DNA variants in the dihydrofolate reductase gene and outcome in childhood ALL.

Dihydrofolate reductase (DHFR) is the major target of methotrexate (MTX), a key component in childhood acute lymphoblastic leukemia (ALL) treatment. A total of 15 polymorphisms in DHFR promoter were analyzed, and 3 sites (C-1610G/T, C-680A, and A-317G) were identified as sufficient to define observed haplotypes (tag single nucleotide polymorphisms [tagSNPs]). These polymorphisms were investigated for association with treatment response in 277 children with ALL. Lower event-free survival (EFS) was associated with homozygosity for the allele A-317 and C-1610 (P=.03 and .02), and with the haplotype *1, defined by both C-1610 and A-317 alleles (P=.03). The haplotype *1 conferred higher transcriptional activity (P<.01 compared with haplotypes generating minimal luciferase expression). Quantitative mRNA analysis showed higher DHFR levels for particular haplotype *1 carriers (P<.01). The analysis combining haplotype *1 with thymidylate synthase (TS) and cyclin D1 (CCND1) genotypes previously shown to affect ALL outcome showed that the number of event-predisposing genotypes was associated with increasingly lower EFS (P<.001). In conclusion, DHFR promoter polymorphisms are associated with worse ALL outcome, likely due to a higher DHFR expression. Combined effects among genes of the folate cycle can further accentuate differences in the response to the treatment.