You may lack the enzyme thiopurine methyltransferase (TPMT) and not even know it.
Serious Side Effects
For most of those who lack the enzyme, it will never be a problem, but, for those who are prescribed thiopurine medication, the lack of functional TPMT enzyme could lead to serious side effects.
Thiopurines are a family of drugs that are used to “turn down” or suppress the immune system, which can be useful when you want to reduce inflammation, for instance.
They’re used in a variety of treatments for such conditions as leukemia, organ transplant rejection, rheumatoid arthritis, Crohn’s disease, and inflammatory bowel disease.
While these are important drugs for many individuals, for those who lack functional TPMT enzyme, thiopurines cause serious side effects, such as bleeding and low blood count. Because of these potential side effects, a doctor can test for TPMT enzyme activity before prescribing thiopurine drugs, but a test isn’t always done.
There are a couple of different types of tests available. One type of test looks at the levels of TPMT enzyme directly. Another type of test looks for mutations in the gene that encodes the TPMT enzyme. Approximately 10 percent of people with European ancestry have reduced TPMT activity due to having inherited one mutant TPMT gene. About 0.3 percent have none at all due to having inherited two mutant copies.