Phenotype and Genotype of 22 Subjects with Heritable Dysfibrinogenaemia

Abstract 1212

The heritable dysfibrinogenaemias (HD) are a heterogenous group of qualitative disorders of fibrinogen in which there is abnormal fibrin clot formation. Affected individuals usually display prolonged thrombin (TCT) and reptilase clotting times (RCT) and reduced plasma fibrinogen activity. The plasma fibrinogen concentration is normal or is reduced less than the fibrinogen activity. HD is associated with pathogenic mutations in FGA, FGB or FGG genes that encode the fibrinogen Aα, Bβ and g chains respectively. However, most clinical descriptions of HD comprise single case reports and the range of phenotype and genotype in the wider population is difficult to estimate. In order to improve understanding of this disorder, we now report a cross-sectional survey of 22 subjects with HD identified at 2 UK regional Haemostasis centres from 2001–11.

We recruited index cases from our Haemostasis laboratory referral practice who satisfied all the following criteria- 1) prolonged TCT; 2) prolonged TCT with protamine (TCTP) and/or prolonged RCT; 3) reduced Clauss fibrinogen and 4) normal fibrinogen dry clot weight. Demographic and clinical characteristics were recorded using a standardised case report form. Genetic analysis was performed by Sanger sequencing of FGA exon 2, FGB exon 2 and FGG exon 8 which we identified as mutation hotspots from the fibrinogen mutation database (http://www.geht.org/databaseang/fibrinogen/). Phenotype and genotype was also determined in available first degree family members.

We identified 16 index cases and 6 family members with HD (median age 41, range 12–78; 9 males). HD was identified in plasma submitted to our Haemostasis laboratories for the investigation of bleeding in 8/16 (50%) index cases, for stroke in 1/16 (6%) and for ‘routine coagulation screening’ in 7/16 (44%). Mild mucocutaneous bleeding or menorrhagia was documented in the clinical records of 13/22 (59%) subjects and thrombotic stroke in 1/22 (5%). 8/22 (36%) subjects were asymptomatic.

The prothrombin time (PT) was prolonged in 21/22 (95%) subjects (median 15.3 s; range 11.4–16.7; reference interval (RI) 9.6–11.6) but the activated partial thromboplastin time (aPTT) was prolonged in only 1/22 (5%; median 23.8 s; range 22.8–37.0, RI 24.0–32.0). The median TCT was 28.6 s (range 20.6–45.7; RI 13–19) and TCTP 31.9 s (n=11; range 20.0–35.5; RI 13–19). The median RCT was 37s (n=12; range 19–68; RI 15–19), Clauss fibrinogen 0.49 g/dL (range 0.29–1.3; RI 1.8–3.6) and dry clot weight 2.3 g/dL (range 1.9–4.2; RI 1.8–3.6). Heterozygous missense nucleotide variations were identified in the candidate exons of the fibrinogen genes in all 16/16 (100%) index cases and segregated with the HD phenotype in all available family members. These variations predicted aminoacid substitutions at the fibrinopeptide A cleavage site in the fibrinogen Aa chain in 10/16 (63%) index cases (FGA p.R35H (n=4); p.R35S (n=3); p.R35P (n=2) and p.G36S (n=1); numbered from RefSeq NP_000499.1) or at nearby residues (FGA p.V39D (n=1) and p.F27C (n=1)). FGG variations were identified in 3 index cases that predicted aminoacid substitutions in the fibrinogen g chain region necessary for D:D domain interactions during fibrin monomer assembly (FGG p.R301H (n=1), p.D342G (n=1) and p.A367T (n=1); RefSeq NP_000499.1). One subject harboured a FGB variation that predicted a p.R44C substitution (Refseq NP_005132.2) at the fibrinopeptide B cleavage site in the fibrinogen Bb chain. The FGA p.R35S, p.R35H, FGB p.R44C and FGG p.R301H and p.A367T substitutions have been associated previously with HD.

We show that the HD laboratory phenotype is rare and was frequently identified in subjects with mild bleeding. However, HD was also commonly asymptomatic and did not segregate with bleeding symptoms within affected families suggesting that the HD variants identified in this study do not confer significant bleeding risk. The prevalence of mild bleeding in some study subjects may reflect recruitment bias. We also show that HD is strongly associated with nucleotide variations in the FGA, FGB or FGG genes. All the observed nucleotide variations have been previously associated with HD and/or predict aminoacid substitutions in critical regions for normal fibrin polymerization. Defective fibrin polymerization may affect endpoint determination in some automated clotting assays and is likely to account for the consistent finding of prolonged PT in the study subjects.