Factor Vii Deficiency

The rare inherited disorders of coagulation are a fascinating group of diseases that have provided us with important insights into the structure and function of their respective deficient protein(s). Factor VII (FVII) deficiency is the commonest of the ‘rare inherited disorders of coagulation’ and this review summarizes current knowledge on the prevalence, diagnosis, management and the molecular pathology of factor VII deficiency. Vascular injury results in the binding of FVII to Tissue Factor (TF), a sequence of events that initiates coagulation and ultimately generates a massive but highly focused burst of thrombin at the site of vascular damage. FVII bound to TF is activated to generate the active serine protease FVIIa and it is the TF–VIIa complex that, through limited proteolytic cleavage, activates factors X and IX (Wildgoose et al, 1992a). The activated Factors Xa, VIIa, thrombin (IIa), IXa and XIIa have all been shown to activate FVII, but factor IXa in association with phospholipids appears to be most efficient at activating factor VII (Wildgoose & Kisiel, 1989; Butenas & Mann, 1996). Factor VII is a vitamin K-dependent glycoprotein and comprises 406 amino acids with a molecular weight of ∼50 kDa. FVII circulates in plasma in two forms – the majority as a single-chain inactive zymogen with a concentration of 10 nmol/l (0·5 µg/ml) and a much smaller amount (∼10–110 pmol/l) as the active two-chain form (Wildgoose et al, 1992a). The conversion of the single-chain form of FVII to the two-chain form occurs by cleavage of a single peptide bond between Arginine 152 and Isoleucine 153, resulting in a light chain of 20 kDa (residues 1–152) and a heavy chain of 30 kDa (residues 153–406 – see Fig 1) and which contain the NH2- and COOH-terminal ends of the parent molecule respectively. Organization of the human factor VII gene and its encoded polypeptide. The upper part of the illustration represents the factor VII gene with the 9 exons shown by filled boxes. The position of the first and last nucleotides of each exon are shown (numbered according to O'Hara et al, 1987). The lower part of the illustration shows the polypeptide structure and the various functional domains encoded by specific exons. Darker hatching indicates the prepro-peptide and lighter hatching the mature protein. Codons initiating each exon are shown (residue +1 is the first amino acid of the mature protein). Activation of factor VII occurs through cleavage at Arg152–Ile153 to generate a two-chain molecule. The light chain is encoded by residues +1 to 152 and the heavy chain by residues 153 to 406. The heavy and light chains of FVIIa are joined by a disulphide bond between Cys135 and Cys236. His193, Asp242 and Ser344 are the residues which constitute the catalytic triad. () shows the position of the 10 Gla residues and () the position of O- and N-linked glycosylation sites. FVII contains 10 glutamic acid residues located towards the N-terminus of the molecule at residues 6, 7, 14, 16, 19, 20, 25, 26, 29 and 35. Post-translational modification of these residues to γ-carboxyglutamic acid (Gla) residues allows the binding of calcium, which causes a conformational change in the molecule, exposing novel epitopes that facilitate its subsequent binding to TF and phospholipid (Wildgoose et al, 1992b). FVII is a glycoprotein with O-linked glycosylation sites at Serine 52 and 60 and N-linked sites at Asn145 and 322. Glycosylation of FVII may be important for both its function and for its plasma half-life. The triad of amino acids Serine 344, Aspartate 242 and Histidine 193, which constitute the catalytic centre of FVIIa, are located on the heavy chain. The most important physiological inhibitor of the TF–VIIa complex is Tissue Factor Pathway Inhibitor (TFPI), a member of the Kunitz family of inhibitors (van der Logt et al, 1991). Human TFPI is a 276-amino-acid molecule with a molecular weight of 32 kDa which circulates in plasma associated with lipoproteins, at a low concentration of 60–180 ng/ml. TFPI is synthesized by megakaryocytes and endothelial cells and its release from the surface of endothelial cells is increased both by heparin and various platelet agonists. TFPI forms an inactive quaternary complex comprising TF–VIIa, factor Xa and TFPI which rapidly inhibits the extrinsic pathway of coagulation. Antithrombin in the presence of heparin may also be involved in the regulation of FVIIa but its precise role remains controversial (Jesty et al, 1996). The factor VII gene (F7) maps to the long arm of chromosome 13 at 13q34, approximately 2·8 kb telomeric to the factor X gene (Miao et al, 1992). The F7 gene spans approximately 12kb of DNA and consists of nine exons encoding a mature protein of 406 amino acids (Fig 1). FVII is synthesized with either a 38-amino-acid or a 60-amino-acid prepro-leader sequence containing a hydrophobic region (residues −36 to 24) which targets the protein for secretion and a pro sequence (residues −17 to −1) which is important for vitamin K-dependent gamma-carboxylation and which is highly conserved among other vitamin K-dependent proteins. The difference in the length of the prepro-leader sequence arises from alternative splicing of exon 1a/1b (Berkner et al, 1986). Exon 1b (66 bp of DNA encoding amino acid residues −39 to −18) is absent in approximately 90% of factor VII mRNA transcripts (Berkner et al, 1986). The pro-peptide and the Gla domain are encoded by exon 2. The pro-peptide is released from the mature protein by cleavage between Arginine −1 and Alanine +1. Exon 3 (residues 38–45) encodes the hydrophobic aromatic stack; exons 4 (residues 46–83) and 5 (residues 84–130) encode the two epidermal-like growth factor (EGF) domains, exons 6 (residues 131–167) and 7 (residues 168–208) encode the activation domain, and exon 8 (residues 209–406) encodes the catalytic domain and 1026 nucleotides of the 3′ non-coding sequence including the poly(A) tail. The F7 gene contains a number of polymorphisms, five (and possibly six) of which have also been shown to influence FVII activity (Table I). Other polymorphisms within the F7 gene have been reported but appear to be silent in terms of their effect upon protein function and/or secretion. In contrast to many eukaryotic promoters and both factor X and factor IX, the FVII promoter lacks both a TATA and CAAT box. The major transcription initiation site for FVII is located 50 bp upstream from the initiation codon– Methionine +1. Analysis of the FVII promoter shows that maximal FVII activity resides within a 185-bp fragment and DNAse I footprint analysis has identified protein binding sites at −51 to −32, −63 to −58, −108 to −84 and −233 to −215 (Pollak et al, 1996). In addition, binding sites for both HNF-4 and Sp1 have been demonstrated and disruption of either of these sites results in a loss of promoter activity (Pollak et al, 1996) and has been reported as a cause of factor VII deficiency (Arbini et al, 1997; Carew et al, 2000). Factor VII plasma levels are determined by both environmental and genetic factors with the latter accounting for up to one-third of the variation in plasma FVII levels (Bernardi et al, 1996). Among environmental factors, dietary fat intake and the levels of plasma triglycerides are positively correlated with factor VII:C levels, but other factors such as age, obesity, diabetes (Heywood et al, 1996) and, in women, the use of sex hormones can all affect FVII levels (Meade, 1988; Habiba et al, 1996). Five and possibly six polymorphisms within the human F7 gene have been shown to affect both plasma FVII:C levels and, more recently, plasma VIIa levels (Bernardi et al, 1997). The first polymorphism to be reported within the F7 gene that affects factor VII levels was the Arg353Gln polymorphism within exon 7, which arises from a G→A substitution at position 10976 (Green et al, 1991). This substitution has a frequency in the UK population of approximately 0·2 and heterozygosity for this polymorphism is associated with an approximately 25% reduction in factor VII coagulant (or functional) activity (FVII:C) and factor VII antigen (FVII:Ag) levels. Individuals homozygous for this polymorphism have an approximately 50% reduction in circulating plasma factor VII. The Arg353Gln polymorphism is commonly found in association with a second polymorphism – the insertion of a 10-bp sequence (decanucleotide) within the 5′ untranslated region of the factor VII gene at position −323 (Marchetti et al, 1992). Until recently it was unclear which of these two polymorphisms was responsible for the alteration in plasma factor VII levels. However, studies of a group of Polish blood donors in which the Arg353Gln polymorphism is not in strong allelic association with the 10 bp promoter insertion has shown that both the Arg353Gln polymorphism and the 10 bp promoter polymorphism independently affect circulating factor VII plasma levels (Hunault et al, 1997). Two additional polymorphisms within the FVII promoter at positions −401 (G→T) and −402 (G→A) have also been shown to affect factor VII levels (Marchetti et al, 1993; van't Hooft et al, 1999). A third polymorphism is located within intron 7 (IVS7) of the factor VII gene and is characterized by the presence of a variable number of a 37-base pair repeat sequence (Marchetti et al, 1991, 1992). In the first repeat, which also contains the IVS7 donor splice site, sequence variations have also been identified. Quantitative mRNA analysis has shown that the higher numbers of repeats are associated with relatively higher mRNA expression and suggests that the IVS7 polymorphism contributes to plasma FVII levels (Pinotti et al, 2000). Finally, a recently identified G to A polymorphism within intron 1a of the FVII gene at position +74 has also been shown to affect FVII levels (Peyvandi et al, 2000a), although it appears to be in strong linkage disequilibrium with both the 10-bp decanucleotide insertion and the Arg353Gln alleles. The diagnosis of factor VII deficiency is usually suspected following the identification of a prolonged prothrombin time which corrects, unless an inhibitor is present, in a 50:50 mix with normal plasma. The activated partial thromboplastin time (APTT), thrombin time and fibrinogen concentration are usually normal. Specific assays of factor VII are undertaken to confirm the deficiency. It is important to exclude vitamin K deficiency or other acquired causes of a clotting disorder before the diagnosis of factor VII deficiency is made. Family studies may also be of value in establishing the diagnosis of factor VII deficiency. Functional factor VII activity (FVII:C) is frequently measured using a one-stage prothrombin time (PT)-based assay et al, 1991). However, the of thromboplastin in the assay can have a effect upon the FVII functional assays and for these a of are with suspected factor VII deficiency et al, 1991). are by Factor VII a molecule characterized by a prolonged prothrombin a normal clotting time and a normal Factor VII activity is low using thromboplastin but is normal using Factor VII antigen is normal and is usually et al, of limited conversion of FVII to FVIIa functional FVII FVII:C both the inactive FVII and FVII:C assays using thromboplastin are more to FVIIa with zymogen assays on human or and of the FVII:C activity of assays on or thromboplastin is to FVIIa, the is to zymogen FVII 1996). is frequently measured using an assay or assay and either or et al, et al, et al, 1992). assays can as as of factor VII et al, 1992). In in which has been activation of FVII, assays for FVII:C and be However, has been activation these results The assay of FVIIa was reported using a Tissue Factor molecule in which the and domains resulting in a Tissue Factor which was deficient in the conversion of FVII to FVIIa but which activity factor VIIa in a one-stage clotting assay & et al, The FVIIa in normal plasma measured using this is approximately et al, of the circulating FVII A second for the assay for FVIIa levels in plasma has been reported that not upon its functional activity but a specific using a with a for two-chain FVIIa et al, 1997). The assay for FVIIa and the factor VII to plasma et al, 1997). However, results from the two assays to the levels of FVIIa in plasma. The that normal plasma contains approximately of FVIIa, approximately lower that measured by the activity The for the between the two assays is unclear although various have been It is that the of the light towards which the in the is may in the This FVIIa to to the but have effect upon the activity alternative is that the assay using the activity assay is not and activation of FVII by can & 1992). Activation of FVII by the complex can also et al, a of the FVII is the of the assay be as the zymogen is in factor VII deficiency is the most of the ‘rare inherited coagulation with an of between and Factor VII deficiency is inherited in an and its frequency is increased in is (Table which are is a relatively between factor VII levels and the of with low levels of factor VII with much higher levels have a et al, In with A or the is characterized by into and with in the of In factor VII the of is variable (Fig (Peyvandi et al, 1997; et al, In with and and other is and deficiency is in with factor VII deficiency. are to in with inherited disorders of platelet The for the in FVII deficiency is although a of the time has been reported (Bernardi et al, of in with inherited of factor VII (Peyvandi et al, 1997). Factor VII:C 10 In with factor VII into the is and reported in between and of (Peyvandi et al, 1997; et al, and this is associated with a and have been reported in with factor VII deficiency (Peyvandi et al, 1997; et al, and, in of these the of and the was to that in & However, are not a and other studies have to this as a in with FVII deficiency (Peyvandi et al, 1997). The current of blood coagulation in suggests that a to coagulation through the binding of factor to factor VII is to be with This is by the FVII although to from major and et al, 1997). of the that have been reported within the human factor VII has recently been a of that in of factor VII et have reported a 5′ splice site within intron 4 of the FVII gene that to a of exon 4 from FVII in expression studies to of FVII protein and the that the presence of this homozygous in the to a of FVII in the plasma. The by et at the of 10 with massive from which In addition, within the a from a at the of with and It that an to However, et recently reported the of a of was with FVII deficiency at the of 3 with – and and from the of these or in to a prolonged prothrombin time and plasma factor VII activity and antigen levels in plasma at with factor VII deficiency. The was with plasma and with FVII and, to remains analysis identified a homozygous bp within exon 1a of the FVII gene at a region that encodes part of the prepro-peptide of analysis of both that both for this bp This results in a the of a and the of circulating factor VII. The for the in the of this and that reported by et is but the indicates that a of FVII is with The of between FVII:C and the may the that of FVIIa are to coagulation in In assays to between a and that low but not FVII:C levels. in association with factor VII deficiency has been reported & et al, et al, et al, although the is can that, in the may be that to FVII:C results with and that the not have FVII deficiency but an – to that with Factor VII et al, In such the diagnosis of factor VII deficiency is and other factors for more alternative to the association between factor VII deficiency and is that the to the deficient results in a factor VII molecule that to TFPI and, the TF–VIIa initiation pathway of coagulation remains A reduction in FVII levels has also been reported in association with et al, & and, in at on a low with et al, of the other vitamin K-dependent clotting factors have not been The low FVII levels in has also been as a to the rare of factor VII deficiency associated with et al, analysis in with inherited FVII deficiency associated with has not been reported and remains levels of factor VII also appear to an important role in from the suggests that factor VII levels in the upper part of the normal factor for the of et al, 1986). FVII levels also with age, sex and et al, are also a number of polymorphisms within the FVII gene that can influence FVII levels and these have been in an factor VII levels as a factor for have been as part of the et al, The with a first and to a between factor VII levels and various F7 gene polymorphisms, to between factor VII levels and In the last 5 has been an in the number of of FVII deficiency reported in the The factor VII site the factor VII gene and these are in The majority of the are located within the catalytic domain of factor the functional of this However, are located the that all domains are important in the structure and function of factor VII. In with many of within the factor VII gene have been but these are not shown in In a number of has shown that with a an for the (Bernardi et al, 1993; & 2000). a relatively number of factor VII have been in and, the by which many in the is has been to and the of specific but such (Peyvandi et al, et the FVII gene in with FVII deficiency from of A of of which novel and 7 been of the novel located in the Gla domain the second growth factor domain and the catalytic serine protease domain of which five novel splice identified in intron 1a intron and intron 6 of for FVIIa and the factor complex provided for the effect of the on FVIIa secretion or function (Fig and In the majority of the located on the serine protease domain, to the region between the catalytic triad and the surface with that of the serine protease domain to bound factor in the complex is for functional of the structure of the complex between FVIIa and The of the novel identified in the of et are shown in and identified in The catalytic triad is by and shown in the Gla domain in the and domains in and the and serine protease in light and respectively. The two domains in factor are identified in of the of the catalytic serine protease domain of The of the catalytic triad and are shown as at the between the two of the serine protease domains, shown in light and respectively. identified in the of et are shown in and identified in of vitamin K are in a of disorders but most are to or vitamin K is a vitamin K and the although both have upon coagulation. inhibits two vitamin K-dependent within the – a vitamin K-dependent and a vitamin K-dependent This efficient of vitamin K to its active form and the activity of a vitamin K-dependent Other vitamin K and is the most in the and UK but and are in other is an a although it appears to vitamin K in a to the In with the levels of factor VII are lower other vitamin K-dependent factors, prothrombin and factor X Other and may be associated with factor VII levels & The clotting in are and the of this A all the vitamin K-dependent clotting factors has been reported and shown to be to a within the gene et al, 2000). rare to of the with vitamin of factor VII deficiency to other et al, and the et al, have been factor VII inhibitors have also been reported either (Meade, 1988; et al, 1989; et al, et al, or in association with other et al, Factor VII deficiency has been reported in association with an et al, in with et al, in association with the use of et al, in et al, and in with et al, factor VII is a rare disorder and are for the management of this and with FVII factor VIIa is the of FVII has a in of approximately 5 although this may be a et al, In contrast to of factor and IX in which a of is efficient can be with levels of FVII in the of et al, et al, et al, to inherited FVII deficiency factor IX complex factor VII and VIIa The of factor VII in normal plasma by plasma has been in the management of factor VII is on its has been to various either by or in with FVII & et al, However, in in which prolonged is and of the of FVII, it is to plasma and with may be & 1987). plasma is to FVII deficiency it is to use a plasma to the of factor IX complex that are not by either or contain variable of factor VII and have been to with factor VII deficiency et al, et al, The amount of factor VII in each of these is variable but is usually by the and this can be to the amount for of these factor IX contain activated forms of factors IX and X and be with as are of both and associated with their use & et al, et al, et al, these it is also to use these in the presence of in of major or in are relatively A number of factor VII have been and have been to with inherited factor VII deficiency with or a of et al, et al, et al, et al, et al, et al, et al, Factor VII have also been for in with factor VII deficiency and for the of factor VII for are in the of to a this the of factor in it et al, from 8 to have been & & et al, et al, et al, of FVII is to and major factor VII levels not 20 A number of studies have shown that with FVII deficiency can be using FVIIa has a that of plasma FVII et al, 1993; et al, in in an increased has been demonstrated et al, and in et al, 2000). In such more or a may be to active FVII levels. et reported their with in with in addition, also 7 major and et al, 1999). The majority of the deficient with FVII levels of with a single of and in was major in of and either or In the of was at for the first by for the the also acid with the In the was following the of an other have reported the use of with a of et al, 1997; et al, et al, 2000). In a of appears to be in the majority of with FVII deficiency are either or to and following The of is much upon the for at in a single may be In 20 of 6 this to FVII levels of and levels of following with have been reported 1996). of has been in at with factor VII deficiency to for an et al, 2000). studies to the a and a which the to the The an or of by a at a of for and at for a factor VII and VIIa levels between and and factor VII with of the other vitamin K-dependent clotting factors, is most commonly in In major of these can be using prothrombin complex & plasma has also been to with to although are that the is not to in factor IX that may be important in the of a & A review of the management of to has recently been & In with factor VII deficiency to the use of prothrombin complex is of the of In such vitamin K and may be to the factor VIIa has been to to and may be a in to to other factor VII deficiency in association with other disorders is In in the and is as to such be or of the this can be is In are the use of factor VII or may be of in such rare the use of and may be of Finally, acid and may be in and