Introduction: Inherited macrothrombocytopenias comprise a heterogeneous group of rare inherited disorders characterized by decreased platelet count with enlarged platelet size. Glycoprotein Ib (GP Ib) is a platelet surface membrane glycoprotein that is encoded by GP1BA gene, and functions as a receptor for von Willebrand factor (VWF). Mutations in the GP1BA gene are seen in Bernard-Soulier syndrome (BSS). Here we describe a family with an isolated giant platelet disorder and a novel variant in the GP1BA gene following an autosomal dominant mode of inheritance. In our kindred this variant was not associated with clinical history or specific laboratory evidence of BSS.
Case Presentation: 34-year-old female reported first being diagnosed with macrothrombocytopenia at the age of 13 on routine blood work. The patient and 11 of her relatives spanning 5 generations have reported asymptomatic macrothrombocytopenia (Figure 1), described as "Magic Blood" by her family. Her platelet count fell below 20,000/microL during one of her pregnancies. She was treated as idiopathic thrombocytopenic purpura (ITP) and was given corticosteroids to increase platelet count without improvement. On presentation, patient's complete blood count was significant for low platelet at 55,000/microL and macrothrombocytes were observed on blood smear.
Methods/Results: VWF assays, including Factor VIII activity, VWF Ag, and Ristocetin Cofactor, were within normal limits. VWF multimer analysis revealed a normal pattern and distribution of bands. Patient had a normal platelet aggregation in response to ADP, Collagen, Epinephrine, Ristocetin and Arachidonate. Flow Cytometry detected normal GP Ib and GP IIb/ IIIa expression. Whole exome sequencing and copy number analysis of 29 genes associated with thrombocytopenia revealed a c.97T>G substitution in the GP1BA gene predicted to result in the amino acid substitution p.Cys33Gly (Figure 2). To our knowledge, this variant has not been reported in the literature or public databases. To confirm this novel variant is the cause of the familiar macrothrombocytopenia, two relatives with macrothrombocytopenia, a maternal uncle and a first cousin, also underwent genetic testing and were found to have the same variant.
Discussion: Variants in GP1BA are associated with both autosomal dominant and recessive forms of BSS and with autosomal dominant platelet-type VWD. Our kindred is surprisingly asymptomatic given the location and specific amino acid substitution generated by the variant. Amino acid residue p.Cys33 resides in an extracellular N-terminal domain that is critical for VWF binding and proper assembly of the GP1B-IX complex. A heterozygous substitution involving the same amino acid residue defined as p.Cys33Arg was observed in two patients with macrothrombocytopenia with no reported bleeding complications (MC, unpublished data). Substitution of a nearby cysteine residue defined as p.Cys20Gly has been reported in a case of monoallelic chronic macrothrombocytopenia without bleeding diathesis similar to our patient. Amino acid residues p.Cys20 and p.Cyc33 are highly conserved among divergent species and substitution of these amino acid residues appears to not be tolerated. Substitution of other cysteine amino acid residues in the extracellular domain of the GP1BA protein (p.Cys81 and p.Cys225) has also been reported in patients with biallelic BSS, suggesting that perturbation of cysteine amino acid residues is likely to affect protein structure and function.
Conclusion:We think the p.Cys33Gly substitution found in our patient and her relatives is likely to be a primary cause of monoallelic GP1BA-associated macrothrombocytopenia. It is important to distinguish inherited macrothrombocytopenia from ITP in order to avoid unnecessary and potentially toxic treatment.
No relevant conflicts of interest to declare.
