DNA sequence of the canine platelet beta3 gene from cDNA: comparison of canine and mouse beta3 to segments that encode alloantigenic sites and functional domains of beta3 in human beings

Significant differences in single-platelet biophysics exist across species but attenuate during clot formation

Human, canine, ovine, and porcine platelets exhibit disparate biophysical signatures, whereas human and murine platelets are similar. Multiple biophysical parameters integrate during clot formation, measured by bulk clot contraction, and attenuate biophysical differences.

Clinical, Biochemical, and Molecular Aspects of Glanzmann's Thrombasthenia in Humans and Dogs

Glanzmann's thrombasthenia (GT) is an inherited, intrinsic platelet function defect that involves the platelet glycoprotein complex IIb–IIIa, also known as the fibrinogen receptor and the integrin αIIbβ3. The defect was originally described by Dr. Glanzmann in humans in 1918 as a bleeding disorder that differed clinically from other known coagulopathies. Over the decades that followed, researchers determined the biochemical and molecular basis for the disease in humans. Otterhounds with thrombasthenic thrombopathia, described in the 1960s, were the only animal model that closely resembled the disease described in humans until 1996. At that time, a Great Pyrenees dog was identified with unequivocal clinical and biochemical features of Type I GT. The cDNA encoding for glycoproteins IIb and IIIa were sequenced in normal dogs in 1999, allowing for identification of specific mutations causing Type I GT in both Otterhounds and Great Pyrenees dogs. Knowing the molecular basis for Type I GT in dogs as well as the cDNA sequences in normal dogs should enhance the understanding of structure/function relationships of the αIIbβ3 integrin and provide an excellent animal model for studies aimed at correction of GT in humans. The following review focuses on the structure and function of this platelet receptor and reviews the molecular, biochemical, and clinical aspects of Glanzmann's thrombasthenia in humans and dogs.

Characterization of the cDNA Encoding alphaIIb and beta3 in normal horses and two horses with Glanzmann thrombasthenia

Glanzmann thrombasthenia (GT) is an inherited, intrinsic platelet defect characterized by a quantitative or qualitative change in the platelet glycoprotein complex IIb-IIIa (integrin alpha(IIb)beta3). The subunits are encoded by separate genes and both subunits must be expressed for a stable complex to form on the platelet surface; therefore, a defect in either gene can result in GT.

Integrin beta3 regions controlling binding of murine mAb 7E3: implications for the mechanism of integrin alphaIIbbeta3 activation

Abciximab, a derivative of the murine mAb 7E3, protects against ischemic complications of percutaneous coronary interventions by inhibiting ligand binding to the alphaIIbbeta3 receptor. In this study we identified regions on integrin beta3 that control 7E3 binding. Murine/human amino acid substitutions were created in two regions of the betaA domain that previous studies found to influence 7E3 binding: the C177-C184 loop and K125-N133. The T182N substitution and a K125Q mutation reduced 7E3 binding to human beta3 in complex with alphaIIb. The introduction of both the human C177-C184 region and human W129 into murine beta3 was necessary and sufficient to permit 7E3 binding to the human alphaIIb/murine beta3 complex. Although we cannot exclude allosteric effects, we propose that 7E3 binds between C177-C184 and W129, which are within 15 A of each other in the crystal structure and close to the beta3 metal ion-dependent adhesion site. We previously demonstrated that 7E3 binds more rapidly to activated than unactivated platelets. Because it has been proposed that alphaIIbbeta3 changes from a bent to an extended conformation upon activation, we hypothesized that 7E3 binds less well to the bent than the extended conformation. In support of this hypothesis we found that 7E3 bound less well to an alphaIIbbeta3 construct locked in a bent conformation, and unlocking the conformation restored 7E3 binding. Thus, our data are consistent with alphaIIbbeta3 existing in variably bent conformations in equilibrium with each other on unactivated platelets, and activation resulting in alphaIIbbeta3 adopting a more extended conformation.

Gene therapy for platelet disorders: studies with Glanzmann's thrombasthenia

Current research aimed at correcting platelet defects are designed to further our knowledge in the use of hematopoietic stem cells for gene therapies of hemorrhagic disorders. Information gained from these studies may be directly applicable to treatment of disorders affecting platelets (e.g. Glanzmann's thrombasthenia, Bernard Soulier syndrome, gray platelet syndrome, and von Willebrand disease) as well as other disorders affecting distinct hematopoietic cell lineages. This work specifically addresses three questions: (i) can bone marrow stem cells be given sufficient genetic information to induce abnormal megakaryocytes to synthesize transgene products that help newly formed platelets to participate in normal hemostasis? (ii) can the newly synthesized receptor be maintained as a platelet-specific protein at therapeutic levels for a reasonable period of time? and (iii) will newly expressed proteins be tolerated by the immune system or become a target for B- and T-cell mediated immunity resulting in the premature destruction and clearing of the genetically altered megakaryocytes and platelets? Answers to these questions should indicate the feasibility of targeting platelets with genetic therapies that will in turn enable better management of patients with inherited bleeding disorders. The long-range benefit of this research will be an improved understanding of the regulation of protein expression during normal megakaryocytopoiesis, and the accumulation of additional scientific knowledge about normal platelet function and the way in which platelets and other cells recognize and interact with each other.

A functional platelet fibrinogen receptor with a deletion in the cysteine-rich repeat region of the beta(3) integrin: the Oe(a) alloantigen in neonatal alloimmune thrombocytopenia

This report describes a new low-frequency alloantigen, Oe(a), responsible for a case of neonatal alloimmune thrombocytopenia (NAIT). In a population study none of 600 unrelated blood donors was an Oe(a) carrier. By immunochemical studies the Oe(a) antigen could be assigned to platelet glycoprotein (GP) IIIa. Sequencing of GPIIIa complementary DNA from an Oe(a) (+) individual showed deletion of a lysine residue at position 611 (DeltaLys(611)). Analysis of 20 Oe(a) (-) and 3 Oe(a) (+) individuals showed that the DeltaLys(611) form of GPIIIa was related to the phenotype. Anti-Oe(a) reacted with the DeltaLys(611), but not with the wild-type isoforms on stable transfectants expressing GPIIIa, indicating that DeltaLys(611) directly induces the expression of Oe(a) epitopes. Under nonreducing conditions the Pro(33)DeltaLys(611) variant migrated with a slightly decreased molecular weight compared to the Pro(33)Lys(611) isoform suggesting that DeltaLys(611) has an influence on the disulfide bonds of GPIIIa. The Pro(33)DeltaLys(611) GPIIIa could undergo conformational changes and bind to fibrinogen in a similar manner as the Pro(33)Lys(611) isoform. No difference was found in the tyrosine phosphorylation of pp125(FAK), suggesting that DeltaLys(611) has no effect on integrin function. In contrast to all other low-frequency antigens, the DeltaLys(611) isoform was associated with the HPA-1b, but not with the high frequency HPA-1a allele. Comparison with GPIIIa DNA from nonhuman primates indicated that the HPA-1a allele represents the ancestral form of GPIIIa. It can be assumed that the Oe(a) form did arise as a result of a mutational event from an already mutated GPIIIa allele.

Molecular and genetic basis for thrombasthenic thrombopathia in otterhounds

OBJECTIVES

To determine the molecular and genetic basis for thrombasthenic thrombopathia in Otterhounds and establish whether the defect would be best classified as type-I Glanzmann's thrombasthenia.

ANIMALS

57 dogs, including 13 affected Otterhounds, 23 carrier Otterhounds, 17 unaffected Otterhounds, and 4 clinically normal unrelated dogs of other breeds.

PROCEDURE

Functional (platelet aggregation, clot retraction, buccal mucosa bleeding time) and biochemical (electrophoresis, flow cytometry, fibrinogen content) analyses were conducted. In addition, first-strand cDNA synthesis from platelet total RNA was performed. Exons of the genes encoding for glycoproteins (GP) IIb and IIIa were amplified in overlapping fashion. The resulting products were excised from agarose gels and sequenced. The sequences obtained were compared with known cDNA sequences for canine GPIIb and GPIIIa.

RESULTS

A single nucleotide change at position G1193 (1100) was detected in exon 12 of the gene encoding for platelet GPIIb in 2 affected Otterhounds. Carrier Otterhounds were heterozygous at this position, and 2 unaffected Otterhounds were unchanged. This nucleotide change would result in substitution of histidine for aspartic acid at position 398 (367) within the third calcium-binding domain of GPIIb.

CONCLUSIONS AND CLINICAL RELEVANCE

These studies suggest that thrombasthenic thrombopathia of Otterhounds is homologous phenotypically and has a similar molecular basis to type-I Glanzmann's thrombasthenia in humans.

Nucleotide sequence of the canine alphaIIb gene from platelet-derived cDNA

OBJECTIVE

To determine the nucleotide sequence of the alphaIIb gene from canine platelet-derived cDNA.

ANIMALS

3 adult dogs.

PROCEDURE

First-strand cDNA was prepared from total RNA isolated from canine platelets. The cDNA was amplified, using specific primers in polymerase chain reaction (PCR), and the nucleotide sequence was obtained from purified PCR products.

RESULTS

Except for the nucleotide at position 694, results of all sequencing reactions of alphaIIb were identical for canine platelet-derived cDNA. Canine alphaIIb had 3 fewer codons than alphaIIb of humans. The nucleotide and deduced amino acid sequences of full-length canine alphaIIb shared > or = 83% similarity with the sequences established for humans. Segments of canine alphaIIb nucleotide and deduced amino acid sequences were > or = 78% similar to alphaIIb associated with 7 functional domains (extracellular, transmembrane, cytoplasmic, and 4 calcium-binding domains) in humans, with the highest degree of similarity correlating with the sequences of the 4 calcium-binding domains. Amino acid residues associated with development of alloantibodies in humans (Met837, Val837, Ile843, Ser843) are not encoded by canine alphaIIb.

CONCLUSIONS AND CLINICAL RELEVANCE

The nucleotide variation at position 694 of canine alphaIIb may represent a polymorphism. The species differences in the alphaIIb sequence may contribute to variations in receptor-li gand interactions. The high degree of alphaIIb sequence conservation of the 4 calcium-binding domains implies functional importance. Some disorders associated with alphaIIbbeta3 in dogs are clinically analogous to diseases in humans, and results indicate that dogs are an appropriate model for the evaluation of gene therapy and other treatments of platelet-associated disorders.

Two genetic defects in alphaIIb are associated with type I Glanzmann's thrombasthenia in a Great Pyrenees dog: a 14-base insertion in exon 13 and a splicing defect of intron 13

Glannzmann's thrombasthenia (GT) is an autosomal recessive bleeding disorder caused by qualitative or quantitative deficiencies of the platelet membrane glycoprotein alphaIIbbeta3. This is the first report of a molecular genetic basis for type I GT in dogs. As previously reported, a thrombasthenic Great Pyrenees dog (dog No. 1) experienced uncontrolled epistaxis despite results of coagulation screening tests, platelet quantitation, and von Willebrand factor quantitation that were within reference ranges. Platelet aggregation was minimal in response to agonists. Flow cytometry, autoradiography, and immunoblot experiments demonstrated either marked reduction or absence of glycoproteins alphaIIb and beta3. In this study, we report the presence of a 14-base insertion in exon 13 and defective splicing of intron 13 in the alphaIIb gene of two thrombasthenic dogs (Nos. 1 and 8). The insertion disrupted the fourth alphaIIb calcium-binding domain, caused a shift in the reading frame and resulted in a premature termination codon. Possible consequences of this mutation include decreased alphaIIb mRNA stability and production of truncated alphaIIb protein that lacks the transmembrane and cytoplasmic domains and a large portion of the extracellular domain. We identified the dam, sire, and three littermates of dog No. 8 as carriers of the alphaIIb mutation. Canine alphaIIb and beta3 genes share significant homology with the genes in human beings, making canine GT an excellent translational model for human GT. A defined molecular basis for canine GT will enhance ongoing gene therapy research and increase the understanding of structure-function relationships of this integrin.