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FeedbackPolymerase Chain Reaction (PCR)/Fluorescence Monitoring
TPMT and NUDT15 are also available as standalone tests:
Polymerase Chain Reaction (PCR)/Flourescence Monitoring
Polymerase Chain Reaction (PCR)/Fluorescence Monitoring
Thiopurine drug therapy is used to treat autoimmune diseases, inflammatory bowel disease, acute lymphoblastic leukemia, and to prevent rejection after solid organ transplant. The inactivation of thiopurine drugs is catalyzed in part by thiopurine methyltransferase (TPMT) and nudix hydrolase 15 (NUDT15). , , Variants in the TPMT and/or NUDT15 genes are associated with an accumulation of cytotoxic metabolites leading to increased risk of drug-related toxicity with standard doses of thiopurine drugs, , and the effects on thiopurine catabolism can be additive.
This Test Fact Sheet focuses on genetic testing for TPMT and NUDT15 variants, which can be performed prior to or during thiopurine therapy. The enzyme activity phenotype of TPMT can be measured directly prior to drug administration. After the initiation of therapy, concentrations of thiopurines and metabolites can be measured to optimize thiopurine therapy dosing. For more information about these tests, refer to the Thiopurine Methyltransferase, RBC and Thiopurine Metabolites in Red Blood Cells Test Fact Sheets.
Disease Overview
Thiopurine drugs include azathioprine, mercaptopurine, and thioguanine. , These are inactive prodrugs that must be metabolized to 6-thioguanine nucleotides (6-TGNs) to function. , The primary metabolic route for inactivation of thiopurine drugs is catalyzed by TPMT. TPMT can be inhibited by common drugs, including NSAIDs, diuretics, and ulcerative colitis treatments such as mesalamine.
When TPMT activity is low, more 6-mercaptopurine (6-MP) may be converted into active (cytotoxic) 6-TGN, which accumulates in the body. Excess 6-TGN in bone marrow (BM) inhibits purine synthesis, which in turn inhibits cell proliferation and contributes to excessive myelosuppression. Specific TPMT variants (seen with higher frequency among individuals of African and European descent) have been associated with TPMT deficiency of varying severity. , In individuals with no or very low TPMT activity, severe myelosuppression occurs with conventional thiopurine doses. , Thirty to sixty percent of individuals with intermediate TPMT activity experience moderate to severe myelosuppression with conventional thiopurine doses. ,
NUDT15 catalyzes the conversion of cytotoxic 6-TG triphosphate metabolites to the less toxic 6-TG monophosphate. NUDT15 variants (seen with higher frequency among individuals of Asian and Hispanic descent) reduce enzyme activity and contribute to excessive myelosuppression. ,
Thiopurine dosing should rely on disease-specific guidelines and the degree of myelosuppression; refer to the Clinical Pharmacogenetics Implementation Consortium (CPIC) Guidelines for thiopurine dosing based on TPMT and NUDT15 genotypes.
Genetics
Genes
TPMT, NUDT15
Inheritance
NUDT15: Autosomal codominant
Test Interpretation
Variants Tested
| Gene (Transcript) | Allele |
|---|---|
| TPMT (NM_000367) | TPMT*2: rs1800462, c.238G>C |
| TPMT*3A: rs1800460, c.460G>A; rs1142345, c.719A>G | |
| TPMT*3B: rs1800460, c.460G>A | |
| TPMT*3C: rs1142345, c.719A>G | |
| TPMT*4: rs1800584, c.626-1G>A | |
| NUDT15 (NM_018283) | NUDT15*2 or *3: rs116855232, c.415C>T |
| NUDT15*4: rs147390019, c.416G>A |
Allele frequencies and phenotype predictions are available at www.pharmvar.org or www.pharmgkb.org.
Sensitivity/Specificity
Results
Metabolizer status is reported separately for each gene analyzed.
TPMT Results | |
|---|---|
| Number of TPMT Variants Detected | Interpretation |
| One | Intermediate metabolizer phenotype predicted Possible susceptibility to dose-related toxicity from standard doses of thiopurine drugs |
| Two | Poor metabolizer phenotype predicted Probable susceptibility to dose-related toxicity from standard doses of thiopurine drugs |
| Zero (reported as Negative) | Normal metabolizer phenotype predicted Standard doses of thiopurine drugs are likely appropriate |
NUDT15 Results | |
| Number of NUDT15 Variants Detecteda | Interpretation |
| One | Intermediate metabolizer phenotype predicted Possible susceptibility to dose-related toxicity from standard doses of thiopurine drugs |
| Two | Poor metabolizer phenotype predicted Probable susceptibility to dose-related toxicity from standard doses of thiopurine drugs |
| Zero (reported as Negative) | Normal metabolizer phenotype predicted Standard doses of thiopurine drugs are likely appropriate |
| aThe NUDT15*4 variant is of uncertain function and can result in an indeterminate metabolizer status. | |
Limitations
- Only targeted TPMT and NUDT15 variants will be detected by this test.
- Diagnostic errors can occur due to rare sequence variations.
- Genotyping in individuals who have received an allogeneic stem cell or bone marrow transplant will reflect donor status and is not recommended.
- Because the complex TPMT*3A allele contains the variants found in the *3B and *3C alleles, genotyping cannot distinguish the *3A/negative genotype (intermediate TPMT activity) from the rare *3B/*3C genotype (no or low TPMT activity); the *3A/negative genotype is assumed when both *3B and *3C are detected.
- Thiopurine drug metabolism and risk for adverse reactions to thiopurines may be affected by genetic and nongenetic factors that are not evaluated by this test.
- This test does not assess for TPMT allele variants associated with ultrahigh enzyme activity.
- Genotyping does not replace the need for therapeutic drug monitoring and clinical observation.
References
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Bayoumy AB, Crouwel F, Chanda N, et al. Advances in thiopurine drug delivery: the current state-of-the-art. Eur J Drug Metab Pharmacokinet. 2021;46(6):743-758.
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Relling MV, Schwab M, Whirl-Carrillo M, et al. Clinical Pharmacogenetics Implementation Consortium guideline for thiopurine dosing based on TPMT and NUDT15 genotypes: 2018 update. Clin Pharmacol Ther. 2019;105(5):1095-1105.
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35931343
Pratt VM, Cavallari LH, Fulmer ML, et al. TPMT and NUDT15 genotyping recommendations: a joint consensus recommendation of the Association for Molecular Pathology, Clinical Pharmacogenetics Implementation Consortium, College of American Pathologists, Dutch Pharmacogenetics Working Group of the Royal Dutch Pharmacists Association, European Society for Pharmacogenomics and Personalized Therapy, and Pharmacogenomics Knowledgebase. J Mol Diagn. 2022;24(10):1051-1063.
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Evans WE. Pharmacogenetics of thiopurine S-methyltransferase and thiopurine therapy. Ther Drug Monit. 2004;26(2):186-191.
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Yates CR, Krynetski EY, Loennechen T, et al. Molecular diagnosis of thiopurine S-methyltransferase deficiency: genetic basis for azathioprine and mercaptopurine intolerance. Ann Intern Med. 1997;126(8):608-614.
Use this genotyping test to assess genetic risk for severe myelosuppression with standard dosing of thiopurine drugs in individuals for whom thiopurine therapy is being considered or who have had an adverse reaction to thiopurine therapy. This test can be performed irrespective of whether thiopurine therapy is currently being administered.