Platelets with a W127X mutation in GPIX express sufficient residual amounts of GPIbα to support adhesion to von Willebrand factor and collagen

Abstract

Bernard-Soulier syndrome (BSS) is an inherited bleeding disorder caused by a defect in the platelet glycoprotein (GP) Ib/IX complex. The GPIX W127X mutation is the most common genetic defect in Japanese patients with BSS, which is often misdiagnosed as immune thrombocytopenic purpura, presumably due to residual expression of GPIbα. Neither the mechanism by which this mutation leads to a mild bleeding diathesis, nor whether functional GPIbα is expressed on platelet surfaces is known. We investigated GPIbα expression and function in platelets with a GPIX W127X mutation (GPIXW127X). GPIbα complexed with GPIbβ by disulfide bonding was expressed on GPIXW127X platelets and stable CHO-K1 cells lacking GPIX but expressing GPIbα and GPIbβ. Expression of GPIbα/β on GPIXW127X platelets was sufficient to support adhesion to immobilized von Willebrand factor and type III collagen and ristocetin-induced platelet agglutination. A residual amount of functional GPIbα/β heteromer expressed on GPIXW127X platelets partially compensates for the absence of the GPIb/IX complex. This may account for the mild bleeding phenotype of the BSS variant characterized by a non-sense mutation in GPIX.

Bernard-Soulier Syndrome: An Update

“…58,68 Even so, platelets with the GPIX nonsense mutation corresponding to position Trp127, common in the Japanese BSS population, express residual functional GPIbα as a heterodimeric complex with GPIbβ in the absence of GPIX. 69 Initial laboratory testing for BSS involves examination of blood smears to identify morphological abnormalities of platelets, and checking for a selective defect in VWF-dependent agglutination, which is uncorrected by addition of normal plasma as a source of VWF. Platelet response to collagen and ADP is typically normal.…”

Non‐myeloablative conditioning with busulfan before hematopoietic stem cell transplantation leads to phenotypic correction of murine Bernard‐Soulier syndrome

“…Thus, it is speculated that a fully normal amount of platelet GPIbα expression may not be required to maintain hemostasis in vivo . Consistent with this idea, BSS patients having a homozygous mutation in GPIX are reported to still express residual GPIbα/β complex without GPIX on platelets, which partially supports platelet adhesion, leading to only a mild bleeding phenotype . Because a fully normal complement of platelets displaying the GPIb‐IX surface complex does not seem to be necessary for phenotypic improvement, partial hematopoietic replacement shows promise as a gene therapy approach to treat genetic platelet protein defects.…”

Lentiviral gene therapy reverts GPIX expression and phenotype in Bernard-Soulier syndrome type C

“…quantified GPIbα relative MFI expression compared with healthy controls, revealing that it ranged from 7% to 11% in this group of BSS patients. 45 Despite the reduced expression level of GPIbα, platelets from W127X patients were capable of binding to immobilized vWF under high-shear conditions in vitro .…”

“… 38 As a result, they have emerged as the primary platform for analyzing GPIb-V-IX receptor assembly and the effects of diverse mutations identified in BSS patients. 39 , 40 , 41 , 42 , 43 , 44 , 45 …”