Does CALU SNP rs1043550 contribute variability to therapeutic warfarin dosing requirements?

Abstract

Objectives: Calumenin, a molecular chaperone, exerts a regulatory effect on the vitamin K-dependent γ-carboxylation redox cycle that inhibits transfer of the reduced vitamin K from VKORC1, the pharmacological target of warfarin, to the γ-carboxylase. Because of its polymorphic structure and central role in the warfarin metabolic pathway, a contributory role for calumenin to warfarin dose variability has been posited. The current study sought to validate modulation of therapeutic dosing requirements by a single nucleotide polymorphisms (SNP) occurring in the calumenin gene (CALU) reported in previous studies. The CALU SNP was further modeled to detect interaction with SNPs occurring in VKORC1, CYP2C9, and CYP4F2 genes and characterize any additional contribution to variability in therapeutic warfarin dose requirement.

Setting: The study was undertaken in an established, well-characterized cohort of subjects treated with warfarin in the Anticoagulation Clinic of Marshfield Clinic in Marshfield, Wisconsin.

Methods: Subjects (N=491) previously genotyped for SNPS known to contribute variability to therapeutic warfarin dose requirement were genotyped for CALU SNP rs1043550, using TaqMan assays. Contribution of CALU SNP rs1043550 was modeled relative to other genotypic and phenotypic characteristics including gender, diagnosis, age, body surface area, underlying indication for warfarin, comorbidities, and pharmacological exposures. Interaction between SNPs impacting on warfarin dose requirements and calumenin SNPs was also modeled.

Results: Small differences in warfarin dosing requirements detected among individuals encoding the mutant G allele in the calumenin SNP were not statistically or clinically significant relative to therapeutic warfarin dose requirement and did not independently contribute significantly to the warfarin dosing model. Interaction between calumenin and VKORC1 SNPs contributed only minor additional variability to that ascribed to the wild type VKORC1 genotype.

Conclusions: The impact of the CALU SNP on warfarin dose variability was minor and did not contribute significantly to therapeutic warfarin dose requirement in our study cohort. While no contribution was noted for the SNP examined in the present study, further examination of interaction between genetic elements contributing major impact on therapeutic warfarin dose requirements and genes exhibiting a lesser contribution is warranted.

Keywords: Anticoagulation; Calumenin; VKORC1; Vitamin K; Warfarin.

Figure 1

Diagram of the interactions between overlapping enzymatic pathways regulating clotting. Reduction in GGCX activity through in the presence of increased calumenin expression perturbs the vitamin K-dependent activity of VKORC1 whose enzymatic action regenerates vitamin K to its reduced state. Compensatory overexpression of VKORC1 results and is associated with increased resistance to warfarin.

Figure 2

Scatter plot showing therapeutic dose of warfarin for groups defined by calumenin variant (AA, AG, and GG). Each black dot corresponds to one patient. The red line indicates the median dosage of warfarin required for therapeutic anticoagulation.

Figure 3

Scatter plot of prediction errors (actual therapeutic dose minus predicted dose) in the subset of patients with CYP2C9 *1/*1 for groups defined by calumenin (AA, AG, and GG) within VKORC1 (GG, CG, and CC). Red lines indicate the median error. Prediction errors by both calumenin and VKORC1 did not differ significantly by genotype.