Hepatic fibrinogen storage disease: identification of two novel mutations (p.Asp316ASn, fibrinogen Pisa and p.Gly366Ser, fibrinogen Beograd) impacting on the fibrinogen gamma-module

Summary. Background: Quantitative fibrinogen deficiencies (hypofibrinogenemia and afibrinogenemia) are rare congenital disorders characterized by low/unmeasurableplasma fibrinogen antigen levels. Their genetic basis is invariably represented by mutations within the fibrinogengenes (FGA, FGB and FGG coding for the Aa, Bb and c chains). Currently, only four mutations (p.Gly284Arg, p.Arg375Trp, delGVYYQ 346-350, p.Thr314Pro), all affecting the fibrinogen c chain, have been reported to cause fibrinogen storage disease (FSD), a disorder characterizedby protein aggregation, endoplasmic reticulumretention and hypofibrinogenemia. Objectives: To investigate the genetic basis of FSD in two hypofibrinogenemic patients. Methods: The mutational screening of the fibrinogen genes was performed by direct DNA sequencing. The impact of identified mutations on fibrinogen structure was investigated by in-silico molecular modeling. Liver histology was evaluated by light microscopy, electron microscopy and immunocytochemistry. Results: Here, we describe two hypofibrinogenemic children with persistent abnormal liver function parameters. Direct sequencing of the coding portion of fibrinogen genes disclosed two novel FGG missense variants (p.Asp316Asn, fibrinogen Pisa; p.Gly366Ser, fibrinogen Beograd), both present in the heterozygous state and affecting residues located in the fibrinogen C-terminal c-module. Liver sections derived from biopsies of the two patients were examined by immunocytochemical analyses, revealing hepatocytecytoplasmic inclusions immunoreactive to anti-fibrinogen antibodies. Conclusions: Our work strongly confirms the clustering of mutations causing FSD in the fibrinogen chain between residues 284 and 375. Based on an in-depth structural analysis of all FSD-causing mutations and on their resemblance to mutations leading to serpinopathies, we also comment on a possible mechanism explaining fibrinogen polymerization within hepatocytes.