Human platelet glycoprotein IX. Characterization of cDNA and localization of the gene to chromosome 3

Molecular basis of Bernard-Soulier syndrome in 27 patients from India

BACKGROUND

Bernard-Soulier syndrome (BSS) is an extremely rare (1:1 million) bleeding disorder of platelet adhesion, caused by defects in the glycoprotein (GP)Ib/IX/V complex.

PATIENTS AND METHODS

The diagnosis in 27 patients was based on low platelet count, presence of giant platelets and aggregometry studies. Flow cytometry to assess the surface GPIb/IX/V complex showed reduced (7.7-57%) expression. gDNA was screened for mutations in the GPIBA, GPIBB, GP9 genes using PCR-conformation sensitive gel electrophoresis (CSGE).

RESULTS

Thirteen different disease-causing mutations, including missense (54%), frameshifts (38%) and nonsense (8%) mutations, were identified in 27 patients. Eleven of them were novel including five novel frameshifts (GPIbα: p.Gln97_98fsX113, p.Pro402_403fsX52; GPIbβ: p.Arg17fsX14; GPIX: p.Gly24fsX43, p. Pro130fs, a nonsense mutation (GPIX, p.94, Gln>X) and five novel missense mutations (GPIbα: p.492, Tyr>His; GPIbβ: p.65, Pro>Arg, p.129, Gln>His, p.132, Leu>Pro; GPIX: p.55, Phe>Cys). Interestingly, four common mutations, Cys8Arg (n = 6) and Phe55Ser (n = 2), Phe55Cys (n = 2) in GPIX and a novel 22-bp deletion in the GPIBB gene predicting p.Arg17fsX 14 (n = 10) were seen in 20 patients.

CONCLUSION

The molecular data presented here is the largest series of BSS patients to be reported so far, adding significantly to the mutation database of this condition and also useful for its genetic diagnosis in India.

Bernard-Soulier syndrome (hemorrhagiparous thrombocytic dystrophy)

Bernard-Soulier syndrome (BSS), also known as Hemorrhagiparous thrombocytic dystrophy, is a hereditary bleeding disorder affecting the megakaryocyte/platelet lineage and characterized by bleeding tendency, giant blood platelets and low platelet counts. This syndrome is extremely rare as only approximately 100 cases have been reported in the literature. Clinical manifestations usually include purpura, epistaxis, menorrhagia, gingival and gastrointestinal bleeding. The syndrome is transmitted as an autosomal recessive trait. The underlying defect is a deficiency or dysfunction of the glycoprotein GPIb-V-IX complex, a platelet-restricted multisubunit receptor required for normal primary hemostasis. The GPIb-V-IX complex binds von Willebrand factor, allowing platelet adhesion and platelet plug formation at sites of vascular injury. Genes coding for the four subunits of the receptor, GPIBA, GPIBB, GP5 and GP9, map to chromosomes 17p12, 22q11.2, 3q29, and 3q21, respectively. Defects have been identified in GPIBA, GPIBB, and GP9 but not in GP5. Diagnosis is based on a prolonged skin bleeding time, the presence of a small number of very large platelets (macrothrombocytopenia), defective ristocetin-induced platelet agglutination and low or absent expression of the GPIb-V-IX complex. Prothrombin consumption is markedly reduced. The prognosis is usually good with adequate supportive care but severe bleeding episodes can occur with menses, trauma and surgical procedures. Treatment of bleeding or prophylaxis during surgical procedures usually requires platelet transfusion.

Synthesis of GPIb beta with novel transmembrane and cytoplasmic sequences in a Bernard-Soulier patient resulting in GPIb-defective signaling in CHO cells

The molecular defect of a new Bernard-Soulier patient, originating from Morocco and presenting thrombocytopenia with large platelets and an absence of ristocetin-induced platelet agglutination, has been identified and reproduced in transfected heterologous cells. Gene sequencing revealed insertion of a guanine in the domain coding for the transmembrane region of the glycoprotein (GP) Ib beta subunit. This mutation causes a translational frame shift, which creates putative novel transmembrane and cytoplasmic 37 and 125 amino acids domains, respectively. A 34 kDa immunoreactive GPIb beta band, instead of the normal 26 kDa subunit, was detected by Western blotting in lysates from the patient's platelets and from transfected cells and in immunoprecipitates of metabolically labeled cells. The abnormal subunit did not associate with GPIb alpha and was mainly intracellular, although a significant fraction could reach the cell surface. Cells expressing the mutant GPIb-IX complex adhered to a von Willebrand factor matrix but were unable to change shape, unlike cells expressing the wild-type receptor. These results strongly suggest a novel role of the GPIb beta subunit and its transmembrane-intracellular region in GPIb-VWF-dependent signaling, in addition to a role in correct assembly and cell surface targeting of the GPIb-V-IX complex.

Thrombocytopenia in patients with 22q11.2 deletion syndrome and its association with glycoprotein Ib-beta

PURPOSE

To elucidate whether thrombocytopenia in 22q11.2 deletion syndrome patients is associated with the hemizygosity of glycoprotein Ib-beta and to clarify the correlation of phenotype and genotype of this gene in 22q11.2 deletion syndrome patients with thrombocytopenia.

METHODS

Platelet number, mean platelet volume, platelet agglutination, and the protein level of glycoprotein Ib-beta were measured in 22q11.2 deletion syndrome patients and controls. Phenotypes other than that of thrombocytopenia were also analyzed in these patients.

RESULTS

The 22q11.2 deletion syndrome patients with thrombocytopenia had a larger mean platelet volume, lower agglutination to ristocetin, and lower protein level of glycoprotein Ib-beta than control patients. The 22q11.2 deletion syndrome patients with thrombocytopenia showed an increased risk of developing schizophrenia.

CONCLUSIONS

Thrombocytopenia in 22q11.2 deletion syndrome patients is associated with decreased expression of glycoprotein Ib-beta because of the hemizygosity. 22q11.2 deletion syndrome patients with thrombocytopenia require total management, especially for schizophrenia.

The cysteine knot of platelet glycoprotein Ib beta (GPIb beta) is critical for the interaction of GPIb beta with GPIX

The glycoprotein Ib (GPIb) complex is composed of GPIb alpha covalently attached to GPIb beta and noncovalently complexed with GPIX and GPV. Patients with Bernard-Soulier syndrome demonstrate that mutations in either GPIb beta or GPIX result in an absence of platelet GPIb alpha. This occurs through the interaction of GPIX with GPIb beta. The precise sites of interaction of GPIb beta with GPIX are not known. To characterize the interaction of GPIb beta and GPIX, we developed an anti-GPIb beta monoclonal antibody MBC 257.4, whose epitope was in the N-terminal region of GPIb beta. N-terminal truncations of GPIb beta were expressed in mammalian cells. N-terminal truncations of GPIb beta, missing the first 14, 26, or 31 amino acids, were surface-expressed but did not enable coexpressed GPIX to be surface expressed, suggesting that the site of interaction with GPIX was modified by these deletions. GPIb beta and GPIX chimeras corresponding to predicted boundaries were used to define the sites of interaction of GPIb beta with GPIX. Replacing the N-terminal disulfide loops of GPIb beta (amino acids 1-14) with the corresponding disulfide loops of GPIX (amino acids 1-22) resulted in surface expression of coexpressed wildtype GPIX. However, when the N terminus of GPIb beta was replaced to residue 32 with the N terminus of GPIX (amino acids 1-36), GPIX did not surface express with this chimera. These results suggest that the cysteine knot region of GPIb beta in the N terminus is critical for the conformation of GPIb beta that interacts with GPIX and further suggests that a critical interaction of GPIb beta with GPIX involve residues 15 through 32 of GPIb beta

Rifampicin-dependent antibodies bind a similar or identical epitope to glycoprotein IX–specific quinine-dependent antibodies

The drug-dependent antibody of a patient with rifampicin-induced thrombocytopenia was characterized using the antigen-capture enzyme-linked immunosorbent assay (MAIPA assay), flow cytometry, and immunoprecipitation. The antibody was found to bind glycoprotein (GP) Ib-IX but not GPIIb-IIIa because (1) it immunoprecipitated drug-dependently the former but not the latter glycoprotein complex and (2) the MAIPA assay showed strong rifampicin-dependent antibody binding when anti-GPIb-IX monoclonal antibodies (mAbs) (AK2 and FMC25) but not anti-GPIIb-IIIa mAbs (AP2, SZ21, and SZ22) were used to capture the antigen. The antibody binding site was further localized to the GPIX subunit of the GPIb-IX complex because flow cytometric analysis revealed drug-dependent antibody binding to L cells transfected with human GPIbβ and GPIX complementary DNA (L βIX cells) but not with human GPIb and GPIbβ complementary DNA (L β cells). Finally, in the MAIPA assay, the rifampicin-dependent antibody almost completely cross-blocked the binding of the anti-GPIX mAb (SZ1) to platelets. Similar cross-blocking of SZ1binding to platelets by the quinine-dependent antibodies was also observed. This finding not only confirms that the epitope of the rifampicin-dependent antibody is on GPIX but it is also identical to or located in close proximity to that of the quinine-dependent antibody and SZ1. Further characterization of the epitopes of these antibodies may have important implications for a general understanding of the mechanism of drug-induced thrombocytopenia.

Single amino acid substitution in human platelet glycoprotein Ibβ is responsible for the formation of the platelet-specific alloantigen Iya

We recently described a new low-frequency platelet alloantigen on the human platelet glycoprotein (GP) Ib-IX complex, termed Iya, which was implicated in a severe case of neonatal alloimmune thrombocytopenia. Immunoprecipitation studies with trypsin-treated platelets indicated that the Iyaalloantigenic determinants are formed by the membrane-associated remnant moiety of GP Ib (GP Ibr) together with GP Ibβ and GP IX. To elucidate the molecular basis underlying the Iya alloantigen, we amplifiedGPIbr, GPIbβ, andGPIX genes by polymerase chain reaction (PCR). Nucleotide-sequence analysis of these 3 genes showed a G to A transition at position 141 on GPIbβ gene in a subject positive for Iya. This transition resulted in a Gly15Glu dimorphism on the N-terminal domain ofGPIbβ. This finding was confirmed by genotyping analysis of 6 Iya-positive subjects by restriction fragment length polymorphism (RFLP) studies using NarI endonuclease. In 300 randomly selected healthy blood donors, one Iya-positive individual was found. Phenotypes determined by monoclonal antibody-specific immobilization of platelet antigens assay and genotypes determined by RFLP were identical in this population. Analysis of Iya-positive platelets showed that the point mutation affected neither the degree of surface expression nor the function of the GP Ib-GP Ibβ-IX complex on the platelet surface. Transient expression of the GP Ib-IX complex in CHO cells using wild-type GP Ibβ (Gly15) or mutant GP Ibβ (Glu15) allowed us to demonstrate that this single amino acid substitution is sufficient to induce Iya epitope(s).

The Critical Interaction of Glycoprotein (GP) Ibβ With GPIX—A Genetic Cause of Bernard-Soulier Syndrome

Bernard-Soulier syndrome is an uncommon bleeding disorder caused by a quantitative or qualitative defect in the platelet glycoprotein (GP)Ib/IX complex. The complex is composed of four subunits, GPIb, GPIbβ, GPIX, and GPV. Here we describe the molecular basis of a novel Bernard-Soulier syndrome variant in a patient in whom GPIb and GPIX were undetectable on the platelet surface. DNA sequence analysis showed normal sequence for GPIb, GPIX, and GPV. The GPIbβ gene has been mapped to the 22q11.2 region of chromosome 22 which was deleted from one chromosome of this patient. There was a single nucleotide deletion within the codon for Ala 80 in GPIbβ within the other allele. This mutation causes a translational frame shift that encodes for 86 altered amino acids and predicts a premature stop 15 amino acids short of the length of the wild-type protein. Transient coexpression of the mutant GPIbβ in 293T cells with wild-type GPIb and GPIX resulted in the surface expression of GPIb, but the absence of GPIX. Moreover, when a plasmid encoding the wild-type GPIbβ was transiently transfected into Chinese hamster ovary cells stably expressing GP, which retain the capacity to reexpress GPIX, there was a significant increase in the surface expression of GPIX. In contrast, when the mutant GPIbβ was transiently transfected into these cells, GPIX was not reexpressed on the plasma surface. Thus, a deletion of one copy of GPIbβ and a single nucleotide deletion in the codon for Ala 80 within the remaining GPIbβ allele causes the Bernard-Soulier phenotype through an interaction of GPIbβ with GPIX resulting in the absence of GPIb on the plasma membrane. The interaction of GPIbβ with GPIX is essential for the functional expression of GPIb.

The Critical Interaction of Glycoprotein (GP) Ibβ With GPIX—A Genetic Cause of Bernard-Soulier Syndrome

Bernard-Soulier syndrome is an uncommon bleeding disorder caused by a quantitative or qualitative defect in the platelet glycoprotein (GP)Ib/IX complex. The complex is composed of four subunits, GPIb, GPIbβ, GPIX, and GPV. Here we describe the molecular basis of a novel Bernard-Soulier syndrome variant in a patient in whom GPIb and GPIX were undetectable on the platelet surface. DNA sequence analysis showed normal sequence for GPIb, GPIX, and GPV. The GPIbβ gene has been mapped to the 22q11.2 region of chromosome 22 which was deleted from one chromosome of this patient. There was a single nucleotide deletion within the codon for Ala 80 in GPIbβ within the other allele. This mutation causes a translational frame shift that encodes for 86 altered amino acids and predicts a premature stop 15 amino acids short of the length of the wild-type protein. Transient coexpression of the mutant GPIbβ in 293T cells with wild-type GPIb and GPIX resulted in the surface expression of GPIb, but the absence of GPIX. Moreover, when a plasmid encoding the wild-type GPIbβ was transiently transfected into Chinese hamster ovary cells stably expressing GP, which retain the capacity to reexpress GPIX, there was a significant increase in the surface expression of GPIX. In contrast, when the mutant GPIbβ was transiently transfected into these cells, GPIX was not reexpressed on the plasma surface. Thus, a deletion of one copy of GPIbβ and a single nucleotide deletion in the codon for Ala 80 within the remaining GPIbβ allele causes the Bernard-Soulier phenotype through an interaction of GPIbβ with GPIX resulting in the absence of GPIb on the plasma membrane. The interaction of GPIbβ with GPIX is essential for the functional expression of GPIb.

Quinine-Dependent Antibodies Bind a Restricted Set of Epitopes on the Glycoprotein Ib-IX Complex: Characterization of the Epitopes

Severe immune thrombocytopenia is an idiosyncratic complication of quinine therapy. Although in most cases the responsible antibody is directed against platelet membrane glycoprotein (GP) Ib-IX, specificity for GPIIb-IIIa or both epitopes has also been reported. The objective of this study was to characterize the binding site of GPIb-IX–specific quinine-dependent antibodies. Antibody binding to Chinese hamster ovary cells or mouse L cells stably transfected with various combinations of the three genes (Ibα, Ibβ, or IX) that encode this complex was detected using flow cytometry, monoclonal antibody–specific immobilization of platelet antigens assay, and differential adsorption studies. IgG in sera from 15 patients with quinine-induced thrombocytopenia binding to the cells, in the presence of quinine, showed three distinct patterns. Group 1 sera contained at least two antibody populations, one which binds to GPIbα and another which recognizes GPIX. Group 2 sera contained an antibody which binds drug dependently to GPIX, and Group 3 sera contained an antibody which recognizes a quinine-dependent epitope on GPIbα. Thus, the quinine-dependent antibodies fall into two distinct populations that bind to GPIbα and GPIX independently. Using proteases which cleave GPIbα at specific sites, we have shown that the GPIbα-specific antibody binds to an 11–amino acid (283 to 293) region. Peptide inhibition studies provide confirmatory evidence that this region contains the epitope for the GPIbα-specific quinine-dependent antibody.

Quinine-Dependent Antibodies Bind a Restricted Set of Epitopes on the Glycoprotein Ib-IX Complex: Characterization of the Epitopes

Severe immune thrombocytopenia is an idiosyncratic complication of quinine therapy. Although in most cases the responsible antibody is directed against platelet membrane glycoprotein (GP) Ib-IX, specificity for GPIIb-IIIa or both epitopes has also been reported. The objective of this study was to characterize the binding site of GPIb-IX–specific quinine-dependent antibodies. Antibody binding to Chinese hamster ovary cells or mouse L cells stably transfected with various combinations of the three genes (Ibα, Ibβ, or IX) that encode this complex was detected using flow cytometry, monoclonal antibody–specific immobilization of platelet antigens assay, and differential adsorption studies. IgG in sera from 15 patients with quinine-induced thrombocytopenia binding to the cells, in the presence of quinine, showed three distinct patterns. Group 1 sera contained at least two antibody populations, one which binds to GPIbα and another which recognizes GPIX. Group 2 sera contained an antibody which binds drug dependently to GPIX, and Group 3 sera contained an antibody which recognizes a quinine-dependent epitope on GPIbα. Thus, the quinine-dependent antibodies fall into two distinct populations that bind to GPIbα and GPIX independently. Using proteases which cleave GPIbα at specific sites, we have shown that the GPIbα-specific antibody binds to an 11–amino acid (283 to 293) region. Peptide inhibition studies provide confirmatory evidence that this region contains the epitope for the GPIbα-specific quinine-dependent antibody.

The mouse chondroadherin gene: characterization and chromosomal localization

The mouse chondroadherin gene was isolated from a cosmid genomic library by the use of a rat chondroadherin cDNA probe. Southern blot analysis of mouse genomic DNA revealed a simple pattern of hybridization indicating a single copy gene for chondroadherin. The mouse chondroadherin gene encompasses 4.1 kb and consists of four exons separated by one large intron of 1929 bp followed by two smaller introns of 247 and 225 bp, respectively. Most of the translated region, including the start codon and the main part of a leucine-rich region, is contained within the first exon. Two small exons of 164 and 146 bp encode the rest of the protein. Interestingly, 4 bases from the stop codon, in the 3'-UTR, a third intron is located. A putative promoter region of 669 bp was sequenced and shown to contain a potential TATAA-box signal 29 bp upstream of the transcription start site and several recognition sites for transcription factors. The exon/intron organization of the chondroadherin gene differs from those of the other known genes of the leucine-rich repeat (LRR) family in the extracellular matrix. Taken together with comparison of protein sequences of other members of the LRR family in the extracellular matrix, the data suggest that chondroadherin has evolved along a different pathway. The chondroadherin gene was mapped to mouse chromosome 11, near D11Mit14, by single-strand conformation polymorphism linkage analysis.

Human Endothelial Cells in Culture and In Vivo Express on Their Surface All Four Components of the Glycoprotein Ib/IX/V Complex

The platelet glycoprotein Ib (GpIb) complex is composed of four polypeptides: the disulfide-linked GpIbα and GpIbβ and the noncovalently associated GpIX and GpV. GpIbα contains binding sites for von Willebrand factor and for thrombin and mediates platelet adhesion to the subendothelium under conditions of high shear stress. We have previously shown the presence of GpIbα and GpIbβ mRNA and protein in cultured human umbilical vein endothelial cells (HUVECs) as well as the presence of GpIbα mRNA and protein in tonsillar endothelium. We, therefore, probed ECs for the presence of the other components of the GpIb/IX/V complex. We have identified the presence of GpIX and GpV mRNA in cultured HUVEC monolayers. The sequence of HUVEC GpIX cDNA was identical to the previously published human erythroleukemia (HEL) cell GpIX cDNA sequence. Two species of GpV mRNA, one of 3 kb and one of 4.4 kb, were found in HUVECs, whereas HEL cells displayed only the 4.4-kb species and the megakaryocytic cell line CHRF-288 contained only the 3-kb species. We previously showed that EC GpIbα protein is identical in molecular weight to platelet GpIbα. HUVEC GpIbβ, in contrast to its platelet counterpart, has a molecular weight of 50 kD and forms a correspondingly larger disulfide-bonded complex with EC GpIbα. The molecular weights of GpIX and GpV were 22 and 88 kD, respectively, identical to the corresponding platelet polypeptides. Furthermore, we have identified all four components of the complex in tonsillar vessels. Using flow cytometry, we have established that all four polypeptides of the GpIb/IX/V complex are expressed on the surface membranes of cultured HUVECs and adult aortic ECs. Furthermore, using two-color fluorescence, we have shown that all ECs expressing GpIbα also express GpIX and GpV on their surface. The ratio of GpIbα:GpIX:GpV is 1:1:0.5, which is identical to the ratio present in platelets. None of the polypeptides of the GpIb complex could be identified on the surface of human smooth muscle cells or lymphocytes. The presence of all members of the GpIb complex in the EC membrane suggests that this complex may play a role in endothelial function in vivo.

Human Endothelial Cells in Culture and In Vivo Express on Their Surface All Four Components of the Glycoprotein Ib/IX/V Complex

The platelet glycoprotein Ib (GpIb) complex is composed of four polypeptides: the disulfide-linked GpIbα and GpIbβ and the noncovalently associated GpIX and GpV. GpIbα contains binding sites for von Willebrand factor and for thrombin and mediates platelet adhesion to the subendothelium under conditions of high shear stress. We have previously shown the presence of GpIbα and GpIbβ mRNA and protein in cultured human umbilical vein endothelial cells (HUVECs) as well as the presence of GpIbα mRNA and protein in tonsillar endothelium. We, therefore, probed ECs for the presence of the other components of the GpIb/IX/V complex. We have identified the presence of GpIX and GpV mRNA in cultured HUVEC monolayers. The sequence of HUVEC GpIX cDNA was identical to the previously published human erythroleukemia (HEL) cell GpIX cDNA sequence. Two species of GpV mRNA, one of 3 kb and one of 4.4 kb, were found in HUVECs, whereas HEL cells displayed only the 4.4-kb species and the megakaryocytic cell line CHRF-288 contained only the 3-kb species. We previously showed that EC GpIbα protein is identical in molecular weight to platelet GpIbα. HUVEC GpIbβ, in contrast to its platelet counterpart, has a molecular weight of 50 kD and forms a correspondingly larger disulfide-bonded complex with EC GpIbα. The molecular weights of GpIX and GpV were 22 and 88 kD, respectively, identical to the corresponding platelet polypeptides. Furthermore, we have identified all four components of the complex in tonsillar vessels. Using flow cytometry, we have established that all four polypeptides of the GpIb/IX/V complex are expressed on the surface membranes of cultured HUVECs and adult aortic ECs. Furthermore, using two-color fluorescence, we have shown that all ECs expressing GpIbα also express GpIX and GpV on their surface. The ratio of GpIbα:GpIX:GpV is 1:1:0.5, which is identical to the ratio present in platelets. None of the polypeptides of the GpIb complex could be identified on the surface of human smooth muscle cells or lymphocytes. The presence of all members of the GpIb complex in the EC membrane suggests that this complex may play a role in endothelial function in vivo.

Alternative expression of platelet glycoprotein Ib(beta) mRNA from an adjacent 5' gene with an imperfect polyadenylation signal sequence

Glycoprotein (GP) Ib is a major component of the platelet membrane receptor for von Willebrand factor, designated the GP Ib-IX-V complex. GP Ib is composed of two subunits (GP Ib(alpha) and GP Ib(beta)) each synthesized from separate genes. The 206 amino acid precursor of GP Ib(beta) is synthesized from a 1.0-kb mRNA expressed by megakaryocytes and was originally characterized from cDNA clones of human erythroleukemic (HEL) cell mRNA, a cell line exhibiting megakaryocytic-like properties. The cell line CHRF-288-11 also exhibits megakaryocytic-like properties, but synthesizes two related GP Ib(beta) mRNA species of 3.5 and 1.0 kb. We performed cDNA cloning experiments to identify the origin of the 3.5-kb transcript and determine its relationship to the 1.0-kb GP Ib(beta) mRNA found in megakaryocytes, platelets, and HEL cells. Our cloning experiments demonstrate that the longer transcript results from a nonconsensus polyadenylation recognition sequence, 5'AACAAT3', within a separate gene located upstream to the platelet GP Ib(beta) gene. In the absence of normal polyadenylation the more 5' gene uses the polyadenylation site within its 3' neighbor, the platelet GP Ib(beta) gene. This newly identified 5' gene contains an open reading frame encoding 369 amino acids with a high degree of sequence similarity to an expanding family of GTP-binding proteins.

Disorders of platelet function

Qualitative platelet disorders are described and reviewed above. The acquired platelet function defects are very common, and sometimes result in hemorrhage, especially in association with trauma or surgery. However, the specific biochemical defect is absent, and no characterized platelet abnormalities have been recognized. On the other hand, the hereditary qualitative platelet defects are rare, but the platelet abnormalities are characteristic. The study of these patients had led to an increased understanding of the normal primary hemostatic mechanism. Recently, the molecular basis analysis of the platelet defects has been developed. This will help us understand the molecular events involved in platelet adhesion and aggregation.

A three-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.

Inherited giant platelet disorders

Giant platelet disorders (GPD) refer to rare, usually inherited states characterized by abnormally large platelets, thrombocytopenia and bleeding tendency of variable severity. This review summarizes major clinical and laboratory features of three GPDs (Bernard-Soulier syndrome, May-Hegglin anomaly and gray platelet syndrome). Differential diagnosis between immunological thrombocytopenia and GPDs is important. Although rare, giant platelet disorders should be borne in mind, since bleeding tendency in some individuals may be severe and knowledge of bleeding diathesis is of importance before delivery or surgical procedures also in less symptomatic individuals.

Complementary DNA cloning of the alternatively expressed endothelial cell glycoprotein Ib beta (GPIb beta) and localization of the GPIb beta gene to chromosome 22

Glycoprotein Ib beta (GPIb beta) exists in platelets disulfide-linked to glycoprotein Ib alpha (GPIb alpha), a major receptor for von Willebrand factor. Both GPIb alpha and GPIb beta are expressed in endothelial cells (EC). While the GPIb alpha mRNA and protein appear similar in platelets and EC, EC GPIb beta mRNA is larger than platelet GPIb beta and encodes a larger protein. We have cloned and sequenced EC GPIb beta cDNA and report a 2793-nucleotide sequence which contains a 411-amino acid open reading frame. The EC sequence contains all of the platelet cDNA sequence and all but three amino acids of the primary translation product. Like the genes encoding GPIb alpha, GPIX, and GPV, the GPIb beta gene appears simple in structure. Using human hamster hybrids, we have localized the GPIb beta gene to chromosome 22pter-->22q11.2. When we examined poly (A)+ RNA from several human tissues for GPIb beta mRNA expression, we found that GPIb beta mRNA was expressed in a variety of tissues but was most abundant in heart and brain, while GPIb alpha and GPIX mRNA expression was found only in lung and placenta at very low levels. The broad distribution of GPIb beta mRNA suggests that it may be playing a role different than or additional to its function in platelets.

Human platelet glycoprotein V: characterization of the polypeptide and the related Ib-V-IX receptor system of adhesive, leucine-rich glycoproteins

Human platelet glycoprotein (GP) V (M(r) 83,300), whose primary structure is reported here, is a part of the Ib-V-IX system of surface glycoproteins (GPs Ib alpha, Ib beta, V, IX) that constitute the receptor for von Willebrand factor (vWf) and mediate the adhesion of platelets to injured vascular surfaces in the arterial circulation, a critical initiating event in hemostasis. System members share physical associations, leucine-rich glycoprotein (LRG) structures, and a congenital deficiency state, Bernard-Soulier syndrome. With PCR techniques and platelet cDNA templates, 1.4 kb of GP V cDNA sequence was obtained that encodes 469 GP V amino acids. A genomic 3.5-kb BamHI fragment was then isolated that includes 3.46 kb of GP V cDNA sequence: the 1.7-kb open reading frame plus 2 bases of the 5' and 1.8 kb of the 3' untranslated regions. Northern blot analysis reveals three GP V platelet transcripts of 3.8, 4.2, and 5.2 kb. A 16-amino acid signal peptide is present. Mature GP V is a 544-amino acid transmembrane protein with a 504-amino acid extracellular domain that encompasses a set of 15 tandem LRG repeats in a "flank-LRG center-flank" array [Roth, G. J. (1991) Blood 77, 5-19] along with eight putative N-linked glycosylation sites and cleavage sites for thrombin and calpain. GP V is a transmembrane, adhesive LRG protein that plays an undefined, but potentially critical, role in the expression and/or function of the Ib-V-IX receptor for vWf/shear-dependent platelet adhesion in arteries.

A review of the role of platelet membrane glycoproteins in the platelet-vessel wall interaction

This review concerns our understanding of the molecular basis of platelet function in haemostasis. In particular, we indicate how research into platelet membrane glycoprotein (GP) receptors is yielding vital information on the mechanisms of platelet adhesion and aggregation. These receptors, nearly always complexes of two or more subunits, are now known to belong to distinct gene families, some of which are unique to platelets while others are widely distributed in mammalian tissues. GP Ib-IX complexes are responsible for the high-shear-rate-dependent adhesion of platelets to von Willebrand factor (vWF) exposed within the subendothelium of damaged vessels. Other adhesion receptors include members of the VLA subclass of the integrin family: VLA-2, VLA-5 and VLA-6, which mediate platelet adhesion to collagen, fibronectin and laminin, respectively. Platelet aggregation is initiated by distinct populations of receptors specific for each physiological agonist. Many of these receptors, including the highly important and recently cloned thrombin receptor, have seven transmembrane domains and possess highly selective agonist-binding determinants. Finally, we highlight platelet aggregation and the role of GP IIb-IIIa complexes which, following platelet activation, bind fibrinogen and other adhesive proteins. The latter, through being polyvalent for GP IIb-IIIa, then form the bridges linking adjoining platelets. The 'ligand-binding pocket' of GP IIb-IIIa contains at least three sequences essential for ligand binding; fibrinogen also binds to the activated complex through identified domains, one of which, the Arg-Gly-Asp (RGD) sequence, is also found in vWF and the other adhesive proteins able to support platelet aggregation. Finally, we further describe how these, and other glycoproteins in both surface and internal membrane systems, constitute a complex receptor network capable of translocation and reorganization after platelet activation. In cardiovascular disease, platelets accumulate within arteries whose luminal surface has been modified through atherosclerosis. Recent molecular advances are yielding exciting opportunities for the development of new, and more powerful, drugs acting as specific inhibitors of thrombotic processes.