The limitation of genetic testing in diagnosing patients suspected for congenital platelet defects

Idiopathic Mild Platelet Dysfunction: Baseline Characteristics and Clinical Courses

INTRODUCTION

The causes of nonsyndromic platelet storage pool disease are still unclear, and whether they are of genetic or acquired origin remains to be defined. The study aimed to describe the characteristics and natural history of this disorder.

METHODS

This mostly retrospective cohort enrolled adults presenting with bleeding from platelet dysfunction. Platelet glycoprotein defects, von Willebrand disease, syndromic inherited platelet disorders and known acquired platelet dysfunctions were excluded. Available patients were retested by lumiaggregometry (Chrono-Log) over 1 year after the initial diagnosis.

RESULTS

There was a total of 56 patients; 91% female, with a median diagnostic age of 28 years (interquartile range [IQR]: 24.5-38.5). The subnormal responses to ADP, epinephrine, collagen, and arachidonate were found in 91%, 82%, 55%, and 34%, respectively. Nineteen patients had von Willebrand factor levels measured. Twenty-three subjects underwent repeat tests. Twenty-one of them were female (91%), with a median age and follow-up time of 37 years (IQR: 28-55) and 6 years (IQR: 3-12), respectively. Median ISTH-BAT bleeding scores at diagnosis and follow-up were 5 (IQR: 3-8) and 1 (IQR: 0-2), respectively. The common abnormalities were reduced responses to ADP combined with other agonists (83%). Twelve (52%) and five (22%) showed complete and partial platelet function recovery, respectively. None of the partial and non-recovery groups had a bleeding score over 4 at follow-up.

CONCLUSIONS

Idiopathic mild platelet dysfunction was female-predominant and showed spontaneous symptom resolution after a long follow-up. Platelet function recovery was observed in most cases. Exogenous factors triggering this condition remain to be identified.

Functional characterization of a nanobody-based glycoprotein VI-specific platelet agonist

Background

Glycoprotein (GP)VI is a platelet-specific collagen receptor required for platelet activation during hemostasis. Platelet reactivity toward collagen is routinely assessed during diagnostic workup of platelet disorders. GPVI can be activated by inducing receptor clustering with suspensions of fibrillar collagen or synthetic cross-linked collagen-related peptide (CRP-XL). However, these suspensions are poorly standardized or difficult to produce. Nanobodies are small recombinant camelid-derived heavy-chain antibody variable regions. They are highly stable, specific, and ideal candidates for developing a stable GPVI agonist for diagnostic assays.

Objectives

Develop a stable nanobody-based GPVI agonist.

Methods

Nanobody D2 (NbD2) was produced as dimers and purified. Tetramers were generated via C-terminal fusion of dimers with click chemistry. Nanobody constructs were functionally characterized with light transmission aggregometry (LTA) in platelet-rich plasma and whole blood flow cytometry. Diagnostic performance was assessed in patients with inherited platelet function disorders with LTA and flow cytometry.

Results

NbD2 was specific for human platelet GPVI. Dimers did not result in platelet activation in LTA or flow cytometry settings and fully inhibited CRP-XL-induced P-selectin expression and fibrinogen binding in whole blood and attenuated collagen-induced platelet aggregation in platelet-rich plasma. However, NbD2 tetramers caused full platelet aggregation, as well as P-selectin expression and fibrinogen binding. NbD2 tetramers were able to discriminate between inherited platelet function disorder patients and healthy controls based on fibrinogen binding, similar to CRP-XL.

Conclusion

Nanobody tetramers to GPVI induce platelet activation and can be used to assess the GPVI pathway in diagnostic assays.

A bispecific antibody approach for the potential prophylactic treatment of inherited bleeding disorders

Inherited bleeding disorders such as Glanzmann thrombasthenia (GT) lack prophylactic treatment options. As a result, serious bleeding episodes are treated acutely with blood product transfusions or frequent, repeated intravenous administration of recombinant activated coagulation factor VII (rFVIIa). Here we describe HMB-001, a bispecific antibody designed to bind and accumulate endogenous FVIIa and deliver it to sites of vascular injury by targeting it to the TREM (triggering receptor expressed on myeloid cells)-like transcript-1 (TLT-1) receptor that is selectively expressed on activated platelets. In healthy nonhuman primates, HMB-001 prolonged the half-life of endogenous FVIIa, resulting in its accumulation. Mouse bleeding studies confirmed antibody-mediated potentiation of FVIIa hemostatic activity by TLT-1 targeting. In ex vivo models of GT, HMB-001 localized FVIIa on activated platelets and potentiated fibrin-dependent platelet aggregation. Taken together, these results indicate that HMB-001 has the potential to offer subcutaneous prophylactic treatment to prevent bleeds in people with GT and other inherited bleeding disorders, with a low-frequency dosing regimen.

Platelet genetic testing by next-generation sequencing: A practical update

Inherited platelet disorders (IPDs) are a heterogeneous group of disorders characterized by normal or reduced platelet counts, bleeding diatheses of varying severities, and the presence (syndromic) or absence (non-syndromic) of involvement of other organs. Due to the lack of highly specific platelet function tests and overlapping clinical and laboratory features, diagnosing the underlying cause of IPDs remains challenging. In recent years, genetic testing via next-generation sequencing (NGS) technologies to rapidly analyze multiple genes has gradually emerged as an important part of the laboratory investigation of patients with IPDs. A systemic clinical and laboratory testing approach and thorough phenotype and genotype correlation studies of both patients and their family members are crucial for accurate diagnoses of IPDs.

SYMPHONY consortium: Orchestrating personalized treatment for patients with bleeding disorders

BACKGROUND

Treatment choices for individual patients with an inborn bleeding disorder are increasingly challenging due to increasing options and rising costs for society. We have initiated an integrated interdisciplinary national research programme.

OBJECTIVES

The SYMPHONY consortium strives to orchestrate personalized treatment in patients with an inborn bleeding disorder, by unravelling the mechanisms behind inter-individual variations of bleeding phenotype.

PATIENTS

The SYMPHONY consortium will investigate patients with an inborn bleeding disorder, both diagnosed and not yet diagnosed.

RESULTS

Research questions are categorized under the themes: 1) Diagnosis; 2) Treatment; and 3) Fundamental research and consist of workpackages addressing specific domains. Importantly, collaborations between patients and talented researchers from different areas of expertise promise to augment the impact of the SYMPHONY consortium, leading to unique interactions and intellectual property.

CONCLUSIONS

SYMPHONY will perform research on all aspects of care, treatment individualization in patients with inborn bleeding disorders as well as diagnostic innovations and results of molecular genetics and cellular model technology with regard to the hemostatic process. We believe that these research investments will lead to health care innovations with long-term clinical and societal impact. This consortium has been made possible by a governmental, competitive grant from the Netherlands Organization for Scientific Research (NWO) within the framework of the NWA-ORC Call grant agreement NWA.1160.18.038.

Experiences in Routine Genetic Analysis of Hereditary Hemorrhagic, Thrombotic, and Platelet Disorders

Hemostasis is a complex and tightly regulated system that attempts to maintain a homeostatic balance to permit normal blood flow, without bleeding or thrombosis. Hemostasis reflects the subtle balance between procoagulant and anticoagulant factors in the pathways of primary hemostasis, secondary hemostasis, and fibrinolysis. The major components in this interplay include the vascular endothelium, platelets, coagulation factors, and fibrinolytic factors. After vessel wall injury, the subendothelium is exposed to the blood stream, followed by rapid activation of platelets via collagen binding and von Willebrand factor-mediated platelet adhesion to the damaged vessel wall through platelet glycoprotein receptor Ib/IX/V. Activated platelets change their shape, release bioactive molecules from their granules, and expose negatively charged phospholipids on their surface. For a proper function of this process, an adequate number of functional platelets are required. Subsequently, a rapid generation of sufficient amounts of thrombin begins; followed by activation of the coagulation system and its coagulation factors (secondary hemostasis), generating fibrin that consolidates the platelet plug. To maintain equilibrium between coagulation and anticoagulation, the naturally occurring anticoagulants such as protein C, protein S, and antithrombin keep this process in balance. Deficiencies (inherited or acquired) at any level of this fine-tuned system result in pathologic bleedings or increased hypercoagulability states leading to thrombosis. This review will focus on genetic diagnosis of inherited bleeding, thrombotic, and platelet disorders, discussing strengths and limitations of existing diagnostic settings and genetic tools and highlight some important considerations necessary for clinical application.

Diagnostic approach to the patient with a suspected inherited platelet disorder: Who and how to test

Bleeding and thrombocytopenia are common referrals to the pediatric and adult hematology practice. The differential diagnosis encompasses a wide spectrum of entities that vary in acuity, severity, and etiology. Most will be acquired (especially in adult patients), but many can be inherited, and some may have manifestations affecting other organ systems. The first step: defining whether the symptoms and/or laboratory findings are clinically significant and warrant additional work-up, can be equally as challenging as reaching the diagnosis itself. How much bleeding is too much to be considered normal? How low of a platelet count is too low? Once the decision has been made to pursue additional studies, considering the increasing number of laboratory tests available, the diagnostic process can be complex. In this article, we outline a general approach for the evaluation of patients in whom an inherited platelet disorder is being considered. We present two clinical vignettes as introduction to the diagnostic approach to inherited platelet disorders. We describe the rationale for the different types of tests that are clinically available, their limitations, and finally the challenges that are frequently encountered in the interpretation of results. We also intend to provide some guidance on the expected phenotype in terms of severity of bleeding and/or thrombocytopenia according to the etiology of the inherited disorder. Our goal is to provide the practicing hematologist with a practical framework that is clinically applicable in their daily practice.

Major differences in clinical presentation, diagnosis and management of men and women with autosomal inherited bleeding disorders

BACKGROUND

In recent years, more awareness is raised about sex-specific dilemmas in inherited bleeding disorders. However, no large studies have been performed to assess differences in diagnosis, bleeding phenotype and management of men and women with bleeding disorders. Therefore, we investigated sex differences in a large cohort of well-defined patients with autosomal inherited bleeding disorders (von Willebrand disease (VWD), rare bleeding disorders (RBDs) and congenital platelet defects (CPDs)).

METHODS

We included patients from three nationwide cross-sectional studies on VWD, RBDs and CPDs in the Netherlands, respectively the WiN, RBiN and TiN study. In all studies a bleeding score (BS) was obtained, and patients filled in an extensive questionnaire on the management and burden of their disorder.

FINDINGS

We included 1092 patients (834 VWD; 196 RBD; 62 CPD), of whom 665 (60.9%) were women. Women were more often referred because of a bleeding diathesis than men (47.9% vs 36.6%, p = 0.002). Age of first bleeding was similar between men and women, respectively 8.9 ± 13.6 (mean ±sd) years and 10.6 ± 11.3 years (p = 0.075). However, the diagnostic delay, which was defined as time from first bleeding to diagnosis, was longer in women (11.6 ± 16.4 years) than men (7.7 ± 16.6 years, p = 0.002). Similar results were found when patients referred for bleeding were analyzed separately. Of women aging 12 years or older, 469 (77.1%) had received treatment because of sex-specific bleeding.

INTERPRETATION

Women with autosomal inherited bleeding disorders are more often referred for bleeding, have a longer diagnostic delay, and often require treatment because of sex-specific bleeding.

FUNDING

The WiN study was supported (in part) by research funding from the Dutch Hemophilia Foundation (Stichting Haemophilia), Shire (Takeda), and CSL Behring (unrestricted grant).