Isolation and characterization of a thymidylate synthase-deficient human colon tumor cell line

Following mutagenesis of the human colorectal tumor cell line HCT C with et hyl methanesulfonate, clonal sublines were isolated that survived on medium toxic to cells expressing thymidylate synthase (TS). The subline exhibitin g the lowest TS activity, designated as C18, was characterized. Extracts fr om C18 cells were mixed with extracts from parental C cells to determine wh ether the TS-deficient phenotype is trans-acting. No effect was observed on the activity of TS in parental extracts. The levels of functional TS in C1 8 cells were analyzed by the binding of the mechanism-based inhibitor 5-flu oro-2'-deoxyuridylate (FdUMP) under conditions that allowed for the detecti on of 10 fmol of TS. Only a low level of FdUMP-TS complexes was detected in C18 extracts. The level of TS expression in C18 cells was similar to that in parental C cells, as indicated by immunoblot and RNA analyses. DNA seque nce analysis of TS cDNA from C18 cells revealed the existence of a point mu tation (C-->T) at nucleotide 647 chat predicts the replacement of Ser216 by a leucine residue. That the C18 cell line was homozygous for this mutation was indicated by restriction fragment-length polymorphism analysis and by primer extension analysis. To provide additional evidence that substitution of Ser216 by a leucine residue created a defective protein, a TS-deficient bacterial strain was transformed with an expression vector containing the mutated human TS cDNA. The transformed strain exhibited thymidine auxotroph y, indicating that the mutant TS (Leu216) is nonfunctional. (C) 1999 Elsevi er Science Inc.