A novel D235Y mutation in the GP1BA gene enhances platelet interaction with von Willebrand factor in an Iranian family with platelet-type von Willebrand disease

Abstract

Platelet-type von Willebrand disease (PT-VWD) is a rare bleeding disorder with an intrinsic defect in platelets rather than von Willebrand factor (VWF), but has clinical and laboratory features similar to the more common type 2B VWD. The intriguing nature of the pathophysiology and molecular genetics of PT-VWD has created lengthy debate in literature regarding its discrimination from type 2B VWD, and essentially confirming DNA analysis as the gold standard in diagnosis and revealing pathologic mutations. In this report we identify a novel Asp235Tyrmutation in the GP1BA gene of two Iranian patients showing the PT-VWD phenotype who were originally misdiagnosed as type 2B VWD. By structural modelling of the mutant by introducing Tyr235 into the available crystal structure of the glycoprotein (GP)Ibα N-terminal domain, we observed the mutant Tyr235 generates a hydrophobic tip to the extended β-switch loop of GPIbα. Further modelling of the resulting complex with VWFA1 indicates this could result in an enhanced interface compared to wild-type Asp235. This data provides an update to the present knowledge about this rare disorder, and confirms the necessity of genetic testing for accurate diagnosis, and the importance of studying natural mutations to better understand molecular aspects of GPIbα-VWFA1 interaction.

Enhanced Local Disorder in a Clinically Elusive von Willebrand Factor Provokes High-Affinity Platelet Clumping

“…GPIbα preferentially interacts with the disordered RCAM A1 conformational state with high-affinity. Gain-of-function PT-VWD mutations, G233V, M239V and D235Y [37, 38], increase the opposable thumb's hydrophobicity from a grand average hydropathy of −0.006 to 0.265, 0.129 and 0.124, respectively [39]. The search for high-affinity contacts by GPIbα's disordered opposable thumb suggests that increased hydrophobicity of the thumb might allow for a greater sampling of dynamically exposed hydrophobic regions of A1 enabling the formation of hydrophobic contacts that stabilize the complex through an induced fit.…”

Congenital platelet disorders and understanding of platelet function

“…Mutations of GP 1 BA that result in plt‐ VWD are in green. The mutation list has been updated from Lanza () and Enayat et al (). Abbreviations: fs, frameshift; dup, duplication; del, deletion; ins, insertion; stop, stop codon.…”

“…A limited number of amino acid substitutions in the GPIbα N‐terminal domain (Fig ) change the conformation of the so‐called “thumb”, a disulphide‐bonded loop that controls the accessibility of VWF multimers to their binding site. As a result, soluble VWF binds directly, changes also introduced by a long range p.Pro449_Ser457 deletion in the macroglycopeptide‐coding region of GP1BA (Nurden & Nurden, ; Enayat et al , ). This clinical condition resembles type 2B VWD (VWD2B) and diagnosis requires care (Othman, ).…”

Progress in understanding the diagnosis and molecular genetics of macrothrombocytopenias

“…Other mutations in the GP1BA gene have been described so far, M239V (Russell & Roth, ), G233S (Matsubara et al , ), a 27‐bp in‐frame deletion (Othman et al , ), D235Y (Enayat et al , ) and W246L (Woods et al , ). All of these mutations induce a gain of function resulting in an enhanced affinity for VWF and enhanced platelet aggregation to a low dose of ristocetin (0·5 mg/ml), which normally does not induce platelet agglutination.…”