Gene of the issue: GP1BA gene mutations associated with bleeding

The GP1BA gene codes for the platelet GPIbα subunit of the glycoprotein GPIb-IX complex, which comprises four transmembrane polypeptides in total with GPIbα disulfide linked to GPIbβ and GPIX and GPV each non-covalently associated with the complex with a stoichiometry revised recently of 2:4:2 [1] GPIbα carries the binding site for the adhesive protein von Willebrand factor (VWF) in its N-terminal domain, and this interaction is critical to primary hemostasis and thrombus formation. Mutations in the platelet GP1BA gene coding for the GPIbα protein result in two known bleeding disorders, Bernard–Soulier Syndrome (BSS) and platelet type von Willebrand disease (PTVWD). The two disorders have distinct clinical, laboratory, and molecular genetics features. Bernard–Soulier Syndrome [2] is a rare autosomal recessive disorder, with examples of dominance, resulting from mutations in the genes coding for proteins forming the GPIb-IX complex. The disease is characterized by giant platelets and thrombocytopenia, an increased bleeding tendency, defective ristocetininduced platelet agglutination, and little or reduced expression of GPIb-IX on the surface of platelets [3,4]. BSS tends to present itself soon after birth or in early childhood with epistaxis, ecchymosis, and gingival bleeding. Menorrhagia tends to be a problem in young women at puberty while gastrointestinal bleeding and hematuria occur more rarely. Severe bleeding can occur with trauma and surgical procedures. The extent, severity, and frequency of bleeding vary between patients and can range from mild to life threatening [5]. The extent of thrombocytopenia is also variable between patients, however, the platelet count characteristically ranges from less than 30 to 200 × 10/μL with prolonged bleeding time, normal clot retraction shorter platelet survival [6] but a constant characteristic is macrothrombocytopenia. BSS mutations have been mapped to genes coding for GPIbα, GPIbβ, and GPIX and are categorized as missense mutations, nonsense mutations, frameshift mutations, and insertions or deletions. This review will focus on the GP1BA gene mutations. In general, BSS mutations tend to be restricted to the extracellular and transmembrane regions of GPIbα (as opposed to the cytoplasmic domain), which are involved in the assembly of the GPIbIX complex and ligand binding [7]. Missense mutations disturb GPIb-IX assembly and this can occur by disrupting the folding of the N-terminal domain or considerably destabilizing the association with other subunits in the complex [7]. Figure 1 shows the crystal structure of the GPIbα N-terminal domain which features the folded leucine rich repeats shown in grey, whereas the PT-VWD mutations reside in the flexible loop regions colored red. Residues altered in BSS are colored in orange, showing these are all buried on the interior of the protein, which contrasts with PT-VWD mutations that are surface exposed [8]. The nonconservative alterations of buried GPIbα residues affected in BSS such as Asn126 and Asn57 (to Asp and His in BSS respectively) are likely to be highly disruptive to the overall fold of the leucine rich repeat solenoid structure. Mutations that modify specific cysteine residues such as GPIbα Cys225 and Cys81 (to Ser and Arg in BSS respectively) will interfere with disulfide bond formation and result in misfolding. Despite classically being described as a biallelic autosomal recessive disease, it has been recognized that there are cases in which patients are affected by a mild monoallelic autosomal dominant form. In these cases, laboratory and clinical findings are inadequately suggestive of BSS and the diagnosis can easily be missed [9]. One of the best-investigated cases in this regard is the Bolzano mutation (Ala172Val or Ala156Val in numbering that excludes the signal sequence); a more conservative variant described in Italian patients [10]. In this variant a monoallelic c.515 C > T transition occurs in the GPIBA gene resulting in macrothrombocytopenia and exhibits a mutant protein which is present on the surface of platelets in a form that binds thrombin but is unable to bind VWF, suggesting a selective disruption of the leucine rich repeat region [10,11]. Dominant inheritance of BSS mutations has been previously reported [12] and is not Color versions of one or more of the figures in the article can be found online at www.tandfonline. com/iplt. Correspondence: Maha Othman, Department of Biomedical and Molecular Sciences, Faculty of Medicine, Queen’s University, Kingston, Ontario, Canada. None othman@queensu.ca; Jonas Emsley, Centre for Biomolecular Sciences, School of Pharmacy, University of Nottingham, University Park, Nottingham NG72RD, UK. E-mail: jonas.emsley@nottingham.ac. uk http://www.tandfonline. com/iplt ISSN: 0953-7104 (print), 1369-1635 (electronic)