Association of thymidylate synthase variants with 5-fluorouracil cytotoxicity.

Identifying relevant cytotoxicity genes using an ex-vivo lymphoblastoid cell line (LCLs) model has distinct advantages for pharmacogenomic discovery studies of cancer chemotherapy, including standardized treatment conditions, availability of large numbers of samples, and publicly available genotypic data. However, there is little proof of principal data to confirm the promise of this approach. One of the known targets of 5-fluorouracil (5-FU) treatment is thymidylate synthase (TYMS). We hypothesized that genetic variants in TYMS would alter cytotoxicity because of 5-FU treatment using a LCL model system. LCLs from the Centre d'Etude du Polymorphisme Humain (CEPH) pedigrees (N=427) were treated with eight concentrations of 5-FU for 72 h, and cytotoxicity was determined using an Alamar Blue assay. For a subset of the 30 International Haplotype Mapping project (HapMap) trios, genotype data for 46 single-nucleotide polymorphism (SNP) variants encompassing the TYMS gene were downloaded from the HapMap website. Using a mixed models approach, each SNP was tested for association to 5-FU cytotoxicity in the subset of HapMap trios. Putatively associated SNPs (P<0.01), were then genotyped in the remaining LCLs in the CEPH pedigrees and tested for association. Two intronic SNPs in TYMS (rs2847153 and rs2853533) were significantly associated (P<0.01) with 5-FU cytotoxicity in the HapMap subset using the mixed models approach. After genotyping these SNPs in the full CEPH pedigrees, the associations with cytotoxicity showed a more reliable significance (P<0.0005), as a result of the increase in sample size. These results highlight the importance of the TYMS gene variants in response to 5-FU treatment. Furthermore, they provide additional biological validation of the relevance of LCLs as a model for pharmacogenomic gene discovery in cancer chemotherapy.

Race does not explain genetic heterogeneity in pharmacogenomic pathways.

INTRODUCTION: Polymorphic alleles in the human genome have been identified as affecting numerous drug responses. Currently, genotyping of all patients before starting a drug regimen is impractical. Since many polymorphisms occur at varying rates in different racial groups, we investigated whether a patient's race could predict presence of drug-relevant genetic variants well enough to be used as a substitute for individual genotyping. METHODS: We performed hierarchical clustering and principal components analysis on tagSNPs from three pathways (irinotecan, 5-fluorouracil and insulin) across 270 individuals from four racial groups available from the International HapMap Project. RESULTS: For the drug pathways, irinotecan and 5-fluorouracil, individuals from each race were widely dispersed, although several subclusters consisted entirely of individuals from a single racial group. Principal components analysis confirmed race was not a major contributor to the SNP data variance. Interestingly, individuals tended to cluster more by race across the endogenous insulin signaling pathway SNPs. CONCLUSIONS: Most genetic variation was determined by individual variation, not racial grouping, indicating race is not adequate as a surrogate to individualized therapy.

Can inhibiting dihydropyrimidine dehydrogenase limit hand-foot syndrome caused by fluoropyrimidines?

Hand-foot syndrome (HFS) is a cutaneous adverse event that occurs in some patients treated with fluoropyrimidines. Although it is not life threatening, HFS can severely disrupt the daily lives of patients. HFS appears more frequently with 5-fluorouracil (5-FU) delivered by continuous infusion or with the 5-FU oral derivative capecitabine than with bolus 5-FU therapy. HFS is a leading cause of treatment interruption, dosage reduction, or, even, therapy discontinuation for patients on a capecitabine regimen. Interestingly, addition of a dihydropyrimidine dehydrogenase (DPD) inhibitor, such as uracil, 5-chloro-2,4-dihydroxypyridine, or eniluracil, to the fluoropyrimidine treatment regimen significantly diminishes the incidence of HFS. DPD inhibitors were initially combined with fluoropyrimidines to increase the efficacy of the drugs by impairing the DPD-mediated catabolism of 5-FU. However, with the accumulating findings from clinical trials that show the benefits of DPD inhibition on decreasing the risk of HFS, consideration should be given to changing the recommendations for the treatment of cancer patients with fluoropyrimidines to include DPD inhibitor components as standard therapy.