A Novel 3-Base Pair 1834–1836 AAG-Deletion (Lys570Del) Mutation In Factor XIII A Subunit Associated with Factor XIII Deficiency

Abstract 1412

Factor XIII is a transglutaminase that cross-links proteins in plasma, vascular matrix, endothelial cells, platelets and monocytes, and plays a role in atherosclerosis, wound healing, and inflammation. Plasma FXIII molecule is a hetero-tetramer consisting of two catalytic A-subunits and two B-subunits that act as carrier molecules. The gene encoding FXIII A subunit comprises of 15 exons spanning 160 kb and the mature protein contains 731 amino acids. FXIII deficiency is a rare autosomal recessive disorder affecting ∼1 in 1–3 million people. It is characterized by bleeding, impaired wound repair and spontaneous abortions. We report studies from a family where two children son (13 yrs) and daughter (11 yrs) have had a lifelong bleeding tendency and spontaneous intracranial hemorrhages. Both parents were asymptomatic and there was no consanguinity. The results of routine laboratory tests, prothrombin time and activated partial thromboplastin time were normal in all subjects. The plasma FXIII activity by a commercially available chromogenic assay was 5% in the son and <3% in the daughter (normal range 57–192%). The FXIII activity in the father and mother were 198% and 74%, respectively. We have identified a novel deletion mutation, which has not been reported so far in FXIII deficiency. Leukocyte RNA was isolated from the buffy-coat and cDNA was obtained by reverse-transcription PCR using SuperScript First-Strand Synthesis System. The amplified products were cloned in pGEM-T vector (Promega) and sequenced on an automated gene-sequencer. Both children and the father have a novel 3 bp AAG-deletion position 1834–1836 nt in FXIII A chain. This mutation causes a lysine 570 deletion in the ß-barrel 1 of Factor XIII A subunit and has not been reported so far. It may lead to protein misfolding resulting in an unstable protein, and low levels of FXIII. The second major change detected in the two siblings was a A/T substitution at position 737 nt causing Tyr204Phe substitution in the two siblings; this was present in the mother in a heterozygous condition. This mutation has been previously reported in FXIII deficiency and linked to increased risk of haemorrhagic stroke in young women and of miscarriages. The compound heterozygosity for Lys570Del and Tyr204Phe substitution observed in both children is the likely cause of Factor XIII deficiency leading to lifelong bleeding condition. In addition to above, the father had Val34Leu polymorphism, previously reported to be associated with resistance to myocardial infarction. This polymorphism is present in ∼20% of white European, 40% of Pima Native American and 13% of South Asian populations. The mother also had a known A/C polymorphism at 1119 nt position for a synonymous Pro332Pro change. We also found 3 other variations in FXIII A chain in this family. The daughter has Glu216Gly and Asp267Asn change in the protein corresponding to alterations at nucleotide 773 (A/G) and 925 (A/G), respectively. The son and mother had a substitution at 1442 nt (T/C) leading to a Leu439Pro change. These variations, Glu216Gly, Asp267Asn and Leu439Pro found in the two children (Leu439Pro also in mother) are present in the catalytic core domain of the Factor XIII A chain. All of the polymorphisms or mutations reported in this study were heterozygous in the studied subjects. FXIII gene mutations and polymorphisms result in a high level of heterogeneity of disease presentation. Other point mutations in the FXIII A catalytic core as well as mutations in ß-barrel 1 region have been described in association with a hemorrhagic state in FXIII deficiency. Our study documents a new 3-bp 1834–1836 nt AAG-deletion (Lys570Del) in association with FXIII deficiency. We suggest that compound heterozygosity for Lys570Del and Tyr204Phe is the cause of FXIII deficiency in our patients. Further structure-function studies will aid in understanding the impact of these amino acid substitutions or deletions on FXIII function and on the associated bleeding diathesis.