Genomic Sequence and Transcription Start Site for the Human γ-Glutamyl Carboxylase

Role of Vitamin K in Bone and Muscle Metabolism

Vitamin K, a cofactor for the γ-glutamyl carboxylase enzyme, is required for the post-translational activation of osteocalcin and matrix Gla protein, which play a key role in bone and muscle homeostasis. In vivo and in vitro models for osteoporosis and sarcopenia suggest the vitamin K could exert a positive effect in both conditions. In bone, it increases osteoblastogenesis, whilst decreases osteoclast formation and function. In muscle, it is associated with increased satellite cell proliferation and migration and might play a role in energy metabolism. Observational trials suggest that high levels of vitamin K are associated with increased bone mineral density and reduced fracture risk. However, interventional studies for vitamin K supplementation yielded conflicting results. Clinical trials in sarcopenia suggest that vitamin K supplementation could improve muscle mass and function. One of the main limitations on the vitamin K studies are the technical challenges to measure its levels in serum. Thus, they are obtained from indirect sources like food questionnaires, or levels of undercarboxylated proteins, which can be affected by other environmental or biological processes. Although current research appoints to a beneficial effect of vitamin K in bone and muscle, further studies overcoming the current limitations are required in order to incorporate this supplementation in the clinical management of patients with osteosarcopenia.

Impact of GGCX polymorphisms on warfarin dose requirements in atrial fibrillation patients

Background/aim: Warfarin is a common anticoagulant with large interindividual differences and a narrow therapeutic range. The polymorphisms of gamma-glutamyl carboxylase (GGCX) are important genetic factors for warfarin dose requirements. Materials and methods: Polymerase chain reaction and direct sequencing methods were used to detect the GGCX rs699664 genotype in 215 atrial fibrillation (AF) patients with warfarin administration. The effects on warfarin dose by different genotypes were analyzed. A warfarin dosing algorithm was developed based on age, height, CYP2C9, VKORC1, and GGCX genotype. Results: In 215 AF patients, there were 104 cases of wild-type GG genotype (48.4%), 92 cases of GA genotype (42.8%), and 19 cases of AA genotype (8.8%). Patients with the GGCX rs699664 A allele (GA or AA genotypes) needed higher warfarin doses than those with the GG genotype (P < 0.05). A warfarin dosing algorithm showed that age, height, CYP2C9, VKORC1, and GGCX genotype were the best variables for estimating warfarin dose (R2 = 41.2%). Another independent cohort of 60 AF patients showed a significant linear correlation between predicted warfarin maintenance dose and actual dose (R = 0.660, P < 0.01). Conclusion: AF patients with the GA and AA genotypes in GGCX rs699664 required significantly higher warfarin doses. GGCX rs699664 is a potential predictor for the warfarin dose of AF patients.

The association between GGCX, miR-133 genetic polymorphisms and warfarin stable dosage in Han Chinese patients with mechanical heart valve replacement

WHAT IS KNOWN AND OBJECTIVE

Warfarin is a widely used anticoagulant with a narrow therapeutic index. Polymorphisms in the VKORC1, CYP2C9 and CYP4F2 genes have been verified to correlate with warfarin stable dosage (WSD). Whether any other genes or variants affect the dosage is unknown. The aim of our study was to investigate the relationship between GGCX, miR-133 variants and the WSD in Han Chinese patients with mechanical heart valve replacement (MHVR).

METHODS

A total of 231 patients were enrolled in the study. Blood samples were collected for genotyping. The average WSD among subjects with different GGCX or miR-133 genotypes was compared. Regression analyses were performed to test for any association of genetic polymorphisms with WSD.

RESULTS AND DISCUSSION

The warfarin dosage in patients with the GGCX rs699664 TT and rs12714145 TT genotypes was 3.77±0.93 (95% CI: 3.35-4.19) mg/d and 3.70±1.00 (95% CI: 3.32-4.09) mg/d, respectively. The GGCX rs699664 and rs12714145 genotypes were significantly associated with WSD (P<.05). But they were ruled out in the multivariate regression analysis. There were no significant differences in the average warfarin stable dosage between subjects with MIR133B rs142410335 wild-type and variant genotypes (P>.05).

WHAT IS NEW AND CONCLUSION

The genotypes of GGCX rs699644 and rs12714145 were significantly associated with WSD (P<.05), but their contributions were not significant after accounting for other factors. MIR133B rs142410335 makes no significant contributions to warfarin stable dosage in Han Chinese patients with MHVR neither in univariate regression nor in multivariate regression analyses.

Uniparental disomy causes deficiencies of vitamin K-dependent proteins

Essentials Vitamin K-dependent coagulant factor deficiency (VKCFD) is a rare autosomal recessive disorder. We describe a case of inherited VKCFD due to uniparental disomy. The homozygous mutation caused the absence of GGCX isoform 1 and overexpression of Δ2GGCX. Hepatic and non-hepatic vitamin K-dependent proteins must be assayed to monitor VKCFD treatment.

SUMMARY

Background Inherited deficiency of all vitamin K-dependent coagulant factors (VKCFD) is a rare autosomal recessive disorder caused by mutations in the γ-glutamyl carboxylase gene (GGCX) or the vitamin K epoxide reductase gene (VKORC1), with great heterogeneity in terms of both clinical presentation and response to treatment. Objective To characterize the molecular basis of VKCFD in a Spanish family. Methods and Results Sequencing of candidate genes, comparative genomic hybridization and massive sequencing identified a new mechanism causing VKCFD in the proband. Uniparental disomy (UPD) of chromosome 2 caused homozygosity of a mutation (c.44-1G>A) resulting in aberrant GGCX splicing. This change contributed to absent expression of the mRNA coding for the full-length protein, and to four-fold overexpression of the smaller mRNA isoform lacking exon 2 (Δ2GGCX). Δ2GGCX might be responsible for two unexpected clinical observations in the patient: (i) increased plasma osteocalcin levels following vitamin K1 supplementation; and (ii) a mild non-bleeding phenotype. Conclusions Our study identifies a new autosomal disease, VKCFD1, caused by UPD. These data suggest that the Δ2GGCX isoform may retain enzymatic activity, and strongly encourage the evaluation of both hepatic and non-hepatic vitamin K-dependent proteins to assess differing responses to vitamin K supplementation in VKCFD patients.

Structural and functional insights into enzymes of the vitamin K cycle

Vitamin K-dependent proteins require carboxylation of certain glutamates for their biological functions. The enzymes involved in the vitamin K-dependent carboxylation include: gamma-glutamyl carboxylase (GGCX), vitamin K epoxide reductase (VKOR) and an as-yet-unidentified vitamin K reductase (VKR). Due to the hydrophobicity of vitamin K, these enzymes are likely to be integral membrane proteins that reside in the endoplasmic reticulum. Therefore, structure-function studies on these enzymes have been challenging, and some of the results are notably controversial. Patients with naturally occurring mutations in these enzymes, who mainly exhibit bleeding disorders or are resistant to oral anticoagulant treatment, provide valuable information for the functional study of the vitamin K cycle enzymes. In this review, we discuss: (i) the discovery of the enzymatic activities and gene identifications of the vitamin K cycle enzymes; (ii) the identification of their functionally important regions and their active site residues; (iii) the membrane topology studies of GGCX and VKOR; and (iv) the controversial issues regarding the structure and function studies of these enzymes, particularly, the membrane topology, the role of the conserved cysteines and the mechanism of active site regeneration of VKOR. We also discuss the possibility that a paralogous protein of VKOR, VKOR-like 1 (VKORL1), is involved in the vitamin K cycle, and the importance of and possible approaches for identifying the unknown VKR. Overall, we describe the accomplishments and the remaining questions in regard to the structure and function studies of the enzymes in the vitamin K cycle.

γ-Glutamyl carboxylase in osteoblasts regulates glucose metabolism in mice

Vitamin K-dependent γ-glutamyl carboxylase (GGCX) is an enzyme that catalyzes the conversion of glutamic acid to gamma-carboxyglutamic acid in substrate proteins. Among GGCX target proteins, recent evidence indicates that osteocalcin regulates insulin sensitivity and secretion. However, the precise contribution of GGCX to glucose metabolism remains to be clarified. To address this question, we generated osteoblast-specific Ggcx-deficient (i.e., conditional knockout [cKO]) mice using collagen type 1 α1 (Col1)-Cre mice. Ggcx cKO mice exhibited altered metabolism compared with their controls; serum glucose levels could be maintained with low amounts of insulin, and the weight of white adipose tissue (WAT) significantly decreased in Ggcx cKO mice. Our findings suggest that GGCX expressed in osteoblasts is critical for the maintenance of blood glucose and WAT.

Association of genetic polymorphisms with warfarin dose requirements in Chinese patients

OBJECTIVE

Warfarin is a commonly used anticoagulant with a narrow therapeutic range and large interindividual differences in dosing requirements. Previously, studies have identified that the interindividual variability was influenced by varieties of factors, including age, body size, vitamin K intake, interacting medications, as well as genetic variants. We aimed to investigate the effect of single-nucleotide polymorphisms (SNPs) on the interindividual variability of warfarin dose requirements in Chinese patients.

METHODS

The study population consisted of 300 patients with a stable maintenance dose of warfarin. We examined SNPs in eight genes involving in the biotransformation and mode of action of warfarin (i.e., CYP4F2, CYP2C19, APOE, CALU, EPHX1, PROC, CYP2C9, and GGCX) using the SNaPshot assay.

RESULTS

The mean daily warfarin dose in patients carrying CYP2C19 rs3814637CC, CYP2C9 rs1057910AA, and GGCX rs699664AA genotype was 3.39, 3.34, and 3.51 mg/day, respectively, which was higher than those carrying CYP2C19 rs3814637TT, CYP2C9 rs1057910CC, and rs699664GG genotype (2.00, 0.81, and 3.09 mg/day, respectively).

CONCLUSION

These findings indicate that individuals carrying the CYP2C19 rs3814637CC or CYP2C9 rs1057910AA or GGCX rs699664AA genotype needed higher warfarin doses in the Chinese population.

Effect of GGCX gene polymorphism on the responses of serum undercarboxylated osteocalcin and bone turnover markers after treatment with vitamin K2 (menatetrenone) among postmenopausal Thai women

The purpose of this study was to evaluate the influence of gamma-glutamyl transferase (GGCX) gene polymorphisms on the response of serum undercarboxylated osteocalcin (ucOC) and bone turnover markers 3 months after treatment with menatetrenone. One hundred and forty postmenopausal Thai women were enrolled and assigned to receive 45 mg/day treatment of menatetrenone (MK-4) concurrently with calcium 1.2 g and vitamin D 400 IU for 3 months. Demographic characteristics, GGCX genotyping, serum bone turnover markers and ucOC levels were obtained from all participants at baseline. We evaluated the reduction of ucOC at 3 months and the reduction of beta-CTx and P1NP at 1 and 3 months. The responses were compared between the different genotypes of GG and GA + AA groups. There was a significant reduction of serum ucOC, beta-CTx and P1NP from the baseline at 3 months (p < 0.001) though there was no significant difference between genotypes (GG vs. GA + AA; p > 0.05). Nonetheless, a subgroup analysis of postmenopausal women who 65 years of age or over (N = 37) revealed a significant difference between the two groups in the reduction of ucOC. Menatetrenone significantly reduced serum ucOC as well as beta-CTX and P1NP from the baseline. GGCX polymorphism appeared to have an influence over the reduction of ucOC especially in older women (age ≥65). Furthermore, the groups which have "A" allele trend to being more efficient in reducing the serum ucOC level than the group which does not have it.

Hereditary combined deficiency of the vitamin K-dependent clotting factors

Hereditary combined vitamin K-dependent clotting factors deficiency (VKCFD) is a rare congenital bleeding disorder resulting from variably decreased levels of coagulation factors II, VII, IX and X as well as natural anticoagulants protein C, protein S and protein Z. The spectrum of bleeding symptoms ranges from mild to severe with onset in the neonatal period in severe cases. The bleeding symptoms are often life-threatening, occur both spontaneously and in a surgical setting, and usually involve the skin and mucosae. A range of non-haemostatic symptoms are often present, including developmental and skeletal anomalies. VKCFD is an autosomal recessive disorder caused by mutations in the genes of either gamma-glutamyl carboxylase or vitamin K2,3-epoxide reductase complex. These two proteins are necessary for gamma-carboxylation, a post-synthetic modification that allows coagulation proteins to display their proper function. The developmental and skeletal anomalies seen in VKCFD are the result of defective gamma-carboxylation of a number of non-haemostatic proteins. Diagnostic differentiation from other conditions, both congenital and acquired, is mandatory and genotype analysis is needed to confirm the defect. Vitamin K administration is the mainstay of therapy in VKCFD, with plasma supplementation during surgery or severe bleeding episodes. In addition, prothrombin complex concentrates and combination therapy with recombinant activated FVII and vitamin K supplementation may constitute alternative treatment options. The overall prognosis is good and with the availability of several effective therapeutic options, VKCFD has only a small impact on the quality of life of affected patients.

Exon 2 deletion splice variant of gamma-glutamyl carboxylase causes des-gamma-carboxy prothrombin production in hepatocellular carcinoma cell lines

Using GGCX gene-specific real-time PCR, exon 2 deletion splice variant of vitamin K-dependent gamma-glutamyl carboxylase (GGCX) mRNA was identified in HCC cell lines. Expressions of wild type and exon 2 deletion variant of GGCX were analyzed with relevance to DCP production in HCC cell lines. Hep3B, HepG2, HuH1, HuH7, and PLC/PRF/5 produced DCP, while SK-Hep-1, HLE, HLF, and JHH1 produced no detectable level of DCP. DCP-producing cells expressed exon 2 deletion variant of GGCX mRNA and protein, while DCP-negative cells expressed no detectable level of exon 2 deletion variant of GGCX. These results suggest that exon 2 deletion splice variant of GGCX causes dysfunction of GGCX enzyme activity resulting in DCP production in HCC cell lines.

Vitamin K-dependent carboxylation

Vitamin K-dependent (VKD) protein carboxylation uses vitamin K epoxidation to convert Glus to carboxylated Glus (Glas), rendering VKD proteins active in physiologies that include hemostasis, apoptosis, bone mineralization, calcium homeostasis, growth control, and signal transduction. Clusters of Glus are modified by a processive carboxylase, generating a calcium-binding module that allows binding to either hydroxyapatite in the extracellular matrices or cell surfaces where anionic phospholipids become exposed, for example, during apoptosis or cell activation. Naturally occurring carboxylase mutations have been informative for function and are associated with bleeding complications and, surprisingly, a pseudoxanthoma elasticum (PXE)-like phenotype. A major advance in defining carboxylase function is the identification of the base that initiates carboxylation, which raises interesting possibilities for how vitamin K epoxidation is regulated by Glu substrate and carboxylase membrane topology. Vitamin K oxidoreductase (VKOR), the target of warfarin, generates the reduced vitamin K cofactor used by the carboxylase. Oxidation of active site thiols during vitamin K reduction inactivates VKOR, and activity is regenerated by an unknown reductase. The amounts of reduced vitamin K limit the capacity for carboxylation in cells, and overexpression of VKOR, but not carboxylase, improves carboxylation. However, the effect of VKOR overexpression is small, possibly because the reductase that regenerates VKOR activity is saturated. The review discusses these advances, as well as the potential impact of secretory components on carboxylation, which occurs during VKD protein secretion. Also discussed is the role of the carboxylase in mammals and lower organisms, including the bacterial pathogen Leptospira interrogans that has acquired a VKD carboxylase by horizontal transfer.

Vitamin K-dependent gamma-glutamylcarboxylation: an ancient posttranslational modification

The vitamin K-dependent carboxylase carries out the posttranslational modification of specific glutamate residues in proteins to gamma-carboxy glutamic acid (Gla) in the presence of reduced vitamin K, molecular oxygen, and carbon dioxide. In the process, reduced vitamin K is converted to vitamin K epoxide, which is subsequently reduced to vitamin K, by vitamin K epoxide reductase (VKOR) for use in the carboxylation reaction. The modification has a wide range of physiological implications, including hemostasis, bone calcification, and signal transduction. The enzyme interacts with a high affinity gamma-carboxylation recognition sequence (gamma-CRS) of the substrate and carries out multiple modifications of the substrate before the product is released. This mechanism ensures complete carboxylation of the Gla domain of the coagulation factors, which is essential for their biological activity. gamma-Carboxylation, originally discovered in mammals, is widely distributed in the animal kingdom. It has been characterized in sea squirt (Ciona intestinalis), in flies (Drosophila melanogaster), and in marine snails (Conus textile), none of which have a blood coagulation system similar to mammals. The cone snails express a large array of gamma-carboxylated peptides that modulate the activity of ion channels. These findings have led to the suggestion that gamma-carboxylation is an extracellular posttranslational modification that antedates the divergence of molluscs, arthropods, and chordates. I will first summarize recent understanding of gamma-carboxylase and gamma-carboxylation gleaned from experiments using the mammalian enzyme, and then I will briefly describe the available information on gamma-carboxylation in D. melanogaster and C. textile.

High-resolution SNP and haplotype maps of the human gamma-glutamyl carboxylase gene (GGCX) and association study between polymorphisms in GGCX and the warfarin maintenance dose requirement of the Japanese population

Gamma-glutamyl carboxylase (GGCX) plays an important role in blood coagulation through post-translational carboxylation of vitamin K-dependent blood-clotting proteins. This carboxylation process is impaired in the presence of warfarin, a vitamin K antagonist. Recent studies on GGCX have provided insights into association of polymorphisms in this gene, with inter-individual differences in the required warfarin maintenance dose. In order to provide a useful resource for further elucidating this association, we here report a high-resolution single nucleotide polymorphism (SNP) and haplotype maps of an 18-kb genomic region corresponding to the GGCX locus in the Japanese population. Among 41 SNPs, seven insertion/deletion polymorphisms, and a microsatellite polymorphism that we detected by direct sequencing of the DNAs of 96 Japanese individuals who were treated with warfarin, 32 genetic variations have not been reported. Using genotype information from 12 SNPs and the EM algorithm, we estimated haplotypes for this genomic region. Subsequently, we investigated associations of each of these polymorphisms with the warfarin maintenance-dose requirements of 828 Japanese patients, including the 96 patients that were used for DNA sequencing. We found no significant association between the polymorphisms in GGCX and the dose requirement.

Association of warfarin dose with genes involved in its action and metabolism

We report an extensive study of variability in genes encoding proteins that are believed to be involved in the action and biotransformation of warfarin. Warfarin is a commonly prescribed anticoagulant that is difficult to use because of the wide interindividual variation in dose requirements, the narrow therapeutic range and the risk of serious bleeding. We genotyped 201 patients for polymorphisms in 29 genes in the warfarin interactive pathways and tested them for association with dose requirement. In our study, polymorphisms in or flanking the genes VKORC1, CYP2C9, CYP2C18, CYP2C19, PROC, APOE, EPHX1, CALU, GGCX and ORM1-ORM2 and haplotypes of VKORC1, CYP2C9, CYP2C8, CYP2C19, PROC, F7, GGCX, PROZ, F9, NR1I2 and ORM1-ORM2 were associated with dose (P < 0.05). VKORC1, CYP2C9, CYP2C18 and CYP2C19 were significant after experiment-wise correction for multiple testing (P < 0.000175), however, the association of CYP2C18 and CYP2C19 was fully explained by linkage disequilibrium with CYP2C9*2 and/or *3. PROC and APOE were both significantly associated with dose after correction within each gene. A multiple regression model with VKORC1, CYP2C9, PROC and the non-genetic predictors age, bodyweight, drug interactions and indication for treatment jointly accounted for 62% of variance in warfarin dose. Weaker associations observed for other genes could explain up to approximately 10% additional dose variance, but require testing and validation in an independent and larger data set. Translation of this knowledge into clinical guidelines for warfarin prescription will be likely to have a major impact on the safety and efficacy of warfarin.

Fatal hemorrhage in mice lacking gamma-glutamyl carboxylase

The carboxylation of glutamic acid residues to gamma-carboxyglutamic acid (Gla) by the vitamin K-dependent gamma-glutamyl carboxylase (gamma-carboxylase) is an essential posttranslational modification required for the biological activity of a number of proteins, including proteins involved in blood coagulation and its regulation. Heterozygous mice carrying a null mutation at the gamma-carboxylase (Ggcx) gene exhibit normal development and survival with no evidence of hemorrhage and normal functional activity of the vitamin K-dependent clotting factors IX, X, and prothrombin. Analysis of a Ggcx(+/-) intercross revealed a partial developmental block with only 50% of expected Ggcx(-/-) offspring surviving to term, with the latter animals dying uniformly at birth of massive intra-abdominal hemorrhage. This phenotype closely resembles the partial midembryonic loss and postnatal hemorrhage previously reported for both prothrombin- and factor V (F5)-deficient mice. These data exclude the existence of a redundant carboxylase pathway and suggest that functionally critical substrates for gamma-carboxylation, at least in the developing embryo and neonate, are primarily restricted to components of the blood coagulation cascade.

A functional single nucleotide polymorphism in the vitamin-K-dependent gamma-glutamyl carboxylase gene (Arg325Gln) is associated with bone mineral density in elderly Japanese women

The vitamin-K-dependent gamma-glutamyl carboxylase (GGCX) carboxylates vitamin-K-dependent proteins including bone Gla protein (osteocalcin) and matrix Gla protein, which play important roles in bone metabolism. Therefore, GGCX polymorphism might explain in part individual susceptibility to osteoporosis. In the present study, polymorphisms in the exons of this gene were screened in Japanese elderly women and a non-synonymous single nucleotide polymorphisms (SNP) were found; c.8762 G>A; (Arg325Gln). When the kinetic parameters of GGCX325-Gln and GGCX325-Arg were compared in vitro, Vmax/Km was significantly higher for GGCX325-Gln (944.4+/-9.21 pmol/30 min/mg/mM FLEEL) than for GGCX325-Arg (671.9+10.79 pmol/30 min/mg/mM FLEEL) (p=0.018). Then, association study of this polymorphism with forearm bone mineral density (BMD) of Japanese postmenopausal women (n=500, age 73.6+/-5.74) was conducted. As a result, the body mass index (BMI)-adjusted Z score in the subpopulation older than 75 years (n=207) was higher in those with 325-Gln (0.650+/-0.883, mean+/-SD) than those with 325-Arg/Gln or 325-Arg (0.133+/-0.650) (p=0.0383). This is the first report to demonstrate the different activities of GGCX between the common genotypes and their association with BMD.

Pharmacogenetics of warfarin: current status and future challenges

Warfarin is an anticoagulant that is difficult to use because of the wide variation in dose required to achieve a therapeutic effect, and the risk of serious bleeding. Warfarin acts by interfering with the recycling of vitamin K in the liver, which leads to reduced activation of several clotting factors. Thirty genes that may be involved in the biotransformation and mode of action of warfarin are discussed in this review. The most important genes affecting the pharmacokinetic and pharmacodynamic parameters of warfarin are CYP2C9 (cytochrome P(450) 2C9) and VKORC1 (vitamin K epoxide reductase complex subunit 1). These two genes, together with environmental factors, partly explain the interindividual variation in warfarin dose requirements. Large ongoing studies of genes involved in the actions of warfarin, together with prospective assessment of environmental factors, will undoubtedly increase the capacity to accurately predict warfarin dose. Implementation of pre-prescription genotyping and individualized warfarin therapy represents an opportunity to minimize the risk of haemorrhage without compromising effectiveness.

Compound heterozygosity of novel missense mutations in the gamma-glutamyl-carboxylase gene causes hereditary combined vitamin K-dependent coagulation factor deficiency

Hereditary combined vitamin K-dependent (VKD) coagulation factor deficiency is an autosomal recessive bleeding disorder associated with defects in either the gamma-carboxylase, which carboxylates VKD proteins to render them active, or the vitamin K epoxide reductase (VKORC1), which supplies the reduced vitamin K cofactor required for carboxylation. Such deficiencies are rare, and we report the fourth case resulting from mutations in the carboxylase gene, identified in a Tunisian girl who exhibited impaired function in hemostatic VKD factors that was not restored by vitamin K administration. Sequence analysis of the proposita did not identify any mutations in the VKORC1 gene but, remarkably, revealed 3 heterozygous mutations in the carboxylase gene that caused the substitutions Asp31Asn, Trp157Arg, and Thr591Lys. None of these mutations have previously been reported. Family analysis showed that Asp31Asn and Thr591Lys were coallelic and maternally transmitted while Trp157Arg was transmitted by the father, and a genomic screen of 100 healthy individuals ruled out frequent polymorphisms. Mutational analysis indicated wild-type activity for the Asp31Asn carboxylase. In contrast, the respective Trp157Arg and Thr591Lys activities were 8% and 0% that of wild-type carboxylase, and their compound heterozygosity can therefore account for functional VKD factor deficiency. The implications for carboxylase mechanism are discussed.

Common VKORC1 and GGCX polymorphisms associated with warfarin dose

We report a novel combination of factors that explains almost 60% of variable response to warfarin. Warfarin is a widely used anticoagulant, which acts through interference with vitamin K epoxide reductase that is encoded by VKORC1. In the next step of the vitamin K cycle, gamma-glutamyl carboxylase encoded by GGCX uses reduced vitamin K to activate clotting factors. We genotyped 201 warfarin-treated patients for common polymorphisms in VKORC1 and GGCX. All the five VKORC1 single-nucleotide polymorphisms covary significantly with warfarin dose, and explain 29-30% of variance in dose. Thus, VKORC1 has a larger impact than cytochrome P450 2C9, which explains 12% of variance in dose. In addition, one GGCX SNP showed a small but significant effect on warfarin dose. Incorrect dosage, especially during the initial phase of treatment, carries a high risk of either severe bleeding or failure to prevent thromboembolism. Genotype-based dose predictions may in future enable personalised drug treatment from the start of warfarin therapy.

Factor IX variants improve gene therapy efficacy for hemophilia B

Intramuscular injection of adeno-associated viral (AAV) vector to skeletal muscle of humans with hemophilia B is safe, but higher doses are required to achieve therapeutic factor IX (F.IX) levels. The efficacy of this approach is hampered by the retention of F.IX in muscle extracellular spaces and by the limiting capacity of muscle to synthesize fully active F.IX at high expression rates. To overcome these limitations, we constructed AAV vectors encoding F.IX variants for muscle- or liver-directed expression in hemophilia B mice. Circulating F.IX levels following intramuscular injection of AAV-F.IX-K5A/V10K, a variant with low-affinity to extracellular matrix, were 2-5 fold higher compared with wild-type (WT) F.IX, while the protein-specific activities remained similar. Expression of F.IX-R338A generated a protein with 2- or 6-fold higher specific activity than F.IX-WT following vector delivery to skeletal muscle or liver, respectively. F.IX-WT and variant forms provide effective hemostasis in vivo upon challenge by tail-clipping assay. Importantly, intramuscular injection of AAV-F.IX variants did not trigger antibody formation to F.IX in mice tolerant to F.IX-WT. These studies demonstrate that F.IX variants provide a promising strategy to improve the efficacy for a variety of gene-based therapies for hemophilia B.

Compound heterozygous mutations in the gamma-glutamyl carboxylase gene cause combined deficiency of all vitamin K-dependent blood coagulation factors

Hereditary combined deficiency of the vitamin K-dependent coagulation factors II, VII, IX, X, protein C, S and protein Z (VKCFD) is a very rare autosomal recessive inherited bleeding disorder. The phenotype may result from functional deficiency of either the gamma-glutamyl carboxylase (GGCX) or the vitamin K epoxide reductase (VKOR) complex. We report on the third case of VKCFD1 with mutations in the gamma-glutamyl carboxylase gene, which is remarkable because of compound heterozygosity. Two mutations were identified: a splice site mutation of exon 3 and a point mutation in exon 11, resulting in the replacement of arginine 485 by proline. Screening of 100 unrelated normal chromosomes by restriction fragment length polymorphism and denaturing high-performance liquid chromatography analysis excluded either mutation as a frequent polymorphism. Substitution of vitamin K could only partially normalize the levels of coagulation factors. It is suggested that the missense mutation affects either the propeptide binding site or the vitamin K binding site of GGCX.

AAV-mediated gene transfer for hemophilia

The goal of our work has been to establish an experimental basis for gene transfer as a method of treating hemophilia, an inherited bleeding disorder that results from the absence of functional factor VIII or factor IX. Using an adeno-associated viral vector derived from AAV serotype 2, we have shown in mice and in hemophilic dogs that we can achieve long-term expression (>3 years) of clotting factor at levels that would result in an improvement of clinical symptoms of the disease. A phase I trial of intramuscular injection of AAV-F.IX showed no evidence of local or systemic toxicity in any of the subjects. Muscle biopsies showed evidence for gene transfer and expression by polymerase chain reaction, Southern blot, and immunohistochemistry. We have also shown that AAV-F.IX can be delivered into the portal veins of hemophilic dogs and that this results in high circulating levels of factor IX, on the order of 5% to 14%, whereas delivery of similar doses to skeletal muscle results in factor levels of only 1% to 2%. Based on these results, a trial of AAV-mediated liver-directed gene transfer for hemophilia B has been proposed and is reviewed here.

gamma -Glutamyl carboxylation: An extracellular posttranslational modification that antedates the divergence of molluscs, arthropods, and chordates

The posttranslational gamma-carboxylation of glutamate residues in secreted proteins to gamma-carboxyglutamate is carried out by the vitamin K-dependent enzyme gamma-glutamyl carboxylase. gamma-Carboxylation has long been thought to be a biochemical specialization of vertebrates, essential for blood clotting. Recently, a gamma-carboxylase was shown to be expressed in Drosophila, although its function remains undefined in this organism. We have characterized both cDNA and genomic clones for the gamma-glutamyl carboxylase from the marine mollusc, Conus, the only nonvertebrate organism for which gamma-carboxyglutamate-containing proteins have been biochemically and physiologically characterized. The predicted amino acid sequence has a high degree of sequence similarity to the Drosophila and vertebrate enzymes. Although gamma-carboxylases are highly conserved, the Conus and mammalian enzymes have divergent substrate specificity. There are striking parallels in the gene organization of Conus and human gamma-carboxylases. Of the 10 Conus introns identified, 8 are in precisely the same position as the corresponding introns in the human enzyme. This remarkable conservation of intron/exon boundaries reveals that an intron-rich gamma-carboxylase was present early in the evolution of the animal phyla; although specialized adaptations in mammals and molluscs that require this extracellular modification have been identified, the ancestral function(s) and wider biological roles of gamma-carboxylation still need to be defined. The data raise the possibility that most introns in the genes of both mammals and molluscs antedate the divergence of these phyla.

On a potential global role for vitamin K-dependent gamma-carboxylation in animal systems. Evidence for a gamma-glutamyl carboxylase in Drosophila

The vitamin K-dependent gamma-carboxylation of glutamate to gamma-carboxyglutamate was originally well characterized in the mammalian blood clotting cascade. gamma-Carboxyglutamate has also been found in a number of other mammalian proteins and in neuropeptides from the venoms of marine snails belonging to the genus Conus, suggesting wider prevalence of gamma-carboxylation. We demonstrate that an open reading frame from a Drosophila melanogaster cDNA clone encodes a protein with vitamin K-dependent gamma-carboxylase activity. The open reading frame, 670 amino acids in length, is truncated at the C-terminal end compared with mammalian gamma-carboxylase, which is 758 amino acids. The mammalian gene has 14 introns; in Drosophila there are two much shorter introns but in positions precisely homologous to two of the mammalian introns. In addition, a deletion of 6 nucleotides is observed when cDNA and genomic sequences are compared. In situ hybridization to fixed embryos indicated ubiquitous presence of carboxylase mRNA throughout embryogenesis. Northern blot analysis revealed increased mRNA levels in 12-24-h embryos. The continued presence of carboxylase mRNA suggests that it plays an important role during embryogenesis. Although the model substrate FLEEL is carboxylated by the enzyme, a substrate containing the propeptide of a Conus carboxylase substrate, conantokin G, is poorly carboxylated. Its occurrence in vertebrates, molluscan systems (i.e. Conus), and Drosophila and the apparently strong homology between the three systems suggest that this is a highly conserved and widely distributed post-translational modification in biological systems.

Posttranslational modifications of recombinant myotube-synthesized human factor IX

Recent data demonstrate that the introduction into skeletal muscle of an adeno-associated viral (AAV) vector expressing blood coagulation factor IX (F.IX) can result in long-term expression of the transgene product and amelioration of the bleeding diathesis in animals with hemophilia B. These data suggest that biologically active F.IX can be synthesized in skeletal muscle. Factor IX undergoes extensive posttranslational modifications in the liver, the normal site of synthesis. In addition to affecting specific activity, these posttranslational modifications can also affect recovery, half-life in the circulation, and the immunogenicity of the protein. Before initiating a human trial of an AAV-mediated, muscle-directed approach for treating hemophilia B, a detailed biochemical analysis of F.IX synthesized in skeletal muscle was carried out. As a model system, human myotubes transduced with an AAV vector expressing F.IX was used. F.IX was purified from conditioned medium using a novel strategy designed to purify material representative of all species of rF.IX in the medium. Purified F.IX was analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), N-terminal sequence analysis, chemical gamma-carboxyglutamyl analysis, carbohydrate analysis, assays for tyrosine sulfation, and serine phosphorylation, and for specific activity. Results show that myotube-synthesized F.IX has specific activity similar to that of liver-synthesized F.IX. Posttranslational modifications critical for specific activity, including removal of the signal sequence and propeptide, and gamma-carboxylation of the N-terminal glutamic acid residues, are also similar, but carbohydrate analysis and assessment of tyrosine sulfation and serine phosphorylation disclose differences. In vivo experiments in mice showed that these differences affect recovery but not half-life of muscle-synthesized F.IX.

The Rhesus Macaque as an Animal Model for Hemophilia B Gene Therapy

We have determined the 2905 nucleotide sequence of the rhesus macaque factor IX complementary DNA (cDNA) and found it to be greater than 95% identical to that of the human factor IX cDNA. The cDNA has a large 3′ untranslated region like the human cDNA, but unlike the human cDNA has two polyadenylation sites 224 nucleotides apart that are used for transcription of the messenger RNA. The deduced amino acid sequence is greater than 97% identical to that of human factor IX, differing in only 11 of 461 amino acids in the complete precursor protein. We found a single silent polymorphism in the nucleotide sequence at the third position of the codon for asparagine at position 167 in the secreted protein (AAC/AAT). All residues subject to posttranslational modifications in the human protein are also found in the rhesus factor IX sequence. The high degree of homology between the rhesus and human factor IX proteins suggested the possibility that the human factor IX protein might be nonimmunogenic in the rhesus. We tested the immunogenicity of human factor IX in three rhesus macaques by repeated intravenous injections of monoclonal antibody–purified, plasma-derived human factor IX over the course of more than a year and assessed the recovery and half-life of the infused protein, as well as in vitro indicators of antihuman factor IX antibodies. Human factor IX recovery and half-life remained unchanged over the course of a year in the three animals studied, and aPTT mixing studies showed no evidence for neutralizing antihuman factor IX antibodies. An outbred, nonhuman primate model that permits assessment of the level and duration of factor IX expression as well as vector safety would complement the use of other (mouse and canine) hemophilia B animal models in current use for the development of gene therapy for hemophilia B.

A Missense Mutation in γ-Glutamyl Carboxylase Gene Causes Combined Deficiency of All Vitamin K-Dependent Blood Coagulation Factors

To identify potential mutations in the γ-glutamyl carboxylase gene, the sequence of all exons and intron/exon borders was determined in 4 patients from a consanguineous kindred with combined deficiency of all vitamin K-dependent procoagulants and anticoagulants and results were compared with normal genomic sequence. All 4 patients were homozygous for a point mutation in exon 9 that resulted in the conversion of an arginine codon (CTG) to leucine codon (CGG) at residue 394. Screening of this mutation based on introduction of Alu I site in amplified fragment from normal allele but not from the mutated allele showed that 13 asymptomatic members of the kindred were heterozygous for the mutation. The mutation was not found in 340 unrelated normal chromosomes. The segregation pattern of the mutation which is the first reported in the γ-glutamyl carboxylase gene fits perfectly with phenotype of the disorder and confirms the suggested autosomal recessive pattern of inheritance of combined deficiency of all vitamin K-dependent procoagulants and anticoagulants in this kindred. The mutated carboxylase protein expressed in Drosophila cells was stable but demonstrated threefold reduced activity compared with WT carboxylase, confirming that the L394R mutation results in a defective carboxylase.

A Missense Mutation in γ-Glutamyl Carboxylase Gene Causes Combined Deficiency of All Vitamin K-Dependent Blood Coagulation Factors

To identify potential mutations in the γ-glutamyl carboxylase gene, the sequence of all exons and intron/exon borders was determined in 4 patients from a consanguineous kindred with combined deficiency of all vitamin K-dependent procoagulants and anticoagulants and results were compared with normal genomic sequence. All 4 patients were homozygous for a point mutation in exon 9 that resulted in the conversion of an arginine codon (CTG) to leucine codon (CGG) at residue 394. Screening of this mutation based on introduction of Alu I site in amplified fragment from normal allele but not from the mutated allele showed that 13 asymptomatic members of the kindred were heterozygous for the mutation. The mutation was not found in 340 unrelated normal chromosomes. The segregation pattern of the mutation which is the first reported in the γ-glutamyl carboxylase gene fits perfectly with phenotype of the disorder and confirms the suggested autosomal recessive pattern of inheritance of combined deficiency of all vitamin K-dependent procoagulants and anticoagulants in this kindred. The mutated carboxylase protein expressed in Drosophila cells was stable but demonstrated threefold reduced activity compared with WT carboxylase, confirming that the L394R mutation results in a defective carboxylase.

Cloning of rat vitamin K-dependent gamma-glutamyl carboxylase and developmentally regulated gene expression in postimplantation embryos

Vitamin K-dependent carboxylase catalyzes the posttranslational modification of glutamate to gamma-carboxyglutamate (Gla) in its substrates, the vitamin K-dependent proteins (VKDPs). This modification is required for the activities of the VKDPs. Recent evidence demonstrates previously unrecognized roles for VKDPs as signaling molecules important in the regulation of cell growth, adhesion, and apoptosis, suggesting developmental functions for VKDPs and hence the carboxylase. The tissue distribution and functions of carboxylase in development are unknown. In this study, we isolated and characterized the full-length cDNA encoding the rat carboxylase and analyzed, at the cellular level, the expression of this gene in rat embryos by in situ hybridization. We demonstrate that the expression of this gene is highly regulated in a developmental and tissue-specific manner. Hepatocytes, the major site of synthesis of VKDPs of blood coagulation, express carboxylase mRNA late in gestation, in contrast to the central nervous system, mesenchymal, and skeletal tissues which express carboxylase mRNA early during rat embryogenesis. The tissue-specific temporal expression of the carboxylase gene during embryogenesis indicates that vitamin K-dependent carboxylation and the formation of Gla is developmentally regulated. These studies suggest that vitamin K-dependent carboxylation is an important modulator of embryonic VKDP function.