Role of topoisomerase I and thymidylate synthase expression in sporadic colorectal cancer: Associations with clinicopathological and molecular features

Abstract

Topoisomerase I (Topo I) and thymidylate synthase (TS) are essential enzymes for the replication, transcription and repair of DNA, and are potential biomarkers in colorectal cancer (CRC). The aim of the study was to correlate the tissue expression of Topo I and TS in sporadic CRCs with relevant pathological and molecular features and patients’ outcome.

Topo I and TS expression was assessed by immunostaining in 112 consecutive primary CRCs. Increased expression of Topo I was found in 36% of tumors, preferentially rectal (50%) and with not otherwise specified (NOS) histology (44%). Topo I expression was associated with 18q allelic loss (LOH), (p = 0.013), microsatellite stable phenotype (p = 0.002) and normal expression of mismatch proteins hMLH1 and hMSH2 (p = 0.0012 and p = 0.02, respectively). High TS expression was found in 60% of tumors, more frequently in distal sites (62%) and with NOS histology (66%); no association with microsatellite instability was observed.

Topo I seems to be involved in the chromosomal instability pathway of sporadic CRCs. Conversely, high TS expression is unlikely to affect the clinical behavior of microsatellite unstable CRCs.

Introduction

Colorectal cancer (CRC) is the third most common cancer in men and the second in women worldwide. According to WHO, there were 1.2 million new diagnoses of CRC in 2008, and more than 608,000 deaths [1], [2]. CRC development follows two major pathways of genetic instability: chromosomal instability (CIN), observed in 85% of sporadic CRCs, and microsatellite instability (MSI), accounting for approximately 15% of cases [3]. CIN-related CRCs are characterized by gross chromosomal rearrangements, aneuploidy, defects in checkpoints for G1/S entry, and loss of heterozygosity (LOH) in particular at chromosomal arm 18q [3], [4], [5]. Conversely, MSI-related CRCs have diploid or nearly diploid karyotypes and show defects of the DNA mismatch repair system (MMR) genes [3]. CRCs referred to CIN or MSI pathway display relevant clinic-pathological differences as to tumor site, histology and response to adjuvant therapy [6]. Several studies have also demonstrated that the molecular phenotype may affect CRC outcome [7], [8], [9], [10]. In particular, CRCs with MSI have been associated with improved survival [11], whereas 18qLOH is a molecular marker of adverse prognosis [7], [8], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21].

Several biological markers have been investigated for a prognostic role in CRC [22], among which are topoisomerase I (Topo I) and thymidylate synthase (TS), whose expression levels correlate with survival although this evidence is controversial among different studies [23], [24], [25], [26], [27], [28].

Topo I is an essential enzyme in regulating the topology of supercoiled DNA by transiently cleaving of one of the two strands [29]. Antineoplastic drugs targeting Topo I, such as irinotecan and camptothecins, form stable Topo I-DNA cleavage complexes and inhibit Topo I activity, thus preventing DNA religation [30], [31]. Topo I is expressed in primary CRCs and metastases, but it is debated whether its expression can predict the response to anti-Topo I treatments [32], [33], [34], [35].

TS catalyzes the conversion of dUMP to dTMP and is essential for ‘de novo’ DNA synthesis [36], [37]. The expression of TS may affect tumor sensitivity to fluoropyrimidines, such as 5-fluorouracil (5-FU) [38]. 5-FU-based treatment is the standard of care for adjuvant therapy of CRC in combination with oxaliplatin [39] and for the treatment of metastatic disease in association with oxaliplatin or irinotecan [40], [41], [42], [43].

The aim of the present study was to assess the expression of Topo I and TS in tumor tissue by immunostaining and to correlate it with pathological and clinical variables, patients’ outcomes and molecular characteristics, such as MSI status, LOH at different loci and other markers implicated in CRC carcinogenesis.