In vitro characterization of missense mutations associated with quantitative protein S deficiency

Protein S gene mutation c.946C > T (p.R316C) contributed to ischemic stroke in a man with von Willebrand disease type 3 caused by two novel VWF gene mutations, c.2328delT (p.A778Lfs* 23) and c.6521G > T (p.C2174F)

The risk factors for a family with VWD presenting with an ischemic stroke (IS) were explored. FVIII activity (FVIII:C), VWF antigen (VWF:Ag), and protein S activity were measured. Next generation sequencing (NGS) was performed targeting F8, F9, VWF, PROC, and PROS1. Sanger sequencing validation was performed on family members. The proband and his sister both had low FVIII:C (1 IU/dL) and VWF:Ag (3 IU/dL) levels, confirming the diagnosis of type 3 VWD. His father had nearly normal levels of FVIII:C (58 IU/dL) and VWF:Ag (57 IU/dL). His daughter had type 1 VWD with decreased FVIII:C (46 IU/dL) and VWF:Ag (19 IU/dL). NGS identified a heterozygous VWF c.2328delT (p.A778Lfs*23) frame shift mutation only in the proband and his sister. Another VWF missense mutation, c.6521G > T (p.C2174F), was found heterozygous in all members studied. A PROS1 mutation, c.946C > T (p.R316C), previously reported to relate to ischemic stroke, was found heterozygous in the patient, his father, and his daughter. Only the proband and daughter have a slightly decreased plasma protein S level. This may be the first case with type 3 VWD with severe VWF/FVIII deficiency presented with ischemic stroke contributed to by a protein S defect.

Carrier frequencies of antithrombin, protein C, and protein S deficiency variants estimated using a public database and expression experiments

BACKGROUND

Genetic deficiencies of antithrombin (AT), protein C (PC), and protein S (PS) are risk factors for venous thromboembolism. In the general population, the prevalence of heterozygous deficiency of AT, PC, and PS are reported as approximately 0.02%-0.2%, 0.2%-0.4%, and 0.03%-0.5%, respectively. The Exome Aggregation Consortium (ExAC) provides a public database containing reference data for over 60 000 exomes.

OBJECTIVE

This study aimed to determine the frequency of AT, PC, and PS deficiencies using the ExAC database and transient expression experiments.

METHODS

In total, 133, 157, and 221 variants of SERPIN1 (encoding AT), PROC (PC), and PROS1 (PS), respectively, were registered as missense and putative loss-of-function variants in the ExAC database. Variants with relatively high allele frequencies were selected and randomly sampled. Recombinant proteins were expressed in human embryo kidney 293 cells and their secretion and anticoagulant activities examined.

RESULTS AND CONCLUSION

We assessed 9 AT, 4 PC, and 14 PS variants with relatively high allele frequencies and randomly sampled 12 AT, 15 PC, and 19 PS missense variants. All 21 AT variants showed normal or mildly reduced secretion, and 6 showed reduced total activity (specific activity × antigen level). Of the 19 PC variants, 11 showed impaired total activity. All 33 PS variants showed normal or mildly reduced secretion, and 4 showed reduced total activity. Based on allele frequencies in the ExAC database, we calculated the frequencies of AT, PC, and PS genetic deficiency as 0.36%, 0.63%, and 0.39%, respectively.

Whole Exome Sequencing Reveals Severe Thrombophilia in Acute Unprovoked Idiopathic Fatal Pulmonary Embolism

BACKGROUND

Acute unprovoked idiopathic fatal pulmonary embolism (IFPE) causes sudden death without an identifiable thrombogenic risk. We aimed to investigate the underlying genomic risks of IFPE through whole exome sequencing (WES).

METHODS

We reviewed 14years of consecutive out-of-hospital fatal pulmonary embolism records (n=1478) from the ethnically diverse population of New York City. We selected 68 qualifying IFPE cases for WES. We compared the WES data of IFPE cases to those of 9332 controls to determine if there is an excess of rare damaging variants in the genome using ethnicity-matched controls in collapsing analyses.

FINDINGS

We found nine of the 68 decedents (13·2%) who died of IFPE had at least one pathogenic or likely pathogenic variant in one of the three anti-coagulant genes: SERPINC1 (Antithrombin III), PROC, and PROS1. The odds ratio of developing IFPE as a variant carrier for SERPINC1 is 144·2 (95% CI, 26·3-779·4; P=1·7×10-7), for PROC is 85·6 (95% CI, 13·0-448·9; P=2.0×10-5), and for PROS1 is 56·4 (95% CI, 5·3-351·1; P=0·001). The average age-at-death of anti-coagulant gene variant carriers is significantly younger than that of non-carriers (28·56years versus 38·02years; P=0·01).

INTERPRETATION

This study showed the important role of severe thrombophilia due to natural anti-coagulant deficiency in IFPE. Evaluating severe thrombophilia in out-of-hospital fatal PE beyond IFPE is warranted.

Hereditary protein S deficiency leads to ischemic stroke

Hereditary protein S (PS) deficiency is an independent risk factor for venous thromboembolism. However, the correlation between PS and arterial thrombotic disease, such as cerebral thrombosis, is not clear. The present study focused on the molecular mechanisms underlying ischemic stroke caused by a PS gene mutation in one family. The activity of antithrombin, protein C and PS in the plasma of the proband was measured, and the genes encoding PS were amplified and sequenced. The cellular localization and expression of PS were analyzed in HEK‑293 cells. The proband was a 50‑year‑old male. Plasma PS activity of the proband was 38.9%, which was significantly decreased compared with normal levels. Sequencing analysis revealed a PROS1 c.1486_1490delGATTA mutation on exon 12. This frameshift mutation converts Asp496 in the precursor PS into the termination codon. In addition, the PROS1 mutation was correlated with low PS activity in the family. Functional tests revealed that the mutant protein aggregated in the cytoplasm and its secretion and expression decreased. In conclusion, protein S mutation appeared to be the primary cause of thrombosis in the family of the present study. However, the correlation between PS deficiency and ischemic stroke requires further investigation.

Activated protein C anticoagulant system dysfunction and thrombophilia in Asia

Thrombophilia that is common among Caucasians is caused by genetic polymorphisms of coagulation factor V Leiden (R506Q) and prothrombin G20210A. Unlike that in Caucasians, thrombophilia that is common in the Japanese and Chinese involve dysfunction of the activated protein C (APC) anticoagulant system caused by abnormal protein S and protein C molecules. Approximately 50% of Japanese and Chinese individuals who develop venous thrombosis have reduced activities of protein S. The abnormal sites causing the protein S molecule abnormalities are distributed throughout the protein S gene, PROS1. One of the most common abnormalities is protein S Tokushima (K155E), which accounts for about 30% of the protein S molecule abnormalities in the Japanese. Whether APC dysfunction occurs in other Asian countries is an important aspect of mapping thrombophilia among Asians. International surveys using an accurate assay system are needed to determine this.

PROC c.574_576del polymorphism: a common genetic risk factor for venous thrombosis in the Chinese population

BACKGROUND

There are ethnic differences in the genetic risk factors for venous thrombosis (VT). The genetic causes of VT in the Chinese population are not fully understood.

OBJECTIVES

To identify possible common abnormal factors that could contribute to thrombosis susceptibility.

METHODS/RESULTS

We measured the levels of nine types of plasma coagulation factor, three types of anticoagulation factor and two types of fibrinolytic factor in 310 VT patients. Factor V activity was higher in 32 cases. Eleven of the 32 cases also had low protein C (PC) or protein S (PS) activities, indicating PC or PS deficiency. No other abnormalities were observed in the other 21 cases. All of the samples were sensitive to activated PC inactivation. Therefore, the abnormal factor involved may be FV inactivator or its cofactor rather than FV itself. Resequencing identified a common PROC c.574_576del variant in 10 of the 32 subjects. In a case-control study, this variant was detected in 68 of the 1003 patients and in 25 of the 1031 controls. It had an adjusted odds ratio of 2.71 (95% confidence interval [CI] 1.68-4.36). PC amidolytic activities of most variant carriers were similar to those of non-carriers, but the mean anticoagulant activity was only 72.7 U dL(-1). Expression studies in vitro showed that the anticoagulant activity of the mutant PC was 43.6% of that of the wild-type PC.

CONCLUSIONS

We identified what is, so far, the most common genetic risk factor for VT in the Chinese population, with its prevalence being approximately 2.36%.

Impaired secretion of carboxyl-terminal truncated factor VII due to an F7 nonsense mutation associated with FVII deficiency

INTRODUCTION

Factor VII (FVII) is a vitamin K-dependent glycoprotein secreted into the blood circulation from hepatic cells. We investigated the molecular basis of the congenital FVII deficiency found in a Japanese patient.

MATERIALS AND METHODS

We analyzed the F7 gene of the patient, who was diagnosed with a FVII deficiency at pregnancy. We expressed a carboxyl-terminal truncated FVII (Arg462X FVII) corresponding to the identified mutation in CHO-K1 cells. To study roles of the carboxyl-terminus in the secretion of FVII, we also expressed a series of recombinant FVIIs deleted of limited numbers of carboxyl-terminal amino acids (462Arg-466Pro).

RESULTS

We identified a nonsense mutation (c.1384C>T: p.Arg462X) in F7, leading to a lack of five amino acids in the carboxyl-terminus. In expression experiments, Arg462X FVII was undetectable not only by Western blotting, but also by ELISA. A Western blot analysis of the truncated FVIIs revealed that all mutants were expressed in the cells the same as the wild type, but were secreted into the culture medium in lesser amounts than the wild type depending on the length of the deletion, which was confirmed by ELISA. Arg462X FVII did not colocalize with the Golgi on immunofluorescence staining, suggesting that it might be retained in the ER and degraded in the cell.

CONCLUSION

The carboxyl-terminal amino acids of FVII play an important role in its secretion, and the p.Arg462X mutation was likely to have caused the FVII deficiency in this patient.