Dysfibrinogenemia

The dysfibrinogenemias consist of three types of fibrinogen disorders in which a critical blood clotting factor, fibrinogen, circulates at normal levels but is dysfunctional. Congenital dysfibrinogenemia is an inherited disorder in which one of the parental genes produces an abnormal fibrinogen. This fibrinogen interferes with normal blood clotting and/or lyses of blood clots. The condition therefore may cause pathological bleeding and/or thrombosis. Acquired dysfibrinogenemia is a non-hereditary disorder in which fibrinogen is dysfunctional due to the presence of liver disease, autoimmune disease, a plasma cell dyscrasias, or certain cancers. It is associated primarily with pathological bleeding. Hereditary fibrinogen Aα-Chain amyloidosis is a sub-category of congenital dysfibrinogenemia in which the dysfunctional fibrinogen does not cause bleeding or thrombosis but rather gradually accumulates in, and disrupts the function of, the kidney. The dysfibrinogenemias consist of three types of fibrinogen disorders in which a critical blood clotting factor, fibrinogen, circulates at normal levels but is dysfunctional. Congenital dysfibrinogenemia is an inherited disorder in which one of the parental genes produces an abnormal fibrinogen. This fibrinogen interferes with normal blood clotting and/or lyses of blood clots. The condition therefore may cause pathological bleeding and/or thrombosis. Acquired dysfibrinogenemia is a non-hereditary disorder in which fibrinogen is dysfunctional due to the presence of liver disease, autoimmune disease, a plasma cell dyscrasias, or certain cancers. It is associated primarily with pathological bleeding. Hereditary fibrinogen Aα-Chain amyloidosis is a sub-category of congenital dysfibrinogenemia in which the dysfunctional fibrinogen does not cause bleeding or thrombosis but rather gradually accumulates in, and disrupts the function of, the kidney. Congenital dysfibrinogenmia is the commonest of these three disorders. Some 100 different genetic mutations occurring in more than 400 families have been found to cause it. All of these mutations as well as those causing hereditary fibrinogen Aα-Chain amyloidosis exhibit partial penetrance, i.e. only some family members with one of these mutant genes develop dysfibrinogenemia-related symptoms. While both of these congenital disorders as well as acquired dysfibrinogenemia are considered very rare, it is estimated that ~0.8% of individuals with venous thrombosis have either a congenital or acquired dysfibrinogenemia. Hence, the dysfibrinogenemia disorders may be highly under-diagnosed conditions due to isolated thrombotic events that are not appreciated as reflecting an underlying fibrinogen disorder. Congenital dysfibrinogenemia is distinguished from a similar inherited disorder, congenital hypodysfibrinogenemia. Both disorders involve the circulation of dysfunctional fibrinogen but in congenital hypodysfibrinogenemia plasma fibrinogen levels are low while in congenital dysfibrinogenemia they are normal. Furthermore, the two disorders involve different gene mutations and inheritance patterns as well as somewhat different symptoms. Fibrinogen is a glycoprotein made and secreted into the blood primarily by liver hepatocyte cells. Endothelium cells are also make what appears to be small amounts of fibrinogen but this fibrinogen has not been fully characterized; blood platelets and their precursors, bone marrow megakaryocytes, although once thought to make fibrinogen, are now known to take up and store but not make the glycoprotein. The final secreted, hepatocyte-derived glycoprotein is made of two trimers each of which is composed of three polypeptide chains, Aα (also termed α) encoded by the FGA gene, Bβ (also termed β) encoded by the FGB gene, and γ encoded by the FGG gene. All three genes are located on the long (i.e. 'p') arm of human chromosome 4 (at positions 4q31.3, 4q31.3, and 4q32.1, respectively) and may contain mutations that are the cause of congenital dysfibrinogenemia. The heximer is assembled as a protein in the endoplasmic reticulum of hepatocytes and then transferred to the Golgi where Polysaccharides (i.e. complex sugars) and sialic acid are added by respective glycosylation and sialylation enzyme pathways thereby converting the heximer to a functional fibrinogen glycoprotein. The final circulating glycoprotein (notated as (AαBβγ)2, (αβγ)2, Aα2Bβ2γ2, or α2β2γ2) is arranged as a long flexible rod with nodules at both ends termed D domains and central nodule termed the E domain. The normal process of blood clot formation involves the coordinated operation of two separate pathways that feed into a final common pathway: 1) primary hemostasis, i.e. the adhesion, activation, and aggregation of circulating blood platelets at sites of vascular injury and 2) secondary hemostasis, i.e. cleavage of the Aα and Bβ chains of fibrinogen by thrombin to form individual fibrin strands plus the respective fibrinopeptides A and B formed from this cleavage. In the final common pathway fibrin is cross-linked by activated clotting factor XIII (termed factor XIIIa) to form mature gel-like fibrin clots. Subsequent fibrinolysis pathways act to limit clot formation and dissolve clots no longer needed. Fibrinogen and its Aα fibrin chain have several functions in this process: Based on these fibrinogen functions, a fibrinogen mutation may act either to inhibit or promote blood clot formation and/or lysis to thereby produce in individuals a diathesis to develop pathological bleeding, thrombosis, or both conditions. Many cases of congenital dysfibrinogenemia are asymptomatic. Since manifestations of the disorder generally occur in early adulthood or middle-age, younger individuals with a gene mutation causing it may not have had time to develop symptoms while previously asymptomatic individuals of advanced age with such a mutation are unlikely to develop symptoms. Bleeding episodes in most cases of this disorder are mild and commonly involve easy bruising and menorrhagia. Less common manifestations of bleeding may be severe or even life-threatening; these include excessive bleeding after tooth extraction, surgery, vaginal birth, and miscarriage. Rarely, these individuals may suffer hemarthrosis or cerebral hemorrhage. In one study of 37 individuals >50 years old afflicted with this disorder, 19% had a history of thrombosis. Thrombotic complications occur in both arteries and veins and include transient ischemic attack, ischemic stroke, myocardial infarction, retinal artery thrombosis, peripheral artery thrombosis, and deep vein thrombosis. In one series of 33 individuals with a history of thrombosis due to congenital dysfibrinogenemia, five developed chronic pulmonary hypertension due to ongoing pulmonary embolism probably stemming form deep vein thrombosis. About 26% of individuals with the disorder suffer both bleeding and thrombosis complications. Congenital dysfibrinogenemia is most often caused by a single autosomal dominant missense mutation in the Aα, Bβ, or γ gene; rarely, it is caused by a homozygous or compound heterozygous missense mutation, a deletion, frameshift mutation, insert mutation, or splice site mutation in one of these genes. The most frequent sites for these mutations code for the N-terminus of the Aα chain or the C-terminus of the γ chain that lead to defective assembly of fibrin in early clot formation and thereby a bleeding predisposition. Two particular missense mutations represent the majority (74% in one study of 101 individuals) of all mutations associated with dysfibrinogenemia and therefore represent prime sites to examine in the initial testing of individuals having a congenital dysfibrinogenmia bleeding disorder. These mutations alter the codon coded for the amino acid arginine at either the 35th position of FGA (termed Arg35; see fibrinogen Metz1 and fibrinogen Bicetre in the Table below) and or the 301st position of FGG (termed Arg301; see fibrinogen Baltimore IV in the Table below). The following Table lists examples of mutations causing congenital dysfibrinogenemias. It gives: a) the mutated protein's trivial name; b) the gene mutated (i.e. FGA, FGB, or FGG), its mutation site (i.e. numbered nucleotide in the cloned gene), and the names of the nucleotides (i.e. C, T, A, G) at these sites before>after the mutation; c) the altered fibrinogen peptide (Aα, Bβ, or λ) and the amino acids (using standard abbreviations) found in the normal-mutated circulating fibrinogen; d) the cause of the mutated fibrinogen's misfunction(s); e) the clinical consequence(s) of the mutation; and f) comments. Unless noted as a deletion (del), frame shift (fs), or homozygous mutation, all mutations are heterozygous, missense mutations.