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

Abstract

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. © Springer-Verlag 2011.