A 3-BASE DELETION REMOVING A LEUCINE RESIDUE IN A LEUCINE-RICH REPEAT OF PLATELET GLYCOPROTEIN-IB-ALPHA ASSOCIATED WITH A VARIANT OF BERNARD-SOULIER-SYNDROME (NANCY-I)

Leucine-rich repeats are conserved structural motifs present in the four components of the human platelet glycoprotein Ib/IX/V complex receptor for the adhesive protein von Willebrand factor. The absence or abnormality of this complex is responsible for Bernard-Soulier disease, an autosomal recessive bleeding disorder. We report a deletion of leucine 179, located in a highly conserved position of the seventh leucine-rich repeat of GPIb alpha, found in a variant form of Bernard-Soulier disease (Bernard-Soulier Nancy I). Three affected siblings of a family were characterized by absence of ristocetin-induced platelet agglutination, although ADP aggregation was normal, Flow cytometry studies showed detectable amounts of all four members of the GPIb/IX/V complex on the surface of the patients' platelets. Western blotting revealed normal levels of GPIX, decreased levels of GPIb beta and GPV, and <1% of GPIb alpha. RT-PCR studies showed the presence of mRNA coding for GPIb alpha, GPIb beta, GPIX and GPV. Sequencing showed a three-base deletion which results in the absence of a leucine residue, highly conserved across the seven leucine-rich repeats of GPIb alpha and also within the other members of the leucine-rich glycoprotein family. The absence of the leucine 179 in a patient's GPIb alpha is believed to cause a conformational change in the protein which would account for the lack of binding of most of the MoAbs tested and would be responsible for the absence of von Willebrand factor binding. These results point to the leucine-rich region of GPIb alpha as being required for the correct exposure of the von Willebrand binding site as well as for the correct assembly and stability of the GPIb/IX/V complex on the platelet surface.