PGx AZATHIOPRINE

Scientifically reviewed | Last updated July 14, 2025
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NUDT15, TPMT

The enzyme thiopurine S-methyltransferase (TPMT) catalyzes the addition of a methyl group to the respective sulfhydryl group of thiopurines such as azathioprine, mercaptopurine (6-MP), and thioguanine (6-TG). This methylation prevents the incorporation of nucleotide analogs into DNA or RNA during nucleic acid synthesis, and is therefore a crucial reaction for the inactivation of cytotoxic compounds.

 

Variants in the TPMT gene lead to enzyme deficiency, impairing the inactivation of thiopurines. Accumulation of thioguanine nucleotides in hematopoietic tissue can result in myelosuppression, potentially with fatal outcomes. The allele variants TPMT*2, *3A, *3B, and *3C are the most common causes of genetically determined TPMT deficiency. The test also detects the alleles TPMT*4, *7, *8, *9, *10, *15, *16, *19, *20, *25, *27, *28, *32, *37, *40, *41, thereby identifying over 95% of the causative deficiency alleles.

 

Determining the TPMT genotype before starting therapy allows for individualized dose adjustment and avoidance of adverse effects. The FDA has already included the relationship between enzyme activity, genotype, and dosage in the warnings section of drug labeling. Further information can be found in the CPIC and DPWG guidelines on TPMT and azathioprine, thioguanine, and mercaptopurine.

 

References

Relling et al., Clin Pharmacol Ther. 2013 Apr;93(4):324-5 / Swen et al. 2011, Clin Pharmacol Ther 89:662

GENES
NUDT15, TPMT
HOW CAN YOU GET TESTED?
Step 1: Visit healthcare professional
Step 2: Sample collection (blood or buccal swab)
Step 3: Sequencing performed at our accredited laboratory
Step 4: Medical report
Step 5: Genetic counselling
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