Portal de Periódicos da CAPES

To aggregate or not to aggregate…

2007; Elsevier BV; Volume: 5; Issue: 10 Linguagem: Inglês

10.1111/j.1538-7836.2007.02608.x

1538-7933

Marguerite Neerman‐Arbez,

Erythrocyte Function and Pathophysiology

Congenital fibrinogen deficiencies (i.e. afibrinogenemia and hypofibrinogenemia) are caused by mutations in one of the three fibrinogen-encoding genes (i.e. FGB, FGA and FGG on chromosome 4). Causative mutations can be divided into two main classes: null mutations with no protein production at all, and mutations producing abnormal protein chains that are retained inside the cell, because of impaired hexamer assembly or secretion. In the majority of these patients with either complete absence of fibrinogen in the circulation (afibrinogenemia) or reduced fibrinogen levels (hypofibrinogenemia) there is no evidence of intracellular accumulation of the mutant fibrinogen chain. This implies the existence of an efficient degradation pathway for fibrinogen mutants that allow individual chain synthesis and assembly but not secretion. Before the study by Dib et al. [1] in this issue, only two fibrinogen gene mutations were known to cause hepatic storage disease. These are two missense mutations in FGG that cause fibrinogen deficiency in the heterozygous state because of the absence of the mutant γ chain in patient plasma, but also progressive liver disease associated with hepatocellular cytoplasmic inclusions [2, 3]. The fibrinogen Brescia mutation (FGG p.Gly284Arg, numbered without the signal peptide as in the article by Dib et al.) was the first to be identified in heterozygosity in a patient suffering from liver cirrhosis. The globular inclusions in the liver of the proband corresponded to dilated rough ER cisternae filled with densely packed tubular structures. Intracisternal material selectively and exclusively reacted with antifibrinogen antibodies [2]. The second case of hypofibrinogenemia associated with hepatic inclusion bodies with fibrinogen aggregates was reported in a young girl, heterozygous for a FGG p.Arg375Trp mutation (fibrinogen Aguadilla). The patient was asymptomatic and only presented chronically elevated liver function test results [3]. We identified the same mutation in a 61-year-old Swiss man with progressive liver disease [4]. Examination of the liver biopsy revealed chronic hepatitis complicated by cirrhosis and weakly eosinophilic globular cytoplasmic inclusions within the hepatocytes. Both the proposita and his two sons showed low functional and antigenic fibrinogen concentrations; all three were heterozygous for the Aguadilla mutation. A liver biopsy performed on the elder son demonstrated the same globular cytoplasmic inclusions, albeit without associated chronic liver disease. This mutation was also identified in heterozygosity in a young boy with liver storage disease and hypofibrinogenemia [5]. In this issue Dib et al. [1] describe the identification of a third mutation in FGG (Fibrinogen Angers), which causes hypofibrinogenemia and liver disease. The patient is a woman with chronic abnormal liver function tests despite cessation of alcohol abuse. Three other family members (i.e. the proposita's mother, brother and daughter) have hypofibrinogenemia. Here the mutation, identified in heterozygosity in the proposita and her brother (the other family members were not screened for the mutation) is a 14-bp deletion at the very end of FGG exon 8, thus creating a new exon 8-intron 8 junction and donor splice site. Usage of this splice-site was predicted to result in an aberrant mRNA with an in-frame deletion of five amino acids (γ346Val-Tyr-Tyr-Glu-Gly350). This mRNA product was confirmed by RT-PCR on a liver biopsy obtained from the proposita's brother (no RNA was available from the original liver sample from the patient). The five amino acids missing in fibrinogen Angers are located in the highly conserved γD domain, more precisely in the 'a' binding pocket or 'a' polymerization site (defined by residues 337–379), which is crucial for fibrin assembly. Indeed several mutations in this region have been identified in patients with dysfibrinogenemia (i.e. patients with normal levels of dysfunctional fibrinogen in circulation). Fibrin polymerization occurs after secretion of the fibrinogen hexamer into the circulation and cleavage of fibrinopeptides A and B by thrombin; one can understand how mutations in this fibrin polymerization site cause dysfibrinogenemia. In contrast, the molecular mechanism by which the fibrinogen Angers mutation, but also the fibrinogen Brescia and Aguadilla mutations, leads to impaired secretion, retention in the ER and formation of aggregates is not known. All three mutations reside in the globular γD domain: the residue mutated in fibrinogen Aguadilla (Arg375) is also localized in the 'a' polymerization site, while the residue mutated in fibrinogen Brescia (Gly284) is localized in the 5-stranded beta sheet of γD. In a simple model, the positions of these three mutations could delimit a region in the γD domain that, if mutated, would cause retention in the cell, but would escape degradation, which is the fate for most mutant fibrinogen molecules. However, this is not the case (we do not live in a simple world!) because, as previously mentioned, there are dysfibrinogenemia mutations occurring in this region that are efficiently assembled and secreted into the circulation, but also hypofibrinogenemia mutations in this region that impair assembly or secretion but do not lead to ER inclusion bodies. The identification of the fibrinogen Angers mutation adds a new member to the intriguing group of mutations that lead to fibrinogen deficiency and liver disease because of aggregation in the ER. Many questions regarding the underlying molecular mechanism remain unanswered. Why do these mutants aggregate while molecules with mutations in neighboring residues do not? Why is it that all individuals with these mutations have hypofibrinogenemia but not all have liver disease because of ER inclusion bodies? Is there a genetic basis for this heterogeneity similar to alpha-1-antitrypsin deficiency? I look forward to reading (or discovering) the answers to these questions in the years to come. The author states that she has no conflict of interest.