Thiopurines, Poor Metabolism Of, 2

A number sign (#) is used with this entry because of evidence that thiopurine toxicity (THPM2) is caused by variation in the NUDT15 gene (615792) on chromosome 13q14. The severity of the phenotype is proportional to the cumulative number of risk alleles.

Description

THPM2 is associated with severe hematopoietic toxicity when patients are treated with standard doses of thiopurines, a class of antineoplastic/immunosuppressant agents that consists of mercaptopurine, thioguanine, and azathioprine. Thiopurines are prodrugs that require extensive metabolism in order to exert their cytotoxic action. Thiopurines are converted into cytotoxic thioguanine nucleotides (TG), which are incorporated into DNA and cause cell death. NUDT15 inactivates thiopurine metabolites and negatively regulates cytotoxicity (summary by Moriyama et al., 2016). The NUDT15 deficiency trait follows an additive genetic mode of inheritance, with the severity of the phenotype proportional to the cumulative number of risk alleles in NUDT15.

For a discussion of genetic heterogeneity of poor thiopurine metabolism, see THPM1 (610460).

Molecular Genetics

Yang et al. (2014) identified a missense variation in the NUDT15 gene (R139C; 615792.0001), resulting in THPM2, that was significantly associated with thiopurine-induced leukopenia in a cohort of 978 Korean individuals. Among the 978 individuals, 66 developed leukopenia within 6 weeks of starting therapy, 280 developed leukopenia within 8 weeks of starting therapy, and 632 did not develop leukopenia for over 8 weeks. In Koreans, this variant demonstrated sensitivity and specificity of 89.4% and 93.2%, respectively, for thiopurine-induced early leukopenia. This SNP was also strongly associated with thiopurine-induced leukopenia in European individuals with inflammatory bowel disease (OR of 9.50; p = 4.64 x 10(-4)). There was a gene-dose effect for the risk allele. Jurkat cells transfected with the variant showed increased thiopurine-induced toxicity and signs of apoptosis compared to cells transfected with wildtype NUDT15. These findings suggested that NUDT15 is a pharmacogenetic determinant for thiopurine-induced leukopenia in diverse populations.

Among 270 children with acute lymphoblastic leukemia (ALL; 613065) from Guatemala, Singapore, and Japan, Moriyama et al. (2016) identified 4 different polymorphisms in the NUDT15 gene (615792.0001-615792.0004) that affected coding regions and resulted in a 74.4 to 100% loss of nucleotide diphosphatase activity. These loss-of-function variants were consistently associated with thiopurine intolerance and excessive cytotoxicity across the 3 cohorts (metaanalysis p = 4.45 x 10(-8)). All variant proteins also showed lower protein unfolding temperature sensitivities compared to wildtype, consistent with perturbation of the protein conformation. Five NUDT15 haplotypes representing different combinations of the variants were inferred (haplotypes *1-*5), and the prevalence of each haplotype varied substantially by ancestry. The patients could be classified into diplotypic groups based on the haplotypes, and these groups had different enzymatic activities. Mercaptopurine tolerance was highest in patients with normal NUDT15 activity and lowest in patients with low NUDT15 activity. There was evidence for a cumulative effect of the variants on enzymatic activity and drug tolerance. In vitro functional expression studies confirmed that wildtype NUDT15 inactivates thiopurine metabolites and decreases thiopurine cytotoxicity. Direct monitoring of DNA-TG levels in patient white blood cells showed that the ratio of DNA-TG levels to mercaptopurine dosage varied by NUDT15 genotype, such that those with low-activity diplotypes had higher DNA-TG content. These in vivo findings thus confirmed the in vitro studies. Moriyama et al. (2016) emphasized that the NUDT15 variation is substantially overrepresented in individuals of Asian origin, which has major implications for patients who are treated with thiopurines.