Glanzmann thrombasthenia (GT)

GT is an autosomal recessive bleeding syndrome affecting the megakaryocyte lineage and characterized by a lack of platelet aggregation. It is a moderate to severe hemorrhagic disorder with mainly mucocutaneous bleeding. The molecular basis is linked to quantitative and/or qualitative abnormalities of αIIbβ3 integrin, the receptor that mediates the incorporation of platelets into an aggregate or thrombus at sites of vessel injury.

Glanzmann first described this disease in 1918 as "hereditary hemorrhagic thrombasthenia" 1. A prolonged bleeding time and an isolated, rather than clumped, appearance of platelets on a peripheral blood smear were early diagnostic criteria. In 1956, Braunsteiner and Pakesch reviewed disorders of platelet function and described thrombasthenia as an inherited disease characterized by platelets of normal size that failed to spread onto a surface and did not support clot retraction 2. The diagnostic features of GT including the absence of platelet aggregation as the primary feature were clearly established in 1964 by the classic report on 15 French patients by Caen et al. 3. Those patients with absent platelet aggregation and absent clot retraction were subsequently termed as having type I disease; those with absent aggregation but residual clot retraction, type II disease; while variant disease was first established in 1987 (reviewed in ref 4).

Platelet aggregation defects specific to adenosine diphosphate (ADP) or collagen, imply abnormalities of their primary receptors or of signaling pathways. Defects in the second wave of aggregation to ADP or in the response to collagen can imply storage pool disease and an absence of the secretory stores of ADP in dense granules. Deficiencies in the platelet response to arachidonic acid can point either to an inherited abnormality in thromboxane A2 formation or a platelet function defect temporarily acquired through aspirin ingestion. GT is the only disease in which platelet aggregation is defective to all agonists, while absent clot retraction is another frequent characteristic.

Normal ristocetin-induced platelet agglutination and normal platelet size clearly rule out the Bernard-Soulier syndrome, a disorder of platelet adhesion. Inherited thrombocytopenias are eliminated by a normal platelet count. Normal coagulation parameters rule out clotting disorders that can also affect platelet function such as congenital afibrinogenemia and von Willebrand disease. Acquired thrombasthenia must be eliminated in the absence of a family history of the disease. Platelet αIIbβ3 deficiency and abnormal platelet aggregation have been reported in patients with acute promyelocytic leukemia 5; the etiology of this acquired disorder is probably a chromosome 15–17 translocation. Although the breakpoint region on chromosome 17 is heterogeneous in acute promyelocytic leukemia, in some patients it occurs at 17q21, and this is the location of the genes for αIIb and β3 6. Another problem in diagnosing GT is to eliminate patients with acquired autoantibodies that block aggregation, although these patients would often be thrombocytopenic 7. These antibodies can be detected immunologically by their binding to αIIbβ3 of control platelets during incubation with the patient's serum 8.

Early reports emphasized the clinical variability of this bleeding syndrome: some patients had only minimal bruising, while others had frequent, severe and potentially fatal hemorrhages. Hemorrhagic symptoms occur only in patients homozygous for mutations causing GT; the heterozygous condition is mostly asymptomatic, even though these subjects have only a half-normal concentration of platelet αIIbβ3 4. The sites of bleeding in GT are clearly defined: purpura, epistaxis, gingival hemorrhage, and menorrhagia are nearly constant features; gastrointestinal bleeding and hematuria are less common but can cause serious complications 3940. It is important to note that deep visceral hematomas, a characteristic of coagulation disorders such as hemophilia, are not usually seen. In most cases, bleeding symptoms manifest rapidly after birth, even if occasionally GT is only diagnosed later in life. Epistaxis is a common cause of severe bleeding, and is typically more severe in childhood. In general, the bleeding tendency in GT decreases with age. The rare co-existence of GT with other inherited diseases, such as mild von Willebrand disease, may accentuate the clinical severity of bleeding 2739.

Although GT can be a severe hemorrhagic disease, the prognosis is excellent with careful supportive care. Most adult patients are in good health and their disease has a limited effect on their daily lives. Death from hemorrhage in diagnosed patients is rare unless associated with trauma or other disease (e.g. cancer). In contrast, families often report deceased siblings on diagnosis of GT.

A review of 177 patients with GT (of which 113 were literature reports and 64 were seen at the Hôpital Lariboisière, Paris) showed that 102 (58%) of the patients were female 4. This finding may reflect the added problem of menorrhagia. The average age of the 113 patients for whom age was given was only 20. This may reflect improvements in diagnostic procedures. The frequency of consanguinity in affected families is noticeable, and GT has an increased incidence in populations in whom marriage among close relatives is an accepted custom. For example, in the review of 177 patients 4, only 12 were from the United States. In contrast, 55 patients were from Israel and Jordan, and 42 were from South India. In certain ethnic groups, such as South Indian Hindus, Iraqi Jews, French gypsies and Jordanian nomadic tribes, thrombasthenia may actually be a common hereditary hemorrhagic disorder. This has recently been borne out by a report of 382 patients in Iran 47. For these reasons, it would be imprudent to give an estimation of worldwide prevalence.

Mucocutaneous bleeding with absent platelet aggregation in response to all physiologic stimuli is pathognomonic for GT, and abnormal clot retraction is rarely observed in other disorders 39. When these two signs are associated with a normal platelet count and morphology, the diagnosis of GT is clear-cut. Use of the PFA-100 system (Dade-Behring, Miami, USA) can replace bleeding time tests for GT 48. The PFA-100 measures the closure time when blood is passed through collagen-based filters under flow; blood from GT patients fails to plug the filter. Platelet αIIbβ3 deficiency should always be confirmed in new patients, and this can be done with monoclonal antibodies and flow cytometry 2435. The detection of trace amounts of intracellular αIIb or β3 in a patient's platelets by western blotting 2728, can give clues to the identity of the affected gene, while the presence of nonprocessed precursor pro-αIIb will suggest a block in integrin biosynthesis 2526.

In the detailed analysis of experience gained at Hôpital Lariboisière in Paris, 50 out of 64 patients (78%) had type I disease, nine patients (14%) had type II disease with residual αIIbβ3, and five patients (8%) were classified as variant GT 4, a subgroup diagnosed by the inability of αIIbβ3 to express activation-dependent epitopes (recognized by the absence of binding of monoclonal antibodies, such as PAC-1 or FITC-fibrinogen, in flow cytometry) 293031323334.

Procedures permitting the rapid screening of given mutations can only be used in ethnic groups with high consanguinity. This is the case for the Iraqi-Jewish and Arab groups in Israel and for a Manouche gypsy population in France 495051. Here, rapid screening procedures include allele-specific restriction enzyme analysis (ASRA) 51. Otherwise, the patient needs to be referred to a specialist laboratory, for direct sequencing of the relevant genes.

Initial studies attempted to diagnose carriers of GT by measuring the number of αIIbβ3 receptors on platelets. However, an occasional overlap between obligate heterozygotes and ostensibly normal donors, whose platelets had a low αIIbβ3 expression, made interpretation of the results difficult. Prenatal diagnosis performed by measuring αIIbβ3 on platelets isolated from cord blood has been attempted, but is accompanied by a high risk of bleeding and of spontaneous abortion 52. It is therefore best to first identify the genetic lesion within the family in question. Carrier diagnosis can then be determined with relative ease particularly if sites are created (or lost) for restriction enzymes. Restriction digest analysis of polymerase chain reaction (PCR)-amplified fragments from DNA isolated from blood or urine was first used to screen subjects for the Iraqi-Jewish and Arab mutations 49. The presence of these mutations could also be confirmed in prenatal diagnosis using DNA extracted from chorionic villi (discussed by French 20). Carriers have also been detected within the French gypsy populations using ASRA methodology 51. Finally, prenatal diagnosis in GT has been achieved using the polymorphic markers BRCA1 and THRA1 on chromosome 17 53.

Genetic counseling can be given with the following reservations: (i) when screening is followed for a single mutation within an ethnic population, the presence of a second Glanzmann's defect may go undetected and (ii) that individuals with the same mutation may differ widely in the frequency and severity of bleeding.

Despite variations in the severity and frequency of bleeding episodes, most GT patients receive blood transfusions 3940. Local bleeding can be treated by local measures, such as fibrin sealants. Epistaxis and gingival bleeding are successfully controlled in most patients by nasal packing or the application of gel foam soaked in topical thrombin. Regular dental care is essential to prevent gingival bleeding. For teeth extractions, or for hemorrhage accompanying the loss of deciduous teeth, hemostasis can be significantly improved by the application of individually prepared plastic splints that provide physical support for hemostasis.

Severe menorrhagia is a frequent clinical problem and is usually associated with an excessively proliferative endometrium caused by estrogen dominance. It can be effectively treated with high doses of progesterone. Maintenance treatment with birth control pills should follow. Severe gastrointestinal bleeding is a problem in isolated cases. Iron deficiency anemia, which can develop insidiously with gingival oozing or minor menorrhagia, is a frequent problem.

Bleeding following trauma or surgical procedures can be severe and transfusions are often given by precaution or should be available on standby. Pregnancy and in particular, delivery, represent a particularly severe hemorrhagic risk. Platelet transfusions are required not only prior to delivery, but sometimes should be continued for at least a week 39. Successful delivery by Cesarean section, with platelet transfusions, has been reported. Note that if platelet transfusions are required, the most HLA-compatible platelet concentrates must be chosen in order to avoid platelet anti-HLA alloimmunization.

The fact that most patients receive red cell and/or platelet transfusions on more than one occasion, makes the production of isoantibodies reactive with αIIbβ3 likely 439. Such antibodies are antigen-driven and are produced against different epitopes on the integrin 54. They may block platelet aggregation, and lead to the rapid removal of transfused platelets by immune mechanisms. Whether a particular category of patient is more likely to form isoantibodies is as yet unknown. When present at high titer, the antibodies cause patients to become refractory to further transfusions. Antibodies have been successfully removed prior to surgery by immunoadsorption on Protein A Sepharose, although this is a complex procedure whose use is restricted to specialized centers 55. Recently, recombinant factor VIIa (NovoSeven^®; Novo Nordisk A/S, Malov, Denmark) has been successfully used in GT and represents an alternative approach for early cessation of bleeding, especially for patients with antibodies and/or a history of refractoriness to transfusion 56. It is often used in association with anti-fibrinolytic agents. Thromboembolic events are a rare but potential hazard. Recombinant factor VIIa appears to enhance deposition of the αIIbβ3-deficient platelets on the subendothelial matrix through their interaction with fibrin formed as a result of increased thrombin generation 19. The stability of the newly-formed clot is increased and its permeability decreased 57. Nevertheless, the efficacity of recombinant factor VIIa in children with GT has been questioned 58. Rarely, in some patients, the condition has been thought to be sufficiently serious for allogeneic bone-marrow transplantation to be performed 3959. In the first report, donors were siblings and the transplantation was successful 59.

Awaiting discovery, perhaps, are abnormalities of the cytoplasmic proteins now thought to regulate the activation state of αIIbβ3 for adhesive proteins 60. The observation that a series of platelet receptor gene haplotypes can markedly affect bleeding severity and bleeding times in patients with von Willebrand disease type I 61, points to the bleeding tendency in congenital disorders being governed by the score of an ensemble of risk factors. As well as gene polymorphisms affecting platelet receptors, those influencing coagulation factors and the functioning of vascular cells may also be involved. The application of proteomic and gene microarray technologies to platelet disorders such as GT may help determine whether bleeding risk in individual patients can be predicted. Although the work is at an early stage, animal models of gene therapy show that GT may be an appropriate disease for such an approach and research is progressing in this direction 62.

Finally, as national and international networks are set up, and sequencing centers become involved in genotyping, especially among ethnic groups and third world countries, healthcare of GT will improve on a worldwide basis.