Disulfide bond disruption by a β3-Cys549Arg mutation in six Jordanian families with Glanzmann thrombasthenia causes diminished production of constitutively active αIIbβ3

αllbβ3 integrin mediates platelet aggregation following its activation. Its absence or dysfunction causes Glanzmann thrombasthenia (GT), an inherited bleeding disorder that is rare world-wide but relatively frequent in several populations with high rates of consanguinity, including Arabs in Israel and Jordan. Cysteine residues in the β3 epidermal growth factor (EGF) domains are involved in αllbβ3 formation and activation. In this study we present a novel Cys549Arg mutation in β3 identified in six Jordanian families, which in the homozygous state is manifested by severe GT. The mutation is located in EGF-3 of β3 predicting disruption of a conserved disulfide bond between Cys549 and Cys558. Haplotype analysis disclosed a common founder whose age estimate was 120-150 years. Flow cytometry revealed 1-14% of normal αllbβ3 expression at the patients' platelet surface. The Cys549Arg or artificial Cys549Ser mutations were introduced into a β3 expression vector. Co-transfection of baby hamster kidney cells with normal or mutant β3 along with normal αllb demonstrated reduced surface expression of αllbβ3 by both mutants. The mutants were constitutively active as demonstrated by 20-fold increased binding of the ligand-mimetic antibody PAC- I. Immunoblotting and immunoprecipitation experiments showed reduced β3 and αllbβ3 expression and a higher than normal ratio of pro-αllb to mature αllb. Immunofluorescence experiments showed that β3 and αllbβ3 were mostly retained in the endoplasmic reticulum. In conclusion, the novel ancestral mutation found in a cluster of Jordanian GT patients disrupts a conserved Cys549-Cys558 bond which results in reduced production of constitutively active αllbβ3.