ABSTRACT
Title
Genetic determinants of severe gastrointestinal toxicity during therapy with 6-mercaptopurine and methotrexate in pediatric acute lymphoblastic leukemia
Authors
G. Stocco1,2, W. Yang1, K.R. Crews1, W.E. Thierfelder1, S.W. Paugh1, B. Diouf1, R. McCorkle1, G. Decorti2, M. Londero2,3, R. Franca3, M.G. Valsecchi4, M. Rabusin3, M.V. Relling1, C.H. Pui1, W.E. Evans1
1: St. Jude Children’s Research Hospital, Memphis, USA
2: University of Trieste, Trieste, Italy
3: I.R.C.C.S. “Burlo Garofolo”, Trieste, Italy
4: University Milano Bicocca, Milan, Italy
1: St. Jude Children’s Research Hospital, Memphis, USA
2: University of Trieste, Trieste, Italy
3: I.R.C.C.S. “Burlo Garofolo”, Trieste, Italy
4: University Milano Bicocca, Milan, Italy
Abstract
Approximately 80% of children with acute lymphoblastic leukemia (ALL) can be cured with combination chemotherapy; 6-mercaptopurine (MP) and methotrexate (MTX) are the cornerstones of continuation therapy for ALL. However, treatment-related toxicity can be life-threatening and is the primary cause of interruption or discontinuation of chemotherapy, leading to an increase in relapse risk. Germline polymorphisms in drug-metabolizing genes can significantly influence efficacy and toxicity of antileukemic therapy. Although genetic polymorphisms in the gene encoding human thiopurine methyltransferase (TPMT) are known to have a marked effect on MP metabolism and toxicity, there are patients with wild-type TPMT who develop toxicity. To increase the understanding of genetic variability leading to differences in TPMT activity, a genome-wide analysis was performed to identify trans-genes whose expression or SNPs are related to TPMT activity in a panel of HapMap cell lines, to be validated in patients with relevant phenotypes of toxicity.
We first selected the highest ranking gene for which the mRNA expression and SNPs within the genes were associated with TPMT activity in HapMap cell lines, identifying PACSIN2 and its intronic SNP rs2413739. We then verified the effect of the PACSIN2 SNP on TPMT activity in patients being treated for ALL considering the phenotypes: TPMT activity and Grade 3 or 4 gastrointestinal (GI) toxicity during consolidation therapy containing MP and MTX according to St. Jude Total 13B protocol (patient discovery cohort). This analysis confirmed that the most significant SNP in PACSIN2 was significantly associated with TPMT activity and GI toxicity in the patient discovery cohort even after adjusting for TPMT genotype. To further validate this finding, the same SNP together with inactivating SNPs in the candidate genes TPMT, SLCO1B1, ITPA and RFC were genotyped in a separate validation cohort of pediatric patients with ALL from the Italian LLA 2000 protocol, in which the same toxicity phenotype was assessed (GI toxicity during consolidation therapy containing MP and MTX); in the Italian validation cohort the PACSIN2 SNP was associated with GI toxicity, together with SNPs in ITPA, but not SNPs in RFC, TPMT, or SLCO1B1.
These results indicate that SNPs in PACSIN2 are significant modulators of TPMT activity and GI toxicity associated with MP and MTX therapy. Differences in MP and MTX dosing between cohorts may be responsible for the different pattern of associations between the two cohorts observed for the TPMT genotype alone; in particular, patients in the validation cohort were treated with a lower dose of MP as compared to patients in the discovery cohort (25 mg/m2/day vs 75 mg/m2/day). On this basis, the finding that PACSIN2 was associated with GI toxicity in both cohorts may indicate that other molecular mechanisms in addition to modulation of TPMT activity are involved. Further studies are ongoing to identify the molecular mechanism associating PACSIN2 with TPMT activity and GI toxicity during therapy with MP and MTX. These results indicate that genetic polymorphisms are consistently associated with GI toxicity during therapy with MP and MTX for ALL, even with different dosing regimens.
We first selected the highest ranking gene for which the mRNA expression and SNPs within the genes were associated with TPMT activity in HapMap cell lines, identifying PACSIN2 and its intronic SNP rs2413739. We then verified the effect of the PACSIN2 SNP on TPMT activity in patients being treated for ALL considering the phenotypes: TPMT activity and Grade 3 or 4 gastrointestinal (GI) toxicity during consolidation therapy containing MP and MTX according to St. Jude Total 13B protocol (patient discovery cohort). This analysis confirmed that the most significant SNP in PACSIN2 was significantly associated with TPMT activity and GI toxicity in the patient discovery cohort even after adjusting for TPMT genotype. To further validate this finding, the same SNP together with inactivating SNPs in the candidate genes TPMT, SLCO1B1, ITPA and RFC were genotyped in a separate validation cohort of pediatric patients with ALL from the Italian LLA 2000 protocol, in which the same toxicity phenotype was assessed (GI toxicity during consolidation therapy containing MP and MTX); in the Italian validation cohort the PACSIN2 SNP was associated with GI toxicity, together with SNPs in ITPA, but not SNPs in RFC, TPMT, or SLCO1B1.
These results indicate that SNPs in PACSIN2 are significant modulators of TPMT activity and GI toxicity associated with MP and MTX therapy. Differences in MP and MTX dosing between cohorts may be responsible for the different pattern of associations between the two cohorts observed for the TPMT genotype alone; in particular, patients in the validation cohort were treated with a lower dose of MP as compared to patients in the discovery cohort (25 mg/m2/day vs 75 mg/m2/day). On this basis, the finding that PACSIN2 was associated with GI toxicity in both cohorts may indicate that other molecular mechanisms in addition to modulation of TPMT activity are involved. Further studies are ongoing to identify the molecular mechanism associating PACSIN2 with TPMT activity and GI toxicity during therapy with MP and MTX. These results indicate that genetic polymorphisms are consistently associated with GI toxicity during therapy with MP and MTX for ALL, even with different dosing regimens.