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Abstract Background Antithrombin (AT) is an important anticoagulant in hemostasis. We describe here the characterization of a novel AT mutation associated with clinically relevant thrombosis. A pair of sisters with confirmed type I AT protein deficiency was genetically analyzed on suspicion of an inherited SERPINC1 mutation. A frameshift mutation, c.1247dupC, was identified and the effect of this mutation was examined on the cellular and molecular level. Methods Plasmids for the expression of wild-type (WT) and mutated SERPINC1 coding sequence (CDS) fused to green fluorescent protein (GFP) or hemagglutinin (HA) tag were transfected into HEK293T cells. Subcellular localization and secretion of the respective fusion proteins were analyzed by confocal laser scanning microscopy and Western blot. Results The c.1247dupC mutation results in a frameshift in the CDS of the SERPINC1 gene and a subsequently altered amino acid sequence (p.Ser417LysfsTer48). This alteration affects the C-terminus of the AT antigen and results in impaired secretion as confirmed by GFP- and HA-tagged mutant AT analyzed in HEK293T cells. Conclusion The p.Ser417LysfsTer48 mutation leads to impaired secretion, thus resulting in a quantitative AT deficiency. This is in line with the type I AT deficiency observed in the patients.

Ana Marco-Rico,1,2 Pascual Marco-Vera2,3 1Hemostasis and Thrombosis Department, Hematology Service, University General Hospital Dr. Balmis, Alicante, Spain; 2Biomedical Research Institute (ISABIAL), Alicante, Spain; 3Clinical Medicine Department, Miguel Hernández University, Alicante, SpainCorrespondence: Ana Marco-Rico, Hemostasis and Thrombosis Department, Hematology Service, University General Hospital Dr. Balmis, Pintor Baeza Avenue, 12, Alicante, 03010, Spain, Tel +34 965913863, Fax +34 965913869, Email marco_anaric@gva.esAbstract: Congenital antithrombin (AT) deficiency represents the form of thrombophilia with the highest thrombotic risk. It is characterized by a heterogeneous clinical presentation, depending mostly on the family history of thrombosis and type of genetic mutation. Inherited AT deficiency promotes idiopathic thrombosis at an early age (even in the pediatric population) and at atypical sites. Therefore, a positive family background necessitates ruling out this high-risk thrombophilia at a young age. Studying first-degree relatives, even if they are asymptomatic, is essential to establish thromboprophylaxis and a proper therapeutic approach in case of thrombosis. Patients with congenital AT deficiency require indefinite anticoagulation owing to the high thrombotic recurrence rate. Here, we present four unrelated cases reported in our institution who were diagnosed with hereditary AT deficiency, with a contrasting clinical evolution.Keywords: hereditary antithrombin deficiency, thrombosis, anticoagulation, heparin resistance, SERPINC1 gene mutation