Thiopurine S-Methyltransferase Genotype - Black Country Pathology Services

Investigation Name:Thiopurine S-Methyltransferase Genotype

Alias or Abbreviation:TPMT genotype

Accreditation:Awaiting accreditation due to relocation.

Intro:

Thiopurine drugs, such as mercaptopurine and azathioprine, are widely used to treat inflammatory and autoimmune disease, leukaemia and to prevent rejection post organ transplant. These drugs are catabolised to inactive metabolites by TPMT, which in effect reduces concentrations of the active metabolite, 6-thioguanine nucleotides (6TGN). TPMT activity exhibits autosomal co-dominant polymorphism.

In a Caucasian population approximately 89% have normal enzyme activity, 11% low activity and 0.3% undetectable levels (deficient).4 Measurement of TPMT activity prior to starting thiopurine drugs is recommended and allows dosing of thiopurine drugs to be adjusted appropriately.
Patients with undetectable TPMT activity are generally not treated with thiopurine drugs due to increased risk of severe side effects, e.g., myelosuppression. Those with low activity usually receive a reduced dose. In patients with high activity, an increased dose may lead to accumulation of inactive metabolites and increased risk of hepatotoxicity.
Eight TPMT alleles have been identified, including three alleles (TPMT*2, TPMT*3A and TPMT*3C) which account for 80-95% of intermediate or low enzyme activity cases (McLeod et al): The mutant allele TPMT*2 is defined by a single nucleotide transversion (G238C) in the open reading frame, leading to an amino acid substitution at codon 80 (Ala>Pro). This mutation leads to a >100-fold reduction in TPMT activity relative to wild-type cDNA.
The second and more prevalent mutant allele, TPMT*3A, contains two nucleotide transition mutations (G460A and A719G) in the open reading frame, leading to amino acid substitutions at codon 154 (Ala>Thr) and codon 240 (Tyr>Cys). TPMT*3A has >200-fold lower TPMT activity. Studies suggest that TPMT*3A is the most common low-activity allele in whites, while TPMT*3C is the most common low-activity allele in Asians, Africans, and African Americans (Hon et al).
TPMT genotype analysis will be performed to confirm TPMT status for selected samples:
Deficient/potential deficient TPMT activity
Low TPMT activity with normal
Clinical details: recent blood transfusion, previous severe reaction to thiopurine drugs, ALL, pancytopenia, neutropenia, myelosuppression, change in TPMT activity status.

Pathology Laboratory:TPMT Laboratory

Requestable Seperately?Yes

Units:Qualitative

Minimum Sample Volume:1mL

Expected Turnaround Time:7 working days

Test Code:TPGR

Sample Type:

EDTA whole blood

Complex Reference Range:Results are expressed as follows:
TPMT *1/*1 (wild-type)
TPMT *1/*2 (heterozygous)
TPMT *1/*3 (heterozygous)
TPMT *2/*2 (homozygous)
TPMT *2/*3 (compound heterozygous)
TPMT *3/*3 (homozygous)

Collection Conditions / Other Information:Patients must give consent for genetic testing prior to sample collection.
Our current method does not screen for all variants, and a wild-type result does not exclude rare variants.

Referred to Another Laboratory?No

Storage Requirements:Do not freeze, store at 4°C prior to dispatch.

Posting Address / Requirements:First class post at ambient temperature.
Sandwell Health Campus,
Specialist Chemistry,
Pathology Department,
Lyndon, B71 4HJ

NPEX / PDF Reporting Available:Yes

EQA Scheme:We participate in interlaboratory sample exchange. Equalis: 527- TPMT genotype, phenotype and thiopurine metabolites GenQa: Pharmacogenomics - TPMT and NUDT15 INSTAND: Molecular Genetics 30

Lead Contact Details:Consultant Clinical Scientist, Hayley Sharrod- Cole. Email: hayley.sharrod.cole@nhs.net

Email Address For Chasing Results:rwh-tr.bcpsspecialistchemistryenquiries@nhs.net

Site Sample Tested:Sandwell Health Campus

Cost:Please email: bcpspathology.info@nhs.net for further details

Contact Number:Tel: 0121 507 5162