Hereditary coagulation factor V deficiency is a rare bleeding disorder characterized by extremely low levels of functional and immunoreactive factor V in plasma associated with moderate-to-severe bleeding symptoms. The genetic bases of factor V deficiency have been characterized only in a limited number of cases and the majority of causative mutations are truncating mutations providing only limited information about the function of subdomains and of individual residues. Here, we present the first report on a Korean family with inherited factor V deficiency. The proband is a 25-year-old man who showed normal coagulation factor levels except those for factor V antigen and activity (3 and 4%, respectively), and was only suffering from bleeding after tooth extraction. Direct sequencing of the factor V gene disclosed the already known nonsense mutation (R1133X) and a novel in-frame 6-bp deletion (6116-6121 delGAACAG, corresponding to the amino-acid deletion N1982-S1983) located in the factor V C1 domain; both mutations were found in the heterozygous state. The structural and functional importance of the in-frame deletion was examined by constructing a molecular model based on the crystal structure of bovine activated factor V that has been inactivated by activated protein C. N1982 and S1983 are located on a loop that is exposed on the surface of the C1 domain, and are in close contact with another loop belonging to the A3 domain. Even though the detailed mechanism of the association of the in-frame deletion of our patient and factor V deficiency needs further investigation, this model suggests the possibility that the N1982-S1983 deletion could destabilize the C1 -A3 interaction by preventing the potential formation of hydrogen bonds between K1980 and N1986 of the C1 domain with D1604 of the A3 domain. Also, because N1982 is strongly expected to be N-glycosylated judging from its structural homology to factor VIII, loss of this residue can influence proper folding of factor V, resulting in unstable structure, which is vulnerable to intracellular degradation.
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