The present study is one of the largest to investigate the prognostic value of three major enzymes involved in the fluoropyrimidine metabolism. The intensity of cytoplasmic staining was used to provide a semi-quantitative estimate of TS, DPD and TP protein expression levels. For TS, this has been validated as an accurate reflection of protein levels [18
]. Colorectal tumours were derived from a population-based cohort with long patient follow-up periods and well-documented pathological and clinical information, including the use of 5-FU-based adjuvant chemotherapy [17
]. This allowed us to determine the prognostic value of TS, DPD and TP expression in clearly defined stage and treatment groups ().
High TS expression levels have generally been associated with worse overall survival in CRC [1
]. A recent meta-analysis found an hazard ratio (HR) of 1.35 for high TS expression in 2610 patients with localised CRC [19
], although evidence of heterogeneity and possible publication bias was observed. The adjuvant treatment status of patients in many studies has not always been well defined, leaving open the possibility that 5-FU might differentially influence the outcome of low or high TS expression groups. In the present study, we found that low TS expression was a prognostic marker for worse survival in stage II CRC patients treated by surgery alone (). The same trend was also apparent for stage III cases. In contrast, TS expression showed no prognostic significance in stage III CRC patients treated with 5-FU. The latter result concurs with several other recent studies on 5-FU-treated CRC patient cohorts [20
]. One explanation for these results may be that low TS expression is associated with a more aggressive tumour phenotype and hence the worse prognosis observed for patients treated by surgery alone. If low TS tumours, however, are more sensitive to 5-FU than high expressing tumours, patients with these tumours would derive preferential benefit, thus explaining the lack of prognostic value observed for TS in 5-FU-treated patients ().
results with tumour cell lines have indicated that low expression or activity of TS was associated with good response to 5-FU [23
]. This was subsequently confirmed by several clinical studies [15
]; however, others have reported discordant results [29
]. The results of the current study on the prognostic significance of TS expression in CRC patients treated with or without chemotherapy () indirectly support the contention that low TS expression level is a predictive marker for survival benefit from 5-FU chemotherapy. Although patients in this study were not randomly allocated to the study, exploratory analysis revealed the HR associated with 5-FU chemotherapy was 0.70 [95% confidence interval (CI) 0.48–1.02] for stage III patients with low TS expression compared with 1.2 (95% CI 0.70–2.05) for those with TS high expression.
In comparison to TS, considerably less work has been carried out on the prognostic and predictive values of DPD and TP expression in CRC. The present results demonstrate that low DPD expression was associated with worse survival of both stages II and III CRC treated by surgery alone (). This concurs with the strong correlation observed here between low DPD expression and more advanced tumour stage (), as well as previous work showing that low DPD expression was associated with worse prognosis for stages II and III CRC patients treated by surgery alone [33
]. Most published studies have reported no significant associations between DPD expression and prognosis [1
], although many of the findings are difficult to interpret because study groups were not clearly defined for adjuvant treatment status. Similar to TS expression, exploratory analysis revealed that the use of 5-FU chemotherapy was associated with good survival for stage III CRC patients with low DPD expression (HR = 0.54, 95% CI 0.34–0.86) but not for those with high DPD expression (HR = 1.16, 95% CI 0.76–1.76). These results support those of a recent study using oral 5-FU-based chemotherapy in stages II and III CRC [33
There is some evidence in the literature to indicate that high TP expression is associated with worse prognosis [1
]. This was not supported by the current results, however, where the study design was aimed at investigating prognostic significance in the absence of 5-FU chemotherapy. In contrast to TS and DPD, low expression of TP did not have significant prognostic value in either stage II or stage III CRC treated by surgery alone (). Low TP expression was similar to low TS and DPD expression in that it was associated with a trend for better survival in stage III CRC treated with 5-FU, again indicative of response to chemotherapy in these patients. This was confirmed by the finding that chemotherapy was associated with good survival in patients with low TP expression (HR = 0.59, 95% CI 0.34–1.01) but not in those with high TP expression (HR = 0.93, 95% CI 0.63–1.35).
In summary, the major finding of the present study was that low TS and DPD protein expression were prognostic markers of poor survival in stages II and III CRC patients treated by surgery alone. The trends observed here for association of low TS, DPD and TP expression with better survival in patients receiving 5-FU indicates that they are also predictive markers of good response to this treatment. Confirmation of this was obtained by exploratory analyses that compared the survival of patients treated with and without 5-FU in low and high expression groups. The current results support the findings of Salonga et al. [15
] who used tumour response as the end point rather than disease-specific survival and who evaluated mRNA rather than protein levels. Together, these results indicate that assessment of protein or mRNA levels for TS, DPD or TP may allow improved targeting of 5-FU-based treatments towards patients who are most likely to benefit. Validation of their clinical utility will, however, require further prospective trials that are designed specifically to evaluate the predictive significance of these markers [4
]. Since TS, DPD and TP are involved in the mechanism of response to 5-FU, any predictive value associated with these markers should not be affected by the simultaneous use of additional agents that do not target the fluoropyrimidine pathway.