REVIEWS Inherited risk factors for venous thromboembolism Ida Martinelli, Valerio De Stefano and Pier M. Mannucci Abstract | Venous thromboembolism (VTE) has important heritable components. In the past 20 years, knowledge in this field has greatly increased with the identification of a number of gene variants causing hypercoagulability. The two main mechanisms are loss-of-function of anticoagulant proteins and gain-offunction of procoagulants, the latter owing to increased synthesis or impaired downregulation of a normal protein or, more rarely, to synthesis of a functionally hyperactive molecule. Diagnosis of thrombophilia is useful to determine the causes of VTE, recognizing that this multifactorial disease can also be influenced by various acquired factors including cancer, surgery, trauma, prolonged immobilization, or reproduction-associated risk factors. Diagnosis of inherited thrombophilia rarely affects the acute or long-term management of VTE. However, the risk of recurrent VTE is increased in anticoagulant-deficient patients and in homozygotes for gain-of-function mutations. Screening for inherited thrombophilia in thrombosis-free individuals is indicated only for relatives of a proband who is anticoagulant-deficient or has a family history of VTE. In families with thrombophilia and VTE, primary antithrombotic prophylaxis during risk situations lowers the rate of incident VTE. In this Review, we discuss the main causes of inherited thrombophilia, the associated clinical manifestations, and the implications for antithrombotic prophylaxis in the affected individuals. Martinelli, I. et al. Nat. Rev. Cardiol. 11, 140–156 (2014); published online 14 January 2014; doi:10.1038/nrcardio.2013.211
Introduction
Angelo Bianchi Bonomi Hemophilia and Thrombosis Center, Fondazione IRCCS Ca’ Granda, Ospedale Maggiore Policlinico, Via Pace 9, 20122 Milan, Italy (I. Martinelli). Institute of Hematology, Catholic University, Largo A. Gemelli 8, I‑00168 Rome, Italy (V. De Stefano). Scientific Direction, Fondazione IRCCS Ca’ Granda, Ospedale Maggiore Policlinico, Via F. Sforza 28, 20122 Milan, Italy (P. M. Mannucci). Correspondence to: P. M. Mannucci piermannuccio. mannucci@ policlinico.mi.it
Venous thromboembolism (VTE), which encompasses deep-vein thrombosis (DVT) and pulmonary embolism, is the most-common vascular disease after coronary artery and cerebrovascular diseases.1 The clinical consequences of VTE include death, VTE recurrence, post-thrombotic syndrome, and bleeding complications owing to anticoagulant therapy. In the USA, the incidence of first VTE per 1,000 person-years has been estimated to be 1.61 (1.17 for DVT alone and 0.45 for pulmonary embolism, with or without associated DVT).2 In Europe, a similar incidence of VTE of 1.5 per 1,000 person-years has been reported in community studies.3–5 VTE is predominant in older age-groups—the incidence of VTE per 1,000 person-years is 0.08 for men and 0.12 for women aged 80 years.3 VTE is a multifactorial disorder resulting from the interaction between an array of acquired and genetic factors. In addition to ageing, the main acquired risk factors for VTE (Box 1) include history of VTE, superficial vein thrombosis (SVT), surgery, trauma, confinement to bed, malignant neoplasms with or without chemotherapy, major illnesses, insertion of a central venous catheter or pacemaker, and limb paresis, as well as pregnancy, oral contraceptive use and hormone Competing interests The authors declare no competing interests.
140 | MARCH 2014 | VOLUME 11
replacement therapy (HRT).2,6–11 The familial segregation of VTE as a heritable phenotype has been estimated to be between 52% and 62% under non-Mendelian and Mendelian assumptions, respectively.12 A family history of VTE has been consistently reported to be a risk factor for VTE per se.13–15 On the other hand, documented inherited thrombophilia (hypercoagulability), and particularly its interaction with acquired factors, is an important determinant of VTE.16 In this Review, we examine the main causes of inherited thrombophilia, and their associated clinical manifestations. We also discuss the indications for laboratory screening to identify individuals who carry thrombophiliacausing mutations, and the implications for primary and secondary antithrombotic prophylaxis.
Definition of thrombophilia The term thrombophilia describes a tendency to develop VTE on the basis of a hypercoagulable state, owing to inherited or acquired disorders of blood coagulation or fibrinolysis. In 1937, Nygaard and Brown introduced the term “essential thrombophilia” in a report on five patients with vascular disease.17 After the connection between thrombophilia and heritability of VTE was clinically described in 1956,18 the deficiencies of antithrombin, protein C, and protein S were documented as the first rare causes of inherited thrombophilia.19–22 These plasmatic defects hamper the two main regulatory pathways of coagulation: the inhibition of serine proteases by antithrombin, and that of the nonenzymatic cofactors VIIIa and Va by
www.nature.com/nrcardio © 2014 Macmillan Publishers Limited. All rights reserved
REVIEWS Key points
Box 1 | Acquired risk factors for VTE
■■ Knowledge about the spectrum of genetic abnormalities causing thrombophilia has greatly expanded in the past 20 years ■■ These abnormalities increase the risk of venous thromboembolism (VTE) by causing blood hypercoagulability through the impairment of natural anticoagulant pathways or the potentiation of procoagulants ■■ VTE risk is higher in carriers of natural anticoagulant deficiencies, homozygous defects, and multiple abnormalities (severe thrombophilia) than in heterozygotes for factor V Leiden and prothrombin 20210A (mild thrombophilia) ■■ Family history of VTE is a strong risk factor for VTE regardless of the presence of known VTE susceptibility genes ■■ Thrombophilia screening is useful in some instances to inform the optimum duration of secondary prophylaxis in patients who have developed VTE and are at high risk of recurrence ■■ Thrombosis-free individuals in risk-enhancing situations (pregnancy, oral contraceptive use, hormone replacement therapy, orthopaedic surgery) do not require thrombophilia screening, except in families with natural anticoagulant deficiencies or history of VTE
Permanent ■■ Increasing age ■■ Family history of VTE ■■ History of VTE or superficial vein thrombosis ■■ Obesity (BMI >30 kg/m2) ■■ Cancer (brain, breast, gastrointestinal, lung, lymphoma, pancreatic, renal, prostate) with or without surgery and chemotherapy ■■ Myeloproliferative neoplasms ■■ Major illness (autoimmune diseases, Behçet disease, chronic renal disease, COPD, inflammatory bowel disease, multiple sclerosis, neurological disease with limb paresis, rheumatoid arthritis) ■■ Congestive cardiac failure ■■ Cardiovascular disease (CHD, stroke, TIA) ■■ Lupus anticoagulant and antiphospholipid antibodies ■■ Paroxysmal nocturnal haemoglobinuria and other haemolytic anaemias ■■ Varicose veins
activated protein C (APC) and its cofactor protein S.23,24 In the 1990s, two gene polymorphisms—factor V G1691A Leiden (FVL), which causes resistance to the anticoagulant action of APC, and prothrombin G20210A (PT20210A), which is often associated with a high plasma level of prothrombin, were recognized as more-common causes of thrombophilia than deficiencies in antithrombin, protein C, or protein S.25–27
Known mechanisms of thrombophilia Inherited thrombophilia can be caused by two main mechanisms: the loss-of-function of endogenous anticoagulants (including deficiency or dysfunction of antithrombin, protein C, or protein S) and gain-offunction of procoagulant factors. Gain-of-function in blood coagulation can be caused by increased synthesis of a normal protein (such as PT20210A, and also fibrin ogen and factors VIII, IX, X, and XI, the genetic determinants of which are only partially known), impaired downregulation of a normal protein (such as FVL) or, rarely, by synthesis of a functionally hyperactive molecule (for example, factor IX Padua). A third mechanism of thrombophilia is hypofibrinolysis (Box 2).
Loss-of-function mechanisms Antithrombin deficiency Antithrombin, a member of the serine protease inhibitor (serpin) superfamily synthesized in the liver, regulates coagulation by forming a 1:1 covalent complex with thrombin, factor Xa, and other activated procoagulants, including factors XIIa, XIa, IXa, and VIIa. The rate of interaction with target proteases is accelerated by heparin (Figure 1).23 In the general population, the estimated prevalence of antithrombin deficiency ranges from five to 17 per 10,000 individuals,28,29 and in patients with VTE is around 1%.30 More than 250 gene variations have been identified in antithrombin deficiency, including missense and nonsense point mutations, insertions, and deletions.31 Two types of antithrombin deficiency can be distinguished, both inherited in an autosomal-dominant fashion
Transient ■■ Confinement to bed ■■ Current smoking status ■■ Standing for >6 h per day ■■ Long-distance travel (>6 h) ■■ Violent effort or muscular trauma ■■ Plaster cast of the lower extremities ■■ Infectious disease ■■ Recent* hospital admission ■■ Recent* hip fracture, hip surgery, or both ■■ Recent* major surgery (abdominal, neurological, pelvic or thoracic) ■■ Recent* splenectomy ■■ Central venous catheter or pacemaker ■■ Current use of antipsychotic drugs, tamoxifen, l‑asparaginase, or thalidomide ■■ Exposure to ambient air pollution Sex-associated ■■ Current use of oral contraceptives ■■ Current use of hormone replacement therapy ■■ Pregnancy, particularly in women aged >35 years, parity >1, or weight gain >21 kg ■■ Pre-eclampsia (with or without foetal growth restriction) ■■ Assisted reproductive techniques ■■ Twin pregnancy ■■ Puerperium, particularly in women with postpartum haemorrhage or Caesarean section *The term recent usually refers to the past 3 months, although some epidemiological studies define triggering events as those occurring within 3–6 weeks prior to VTE. Abbreviations: CHD, coronary heart disease; COPD, chronic obstructive pulmonary disease; TIA, transient ischaemic attack; VTE, venous thromboembolism.
(Table 1).31 In type I, both antithrombin activity and antigen level are low in plasma owing to a lack of protein production or secretion by the mutant allele. In type II, low antithrombin activity contrasts with normal antigen levels, indicating functional defects in the molecule. Type II can be further subdivided into three subtypes characterized by impairment of the enzyme reactive site, of the heparin-binding site, or by pleiotropic defects affecting antigen concentration and heparin binding or enzyme activity.31 The only individuals homozygotic for antithrombin deficiency described so far carry
NATURE REVIEWS | CARDIOLOGY
VOLUME 11 | MARCH 2014 | 141 © 2014 Macmillan Publishers Limited. All rights reserved
REVIEWS Box 2 | Mechanisms associated with thrombophilia Known mechanisms Loss-of-function ■■ Antithrombin deficiency ■■ Protein C deficiency ■■ Protein S deficiency Gain-of-function ■■ Factor V Leiden ■■ Prothrombin G20210A ■■ High factor VIII level ■■ Non‑O blood group ■■ Dysfibrinogenaemia
Postulated mechanisms ■■ Low tissue factor pathway inhibitor level ■■ High fibrinogen level ■■ High factor IX level ■■ High factor X level ■■ High factor XI level ■■ Resistance to antithrombin ■■ Global hypofibrinolysis ■■ High thrombin activatable fibrinolysis inhibitor level ■■ Hyperhomocysteinaemia
XII
XIIa XI
XIa IX
IXa Heparin
VIIIa X VII
Antithrombin
Xa VIIa Va
Prothrombin (II)
Thrombin (IIa)
Endothelium
Figure 1 | Anticoagulant mechanisms of antithrombin, which mainly inhibits factor IIa and factor Xa, but also factors VIIa, IXa, XIa, and XIIa. The rate of interaction with target proteases is accelerated by heparin. Solid lines denote activation and broken lines inhibition (dashed lines, strong inhibition; dotted lines, weak inhibition).
heparin-binding site defects, suggesting that the other subtypes are associated with embryonic lethality.32,33 Protein C deficiency Protein C is a natural anticoagulant protein that is activated when thrombin is generated, this process being accelerated by the complex formed by thrombin with an endothelial protein C receptor and thrombomodulin (Figure 2).24 Protein C inactivates factor Va through an initial cleavage at Arg506, which is required for exposure of the cleavage sites at Arg306 and Arg679.24 Protein C deficiency is inherited as an autosomal-dominant trait,20 and heterozygous deficiency has been found in 14–50 per 10,000 adult individuals in the general population34,35 and in 3% of patients with VTE.30 In contrast to antit hrombin deficiency, homozygous or double 142 | MARCH 2014 | VOLUME 11
heterozygous protein C deficiency does not cause embryonic death, although newborns with these disorders often develop purpura fulminans characterized by severe thrombosis of the small vessels, resulting in cutaneous and subcutaneous ischaemic necrosis.36 Similarly to antithrombin deficiency, protein C def iciency can be divided into two subtypes (Table 1). 37 Type I is the most common, and is characterized by a parallel reduction in plasma antigen level and activity, reflecting a reduced synthesis of functional protein. The rarer type II is characterized by normal antigen level with reduced functional activity, reflecting normal synthesis of a dysfunctional protein. More than 200 mutations in the PROC gene have been reported.37 In type I, the majority of mutations are of the missense variety, leading to premature termination of synthesis or disruption of protein folding; deletions and insertions occur with lower frequency (~10%).37 In type II, missense mutations are located mainly in the γ‑carboxyglutamic acid and protease domains.37 Protein S deficiency Protein S acts as a cofactor for APC, enhancing its capacity to inactivate factors Va and VIIIa.24 Approximately 60% of protein S is bound to the complement C4bbinding protein, and only the remaining 40% is functionally active.24 In addition, protein S acts as cofactor of tissue factor pathway inhibitor (TFPI) in the inhib ition of factor Xa.38 Protein S and TFPI circulate as a complex in plasma,38 which could explain the covariance of protein S and TFPI levels39 and the low levels of TFPI in patients who are protein S deficient.40 In the general population, protein S deficiency has been detected in up to 10 in 10,000 individuals,41 whereas the prevalence in patients with VTE is 2%, similar to that of protein C deficiency.30 Three subtypes of inherited protein S deficiency have been identified (Table 1): type I (low total and free antigen levels, reduced activity), type II (normal total and free antigen levels, reduced activity), and type III (normal total antigen level, and reduced free antigen level and activity). 42 The PROS1 gene database lists almost 200 different mutations associated with protein S deficiency, the majority being missense mutations or short deletions or insertions. Large deletions or insertions account for
Introduction
Angelo Bianchi Bonomi Hemophilia and Thrombosis Center, Fondazione IRCCS Ca’ Granda, Ospedale Maggiore Policlinico, Via Pace 9, 20122 Milan, Italy (I. Martinelli). Institute of Hematology, Catholic University, Largo A. Gemelli 8, I‑00168 Rome, Italy (V. De Stefano). Scientific Direction, Fondazione IRCCS Ca’ Granda, Ospedale Maggiore Policlinico, Via F. Sforza 28, 20122 Milan, Italy (P. M. Mannucci). Correspondence to: P. M. Mannucci piermannuccio. mannucci@ policlinico.mi.it
Venous thromboembolism (VTE), which encompasses deep-vein thrombosis (DVT) and pulmonary embolism, is the most-common vascular disease after coronary artery and cerebrovascular diseases.1 The clinical consequences of VTE include death, VTE recurrence, post-thrombotic syndrome, and bleeding complications owing to anticoagulant therapy. In the USA, the incidence of first VTE per 1,000 person-years has been estimated to be 1.61 (1.17 for DVT alone and 0.45 for pulmonary embolism, with or without associated DVT).2 In Europe, a similar incidence of VTE of 1.5 per 1,000 person-years has been reported in community studies.3–5 VTE is predominant in older age-groups—the incidence of VTE per 1,000 person-years is 0.08 for men and 0.12 for women aged 80 years.3 VTE is a multifactorial disorder resulting from the interaction between an array of acquired and genetic factors. In addition to ageing, the main acquired risk factors for VTE (Box 1) include history of VTE, superficial vein thrombosis (SVT), surgery, trauma, confinement to bed, malignant neoplasms with or without chemotherapy, major illnesses, insertion of a central venous catheter or pacemaker, and limb paresis, as well as pregnancy, oral contraceptive use and hormone Competing interests The authors declare no competing interests.
140 | MARCH 2014 | VOLUME 11
replacement therapy (HRT).2,6–11 The familial segregation of VTE as a heritable phenotype has been estimated to be between 52% and 62% under non-Mendelian and Mendelian assumptions, respectively.12 A family history of VTE has been consistently reported to be a risk factor for VTE per se.13–15 On the other hand, documented inherited thrombophilia (hypercoagulability), and particularly its interaction with acquired factors, is an important determinant of VTE.16 In this Review, we examine the main causes of inherited thrombophilia, and their associated clinical manifestations. We also discuss the indications for laboratory screening to identify individuals who carry thrombophiliacausing mutations, and the implications for primary and secondary antithrombotic prophylaxis.
Definition of thrombophilia The term thrombophilia describes a tendency to develop VTE on the basis of a hypercoagulable state, owing to inherited or acquired disorders of blood coagulation or fibrinolysis. In 1937, Nygaard and Brown introduced the term “essential thrombophilia” in a report on five patients with vascular disease.17 After the connection between thrombophilia and heritability of VTE was clinically described in 1956,18 the deficiencies of antithrombin, protein C, and protein S were documented as the first rare causes of inherited thrombophilia.19–22 These plasmatic defects hamper the two main regulatory pathways of coagulation: the inhibition of serine proteases by antithrombin, and that of the nonenzymatic cofactors VIIIa and Va by
www.nature.com/nrcardio © 2014 Macmillan Publishers Limited. All rights reserved
REVIEWS Key points
Box 1 | Acquired risk factors for VTE
■■ Knowledge about the spectrum of genetic abnormalities causing thrombophilia has greatly expanded in the past 20 years ■■ These abnormalities increase the risk of venous thromboembolism (VTE) by causing blood hypercoagulability through the impairment of natural anticoagulant pathways or the potentiation of procoagulants ■■ VTE risk is higher in carriers of natural anticoagulant deficiencies, homozygous defects, and multiple abnormalities (severe thrombophilia) than in heterozygotes for factor V Leiden and prothrombin 20210A (mild thrombophilia) ■■ Family history of VTE is a strong risk factor for VTE regardless of the presence of known VTE susceptibility genes ■■ Thrombophilia screening is useful in some instances to inform the optimum duration of secondary prophylaxis in patients who have developed VTE and are at high risk of recurrence ■■ Thrombosis-free individuals in risk-enhancing situations (pregnancy, oral contraceptive use, hormone replacement therapy, orthopaedic surgery) do not require thrombophilia screening, except in families with natural anticoagulant deficiencies or history of VTE
Permanent ■■ Increasing age ■■ Family history of VTE ■■ History of VTE or superficial vein thrombosis ■■ Obesity (BMI >30 kg/m2) ■■ Cancer (brain, breast, gastrointestinal, lung, lymphoma, pancreatic, renal, prostate) with or without surgery and chemotherapy ■■ Myeloproliferative neoplasms ■■ Major illness (autoimmune diseases, Behçet disease, chronic renal disease, COPD, inflammatory bowel disease, multiple sclerosis, neurological disease with limb paresis, rheumatoid arthritis) ■■ Congestive cardiac failure ■■ Cardiovascular disease (CHD, stroke, TIA) ■■ Lupus anticoagulant and antiphospholipid antibodies ■■ Paroxysmal nocturnal haemoglobinuria and other haemolytic anaemias ■■ Varicose veins
activated protein C (APC) and its cofactor protein S.23,24 In the 1990s, two gene polymorphisms—factor V G1691A Leiden (FVL), which causes resistance to the anticoagulant action of APC, and prothrombin G20210A (PT20210A), which is often associated with a high plasma level of prothrombin, were recognized as more-common causes of thrombophilia than deficiencies in antithrombin, protein C, or protein S.25–27
Known mechanisms of thrombophilia Inherited thrombophilia can be caused by two main mechanisms: the loss-of-function of endogenous anticoagulants (including deficiency or dysfunction of antithrombin, protein C, or protein S) and gain-offunction of procoagulant factors. Gain-of-function in blood coagulation can be caused by increased synthesis of a normal protein (such as PT20210A, and also fibrin ogen and factors VIII, IX, X, and XI, the genetic determinants of which are only partially known), impaired downregulation of a normal protein (such as FVL) or, rarely, by synthesis of a functionally hyperactive molecule (for example, factor IX Padua). A third mechanism of thrombophilia is hypofibrinolysis (Box 2).
Loss-of-function mechanisms Antithrombin deficiency Antithrombin, a member of the serine protease inhibitor (serpin) superfamily synthesized in the liver, regulates coagulation by forming a 1:1 covalent complex with thrombin, factor Xa, and other activated procoagulants, including factors XIIa, XIa, IXa, and VIIa. The rate of interaction with target proteases is accelerated by heparin (Figure 1).23 In the general population, the estimated prevalence of antithrombin deficiency ranges from five to 17 per 10,000 individuals,28,29 and in patients with VTE is around 1%.30 More than 250 gene variations have been identified in antithrombin deficiency, including missense and nonsense point mutations, insertions, and deletions.31 Two types of antithrombin deficiency can be distinguished, both inherited in an autosomal-dominant fashion
Transient ■■ Confinement to bed ■■ Current smoking status ■■ Standing for >6 h per day ■■ Long-distance travel (>6 h) ■■ Violent effort or muscular trauma ■■ Plaster cast of the lower extremities ■■ Infectious disease ■■ Recent* hospital admission ■■ Recent* hip fracture, hip surgery, or both ■■ Recent* major surgery (abdominal, neurological, pelvic or thoracic) ■■ Recent* splenectomy ■■ Central venous catheter or pacemaker ■■ Current use of antipsychotic drugs, tamoxifen, l‑asparaginase, or thalidomide ■■ Exposure to ambient air pollution Sex-associated ■■ Current use of oral contraceptives ■■ Current use of hormone replacement therapy ■■ Pregnancy, particularly in women aged >35 years, parity >1, or weight gain >21 kg ■■ Pre-eclampsia (with or without foetal growth restriction) ■■ Assisted reproductive techniques ■■ Twin pregnancy ■■ Puerperium, particularly in women with postpartum haemorrhage or Caesarean section *The term recent usually refers to the past 3 months, although some epidemiological studies define triggering events as those occurring within 3–6 weeks prior to VTE. Abbreviations: CHD, coronary heart disease; COPD, chronic obstructive pulmonary disease; TIA, transient ischaemic attack; VTE, venous thromboembolism.
(Table 1).31 In type I, both antithrombin activity and antigen level are low in plasma owing to a lack of protein production or secretion by the mutant allele. In type II, low antithrombin activity contrasts with normal antigen levels, indicating functional defects in the molecule. Type II can be further subdivided into three subtypes characterized by impairment of the enzyme reactive site, of the heparin-binding site, or by pleiotropic defects affecting antigen concentration and heparin binding or enzyme activity.31 The only individuals homozygotic for antithrombin deficiency described so far carry
NATURE REVIEWS | CARDIOLOGY
VOLUME 11 | MARCH 2014 | 141 © 2014 Macmillan Publishers Limited. All rights reserved
REVIEWS Box 2 | Mechanisms associated with thrombophilia Known mechanisms Loss-of-function ■■ Antithrombin deficiency ■■ Protein C deficiency ■■ Protein S deficiency Gain-of-function ■■ Factor V Leiden ■■ Prothrombin G20210A ■■ High factor VIII level ■■ Non‑O blood group ■■ Dysfibrinogenaemia
Postulated mechanisms ■■ Low tissue factor pathway inhibitor level ■■ High fibrinogen level ■■ High factor IX level ■■ High factor X level ■■ High factor XI level ■■ Resistance to antithrombin ■■ Global hypofibrinolysis ■■ High thrombin activatable fibrinolysis inhibitor level ■■ Hyperhomocysteinaemia
XII
XIIa XI
XIa IX
IXa Heparin
VIIIa X VII
Antithrombin
Xa VIIa Va
Prothrombin (II)
Thrombin (IIa)
Endothelium
Figure 1 | Anticoagulant mechanisms of antithrombin, which mainly inhibits factor IIa and factor Xa, but also factors VIIa, IXa, XIa, and XIIa. The rate of interaction with target proteases is accelerated by heparin. Solid lines denote activation and broken lines inhibition (dashed lines, strong inhibition; dotted lines, weak inhibition).
heparin-binding site defects, suggesting that the other subtypes are associated with embryonic lethality.32,33 Protein C deficiency Protein C is a natural anticoagulant protein that is activated when thrombin is generated, this process being accelerated by the complex formed by thrombin with an endothelial protein C receptor and thrombomodulin (Figure 2).24 Protein C inactivates factor Va through an initial cleavage at Arg506, which is required for exposure of the cleavage sites at Arg306 and Arg679.24 Protein C deficiency is inherited as an autosomal-dominant trait,20 and heterozygous deficiency has been found in 14–50 per 10,000 adult individuals in the general population34,35 and in 3% of patients with VTE.30 In contrast to antit hrombin deficiency, homozygous or double 142 | MARCH 2014 | VOLUME 11
heterozygous protein C deficiency does not cause embryonic death, although newborns with these disorders often develop purpura fulminans characterized by severe thrombosis of the small vessels, resulting in cutaneous and subcutaneous ischaemic necrosis.36 Similarly to antithrombin deficiency, protein C def iciency can be divided into two subtypes (Table 1). 37 Type I is the most common, and is characterized by a parallel reduction in plasma antigen level and activity, reflecting a reduced synthesis of functional protein. The rarer type II is characterized by normal antigen level with reduced functional activity, reflecting normal synthesis of a dysfunctional protein. More than 200 mutations in the PROC gene have been reported.37 In type I, the majority of mutations are of the missense variety, leading to premature termination of synthesis or disruption of protein folding; deletions and insertions occur with lower frequency (~10%).37 In type II, missense mutations are located mainly in the γ‑carboxyglutamic acid and protease domains.37 Protein S deficiency Protein S acts as a cofactor for APC, enhancing its capacity to inactivate factors Va and VIIIa.24 Approximately 60% of protein S is bound to the complement C4bbinding protein, and only the remaining 40% is functionally active.24 In addition, protein S acts as cofactor of tissue factor pathway inhibitor (TFPI) in the inhib ition of factor Xa.38 Protein S and TFPI circulate as a complex in plasma,38 which could explain the covariance of protein S and TFPI levels39 and the low levels of TFPI in patients who are protein S deficient.40 In the general population, protein S deficiency has been detected in up to 10 in 10,000 individuals,41 whereas the prevalence in patients with VTE is 2%, similar to that of protein C deficiency.30 Three subtypes of inherited protein S deficiency have been identified (Table 1): type I (low total and free antigen levels, reduced activity), type II (normal total and free antigen levels, reduced activity), and type III (normal total antigen level, and reduced free antigen level and activity). 42 The PROS1 gene database lists almost 200 different mutations associated with protein S deficiency, the majority being missense mutations or short deletions or insertions. Large deletions or insertions account for
