Modelling Studies to Characterize a Novel Diseasecausing Variant in the GP1BA Gene Related to Bernard Soulier Syndrome

ALBERTO MF; CASINELLI MM; PRIMROSE, DEBORA M.; PAIVA J; BLANCO AN; WOODS AI; ASENSIO M; SANCHEZ LUCEROS ANALIA

Philadelphia

Congreso; ISTH 2021 VIRTUAL CONGRESS; 2021

International Society on Thrombosis and Haemostasis

Background: We described a novel disease-causing variant (DCV) p.Tyr231Cys inthe GP1BA gene related to recessive BSS in a patient with no bleeding symptomsand 50% CD42b-expression. Homology modelling was used to describe the effectsof this DCV on protein-protein interaction.Aims: To study effect of the GPIbα-p.Tyr231Cys on the interaction of GPIbα withVWF-A1 domain by homology modelling.Methods: The model GPIbα-p.Tyr231Cys was made by replacing Tyr231 by Cys inwild-type-GPIbα (Swiss-PDBviewer). Docking between VWF-A1 and GPIbα-p.Tyr231Cys was obtained (PatchDock) and compared to WT-GPIbα-VWF-A1.Hydrogen-bonds and root-mean-square deviation (RMSD) between α-carbonchains were obtained (UCSF Chimera).Results: GPIbα-Tyr231 is found in the a2-helix, nearby two disulphide-bonds:C225-C264; C227-C280. No new disulphide-bonds are predicted in GPIbα-p.Tyr231Cys. GPIbα-Tyr231 forms intramolecular hydrogen-bonds with Phe208,Trp235 and Glu228.In p.Cys231Tyr, hydrogen-bond with Phe208 is lost and a newbond is formed with Ile203. The bond with Trp235 is maintained and is stillpresent with Glu228 but at a larger distance, consequently, weaker.In silico-model of wild-type-GPIbα-VWF-A1 (Fig-1A) shows differences withGPIbα-p.Tyr231Cys-VWF-A1(Fig-1B). Total RMSD between models is 38.05Å(0.65Å between GPIbα portions; 15.07Å between VWF-A1 domains) (Fig-1C). Thechange Tyr231Cys causes a slight alteration in the conformation of GPIba (0.65Å)respecting wild-type-GPIbα. This change does not affect neighboring residuesbut alters the structure of GPIbα at the site that acts as a hinge, increasing thedistance of the b-switch region of GPIba (flexible loop at Val243-Ser257). Thismodifies the interaction with VWF-A1, changing hydrogen-bonds between thesemolecules (Table 1).Conclusions: In-silico predictions show that GP1BA-p.Tyr231Cys modifies thetertiary structure of GPIba affecting the interaction with VWF-A1, explaining itsnegative effect that influences in both platelet count and size and in 50%expression of GPIb as observed in our patient. The change Tyr231Cys could bepreventing the conformational change of the b-switch from compact to extendedthus altering the VWF-GPIbα binding.