Background: Gain-of-function variants in GP1BA cause platelet-type von Willebrand disease (PT-VWD), a rare inherited autosomal dominant bleeding disorder characterized by enhanced platelet GPIbα-von Willebrand factor (VWF) interaction and thrombocytopenia. To date, only 6 variants causing PT-VWD have been described, 5 in the VWF-binding domain (β‐switch) of GPIbα, and one in the macroglycopeptide. We have studied a boy with easy bruising, enhanced ristocetin-induced platelet agglutination (RIPA=0.6 mg/ml) and normal platelet count, characteristics that oriented towards PT-VWD or type 2B-VWD.

Aims: To perform differential diagnosis between PT-VWD and type 2B-VWD, to identify the gene variant and to investigate its impact on GPIbα.

Methods: GP1BA gene was analyzed by PCR and direct sequencing. CHO cells expressing the GPIbβ and GPIX subunits were transfected with plasmids expressing WT GPIbα, p.G233V mutant GPIbα (a typical β‐switch gain-of-function variant) or the novel variant. Binding of VWF to platelets and GPIbα-transfected CHO cells induced by ristocetin was assessed by flow cytometry; rolling of GPIbα-transfected CHO cells on immobilized VWF was assessed by microcinematography in a flow chamber.

Results: Patient platelets mixed with control plasma showed enhanced VWF-binding, orienting towards PT-VWD. GP1BA sequencing revealed a novel heterozygous c.C455A substitution, corresponding to a p.R111Q in the leucine-rich-repeat (LRR) 5 domain of GPIbα. CHO cells expressing WT GPIbα barely bound VWF (5.8±3.2%), while those expressing p.G233V and p.R111Q GPIbα showed enhanced VWF-binding (G233V: 23.6±4.9%, p< 0.05 vs WT; R111Q: 11.8±1.9%, p< 0.05 vs WT)*. CHO cells expressing WT GPIbα rolled significantly faster on a VWF-coated surface (average speed:14.1±2.7 mm/sec), then those expressing p.G233V and p.R111Q GPIbα (G233V:4.9±0.4 mm/sec, p< 0.05 vs WT; R111Q:9.1±0.5 mm/sec, p< 0.05 vs WT and G233V).

Conclusions: We describe the first p.R111Q variant in the LLR5 domain of GPIbα inducing PT-VWD, demonstrating that gain-of-function variants outside the β‐switch may confer increased VWF-binding activity to GPIbα and generate a mild PT-VWD phenotype.

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ISTH Congress Abstracts - https://abstracts.isth.org/abstract/variants-in-the-llr5-domain-confer-to-gpib%ce%b1-increased-affinity-for-vwf-a-novel-case-of-platelet-type-von-willebrand-disease/