A mutation of the active protein S gene leading to an EGF1-lacking protein in a family with qualitative (type II) deficiency

The genomic analysis of a 70-year-old man with recurrent deep venous thrombosis having a protein S (PS)-deficient phenotype corresponding to both type III and type II evidenced two different mutations: a +5 g→a mutation in the donor splice site of intron e (ivs e) and a ser 460 to Pro mutation. The propositus' son, who had a type II PS deficiency phenotype, only bore the ivs e +5 g→a mutation. The study of platelet PS mRNA prepared from this subject showed that the ivs e, +5 g→a mutation led to the generation of two abnormal transcripts, one lacking exon 5 and the other lacking exons 5 and 6. The presence of an additional PS band with a decreased molecular mass on immunoblots performed in reducing conditions suggested the presence of truncated PS lacking EGF1 (encoded by exon 5). Two monoclonal antibodies (MoAbs) were used to further characterize the nonfunctional plasma PS. Comparison of PS levels measured with each of these MoAbs and PS levels in conventional assays was consistent with the presence of an abnormal inactive protein in the plasma of both patients bearing the ivs e, +5 g→a mutation, suggesting that variant PS lacking EGF1 is secreted but is devoid of activated protein C cofactor activity.