The present study intended to crosslink susceptibility variants of nutrition-related complex diseases to CRC. In fact, the results in the Czech hospital-based case–control study suggested that polymorphisms in AGT, CYP3A7 and ENPP1 may be associated with the risk of CRC. However, replication in the population-based German DACHS population did not confirm the associations.
From the 246 SNPs that have been reported to be associated with a nutrition-related disease, 130 showed ancestral susceptibility to overall risk of obesity, T2D, metabolic syndrome or hypertension. However, only 29 SNPs fulfilled the initial selection criterion of ≥45% allele frequency difference between the YRI and any HapMap population, indicating selective pressure. Except ABCA1 that has been found to be mutated in CRC tumour samples [38, 39], none of the 15 genes of the present study has previously been associated with the risk of CRC (http://www.ncbi.nlm.nih.gov/; http://www.hugenavigator.net/CancerGEMKB/home.do) .
The association study in the Czech population indicated ancestral susceptibility to the risk of CRC for the missense AGT SNP rs699 and to the 3’UTR SNP rs10211 in CYP3A7. Interestingly, SNPs in these two genes feature similar phenotypic effects, such as predisposing to hypertension and salt sensitivity .
Published data about AGT suggests that the ancestral allele of the probable pathogenic SNP rs699 (M268T), as well as the ancestral alleles of the missense SNP rs4762 (T207M) and the 5′UTR SNP rs5051, predispose to essential hypertension, increased plasma angiotensinogen and increased frequency of preeclampsia (OMIM ID: 189800) [18, 41, 42]. Additionally, rs5051 (r2=0.95 to rs699) has been demonstrated to affect the transcription rate of AGT[41, 42]. AGT (angiotensinogen [serpin peptidase inhibitor, clade A, member 8]) is an important member of the renin-angiotensin system that regulates blood pressure and fluid homeostasis probably through influencing sodium sensitivity [41, 42].
Also the intronic CYP3A5 SNP rs776746 has previously been associated with hypertension and salt sensitivity [9, 18]. This SNP has been reported to result in an incorrectly spliced mRNA and in a truncated non-functional protein. In the Czech population, rs776746 was monomorphic. However, the CYP3A7 SNP rs10211 - also located within the same cytochrome P450 gene cluster and linked to rs776746 in the HapMap CEU population (r2=0.82) - showed nominally significant association with the risk of CRC. The genes of the cytochrome P450 gene family encode for some of the most important enzymes involved in the metabolism of various xenobiotics and endogenous substrates such as cholesterol, steroids, environmental carcinogens and drugs . In particular, CYP3 enzymes are responsible for the metabolism of eicosanoids.
The allele frequencies of the genotyped SNPs rs699 and rs10211 and the two functional SNPs rs5051 and rs776746 are highly variable among worldwide populations, with higher frequencies of the derived alleles in non-African populations while the ancestral alleles predominate in the African population. The values of |iHS| determined for SNPs that are fully linked to rs699 and rs10211 provided conclusive evidence for natural selection in the population with European ancestry. Additionally, rs2687075 (r2=1.0 to rs10211) in CYP3A7 showed strong negative values of Fay-Wu’s H that were considered as signatures for a selective sweep in non-African populations [12, 15]. Previous studies have already suggested AGT and CYP3A5 as targets of selection, and have additionally connected the two genes directly by their related function [18, 42]. In AGT, selection was particularly suggested to work on the promoter that contains rs699 and on SNPs in high LD with it. This selection was attributed to altered requirements for the human to maintain sodium homeostasis . Since the derived allele, that predisposes to salt tolerance, is not yet fixed in non-African populations, the remaining ancestral allele, that predisposes to salt sensitivity, shows ancestral susceptibility to related diseases such as hypertension, preeclampsia [41, 42] and CRC.
A possible association with the risk of CRC and signatures of recent selection were also observed for one polymorphism in ENPP1. However, in contrast to the initial hypothesis, the ancestral allele of rs1044498 was associated with a decreased risk of CRC in the Czech population. ENPP1 is a member of the ecto-nucleotide pyrophosphatase/phosphodiesterase (ENPP) family. The encoded protein interacts with the insulin receptor thereby inhibiting subsequent signalling. In previous studies, the ancestral allele of the missense polymorphism rs1044498 (Q121K) has been associated with more vivid insulin receptor binding, stronger inhibition of insulin signalling, insulin resistance, an increased risk of T2D and an increased risk of myocardial infarction (OMIM ID: 608446) [9, 44, 45]. The associations were most pronounced in cohorts that underwent lifestyle interventions to improve an individual’s weight or cholesterol level . It is possible that the effect of ENPP1 on the risk of metabolic syndrome and the risk of CRC is highly dependent on additional environmental factors or modifiers.
Unfortunately, we were not able to validate our results in the independent German case–control study. Since the Czech and the German populations should not differ significantly in their genetic constitution, differences in nutrition or other environmental factors may contribute to the observed results or the associations may be a chance finding [46, 47]. Since the selection of candidate SNPs was based on complex gene-environmental interactions - with the SNP contributing to a phenotype that predisposes to CRC - the detected associations are expected to be weaker than the associations for the original intermediate phenotype. Already in the original reports about the associations of SNPs with nutrition-related diseases, low ORs were detected in most of the cases. As we studied only a few polymorphisms per gene, other polymorphisms with low LD (r2<0.8) or rare SNPs (MAF<0.05) that may contribute to the risk of CRC might have been missed. However, considering previously reported associations of several SNPs in the three described genes with components of the metabolic syndrome, the ancestral nature of the risk alleles, and the detected signatures of selection, a true nature of the modest effects on CRC risk in the Czech population cannot be excluded . Especially the close resemblance of the detected associations and function of SNPs in AGT and CYP3A7 may indicate a true effect of the polymorphisms on CRC susceptibility.