The TPMT gene helps your body break down certain medicines called thiopurines. If it works less well or not at all, these drugs can build up and cause very low blood counts, mouth sores, or other side effects. This gene can play a role in how your body responds to azathioprine, mercaptopurine, and thioguanine. Some people learn about the TPMT gene before starting treatment so clinicians can choose a safer plan.
What is a gene?
Genes are sections of DNA that carry instructions your cells use to make molecules or regulate processes in the body. Some genes provide the directions for making proteins, the parts of cells that do much of the work. Other genes mostly help control when certain processes take place, where they occur, and how active they are. Together, genes affect how the body develops and works, and they can influence how you respond to food, stress, or medications. For a more detailed look at this topic, you can find more in the LearnHub.
Overview
TPMTGene function
What if the gene works
The TPMT gene helps your body process thiopurine medicines, including azathioprine and mercaptopurine. Its protein breaks down part of these drugs, which helps control how much active drug stays in your cells.
Risk
What if the gene does not work
Variants in the TPMT gene can reduce or nearly eliminate this enzyme’s activity. Milder variants may leave some activity, while more severe variants leave very little, so cells break down thiopurine compounds more slowly and active byproducts can build up.
Treatment
What does that mean for your treatment
A pathogenic or loss-of-function variant in the TPMT gene can make it harder to break down thiopurine medicines such as azathioprine, mercaptopurine, and thioguanine. If you carry this kind of variant, clinicians often use a much lower dose or choose a different medicine to reduce the chance of severe bone marrow suppression and low blood counts.
Conditions associated with TPMT
Gene function
What if the gene works
Your body uses the TPMT gene to handle a small group of medicines in a measured way. Its protein adds a tiny chemical tag, called a methyl group, to thiopurine compounds, which changes how active they are. This is the core of how the TPMT gene works in the body.
Inside cells, the TPMT gene makes an enzyme that attaches a small chemical tag called a methyl group to thiopurine compounds and similar substances. That tag changes how those compounds behave and helps your body handle them.
One of its main jobs is helping process thiopurine medicines such as azathioprine, mercaptopurine, and thioguanine. This is a key part of how the TPMT gene affects medication processing in the body.
By modifying thiopurine compounds, this gene helps control the balance between more active and less active forms inside cells. That supports a steadier overall level of drug activity as the body handles treatment.
What is a variant?
A genetic variant is a minor change in the DNA sequence within a gene. These changes are common and help shape normal human diversity, including differences in appearance, metabolism, and the way the body reacts to environmental influences. Many variants do not cause any noticeable effects and are simply part of natural genetic variation. Sometimes, though, a variant can change how a gene works more significantly and may contribute to or lead to a genetic condition.
Genetic variants
What if the gene does not work
Variants in the TPMT gene can make the TPMT enzyme less stable, less active, or, in some cases, nearly absent inside cells. In practice, how TPMT gene variants affect thiopurine drugs depends on whether the change causes a mild drop in enzyme activity or a more severe loss of function. Some variants leave partial activity, while others greatly reduce the cell’s ability to chemically inactivate these medicines.
How variants are linked to conditions
In discussions of the TPMT gene variants and related health conditions, the main well-established link is thiopurine intolerance rather than a separate inherited illness. People with certain variants in the TPMT gene have a higher risk of very low blood counts, infections, and sometimes liver problems when treated with thiopurine medicines such as azathioprine, mercaptopurine, or thioguanine. Many people with these variants never have related problems unless they take one of these medicines.
A variant is not necessarily a diagnosis
Genetic variants are common across the general population and are a normal feature of human biology. Most do not directly affect health and simply reflect natural variation among people. For many genes, having a variant associated with a condition points to a possible link rather than confirming that condition, particularly when several genes and environmental influences play a role. Still, in certain situations, one well-characterized variant in a particular gene is enough to establish a diagnosis.
Are there things that can increase or change my risk?
For TPMT, risk mainly changes if you take certain medicines such as azathioprine, mercaptopurine, or thioguanine. Inherited TPMT changes that lower enzyme activity greatly raise the chance of serious side effects, especially very low blood counts, and this risk can be even higher with higher doses or with changes in the NUDT15 gene.
Inheritance of TPMT variants
For inherited TPMT deficiency, variants in the TPMT gene are usually passed down in an autosomal codominant pattern. This TPMT gene inheritance pattern means you receive one copy of the gene from each parent, and the effect often depends on whether you inherit one reduced-function variant or two. People with one reduced-function copy often have intermediate TPMT activity, while those with two reduced-function copies usually have low or absent TPMT activity. Parents can pass these variants to sons and daughters equally.
What does my TPMT result mean for my family?
TPMT results often run in families, so close relatives may carry the same change. This can matter because family members with a TPMT change may process certain medicines, especially thiopurines, more slowly or more quickly than expected. Parents, brothers, sisters, and children may want to share this result with their own doctor if thiopurine treatment is ever being considered.
Genetic information
Genetic testing
How you find out about variants
TPMT genetic testing is usually done on blood, saliva, or a cheek swab. A lab reads the DNA code in the TPMT gene to look for inherited variants. Results may show whether you carry common variants linked with reduced TPMT activity.
When should I consider getting a genetic test?
Consider TPMT testing before starting certain medicines, especially azathioprine, mercaptopurine, or thioguanine, because some gene changes can raise the chance of serious side effects. It can also help explain unexpectedly strong reactions, such as very low blood counts, after starting one of these treatments.
A genetic result is only one part of your health story
Genetic test results can be informative, but how much they reveal varies by the condition and the specific type of variant. Over time, many factors also influence how the body functions, including lifestyle, environment, age, and medical history. In some conditions, especially those shaped by many genes, genetic information offers context rather than certainty and is most useful when considered together with other parts of care. In other situations, a genetic result can provide a clear answer. In areas like medication response, genetic results can also help put personal experiences into context.
Pharmacogenetics
How variants shape your response to medicine
Variants in the TPMT gene can strongly affect how your body breaks down thiopurine medicines such as azathioprine, mercaptopurine, and thioguanine. If you carry a TPMT variant that lowers gene activity, these medicines can build up more easily and increase the chance of side effects, including very low blood cell counts. This helps explain why the same medication can work differently in different people, because inherited differences can change how quickly the body processes a drug. Genetic variation in medication response is common and can influence both how well a treatment works and how sensitive someone is to it.
What is Pharmacogenetics?
Pharmacogenetics is the area of medicine that examines how genetic differences affect a person’s response to medications. It looks at the relationship between genes and drugs to better explain why the same treatment may work differently from one person to another. For many medicines, genes can influence both how strongly a drug works and how the body handles it. This knowledge can help guide treatment decisions that are more informed and more individualized. More on this topic is available in the LearnHub.
Drugs influenced by the TPMT gene
When should I seek medical advice?
If you are about to start or change certain medicines called thiopurines, it is worth discussing the TPMT gene with your doctor because it can strongly affect how your body handles these treatments. People with lower TPMT activity can have a much higher chance of serious side effects, especially very low blood counts. Seek medical advice promptly if you develop unusual bruising, fever, mouth sores, or marked tiredness while taking one of these medicines.
History
Interest in the TPMT gene grew as doctors noticed that people could respond very differently to the same thiopurine medicines. In recent decades, awareness of this gene’s role has grown. A major step came in the 1980s, when researchers showed that these treatment differences often followed an inherited pattern, helping launch modern pharmacogenetics, the study of how genes affect medicine response. Building on this early work, scientists uncovered more about the TPMT gene and linked certain variants to a higher chance of treatment side effects. This research helped move genetic testing into routine care for some patients before thiopurine treatment. Today, the TPMT gene is often cited as one of the clearest early examples of genetics improving safer prescribing.