Reexamining a proposal: Thymidylate synthase 5′-untranslated region as a regulator of translation efficiency

The DNA replicative gene, thymidylate synthase (TYMS), is inhibited upon treatment with the anticancer drug 5-fluorouracil (5-FU). TYMS has 28-bp tandem repeat sequences or VNTR (variable numbers of tandem repeats) in the 5′-untranslated region (5′-UTR). The number of these repeats is variable in any given population, but the most prevalent are double (2R) and triple (3R) repeat sequences. A single G/C nucleotide polymorphism in the triple repeat sequence gives rise to a 3Rc or a 3Rg triple repeat structure. A widely cited literature used plasmid constructs of the 5′-UTR and proposed that genotyping the TYMS UTRs would predict the efficiency of Tyms protein translation, justifying altered therapeutic dosage of 5-FU. Prior studies had unusual features in experimental design, such as using the firefly Kozak sequence in place of the native human TYMS Kozak sequence to determine the ribosomal translational efficiency of TYMS mRNA. Our results using transient transfection, antibiotic-selected pools of transfected cells, and stably transfected clones, while using plasmids having native human Kozak sequence, refute the earlier results.