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TPMT gene
Reviewed by Medicinsk Expert•Last updated: 2026-10-22•Gene

TPMT gene

The TPMT gene provides instructions for an enzyme that metabolizes thiopurine drugs, and genetic variations in this gene can significantly affect a patient's risk of drug toxicity.

Normal Function

The TPMT gene provides instructions for making an enzyme called thiopurine S-methyltransferase (TPMT). This enzyme is critical for breaking down (metabolizing) drugs called thiopurines, which include 6-thioguanine, 6-mercaptopurine, and azathioprine. These drugs are used to suppress the immune system in patients with cancer, Crohn's disease, rheumatoid arthritis, or those who have received an organ transplant.

Thiopurine drugs are converted into toxic compounds that kill immune system cells in the bone marrow. The TPMT enzyme "turns off" these drugs by metabolizing them into inactive, nontoxic compounds.

Health Conditions Related to Genetic Changes

Thiopurine S-methyltransferase deficiency

Changes in the TPMT gene cause TPMT deficiency, a reduction in the activity of the TPMT enzyme. Without enough of this enzyme, the body cannot effectively "turn off" thiopurine drugs. The drugs remain in the body longer and continue to destroy cells, leading to myelosuppression—an inability of the bone marrow to produce enough red blood cells, white blood cells, and platelets. This can result in abnormal bleeding and an increased risk of life-threatening infections.

TPMT gene activity is classified as either high-activity or low-activity. Individuals with two low-activity copies of the gene are at the greatest risk of hematopoietic toxicity when treated with thiopurines. Those with one high-activity and one low-activity copy have moderately reduced enzyme activity and also face an increased risk unless given a significantly lower dose of the medication.

Other Names for This Gene

  • S-adenosyl-L-methionine:thiopurine S-methyltransferase
  • thiopurine methyltransferase
  • TPMT_HUMAN

Frequently Asked Questions

While on thiopurine medications, it's important to be vigilant for signs of myelosuppression. These can include unexplained fever, unusual bruising or bleeding, extreme fatigue, and frequent or severe infections. Regular blood tests (complete blood counts) are essential to monitor for these changes, even if you don't have obvious symptoms, as early detection allows for timely dose adjustments or treatment changes.
Yes, TPMT deficiency is an inherited condition. It follows an autosomal recessive pattern, meaning a person must inherit two copies of a low-activity TPMT gene (one from each parent) to have severely reduced enzyme activity. Individuals who inherit one low-activity gene and one high-activity gene will have moderately reduced activity. If you have TPMT deficiency, it's important for family members to be aware, especially if they might need thiopurine drugs in the future, as they may also be carriers or affected.
TPMT gene testing is crucial because it identifies individuals who have reduced or absent activity of the TPMT enzyme. This enzyme is responsible for breaking down thiopurine drugs. If a person has low TPMT activity, the drugs are metabolized slowly, leading to a buildup of toxic compounds in the body. This can cause severe side effects, particularly myelosuppression (a dangerous decrease in blood cell production), which can be life-threatening.
If your TPMT test shows low enzyme activity, your doctor will likely significantly reduce your thiopurine medication dosage, often to 10-15% of the standard dose, or consider an alternative treatment. Individuals with moderately reduced activity may receive a lower dose, while those with very low or absent activity require extreme caution or complete avoidance of these drugs. This adjustment helps prevent severe toxicity while still allowing for therapeutic benefits.
Source: MedlinePlus, National Library of Medicine

MedlinePlus brings together authoritative health information from the National Library of Medicine (NLM), the National Institutes of Health (NIH), and other government agencies and health-related organizations.

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