What are other Names for this Test? (Equivalent Terms)
- S-Adenosyl-L-Methionine:Thiopurine S-Methyltransferase Mutation Analysis Test
- Thiopurine Methyltransferase Mutation Analysis Test
- TPMT_HUMAN Mutation Analysis Test
What is TPMT Mutation Analysis Test? (Background Information)
- TPMT mutation refers to an alteration in the TPMT gene. This is associated with increased risk of toxicity when undergoing immunosuppressive therapy
- Immunosuppressive therapy involves the use of drugs that decrease the activity of the immune system. Examples of such drugs include:
- Though normally unhealthy, such an action is desired when the immune system is overactive, or when the normal action of the immune system may complicate certain conditions or procedures. Immunosuppression is desired in the following situations:
- During organ transplantation
- Certain immune system cancers (leukemia and lymphoma)
- Autoimmune disorders (Crohn’s disease and rheumatoid arthritis)
- The TPMT gene gives instructions for the TPMT protein (TPMT stands for thiopurine S-methyltransferase). As the name implies, TPMT is an enzyme that performs a chemical alteration: S-methylation of a class of compounds, called thiopurines, that consist of immunosuppressive drugs
- By S-methylating the drugs, TPMT deactivates them. Normal drug deactivation is factored into the half-life of a drug (the time it takes for half the drug to be metabolized in the body) and carefully considered to avoid toxicity
- If an individual possesses a mutation in the TPMT gene, he/she may possess a TPMT protein that is unable to properly metabolize thiopurines. This may be problematic if the individual is undergoing immunosuppressive therapy
- In such a case, the immunosuppressive drugs may linger in the body far longer than is anticipated by their half-life in normal TPMT-possessing individuals. This may damage the bone marrow and cause hematopoietic toxicity
- The TPMT Mutation Analysis Test is a genetic test that detects mutation in the TPMT gene. It is used to determine if an individual will have trouble metabolizing certain immunosuppressive drugs
Molecular testing, in general, can be performed using a variety of methods. Some of these methods include:
- In situ hybridization techniques, such as fluorescence in situ hybridization (FISH)
- Immunohistochemistry (IHC)
- Next-generation sequencing (NGS)
- Methylation profiling
- Polymerase chain reaction (PCR)
- Comparative genomic hybridization (CGH)
- Karyotyping including spectral karyotyping
- mRNA analysis
- Tissue microarrays (TMAs)
- Southern blot test
- Northern blot test
- Western blot test
- Eastern blot test
The methodology used for the test may vary from one laboratory to another.
Note: Molecular testing has limitations depending on the method being used, and genetic mutational abnormalities being tested. This can affect the results on a case-by-case basis. Consultation with your healthcare provider will help in determining the right test and right molecular method, based on individual circumstances.
What are the Clinical Indications for performing the TPMT Mutation Analysis Test?
Following are the clinical indications for performing the TPMT Mutation Analysis Test:
- Monitoring chemotherapy
- Pale or yellowish skin
- Irregular heartbeats
- Shortness of breath
- Dizziness or lightheadedness
- Chest pain
- Cold hands and feet
In general, the molecular genetic testing is undertaken in the following situations:
- To assist (and in some cases, confirm) the initial diagnosis
- To check for or ascertain a family history of the condition
- To distinguish other conditions that have similar features (signs and symptoms)
- To help determine treatment options
How is the Specimen Collected for TPMT Mutation Analysis Test?
The type and source and specimen sample requirements will depend on the preference of the individuals and the preference of the testing lab. Thus, it may vary from one individual to another and from one lab to another. Therefore, it is important to contact the testing lab for exact specimen requirements, before initiating the collecting and testing process.
Following is the specimen collection process for TPMT Mutation Analysis Test:
- Sample on which the test is performed may include:
- Peripheral blood in individuals showing signs and symptoms suspected of the condition
- Bone marrow biopsy specimen
- For mitochondrial DNA testing, usually a muscle biopsy or a liver biopsy is preferred
- In case of expectant mothers, prenatal testing through amniotic fluid and chorionic villi sampling
- Fetal cord blood
- Fresh tissue from biopsy
- Fresh tissue from autopsy sample
- Fresh tissue from fetal demise
- Buccal brushes: Using the kit that is provided by the testing laboratory, buccal brushes can be used to collect the specimen, by scraping the inner cheek lining (buccal mucosa)
- Oral rinse specimens
- Body fluids such as saliva, tears, and semen
- Dried blood spots: This specimen type is usually requested in situations where the collection and/or shipping of whole blood is not practical
- In some cases, hair samples (with attached roots), finger nails, and buccal swabs, may be acceptable
- Formalin-fixed paraffin-embedded solid tumor tissue (FFPE tumor tissue), often referred to as paraffin block of the tumor
- Products of conception sample from aborted pregnancy
- Process of obtaining the sample: As outlined by the laboratory testing facility
- Preparation required: As outlined by the laboratory testing facility
Limitations of specimen while testing for the condition
- For blood specimens: Individuals, who have received platelet transfusions, red blood transfusion, or white blood (leukocyte) transfusion, should wait at least 4 weeks before providing a blood specimen
- The following specimens may not be acceptable in individuals who have received heterologous bone marrow transplant (in the past):
- Peripheral blood samples
- Oral rinse specimens
- Bone marrow biopsy specimens
- Testing should not be performed on a transplanted organ/specimen, since the genetic material belongs to the donor and not to the individual being tested
- Formalin-fixed paraffin-embedded solid tumor tissue: In many cases, FFPE tissue blocks are usually not acceptable. Please contact the testing lab to ascertain, if it is an acceptable sample specimen
- In some cases, a different source of specimen may be acceptable to the laboratory performing the test
Occasionally, additional samples may be required to either repeat the test or to perform follow-up testing.
Turnaround time for test results
- Depending on the location of testing, it may take from 2 to 8 weeks from the time of sample collection, to obtain the test results
Sample storage information
- Many hospitals preserve the paraffin blocks for at least 7 years
- In general, older paraffin blocks (over 5 years) may affect the detection of specific mutations, due to degradation of the tissue specimen over time
Cost of TPMT Mutation Analysis Test:
- The cost of the test procedure depends on a variety of factors, such as the type of your health insurance, annual deductibles, co-pay requirements, whether your healthcare provider/facility is in-network or out-of-network of your insurance company
- In many cases, an estimate may be provided before the test is conducted. The final amount may depend upon the findings during the test procedure and post-operative care, if required
What is the Significance of the TPMT Mutation Analysis Test Result?
- The presence of a mutation in the TPMT gene indicates a positive result for the TPMT Mutation Analysis Test
- This may necessitate adjustments to the anticipated dose of immunosuppressive therapy medication, if thiopurines are to be used
- It may point to a diagnosis of hematapoietic toxicity
The laboratory test results are NOT to be interpreted as results of a "stand-alone" test. The test results have to be interpreted after correlating with suitable clinical findings and additional supplemental tests/information. Your healthcare providers will explain the meaning of your tests results, based on the overall clinical scenario.
Additional and Relevant Useful Information:
- The TPMT gene resides on a position of the chromosome called 6p22.3 i.e., the short (p) arm of chromosome 6 in location 22.3
- Many laboratories may not have the capability to perform this test. Only highly-specialized labs with advanced facilities and testing procedures may offer this test
Certain medications that you may be currently taking may influence the outcome of the test. Hence, it is important to inform your healthcare provider, the complete list of medications (including any herbal supplements) you are currently taking. This will help the healthcare provider interpret your test results more accurately and avoid unnecessary chances of a misdiagnosis.
What are some Useful Resources for Additional Information?
Please visit our Laboratory Procedures Center for more physician-approved health information:
References and Information Sources used for the Article:
https://ghr.nlm.nih.gov/primer/testing/genetictesting (accessed on 05/10/2017)
https://www.cdc.gov/mmwr/preview/mmwrhtml/rr5806a1.htm (accessed on 05/10/2017)
http://www.nature.com/gim/journal/v10/n5/full/gim200852a.html (accessed on 05/10/2017)
http://pediatrics.aappublications.org/content/106/6/1494 (accessed on 05/10/2017)
Kuhn, J. G. (2002). Chemotherapy-associated hematopoietic toxicity. American Journal of Health-System Pharmacy, 1(59), S4-7.
Symptoms and causes - Anemia - Mayo Clinic. (2016, August 6). Retrieved from http://www.mayoclinic.org/diseases-conditions/anemia/symptoms-causes/dxc-20183157
TPMT gene - Genetics Home Reference. (n.d.). Retrieved from https://ghr.nlm.nih.gov/gene/TPMT#location
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