In this cohort of Singapore Chinese, we reported a statistically significant effect of the COX-2 −765G>C gene polymorphism on colon cancer risk among subjects with high intake of dietary n-6 PUFAs. Our data support the hypothesis that COX-2 exerts its effects on colon carcinogenesis through its influence on prostaglandin synthesis from n-6 PUFAs.
The current study has several strengths. (1) Our prospective study design precludes the possibility of recall bias. Furthermore, reliable dietary nutrient estimates including n-6 PUFAs were assessed using a validated food frequency questionnaire. (2) The nationwide cancer registry has been in place in Singapore since 1968 (Parkin et al, 2002
), and migration out of Singapore has been negligible since inception of the study. This relatively complete ascertainment of cancer and death outcomes eliminates a concern for potential selection bias. (3) Study subjects originated from two contiguous regions in Southern China, resulting in a genetically homogeneous study population that facilitated the investigation of gene–disease associations. (4) Exposure information on other known/suspected risk factors for colorectal cancer was collected and accounted for in all analyses of gene–diet–cancer risk associations.
The chief limitation of our study is the lack of information on use of COX-2 inhibitors, which may bias the effect of COX-2 genotype on risk. However, if use of COX-2 inhibitors were to exert a confounding effect on the observed COX-2 genotype/colon cancer association, our inability to control for such confounding is likely to lead to an underestimation, rather than an overestimation, of the risk associated with the putative high-activity genotype. This is because use of COX-2 inhibitors is likely to be more common among subjects with more severe symptoms of inflammation, possibly due to the possession of the high activity COX-2 genotype. Another weakness of the present study is our relatively small number of cancer cases, which may result in less precise estimation of risk factor–disease associations.
The major n-6 PUFA in most diet is linoleic acid, the precursor of arachidonic acid. The latter is consequently converted to various prostaglandins, and COX is the crucial and rate-limiting enzyme for this conversion. There is compelling evidence that prostaglandins play important roles in colorectal carcinogenesis by enhancing cell proliferation and growth, promoting angiogenesis and inhibiting apoptosis (Cao and Prescott, 2002
; Stoehlmacher and Lenz, 2003
gene expression and its mRNA and protein levels were markedly elevated in most human colorectal cancers relative to adjacent normal mucosa (Kargman et al, 1995
; Sano et al, 1995
). It is hypothesised that the COX-2-associated effect on colorectal carcinogenesis is due to the increased production of prostaglandins from dietary n-6 PUFAs (Eberhart and Dubois, 1995
). In support of this hypothesis, high dietary n-6 PUFAs has been shown to promote colon tumorigenesis by upregulating COX-2 expression in animal studies (Singh et al, 1997
The human COX-2
gene is mapped to chromosome 1q25.2–q25.3 and spans about 8.3
kb pairs with 10 exons (Kosaka et al, 1994
). Previous studies on the 5′ flanking region of the human COX-2
gene show that this region contains a canonical TATA box as well as several putative factor elements that are critical in inducing COX-2
gene transcription, such as Sp1, NF-κ
B, GRE (glucocorticoid) and IRE (insulin) elements (Tazawa et al, 1994
; Yang et al, 1997
). The region from nucleotide −827 to −454 has been described as a negative region since deletion of this region led to increased luciferase activity in reporter expression studies. The −765G
mutation lies within this region, and is also within one of the five putative Sp1 elements (Yang et al, 1997
). At present, data on the functionality of the −765G
polymorphism and the direction/magnitude of change in protein expression/activity between the C
alleles are limited and mixed (Papafili et al, 2002
; Schneider et al, 2003
In summary, the present study provides the first epidemiological evidence for a possible cause-and-effect connection between the production of prostaglandins from n-6 PUFAs through the enzymatic activity of COX-2, and increased risk of tumour development in the colon. Our novel findings require confirmation in larger studies with varying levels of substrate intake and genotype frequency.