AIM: To investigate the association of variations in the cyclooxygenase-2 (COX2) and uridine diphosphate glucuronosyltransferase 1A6 (UGT1A6) genes and non-steroidal anti-inflammatory drugs (NSAIDs) use with risk of colon cancer.

METHODS: NSAIDs, which are known to reduce the risk of colon cancer, act directly on COX2 and reduce its activity. Epidemiological studies have associated variations in the COX2 gene with colon cancer risk, but others were unable to replicate this finding. Similarly, enzymes in the UGT1A6 gene have been demonstrated to modify the therapeutic effect of NSAIDs on colon adenomas. Polymorphisms in the UGT1A6 gene have been statistically shown to interact with NSAID intake to influence risk of developing colon adenomas, but not colon cancer. Here we examined the association of tagging single nucleotide polymorphisms (SNPs) in the COX2 and UGT1A6 genes, and their interaction with NSAID consumption, on risk of colon cancer in a population of 422 colon cancer cases and 481 population controls.

RESULTS: No SNP in either gene was individually statistically significantly associated with colon cancer, nor did they statistically significantly change the protective effect of NSAID consumption in our sample. Like others, we were unable to replicate the association of variants in the COX2 gene with colon cancer risk (P > 0.05), and we did not observe that these variants modify the protective effect of NSAIDs (P > 0.05). We were able to confirm the lack of association of variants in UGT1A6 with colon cancer risk, although further studies will have to be conducted to confirm the association of these variants with colon adenomas.

CONCLUSION: Our study does not support a role of COX2 and UGT1A6 genetic variations in the development of colon cancer.

Almost 150 000 new colorectal cancer cases are estimated to be diagnosed in 2008, resulting in almost 50 000 deaths (National Cancer Institute-http://www.cancer.gov). Colon adenomas (polyps) are a well established precursor of colon cancer. The genetic and environmental factors that cause the development of colon adenomas or their subsequent progression into cancer are not entirely known. Genetics are known to be a large risk factor for colon cancer, and indeed having a family history of colon cancer increases your risk of developing it yourself substantially. However, the known genetic susceptibility loci for colon cancer make up only a small fraction of this risk.

Cyclooxygenase-2 (COX-2) is a pro-inflammatory enzyme that converts arachidonic acid to prostaglandins. COX2 has been shown to be upregulated in a high proportion (about 86%) of human colorectal cancer[1]. Numerous additional findings have indicated a likely role for the cycloxygenases and inflammation in the development of colon cancer[2].

Non-steroidal anti-inflammatory drugs (NSAIDs), including aspirin and ibuprofen, act directly on COX2 as well as other targets to reduce activity. A substantial body of epidemiologic and randomized clinical trial evidence suggests that regular NSAID use and selective COX-2 inhibitors reduce the risks of colorectal cancer or the recurrence of adenomatous polyps[3], which is, at least in part, attributed to their known anti-inflammatory effects. The COX-2 gene is thus a good candidate gene for colorectal carcinogenesis and its genetic variants may affect the susceptibility to the development of this colorectal cancer by altering the effects of this enzyme on the inflammatory response. To date, a few studies have evaluated a limited number of polymorphisms in the COX2 gene in relation to risk of colorectal cancer and adenomatous polyps[4–7]. The findings have yielded inconsistent results, with some[4–6], but not all[7], reporting an association with the risk of colorectal cancer or polyps. Additionally, a sib-pair study of colon neoplasia with relatively small sample size found no linkage to the COX2 locus[8].

Furthermore, genetic variation in the uridine diphosphate glucuronosyltransferase 1A6 (UGT1A6) gene have been associated with differences in aspirin metabolism, and those with the less frequent variants have a 30%-50% lower enzyme activity[9]. Additionally, a few studies have examined the association of genetic polymorphisms in UGT1A6, a rate-limiting enzyme directly involved in aspirin metabolism, and their interactions with aspirin or NSAIDs in relation to colorectal cancer and polyps[10–13]. Although two studies of colon cancer observed no association with genetic variants in UGT1A6[1213], two studies of colon adenomas identified variants in UGT1A6 that modified the protective effect of aspirin[1011]. Others have identified variants that influence the risk of adenoma recurrence[14]. To further investigate the role of COX2 and UGT1A6 in relation to the risk of colon cancer, we tested nine tag single nucleotide polymorphisms (SNPs) in COX2 and two functional SNPs in UGT1A6[13] in a population-based case-control study of colon cancer.

The characteristics and genotypic distributions of this predominantly Caucasian study population are summarized in Table 1. All SNPs were found to be in Hardy-Weinberg equilibrium both in controls alone and in the entire sample. We found no statistically significant association between any of the nine COX2 SNPs and two functional UGT1A6 SNPs and colon cancer risk, regardless of the mode of inheritance (Table 2).

We further explored potential effect modification of the association by regular NSAID use, and found no evidence for interaction of any SNP (Table 2) in either gene (P > 0.05).

Since others[10] have reported that UGT1A6 variants modify the therapeutic effects of aspirin in a population of women only, we stratified our analyses by gender. We found very little differences in results with nothing significant in females or males only as well (results not shown).

In our analysis, we examined potential effect modification by regular NSAID use rather than aspirin alone, as other groups have done, due to the smaller number of aspirin alone users. To account for this, we repeated our analysis using aspirin use only (115 cases and 157 controls), and the results did not change materially (not shown), with no significant findings. However, the lack of association with aspirin alone may be due to the small sample size and thus lack of statistical power.

Our study had over 90% power to detect an OR ≥ 1.75 at a two-sided α = 0.05 for the polymorphisms studied here, and over 80% power to detect an OR ≥1.60, assuming a dominant model of inheritance and allele frequency of 0.1. While we did comprehensively capture the common genetic variation across the COX2 gene, we only evaluated two putative functional SNPs in UGT1A6, and were limited in our ability to make direct conclusions about the effect of other genetic variants in UGT1A6. It is possible that these variants have smaller effects on colorectal cancer susceptibility or the therapeutic effects of NSAID use that we were unable to detect with this study.

It is important to note that NSAID use was based on self-report. Individuals may not accurately recall duration or frequency of NSAID intake. Nevertheless, our finding of a protective effect of NSAID use (OR = 0.69, 95% CI = 0.50-0.96, P = 0.02) is in agreement with its well-documented association with colon cancer[17], lending credibility to our questionnaire data.

In conclusion, this moderately large population-based case-control study did not detect a direct association between variants in the UGT1A6 and COX2 genes and risk of colon cancer nor an effect modification by NSAIDs. The results of our study are in line with the two studies of colon cancer showing null results[1213]. Taken together with the studies showing an association with polyps[1011], our results suggest genetic variation of UGT1A6 may affect the early stages of colon tumorigenesis, but has little influence on the progression from adenomatous polyps to colon cancer, although we are unable to test that hypothesis directly.