TS gene

In vitro studies have shown that increasing the number of repeats leads to stepwise increases of TS gene expression with the presence of a triple repeat resulting in a 2.6-fold greater TS expression than a double repeat [70, 80]. In vivo studies in human gastrointestinal tumors have shown a significant increase in TS protein levels and functional activity in patients with TSER 3 compared to individuals with TSER 2 [81]. As TS tumor levels are important for resistance and survival prediction, this may have important implications for TS-based chemotherapy. 

An important factor in the resistance to chemotherapy drugs such as 5-fhiorour-acil and Raltitrexed is an increase in TS expression [72, 37]. The TS enhancer region polymorphism may be one mechanism responsible for increasing TS gene expression. 

Hori T, Takahashi E, Ayusawa D, Takeishi K, Kaneda S, Seno T. Regional assignment of the human thymidylate synthase (TS) gene to chromosome band 18pll.32 by nonisotopic in situ hybridization. Hum Genet 1990 8 576-580. 

These three different polymorphisms in the same gene obviously complicate efforts aimed at imderstanding the effects of each individual polymorphism. TS expression levels, tumor response, and toxicity are functions of multiple TS gene alterations, rather than the result of one single polymorphism. This review will provide an update of the most recent data on 5-FU metabolism and TS gene variations in CRC. 

Chapter 10 / TS Gene Variation in Colorectal Cancer Patients... 

One possible explanation for the differences in MTD is the significant discrepancy in frequencies of polymorphisms in the TS gene reported in Asians compared with Caucasians (42). 

Fig. 2. Regulation of thymidylate synthase (TS) gene expression. Abbreviations.- TS (thymidylate synthase) USF-1 (upstream stimulating factor 1) USF-2 (upstream stimulating factor 2) SNP (single nucleotide polymorphism). (Reprinted with permission of El-Khoueiry et al. Pharmacogenomics and molecular biology of gastrointestinal cancers, Atlas of Gastrointestinal Cancers, Current Medicine Group, LLC, Philadelphia, 2007).

Fig. 3. Polymorphisms in the TS gene. Abbreviations.- TSER (thymidylate synthase enhancer region) 2R (two-tandem repeats) 3R (three-tandem repeats) TSER 3G (single nucleotide polymorphism of TSER 3R) -6bp (TS 3 -UTR 6bp deletion polymorphism). (Adapted to Marsh et al. Thymidylate synthase pharmacogenetics. Invest New Drugs 2005).

Patients with metastatic colon cancer homozygous for the triple tandem repeat (TSER-3R/3R) had significantly higher intratumoral TS gene expression compared with those with double tandem repeats (TSER-2R/2R) within the 5 -UTR region (3,57). 

In a recent study, Mandola et al. showed that the 28 bp TSER tandem repeats contain elements that bind upstream stimulating factor (USF), and also that ligand binding by USF-1 and USF-2 enhances the transcriptional activity of the TS gene (Fig. 2) (42). Electrophoretic mobility shift analysis has shown that the presence of a G-to-C single nucleotide polymorphism (SNP) within the second repeat of the 3R allele leads to decreased ability of upstream stimulatory factor (USF) to bind within the repeat and therefore sequentially result in decreased transcriptional activity of the 3R TS gene variant (42). 

The TS gene is localized to the short arm of chromosome 18 at chromosome band 18p 11.32 (64). Chromosome 18 is generally known to be a site of frequent deletions in colorectal cancer tissues (64). Therefore it is highly probably that allelic imbalance occurs at the TS locus in some colorectal tumors. Zinzindohoue et al. were the first to report on the idea of LOH at the TS locus. The authors showed that the TS genotype from 2R/3R heterozygotes differed in ratio between 2R and the 3R bands. The observed LOH frequency at the TS locus was 63% (31 of 50) (65). 

Lenz H-J, Zhang W, Zahedy S et al. A 6 base-pair deletion in the 3 UTR of the thymidylate synthase (TS) gene prediets TS mRNA expression in colorectal tumors a possible eandidate gene for eoloreetal cancer risk. Proc Am Assoc Cancer Res (Abstract) 2002 43. 

Uehida K, Flayashi K, Kawakami K et al. Loss of heterozygosity at the thymidylate synthase (TS) loeus on ehromosome 18 affeets tumor response and survival in individuals heterozygous for a 28-bp polymorphism in the TS gene. Clin Cancer Res 2004 0 A33-439. 

Costea 1, Moghrabi A, Krajinovie M. The influence of cyclin D1 (CCNDl) 870 A>G polymorphism and CCNDl-thymidylate synthase (TS) gene-gene interaction on the outcome of childhood acute lymphoblastic leukaemia. Pharmacogenetics 2003 13 577-580. 

---