Page 47 - Haematologica April 2020

Clinical and molecular aspects of GT
wide distribution; however, a large proportion of the cases have been described in selected populations such as the French Romani,7 South Indian Hindus, Iraqi Jews, and Jordanian nomadic tribes,8 in all of which consanguinity is common. Type I is the most common subtype and accounts for around 78% of patients with GT type II and type III (functional variant in receptor) constituting around 14% and 8% of cases, respectively.9
GPIIb/IIIa (integrin αIIbβ3) is a heterodimeric receptor present in large quantities in the plasma membrane of platelets. Activation of this integrin and binding of soluble ligands [primarily fibrinogen, but also von Willebrand fac- tor (vWF) and fibronectin] are essential for platelet aggre- gation. In the resting state, the integrin has low affinity for ligands. During platelet activation, driven by exposure to soluble agonists or the subendothelial matrix, “inside out” cellular signaling generates a conformational change in GPIIb/IIIa that allows high affinity binding for fibrinogen, which serves as a “bridge” to other activated platelets, eventually forming the platelet plug. Protein kinase C (PKC), diaglycerol-regulated guanine nucleotide exchange factor I (CalDAG-GEFI or RASGRP2), and phosphoinosi- tide 3-kinase (PI3K) participate in this signaling pathway. Subsequent “outside in” signaling triggers additional gran- ule secretion, cytoskeletal interactions (that allow for platelet spreading), stabilization and clot retraction to con- solidate the fibrin clot. Kindlins (including kindlin-3) and talins are key regulators of integrin activation.10, 11
Glanzmann thrombasthenia is usually caused by decreased or absent expression of αIIb or β3, abnormali- ties in protein folding, defective post-translational pro- cessing or transport of either integrin subunit causing decreased surface expression, or abnormalities affecting protein function. Other defects change integrin function by altering the ligand binding pocket (interface between αIIb and β3), which modifies the cytoplasmic domain and affects binding of regulators, or locks the integrin in the activated form.
Molecular basis of Glanzmann thrombasthenia
The ITGA2B and ITGB3 genes are located on chromo- somes 17q21.31 and 17q21.32, respectively, and are inde- pendently expressed. GT is caused by pathogenic variants in both alleles of either of the two genes; concomitant pathogenic variants in both genes, but affecting only one allele of each, is not known to cause GT. Due to the auto- somal recessive inheritance, compound heterozygosity is frequent, except in selected ethnic groups where homozygosity is more likely due to consanguinity. A higher percentage of pathogenic variants occur in ITGA2B, likely because of the larger size of this gene with 30 exons encoding 1,039 amino acids, compared to ITGB3 which is composed of 15 exons with 788 amino acids.7 The clinical phenotypes associated with either gene are indistinguishable.12
Pathogenic nonsense, missense and splice site variants are common and large deletions and duplications, although rare, have also been described.13 Pathogenic mis- sense variants impair subunit biosynthesis in megakary- ocytes or inhibit the transport of the pro-αIIbβ3 complex- es from the endoplasmic reticulum (ER) to the Golgi apparatus or the export of the mature complexes to the cell surface. A large proportion of variants affect the β-
propeller region of αIIb and the epithelial growth factor domains of β3.14
A different type of variant affecting specific regions of these genes has been more recently described to cause a mild autosomal dominant macrothrombocytopenia15 by interfering with proplatelet formation.16 These “gain of function” variants cause spontaneous activation of GPIIb/IIIa (αIIbβ3) by affecting the cytoplasmic domains or the membrane proximal residues in the extracellular domains. The majority of these are in ITGB3 and affect the MIDAS (metal ion dependent adhesion site), ADMI- DAS (adjacent to MIDAS) or SyMBS (synergistic metal ion binding site) regions. A small proportion have been reported in ITGA2B and affect the conserved intracellular GFFKR sequence.17
Clinical manifestations and diagnosis
Bleeding phenotype
With integrins αIIb and β3 participating in primary hemostasis, the bleeding manifestations are typically pur- pura, epistaxis (60-80%), gum bleeding (20-60%), and menorrhagia (60-90%). Gastrointestinal bleeding in the form of melena or hematochezia is present in 10-20%, and 1-2% develop intracranial hemorrhage.9 Mucocutaneous bleeding can be spontaneous or occur after minimal trauma. Epistaxis is the most common cause of severe bleeding, especially in the pediatric popu- lation, and risk of severe nosebleeds decreases with age as the septal arterial plexus becomes less friable and children grow out of the habit of nose picking. Menorrhagia is highly prevalent in affected females and there is a higher risk of severe bleeding at the time of menarche due to the prolonged estrogen influence on the proliferative endometrium that occurs during anovulatory cycles. Bleeding complications during pregnancy are uncommon; however, the risk of obstetric hemorrhage at the time of delivery and postpartum is high. Hematuria and sponta- neous hemarthrosis have been described in some cases but are not usually part of the bleeding phenotype.
Several bleeding scores have been developed with the goal of standardizing the assessment of bleeding and facilitating the diagnosis of patients with a suspected inherited bleeding disorder. These are useful tools that assist in communication of the bleeding phenotype in the clinical and research setting; however, they have not been widely validated for patients with inherited platelet func- tion disorders. Therefore, specific cutoffs to define a pos- itive bleeding score have not been established for this population.18 This is of particular relevance in GT, because while the types of bleeding are consistent among individuals, the degree of bleeding is highly variable. Given the severity of this disorder, historically, most patients have been diagnosed in childhood (before 5 years of age), but there are some patients that reach adulthood without having severe bleeding.9 In general, the severity of bleeding (except for menorrhagia and pregnancy-asso- ciated bleeding) decreases with age.
Laboratory phenotype
Complete blood count
There should be normal platelet count and size with normal granularity on evaluation of the peripheral blood smear by light microscopy. If bleeding is severe and/or
haematologica | 2020; 105(4)
889