ITGA2B and ITGB3 gene mutations associated with Glanzmann thrombasthenia

Studies on Glanzmann thrombasthenia (GT) helped discover the αIIbβ3 integrin and led to successful antithrombotic therapy [1,2]. GT is defined by a lack of platelet aggregation with physiological agonists, but a normal response to ristocetin. Historically, patients have been segregated into three groups: type I where platelets lack αIIbβ3 (by definition <5% of the normal platelet content), type II with residual αIIbβ3 (usually 5–15%) that may be functional or not, while in type III or variant GT αIIbβ3 fails to function despite platelets having up to 100% of normal levels [3]. αIIbβ3 assures platelet aggregation by changing from a bent to an extended and open form on platelet activation, conformational changes that allow fibrinogen (Fg) or other adhesive proteins to bind and crosslink platelets [4]. The absence of this function causes severe mucocutaneous bleeding and perhaps underestimated nonhemostatic defects often linked to loss of the wider biological expression of αvβ3 [5]. Inherited thrombocytopenia and increased platelet size associate with a GT-like phenotype in rare families [6]. GT is caused by autosomal recessive mutations in the ITGA2B or ITGB3 genes, both are large composed of 30 and 15 exons, respectively; they are near-neighbors localizing to chromosome 17q21.31 and 17q21.32 (Ensembl database; www.ensembl.org). Autosomal recessive inheritance is the general rule. The landmark publication for molecular genetic studies in GT was the detection of homozygous 11-bp and 13bp deletions on sequencing mRNA from Israeli families of differing ethnic origins [7]. These located to exon 12 of ITGB3 (Iraqi Jews) and the splice acceptor of exon 4 of ITGA2B (Arab kindred), both resulted in loss of αIIbβ3 expression. Polymerase chain reaction-single conformation polymorphism analysis of genomic DNA fragments followed by Sanger sequencing was the next advance but did not guarantee success as some mutations failed to give migration changes during the initial screening [8]. Direct Sanger sequencing of exons and splice sites became the state of the art of GT genotyping; the results from many groups including several large epidemiological studies were reviewed in 2011 [6]. The largest international study so far is from France, involving 76 unrelated families (“Genoscope” cohort) [9]. Genetic analysis of GT worldwide is accelerating with the availability of whole exome or whole genome sequencing and the application of high-throughput sequencing platforms both to patients and to mass screening of populations [10,11]. A frequent cause of GT are nonsynonymous variants of ITGB3 and ITGA2B; splice defects and stop codons truncate or lead to the loss mRNA synthesis while small deletions (del) and insertions (ins) with frameshifts (fs) are also common [reviewed in 6]. Large gene deletions or duplications occur but are rare [6,9]. Genetic variants deeper within introns can be disease causing, an example is an enigmatic heterozygous c.1440-13_c.1440-1del in intron 14 of ITGA2B shown to cause exon skipping in a series of unrelated European families [9]. Figure 1 highlights the wide range of missense mutations reported for GT. They involve all of the principle extracellular, transmembrane and cytoplasmic domains. Some mutations re-occur in non-related families across continents and reflect mutation hotspots, an example being the αIIb β-propeller missense substitution E355K* that gives type I GT [reviewed in 6]. Occasional exon missense mutations also cause splicing and exon skipping (e.g. ITGA2B Q626H and F191V) [9]. Homozygous mutations often reflect consanguinity in the family; some characterize ethnic groups of which the Israeli Jewish and Arab kindreds and the French Manouche travelers are prime examples [7,12]. Type I GT is by far the most frequently encountered with the malformed proteins, often as pro-αIIbβ3, retained in the endoplasmic reticulum and/or Golgi apparatus prior to degradation by the proteasome [13]. Shown in Figure 2 are mutations that permit αIIbβ3 expression but which give rise to variant forms of GT (with loss of function). These include classic D145N, R240Q or W substitutions within the β3 β1(or βA) extracellular domain that prevent Fg binding to the αIIbβ3 ligand-binding pocket while β3 cytoplasmic domain S778P and R750ter mutations