Thymidylate synthase expression and mismatch repair protein expression in colorectal cancer

Abstract

The tumour, node, metastasis (TNM) system, with the current staging and risk stratification methods for prognostication in colorectal cancer (CRC) has its limitations. The need for additional validated prognostic and predictive markers is particularly important in CRC stage II and III as some of these patients can be cured by surgery alone.

In international tumour marker guidelines there is still not enough evidence to recommend the routine use of any tissue-based tumour markers in the treatment or surveillance of CRC. The problems are that the majority of studies evaluating tumour markers lack standardized methods for assessment, are non-randomized and involve a limited number of patients. Another main concern is the intrapatient heterogeneity of a certain tumour marker between the primary tumour and the corresponding metastases and even between different metastatic sites.

Two potential prognostic and predictive markers are: 1) Thymidylate synthase (TS), a rate-limiting enzyme involved in DNA synthesis and the target enzyme for 5-Fluorouracil (5-FU), which is the standard treatment used in CRC. 2) Microsatellite instability (MSI), the hallmark of a defective DNA mismatch repair (MMR) that occurs in about 15 % of sporadic colorectal cancer (CRC).

In this thesis we have compared the expression of TS assessed with immunohistochemistry (IHC) in lymph node metastases as well as lung- and liver metastases of CRC with TS expression in matched primary tumours. Furthermore, we have evaluated the prognostic and predictive role of TS expression and MMR protein expression using IHC in stage II and III CRC.

A significant correlation was found between expression of TS in lymph node metastases and TS expression in their matched primary tumours. TS expression assessed in lymph node metastases did significantly improve the prognostic precision compared with TS expression in primary tumours but did not predict response to 5-FU-based adjuvant chemotherapy. A low TS expression in lymph node metastases was associated with a longer OS and DFS.

TS expression was analyzed in liver metastases (n=38) and lung metastases (n=10) as well as in their matched primary tumours (n=45) There was no significant correlation between TS expression in distant metastases and their matched primary tumours. A tendency to a higher TS expression was seen in liver metastases (84%) compared to lung metastases (70%).

TS expression was a significant prognostic marker in patients treated with surgery alone where an improved survival was found in patients with a low TS. Patients with the highest TS expression (grade 3) had a significantly improved survival when treated with adjuvant 5-FUbased chemotherapy independently of the dose of 5-FU.

MMR protein expression was found to be a significant prognostic marker for survival in univariate analysis as well as in multivariate analysis adjusted for gender, age, grade of differentiation, stage of disease and numbers of analyzed lymph nodes. Patients with MMR protein negative tumours had an improved survival compared to patients with MMR protein positive tumours. MMR protein expression did not predict benefit of adjuvant 5-FU-based chemotherapy with respect to survival.

In a combined analysis no significant correlation was revealed between MMR protein expression and TS expression in colon cancer. There was a significantly improved survival in stage III patients with MMR protein positive colon tumours expressing high TS when receiving adjuvant 5-FU-based chemotherapy compared to treatment with surgery alone.

It is questionable whether a single molecular marker may play a relevant prognostic and predictive role in a complex and heterogenous disease such as CRC. In the future we rather have to define different subtypes of CRC based on molecular, clinical and morphological features in order to tailor the optimal treatment for each individual patient maximizing the therapeutic effects whilst minimizing toxicity.