Thiopurine methyltransferase genotype–phenotype discordance and thiopurine active metabolite formation in childhood acute lymphoblastic leukaemia

Abstract

AIMSIn children with acute lymphoblastic leukaemia (ALL) bone marrow activity can influence red blood cell (RBC) kinetics, the surrogate tissue for thiopurine methyltransferase (TPMT) measurements. The aim of this study was to investigate TPMT phenotype-genotype concordance in ALL, and the influence of TPMT on thiopurine metabolite formation. METHODSWe measured TPMT (activity, as units ml -1 packed RBCs and genotype) at diagnosis (n = 1150) and TPMT and thioguanine nucleotide (TGN) and methylmercaptopurine nucleotide (MeMPN) metabolites (pmol/8 ¥ 10 8 RBCs) during chemotherapy (n = 1131) in children randomized to thioguanine or mercaptopurine on the United Kingdom trial ALL97. RESULTSMedian TPMT activity at diagnosis (8.5 units) was significantly lower than during chemotherapy (13.8 units, median difference 5.1 units, 95% confidence interval (CI) 4.8, 5.4, P < 0.0001). At diagnosis genotype-phenotype was discordant. During chemotherapy the overall concordance was 92%, but this fell to 55% in the intermediate activity cohort (45% had wild-type genotypes). For both thiopurines TGN concentrations differed by TPMT status. For mercaptopurine, median TGNs were higher in TPMT heterozygous genotype (754 pmol) than wild-type (360 pmol) patients (median difference 406 pmol, 95% CI 332, 478, P < 0.0001), whilst median MeMPNs, products of the TPMT reaction, were higher in wild-type (10 650 pmol) than heterozygous patients (3868 pmol) (P < 0.0001). In TPMT intermediate activity patients with a wild-type genotype, TGN (median 366 pmol) and MeMPN (median 8590 pmol) concentrations were similar to those in wild-type, high activity patients. CONCLUSIONSIn childhood ALL, TPMT activity should not be used to predict heterozygosity particularly in blood samples obtained at disease diagnosis. Genotype is a better predictor of TGN accumulation during chemotherapy. WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT• In healthy children and adults thiopurine methyltransferase (TPMT) activities have a trimodal frequency distribution. In adult populations the concordance between TPMT genotype and phenotype is over 98%. • In childhood acute lymphoblastic leukaemia (ALL) the disease process and subsequent chemotherapy can influence TPMT activity measurements. WHAT THIS STUDY ADDS• In childhood ALL, at disease diagnosis, the extent of the reduction in measured TPMT activity and the resulting TPMT genotype-phenotype discordance, previously reported in small patient cohorts with low numbers of variant alleles, has been confirmed in a large population of children with ALL. TPMT activity should not be used to predict heterozygosity. • During thiopurine chemotherapy, TPMT discordance is 45% in children with intermediate activity. TPMT genotype more accurately predicts mercaptopurine active metabolite accumulation and therefore should be used in preference to phenotyping for dosage recommendations.

Methotrexate binds to recombinant thiopurine S-methyltransferase and inhibits enzyme activity after high-dose infusions in childhood leukaemia

“…Preferably, TPMT genotyping should be performed using techniques that allow for the detection of novel/rare variants, however, most laboratories only genotype for the three most common TPMT sequence variants. Our results is in accordance with a recently published paper in a large cohort of ALL patients in UK [16].…”

Augmenting Clinical Interpretability of Thiopurine Methyltransferase Laboratory Evaluation

“…Some studies on TPMT genotype/phenotype correlations report similar concordance as we do, but others report concordance as high as 95% [27,28]. As discussed above, the methodology of activity determination may have been a source of variability in the reports.…”

“…We propose a clinical algorithm to perform the TPMTcombo testing at diagnosis with following interpretation - activity measurement is to inform clinicians on the potential pharmacokinetic-related adverse events and/or hypermetabolism [32] and, conversely, genotyping may be indicative of the rate of pharmacodynamic 6-thioguanine nucleotide accumulation due to the slower overall metabolism of thiopurines, which is not well reflected in the surrogate tissue [10,28,33]. However, these encouraging results await further support by clinical trial data.…”

Thiopurine S-methyltransferase testing for averting drug toxicity: a meta-analysis of diagnostic test accuracy

“…This resulted in two strata for the test of deficient TPMT enzyme activity/homozygous TPMT mutation versus the rest of the population: (1) studies where the genotype test only considered TPMT*2 and *3, [33][34][35][36][37][38][39][40][41][42] and (2) studies where the genotype test considered TPMT*2, TPMT*3 and additional polymorphisms. [43][44][45] There were three strata for the test of low or intermediate TPMT enzyme activity/ homozygous or heterozygous TPMT mutation versus the rest of the population: (1) studies where the genotype test only considered TPMT*3, 46,47 (2) studies where the genotype test considered TPMT*2 and TPMT*3, 6,[33][34][35][36][37][38][39][40][41][42][48][49][50][51][52][53][54][55][56] and (3) studies where the genotype test considered TPMT*2, TPMT*3 and additional polymorphisms. [42][43][44][45]57,58 The meta-analysis used a Bayesian model.…”