Association between polymorphisms of TAS2R16 and susceptibility to colorectal cancer

Colorectal cancer (CRC) is the third most commonly diagnosed cancer worldwide [1]. Several environmental/life style risk factors have been identified for CRC, such as obesity, sedentary behavior, high red meat consumption, high intake of hyper caloric foods, diets rich in fat and poor in fiber, tobacco smoking, alcohol consumption and diabetes [2]. In addition gut inflammation, due to chronic conditions such as ulcerative colitis and Crohn’s disease, is an established risk factor [2]. Also genetics is hypothesized to play an important role for the sporadic form. In the last 10 years genome-wide association studies (GWAS) have identified over 40 independent low penetrance polymorphic variants [3]. Besides GWAS, an overwhelming number of hypothesis-driven studies carried out on specific candidate genes highlighted variants belonging to multiple pathways to be connected to CRC risk. The majority of the studies focused on SNPs within DNA repair genes [4], xenobiotic metabolism and transport genes [5‐14], inflammatory response genes [15‐17], hormone metabolisms [18, 19] and more recently miRNAs [20, 21]. Our group and others have also analyzed the possible involvement of taste receptor genes in CRC risk [22‐24]. In the human genome, there are 25 functional genes and 11 pseudogenes belonging to the TAS2Rs gene family. It has already been ascertained that polymorphic variants in these genes are responsible for the variability observed in several human phenotypes and life habits such as alcohol consumption [25, 26], and nicotine dependence [27]. However, taste receptors seem to have also less obvious functions. In fact taste receptors certainly do not only have a role in taste perception, as documented by their ectopic expression in a variety of extra-oral tissues including gut [28‐31]. In this study we focused our attention on the genetic variability of the TAS2R16 gene whose encoded receptor selectively binds to salicin [32], a natural anti-inflammatory and analgesic agent extracted from willow bark and resembling aspirin [33]. Given the similarities between salicin and aspirin it is reasonable to postulate that variants within the TAS2R16 gene could affect the receptor activity following its binding with ligands such as salicin or aspirin. In particular, aspirin intake is an established factor reducing the risk to develop CRC. In fact, there are evidences from case-control and cohort studies indicating benefit from long term use of aspirin at low dose in colorectal cancer chemoprevention [34]. This effect could be elicited through an anti-inflammatory activity or to the binding to specific receptors in the gut. Furthermore, there are indications that regular use of aspirin is associated with improved survival for patients with colorectal cancer [35]. The hypothesis is corroborated by a previous study (Campbell and colleagues) conducted on two different African populations, describing that the polymorphic variant rs846664 within TAS2R16 confers different sensitivities to salicin [36]. Unfortunately, this finding could not be replicated among Caucasians since this SNP is monomorphic in this population. However, according to this base of knowledge, it could be hypothesized that TAS2R16 could modulate chronic inflammation in the gut. Alternatively, the receptor, upon binding with its natural ligands, could affect the individual risk of cancer with mechanisms similar to those observed following a long term aspirin intake. Thus, in the present work we tested whether polymorphic variants within TAS2R16 could affect the risk of developing CRC. The study was carried out in a large cohort of European individuals.

To further expand our knowledge on CRC genetic susceptibility we investigated whether polymorphic variants within TAS2R16 could have a role in the etiology of the disease. Our hypothesis finds its rationale on several epidemiologic evidences that polymorphic variants within taste receptor genes could be associated with risk of cancer development. For instance TAS2R38 SNPs were found to be associated with risk of developing CRC in two different populations of Caucasian origin [23], while SNPs within TAS2R38 and TAS1Rs were associated with increased risk of gastric cancer [44, 45]. In this study we examined the genetic variability of TAS2R16 gene because the receptor binds different compounds including salicin, a natural anti-inflammatory substance which is very similar to aspirin [32, 33]. One of the strongest risk factors for CRC is represented by chronic inflammation especially arisen from diseases that trigger a continuous inflammatory responses such as ulcerative colitis and Crohn’s disease [2]. Aspirin treatment reduces the risk of CRC [2] and therefore genetic variants that modulate the efficiency or the expression of the receptor may play a role in CRC development. Since polymorphisms in taste genes are generally functional as reflected in their effect in a multitude of phenotypes, it is reasonable to hypothesize a possible link between TAS2R16 allelic variants and CRC risk. To perform our analysis 6 SNPs tagging the common variants present in the region of the gene under examination were chosen. The selection was mainly based on linkage disequilibrium scores between all the variants of the region. Our results seem to suggest no association between the selected polymorphism and risk of developing CRC. The signal closest to the significance threshold of P < 0.05 was observed for rs1525489. For this SNP stratifying the analysis for country of origin we found in the Lithuanian and in the Spanish sub-populations a tendency for individuals with at least 1 C allele of the rs1525489 polymorphism to have and increased risk of developing CRC (P 0.047 and P = 0.051, respectively). However, these associations were not statistically significant following Bonferroni’s correction. A possible explanation for this association is that this SNP interact with a lifestyle factor or a dietary habit a factor that we do not account for in the analysis. However, this difference maybe just due to statistical fluctuations. Schembre and colleagues have performed a study investigating two TAS2R16 polymorphic variants (rs846672 and rs846664) in relation to the risk of developing colorectal adenoma [24]. Even though the study is rather small (914 cases of three different ethnic groups) their results are concordant with ours, suggesting an overall no effect of the two variants in the disease. The rs846664 SNP is monomorphic in Caucasian and therefore was not typed in our study while rs846672 is in complete LD with rs860170 that was used in the present study (r² = 1 in European Hapmap Ceu). When analyzing separately colon and rectum, we observed an association between the minor allele of the rs1525489 and an increased risk of developing specifically rectal cancer. This association was showed to have a marginal/borderline statistically significance also after Bonferroni’s correction (P = 0.0071). However, this finding must be taken with caution since it is the result of a stratified analysis, on a limited sample size, and it is also difficult to explain from a biological point of view. In fact, the bioinformatics tools did not reveal any possible functional effect for this SNP nor for the variants in LD with it. We acknowledge that the present study has several limitations. We could not collect information on aspirin use and therefore we could not analyze any possible interaction with the SNP analyzed. Moreover, cases and controls are not individually matched for age and gender, however we have adjusted all the analyses for these two variables to minimize their possible effect. We had a power greater than 80% (considering an alpha of 0.05/6 = 0.008) to find an association with an OR of 1.25 or higher for all the SNPs in analysis with the exception of rs1525489 that had a low MAF and for which we had the power to find OR greater than 1.53. An OR of 1.25 is compatible with the ORs found by GWAS in CRC [3]. Using a tagging approach, we have covered most of the common variability of the gene region excluding the possibility of having missed an association with a common but un-typed variant.