Gly74Ser mutation in protein C causes thrombosis due to a defect in protein S-dependent anticoagulant function

Analysis of four hereditary protein C deficiencies associated with vascular thromboembolism

OBJECTIVE

To analyze the clinical features and gene mutations in four families with hereditary protein C (PC) deficiency and explore their association with vascular thromboembolism.

METHODS

The clinical data of four patients with PC deficiency were retrospectively analyzed. Venous blood samples were collected from the four affected patients and their family members, and relevant coagulation indexes and thrombin production and inhibition tests were performed. PCR was used to amplify and directly sequence the PROC gene of the probands. Software analysis was conducted to assess the conservativeness and pathogenicity of the mutated loci. Protein models were constructed to analyze the spatial structure before and after the mutation.

RESULTS

Thrombin generation and inhibition assays demonstrated impaired anticoagulation in all four probands. Proband 1 and 4 presented clinically with pulmonary embolism and lower extremity deep vein thrombosis (DVT), Proband 2 with cerebral infarction, and Proband 3 with DVT. Genetic analysis revealed the presence of the following mutations: c.541T > G heterozygous missense mutation, c.577-579delAAG heterozygous deletion mutation, c.247-248insCT heterozygous insertion mutation, c.659G > A heterozygous missense mutation, and a new variant locus c.1146_1146delT heterozygous deletion mutation in the four probands, respectively. In particular, c.1146_1146delT heterozygous deletion mutations not reported previously. Conservativeness and pathogenicity analyses confirmed that most of these amino acid residues were conserved, and all the mutations were found to be pathogenic. Analysis of protein modeling revealed that these mutations induced structural alterations in the protein or led to the formation of truncated proteins. According to the American College of Medical Genetics and Genomics (ACMG) classification criteria and guidelines for genetic variants, c.1146_1146delT was rated as pathogenic (PVS1 + M2 + PM4 + PP1 + PP3 + PP4).

CONCLUSION

The identified mutations are likely associated with decreased PC levels in each of the four families. The clinical manifestations of hereditary PC deficiency exhibit considerable diversity.

Ser252Asn Mutation Introduces a New N-Linked Glycosylation Site and Causes Type IIb Protein C Deficiency

BACKGROUND

 Protein C (PC) is a vitamin K-dependent anticoagulant serine protease zymogen which upon activation by the thrombin-thrombomodulin (TM) complex downregulates the coagulation cascade by degrading cofactors Va and VIIIa by limited proteolysis. We identified a thrombosis patient who carried a heterozygous mutation c.881G > A, p.Ser252Asn (S252N) in PROC. This mutation was originally described in a report of novel mutations in patients presenting with defective PC anticoagulant activity in Paris. The research identified PC-S252N (the "Paris" mutation) in a propositus and her family members and highlighted the critical role of Ser252 in the anticoagulation process of activated PC (APC).

MATERIAL AND METHODS

 We expressed the PC-S252N mutant in mammalian cells and characterized the properties in coagulation assays to decipher the molecular basis of anticoagulant defect of this mutation.

RESULTS

 We demonstrated that PC-S252N had a diminished ability to TM binding, which resulted in its impaired activation by the thrombin-TM complex. However, APC-S252N exhibited a slightly stronger cleavage capacity for the chromogenic substrate. Meanwhile, the catalytic activity of APC-S252N toward FVa was significantly reduced. Sequence analysis revealed that Ser252 to Asn substitution introduced a new potential N-linked glycosylation site (252NTT254) in the catalytic domain of PC, which adversely affected both the activation process of PC and anticoagulant activity of APC.

CONCLUSION

 The new N-glycosylation site (252NTT254) resulting from the mutation of Ser252 to Asn252 in PROC affects the overall structure of the protease, thereby adversely affecting the anticoagulant function of protein C. This modification has a negative impact on both TM-promoted activation of protein C and APC cleavage of FVa, ultimately leading to thrombosis in the patient.

Role of Gly197 in the structure and function of protein C

We previously demonstrated that heterozygous Gly197 to Arg mutation in PROC is associated with venous thrombosis due to the mutation abrogating both zymogenic and enzymatic activities of protein C and activated protein C (APC). In this study, we investigated the role of Gly197 on the structure and function of protein C by replacing it with Ala, Lys and Glu in separate constructs. Characterization of protein C mutants indicated their activation by thrombin is improved ~5-20-fold with the order of PC-G197K > PC-G197E > PC-G197A > PC-WT. Interestingly, the cofactor function of thrombomodulin (TM) in promoting the activation of zymogens by thrombin followed the reverse order of PC-WT > PC-G197A > PC-G197E > PC-G197K. The thrombin-generation inhibitory profiles of zymogens in a tissue factor-mediated thrombin generation assay using protein C-deficient plasma with or without supplementation with TM followed the same order of zymogen activation in the purified system. Evaluation of anticoagulant activities of APC derivatives by prothrombinase and aPTT assays revealed a normal activity for APC-G197A but dramatically impaired activity for the other two mutants. In the endothelial cell permeability assay, APC-G197A exhibited normal antiinflammatory activity, but the other two mutants were nearly inactive. These results suggest that Gly197 plays a key role in TM cofactor-dependent protein C activation by thrombin. It facilitates the recognition of protein C by thrombin in the presence of TM but impedes it in the absence of the cofactor. In APC, a small residue at this position is required for the proper folding/reactivity of the active-site pocket of the protease, a hypothesis supported by structural modeling.

The genetics of venous thromboembolism: a systematic review of thrombophilia families

Genetic risk factors are important for the occurrence and prognosis of venous thromboembolism (VTE). The studies of thrombophilia families are important for dissecting the genetic background of the thrombotic disease. We conducted the systematic review of all published family-based studies on VTE genetics across all racial groups through PubMed and Embase prior to 13th April 2020. This systematic review of 287 families (including 225 Caucasian families, 52 East Asian families, and families of other ethnicities) revealed a total of 21 different genes; the five most reported mutated genes were F5 (88/287, 30.7%), SERPINC1 (67/287, 23.3%), PROC (65/287, 22.6%), F2 (40/287, 13.9%) and PROS1 (48/287, 16.7%). For Caucasian families, F5 mutations were most frequently reported at 37.8% (85/225), while PROS1 mutations were most frequently reported, at 40.4% (21/52), for East Asian families (Chinese, Japanese and Korean). Factor V Leiden was reported more frequently in Caucasians than in East Asians. Missense mutations were reported frequently in the SERPINC1, PROC and PROS1 genes. In conclusion, our study found the most likely mutated genes associated with VTE among different ethnic groups and provided indications for VTE genetic testing and research in the future.

Gly197Arg mutation in protein C causes recurrent thrombosis in a heterozygous carrier

BACKGROUND

Activated protein C (APC) downregulates thrombin generation by inactivating procoagulant cofactors Va and VIIIa by limited proteolysis. We identified two protein C-deficient patients both of whom carry a heterozygous Gly197 to Arg (G197R) mutation in PROC and experience venous thrombosis.

OBJECTIVE

The objective of this study was to determine the molecular basis of the clotting defect in patients carrying the G197R mutation.

METHODS

We expressed protein C-G197R in mammalian cells and characterized its properties in established coagulation and anti-inflammatory assay systems.

RESULTS

The activation of protein C-G197R by thrombin was improved ~10-fold; however, its activation by thrombin was not promoted by thrombomodulin (TM). In a tissue factor-mediated thrombin generation assay, the addition of soluble TM to protein C-deficient plasma, supplemented with protein C-G197R, did not have a significant inhibitory effect on thrombin generation parameters. APC-G197R did not exhibit a significant anticoagulant activity in either purified or plasma-based assay systems. APC-G197R was essentially inactive because it showed no activity in an aPTT assay. Anti-inflammatory activity of APC-G197R was also dramatically impaired as determined by an endothelial cell permeability assay. Structural modeling predicted that the side-chain of Arg cannot be accommodated at this site of APC without a major distortion of the local structure that appears to propagate and adversely affect the reactivity/folding of the catalytic pocket.

CONCLUSION

The G197R mutation in patients appears to be functionally equivalent to a heterozygous protein C knockout with half of the protein having no significant activity and thus causing thrombosis.

C-terminal residues of activated protein C light chain contribute to its anticoagulant and cytoprotective activities

BACKGROUND

Activated protein C (APC) is an important homeostatic blood coagulation protease that conveys anticoagulant and cytoprotective activities. Proteolytic inactivation of factors Va and VIIIa facilitated by cofactor protein S is responsible for APC's anticoagulant effects, whereas cytoprotective effects of APC involve primarily the endothelial protein C receptor (EPCR), protease activated receptor (PAR)1 and PAR3.

OBJECTIVE

To date, several binding exosites in the protease domain of APC have been identified that contribute to APC's interaction with its substrates but potential contributions of the C-terminus of the light chain have not been studied in detail.

METHODS

Site-directed Ala-scanning mutagenesis of six positively charged residues within G142-L155 was used to characterize their contributions to APC's anticoagulant and cytoprotective activities.

RESULTS AND CONCLUSIONS

K151 was involved in protein S dependent-anticoagulant activity of APC with some contribution of K150. 3D structural analysis supported that these two residues were exposed in an extended protein S binding site on one face of APC. Both K150 and K151 were important for PAR1 and PAR3 cleavage by APC, suggesting that this region may also mediate interactions with PARs. Accordingly, APC's cytoprotective activity as determined by endothelial barrier protection was impaired by Ala substitutions of these residues. Thus, both K150 and K151 are involved in APC's anticoagulant and cytoprotective activities. The differential contribution of K150 relative to K151 for protein S-dependent anticoagulant activity and PAR cleavage highlights that binding exosites for protein S binding and for PAR cleavage in the C-terminal region of APC's light chain overlap.

Ile73Asn mutation in protein C introduces a new N-linked glycosylation site on the first EGF-domain of protein C and causes thrombosis

Activated protein C exerts its anticoagulant activity by protein S-dependent inactivation of factors Va and VIIIa by limited proteolysis. We identified a venous thrombosis patient who has plasma protein C antigen level of 63% and activity levels of 44% and 23%, as monitored by chromogenic and clotting assays. Genetic analysis revealed the proband carries compound heterozygous mutations (c.344T>A, p.I73N and c.1181G>A, p.R352Q) in PROC. We individually expressed protein C mutations and discovered that thrombin-thrombomodulin activates both variants normally and the resulting activated protein C mutants exhibit normal amidolytic and proteolytic activities. However, while protein S-dependent catalytic activity of activated protein C-R352Q toward factor Va was normal, it was significantly impaired for activated protein C-I73N. These results suggest that the Ile to Asn substitution impairs interaction of activated protein C-I73N with protein S. This conclusion was supported by a normal anticoagulant activity for activated protein C-I73N in protein S-deficient but not in normal plasma. Further analysis revealed Ile to Asn substitution introduces a new glycosylation site on first EGF-like domain of protein C, thereby adversely affecting interaction of activated protein C with protein S. Activated protein C-R352Q only exhibited reduced activity in sub-physiological concentrations of Na⁺ and Ca²⁺, suggesting that this residue contributes to metal ion-binding affinity of the protease, with no apparent adverse effect on its function in the presence of physiological levels of metal ions. These results provide insight into the mechanism by which I73N/R352Q mutations in activated protein C cause thrombosis in proband carrying this compound heterozygous mutation.