| Background and ObjectivesColorectal cancer (CRC) is one of the most commonly diagnosed cancer types worldwide, with an estimation of more than1.36million new cases and693,881deaths to have occurred in2012. The incidence rates of colorectal cancer are increasing rapidly in a number of countries historically at low risk, including Czech, Japan and China. In Shanghai for instance, the incidence rates of colorectal cancer in both males and females increased by50%from1987to2002.Environmental risk factors as well as acquired and constitutional genetic variations are considered as predominant contributors in the etiology of colorectal cancer. Modifiable risk factors include dietary factors and lifestyle-related factors such as dietary intake, physical activities, smoking and moderate-to-heavy alcohol drinking. In addition to these environmental factors, genetic variations of colorectal cancer-related genes contribute substantially to the risk as well. Regulation of protein synthesis plays a pivotal role in cellular homeostasis, proliferation and malignant transformation. Phosphorylation of eukaryotic translation initiation factor2(eIF2) by eIF2kinases is a potent rate-limiting step of translational control. Disease may arise when dysregulation of protein synthesis occurs. Recently, growing evidence has indicated that deregulation of eIF2and eIF2kinases contributes importantly to the development of various types of malignancies, including colorectal cancer.In the current study, we hypothesized that genetic variations in eukaryotic translation initiation pathway genes may be associated with an individual’s susceptibility to colorectal cancer. We conducted a population-based case-control study in Jiashan County to explore the associations between single nucleotide polymorphisms (SNPs) in these genes and risk of colorectal cancer as well as gene-environment interactions. We also searched publicly available microarray databases to explore the expression level of these genes in colorectal cancer tissue relative to controls.Materials and MethodsThe study population consists of subjects enrolled in a population-based case-control study conducted in Jiashan County from May1990to May2010. The study was approved by the Medical Ethical Committee of Zhejiang University School of Medicine. Patients with colorectal cancer were identified from the Cancer Surveillance and Registry System established since1989in Jiashan County, Zhejiang Province. Eligible cases were incident and histologically confirmed colorectal cancer, living in the study geographic area, mentally competent to complete the interview and with no previous diagnosis of familial adenomatous polyposis, ulcerative colitis or Crohn’s disease. Healthy controls with no previous history of cancer were recruited in parallel from the same population and were matched to cases by age (±5years), gender and residential area. Before participation, written informed consent was obtained from all the study subjects. Face-to-face interviews were conducted by trained interviewers, who administered a structured questionnaire relating to demographic characteristics (e.g., age, gender, former body mass index (BMI), marital status, occupation and education level), family history of cancer, menopausal history, and life style (e.g., diet, smoking, alcohol consumption and tea consumption). After interview,2ml blood sample was collected into sodium citrate anticoagulant tubes and stored at-80℃for DNA isolation. A total of554cases and566controls with DNA samples available for subsequent analyses were included in the current study. Genomic DNA was isolated from peripheral blood samples for each study subject using the modified salting-out procedure. Single nucleotide polymorphisms were selected and genotyped using the SNPscan Genotyping System.All statistical analyses were performed using SAS9.2, unless otherwise noted. Differences in the distribution of demographic and lifestyle characteristics between cases and controls were evaluated using Student t-test for continuous variables and Pearson χ2test for categorical variables. Departures from Hardy-Weinberg equilibrium (HWE) for all SNPs were tested among controls using goodness-of-fit/2analysis. SNPs not in HWE (P<0.01) were exclued from subsequent analyses. Odds ratios (ORs) with95%confidence intervals (CIs) and two-sided P values were determined for each SNP-disease association by uncoditional univariate and multivariate logistic regression models. The covariants included in the model were:age (continuous), gender, former BMI, marital status, education level, occupation, smoking status, alcohol consumption and tea consumption. Linkage disequilibrium (LD) and haplotype analysis was performed using the SHEsis software and R.13.0Haplo.Stat1.4.3package. Potential gene-environment interactions were explored using stratified analysis. We also searched the public microarray database Oncomine to further explore the expression level of these genes in colorectal cancer tissue relative to controls.ResultsCompared with controls, colorectal cancer cases in our study were less likely to be farmers (P=0.044) and currently married (P=0.006), but no other demographic characteristics differed significantly between cases and controls. Tea consumption was associated with a decreased risk of colorectal cancer (OR=0.563,95%CI:0.399-0.793). Similar results were obtained for both colon cancer (OR=0.626,95%CI:0.408-0.960) and rectal cancer (OR=0.507,95%CI:0.335-0.767), respectively.Twenty-six SNPs were selected and susccefully genotyed. Two SNPs (rs2235596in EIF2S2and rs4390116in EIF2S3) not in HWE (P<0.01) were excluded from further analyses. The missense mutation of EIF2AK3rs13045was associated with an increased risk of colorectal cancer under the dominant (CT+TT vs. CC:OR=1.324,95%CI:1.005-1.743) or the co-dominant model (CT vs. CC:OR=1.398,95%CI:1.042-1.876). The homozygous variant genotype of rs2254958located in the5’-UTR of EIF2AK2was associated with an increased risk of colorectal cancer (GG vs. AA:OR=1.922,95%CI:1.067-3.461). When stratified by cancer site, the association was more profound between rs2254958and risk of colon cancer (GG vs. AA:OR=3.059,95%CI:1.593-5.872), but no statistical evidence of association was observed for rectal cancer. In addition, the variant allele of rs17136342in EIF2AK1was associated with a decreased risk of colon cancer (GA vs. GG:OR=0.448,95%CI:0.232-0.866; GA+AA vs. GG:OR=0.466,95%CI:0.242-0.894). EIF2AK4haplotype CGAGA was associated with increased rectal cancer risk (OR=1.354,95%CI:1.089-1.684), while CGAGG (OR=0.693,95%CI:0.530-0.907), AGGAG (OR=0.545,95%CI:0.303-0.982) and AAAGA (OR=0.551,95%CI:0.312-0.976) were associated with reduced risk of rectal cancer. Stratified analyses revealed significant gene-environment interactions on colorectal cancer risk. Moreover, public microarray expression data mining showed except that EIF2AK3exhibiting decreased expression in colorectal cancer tissue, EIF2S1, EIF2S2, EIF2S3, EIF2AK1, EIF2AK2as well as EIF2AK4showed elevated expression levels in colorectal cancer tissue compared with controls (P<0.001).ConclusionsThe current study explored the associations between genetic polymorphisms in eurokaryotic translation initiation pathway genes and risk of colorectal cancer. The findings of our study are as follows:(1) EIF2AK3rs13045was associated with an increased risk of colorectal cancer under the dominant or the co-dominant model. The homozygous variant genotype of rs2254958located in the5’-UTR of EIF2AK2was associated with an increased risk of colorectal cancer. The variant allele of rs17136342in EIF2AK1was associated with a decreased risk of colon cancer. Haplotypes of EIF2AK4were associated with altered risk of rectal cancer.(2) Significant gene-environment interactions were observed between genetic polymorphisms in eurokaryotic translation initiation pathway genes and smoking, alcohol drinking and tea drinking on colorectal cancer risk.(3) Except that EIF2AK3exhibiting decreased expression in colorectal cancer tissue, EIF2S1, EIF2S2, EIF2S3, EIF2AK1, EIF2AK2as well as EIF2AK4showed elevated expression levels in colorectal cancer tissue compared with controls. In summary, the findings from our population-based genetic association analysis provide the first evidence of a link between genetic variations in eIF2kinases and colorectal carcinogenesis. Confirmation studies with larger sample size and further mechanistic investigations into the function of these biomarkers are warranted to advance our understanding of their roles in colorectal tumorigenesis, and to aid in the development of novel and targeted therapeutic strategies. |
