Nonsense mutation in the glycoprotein Ib alpha coding sequence associated with Bernard-Soulier syndrome

Characterization of zebrafish gp1ba mutant and modelling Bernard Soulier syndrome

The aim of this study is to model classical Bernard Soulier Syndrome in the zebrafish by targeting Gp1ba. We obtained gp1ba mutant embryos from Zebrafish International Resource Center and grew them to adulthood. The tail clips from these fish were used to prepare DNA and sequenced to identify heterozygotes. They were then bred to obtain homozygotes. The mutation was confirmed by DNA sequencing as a termination codon UAA in place of AAA codon at position 886 in the gp1ba transcript. Thus, at the Pro-295, the Gp1ba protein could be terminated. The blood from gp1ba homozygous and heterozygous mutants showed decreased ristocetin-mediated agglutination in the whole blood agglutination assay. The gp1ba heterozygous and homozygous larvae were subjected to a laser-assisted arterial thrombosis assay, and the results showed the prolonged occlusion in the caudal artery. These results suggested that the gp1ba mutant had a bleeding phenotype. The blood smears from the adult gp1ba, heterozygous and homozygous mutants, showed macrothrombocytes, similar to the human GP1BA deficiency that showed giant platelets. The bleeding assay on these heterozygous and homozygous mutants showed greater bleeding than wildtype, confirming the above findings. Taken together, the characterization of gp1ba zebrafish mutant suggested an autosomal dominant mode of inheritance. The zebrafish gp1ba mutant models classical Bernard Soulier Syndrome and could be used for reversing this phenotype to identify novel factors by the genome-wide piggyback knockdown method.

Genetics of inherited thrombocytopenias

The inherited thrombocytopenia syndromes are a group of disorders characterized primarily by quantitative defects in platelet number, though with a variety demonstrating qualitative defects and/or extrahematopoietic findings. Through collaborative international efforts applying next-generation sequencing approaches, the list of genetic syndromes that cause thrombocytopenia has expanded significantly in recent years, now with over 40 genes implicated. In this review, we focus on what is known about the genetic etiology of inherited thrombocytopenia syndromes and how the field has worked to validate new genetic discoveries. We highlight the important role for the clinician in identifying a germline genetic diagnosis and strategies for identifying novel causes through research-based endeavors.

Pathogenic and likely pathogenic variants in at least five genes account for approximately 3% of mild isolated nonsyndromic thrombocytopenia

BACKGROUND

Thrombocytopenia has a variety of different etiologies, both acquired and hereditary. Inherited thrombocytopenia may be associated with other symptoms (syndromic forms) or may be strictly isolated. To date, only about half of all the familial forms of thrombocytopenia have been accounted for in terms of well-defined genetic abnormalities. However, data are limited on the nature and frequency of the underlying causative genetic variants in individuals with mild isolated nonsyndromic thrombocytopenia.

STUDY DESIGN AND METHODS

Thirteen known or candidate genes for isolated thrombocytopenia were included in a gene panel analysis in which targeted next-generation sequencing was performed on 448 French blood donors with mild isolated nonsyndromic thrombocytopenia.

RESULTS

A total of 68 rare variants, including missense, splice site, frameshift, nonsense, and in-frame variants (all heterozygous) were identified in 11 of the 13 genes screened. Twenty-nine percent (N = 20) of the variants detected were absent from both the French Exome Project and gnomAD exome databases. Using stringent criteria and an unbiased approach, we classified seven predicted loss-of-function variants (three in ITGA2B and four in TUBB1) and four missense variants (one in GP1BA, two in ITGB3 and one in ACTN1) as being pathogenic or likely pathogenic. Altogether, they were found in 13 members (approx. 3%) of our studied cohort.

CONCLUSION

We present the results of gene panel sequencing of known and candidate thrombocytopenia genes in mild isolated nonsyndromic thrombocytopenia. Pathogenic and likely pathogenic variants in five known thrombocytopenia genes were identified, accounting for approximately 3% of individuals with the condition.

Bernard Soulier Syndrome: 10 years' experience at a tertiary care hospital

OBJECTIVE

To determine clinical manifestations and laboratory findings in patients with BSS diagnosed through platelet aggregometry followed in a tertiary care hospital in Lahore, Pakistan.

METHODS

The retrospective study comprised patients who presented in Hematology and Transfusion Medicine Department of The Children Hospital & Institute of Child Health, Lahore with the relevant diagnosis from 2006 to 2013. The result of all the patients were collected on a predesigned proforma. Medical data was scrutinized to collect age, gender, clinical findings along with results of complete blood count, bleeding time and platelet aggregation studies for the diagnosis of Bernard Soulier Syndrome.

RESULTS

Among 49 patients, 26 patients were females and 23 males. The mean age of the patients was 5±2.5 years. 81% had a family history of consanguinity. The most common presenting symptom included epistaxis seen in 73.4% patients. Complete blood count demonstrated decreased platelets in 85.7% of patients ranging from 20 X 109/L to 130 X 109/L. Anemia was seen in 67.3% and 93.8% had prolonged bleeding time. Peripheral blood smears demonstrated giant platelets in all patients. The majority of patients 65.3% had mild bleeding episodes. Platelet aggregation studies showed normal aggregation with ADP, Collagen and Epinephrine in 100% of our patients whereas all showed no response of aggregation with Ristocetin.

CONCLUSION

Our data is consistent with other reports regarding clinical presentation of BSS, but we report large number of BSS patients from our area, emphasizing significance to provide diagnostic services in Pakistan to find out exact magnitude of disease.

Whole exome sequencing in the diagnostic workup of patients with a bleeding diathesis

INTRODUCTION

Bleeding assessment tools and laboratory phenotyping often remain inconclusive in patients with a haemorrhagic diathesis.

AIM

To describe the phenotype and genetic profile of patients with a bleeding tendency.

METHODS

Whole exome sequencing (WES) was incorporated in the routine diagnostic pathway of patients with thrombocytopenia (n = 17), platelet function disorders (n = 19) and an unexplained bleeding tendency (n = 51). The analysis of a panel of 126 OMIM (Online Mendelian Inheritance in Man) genes involved in thrombosis and haemostasis was conducted, and if negative, further exome-wide analysis was performed if informed consent given.

RESULTS

Eighteen variants were detected in 15 patients from a total of 87 patients (17%). Causative variants were observed in MYH9 (two cases), SLFN14, P2RY12 and GP9. In addition, one case was considered solved due to combined carriership of F7 and F13A1 variants and one with combined carriership of F2, F8 and VWF, all variants related to secondary haemostasis protein aberrations. Two variants of uncertain significance (VUS) were found in two primary haemostasis genes: GFI1B and VWF. Eight patients were carriers of autosomal recessive disorders. Exome-wide analysis was performed in 54 cases and identified three variants in candidate genes.

CONCLUSION

Based on our findings, we conclude that performing WES at the end of the diagnostic trajectory can be of additive value to explain the complete bleeding phenotype in patients without a definite diagnosis after conventional laboratory tests. Discovery of combinations of (novel) genes that predispose to bleeding will increase the diagnostic yield in patients with an unexplained bleeding diathesis.

A Brazilian case of Bernard-Soulier syndrome with two distinct founder mutations

Bernard-Soulier syndrome (BSS) is a rare bleeding disorder of autosomal recessive inheritance characterized by macrothrombocytopenia. We report the case of a 14-year-old girl diagnosed with BSS who is a fourth-generation Brazilian of Japanese descent and has a compound heterozygote mutation as the responsible gene. The compound heterozygosity would have occurred from the global and long-term racial migration that brought about an accidental encounter of two rare mutant alleles of different origins.

Inherited thrombocytopenia: novel insights into megakaryocyte maturation, proplatelet formation and platelet lifespan

The study of patients with inherited bleeding problems is a powerful approach in determining the function and regulation of important proteins in human platelets and their precursor, the megakaryocyte. The normal range of platelet counts in the bloodstream ranges from 150 000 to 400 000 platelets per microliter and is normally maintained within a narrow range for each individual. This requires a constant balance between thrombopoiesis, which is primarily controlled by the cytokine thrombopoietin (TPO), and platelet senescence and consumption. Thrombocytopenia can be defined as a platelet count of less than 150 000 per microliter and can be acquired or inherited. Heritable forms of thrombocytopenia are caused by mutations in genes involved in megakaryocyte differentiation, platelet production and platelet removal. In this review, we will discuss the main causative genes known for inherited thrombocytopenia and highlight their diverse functions and whether these give clues on the processes of platelet production, platelet function and platelet lifespan. Additionally, we will highlight the recent advances in novel genes identified for inherited thrombocytopenia and their suggested function.

Spectrum of the mutations in Bernard-Soulier syndrome

Bernard-Soulier syndrome (BSS) is a rare autosomal recessive bleeding disorder characterized by defects of the GPIb-IX-V complex, a platelet receptor for von Willebrand factor (VWF). Most of the mutations identified in the genes encoding for the GP1BA (GPIbα), GP1BB (GPIbβ), and GP9 (GPIX) subunits prevent expression of the complex at the platelet membrane or more rarely its interaction with VWF. As a consequence, platelets are unable to adhere to the vascular subendothelium and agglutinate in response to ristocetin. In order to collect information on BSS patients, we established an International Consortium for the study of BSS, allowing us to enrol and genotype 132 families (56 previously unreported). With 79 additional families for which molecular data were gleaned from the literature, the 211 families characterized so far have mutations in the GP1BA (28%), GP1BB (28%), or GP9 (44%) genes. There is a wide spectrum of mutations with 112 different variants, including 22 novel alterations. Consistent with the rarity of the disease, 85% of the probands carry homozygous mutations with evidence of founder effects in some geographical areas. This overview provides the first global picture of the molecular basis of BSS and will lead to improve patient diagnosis and management.

GFI1B mutation causes a bleeding disorder with abnormal platelet function

BACKGROUND

GFI1B is a transcription factor important for erythropoiesis and megakaryocyte development but previously unknown to be associated with human disease.

METHODS

A family with a novel bleeding disorder was identified and characterized. Genetic linkage analysis and massively parallel sequencing were used to localize the mutation causing the disease phenotype on chromosome 9. Functional studies were then performed in megakaryocytic cell lines to determine the biological effects of the mutant transcript.

RESULTS

We have identified a family with an autosomal dominant bleeding disorder associated with macrothrombocytopenia, red cell anisopoikilocytosis, and platelet dysfunction. The severity of bleeding is variable with some affected individuals experiencing spontaneous bleeding while other family members exhibit only abnormal bleeding with surgery. A single nucleotide insertion was identified in GFI1B that predicts a frameshift mutation in the fifth zinc finger DNA-binding domain. This mutation alters the transcriptional activity of the protein, resulting in a reduction in platelet α-granule content and aberrant expression of key platelet proteins.

CONCLUSIONS

GFI1B mutation represents a novel human bleeding disorder, and the described phenotype identifies GFI1B as a critical regulator of platelet shape, number, and function.

Clinical and genetic aspects of Bernard-Soulier syndrome: searching for genotype/phenotype correlations

BACKGROUND

Bernard-Soulier syndrome is a severe bleeding disease due to a defect of GPIb/IX/V, a platelet complex that binds the von Willebrand factor. Due to the rarity of the disease, there are reports only on a few cases compromising any attempt to establish correlations between genotype and phenotype. In order to identify any associations, we describe the largest case series ever reported, which was evaluated systematically at the same center.

DESIGN AND METHODS

Thirteen patients with the disease and seven obligate carriers were enrolled. We collected clinical aspects and determined platelet features, including number and size, expression of membrane glycoproteins, and ristocetin induced platelet aggregation. Mutations were identified by direct sequencing of the GP1BA, GP1BB, and GP9 genes and their effect was shown by molecular modeling analyses.

RESULTS

Patients all had a moderate thrombocytopenia with giant platelets and a bleeding tendency whose severity varied among individuals. Consistent with expression levels of GPIbα always lower than 10% of control values, platelet aggregation was absent or severely reduced. Homozygous mutations were identified in the GP1BA, GP1BB and GP9 genes; six were novel alterations expected to destabilize the conformation of the respective protein. Except for obligate carriers of a GP9 mutation with a reduced GPIb/IX/V expression and defective aggregation, all the other carriers had no obvious anomalies.

CONCLUSIONS

Regardless of mutations identified, the patients' bleeding diathesis did not correlate with thrombocytopenia, which was always moderate, and platelet GPIbα expression, which was always severely impaired. Obligate carriers had features similar to controls though their GPIb/IX/V expression showed discrepancies. Aware of the limitations of our cohort, we cannot define any correlations. However, further investigations should be encouraged to better understand the causes of this rare and underestimated disease.

Genome-wide association study of hematological and biochemical traits in a Japanese population

We report genome-wide association studies for hematological and biochemical traits from approximately 14,700 Japanese individuals. We identified 60 associations for 8 hematological traits and 29 associations for 12 biochemical traits at genome-wide significance levels (P < 5 x 10(-8)). Of these, 46 associations were new to this study and 43 replicated previous reports. We compared these associated loci with those reported in similar GWAS in European populations. When the minor allele frequency was >10% in the Japanese population, 32 (94.1%) and 31 (91.2%) of the 34 hematological loci previously reported to be associated in a European population were replicated with P-values less than 0.05 and 0.01, respectively, and 31 (73.8%) and 27 (64.3%) of the 42 European biochemical loci were replicated.

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.

Molecular genetics and transfusion management in a child with Bernard Soulier syndrome

We present a case of Bernard Soulier syndrome in a 9-year-old boy caused by a novel genetic mutation. This child was shown to be homozygous for a single nucleotide deletion (c.1077delG) in the GP1BA gene not previously reported. Clinically, the boy has become refractory to platelet transfusions with both allo-antibodies and iso-antibodies and a massive transfusion requirement for ongoing haemorrhage. We describe the critical role that the blood product transfusion continues to play in the management of Bernard Soulier syndrome and discuss therapeutic options in these patients.

Protease-activated receptors in hemostasis, thrombosis and vascular biology

The coagulation cascade and protease-activated receptors (PARs) together provide an elegant mechanism that links mechanical information in the form of tissue injury to cellular responses. These receptors appear to largely account for the cellular effects of thrombin and can mediate signaling to other trypsin-like proteases. An important role for PARs in hemostasis and thrombosis is established in animal models, and studies in knockout mice and nonhuman primates raise the question of whether PAR inhibition might offer an appealing new approach to the prevention and treatment of thrombosis. PARs may also trigger inflammatory responses to tissue injury. For example, PAR activation on endothelial cells and perhaps sensory afferents can trigger local accumulation of leukocytes and platelets and transudation of plasma. However, panoply of signaling systems and cell types orchestrates inflammatory responses, and efforts to define the relative importance and roles of PARs in various inflammatory processes are just beginning. Lastly, roles for PARs in blood vessel formation and other processes during embryonic development are emerging, and whether these reflect new roles for the coagulation cascade and/or PAR signaling to other proteases remains to be explored.

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

Bernard-Soulier syndrome (BSS) is a rare inherited bleeding disorder due to quantitative or qualitative abnormalities in the platelet glycoprotein (GP) Ib/IX/V complex, the major von Willebrand factor receptor. The complex comprises four subunits, each encoded by a separate gene. Several mutations have been described for each of the subunits, except for GPV, as a cause of BSS. We describe here the genetic basis of the disorder in a child with BSS. Flow-cytometric analysis of the patient's platelets showed a markedly reduced surface expression of all three glycoproteins of the GPIb/IX/V complex. DNA sequencing analysis showed the patient to be a compound heterozygote for two mutations in the GPIX gene, a novel nine-nucleotide deletion starting at position 1952 of the gene that changes asparagine 86 for alanine and eliminates amino acids 87, 88, and 89 (arginine, threonine, and proline) and a previously reported point mutation that changes the codon asparagine (AAC) for serine (AGC) at residue 45. Her mother was heterozygous for the Asn45Ser mutation, and her father, for the nine-nucleotide deletion. Our findings suggest that the additive effects of both mutations in the GPIX gene are responsible for the BSS phenotype of the patient.

Genetic abnormalities of Bernard-Soulier syndrome

Bernard-Soulier Syndrome (BSS) is an autosomal recessive bleeding disorder due to quantitative or qualitative abnormalities in the glycoprotein (GP) Ib/IX/V complex, the platelet receptor for von Willebrand factor. BSS is characterized by giant platelets, thrombocytopenia, and prolonged bleeding time, and the hallmark of this disorder is the absence of ristocetin-induced platelet agglutination. In the last 10 years, the molecular and genetic bases of many GPIb/IX/V defects have been elucidated, providing a better understanding of primary hemostasis and structure-function relations of the complex. Thus far, more than 30 mutations of the GPIbalpha, GPIbbeta, or GPIX genes have been described in BSS. Recent studies also have shown that the phenotypes caused by mutations in the subunits of the GPIb/IX/V span a wide spectrum, from the normal phenotype, to isolated giant platelet disorders/macrothrombocytopenia, to full-blown BSS and platelet-type von Willebrand disease. Although recent progress in molecular biology has clarified the genotype-phenotype relationships of the GPIb/IX/V disorders, a close examination of platelet morphology on blood smears is still indispensable for a proper diagnosis. In this review, we summarize recent advances in the molecular basis of BSS with special emphasis on giant platelets and the genetic characteristics of Japanese BSS.

Amelioration of the macrothrombocytopenia associated with the murine Bernard-Soulier syndrome

An absent platelet glycoprotein (GP) Ib-IX receptor results in the Bernard-Soulier syndrome and is characterized by severe bleeding and the laboratory presentation of macrothrombocytopenia. Although the macrothrombocytopenic phenotype is directly linked to an absent GP Ib-IX complex, the disrupted molecular mechanisms that produce the macrothrombocytopenia are unknown. We have utilized a mouse model of the Bernard-Soulier syndrome to engineer platelets expressing an alpha-subunit of GP Ib (GP Ibalpha) in which most of the extracytoplasmic sequence has been replaced by an isolated domain of the alpha-subunit of the human interleukin-4 receptor (IL-4Ralpha). The IL-4Ralpha/GP Ibalpha fusion is membrane expressed in Chinese hamster ovary (CHO) cells, and its expression is facilitated by the presence of human GP IX and the beta-subunit of GP Ib. Transgenic animals expressing a chimeric receptor were generated and bred into the murine Bernard-Soulier syndrome-producing animals devoid of mouse GP Ibalpha but expressing the IL-4Ralpha/GP Ibalpha fusion sequence. The characterization of these mice revealed a 2-fold increase in circulating platelet count and a 50% reduction in platelet size when compared with platelets from the mouse model of the Bernard-Soulier syndrome. Immunoprecipitation confirmed that the IL-4Ralpha/GP Ibalpha subunit interacts with filamin-1 and 14-3-3zeta, known binding proteins to the GP Ibalpha cytoplasmic tail. Mice expressing the chimeric receptor retain a severe bleeding phenotype, confirming a critical role for the GP Ibalpha extracytoplasmic domain in hemostasis. These results provide in vivo insights into the structural elements of the GP Ibalpha subunit that contribute to normal megakaryocyte maturation and thrombopoiesis.

Role of thrombin signalling in platelets in haemostasis and thrombosis

Platelets are critical in haemostasis and in arterial thrombosis, which causes heart attacks and other events triggered by abnormal clotting. The coagulation protease thrombin is a potent activator of platelets ex vivo. However, because thrombin also mediates fibrin deposition and because multiple agonists can trigger platelet activation, the relative importance of platelet activation by thrombin in haemostasis and thrombosis is unknown. Thrombin triggers cellular responses at least in part through protease-activated receptors (PARs). Mouse platelets express PAR3 and PAR4 (ref. 9). Here we show that platelets from PAR4-deficient mice failed to change shape, mobilize calcium, secrete ATP or aggregate in response to thrombin. This result demonstrates that PAR signalling is necessary for mouse platelet activation by thrombin and supports the model that mouse PAR3 (mPAR3) does not by itself mediate transmembrane signalling but instead acts as a cofactor for thrombin cleavage and activation of mPAR4 (ref. 10). Importantly, PAR4-deficient mice had markedly prolonged bleeding times and were protected in a model of arteriolar thrombosis. Thus platelet activation by thrombin is necessary for normal haemostasis and may be an important target in the treatment of thrombosis.

Inherited giant platelet disorders. Classification and literature review

Inherited giant platelet disorders are extremely rare. The aim of this article is to review the clinical and laboratory features of this heterogeneous group and to arrive at a working classification. We conducted our literature search using the National Library of Medicine database. A total of 12 clinical entities were described. We classified them into 4 groups depending on the clinical and structural abnormalities. The pathophysiology of these disorders is largely unknown, and more research is needed, particularly in the light of recent advances in laboratory medicine. This review may provide a valuable reference for clinicians and may form a basis for future classification and research.

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.

Variant Bernard-Soulier syndrome due to homozygous Asn45Ser mutation in the platelet glycoprotein (GP) IX in seven patients of five unrelated Finnish families

Bernard-Soulier syndrome (BSS), a rare bleeding disorder with macrothrombocytopenia, is caused by a defect of the platelet glycoprotein (GP) Ib/IX/V complex. Here we report a variant form of BSS in eleven patients of five unrelated families who originate from a particular area of Finland. The differential diagnosis from idiopathic thrombocytopenic purpura was difficult. Bleeding symptoms were epistaxis and haematomas debuting in childhood, but no spontaneous, severe bleeding episodes were reported. The platelet count varied from 43 to 81x10(9)/l. Screening the entire GP Ibalpha, GP Ibbeta, GP IX and GP V genes revealed a recurrent homozygous Asn45Ser mutation in GP IX in all probands. Flow cytometry showed markedly reduced expression of GP Ib (<10%), and only moderately reduced expression of GP IX (24-36%) and GP V (38-49%). The expression of subunits seemed to vary independently from the normal polymorphisms. Heterozygotes did not differ significantly from controls by their GP Ib/IX/V expression. Since the Asn45Ser mutation has also been reported in three other kindreds of northern and central European origin, this study reveals that instead of being a mutation hot spot, it may be ancient and scattered in Europe. Moderate, chronic thrombocytopenia should be carefully studied to diagnose variant BSS correctly from treatment resistant idiopathic thrombocytopenia.

Molecular characterization of two mutations in platelet glycoprotein (GP) Ib alpha in two Finnish Bernard-Soulier syndrome families

Bernard-Soulier syndrome (BSS) is a rare hereditary bleeding disorder and macrothrombocytopenia which is caused by a defect in the platelet glycoprotein Ib/IX/V (GP Ib/IX/V) complex, the receptor for von Willebrand factor and thrombin. Here we report the molecular basis of the classical form of BSS in two unrelated Finnish patients, both with a life-long history of severe bleeding. Flow cytometry and immunoblotting showed no expression of GP Ib/IX, GP Ib alpha, GP Ib beta or GP IX (less than 10%) in the patients' platelets. No expression of GP V (< 10%) was observed in propositus 1, but a residual amount was found in propositus 2 (24%). DNA sequencing analysis revealed that propositus 1 was compound heterozygous for a two-base-pair deletion at Tyr505(TAT) and a point mutation Leu129(CTC)Pro(CCC) in the GP Ib alpha gene. Propositus 2 was homozygous for the Tyr505(TAT) deletion. The nine relatives who were heterozygous for either of the mutations also had low levels of GP Ib alpha (74-90%). Hence, Bernard-Soulier patients homozygous or compound heterozygous for Tyr505(TAT) are severely affected. Interestingly, both mutations have independently been found in three other families in previous reports, suggesting their ancient age or mutational 'hot spot'.

Mutations in filamin 1 prevent migration of cerebral cortical neurons in human periventricular heterotopia

Long-range, directed migration is particularly dramatic in the cerebral cortex, where postmitotic neurons generated deep in the brain migrate to form layers with distinct form and function. In the X-linked dominant human disorder periventricular heterotopia (PH), many neurons fail to migrate and persist as nodules lining the ventricular surface. Females with PH present with epilepsy and other signs, including patent ductus arteriosus and coagulopathy, while hemizygous males die embryonically. We have identified the PH gene as filamin 1 (FLN1), which encodes an actin-cross-linking phosphoprotein that transduces ligand-receptor binding into actin reorganization, and which is required for locomotion of many cell types. FLN1 shows previously unrecognized, high-level expression in the developing cortex, is required for neuronal migration to the cortex, and is essential for embryogenesis.

A new variant of Bernard-Soulier syndrome characterized by dysfunctional glycoprotein (GP) Ib and severely reduced amounts of GPIX and GPV

We describe a new variant of Bernard-Soulier syndrome characterized by almost normal amounts of GPIb and severely reduced GPIX and GPV. Despite surface expression, GPIbalpha failed to support ristocetin-induced platelet agglutination and to bind two conformation-dependent monoclonal antibodies, suggesting a qualitative defect. Sequence analysis of the gene coding for GPIX revealed a T-to-C substitution at base 1811, leading to a Leu40Pro conversion, whereas no defects were found in the coding region of the GPIbalpha gene. Allele-specific restriction enzyme analysis showed that the propositus and one of his sisters. both with severe bleeding diathesis. were homozygous for the GPIX mutation: the members of the family with mild bleeding diathesis and/or giant platelets in the peripheral blood were heterozygous, whereas the healthy ones were homozygous for the normal allele. Infusion of 1-desamino-8-D-arginine vasopressin normalized bleeding time in the two severely affected patients, although it did not modify ristocetin-induced platelet agglutination or membrane expression of GPIbalpha, GPIX, GPIIb-IIIa and GMP-140. Moreover, in one patient, normalization of bleeding time and rise of von Willebrand factor plasma concentration did not seem to be directly related.

Naturally Occurring Mutations in Glycoprotein Ibα That Result in Defective Ligand Binding and Synthesis of a Truncated Protein

The platelet GPIb-V-IX complex is the receptor for the initial binding of von Willebrand factor (vWF) mediating platelet adhesion. The complex is composed of four membrane-spanning glycoproteins (GP): GPIbα, GPIbβ, GPIX, and GPV. Bernard-Soulier syndrome results from a qualitative or quantitative defect in one or more components of the platelet membrane GPIb-V-IX complex. We describe the molecular basis of a novel Bernard-Soulier syndrome variant in two siblings in whom GPIbα was not detected on the platelet surface but that was present in a soluble form in plasma. DNA sequence analysis showed that the affected individuals were compound heterozygotes for two mutations. One, inherited from a maternal allele, a T777 → C point mutation in GPIbα converting Cys65 → Arg within the second leucine rich repeat, the other, a single nucleotide substitution (G2078 → A) for the tryptophan codon (TGG) causing a nonsense codon (TGA) at residue 498 within the transmembrane region of GPIbα, inherited from a mutant paternal allele. The Bernard-Soulier phenotype was observed in siblings who were compound heterozygotes for these two mutations. Although GPIbα was not detected on the surface of the patient's platelets, soluble GPIbα could be immunoprecipitated from plasma. When plasmids encoding GPIbα containing the Cys65 → Arg mutation were transiently transfected into Chinese hamster ovary (CHO) cells stably expressing the GPβ-IX complex (CHOβIX), the expression of GPIbα was similar to the wild-type (WT) GPIbα, but did not bind vWF. When plasmids encoding GPIbα containing the Trp498 → stop were transiently transfected into CHOβIX, the surface expression of GPIbα was barely detectable compared with the WT GPIbα. Thus, this newly described compound heterozygous defect produces Bernard-Soulier syndrome by a combination of synthesis of a nonfunctional protein and of a truncated protein that fails to insert into the platelet membrane and is found circulating in plasma.

Severe bleeding tendency in a patient with Bernard-Soulier syndrome associated with a homozygous single base pair deletion in the gene coding for the human platelet glycoprotein Ibalpha

PURPOSE

The genetic basis of Bernard-Soulier syndrome (BSS) was studied to clarify a relationship between severe clinical manifestations and gene abnormality.

PATIENT AND METHODS

A patient with BSS had a severe bleeding tendency that was sometimes life threatening. Flow cytometric analysis of the patient's and normal control platelets was performed to study which glycoprotein (GP) was impaired in glycoprotein Ib/V/IX complex. The genes encoding GPIbalpha from the patient's and control genomic DNA were amplified and directly sequenced.

RESULTS

Flow cytometric analysis revealed a defect of GPIbalpha on the surface of the patient's platelets. A homozygous single base pair deletion was identified in seven repeats of adenine at positions 1932 to 1938 in the GPIbalpha gene. This mutation, which has been previously reported, results in a frameshift and predicts a premature stop codon leading to a truncated peptide that cannot fix on the platelet membrane.

CONCLUSION

This patient's severe clinical phenotype would be explained by this mutation in the GPIbalpha gene.

Bernard-Soulier Syndrome Caused by a Dinucleotide Deletion and Reading Frameshift in the Region Encoding the Glycoprotein Ibα Transmembrane Domain

We investigated the molecular genetic and biosynthetic basis of Bernard-Soulier syndrome in a severely affected white woman. Flow cytometric analysis showed a severe deficiency of glycoprotein (GP) Ib, GP IX, and GP V on the surface of her platelets. Similarly, GP Ibα was undetectable by immunoblot analysis of platelet lysates. Surprisingly, a large quantity of a 70-kD protein (which probably represents a GP Ibα degradation product) was found in the patient's plasma in much greater quantities than in the plasma of an unaffected individual. To analyze the molecular lesion responsible for the disorder, we amplified and sequenced gene segments corresponding to the entire coding regions of the GP Ibα, GP Ibβ, and GP IX genes. The patient was homozygous for a specific GP Ibα allele that contained two tandem VNTR repeats in the region encoding the macroglycopeptide (C variant) and three differences from the published GP Ibα gene sequence. Two mutations were unlikely to be involved in the disorder: the substitution of a single base (T → C) in the second nucleotide of exon 2, which is in the 5′ untranslated region of the GP Ibα transcript, and a silent mutation in the third base of the codon for Arg342 (A → G) that does not change the amino acid sequence. The third mutation was a deletion of the last two bases of the codon for Tyr492 (TAT). This mutation causes a frameshift that alters the GP Ibα amino acid sequence, beginning within its transmembrane region. The mutant polypeptide contains 81 novel amino acids and is 38 amino acids shorter than its wild-type counterpart. The new sequence changes the hydrophobic nature of the transmembrane domain and greatly decreases the net positive charge of what had been the cytoplasmic domain. The deletion mutation was introduced into the GP Ibα cDNA, alone and in combination with the 5′ mutation, and expressed in Chinese hamster ovary (CHO) cells. The deletion alone severely reduced GP Ibα expression on the cell surface. Expression was not decreased further by addition of the 5′ mutation, confirming that the deletion was the cause of the Bernard-Soulier phenotype. Stable cell lines expressing the mutant polypeptide secreted large amounts of the polypeptide into the medium, suggesting that the mutant anchors poorly in the plasma membrane. Nevertheless, a fraction of the mutant was able to associate with GP Ibβ, as demonstrated by their coimmunoprecipitation with a GP Ibβ antibody.

A Dinucleotide Deletion Results in Defective Membrane Anchoring and Circulating Soluble Glycoprotein Ibα in a Novel Form of Bernard-Soulier Syndrome

The platelet membrane glycoprotein (GP)Ib-V-IX complex is the receptor for von Willebrand factor and is composed of four membrane-spanning polypeptides: GPIbα, GPIbβ, GPIX, and GPV. A qualitative or quantitative deficiency in the GPIb-V-IX complex on the platelet membrane is the cause of the congenital platelet disorder Bernard-Soulier syndrome (BSS). We describe the molecular basis of a novel variant BSS in a patient in which GPIbα was absent from the platelet surface but present in a soluble form in the plasma. DNA sequence analysis showed a homozygous dinucleotide deletion in the codon for Tyr 508 (TAT) in GPIbα. This mutation (GPIbαΔAT) causes a frame shift that alters the amino acid sequence of GPIbα within its transmembrane region. The hydrophobic nature of the predicted transmembrane region and the cytoplasmic tail at the COOH terminal are altered before reaching a new premature stop codon 38 amino acids short of the wild-type peptide. Although GPIbαΔAT was not detectable on the platelet surface, immunoprecipitation of plasma with specific monoclonal antibodies (MoAbs) identified circulating GPIbα. Transient expression of recombinant GPIbαΔAT in 293T cells also generated a soluble form of the protein. Moreover, when a plasmid encoding GPIbαΔAT was transiently transfected into Chinese hamster ovary (CHO) cells stably expressing the GPβ-IX complex, it failed to be expressed on the cell surface. Thus, a dinucleotide deletion in the codon for Tyr 508 causes a frameshift that alters the amino acid sequence of GPIbα starting within its transmembrane region, changes the hydrophobicity of the normal transmembrane region, and truncates the cytoplasmic domain affecting binding to the cytoskeleton and cytoplasmic proteins. This mutation affects anchoring of the GPIbα polypeptide in platelets and causes the observed BSS phenotype with circulating soluble GPIbα.

Bernard-Soulier Syndrome Caused by a Dinucleotide Deletion and Reading Frameshift in the Region Encoding the Glycoprotein Ibα Transmembrane Domain

We investigated the molecular genetic and biosynthetic basis of Bernard-Soulier syndrome in a severely affected white woman. Flow cytometric analysis showed a severe deficiency of glycoprotein (GP) Ib, GP IX, and GP V on the surface of her platelets. Similarly, GP Ibα was undetectable by immunoblot analysis of platelet lysates. Surprisingly, a large quantity of a 70-kD protein (which probably represents a GP Ibα degradation product) was found in the patient's plasma in much greater quantities than in the plasma of an unaffected individual. To analyze the molecular lesion responsible for the disorder, we amplified and sequenced gene segments corresponding to the entire coding regions of the GP Ibα, GP Ibβ, and GP IX genes. The patient was homozygous for a specific GP Ibα allele that contained two tandem VNTR repeats in the region encoding the macroglycopeptide (C variant) and three differences from the published GP Ibα gene sequence. Two mutations were unlikely to be involved in the disorder: the substitution of a single base (T → C) in the second nucleotide of exon 2, which is in the 5′ untranslated region of the GP Ibα transcript, and a silent mutation in the third base of the codon for Arg342 (A → G) that does not change the amino acid sequence. The third mutation was a deletion of the last two bases of the codon for Tyr492 (TAT). This mutation causes a frameshift that alters the GP Ibα amino acid sequence, beginning within its transmembrane region. The mutant polypeptide contains 81 novel amino acids and is 38 amino acids shorter than its wild-type counterpart. The new sequence changes the hydrophobic nature of the transmembrane domain and greatly decreases the net positive charge of what had been the cytoplasmic domain. The deletion mutation was introduced into the GP Ibα cDNA, alone and in combination with the 5′ mutation, and expressed in Chinese hamster ovary (CHO) cells. The deletion alone severely reduced GP Ibα expression on the cell surface. Expression was not decreased further by addition of the 5′ mutation, confirming that the deletion was the cause of the Bernard-Soulier phenotype. Stable cell lines expressing the mutant polypeptide secreted large amounts of the polypeptide into the medium, suggesting that the mutant anchors poorly in the plasma membrane. Nevertheless, a fraction of the mutant was able to associate with GP Ibβ, as demonstrated by their coimmunoprecipitation with a GP Ibβ antibody.

A Dinucleotide Deletion Results in Defective Membrane Anchoring and Circulating Soluble Glycoprotein Ibα in a Novel Form of Bernard-Soulier Syndrome

The platelet membrane glycoprotein (GP)Ib-V-IX complex is the receptor for von Willebrand factor and is composed of four membrane-spanning polypeptides: GPIbα, GPIbβ, GPIX, and GPV. A qualitative or quantitative deficiency in the GPIb-V-IX complex on the platelet membrane is the cause of the congenital platelet disorder Bernard-Soulier syndrome (BSS). We describe the molecular basis of a novel variant BSS in a patient in which GPIbα was absent from the platelet surface but present in a soluble form in the plasma. DNA sequence analysis showed a homozygous dinucleotide deletion in the codon for Tyr 508 (TAT) in GPIbα. This mutation (GPIbαΔAT) causes a frame shift that alters the amino acid sequence of GPIbα within its transmembrane region. The hydrophobic nature of the predicted transmembrane region and the cytoplasmic tail at the COOH terminal are altered before reaching a new premature stop codon 38 amino acids short of the wild-type peptide. Although GPIbαΔAT was not detectable on the platelet surface, immunoprecipitation of plasma with specific monoclonal antibodies (MoAbs) identified circulating GPIbα. Transient expression of recombinant GPIbαΔAT in 293T cells also generated a soluble form of the protein. Moreover, when a plasmid encoding GPIbαΔAT was transiently transfected into Chinese hamster ovary (CHO) cells stably expressing the GPβ-IX complex, it failed to be expressed on the cell surface. Thus, a dinucleotide deletion in the codon for Tyr 508 causes a frameshift that alters the amino acid sequence of GPIbα starting within its transmembrane region, changes the hydrophobicity of the normal transmembrane region, and truncates the cytoplasmic domain affecting binding to the cytoskeleton and cytoplasmic proteins. This mutation affects anchoring of the GPIbα polypeptide in platelets and causes the observed BSS phenotype with circulating soluble GPIbα.

Missense Mutations of the Glycoprotein (GP) Ibβ Gene Impairing the GPIb α/β Disulfide Linkage in a Family With Giant Platelet Disorder

We describe here the molecular basis of an isolated hereditary giant platelet disorder (GPD) which is not accompanied with thrombocytopenia or leukocyte inclusion. Platelet aggregation with ristocetin and botrocetin was almost normal in this patient. Flow cytometric analysis showed that the glycoprotein (GP) Ib/IX complex was expressed on the platelet membranes at decreased levels. The amount of platelet GPIbα and the plasma glycocalicin concentration, the water-soluble extracellular portion of GPIbα, were also decreased. The anti-GPIbα antibody coprecipitated GPIbβ and GPIX, although the ratios of these polypeptides to GPIbα was greatly decreased compared with the ratio in normal platelets. Immunoblot analysis under nonreduced conditions showed that most of the GPIbα in the patient's platelets was not disulfide linked with GPIbβ. DNA sequencing analysis showed compound heterozygosity for two independent single nucleotide substitutions: from Tyr (TAC) to Cys (TGC) at residue 88, and from Ala (GCC) to Pro (CCC) at residue 108 in her GPIbβ gene. These substitutions were not found in genomic DNA samples from 108 normal individuals. These mutations might result in decreased expression of the GPIb/IX complex and may influence the association of the complex with the membrane skeleton, consequently impairing normal platelet morphology. Furthermore, the phenotype caused by mutations in the subunits of the GPIb/IX complex could span the spectrum from a normal phenotype, to isolated GPD, to a full-blown bleeding disorder, such as Bernard-Soulier syndrome.

Identification of a mutation in a GATA binding site of the platelet glycoprotein Ibbeta promoter resulting in the Bernard-Soulier syndrome

Bernard-Soulier Syndrome (BSS) is a rare congenital bleeding disorder due to absent or decreased expression of the glycoprotein Ib-IX-V (GpIb-IX-V) receptor complex on the platelet surface. To date, only mutations in GpIbalpha or GpIX have been reported in patients with BSS. GpIbbeta differs from the other proteins in this receptor in that the gene is more complex, and an alternative form is expressed in cells of non-megakaryocytic lineage, including endothelial cells. It appears that the megakaryocytic and endothelial cell mRNA species are transcribed from different start sites and have different proximal promoter regions. We have identified a patient with BSS who has a deletion on one chromosome 22, resulting in velocardiofacial syndrome. The GpIbbeta gene has been mapped to this deleted (22q11.2) region of chromosome 22. The patient has greatly reduced levels of GpIbbeta mRNA and no detectable platelet GpIbbeta protein, suggesting that his BSS results from a mutation in his remaining GpIbbeta allele. Sequence analysis revealed that the coding region of GpIbbeta is normal, but the 5'-upstream region contains a C to G transversion at base -133 from the transcription start site used in megakaryocytes. The mutation changes a GATA consensus binding site, disrupts GATA-1 binding to the mutated site, and decreases promoter activity by 84%. Thus, in this patient, Bernard-Soulier syndrome results from a deletion of one copy of GpIbbeta and a mutated GATA binding site in the promoter of the remaining allele, resulting in decreased promoter function and GpIbbeta gene transcription.