VKORC1: Warfarin's Drug Target Gene

What Is VKORC1?

VKORC1 (vitamin K epoxide reductase complex subunit 1) encodes an enzyme that plays an essential role in the blood clotting process. The VKORC1 enzyme recycles vitamin K, which is required for the activation of several clotting factors (II, VII, IX, and X). Without functional VKORC1, these clotting factors cannot be activated, and blood clotting is impaired.

VKORC1 is different from the CYP pharmacogenes (like CYP2C9, CYP2C19, and CYP2D6) in an important way: it is not a drug-metabolizing enzyme. It is the pharmacological target of warfarin. Warfarin works by inhibiting VKORC1, thereby reducing the activation of vitamin K-dependent clotting factors and preventing excessive blood clot formation. Understanding this distinction is fundamental to pharmacogenomics.

VKORC1 and Warfarin Sensitivity

Genetic variants in the VKORC1 promoter region affect how much VKORC1 protein your body produces. The key variant — rs9923231, also known as VKORC1 -1639G>A — directly influences gene expression levels. Individuals with the A allele produce less VKORC1 enzyme, making them more sensitive to warfarin's inhibitory effect.

This relationship is straightforward: if you produce less of warfarin's target enzyme, you need less warfarin to achieve the same anticoagulant effect. Patients with the AA genotype (homozygous for the low-expression allele) typically require warfarin doses that are 25-50% lower than patients with the GG genotype. The GA genotype falls in between.

This is why the CPIC warfarin dosing guideline incorporates VKORC1 genotype alongside CYP2C9 metabolizer status. CYP2C9 determines how fast you clear warfarin; VKORC1 determines how sensitive you are to it. Both factors together provide a more complete picture for dose estimation.

VKORC1 Genotype Groups

Unlike CYP enzymes, VKORC1 does not use the metabolizer phenotype nomenclature (Normal/Intermediate/Poor/Ultrarapid). Instead, CPIC guidelines classify VKORC1 genotype into sensitivity groups based on the rs9923231 variant:

• High Warfarin Sensitivity (AA): Homozygous for the low-expression allele. Lowest VKORC1 enzyme production. Patients typically require the lowest warfarin doses (often 25-50% less than average).
• Variable Warfarin Sensitivity (GA): Heterozygous. Intermediate VKORC1 enzyme production. Patients may need moderately reduced warfarin doses compared to those with the GG genotype.
• Normal Warfarin Sensitivity (GG): Homozygous for the normal-expression allele. Standard VKORC1 enzyme production. Standard warfarin dosing considerations apply (with CYP2C9 status still relevant).

The rs9923231 Variant

rs9923231 is a single nucleotide polymorphism (SNP) located in the promoter region of the VKORC1 gene, at position -1639 relative to the transcription start site. The G allele (reference) is associated with normal VKORC1 expression, while the A allele reduces promoter activity, leading to lower mRNA levels and less VKORC1 protein production.

This variant is highly informative for warfarin dosing prediction. Studies have estimated that VKORC1 genotype alone accounts for approximately 20-25% of the variability in warfarin dose requirements, making it the single largest genetic contributor to warfarin dose variability. When combined with CYP2C9 genotype and clinical factors, genetics can explain up to 40-50% of the variability in warfarin dosing requirements.

Population Frequency Differences

The frequency of the VKORC1 rs9923231 A allele varies substantially across populations, which partly explains observed population-level differences in average warfarin dose requirements:

• East Asian: A allele frequency approximately 89-95%. The majority of individuals in East Asian populations carry the high-sensitivity genotype, which is consistent with the clinical observation that East Asian patients generally require lower warfarin doses.
• European: A allele frequency approximately 37-44%. Genotype distribution is more balanced, with all three genotypes well-represented.
• African: A allele frequency approximately 10-20%. The GG (normal-sensitivity) genotype is most common. Additional VKORC1 variants beyond rs9923231 may also contribute to warfarin dose variability in African populations.

CPIC Clinical Guidelines

The CPIC warfarin guideline (Level A — strongest evidence) integrates VKORC1 genotype with CYP2C9 metabolizer status to produce genotype-guided initial dose recommendations. The guideline provides both a pharmacogenetic dosing algorithm and a clinical dosing table as alternative approaches.

VKORC1 genotype has the largest effect on estimated dose: an AA patient may have an estimated maintenance dose 2-3 mg/day lower than a GG patient with the same CYP2C9 status. The clinical value of genotype-guided dosing is best established during warfarin initiation, when the risk of over- or under-anticoagulation is highest. For more on how these guidelines translate to clinical practice, see the warfarin and CYP2C9 page.

Get Your VKORC1 Results

The key VKORC1 variant rs9923231 is well-covered on consumer genotyping arrays from 23andMe, AncestryDNA, and other services. Upload your raw data to DecodeMyBio to learn your VKORC1 genotype group. Your Medication Safety Report will include your VKORC1 genotype, warfarin sensitivity classification, and the combined CYP2C9-VKORC1 warfarin dosing context. You can view a sample report to preview the format.

For details on how pharmacogenomic testing works from consumer raw data, and for information on the limitations of consumer-grade analysis, see our reference pages. For an overview of how consumer raw data translates to medication insights, see pharmacogenomic insights from raw DNA data.

Frequently Asked Questions

Beyond Medications: Nutrition and Methylation

If you're exploring how your DNA affects nutrition or methylation, your raw data also contains variants in genes like MTHFR, FUT2, VDR, and others that influence nutrient metabolism. See our Nutrition & Methylation Report for a nutrigenomic analysis from the same DNA data.