ABSTRACT
BACKGROUND: Factor XII (FXII) deficiency is an interesting condition that causes prolonged activated partial thromboplastin time without bleeding diathesis. FXII may be not important in hemostasis, but still plays roles in thrombosis and inflammation. In order to raise clinical awareness about this condition, we studied patients with severe FXII deficiency and their relatives. METHODS: Consecutive severely FXII deficient patients presenting from 1995 to 2020 were recruited from two medical centers in Taiwan. Index patients and their families were tested for FXII function, antigen and F12 gene. F12 variants were constructed into the pIRES-hrGFP vector and expressed on human embryonic kidney cells (HEK293T). FXII antigen and activity were analyzed. RESULTS: We found five severely FXII deficient patients, three women and two men, aged 44-71 years. FXII antigen results ranged from undetectable to 43.7%. Three different mutations were identified: c.1681C>A (p.Gly542Ser), c.1561G>A (p.Glu502Lys), and a novel mutation c.1556T>A (p.Leu500Gln). HEK293T cells expressed consistently low FXII activity with all mutations. FXII antigen expression was similar to the wild type in c.1681C>A (p.Gly542Ser), but reduced in c.1556T>A (p.Leu500Gln) and c.1561G>A (p.Glu502Lys). CONCLUSIONS: We report five unrelated patients with severe FXII deficiency, one of whom carried a novel, cross-reacting material negative mutation c.1556T>A (p.Leu500Gln).