Epistatic interactions between thiopurine methyltransferase (TPMT) and inosine triphosphate pyrophosphatase (ITPA) variations determine 6-mercaptopurine toxicity in Indian children with acute lymphoblastic leukemia.

PURPOSE: To explore the role of genetic variants of thiopurine methyltransferase (TPMT) and inosine triphosphate pyrophosphatase (ITPA) in 6-mercaptopurine (6-MP)-induced toxicity in Indian children with acute lymphoblastic leukemia (ALL). METHODS: Children with ALL receiving 6-MP in maintenance phase of treatment (n = 90) were enrolled in the study. Bidirectional sequencing of TPMT (whole gene) and ITPA (exon 2, exon 3, and intron 2) was undertaken, and correlation between genotype and 6-MP toxicity was assessed. RESULTS: Five variations were observed in TPMT, including two exonic variations, TPMT*12 (374 C > T) and TPMT*3C (719A > G), and three intronic, intron 3 (12356 C > T), intron 4 (16638 C > T), and TPMT rs2842949. Two exonic, ITPA exon -2 (94 C → A) and exon 3 of ITPA (138 G > A), and one intronic, ITPA intron 2 (A→C), variations were observed in ITPA. Multifactor dimensionality reduction analysis of all the genetic variants showed independent association of ITPA 94 C→A as well as synergic epistatic interactions, i.e., TPMT*12 × ITPA ex3, ITPA ex2 × TPMT*12 × ITPA ex3, and TPMT*3C × ITPA ex2 × TPMT*12 × ITPA ex3, in determining hematological toxicity. This is further substantiated by a multiple linear regression model, which showed moderate predictability of toxicity with these variants (area under the curve = 0.70, p = 0.004). CONCLUSION: Our results suggest that apart from the individual effect of ITPA 94 C→A, epistatic interactions between the variations of TPMT (*3C, *12) and ITPA (ex2, ex3) are associated with the 6-MP toxicity. Testing these variants facilitates tailoring of the 6-MP therapy in children with ALL.

Genetic variants of thiopurine and folate metabolic pathways determine 6-MP-mediated hematological toxicity in childhood ALL.

AIM: The rationale of this study was to explore the contribution of genetic variants of the folate pathway to toxicity of 6-mercaptopurine (6-MP)-mediated hematological toxicity in children with acute lymphoblastic leukemia (ALL) and to explore the interaction of these variants with TPMT and ITPA haplotypes using multifactor dimensionality reduction analysis. MATERIALS & METHODS: Children with ALL (n = 96) were screened for GCPII C1561T, RFC1 G80A, cSHMT C1420T, TYMS 5´-UTR 2R3R, TYMS 3´-UTR ins6/del6, MTHFR C677T, MTR A2756G polymorphisms using PCR-RFLP and PCR-amplified fragment length polymorphism techniques. RESULTS: GCPII C1561T showed independent association with toxicity. The following synergetic interactions appeared to increase the toxicity of 6-mercaptopurine: TPMT*12 × RFC1 G80A; TPMT CTTAT haplotype × RFC1 G80A; TPMT CTTAT haplotype × RFC1 G80A × TYMS 2R3R. The genetic variants of thiopurine and folate pathway cumulatively appeared to increase the predictability of toxicity (r(2) = 0.41) in a multiple linear regression model. For the observed toxicity grades of 1, 2, 3 and 4, the respective predicted toxicity grades were 1.65 ± 0.29, 1.68 ± 0.24, 2.56 ± 0.58 and 2.99 ± 1.03, p(trend) < 0.0001. CONCLUSION: Gene-gene interaction between thiopurine and folate pathways inflate the 6-MP-mediated toxicity in Indian children with ALL illustrating the importance of ethnicity in the toxicity of 6-MP.