| [Background]Esophageal cancer and gastric cancer are two common gastrointestinal cancers worldwide.The majority of GC is gastric adenocarcinoma,whereas EC consists of two histopathological types,esophageal squamous cell carcinoma（ESCC）and esophageal adenocarcinoma（EAC）.ESCC is predominant around the world especially in China,whereas EAC subtype is a major type in the European and American countries.The anatomic location of esophageal cancer is similar to gastric cancer,and the risk factors are both specific and common.Tobacco use was associated with increased risk of both ESCC and EAC.Alcohol consumption is a specific risk factor for ESCC,whereas gastroesophageal reflux disease,obesity,and Barrett’s esophagus were associated with increased susceptibility of EAC.The known environmental risk factors for GC are Helicobacter pylori infection,smoking,obesity,low intake of fresh fruits and vegetables,and high consumption of salted foods.Genome-wide association studies（GWAS）have been conducted to explore genetic variants influencing the susceptibility of EC and GC over the past few decades.A missense mutation located in PLCE1 named rs2274223 was found to be associated with risk of ESCC and gastric cancer.we hypothesized that the genetic basis of developing EC and GC might have something in common.To test whether single EC risk variant or cumulative genetic risk score computed using established EC risk loci were also associated with GC risk,we utilized risk loci reported in the published EC GWAS and tested whether they were associated with GC in our large case-control studies.[Methods]（1）We searched the GWAS catalog for genetic variants associated with EC risk.Besides,we searched Pub Med database for recently published EC GWAS.The reported risk loci for esophageal cancer were screened according to the following criteria:1)the significant level of Association reached 5.00×10-8;2)the allele frequency（MAF）in Chinese population was not less than 1%.（3）There was no significant linkage disequilibrium（r2>0.1）among the genetic variation loci.For loci with linkage disequilibrium,the genetic variation loci with the lowest P value were included.Three GWAS of gastric cancer originated from Nanjing（NJ-GWAS）,Beijing（BJ-GWAS）and the National Cancer Institute（NCI-GWAS）.For GWAS of gastric cancer,after carrying out the basic standardized quality control,we filled GWAS with the reference of the Chinese Han population of 1000 Genomes Project Phase III.After filling,we further excluded the SNPs with poor filling quality（filling score<0.3）,and restored the focus of standardized quality control procedures.Finally,7192774 loci of 550 cases of NJ-GWAS and 1155 cases of control,7192774 loci of456 cases of BJ-GWAS and 1118 cases of control,7641942 loci of 1625 cases of NCI-GWAS and 2100 cases of control passed quality control.（2）We analyzed the association between single locus of esophageal cancer,cumulative effect of genetic locus of esophageal cancer and the risk of gastric cancer with weighted genetic score（WGS）.Genetic association analysis was conducted by using logistic regression models.When dealing with association between single locus and GC risk,we assumed an additive genetic model in logistic regression.For GC risk-associated variants,we estimated the cumulative effect based on the number of risk alleles.We included the WGS in the logistic regression model both as a continuous variable and categorical variable.For the Nanjing and the Beijing GWAS,we adjusted for age,sex,smoking,drinking status,and principal component analysis（PCA）for population stratification and for the NCI GWAS,we adjusted for age,sex and PCA in the regression models.Meta-analysis was used to combine results from the three GWAS,and Cochran’s Q was used for heterogeneity test.Fixed-effect model was applied to assume the combined effect,whereas random effect model was repeated if I2（calculated by 100%×（Q–（n–1））/Q）was>75%.Haplo Reg website was used to annotate the candidate gastric cancer associated loci and their genetic variation loci with high linkage imbalance（r2>0.6）.SIFT,Poly Phen-2 and Clin Var were used to predict the function of exon loci.（3）The Cancer Genome Atlas（TCGA）database was used to analyze the difference of gene expression between gastric cancer and paracancerous tissue,and between esophageal cancer and paracancerous tissue.Two-sample t-test or paired t-test were used to analyze the difference of gene expression between gastric cancer and normal esophageal tissues.Using the data of 415 cases of total transcriptome of gastric cancer in TCGA database,we analyzed the gene groups with significant co-expression relationship with candidate target genes.Finally,we selected the genes corrected by FDR（P<0.01）and enriched the KEGG pathway of co-expression genes by R package of"cluster Profiler".[Results]（1）Nine GWAS（eight studies for ESCC,one study for EAC）were included in our current study.A total of 21 SNPs（18 SNPs for ESCC,3 SNPs for EAC）associated with EC in genome-wide association studies were evaluated with association of GC risk.Genetic variants of rs2274223 in PLCE1 at 10q23.33(per G allele:odds ratio（OR）=1.26,95%confidence interval（CI）:1.16-1.38,P=6.51×10-8),rs10052657 in PDE4D at 5q11.2(per C allele:OR=1.12,95%CI:1.01-1.25,P=3.28×10-2)and rs671 in ALDH2 at 12q24.12(per A-allele:OR=0.83,95%CI:0.75-0.91,P=1.14×10-4)were significantly associated with GC risk.The combination of the three loci（rs2274223,rs10052657 and rs671）showed a significant dose-response relationship with the risk of gastric cancer.With the increase of the number of risk alleles,the risk of gastric cancer increased significantly(OR=1.31,95%CI:1.19-1.44,P=2.34×10-8).（2）Weighted genetic scores（WGS）for esophageal squamous cell carcinoma（ESCC）,esophageal adenocarcinoma（EAC）,and overall EC were analyzed with GC risk,respectively.We found that the EC WGS was significantly associated with increased risk of GC(OR=1.15,95%CI:1.06-1.25,P=1.20×10-3 for continuous WGS and OR=1.08,95%CI:1.03-1.13,P=9.11×10-4 for trend test for WGS categories).This association was mainly restricted to ESCC(OR=1.16,95%CI:1.07-1.27,P=5.52×10-4for continuous WGS and OR=1.09,95%CI:1.04-1.14,P=2.71×10-4 for trend test for WGS categories)but not to EAC（OR=1.02,95%CI:0.92-1.13,P=0.66 for continuous WGS and OR=0.99,95%CI:0.95-1.04,P=0.70 for trend test for WGS categories）.（3）Functional annotation of three candidate gastric cancer related sites showed that the sites with high linkage imbalance were mainly located in the intron regions of PLCE1 and NOC3L.For rs10052657,the sites with high linkage imbalance were located in the intron regions of PDE4D,and rs78423157 loci coexisted.The enrichment of promoter,enhancer and DHS,while rs671 located in 12q24.12 region,has five other sites with high linkage imbalance.rs671 itself is a missense mutation of exon of ALDH2（Glu>Lys）.We further analyzed the differences in the expression levels of the three susceptibility genes related to gastric cancer risk（rs2274223,rs10052657 and rs671）in esophageal and gastric cancers and adjacent tissues.PDE4D and ALDH2 were downregulated in both ESCC and GC tissues.However,we did not observe differential expression for PLCE1 in either ESCC or GC tissues.Pathway enrichment analysis showed that PDE4D were mainly enriched in Ras pathway,c GMP-PKG pathway,PI3K-Akt pathway and T-cell receptor signaling pathway,while those associated with ALDH2 were mainly enriched in glyceride metabolism and fatty acid metabolism.[Conclusions]This study explored the relationship between the genetic locus of esophageal cancer and the risk of gastric cancer.We found that the genetic locus located in the regions of chromosome 10q23.33,5q11.2 and 12q24.12 may affect the risk of ESCC and gastric cancer at the same time.The genetic locus of ESCC has a combined effect with the risk of gastric cancer.The results of this study further confirmed that gastric cancer and ESCC have a common genetic basis,and their genetic susceptibility is similar,which provides new evidence for the identification of high-risk groups of upper gastrointestinal cancer. |
