Platelet structure 1 Membrane glycoproteins –IIb-IIIa: integrin, cryptic in resting platelet, after platelet activation binds fibrinogen and other adhesive proteins, necessary for aggregation –Ib-IX-V: binds VWF, necessary for platelet adhesion at high shear rates –Ia-IIa: integrin, binds collagen, mediates adhesion at low shear rates and platelet spreading (also acts as receptor)

Platelet structure 2 Membrane receptors –Thrombin receptors (2): cleaved and activated by thrombin –Thromboxane A2 receptor –ADP receptors (3) –Epinephrine receptor –Serotonin receptor –Cytokine, chemokine receptors –Fc receptor

Platelet structure 3 Granules –Dense granules: small molecules involved in platelet activation (ATP/ADP, serotonin) – Alpha granules: fibrinogen, fibronectin, thrombospondin, P-selectin, plasminogen, alpha-2 antiplasmin, factor V, PF4, PDGF, TGF-alpha and beta, ECGF

Bernard-Soulier syndrome Pathophysiology: –Deficiency of platelet membrane glycoprotein Ib-IX (VWF “receptor”) –Defective platelet adhesion Clinical: Moderate to severe bleeding Inheritance: autosomal recessive Morphology: –Giant platelets –Thrombocytopenia (20-100K) Diagnosis: –No agglutination with ristocetin, decr thrombin response, responses to other agonists intact –Morphology –Decreased GP Ib expression

Bernard-Soulier syndrome

Glanzmann thrombasthenia Pathophysiology: –Deficiency of platelet membrane GPIIb-IIIa –Absent platelet aggregation with all agonists; agglutination by ristocetin intact Clinical: Moderate to severe bleeding Inheritance: autosomal recessive Morphology: normal Diagnosis: –Defective platelet aggregation –Decreased GP IIb-IIIa expression

Gray platelet syndrome Pathophysiology: Empty platelet alpha granules Clinical: Mild bleeding Inheritance: Autosomal dominant or recessive Morphology: –Hypogranular platelets –Giant platelets –Thrombocytopenia (30-100K) –Myelofibrosis in some patients Diagnosis –Variably abnormal platelet aggregation (can be normal) –Abnormal platelet appearance on blood smear –Electron microscopy showing absent alpha granules

Gray platelet syndrome

Giant platelet syndromes associated with MYH9 mutations 1.May-Hegglin anomaly 2.Fechtner syndrome 3.Sebastian syndrome 4.Epstein syndrome All associated with mutations in the non-muscle myosin heavy chain gene MYH9 Thrombocytopenia with giant platelets, but mild bleeding Autosomal dominant inheritance No consistent defects of platelet function detectable in the clinical laboratory Diagnosis usually based on clinical picture, family history, examination of blood smear for neutrophil inclusions

Giant platelet syndromes associated with MYH9 mutations SyndromeNeutrophil inclusions Hereditary nephritis Deafness May- Hegglin YesNo FechtnerYes SebastianYes*No EpsteinNoYes *Neutrophil inclusions have different structure from those in May-Hegglin

Neutrophil inclusions in May-Hegglin anomaly

Neutrophil inclusions in MYH9 giant platelet syndromes May-HegglinSebastian In both cases an oval cytoplasmic inclusion (*) not bounded by a membrane and lacking specific granules is evident (original magnification 7,000). At higher magnification (insets, original magnification 13,400), inclusion bodies in MHA contain clusters of ribosomes oriented along parallel filaments 7?10 nm in diameter, whereas in SBS they are composed of highly dispersed filaments and randomly distributed ribosomes.

Wiskott-Aldrich syndrome Pathophysiology –Mutation in WASP signaling protein –Decreased secretion and aggregation with multiple agonists; defective T-cell function Clinical: –Mild to severe bleeding –Eczema, immunodeficiency Inheritance: X-linked Morphology: –Thrombocytopenia (20-100K) –Small platelets with few granules Diagnosis: Family hx, clinical picture, genetic testing

Wiskott-Aldrich syndrome

Hermansky Pudlak syndrome Chédiak-Higashi syndrome Pathophysiology: –Platelet dense granule deficiency: decreased aggregation & secretion with multiple agonists –Defective pigmentation –Defective lysosomal function in other cells Clinical: –Mild to moderate bleeding –Oculocutaneous albinism (HPS) –Lysosomal storage disorder with ceroid deposition, lung & GI disease (HPS) –Immunodeficiency, lymphomas (CHS) Inheritance: autosomal recessive Morphology –Reduced dense granules –Abnormal neutrophil granules (CHS) Diagnosis: clinical picture, neutrophil inclusions (CHS), genetic testing

Chédiak-Higashi, showing neutrophil inclusions HPS, with oculocutaneous albinism

Hermansky-Pudlak syndrome Disaggregation after primary aggregation with ADP Dense granule deficiencyControl platelet Br J Haematol 2007;138:671

Platelet type von Willebrand disease Pathophysiology: Gain of function mutation in GP Ib, with enhanced binding to VWF and clearance of largest multimers from blood Clinical: Mild to moderate bleeding Inheritance: Autosomal dominant Morphology: Normal, but platelet count often low Diagnosis: Variably low VWF antigen, disproportionately low ristocetin cofactor activity, loss of largest VWF multimers on electrophoresis, enhanced platelet agglutination by low dose ristocetin (indistinguishable from type 2B VWD) Can distinguish from 2B VWD by mixing studies with normal/pt platelets and plasma and low dose ristocetin, or by genetic testing

Von Willebrand multimer analysis