Page 56 - Haematologica May 2022

Ferrata Storti Foundation
Haematologica 2022 Volume 107(5):1064-1071
A homozygous duplication of the FGG exon 8-intron 8 junction causes congenital afibrinogenemia. Lessons learned from
the study of a large consanguineous Turkish family
Michel Guipponi,1,2 Frédéric Masclaux,1 Frédérique Sloan-Béna,1 Corinne Di Sanza,2 Namik Özbek,3 Flora Peyvandi,4,5 Marzia Menegatti,4 Alessandro Casini,6 Baris Malbora7 and Marguerite Neerman-Arbez2
1Medical Genetics Service, University Hospitals of Geneva, Geneva, Switzerland; 2Department of Genetic Medicine and Development, Faculty of Medicine, University of Geneva, Geneva, Switzerland; 3Department of Pediatric Hematology, Ankara City Hospital, Ankara, Turkey; 4Fondazione IRCCS Ca’ Granda Ospedale Maggiore Policlinico, Angelo Bianchi Bonomi Hemophilia and Thrombosis Center and Fondazione Luigi Villa, Milan, Italy; 5Università degli Studi di Milano, Department of Pathophysiology and Transplantation, Milan, Italy; 6Division of Angiology and Hemostasis, University Hospitals of Geneva, Geneva, Switzerland and 7Department of Pediatric Hematology and Oncology, Istanbul Yeni Yuzyil University, Istanbul, Turkey
ABSTRACT
Congenital afibrinogenemia is the most severe congenital fibrino- gen disorder, characterized by undetectable fibrinogen in circula- tion. Causative mutations can be divided into two main classes: null mutations with no protein production at all and missense mutations producing abnormal protein chains that are retained inside the cell. The vast majority of cases are due to single base pair mutations or small inser- tions or deletions in the coding regions or intron-exon junctions of FGB, FGA and FGG. Only a few large rearrangements have been described, all deletions involving FGA. Here we report the characterization of a 403 bp duplication of the FGG exon 8-intron 8 junction accounting for congen- ital afibrinogenemia in a large consanguineous family from Turkey. This mutation, which had escaped detection by Sanger sequencing of short polymerase chain reaction (PCR) amplicons of coding sequences and splice sites, was identified by studying multiple alignments of reads obtained from whole exome sequencing of a heterozygous individual followed by PCR amplification and sequencing of a larger portion of FGG. Because the mutation duplicates the donor splice site of intron 8, we predicted that the impact of the mutation would be on FGG tran- script splicing. Analysis of mRNA produced by cells transiently transfect- ed with normal or mutant minigene constructs showed that the duplica- tion causes production of several aberrant FGG transcripts generating premature truncating codons.
Introduction
The ultimate goal of the coagulation cascade is the controlled conversion by thrombin of fibrinogen into fibrin which forms a polymer to give stability, strength and adhesive surfaces to growing blood clots containing platelets and red blood cells. Human fibrinogen1,2 is produced in the liver from three homologous polypeptide chains, Bβ, Aa and g encoded by the fibrinogen gene cluster FGB, FGA and FGG, on human chromosome 4. Two copies of each polypeptide chain assemble to form a hexamer (AaBβg)2 held together by disulphide bonds. Alternative spliced transcripts are produced for FGA and FGG, these are AaE and g’ respectively. While AaE chains are present in only 1-2% of circulat- ing fibrinogen, g’ chains are present in 8-15% of circulating fibrinogen, in het-
Coagulation & its Disorders
Correspondence:
MARGUERITE NEERMAN-ARBEZ
Marguerite.Neerman-Arbez@unige.ch
Received: April 9, 2021. Accepted: June 21, 2021. Pre-published: July 1, 2021.
https://doi.org/10.3324/haematol.2021.278945 ©2022 Ferrata Storti Foundation
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