Five Novel Mutations in F13B Gene Resulting in FXIII Deficiency.
Authors: Ivaskevicius Vytautas , Rott Hannelore , Scharrer Inge , Krause Manuela , Seifried Erhard , Oldenburg Johannes 
Published: April 2020

Abstract
Introduction. FXIII deficiency is a rare autosomal recessive disorder affecting approximatly 1 out of 1–3 million inhabitans. The disease is characterized by bleeding, impaired wound repair and spontaneous abortions in females. Extracellular FXIII molecule has a tetramer structure composing of two catalytic A-subunits and two B-subunits that act as a carrier molecules. Based on genotype there are two types of FXIII deficiency: A-subunit deficiency (XIIIA) when mutations affect F13A gene and much rarely B-subunit deficiency (XIIIB) when mutations affect F13B gene. Both types result in absence of FXIII catalytical activity in plasma. To date, only five families with isolated B-subunit deficiency have been described in the literature. Here we report five novel mutations, affecting F13B gene and resulting in FXIII deficiency.
Materials and Methods. FXIII activity was measured by photometric assay according to Fickenscher et al. (1991). Fiveteen exons of F13A gene as well as 12 exons of F13B gene and their flanking regions were amplified by PCR. Sequencing was performed on an automated sequencing system.
Results and Discussion. Five german origin patients with mildly reduced FXIII deficiency have been routinely investigated in order to identify genetic defects within F13A and F13B genes. All patients have shown single heterozygous mutations in F13B gene, revealing two missense, one deletion, one insertion, and one splice site mutation. There were no genetic defects in F13A gene. Patient E.M. (female, born 1947, FXIII activity 47–53%, postoperative bleeding) had a missense mutation in exon 2 (c.73 T&gt;C, Cys5Arg) resulting in disruption of disulfide bond between amino acids Cys5 and Cys56. Patient H.D. (male, born 1983, FXIII activity 54%, epistaxis) showed a missense mutation in exon 3 (c.406C&gt;T, Leu116Phe). It may cause instability of disulfide bond between neighbouring Cys117 and Cys71 in the second domain. Patient E.R. (female, born 1989, FXIII activity 53%, increased bleeding during menstruation) was heterozygous for an in-frame deletion (c.471–473delATT) resulting in deletion of Leu138. Patient O.S. (female, born 1983, FXIII activity 46–57%, bleeding after tonsillectomy) had a small insertion (c.1959insT) in the exon 12 changing the distal amino acid (634–641) sequence and predisposing a synthesis of additional 21 new amino acids at the Carboxy-termini of FXIIIB-Subunit due to disruption of Stop codon at position 641. Patient P.G. (male, born 1974, FXIII activity 43%, bleeding after tooth extraction) was heterozygous for a splice site mutation in intron 3 (IVS3-1 G&gt;C). Interestingly this patient turned out also to have a mild von Willebrand Syndrome (FVIII activity 76%, vWF:RiCo 48%, vWF:Ag 56%, vWF-Collagen-Binding activity 0.54, AB0 blood group A).
In conclusion,
our results suggest higher prevalence of FXIIIB deficiency as it was thought before, Patients having mild (heterozygous) FXIIIB deficiency may bleed upon provocations.