Searching expressed sequence tag databases: discovery and confirmation of a common polymorphism in the thymidylate synthase gene.

Databases of expressed sequence tags (EST) can be used to screen rapidly for potential polymorphisms in candidate proteins. As part of this study, we screened the gene for the enzyme thymidylate synthase (TS). TS is important physiologically because it is essential for the synthesis of deoxythymidylate, a nucleotide required for DNA synthesis and repair. TS is also a major target for cancer chemotherapeutic drugs, especially the widely used 5-fluorouracil. Using sequence alignment of ESTs, we identified a candidate 6-bp variation at bp 1494 in the 3'-untranslated region of the TS mRNA. This sequence variation occurred in 21 of 34 aligned ESTs at this location, including ESTs from various tissue sources. The presence of this polymorphism was confirmed in a Caucasian population (n = 95) by polymerase chain restriction amplification/RFLP analysis. The allele frequency of the 6-bp deletion was found to be 0.29 (wildtype +6 bp/+6 bp, 48%; +6 bp/-6 bp, 44%; -6 bp/-6 bp, 7%). Although the function of this polymorphism has not yet been investigated, the 3'-untranslated region of a gene can play a role in mRNA stability and translation. This study illustrates an approach to polymorphism discovery in candidate enzymes of physiological interest by searches of publicly available sequence data, a rapid and inexpensive method. The potential functional relevance of the common 6-bp deletion in the TS gene needs to be investigated, because this enzyme is plausibly of major importance not only in cancer treatment but also in cancer prevention.