Diagnosis of rare bleeding disorders.

Rare bleeding disorders result in significant morbidity but are globally underdiagnosed. Advances in genomic testing and specialist laboratory assays have greatly increased the diagnostic armamentarium. This has resulted in the discovery of new genetic causes for rare diseases and a better understanding of the underlying molecular pathology.

Whole-exome analysis of adolescents with low VWF and heavy menstrual bleeding identifies novel genetic associations.

Adolescents with low von Willebrand factor (VWF) levels and heavy menstrual bleeding (HMB) experience significant morbidity. There is a need to better characterize these patients genetically and improve our understanding of the pathophysiology of bleeding. We performed whole-exome sequencing on 86 postmenarchal patients diagnosed with low VWF levels (30-50 IU/dL) and HMB and compared them with 660 in-house controls. We compared the number of rare stop-gain/stop-loss and rare ClinVar "pathogenic" variants between cases and controls, as well as performed gene burden and gene-set burden analyses. We found an enrichment in cases of rare stop-gain/stop-loss variants in genes involved in bleeding disorders and an enrichment of rare ClinVar "pathogenic" variants in genes involved in anemias. The 2 most significant genes in the gene burden analysis, CFB and DNASE2, are associated with atypical hemolytic uremia and severe anemia, respectively. VWF also surpassed exome-wide significance in the gene burden analysis (P = 7.31 × 10-6). Gene-set burden analysis revealed an enrichment of rare nonsynonymous variants in cases in several hematologically relevant pathways. Further, common variants in FERMT2, a gene involved in the regulation of hemostasis and angiogenesis, surpassed genome-wide significance. We demonstrate that adolescents with HMB and low VWF have an excess of rare nonsynonymous and pathogenic variants in genes involved in bleeding disorders and anemia. Variants of variable penetrance in these genes may contribute to the spectrum of phenotypes observed in patients with HMB and could partially explain the bleeding phenotype. By identifying patients with HMB who possess these variants, we may be able to improve risk stratification and patient outcomes.

Important roles of the human leukocyte antigen class I and II molecules and their associated genes in the autoimmune coagulation factor XIII deficiency via whole-exome sequencing analysis.

Autoimmune coagulation factor XIII deficiency is a bleeding disorder caused by the formation of autoantibodies against the coagulation factor XIII (FXIII); however, the molecular mechanism underlying this process is unknown. Therefore, in the present study, we aimed to elucidate this mechanism by performing whole-exome sequencing analysis of 20 cases of autoimmune FXIII deficiency. We identified approximately 21,788-23,916 variants in each case. In addition to their ability to activate T cells, present antigens, and immune tolerance, the candidate alleles were further narrowed down according to their allelic frequencies and the magnitude of damage caused by the substitution of amino acids. After selecting 44 candidate alleles, we investigated whether they were associated with the FXIII inhibitory titers and/or the anti-FXIII autoantibodies. We found that two polymorphisms whose variant allele frequencies were significantly lower in the patients tended to decrease FXIII inhibitory titers as the number of variant alleles increased. We also found that five polymorphisms whose variant allele frequencies were significantly higher in the patients tended to increase the levels of the anti-FXIII autoantibodies as the number of variant alleles increased. All of these polymorphisms were found in the human leukocyte antigen (HLA) class I and II molecules and their associated genes. In particular, the HLA class II molecule and its associated genes were found to be involved in the presentation of foreign antigens as well as the negative regulation of the proliferation of T-cells and the release of cytokines. Polymorphisms in the HLA class II molecules and the cytotoxic T lymphocyte antigen 4 have been reported to be associated with the development of autoantibodies in acquired hemophilia A. Therefore, we hypothesized that these polymorphisms may be associated with the development of autoantibodies in autoimmune FXIII deficiency.

Genome sequencing for detection of pathogenic deep intronic variation: A clinical case report illustrating opportunities and challenges.

Variants in JAM3 have been reported in four families manifesting a severe autosomal recessive disorder characterized by hemorrhagic destruction of the brain, subependymal calcification, and cataracts. We describe a 7-year-old male with a similar presentation found by research-based quad genome sequencing to have two novel splicing variants in trans in JAM3, including one deep intronic variant (NM_032801.4: c.256+1260G>C) not detectable by standard exome sequencing. Targeted sequencing of RNA isolated from transformed lymphoblastoid cell lines confirmed that each of the two variants has a deleterious effect on JAM3 mRNA splicing. The role for genome sequencing as a clinical diagnostic test extends to those patients with phenotypes strongly suggestive of a specific Mendelian disorder, especially when the causal genetic variant(s) are not found by a more targeted approach. Barriers to diagnosis via identification of pathogenic deep intronic variation include lack of laboratory consensus regarding in silico splicing prediction tools and limited access to clinically validated confirmatory RNA experiments.

Thrombospondin-1 promotes hemostasis through modulation of cAMP signaling in blood platelets.

Thrombospondin-1 (TSP-1) is released by platelets upon activation and can increase platelet activation, but its role in hemostasis in vivo is unclear. We show that TSP-1 is a critical mediator of hemostasis that promotes platelet activation by modulating inhibitory cyclic adenosine monophosphate (cAMP) signaling. Genetic deletion of TSP-1 did not affect platelet activation in vitro, but in vivo models of hemostasis and thrombosis showed that TSP-1-deficient mice had prolonged bleeding, defective thrombosis, and increased sensitivity to the prostacyclin mimetic iloprost. Adoptive transfer of wild-type (WT) but not TSP-1-/- platelets ameliorated the thrombotic phenotype, suggesting a key role for platelet-derived TSP-1. In functional assays, TSP-1-deficient platelets showed an increased sensitivity to cAMP signaling, inhibition of platelet aggregation, and arrest under flow by prostacyclin (PGI2). Plasma swap experiments showed that plasma TSP-1 did not correct PGI2 hypersensitivity in TSP-1-/- platelets. By contrast, incubation of TSP-1-/- platelets with releasates from WT platelets or purified TSP-1, but not releasates from TSP-1-/- platelets, reduced the inhibitory effects of PGI2. Activation of WT platelets resulted in diminished cAMP accumulation and downstream signaling, which was associated with increased activity of the cAMP hydrolyzing enzyme phosphodiesterase 3A (PDE3A). PDE3A activity and cAMP accumulation were unaffected in platelets from TSP-1-/- mice. Platelets deficient in CD36, a TSP-1 receptor, showed increased sensitivity to PGI2/cAMP signaling and diminished PDE3A activity, which was unaffected by platelet-derived or purified TSP-1. This scenario suggests that the release of TSP-1 regulates hemostasis in vivo through modulation of platelet cAMP signaling at sites of vascular injury.

γ-Glutamyl carboxylase mutations differentially affect the biological function of vitamin K-dependent proteins.

γ-Glutamyl carboxylase (GGCX) is an integral membrane protein that catalyzes posttranslational carboxylation of a number of vitamin K-dependent (VKD) proteins involved in a wide variety of physiologic processes, including blood coagulation, vascular calcification, and bone metabolism. Naturally occurring GGCX mutations are associated with multiple distinct clinical phenotypes. However, the genotype-phenotype correlation of GGCX remains elusive. Here, we systematically examined the effect of all naturally occurring GGCX mutations on the carboxylation of 3 structure-function distinct VKD proteins in a cellular environment. GGCX mutations were transiently introduced into GGCX-deficient human embryonic kidney 293 cells stably expressing chimeric coagulation factor, matrix Gla protein (MGP), or osteocalcin as VKD reporter proteins, and then the carboxylation efficiency of these reporter proteins was evaluated. Our results show that GGCX mutations differentially affect the carboxylation of these reporter proteins and the efficiency of using vitamin K as a cofactor. Carboxylation of these reporter proteins by a C-terminal truncation mutation (R704X) implies that GGCX's C terminus plays a critical role in the binding of osteocalcin but not in the binding of coagulation factors and MGP. This has been confirmed by probing the protein-protein interaction between GGCX and its protein substrates in live cells using bimolecular fluorescence complementation and chemical cross-linking assays. Additionally, using a minigene splicing assay, we demonstrated that several GGCX missense mutations affect GGCX's pre-messenger RNA splicing rather than altering the corresponding amino acid residues. Results from this study interpreted the correlation of GGCX's genotype and its clinical phenotypes and clarified why vitamin K administration rectified bleeding disorders but not nonbleeding disorders.

Diagnosis of rare bleeding disorders.

Rare bleeding disorders result in significant morbidity but are globally underdiagnosed. Advances in genomic testing and specialist laboratory assays have greatly increased the diagnostic armamentarium. This has resulted in the discovery of new genetic causes for rare diseases and a better understanding of the underlying molecular pathology.

A novel missense variant in the RASGRP2 gene in patients with moderate to severe bleeding disorder.

Inherited platelet function disorder-18 (IPD-18) is a relatively new non-syndromic autosomal recessive bleeding disorder. It is characterized by deficient or dysfunctional CalDAG-GEFI protein. The distinctive feature of the disease is impaired platelet aggregation in response to multiple physiologic agonists. We here report a family with a platelet-type bleeding disorder and a novel mutation in the RASGRP2 gene. The overall bleeding score for the affected individuals was 15 and 12. Based on the initial diagnosis of Glanzmann thrombasthenia, targeted sequencing of integrin subunit alpha 2b and integrin subunit beta 3 encoding genes ITGA2B and ITGB3 were carried out in both affected members of a family. Sequence alignment failed to identify the disease-causing variant(s) in both genes. Therefore, whole exome sequencing in one affected individual was performed. Data analysis detected a novel homozygous missense variant (c.956C>T; p.Pro319Leu) in the exon 9 of the RASGRP2 gene. Five additional individuals of a family including both parents, an affected individual and two asymptomatic individuals were Sanger sequenced for the variant (c.956C>T). The variant segregates in the family in an autosomal recessive manner. Several in silico tools predicted the variant as pathogenic. Protein modeling studies suggest that the mutation (p.Pro319Leu) cause a conformational change in the loop structure of the RasGEF domain of the CalDAG-GEFI protein. Reported variants in the RasGEF domain impair expression and/or nucleotide exchange activity of CalDAG-GEFI protein and thus inhibit the activation of Rap1 protein required for platelet adhesion and hemostatic plug formation.

Complications in Children with Ehlers-Danlos Syndrome Following Spine Surgery: Analysis of the Pediatric National Surgery Quality Improvement Program Database.

BACKGROUND: Ehlers-Danlos syndrome (EDS) is a group of rare congenital disorders of connective tissue that result in tissue fragility and joint hyperextensibility. Owing to its rarity, outcomes of pediatric spine surgery in patients with EDS are poorly characterized. Although it has been suggested that complication rates are high, few studies have characterized these complications. METHODS: Pediatric National Surgery Quality Improvement Program data from 2012-2016 were analyzed. Patients with EDS undergoing spine surgery were identified along with patients without EDS undergoing the same surgeries using International Classification of Diseases, Ninth Revision and Current Procedural Terminology codes. RESULTS: Of 369,176 total patients, 279 were determined to have EDS. Of these, 56 patients underwent spine surgery; 46% were male and 54% were female (P = 0.108). Mean age at surgery was 11.59 years (P = 0.888) with a range of 1.77-17.33 years. The most common procedure was arthrodesis (n = 37). There were no differences in unplanned reoperations (n = 4, P = 0.119), wound infections or disruptions (n = 2, P = 0.670), or overall complications (n = 25, P = 0.751). Blood transfusions were required in 41% of patients with EDS, but this was not significant compared with patients without EDS undergoing the same procedures (n = 23, P = 0.580). The total amount of blood transfused (P = 0.508), length of hospital stay (P = 0.396), and total operative time (P = 0.357) were not different from control subjects. CONCLUSIONS: Pediatric patients with EDS do not appear to be at a higher risk of bleeding or other complications during spine surgery as reported in past case series. This is the largest retrospective review of its kind that has been performed in this patient population.

Platelet dysfunction caused by a novel thromboxane A2 receptor mutation and congenital thrombocytopenia in a case of mild bleeding.

Chronic hemorrhagic diathesis in patients showing normal levels of plasmatic clotting factors strongly suggests for congenital platelet disorders. We report on a pediatric patient (male, 3 years, D1) with mild bleeding. A sibling (D2), his mother (D3) and father (D4) were included for laboratory investigation. Platelet counts in D1, D2 and D4 indicated mild thrombocytopenia (100 Gpt/L). D1 and D3 platelets showed significantly diminished aggregation response on arachidonic acid and U46619 stimulation. Immunostaining for platelet proteins on blood smears of D1 and D2 indicated defects in ß1-tubulin. Exon sequencing of TBXA2R and TUBB1 revealed heterozygosity for the novel TBXA2R*c.908T>C (p.L303P) mutation in D1 and D3. TUBB1 was either wild type (D2, D3) or heterozygous (D1, D4) for the common polymorphism TUBB1*c.920G>A (rs6070697; p.R307H). In conclusion, the bleeding phenotype of the index patient can be explained by a diminished platelet function caused by the TBXA2R*c.908T>C mutation inherited from the mother and a mild thrombocytopenia with unknown molecular basis that is inherited from the father.

Novel variants in FERMT3 and RASGRP2-Genetic linkage in Glanzmann-like bleeding disorders.

Defects of platelet intracellular signaling can result in severe platelet dysfunction. Several mutations in each of the linked genes FERMT3 and RASGRP2 on chromosome 11 causing a Glanzmann-like bleeding phenotype have been identified so far. We report on novel variants in two unrelated pediatric patients with severe bleeding diathesis-one with leukocyte adhesion deficiency type III due to a homozygous frameshift in FERMT3 and the other with homozygous variants in both, FERMT3 and RASGRP2. We focus on the challenging genetic and functional variant assessment and aim to accentuate the risk of obtaining misleading results due to the phenomenon of genetic linkage.

Thrombin and plasmin generation in patients with plasminogen or plasminogen activator inhibitor type 1 deficiency.

INTRODUCTION: Deficiencies of plasminogen and plasminogen activator inhibitor type 1 (PAI-1) are rare disorders of fibrinolysis. Current laboratory assays for analysis of activity of plasminogen and PAI-1 do not provide an accurate correlation with clinical phenotype. METHODS: The Nijmegen Hemostasis Assay (NHA) was used to simultaneously measure thrombin and plasmin generation in 5 patients with plasminogen deficiency (PLGD) and 10 patients with complete PAI-1 deficiency. Parameters analysed included: lag time ratio, thrombin peak time ratio, thrombin peak height, thrombin potential (AUC), fibrin lysis time, plasmin peak height and plasmin potential. Parameters were expressed as a percentage compared to a reference value of 53 healthy normal controls. RESULTS: Patients with PLGD demonstrated a short lag time and thrombin peak time, with normal thrombin peak height but an increased AUC. Plasmin generation was able to be detected in only one (23% plasminogen activity) of the five PLGD patients. All ten PAI-1 deficient patients demonstrated a short lag and thrombin peak time, low thrombin peak height with normal AUC. Plasmin generation revealed an increased plasmin peak and plasmin potential; interestingly, there was a large variation between individual patients despite all patients having the same homozygous defect. CONCLUSION: Patients with either PLGD or PAI-1 deficiency show distinct abnormalities in plasmin and thrombin generation in the NHA. The differences observed in the propagation phase of thrombin generation may be explained by plasmin generation. These results suggest that disorders of fibrinolysis also influence coagulation and a global assay measuring both activities may better correlate with clinical outcome.

Inherited disorders of the fibrinolytic pathway.

Deficiencies or excessive activation of the fibrinolytic system can result in severe, lifelong bleeding disorders. The most severe clinical phenotype is caused by α2-Antiplasmin (α2-AP) deficiency which results in excess fibrinolysis due to the inability to inhibit plasmin. Another bleeding disorder due to a defect in the fibrinolytic pathway results from Plasminogen activator inhibitor-1 (PAI-1) deficiency causing enhanced fibrinolysis due to the decreased inhibition of plasminogen activators resulting in increased conversion of plasminogen to plasmin. Both these disorders are rare and have an autosomal recessive pattern of inheritance. They can remain undetected as routine coagulation and platelet function tests are normal. A unique gain-of-function defect in fibrinolysis causes the Quebec platelet disorder (QPD) which is characterized by profibrinolytic platelets containing increased urokinase-type plasminogen activator (uPA) in the α-granules. A high index of suspicion based on clinical phenotype along with the availability of specialized hemostasis testing is required for timely and accurate diagnosis. Antifibrinolytic agents, such as tranexamic acid or ε-aminocaproic acid, are the mainstays of treatment which inhibit fibrinolysis by preventing the binding of plasminogen to fibrin and thereby stabilizing the fibrin clot. The purpose of this review is to summarize the pathogenesis, clinical phenotype, approaches to diagnosis and treatment for these three major disorders of fibrinolysis.

Molecular Diagnosis of Inherited Coagulation and Bleeding Disorders.

Diagnosis of inherited bleeding disorders (IBDs) remains challenging, especially in the case of inherited platelet disorders, due to the heterogeneity of the clinical and laboratory phenotype, the limited specificity of platelet function tests, and the large number of potential culprit genes. Unraveling the underlying molecular defect provides the definitive diagnosis of IBDs, facilitating prognosis and clinical care, which are especially important for severe clinical syndromes and those that may be associated with an increased risk of malignancy. Until recently, Sanger sequencing of candidate genes has been the only method of molecular diagnosis, but this approach is time-consuming and costly and requires phenotype-based identification of any obvious candidate gene(s). Nowadays, high-throughput sequencing (HTS) allows the simultaneous and rapid investigation of multiple genes at a manageable cost. This HTS technology that includes targeted sequencing of prespecified genes, whole-exome sequencing, or whole-genome sequencing, is revolutionizing the genetic diagnosis of human diseases. Through its extensive implementation in research and clinical practice, HTS is rapidly improving the molecular characterization of IBDs. However, despite the availability of this powerful approach, many patients still do not receive a diagnosis. As IBDs are complex and rare diseases, development of more advanced laboratory assays, improvements in bioinformatic pipelines, and the formation of multidisciplinary teams are encouraged to advance our understanding of IBDs.

ATP6V0A2-related cutis laxa in 10 novel patients: Focus on clinical variability and expansion of the phenotype.

In ATP6V0A2-related cutis laxa, the skin phenotype varies from a wrinkly skin to prominent cutis laxa and typically associates with skeletal and neurological manifestations. The phenotype remains incompletely characterized, especially in adult patients. Glycosylation defects and reduced acidification of secretory vesicles contribute to the pathogenesis, but the consequences at the clinical level remain to be determined. Moreover, the morphology of the elastic fibres has not been studied in ATP6V0A2-related cutis laxa, nor its relation with potential clinical risks. We report on the extreme variability in ATP6V0A2-related cutis laxa in 10 novel patients, expand the phenotype with emphysema and von Willebrand disease and hypothesize on the pathogenesis that might link both with deficiency of glycosylation and with elastic fibre anomalies. Our data will affect clinical management of patients with ATP6V0A2-related cutis laxa.

Whole exome sequencing in the diagnostic workup of patients with a bleeding diathesis.

INTRODUCTION: Bleeding assessment tools and laboratory phenotyping often remain inconclusive in patients with a haemorrhagic diathesis. AIM: To describe the phenotype and genetic profile of patients with a bleeding tendency. METHODS: Whole exome sequencing (WES) was incorporated in the routine diagnostic pathway of patients with thrombocytopenia (n = 17), platelet function disorders (n = 19) and an unexplained bleeding tendency (n = 51). The analysis of a panel of 126 OMIM (Online Mendelian Inheritance in Man) genes involved in thrombosis and haemostasis was conducted, and if negative, further exome-wide analysis was performed if informed consent given. RESULTS: Eighteen variants were detected in 15 patients from a total of 87 patients (17%). Causative variants were observed in MYH9 (two cases), SLFN14, P2RY12 and GP9. In addition, one case was considered solved due to combined carriership of F7 and F13A1 variants and one with combined carriership of F2, F8 and VWF, all variants related to secondary haemostasis protein aberrations. Two variants of uncertain significance (VUS) were found in two primary haemostasis genes: GFI1B and VWF. Eight patients were carriers of autosomal recessive disorders. Exome-wide analysis was performed in 54 cases and identified three variants in candidate genes. CONCLUSION: Based on our findings, we conclude that performing WES at the end of the diagnostic trajectory can be of additive value to explain the complete bleeding phenotype in patients without a definite diagnosis after conventional laboratory tests. Discovery of combinations of (novel) genes that predispose to bleeding will increase the diagnostic yield in patients with an unexplained bleeding diathesis.