26569 Thiopurine Methyltransferase (TPMT) & Nudix Hydrolase (NUDT15) Genotyping, Varies (TPNUQ)

6-Mercaptopurine; 6-Thioguanine; Acute Lymphoblastic Leukemia (ALL); Azathioprine; Crohn's Disease; Inflammatory Bowel Disease (IBD); NUDT15; Rheumatoid Arthritis; Thiopurine; TPMT; Ulcerative Colitis; Mercaptopurine; thioguanine; tmpt​

​Predicting potential for toxicity to thiopurine drugs (6-mercaptopurine, 6-thioguanine, and azathioprine)

​Whole blood: Invert several times to mix blood. Send whole blood specimen in original tube. Do not aliquot.
Saliva: Patient should not eat, drink, smoke or chew gum 30 minutes prior to collection. Collect and send specimen per kit instructions.

All specimens will be evaluated at Mayo Clinic Laboratories for test suitability.

​Rare variants may be present that could lead to false-negative or false-positive results. If no TPMT variant alleles are detected by this assay the most likely genotype is that of TPMT*1/*1, although the presence of other rarer alleles cannot be excluded. In addition, if no NUDT15 variant alleles are detected by this assay, the most likely genotype is that of NUDT15*1/*1, although the presence of other rarer alleles cannot be excluded.

If genotype results obtained do not match the clinical findings, additional testing should be considered for thiopurine methyltransferase enzyme activity (TPMT3 / Thiopurine Methyltransferase Activity Profile, Erythrocytes). A corresponding activity assay for NUDT15 is not currently available.

Samples may contain donor DNA if obtained from patients who received non-leukoreduced blood transfusions or allogeneic hematopoietic stem cell transplantation. Results from samples obtained under these circumstances may not accurately reflect the recipient's genotype. For individuals who have received blood transfusions, the genotype usually reverts to that of the recipient within 6 weeks. For individuals who have received allogeneic hematopoietic stem cell transplantation, a pretransplant DNA specimen is recommended for testing.

The results do not rule out the possibility that a patient harbors another variant in TPMT, NUDT15, or another gene that can impact drug response or side effects. These genotyping procedures will not distinguish between heterozygous TPMT*3A from the rare TPMT*3B/*3C, which has an estimated frequency of 1:120,890. This rare genotype is associated with low enzyme activity. Enzyme activity evaluation is necessary to definitively identify this rare genotype (TPMT3 / Thiopurine Methyltransferase Activity Profile, Erythrocytes).

This test will not detect all TPMT or NUDT15 genetic variants. A negative result does not rule out the possibility of toxicity if thiopurines are used, since multiple factors (eg, other genetic factors, drug-drug interactions) are known to play a role. Co-prescription of allopurinol might inhibit TPMT activity. Other drugs that have been shown to inhibit TPMT activity include naproxen, ibuprofen, ketoprofen, furosemide, sulfasalazine, mesalamine, olsalazine, mefenamic acid, thiazide diuretics, and benzoic acid inhibitors.

​The thiopurine drugs are purine antimetabolites that are useful in the treatment of acute lymphoblastic leukemia, autoimmune disorders (eg, Crohn disease, rheumatoid arthritis), and organ transplant recipients. The thiopurine drugs, 6-mercaptopurine, 6-thioguanine, and azathioprine are prodrugs that require intracellular activation to 6-thioguanine nucleotides (6-TGN). This activation is catalyzed by multiple enzymes. The cytotoxic effects of thiopurine drugs are achieved mainly through incorporation of 6-TGN into DNA and RNA. The pathway that leads to synthesis of active cytotoxic 6-TGN is in competition with inactivation pathways catalyzed by thiopurine methyltransferase (TPMT). Evaluation of this pathway is important because the level of 6-TGN measured in red blood cells have been correlated with both thiopurine therapeutic efficacy and toxicity such as myelosuppression.

TPMT activity is inherited as a monogenic codominant trait, and variable TPMT activity is associated with TPMT genetic variants. The distribution of TPMT activity in red blood cells is trimodal in the White population, with approximately 0.3% of people having deficient (undetectable) TPMT activity, 11% low (intermediate) activity, and 89% normal TPMT activity. The allele for normal TPMT activity (wild type) has been designated TPMT*1. Four TPMT alleles, comprised of a combination of 3 different single-nucleotide variants, account for the majority of inactivating alleles in some ethnicities, including whites: TPMT*2, TPMT*3A, and TPMT*3C. Less frequently occurring TPMT alleles TPMT*4, TPMT*5, TPMT*8, and TPMT*12 also have been implicated as deficiency alleles. If no TPMT variant alleles are detected by this assay, the most likely genotype is that of TPMT*1/*1, although the presence of other rarer alleles cannot be excluded.

Nudix hydrolase (NUDT15) is thought to dephosphorylate the active metabolites of thiopurines, TGTP, and TdGTP, which prevents their incorporation into DNA and decreases their cytotoxic effects. Genetic variants in NUDT15 that decrease this activity are strongly associated with thiopurine-related myelosuppression. NUDT15 deficiency is most common among East Asian (22.6%), South Asian (13.6%), and Native American populations (12.5%-21.2%). Studies in other populations are ongoing. This test evaluates variants associated with NUDT15*2, NUDT15*3, NUDT15*4, and NUDT15 *5. If no NUDT15 variant alleles are detected by this assay, the most likely genotype is that of NUDT15*1/*1, although the presence of other rarer alleles cannot be excluded. Individuals with variants in both TPMT and NUDT15 have been identified and were significantly more sensitive to mercaptopurine than individuals heterozygous for a variant in only one gene. Integration of both TPMT and NUDT15 testing may allow for more accurate prediction of thiopurine-related toxicity risk to guide dosing, particularly among patients from diverse populations.

Table 1. TPMT Enzyme Activity of Individual Star Alleles

 
Table 2. NUDT15 Enzyme Activity of Individual Star Alleles

 
The US Food and Drug Administration, the Clinical Pharmacogenetics Implementation Consortium, and some professional societies recommend consideration of TPMT and NUDT15 genotype testing or TPMT enzyme activity testing along with NUDT15 genotype testing prior to the initiation of therapy with thiopurine drugs. There is substantial evidence linking TPMT and NUDT15 genotypes to phenotypic variability. Dose adjustments based upon TPMT and NUDT15 genotypes have reduced thiopurine-induced adverse effects without compromising desired antitumor and immunosuppressive therapeutic effects in several clinical settings.

Genotyping is not impacted by other medications known to inhibit TPMT activity. Complementary clinical testing is available to measure TPMT enzymatic activity in erythrocytes (TPMT3 / Thiopurine Methyltransferase Activity Profile, Erythrocytes) if the clinician wants to check for lower TPMT enzyme activity, regardless of cause. Testing for TPMT enzyme activity is not impacted by variants in NUDT15.

An interpretive report will be provided

​An interpretive report will be provided