Platelet Function and Its Defects

Question 1

How are platelets made?

Answer 1

• from megakaryocytic in bone marrow
• undergo endomitosis-repeated cycles of DNA replication without cell division
• Each meg forms 10-20 proplatelets, and each pro platelet gives rise to 1000-2000 mature platelets

Question 2

What are the contents of a platelet?

Answer 2

Granules
-alpha granules: larger, more
includes: adhesive proteins e.g. fibrinogen, growth factors, coagulation factors
-Dense granules: smaller, fewer
includes platelet agonists i.e. ATP, ADP, Serotonin, Ca2+
-lysosomal granules- minor role
Open Cannicular system (OCS)
-allows internal pathway for release of granules
-increases platelet surface when activated
Cytoplasmic proteins
-COX1 mediates production of thromboxane A2
-cytoskeletal proteins
-Calcium, signaling enzymes

Question 3

In general, what are the three As of platelet function?

Answer 3

Adhesion
Activation
Aggregation

Question 4

What are the steps in platelet adhesion?

Answer 4

1. Endothelial injury–>release of vWF
   - vWf adheres to subendothelial colllagen
   - vWf binds to platelet GP Ib/V/IX
2. Stable binding of platelet GP VI to collagen
   - high shear stress in arteries–>conformation change in vWf, leads to better binding to platelet
3. Platelet expresses GP Ia/IIa, which stabilizes adhesion to exposed collagen

Question 5

Describe von Willebrand Factor. Where is it made, stored, and how is it broken down?

Answer 5

• made in endothelial cells mainly, some in megakaryocytes
• Stored in Weibel Palade bodies in endothelium and in platelets as large sticky multimers
• Broken down and cleaved by ADAMTS13 into smaller less active forms in circulation
• Its binding domains: GP1b/V/IX, collagen, factor VIII, GPIIB/IIIa, ADAMTS13

Question 6

What are agonists of platelet activation? What is activation suppressed by?

Answer 6

Agonists:
-collagen binding to GP VI
-vWf binding to Gp1b/v/IX (platelet)
-thrombin (activated by a developing clot)
-ADP
-Epinephrine
Suppressed by:
-Endothelial NO
-Endothelial Prostacyclin
-CD39 (breaks down ADP)

Question 7

What are the results of platelet activation?

Answer 7

ADP release
Platelet shape change
Expression of GP IIb/IIIa (increase aggregation)
Thromboxane A2 production and release
Inhibition of cAMP (platelet antagonist)

Question 8

How does platelet aggregation occur?

Answer 8

Activated platelets will recruit and activate other platelets

• expression of GP IIb/IIIa - binds fibrinogen and crosslinks platelets
• release ADP, TXA2, fibrinogen
• is surface for thrombin generation by coat proteins–>clot propagation
• phospholipid moved from inner to outer membrane by scramblase–> micro vesicles that are procoag–>thrombin generation

Question 9

List the disease of platelet adhesion defects.

Answer 9

1. Defective vWF
2. BSS Bernard Soulier Syndrome-absent GP Ib/V/IX
3. Platelet type vWD -defect in GP Ib/V/IX enhanced binding and clearance of vWf
4. Defective collagen or GP VI

Question 10

What laboratory tests can you use to detect platelet adhesion defects?

Answer 10

1. Bleeding Time, PFA-100
2. Ristocetin Cofactor Assay -enhances vWF interaction with Plt GP Ib/V/IX - detects problems with vWf
3. Collagen Binding Assay - measures interaction between vWf and collagen

Question 11

What are the challenges in diagnosing Von Willebrand’s Disease?

Answer 11

• vWf levels increased by infection, pregnancy, or estrogen intake
• vWf levels decreased in Type O blood groups
• variable clinical expression (bleeding of mucus membranes depends on subtype and heterogeneity)
• bleeding time and PFA-100 lack high sensitivity and specificity

Question 12

What are the subtypes of von Willebrand’s disease?

Answer 12

Type 1: partial deficiency of vWF
Type 3: complete deficiency of vWF
Type 2: qualitative variants
A: can only make short chains, no high molecular weight multimers
B: same as 2A with increased affinity for plt 1b, slight thrombocytopenia bc binds pot
M: not completely understood
N: decreased affinity for factor VIII, ristocetin factor low, factor VIII low, normal vWF level

Question 13

What are tests for von Willebrand’s disease?

Answer 13

1. vWf: Ag -amt of antigen (normal 50-150IU/dL), low levels of platelets and plasma, increases amt of ristocetin

Question 14

How id von Willebrand’s disease difference from hemophilia A?

Answer 14

Hemophilia A

• clinical expression: severe, joint bleeding, deep tissue bleeding. SPONTANEOUS bleeding
• inheritance: sex-linked recessive
• bleeding time: normal
• plt agglutination with ristocetin: normal
• factor VIII clotting: decreased and constant
• vWF: Ag: normal
• multimer analysis: normal

von Willebrand’s disease

• clinical expression:mild, mucous membrane bleeding, joint bleeding rare
• inheritance: autosomal dominant usually
• bleeding time: prolonged
• plt agglutination with ristocetin: abnormal
• factor VIII clotting: decreased and variable
• vWF: Ag: decreased
• multimer analysis: decreased

Question 15

What are treatment options for vWD?

Answer 15

1. desmopression (DDAVP) -stimulates endothelial release of vWF
   - response of subsequent doses decreases
   - can cause hyponatremia b/c is natural antidiuretic, if given with lots of fluid like in a surgery
2. Cryoprecipitate -donor plasma w/ VIII and vWf (not recommended)
3. Purified vWf:VIII from donors- humane P, alphanate.
   - can be used for surgical prophylaxis and active bleeding
4. Anti fibrinolytic agents to help control mucosal bleeding
5. hormones or nasal DDAVP for menorrhagia

Question 16

Describe Bernard Soulier Syndrome.

Answer 16

• inherited autosomal recessive disorder
• deficiency in Plt GP Ib/V/IX
• smear: mild to moderate thrombocytopenia, large platelets
• RIPA is low

Question 17

List the defects associated with platelet activation.

Answer 17

1. storage pool defects
   a. gray platelet syndrome-Autosomal recessive deficiency of alpha granules-
   b. quebec platelet disorder-Autosomal dominant (but rare) proteolysis of alpha granules due to excess platelet plasminogen activator
2. Inherited platelet receptor defects for ADP, collagen, TXA2, epinephrine
3. Inherited defects in cyclooxygenase or arachidonic acid (can’t make TXA2)
4. Inherited defects in intracellular signalling

Question 18

Describe Glanzmann’s thrombasthenia. How is it diagnosed?

Answer 18

-autosomal recessive defect in platelet GP IIb/IIIa
-normal plt count, iron def anemia
CLINICAL
Excessive bleeding extremely variable even within same individual and same family (same genotype)
Moderate to severe spontaneous mucous membrane and cutaneous bleeding
Menorrhagia common
Severe bleeding after trauma or surgery
Severe postpartum bleeding
LAB TESTING
-prolonged BT, absence of aggregation to any agonist except ristocetin
-diagnosis: confirmed by flow cytometry with antibodies to GP IIb/IIIa

Question 19

What are treatments for Glanzmann’s thrombasthenia?

Answer 19

Hormonal treatment in women
Platelet transfusions after trauma or before surgery
Fibrinolytic inhibitors for mucous membrane bleeding
Recombinant factor VIIa

Question 20

What are acquired platelet function defects?

Answer 20

Medications:
-aspirin inhibiting COX, preventing formation of TXA2
-Clopidogrel, Dipyridamole, stop activation
-IIb/IIIa antagonist stops aggregation
Renal Failure: Uremia inhibits platelet function
Paraproteinemia:
-Waldenstrom’s IgM interfere with platelet interactions with fibrinogen/vWf
Cardiopulmonary Bypass:
-Platelets activated in the pump, dysfunctional thereafter
Acquired vonWillebrand’s of Aortic valve disease: -Consumption of HMW multimers (usually 2A)
-Pulls platelets out of circulation–>bleeding