Glanzmann thrombasthenia in two Iraqi-Jewish siblings is caused by a novel splice junction mutation in the glycoprotein IIb

An A-->G transition in the acceptor splice site at the intron 19/exon 20 junction of the glycoprotein IIb gene was defined as a novel mutation causing Glanzmann thrombasthenia in two Iraqi-Jewish siblings. This mutant DNA was transcribed into four distinct species of mRNA, one of which resulted in a premature termination codon and the other three predicting deletions of 50, 61 or 72 amino acids, respectively.

Rapid heterozygote detection in Glanzmann's thrombasthenia

To assess the potential of using flow cytometric analysis of platelet glycoprotein IIb in the detection of heterozygotes in Glanzmann's thrombasthenia (GT) we compared the fluorescence intensity of anti-CD41-labelled platelets of obligatory heterozygotes with that of normal subjects. This study showed that normal subjects had relative fluorescences occasionally overlapping with heterozygotes. Use of an independent factor, the mean platelet volume, as a correction factor for platelet size differences eliminated this overlap. This study indicates that it is possible to confidently and rapidly predict the carrier status of family members of GT sufferers, both type I and type II, by flow cytometric means.

Glycoprotein IIb Leu214Pro mutation produces glanzmann thrombasthenia with both quantitative and qualitative abnormalities in GPIIb/IIIa

Glanzmann thrombasthenia is an inherited bleeding disorder due to a functional reduction or absence of platelet GPIIb/IIIa (alphaIIbbeta3) integrin receptors. Based on a prolonged bleeding time and absence of platelet aggregation in response to physiologic agonists, a 55-year-old white man was diagnosed as having Glanzmann thrombasthenia. The patient's platelet fibrinogen level was approximately 5% of normal. As judged by complex-dependent monoclonal antibody (MoAb) binding, surface expression of platelet GPIIb/IIIa receptors was less than 5.5% of normal, whereas the binding of an anti-GPIIIa specific MoAb (7H2) was approximately 12% of normal. Immunoblot analysis of the patient's platelet lysates showed approximately 35% of normal levels of GPIIIa, approximately 30% of normal levels of GPIIb, and an abnormally migrating fragment of GPIIb. Biotinylation of the surface proteins on the patient's platelets followed by immunoprecipitation and sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis showed only GPIIb and GPIIIa subunits of normal size. Surface expression of platelet alphavbeta3 receptors was 192% of normal, suggesting that the patient's' defect was in GPIIb. Sequence analysis of the patient's GPIIb cDNA identified a T to C transition at nucleotide 643, predicting a Leu214Pro substitution. Direct sequencing of GPIIb exon 6 indicated that the patient is homozygous for the mutation. The nature of the Leu214Pro mutation was analyzed by expression in Chinese hamster ovary (CHO) cells. As judged by subunit-specific MoAb binding, surface expression of mutant receptors was approximately 60% of normal, but these receptors were not recognized by the complex-dependent monoclonal antibodies, 10E5 and 7E3. In addition, mutant receptors pretreated with the ligand-induced binding site MoAb AP5 were not recognized by the activation-dependent MoAb PAC-1 and mutant expressing CHO cells did not adhere to immobilized fibrinogen. These data suggest that the Leu214Pro mutation in GPIIb disrupts the structural conformation, and either directly or indirectly, the ligand binding properties of the heterodimeric complex. This is in accord with studies from other integrins that have implicated a beta-turn in a homologous region as important in ligand binding. Thus, the Leu214Pro mutation appears to produce the Glanzmann thrombasthenia phenotype by both qualitative and quantitative abnormalities. In addition, the mutation appears to confer susceptibility of the GPIIb subunit to proteolysis.

Severe juvenile vaginal bleeding due to Glanzmann's thrombasthenia: case report and review of the literature

Glanzmann's thrombasthenia is a rare inherited hematological disorder defined by deficiency or abnormality of the glycoprotein (GP) IIb-IIIa complex. Presenting symptoms are hemorrhagic events, mainly epistaxis, purpura, or menorrhagia. We describe the clinical course and management of a 14-year-old girl with Glanzmann's thrombasthenia and severe menorrhagia. Following treatment with 20 U of packed red blood cells, 37 U of platelets, 7 U of fresh frozen plasma, cryoprecipitate, intravenous estrogens, and methylergotrine maleate with no improvement, the uterine cavity was packed for 48 hr. This unusual procedure halted the bleeding and avoided the necessity for a hysterectomy. When treating acute menorrhagia in patients with Glanzmann's thrombasthenia, the physician should be familiar with the characteristics and all treatment modalities for this disorder.

Mutations in the alphaIIb and beta3 genes that cause Glanzmann thrombasthenia can be distinguished by a simple procedure using transformed B-lymphocytes

Glanzmann thrombasthenia (GT) is caused by a defect in either glycoprotein (GP)IIb (alphaIIb) or GPIIIa (beta3) genes and therefore screening of both genes is required for mutation identification. The beta subunit of the GPIIb/IIIa complex (beta3) forms a complex with another alpha subunit (alpha(v)) yielding the alpha(v)beta3 vitronectin receptor (VnR). GT patients with mutations in the GPIIIa gene that cause diminished synthesis of GPIIIa are deficient in both GPIIb/IIIa and VnR, whereas patients with mutations in the GPIIb gene are deficient in GPIIb/IIIa, yet express normal or increased VnR in their platelets. The presence or absence of VnR in platelet membranes of GT patients has therefore been used for distinguishing between mutations in the GPIIb gene and mutations in the GPIIIa gene. However, the method of assessing VnR in platelets is cumbersome and use of fresh platelets is indispensible. In the present work we devised a procedure for detection of the VnR in B-lymphocytes transformed by Epstein-Bar virus (EBV). The transformed lymphocytes transcribed GPIIIa mRNA but not GPIIb mRNA and expressed VnR on their surface. Using flow cytometry analysis or immuno-precipitation and western blotting VnR was found in B-lymphocytes of GT patients bearing a well characterized mutation in the GPIIb gene. In contrast, in B-lymphocytes of GT patients bearing 2 different mutations in the GPIIIa gene no VnR was detectable. Thus, for determining which gene is mutated in a GT patient, EBV-transformed B-lymphocytes are useful and can as well be used for analyses of GPIIIa mRNA and genomic DNA. Ten ml of blood are sufficient for the procedure.

Illegitimate transcription: its use for studying genetic abnormalities in lymphoblastoid cells from patients with Glanzmann thrombasthenia

Glanzmann thrombasthenia is the most common inherited disorder of platelets that may induce severe bleeding complications. Molecular biology techniques have offered the possibility to assess the basis of this chronic haemorrhagic disease at the molecular level. However, the accessibility of mRNA in platelets is limited by the availability of the patient's blood samples and the relatively weak amount of this material in these cells. Taking advantage of the genetic phenomenon of illegitimate transcription, we have demonstrated that glycoprotein IIb and glycoprotein IIIa mRNA could be detected in lymphoblastoid cell lines issued from normal EBV-transformed lymphoblasts. We further analysed the sequences of the two glycoprotein transcripts in lymphoblastoid cell lines from two previously characterized patients presenting with Glanzmann thrombasthenia. The results showed that illegitimate transcripts presented similar molecular abnormalities to those found in platelets. These data demonstrated that the nucleotide sequences of illegitimate transcripts were identical to tissue-specific mRNA found in platelets. We applied this methodology to screen for the genetic defect in a new thrombasthenic patient, and found a homozygous nonsense mutation GCA-->TGA converting Arg8 to stop in the glycoprotein IIIa gene. This immortalized source of genetic material is therefore particularly useful for molecular genetic studies in inherited platelet disorders, avoiding repetitive and large blood samplings in frequently anaemic patients.

Truncation of the cytoplasmic domain of beta3 in a variant form of Glanzmann thrombasthenia abrogates signaling through the integrin alpha(IIb)beta3 complex

Glanzmann thrombasthenia is an inherited bleeding disorder characterized by absence or dysfunction of the platelet integrin alpha(IIb)beta3. Patient RM is a thrombasthenic variant whose platelets fail to aggregate in response to physiological agonists, despite the fact that they express abundant levels of alpha(IIb)beta3 on their surface. Binding of soluble fibrinogen or fibrinogen mimetic antibodies to RM platelets did not occur, except in the presence of ligand-induced binding site (LIBS) antibodies that transformed the RM integrin complex into an active conformation from outside the cell. Sequence analysis of PCR-amplified genomic DNA and platelet mRNA revealed a C2268T nucleotide substitution in the gene encoding the integrin beta3 subunit that resulted in an Arg724Ter mutation, producing a truncated protein containing only the first eight of the 47 amino acids normally present in the cytoplasmic domain. Functional analysis of both RM platelets and CHO cells stably expressing this truncated integrin revealed that the alpha(IIb)beta3Arg724Ter complex is able to mediate binding to immobilized fibrinogen, though downstream events, including cytoskeletally-mediated cell spreading and tyrosine phosphorylation of focal adhesion kinase, pp125FAK, fail to occur. These studies establish the importance of the membrane-distal portion of the integrin beta3 cytoplasmic domain in bidirectional transmembrane signaling in human platelets, and the role of integrin signaling in maintaining normal hemostasis in vivo.

A Leu117-->Trp mutation within the RGD-peptide cross-linking region of beta3 results in Glanzmann thrombasthenia by preventing alphaIIb beta3 export to the platelet surface

We report a case of Glanzmann thrombasthenia in a Pakistani child whose platelets express less than 10% of the normal amount of alphaIIb beta3 on their surface. Single-stranded conformation polymorphism analysis of the exons of the patient's alphaIIb and beta3 genes showed an abnormality in exon 4 of the beta3 gene. Direct sequence analysis showed that the patient was homozygous for a T --> G nucleotide substitution in this exon, resulting in the replacement of a highly conserved Leu at position 117 with Trp. Heterologous expression of alphaIIb beta3 containing the beta3 mutation in COS-1 cells confirmed the pathogenicity of the Leu117 --> Trp substitution and showed that it resulted in the intracellular retention of malfolded alphaIIb beta3 heterodimers. Additional site-directed mutagenesis at position 117 indicated that, although the smaller hydrophobic amino acid Val could be substituted for the wild-type Leu, the larger hydrophobic amino acids Trp and Phe or the charged amino acids Asp and Lys were not tolerated. These studies indicate that Leu117 in beta3 plays a critical role in attaining the correct folded conformation of alphaIIb beta3. These studies also suggest that the hydrophobic side chain of Leu117 is likely folded into the interior of beta3, where it serves to stabilize internal packing of the protein and determines its overall shape.

A three amino acid deletion in glycoprotein IIIa is responsible for type I Glanzmann's thrombasthenia: importance of residues Ile325Pro326Gly327 for beta3 integrin subunit association

Glanzmann's thrombasthenia (GT) is a recessive autosomal bleeding disorder characterized by abnormal platelet aggregation due to a qualitative or quantitative defect of the glycoprotein (GP) IIb-IIIa complex (integrin alphaIIb beta3). We describe a new mutation in the GPIIIa gene responsible for type I GT in a consanguineous Algerian family. A discordance between phenotyping and genotyping of the GPIIIa-related HPA-1 platelet alloantigen system in three family members heterozygous for the disease suggested a genetic defect in the GPIIIa gene and a normal GPIIb gene. Sequence analysis of amplified genomic DNA fragments showed a 6-bp deletion in exon 7 of the GPIIIa gene resulting in the amino acid deletion/substitution (Ile325pro326Gly327 --> Met) and creating a new BspHI restriction site. Expression of the mutated integrin beta3 subunit cDNA in Chinese hamster ovary cells showed that the cDNA gene was transcribed into a full-length beta3 protein with an apparent molecular weight identical to wild-type beta3 and accumulated as a single-chain molecule in the cell cytoplasm. The absence of heterodimeric complex formation of the mutant beta3 protein with endogenous alpha v was shown by immunoprecipitation experiments, intracellular immunofluorescent labeling, and a semiquantitative enzyme-linked immunosorbent assay using the alpha vbeta3 complex-specific monoclonal antibodies LM609 and 23C6. Substitution of the methionine residue by a proline, present at position 326 of wild-type beta3, did not restore the ability of the recombinant mutant beta3 protein to associate with alpha v , suggesting that the Ile-Pro-Gly motif is located in a beta3 domain important for integrin subunit interaction. The association of a BspHI restriction site with this newly identified mutation has allowed allele-specific restriction analysis of Algerian GT individuals and the identification of two new unrelated type I patients exhibiting the same mutation, suggesting that the described mutation might be significant in this population and that BspHI restriction analysis will provide a useful screening assay for antenatal diagnosis and genetic counselling.

Glanzmann thrombasthenia caused by an 11.2-kb deletion in the glycoprotein IIIa (beta3) is a second mutation in Iraqi Jews that stemmed from a distinct founder

Glanzmann thrombasthenia (GT) is a rare bleeding disorder resulting from mutations in either glycoprotein (GP) IIb or GPIIIa genes. The disease is relatively frequent in highly inbred populations such as Iraqi Jews. The molecular basis of GT in 6 unrelated Iraqi-Jewish patients was previously identified as an 11-bp deletion in exon 12 of the GPIIIa gene. We now describe a second mutation found in 3 unrelated Iraqi-Jewish families that consists of an 11.2-kb deletion between an Alu repeat in intron 9 and exon 13 of the GPIIIa gene. The mutant DNA is transcribed into mRNA in which exons 10 through 13 are absent. Splicing of exon 9 directly to exon 14 leads to a shift in the reading frame resulting in a stop codon. The predicted protein is truncated in the middle of the third cysteine-rich domain before the transmembrane domain. Simple DNA-based methods were devised for identification of both mutations in Iraqi Jews for the purpose of carrier detection and prenatal diagnosis enabling prevention of GT. A survey of the general Iraqi-Jewish population for the first 11-bp deletion and the second 11.2-kb deletion disclosed that the allele frequency of the first mutation was 0.0043, whereas none of 700 individuals examined bore the second mutation (allele frequency <0.0007). Among 40 GT patients of Iraqi-Jewish origin 31 were homozygous for the first mutation, 4 were compound heterozygotes for the first and second mutations, and 2 were homozygous for the second mutation. Haplotype analyses using 4 polymorphic markers in the GPIIIa gene showed that each mutation originated in a distinct founder.

A dinucleotide deletion in exon 4 of the PlA2 allelic form of glycoprotein IIIa: implications for the correlation of serologic versus genotypic analysis of human platelet alloantigens

Platelets from a patient with a suspected case of posttransfusion purpura were subjected to alloantigen phenotyping and found to express the PlA1, but not the PlA2, allelic form of human platelet membrane glycoprotein (GP) IIIa on the platelet surface. However, genotyping showed unambiguously that the patient carried the genes for both of these GPIIa alleles. Based on these results, we postulated that the PlA2 allele was silent, ie, that this patient was a carrier for Glanzmann thrombasthenia (GT). Quantitative analysis of GPIIb-IIIa surface expression showed only 20,000 GPIIb-IIIa receptors/platelet, approximately half of the value obtained with control platelets. Southern blot analysis showed no large deletions or insertions within the GPIIIa gene, and amplification of all 14 exons encoding GPIIIa resulted in the production of normal sized polymerase chain reaction (PCR) products in all cases. DNA-sequence analysis showed an AG dinucleotide deletion affecting codons 210 and 211 within exon 4 of the GPIIIa gene, leading to a change in reading frame and the creation of a stop codon 38 nucleotides down-stream. The predicted truncated protein consists of only the first 223 of the normal 762 amino acids, thus accounting for the failure to express the PlA2 allele on the platelet surface. While encountered only rarely, carriers of either GT or Bernard Soulier syndrome that are at the same time heterozygous for human platelet alloantigenic epitopes found on GPIb, GPIIb, or GPIIIa have the possibility to give discrepant results when comparing genotypic versus phenotypic analysis. In such situations, the combination of serologic and DNA-based evaluation contributes complementary and beneficial diagnostic information than either one alone are able to provide.

A nonsense mutation in the GPIIb heavy chain (Ser 870-->stop) impairs platelet GPIIb-IIIa expression

Glanzmann thrombasthenia (GT) is a rare autosomal recessive bleeding disorder, caused by a quantitative or qualitative defect of the GPIIb-IIIa integrin (alpha IIb beta 3), which functions as the platelet fibrinogen receptor. We report a case of type I GT due to a homozygous mutation resulting in Ser 870 to stop codon substitution. This residue is located near the proteolytic cleavage site of proGPIIb. The mutation results in a GPIIb truncated of 138 amino acids, including transmembrane and intracytoplasmic domains. Cotransfection of an expression vector containing the mutant GPIIb and wild-type GPIIIa showed that the mutant Ser 870-->stop GPIIb was able to associate to GPIIIa. However, this heterodimer failed to mature as shown by endoglycosidase-H digestion and was therefore not expressed at the COS-7 cell surface. This report is the first description of a homozygous nonsense mutation in the GPIIb gene and highlights the role of the GPIIb light chain.

Glanzmann thrombasthenia. Cooperation between sequence variants in cis during splice site selection

Glanzmann thrombasthenia (GT), an autosomal recessive bleeding disorder, results from abnormalities in the platelet fibrinogen receptor, GP(IIb)-IIIa (integrin alpha(IIb)beta3). A patient with GT was identified as homozygous for a G-->A mutation 6 bp upstream of the GP(IIIa) exon 9 splice donor site. Patient platelet GP(IIIa) transcripts lacked exon 9 despite normal DNA sequence in all of the cis-acting sequences known to regulate splice site selection. In vitro analysis of transcripts generated from mini-gene constructs demonstrated that exon skipping occurred only when the G-->A mutation was cis to a polymorphism 116 bp upstream, providing precedence that two sequence variations in the same exon which do not alter consensus splice sites and do not generate missense or nonsense mutations, can affect splice site selection. The mutant transcript resulted from utilization of a cryptic splice acceptor site and returned the open reading frame. These data support the hypothesis that pre-mRNA secondary structure and allelic sequence variants can influence splicing and provide new insight into the regulated control of RNA processing. In addition, haplotype analysis suggested that the patient has two identical copies of chromosome 17. Markers studied on three other chromosomes suggested this finding was not due to consanguinity. The restricted phenotype in this patient may provide information regarding the expression of potentially imprinted genes on chromosome 17.

A Cys374Tyr homozygous mutation of platelet glycoprotein IIIa (beta 3) in a Chinese patient with Glanzmann's thrombasthenia

A 20-year-old woman from a consanguineous family in the Hunan Province of the People's Republic of China was diagnosed as having Glanzmann's thrombasthenia based on (1) nearly a lifelong history of epistaxis, gum bleeding, petechiae, and purpura; (2) severe menorrhagia resulting in anemia and need for whole-blood transfusion; (3) normal coagulation assays; (4) prolonged bleeding time; (5) absent clot retraction; (6) decreased glass bead retention; (7) absent platelet aggregation in response to adenine diphosphate, epinephrine, and collagen; and (8) normal initial slope of platelet aggregation in response to ristocetin, but with a diminished maximal extent. The patient's platelets had a decreased level of platelet fibrinogen, but the deficiency was not as severe as in other Glanzmann's thrombasthenia patients. As judged by monoclonal antibody binding studies, surface glycoprotein (GP) IIb/IIIa (alpha IIb beta 3) expression was less than 15% of normal and alpha v beta 3 vitronectin receptor expression was 15% to 19% of normal, suggesting that the defect was in GPIIIa (beta 3). Immunoblotting of platelet lysates demonstrated decreased levels of GPIIb (approximately 30% to 35% of normal) and GPIIIa (approximately 10% of normal), and the GPIIb had undergone normal maturational processing into GPIIb heavy and light chains. Sequence analysis of the patient's GPIIIa RNA identified a G to A mutation at nucleotide 1219, predicting a Cys to Tyr substitution at residue 374. The patient's parents, who are first cousins, are asymptomatic and have only minor reductions in platelet aggregation. Direct sequencing of polymerase chain reaction-amplified cDNA and GPIIIa exon VIII indicated that the patient is homozygous and her parents are heterozygous for the mutation. Transient transfection studies in Chinese hamster ovary cells indicated that the mutation results in an 85% to 90% reduction in GPIIb/IIIa surface expression, but these cells retain the ability to mediate adhesion to immobilized fibrinogen. The relative preservation of platelet fibrinogen despite the very low level of platelet surface GPIIb/IIIa expression in this patient raises some interesting questions regarding the mechanism of fibrinogen uptake and the pathophysiology of Glanzmann's thrombasthenia.

A mutant (Arg327-->His) GPIIb associated to thrombasthenia exerts a dominant negative effect in stably transfected CHO cells

This work reports the structural and functional characterization of the platelet glycoprotein complex GPIIb-IIIa (integrin alpha IIb beta 3) in a patient of type II Glanzmann thrombasthenia, bearing a homozygous G-->A base transition at position 1074 of GPIIb that results in an Arg327-->His substitution. CHO cells stably transfected with cDNA encoding His327GPIIb showed a drastic reduction in the surface expression of alpha IIb beta 3 complex relative to control cells transfected with wild type GPIIb. Immunoprecipitation analysis demonstrated that GPIIb synthesis, heterodimerization, and short term maturation were not impeded, suggesting that conformational changes dependent on Arg327 of GPIIb may play an essential role in either the rate of maturation and/or transport of heterodimers to the cell surface. Cotransfection of CHO cells with equimolar amounts of cDNAs encoding wild type and mutant His327-GPIIb led to a marked reduction in the surface expression of alpha IIb beta 3. This novel observation of a dominant-negative effect of the mutant His 327 alpha IIb subunit provides a molecular basis for the reduced platelet alpha IIb beta 3 content observed in the heterozygous offspring.

Critical residues of integrin alphaIIb subunit for binding of alphaIIbbeta3 (glycoprotein IIb-IIIa) to fibrinogen and ligand-mimetic antibodies (PAC-1, OP-G2, and LJ-CP3)

Integrin alphaIIbbeta3 plays a critical role in platelet aggregation through its interaction with fibrinogen. Elucidation of the mechanisms of alphaIIbbeta3-fibrinogen interaction is critical to understanding hemostasis and thrombosis. Here we report that mutations of Gly-184, Tyr-189, Tyr-190, Phe-191, and Gly-193 within the predicted turn structure of the third amino-terminal repeat of alphaIIb significantly block binding of alphaIIbbeta3 to soluble fibrinogen. These mutations also block binding of alphaIIbbeta3 to ligand-mimetic monoclonal antibodies PAC-1, OP-G2, LJ-CP3, which have an RGD-related RYD sequence in their antigen-binding sites. These mutations do not significantly affect the expression of alphaIIbbeta3, in contrast to most of the natural alphaIIb mutations occurring in Glanzmann's thrombasthenic patients. The data suggest that these residues are critically involved in alphaIIbbeta3-ligand interactions.

Periodontal management of Glanzmann's thrombasthenia: report of 3 cases

Glanzmann's thrombasthenia was reported and described as a bleeding diathesis seen in children and characterized by diminished clot retraction. It is an autosomal recessive bleeding disorder. The disease is marked by frequent mucocutaneous hemorrhages either due to defective function of the platelets or lack of fibrinogen binding membrane receptor glycoproteins IIb/IIIa which are located on the surface of the platelets. Case reports on 3 siblings, a girl of 11, and 2 boys of 12 and 16 years old with Glanzmann's thrombasthenia are reviewed. The major complaint of the patients was gingival bleeding. Periodontal treatment was performed under platelet transfusion and proper oral hygiene instruction was given. The patients were followed for 6 months and no periodontal complications developed during this time. Proper periodontal care for such patients is essential both for local and systemic health.

Serine 752 in the cytoplasmic domain of the beta 3 integrin subunit is not required for alpha v beta 3 postreceptor signaling events

A naturally occurring point mutation (Ser752Pro substitution) in the beta subunit cytoplasmic domain of the platelet fibrinogen receptor GPIIb-IIIa (integrin alpha IIb beta 3), causing Glanzmann's thrombasthenia, has been shown to abrogate bidirectional transmembrane signaling of GPIIb-IIIa when expressed in heterologous cells (Chen YP, 1994, Blood 84, 1857-1865). As the vitronectin receptor alpha v beta 3 constitutively mediates cell attachment to RGD containing extracellular matrix proteins, the purpose of this study was to explore the regulatory role of Ser752 in alpha v beta 3 vitronectin receptor function, by cotransfecting recombinant human alpha v cDNA together with human beta 3 mutant cDNA into Chinese hamster ovary (CHO) cells. CHO cells expressing wild type human alpha v beta 3 acquired the ability to attach and spread on fibrinogen and von Willebrand factor, in contrast to non transfected CHO cells that only bound to vitronectin and fibronectin. Overexpression of a truncated recombinant beta 3 subunit (beta 3 delta 744) generated alpha v (hamster) beta 3 (human) chimers that mediated attachment but lost the ability to promote cell spreading on vitronectin, von Willebrand factor and fibrinogen, and to concentrate in focal contact sites, demonstrating a negative effect of beta 3 delta 744 on alpha v beta 3 dependent postreceptor occupancy events. Transfection of beta 3Ser752Pro reproduced the same negative effect as beta 3 delta 744, whereas beta 3Ser752Ala restored normal receptor function by allowing pronounced attachment and spreading on fibrinogen and von Willebrand factor. Our results provide evidence that (1) the C-terminal cytoplasmic domain of beta 3 (amino acids 744-762) is essential for alpha v beta 3 integrin postreceptor occupancy events; (2) within this domain, the Ser752Pro mutation affects alpha v beta 3 postreceptor occupancy events by preventing cell spreading and focal contact localization; (3) the defective receptor function of the vitronectin receptor alpha v beta 3 is due to the presence of Pro752, rather than the absence of Ser752, as a Ser to Ala substitution at position 752 restores normal beta 3 integrin cell spreading and adhesive plaque formation.

Glanzmann thrombasthenia due to a two amino acid deletion in the fourth calcium-binding domain of alpha IIb: demonstration of the importance of calcium-binding domains in the conformation of alpha IIb beta 3

The integrin alpha IIb beta 3, a calcium-dependent heterodimer, plays a critical role in platelet aggregation. The alpha IIb subunit of the heterodimer contains four highly conserved putative calcium-binding domains in its extracellular portion. During studies of the molecular basis of Glanzmann thrombasthenia in a child of mixed Caucasian background whose platelets expressed little alpha IIb beta 3 on their surface, we found the patient heterozygous for a two amino acid deletion in the fourth alpha IIb calcium-binding domain. When this alpha IIb mutant was expressed in COS-1 cells, we found that the deletion did not interfere with the assembly of alpha IIb beta 3 heterodimers, but altered their conformation such that they were neither recognized by the heterodimer-specific antibody A2A9 nor able to undergo further intracellular processing or transport to the cell surface. These results suggest that the calcium-binding domains in alpha IIb play an important role maintaining the overall conformation of alpha IIb beta 3. To confirm this suggestion, we deleted each of the four 12 amino acid calcium-binding domains in alpha IIb by in vitro mutagenesis and expressed the mutants along with beta 3 in COS-1 cells. Each construct formed a heterodimer with beta 3, but none of the heterodimers interacted with A2A9 or underwent further intracellular processing. These data indicate that the calcium-binding domains in alpha IIb are not involved in alpha IIb beta 3 heterodimer formation, but their presence is required for the intracellular transport of alpha IIb beta 3 to the cell surface.

Glanzmann thrombasthenia in children from Argentina

PURPOSE

Glanzmann thrombasthenia is a well-defined inherited disorder of platelet function characterized by a decrease or absence of functional platelet glycoprotein (GP) GPIIbIIIa. The diagnosis must be considered in patients presenting with mucocutaneous bleeding, purpura, a normal platelet count, abnormal platelet aggregation, and a prolonged bleeding time. In most of the patients, the presence of small amounts of either GPIIb or GPIIIa was detected in their platelets. These observations could provide a basis for determining the clinical and laboratory heterogeneity of the disease.

PATIENTS AND METHODS

We studied 10 patients of seven unrelated families with the usual methods and an immunoalkaline phosphatase technique (APAAP) to analyze the biosynthesis of GP in megakaryocytes.

RESULTS

The results allowed us to classify six patients as GT type I, three as type II, and one as a variant.

CONCLUSION

The nature and severity of the bleeding manifestations, in our patients, were not predictible by the laboratory findings. These confirm the clinical and laboratory heterogeneity of the disease.

Glanzmann's thrombasthenia in 11 cases of one family

We report here the largest family in Bulgaria with Glanzmann's thrombasthenia which is inherited in an autosomal recessive manner. Eleven children in four successive generations were followed up. Clinically, the disease manifested in mucocutaneous haemorrhages. Five of the children died due to severe haemorrhagic episodes. The diagnosis was confirmed by haemostasic investigation; in some of the children the thrombocytes were examined by electron microscopy. Necessity of prenatal diagnosis is suggested.

Dystrophin-related protein in the platelet membrane skeleton. Integrin-induced change in detergent-insolubility and cleavage by calpain in aggregating platelets

The platelet membrane is lined with a membrane skeleton that associates with transmembrane adhesion receptors and is thought to play a role in regulating the stability of the membrane, distribution and function of adhesive receptors, and adhesive receptor-induced transmembrane signaling. When platelets are lysed with Triton X-100, cytoplasmic actin filaments can be sedimented by centrifugation at low g-forces (15,600 x g) but the membrane skeleton requires 100,000 x g. The present study shows that DRP (dystrophin-related protein) sediments from lysed platelets along with membrane skeleton proteins. Sedimentation results from association with the membrane skeleton because DRP was released into the detergent-soluble fraction when actin filaments were depolymerized. Interaction of fibrinogen with the integrin alpha IIb beta 3 induces platelet aggregation, transmembrane signaling, and the formation of integrin-rich cytoskeletal complexes that can be sedimented from detergent lysates at low g-forces. Like other membrane skeleton proteins, DRP redistributed from the high-speed pellet to the integrin-rich low-speed pellet of aggregating platelets. One of the signaling enzymes that is activated following alpha IIb beta 3-ligand interactions in a platelet aggregate is calpain; DRP was cleaved by calpain to generate an approximately 140-kDa fragment that remained associated with the low-speed detergent-insoluble fraction. These studies show that DRP is part of the platelet membrane skeleton and indicate that DRP participates in the cytoskeletal reorganizations resulting from signal transmission between extracellular adhesive ligand and the interior of the cell.

Bilateral linkage between a new deletion polymorphism in intron 21 of the GP IIb gene and the HPA-3b (Bakb) determinant

Glycoproteins (GP) IIb (alpha IIb) and GP IIIa (beta 3) form heterodimeric complexes (GP IIb-IIIa) at the platelet surface and mediate platelet aggregation by binding fibrinogen after platelet activation. The structures and DNA sequences of the GP IIb and GP IIIa genes are known. Punctual mutations resulting in alloantigen systems (HPA) have been described on both genes, as have a series of genetic defects giving rise to Glanzmann's thrombasthenia (GT). We now report a nine base pair deletion located in intron 21 of the GP IIb gene. This was found both in unrelated GT patients and in normal individuals. Subsequent studies showed that the deletion polymorphism and the mutation responsible for the platelet alloantigen. HPA-3b, were linked together. The deletion was always present when the gene carried the HPA-3b genotype, but was never observed in association with the HPA-3a polymorphism. Analysis of 60 independent alleles from 30 unrelated caucasian individuals revealed no exceptions to this linkage. It is the first time that two genetic markers have been reported to be linked to each other on the GP IIb gene.

Molecular study of Glanzmann thrombasthenia in 3 patients issued from 2 different families

In an effort to further understand Glanzmann thrombasthenia (GT) 3 patients from 2 different families were studied. After biochemical and immunological analysis these patients were classified as type I. We observed in the first family a new restriction site for Stu I in exon II of the glycoprotein (GP) IIIa gene caused by a homozygous nonsense mutation: 62 Arg to stop codon. The parents were heterozygotes for this mutation. We found in the second family a previously described nonsense mutation: 584 Arg to stop codon in exon 17 of the GPIIb gene. The father and his two affected sons were heterozygous for this genetic defect. This mutation 62 Arg to stop codon is a new description of a genetic defect associated with GT. Furthermore, the discovery of the same mutation in 3 affected families from different ethnic groups raises the possibility of either a hot spot mutation in the CG dinucleotide region of GPIIb gene, or an ancient mutant allele present in diffuse populations at a relatively high frequency.

The molecular genetic basis of Glanzmann's thrombasthenia in a gypsy population in France: identification of a new mutation on the alpha IIb gene

Glanzmann's thrombasthenia is a rare inherited bleeding disorder caused by a qualitative or quantitative defect of platelet alpha IIb beta 3. We describe here a new mutation that is the molecular genetic basis of Glanzmann's thrombasthenia in two gypsy families. Our investigation was focused on the alpha IIb gene as a result of biochemical and immunologic analysis of patients' platelets showing undetectable alpha IIb but residual beta 3 levels. The entire alpha IIb cDNA was polymerase chain reaction (PCR) amplified using patients platelet RNA. Sequence analysis showed an 8-bp deletion located at the 3' end of exon 15. This deletion causes a reading-frame shift leading to a premature stop codon and the synthesis of a severely truncated form of alpha IIb. Genomic DNA study showed a G-->A substitution, the Gypsy mutation, at the splice donor site of intron 15. This mutation results in an abnormal splicing occurring at an alternative donor site located 8 bp upstream from the mutation. Based on those results, an allele-specific PCR analysis was developed to allow a rapid identification of the mutation in patients and potential carriers of the gypsy community. This PCR analysis can also be used for genetic counseling and antenatal diagnosis.

Platelet-vessel wall interaction: from the bedside to molecules

This paper is an overview of the progress made in the field of platelet-vessel wall interaction during the past three quarters of a century. (I) "Prehistoric" era (1918-1948): description of Thrombasthenia by 3 European clinicians (Glanzmann, Switzerland, 1918), Hereditary Pseudohemophila (von Willebrand, Finland 1926) and Congenital Hereditary Platelet Dystrophy (Bernard and Soulier, France 1948). (II) Physiopathological era (1957-1972) based on the understanding of these 3 disorders: abnormality of platelet adhesion to the vessel wall in von Willebrand (vW) and Bernard Soulier syndrome (BSS), abnormality of platelet aggregation in thrombasthenias (GT). Breakthrough was made by I.M. Nilsson et al in vWD in Aåland Islands with the recognition of a plasmatic anomaly, later confirmed by Cornu et al to be different from factor VIII deficiency. (III) The third and crucial development (glycoprotein era) (1974-1981) came from the discovery by Nurden & Caen (1974-1975) of GPIIb-IIIa defect in GT and of GPIb-IX in BSS. Polyclonal antibodies against GPIIb-IIIa and GPIb-IX inhibit clearly ex vivo platelet aggregation and adhesion respectively. On everted rabbit subendothelium platelet, adhesion was abnormal in BSS whichever shear stress and at high shear stress in vWD. (IV) The molecular biology era (1984-1995) with the exquisite recognition of gene or protein anomalies in the above mentioned disorders together with the cloning of the 3 genes [for vW factor (vWF) (GPIb-IX, GPIIb, and GPIIIa]. (V) We are at the dawn of the more crucial era, the antithrombotic therapy acting either on the GPIb-vWF complex or on the GPIIb-IIIa.(ABSTRACT TRUNCATED AT 250 WORDS)

[Rapid detection of platelet glycoprotein IIb, IIIA gene variety by single strand conformation polymorphism analysis]

Glanzmann thrombasthenia (GT) is the most common inherited disorder of platelet defective function. Most of the molecular defects of GT identified in recent years have been caused by point mutation in the platelet glycoprotein (GP) IIb, IIIa genes. Trying to rapidly detect point mutation (or subtle variety) in GPIIb, IIIa genes, we have used single strand conformation polymorphism analysis (SSCPA) and studied the gene structure of platelet GPIIb. IIIa from normal controls and patients with Glanzmann thrombasthenia. After DNA preparation from peripheral blood, all the exons of GPI-Ib, IIIa were amplified by PCR. Denaturated PCR products were separated on mini polyacrylamide gel and stained by silver staining method in the Phast System. Only the PCR products with different migration on the gel were sequenced. 2 GT patients with point mutations and 1 GT patient with 16 bp deletion were found in our study. At the same time, we determined 3 polymorphisms in GPIIb gene. We conclude that SSCPA is a rapid, sensitive and effective method for detecting platelet IIb, IIIa gene variety.

Platelet phenotyping in carriers for Glanzmann's thrombasthenia: a simple screening test for assessment of the molecular defect

Glanzmann's thrombasthenia (GT) is a recessive autosomal bleeding disorder characterized by the abnormality of aggregation due to a platelet glycoprotein (GP) IIb-IIIa deficiency or a dysfunctional complex. Molecular abnormalities have been localized on the gene coding for GP IIb or IIIa. The aim of our work was an attempt to obtain indirectly information on the putative localization of the molecular defect in patients with GT type I or II by the determination of the HPA-1 (GP IIIa) and HPA-3 (GP IIb) alloantigenic systems' expression in GT carriers. If GT results from a defective GP IIb gene, a GT carrier would appear homozygous for HPA-3 by serology, because the normal gene product will be expressed while the abnormal GP IIb gene product will not be present. Conversely, if the abnormality is in the GP IIIa gene, such an individual would appear homozygous for HPA-1. Therefore, the heterozygous status for HPA would result from the normal expression of the two genes for the considered alloantigenic system. Among the four families studied with informative members, our presumptions were strengthened by the preliminary genetic results in one family showing a mutation in the GP IIb gene. Thus, serology could be a simple screening test for the possible defective gene responsible for GT allowing molecular investigation focusing only on GP IIb or IIIa gene.

Abnormal processing of the glycoprotein IIb transcript due to a nonsense mutation in exon 17 associated with Glanzmann's thrombasthenia

We analyzed the molecular genetic defect responsible for type I Glanzmann's thrombasthenia in a Japanese patient. In an immunoblot assay using polyclonal anti-GpIIb-IIIa antibodies, some GPIIIa (15% of normal amount) could be detected in the patient's platelets, whereas GPIIb could not (< 2% of normal amount). Nucleotide sequence analysis of platelet GPIIb mRNA-derived polymerase chain reaction (PCR) products revealed that patient's GPIIb cDNA had a 75-bp deletion in the 3' boundary of exon 17 resulting in an in-frame deletion of 25 amino acids. DNA analysis and family study revealed that the patient was a compound heterozygote of two GPIIb gene defects. One allele derived from her father was not expressed in platelets, and the other allele derived from her mother had a 9644C--> T mutation which was located at the position -3 of the splice donor junction of exon 17 and resulted in a termination codon (TGA). Moreover, quantitative analysis demonstrated that the amount of the abnormal GPIIb transcript in the patient's platelets was markedly reduced. Thus, the C --> T mutation resulting in the abnormal splicing of GPIIb transcript and the reduction in its amount is responsible for Glanzmann's thrombasthenia.

Glanzmann thrombasthenia resulting from a single amino acid substitution between the second and third calcium-binding domains of GPIIb. Role of the GPIIb amino terminus in integrin subunit association

To gain insight into region of the platelet GPIIb-IIIa complex involved in receptor biogenesis and function, we examined the biochemical properties of a defective GPIIb-IIIa complex from patient suffering from type II Glanzmann thrombasthenia. Flow cytometric as well as immunoblot analysis of patient platelets showed significantly reduced levels of GPIIb and GPIIIa compared with a normal control. Patient platelets, however, retained the ability to retract a fibrin clot. Sequence analysis of PCR-amplified platelet GPIIb mRNA revealed an Arg327-->His amino acid substitution between the second and third calcium-binding domains of the GPIIb heavy chain, a residue that is highly conserved among integrin alpha-subunits. The recombinant His327 form of GPIIb was found to be fully capable of associating with GPIIIa, therefore the role of the calcium-binding domains in intersubunit association was further examined by constructing amino-terminal segments of GPIIb that ended before the first, second, and third calcium-binding domains. All three fragments were found to associate with GPIIIa, demonstrating that the calcium-binding domains of GPIIb are not necessary for initial complex formation. Regions amino-terminal to the calcium-binding domains of GPIIb may play a heretofore unappreciated role in integrin subunit association.

Glanzmann's thrombasthenia associated with deletion-insertion and alternative splicing in the glycoprotein IIb gene

Glanzmann's thrombasthenia is a bleeding disorder characterized by a decrease or absence of the functional platelet membrane glycoprotein (GP) complex, GPIIb/IIIa (alpha IIb beta 3). We describe a new deletion-insertion mutation in the GPIIb gene causing type I Glanzmann's thrombasthenia in two siblings of a consanguineous Iranian-Jewish family. The proband's platelets bound more antibodies against the vitronectin receptor-alpha V beta 3 than normal platelets, suggesting a normal GPIIIa (beta 3) gene and a defect in the GPIIb gene. Sequencing of amplified cDNA and genomic DNA fragments showed a 6-bp deletion and 31-bp insertion in exon 25 of the GPIIb gene. The predominant platelet GPIIb mRNA of the proband was a product of the splicing of exon 24 to a cryptic AG acceptor site in the insertion and encoded for deletion of amino acids Leu817-Asn826 and insertion of eight different amino acids. Cotransfection of COS-7 cells with expression vectors containing wild-type GPIIIa cDNA and the mutated GPIIb cDNA failed to produce detectable amounts of GPIIb/IIIa on the surface of the cells. Allele-specific restriction analysis of genomic DNA of family members showed homozygosity for the mutation in the affected siblings, heterozygosity in the parents, and homozygosity for the normal allele in an unaffected sibling. The observed mutation is in a region that is conserved from rodents to humans and has been suggested to be involved in the interaction between GPIIb and GPIIIa when these GPs are complexed in solution.

[The fluorescence flow cytometry technique in the diagnosis of hereditary platelet disorders--a case of Glanzmann's thrombasthenia]

The authors describe a method for the diagnosis of heredital platelet membranopathies by means of monoclonal antibodies against the main membrane glycoproteins of thrombocytes, glycoprotein Ib and IIIa. The platelets are differentiated in the flow fluorocytometer from other blood cells by the typical optic profile caused by their size and granular character. Monoclonal antibodies are bound to the appropriate membrane glycoprotein and their amount is then detected by means of a secondary antibody labelled with fluorescein. The intensity of fluorescence of individual platelets is proportional to the number of molecules of the appropriate glycoprotein on their surface. By the above technique a case of Glanzmann's thrombasthenia was diagnosed, a rare hereditary haemorrhagic disease, characterized by the absence or abnormal function of glycoprotein complex IIb/IIIa the platelet receptor for fibrinogen.

Inherited diseases of platelet glycoproteins: considerations for rapid molecular characterization

The characterization of inherited diseases of platelets has provided valuable information about platelet physiology and platelet protein function. Genetic studies on patients with Glanzmann thrombasthenia, the Bernard-Soulier syndrome, and platelet-type von Willebrand disease have been confined to abnormalities of the GPIIb-IIIa and GPIb-IX receptor complexes. The primary molecular technique used in these analyses has been the polymerase chain reaction (PCR). The amplified PCR products are either directly sequenced, or used to screen for abnormal regions of the genes which are then sequenced. This review examines the known mutations in GPIIb-IIIa and GPIb-IX, focusing on those genetic issues which should dictate decisions regarding the approach to identifying molecular defects. The techniques for characterizing mutant alleles in Glanzmann thrombasthenia and Bernard-Soulier syndrome are described and a general strategy is offered. Because mutations resulting in reduced levels of transcripts can be missed when screening RNA, an argument is made for using genomic DNA as the primary material for mutation detection.

A point mutation in the integrin beta 3 cytoplasmic domain (S752-->P) impairs bidirectional signaling through alpha IIb beta 3 (platelet glycoprotein IIb-IIIa)

Agonist-induced inside-out signaling results in an increased affinity of integrin alpha IIb beta 3 (platelet glycoprotein IIb-IIIa) for soluble ligands (fibrinogen [Fg] and PAC1). Ligand binding to integrins initiates outside-in signaling that leads to cellular responses such as cell spreading and focal adhesion formation. A point mutation in the beta 3 cytoplasmic domain (S752-->P) is associated with blocked inside-out alpha IIb beta 3 signaling in a variant Glanzmann's thrombasthenia. This mutation was introduced into beta 3 and cotransfected into Chinese hamster ovary cells with a chimeric alpha subunit consisting of the alpha IIb extracellular and transmembrane domains and the alpha 6B cytoplasmic domain. The substitution of the alpha IIb cytoplasmic domain with that of alpha 6 led to activation of alpha IIb beta 3 to bind PAC1, mimicking inside-out signaling. This effect was reversed by the S752-->P mutation, indicating a disruption of inside-out signaling by the mutation. In addition, transfectants expressing this beta 3 variant showed reduced alpha IIb beta 3-mediated cell spreading on immobilized Fg, focal adhesion, and fibrin clot retraction, suggesting an impairment in outside-in alpha IIb beta 3 signaling. Therefore, a single point mutation in the beta 3 cytoplasmic domain impaired bidirectional signaling through alpha IIb beta 3.

Bone marrow transplantation for Glanzmann's thrombasthenia

Allogeneic matched bone marrow transplantation (BMT) was performed in a patient with type I Glanzmann's thrombasthenia, a rare, inherited bleeding disorder caused by a deficiency in the platelet membrane glycoprotein IIb-IIIa complex. The patient was a 2-year-old girl with a history of frequent hospitalisation. She was successfully transplanted with BM from her HLA-identical sibling. Engraftment was monitored by analysis of the platelet GPIIb-IIIa complex and by RFLP analysis using a minisatellite probe. Complete engraftment was seen at day +25. The patient has been clinically stable for 19 months. It is proposed that BMT is a suitable treatment for this condition where a matched, related donor is available and at an early stage, before the development of anti-platelet antibodies as a result of repeated transfusions.

Molecular genetics aspects of factor XI deficiency and Glanzmann thrombasthenia

Factor XI deficiency and Glanzmann thrombasthenia are among the hereditary disorders frequently encountered in Israel. Factor XI deficiency is particularly frequent in Ashkenazi (European) Jews with 1:190 individuals affected by the severe deficiency and 8.1% of the population being heterozygotes. So far 4 mutations causing factor XI deficiency have been identified of which the type II (a non-sense mutation) and type III (a missense mutation) are predominant and type I and IV observed only in 5 families. Recently, the type II mutation was observed in Iraqui-Jews as well with 3.7% of 400 unrelated subjects being heterozygotes and with the type III mutation completely absent. Since Iraqui-Jews represent the original gene pool of Jews who lived in Babylon 2500 years ago we hypothesize that the type II mutation is ancient and that the type III mutation occurred more recently, after the divergence of the original Babylonian Jews into Ashkenazi, Sephardic (Spanish) and Middle Eastern Jews. Preliminary data on factor XI intragenic polymorphic markers indeed indicate that type II and type III mutations reside on chromosomes each characterized by a different specific haplotype. Fifty living patients with type I Glanzmann thrombasthenia (28 families) have been observed in Israel. Most of them are Iraqui-Jewish and the rest are Arabs (5 families) and one Iranian Jewish. All Iraqui-Jewish patients have an IIbp deletion within exon 12 of the glycoprotein (GP) IIIa resulting in a shift of the reading frame that leads to premature termination of the GPIIIa synthesis.(ABSTRACT TRUNCATED AT 250 WORDS)

Human platelet alloantigen typing: PCR analysis is not a substitute for serological methods

Currently, five platelet alloantigen (alloAg) systems have been established (HPA-1, -2, -3, -4, -5). Three of these are expressed on the glycoprotein (GP) IIb-IIIa complex, HPA-1, HPA-3 and HPA-4, inherited in an autosomal codominant mode. Recent investigations of the molecular basis of these platelet alloantigen systems have shown that only one nucleic acid base substitution in the genes encoding for GP IIb and GP IIIa is responsible for the polymorphism. This substitution is reflected in a difference in restriction enzyme recognition allowing platelet alloantigen typing by restriction fragment length polymorphism (RFLP) analysis of DNA amplified by the polymerase chain reaction (PCR). To validate the PCR technology for platelet typing, we have compared PCR-RFLP with monoclonal-antibody-specific immobilization of platelet antigens (MAIPA). For this purpose, we have studied different Glanzmann thrombasthenic families and particularly heterozygous individuals, who are not lacking GP IIb-IIIa, as a model to detect the occurrence of discrepancies between these two technologies. In two families, we have found differences between molecular biology and serological methods with the lack of expression of one antigen on the platelet membrane surface. In the first family, the abnormality is related to the HPA-1 alloantigen system with three informative members; in the second, the HPA-3 alloantigen system is concerned with two informative members. Considering these results, there may not always be a perfect correlation between molecular biology and serological methods, as an unknown molecular defect could interfere with the PCR results and lead to false platelet typing.(ABSTRACT TRUNCATED AT 250 WORDS)

An exon 28 mutation resulting in alternative splicing of the glycoprotein IIb transcript and Glanzmann's thrombasthenia

The alternatively spliced from mRNA of platelet glycoprotein IIb (GPIIb) with a deletion of exon 28 (GPIIb-28) has been isolated from the HEL cell cDNA library. The defective expression on the surface of DNA cotransfected COS-1 cells with GPIIb-28 and GPIIIa cDNAs was described in an earlier report. We studied siblings with Glanzmann's thrombasthenia who expressed only the GPIIb-28 mRNA in their platelets. Flow cytometry showed that the patients' platelets failed to bind GPIIb/IIIa complex-specific and GPIIb-specific monoclonal antibody. Western blotting showed that the patients' platelets had defective GPIIb and have trace amounts of GPIIIa. Sequence analysis was performed after polymerase chain amplification of the patients' GPIIb and GPIIIa mRNAs. The patients' GPIIb cDNA had a deletion of the exon 28 nucleotides. The polymerase chain reaction (PCR) from exon 27 to 29 showed that the GPIIb-28 mRNA was 3% +/- 1.6% of the normally, spliced form in control platelets, and 61% in the megakaryoblastic cell line UT-7. The patients' platelets showed only the GPIIb-28. Family study and quantitative PCR studies showed that these patients were compound heterozygotes of two GPIIb gene defects. The father's allele is described in this report and involves skipping exon 28 secondary to a base substitution at codon Gln948, CAG-->TAG. The mother's allele appears to involve decreasing GPIIb mRNA levels in platelets. Our results indicate that the GPIIb-28 is not expressed on platelet membranes as a stable GPIIb/IIIa heterodimer or left as a monomer in platelets. Our studies confirm the previous data observed in COS-1 cells expressing recombinant GPIIb-28.

A single amino acid substitution flanking the fourth calcium binding domain of alpha IIb prevents maturation of the alpha IIb beta 3 integrin complex

To define specific structural domains involved in the biosynthesis and processing of integrin subunits, we examined the biosynthesis of normal and mutated forms of the platelet-specific integrin alpha IIb beta 3. Platelet mRNA was isolated from a Glanzmann thrombasthenic patient who failed to express significant levels of the glycoprotein (GP) IIb-IIIa complex on the platelet surface. Sequence analysis of polymerase chain reaction-amplified platelet GPIIb mRNA revealed a Gly418-->Asp amino acid substitution in GPIIb. Gly418 is a highly conserved residue that flanks the fourth calcium binding domain of GPIIb. Cotransfection of Asp418 GPIIb and GPIIIa plasmid constructs into COS-7 cells resulted in the accumulation of a pre-GPIIb-IIIa complex that failed to reach the cell surface, in effect recreating the thrombasthenic phenotype. Pulse-chase and endoglycosidase studies demonstrated that the biosynthetic blockade occurred in a pre-Golgi compartment. Removal of the negatively charged carboxyl group of Asp418 GPIIb, creating Ala418 GPIIb, rescued intracellular transport and surface expression of the integrin complex. Mutagenesis of a homologous Gly within the integrin alpha subunit alpha v also resulted in the failure to express alpha v beta 3 on the cell surface. These results suggest that the presence of a small, uncharged amino acid 6-8 residues amino-terminal to the calcium coordination complex is crucial for the proper folding and maturation of integrin complexes.

Glanzmann thrombasthenia secondary to a Gly273-->Asp mutation adjacent to the first calcium-binding domain of platelet glycoprotein IIb

We studied the defect responsible for Glanzmann thrombasthenia in a patient whose platelets expressed < 5% of the normal amount of GPIIb-IIIa. Genetic and biochemical evidence indicated that the patient's GPIIIa genes were normal. However, DNA analysis revealed the patient homozygous for a G818-->A substitution in her GPIIb genes, resulting in a Gly273-->Asp substitution adjacent to the first GPIIb calcium-binding domain. To determine how this mutation impaired GPIIb-IIIa expression, recombinant GPIIb containing the mutation was coexpressed with GPIIIa in COS-1 cells. The GPIIb mutant formed stable GPIIb-IIIa heterodimers that were not immunoprecipitated by either of two heterodimer-specific monoclonal antibodies, indicating that the mutation disrupted the epitopes for these antibodies. Moreover, the GPIIb in the heterodimers was not cleaved into heavy and light chains, indicating that the heterodimers were not transported from the endoplasmic reticulum to the Golgi complex where GPIIb cleavage occurs, nor were the mutant heterodimers expressed on the cell surface. These studies demonstrate that a Gly273-->Asp mutation in GPIIb does not prevent the assembly of GPIIb-IIIa heterodimers, but alters the conformation of these heterodimers sufficiently to impair their intracellular transport. The impaired GPIIb-IIIa transport is responsible for the thrombasthenia in this patient.

Glanzmann's thrombasthenia with mild von Willebrand's disease

A Saudi Arabian family is reported in which Glanzmann's thrombasthenia and von Willebrand's disease occurred simultaneously. The daughter presented with menorrhagia and gave a history of gastrointestinal bleeding and a strong family history of bleeding disorder. Full haematological investigations were performed on the propositus, parents, and siblings, including complete blood count, bleeding time, prothrombin time, partial thromboplastin time, factor VIII:C, von Willebrand factor, ristocetin cofactor, platelet aggregometry, platelet glycoprotein Ib and IIb/IIIa and platelet antigen PLT-1 (Coulter Clone). The propositus had Glanzmann's thrombasthenia, both parents had mild von Willebrand's disease and were carriers of Glanzmann's thrombasthenia. Three symptomatic brothers had both Glanzmann's thrombasthenia and von Willebrand's disease; two asymptomatic brothers had von Willebrand's disease only and one had completely normal results. Those family members with both diseases were more severely affected than those with just one disease. In areas where consanguineous marriage is common, such as Saudi Arabia, multiple haemostatic abnormalities may occur, and investigation should not stop with the discovery of a single abnormality. The increased clinical severity of bleeding, including haemarthroses, in those patients having both congenital defects emphasises the importance of von Willebrand factor in glycoprotein Ib-mediated platelet adhesion.