Congenital Deficiency of Vitamin K-Dependent Clotting Factors
Congenital combined deficiency of vitamin K-dependent clotting factors (VKCFD) is a rare autosomal recessive bleeding disorder. The clinical presentation of combined deficient activity of clotting factors II (FII), FVII, FIX and FX (and proteins C, S and Z) occurs most commonly due to acquired vitamin K deficiency, in particular resulting from liver disease, malabsorption or malnutrition, or associated with a number of medications, most notably overdose of warfarin or warfarin derivatives.
The function of each of the vitamin K-dependent proteins depends upon normal Ƴ-carboxylation of a number of glutamic acid residues in the amino-terminal region of the proteins (the Gla Domain). This reaction requires normal cycling of vitamin K from its reduced form to the oxidized state, followed by regeneration of reduced vitamin K.1,2 These reactions are accomplished via the action of two enzymes, the Ƴ-glutamyl carboxylase (GGCX) and the vitamin K epoxide reductase (VKOR). The two variants of VKCFD arise due to defects in the genes that encode these two proteins and not due to intrinsic defects in the clotting factors. The most common clinical presentation of VKCFD is severe hemorrhage during infancy, although the degree of deficiency is highly variable and presentation later in life also occurs.3
McMillan and Roberts4 described the first case of clinical hemorrhage resulting from congenitally deficient Ƴ-carboxylation in 1966. The female infant of an uncomplicated pregnancy manifested bruising beginning at age 1 week, with recurrent serosanguinous oozing from the umbilical stump throughout the first months of life. At presentation her prothrombin time (PT) was 95 seconds and her partial thromboplastin time (PTT) was 305 seconds. Upon mixing the patient’s plasma in a 1:1 ratio with normal plasma, each of these tests corrected completely to the normal range and clinically her umbilical bleeding stopped after a single infusion of fresh frozen plasma (FFP). While her platelet count, fibrinogen, and FV and FVIII were normal, activity of FII, FVII, FIX and FX were all below the lower limit of detection (<3%). On institution of a program of daily enteral vitamin K1 therapy, the child grew and developed normally with only a mild bruising tendency. An interruption of vitamin K1 therapy for a 9-day period resulted in the return of marked severe bruising.
Fewer than 30 kindreds of VKCFD have been described in the ensuing half century, and the degree of deficiency is variable among the reported cases. The particular combination of deficient FII, FVII, FIX and FX results in multiple levels of defective thrombin generation. Activity of the substrate prothrombin is itself deficient and additionally thrombin generation via both the Tissue Factor/FVIIa and via the tenase complex is impaired. Likewise, deficiencies of proteins in each of the intrinsic, extrinsic, and common pathways of the coagulation cascade are reflected in prolongation of both the PT and aPTT screening tests. The prolongation of the PT may be more marked than the prolongation of the aPTT in milder cases. Ƴ-carboxylation is required also for the normal function the anticoagulant proteins C, S, and Z, and a tendency to thrombosis has been implicated in one case.5
The full etiology of VKCFD became clear after the cloning and characterization of the gene encoding the GGCX protein by Stafford and colleagues in 19916,7 and the gene encoding the VKOR complex by Stafford and colleagues and Oldenburg and colleagues in 2004.8, 9,10 Subsequent pedigree studies have led to the designation of two variants: VKCFD1, resulting from mutations in the Ƴ-glutamyl carboxylase gene (GGCX), and VKCFD2, resulting from mutations in the VKOR complex gene (VKORC, or VKOR).11,12,13
Vitamin K exists in 3 forms:
Vitamin K1 (phylloquinone), an important dietary vitamin, is produced by plants and algae and present in large quantities in green and leafy vegetables. Along with vitamins A, D, and E, vitamin K1 is a fat-soluble vitamin.
Normal intestinal microbial flora produce vitamin K2 (menaquinones).
A synthetic form of vitamin K has been derived as a pharmacologic agent with greater water solubility and designated vitamin K3 (menadione).
The vitamin K dependent proteins all contain a Ƴ-carboxyglutamic acid-rich Gla domain, which is critical to their normal function. In addition to the proteins involved in coagulation, there are a number of other vitamin K-dependent proteins, including osteocalcin and matrix Gla protein (MGP), the Growth Arrest Specific gene 6 (GAS-6) protein (a cell growth regulating protein), nephrocalcin A-D and additional transmembrane Gla proteins (PRGP1, PRGP2, TMGP3 and TMGP4), for which a function has not yet been defined.14,1,15,16,17
Clotting factors FII, FVII, FIX and FX and proteins C, S and Z contain anywhere from 9 to 13 glutamates (Glu) in the amino acid sequence of their respective Gla domains. Insufficient posttranslational modification of these glutamates to Ƴ-carboxyglutamic acid (Gla) is the enzymatic step that is defective in both variants of VKCFD. The carboxylation of the Gla domain confers a calcium-dependent conformation that is critical to normal interaction of the proteins with phospholipids (e.g., with the phospholipid surface of the activated platelet) or with endothelial cells. The GGCX is the protein that performs this posttranslational modification; however, vitamin K in its reduced form is required as well to act as a cofactor. Vitamin K reduced is converted to vitamin K epoxide in the course of the reaction, and the reduced form must be regenerated for additional cycles of catalysis via the action of VKOR. VKOR is opposed by the anticoagulant warfarin, so that an individual taking warfarin has impaired regeneration of vitamin K reduced, with a controlled result similar to vitamin K deficiency or VKCFD: excess undercarboxylated and functionally deficient FII, FVII, FIX, and FX.1,2 The vitamin K cycles can be visualized as shown in Figure 1.
Figure 1. The Vitamin K Cycle. Carboxylation of glutamic acid residues of the zymogen vitamin K-dependent proteins is necessary for their proper function. The carboxylase (GGCX) adds a carbon dioxide molecule to Glu to form Gla in a reaction for which vitamin K is an essential co-factor. The reduced form of vitamin K (hydroquinone) is oxidized in the process, yielding vitamin K 2,3-epoxide. Vitamin K reduced must be regenerated from the epoxide by the enzymatic action of VKOR so that Vitamin K reduced is available to serve as a cofactor for subsequent carboxylation reactions. Deficient activity of either of the enzymes VKOR or GGCX (see hatched boxes in figure) results in VKCFD.
It is evident from this discussion that there is no synthetic or secretory deficiency of the vitamin K dependent coagulation proteins. It is the function of the proteins that is abnormal as a result of insufficient carboxylation.
VKCFD arises due to mutations in the GGCX gene (VKCFD1 variant) or the VKOR gene (VKCFD2 variant). Causative mutations in the GGCX gene are usually missense mutations, with both homozygous and compound heterozygous mutations described; splice site mutations and a 14 bp deletion mutation in intron 1 have been reported.18,19–20
A recent report attempts to link a clinical bleeding phenotype to the heterozygous inheritance of a single nucleotide polymorphism (p.R325Q), although interpretation of the case is difficult because levels of the activity of FII, FVII,FIX and FX were not reported.21 One causative mutation in the VKOR gene has been reported, which was a homozygous mutation of VKOR complex subunit 1 (VKORC1) nucleotide 292CàT, resulting in a substitution of tryptophan for arginine at amino acid 98.9
Interestingly, in at least one well-studied case, no defect in the VKOR or GGCX gene was identified. The possibility exists that additional genes that modify the action of these proteins may as yet be unidentified. An independent, redundant carboxylase pathway does not exist, however.22
VKCFD is very rare, with fewer than 30 kindreds reported worldwide. No influence of race or ethnicity has been described, and cases have been reported from Africa, Asia, Europe and North America.