We have identified 2 PROS1 missense mutations in the exon that encodes the
vitamin K-dependent Gla domain of
protein S (Gly11Asp and Thr37Met) in kindred with phenotypic
protein S deficiency and
thrombosis. In studies using
recombinant proteins, substitution of Gly11Asp did not affect production of
protein S but resulted in 15.2-fold reduced
protein S activity in
a factor Va inactivation assay. Substitution of Thr37Met reduced expression by 33.2% (P <.001) and activity by 3.6-fold. The Gly11Asp variant had 5.4-fold reduced affinity for anionic
phospholipid vesicles (P <.0001) and decreased affinity for an antibody specific for the Ca(2+)-dependent conformation of the
protein S Gla domain (HPS21). Examination of a molecular model suggested that this could be due to repositioning of Gla29. In contrast, the Thr37Met variant had only a modest 1.5-fold (P <.001), reduced affinities for
phospholipid and HPS21. This mutation seems to disrupt the aromatic stack region. The proposita was a compound heterozygote with free
protein S antigen levels just below the lower limit of the normal range, and this is now attributed to the partial expression defect of the Thr37Met mutation. The activity levels were strongly reduced to 15% of normal, probably reflecting the functional deficit of both
protein S variants. Her son (who was heterozygous only for Thr37Met) had borderline levels of
protein S antigen and activity, reflecting the partial secretion and functional defect associated with this mutation. This first characterization of natural
protein S Gla-domain variants highlights the importance of the high affinity
protein S-
phospholipid interaction for its
anticoagulant role.