Non-radioactive SSCP for genotyping human platelet alloantigens

Real-time PCR genotyping of human platelet alloantigens HPA-1, HPA-2, HPA-3 and HPA-5 is superior to the standard PCR-SSP method

Genotyping of the human platelet alloantigens (HPA) is useful for the diagnosis and therapy of the patients with alloimmune thrombocytopenic syndromes, such as post-transfusion refractoriness to platelets, post-transfusion thrombocytopenic purpura and foetomaternal alloimmune thrombocytopenia. We have developed, optimized and validated a new method for simultaneous genotyping of HPAs - HPA-1, HPA-2, HPA-3 and HPA-5 - by using the real-time polymerase chain reaction (PCR) based on TaqMan technology. Its performances were compared to those of the standard PCR-sequence-specific primers (SSP) method by testing 120 DNA samples. Several discrepancies between the two methods have been observed, especially in the HPA-3 genotyping. Evidently, the PCR-SSP method produced several false positive results due to its technical drawbacks. Based on our comparison, we believe that the new real-time TaqMan PCR assay for the HPA-1, HPA-2, HPA-3 and HPA-5 genotyping is faster, more reliable and reproducible, compared to the standard PCR-SSP.

Genotyping for platelet-specific antigens: techniques for the detection of single nucleotide polymorphisms

Accurate typing of patients for platelet-specific (human platelet) antigens (HPA) is required in several different clinical situations, and blood services need to maintain panels of HPA-typed apheresis platelet donors and whole-blood donors to support HPA alloimmunized patients. Six clinically relevant HPA alloantigen systems have been described and, in addition, a significant number of HPA alloantigens with a highly skewed allele frequency or of very low immunogenicity have been reported. Certain well-characterized biallelic systems such as Gov have not as yet been included in the HPA nomenclature but are included in this review. Biochemical studies have identified the platelet membrane proteins on which the HPA antigens are localized. Cloning of the genes encoding these proteins and the realization that there is adequate mRNA in fresh platelets has led to identification of the molecular basis of HPA antigens over the last decade. All but one of the biallelic platelet-specific alloantigen systems are based on a single nucleotide polymorphism in the DNA sequence, corresponding to a single amino acid substitution in the encoded primary protein sequence. The discovery of the genetic basis of the alloantigens has allowed the development of polymerase chain reaction-based techniques for HPA genotyping using genomic DNA. The genetic basis of the HPA alloantigens, the most commonly used genome typing techniques and their pitfalls, and future developments, are discussed in this review.

Genotyping of human platelet antigen-1 by gene amplification and labelling in one system and automated fluorescence correlation spectroscopy

Genotyping of human platelet antigen-1 (HPA-1) is required for the diagnosis and appropriate therapy of alloimmunization. Recently, the HPA-1 polymorphism has been identified as an inherited risk factor for thrombosis. Most currently used methods for HPA-1 genotyping have the disadvantage of time-consuming post-polymerase chain reaction (PCR) processes such as ligation (oligonucleotide ligation assay), restriction enzyme digestion (allele-specific restriction enzyme analysis) and electrophoresis (single-strand conformation polymorphism). We present a novel method for HPA-1 genotyping based on a homogeneous PCR strategy (GALIOS, gene amplification and labelling in one system) combined with automated fluorescence correlation spectroscopy (FCS). The PCR uses one pair of gene-specific amplification primers and two allele-specific, semi-nested labelling primers. The allele-specific labelling primers differ in a single nucleotide (T for HPA-1a/1a, C for HPA-1b/1b) and are coupled to different fluorescent dyes. The quantities of generated fluorescent PCR products are analysed by FCS at 543 nm and 633 nm excitation wavelength respectively. The genotypes determined using this method were in 100% concordance with the results obtained by allele-specific restriction analysis (n = 380 samples). The assay was validated for specificity, reliability and the dynamic range. This innovative method of rapid HPA-1 genotyping offers a specific and robust system, which is applicable for routine HPA-1 genotyping.

A novel (288delC) mutation in exon 2 of GPIIb associated with type I Glanzmann's thrombasthenia

This work reports the molecular genetic analysis of two patients who suffer mucocutaneous haemorrhages, prolonged bleeding time and failure of platelets to aggregate, either spontaneously or in response to agonists. The absence of platelet surface glycoprotein (GP)IIb-IIIa complexes confirmed the clinical diagnosis of Glanzmann's thrombasthenia (GT). Polymerase chain reaction single-strand conformation polymorphism (PCR-SSCP) analysis of exon 2 of GPIIb showed polymorphic bands caused by the homozygous deletion of a cytosine at position 288 relative to the translation start site. causing a shifting of the reading frame and appearance of a premature termination codon. The heterozygous relatives showed a reduced platelet content of GPIIb-IIIa, and a correlation was found between the levels of GPIIb mRNA and surface expression of GPIIb-IIIa complexes. Unlike other mRNAs carrying a nonsense mutation, (288Cdel)GPIIb does not force alternative splicing of GPIIb mRNA. As expected, co-transfection of Chinese hamster ovary (CHO) cells with cDNAs encoding GPIIIa and (288delC)GPIIb failed to enhance the surface exposure of GPIIIa. It is concluded that the (288delC)GPIIb mutation is responsible for the thrombasthenic phenotype of the patients. In addition, it has also been determined that heterodimerization of GPIIb-IIIa requires the integrity of exons 2 and 3 of GPIIb.

Rapid genotyping of human platelet antigen 1 (HPA-1) with fluorophore-labelled hybridization probes on the LightCycler

Genotyping of human platelet alloantigens (HPA) has become an important procedure in the diagnosis and prevention of disorders such as neonatal alloimmune thrombocytopenic purpura, post-transfusion purpura, and refractoriness to platelet transfusion therapy. We present a single-tube method for HPA-1 genotyping that combines rapid-cycle PCR with allele-specific fluorescent probe melting profiles for product genotyping. A fragment covering the polymorphic site is amplified in the presence of two fluorescently-labelled hybridization probes. During the annealing step of the thermal cycling, both probes bind to their complementary sequences in the amplicon resulting in resonance energy transfer, thus providing real-time fluorescence monitoring of PCR. Continuous aquisition of fluorescence data during a melting curve analysis at the completion of PCR revealed that loss of fluorescence occurred in an allele-specific manner as the detection probe, which was fully complementary to the HPA-1b allele, melted off the template. By determining the temperature at which maximum melting of the hybrids occurred, the two alleles were readily distinguishable. Using this method, genotyping of 32 samples was completed within 30 min without the need for any post-PCR sample manipulation, thereby eliminating the risks of end-product contamination and sample tracking errors. The genotypes determined with the LightCyclerTM were identical when compared with a conventional PCR and restriction fragment length polymorphism technique. The genotyping of HPA-1 on the LightCycler is a rapid and reliable method that is suitable for typing both small and large numbers of samples.

Expression of markers of platelet activation and the interpatient variation in response to abciximab

Our study concerns the biological effects of abciximab (c7E3 Fab, ReoPro), a powerful new antiplatelet drug that blocks glycoprotein (GP) IIb-IIIa complexes. Samples were examined from 6 patients with coronary artery disease who received a bolus of abciximab followed by a 10- microg/min infusion for at least 18 hours before percutaneous transluminal coronary angioplasty. Inhibition of ADP-induced PA was maximal for 4 patients but partial (79% and 53%) for 2 others during the infusion. Flow cytometry performed with monoclonal antibodies (PAC-1, AP-6, and F26) specific for the "activated" GP IIb-IIIa complex revealed large decreases in the expression of activation markers on platelets during therapy, but these decreases were less marked when inhibition of ADP-induced PA was incomplete. Residual aggregation was seen for all patients during the infusion when TRAP 14-mer peptide or thrombin was the stimulus. Unblocked GP IIb-IIIa complexes were detected on thrombin-stimulated platelets from the patients by immunoelectron microscopy performed using the monoclonal antibody AP-2. Unblocked GP IIb-IIIa complexes were also detected by flow cytometry when platelets preincubated for 1 hour in vitro with abciximab under saturating conditions were (1) incubated with TRAP 14-mer or (2) permeabilized with Triton X-100. In confirming interpatient variation in the platelet response to a standard dose of abciximab, our results also show that an uninhibited internal pool of GP IIb-IIIa complexes may mediate a residual response to strong agonists.

R to Q Amino Acid Substitution in the GFFKR Sequence of the Cytoplasmic Domain of the Integrin IIb Subunit in a Patient With a Glanzmann’s Thrombasthenia-Like Syndrome

The integrin IIbβ3 mediates platelet aggregation through its fibrinogen and adhesive protein-binding properties. Particular interest concerns the role of the cytoplasmic domains of IIb and β3. We now report the molecular analysis of IIbβ3 from a patient with a Glanzmann’s thrombasthenia-like syndrome for whom the principal characteristics are an approximate 50% total platelet content of IIbβ3 but with a much lower proportion in the surface pool (Hardisty et al, Blood 80:696, 1992). Polymerase chain reaction (PCR) single-strand conformational polymorphism and DNA sequencing showed a heterozygous mutation giving rise to amino acid substitution R995 to Q in the GFFKR sequence of the cytoplasmic domain of IIb. Reverse transcriptase-PCR and polymorphism analysis only detected mRNA for the mutated allele of the IIb gene and a single allele of the β3 gene in his platelets, suggesting other unidentified defects. Site-directed mutagenesis followed by transient expression of the mutated IIb together with wild-type β3 in Cos-7 cells resulted in a markedly decreased expression of the complex at the cell surface when compared with cells transfected with wild-type IIb and β3. Flow cytometry with PAC-1 and a stable Chinese hamster ovary–transfected cell line showed that the mutated receptor was not locked into a high activation state, although it became so in the presence of the activating antibody, anti-LIBS6. This is the first reported natural mutation in the highly conserved GFFKR sequence of the IIb cytoplasmic domain.

R to Q Amino Acid Substitution in the GFFKR Sequence of the Cytoplasmic Domain of the Integrin IIb Subunit in a Patient With a Glanzmann’s Thrombasthenia-Like Syndrome

The integrin IIbβ3 mediates platelet aggregation through its fibrinogen and adhesive protein-binding properties. Particular interest concerns the role of the cytoplasmic domains of IIb and β3. We now report the molecular analysis of IIbβ3 from a patient with a Glanzmann’s thrombasthenia-like syndrome for whom the principal characteristics are an approximate 50% total platelet content of IIbβ3 but with a much lower proportion in the surface pool (Hardisty et al, Blood 80:696, 1992). Polymerase chain reaction (PCR) single-strand conformational polymorphism and DNA sequencing showed a heterozygous mutation giving rise to amino acid substitution R995 to Q in the GFFKR sequence of the cytoplasmic domain of IIb. Reverse transcriptase-PCR and polymorphism analysis only detected mRNA for the mutated allele of the IIb gene and a single allele of the β3 gene in his platelets, suggesting other unidentified defects. Site-directed mutagenesis followed by transient expression of the mutated IIb together with wild-type β3 in Cos-7 cells resulted in a markedly decreased expression of the complex at the cell surface when compared with cells transfected with wild-type IIb and β3. Flow cytometry with PAC-1 and a stable Chinese hamster ovary–transfected cell line showed that the mutated receptor was not locked into a high activation state, although it became so in the presence of the activating antibody, anti-LIBS6. This is the first reported natural mutation in the highly conserved GFFKR sequence of the IIb cytoplasmic domain.

Human platelet antigen genotyping using a fluorescent SSCP technique with an automatic sequencer

The typing of human platelet antigens (HPA) can be useful in many clinical situations such as neonatal alloimmune thrombocytopenia, post-transfusion purpura, and platelet transfusion refractoriness. The fluorescent-based single-strand conformation polymorphism (F-SSCP) technique is a fast and convenient way to perform HPA genotyping. Universal sequences from phage M13 were introduced at both ends of specific PCR-products by using 5'-tailed primers. A short second round of PCR with universal primers coupled to Cy-5 enabled the PCR-products to be fluorescently labelled. F-SSCP was performed by gel electrophoresis on an automated fluorescent DNA analyser. Genotyping of the three major HPA systems carried by the GP IIb-IIIa complex showed the F-SSCP technique to be accurate and reliable. A single gel procedure has been sufficient to detect HPA genetic polymorphisms tested to date. Neither restriction enzyme, radioactive material, nor any other hazardous chemicals such as ethidium bromide were required. This technique enabled us to genotype HPA-1, -3 and -4 alleles easily and to diagnose materno-fetal incompatibility in a rare alloantigenic system. F-SSCP is a promising technique for the detection of new mutations and/or DNA polymorphisms on a large scale.

Polymorphisms of Platelet Membrane Glycoprotein Ib Associated With Arterial Thrombotic Disease

Platelet membrane glycoprotein Ib (GPIb) is a major receptor for von Willebrand factor and thrombin, which plays a key role in the initial development of thrombi. Two polymorphisms (HPA-2 and VNTR) that affect phenotype have been described in GPIb. The relevance of these polymorphisms to thrombotic disease was investigated by genotypic identification in three case-control studies: 104 case patients with acute cerebrovascular disease (CVD), 101 case patients with acute coronary heart disease (CHD), 95 patients with deep venous thrombosis (DVT), and one control age-, sex-, and race-matched for each case patient. Results show that the C/B genotype of the VNTR and the HPA-2b polymorphisms of GPIb are strongly associated with increased risk of coronary heart disease and cerebral vascular disease but not with deep vein thrombosis. These two polymorphisms of GPIb may represent newly identified risk factors for arterial thrombotic disease, but not for venous thrombosis.

© 1998 by The American Society of Hematology.

Polymorphisms of Platelet Membrane Glycoprotein Ib Associated With Arterial Thrombotic Disease

Platelet membrane glycoprotein Ib (GPIb) is a major receptor for von Willebrand factor and thrombin, which plays a key role in the initial development of thrombi. Two polymorphisms (HPA-2 and VNTR) that affect phenotype have been described in GPIb. The relevance of these polymorphisms to thrombotic disease was investigated by genotypic identification in three case-control studies: 104 case patients with acute cerebrovascular disease (CVD), 101 case patients with acute coronary heart disease (CHD), 95 patients with deep venous thrombosis (DVT), and one control age-, sex-, and race-matched for each case patient. Results show that the C/B genotype of the VNTR and the HPA-2b polymorphisms of GPIb are strongly associated with increased risk of coronary heart disease and cerebral vascular disease but not with deep vein thrombosis. These two polymorphisms of GPIb may represent newly identified risk factors for arterial thrombotic disease, but not for venous thrombosis.

© 1998 by The American Society of Hematology.

Double heterozygosity of the GPIIb gene in a Swiss patient with Glanzmann's thrombasthenia

Glanzmann's thrombasthenia (GT) results from a qualitative or quantitative defect of GPIIb-IIIa complexes (integrin alphaIIbbeta3). the fibrinogen receptor on platelets. This integrin plays a critical role in platelet aggregation. In this report we describe the molecular abnormalities of a patient with clinical and laboratory findings typical of type I Glanzmann's thrombasthenia. SDS-PAGE with Western blotting revealed an absence of GPIIb but small amounts of normally migrating GPIIIa in his platelets. A non-radioactive PCR-SSCP procedure and direct sequence analysis of PCR-amplified DNA fragments showed the patient to be a compound heterozygote for mutations in the GPIIb gene. A single point mutation (G to A) at nucleotide 1064 of the cDNA derived from the mother's allele led to a Glu324 to Lys amino acid substitution in GPIIb. It was responsible for a MscI restriction site in exon 12 of the GPIIb gene. This amino acid substitution changes the electric charge between the second and third Ca++-binding domains of GPIIb. The second mutation was inherited from his father and is in exon 18 of the GPIIb gene. It was a T --> C base transition at position 1787 of GPIIb cDNA and results in a Ile565 to Thr substitution. The two GPIIb mutations identified in this study will provide new information on GPIIb-IIIa structure and biosynthesis.

HPA-10wb (Laa): Genetic Determination of a New Platelet-Specific Alloantigen on Glycoprotein IIIa and Its Expression in COS-7 Cells

The heterodimeric complex glycoprotein (GP)IIb-IIIa, the fibrinogen receptor of platelets, carries numerous alloantigen systems. These polymorphisms are responsible for the immune response after transfusion or during pregnancy. In the latter case, the mother develops an antibody against an epitope present on fetal platelets, and this results in platelet destruction in the fetus. In this report, we describe the molecular characterization of a new alloantigen (Laa) on GPIIIa responsible for neonatal alloimmune thrombocytopenia (NAIT). Using polymerase chain reaction (PCR)–singlestrand conformation polymorphism (SSCP) and DNA sequencing, we found a point mutation (G to A) in a heterozygous state on the GPIIIa gene leading to amino acid substitution Arg to Gln at position 62 of the mature protein. Transient expression of GPIIb-IIIa complexes in Cos-7 cells using wild-type or mutated GPIIIa cDNA allowed us to demonstrate that this mutation was responsible for expression of the Laa epitope.

Simultaneous genotyping of human platelet antigens by hot start sequence-specific polymerase chain reaction with DNA polymerase AmpliTaq Gold

OBJECTIVES

Human platelet alloantigen (HPA) typing has potential clinical relevance in a variety of contexts. We can improve methods for HPA genotyping by complementing our knowledge of the DNA sequence polymorphisms of HPA genes and experience with various DNA-based HPA typing techniques.

METHODS

A newly available DNA polymerase, AmpliTaq Gold (Perkin Elmer), provided in an inactive state and activated by heat, makes it possible to perform a hot start polymerase chain reaction (PCR) in order to prevent nonspecific amplification during the setup of PCR. To establish a practical procedure for HPA-1, 2, 3 and 5 genotyping, we applied the AmpliTaq Gold for a hot start PCR and employed 8 pairs of published sequence-specific primers (SSP). A simple simultaneous genotyping of these 4 HPA systems could be rapidly achieved with high specificity.

RESULTS

The HPA gene frequencies observed in 126 randomly selected German blood donors were 0.82 and 0.18 for HPA-1a and 1b, 0.92 and 0.08 for HPA-2a and 2b, 0.63 and 0.37 for HPA-3a and 3b and 0.90 and 0.10 for HPA-5a and Sb, respectively.

CONCLUSION

Using our hot start PCR-SSP procedure with AmpliTaq Gold a simple, rapid and reproducible genotyping for HPA-1, 2, 3 and S systems could be achieved.

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

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