A Point Mutation in Glycoprotein IX Coding Sequence (Cys73(TGT) to Tyr(TAT)) Causes Impaired Surface Expression of GPIb/IX/V Complex in Two Families with Bernard-Souiier Syndrome

Abstract

SummaryBernard-Soulier syndrome (BSS) is a rare inherited bleeding disorder which is caused by abnormal expression or function of the glycoprotein (GP) Ib/IX/V complex, a platelet major receptor for von Wille-brand factor. We studied four BSS patients in two unrelated families in which the same and novel mutation was found. Flow cytometric analysis showed that GPIX was completely absent but residual amounts of GPIbα and GPV were detectable in these patients. We analyzed all coding regions of GPIbα, GPIbβ, GPV and GPIX which were amplified from the patients’ genomic DNA by the polymerase chain reaction (PCR). In all four cases, we identified a point mutation in the GPIX coding region that changes the codon for cysteine 73 (TGT) to a codon for tyrosine (TAT). Furthermore, we confirmed by a transient expression study that the mutation caused the loss of adequate surface expression of GPIX. Since cysteine might be important for the secondary structure, this mutation of GPIX gene would lead to a dramatic conformational change of GPIX protein, resulting in the reduced surface expression. We concluded that this novel point mutation of the GPIX gene was responsible for BSS in these families

Compound heterozygosity for a novel nine-nucleotide deletion and the Asn45Ser missense mutation in the glycoprotein IX gene in a patient with Bernard-Soulier syndrome

“…The sequences flanking the LRM of GPIbb, GPIX, and GPIba are very similar, these regions showing conserved cysteine residues that form disulfide loops within the polypeptide, as observed in GPIba by crystallography [22]. It has been suggested that GPIX gene mutations affecting cysteine residues [13,14,21] prevent the formation of intramolecular disulfide-loop structures that are critical for the interaction of the polypeptide with the GPIbb subunit. A mutation involving the GPIX signal peptide was recently shown to drastically affect the biosynthesis of both GPIba and GPIX in affected platelets [23].…”

The cysteine knot of platelet glycoprotein Ibβ (GPIbβ) is critical for the interaction of GPIbβ with GPIX

“…Mutations in GPIX that result from changes in the codons for cysteines on either flanking region of the LRR resulted in BSS. 23,25 This mutation probably disrupts the disulfide bonding pattern in the amino terminus and hence altered the secondary structure of GPIX, which disrupts the proper assembly and anchoring of the complex. 23 In the present investigation, we used chimeras where the cysteine knots of both GPIb␤ and GPIX were preserved within each of the subunits by swapping precise structural domains.…”

Novel heterozygous missense mutation in the platelet glycoprotein Ibβ gene associated with isolated giant platelet disorder

“…In the patients' platelets, there was no surface expression of the aected subunit, whereas a residual amount of unaected polypeptides was detected. Transfection studies have shown that GPIX Cys73Tyr and GPIX Cys97Tyr were not expressed on the transfected CHO cells [20,21]. In addition, the heterozygous GPIbb Tyr88Cys mutation, found in a patient with GPD, also introduces a Cys residue [11].…”