Mayo Clinic researchers found that a gene variant, the TPMT ∗ 8 allele, is associated with reduced metabolism of the thiopurine class of drugs. TPMT ∗ 8 is relatively common in individuals of African or African American descent. A novel study in The Journal of Molecular Diagnostics, published by Elsevier, details the researchers' goal to provide pharmacogenomic testing that can be used clinically to predict drug response that is beneficial to all patients, regardless of their ancestral background.
Thiopurines are a class of drugs that can be used to treat a variety of medical conditions including leukemia, inflammatory bowel disease, and autoimmune disorders. TPMT is an enzyme important in metabolizing thiopurine medications that limits the levels of active metabolite, which contributes to toxicity.
Variants in the TPMT gene can result in decreased enzyme activity. Thus, to prevent thiopurine adverse reactions, individuals are tested to see if they have genetic variants prior to initiation of treatment, so dosing can be adjusted according to enzyme activity. Currently, guidelines do not recommend testing for the TPMT∗8 variant, which is a variant common among individuals of African or African American ancestry (2.3% minor allele frequency) as its function has been considered uncertain."
Rosalie M. Sterner, Department of Laboratory Medicine and Pathology, Mayo Clinic
Researchers combined data from two assays: one to measure TPMT activity and the other that performs genotyping. They compared the TPMT activity of individuals who carry one copy of the TPMT ∗ 8 allele (and one normal copy) to individuals who are considered intermediate metabolizers (they have one normal copy of TPMT and one no function copy of TPMT). They found that individuals who carry TPMT ∗ 8 metabolized the chemotherapeutic 6-mercaptopurine to 6-methylmercapture purine slower than normal metabolizers (two normal copies of TPMT), but faster than the intermediate metabolizers.
Co-lead investigator Ann M. Moyer, MD, PhD, Department of Laboratory Medicine and Pathology, Mayo Clinic in Rochester, explains, "This is important because a competing reaction converts 6-mercaptopurine to the active metabolites that are responsible for toxicity, and reduced TPMT activity is associated with thiopurine toxicity. In addition, we found that the TPMT activity differed with respect to three substrates (6-MP, 6-MPR, and 6-TGR)."
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Co-lead investigator John L. Black, MD, Department of Laboratory Medicine and Pathology, Mayo Clinic in Rochester, notes, "Genotyping will only detect the alleles that are targeted by a test. Some laboratories, including ours, are moving to sequencing pharmacogenes by next-generation sequencing (NGS), which allows the detection of all alleles. This sometimes yields results that can be useful when the rare allele has a known function. Equally important, this approach is superior in providing service to individuals of all ancestries because alleles, including rare alleles, often cluster within ancestral groups. NGS sequencing reduces the ethnocentric approach driven by the fact that most tests have been designed for only the most commonly studied ancestral groups."
This study showing that TPMT ∗ 8 is a reduced function allele could result in recognition of reduced function alleles and more accurate and granular phenotype prediction. However, this would require further study to understand the clinical significance (thiopurine dose tolerated) of reduced function alleles, incorporation into clinical dosing guidelines, and inclusion in allele selection recommendations. These findings have the potential to help shape clinical practice.
Dr. Sterner concludes, "The commonly tested TPMT variants result in a protein with no activity, so as a next step, it will be important to determine whether a dose reduction is warranted in individuals with reduced function alleles, alone or in combination with other variants."
Journal reference:
Sterner, R. M., et al. (2024) Genotype and Phenotype Correlation of the TPMT∗8 Allele in Thiopurine Metabolism. The Journal of Molecular Diagnostics. doi.org/10.1016/j.jmoldx.2024.07.005.