Modulation of clinical phenotype of Glanzmann's thrombasthenia by thrombogenic mutations

Perspective - The case for zero bleeds and drug bioequivalence in the treatment of congenital hemophilia A in 2021

Not all patients with severe hemophilia A (HA) respond optimally to a given dose of a given product. Within-individual variance in cross-over studies makes each patient unique in the response to each standard half-life (SHL) factor VIII (FVIII) product in pharmacokinetic (PK) terms. This hampers the prediction of efficacy when a SHL FVIII product is employed. PK data showing that half-lives of SHL rFVIII are unsatisfactory to achieve zero bleeding in individual HA patients provide the rationale for switching from SHL to extended half-life (EHL) products. However, not all subjects receiving prophylaxis with EHL products achieve zero bleeding, the most cogent objective of personalized prophylaxis. Known determinants of FVIII half-life (age, von Willebrand factor [VWF] levels, blood group) cumulatively account for one third of the total inter-individual variation in FVIII clearance in subjects with severe HA. Investigations into precision, and accuracy of laboratory measurement to be employed; newer pathways for the clearance of both free-FVIII and VWF-bound FVIII, and adequately powered studies on omics and phenotypic heterogeneity, are likely to provide additional information on the remaining two thirds of inter-individual variation in FVIII clearance in HA. Variability in the clinical response has also been documented in patients when FVIII activity is mimicked by fixed subcutaneous doses of the bispecific antibody emicizumab. National registries that collect PK data of available FVIII products and ad hoc information on the individual response to emicizumab should be encouraged, to establish newer standards of care and ease personalized clinical decisions to achieve zero bleeding.

Paradigm shift for the treatment of hereditary haemophilia: Towards precision medicine

Patients with haemophilia A (HA) or B (HB) experience spontaneous limb- or life-threatening bleedings which are prevented by regular prophylactic intravenous infusions of the deficient coagulation factor (FVIII or FIX). Prophylaxis with subcutaneous long-acting non-factor products that improve in vivo thrombin generation is now under intensive investigation (concizumab, fitusiran) or successfully employed (emicizumab) in haemophilia patients. Both haemophilia patients with/without inhibitors take advantage of non-factor products employed alone. In those who also need bypassing agents (or FVIII concentrates) for breakthrough bleeds, thromboembolic events and/or thrombotic microangiopathy may occur. By enhancing thrombin generation, prothrombotic mutations co-segregating with FVIII/FIX gene mutations may trigger thrombotic episodes in HA patients carrying acquired thrombogenic factors (e.g. venous catheters). A thorough knowledge of individual needs increasingly contributed to improve comprehensive care and personalize treatments in haemophilia. Integrating genomics, lifestyle and environmental data is expected to be key to: 1) identify which haemophilia patients are less likely to benefit from a given intervention; 2) define optimal dosing and scheduling of bypassing agents (or FVIII) to employ in combination with non-factor products; 3) establish tests to monitor in vivo thrombin generation; 4) improve communication and deliver results to individuals. As individual outcomes will be improved and the risk of adverse events minimized, non-factor products will come into wider use within the haemophilia community, and patients will hopefully have no more risks of breakthrough bleeds. The risks of a normal life for a "former haemophilia patient" is likely to change the treatment landscape and the structure of haemophilia Centers.

Clinical and molecular insights into Glanzmann's thrombasthenia in China

Glanzmann's thrombasthenia (GT) is a rare bleeding disorder characterized by spontaneous mucocutaneous bleeding. The disorder is caused by quantitative or qualitative defects in integrin αIIbβ3 (encoded by ITGA2B and ITGB3) on the platelet and is more common in consanguineous populations. However, the prevalence rate and clinical characteristics of GT in non-consanguineous populations have been unclear. We analyzed 97 patients from 93 families with GT in the Han population in China. This analysis showed lower consanguinity (18.3%) in Han patients than other ethnic populations in GT-prone countries. Compared with other ethnic populations, there was no significant difference in the distribution of GT types. Han females suffered more severe bleeding and had a poorer prognosis. We identified a total of 43 different ITGA2B and ITGB3 variants, including 25 previously unidentified, in 45 patients. These variants included 14 missense, 4 nonsense, 4 frameshift, and 3 splicing site variants. Patients with the same genotype generally manifested the same GT type but presented with different bleeding severities. This suggests that GT clinical phenotype does not solely depend on genotype. Our study provides an initial, yet important, clinical and molecular characterization of GT heterogeneity in China.

Identification of Clinicopathological Spectrum, Platelet Glycoprotein IIb/IIIa complex and Platelet Antibodies in Egyptian Children with Glanzmann's Thrombasthenia

Glanzmann's thrombasthenia (GT) is a rare genetic bleeding disorder. The aim of our study was to evaluate the clinicopathological spectrum of this syndrome and to study the platelet glycoprotein IIb/IIIa complex and platelet antibodies by flow cytometry in a cohort of children with GT in a tertiary care center in Upper Egypt. Forty children with GT were assessed for the expression of GPIIb-IIIa on the platelet surface and platelet antibodies by using flow cytometry, to determine the most common GT subtypes among Egyptian children. By analysis of platelet GP IIb-IIIa by flow cytometry the classification of patients with GT in our study was type I GT (47.5%), type II GT (32.5%) and type III GT (20%). In this study, we have delineated that type I is the most common type of GT in Upper Egypt. Our data suggested that there is a good correlation between quantitative changes in the surface expression of platelet membrane glycoproteins detected by flow cytometry and the clinical severity of bleeding. Therefore, classifying of severity of bleeding in patients with GT could possibly aid the pediatricians and hematologists in the implementation of ideal prophylactic measures.

Modifiers of clinical phenotype in severe congenital hemophilia

Patients with inherited hemophilia A and B usually exhibit a bleeding tendency of a severity proportional to the degree of plasmatic deficiency of the coagulant activity of factor VIII (FVIII:C) and factor IX (FIX:C). Although patients with severe hemophilia (i.e., with FVIII:C and FIX:C levels <1IU/dL) are generally those with the most severe bleeding phenotype, it is common experience that a variable proportion of them experiences a milder bleeding tendency. In this review, we summarize the current knowledge on the possible mechanisms at the basis of the phenotypic heterogeneity of severe hemophilia, focusing in particular on the role of FVIII/FIX gene mutations and thrombophilic polymorphisms. Finally, the possible therapeutic implications of such modifiers will be analyzed.

Should studies on Glanzmann thrombasthenia not be telling us more about cardiovascular disease and other major illnesses?

Glanzmann thrombasthenia (GT) is a rare inherited bleeding disorder caused by loss of αIIbβ3 integrin function in platelets. Most genetic variants of β3 also affect the widely expressed αvβ3 integrin. With brief mention of mouse models, I now look at the consequences of disease-causing ITGA2B and ITGB3 mutations on the non-hemostatic functions of platelets and other cells. Reports of arterial thrombosis in GT patients are rare, but other aspects of cardiovascular disease do occur including deep vein thrombosis and congenital heart defects. Thrombophilic and other risk factors for thrombosis and lessons from heterozygotes and variant forms of GT are discussed. Assessed for GT patients are reports of leukemia and cancer, loss of fertility, bone pathology, inflammation and wound repair, infections, kidney disease, autism and respiratory disease. This survey shows an urgent need for a concerted international effort to better determine how loss of αIIbβ3 and αvβ3 influences health and disease.

Classification of Iranian patients with Glanzmann's Thrombasthenia using a flow cytometric method

Glanzmann's Thrombasthenia (GT) is a rare inherited autosomal recessive platelet disorder caused by a deficiency or dysfunction of the GPIIb-IIIa receptor on platelets, which is characterized by a lack of platelet aggregation in response to multiple physiologic agonists and a life-long bleeding disorder. Flow cytometry is a rapid and highly sensitive method that can detect reduced levels of receptors, as well as absolute deficiency. The aim of this study was to classify Iranian GT patients by a flow cytometric method, and to correlate these findings with the severity of clinical bleeding. The expression of GPIIb-IIIa on the platelet surface was assessed in 123 GT patients using quantitative flow cytometry to determine the most common subtype among these patients. We used a panel of antibodies to detect the expression of glycoproteins GPIb, GPIIb, GPIIIa, as well as Integrin αv. Patients were also interviewed with regard to the severity and frequency of bleeding, according to history and gender, in order to evaluate the nature of their bleeding phenotype, and classify them as mild, moderate or severe bleeders, in accordance with the Glanzmann's Thrombasthenia Italian Team (GLATIT) protocol. In the detailed analysis of the results of our investigation, 95 out of 123 (77.5%) were classified as type I; 20 (16%) as type II with residual GPIIb-IIIa, and eight (6.5%) as GT variants. The variant type was diagnosed by the inability of GPIIb-IIIa to bind fibrinogen, as evidenced by the absence of platelet aggregation in response to physiologic agonists. There was no significant correlation between bleeding severity and different subtypes of GT. This study demonstrates that GT type I is the most common subtype among Iranian patients. There was no correlation between severity of symptoms and cytometric phenotype of the disease. The identification of families at risk may significantly decrease the incidence of the severe form of the disorder if genetic counseling is provided.