Warfarin resistance is a condition in which individuals have a high tolerance for the drug warfarin. Warfarin is an anticoagulant, which means that it thins the blood, preventing blood clots from forming. Warfarin is often prescribed to prevent blood clots in people with heart valve disease who have replacement heart valves, people with an irregular heart beat (atrial fibrillation), or those with a history of heart attack, stroke, or a prior blood clot in the deep veins of the arms or legs (deep vein thrombosis).

There are two types of warfarin resistance: incomplete and complete. Those with incomplete warfarin resistance can achieve the benefits of warfarin treatment with a high dose of warfarin. Individuals with complete warfarin resistance do not respond to warfarin treatment, no matter how high the dose. If people with warfarin resistance require anticoagulant therapy and take the average warfarin dose, they will remain at risk of developing a potentially harmful blood clot.

Both types of warfarin resistance are related to how the body processes warfarin. In some people with warfarin resistance, their blood-clotting process does not react effectively to the drug. Others rapidly break down (metabolize) warfarin, so the medication is quickly processed by their bodies; these individuals are classified as "fast metabolizers" or "rapid metabolizers" of warfarin. The severity of these abnormal processes determines whether the warfarin resistance is complete or incomplete.

Warfarin resistance does not appear to cause any health problems other than those associated with warfarin drug treatment.

Many genes are involved in the metabolism of warfarin and in determining the drug's effects in the body. Certain common changes (polymorphisms) in the VKORC1 gene account for 20 percent of the variation in warfarin metabolism due to genetic factors. Polymorphisms in other genes, some of which have not been identified, have a smaller effect on warfarin metabolism. The polymorphisms associated with warfarin resistance often differ by population and ethnic background.

The VKORC1 gene provides instructions for making a vitamin K epoxide reductase enzyme. The VKORC1 enzyme helps turn on (activate) clotting proteins in the pathway that forms blood clots. Warfarin prevents (inhibits) the action of the VKORC1 enzyme by binding to the enzyme and preventing it from binding to and activating the clotting proteins, stopping clot formation. Certain VKORC1 gene polymorphisms lead to the formation of a VKORC1 enzyme with a decreased ability to bind to warfarin. This reduction in warfarin binding causes incomplete warfarin resistance and results in a higher dose of warfarin needed to inhibit the VKORC1 enzyme and stop the clotting process. If the VKORC1 enzyme cannot bind to any warfarin, the result is complete warfarin resistance.

While changes in specific genes affect how the body reacts to warfarin, many other factors, including sex, age, weight, diet, and other medications, also play a role in the body's interaction with this drug.

Normal Function

The UGT1A1 gene belongs to a family of genes that provide instructions for making enzymes called UDP-glucuronosyltransferases. These enzymes perform a chemical reaction called glucuronidation, in which a compound called glucuronic acid is attached (conjugated) to one of a number of different substances.

The protein produced from the UGT1A1 gene, called the bilirubin uridine diphosphate glucuronosyl transferase (bilirubin-UGT) enzyme, is the only enzyme that glucuronidates bilirubin, a substance produced when red blood cells are broken down. This enzyme converts the toxic form of bilirubin (unconjugated bilirubin) to its nontoxic form (conjugated bilirubin), making it able to be dissolved and removed from the body.

The bilirubin-UGT enzyme is primarily found in cells of the liver, where bilirubin glucuronidation takes place. Conjugated bilirubin is dissolved in bile, a fluid produced in the liver, and excreted with solid waste.

Health Conditions Related to Genetic Changes

Crigler-Najjar syndrome

At least 85 mutations in the UGT1A1 gene that cause Crigler-Najjar syndrome have been identified. This condition occurs when both copies of the UGT1A1 gene in each cell are altered. Crigler-Najjar syndrome is characterized by high levels of unconjugated bilirubin in the blood (unconjugated hyperbilirubinemia) and yellowing of the skin and eyes (jaundice). Some affected individuals develop a form of brain damage called kernicterus due to the accumulation of unconjugated bilirubin in the brain, which can be lethal.

Mutations in the UGT1A1 gene that cause Crigler-Najjar syndrome result in reduced or absent function of the bilirubin-UGT enzyme. People with Crigler-Najjar syndrome type 1 (CN1) have no enzyme function, while people with Crigler-Najjar syndrome type 2 (CN2) have less than 20 percent of normal function. The signs and symptoms of CN1 are more severe than those of CN2. The loss of bilirubin-UGT function decreases glucuronidation of unconjugated bilirubin. This toxic substance then builds up in the body, causing hyperbilirubinemia, jaundice, and sometimes, kernicterus.

Gilbert syndrome

Changes in the UGT1A1 gene can cause Gilbert syndrome. This condition is characterized by periods of mild unconjugated hyperbilirubinemia, which rarely leads to episodes of jaundice.

Gilbert syndrome occurs worldwide, but some mutations are seen more often in particular populations. In many populations, the most common genetic change that causes Gilbert syndrome occurs in an area near the UGT1A1 gene called the promoter region, which controls the production of the bilirubin-UGT enzyme. This change must occur in both copies of the UGT1A1 gene to cause Gilbert syndrome. The common genetic change involved in Gilbert syndrome, called UGT1A1*28, results from the addition of two DNA building blocks (nucleotides) to an important sequence in the promoter region known as the TATA box. The normal UGT1A1 TATA box sequence is written as A(TA)6TAA. The UGT1A1*28 sequence includes an extra TA nucleotide pair and is written as A(TA)7TAA. This genetic change creates a longer than normal TATA box and impairs protein production.

The UGT1A1*28 change, however, is uncommon in Asian populations. Asians with Gilbert syndrome often have a mutation in one copy of the UGT1A1 gene that results in the change of a single protein building block (amino acid) in the bilirubin-UGT enzyme. The most common mutation in this population replaces the amino acid glycine with the amino acid arginine at position 71 of the enzyme (written as Gly71Arg or G71R). This type of mutation, known as a missense mutation, results in reduced enzyme function.

People with Gilbert syndrome have approximately 30 percent of normal bilirubin-UGT enzyme function. As a result, unconjugated bilirubin is not glucuronidated quickly enough, and it builds up in the body, causing mild hyperbilirubinemia.

Warfarin resistance

MedlinePlus Genetics provides information about Warfarin resistance

Other disorders

Although jaundice is common in newborns, mutations in the UGT1A1 gene increase the risk of developing a more severe condition called transient familial neonatal hyperbilirubinemia. In this condition, severe unconjugated hyperbilirubinemia and jaundice occur in newborns and usually disappear in 1 to 2 weeks. Some babies develop kernicterus (which can be lethal), hearing loss, or other neurological problems. The G71R mutation is the most common mutation associated with transient familial neonatal hyperbilirubinemia. Asian but not white newborns with a UGT1A1 gene mutation seem to be at risk of developing this condition.

Sometimes newborn jaundice is associated with breastfeeding: Unconjugated bilirubin levels increase when the baby is breastfed, causing jaundice, and return to normal when breastfeeding is stopped for a prolonged period. This condition, often called breast milk jaundice, appears 5 or 10 days after birth and disappears at around 4 months of age. Kernicterus is not typically seen in infants with breast milk jaundice. Research suggests that a substance in the breast milk of mothers of affected infants blocks glucuronidation. In addition, many affected infants have a mutation in one copy of the UGT1A1 gene, most commonly the G71R mutation, and the mutation is thought to underlie the unconjugated hyperbilirubinemia. The substance in the breast milk may trigger the buildup of unconjugated bilirubin in infants with already impaired bilirubin-UGT enzyme function.

Other Names for This Gene

• BILIQTL1
• bilirubin UDP-glucuronosyltransferase 1-1
• bilirubin-specific UDPGT isozyme 1
• GNT1
• HUG-BR1
• UD11_HUMAN
• UDP glucuronosyltransferase 1 family, polypeptide A1
• UDP glycosyltransferase 1 family, polypeptide A1
• UDP-glucuronosyltransferase 1-A
• UDP-glucuronosyltransferase 1A1
• UDPGT
• UDPGT 1-1
• UGT-1A
• UGT1
• UGT1*1
• UGT1-01
• UGT1.1
• UGT1A

Genomic Location

Normal Function

The VKORC1 gene provides instructions for making a vitamin K epoxide reductase enzyme. The VKORC1 enzyme is made primarily in the liver. It spans the membrane of a cellular structure called the endoplasmic reticulum, which is involved with protein processing and transport. The VKORC1 enzyme helps turn on (activate) clotting proteins in the pathway that forms blood clots. Specifically, the VKORC1 enzyme converts one form of vitamin K into a different form of vitamin K that assists in activating clotting proteins.

Health Conditions Related to Genetic Changes

Warfarin resistance

Multiple variations (polymorphisms) in the VKORC1 gene have been associated with warfarin resistance, a condition in which individuals require higher doses of the drug warfarin than are usually prescribed. Warfarin is an anticoagulant, which means that it thins the blood and prevents blood clots from forming. It acts by attaching (binding) to the VKORC1 enzyme and blocking (inhibiting) the activation of the clotting proteins.

The most common VKORC1 gene polymorphism in people with warfarin resistance changes a single protein building block (amino acid) in the VKORC1 enzyme. Specifically, the amino acid aspartic acid is replaced with the amino acid tyrosine at position 36 (written as Asp36Tyr or D36Y). This polymorphism leads to the formation of a VKORC1 enzyme with a decreased ability to bind to warfarin. As a result, a higher dose of warfarin is needed to inhibit the VKORC1 enzyme and stop the clotting process. If people with warfarin resistance require anticoagulant treatment and take the average warfarin dose (or less), they will remain at risk of developing a potentially harmful blood clot.

Warfarin sensitivity

Multiple polymorphisms in the VKORC1 gene have been associated with warfarin sensitivity, a condition in which individuals require lower doses of the drug warfarin than are usually prescribed.

The most common VKORC1 gene polymorphism in people with warfarin sensitivity, known as VKORC1A, changes a single DNA building block (nucleotide) in an area near the VKORC1 gene, which controls the production of the enzyme. Specifically, the nucleotide guanine is replaced with the nucleotide adenine (written as -1639G>A). The VKORC1A polymorphism is particularly common in individuals of Asian and European descent. This change reduces the amount of VKORC1 enzyme that is available to convert vitamin K into a form that can activate clotting proteins. Because there is a decreased amount of VKORC1 enzyme, a lower dose of warfarin is needed to inhibit the actions of the enzyme, resulting in warfarin sensitivity. If people with warfarin sensitivity take the average warfarin dose (or more), they are at risk of an overdose, which can cause abnormal bleeding in the brain, gastrointestinal tract, or other tissues, and may lead to serious health problems or death.

Other Names for This Gene

• FLJ00289
• vitamin K 1 2,3-epoxide reductase subunit 1
• vitamin K epoxide reductase complex, subunit 1
• VKOR

Genomic Location

The polymorphisms associated with this condition are inherited in an autosomal dominant pattern, which means one copy of the altered gene in each cell is sufficient to result in warfarin resistance. However, different polymorphisms affect the activity of warfarin to varying degrees. Additionally, people who have more than one polymorphism in a gene or polymorphisms in multiple genes associated with warfarin resistance have a higher tolerance for the drug's effect or are able to process the drug more quickly.