THE GENETIC-DEFECT IN 2 WELL-STUDIED CASES OF BERNARD-SOULIER-SYNDROME - A POINT MUTATION IN THE 5TH LEUCINE-RICH REPEAT OF PLATELET GLYCOPROTEIN-IB-ALPHA

Bernard-Soulier syndrome (B-Ss) is a rare congenital bleeding disorder caused by abnormal giant platelets, thrombocytopenia, and defective g lycoprotein (GP) Ib-V-IX, the adhesion receptor for von Willebrand fac tor (vWF), This report describes the molecular defect in two related i ndividuals with well-established B-Ss whose platelets exhibit decrease d GPIb-IX and normal GPV on their surfaces. The GPIb-V-IX genes of the two patients were analyzed by Southern blotting, hetero-duplex analys is, and polymerase chain reaction (PCR) amplification/sequencing. A po int mutation was found in codon 129 of the GPIb alpha gene that result s in the substitution of proline for leucine in the first position of the fifth leucine-rich glycoprotein repeat of the mature gene product. The mutation (CTC: leucine, wild-type to CCC:proline, mutant) elimina tes a Sac I restriction site, facilitating analysis of the mutation in the propositi (both homozygotes), unaffected family members (8 hetero zygotes and 8 wild-type), and 58 normal controls (116 wild-type allele s). The status of the genomes was confirmed by the sequencing of plate let cDNA, The mutation does not affect transcription of the Ib-IX gene s, as estimated by PCR and Northern blot analysis, but it does inhibit surface expression of the receptor as assessed by transient transfect ion of mutant and wildtype GPIb alpha genes into mouse Ib beta-IX L ce lls. Many of the cells (43%) transfected with the normal gene express surface GPIb alpha, whereas untransfected cells and those transfected with the mutant gene lack surface GPIb alpha entirely. Patient platele ts were tested both for VWF binding in the presence of ristocetin and for surface GPIb-IX expression. In these instances, despite their inab ility to agglutinate with ristocetin and vWF, patient platelets exhibi t about 40% of normal vWF binding and 40% of normal Ib-IX surface anti gens. The results suggest that the described mutation (GPIb alpha: Leu 129 --> Pro) affects the conformation of the GPIb-V-IX receptor, alter s its availability on platelet surfaces, and causes the observed Berna rd-Soulier phenotype. (C) 1995 by The American Society of Hematology.