A 29-year-old female patient with heterozygous congenital
protein S deficiency suffering from thrombotic disease had normal levels of both total and free
protein S antigen (70% and 65%, respectively), but low cofactor activity (31%) for activated
protein C, indicating that she had a variant of
protein S,
protein S Tokushima. Western blotting using the polyclonal anti-
protein S antibody showed that approximately half of the patient's
protein S appeared to be the variant with a higher molecular weight than normal
protein S. The partially purified variant
protein S bound neither to the
monoclonal antibody recognizing
calcium-dependent conformation of
protein S nor to the antibody recognizing the
thrombin-sensitive domain of
protein S. Among the exons from II to XV of the patient's
protein S gene encoding from the NH2-terminal end to the COOH-terminal end of
protein S, only one missense mutation (A to G) was found in exon VI of the
protein S alpha-gene, which results in amino acid substitution of Glu(GAG) for Lys-155(AAG) in the second
epidermal growth factor-like domain of
protein S. The
recombinant protein S Tokushima expressed in BHK cells had a slightly higher molecular weight than the recombinant normal one, did not bind to the antibody specific for the
thrombin-sensitive domain, and did not show the cofactor activity. These findings suggest that the
protein S Tokushima molecule is structurally and functionally a variant of
protein S, and that this variant
protein S is the cause of severe
thrombosis in this patient.