An intronic mutation within FGB (IVS1+2076 a→g) is associated with afibrinogenemia and recurrent transient ischemic attacks

Abstract

To cite this article: Dear A, Daly J, Brennan SO, Tuckfield A, George PM. An intronic mutation within FGB (IVS1+2076 a fi g) is associated with afibrinogenemia and recurrent transient ischemic attacks. J Thromb Haemost 2006; 4: 471-2. Afibrinogenemia is a rare autosomal recessive disorder caused by homozygous or compound heterozygous inheritance of fibrinogen mutations. It is characterized by an absence of clottable plasma fibrinogen, although sensitive immunoassays can often detect trace amounts of the molecule. Common symptoms include joint and mucosal bleeding, but gastrointestinal and central nervous system bleeding have also been described [1]. While seeming contradictory, thrombosis is also a rare manifestation of afibrinogenemia [2]. The proband in this report, a 41-year-old man of Sri Lankan descent, first presented at the age of 29 with hematemesis and melena from a MalloryWeiss tear. Coagulation studies revealed a functional (Clauss) fibrinogen concentration of <0.1 g L )1 [normal range (NR):], a thrombin time of >300 s (NR: 18-26 s) and an international normalized ratio of >8.5 (NR: 0.8-1.2). No fibrinogen was detected by radial immunodiffusion, heat precipitation or plasma protein electrophoresis. A diagnosis of afibrinogenemia was made, and the hematemesis managed with cryoprecipitate.He remained well until age 34 when he presented with a cerebral transient ischemic attack. Further investigations suggested a prior apical myocardial infarction with a left ventricular mural thrombus. From age 35-40, the proband experienced recurrent bilateral weakness because of transient ischemic attack and an echocardiogram revealed a persistent mural thrombus. A coagulation screen showed a normal plasminogen radial immunodiffusion assay with a low functional (chromogenic) plasminogen of 39% (NR: 80-120%), most likely because of low fibrin levels causing diminished tissue plasminogen activator activity. All other tests, including antithrombin III, protein C, protein S, activated protein C resistance ratio, lupus anticoagulant, and anticardiolipin antibodies, were normal.Treatment with low-dose warfarin was ineffective and therapeutic dose low-molecular weight heparin (enoxaparin) resulted in bleeding complications, including melena that required transfusion, as did antiplatelet therapy in combination with low-dose enoxaparin. The reason for the bleeding is obvious, as no significant fibrin clot can be formed. The thromboembolic events are probably because of fibrin being a cofactor of platelet aggregation, such that large, loosely packed and unstable platelet thrombi form in the absence of fibrin. It was concluded that the proband was best managed on lowdose (20 mg) enoxaparin twice daily, which he has taken for 8 months without any further bleeding or thrombotic episodes.Quantitative Western blot of purified fibrinogen revealed a normal pattern of Aa, Bb, and c chains, and indicated a fibrinogen concentration approximately one 50th of normal levels, or 0.05 g L )1 (Fig. 1). It was concluded that the proband had ver...

A deep intronic mutation inFGBcreates a consensus exonic splicing enhancer motif that results in afibrinogenemia caused by aberrant mRNA splicing, which can be corrected in vitro with antisense oligonucleotide treatment

“…We previously described a patient with persistent cerebral transient ischemic attacks associated with afibrinogenemia. Screening of the entire coding and intronic regions of the three fibrinogen genes revealed a single homozygous point mutation deep within intron 1 of the fibrinogen beta gene (FGB), which we named fibrinogen Sri Lanka (FGB, c.115-600A4G) [Dear et al, 2006]. This mutation did not create or destroy any consensus splice-sites, but despite its benign appearance it was the only candidate mutation for the patient's persistent phenotype.…”

Prediction of single‐nucleotide substitutions that result in exon skipping: identification of a splicing silencer inBRCA1exon 6

“…Similarly, mutations introducing guanine at position 195-197 of the BRCA2 exon 18 were ranked by HOT-SKIP among the first 5 exon-skipping mutations, just 6 nt from the disease-causing substitution at position 189 [Fackenthal et al, 2002]. In cryptic exons, identical positions were in the top quartile if mutated to other nucleotides [Dear et al, 2006;Ishii et al, 2002]. These observations suggest that in a subset of cases, the ESS/ESE profile did not correctly predict mutations that are likely to reside within a shared secondary structure.…”

In vitro transcription of compound heterozygous hypofibrinogenemia Matsumoto IX; first identification of FGB IVS6 deletion of 4 nucleotides and FGG IVS3-2A>G causing abnormal RNA splicing

“…Reports on autosomal recessive disorder with a molecular base of congenital afibrinogenemia indicated association with the aberrant splicing caused by inactivation of physiologic splice sites [11][12][13][14][15][16][17][18][19][20][21][22][23][24]. Most abnormal RNA splicing was reported to occur near the donor splice site, with five kinds of mutation in FGA, four in FGB and three in the IVS3 +1 to +4 deletions, which were another FGA donor splice site mutation, showed exon 3 skipping in 99% of transcripts, and exon 2 and 3 skipping in 1% of transcripts [23].…”

“…Fibrinogen Matsumoto IX: FGB and FGG gene splicing mutations Some cases of afibrinogenemia caused by aberrant mRNA have been reported to be associated with large deletions [8][9][10] or other genetic mutations that affect transcription [11][12][13][14][15][16][17][18][19][20][21][22][23][24]. We assumed that abnormal transcription of mRNA for FGB IVS6Δ4b and/or FGG IVS3-2G caused hypofibrinogenemia.…”