An adenine insertion in exon 6 of human GP6 generates a truncated protein associated with a bleeding disorder in four Chilean families

Abstract

Summary. Background: Glycoprotein VI (GPVI), 60-65 kDa, is a major collagen receptor on platelet membranes involved in adhesive and signaling responses. Mice lacking GPVI have impaired platelet response to collagen and defective primary adhesion and subsequent thrombus formation. Complete or partial deficiency of GPVI in humans is a rare condition presenting as a mild bleeding disorder. The defect in most of the reported patients is acquired and associated with other diseases. To date, only two patients have been characterized at the molecular level who carry different compound heterozygous mutations in the GP6 gene. Objective: To report four unrelated patients from non-consanguineous families who presented with mucocutaneous bleeding. They had absent platelet aggregation and 14 C-5-HT secretion with collagen, convulxin and collagen-related peptide. Results: Flow cytometry and immunofluorescence-confocal microscopy showed an absence of GPVI in non-permeabilized platelets. All the patients had an adenine insertion in exon 6 (c.711_712insA), changing the reading frame and generating a premature 'stop codon' in site 242 of the protein.The mutation predicts the synthesis of the truncated protein before the trans-membrane domain, corresponding to a band of %49 kDa observed in western blots and in permeabilized platelets by immunofluorescence. Platelet mRNA from all the patients was sequenced and contained the corresponding adenine insertion. Heterozygous relatives had no pathological bleeding, normal response to collagen and convulxin and intermediate membrane expression of GPVI. Conclusions: The identification of four unrelated homozygous patients with an identical defect suggests that inherited GPVI deficiency is more frequent than previously suspected, at least in Chile.

High proportion of patients with bleeding of unknown cause in persons with a mild‐to‐moderate bleeding tendency: Results from the Vienna Bleeding Biobank (VIBB)

“…None of the patients had GP VI expression below 50%. Of patients with GP VI between 50 and 70%, none had pathologies in the collagen‐induced platelet activation …”

Fibrin and D-dimer bind to monomeric GPVI

“…1,2 This provides an explanation for the paradoxical observation that mice deficient in GPVI fail to form an occlusive thrombus in a FeCl 3 injury model, even though the onset of thrombosis, which is where collagen is exposed, is not altered. 3 Given the relatively mild bleeding diathesis of mice and patients deficient in GPVI, 1,4,5 these findings raise the possibility that activation of GPVI by fibrin represents a target for development of a new class of antithrombotic drug that may have a reduced effect on bleeding relative to current antiplatelet drugs.…”

“…The 2 patients are homozygous for an adenine insertion in exon 6 at position 242, which generates a stop codon before the transmembrane domain and therefore prevents surface expression of GPVI. 4 Platelets from either patient do not aggregate to collagen and do not express detectable GPVI when measured by flow cytometry ( Figure 1B; data not shown).…”

Fibrin activates GPVI in human and mouse platelets

“…This unexpected role, coupled with its relatively minor role in hemostasis as shown by the mild bleeding seen in GPVIdeficient human and mouse platelets, 22,42,43 indicates that targeting the binding of fibrin to GPVI may prevent vessel occlusion at sites of arterial thrombosis, without causing a major bleeding diathesis.…”