Charles R. Yates, BS; Eugene Y. Krynetski, PhD; Thrina Loennechen, PhD; Michael Y. Fessing, PhD; Hung-Liang Tai, PhD; Ching-Hon Pui, MD; Mary V. Relling, PharmD; William E. Evans, PharmD
Thiopurine S-methyltransferase (TPM) catalyzes the S-methylation (that is, inactivation) of mercaptopurine, azathioprine, and thioguanine and exhibits genetic polymorphism. About 10% of patients have intermediate TPM activity because of heterozygosity, and about 1 in 300 inherit TPM deficiency as an autosomal recessive trait. If they receive standard doses of thiopurine medications (for example, 75 mg/m2 body surface area per day), TPM-deficient patients accumulate excessive thioguanine nucleotides in hematopoietic tissues, which leads to severe and possibly fatal myelosuppression.
To elucidate the genetic basis and develop molecular methods for the diagnosis of TPM deficiency and heterozygosity.
Diagnostic test evaluation.
The TPM phenotype was determined in 282 unrelated white persons, and TPM genotype was determined in all persons who had intermediate TPM activity (heterozygotes) and a randomly selected, equal number of persons who had high activity. In addition, genotype was determined in 6 TPM-deficient patients.
Polymerase chain reaction (PCR) assays were developed to detect the G238C transversion in TPM*2 and the G460A and A719G transitions in TPM*3 alleles. Radiochemical assay was used to measure TPM activity. Mutations of TPM were identified in genomic DNA, and the concordance of TPM genotype and phenotype was determined.
21 patients who had a heterozygous phenotype were identified (7.4% of sample [95% CI, 4.7% to 11.2%]). TPM*3A was the most prevalent mutant allele (18 of 21 mutant alleles in heterozygotes; 85%); TPM*2 and TPM*3C were more rare (about 5% each). All 6 patients who had TPM deficiency had two mutant alleles, 20 of 21 patients (95% [CI, 76% to 99.9%]) who had intermediate TPM activity had one mutant allele, and 21 of 21 patients (100% [CI, 83% to 100%]) who had high activity had no known TPM mutation. Detection of TPM mutations in genomic DNA by PCR coincided perfectly with genotypes detected by complementary DNA sequencing.
The major inactivating mutations at the human TPM locus have been identified and can be reliably detected by PCR-based methods, which show an excellent concordance between genotype and phenotype. The detection of TPM mutations provides a molecular diagnostic method for prospectively identifying TPM-deficient and heterozygous patients.
Yates CR, Krynetski EY, Loennechen T, Fessing MY, Tai H, Pui C, et al. Molecular Diagnosis of Thiopurine S-Methyltransferase Deficiency: Genetic Basis for Azathioprine and Mercaptopurine Intolerance. Ann Intern Med. ;126:608–614. doi: 10.7326/0003-4819-126-8-199704150-00003
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Published: Ann Intern Med. 1997;126(8):608-614.
Emergency Medicine, Hematology/Oncology, Hospital Medicine.
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Print ISSN: 0003-4819 | Online ISSN: 1539-3704
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