disorders of platelets and

megakaryocytes. Thromb Haemost 2005; 3: 1773–82.

2 Nurden P, Nurden AT. Congenital disorders associatedwith platelet dysfunctions. Thromb Haemost 2008; 99:253–63.

3 Caen JP, Rosa JP. Platelet-vessel wall interaction: fromthe bedside to molecules. Thromb Haemost 1995; 74:18–24.

4 Kunicki TJ. Platelet membrane glycoproteins and theirfunction: an overview. Blut 1989; 59 : 30–4.

5 Nurden AT, Dupuis D, Kunicki TJ, Caen JP. Analysisof the glycoprotein and protein composition of Ber-nard–Soulier platelets by single and two-dimensionalsodium dodecyl sulfate-polyacrylamide gel electro-phoresis. J Clin Invest 1981; 67: 1431–40.

6 Weiss HJ, Lages BA. Platelet prothrombinase activityand intracellular calcium responses with storage pooldeciency, glycoprotein IIb–IIIa deciency, or impairedplatelet coagulant activity – a comparison with ScottSyndrome. Blood 1997; 89: 1599–611.

7 Salles II, Feys HB, Iserbyt BF, De Meyer SF,Vanhoorelbeke K, Deckmyn H. Inherited traits affect-ing platelet function. Blood Rev 2008; 22: 155–72.

8 Zwaal RF, Comfurius P, Bevers EM. Scott Syndrome, ableeding disorder caused by defective scrambling of membrane phospholipids. Biochim Biophys Acta 2004;1636 : 119–28.

9 Albrecht C, McVey JH, Elliott JI et al. A novel mis-sense mutation ibn ABCA1 results in altered proteintrafcking and reduced phosphatidlyserine transloca-tion in a patient with Scott Syndrome. Blood 2005;106 : 542–9.

10 George JN, Caen JP, Nurden AT. Glanzmannthrombasthenia: the spectrum of clinical disease. Blood 1990; 75: 1383–95.

11 Nurden AT, Nurden P. Inherited thrombocytopenias.Haematologica 2007; 92: 1158–64.12 Harker LA, Slichter SJ. The bleeding time as a screen-

ing test for evaluation of platelet function. N Engl J Med 1972; 287 : 155–9.

13 Favaloro EJ. Utility of the PFA-100 for assessingbleeding disorders and monitoring therapy: a review of analytical variables, benets and limitations. Haemo- philia 2001; 7: 170–9.

14 Zhou L, Schmaier AH. Platelet aggregation testing inplatelet-rich plasma: description of procedures with theaim to develop standards in the eld. Am J Clin Pathol 2005; 123 : 172–83.

15 Moffat KA, Ledford-Kraemer MR, Nichols WL,Hayward CP. Variability in clinical laboratory practicein testing for disorders of platelet function: results of two surveys of the North American SpecializedCoagulation Laboratory Association. Thromb Hae-most 2005; 93: 549–53.

16 McCloskey DJ, Postolache TT, Vittone BJ et al.Selective serotonin reuptake inhibitors: measurementof effect on platelet function. Transl Res 2008; 151 :168–72.

17 Demers C, Derzko C, David M, Douglas J, Society of Obstetricians and Gynaecologists of Canada. Gynae-cological and obstetric management of women withinherited bleeding disorders. Int J Gynaecol Obstet

2006; 95: 75–87.18 Rodeghiero F. Management of menorrhagia in womenwith inherited bleeding disorders: general principlesand use of desmopressin. Haemophilia 2008; 14(Suppl.1): 21–30.

19 DiMichele DM, Hathaway WE. Use of DDAVP ininherited and acquired platelet dysfunction. Am J Hematol 1990; 33: 39–45.

20 Franchini M, Lippi G, Guidi GC. The use of re-combinant activated factor VII in platelet-associatedbleeding. Hematology 2008; 13: 41–5.





21 Poon MC. The evidence for the use of recombinanthuman activated factor VII in the treatment of bleedingpatients with quantitative and qualitative platelet dis-orders. Transfus Med Rev 2007; 3: 223–36.

22 Nurden AT, Didry D, Rosa JP. Molecular defects of platelets in Bernard–Soulier syndrome. Blood Cells1983; 9: 333–58.

23 Clemetson KJ, Clemetson JM. Molecular abnormalitiesin Glanzmann Õs thrombasthenia, Bernard–Soulier syn-drome, and platelet-type von Willebrand Õs disease.Curr Opin Hematol 1994; 1: 388–93.

24 Kenny D, Jo ´nsson OG, Morateck PA, MontgomeryRR. Naturally occurring mutations in glycoprotein Ibalpha that result in defective ligand binding andsynthesis of a truncated protein. Blood 1998; 92: 175–83.

25 Rao AK. Congenital disorders of platelet function:disorders of signal transduction and secretion. Am J Med Sci 1998; 16 : 69–76.

26 Nurden AT, Nurden P. The gray platelet syndrome:

clinical spectrum of the disease. Blood Rev 2007; 21:21–36.27 Weiss HJ, Witte LD, Kaplan KL et al. Heterogeneity in

storage pool deciency: studies on granule boundsubstances in 18 patients including variants decient ina -granules, platelet factor 4, b-thromboglobulin andplatelet-derived growth factor. Blood 1979; 54: 1296–319.

28 Gebrane-Younes J, Cramer EM, Orcel L et al. Grayplatelet syndrome: dissociation between abnormalsorting in megakaryocyte alpha-granules and normalsorting in Weibel–Palade bodies of endothelial cells. J Clin Invest 1993; 92: 3023–8.

29 Summereld GP, Keenan JP, Brodie NJ, Bellingham AJ.Bioluminescent assay of adenine nucleotides: rapidanalysis of ATP and ADP in red cells and plateletsusing the LKB luminometer. Clin Lab Haematol 1981;3: 257–71.

30 White JG. Use of the electron microscope for diagnosisof platelet disorders. Semin Thromb Hemost 1998; 24:163–8.

31 Nieuwenhuis HK, Akkerman JW, Sixma JJ. Patientswith prolonged bleeding time and normal aggregationtests may have storage pool deciency: studies on onehundred six patients. Blood 1987; 70: 620–3.

32 Rendu F, Nurden AT, Lebret M, Caen JP. Relationshipbetween mepacrine-labelled dense body number,

platelet capacity to accumulate14

C-5-HT and platelet

density in Bernard–Soulier and Hermansky–Pudlaksyndromes. Thromb Haemost 1979; 42: 694–704.

33 Gordon N, Thom J, Cole C, Baker R. Rapid detectionof hereditary and acquired platelet storage pool de-ciency by ow cytometry. Br J Haematol 1995; 89:117–23.

34 Weiss HJ, Lages B, Hoffmann T, Turitto VT. Correc-tion of the platelet adhesion defect in d-storage pooldeciency at elevated hematocrit-possible role of adenosine diphosphate. Blood 1996; 87: 4214–22.

35 Huizing M, Anikster Y, Gahl WA. Hermansky–Pudlaksyndrome and Chediak–Higashi syndrome: disordersof vesical formation and trafcking. Thromb Haemost 2001; 86: 233–45.

36 Eapen M, DeLaat CA, Baker KS et al. Hematopoieticcell transplantation for Chediak–Higashi syndrome.Bone Marrow Transplant 2007; 39: 411–5.

37 Biddle DA, Neto TG, Nguyen AN. Platelet storagepool deciency of alpha and delta granules. ArchPathol Lab Med 2001; 125 : 1125–6.

38 Gachet C. P2 receptors, platelet function and phar-macological implications. Thromb Haemost 2008; 99:466–72.

39 French DL. The molecular genetics of Glanzmannthrombasthenia. Platelets 1998; 9: 5–20.

40 Belluci S, Caen J. Molecular basis of Glanzmannthrombasthenia and current strategies in treatment.Blood 2002; 16: 193–202.

41 Belluci S, Devergie A, Gluckman E et al. Completecorrection of Glanzmann thrombasthenia by allogeneicbone-marrow transplantation. Br J Haematol 1985;59: 635–41.

Links to organizations with interest in theseareas

1. National Hemophilia Foundation: http://www.hemophilia.org

2. International Society on Thrombosis andHaemostasis Scientic and StandardizationCommittee (ISTH/SSC): http://www.med.unc.edu/ isth/ssc_home.htm

3. National Organization for Rare Diseases, Inc.:http://www.rarediseases.org



platelet function defects

Documents