A Mutation of the Active Protein S Gene Leading to an EGF1-Lacking Protein in a Family With Qualitative (Type II) Deficiency

Venenastverschluss und Abduzensparese bei Protein-S-Mangel

BACKGROUND

Protein S deficiency, which exists in 0.7% of the population, is a risk factor for retinal vein branch occlusions and is inherited in an autosomal dominant manner.

METHODS

A genealogical study was carried out on three generations of one family who exhibited different venous occlusions and subsequent complications.

RESULTS

Four members of the family, spanning three generations, suffered from complications of venous thrombosis. In the first generation a great uncle died of complications from a deep leg venous thrombosis. In the second generation, the mother underwent a venous branch thrombosis at the age of 41 with a protein S activity of 18%. Subsequently, a palsy of the N. abducens developed with multiple cerebral lesions (presumably post-thrombotic) in the MRI. Fluorescein angiography showed a typical picture of a venous branch occlusion which had been treated by laser. In the third generation, the 16-year-old daughter developed iliac venous thrombosis and a pulmonary embolism with a protein S activity of 0%. The fluorescein angiography showed distinctively engorged veins. A 28-year-old daughter, with a protein S activity of 16%, remained asymptomatic, although fluorescein angiography demonstrated engorged veins. Protein C activity and APC resistance of all family members were normal. The chromosomal analysis of the family members revealed no morphological aberrations.

CONCLUSION

Protein S deficiency increases the risk of congenital thrombosis in young and middle-aged heterozygous individuals.

Molecular diversity and thrombotic risk in protein S deficiency: The PROSIT study

The Protein S Italian Team (PROSIT) enrolled 79 protein S (PS) deficient families and found 38 PROS1 variations (19 novel) in 53 probands. Of these, 23 variants were selected for expression in'vitro, to evaluate their role as possible causative variants. Transient expression showed high secretion levels (>75%) for three variants, which were considered neutral. Seven missense and five nonsense variants showed low (<or=11%) expression levels and were classified as severe defects. Intermediate expression was observed for eight variants, which were evaluated by factor Va inactivation assay in order to be globally classified as severe or intermediate. Based on the cumulative data, the hazard ratio associated with causative variants was 4.9 (95% CI: 1.4-17.7) for deep vein thrombosis and/or pulmonary embolism, 5.1 (95% CI: 1.1-23.9) for superficial thrombophlebitis, and 4.8 (95% CI: 1.8-13.0) for any venous thrombosis. The hazard ratio for deep vein thrombosis and/or pulmonary embolism in carriers of severe defects only was 7.4 (95% CI: 1.6-24.1). PROSIT showed that dysfunctional variants causing PS deficiency are more common than expected and confirmed that PS deficiency is associated with increased thrombotic risk, although risk assessment is complicated by molecular heterogeneity.

Coagulation, inflammation, and apoptosis: different roles for protein S and the protein S-C4b binding protein complex

Protein S (PS) has an established role as an important cofactor to activated protein C (APC) in the degradation of coagulation cofactors Va and VIIIa. This anticoagulant role is evident from the consequences of its deficiency, when there is an increased risk of venous thromboembolism. In human plasma, PS circulates approximately 40% as free PS (FPS) and 60% in complex with C4b-binding protein (C4BP). Formation of this complex results in loss of PS cofactor function, and C4BP can then modulate the anticoagulant activity of APC. It had long been predicted that the complex could act as a bridge between coagulation and inflammation due to the involvement of C4BP in regulating complement activation. This prediction was recently supported by the demonstration of binding of the PS-C4BP complex to apoptotic cells. This review aims to summarize recent findings on the structure and functions of PS, the basis and importance of its deficiency, its interaction with C4BP, and the possible physiologic and pathologic importance of the PS-C4BP interaction.

The APC-independent anticoagulant activity of protein S in plasma is decreased by elevated prothrombin levels due to the prothrombin G20210A mutation

Protein S exhibits anticoagulant activity independent of activated protein C (APC). An automated factor Xa-based one-stage clotting assay was developed that enables quantification of the APC-independent activity of protein S in plasma from the ratio of clotting times (protein S ratio [pSR]) determined in the absence and presence of neutralizing antibodies against protein S. The pSR was 1.62 +/- 0.16 (mean +/- SD) in a healthy population (n = 60), independent of plasma levels of factors V, VIII, IX, and X; protein C; and antithrombin, and not affected by the presence of factor V Leiden. The pSR strongly correlates with the plasma level of protein S and is modulated by the plasma prothrombin concentration. In a group of 16 heterozygous protein S-deficient patients, the observed mean pSR (1.31 +/- 0.09) was significantly lower than the mean pSR of the healthy population, as was the pSR of plasma from carriers of the prothrombin G20210A mutation (1.47 +/- 0.21; n = 46). We propose that the decreased APC-independent anticoagulant activity of protein S in plasma with elevated prothrombin levels may contribute to the thrombotic risk associated with the prothrombin G20210A mutation.

Functional properties of the sex-hormone-binding globulin (SHBG)-like domain of the anticoagulant protein S

Protein S (PS) possesses a sex-hormone-binding globulin (SHBG)-like domain in place of the serine-protease domain found in other vitamin K-dependent plasma proteins. This SHBG-like domain is able to bind a complement fraction, C4b-binding protein (C4b-BP). To establish whether the PS SHBG-like domain can fold normally in the absence of other domains, and to obtain information on the specific functions of this region, we expressed the PS SHBG-like domain alone or together with its adjacent domain EGF4. The folding of the two recombinant modules was studied by analyzing their binding to C4b-BP. The apparent dissociation constants of this interaction indicated that both recombinant modules adopted the conformation of native PS, indicating that the PS SHBG-like region is an independent folding unit. We also obtained the first direct evidence that the SHBG-like domain alone is sufficient to support the interaction with C4b-BP. In addition, both recombinant modules were able to bind Ca2+ directly, as shown by the migration shift in agarose gel electrophoresis in the presence of Ca2+, together with the results of equilibrium dialysis and the functional effect of Ca2+ on the C4b-BP/PS interaction, confirming the presence of one Ca2+ binding site within the SHBG-like domain. Neither recombinant module exhibited activated protein C (aPC) cofactor activity in a clotting assay, suggesting that the PS SHBG-like region must be part of the intact molecule for it to contribute to aPC cofactor activity, possibly by constraining the different domains in a conformation that permits optimal interaction with aPC.

Severe factor VII deficiency with recurrent intracranial haemorrhages owing to double heterozygosity for a splice site mutation of an IVS4 and a novel nonsense mutation in exon 8 (Gln211-->Term)

Genetic analysis of a 10-month-old Japanese baby boy with recurrent intrathoracic bleeding, cerebral haemorrhages and gastrointestinal bleeding secondary to severe factor VII (FVII) deficiency revealed evidence of two distinct mutations of FVII: a splice site mutation of G-->A at nucleotide 6071 in the IVS4 splice site and a novel nonsense mutation (Gln211-->Term) in exon 8. His bleeding was difficult to control without prophylactic infusion of FVII. We detected a heterozygous splice site mutation of the IVS4 in his mother and a heterozygous nonsense mutation in exon 8 (Gln211-->Term) in his father. The parents' FVII levels are both 50% of normal controls. The FVII:C in plasma from the proband was < 1.5% of normal controls. FVII:antigen (Ag) was < 1% of normal controls, using a monoclonal antibody (mAb) hVII-B101/1 that specifically reacts with FVII epidermal growth factor 1 (EGF-1), and 5% of normal controls, using a rabbit polyclonal antibody against human FVII. After immunoadsorption with mAb hVII-B101/B1-Sepharose 4B, FVII levels of both the proband and his mother were 5% of normal controls; after immunoadsorption the FVII levels of normal subjects were < 1%. We hypothesize that secretion of a small amount of dysfunctional FVII lacking EGF-1 into the circulation accounts for this observation.

Deficient APC-cofactor activity of protein S Heerlen in degradation of factor Va Leiden: a possible mechanism of synergism between thrombophilic risk factors

In protein S Heerlen, an S-to-P (single-letter amino acid codes) mutation at position 460 results in the loss of glycosylation of N458. This polymorphism has been found to be slightly more prevalent in thrombophilic populations than in normal controls, particularly in cohorts of patients having free protein S deficiency. This suggests that carriers of the Heerlen allele may have an increased risk of thrombosis. We have now characterized the expression in cell cultures of recombinant protein S Heerlen and investigated the anticoagulant functions of the purified recombinant protein in vitro. Protein S Heerlen was synthesized and secreted equally well as wild-type protein S by transiently transfected COS-1 cells. The recombinant protein S Heerlen interacted with conformation-dependent monoclonal antibodies and bound C4b-binding protein to the same extent as wild-type protein S. Protein S Heerlen displayed reduced anticoagulant activity as cofactor to activated protein C (APC) in plasma-based assays, as well as in a factor VIIIa–degradation system. In contrast, protein S Heerlen functioned equally well as an APC cofactor in the degradation of factor Va as wild-type protein S did. However, when recombinant activated factor V Leiden (FVa:Q506) was used as APC substrate, protein S Heerlen was found to be a poor APC cofactor as compared with wild-type protein S. These in vitro results suggest a possible mechanism of synergy between protein S Heerlen and factor V Leiden that might be involved in the pathogenesis of thrombosis in individuals carrying both genetic traits.

Deficient APC-cofactor activity of protein S Heerlen in degradation of factor Va Leiden: a possible mechanism of synergism between thrombophilic risk factors

In protein S Heerlen, an S-to-P (single-letter amino acid codes) mutation at position 460 results in the loss of glycosylation of N458. This polymorphism has been found to be slightly more prevalent in thrombophilic populations than in normal controls, particularly in cohorts of patients having free protein S deficiency. This suggests that carriers of the Heerlen allele may have an increased risk of thrombosis. We have now characterized the expression in cell cultures of recombinant protein S Heerlen and investigated the anticoagulant functions of the purified recombinant protein in vitro. Protein S Heerlen was synthesized and secreted equally well as wild-type protein S by transiently transfected COS-1 cells. The recombinant protein S Heerlen interacted with conformation-dependent monoclonal antibodies and bound C4b-binding protein to the same extent as wild-type protein S. Protein S Heerlen displayed reduced anticoagulant activity as cofactor to activated protein C (APC) in plasma-based assays, as well as in a factor VIIIa–degradation system. In contrast, protein S Heerlen functioned equally well as an APC cofactor in the degradation of factor Va as wild-type protein S did. However, when recombinant activated factor V Leiden (FVa:Q506) was used as APC substrate, protein S Heerlen was found to be a poor APC cofactor as compared with wild-type protein S. These in vitro results suggest a possible mechanism of synergy between protein S Heerlen and factor V Leiden that might be involved in the pathogenesis of thrombosis in individuals carrying both genetic traits.

Protein S secretion differences of missense mutants account for phenotypic heterogeneity

To elucidate the molecular background for the heterogeneity in protein S plasma concentrations observed in protein S deficient individuals, the in vitro synthesis of recombinant protein S missense mutants was investigated. Six different naturally occurring mutations identified in the protein S gene (PROS1) of thrombosis patients were reproduced in protein S cDNA by site directed mutagenesis. Two mutants, G441C and Y444C (group A), were associated with low total plasma concentration of protein S. Modestly low protein S was found in families with R520G and P626L (group B) mutants. T57S and I518M (group C), which was associated with marginally low protein S, did not segregate with protein S deficiency in the respective families, raising doubts as to whether they were causative mutations or rare neutral variants. The 6 protein S mutants were transiently expressed in COS 1 cells. The Y444C mutant showed the lowest level of secretion (2.5%) followed by the G441C mutant (40%). Group B demonstrated around 50% reduction in secretion, whereas group C mutants showed normal secretion. Pulse-chase experiments demonstrated impaired protein S processing with intracellular degradation and decreased secretion into the culture media of group A and B mutants. Interestingly, there was a good correlation between in vitro secretion and the concentration of free protein S in the plasma of heterozygous carriers. These results demonstrate impaired protein S secretion to be an important mechanism underlying hereditary protein S deficiency and that variations in protein secretion is a major determinant of the phenotypic heterogeneity observed in protein S deficiency. (Blood. 2000;95:173-179)

Protein S secretion differences of missense mutants account for phenotypic heterogeneity

To elucidate the molecular background for the heterogeneity in protein S plasma concentrations observed in protein S deficient individuals, the in vitro synthesis of recombinant protein S missense mutants was investigated. Six different naturally occurring mutations identified in the protein S gene (PROS1) of thrombosis patients were reproduced in protein S cDNA by site directed mutagenesis. Two mutants, G441C and Y444C (group A), were associated with low total plasma concentration of protein S. Modestly low protein S was found in families with R520G and P626L (group B) mutants. T57S and I518M (group C), which was associated with marginally low protein S, did not segregate with protein S deficiency in the respective families, raising doubts as to whether they were causative mutations or rare neutral variants. The 6 protein S mutants were transiently expressed in COS 1 cells. The Y444C mutant showed the lowest level of secretion (2.5%) followed by the G441C mutant (40%). Group B demonstrated around 50% reduction in secretion, whereas group C mutants showed normal secretion. Pulse-chase experiments demonstrated impaired protein S processing with intracellular degradation and decreased secretion into the culture media of group A and B mutants. Interestingly, there was a good correlation between in vitro secretion and the concentration of free protein S in the plasma of heterozygous carriers. These results demonstrate impaired protein S secretion to be an important mechanism underlying hereditary protein S deficiency and that variations in protein secretion is a major determinant of the phenotypic heterogeneity observed in protein S deficiency. (Blood. 2000;95:173-179)

Amino acid residues in thrombin-sensitive region and first epidermal growth factor domain of vitamin K-dependent protein S determining specificity of the activated protein C cofactor function

Human protein S (PS) potentiates the anticoagulant activity of human but not bovine activated protein C (APC), whereas bovine PS is a cofactor to APC from both species. The structural requirements for the specificity of the APC cofactor function of human PS are located in its thrombin-sensitive region (TSR) and the first epidermal growth factor (EGF1)-like module. To elucidate which residues in these two modules determine the specificity of the APC cofactor activity, 41 human PS mutants were expressed. All mutants were cofactors to human APC and some also to bovine APC. Residues in TSR (positions 49 and 52) and EGF1 (residues 97 and 106) together determined the specificity of the APC cofactor function, whereas substitution of individual residues did not change specificity. Bovine PS, and mutants expressing cofactor activity to bovine APC, stimulated phospholipid binding of bovine APC. In contrast, human PS and mutants lacking cofactor activity to bovine APC failed to support binding of bovine APC to phospholipids. These data indicate that residues in TSR and EGF1 cause the specificity of the APC cofactor activity and support the concept that key residues in these two modules interact with APC on the phospholipid surface.