In the present study, both of the studied MDR1 polymorphisms were associated with CRC risk and interacted with meat intake in relation to CRC risk. Carriers of the MDR1 G-rs3789243-A A-allele and homozygous carriers of MDR1 C3435T C-allele were at higher risk of CRC than carriers of the common allele genotypes. Carriers of these two genotypes were, moreover, at higher risk of CRC in relation to intake of red and processed meat whereas carriers of the respective common allele genotypes were not at higher risk of CRC in relation to meat intake. COX-2 and BCRP gene polymorphisms were not associated with CRC risk. Furthermore, there was interaction between NSAID use and the MDR1 C3435T and COX-2 T8473C polymorphisms in relation to risk of CRC. No gene-smoking interactions were found.
The present study design has pros and cons. Prospective studies have the advantage in relation to examining gene-environmental interactions that they are not encumbered by recall bias. In the present study, cases and cohort sample were selected from the same cohort, which together with complete follow up of the participants, minimised the risk of selection bias. Information on lifestyle factors were collected at enrolment for all participants which minimised the risk of differential misclassification of cases and comparison group. However, lifestyle factors were only collected once, and may thus not be representative for the lifestyle during follow-up. This is, however, not expected to result in differential misclassification. Furthermore, information on food intake was based on a semi-quantitatively food frequency questionnaire [49, 53], which was, however, evaluated and found useable .
Known life-style factors affecting the risk of CRC include diet, physical activity, body mass index (BMI), alcohol, smoking and NSAID use . In this study the results were adjusted for relevant confounding factors. Physical activity (habitual exercise) has been shown not to be a risk factor in this population and hence, was not adjusted for .
Heterozygous and homozygous variant genotype carriers were combined for the analyses of interactions due to power-considerations. Therefore, in the light of the obtained P-values and the number of statistical testes performed, we cannot exclude that our positive findings are due to chance. On the other hand, the fact that we found interaction between both of the studied MDR1 polymorphisms and meat intake in relation to CRC risk makes a chance finding less likely.
In contrast to the finding in the present study, MDR1 polymorphisms have not been associated with overall risk of CRC in previous studies [31–33]. The only other larger study, a case-cohort study in a Norwegian population of the polymorphism G-rs3789243A [29, 30] found no association between this SNP and the development of intestinal adenomas and carcinomas . Smaller studies, including up to 285 cases, have not found associations with overall CRC risk among Caucasians or Koreans [31–33], although an association was found between the MDR1 C3435T variant allele and CRC risk among patients diagnosed before the age of 50 years . Moreover, we found no interaction between the MDR1 polymorphism and smoking status which is in contrast to a previous study showing an association between MDR1 C3435T variant allele and CRC risk among life-long nonsmokers of more than 63 years of age . Relative to our study, the mentioned studies have a weaker design.
We found a relative risk by meat intake of 1.03 (95% CI: 0.98-1.09, p for interaction = 0.01) and 1.08 (95% CI: 1.00-1.16, p for interaction 0.02) per 25 g red and processed meat per day for the two identified risk MDR1 genotypes. The risk is in line with a previous finding of a relative risk of 1.29 per 100 g per day . Moreover, meat intake was not associated with CRC risk for carriers of the other genotypes. The found interactions between MDR1 polymorphisms and intake of meat in the present study suggest that the MDR1 polymorphisms may be of minor importance in study populations with a low intake of red and processed meat.
We observed an interaction between MDR1 C3435T and NSAID use such that NSAID use was associated with increased CRC risk among homozygous carriers of the wild type allele only. In cell lines, COX-2 has been shown to induce P-glycoprotein  whereas COX-2 inhibition prevented the expression and function of P-glycoprotein , thereby affecting apoptosis . A similar relationship has been found in vivo . Thus, NSAID use seems to affect P-glycoprotein tumorigenesis in an incompletely understood manner. While MDR1 seems to play a role in CRC carcinogenesis, much is to be learned about the function of P-glycoprotein in relation to carcinogenesis.
The silent MDR1 C3435T polymorphism has been suggested to lead to a more unstable mRNA and consequently, lower overall activity of the variant allele should be expected . Therefore, we expected that red and processed meat would be most carcinogenic among T-allele carriers of MDR1 C3435T, whereas the opposite was found. However, in a very recent review, Fung et al suggest that the silent C3435 polymorphism induces a conformational change in P-glycoprotein due to ribosome stalling during translation, whereas no effect on mRNA stability was detected . The two polymorphic P-glycoproteins were shown to differ in their substrate specificity, since transport of varapamil but not rapamycin was changed by the polymorphism . Our study suggestthat red and processed meat were most carcinogenic among C-allele carriers of MDR1 C3435T. The very large MDR1 gene includes 28 exons and is highly polymorphic which makes it difficult to identify causal polymorphisms. In addition, linkage patterns and allele frequencies in MDR1 are highly variable between different ethnic groups and thus between the studied populations . Hence, case-control studies with assessment of multiple polymorphisms, enabling comprehensive haplotype analysis, in parallel with P-glycoprotein activity, mRNA and protein level measurements, will be required to understand MDR1 genotype-phenotype causality.
We found no association between BCRP genotypes and risk of CRC and on interaction between BCRP genotypes and meat, smoking or NSAID. Our result is in accordance with a recent gene-wide association study . Our study suggests that P-glycoprotein, but not BCRP, transport certain carcinogens, which are relevant in relation to meat intake. Therefore, elucidating substrate differences between P-glycoprotein and BCRP may help to identify possible mechanisms behind meat-related carcinogenesis .
We found no association between COX-2 genotypes and risk of CRC. The haplotype pattern was similar to what has previously been found for Danes . In accordance with our results, no association between COX -2 polymorphisms and CRC was demonstrated in a large French case control study  and smaller studies [65, 66]. Variant allele carriers of COX-2 G-765C have been reported to be at higher CRC risk among Han Chinese . Other studies suggest that the effects of polymorphisms on COX-2 expression levels are large enough to have biological impact provided that COX-2 expression is important in CRC . On this basis, our results suggest that COX-2 plays a limited role in colon carcinogenesis in the present study population.
Interaction between NSAID and COX-2 polymorphisms in relation to risk of CRC and colorectal adenomas has been investigated previously [65, 67, 68]. Although some studies on colorectal adenomas may suggest that the largest risk-reducing effect by NSAID use is observed among the genotypes related to high COX-2 levels, results are not consistent [68–70]. We observed interaction between COX-2 T8473C and NSAID use. NSAID use was associated with CRC risk among homozygous carriers of the COX-2 T8473C genotype, which is assumed to be associated with low expression levels. In accordance with our result, [45, 61] aspirin has been shown to induce COX-2 transcription in some tissues, among others intestinal myofibroblasts, especially in the presence of IL-1β . On the other hand, induction of COX-2 gene expression by inhibition of COX-2 have been demonstrated in both liver and colon cancer cell lines [45, 61]. This seem to be mediated by a COX-2 independent mechanism , These in vitro results indicate that the tissue specific effects of NSAIDs are far from clear. Thus, the observed interaction may be a chance finding. In the 'Diet, Cancer and Health cohort, used in the present study, long term use of NSAID was associated with a protective effect against CRC . However, using a higher intake of NSAID as cut-off value (weekly use) did not change our results regarding NSAID use (results not shown).