Hemostatic phenotypes and genetic disorders

Functional assessment of genetic variants in thrombomodulin detected in patients with bleeding and thrombosis

ABSTRACT

Thrombomodulin (TM) expressed on endothelial cells regulates coagulation. Specific nonsense variants in the TM gene, THBD, result in high soluble TM levels causing rare bleeding disorders. In contrast, although THBD variants have been associated with venous thromboembolism, this association remains controversial. A multigene panel was used to diagnose 601 patients with inherited bleeding or thrombotic disorders. This resulted in the identification of 8 THBD variants for 6 patients with a thrombotic (C175S, A282P, L433P, P501L, G502R, and P508L) and 2 patients with a bleeding (P260A and T478I) phenotype. These were all classified as variants of uncertain significance, and we here aimed to assess their functional role in coagulation. For this purpose, soluble and cell membrane-bound recombinant TM were produced in Expi293F cells. L433P TM showed a marked decrease in the inhibition of thrombin generation and complete inhibition of protein C and thrombin activatable fibrinolysis inhibitor (TAFI) activation. Soluble C175S TM showed decreased inhibition of thrombin generation and protein C activation, whereas no effect was observed for cell membrane-bound recombinant TM. For the other TM variants, no effect on thrombin generation, protein C, or TAFI activation could be observed. Surface plasmon resonance analysis showed no thrombin-TM binding in the presence of L433P because this residue is located at their interaction site. In conclusion, our study shows the functional effects of L433P TM and potentially C175S TM, which are compatible with an increased thrombosis risk. THBD variants are rare but can be relevant to both bleeding and thrombosis. Functional assays for these variants are critical to understand their roles.

Gaining Insights into Inherited Bleeding Disorders of Complex Etiology in Pediatric Patients: Whole-Exome Sequencing as First-Line Investigation Tool

INTRODUCTION

 Investigation of the molecular basis of inherited bleeding disorders (IBD) is mostly performed with gene panel sequencing. However, the continuous discovery of new related genes underlies the limitation of this approach. This study aimed to identify genetic variants responsible for IBD in pediatric patients using whole-exome sequencing (WES), and to provide a detailed description and reclassification of candidate variants.

MATERIAL AND METHODS

 WES was performed for 18 pediatric patients, and variants were filtered using a first-line list of 290 genes. Variant prioritization was discussed in a multidisciplinary team based on genotype-phenotype correlation, and segregation studies were performed with available family members.

RESULTS

 The study identified 22 candidate variants in 17 out of 18 patients (94%). Eleven patients had complete genotype-phenotype correlation, resulting in a diagnostic yield of 61%, 5 (28%) were classified as partially solved, and 2 (11%) remained unsolved. Variants were identified in platelet (ACTN1, ANKRD26, CYCS, GATA1, GFI1B, ITGA2, NBEAL2, RUNX1, SRC, TUBB1), bleeding (APOLD1), and coagulation (F7, F8, F11, VWF) genes. Notably, 9 out of 22 (41%) variants were previously unreported. Variant pathogenicity was assessed according to the American College of Medical Genetics and Genomics guidelines and reclassification of three variants based on family segregation evidence, resulting in the identification of 10 pathogenic or likely pathogenic variants, 6 variants of uncertain significance, and 6 benign or likely benign variants.

CONCLUSION

 This study demonstrated the high potential of WES in identifying rare molecular defects causing IBD in pediatric patients, improving their management, prognosis, and treatment, particularly for patients at risk of malignancy and/or bleeding due to invasive procedures.

Evaluating the clinical validity of genes related to hemostasis and thrombosis using the Clinical Genome Resource gene curation framework

BACKGROUND

Inherited bleeding, thrombotic, and platelet disorders (BTPDs) are a heterogeneous set of diseases, many of which are very rare globally. Over the past 5 decades, the genetic basis of some of these disorders has been identified, and recently, high-throughput sequencing has become the primary means of identifying disease-causing genetic variants.

OBJECTIVES

Knowledge of the clinical validity of a gene-disease relationship is essential to provide an accurate diagnosis based on results of diagnostic gene panel tests and inform the construction of such panels. The Scientific and Standardization Committee for Genetics in Thrombosis and Hemostasis undertook a curation process for selecting 96 TIER1 genes for BTPDs. The purpose of the process was to evaluate the evidence supporting each gene-disease relationship and provide an expert-reviewed classification for the clinical validity of genes associated with BTPDs.

METHODS

The Clinical Genome Resource (ClinGen) Hemostasis/Thrombosis Gene Curation Expert Panel assessed the strength of evidence for TIER1 genes using the semiquantitative ClinGen gene-disease clinical validity framework. ClinGen Lumping and Splitting guidelines were used to determine the appropriate disease entity or entities for each gene, and 101 gene-disease relationships were identified for curation.

RESULTS

The final outcome included 68 Definitive (67%), 26 Moderate (26%), and 7 Limited (7%) classifications. The summary of each curation is available on the ClinGen website.

CONCLUSION

Expert-reviewed assignment of gene-disease relationships by the ClinGen Hemostasis/Thrombosis Gene Curation Expert Panel facilitates accurate molecular diagnoses of BTPDs by clinicians and diagnostic laboratories. These curation efforts can allow genetic testing to focus on genes with a validated role in disease.

[Molecular genetic factors of hemostasis system in assessing the risk of arterial and venous thrombosis in microvascular surgery]

The review is devoted to molecular genetic factors of hemostasis system in assessing the risk of arterial and venous thrombosis in microvascular surgery. In modern reconstructive surgery, one can trace a tendency towards preoperative management taking into account molecular-genetic risk factors of hemostatic disorders for prevention of intravascular thrombosis. The importance of multidisciplinary concept in prevention is undeniable. However, the risk of vascular complications after microsurgical autotransplantation depending on genetic predisposition has not been considered previously.

A frameshift deletion in F8 associated with hemophilia A in Labrador Retriever dogs

Hemophilia A is the most common inherited coagulation factor disorder in dogs. It manifests as excessive bleeding resulting from pathogenic variants in the X-chromosomal F8 gene encoding coagulation factor VIII (FVIII) protein. In this study, we performed careful clinical phenotyping to confirm hemophilia A in two distinct Labrador Retriever (LR) pedigrees. Whole-genome sequencing on an affected dog from litter 1 identified a case-specific frameshift deletion variant in F8 predicted to cause a premature stop codon (c.2923_2924del, p.(E975Kfs*8)). This variant was hemizygous in all the affected males from litter 1 (n = 3), while all the unaffected LRs in the pedigree were heterozygous or wild-type (n = 22). Additionally, screened samples from 199 LRs were all found to be wild-type. As a result of this study, a gene test can now be developed to screen dogs before breeding to prevent further cases. However, it is important to note that the affected LR with decreased FVIII activity from litter 2 was wild-type for the identified deletion variant, and no segregating F8 variants were detected when this dog's DNA sample was whole-genome sequenced. Thus, the cause of decreased FVIII activity in this dog remains to be unraveled in future studies.

Validation of immunofluorescence analysis of blood smears in patients with inherited platelet disorders

BACKGROUND

Inherited platelet disorders (IPDs) are rare diseases characterized by reduced blood platelet counts and/or impaired platelet function. Recognizing IPDs is advisable but often challenging. The diagnostic tools include clinical evaluation, platelet function tests, and molecular analyses. Demonstration of a pathogenic genetic variant confirms IPDs. We established a method to assess the platelet phenotype on blood smears using immunofluorescence microscopy as a diagnostic tool for IPDs.

OBJECTIVES

The aim of the present study was to validate immunofluorescence microscopy as a screening tool for IPDs in comparison with genetic screening.

METHODS

We performed a blinded comparison between the diagnosis made using immunofluorescence microscopy on blood smears and genetic findings in a cohort of 43 families affected with 20 different genetically confirmed IPDs. In total, 76% of the cases had inherited thrombocytopenia.

RESULTS

Immunofluorescence correctly predicted the underlying IPD in the vast majority of patients with 1 of 9 IPDs for which the typical morphologic pattern is known. Thirty of the 43 enrolled families (70%) were affected by 1 of these 9 IPDs. For the other 11 forms of IPD, we describe alterations of platelet structure in 9 disorders and normal findings in 2 disorders.

CONCLUSION

Immunofluorescence microscopy on blood smears is an effective screening tool for 9 forms of IPD, which include the most frequent forms of inherited thrombocytopenia. Using this approach, typical changes in the phenotype may also be identified for other rare IPDs.

Insights into the Molecular Genetic of Hemophilia A and Hemophilia B: The Relevance of Genetic Testing in Routine Clinical Practice

Hemophilia A and hemophilia B are rare congenital, recessive X-linked disorders caused by lack or deficiency of clotting factor VIII (FVIII) or IX (FIX), respectively. The severity of the disease depends on the reduction of coagulation FVIII or FIX activity levels, which is determined by the type of the pathogenic variants in the genes encoding the two factors (F8 and F9, respectively). Molecular genetic analysis is widely applied in inherited bleeding disorders. The outcome of genetic analysis allows genetic counseling of affected families and helps find a link between the genotype and the phenotype. Genetic analysis in hemophilia has tremendously improved in the last decades. Many new techniques and modifications as well as analysis softwares became available, which made the genetic analysis and interpretation of the data faster and more accurate. Advances in genetic variant detection strategies facilitate identification of the causal variants in up to 97% of patients. In this review, we discuss the milestones in genetic analysis of hemophilia and highlight the importance of identification of the causative genetic variants for genetic counseling and particularly for the interpretation of the clinical presentation of hemophilia patients.