- Email: [email protected]
The Bernard-Soulier Syndrome
extrinsic pathway of coagulation. Blood 69: 645-651. Sevinsky JR, Rao LVM, Ruf W: 1996. Ligandinduced protease receptor translocation into caveolae: a mechanism for regulating cell surface proteolysis of the tissue factor-dependent coagulation pathway. J Cell Biol 133: 293-304. WarshawskyI, Broze GJ Jr,Schwartz AL: 1994. The low density lipoprotein receptor-related protein mediates the cellular degradation of tissue factor pathway inhibitor. Proc Natl Acad Sci USA 91:6664-6668. Warshawsfg I, Bu G, Mast A, Saffitz JE, Broze GJJr,SchwartzAL: 1995. The carboxy terminus of tissue factor pathway inhibitor is required for interactingwith hepatoma cells in vitro and in vivo. J Clin Invest95:1773–1781. Warshawsky I, Herz J, Broze GJ JL Schwartz AL: 1996. The low density lipoprotein receptor-related protein can function independently from heparin sulfate proteoglycans in tissue factor pathway inhibitor endocytosis. J Biol Chem 271:25,873-25,879.
Wesselschmidt R, LikertK, Girard T, Wun T-C, Broze GJ Jr: 1992. Tissue factor pathway inhibitor: the carboxy-terminus is required for optimaf inhibition of factor Xa. Blood 79: 2004-2010. Wesselschmidt R, Likert K, Huang Z-F, MacPhail L, Broze GJ Jr: 1993. Structural requirementsfor tissuefactor pathwayinhibitor interactionswith factor Xa and hepann. Blood Coagul Fibrinolysis 4:661. Willnow TE, ArmstrongSA, Hammer RE, Herz J: 1995. Functional expression of low density lipoprotein receptor-related protein is controlled by receptor-associatedprotein in vivo. Proc Natl Acad Sci USA 92:4537+541. Wun T-C, Kretzmer KK, Girard TJ, Miletich JP, Broze GJ Jr: 1988. Cloning and characterization of a cDNA coding for the lipoprotein-associated coagulation inhibitor shows that it consists of three tandem Kunitz-type inhibitory domains. J Biol Chem 263:6001-6004. TCM
The Bernard-Soulier Syndrome Barbara A. Konkle
The Bernard-Soulier syndrome (BSS) is a rare inherited disorder of platelet function due to absence or markedlydecreased expression of the platelet GpIb–V–IX receptoz This ~eceptorplays a critical role in hemostasis by initiating platelet adhesion and subsequent activation at the site of vessel injury. Through its link to the platelet cytoskeleton, the GpIb–V–IXcomplex appears to be ~equiredto maintain norrnalplatelet size and structure. Elucidation of the cDNA and genomic structures of the members of this complex, as well as the mutations responsible for BSS, has greatly advanced our knowledge of the structure and function (Trends Cardiovasc Med of the GpIb–V–IX complex in hemostasis. 1997; 7:239–244). @ 1997, Elsevier Science Inc.
The platelet glycoprotein Ib-V-IX (GpI& V–IX) recepto~ the receptor affected in the Bernard-Soulier syndrome (BSS), mediates initialplateletadhesion to the injured
Barbara A. Konkle is at the Cardeza Foundation for Hematologic Research, Department of Medicine, Jefferson Medical College of Thomas Jefferson University, Philadelphia, PA 19107, USA.
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vessel wall through binding to exposed von Willebrand factor (vWF), particularly under conditions of high shear stress [reviewed by Berndt et al. (1995)]. GpIb is a transmembraneheterodimerthatin platelets is comprised of a 140-kDalpha chain (GpIba;CD42b) disulfide, linked to a smaller 24-kD beta chain (GpIb~;CD42c). In the platelet,membrane GpIb existsin a 1:1 noncovalent complex with GpIX
(20kD;CD42a)and is more loosely associated with GpV (82 kD;CD42d) [reviewed by Lopez (1994)]. In most cases of BSS, all of these proteins are absent or affected, suggesting that they must all be present for stable expression and function of the complex.
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ClinicalManifestationsof Bernard-SoulierSyndrome
Patients with BSS usually present with a mild to moderate bleeding disorder, manifest most notably by mucosal bleeding. Bleeding may, however, be severe and life threatening. Two French physicians, Jean Bernard and JeanPreue Soulier initially described the disorder in 1948 (Bernard and Soulier 1948). The proband presented 15 days after birth with epistaxis and rectal bleeding and continued to have frequent ecchymoses, gastrointestinal bleeding, and trauma-induced bleeding until he died at the age of 28 of a cerebral hemorrhage after a barroom brawl. He had one affected sibling, but his parents and other siblings were phenotypically normal compatible with an autosomal recessive inheritance. In the early 1970s, BSS platelets were shown to be defective in vWF-dependent adhesion (Weiss et al. 1974), and in 1975 Nurden and Caen identified the defect to be in platelet GpIb receptor expression. In 1983, Berndt et al. described the associated deficiencies of GpIb~, GpIX, and GpV in BSS platelets. Patients with BSS are thrombocytopenic, with giant platelets noted on the peripheral smear. Except in the one family with an autosomal dominant form of BSS, obligate heterozygotes do not have bleeding symptoms but have been shown to have increased mean platelet volumes and quantitatively decreased platelet GpIba surface expression (de Marco et al. 1986). Greater than one third of BSS platelets may have a diameter of >3.5 pM, and platelets have been noted to have increased membrane deformability and abnormal filopodia formation with activation. The abnormal platelet morphology is likely secondary to the loss of the GpIb-V-IX–cytoskeletal link and supports a role for the complex in maintaining normal platelet structure. Platelet counts are usually between 50,000 and 100,000 but may be reported to be much lower with automated count-
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ing owing to some platelets being counted as lymphocytes because of their size. The reason for the thrombocytopenia is unclear; possible explanations include decreased platelet release or a normal physiologic response to a normal platelet mass with a decreased number of platelets compensated by their large size. Another explanation has been that the abnormal platelets are removed by the spleen. Because of the large platelets and thrombocytopenia, patients may be initially diagnosed with immune thrombocytopenia purpura (ITP) and undergo splenectomy. This has not, howeve~ been reported to increase platelet number in BSS. At our institution, we follow two brothers with classic BSS due to a homozygous mutation of GpIX (Asn45+ Ser), one of whom had a splenectomy in the past. There is no significant difference in their platelet counts or platelet morphology. Patientswith BSS have a markedlyprolonged bleeding time and reduced or absent ristocetin-induced platelet aggregation and decreased platelet adhesion to the exposed subendothelium, reflecting the lack of platelet GpIb-vWF interaction [reviewed by Berndt et al. (1995)]. Other agonist-inducedplateletaggregation,such as with ADP and epinephrine,are normal. AIIeven rarer disorder of GpIb-IX, termed pseudo- or platelet-type von Willebrand disease results from gain of function mutations in GpIbci that are associated with increased binding to vWF and present clinically similar to type 2B vWD. The gastrointestinal tract is a commonly reported bleeding site in patients with BSS. Recently, there have been two reports of angiodysplasia in patients with BSS (Bellucci et al. 1995, Okamur et al. 1996). Interestingly,this is a defect that has been reported repeatedly in patients with vWD, particularly in inherited or acquired type 2 subtype (Fressinaud and Meyer 1993). The GpIb– V–IX complex is present in endothelial cells (Konkle et al. 1990, Kelly et al. 1994, B.A. Konkle and S.S. Shapiro unpublished observations), and it is interesting to postulate a role for endothelial GpIb with vWF in maintaining normal endothelial structure. The treatment of bleeding in patients with BSS is limited to platelet transfusion if local measures are ineffective, although antifibrinolytic therapy may be useful alone or in conjunction with
240
vWF and Thrombin binding N-linked carbohydrate O-linked carbohydrate
membrane
-
-
*
(
(
1
Actin-binding protein Figure 1.The GpI&V-IX complex in platelets.This drawing depicts GpIba, GpIb~, GpIX and GpV in a 1:1:1:0.5 ratio, respectively,as well as the association of the complex with actin-binding protein. Secondary structuredue to disulfide-bonding is schematized per Lopez (1994).
platelet transfusion in nonurinary tract bleeding. The vasopressin analogue, DDAVP,which is effective in some platelet disorders (Rao et al. 1995), has not been shown to be effective in treating patients with BSS. Because BSS patients may require transfusion over their lifetime, leukocyte-depleted single donor platelets should be used for transfusion to prevent alloimunization, if possible.
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Structureof the GpIb-V-IX Complex
The GpIb-V-IX complex is illustratedin
Figure 1. Approximately 25,000 copies of GpIb and GpIX and one-half that number of GpV molecules are present on the platelet surface, suggesting that at least two GpIb-IX complexes may be linked together on the surface [reviewed by Lopez (1994)]. The GpIba protein extends60 nm from the plateletsurface, significantlyfarther than other major components of the plateletmembrane. This is due to the long macroglycopeptide consisting of serine-, threonine-, and proline-rich repeats that are heavily O-glycosylated and, with the globular N-terminaldomain, give a “palm tree’’-likestructure to the protein. The extended structureof GpIbcxpositions it well to initiate platelet adhesion to the vessel wall.
In nonactivated platelets, the intracytoplasmic tail of GpIba binds actin-binding
protein (ABP) and thereby links the complex to the cytoskeleton (Fox 1985). The site of interaction,recentlylocalized to the region of amino acids 185&2136 in the C-terminal domain of ABP (Meyer et al. 1997), is proximal to the region through which ABP dimerizes, and such dimerization could maintain two GpIb subunits in proximity on the platelet membrane. The site of interaction between the GpIb heterodimer and GpIX is unknown. cDNA cotransfection studies by Lopez et al. [reviewed by Lopez (1994)] suggested that GpIX associated with GpIb(3, whereas studies of Wu et al. (1996), evaluatingsurface antibody binding to members of the complex, suggested a direct interaction between GpIba and GplX. These findings may not be contradictory as one study evaluatedintracytoplasmic, and the other, surface association. The GpIb–V–IX receptor is not an integrin, but is a member of the leucinerich repeat (LRR) glycoprotein family [reviewed by Kobe and Deisenhofer (1994)]. Members of this family share a varying number of 20–29 residue leucine-rich repeats. The crystal structure of porcine ribonuclease inhibitor protein, which contains 15 LRRs, has been determined as consisting of ~-a struc-
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tural units, with short ~-strands and cx-helicesapproximately parallel to each other. GpIba, GpIb~, GpIX, and GpV share in common these LRRs; GpIba has seven, GpV 15, and GpIb~ and GpIX one each. The function of the LRR repeats is unknown, but most proteins in this family are involved in adhesion and/or cell signaling. Although the function of the LRR in the GpIb complex is unknown, as discussed later in this review, the fact that a number of missense mutations in the LRR or the LRR flanking region of GpIba and GpIX result in BSS demonstrates the importance of this domain in maintaining the structure and/or function of the GpIb–V–IX complex. Based on the crystal structure of the porcine ribonuclease inhibitor protein, it is unlikely that a single leucinerich motif would adopt a similar conformation (Kobe and Deisenhoger 1994). Because GpIba and GpIX have only one LRR each they could form part of a complex structure, as described for the ribonuclease inhibitor, through interaction with GpIba and GpV. This would suggest a much more interrelated structure than that proposed in Figure 1. The cDNA and genomic structures of GpIba, GpIb~, GpIX, and GpV have been reported [reviewed by Lopez (1994)]. These proteins are all products of different genes. The GpIba and GpIb(3 genes have been localized to chromosomes 17p12ter and 22ql 1.2, respectively, and the GpIX and GpV genes to different regions of chromosome 3, 3q21 and 3q29, respectively (Yagi et al. 1995). In addition to sharing varying degrees of homology, the genes are similar in their simple structure, having no introns within the coding region of the mature peptide. The GpIb~ gene has been localized to 22ql 1.2, a region of chromosome 22 termed the DiGeorge critical region, as there is a microdeletion of this region on one chromosome 22 allele in 900/0of patients with DiGeorge syndrome and in 85% of patients with the related velocardiofacial syndrome (Kelly et al. 1994, Budarf et al. 1995). The defect responsible for these syndromes is thought to cause abnormal migration of the cephalic neural crest cells. These cells contribute to the development of the thymus, parathyroid, and conotruncus of the heart. Whether the GpIb~ gene could play a role in these disorders is unknown.
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Transgenic mice can express human GpIba on the mouse platelet surface, apparently covalently linked to mouse GpIb(3 (Ware et al. 1993b). Future studies identifying evolutionarily conserved sequences and domains in GpIba, GpIb~, GpIX, and GpV will assist in the identification and further characterization of regions important in the structure and function of these proteins.
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Function of the GpIb-V-IX Complex
thrombin binding. Interestingly, GpV is the only platelet surface protein other than the seven-transmembrane domain thrombin receptor known to be susceptible to thrombin cleavage, although it is not clear whether cleavage of GpV by thrombin has any functional significance. The roles of GpIb(3, GpIX, and GpV in platelets is not well characterized. Patients with BSS with defects in either GpIb~ or GpIX (see Table 1) have markedly decreased expression of GpIbcx, demonstrating the requirement of GpIb~ and GpIX for assembly and/or stable expression of the complex on the platelet surface. This requirement for coordinate expression has also been demonstrated in vitro. In a series of studies, Lopez and coworkers showed that membrane expression of GpIba requires the coexpression of GpIb~, and that optimal surface expression requires, in addition, the presence of GpIX [reviewed by Lopez (1994)]. GpV may enhance the membrane expression of the GpIb–IX complex (Calverley et al. 1995), and stable expression of GpV itself may be dependent on GpIb–IX expression (Meyer and Fox 1995). GpIb~ contains a phosphorylatable serine residue (Ser’66) in its cytoplasmic tail, whose degree of phosphorylation may vary with the state of activation of the platelet, although quantitative studies of this phenomenon have not be reported.
PlateletGpIba binds von Willebrand factor (vWF) and thrombin in its aminoterminal extracytoplasmic region as indicated in Figure 1. Through vWF binding, GpIb plays a critical role in platelet adhesion to the vascular subendothelium, especially under conditions of high shear, where platelet adhesion to damaged subendothelium can be shown to be dependent upon the presence of GpIb and vWF [reviewed by Lopez (1994), Berndt et al. (1995)]. As is well known, abnormalities in the production of vWF lead to the bleeding disorder known as von Willebrand disease. Binding of vWF to GpIba has been shown to initiate a signaling cascade involving intracellular release of Ca2+, phosphorylation of several platelet proteins, ADP release, thromboxane synthesis, and activation of the GpaIIb–~3 complex (Kroll et al. 1991, Clemetson 1995). The components of the signaling pathway ● Characterizationof Mutations are largely unknown, although recently Responsible for Bernard-Soulier a protein of the 14-3-3 superfamily with Syndrome phospholipase A2 activity (PLA2~) has been identified in platelets in association As of Februa~ 1, 1997, mutations rewith the GpIb complex (Du et al. 1994, sponsible for BSS, elucidated in 16 1996). PLA2~seems capable of binding unrelated families and representing 15 to the complex via GpIbci, GpIb~ ancUor independent mutations have been reGpV, according to data recently pre- ported in full manuscript form (see Tasented but not yet published in detail. ble 1). Ten of these mutations are in GpIbci also contains a high-affinity GpIba, 4 are in GpIX, and 1 in GpIb(3.Of thrombin binding site [reviewed by Jan- the 15 mutations, 6 are in the LRR or the drot-Perruset al. (1996)]. The exact role of adjacent flanking regions and 6 produce this binding in platelet function is un- premature stop codons. Of the other known, particularlyin relationship to the three mutations, one in GpIba and one “classic” seven-transmembrane domain in GpIX obliterate cysteines thought to thrombin receptor present on the platelet be involved in intrachain disulfide bindsurface. Studies suggest that thrombin ing and the other is in the promoter binding to the GpIb-V-IX complex may region of platelet GpIb~. The GpIb(3mube of physiologic importance at very low tation occurred in a patient with velocarconcentrations of thrombin, triggering diofacial syndrome with only one GpIb~ platelet activation in this setting. Unlike allele due to a microdeletion of the rethe seven-transmembranedomain throm- gion of the other chromosome 22 allele bin recepto~ GpIba is not cleaved by containing the GpIb~ gene. The pro-
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None
Autosomal recessive; homozygous
Autosomal recessive; homozygous
Autosomal recessive; homozygous Autosomal recessive; homozygous Autosomal recessive; homozygous
Ala’5’ + Val (point mutation) (LRR)
Ser444 + stop (point mutation)
cys*09+ Ser (point mutation) LeuL29+ Pro (point mutation) (LRR) Leu179 (TCC) deletion (LRR)
Trp126 + stop (point mutation)
Absent GpIX; decreased GpIba, GpV
“ Expression of mutation transected into heterologous cells b These expression studies were done by Sae-Tung et al. (1996). LRR, leucine-rich repeat; NA, not applicable; nl, normal; nt, nucleotide.
Absent GpIb(3; markedly decreased GpIba, GpIX, and GpV
Autosomal recessive: compound heterozygous with microdeletion of other 22ql 1 allele
Autosomal recessive: homozygous
Cys73 + Tyr (point mutation)
Absent GpIX; markedly decreased GpIba; GpV
– 133 C + G (mutation of GATA binding site in promoter)
Autosomal recessive; compound heterozygous Autosomal recessive: homozygous Autosomal recessive: homozygous in two unrelated families
Asn45 + Ser (point mutation) (LRR)
Markedly decreased GpIX (2?4.) and GpIba, GpIb(3, and GpIX (all 7~o)
GpIbf3mutation
Autosomal recessive: compound heterozygous
Markedly decreased promoter function in transection assay
No GpIX surface expression when transected with GpIb(3 N/A; premature stop codon
No GpIX surface expression; did not promote G~Iba surface expression As for Asp*’ + Gly mutation
N/A premature stop codon
Autosomal recessive; homozygous
Asp*l+ Gly (point mutation) (LRR)
N/A; premature stop codon
Autosomal recessive; homozygous
Ludlow et al. (1996)
Noda et al. (1995)
Wright et al. (1993) Clemetson et al. (1994) Noda et al. (1996)
Wright et al. (1993)
Li et al. (1996)
Noda et al. (1995)
N/A; premature stop codon
Simsek et al. (1994b) Li et al. (1995)
Kunishima et al. (1994)
Ware et al. (1993)
Autosomal recessive; homozygous
None
Loss of antibody and von Willebrand factor binding N/A; premature stop codon
Miller et al. (1992)
Ware et al. (1990)
Reference(s)
de la Salle et al. (1995) Simsek et al. (1992a)
Absent GpIX; markedly decreased GpIba
GpIX mutations
Absent risto-induced platelet aggregation; protein studies not done
Absent surface GpIba, truncated protein in plasma; moderately decreased GpIb~, GPIX Absent GpIba~moderately decreased GpIX, GpV Moderately decreased GpIba, GpIX, nl GpV Absent G Ibci; decreased GpIb~, GpV; nr GpIX Markedly decreased GpIbct; decreased GpIX, GpV Absent GpIba; markedly decreased GpIX, GpV Leu’6 Autosomal recessive (del T317) stop codon at amino acid 95) Deletion of nucleotides 1932 + 1938; frame-shift, new stop codon at amino acid 509 One base pair deletion, nt 144 (A) in codon 19 producing frame shift and new stop codon a 22
No surface GpIba
Autosomal dominant; heterozygous
Leu57 + Phe (point mutation) (LRR)
Normal protein plus minor truncated species (? increased proteolysis); no GpIX Normal quantity of GpIb-IX; no von Willebrand factor binding
None
N/A; prematurestop codon; mutationin other allelenot determined
Autosomalrecessive;compound heterozygote
Trp343 + stop (point mutation on one allele); mutation in other allele not reported
Truncated GpIbcx protein; absent GpIX
GpIbcimutations
FunctionalanalysiS’
Inheritance
Mutation
Protein
Table 1. Mutations in the Gplb-lX complex in Bernard-Soulier syndrome
meter mutation was in a GATAbinding motif (Ludlow et al. 1996), a motif important in regulating the expression of a number of genes expressed in hematopoetic cells. The only mutation found in more than one family was the Asn45+Ser in GpIX reported in manuscript form in two unrelated families (Wright et al. 1993, Clemetson et al. 1994). In one of these families, they were found to be compound heterozygotes with the Asp21+ Gly mutation on the other allele (Wright et al. 1993). Donner et al. (1996), in a letter to the editor, reported the Asn45+Ser mutation in a third family and in an unreported family; the author’s laboratory has found the same mutation in a homozygous form. Some of the mutations reported in GpIbrx are associated with atypical BSS including a mutation in the vWF binding site resulting in loss of function but not loss of complex expression (Ware et al. 1993a, de Marco et al. 1990) and a mutation in an autosomal dominant form of the disease (Miller et al. 1992); both are within an LRR. It is very interesting that the mutation resulting in normal complex expression with isolated loss of vWF binding function is in an LRR and is not in the adjacent vWF binding region, again speaking to the importance of this region in maintaining complex structure. Also, although some GpIba mutations resulted in classic BSS, with marked decrease or absence of all members of the complex, one was reported with retention of GpIX (de la Salle et al. 1994) expression and another of GpV (Li et al. 1995) expression. The reason for these differences in complex expression is not clear. Future studies evaluating the intracellular processing and surface expression of mutant complex transected into heterologous cells will help to clarify the requirements for individual protein and complex expression. On the other hand, the mutations resulting in loss of GpIX or GpIb(3 expression all had an associated marked decrease in GpIba, GpV, and for the GpIb~ mutation, GpIX. Elucidation of mutations responsible for BSS in additional patients should further increase our understanding of requirements for GpIb–V–IX complex expression and function.
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Acquired GpIb-V-IX Receptor Dysfunction
Acquired Bernard Soulier syndrome has been reported rarely due to antibody formation (Stricker et al. 1985).Patientswith immune thrombocytopenic purpura may have bleeding symptoms out of proportion to their degree of thrombocytopenia due occasionally to antibodies directed against functional platelet receptors, including the GpI&V–IX receptor Induction of GpIb-V-IX dysfunction by a peptide blocking vWF-GpIb binding as antithrombotic therapy in cardiovascular diseasehas been investigatedin an animal model (McGhie et al. 1994),althoughsuch therapies have yet to come into clinical use. The use of such antagonistsis likelyto be focused on preventionof thrombosis or as an adjunct to treatmentof thrombosis, as blocking vWI-GpIb interaction would prevent initial adhesion and platelet activation but not platelet-rich thrombus propagation.
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Conclusion
The platelet GpIb–V–IX complex is a critical platelet receptor initiating platelet adhesion, particularly under conditions of high shear stress, and transducing the signal required for subsequent platelet activation. Elucidation of defects in patients with the BernardSoulier syndrome has greatly advanced our knowledge of the structure and function of this receptor. Future studies to characterize better the extracytoplasmic structure of the complex, as well as to elucidate intracytoplasmic associations and signal transduction pathways, will further define the role of this receptor in normal and pathologic hemostasis.
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Konkle BA, Shapiro SS, Asch AS, Nachman RL: 1990. Cytokine- enhanced expression of glycoprotein Iba in human endothelium. J Biol Chem 265:19,833-19,838. Krolf MH, Harris TS, Moake JL, Handin RI, Schafer AI: 1991.von Willebrand factor binding to plateletGpIb initiatessignals for platelet activation. J Clin Invest 88:1568-1573. Kunishima S, Miura H, Fukutani H, et al.: 1994. Bernard-Soulier syndrome Kagoshima: Ser 444 + stop mutation of glycoprotein (GP) Iba resulting in circulating troncated GpIba and surface expressionof GpIb~ and GPIX Blood 84:3356–3362. Li C, Martin SE, Roth GJ: 1995. The genetic defect in two well-studied cases of BemardSoulier Syndrome: a point mutation in the fifth leucine-rich repeat of platelet glycoprotein Iba. Blood 86:3805–3814. Li C, Pasquale DN, Roth GJ: 1996. BemardSoulier syndrome with severe bleeding: absent platelet glycoprotein Ib alpha due to a homozygous one-base deletion. Thromb Haemost 76:670-674. Lopez JA: 1994. The plateletglycoprotein Ib-IX complex. Blood Coagul Fibrinolysis 5:97– 119. Ludfow LB, Schick BP,Budarf ML, et al.: 1996. Identification of a mutation in a GATAbinding site of the platelet glycoprotein hp promoter resulting in the Bernard-Soulier syndrome. J Biol Chem 271:22,076-22,080. McGhie AI, McNatt J, Ezov N, et af.: 1994. Abolition of cyclic flow variations in stenosed, endothelium-injured coronary arteries innonhuman primates with a peptide fragment (VCL) derived from human plasma von Willebrand factor-glycoprotein Ib binding domain. Circulation 90:2976–2981. Meyer SC, Fox JEB: 1995. Interaction of platelet glycoprotein V with glycoprotein Ib-IX regulatesexpression of the glycoproteins and binding of von Wilfebrand factor to glycoprotein Ib-IX in transected cells. J Biol Chem 270:14,693-14,699. Meyer SC, Zuerbig S, Cunningham CC: 1997. Identification of the region of actin-binding protein that binds to the cytoplasmic domain of glycoprotein Iba. J Biol Chem 272:2914– 2919. Miller JL, LyleVA,Cunningham C: 1992. Mutation of Leucine-57 to Phenykdanine in a platelet glycoprotein Iba leucine tandem repeat occurring in patientswith an autosomal dominant variantof Bemard-Soulier disease. Blood 79:439446. Noda M, Fujimura K, TakafutaT, et al.: 1995. Heterogeneousexpression of glycoprotein Ib, IX and V in plateletsfrom two patients with Bemard-Soulier syndrome caused by different genetic abnormalities. Thromb Haemost 74:1411-1415.
sequence (CYS73[TGT]to TjdTAT]) causes impaired surface expression of GPIb/IX/V complex in two families with BemardSoulier syndrome. Thromb Haemost 76:874878. Nurden AT, Caen JP: 1975. Specific roles for platelet surface glycoproteins in platelet function. Nature 255:72@722. Okamura T, Kanaji T, Osaki K, Kuroiwa M, Yamashita S, Niho Y: 1996. Gastrointestinal angiodysplasia in congenital platelet dysfunction. Int J Hematol 65:79-84. Rao AK, Ghosh S, Sun L, et al.: 1995. Mechanisms of plateletdysfunction and response to DDAVP in patients with congenital platelet function defects. Thromb Haemost 74:10171078. Sae-TungG, Dong JF,Lopez JA: 1996. Biosynthetic defect in platelet glycoprotein IX mutants associated with Bemard-Soulier syndrome. Blood 87:1361-1367. Simsek S, Admiraal LG, Moddennan PW, van der Schoot CE, von dem Borne AE: 1994a. Identification of a homozygous single base pair deletion in the gene coding for the human platelet glycoprotein Iba causing Bernard-Soulier Syndrome. Thromb Haemost 72:444449. Simsek S, Noris P, Lozano M, et aL: 1994b. CYS209Ser mutation in the platelet membrane glycoprotein Iba is associated with Bemard-Soulier syndrome. Br J Haematol 88:839-844.
StrickerRB, Wong D, Saks SR, Corash L, Shuman MA: 1985. Acquired Bemard-Soulier Syndrome. J Clin Invest 76:1274-1278. Ware J, Russell SR, VicenteV, et al.: 1990. Nonsense mutation in the glycoprotein Ibexcoding sequence associated with BemardSoulier syndrome. Proc Natl Acad Sci USA 87:20262030. Ware J, Russell SR, Marchese P, et al.: 1993a. Point mutation in a leucine-rich repeat of plateletglycoprotein Iba resultingin the Bernard-Soulier Syndrome. J Clin Invest 92: 1213-1220. Ware J, Russell SR, Marchese P, Ruggeri ZM. 1993b. Expression of human platelet glycoprotein Iba in transgenic mice. J Biol Chem 268:8376-8382. Weiss HJ, Tschopp TB, Baumgartne~ et al.: 1974. Decreasedadhesion of giant (BemardSoulier) platelets to subendothelium. Am J Med 57:920-925. Wright SD, Michaelides K, Johnson DJD,West NC, Tuddenham EGD: 1993. Double heterozygosity for mutations in the plateletglycoprotein IX gene in three siblings with Bernard-Souliersyndrome. Blood 81:2339–2347. Yagi M, Edelhoff S, Disteche CM, Roth GJ: 1995. Human platelet glycoproteins V and IX: Mapping to two leucine-rich glycoprotein genes to chromosome 3 and analysisof structure. Biochemistry 34:16,132–16,137. TCM
MEETING REPORTS Are you attending a conference that would be of interest to other cardiologists? If you are interested in preparing a report of conference highlights for TCM please contact: Kenneth R. Chien, MD, PhD Editor in Chief Trends in Cardiovascular Medicine Editorial Office 655 Avenue of the Americas New York, NY 1OO1O Phone: 212-633-3916
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Noda M, Fujkimura K, TakafutaT, et al.: 1996. A point mutation in glycoprotein IX coding
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Wesselschmidt R, LikertK, Girard T, Wun T-C, Broze GJ Jr: 1992. Tissue factor pathway inhibitor: the carboxy-terminus is required for optimaf inhibition of factor Xa. Blood 79: 2004-2010. Wesselschmidt R, Likert K, Huang Z-F, MacPhail L, Broze GJ Jr: 1993. Structural requirementsfor tissuefactor pathwayinhibitor interactionswith factor Xa and hepann. Blood Coagul Fibrinolysis 4:661. Willnow TE, ArmstrongSA, Hammer RE, Herz J: 1995. Functional expression of low density lipoprotein receptor-related protein is controlled by receptor-associatedprotein in vivo. Proc Natl Acad Sci USA 92:4537+541. Wun T-C, Kretzmer KK, Girard TJ, Miletich JP, Broze GJ Jr: 1988. Cloning and characterization of a cDNA coding for the lipoprotein-associated coagulation inhibitor shows that it consists of three tandem Kunitz-type inhibitory domains. J Biol Chem 263:6001-6004. TCM
The Bernard-Soulier Syndrome Barbara A. Konkle
The Bernard-Soulier syndrome (BSS) is a rare inherited disorder of platelet function due to absence or markedlydecreased expression of the platelet GpIb–V–IX receptoz This ~eceptorplays a critical role in hemostasis by initiating platelet adhesion and subsequent activation at the site of vessel injury. Through its link to the platelet cytoskeleton, the GpIb–V–IXcomplex appears to be ~equiredto maintain norrnalplatelet size and structure. Elucidation of the cDNA and genomic structures of the members of this complex, as well as the mutations responsible for BSS, has greatly advanced our knowledge of the structure and function (Trends Cardiovasc Med of the GpIb–V–IX complex in hemostasis. 1997; 7:239–244). @ 1997, Elsevier Science Inc.
The platelet glycoprotein Ib-V-IX (GpI& V–IX) recepto~ the receptor affected in the Bernard-Soulier syndrome (BSS), mediates initialplateletadhesion to the injured
Barbara A. Konkle is at the Cardeza Foundation for Hematologic Research, Department of Medicine, Jefferson Medical College of Thomas Jefferson University, Philadelphia, PA 19107, USA.
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vessel wall through binding to exposed von Willebrand factor (vWF), particularly under conditions of high shear stress [reviewed by Berndt et al. (1995)]. GpIb is a transmembraneheterodimerthatin platelets is comprised of a 140-kDalpha chain (GpIba;CD42b) disulfide, linked to a smaller 24-kD beta chain (GpIb~;CD42c). In the platelet,membrane GpIb existsin a 1:1 noncovalent complex with GpIX
(20kD;CD42a)and is more loosely associated with GpV (82 kD;CD42d) [reviewed by Lopez (1994)]. In most cases of BSS, all of these proteins are absent or affected, suggesting that they must all be present for stable expression and function of the complex.
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ClinicalManifestationsof Bernard-SoulierSyndrome
Patients with BSS usually present with a mild to moderate bleeding disorder, manifest most notably by mucosal bleeding. Bleeding may, however, be severe and life threatening. Two French physicians, Jean Bernard and JeanPreue Soulier initially described the disorder in 1948 (Bernard and Soulier 1948). The proband presented 15 days after birth with epistaxis and rectal bleeding and continued to have frequent ecchymoses, gastrointestinal bleeding, and trauma-induced bleeding until he died at the age of 28 of a cerebral hemorrhage after a barroom brawl. He had one affected sibling, but his parents and other siblings were phenotypically normal compatible with an autosomal recessive inheritance. In the early 1970s, BSS platelets were shown to be defective in vWF-dependent adhesion (Weiss et al. 1974), and in 1975 Nurden and Caen identified the defect to be in platelet GpIb receptor expression. In 1983, Berndt et al. described the associated deficiencies of GpIb~, GpIX, and GpV in BSS platelets. Patients with BSS are thrombocytopenic, with giant platelets noted on the peripheral smear. Except in the one family with an autosomal dominant form of BSS, obligate heterozygotes do not have bleeding symptoms but have been shown to have increased mean platelet volumes and quantitatively decreased platelet GpIba surface expression (de Marco et al. 1986). Greater than one third of BSS platelets may have a diameter of >3.5 pM, and platelets have been noted to have increased membrane deformability and abnormal filopodia formation with activation. The abnormal platelet morphology is likely secondary to the loss of the GpIb-V-IX–cytoskeletal link and supports a role for the complex in maintaining normal platelet structure. Platelet counts are usually between 50,000 and 100,000 but may be reported to be much lower with automated count-
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ing owing to some platelets being counted as lymphocytes because of their size. The reason for the thrombocytopenia is unclear; possible explanations include decreased platelet release or a normal physiologic response to a normal platelet mass with a decreased number of platelets compensated by their large size. Another explanation has been that the abnormal platelets are removed by the spleen. Because of the large platelets and thrombocytopenia, patients may be initially diagnosed with immune thrombocytopenia purpura (ITP) and undergo splenectomy. This has not, howeve~ been reported to increase platelet number in BSS. At our institution, we follow two brothers with classic BSS due to a homozygous mutation of GpIX (Asn45+ Ser), one of whom had a splenectomy in the past. There is no significant difference in their platelet counts or platelet morphology. Patientswith BSS have a markedlyprolonged bleeding time and reduced or absent ristocetin-induced platelet aggregation and decreased platelet adhesion to the exposed subendothelium, reflecting the lack of platelet GpIb-vWF interaction [reviewed by Berndt et al. (1995)]. Other agonist-inducedplateletaggregation,such as with ADP and epinephrine,are normal. AIIeven rarer disorder of GpIb-IX, termed pseudo- or platelet-type von Willebrand disease results from gain of function mutations in GpIbci that are associated with increased binding to vWF and present clinically similar to type 2B vWD. The gastrointestinal tract is a commonly reported bleeding site in patients with BSS. Recently, there have been two reports of angiodysplasia in patients with BSS (Bellucci et al. 1995, Okamur et al. 1996). Interestingly,this is a defect that has been reported repeatedly in patients with vWD, particularly in inherited or acquired type 2 subtype (Fressinaud and Meyer 1993). The GpIb– V–IX complex is present in endothelial cells (Konkle et al. 1990, Kelly et al. 1994, B.A. Konkle and S.S. Shapiro unpublished observations), and it is interesting to postulate a role for endothelial GpIb with vWF in maintaining normal endothelial structure. The treatment of bleeding in patients with BSS is limited to platelet transfusion if local measures are ineffective, although antifibrinolytic therapy may be useful alone or in conjunction with
240
vWF and Thrombin binding N-linked carbohydrate O-linked carbohydrate
membrane
-
-
*
(
(
1
Actin-binding protein Figure 1.The GpI&V-IX complex in platelets.This drawing depicts GpIba, GpIb~, GpIX and GpV in a 1:1:1:0.5 ratio, respectively,as well as the association of the complex with actin-binding protein. Secondary structuredue to disulfide-bonding is schematized per Lopez (1994).
platelet transfusion in nonurinary tract bleeding. The vasopressin analogue, DDAVP,which is effective in some platelet disorders (Rao et al. 1995), has not been shown to be effective in treating patients with BSS. Because BSS patients may require transfusion over their lifetime, leukocyte-depleted single donor platelets should be used for transfusion to prevent alloimunization, if possible.
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Structureof the GpIb-V-IX Complex
The GpIb-V-IX complex is illustratedin
Figure 1. Approximately 25,000 copies of GpIb and GpIX and one-half that number of GpV molecules are present on the platelet surface, suggesting that at least two GpIb-IX complexes may be linked together on the surface [reviewed by Lopez (1994)]. The GpIba protein extends60 nm from the plateletsurface, significantlyfarther than other major components of the plateletmembrane. This is due to the long macroglycopeptide consisting of serine-, threonine-, and proline-rich repeats that are heavily O-glycosylated and, with the globular N-terminaldomain, give a “palm tree’’-likestructure to the protein. The extended structureof GpIbcxpositions it well to initiate platelet adhesion to the vessel wall.
In nonactivated platelets, the intracytoplasmic tail of GpIba binds actin-binding
protein (ABP) and thereby links the complex to the cytoskeleton (Fox 1985). The site of interaction,recentlylocalized to the region of amino acids 185&2136 in the C-terminal domain of ABP (Meyer et al. 1997), is proximal to the region through which ABP dimerizes, and such dimerization could maintain two GpIb subunits in proximity on the platelet membrane. The site of interaction between the GpIb heterodimer and GpIX is unknown. cDNA cotransfection studies by Lopez et al. [reviewed by Lopez (1994)] suggested that GpIX associated with GpIb(3, whereas studies of Wu et al. (1996), evaluatingsurface antibody binding to members of the complex, suggested a direct interaction between GpIba and GplX. These findings may not be contradictory as one study evaluatedintracytoplasmic, and the other, surface association. The GpIb–V–IX receptor is not an integrin, but is a member of the leucinerich repeat (LRR) glycoprotein family [reviewed by Kobe and Deisenhofer (1994)]. Members of this family share a varying number of 20–29 residue leucine-rich repeats. The crystal structure of porcine ribonuclease inhibitor protein, which contains 15 LRRs, has been determined as consisting of ~-a struc-
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tural units, with short ~-strands and cx-helicesapproximately parallel to each other. GpIba, GpIb~, GpIX, and GpV share in common these LRRs; GpIba has seven, GpV 15, and GpIb~ and GpIX one each. The function of the LRR repeats is unknown, but most proteins in this family are involved in adhesion and/or cell signaling. Although the function of the LRR in the GpIb complex is unknown, as discussed later in this review, the fact that a number of missense mutations in the LRR or the LRR flanking region of GpIba and GpIX result in BSS demonstrates the importance of this domain in maintaining the structure and/or function of the GpIb–V–IX complex. Based on the crystal structure of the porcine ribonuclease inhibitor protein, it is unlikely that a single leucinerich motif would adopt a similar conformation (Kobe and Deisenhoger 1994). Because GpIba and GpIX have only one LRR each they could form part of a complex structure, as described for the ribonuclease inhibitor, through interaction with GpIba and GpV. This would suggest a much more interrelated structure than that proposed in Figure 1. The cDNA and genomic structures of GpIba, GpIb~, GpIX, and GpV have been reported [reviewed by Lopez (1994)]. These proteins are all products of different genes. The GpIba and GpIb(3 genes have been localized to chromosomes 17p12ter and 22ql 1.2, respectively, and the GpIX and GpV genes to different regions of chromosome 3, 3q21 and 3q29, respectively (Yagi et al. 1995). In addition to sharing varying degrees of homology, the genes are similar in their simple structure, having no introns within the coding region of the mature peptide. The GpIb~ gene has been localized to 22ql 1.2, a region of chromosome 22 termed the DiGeorge critical region, as there is a microdeletion of this region on one chromosome 22 allele in 900/0of patients with DiGeorge syndrome and in 85% of patients with the related velocardiofacial syndrome (Kelly et al. 1994, Budarf et al. 1995). The defect responsible for these syndromes is thought to cause abnormal migration of the cephalic neural crest cells. These cells contribute to the development of the thymus, parathyroid, and conotruncus of the heart. Whether the GpIb~ gene could play a role in these disorders is unknown.
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Transgenic mice can express human GpIba on the mouse platelet surface, apparently covalently linked to mouse GpIb(3 (Ware et al. 1993b). Future studies identifying evolutionarily conserved sequences and domains in GpIba, GpIb~, GpIX, and GpV will assist in the identification and further characterization of regions important in the structure and function of these proteins.
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Function of the GpIb-V-IX Complex
thrombin binding. Interestingly, GpV is the only platelet surface protein other than the seven-transmembrane domain thrombin receptor known to be susceptible to thrombin cleavage, although it is not clear whether cleavage of GpV by thrombin has any functional significance. The roles of GpIb(3, GpIX, and GpV in platelets is not well characterized. Patients with BSS with defects in either GpIb~ or GpIX (see Table 1) have markedly decreased expression of GpIbcx, demonstrating the requirement of GpIb~ and GpIX for assembly and/or stable expression of the complex on the platelet surface. This requirement for coordinate expression has also been demonstrated in vitro. In a series of studies, Lopez and coworkers showed that membrane expression of GpIba requires the coexpression of GpIb~, and that optimal surface expression requires, in addition, the presence of GpIX [reviewed by Lopez (1994)]. GpV may enhance the membrane expression of the GpIb–IX complex (Calverley et al. 1995), and stable expression of GpV itself may be dependent on GpIb–IX expression (Meyer and Fox 1995). GpIb~ contains a phosphorylatable serine residue (Ser’66) in its cytoplasmic tail, whose degree of phosphorylation may vary with the state of activation of the platelet, although quantitative studies of this phenomenon have not be reported.
PlateletGpIba binds von Willebrand factor (vWF) and thrombin in its aminoterminal extracytoplasmic region as indicated in Figure 1. Through vWF binding, GpIb plays a critical role in platelet adhesion to the vascular subendothelium, especially under conditions of high shear, where platelet adhesion to damaged subendothelium can be shown to be dependent upon the presence of GpIb and vWF [reviewed by Lopez (1994), Berndt et al. (1995)]. As is well known, abnormalities in the production of vWF lead to the bleeding disorder known as von Willebrand disease. Binding of vWF to GpIba has been shown to initiate a signaling cascade involving intracellular release of Ca2+, phosphorylation of several platelet proteins, ADP release, thromboxane synthesis, and activation of the GpaIIb–~3 complex (Kroll et al. 1991, Clemetson 1995). The components of the signaling pathway ● Characterizationof Mutations are largely unknown, although recently Responsible for Bernard-Soulier a protein of the 14-3-3 superfamily with Syndrome phospholipase A2 activity (PLA2~) has been identified in platelets in association As of Februa~ 1, 1997, mutations rewith the GpIb complex (Du et al. 1994, sponsible for BSS, elucidated in 16 1996). PLA2~seems capable of binding unrelated families and representing 15 to the complex via GpIbci, GpIb~ ancUor independent mutations have been reGpV, according to data recently pre- ported in full manuscript form (see Tasented but not yet published in detail. ble 1). Ten of these mutations are in GpIbci also contains a high-affinity GpIba, 4 are in GpIX, and 1 in GpIb(3.Of thrombin binding site [reviewed by Jan- the 15 mutations, 6 are in the LRR or the drot-Perruset al. (1996)]. The exact role of adjacent flanking regions and 6 produce this binding in platelet function is un- premature stop codons. Of the other known, particularlyin relationship to the three mutations, one in GpIba and one “classic” seven-transmembrane domain in GpIX obliterate cysteines thought to thrombin receptor present on the platelet be involved in intrachain disulfide bindsurface. Studies suggest that thrombin ing and the other is in the promoter binding to the GpIb-V-IX complex may region of platelet GpIb~. The GpIb(3mube of physiologic importance at very low tation occurred in a patient with velocarconcentrations of thrombin, triggering diofacial syndrome with only one GpIb~ platelet activation in this setting. Unlike allele due to a microdeletion of the rethe seven-transmembranedomain throm- gion of the other chromosome 22 allele bin recepto~ GpIba is not cleaved by containing the GpIb~ gene. The pro-
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None
Autosomal recessive; homozygous
Autosomal recessive; homozygous
Autosomal recessive; homozygous Autosomal recessive; homozygous Autosomal recessive; homozygous
Ala’5’ + Val (point mutation) (LRR)
Ser444 + stop (point mutation)
cys*09+ Ser (point mutation) LeuL29+ Pro (point mutation) (LRR) Leu179 (TCC) deletion (LRR)
Trp126 + stop (point mutation)
Absent GpIX; decreased GpIba, GpV
“ Expression of mutation transected into heterologous cells b These expression studies were done by Sae-Tung et al. (1996). LRR, leucine-rich repeat; NA, not applicable; nl, normal; nt, nucleotide.
Absent GpIb(3; markedly decreased GpIba, GpIX, and GpV
Autosomal recessive: compound heterozygous with microdeletion of other 22ql 1 allele
Autosomal recessive: homozygous
Cys73 + Tyr (point mutation)
Absent GpIX; markedly decreased GpIba; GpV
– 133 C + G (mutation of GATA binding site in promoter)
Autosomal recessive; compound heterozygous Autosomal recessive: homozygous Autosomal recessive: homozygous in two unrelated families
Asn45 + Ser (point mutation) (LRR)
Markedly decreased GpIX (2?4.) and GpIba, GpIb(3, and GpIX (all 7~o)
GpIbf3mutation
Autosomal recessive: compound heterozygous
Markedly decreased promoter function in transection assay
No GpIX surface expression when transected with GpIb(3 N/A; premature stop codon
No GpIX surface expression; did not promote G~Iba surface expression As for Asp*’ + Gly mutation
N/A premature stop codon
Autosomal recessive; homozygous
Asp*l+ Gly (point mutation) (LRR)
N/A; premature stop codon
Autosomal recessive; homozygous
Ludlow et al. (1996)
Noda et al. (1995)
Wright et al. (1993) Clemetson et al. (1994) Noda et al. (1996)
Wright et al. (1993)
Li et al. (1996)
Noda et al. (1995)
N/A; premature stop codon
Simsek et al. (1994b) Li et al. (1995)
Kunishima et al. (1994)
Ware et al. (1993)
Autosomal recessive; homozygous
None
Loss of antibody and von Willebrand factor binding N/A; premature stop codon
Miller et al. (1992)
Ware et al. (1990)
Reference(s)
de la Salle et al. (1995) Simsek et al. (1992a)
Absent GpIX; markedly decreased GpIba
GpIX mutations
Absent risto-induced platelet aggregation; protein studies not done
Absent surface GpIba, truncated protein in plasma; moderately decreased GpIb~, GPIX Absent GpIba~moderately decreased GpIX, GpV Moderately decreased GpIba, GpIX, nl GpV Absent G Ibci; decreased GpIb~, GpV; nr GpIX Markedly decreased GpIbct; decreased GpIX, GpV Absent GpIba; markedly decreased GpIX, GpV Leu’6 Autosomal recessive (del T317) stop codon at amino acid 95) Deletion of nucleotides 1932 + 1938; frame-shift, new stop codon at amino acid 509 One base pair deletion, nt 144 (A) in codon 19 producing frame shift and new stop codon a 22
No surface GpIba
Autosomal dominant; heterozygous
Leu57 + Phe (point mutation) (LRR)
Normal protein plus minor truncated species (? increased proteolysis); no GpIX Normal quantity of GpIb-IX; no von Willebrand factor binding
None
N/A; prematurestop codon; mutationin other allelenot determined
Autosomalrecessive;compound heterozygote
Trp343 + stop (point mutation on one allele); mutation in other allele not reported
Truncated GpIbcx protein; absent GpIX
GpIbcimutations
FunctionalanalysiS’
Inheritance
Mutation
Protein
Table 1. Mutations in the Gplb-lX complex in Bernard-Soulier syndrome
meter mutation was in a GATAbinding motif (Ludlow et al. 1996), a motif important in regulating the expression of a number of genes expressed in hematopoetic cells. The only mutation found in more than one family was the Asn45+Ser in GpIX reported in manuscript form in two unrelated families (Wright et al. 1993, Clemetson et al. 1994). In one of these families, they were found to be compound heterozygotes with the Asp21+ Gly mutation on the other allele (Wright et al. 1993). Donner et al. (1996), in a letter to the editor, reported the Asn45+Ser mutation in a third family and in an unreported family; the author’s laboratory has found the same mutation in a homozygous form. Some of the mutations reported in GpIbrx are associated with atypical BSS including a mutation in the vWF binding site resulting in loss of function but not loss of complex expression (Ware et al. 1993a, de Marco et al. 1990) and a mutation in an autosomal dominant form of the disease (Miller et al. 1992); both are within an LRR. It is very interesting that the mutation resulting in normal complex expression with isolated loss of vWF binding function is in an LRR and is not in the adjacent vWF binding region, again speaking to the importance of this region in maintaining complex structure. Also, although some GpIba mutations resulted in classic BSS, with marked decrease or absence of all members of the complex, one was reported with retention of GpIX (de la Salle et al. 1994) expression and another of GpV (Li et al. 1995) expression. The reason for these differences in complex expression is not clear. Future studies evaluating the intracellular processing and surface expression of mutant complex transected into heterologous cells will help to clarify the requirements for individual protein and complex expression. On the other hand, the mutations resulting in loss of GpIX or GpIb(3 expression all had an associated marked decrease in GpIba, GpV, and for the GpIb~ mutation, GpIX. Elucidation of mutations responsible for BSS in additional patients should further increase our understanding of requirements for GpIb–V–IX complex expression and function.
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Acquired GpIb-V-IX Receptor Dysfunction
Acquired Bernard Soulier syndrome has been reported rarely due to antibody formation (Stricker et al. 1985).Patientswith immune thrombocytopenic purpura may have bleeding symptoms out of proportion to their degree of thrombocytopenia due occasionally to antibodies directed against functional platelet receptors, including the GpI&V–IX receptor Induction of GpIb-V-IX dysfunction by a peptide blocking vWF-GpIb binding as antithrombotic therapy in cardiovascular diseasehas been investigatedin an animal model (McGhie et al. 1994),althoughsuch therapies have yet to come into clinical use. The use of such antagonistsis likelyto be focused on preventionof thrombosis or as an adjunct to treatmentof thrombosis, as blocking vWI-GpIb interaction would prevent initial adhesion and platelet activation but not platelet-rich thrombus propagation.
●
Conclusion
The platelet GpIb–V–IX complex is a critical platelet receptor initiating platelet adhesion, particularly under conditions of high shear stress, and transducing the signal required for subsequent platelet activation. Elucidation of defects in patients with the BernardSoulier syndrome has greatly advanced our knowledge of the structure and function of this receptor. Future studies to characterize better the extracytoplasmic structure of the complex, as well as to elucidate intracytoplasmic associations and signal transduction pathways, will further define the role of this receptor in normal and pathologic hemostasis.
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