Combinations of 4 mutations (FV R506Q, FVH1299R, FVY1702C, PT 20210G/A) affecting the prothrombinase complex in a thrombophilic family

The study of the molecular bases of thrombophilia in a large family with 4 symptomatic members is reported. Three thrombophilic genetic components (FV R506Q, FV H1299R, and PT 20210G/A), all affecting the activity of the prot hrombinase complex, were detected alone and in combination in various famil y members. In addition, a newly identified missense mutation (factor V [FV] Y1702C), causing FV deficiency, was also present in the family and appeare d to enhance activated protein C (APC) resistance in carriers of FV R506Q o r FV H1299R by abolishing the expression of the counterpart FV allele. The relationships between complex genotypes, coagulation laboratory findings, a nd clinical phenotypes were analyzed in the family. All symptomatic family members were carriers of combined defects and showed APC resistance and ele vated F1 + 2 values. Evidence for the causative role of the FV Y1702C mutat ion, which affects a residue absolutely conserved in all 3 A domains of FV, factor VIII, and ceruloplasmin, relies on (I) the absolute cosegregation b etween the mutation and FV deficiency, both in the family and in the genera l population; (2) FV antigen and immunoblot studies indicating the absence of Y1702C FV molecules in plasma of carriers of the mutation, despite norma l levels of the FV Y1702C messenger RNA; and (3) molecular modeling data th at support a crucial role of the mutated residue in the A domain structure. These findings help to interpret the variable penetrance of thrombosis in thrombophilic families and to define the molecular bases of FV deficiency. (Blood. 2000;96:1443-1448) (C) 2000 The American Society of Hematology.