Analysis of genetic polymorphism related to toxicity of mercaptopurine in pediatric acute lymphoblastic leukemia

Abstract

Introduction: Despite of marked improvement in survival rates of pediatric acute lymphoblastic leukemia (ALL), drug resistance and recurrence are remaining problems. Genetic polymorphism has been known to be an important factor affecting the efficacy and toxicity of antileukemic agents, thus pharmacogenetic researches have been actively conducted in pediatric ALL. Mercaptopurine (MP) is one of the main chemotherapeutics for ALL, and constant MP dose titration is essential to maintain steady drug exposure, while minimizing myelosuppression. Throughout previous studies, thiopurine methyltransferase (TPMT) is proved to be one of the most responsible genes in the pharmacogenetics of MP. Recently, mutations of nucleoside diphosphate linked moiety X-type motif 15 (NUDT15) have been shown to be a powerful genetic factor affecting the toxicity and intolerance of MP. In this study, the genetic factors affecting the toxicity of MP were investigated in Korean children with ALL. Patients and Methods: Two-stage pharmacogenetic analyses were performed to identify genetic determinants of MP-related neutropenia in Korean pediatric ALL patients. A total of 185 patients with ALL who were diagnosed and treated at the Seoul National University Children's Hospital were included. First, targeted sequencing was carried out using a novel gene panel of 147 pharmacogenetics related genes and 8 SNPs in 44 patients who received less than 25% of initial dose during maintenance therapy due to MP intolerance. Next, significant genes identified in the first analysis were re-sequenced in a total of 185 patients. The relationship between mutation of each gene identified through sequencing and toxicity or dose of MP were analyzed. Results: As a result of the first stage analysis, the 4 loci which were likely to be most relevant to the toxicity of MP (NUDT15 rs116855232, APEX1 rs2307486, CYP1A1 rs4646422, BAG3 rs78439745) were identified, and 24 variants in 13 genes potentially linked to MP adverse reactions including these 4 loci were selected as final candidates for subsequent validation in a cohort of 185 patients. The distribution of variant alleles were NUDT15 rs116855232 (21.7%), APEX1 rs2307486 (13.5%), ABCC4 rs2274407 (34%), ABCC4 rs3765534 (14.1%), ABCC4 rs11568658 (23.2%), CYP4F2 rs2108622 (51.9%), CYP1A1 rs4646422 (29.1%), SLCO1B1 rs11045879 (55.7%), SLCOB1 rs4149056 (26.5%), ITPA rs1127354 (26%), ITPA rs7270101 (0%), MTHFR rs1801131 (60.5%), MTHFR rs1801133 (28.6%), MTHFR rs1901133 (50.8%), GRIA1 rs4958351 (3.8%), MOCOS rs594445 (48.1%), PACSIN2 rs2413739 (13.5%), BAG3 rs78439745 (7%) and TPMT (* 1 / * 3C, 3.8%). Neutropenia (neutrophil<500/㎕) was observed in 121 patients (65.4%) during the administration of MP. When frequency of neutropenia during MP administration was crosstabulated with variant frequencies, the risk of neutropenia was 4.64 times higher in patients with T allele in ABCC4 rs3765534 (95% CI

1.32 to 25.12, P=0.008). Among 121 patients with neutropenia, 15 patients had neutropenia within 30 days (early neutropenia) after starting MP, and the risk of early neutropenia was significantly higher in patients with G allele in APEX1 rs2307486 (OR=3.44

95% CI

1.04 to 9.89, P=0.02). In addition, the cumulative incidence of early neutropenia was significantly higher in patients with variant forms of APEX1 rs2307486 (AA:AG:GG=66.3%:73%:100%, P<0.001). The cumulative incidence of early neutropenia was also significantly increased in patients with NUDT15 rs116855232 mutations (CC:CT:TT=65.0%:72.2%:100%, P<0.001). Comparing the average doses of MP used in the study, the T allele type of NUDT15 rs116855232 was significantly associated with a lower tolerated dose of MP as 10.5, 19.4 and 28.7 mg/m2/day for TT, CT, and CC genotypes (P<0.001). However, MP doses by TPMT mutations were not significantly different in this study. Conclusion: In conclusion, this study revealed that genetic variation of APEX1 increases the risk of early neutropenia associated with MP, and the variant allele in NUDT15 was related to MP intolerance. In Korean ALL patients, toxicity and tolerance of MP were more affected by APEX1 and NUDT15 than by TPMT, which has a lower variant allele frequency in Asian. Further validation will be needed to confirm the frequency and pharmacogenetic consequences of APEX1 variants in other ethnic groups, including different Asian populations