Glanzmann Thrombasthenia is an inherited bleeding disorder, resulting from a defect in platelet aggregation, which is vital for preventing excess bleeding at the site of a wound. People with this disorder have mutations in the gene code for glycoprotein receptors IIb/IIIa that bind to the platelets. The receptors are required for platelet fibrinogen binding, fibrinogen is essential for platelet aggregation and hemostatic plug formation that arrest bleeding at injured blood vessels, and defective platelet fibrinogen binding leads to bleeding tendency. The GT can result from either qualitative or quantitative defects in integrins aIIb and B3. Variable mutation types targeting ITGA2B and ITGB3 genes which encode for integrins aIIb and B3 respectively.

Patients have variable clinical manifestations ranging from mild bleeding to severe, the majority of patients experience early age serious mucocutaneous bleeding, epistaxis is the most common. The confirmed diagnosis occurs by studying platelets aggregation, platelets glycoprotein expression and detection of genetic mutation.^1-3

Molecular basis is important for diagnosis, prediction of carrier individuals and future suspected carriers, also important for prenatal diagnosis and genetic counselling for management and treatment protocol.⁴

Introduction 

Glanzmann's thrombasthenia is a platelets function disorder, inherited in an autosomal recessive manner, patient needs to receive two genetic mutations one from each parent to develop the disease. The platelets fail to aggregate and patients develop bleeding manifestations, such as purpura, and mucocutaneous bleeding such as gum bleeding and epistaxis mostly and excessive menstrual bleeding in females in childbearing age, gastrointestinal bleeding are also occurs, the bleeding can be spontaneous or following minimal trauma, urinary tract bleeding and hemarthrosis also recognised in some cases.¹

GT is considered a rare disorder with an incidence of 1 in 1,000,000 individuals with worldwide distribution, and both genders are equally affected.⁵

The hemostasis is a group of mechanisms that aim to preserve blood in a fluid state inside blood vessels, prevent bleeding when injury occurs by forming a hemostatic plug at the damage site, localise the plug to prevent thrombosis and complete removal of the plug after healing. The components of the hemostatic system include blood vessels, platelets, coagulation factors, coagulation inhibitors and fibrinolysis agents, any defect in one component can lead either to bleeding or thrombosis.⁶

Pathophysiology of Glanzmann thrombasthenia 

Platelets' structure and function

Platelets are small spherical fragments, formed in the bone marrow by fragmentation of the cytoplasm of megakaryocytes cells. Platelets have granules containing molecules important in hemostasis; these granular materials are released when platelets are activated following injury. 

When injury occurs, platelets adhere to exposed collagen from the blood vessels, binding can be directly or facilitated by molecule known as Von Willebrand factor, adhered platelets get activated and release intracellular granular components that work in vasoconstriction at injury site to slow down the blood flow and recruit white blood cells and platelets at injury site and others molecules. After platelets adhesion others platelets start to aggregate to the firstly adhered platelets, which is facilitated by fibrinogen and Von Willebrand factor, both molecules form cross link between platelets by binding to glycoprotein receptors of platelet.⁷ Integrins IIb/IIIa are the binding site for fibrinogen, platelets aggregates and form localised plug.³ Coagulation factors stabilise the plug at the injury site to prevent its removal, after healing fibrinolytic agents degrade the plug.

Pathological mechanism in GT

Absence or reduction of glycoprotein IIb/IIIa receptors prevent platelet aggregation and consequently leads to spontaneous bleeding or after minimal trauma. 

Platelet aggregation is one of the most important platelet functions; it depends on fibrinogen, which is a protein that links platelets to each other to form a plug at the injury site in order to arrest the bleeding. Integrins aIIb and B3 are principal platelet glycoprotein receptors for fibrinogen. GT occurs either due to qualitative or quantitative defects in integrins IIb/IIIa. Depending on the integrins status GT classified to three subtypes, subtype I shows complete absent integrins IIb/IIIa, subtype II with reduced integrins IIb/IIIa expression, subtype III with normal expressed IIb/IIIa receptors but is nonfunctional. Subtype I is the most relevant, followed by II and III respectively.¹

Classification of the disease is important for treatment and management and for incident and epidemiological studies. 

Genetic mutations associated with glanzmann thrombasthenia

Overview of genes involved 

ITGA2B and ITGB3 genes encoding GP IIb and GP IIIa, and located in chromosome 17q21.31 and 17q21.32, respectively. The patients develop GT if pathologic mutations occur in both alleles of either previous genes. The clinical phenotype of each gene defect is indistinguishable. ITGA2B is more likely to be defected because of its larger size, it encodes for 1,039 amino acids, whereas ITGB3 codes for 788 amino acids.⁵

Types of mutations 

Variable pathogenic mutation types recognised in this disorder, such as single nucleotide polymorphism, missense and nonsense and varied splicing sites are all prevalent, whereas duplication and deletion are less detected.⁸

The adverse effect of pathogenic mutation can disrupt either early synthesis of the glycoprotein subunit or prevent pro aIIbB3 transportation to Golgi apparatus or the receptor exportation on the cell membrane. 

Mutations more commonly affect the β- propeller region of αIIb and the domains of epithelial growth factor of β3.⁹

Genotype-phenotype correlation 

The type and the location of the mutation definitely impact the clinical presentation and disease phenotype,as the clinical severity can be variably ranged from minor, moderate to severe symptoms. Also, pathogenic variants in defined regions recorded to cause dominant macrothrombocytopenia.¹

Molecular diagnostic

Genetic testing

Genetic testing is important for confirming the diagnosis, detecting the carrier individuals and future suspected carriers, also important for prenatal diagnosis and genetic counseling for management and treatment protocol. 

Genetics testing occurs by sequencing of ITGA2B and ITGB3, to detect pathogenic variants.⁴

Functional assays

• Flow Cytometry is used to study platelets glycoprotein expression, which is helpful in diagnosis of quantitative abnormalities in glycoprotein IIb/IIIa expression, but not valuable in detection of qualitative defects as in type III GT¹⁰
• Platelets aggregation test, platelets show failure to aggregate or limited aggregation with all platelets agonist but normally aggregates with ristocetin that mediates platelets aggregation depending on glycoprotein receptors others than IIb/IIIa¹¹

Current and emerging therapeutic approaches

Conventional treatments

Platelet transfusion with or without factor VII, the aim of this is to manage bleeding either acute or chronic bleeding or to avoid consequent complications. 

Recombinant activated VII is a treatment approved by FDA patients who received this treatment and showed improved clinical symptoms. Patients who did not respond to the platelet transfusion showed better hemostatic management. 

Patients repeatedly received platelet transfusions especially those with truncating mutations that cause the absence of glycoprotein IIb/IIIa are vulnerable to developing antibodies against transfused platelets, called anti-HLA antibodies or antibodies against glycoprotein IIb/IIIa. If patients develop antibodies against transfused platelets, they become not able to it any more. So it is important to use platelet transfusions for critical conditions such as major trauma or major surgeries. Females patients are more relevant to develop antibodies against platelets and unfortunately, these antibodies are capable of crossing the placenta and this may develop adverse effects on the embryo.^{12 -17}

Stem cells transplantation 

GT patients received allogeneic stem cells transplantation show good clinical outcomes, but patients who developed antiplatelet antibodies are still challenge, as the antibodies may be directed against the graft.^18-20

Gene therapy prospects

Promising treatment approaches like gene therapy using CRISPR/Cas9 and other gene-editing technologies are potential curative approaches for GT patients and are suspected to offer more safe treatment for patients and to overcome the current challenges.^21-23

Summary 

Glanzmann Thrombasthenia is a rare inherited autosomal recessive platelet disorder, platelets ability to aggregate at the disrupted blood vessel site is impaired due defect in platelet receptors integrins aIIb and B3, which have principal role in mediating platelet aggregation by fibrinogen binding. Platelet aggregation is an essential process for bleeding arrest and plugging injured blood vessels. Different pathologic mutation types target genes encoding for aIIb and B3 integrins, and leads to absence or reduction or function impairment of these platelet receptors.^1-3

As a result of platelet aggregation defects patients develop bleeding episodes in form of purpura, epistaxis, excessive menstrual bleeding commonly, also patients may develop gastrointestinal tract bleeding. Minimal trauma may lead to severe blood loss, patients may develop life-threatening complications if bleeding is not managed.¹

Patients presented to the hospital with clinical bleeding started at an early age of life, despite normal platelet count and structure.¹ Platelets failed or minimally aggregate in platelet aggregation tests,⁴ the confirmation of the diagnosis is made by detection of the pathologic mutations in ITGA2B or ITGB3 both alleles by gene sequencing.¹¹

Conventional treatment depends on the management of severe or chronic bleeding or treatment of hemorrhagic complications using platelet transfusion and factor VII. After repeated platelet transfusions some patients develop antiplatelet antibodies that target and destruct transfused platelets. Recombinant activated factor VII shows better management of clinical bleeding in patients non responded to platelet transfusions.^12-17

Gene therapy approach is a promising treatment for GT, as it is considered a potential curative treatment.^21-23