Author's response to reviews Title: Polymorphisms in the xenobiotic transporter Multidrug Resistance 1 (MDR1) gene and interaction with meat intake in relation to risk of colorectal cancer in a Danish prospective case-cohort study. Authors:

Background: Genetic variation that modifies the gut barrier function or the inflammatory response may modify the risk of colorectal cancer (CRC). The intestinal xenobiotic transporters, Multidrug Resistance 1 (MDR1/ABCB1) and Breast Cancer Resistance Protein (BCRP/ABCG2) contribute to the intestinal homeostasis by exporting a broad spectrum of substrates including potential carcinogens from the intestinal lining into the lumen. Cyclooxygenase-2 (COX-2/PTGS2/PGHS2) plays a key role in the intestinal immune response to luminal antigens. The aim of this study was to investigate if polymorphisms in these genes were associated with CRC risk, and to investigate possible interactions with lifestyle factors such as alcohol consumption, smoking, meat consumption and use of non-steroidal anti-inflammatory drugs (NSAID). Methods: MDR1 C3435T (rs1045642) and intron3 G-rs3789243-A, BCRP C421A (rs2231142), and COX-2 A-1195G (rs689466), G-765C (rs20417), and T8473C (rs5275) polymorphisms were assessed together with lifestyle factors in a nested case-cohort study of 364 cases and a random cohort sample of 772 participants from the Danish prospective Diet, Cancer and Health study. Results: Carriers of the variant allele of MDR1 intron 3 were at 1.52-fold higher risk of CRC compared with homozygous wild type allele carriers (Incidence rate ratio (IRR) =1.52. 95 % Confidence Interval (CI): 1.12-2.06). Homozygous carriers of the T-allele of MDR1 C3435T had a lower risk of CRC compared with homozygous C-allele carriers (IRR= 0.66. 95% CI: 0.45-0.98). There was interaction between the studied MDR1 polymorphisms and intake of red and processed meat in relation to CRC risk. Homozygous MDR1 C3435T Callele carriers were at 8% increased risk pr 25 g meat/day (CI: 1.00-1.16) whereas variant allele carriers were not at increased risk (p for interaction = 0.02). There was interaction between NSAID use and MDR1 C3435T and COX-2 T8473C (p-values for interaction 0.001 and 0.04, respectively). Use of NSAID was associated with an increased risk among homozygous wild type allele carriers (MDR1 C3435T: IRR=2.34 (1.22-4.48) and COX-2 T8473C: IRR=1.38 (0.89-2.14), but not among variant allele carriers. Conclusions: Two polymorphisms in MDR1 were associated with higher risk of CRC and there was interaction between the polymorphisms and meat intake in relation to CRC risk.