Enhancer-gene rewiring in the pathogenesis of Quebec platelet disorder.

Quebec platelet disorder (QPD) is an autosomal dominant bleeding disorder with a unique, platelet-dependent, gain-of-function defect in fibrinolysis, without systemic fibrinolysis. The hallmark feature of QPD is a >100-fold overexpression of PLAU, specifically in megakaryocytes. This overexpression leads to a >100-fold increase in platelet stores of urokinase plasminogen activator (PLAU/uPA); subsequent plasmin-mediated degradation of diverse α-granule proteins; and platelet-dependent, accelerated fibrinolysis. The causative mutation is a 78-kb tandem duplication of PLAU. How this duplication causes megakaryocyte-specific PLAU overexpression is unknown. To investigate the mechanism that causes QPD, we used epigenomic profiling, comparative genomics, and chromatin conformation capture approaches to study PLAU regulation in cultured megakaryocytes from participants with QPD and unaffected controls. QPD duplication led to ectopic interactions between PLAU and a conserved megakaryocyte enhancer found within the same topologically associating domain (TAD). Our results support a unique disease mechanism whereby the reorganization of sub-TAD genome architecture results in a dramatic, cell-type-specific blood disorder phenotype.

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.

Understanding the Impact of Aberrant Splicing in Coagulation Factor V Deficiency.

Rare inherited coagulation disorders (RICDs) are congenital deficiencies of the plasma proteins that are involved in blood coagulation, which generally lead to lifelong bleeding manifestations. These diseases are generally qualitative and/or quantitative defects that are associated with monoallelic or biallelic mutations in the relevant gene. Among RICDs, factor V (FV) deficiency is one of the least characterized at the molecular level. Here, we investigated four unrelated patients with reduced plasma FV levels (three severe, one mild), which were associated with a moderately severe bleeding tendency. Sequence analysis of the FV gene identified seven different variants, five hitherto unknown (p.D1669G, c.5789-11C>A, c.5789-12C>A, c.5789-5T>G, and c.6528G>C), and two previously reported (c.158+1G>A and c.5789G>A). The possible pathogenic role of the newly identified missense variant was studied by in silico approaches. The remaining six genetic defects (all putative splicing mutations) were investigated for their possible effects on pre-mRNA splicing by transient transfection experiments in HeLa cells with plasmids expressing appropriate hybrid minigenes. The preparation of minigene constructs was instrumental to demonstrate that the two adjacent variants c.5789-11C>A and c.5789-12C>A are indeed present in cis in the analyzed FV-deficient patient (thus leading to the c.5789-11_12CC>AA mutation). Ex vivo experiments demonstrated that each variant causes either a skipping of the relevant exon or the activation of cryptic splice sites (exonic or intronic), eventually leading to the introduction of a premature termination codon.

"In vitro" correction of the severe factor V deficiency-related coagulopathy by a novel plasma-derived factor V concentrate.

INTRODUCTION: Severe congenital factor V (FV) deficiency is a rare bleeding disorder characterized by very low/undetectable levels of FV. Fresh frozen plasma is the standard treatment for bleeding manifestations. Recently, a novel plasma-derived FV concentrate has been developed. AIM: To evaluate the "in vitro" ability of the novel FV concentrate to normalize clotting times and generate normal amount of thrombin in plasma collected from patients with severe FV deficiency. METHODS: Prothrombin time (PT), activated partial thromboplastin time (aPTT), FV activity and antigen levels and thrombin generation were measured pre- and postspiking of plasma samples of 10 patients with increasing doses of FV concentrate (from 0 to 100 IU/dL). RESULTS: Prothrombin time and activated partial thromboplastin time ratios as well as all thrombin generation parameters were fully corrected by the addition of FV concentrate at a final concentration of 25 IU/dL. However, the addition of FV at a concentration of 1-3 IU/dL was already sufficient to correct peak height and endogenous thrombin potential (but not lag time and time to peak) after activation with 5 pmol/L tissue factor. FV activity and antigen levels showed a linear response to supplementation with the novel FV concentrate. CONCLUSION: The novel plasma-derived FV concentrate was effective to correct "in vitro" severe FV deficiency in patients. The optimal FV concentration to fully normalize both global clotting times and thrombin generation parameters using the novel plasma-derived FV concentrate was 25 IU/dL.

Endocytosis of exogenous factor V by ex-vivo differentiated megakaryocytes from patients with severe parahaemophilia.

Although human megakaryocytes can synthesize factor V (FV), platelet FV derives largely from endocytosis of plasma FV. Recently, it has been shown that plasma transfusions can replenish the platelet FV pool in parahaemophilic patients. Here we corroborate this finding by showing FV endocytosis by ex vivo differentiated megakaryocytes derived from patients with inherited parahaemophilia. Mononuclear stem cells isolated from peripheral blood of healthy subjects and of three patients with severe parahaemophilia were cultured in the presence of thrombopoietin and interleukin-3 and differentiated into CD41-positive polynucleated megakaryocytes. Exogenous purified FV was added to the culture medium to evaluate FV endocytosis. Immunofluorescence staining revealed abundant FV expression in megakaryocytes derived from healthy donors, but no FV expression in those derived from patients with severe parahaemophilia. However, after the addition of purified FV to the culture medium, megakaryocytes from parahaemophilia patients became positive upon FV immunostaining, suggesting endocytosis of exogenous FV. Endocytosed FV retained factor Xa-co-factor activity as assessed by a prothrombin time-based functional test in megakaryocyte lysates. Addition of exogenous FV to culture medium can restore the FV content of megakaryocytes derived from patients with severe FV defects. This rescue mechanism can have important clinical implications in the management of parahaemophilia patients.

Simultaneous measurement of thrombin and plasmin generation to assess the interplay between coagulation and fibrinolysis.

Normal haemostasis is maintained by a controlled balance between coagulation and fibrinolysis, involving thrombin and plasmin the respective key enzymes. Simultaneous evaluation of both enzymes facilitates, therefore, an overall understanding of normal and pathological haemostasis. Combined thrombin and plasmin generation (T/P-G) assays have been recently described, and we have adapted the technique to investigate the interplay between coagulation and fibrinolysis in patients with various haemostatic disorders. Our modified T/P-G was initiated by the addition of a mixture of optimised lower concentrations of tissue factor and tissue-type plasminogen activator. Thrombin generation (TG) and plasmin generation (PG) were monitored simultaneously using individual fluorescent substrates in separate microtitre wells. The relationship between coagulation and fibrinolysis was demonstrated by analysing the effects of thrombin inhibitors, activated protein C and thrombomodulin. The most evident impairments in TG were observed with plasma samples deficient of coagulation factors participating in the prothrombinase complex. Defects in PG were observed with deficiencies of factor (F)V, FX, fibrinogen, and plasminogen. TG appeared to be a prerequisite for the initiation of PG, and overall PG was governed by fibrinogen concentration. TG in patients with haemophilia A correlated with levels of FVIII activity, but there was no significant relationship between PG and FVIII:C, confirming that the abnormal haemostasis in haemophilia A results in a severe imbalance between coagulation and fibrinolysis. The findings demonstrate that global haemostasis depends on a sensitive balance between coagulation and fibrinolysis, and that the modified T/P-G assay could provide an enhanced understanding of haemorrhage and thrombosis in clinical practice.

Inherited hematological disorders due to defects in coat protein (COP)II complex.

Many diseases attributed to trafficking defects are primary disorders of protein folding and assembly. However, an increasing number of disease states are directly attributable to defects in trafficking machinery. In this context, the cytoplasmic coat protein (COP)II complex plays a pivotal role: it mediates the anterograde transport of correctly folded secretory cargo from the endoplasmic reticulum towards the Golgi apparatus. This review attempts to describe the involvement of COPII complex alteration in the pathogenesis of human genetic disorders; particularly, we will focus on two disorders, the Congenital Dyserythropoietic Anemia type II and the Combined Deficiency of Factor V and VIII.

Use of pharmacokinetic modelling to individualize FFP dosing in factor V deficiency.

Therapy with fresh frozen plasma (FFP) confers serious risks, such as contraction of blood-borne viruses, allergic reaction, volume overload and development of alloantibodies. The aim of this study was to apply principles of pharmacokinetic (PK) modelling to individual factor content of FFP to optimize individualized dosing, while minimizing potential risks of therapy. We used PK modelling to successfully target individual factor replacement in an 8-month-old patient receiving FFP for treatment of a severe congenital factor V (FV) deficiency. The model fit for the FV activity vs. time data was excellent (r = 0.98) and the model accurately predicted FV activity during the intraoperative and postoperative period. Accurate PK modelling of individual factor activity in FFP has the potential to provide better targeted therapy, enabling clinicians to more precisely dose patients requiring coagulation products, while avoiding wasteful and expensive product overtreatment, minimizing potentially life-threatening complications due to undertreatment and limiting harmful product-associated risks.

The COPII pathway and hematologic disease.

Multiple diseases, hematologic and nonhematologic, result from defects in the early secretory pathway. Congenital dyserythropoietic anemia type II (CDAII) and combined deficiency of coagulation factors V and VIII (F5F8D) are the 2 known hematologic diseases that result from defects in the endoplasmic reticulum (ER)-to-Golgi transport system. CDAII is caused by mutations in the SEC23B gene, which encodes a core component of the coat protein complex II (COPII). F5F8D results from mutations in either LMAN1 (lectin mannose-binding protein 1) or MCFD2 (multiple coagulation factor deficiency protein 2), which encode the ER cargo receptor complex LMAN1-MCFD2. These diseases and their molecular pathogenesis are the focus of this review.

Simultaneous measurement of adenosine triphosphate release and aggregation potentiates human platelet aggregation responses for some subjects, including persons with Quebec platelet disorder.

Platelet aggregometry and dense granule adenosine triphosphate (ATP) release assays are helpful to diagnose platelet disorders. Some laboratories simultaneously measure aggregation and ATP release using Chronolume® a commercial reagent containing D-luciferin, firefly luciferase and magnesium. Chronolume® can potentiate sub-maximal aggregation responses, normalising canine platelet disorder findings. We investigated if Chronolume® potentiates human platelet aggregation responses after observing discrepancies suspicious of potentiation. Among patients simultaneously tested by light transmission aggregometry (LTA) on two instruments, 18/43 (42%), including 14/24 (58%) with platelet disorders, showed full secondary aggregation with one or more agonists only in tests with Chronolume®. As subjects with Quebec platelet disorder (QPD) did not show the expected absent secondary aggregation responses to epinephrine in tests with Chronolume®, the reason for the discrepancy was investigated using samples from 10 QPD subjects. Like sub-threshold ADP (0.75 µM), Chronolume® significantly increased QPD LTA responses to epinephrine (p<0.0001) and it increased both initial and secondary aggregation responses, leading to dense granule release. This potentiation was not restricted to QPD and it was mimicked adding 1-2 mM magnesium, but not D-luciferin or firefly luciferase, to LTA assays. Chronolume® potentiated the ADP aggregation responses of QPD subjects with a reduced response. Furthermore, it increased whole blood aggregation responses of healthy control samples to multiple agonists, tested at concentrations used for the diagnosis of platelet disorders (p values <0.05). Laboratories should be aware that measuring ATP release with Chronolume® can potentiate LTA and whole blood aggregation responses, which alters findings for some human platelet disorders, including QPD.

A new look at blood coagulation factor V.

PURPOSE OF REVIEW: Factor V plays an essential role in hemostasis and has a profound influence on thrombin generation. The aim of this review is to highlight recent advances in our understanding of the biology of factor V which shed light on the variable bleeding tendencies in severe factor V deficiency. Furthermore, new mechanistic insights responsible for maintaining factor V as an inactive procofactor will be discussed. RECENT FINDINGS: The bleeding manifestation of severe factor V-deficient patients varies dramatically. Phenotypic modifiers of the bleeding predisposition in these patients have recently been identified. These include platelet factor V and, surprisingly, plasma tissue factor pathway inhibitor, which is significantly reduced in these patients. An important step in robust thrombin generation is the activation of factor V to factor Va. In a mechanism distinct from factor VIII, factor V activation involves proteolytic removal of inhibitory and conserved sequences from the large central B domain which exposes binding sites for factor Xa and possibly prothrombin. Taking advantage of this mechanism, certain Australian snakes have a unique form of factor V in their venom with these inhibitory sequences removed, thereby creating a potent constitutively active procoagulant cofactor. SUMMARY: Basic biochemical and clinical studies continue to move our understanding of factor V forward. It is apparent that there is much to be learned about parahemophilia and factor V activation, two seemingly well studied areas of research. A full understanding of each may provide unanticipated insights into ways to modulate factor V/Va function for therapeutic benefit.

Inherited and acquired factor V deficiency.

The clotting factor V, also known as proaccelerin or labile factor, is synthesized by the liver and possibly by the megakaryocytes. Factor V exerts a pivotal role in hemostasis, as it participates in both procoagulant and anticoagulant pathways, being an essential cofactor of the prothrombinase complex in the former case and participating in the inactivation of factor VIII (FVIII) in the latter. Isolated factor V deficiency due to mutations in the F5 gene is a rare inherited coagulopathy typically associated with a broad spectrum of bleeding symptoms, ranging from easy bruising, delayed bleeding after haemostatic challenges such as trauma or surgery to more severe joint bleeds. The combined deficiency of factor V and FVIII, commonly known as F5F8D, is a recessive disorder not attributable to the association of isolated factor V and FVIII deficiencies, but rather to defective intracellular processing of both proteins due to mutations involving the LMAN1 and MCFD2 genes, which encode two proteins forming an essential cargo receptor complex. Overall, patients affected by F5F8D do not bleed more in terms of both frequency and severity than those carrying specific deficiencies of both factors and the bleeding phenotype is generally mild. Although now increasingly rare, inhibitors directed against factor V may also develop in individuals of any age and are characterized by a very heterogeneous clinical phenotype. The aim of the current review is to provide an overview on the physiopathology, diagnostics, and clinical management of both inherited and acquired factor V deficiency.

Two new mutations at ERGIC-53 gene in a Turkish family.

Combined factor V and factor VIII deficiency (F5F8D) is a rare autosomal recessive coagulation disorder associated with plasma levels of coagulation factors V and VIII approximately 5% to 30% normal. Combined factor V and factor VIII deficiency is caused by mutations in ERGIC-53 (LMAN1) gene. ERGIC-53 and multiple coagulation factor deficiency 2 (MCFD2) form a protein complex that functions as a cargo receptor transport FV and FVIII from the endoplasmic reticulum to the Golgi. The aim of this study was to determine the mutations of ERGIC-53 (endoplasmic reticulum [ER] to the ER-Golgi intermediate compartment) gene and combined F5F8D in a family. In this study, we analyzed a patient in a Turkish family with combined F5F8D. We found a nonsense mutation of C to T at nucleotide 202 in exon 9, resulting in a transition of arginine to stop codon, and in 1 child, we found a timine deletion in exon 4 in ERGIC-53 gene.

Crystal structure of the LMAN1-CRD/MCFD2 transport receptor complex provides insight into combined deficiency of factor V and factor VIII.

LMAN1 is a glycoprotein receptor, mediating transfer from the ER to the ER-Golgi intermediate compartment. Together with the co-receptor MCFD2, it transports coagulation factors V and VIII. Mutations in LMAN1 and MCFD2 can cause combined deficiency of factors V and VIII (F5F8D). We present the crystal structure of the LMAN1/MCFD2 complex and relate it to patient mutations. Circular dichroism data show that the majority of the substitution mutations give rise to a disordered or severely destabilized MCFD2 protein. The few stable mutation variants are found in the binding surface of the complex leading to impaired LMAN1 binding and F5F8D.

Molecular characterization of three novel splicing mutations causing factor V deficiency and analysis of the F5 gene splicing pattern.

BACKGROUND: Factor V deficiency is a rare autosomal recessive hemorrhagic disorder, associated with bleeding manifestations of variable severity. In the present study, we investigated the molecular basis of factor V deficiency in three patients, and performed a comprehensive analysis of the factor V gene (F5) splicing pattern. DESIGN AND METHODS: Mutational screening was performed by DNA sequencing. Wild-type and mutant F5 mRNA were expressed by transient transfection in COS-1 cells, followed by reverse-transcriptase polymerase chain reaction and sequencing. Real-time reverse-transcriptase polymerase chain reaction was used to evaluate degradation of mRNA carrying premature termination codons. RESULTS: Mutational screening identified three hitherto unknown splicing mutations (IVS8+6T>C, IVS21+1G>A, and IVS24+1_+4delGTAG). Production of mutant transcripts in COS-1 cells demonstrated that both IVS21+1G>A and IVS24+1_+4delGTAG cause the activation of cryptic donor splice sites, whereas IVS8+6T>C causes exon-8 skipping (F5-Delta 8-mRNA). Interestingly, F5-Delta 8-mRNA was also detected in wild-type transfected samples, human liver, platelets, and HepG2 cells, demonstrating that F5 exon-8 skipping takes place physiologically. Since F5-Delta 8-mRNA bears a premature termination codons, we investigated whether this transcript is subjected to nonsense-mediated mRNA decay degradation. The results confirmed the involvement of nonsense-mediated mRNA decay in the degradation of F5 PTC(+) mRNA. Moreover, a comprehensive analysis of the F5 splicing pattern led to the identification of two in-frame splicing variants resulting from skipping of exons 3 and 5-6. CONCLUSIONS: The functional consequences of three splicing mutations leading to FV deficiency were elucidated. Furthermore, we report the identification of three alternatively spliced F5 transcripts.