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2154 Part XII: Hemostasis and Thrombosis Chapter 125: Hereditary Fibrinogen Abnormalities 2155

FIBRINOLYSIS The disease, originally described in 1920, has an estimated prev-
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alence of approximately one in 1,000,000. In populations where consan-
Plasminogen and tissue-type plasminogen activator (t-PA) binding sites guineous marriages are common, the prevalence of afibrinogenemia, is
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−1
in the D regions (i.e., γ 337 to 350) (312 to 324), and αC domains (i.e., Aα increased. Because hypofibrinogenemia (fibrinogen levels below 1.5 g L )
167 to 179) (148 to 160), are cryptic in fibrinogen and become exposed is often caused by heterozygosity for a fibrinogen gene mutation, this
during fibrin assembly or during formation of crosslinked fibrinogen is much more frequent than afibrinogenemia. If one applies the Hardy
fibrils (Chap. 135). 29–30 Two phases can be distinguished in the t-PA Weinberg binomial distribution of alleles in the population to afibrino-
induced lysis of a fibrin clot. . In the first, slow, phase, t-PA activates genemia, carriers of fibrinogen deficiency causing mutations could be as
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plasminogen on the intact fibrin surface. The generation of C-terminal frequent as 1 in 500.
lysine residues in partially degraded fibrin (by plasmin) in the second
phase of clot lysis may result in accumulation of plasminogen at the clot ETIOLOGY AND PATHOGENESIS
surface and a concomitant increase in lysis rate. Thrombin-activatable
fibrinolysis inhibitor (TAFI) removes C-terminal lysine residues, result- Since the identification of the first causative mutation for congenital
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ing in a strongly reduced binding of plasminogen and in an inhibition afibrinogenemia in 1999, approximately 100 distinct mutations, the
of the second phase of clot lysis by a reduction of the activation of plas- majority in FGA, have been identified in patients with afibrinogenemia
minogen on the fibrin surface. TAFI, as well as α-antiplasmin, lipopro- (in homozygosity or in compound heterozygosity) or in hypofibrino-
tein(a), and histidine-rich glycoprotein, bind to fibrin and all have an genemia. Causative mutations can be divided into two main classes: null
inhibitory effect on fibrinolysis through various mechanisms. mutations with no protein production at all and mutations producing
abnormal protein chains which are retained inside the cell. 1

ANTITHROMBIN ACTIVITY OF FIBRIN Large Deletions
The first causative mutation for afibrinogenemia was identified in a
Thrombin binds to its substrate, fibrinogen, through a fibrinogen recog- nonconsanguineous Swiss family with two pairs of afibrinogenemic
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nition site in thrombin, referred to as exosite 1. The fibrin clot itself also brothers. In a first step toward establishing whether or not the disease
exhibits significant thrombin-binding potential; this nonsubstrate bind- was linked to the fibrinogen gene cluster on chromosome 4, haplotype
ing potential of fibrin for thrombin is referred to as antithrombin activity data were obtained for five microsatellite markers surrounding this
I. This activity is defined by two classes of nonsubstrate thrombin-bind- locus. One of these, FGAi3, a (TCTT)n polymorphic marker located
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ing sites in fibrin, one of “low-affinity” in the E-region and the other in intron 3 of the FGA gene, was found to be deleted in all four affected
of “high-affinity” in D regions of fibrin(ogen) molecules containing the individuals and was hemizygous in the obligate carriers, implying that
variant γ′ chain. Altogether, heterodimeric γA/γ′ and homodimeric homozygous deletion of at least part of the FGA gene was responsible
molecules γ′/γ′ chains make up 8 to 15 percent of the total γ-chain pop- for the congenital afibrinogenemia in this family. Indeed, the genetic
ulation. Low-affinity thrombin-binding activity reflects thrombin exo- defect was found to be a recurrent deletion of approximately 11 kb of
10
site 1 binding in the E region of fibrin, whereas high-affinity thrombin DNA, with breakpoints in FGA intron 1 and the FGA–FGB intergenic
binding to γ′ chains takes place through exosite 2. The binding affinity region, resulting in an absence of fibrinogen.
of thrombin for γ′-containing fibrin molecules is increased by concom- Three other large deletions in the fibrinogen gene cluster have been
itant fibrin binding to thrombin exosite 1. Antithrombin I (fibrin) is an identified, all involving part of the FGA gene. These are: a deletion of 1.2
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important inhibitor of thrombin generation that functions by sequester- kb eliminating the entire FGA exon 4 in a Japanese patient ; a deletion
ing thrombin in the forming fibrin clot, and also by reducing the cata- of 15 kb, with breakpoints situated in FGA intron 4 and in the FGA–FGB
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lytic activity of fibrin-bound thrombin. Vascular thrombosis may result intergenic region in a Thai patient ; and a 4.1-kb deletion encompassing
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from absence of antithrombin I (as in afibrinogenemia; see “Afibrino- FGA exon 1 in an Italian patient. All patients were homozygous for the
genemia and Hypofibrinogenemia” below), reduced plasma γ′-chain identified deletions except for the Thai patient, for whom complete mater-
content, or defective thrombin binding to fibrin as found in certain nal uniparental disomy was confirmed for the deleted chromosome 4. 39
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dysfibrinogenemias (see “Dysfibrinogenemia and Hypodysfibrinogene-
mia” below). In contrast, an increased susceptibility to arterial throm- Splice-Site Mutations
bosis has been reported when γ′-chain levels are significantly elevated. Several splice-site mutations have been identified in all three fibrino-
Moreover, thrombin bound to γ /γ′-fibrin is protected from inhibition gen genes. In afibrinogenemic patients of European origin, the most
A
by antithrombin to a greater extent than thrombin bound to γ /γ -fibrin. common mutation is a donor splice mutation in intron 4, c.510+1G→T
A
A
1,41
Thus, γ /γ′-fibrin serves as a reservoir of active thrombin, which may (previously described as IVS4+1 G→T). Haplotype data suggest that
A
contribute to the prothrombotic nature of thrombi. 33 this mutation, like the FGA 11-kb deletion, is also recurrent, or a very
ancient mutation, because the c.510+1G→T mutation is found on mul-
tiple discrete haplotypes.
AFIBRINOGENEMIA AND
HYPOFIBRINOGENEMIA Frameshift Mutations
Frameshift mutations have been identified in all three fibrinogen genes.
FGA exon 5, the largest fibrinogen-coding exon has the most frameshift
DEFINITION, HISTORY, AND EPIDEMIOLOGY mutations. Interestingly, seven single base-pair deletions in FGA exon
Type I disorders (afibrinogenemia and hypofibrinogenemia) affect the 5 result in usage of the same new reading frame. All seven mutations
quantity of fibrinogen in circulation. Type II disorders (dysfibrino- are predicted to encode a long stretch of aberrant amino acids before
genemia and hypodysfibrinogenemia) affect the quality of circulating terminating at the same premature stop codon, 69 to 158 codons down-
fibrinogen. While the first dysfibrinogenemia mutation was identified stream. The aberrant amino acid sequence (if the abnormal protein
1
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as early as 1968, the molecular basis of afibrinogenemia was elucidated is synthesized and stable, which remains to be determined) may lead
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much later. This disorder is characterized by autosomal recessive to abnormal folding of the Aα chain, thus affecting fibrinogen chain
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inheritance and the complete absence of fibrinogen in plasma. assembly or secretion.

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