Afibrinogenemia resulting from homozygous nonsense mutation in A alpha chain gene associated with multiple thrombotic episodes

Clinical, biological, and genetic features in an afibrinogenemia patient series in Algeria

Introduction

The incidence of afibrinogenemia had not been previously reported in Algeria. Afibrinogenemia patients are prone to both haemorrhagic and thrombotic complications. Predictive markers of thrombosis in afibrinogenemia patients are not existent.

Aims and methods

Clinical and biological data from 46 afibrinogenemia patients are reported. Biological investigations included routine tests, genetics analysis and thrombin generation.

Results

FGA mutations (four novel and four previously described) and FGB mutations (seven mutations; five novels) were homozygous in all but one family as a result of 28 consanguineous marriages out of 30 discrete families. Incidence of afibrinogenemia in Algeria is at least 3 per million births. Umbilical bleeding was reported in 39/46 cases and was the main discovery circumstance. We also report post trauma or post‐surgery (3/46) bleeding and spontaneous deep vein thrombosis (DVT) in adulthood (1/46), as discovery circumstances. The median age (10.5‐year‐old) of the population reported here explains why there are few hemarthrosis and obstetrical or gynaecological complications in this series. Thrombotic events were reported in seven patients (four spontaneous). Endogenous Thrombin Potential was significantly increased in thrombosis‐prone patients compared to afibrinogenemic patients with and without personal or familial history (1118 vs. 744 and 817 nM IIa × min, respectively).

Conclusion

The incidence of afibrinogenemia in Algeria is the consequence of consanguineous marriage in families carrying private mutations. The thrombin generation test (TGT) could identify, among afibrinogenemic patients, those presenting a thrombotic risk.

Extension of the Human Fibrinogen Database with Detailed Clinical Information—The αC-Connector Segment

Fibrinogen, an abundant plasma glycoprotein, is involved in the final stage of blood coagulation. Decreased fibrinogen levels, which may be caused by mutations, are manifested mainly in bleeding and thrombotic disorders. Clinically relevant mutations of fibrinogen are listed in the Human Fibrinogen Database. For the αC-connector (amino acids Aα240–410, nascent chain numbering), we have extended this database, with detailed descriptions of the clinical manifestations among members of reported families. This includes the specification of bleeding and thrombotic events and results of coagulation assays. Where available, the impact of a mutation on clotting and fibrinolysis is reported. The collected data show that the Human Fibrinogen Database reports considerably fewer missense and synonymous mutations than the general COSMIC and dbSNP databases. Homozygous nonsense or frameshift mutations in the αC-connector are responsible for most clinically relevant symptoms, while heterozygous mutations are often asymptomatic. Symptomatic subjects suffer from bleeding and, less frequently, from thrombotic events. Miscarriages within the first trimester and prolonged wound healing were reported in a few subjects. All mutations inducing thrombotic phenotypes are located at the identical positions within the consensus sequence of the tandem repeats.

Pharmacological Blockade of Glycoprotein VI Promotes Thrombus Disaggregation in the Absence of Thrombin

OBJECTIVE

Atherothrombosis occurs upon rupture of an atherosclerotic plaque and leads to the formation of a mural thrombus. Computational fluid dynamics and numerical models indicated that the mechanical stress applied to a thrombus increases dramatically as a thrombus grows, and that strong inter-platelet interactions are essential to maintain its stability. We investigated whether GPVI (glycoprotein VI)-mediated platelet activation helps to maintain thrombus stability by using real-time video-microscopy. Approach and Results: We showed that GPVI blockade with 2 distinct Fab fragments promoted efficient disaggregation of human thrombi preformed on collagen or on human atherosclerotic plaque material in the absence of thrombin. ACT017-induced disaggregation was achieved under arterial blood flow conditions, and its effect increased with wall shear rate. GPVI regulated platelet activation within a growing thrombus as evidenced by the loss in thrombus contraction when GPVI was blocked, and the absence of the disaggregating effect of an anti-GPVI agent when the thrombi were fully activated with soluble agonists. The GPVI-dependent thrombus stabilizing effect was further supported by the fact that inhibition of any of the 4 key immunoreceptor tyrosine-based motif signalling molecules, src-kinases, Syk, PI3Kβ, or phospholipase C, resulted in kinetics of thrombus disaggregation similar to ACT017. The absence of ACT017-induced disaggregation of thrombi from 2 afibrinogenemic patients suggests that the role of GPVI requires interaction with fibrinogen. Finally, platelet disaggregation of fibrin-rich thrombi was also promoted by ACT017 in combination with r-tPA (recombinant tissue plasminogen activator).

CONCLUSIONS

This work identifies an unrecognized role for GPVI in maintaining thrombus stability and suggests that targeting GPVI could dissolve platelet aggregates with a poor fibrin content.

Thromboembolism in patients with congenital afibrinogenaemia

Frequent arterial and venous thromboembolism in patients with congenital afibrinogenaemia (CA) is neither understood nor is a safe and effective treatment established. It was our objective to report on the clinical observations and laboratory data contributing to the understanding of the frequency, physiopathology, prognosis and treatment of CA. We observed the long-term clinical course and laboratory data in a cohort of four patients with CA and thromboembolic complications, and conducted a systematic review retrieving all available data. Four patients with CA developed recurrent and extensive arterial and venous thromboembolism (TE) from an age of 25–51 years. In two patients, a treatment strategy targeting at maintaining constantly measurable fibrinogen (Fbg) levels (≥0.5 g/l) either by regular Fbg replacement or by orthotopic liver transplantation resulted in long-term remissions. Radiological imaging documented resolved arterial thrombi after 6–12 months. In contrast, recurrent thromboembolic events were observed in two other patients with infrequent Fbg replacement. A systematic review of the literature revealed 48 reports of TE in patients with CA (median age at first event 31 years), and a favourable outcome in most patients with frequent application of Fbg, aimed at constantly measurable trough levels. Present data suggests that patients with CA are at high risk of arterial and venous thromboembolic events, probably caused by thrombin excess owing to lack of thrombin scavenging by Fbg/fibrin. Regular low-dose Fbg replacement might be a safe and effective treatment option in patients with CA and thromboembolic complications.

Note: Preliminary data of this report were presented as oral presentation at the XXV Congress of the International Society on Thrombosis and Haemostasis, June 20th to 25th, Toronto, Canada.

Supplementary Material to this article is available online at www.thrombosis-online.com.

Thrombosis in Inherited Fibrinogen Disorders

Although inherited fibrinogen disorders (IFD) are primarily considered to be bleeding disorders, they are associated with a higher thrombotic complication risk than defects in other clotting factors. Managing IFD patients with thrombosis is challenging as anticoagulant treatment may exacerbate the underlying bleeding risk which can be life-threatening. Due to the low prevalence of IFD, there is little information on pathophysiology or optimal treatment of thrombosis in these patients. We searched the literature for cases of thrombosis among IFD patients and identified a total of 128 patient reports. In approximately half of the cases, thromboses were spontaneous, while in the others trauma, surgery, and parturition contributed to the risk. The true mechanism(s) of thrombosis in IFD patients remain to be elucidated. A variety of anticoagulant treatments have been used in the treatment or prevention of thrombosis, sometimes with concurrent fibrinogen replacement therapy. There is no definite evidence that fibrinogen supplementation increases the risk of thrombosis, and it may potentially be effective in the treatment and prevention of both thrombosis and hemorrhage in IFD patients.

[Hereditary afibrinogenemia: A literature review and clinical observations]

Afibrinogenemia is a rare congenital coagulopathy that leads to life-threatening bleeding. In afibrinogenemia, plasma fibrinogen levels are less than 0.1 g/L. The clinical manifestations of the disease can be both bleeding and thromboses of different localizations, which is determined by the multifunctional role of fibrinogen in hemostasis. The described cases demonstrate different clinical phenotypes of the disease. In both cases the diagnosis was confirmed by genetic examinations that revealed homozygous mutations in the fibrinogen A genes. The nature of the mutations assumes consanguineous marriages, as confirmed by the results of a genealogical analysis. Fibrinogen preparations are promising in treating afibrinogenemia in Russia.

Congenital hypofibrinogenemia associated with novel homozygous fibrinogen Aα and heterozygous Bβ chain mutations

We report the molecular characterisation of two novel cases of inherited hypofibrinogenemia. After sequencing all coding regions and intron-exon boundaries of the three fibrinogen genes (FGA, FGB, and FGG), two different novel mutations were found, one homozygous and one heterozygous. The first patient, with a mild bleeding history and mild discrepancy between functional and immunological fibrinogen, showed a novel homozygous nonsense mutation in exon 5 of FGA (p.Trp373*, p.Trp354* according to the mature protein) caused by a G>A transition at nucleotide position 1,119. The resulting truncation in the Aα chain is likely to reduce the efficiency of fibrinogen assembly and secretion. The second patient, referred after ischemic stroke (functional fibrinogen 77mg/dL), had a novel heterozygous splicing mutation in intron 5 of FGB (IVS5+2T>A or c.832+2T>A), which we demonstrated to cause either exon 5 skipping or the inclusion of 75bp belonging to intron 5. Neither splicing defect alters the reading frame: one results in a 38-residue deletion and the other in a 25-residue insertion in the D domain of fibrinogen Bβ chain. This report confirms that genetically determined partial deficiencies of fibrinogen with levels greater than 50mg/dL are rarely associated with significant bleeding symptoms and that homozygous null mutations removing a significant portion of the Aα chain may be associated with mild fibrinogen deficiency.

The clinical significance of the lack of arterial or venous thrombosis in patients with congenital prothrombin or FX deficiency

Several reports have dealt with the occurrence of both arterial and venous thrombosis in patients with haemophilia A, haemophilia B, and von Willebrand disease. Similar thrombotic events have been occasionally reported also in rare congenital coagulation disorders, particularly in fibrinogen or FVII deficiencies. On the contrary no sure venous or arterial thrombotic event has ever been reported in congenital prothrombin or Factor X deficiency. The significance of this observation is discussed. This discrepancy cannot be explained on the basis of the rarity of the two conditions, since in similarly rare congenital bleeding disorders such as FV or FXIII deficiency a few patients with thrombosis have been described. It appears that only these two defects are able to allow a sure protection from thrombosis. These observations may indirectly support the rationale for the use of direct thrombin or Factor X inhibitors in the prophylaxis and/or therapy of thrombotic manifestations.